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Review Large-Scale Piezoelectric-Based Systems for More Electric Aircraft Applications

Tran Vy Khanh Vo 1 , Tomasz Marek Lubecki 1, Wai Tuck Chow 2, Amit Gupta 1 and King Ho Holden Li 2,*

1 Rolls-Royce@NTU Corporate Lab, Nanyang Technological University, Singapore 637460, Singapore; [email protected] (T.V.K.V.); [email protected] (T.M.L.); [email protected] (A.G.) 2 School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore; [email protected] * Correspondence: [email protected]; Tel.: +65-6790-6398

Abstract: A new approach in the development of aircraft and aerospace industry is geared toward increasing use of electric systems. An electromechanical (EM) piezoelectric-based system is one of the potential technologies that can produce a compactable system with a fast response and a high power density. However, piezoelectric materials generate a small strain, of around 0.1–0.2% of the original actuator length, limiting their potential in large-scale applications. This paper reviews the potential amplification mechanisms for piezoelectric-based systems targeting aerospace applications. The concepts, structural designs, and operation conditions of each method are summarized and compared. This review aims to provide a good understanding of piezoelectric-based systems toward selecting suitable designs for potential aerospace applications and an outlook for novel designs in the near future.



 Keywords: piezoelectric stack; amplification mechanism; quasi-static stepped system; ultrasonic

Citation: Vo, T.V.K.; Lubecki, T.M.; system; piezoelectric-hydraulic; aerospace applications Chow, W.T.; Gupta, A.; Li, K.H.H. Large-Scale Piezoelectric-Based Systems for More Electric Aircraft Applications. Micromachines 2021, 12, 1. Introduction 140. https://doi.org/10.3390/ A concept of more/all-electric aircraft has recently received huge attention in the mi12020140 research and development work in the field of aerospace engineering [1–7]. The intent is to use more electrical systems in aircraft and aerospace applications to bring an impact on Academic Editor: Jose the environment [8]. With the fast development of electrification, more researchers and Luis Sanchez-Rojas manufacturers are shifting to this dynamic trend involving a high demand for increasing Received: 24 December 2020 the load, improving fuel efficiency, reducing emissions, and lowering the total cost of oper- Accepted: 22 January 2021 Published: 28 January 2021 ation. Researchers seek different approaches and technologies to broaden this fashionable concept in a wide range of applications. The choice of actuators in the aircraft is based on

Publisher’s Note: MDPI stays neutral various critical factors, such as power density, reliability, efficiency, control features, and with regard to jurisdictional claims in thermal robustness, as well as the weight, size, and maintenance cost. In a commercial published maps and institutional affil- aircraft, actuators are essential in various applications, such as flight control, engine starter, iations. landing system, brake actuation, and fuel pump [9,10]. The specifications of actuators in an aircraft vary across a wide range. Typical requirements can be listed as 1–320 kN of force, 10–700 mm of stroke, and 10–500 mm/s of speed, with the requirement of both modulated and two-position control methods [4,11]. For these actuation systems in the aircraft engine, the working temperature is from −50 to 150 ◦C at the engine intake; and it is higher for the Copyright: © 2021 by the authors. actuators located toward the high-pressure compressor void (300–400 ◦C) or the tail cone Licensee MDPI, Basel, Switzerland. ◦ This article is an open access article area (500–600 C) [12]. Overall, actuators in an aircraft require both the advantages of mate- distributed under the terms and rials that allow them to deliver the required power in extreme environmental conditions conditions of the Creative Commons and the optimal structural designs to maximize their performance within a constrained Attribution (CC BY) license (https:// weight and space. creativecommons.org/licenses/by/ In the development of signal-by-wire and power-by-wire actuators in aircraft, elec- 4.0/). tromechanical (EM) systems have seen a huge improvement, with significant results from

Micromachines 2021, 12, 140. https://doi.org/10.3390/mi12020140 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, 140 2 of 28 Micromachines 2021, 12, x FOR PEER REVIEW 2 of 29

both researchers and manufacturers. Electrical actuators, which have taken advantage of In the development of signal-by-wire and power-by-wire actuators in aircraft, elec- state-of-the-arttromechanical (EM) motors systems and have power seen screws, a huge improvement, are among these with systems significant [13 results–17]. Thefrom electrical actuatorsboth researchers could and provide manufacturers. a load range Electrical of up actuators, to 90 kN wh, withich have over taken 90% advantage efficiency, of making themstate-of suitable-the-art motors for replacing and power several screws, conventional are among these hydraulic systems or [13 fueldraulic–17]. The electrical systems in the jetactuators engine could [18, 19provide]. These a load systems range bringof up moreto 90 kN advantages, with over in90% terms efficiency, of a compactmaking design (eliminatingthem suitable for pipes replacing and heavy several elements) conventional and hyd power-to-weightraulic or fueldraulic ratio systems (weight in the reducing), enhancingjet engine [18,19]. aircraft These stability systems and bring thus more providing advantages the ability in terms to incorporateof a compact more design functions within(eliminating thecontrol pipes and system heavy toelements) further and enhance power- aircraftto-weight utility. ratio (weight Besides reducing), electrical en- actuators, smart-material-basedhancing aircraft stability actuators and thus are providing also considered the ability a to promising incorporate approach. more functions The develop- mentwithin of the smart control materials, system to such further as enhance piezoelectric aircraft materials utility. Besides [20], shapeelectrical memory actuators, alloys [21], smart-material-based actuators are also considered a promising approach. The develop- magnetostrictive materials [22], and electroactive polymers [23,24], also offers advantages ment of smart materials, such as piezoelectric materials [20], shape memory alloys [21], in the aerospace applications [25,26]. Looking beyond the potential of replacing the con- magnetostrictive materials [22], and electroactive polymers [23,24], also offers advantages ventionalin the aerospace system applications with similar [25,26]. or even Looking better beyond performance the potential actuators, of replacing the smart the con- behavior of suchventional materials system may with offersimilar more or even room better for performance the development actuators, ofnovel the smart systems. behavior For of example, thesuch shape-changing materials may offer ability more of room smart for materials the development can be exploredof novel systems. in morphing For example, aircraft [27,28]. Shapethe shape memory-changing alloy-based ability of smart [29,30 materials] and piezoelectric-based can be explored in morphing bender designs aircraft [ [27,28].31] can be used forShape noise memory reduction alloy- whenbased mounting[29,30] and the piezoelectric bender on-based the bendertrailing designs edge of [31] the can jet enginebe fan nozzleused for and noise the reduction rotor of when the helicopter, mounting the respectively. bender on the Each trailing material edge respondsof the jet engine differently to thefan nozzle stimuli, and and the various rotor of the actuation helicopter, modes respectively. can be achieved Each material with responds distinct differently working concepts andto the geometrical stimuli, and designs. various actuation Among them, modes piezoelectric can be achieved materials with distinct have shownworking great con- potential incepts aircraft and geometrical and spacecraft designs. applications Among them, [32– 36piezoelectric]. The definition materials of have piezoelectric shown great materials is thatpotential they in can aircraft either and generate spacecraft an applications output voltage [32–36]. when The definition subjected of piezoelectric to mechanical ma- stress or terials is that they can either generate an output voltage when subjected to mechanical perform a dimensional change when subjected to an electric field. These phenomena are stress or perform a dimensional change when subjected to an electric field. These phe- knownnomena asare direct known and as direct indirect and modesindirect ofmodes operation, of operation, which which can becan used be used for for generators gen- [37], sensorserators [37], [38], sensors and actuators [38], and [actuators39]. Piezoelectric [39]. Piezoelectric materials materials have the have advantages the advantages of high power density,of high power high efficiency,density, high driving efficiency, force, driving and displacement force, and displacement resolution resolution over electromagnetic over materials.electromagnetic They materials. also do not They generate also do electromagneticnot generate electromagnetic noise and arenoise nonflammable and are non- [40–42]. Piezoelectricflammable [40– materials42]. Piezoelectric come inmaterials different come forms, in different such asforms, sheet, such wafer as sheet, plate, wafer stack, fiber, andplate, composite, stack, fiber, whichand composite, makes themwhich suitablemakes them for suitable diverse for geometrical diverse geometrical designs. de- Despite a minimalsigns. Despite strain a minimal capability, strain piezoelectric capability, actuatorspiezoelectric can actuato deliverrs highcan deliver power high outputs power with high efficiencyoutputs with due high to theirefficiency ability due to to be their cycled ability at veryto be highcycled frequencies at very high as frequencies compared as with other actuatorscompared [with43] (Figureother actuators1). [43] (Figure 1).

Figure 1. 1. PowerPower-to-weight-to-weight ratio ratio versus versus efficiency efficiency from froma database a database of 220 actuators of 220 actuators (from [43] (from). [43]).

Some piezoelectric materials can work in a very large temperature range, making them more promising in aircraft applications. A report from NASA revealed positive results of four piezoelectric ceramics, namely PZT-4, PZT-5A, PZT-5H, and PLZT-9/65/36, from sev- eral tests to evaluate their applicability as sensors and actuators in the intelligent aerospace Micromachines 2021, 12, 140 3 of 28

system over a large temperature range, from −150 to 250 ◦C[44]. More efforts on material development were recorded that would gradually enhance the potential of piezoelectric materials in high-temperature industrial applications [45,46]. Therefore, piezoelectric-based systems are possible for applications located in the cold section of the aircraft engine, in which the temperature varies from −50 to 250 ◦C. However, amplification methods are required to generate sufficient stroke for these applications. The specifications of suitable applications for a compact piezoelectric design should be in the range of up to 5 kN of force, 100 mm of stroke, and 50 mm/s of speed. Thus the high stress and working frequency of piezoelectric actuators can be advantageous within a compact system. Some applications could be variable blow-in doors, booster bleeds, variable inlet guide vanes (IGVs), and variable stator vanes (VSVs). For instance, a piezoelectric-based linear actuator with a crank-slider mechanism was proposed to drive the IGV, which helps to control the flow that enters the jet engine and to improve the efficiency of the compressor [47,48]. Sufficient stroke of the actuator is accumulated over repeated cycles. For the same application (IGV or VSV of the gas turbine jet engine), the piezoelectric system could also be designed in such a way that a rotary motion can deliver directly to the application [49]. This actuator can be mounted on the unison ring, thus eliminating the need for other mechanical structures that add extra weight to the system. Moreover, the ability of power-off holding position of piezoelectric materials allows a design that can maintain the last controlled position in the event of failure, thus enhancing the safety level in aircraft applications. This review paper focuses on the potential of piezoelectric-based systems for large- scale applications in the aircraft and aerospace industry. The structure of this review is as follows: Section2 presents a brief overview of piezoelectric fundamentals, piezoelectric stacks, and classification of amplification methods. The subsequent four sections review the amplification methods for piezoelectric. Section3 introduces direct amplification mech- anisms to produce continuous motion. In Section4, the quasi-static stepped actuators are reviewed and are divided into three concepts: inchworm, inertial, and walking. Section5 reviews the ultrasonic actuators, where the resonant mode of piezoelectric is used. In Section6 , a different approach is described as the piezoelectric stack is coupled with hy- draulic fluid in a pump to power the hydraulic cylinder. Section7 summarizes the reviewed piezoelectric-based systems and discusses their potential in aerospace applications. Finally, the conclusion and future outlook are presented in the last section.

2. Piezoelectric Actuators 2.1. Fundamentals of Piezoelectric Materials The piezoelectric effect on ceramic materials was discovered in 1880 by Nobel laureates Pierre and Jacques Curie. A piezoelectric transducer can be used as both generator [50] and actuator [51]. Specifically, the direct piezoelectric effect is used in the generator, while the indirect piezoelectric effect is used for the actuator [52]. The direct piezoelectric effect refers to the development of electrical charges on applications of mechanical stress, and vice versa (indirect piezoelectric effect). For the actuation applications reviewed in this paper, the piezoelectric material deforms with the applied electric field to produce mechanical energy. The most commonly used piezoelectric materials are piezoelectric ceramic, such as lead zirconate titanate (PZT), barium titanate (BaTiO3), and lead titanate (PbTiO3). With a polycrystalline structure, ceramic materials can be fabricated into a variety of shapes and sizes. Besides, with the effort to reduce and avoid lead (Pb) in piezoelectric materials, lead-free piezoelectric development has been gaining momentum in recent years [20,53–55]. Some of these materials are alkali-metal-based bismuth sodium titanate (BNT), bismuth potassium titanate (BKT) [56], and potassium sodium niobate (KNN) [57]. To increase the potential of piezoelectricity in various working conditions, high-temperature piezoelectric materials have been developed, such as Pb(NbO3)2 and Bi4Ti3O12 [45]. However, the strain and stress of these materials may be reduced. In general, piezoelectric materials have a very small strain, of 0.1–0.2%, but with high stress, in the range of 100–131 MPa. Their specific power density is around 1000 kW/kg, and they have high efficiency, of Micromachines 2021, 12, 140 4 of 28

more than 80% [40,43,58]. However, piezoelectric materials experience some drawbacks, such as substantial hysteresis [59], temperature-dependent properties [60], and fracture behaviors [61]. These phenomena eventually affect the performance of the piezoelectric materials, especially the stroke and accuracy of piezoelectric actuators. The performance of the piezoelectric actuator is determined by the material properties known as the electromechanical coefficients. The most common material properties are the directional piezoelectric charge constants. The mechanical strain (S) of a piezoelectric material can be found by the relation

S = sET + dE (1)

where sE is the compliance or elasticity coefficient, T is the mechanical stress, d is the piezoelectric charge constant, and E is the electric field (E = Φ/t, where Φ is the applied voltage and t is the thickness of the material). The coupling coefficient of the piezoelectric can be divided into three groups corre- sponding to the orientations of the electric field and the displacement. These coefficients are d33, d31, and d15, corresponding to three deformation modes: longitudinal, transversal (Figure2a), and shear modes (Figure2b), respectively. In general, the strain and electrome- chanical conversion efficiency are higher in the longitudinal direction [62,63]. Therefore, this deformation mode is usually used in actuators, especially in the stacked configuration. Table1 below shows examples of some piezoelectric materials and their properties that are commonly used in the piezoelectric-based system.

