Optimal Configuration of the Mechanical Structure and Flap Actuator for Hydrofoils

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DEGREE PROJECT IN THE FIELD OF TECHNOLOGY DESIGN AND PRODUCT REALISATION AND THE MAIN FIELD OF STUDY MECHANICAL ENGINEERING, SECOND CYCLE, 30 CREDITS STOCKHOLM, SWEDEN 2019

Optimal configuration of the mechanical structure and flap actuator for hydrofoils

STANISLAV MINKO

KTH ROYAL INSTITUTE OF TECHNOLOGY SCHOOL OF INDUSTRIAL ENGINEERING AND MANAGEMENT

Examensarbete TRITA-ITM-EX 2019:587

Optimal konfiguration av roder och klaffställdon för bärplan

Stanislav Minko Godkänt Examinator Handledare 2019-09-01 Hans Johansson Fariba Rahimi Uppdragsgivare Kontaktperson KTH Maribot Ivan Stenius, Nicholas Honeth

Sammanfattning Bärplansfartyg blir mer och mer populära och driver marinutvecklingen framåt. Bärplan tillåter högre hastigheter att nås och elektrifiering av båtar eftersom friktionen minskar kraftigt. FoilCart är en elektrisk aktivt kontrollerad bärplansbåt som är framtagen av ett forskningsprojekt på KTH. En mindre version av fartyget är under utveckling och den här studien undersöker valet av optimal design för mast och roderkonstruktionen. Rapporten kan också användas som underlag för bärplans vingdesign och för projekt där små elektromagneter är inblandade.

Designen och dynamiken av bärplan kommer ursprungligen från flygplansbranschen. Idén av en böjlig, morfande vinge har redan implementerats på flygplan och ska nu testas i vatten. Förutom detta evalueras även ett föreslaget elektromagnetiskt ställdon för aktueringen av rodret. Studien hittar den mest optimala konfigurationen av en mekanisk struktur och ett ställdon. Klassisk struktur (gångjärnstypen) jämförs med den morfande strukturen och det klassiska servot jämförs med den elektromagnetiska aktuatorn. I kombination med varandra formar de två strukturerna och två aktuatorerna fyra konfigurationer som evalueras med avseende på responstid, energiförbrukningen, volymen av lösningen och den maximala roderkraften.

Arbetsprocessen är uppdelad i fyra huvuddelar. Hydrodynamiska beräkningar för att få ut momentet som rodret behöver motstå. CAD design av varje konfiguration, inkluderande strukturella- och aktuatorstudier. Tillverkning av prototyper för att verifiera teoretiska resultat. Summering och presentation av informationen med avseende på evalueringsparametrar.

Slutsatserna av studien visar optimal konfiguration mot respektive evalueringsparameter. Servo i kombination med den klassiska gångjärnsstrukturen är mest energieffektiv. Den elektromagnetiska aktuatorn tillsammans med gångjärnsstrukturen har minst volym. Servo tillför den maximala roderkraften och den elektromagnetiska aktuatorn har snabbast responstid. Författaren rekommenderar kombinationen av servot och den mofrande strukturen för implementering vid utveckling av det mindre FoilCart fartyget. Kombinationen har stora hydrodynamiska fördelar, högst maximal roderkraft och liten volym. Implementeringen skulle innebära ett första steg för utvecklingen av morfande struktur inom bärplan. Varje konfiguration har sina för- och nackdelar och därför ska noga undersökas vid val av struktur / aktuator i andra applikationer.