Table 1. Examples of some properties of piezoelectric materials.

d at Room Curie Temp. T Operating Temp. Materials 33 C Reference Temp. (pC/N) (◦C) (◦C) PZT powder 590–610 - - [20] PMN-PT 2000–3500 120–130 Up to 80 [42,60] PZN-PT 1900–2000 160 Up to 110 [42,64] PZT-5H 585 170 −150–125 [44] PZT Navy Type III <300 305 Up to 220 [45] (Hard) 1 PZT-4 225 310 −150–100 [44] PZT Navy Type II <600 340 Up to 200 [45] (Soft) 2 PZT-5A 350 350 Up to 250 [44] PIC series 240–500 160–370 −40–150 [65] Lead-free materials BTBK 58.9–117 170–223 - [56] BNT 91 320 - [56] KNN 80–160 Up to 400 - [57] High-temperature materials

Pb(NbO3)2 81 550 Up to 300 [45]

Bi4Ti3O12 3.5 675 Up to 675 [45]

Bi4Ti2.86Nb0.14O12 20 655 Up to 655 [45] 1 Hard: less hysteresis loss, good stability under high mechanical loads, and operating field strength; thus good for ultrasonic transducers. 2 Soft: large hysteresis loss, large piezoelectric charge coefficient, and easy polarization at low field strength; thus ideal for actuator and sensor applications. Micromachines 2021, 12, x FOR PEER REVIEW 5 of 29

1 Hard: less hysteresis loss, good stability under high mechanical loads, and operating field strength; thus good for ultrasonic transducers. 2 Soft: large hysteresis loss, large piezoelectric charge coefficient, and easy polarization at low field strength; thus ideal for actuator and sensor Micromachines 2021, 12, 140 5 of 28 applications.

Transversal mode Shear mode

Longitudinal E E mode

(a) (b)

Figure 2. DeformationFigure 2. Deformation of the piezoelectric of the piezoelectric actuator. actuator. (a) Longitudinal (a) Longitudinal and and transversal mode; mode; (b) shear(b) mode. shear mode. Piezoelectric elements can come in different geometrical forms, such as thin plate, single layer, multilayer, torsion tube. Besides these designs, macro piezo fiber composite Piezoelectric(MFC) is elements another form can ofcome piezoelectric in different that wasgeomet inventedrical byforms, NASA such back as in thin the 1990s plate, and single layer,has multilayer, been commercialized torsion tube. by Smart Besides Material these since designs, 2002 [66 macro,67]. Constructed piezo fiber of composite piezoelectric- (MFC) is anotherceramic-based form of fibers piezoelectric (usually PZT that 5A was or PZT invented Navy Type by NASA II) sandwiched back in between the 1990s electrodes and has been commercializedand polyimide layers, by Smart MFCs Material can produce since elongation, 2002 [66,67] contraction,. Constructed and bending of piezoelec- motions for tric-ceramicactuation-based fibers [68]. They (usually can also PZT function 5A or PZT in sensitive Navy Type sensor II) and sandwiched vibration harvesting between elec- applica- trodes and tionspolyimide [69,70]. layers, With a MFCs flexible can nature, produce these elongation, piezoelectric contraction, composites have and greater bendingdurability mo- and reliability and can be attached to the surface of or embedded inside the structures. tions for actuation [68]. They can also function in sensitive sensor and vibration harvesting They have been proposed to be used in various aerospace applications, such as aircraft applicationshealth [69,70]. structure With monitoring, a flexible nature, noise and these vibration piezoelectric control of composites helicopter rotors, have and greater surface durability controland reliability of morphing and wings can be [67 attached,69,71]. MFCs to the can surface find more of aerospaceor embedded applications inside ifthe high- structures. temperatureThey have piezoelectricbeen proposed (see Tableto be1 ),used electrode, in various and adhesive aerospace materials applications, are explored such [ 72 ]. as aircraft Tablehealth2 belowstructure summarizes monitoring, the performance noise and vibration of commercial control piezoelectric of helicopter actuators rotors, with and surfacesome control typical of geometricalmorphing wings forms. [67,69,71]. MFCs can find more aerospace appli- cations if high-temperature piezoelectric (see Table 1), electrode, and adhesive materials Table 2. Typical displacement and resonant frequency of typical geometrical forms of commercial piezoelectric actuators. are explored [72]. Table 2 below summarizes the performance of commercial piezoelectric Formactuators with some typical Typical geometrical Size forms. Displacement Range Resonant Frequency A few hundred micrometer Single layer (wafer) Up to 0.1 µm Up to 100 kHz Table 2. Typical displacement and resonant frequencythickness of typical geometrical forms of commercial piezoelectric actuators. Multilayer extension Up to 100 mm2 area & 100 mm Form Typical Size Displacement UpRange to 100 µmResonant Up to Frequency 100 kHz and more (rectangle, round, hollow stacks) in length A few hundred micrometer Up to ∼ Single layer (wafer)Multilayer shear ing Up to 0.1 μUpm to 10 µmUp Up to to 100 100 kHzkHzand more thickness250 mm 2area & 50 mm length

Multilayer extensionPiezo bender 2 Up to 100 mm area<1 mm& 100 thickness 10 µm to 2 mm Up to a few kilohertz (rectangle, round,(unimorph/bimorph) hollow Up to 100 μm Up to 100 kHz and more mm in length stacks) Macrofiber composite Up to 140 mm active length Up to 150 µm 1 From kIlohertz to megahertz (elongation)Up to ~250 mm2 area & 50 Multilayer shearMacrofibering composite Up to 10 μm Up to 100 kHz and more mm lengthUp to 170 mm active length Up to 100 µm 2 From kIlohertz to megahertz (contraction) Piezo bender (unimorph/bi- <1 mm thickness1 Free strain: up to 1050 ppm.10 2 μFreem strain:to 2 upmm to −600 ppm. Up to a few kilohertz morph) Macrofiber composite Up to 140 mmThe piezoelectricactive actuator can be powered by a periodic voltage with sinusoidal, Up to 150 μm 1 From kIlohertz to megahertz (elongation) sawtooth,length or rectangle waveforms. Depending on the required movements, each piezoelec- tric mechanism requires a customized input signal with a particular pattern, amplitude, Macrofiber composite Up to 170 mm active and frequency to maximizeUp its to performance. 100 μm 2 In the eventFrom ofkIlohertz more than to one megahertz piezoelectric (contraction) elementlength being involved in the design, the phase difference of the controlled signal of each 1 Free strain:piezoelectric up to 1050 actuator ppm. 2 needsFree strain: to be designedup to −600 precisely ppm. to obtain the coupling performance. The sinusoidal, square/rectangle, and sawtooth waveforms are commonly used in the stepped-motion piezoelectric system (Table3). Power consumption during the operation

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The piezoelectric actuator can be powered by a periodic voltage with sinusoidal, saw- tooth, or rectangle waveforms. Depending on the required movements, each piezoelectric mechanism requires a customized input signal with a particular pattern, amplitude, and frequency to maximize its performance. In the event of more than one piezoelectric ele- MicromachinesMicromachinesMicromachines 2021 2021, 12,, x 122021 FOR, x ,FOR 12PEER, x PEER FOR REVIEW PEERREVIEW REVIEW 6 of 629 of 296 of 29 ment being involved in the design, the phase difference of the controlled signal of each piezoelectric actuator needs to be designed precisely to obtain the coupling performance. The sinusoidal, square/rectangle, and sawtooth waveforms are commonly used in the ThesteppedThe piezoelectric piezoelectricThe -piezoelectricmotion actuator actuatorpiezoelectric actuator can can be poweredbe can systempowered be powered by ( Tableaby periodic a periodicby 3 ).a periodicPower voltage voltage consumption voltagewith with sinusoidal, withsinusoidal, sinusoidal, during saw- saw- th saw-e operation tooth,tooth, ortooth,of rectangle or piezoelectric rectangle or rectangle waveforms. waveforms. actuators waveforms. Depending Depending is directlyDepending on theon proportional therequired on required the required movements, movements,to the movements, capacitance each each piezoelect eachpiezoelect of piezoelect theric ricdevice ric by the mechanism requires a customized input signal with a particular pattern, amplitude, and mechanismmechanismrelation requires shown requires a in customized Table a customized 3. input input signal signal with witha particular a particular pattern, pattern, amplitude, amplitude, and and frequencyfrequencyfrequency to maximizeto maximize to maximize its performance.its performance. its performance. In theIn theevent In event the of event moreof more of than more than one thanone piezoelectric piezoelectricone piezoelectric ele- ele- ele- mentment beingment being involved being involved involved in thein thedesign, in design,the design,the thephase phase the difference phase difference difference of theof thecontrolled of controlled the controlled signa signal of signal eachof eachl of each Table 3. Typical input signals and related power consumption for piezoelectric actuators. piezoelectricpiezoelectricpiezoelectric actuator actuator actuator needs needs to needs be to designedbe to designed be designed precisely precisely precisely to obtain to obtain to the obtain thecoupling coupling the coupling performance. performance. performance. Micromachines 2021, 12, 140 6 of 28 Sinusoidal WaveformTheThe sinusoidal, The sinusoidal, sinusoidal, square/rectangle, square/rectangle, square/rectangle,Square/Rectangl and and sawtooth andsawtoothe Waveform sawtooth waveforms waveforms waveforms are are commonly commonly are Sawtooth commonly used used in Waveform used the in the in the steppedsteppedstepped-motion-motion- motionpiezoelectric piezoelectric piezoelectric system system (systemTable (Table 3 ().Table Power3). Power 3). consumption Power consumption consumption during during thduringe th operatione operation the operation of piezoelectricof piezoelectricof piezoelectric actuators actuators actuators is directly is directly is directlyproportional proportional proportional to theto thecapacitance to capacitance the capacitance of theof thedevice of device the bydevice bythe the by the relationrelationof relationshown piezoelectric shown inshown Table in Table actuators in 3. Table 3. is 3. directly proportional to the capacitance of the device by the relation shown in Table3. TableTable 3. TableTypical 3. Typical 3. input Typical input signals input signals and signals and related related and power related power consumption power consumption consumption for piezoelectricfor piezoelectric for piezoelectric actuators. actuators. actuators. Table 3. Typical input signals and related power consumption for piezoelectric actuators. 푃 = 휋푓퐶푉 2 푃 = 푓퐶푉 2 2 SinusoidalSinusoidalSinusoidal Waveform Waveform Waveform푝푝 /4 Square/RectanglSquare/RectanglSquare/Rectangle Waveforme Waveforme Waveform푝푝 SawtoothSawtoothSawtooth Waveform Waveform푃 Waveform= 푓퐶푉 푝푝 / √3 Sinusoidal Waveform Square/Rectangle Waveform Sawtooth Waveform where 푓 is the working frequency, 퐶 is the capacitance of the piezoelectric, and 푉푝푝 is the applied peak to peak voltage. The piezoelectric heat dissipation is usually 10% of the power supplied to the load. Therefore, the selection of usage pie- zoelectric materials and operating conditions must be weighed against the consumed power to ensure that the system’s power budget is optimized. √ P =2 f CV2 22 P = f CV2 22 2 = 22 2 2 푃 =푃휋푓퐶= 휋푓퐶푃푉=푉π휋푓퐶/4 /4푉pp /4/4 푃 =푃푓퐶=푉푓퐶푃푝푝=푉 pp푓퐶 푉 푃P=푃푓퐶f= CV푉푓퐶푃pp=푉//푓퐶√33/푉 √3 /√3 푝푝 푝푝 푝푝 2.2. Piezoelectric Stacks 푝푝 푝푝 푝푝 푝푝 푝푝 where f is the working frequency, C is the capacitance of the piezoelectric, and V is the applied peak to peak voltage. The piezoelectric wherewhere 푓where is푓 the is theworking푓 is working the workingfrequency, frequency, frequency, 퐶 is퐶 the is thecapacitance퐶 is capacitance the capacitance of the of thepiezoelectric, of piezoelectric, the piezoelectric,pp and and 푉푝푝 푉andis푝푝 the is푉 푝푝theapplied is applied the peakapplied peak to peak peakto peak voltage. to peakvoltage. voltage. heat dissipation is usually 10% of the powerPiezoelectric supplied to the mat load.erials Therefore, can the be selection stacked of usage together piezoelectric and materials be sandwiched and operating between electrode TheThe piezoelectric piezoelectricTheconditions piezoelectric heat must heat dissipation be dissipation weighedheat dissipation againstis usually is theusually is consumed 10% usually 10% of powerthe of10% thepower toof power ensure the supplied power that supplied the supplied system’sto the to theload. power to load. the Therefore, budget load.Therefore, is Therefore, optimized. the theselection selection the selection of usage of usage ofpie- usage pie- pie- zoelectriczoelectriczoelectric materials materials materials and and operating operatingand operating conditionslayers conditions conditionsto must achieve must be mustweighedbe a weighed higher be weighed against againststroke the against the consumedfor consumed theactuator consumed power power applications to power ensure to ensure to that ensure [73,74].that the thesystem’sthat system’s Adoptingthe system’s the name of powerpower budgetpower budget is budget optimized. is optimized. is optimized. 2.2.the Piezoelectric manufacturing Stacks method, they are known as the piezoelectric stack or the multilayer piezoelectricPiezoelectric (Figure materials 3). can Piezoelectric be stacked together stacks and and be sandwichedpiezoelectric between actuators electrode are manufactured 2.2.2.2. Piezoelectric Piezoelectric2.2. Piezoelectric Stacks Stacks Stacks layersand developed to achieve a higherby various stroke forcompanies, actuator applications such as Physik [73,74]. AdoptingInstrumente the name (PI), of Tokin Corpora- the manufacturing method, they are known as the piezoelectric stack or the multilayer Piezoelectriction,PiezoelectricPiezoelectric Cedrat mat matTechnologies,erialserials mat canerials can be stackedbe can PiezoDrive,stacked be stackedtogether together togetherPiezoMotor, and and be sandwiched beand sandwiched be Piezosystem sandwiched between between betweenJena, electrode electrode and electrode CTS Corpora- layerslayers topiezoelectriclayers achieveto achieve to achievea (Figurehigher a higher a3 stroke).higher Piezoelectric stroke for stroke foractuator actuator stacksfor actuator applications and applications piezoelectric applications [73,74]. [73,74]. actuators [73,74].Adopting Adopting are Adopting manufactured the thename name the of name of of andtion. developed Usually, by the various length companies, of the stack such is as limited Physik Instrumente to 150 mm (PI), and Tokin the Corporation, area is less than 225 mm2. the themanufacturing manufacturingthe manufacturing method, method, method, they they are they areknown knownare knownas theas thepiezoelectric as piezoelectricthe piezoelectric stack stack or stack theor themultilayer or multilayerthe multilayer CedratThe commercial Technologies, piezoelectric PiezoDrive, PiezoMotor, stack usually Piezosystem offers a Jena,stroke and range CTS Corporation.from several micrometers piezoelectricpiezoelectricpiezoelectric (Figure (Figure (Figure3). Piezoelectric3). Piezoelectric 3). Piezoelectric stacks stacks and stacks and piezoelectric piezoelectricand piezoelectric actuators actuators actuators are aremanufactured manufactured are manufactured Usually,to a hundred the length micrometers of the stack is limited(longitudinal to 150 mm mode)and the and area a is blocked less than force 225 mm range2. The from a hundred andand developed developedand developed by byvarious various by variouscompanies, companies, companies, such such as such Physikas Physik as InstrumentePhysik Instrumente Instrumente (PI), (PI), Tokin (PI),Tokin Corpora- Tokin Corpora- Corpora- commercialto a few thousand piezoelectric newtons stack usually. The offers size aand stroke shape range of from a piezoelectric several micrometers stack tocan a be customized tion,tion, Cedrathundred tion,Cedrat Technologies,Cedrat Technologies,micrometers Technologies, PiezoDrive,(longitudinal PiezoDrive, PiezoDrive, PiezoMotor, mode) PiezoMotor, andPiezoMotor, a Piezosystem blocked Piezosystem Piezosystem force Jena, range Jena, and from Jena, and CTS a hundred CTSand Corpora- CTSCorpora- to Corpora- tion.tion. Usuala tion.toUsual few ly,generate Usualthousand thely, thelengthly, lengththe thenewtons of requiredlength the of thestack. Theof stack the force sizeis limitedstack is and limitedand is shape tolimited stroke. 150 to of 150 mm a to piezoelectric In mm150 andthe mmand thecase theandarea stackof area theislarge less can areais less bethanstroke is customized than less 225 applications,than225 mm mm2252. 2.mm 2. an am- TheThe commercial tocommercialTheplification generate commercial piezoelectric the mechanismpiezoelectric required piezoelectric stack force isstack preferred.usually and stack usually stroke. usuallyoffers offers In a thestrokeoffers a stroke case arange stroke of range large from range strokefrom several from several applications, micrometersseveral micrometers micrometers an to ato hundred a hundredamplificationto a hundred micrometers micrometers mechanism micrometers (longitudinal (longitudinal is preferred. (longitudinal mode) mode) and mode) and a blocked aand blocked a blocked force force range force range from range from a hund from a hundred a hundred red to ato few a fewto thousand a thousandfew thousand newtons newtons newtons. The. The size. sizeThe and andsize shape shapeand of shape a of piezoelectric a piezoelectricof a piezoelectric stack stack can stack can be customizedbe can customized be customized to generateto generateto generate the therequired required the required force force and force and stroke. stroke.and Instroke. theIn thecase In case the of largecaseof1 large layer ofstroke large stroke applications, stroke applications, applications, an am-an am- an am- plificationplificationplification mechanism mechanism mechanism is preferred. is preferred. is preferred.

N layers 1 layer1 layer1 layer

N Nlayers layersN layers

FigureFigure 3. 3.The The electrical electrical connection connection of an N -layersof an N piezoelectric-layers piezoelectric stack. stack.

The stroke (∆L) of the stack is scaled with the number of stacking layers (Equation (2)), FigureFigure 3.whileFigure The 3. The theelectrical 3. outputelectrical The electrical connection force connection (Fb )connection is of related an of Nan to- layers ofN the -anlayers active Npiezoelectric-layers piezoelectric area ofpiezoelectric the stack. piezoelectric stack. stack. actuators (Equation (3)).

∆L = Vpp × d33 × N (2)

Fb = Vpp × d33 × YA/L0 (3)

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Micromachines 2021, 12, x FOR PEER REVIEW The stroke (Δ퐿) of the stack is scaled with the number7 of of 29stacking layers (Equation (2)),

while the output force (F푏) is related to the active area of the piezoelectric actuators (Equation (3)). The stroke (Δ퐿) of the stack is scaled with the number of stacking layers (Equation (2)), Δ퐿 = 푉푝푝 × 푑33 × 푁 (2) while the output force (F푏) is related to the active area of the piezoelectric actuators (Equation (3)). F푏 = 푉푝푝 × 푑33 × 푌퐴/퐿0 (3) Δ퐿 = 푉 × 푑 × 푁 Micromachines 2021, 12, 140 푝푝 33 (2) 7 of 28 where, 푑33 is the piezoelectric constant (longitudinal mode), 푁 is the number of stack- (3) ing layers, 푌 isF the푏 = 푉modulus푝푝 × 푑33 × of푌퐴 the/퐿0 piezoelectric material, 퐴 is the area of each layer, and

where, 푑33 is the퐿where, 0piezoelectric is thed33 isinitial the constant piezoelectric thickness (longitudinal constant of each (longitudinal mode), layer. 푁The mode),is theforce numberN isand the numberstroof stack-ke ofof stacking the piezoelectric stack are ing layers, 푌 is theunderlayers, modulusY anis theelectrical of modulus the piezoelectric load of the (applied piezoelectric material, voltage material, 퐴 is the 푉푝푝A areais), theand of area each the of layer, mechanical each layer, and and loadL0 is is shown in 퐿0 is the initial thicknessFigurethe initial 4 of. thickness Wheneach layer. ofthe each Thepiezoelectric layer. force The and force stro stacks andke strokeof theare ofpiezoelectric implemented the piezoelectric stack stackin are a arecyclic under process, they will be under an electricalan load electrical (applied load voltage (applied 푉 voltage), and theVpp ),mechanical and the mechanical load is shown load isin shown in Figure4 . subjected to a severe 푝푝hysteresis characteristic affected by the frequency and magnitude of Figure 4. When theWhen piezoelectric the piezoelectric stacks stacks are implemented are implemented inin a acyclic cyclic process, process, theythey will will be be subjected to a severe hysteresis characteristic affected by the frequency and magnitude of the applied subjected to a severethe appliedhysteresis voltage. characteristic Therefore, affected closedby the frequency-loop control and magnitude is required of to compensate for the hys- teresisvoltage. Therefore,effect in closed-loopprecise positioning control is required applications to compensate [59,75]. for the hysteresis effect in the applied voltage.precise Therefore, positioning closed applications-loop control [59,75 is]. required to compensate for the hys- teresis effect in precise positioning applications [59,75].

Figure 4. Force–stroke characteristic of the piezoelectric stack. Figure 4. Force–stroke characteristic of the piezoelectric stack. 2.3. Classification Figureof Amplification 4. Force Methods–stroke characteristic of the piezoelectric stack. 2.3. Classification of Amplification Methods The microstroke range of a stand-alone piezoelectric stack can be further amplified 2.3. ClassificationThe microstroke of range Amplification of a stand-alone Methods piezoelectric stack can be further amplified to to the required levelthe required using external level using amplifiers, external amplifiers, such as suchmechanical, as mechanical, hydraulic, hydraulic, or other or other kinetic kinetic mechanisms,mechanisms, dependiThe microstrokeng depending on the architecture. on range the architecture. of Severala stand Several conceptual-alone conceptual piezoelectric designs designs are pro- arestack proposed can be further amplified posed and used, toandsuch the used, as required the such amplified as the level amplified mechanism using mechanism external by the bycompliant amplifiers, the compliant structure, such structure, theas inch-mechanical, the inchworm hydraulic, or other worm mechanism,mechanism, the walking the mechanism, walking mechanism, and the andhybrid the hybridelectro electro-hydraulic-hydraulic system. system. In In this kinetic mechanisms, depending on the architecture. Several conceptual designs are pro- this paper, the amplificationpaper, the amplification methods are methods divided are into divided four into groups, four groups, as shown as shown in Figure in Figure 5. 5. Each Each technique willposedtechnique be discussed and will used, be discussedin subsequent such inas subsequent the sections. amplified sections. mechanism by the compliant structure, the inch- worm mechanism, the walking mechanism, and the hybrid electro-hydraulic system. In this paper, the amplification methods are divided into four groups, as shown in Figure 5. Each technique will be discussed in subsequent sections.

Figure 5. Summary of amplificationFigure 5. methods.Summary of amplification methods.

Figure 5. Summary of amplification methods.