Master of Science Thesis TRITA-ITM-EX 2019:587

Optimal configuration of the mechanical structure and flap actuator for hydrofoils

Stanislav Minko Approved Examiner Supervisor 2019-09-01 Hans Johansson Fariba Rahimi Commissioner Contact person KTH Maribot Ivan Stenius, Nicholas Honeth

Abstract Hydrofoil vessels are becoming more popular and driving marine development forward. Since the water friction is greatly reduced, they allow achieving higher speeds and electrification of the boats. FoilCart is a full electric active controlled hydrofoil boat designed as a research project at KTH. The small version of this boat is considered to be designed in this thesis and an optimal configuration for the mast and rudder of the boat is studied. The study is prone to be used as a basis for hydrofoil wing and flap design and includes research on small electromagnets. The design and dynamics of the hydrofoils are adapted from aeronautics. An idea of a morphing airplane wing has been successfully tested and now evaluated for the hydrofoil application. An electromagnetic actuator is proposed for the actuation of the rudder as competitor to typically used once. This study finds the optimal combination of an actuator and mechanical structure comparing classical actuator (servo) with electromagnetic actuator and classical structure (hinge- type) with morphing structure. Those four configurations are evaluated with respect to energy usage, response time, volume of the solution and maximal rudder force. Workflow of the thesis is divided to four main parts: calculations of the hydrodynamic forces to realize the peak torque that the rudder will need to withstand in water, CAD design of each evaluated configuration (including material and actuator studies), prototype manufacturing for the verifications, summary of the information and presentation according to evaluation parameters. The study is concluded with an optimal choice of the configuration with respect to different factors. Servo actuator in combination with hinge-typed structure is most optimal in terms of energy efficiency. The most compact configuration is electromagnetic actuator in combination with hinge-type structure. Maximal rudder force is supplied by the servo meanwhile fastest response is achieved by electromagnetic actuator. Due to better hydrodynamic characteristics, maximal rudder force and compact volume, a configuration of morphing structure and servo actuator is recommended for implementation for miniaturized version of FoilCart vessel. Implementation means a first step to apply morphing structures into marine applications. Each configuration has its own strength and weakness and therefore all four are to be carefully studied before choosing an actuator / structure according to the application.

4 FOREWORD

A thank you to everyone who made this master thesis possible.

I would like to thank …

Hans Johansson for being examiner of this thesis. Fariba Rahimi for being the supervisor and leaving incredibly valuable feedback. Nicholas Honeth and Ivan Stenius for being project stakeholders, for founding the thesis, for supplying me with resources and facilities but also being great help for any questions throughout the thesis. Anton Svensson and Fredrik Löfblom for helping with manufacturing the prototypes and calculating the hydrodynamic forces. Henrik Strömqvist for help on the Simulink model. Family and friends, especially Kajsa and André for continuous support throughout the thesis.

Stanislav Minko

KTH, August 2019

5 NOMENCLATURE

Notations and Abbreviations that are used in this Master thesis.

Notations Symbol Description

훼 Angle of attack (°) 푐 Chord length (m)

푐퐿 Lift coefficient

푐퐷 Drag coefficient 푀 Mach number 푢 Local flow (m3/s) 퐶 Speed of sound (m/s) 푅푒 Reynolds number 퐵 Magnetic field (T) 푁 Amount of turns 퐼 Current (A) 퐿 Flux distance (m) 휇 Magnetic permeability (H/m)

휇0 Permeability of free space (H/m)

휇푟 Relative magnetic permeability 퐹 Electromagnetic force (N) 퐴 Cross section area (m2)

퐶푝 Pressure coefficient

퐶퐻푚푎푠푡 Torque coefficient of the flap

푀퐻 Flap torque (Nm) 푞 Dynamic pressure (Pa)

푐푓푙푎푝 Chord length of the flap (m) 2 푆푓푙푎푝 Flap area (m )

Abbreviations

CAD Computer Aided Design CFD Computational Fluid Dynamics NACA National Advisory Committee for Aeronautics

6 TABLE OF CONTENTS

1 INTRODUCTION 8

1.1 Background 8 1.2 Purpose 11 1.3 Delimitations 11 1.4 Method 13 1.5 Ethics 14

2 FRAME OF REFERENCE 15

2.1 Basic wing nomenclature 15 2.2 Computational fluid dynamics 15 2.3 Electromagnetism 16

3 IMPLEMENTATION 17

3.1 Flap forces calculations 17 3.2 Structural studies 19 3.2.1 Hinge-type 19 3.2.2 Morphing-type 20 3.3 Actuator studies 21 3.3.1 Servo 21 3.3.2 Electromagnetic actuator 22

4 RESULTS 23

4.1 Theoretical results 23 4.2 Validation 24

5 DISCUSSION AND CONCLUSIONS 27

5.1 Discussion 27 5.2 Conclusions 28

6 RECOMMENDATIONS AND FUTURE WORK 29

6.1 Recommendations 29 6.2 Future work 29

7 1 INTRODUCTION

This chapter describes the background, the purpose, the limitations and the methods used in the presented project.