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Micromachines 2021, 12, x FOR PEER REVIEW 8 of 29

Micromachines 2021, 12, 140 8 of 28 3. Continuous Motion 3. ContinuousThe small strokeMotion can be amplified instantly once the piezoelectric element is acti- vated. Compliant mechanisms, cantilever, X-frame mechanism, and unimorph/bimorph 3. ContinuousThe small Motion stroke can be amplified instantly once the piezoelectric element is acti- configurationsvated. Compliant could mechanisms, be classified cantilever, under this X- frame group. mechanism, They are capable and unimorph/bimorph of generating a The small stroke can be amplified instantly once the piezoelectric element is activated. smoothconfigurations and continuous could be motion classified with underless friction this group. and zero They backlash are capable in a compact of generating design. a Compliant mechanisms, cantilever, X-frame mechanism, and unimorph/bimorph config- However,smooth and there continuous are trade -motionoffs between with less the frictionoutput andforce, zero the backlashoverall stiffness, in a compact and thedesign. re- urations could be classified under this group. They are capable of generating a smooth sponseHowever, speed there for theare displacement.trade-offs between The output the output stroke force, is defined the overall by the stiffness, input stroke and thefrom re- and continuous motion with less friction and zero backlash in a compact design. However, thesponse piezoelectric speed for element the displacement. and the amplification The output ratiostroke of is the defined amplifier. by the The input amplification stroke from there are trade-offs between the output force, the overall stiffness, and the response speed ratiothe piezoelectricis usually limited element to tens and due the to amplification geometrical ratioconstraints. of the amplifier.Besides, the The relative amplification size of thefor piezoelectric the displacement. element The should output not stroke be istoo defined large as by it the would input lead stroke to an from oversized the piezoelectric system. elementratio is usually and the limited amplification to tens due ratio to of geometrical the amplifier. constraints. The amplification Besides, the ratio relative is usually size of the Thesepiezoelectric amplification element methods should notcan be generate too large a asmoderate it would stroke lead to or an increase oversized the system. input strokelimited from to tens the duepiezoelectric to geometrical element constraints. for other Besides,designs. the The relative commercial size of amplified the piezoelectric piezo- elementThese should amplification not be too methods large as it can would generate lead toa anmoderate oversized stroke system. or increase the input electricstroke actuatorfrom the of piezoelectric Cedrat Technologies element for company, other designs. for example, The commercial could generate amplified a stroke piezo- of Thesemm amplification methods can generate a moderate stroke or increase the input upelectric to 1 actuator. This ofstroke Cedrat is 10 Technologies times or more company, larger than for example,that of a stand could- alonegenerate piezoelectric a stroke of stackstroke in from a load the-free piezoelectric condition element [33,76]. for For other a broader designs. stroke The commercial range, the stepped amplified-motion piezo- electricup to 1 actuatormm. This of stroke Cedrat is Technologies 10 times or more company, larger for than example, that of coulda stand generate-alone piezoelectric a stroke of mechanisms would offer better solutions [39]. upstack to 1 inmm a load. This-free stroke condition is 10 times [33,76]. or more For a larger broader than stroke that of range, a stand-alone the stepped piezoelec--motion mechanisms would offer better solutions [39]. 3.1.tric Compliant stack in a Mechanism load-free condition [33,76]. For a broader stroke range, the stepped-motion mechanisms would offer better solutions [39]. 3.1. TheCompliant micron Mechanism stroke of a piezoelectric stack can be amplified by up to thousands of micrometers3.1. CompliantThe micron by Mechanism a compliant stroke of mechanisma piezoelectric [77 –stack79]. These can be mechanisms amplified bycan up be constructedto thousands by of rigidmicrometers Thearms micron with by flexure a stroke compliant hinges of a piezoelectricmechanism (Figure 6a [77) stackor– 79].by canthin Thesebe arms mechanisms amplified (Figure by 6b can up). The be to constructed thousandspiezoelectric ofby stackmicrometersrigid applies arms with bya mechanical a flexure compliant hinges force mechanism (toFigure the system [6a77)– or79 ].byand These thin cau mechanismsarmsses the (Figure elastic can6b deformation). be The constructed piezoelectric of the by compliantrigidstack arms applies mechanism. with a mechanical flexure As hinges a result,force (Figure to the the output6 a)system or by motion and thin cau arms is generatedses (Figure the elastic6 withb). Thedeformation an piezoelectricamplification of the Δ푌/Δ푋 ratiostackcompliant of applies mechanism. a mechanicaleither in As a perpendiculara force result, to thethe systemoutput direction andmotion to causes ( Figureis generated the 6 elastic) or in with deformationthe ansame amplification direction of the (compliantFigureratio of 7) Δas푌 mechanism. the/Δ푋 piezoelectric either in As a a perpendicular result,stroke. theOn outputamplifying direction motion the to isstroke,(Figure generated the 6) outputor with in the anforce same amplification decreases. direction ratio(Figure of ∆7)Y as/∆ theX eitherpiezoelectric in a perpendicular stroke. On amplifying direction tothe (Figure stroke,6 )the or output in the same force directiondecreases. (Figure7) as the piezoelectric stroke. On amplifying the stroke, the output force decreases.

(a) (b) Figure 6. The output is perpendicular(a) to the input: (a) rhombus type with rigid arms and flexure(b) hinge; (b) bridge type with thin arms. FigureFigure 6. 6.The The output output is is perpendicular perpendicular to to the the input: input: (a(a)) rhombus rhombus type type with with rigid rigid arms arms and and flexure flexure hinge; hinge; ( b(b)) bridge bridge type type withwith thin thin arms. arms.

(a) (b)

FigureFigure 7 7.. TheThe output output is is in in the the (samea same) dire directionction as as the the input: input: (a) ( alever) lever mechanism mechanism with with a rigid a rigid arm arm and(b) and a flexure a flexure hinge; hinge; (b) (Xb-) frame/scissorsX-frame/scissorsFigure 7. The mechanismoutput mechanism. is in. the same direction as the input: (a) lever mechanism with a rigid arm and a flexure hinge; (b) X- frame/scissors mechanism.

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The amplifying ratio can also be maximized by modifying the structure of compliant mechanisms. Several designs were proposed and used in applications, such as bridge type Micromachines 2021, 12, 140 [80,81], rhombus type [78,82], Scott Russell type [83,84], and honeycomb type 9[85]. of 28 The amplified piezoelectric actuator can also be used to enhance the input stroke. An optimal geometrical design can be achieved to provide a specific stroke and force. The designs with compliantThe amplifying mechanisms ratio can usually also be offer maximized an amplification by modifying ratio the structureof up to a of few compliant tens. mechanisms.Besides the Severalbridge type, designs which were is proposed commonly and seen used in in the applications, commercial such amplified as bridge piezo- electrictype actuators [80,81], rhombus (Figure type 6), the [78 lever,82], Scott mechanism Russell typeoffers [83 simplicity,84], and honeycomb in design, manufacture, type [85]. andThe assembly amplified among piezoelectric amplification actuator methods can also [86,87]. be used A to simple enhance lever the mechanism input stroke. [42] An (Figureoptimal 7a) geometricalcan also be designmodified can to be an achieved X-frame to mechanism/scissor provide a specific stroke mechanism and force. [88] The designs with compliant mechanisms usually offer an amplification ratio of up to a few tens. (Figure 7b). The output motion is parallel to the stroke input from the piezoelectric stack. Besides the bridge type, which is commonly seen in the commercial amplified piezo- Similarelectric to other actuators compliant (Figure 6mechanisms,), the lever mechanism the lever offersmechanism simplicity trades in design, force for manufac- stroke am- plification.ture, and A assembly design of among two- amplificationstage cantilever methods mechanism [86,87]. Acan simple have lever an amplification mechanism [42 factor] of 30,(Figure with7 aa) finalcan alsostroke be modified of 400 μ tom an, to X-frame be used mechanism/scissor as a printed head mechanism [62]. Another [88] (Figure mechanism,7b) . the Theso-called output high motion-bending is parallel-stiffness to the strokeconnector, input which from the was piezoelectric recently proposed stack. Similar and to com- mercializedother compliant (in 2018), mechanisms, can amplify the lever the mechanismoutput stroke trades to forceby two for– strokethree amplification. times, with Aonly a fractionaldesign ofincrement two-stage in cantilever length [89]. mechanism These compliant can have an mechanisms amplification can factor also of 30,be withintegrated a µ intofinal the strokedesign of to 400 gainm ,a to long be used input as astroke printed for head other [62 ].actuators, Another mechanism, such as inchworm the so-called [90,91], high-bending-stiffness connector, which was recently proposed and commercialized (in inertial [92–94], and piezoelectric-hydraulic pump [86]. 2018), can amplify the output stroke to by two–three times, with only a fractional increment in length [89]. These compliant mechanisms can also be integrated into the design to gain 3.2.a Piezoelectric long input strokeBending for other actuators, such as inchworm [90,91], inertial [92–94], and piezoelectric-hydraulicThe piezoelectric pla pumpte can [ 86be]. used to create bending motion for various applications [95–97]. Figure 8a illustrates a schematic of a unimorph configuration with one piezoelec- 3.2. Piezoelectric Bending tric plate and one passive plate. In other applications, a bimorph configuration can be constructedThe piezoelectric from two piezoelectric plate can be used elements to create bendingto create motion a two for-way various bending applications motion[95– 97 ]. Figure8a illustrates a schematic of a unimorph configuration with one piezoelectric plate (Figure 8b). and one passive plate. In other applications, a bimorph configuration can be constructed from two piezoelectric elements to create a two-way bending motion (Figure8b).

(a) (b)

FigureFigure 8. Schematic 8. Schematic of ( ofa) ( aunimorph) unimorph configuration configuration andand (b(b)) bimorph bimorph configuration. configuration.

TheThe bending bending motion motion from from unimorph unimorph andand bimorph bimorph configurations configurations can can be used be used in the in the inertialinertial piezoelectric piezoelectric actuator actuator [[98]98] or or as as an an active active valve valve of the of piezoelectric the piezoelectric micropump micropump [99]. A combination of a piezoelectric bender and a compliant mechanism was also proposed to [99]. A combination of a piezoelectric bender and a compliant mechanism was also pro- control a helicopter rotor blade [100]. posed toThe control amplification a helicopter methods rotor in blade this section [100]. (Section 3) are usually considered in those applicationsThe amplification where the methods stroke is thein this priority section and (Section space is not 3) are constrained. usually considered For applications in those applicationswhere a higher where stroke the stroke is required, is the other priority amplifiers and space can be is selected not constrained. for a compact For design.applications where a higher stroke is required, other amplifiers can be selected for a compact design. 4. Quasi-Static Stepped Motion 4. QuasiAn-Static output Stepped stroke ofMotion up to centimeters could not be achieved directly from the am- plification methods covered in Section3. It requires a further cumulative effect whereby An output stroke of up to centimeters could not be achieved directly from the ampli- the stroke can be accumulated from microstep motion over repeated cycles. These mecha- ficationnisms methods can be classified covered as in inertial, Section inchworm, 3. It requires and walkinga further concepts. cumulative Theoretically, effect whereby these the strokeamplification can be accumulated concepts could from produce microstep unlimited motion output over motion. repeated However, cycles. inThes practice,e mecha- nismsthe can piezoelectric-based be classified as actuators inertial, usinginchworm, such methods and walking are designed concepts. to haveTheoretically, the output these amplificationmotion in theconcepts range ofcould centimeters, produce with unlimited a maximum output speed motion. of tens However, of millimeters in practice, per the

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Micromachines 2021, 12, 140 10 of 28 piezoelectric-based actuators using such methods are designed to have the output motion in the range of centimeters, with a maximum speed of tens of millimeters per second. For more significant stroke requirements, these stepped-motion actuators could be scaled up. However,second. For they more require significant a suitable stroke housing requirements, structure theseto provide stepped-motion the backbone actuators and support. could Thisbe scaled housing up. structure However, must they not require be oversized a suitable as housingcompared structure to the actuator. to provide As thethe backboneworking principleand support. is mostly This based housing on structurefriction, they must require not be tight oversized tolerance as compared of the structural to the actuator.dimen- As the working principle is mostly based on friction, they require tight tolerance of the sion and the frictional holder. For high-force applications, they are prone to mechanical structural dimension and the frictional holder. For high-force applications, they are prone wear and tear over time. Thus, regular maintenance of such systems is required. The pie- to mechanical wear and tear over time. Thus, regular maintenance of such systems is zoelectric stack is usually driven at a low frequency, of less than 1 kHz. Therefore, these required. The piezoelectric stack is usually driven at a low frequency, of less than 1 kHz. actuators can also be considered as a quasi-static system and are different from the ultra- Therefore, these actuators can also be considered as a quasi-static system and are different sonic system discussed in Section 5. from the ultrasonic system discussed in Section5.

4.1.4.1. Inertial Inertial Concept Concept TheThe inertial inertial actuator isis builtbuilt fromfrom one one moving moving block block (piezoelectric (piezoelectric stacks), stacks), fixed fixed at one at oneend, end, and and one one inertial inertial mass mass acting acting as a as friction a friction element eleme (Figurent (Figure9). The 9). workingThe working principle princi- of plethe of inertial the inertial piezoelectric-based piezoelectric- actuatorbased actuator can be can described be described as follows: as follows: First, the First, piezoelectric the pie- zoelectricstack is controlled stack is controlled to extend slowly. to extend During slowly. this process,During this the frictionprocess, element the friction makes element contact makeswith the contact moving with structure; the moving hence, structure; they are hence, moving they forward are moving together forward due to together the frictional due toforce. the frictional After that, force. the piezoelectric After that, the stack piezoelectric contracts quickly stack contracts to create quickly an impulsive to create force. an Dueim- pulsiveto inertia, force. the Due moving to inertia, structure the cannot moving respond structure to thecannot fast retractionrespond to to the return fast toretraction its original to returnposition to its but original remains p inosition its current but remains place. Asin its a result,current the place. moving As a blockresult, returns the moving to get block ready returnsfor a new to get cycle ready while for the a new moving cycle structure while the is broughtmoving structure forward. Theis brought piezoelectric forward. stack The is piezoelectricpowered by stack a sawtooth is powered wave by input a sawtooth signal to wave create input the requiredsignal to motion.create the This required design mo- can tion.be used This for design long-stroke can be andused high-resolution for long-stroke applicationsand high-resolution [101,102 ]applications by slowly increasing [101,102] byvoltage slowly to increasing drive the piezoelectricvoltage to drive stack. the piezoelectric stack.