1.1 Background Hydrofoils, just like airplane wings (airfoils), generate lift that allows to raise hull of the vessel out of the water and reduce drag (water resistance). Reduced drag means lower fuel costs, higher traveling speed and better seakeeping performance. [1] [2]

Figure 1. Boat with hydrofoils, Pinterest (2019). Most of the hydrofoil vessels raise to a fixated height over water once the design speed is reached (passive hydrofoils or also called surface-piercing hydrofoils). Surface-piercing hydrofoils operation is limited. A wave higher than designed fly height can cause an unpleasant ride meanwhile low wave height causes ventilation problem which results a loss of thrust. Active controlled hydrofoils or even known as fully submerged hydrofoils adjusts the fly height and have therefore advantage over passive design [3]. Similar to the wings of an airplane, the height adjustment is performed by flaps. Flap actuator choice coupled to mechanical design determines effectiveness of height adjustment. Two common types of the mechanical design are presented in Figure 2.

Figure 2. Two mechanical designs for flap attachment.

Mechanical design presented in A is commonly known as plain flap type. The flap and the wing are two separate parts, they are connected through the bearings to allow the rotation of the flap. The solution is typically easy to manufacture and allows simple detaching of the flap. The downside is that the mechanical construction results in a gap between main foil and the flap, i.e.

8 hydrodynamic losses. The solution is widely used in simpler aircrafts and Wazp hydrofoil sailboats. Mechanical design presented in B is based on flexible material attached between main foil and the flap. The material is stiff enough to hold the flap straight but will allow it to move when actuated. This solution provides better hydrodynamic characteristics but sets high pressure on the correct material choice due to high risk of fail. The solution is also permanent. Moth class hydrofoil sailboats use this solution [4].

Yet untested mechanical structure in hydrofoil vessels are inspired by nature. Morphing structures can change their shape under actuation, those were investigated in aircraft since NASA launched the morphing project in 1990s [5]. Morphing structures allow material geometry changes, for example: twisting, bending or stretching, to improve aerodynamic performance [6]. Furthermore the structure can improve manoeuvrability and reduce acoustic emissions [5]. Piezoelectric actuators were successfully implemented in flap actuator system and tested on hinge-less system of BK117 helicopter [7]. Construction of Fish Bone Active Camber (FishBAC) morphing structure were successfully completed and aerodynamic properties were evaluated by wind tunnel tests. [8]

Figure 3. Left: BK117 flap actuator system presented by Jaenker in 2008. Right: FishBAC continuous morphing trailing edge presented by Woods and Friswell in 2012.

BK117 tests showed an improved vibration stability by 90% meanwhile FishBAC airfoil improved lift efficiency by 25% compared to a standard mechanically hinged airfoil [7] [8].

An effective structure is not enough to achieve optimal lift values and the actuator choice has a large impact on it. Two common actuator types are: the servomotor, normally used for actuating the mechanical solution A, and a linear actuator, normally used for actuating the mechanical solution B presented in figure 2.

Figure 4. Left: A rotational servomotor. Right: Linear motor.

Electronical actuators as servomotors and linear motors are preferably located outside of the water, in the hull of the vessel, due to water resistance problems. The size of the actuator is a function of force that the flap can handle. The actuator is forced to be placed inside the hull due to its size.

Inspired by airfoil morphing techniques, the idea of using electromagnetic actuation for hydrofoil flap actuation is viable.

Figure 5. Flap actuation concept based on electromagnets.