FigureFigure 9. 9. SchematicSchematic of of the the inertial inertial piezoelectric piezoelectric actuator. actuator.

TheThe concept concept demonstrated demonstrated in in Figure Figure 99 cancan also also be be c calledalled the the stick stick-and-slip-and-slip mecha- mech- nism.anism. The The roles roles of of piezoelectric piezoelectric stack, stack, friction friction element, element, and and moving moving structure structure are are ex- ex- changeablechangeable [92,103 [92,103––105].105]. In In the the impact impact drive drive mechanism, mechanism, the the position position of of the the moving moving unit unit (piezoelectric(piezoelectric stack stack and and friction friction element) element) changes changes in in each each step, step, while while the the moving moving structure structure isis now now fixed fixed [106]. [106]. Flexure Flexure hinges hinges can can be be introduced introduced to to generate generate the the required required motion motion from from piezoelectricpiezoelectric elements elements [92 [92––94,104].94,104]. Moreover, Moreover, these these mechanisms mechanisms also also increase increase the the effective effective displacementdisplacement of of the the piezoelectric piezoelectric el elementement in in the the designed designed direction. direction. Table Table 4 summarizessummarizes somesome inertial inertial piezoelectric piezoelectric actuators actuators with with both both symmetrical symmetrical and and asymmetrical asymmetrical flexure flexure hinges.hinges. A A design design with with one one piezoelectric piezoelectric stack stack and and asymmetrical asymmetrical flexure flexure hinge hinge can can produce produce aa linear linear motion motion with with a a speed speed o off up up to to 15 15 mm/s mm/s [93]. [93].

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Micromachines 2021, 12, 140 Table 4. Performance of a typical piezoelectric-based actuator. 11 of 28

Flexure Hinge Piezoelectric (mm) Voltage (Vp-p) Frequency (Hz) Force (N) Speed (mm/s) Reference Symmetrical 5 × 5 × 20 Table 4. Performance100 of a typical5000 piezoelectric-based 3.43 1 actuator.(1 Hz) 6.057 [105] (Z shaped) (Two) SymmetricalFlexure Hinge5 Piezoelectric× 5 × 20 (mm) Voltage (Vp-p) Frequency (Hz) Force (N) Speed (mm/s) Reference Symmetrical 5 × 5 × 20 100 1000 1.58 (1 Hz) 7.95 [104] (bridge type) (Two) 100 5000 3.43 1 (1 Hz) 6.057 [105] (Z shaped) (Two) Asymmetrical 5 × 5 × 20 Symmetrical 5 × 5 × 20 100 500 2.94 * 5.96 [92] 100 1000 1.58 (1 Hz) 7.95 [104] (nonparallel(bridge type) type) (One)(Two) AsymmetricalAsymmetrical 5 × 5 × 20 5 × 5 × 20 100 2000 3.43 14.25 [107] (parallelogram(nonparallel type) (One) 100 500 2.94 * 5.96 [92] (One) Asymmetricaltype) Asymmetrical 5 × 5 × 20 (four-bar mecha- 5 × 5 × 20 100 490 4.32 15.04 [93] (parallelogram (One) 100 2000 3.43 14.25 [107] (One) nism) type) 1 Force (N) = weight (kg) × 9.81. Asymmetrical 5 × 5 × 20 (four-bar 100 490 4.32 15.04 [93] (One) mechanism) 4.2. Inchworm Concept The piezoelectric 1inchwormForce (N) = weight creates (kg) stepped× 9.81. motion by mimicking the crawling mo- tion of an4.2. inchworm Inchworm Concept[91,108]. Figure 10a shows the basic principle of the inchworm design with three setsThe of piezoelectric stacked piezoelectric inchworm creates actuators: stepped a moving motion by block mimicking (2) is theextended crawling and motion con- tracted ofto anprovide inchworm the[ 91main,108 ].motion Figure 10toa drive shows the the basicmoving principle structure, of the inchwormwhile two design clamping with blocks (1three and sets 3) ofare stacked engaged piezoelectric and disengaged actuators: with a moving the moving block (2) structure is extended one and at contracted the time. With thisto providegeometrical the main relation, motion the to piezoelectric drive the moving inchworm structure, allows while twoa straightforward clamping blocks de- (1 sign forand each 3) arepiezoelectric engaged and block disengaged to achieve with the the required moving structure level of onepushing at the time.and clamping With this force. geometrical relation, the piezoelectric inchworm allows a straightforward design for each piezoelectric block to achieve the required level of pushing and clamping force.

(a) (b)

Figure 10. FigureSchematic 10. Schematic of the inchworm of the inchworm mechanism mechanism with s withtacked stacked piezoelectric piezoelectric actuators. actuators. (a) (aStandard) Standard inchworm inchworm system system with three withpiezoelectric three piezoelectric stacks: stacks stacks: 1 stacksand 3 1are and to 3 create are to createa clamping a clamping force, force, and andstack stack 2 is 2 to is generate to generate the the movement. movement. ( (b) ImprovementImprovement of the inchworm of the inchworm concept conceptwith various with various clamping clamping blocks blocks and andmoving moving blocks blocks integrating integrating into into the the moving moving structure. Bluestructure. block: Blue inactive block: inactivepiezoelectric piezoelectric (contract); (contract); red block: red block: active active piezoelectric piezoelectric (extend); (extend); blackblack block: block: moving moving structure. struc- ture. The working principle of the inchworm actuator can be explained as follows: First, Theclamping working stack principle 3 extends of the and inchworm clamps the actuator moving structurecan be explained below it. Theas follows: moving stackFirst, clamping(stack stack 2) then3 extends extends and and clamps pushes stackthe moving 3 to move structure forward (towardbelow theit. The right moving side) together stack with the moving structure. Next, clamping stack 1 extends down to clamp the moving (stack 2) then extends and pushes stack 3 to move forward (toward the right side) together structure while stack 3 retracts. Stack 2 contracts to drive stack 1 and the moving structure with thefurther moving toward structure. the right Next, side. clamping After that, stack stack 1 1extends retracts down to release to clamp the structure the moving while structurestack while 3 extends stack 3 to retracts. engage Stack again. 2 These contracts steps to are drive repeated stack to 1 accumulate and the moving small stepsstructure into further towardsignificant the motion. right side. In this After design, that, the stack inchworm 1 retracts unit to (three release piezoelectric the structure stacks) while remains stack 3 extendsthe to same engage while again. the movingThese steps structure are repeated is being pushed to accumulat in onee direction small steps only. into Therefore, signif- icant motion. In this design, the inchworm unit (three piezoelectric stacks) remains the same while the moving structure is being pushed in one direction only. Therefore, this

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design can be called a pusher inchworm. In another modification, called the walking inch- Micromachines 2021, 12, 140 worm, the moving structure is now fixed while the inchworm unit moves, which12 of resem- 28 bles the inchworm crawling on the tree branch. The clamping and moving blocks in inch- worm piezoelectric can be divided into several blocks and arranged along with the mov- ingthis structure, design can as be shown called in a pusher Figure inchworm. 10b These In another piezoelectric modification, blocks called can be the controlled walking to achieveinchworm, the desired the moving performance. structure is now fixed while the inchworm unit moves, which resemblesSeveral theworks inchworm are geared crawling toward on the developing tree branch. the The inchworm clamping and design moving from blocks a geomet- in ricalinchworm design to piezoelectric a control method can be for divided various into targeted several blocks applications and arranged [39,108 along–113]. with The the clamp- ingmoving force can structure, be achieved as shown by ineither Figure the 10 bintermittent These piezoelectric [114–116] blocks or canthe becontinuous controlled mecha- to achieve the desired performance. nism [113]. In the intermittent clamping mechanism, the clamping blocks provide the Several works are geared toward developing the inchworm design from a geometrical clampingdesign toforce a control to the method struct forure various in sequence. targeted In applications contrast, [39the,108 continuous–113]. The clamping clamping force mecha- nismcan maintains be achieved contact by either with the intermittent the structure [114 during–116] or the continuous working process. mechanism Therefore, [113]. In the clampingthe intermittent force varies clamping and mechanism,depends on the the clamping load condition blocks provide for the the intermittent clamping force mechanism, to the whilestructure the continuous in sequence. mec In contrast,hanism the can continuous only generate clamping a constant mechanism clamping maintains force. contact The with clamp- ingthe structure structure can during be a thepiezoelectric working process. stack Therefore,[108,115,116] the clamping, electromagnetic force varies in and nature depends [49,117], an inertialon the load mass condition [118,119], for or the a wedge intermittent-typemechanism, clamping mechanism while the continuous [111,120]. mechanism To increase the velocitycan only of generatethe motion, a constant the stroke clamping of the force. moving The clamping block can structure be amplified can be a piezoelectric using a flexure stack [108,115,116], electromagnetic in nature [49,117], an inertial mass [118,119], or a wedge- mechanism, as mentioned earlier, in Section 3.1 [90,91]. The moving block can also be re- type clamping mechanism [111,120]. To increase the velocity of the motion, the stroke of placedthe movingby a magnetostrictive block can be amplified actuation using with a flexure a similar mechanism, or larger as mentionedstroke (0.2% earlier, strain). in An inchwormSection 3.1 actuator[90,91]. with The moving Terfenol block-D as can a moving also be replaced block and by apiezoelectric magnetostrictive stacks actuation as a clamp- ingwith block a similarcan generate or larger a stall stroke load (0.2% of strain).115 N Anand inchworm a no-load actuator speed of with 2.5Terfenol-D cm/s [121]. as a moving block and piezoelectric stacks as a clamping block can generate a stall load of 115 N 4.3.and Walking a no-load Concept speed of 2.5 cm/s [121].

4.3.The Walking main Conceptdifference between the walking concept and the inchworm concept is that the walking type does not require a moving block. Instead, the moving motion is created The main difference between the walking concept and the inchworm concept is that directlythe walking from the type walking does not legs. require The a movinglegs produce block. Instead,an elliptical the moving motion motion on their is created tips to en- gagedirectly and disengage from the walking with the legs. moving The legs structure produce anin ellipticalsequence motion to driv one theirit further. tips to In engage the walk- ingand concept, disengage the position with the movingof the leg structure structure in sequence remains to the drive same. it further. The number In the walking of legs can varyconcept, depending the position on the ofdesign the leg architecture, structure remains with thea minimum same. The of number two legs of legs[122]. can vary dependingAs shown on in the Figure design 11 architecture,, the working with principle a minimum of the of two walking legs [122 concep]. t is described as follows:As Legs shown 1 and in Figure3 (leg group11, the workingI) come in principle contact of with the walkingthe moving concept structure is described and bend as to thefollows: right to Legs drive 1 andit forward 3 (leg group while I) comelegs 2 in and contact 4 (leg with group the moving II) retract structure and andbend bend to the to left. the right to drive it forward while legs 2 and 4 (leg group II) retract and bend to the left. Next, leg group II extends to make contact with the moving structure; then, they bend to Next, leg group II extends to make contact with the moving structure; then, they bend to thethe right right to todrive drive the the moving moving structurestructure further. further. Leg Leg group group I retracts I retracts and bends and bends to the leftto the to left to repeatrepeat thethe previous previous motion motion of leg of groupleg group II. These II. stepsThese are steps repeated are repeated in sequence in tosequence create to createstepped stepped motions motions of the of moving the moving structure. structure.

FigureFigure 11. 11.SchematicSchematic of of a awalking walking piezoelectric piezoelectric actuator. actuator. The bending movement of the leg could be created by piezoelectric in bending mode [123] (FigureThe bending 12a), V-shape movement configuration of the leg [124 could] (Figure be 12createdb), or combining by piezoelectric the longitudinal in bending and mode [123]shear (Figure motions 12a [),125 V]-shape (Figure configuration 12c). The output [124] force (Figure and speeds 12b of), or these combining designs are the compared longitudinal andin shear Table 5motion. Severals [125] products (Figure using 12c walking). The conceptsoutput force are commercialized and speeds of for these small-scale designs are comparedapplications in Table with the5. Several force range products of around using a few walking hundred concepts newtons, are a designed commercialized stroke of for smallless-scale than 100applicationsmm, and a with speed the of lessforce than range 20 mm of/ saround[126,127 ].a Afew series hundred of PiezoWalk newtons, from a de- signedPhysik stroke Instrumente of less than company, 100 formm example,, and a speed could produceof less than a linear 20 motionmm/s with[126,127]. a velocity A series of of PiezoWalk15 mm/s from (PICMAWalk) Physik orInstrumente a maximum company, blocking force for ofexample, 300 N [126 could]. produce a linear mo- tion with a velocity of 15 mm/s (PICMAWalk) or a maximum blocking force of 300 N [126].

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(a)

(b)

(c)

Figure 12.Figure Designs 12. ofDesigns walking of legs: walking (a) bimorph legs: (a) configuration; bimorph configuration; (b) V-shape (b )configuration; V-shape configuration; (c) com- (c) combi- bination ofnation longitudinal of longitudinal and shear and modes shear of modes piezoelectric. of piezoelectric.

Table 5. ComparisonTable 5. Comparison of the output of the force output and forcespeed and performance speed performance of three commercial of three commercial designs us- designs using ing the walking mechanism. the walking mechanism. Configuration of the Configuration ofOutput the Force Speed Resolution Output Force Speed Resolution PiezoelectricPiezoelectric Stack Stack Bending configura- ~10–20 N/50 g Medium 0.03 nm (open loop) Bending ~10–20 N/50 g Medium 0.03 nm (open loop) tion (NEXACT)configuration (20 mm travel range) Up to 10 mm/s - (20 mm travel range) Up to 10 mm/s - V-shape configura-(NEXACT) ~50–60 N/700 g High 0.02 nm (open loop) tion (PICMAWalk)V-shape configuration (20 mm travel~50–60 range) N/700 Up g to 15 mm/sHigh <10 nm (closed0.02 nm loop) (open loop) Combination of(PICMAWalk) longi- (20 mm travel range) Up to 15 mm/s <10 nm (closed loop) Up to 600 N/1250 g Low 0.03 nm (open loop) tudinal and shear Combination(20 of mm travel range) Up to 1 mm/s 5 nm (closed loop) modes (NEXLINE)longitudinal and Up to 600 N/1250 g Low 0.03 nm (open loop) shear modes (20 mm travel range) Up to 1 mm/s 5 nm (closed loop) Similar to (NEXLINE)the other stepped-motion design, flexure hinges can also be employed to create the bending motion for the walking leg from only one piezoelectric stack. This could reduce the number of piezoelectric elements and the input signal of the system. For exam- Similar to the other stepped-motion design, flexure hinges can also be employed ple, two asymmetrical right-angle flexure hinges could generate a linear motion with a to create the bending motion for the walking leg from only one piezoelectric stack. This motion speed of 39.78 μm/s at a frequency of 20 Hz [128]. could reduce the number of piezoelectric elements and the input signal of the system. For example, two asymmetrical right-angle flexure hinges could generate a linear motion with 5. Ultrasonic Stepped Motion a motion speed of 39.78 µm/s at a frequency of 20 Hz [128]. While the quasi-static stepped-motion designs operate at a low frequency, ultrasonic actuators5. use Ultrasonic piezoelectric Stepped resonant Motion vibrations [129]. Therefore, they are capable of pro- ducing high velocitiesWhile the and quasi-static long-range stepped-motion motions. The ultrasonic designs operateactuator atconsists a low of frequency, a pie- ultra- zoelectricsonic-based actuators stator and use a moving piezoelectric structure resonant (Figure vibrations 13). The stator [129]. produces Therefore, the they elliptic are capable motion toof drive producing the moving high velocitiesstructure andthat long-rangeis similar to motions. the piezoelectric The ultrasonic walking actuator design. consists of a piezoelectric-based stator and a moving structure (Figure 13). The stator produces the

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The elliptic motion could be generated from the tip of each piezoelectric element or the combination motions of the whole stator [130]. Based on these driving methods, the ultra- sonicThe actuator elliptic motioncanelliptic be classified motioncould be to intogenerated drive two the groups: from moving standingthe structure tip of wave each that [131–133]piezoelectric is similar (Figure to element the 13a) piezoelectric orand the walking travelingcombination wavedesign. motions[39,134,135] The of ellipticthe (Figure whole motion 13b).statorcould The [130]. ultrasonic be Based generated on concept these from dri thecouldving tip bemethods, of eachused piezoelectricfor the both ultra- element or linearsonic motion actuator [136]the can combination and be classified rotary motion motions into two [135,137]. of groups: the whole standing stator wave [130]. [131 Based–133 on] (Figure these driving 13a) and methods, the traveling waveultrasonic [39,134,135] actuator (Figure can be 13b classified). The ultrasonic into twogroups: concept standing could be wave used [ 131for– both133]( Figure 13a) linear motionand [136] traveling and rotary wave motion [39,134 [135,137].,135] (Figure 13b). The ultrasonic concept could be used for both linear motion [136] and rotary motion [135,137].

(a) (b)

Figure 13. Working principle of the piezoelectric-based ultrasonic actuators: (a) standing wave; (b) traveling wave. (a) (b) Figure 13. WorkingFigure 13. principle Working of principle the piezoelectric-based of the piezoelectric ultrasonic-based actuators:ultrasonic (actuators:a) standing (a wave;) standing (b) traveling wave; (b wave.) 6. travelingHybrid Piezoelectric–Hydraulicwave. System The piezoelectric6. Hybrid mechanisms Piezoelectric–Hydraulic described in previous System sections always require a trade- off6. between Hybrid forcePiezoelectric andThe stroke. piezoelectric–Hydraulic Therefore, mechanisms System hydraulic described energy may in previous be more sections realistic always for appli- require a trade-off cationsThe requiring piezoelectricbetween both massive force mechanisms and stroke stroke. described and Therefore, force in(in hydraulicprevious the range sections energy of a thousand may always be more requirenewtons realistic a tradeand for - applications more).off be Intween such requiringforce cases, and hybrid stroke. both piezoelectric-hydraulic massive Therefore, stroke hydraulic and force systemsenergy (in the may would range be morebe of beneficial. a thousandrealistic forFornewtons appli- ex- and more). ample,cations a piezoelectric requiringIn such both cases,actuator massive hybrid with stroke piezoelectric-hydraulica high ander force energy (in densitythe range systemsand of lowera thousand would power be newtons consump- beneficial. and For example, tionmore). than Inthe such electromagnetica piezoelectric cases, hybrid actuator actuator piezoelectric would with-hydraulic amake higher it a energysystemspromising density would candidate be and beneficial. lowerfor the powerpump For ex- consumption in ample,an electro-hydrostatic a piezoelectricthan the electromagnetic actuator actuator with [33]. aactuator higherHybrid energy wouldpiezoelectric–hydraulic density make it and a promising lower actuatorspower candidate consump- have for the pump in beention researched than thean electromagnetic and electro-hydrostatic used in various actuator fields, actuator would such [33 make as]. Hybridaerospace it a promising piezoelectric–hydraulic [138], automotivecandidate for [139], the actuators pumpand have been mechanicalin an electro engineeringresearched-hydrostatic [140]. and actuator The used basic in [33]. variousconcept Hybrid fields,lies piezoelectricin coupling such as aerospace a– hydraulicpiezoelectric [138 actuators], stack automotive with have [139], and thebeen transmission researchedmechanical of and hydraulic used engineering in fluid various via valve[fields,140]. sy The suchstems. basic as aerospaceThe concept high frequency, lies [138], in couplingautomotive large a force, piezoelectric [139], and and stack with smallmechanical stroke of engineeringthe the transmission piezoelectric [140]. of The actuator hydraulic basic conceptcan fluid be via convertedlies valve in coupling systems. into aa lowerpiezoelectric The high frequency frequency, stack and with large force, and largerthe transmission stroke ofsmall the ofoutput stroke hydraulic ofcylinder the fluid piezoelectric [141–147] via valve actuator(Figure systems. 14a). can The be Piezoelectric high converted frequency, into stacks alarge lower can force, be frequency op- and and larger eratedsmall by stroke hundredsstroke of the of piezoelectric thevolts, output making cylinderactuator it suitable [141can– be147for converted ]available (Figure 14 intoelectrica). Piezoelectrica lower power frequency in stacksaircraft. canand be operated Unlikelarger the stroke conventionalby of hundredsthe output hydraulic of cylinder volts, system making [141– in147] itthe suitable(Figure jet engine, 14a for). available thePiezoelectric hybrid electric piezoelectric-hy- stacks power can inbe aircraft.op- Unlike draulicerated system by hundredsthe only conventional consumes of volts, power hydraulicmaking when it systemsuitable required in for the to a vailable jetmove engine, load. electric the Therefore, hybrid power piezoelectric-hydraulicitin would aircraft. haveUnlike high the energy conventionalsystem efficiency. only consumeshydraulic Besides, powersystemthe piezoelectric-hydraulic when in the required jet engine, to move the pump hybrid load. has Therefore,piezoelectric few moving it would-hy- have high parts;draulic it also system eliminatesenergy only efficiency. consumes the need Besides,for power lubric thewhenation, piezoelectric-hydraulic required hence reducing to move the load. maintenance pump Therefore, has few effort. it moving would parts; it also have high energyeliminates efficiency. the need Besides, for lubrication, the piezoel henceectric reducing-hydraulic the pump maintenance has few effort.moving parts; it also eliminates the need for lubrication, hence reducing the maintenance effort.

(a) (b)

Figure 14. (aFigure) Flowchart 14. (a of) Flowchart the hybrid of piezoelectric–hydraulic the hybrid piezoelectric–hydraulic system. (b) Working system. principle (b) Working of the piezoelectric-hydraulic principle of the pump. piezoelectric-hydraulic pump.(a) (b) Figure 14. (a) Flowchart of the hybrid piezoelectric–hydraulic system. (b) Working principle of the piezoelectric-hydraulic pump.

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The working principle of the piezoelectric–hydraulic pump is shown in Figure 14b. It is described as follows: When the piezoelectric stack expands, the piston is pushed further, thereby decreasing the volume of the pump chamber. Pressure is built up inside the pump chamber, causing the outlet valve to open to release the fluid. When the stack contracts, the volume of the pump chamber increases. The decrease in chamber pressure causes the inlet valve to open, allowing fluid to enter the chamber. This process is repeated to control the fluid flow and regulate the chamber pressure from the piezoelectric-hydraulic pump. The piezoelectric–hydraulic pump has advantages over conventional systems such as hydraulic, pneumatic, and electric actuators in terms of efficiency and power density [139]. Its performance can be characterized by the maximum flow rate of the working fluid and the stall pressure [148]. This fluid circuit will then be extended into a working cylinder via a control valve system and accumulator. Some researchers have worked on developing the pump design to maximize the flow rate and pressure performance [144,149–153] while other researchers have attempted to build a compact system with an integrated hydraulic cylinder [139,150,154,155]. The performance of this hybrid pump relies on both the design of the piezoelectric stack and the pump chamber and the use of hydraulic fluid and the valve system.

6.1. Design of the Piezoelectric Configuration The first publication on the piezoelectric–hydraulic system proposed the use of a piezoelectric stack to drive the hydraulic circuit [149,156]. A piezoelectric stack 55.5 mm in length and 22 mm (stroke of 60 µm) in diameter is driven from −100 V to 500 V at a frequency of 300 Hz and could produce an output power of 34 W. Based on this result, several improvements have been proposed. To increase the performance of the pump, the frequency of the piezoelectric stack in the system was investigated. It was found that the piezoelectric stack self-heating phenomenon is one of the critical issues that occurs in PZT materials [141,157]. Therefore, a maximum pumping frequency of 1 kHz was selected based on the thermal limitations of piezoelectric stacks [154]. For higher-frequency applications, a cooling system was introduced to enhance the performance of the piezoelectric stack. The cooling fluid was introduced so that it could be used to thermally regulate the piezoelectric stack operating at a high frequency [140,158]. Research on a compact piezoelectric–hydraulic system was conducted at the Univer- sity of Maryland [150,154,155]. At a high frequency, significant losses in flow rate were observed. It demonstrated a highly nonlinear variable of the output velocity with pumping frequency. By comparing the performances of the hybrid pumps from different piezo- electric materials [159], they found that higher power output could be achieved from single-crystal PMN-based materials. Besides piezoelectric materials, some hybrid pumps also use magnetostrictive materials, such as Terfenol-D and Galfenol [138,155,160].

6.2. Design of the Pump Chamber

The size (Apiston/chamber) of the piston and the pump chamber is based on the perfor- mance of the piezoelectric stack (Strokepiezo, Forcepiezo, and Frequency) and pump perfor- mance (Flow rate and Pressurechamber) (Equation (4)):

Flow rate Forcepiezo Apiston/chamber = = (4) Strokepiezo × Frequency Pressurechamber

The design of the multipump chamber adopted from micropump designs [99,161,162] can be investigated in the piezoelectric stack pump to increase its performance. The double- piezoelectric pump was reported to have a significant increase in both the flow rate and the output pressure and produce a continuous fluid flow inside the system [163]. The sealing method is also an essential issue in the pump chamber design to prevent fluid leakage. The pump chamber can be sealed by O-rings arranged on the long side piston or a thin diaphragm plate. The first method is used in the pump design with a large input stroke [164]. The second one is commonly used in the design with a small to medium input Micromachines 2021, 12, 140 16 of 28

stroke [147,158,165–167]. However, a thin diaphragm plate will be deformed permanently after repeated cycles. It can lead to a reduction in pump performance. The working fluid selection is also important to reduce the leakage and increase the pressure generated within the chamber. Some working fluids used in the piezoelectric–hydraulic pump are the hydraulic fluid MIL-H5606F [154], the water-based hydraulic fluid Hydrolubric 123 [168], Mobil DTE-24 [163], glycerin solution [169], and AeroShell oils [138]. The ionic liquid (IL) with a higher bulk modulus than other common working fluids was also proposed to increase the output pressure [170]. Besides, the operating temperature range of the fluid is also an essential criterion for fluid selection. They should be sufficient for aerospace applications in which the ambient temperature may vary across a broad range, from −50 to more than 250 ◦C. Therefore, in some cases, thermal solutions are important to assure the stable performance of the system. The properties of typical working fluids are shown in Table6, below.

Table 6. Properties of typical working fluids.

Density Temperature Bulk Modulus Viscosity Fluid 3 ◦ Reference (kg/m ) Range ( C) (GPa) 40 ◦C 100 ◦C Water 997 0–100 2.1 0.7 0.5 [171] 70% Glycerinaquerous 1181 −39–114 1 0.4 22.5 - [172] Hydrolubric 123-B - ~1 (Pour point) - 21.5 3 -[173] Mobile DTE-24 871 −27–220 1 1.7 31.5 5.3 [174] AeroShell 41 874 −41–135 2 - 15.68 6.13 [175] MIL-H5606F 859 −54–135 1.79 15 - [154] IL-EMIM-EtSO4 1241 162 (Flash point) 3.1 39.44 7.66 [170] 1 From pour point to flash point. 2 At the pressurized condition. 3 At 100 F (~38 ◦C).

6.3. Design of the Valve System The valve system is important to determine the performance of the piezoelectric– hydraulic pump. The valves are designed to regulate the fluid flowing into and out of the pump chamber. Therefore, the response of the valves needs to be compatible with the movement of the piston (or piezoelectric stack). The valves used in piezoelectric–hydraulic pumps are reed valve, microreed valve, active valve, and diffuser valve (valve less design). The reed valve is popular among all valve types (Figure 15a). The reed valve structure is simple, and its movement is passively related to the piezoelectric stack’s performance. However, the response of the reed valve is limited by the natural frequency of the geo- metrical design. While the piezoelectric stack can be operated at hundreds to thousands of hertz, the reed valve movement is usually at around a few hundred hertz, depending on the geometry and size. A miniaturized piezo-hydraulic pump was developed with the highest frequency of the reed valve at 400 Hz [176]. Other attempts to change the design of the reed valve with the microarray valves have focused on increasing the frequency but still maintaining the working condition of the pump. The microvalve arrays with a spider- spring (or arm) configuration [158,168,177] (Figure 15b) could enhance the performance of the pump function, especially the flow rates. Micromachines 2021, 12, x FOR PEER REVIEW 17 of 29 Micromachines 2021, 12, 140 17 of 28

(a) (a)

FigureFigure 15. 15.( (aa)) Structure Structure of of cantilever cantilever reed reed valves. valves. ( b(b)) Structure Structure of of micro micro arm arm valves valves (from (from [ 177[177]).]).

TheThe activeactive valvevalve isis formedformed byby aa unimorphunimorph discdisc typetype ofof piezoelectricpiezoelectric forfor fluidfluid flowflow rectificationrectification [ 165[165].]. This This active active valve valve allows allows it it to to open open more more rapidly rapidly than than the the reed reed valve, valve, as as wellwell as as it it can can reduce reduce flow flow resistance. resistance. BackflowBackflow can can also also be be suppressed. suppressed. ByBy controllingcontrolling the the valvevalve operationoperation correspondingcorresponding toto thethe movementmovement ofof thethe piezoelectric piezoelectric stack,stack, thethe delivered delivered fluidfluid volume volume could could be be maximized.maximized. However,However, existing existing research research has has reported reported that that the the actual actual flowflow raterate isislower lowerthan than thetheexpected expected valuevalue duedue toto the the appearance appearanceof of air air entrapment entrapmentand and thethe effective effective control control of of the the active active valve. valve. Later Later research research has has shown shown the the importance importance of of timing timing controlcontrol of of the the active active valve valve in in the the performance performance of theof the system system at a at high a high frequency, frequency, which which was missingwas missing in the in previous the previous study study [178]. [178]. TheThe diffuser diffuser valve valve or or valveless valveles designs design is also is usedalso used in the in piezoelectric the piezoelectric pump, butpump, mostly but inmostly a micropump in a micropump driven by driven a diaphragm by a diaphragm piezoelectric. piezoelectric. This valve typeThis can valve be conicaltype can [179 be, 180con-] orical a Tesla[179,180] valve or [153 a Tesla,181]. Thesevalve designs[153,181]. do These not have designs anymoving do not have parts, any so they moving do not parts, suffer so fromthey fatiguedo not failuresuffer from as compared fatigue failure to other as valves. compared This to valveless other valves. structure This is valveless much smaller struc- thanture designsis much withsmaller valves than as designs it does with not havevalves a flowas it rectificationdoes not have system. a flow Itsrectification performance sys- reliestem. Its on fluidperformance flow from relies a high-pressure on fluid flow source from toa high a low-pressure-pressure source place. to a low-pressure place.Table 7 Summarizes several valve types in the piezoelectric–hydraulic pump. Each valveTable has its 7 prosSummarizes and cons several that can valve still be types developed in the piezoelectric to maximize– thehydraulic performance pump. of Each the piezoelectricvalve has its pump.pros and A cons control that strategy can still can be developed be created forto maximize the active the valve performance to synchronize of the itspiezoelectric movement pump. with the A pumpcontrol function. strategy can Finally, be created the concept for the of active the diffuser valve to valves synchronize can be integratedits movement with with a piezoelectric-stack the pump function. pump. Finally, the concept of the diffuser valves can be integrated with a piezoelectric-stack pump. Table 7. Comparison of valve types in the piezoelectric–hydraulic pump. Table 7. Comparison of valve types in the piezoelectric–hydraulic pump. Type Pros. Cons. ReedType valve SimplerPros structure. Working frequencyCons. limitations Small size and low inertia, WorkingComplex frequency structure andlimita- Reed valve Simpler structure Microreed valve array hence a broader working requirementtions of frequency range micromachining SmallOperates size and at a higherlow iner- Active valve (piezoelectric disc) ComplexComplex structure control and re- Microreed valve array tia, hencefrequency a broader quirement of micromachining No moving part, hence no Diffuser valve—conical shape working frequency range Leakage Operatesfatigue at failurea higher fre- ActiveDiffuser valve valve—Tesla (piezoelectric valve disc) Lesser pressure drop ComplexComplex structure control quency No moving part, hence Diffuser valve—conical shape Leakage The performances of some piezoelectric–hydraulicno fatigue failure pumps are presented in Table8. The experimentalDiffuser valve results—Tesla are based valve on prototypesLesser pressure with a maximum drop piezoelectricComplex inputstructure stroke of less than 100 µm. The reported flow rates were less than 2 L/min, with the stall pressure of a fewThe thousand performances kilopascals. of some For practical piezoelectric applications,–hydraulic a piezoelectric–hydraulic pumps are presented pumpin Table can 8. beThe scaled experimental up to increase results its performance.are based on The prototypes most powerful with a commercial maximum piezoelectric-basedpiezoelectric input pumpsstroke areof less the solid-statethan 100 μ pumpsm. The SSP reported from Kinetic flow rates Ceramics were company,less than with2 L/min reported, with pressure the stall in a stalled condition of 2700 psi (18.6 MPa) and a maximum flow rate of up to 7 L/min,

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using high-voltage piezoelectric stacks (operating voltage 0–1000 V) working at 1 kHz of frequency [164].

Table 8. Summary of the performance of several piezoelectric–hydraulic pumps.

Frequency Pressure Flow Rate Piezo Stack (mm) Voltage (Vp-p) Valve Reference (Hz) (kPa) (mL/min) Commercial PZT stack 19 × 19 × 102 800 60 ball-type 3800 312 [141] check valve Unimorph PZWT100 13 × 20 1000 1000 8300 204 [165] ∅ disc valve P − 885.91 120 60 Reed valve 7.96 10.32 [169] 7 × 7 × 36 APC Pst150 10 × 10 × 81 100 200 Reed valve 550 1140 [139] P 2) 10 × 10 × 18 100 300 Reed valve 1600 180 [154] APC Pst150 3.5 × 3.5 × 18 150 400 Reed valve 125 186 [167] EPCOS 150 400 Reed valve 1724 338 [182] (×3) 6.7 × 6.7 × 30 Reed valve P25 × 60 1000 300 (double piezo 6532 1246 [163] pump) PZWT100 81 (120 µm Reed valve ∅ 1200 1000 6895 1830 [168] stroke) MEMS valve

7. Piezoelectric-Based Systems in Aerospace Applications Harnessing various amplification methods listed earlier, the potential of piezoelectric actuators can be further exploited significantly. Each design has a different performance range (force, stroke, resolution, speed) and requires different structural designs and control strategies. Table9, below, shows a comparison of these piezoelectric-based systems studied in this paper. The designs of continuous motion are the simplest to directly amplify the stroke of the piezoelectric by a certain ratio. Amongst them, the flexure hinge is the most popu- lar mechanism that is used in both research prototypes and commercial products. The amplification ratio can be adjusted by the geometrical design but is usually limited to a few tens. Commercial products usually produce output strokes in the millimeter range and an output force of thousands of newtons. These structures can be used directly for applications with requirements in this range or in combination with other mechanisms [33]. For broader stroke application, the stepped-motion systems are preferred. The output stroke is achieved by accumulating small motions after steps. The resolution, speed, and force abilities vary in each concept. Generally, quasi-static systems are more suitable for larger force and slower speed applications compared to ultrasonic systems. These methods are used in various commercial piezoelectric drives with the designed stroke length in the centimeter range. The available output force and speed are tens to hundreds of newtons and a few to hundreds of milimeters/second, respectively. These systems can be scaled up to achieve larger outputs, but they are limited due to the space constraint of the applications. The performances of some commercial piezoelectric products are summarized in Figure 16 (from Physik Instrumente (NEXLINE, PICMAWalk, NEXAC-walking concept; Inertia Drives; PILine Ultrasonic) [126], PiezoMotor (LEGS linear-walking concept) [127], Cedrat Technologies (APA-continuous motion) [76]). The output force varies in each prod- uct based on the size of the piezoelectric elements. Stepped-motion actuators could provide a maximum force of a few to hundreds of newtons. Amplified piezoelectric has a maxi- mum force of hundreds to a few thousand newtons. The standard piezoelectric stacks are Micromachines 2021, 12, x FOR PEER REVIEW 19 of 29

be scaled up to achieve larger outputs, but they are limited due to the space constraint of the applications.

Table 9. Comparison of piezoelectric-based systems. Concept Structure Force Stroke Resolution Speed Control Continuous motion Flexure mechanism Medium Medium Small - - Simple Lever, X-mechanism Simple Medium Small - - Simple Bimorph configuration Simple Low Very small - - Simple Quasi-static stepped motion Inchworm—intermittent Medium Large Large Medium Low Complex Inchworm—continuous Medium Medium Large Medium Low Complex Inertial Medium Small Large High Medium Simple MicromachinesWalking2021—bending, 12, 140 legs Complex Small Large High 19 of 28High Medium Walking—V-shape legs Complex Small Large High High Medium Walking—combination legs Complex Medium Large High Medium Medium usually manufactured to produce a force in a range of hundreds to thousands of newtons. High-force actuatorsUltrasonic can be achieved stepped by combining motion a number of piezoelectric stacks to Standing wave increase the effectiveMedium cross-sectional Small area. The strokeLarge of each piezoelectricMedium stack is small, inHigh Medium Traveling wave the microrange.Complex However, by usingSmall the stepped-motion Large concepts, the strokeMedium of piezoelectric High Complex devices can be increased up to the centimeter range. The ability to be operated with a high frequencyThe (ultrasonicperformances devices) of allows some the commercial design to gain piezoelectriccentimeters/second productsof output are summarized in velocity. These performances are sufficient for those aircraft applications as mentioned Figurein Section 161. Furthermore,(from Physik with Instrumente a resolution of up(NEXLINE, to micro- and PICMAWalk, nanorange, piezoelectric NEXAC-walking concept; Inertiadevices couldDrives; add PILine an extra advantageUltrasonic) in precise [126], positioning PiezoMotor applications (LEGS if linear required.-walking In concept) [127], Cedratapplications Technologies in which multiple (APA sets of-continuous the stepped-motion motion) piezoelectric [76]). systems The output are used, force varies in each the resolution of each piezoelectric system plays a key role in the control method. Thus productsynchronizing based individual on the performances size of the of piezoelectric piezoelectric sets elements. helps gain high Stepped output force-motion actuators could provideand stroke. a maximum force of a few to hundreds of newtons. Amplified piezoelectric has a

maximumTable 9. forceComparison of hundreds of piezoelectric-based to a few systems. thousand newtons. The standard piezoelectric stacks are usually manufactured to produce a force in a range of hundreds to thousands of Concept Structure Force Stroke Resolution Speed Control newtons. High-force actuators can be achieved by combining a number of piezoelectric Continuous motion stacks to increase the effective cross-sectional area. The stroke of each piezoelectric stack Flexure mechanism Medium Medium Small - - Simple Lever, X-mechanismis small, Simple in the microrange. Medium However, Small by using - the stepped --motion Simple concepts, the stroke of Bimorph configurationpiezoelectric Simple devices Low can be Very incr smalleased up -to the centimeter - range. Simple The ability to be oper- ated with a highQuasi-static frequency stepped motion(ultrasonic devices) allows the design to gain centimeters/ Inchworm—intermittent Medium Large Large Medium Low Complex Inchworm—continuoussecond Medium of output Medium velocity. These Large performances Medium are sufficient Low for Complex those aircraft applications Inertialas mentioned Medium in Secti Smallon 1. Furthermore, Large Highwith a resolution Medium of up Simple to micro- and nanorange, Walking—bending legs Complex Small Large High High Medium Walking—V-shape legspiezoelectric Complex devices Small could add Large an extra High advantage High in precise Medium positioning applications if Walking—combination legsrequired. Complex In applications Medium in Largewhich multiple High sets of Medium the stepped Medium-motion piezoelectric sys- tems are used,Ultrasonic the resolution stepped motion of each piezoelectric system plays a key role in the control Standing wavemethod. Medium Thus synchronizing Small Large individual Medium performances High of piezoelectric Medium sets helps gain high Traveling wave Complex Small Large Medium High Complex output force and stroke.

FigureFigure 16. 16.Summary Summary of the of strokes the strokes and resolution and resolution of some commercial of some piezoelectric-based commercial piezoelectric products. -based products. The piezoelectric-hydraulic pump is another approach to amplifying the performance of the piezoelectric by coupling it with a working fluid. This pump, with a significant flow rate and stalled pressure, has been commercialized by Kinetic Ceramics company [164]. Several products from the solid-state pump series SSP can generate output pressure of 100 to Micromachines 2021, 12, x FOR PEER REVIEW 20 of 29

The piezoelectric-hydraulic pump is another approach to amplifying the perfor- mance of the piezoelectric by coupling it with a working fluid. This pump, with a signifi- cant flow rate and stalled pressure, has been commercialized by Kinetic Ceramics com- Micromachines 2021, 12, 140 20 of 28 pany [164]. Several products from the solid-state pump series SSP can generate output pressure of 100 to 2700 psi (~0.69 to 18.6 MPa) and a maximum flow rate of up to 7.5 L/min (2700Figurepsi (~0.69 17). toThese 18.6 MPa pumps) and a maximumcan work flow with rate ofvarious up to 7.5 Lworking/min (Figure fluids 17). Theseand in extreme environ- mpumpsental can conditions work with various (−40 to working 125 °C), fluids which and in extreme makes environmental it possible conditionsto replace the conventional − ◦ hydraulic( 40 to 125 C),or whichpneumatic makes it possiblepump toin replace aerospace the conventional applications. hydraulic or pneumatic pump in aerospace applications.

FigureFigure 17. 17Summary. Summary of the of performance the performance of commercial of piezoelectric-hydrauliccommercial piezoelectric pumps from-hydraulic Kinetic pumps from Ki- neticCeramics Ceramics company company (solid-state pump (solid SSP)-state (from pump [164]). SSP) (from [164]). Considering the potential of piezoelectric-based systems, it is possible that such sys- tems couldConsidering be developed the further potential and integrated of piezoelectric into various-based aerospace systems, applications. it is The possible that such sys- temsselection could of a conceptual be developed design dependsfurther on and the strokeintegrated and load into range. various For example, aerospace the applications. The bridge-type piezoelectric actuator has also been used to build a tip-tilt mechanism for selectionmicron positioning of a conceptual or a linear stepping design actuator depends using on a stick-slip the stroke concept and [183 load]. In otherrange. For example, the bridgeapplications-type in thepiezoelectric helicopter, a bimorph actuator piezoelectric has alsoactuator been used constructed to build by two a tip piezo--tilt mechanism for mi- cronelectric positioning ceramic plates or was a usedlinear to deflectstepping a trailing actuator edge flap using on the a rotorstick blade-slip [ 100concept] [183]. In other (Figure 18b). To achieve a large stroke, a piezoelectric-stack-based system was proposed applicationsin the design where in the micron helicopter, strokes are a accumulatedbimorph piezoelectric by using a feed-screw actuator for morph-constructed by two piezo- electricing aircraft ceramic structures plates [184]. Piezoelectricwas used stacksto deflect are also a proposedtrailing toedge be embedded flap on inthe rotor blade [100] (theFigure trailing-edge 18b). To flap achieve in the main a large rotor of stroke, the MD900 a piezoelectric helicopter. This-stack system-based allows ac-system was proposed tive control of the flap, thus improving the aerodynamic performance and reducing the invibration, the design noise, andwhere power mi consumptioncron strokes of the are rotor accumulated [74,185]. Progressively, by using amplified- a feed-screw for morphing aircraftpiezoelectric structures actuators [184]. with compliant Piezoelectric mechanisms stacks have are been also used proposed for the active to be flap embedded in the trail- ing(Figure-edge 18a) flap [28,31 in,183 the]. In main another rotor stepped-motion of the MD900 concept, helicopter. a linear inchworm This system piezoelec- allows active control tric actuator has been proposed for positioning engine inlet guide vanes via a crank slider ofmechanism the flap, [47 , thus48] (Figure improving 18c). The the inchworm aerodynamic concept can performance be used to generate and rotary reducing the vibration, noise,motion toand directly power drive consumption the unison ring of to controlthe rotor the inlet[74,185]. guide vanes,Progressively, hence reducing amplified-piezoelectric actuatorsthe transmission with mechanism compliant from the mechanisms previous linear have actuator been [49]. used In the approach for the of active the flap (Figure 18a) hybrid hydraulic system, the piezoelectric pump developed by Kinetic Ceramics Inc. has [28,3been tested1,183] with. In hydraulic another primary stepped flight-motion control inconcept, remotely a piloted linear vehicles inchworm [158,168 piezoelectric]. actuator hasThe solid-statebeen proposed pumps from for this positioning manufacturer engine have been inlet improved guide in terms vanes of performance via a crank slider mechanism [47,48]over the years(Figure for more18c). practical The inchworm applications. concept Following can the be same used working to generate concept as rotary motion to di- that of electro-fluidic components with smart materials, piezoelectric stacks can be used as rectlyprecisely drive controlled the valvesunison for ring a magnetostrictive to control the pump inlet [138 ].guide vanes, hence reducing the transmis- sion mechanism from the previous linear actuator [49]. In the approach of the hybrid hy- draulic system, the piezoelectric pump developed by Kinetic Ceramics Inc. has been tested with hydraulic primary flight control in remotely piloted vehicles [158,168]. The solid- state pumps from this manufacturer have been improved in terms of performance over the years for more practical applications. Following the same working concept as that of electro-fluidic components with smart materials, piezoelectric stacks can be used as pre- cisely controlled valves for a magnetostrictive pump [138].

Micromachines 2021, 12, x FOR PEER REVIEW 21 of 29 Micromachines 2021, 12, 140 21 of 28

(a) (b)

(c)

FigureFigure 18. 18Examples. Examples of theof the piezoelectric piezoelectric system system in aerospacein aerospace applications: applications: (a) ( activea) active flap flap on aon helicopter a helicopter blade blade using using an an amplified-piezoelectricamplified-piezoelectric actuator actuator [183 ];[183] (b) piezoelectric; (b) piezoelectric bender bender for helicopter for helicopter rotor controlrotor control (from [100(from]); ([100]c) linear); (c inchworm) linear inch- piezoelectricworm piezoelectric actuator for actuator application for application in inlet guide in inlet vanes guide (from vanes [47]). (from [47]).

However,However, some some limitations limitations may mayneed needto be considered to be considered when using when piezoelectric-based using piezoelectric- systems.based systems. First, material First, agingmaterial causes aging a changecauses a in change the properties in the properties of materials of materials and loss ofand po- loss larization,of polarization, which results which in results instability in instability over a long over working a long period.working Second, period. the Second, temperature the tem- dependenceperature dependence of properties of limits properties the working limits conditionthe working of the condition piezoelectric of the systems. piezoelectric Hence, sys- piezoelectric-basedtems. Hence, piezoelectric systems may-based be systems suitable may for suchbe suitable applications for such exposed applications to ambient exposed ◦ temperatureto ambient in temperature a range of in− 50–150a range ofC. − In50 the–150 case °C. In of the a higher case of working a higher temperature, working temper- a thermalature, solutiona thermal is solution required. is Moreover,required. Moreover, as a brittle as material, a brittle material, piezoelectric piezoelectric is prone is to prone be easilyto be damaged easily damaged by tension. by Therefore, tension. Therefore, it requires it careful requires design careful and design operation and to operation suppress to unexpectedsuppress tensileunexpected loads. ten Finally,sile loads. the enclosure Finally, the may enclosure be vital may for stepped-motion be vital for stepped actuators-motion to eliminateactuators to the eliminate working the environment’s working environment’s effect on the effect friction on the elements. friction elements. Knowing Knowing these limitations,these limitations, the development the development of futuristic of piezoelectricfuturistic piezoelectric materials ismaterials essential is besides essential the besides con- ceptualthe conceptual designs. Piezoelectric designs. Piezoelectric materials could materials be tailored could to be alter tailored their tomaterial alter their properties material to beproperties better suited to be forbetter aerospace suited for applications. aerospace applications. 8. Conclusions 8. Conclusions Piezoelectric-based systems can be considered as novel electromechanical designs to Piezoelectric-based systems can be considered as novel electromechanical designs to use in aerospace applications, especially toward the concept of more electric aircraft. To increaseuse in theaerosp potentialace applications, of piezoelectric especially systems, toward several the amplification concept of more methods electric and aircraft. related To conceptualincrease designsthe potential have beenof piezoelectric reviewed in systems, this paper. several Understanding amplification the methods mechanisms and related and theirconceptual specifications designs is beneficial have been to reviewed technology in this selection. paper. These Understanding mechanisms the can mechanisms be divided and intotheir four specifications amplification is groups: beneficial continuous-motion, to technology selection. quasi-static These stepped-motion, mechanisms can ultrasonic be divided stepped-motion,into four amplification and piezoelectric–hydraulic groups: continuous systems.-motion, The quasi designs-static in the stepped first three-motion, groups ultra- cansonic directly stepped generate-motion, output and force piezoelectric and stroke,–hydraulic while the systems. piezoelectric The pumpdesigns reviewed in the fi inrst the three

Micromachines 2021, 12, 140 22 of 28

last group produces a fluid flow to power the hydraulic cylinder. Moreover, continuous motion from the first group can be used to enhance the input stroke from the piezoelectric element for other systems. Even though most of the current research prototypes and commercial products based on piezoelectric serve in small-scale applications with moderate force and stroke ranges, the concepts of stepped motion and piezoelectric–hydraulic have the potential to be scaled up and developed for large-scale applications. Some examples of aerospace applications and developmental products have been introduced to highlight these possibilities. How- ever, to scale the actuators for practical applications, new challenges must be overcome. Manufacturing and assembly are the most critical issues for those concepts using frictional elements. For a large prototype, the housing structure needs to be optimized to avoid having an oversized system. Besides, some properties of piezoelectric materials, such as hysteresis characteristics, temperature-dependent properties, or aging, may reduce the overall performance of the systems. Therefore, they need to be considered in the overall design and control process; else, regular maintenance is required.

Author Contributions: Conceptualization, T.V.K.V. and K.H.H.L.; writing—original draft prepara- tion, T.V.K.V.; writing—review and editing, K.H.H.L., W.T.C., and T.M.L.; visualization, T.V.K.V.; supervision, K.H.H.L.; project administration, T.M.L.; funding acquisition, A.G. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Acknowledgments: This research is supported by the Singapore Research Foundation under its Rolls-Royce Electrical (RRE) program and administered by Rolls-Royce@NTU Corporate Lab. Conflicts of Interest: The authors declare no conflict of interest.

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