The concept is based on two electromagnets with one permanent magnet in between them. The stiff rod is fixated to the flap on one side and permanent magnet on the other side. The control system supply more current to one of the electromagnets which creates more actuation force that makes the magnet to move closer to one of the electromagnets and actuates the flap. The gap between the main foil and flap is filled by elastic material that allows the movement and creates morphing structure.

An advantage of the electromagnetic actuators is that they can be exposed to water, even more water will guarantee coil cooling, which is typically a problem when higher current is drawn [9]. A disadvantage of the solution is that the actuation requires continuous current supply to the electromagnets to keep the flap straight. On the other hand, if the flexible material (marked in green in Figure 5) would be stiff enough and act as a spring which pushes the flap to its neutral position then the current would be only needed during actuation. Later, the flexible material acts as a damper during actuation that increases the response time and decreases the maximal flap force. The maximal flap force is further affected by the size of the solution. Based on the length of the lever (rod attached to permanent magnet and the flap) and electromagnets size the flap area can become smaller. A small area results in a lower flap force.

Servomotor is typically used as actuator for design solution that uses a mechanical hinge structure and have a disadvantage regarding response time. Since the servomotor got an inbuilt control loop and mechanical gears, it introduces time delay [10]. Meanwhile, in the electromagnetic actuator, a change in the current directly results in change of the attraction force. Considering maximal flap force it is directly coupled to the size of the servo motor, i.e. the torque it can supply.

The parameters which evaluate the performance of different designs are energy usage, response time, maximal flap force and the volume of the solution. Similar to the structural properties, the actuator choice affects the performance. The mechanical properties are strongly coupled to the actuator choice which forms a mechatronic system that is optimally configured.

1.2 Purpose

The purpose of the study is to find an optimal configuration between actuator and mechanical structure that maximizes lift force that is generated by wings in hydrodynamic environment.

What is the optimal configuration of the mechanical structure and actuator type considering challenges as energy usage, response time, maximal rudder force and the volume of the solution?

1.3 Delimitations

Marine systems department at KTH Royal Institute of Technology has developed an active controlled hydrofoil vessel called FoilCart, presented in Figure 6. FoilCart is unstable and without active control it will tip to the side (roll motion) or forward/backward (pitch motion). One way of controlling vessels pitch motion is by fixating main wing and having adjustable elevator. To steer and control the roll motion, the mast and rudder are used. The physics behind turning the vessel is like a bike, i.e. the rudder will make the hull to fall to the side where you want to turn.

Figure 6. A hydrofoil vessel, FoilCart. 1: Fuselage. 2: Adjustable elevator. 3: Mast. 4: Main wing. 5: Hull. 6: Rudder.

Downscaled version of FoilCart is in high demand due to sonar missions in swallow waters. The construction of the mast and rudder is comparable to wing and a flap actuator and will be used in this study. Following four configurations are used as test cases and evaluated with respect to response time, energy usage, maximal lifting force and the size of the solution.

Figure 7. Four configuration that will be evaluated to find the optimal one.

Response time is the time it takes to actuate the flap to its maximal angle. Maximal lifting force is calculated at a fixated sailing speed from flap angle and area of the flap. All the physical joints are considered in the volume.

Design limitations

• The size of the mast and flap developed for the purpose of this master’s thesis should be such that most components can be 3D printable for rapid prototyping. • The morphing structure (mast and rudder) should be able to handle realistic hydrodynamic forces on the mast/flap configuration. • No energy limitation for the actuator system. • Traveling speed of the vessel as most is 10 m/s. • Cord length of the mast should not exceed 100 mm. • 10° to 12° actuation range for actuators.

12 1.4 Method

Plan of attack To fulfil this Master Thesis the workflow is divided into checkpoints. Those are marked by the blue lines in Figure 8.

Figure 8. Workflow of the process.

Calculation of hydrodynamic forces completed in XFOIL software. It is a simple, yet powerful tool used daily for hydrodynamic calculations. 3D models are developed in Solid Edge. Matlab is used for numerical calculations and modelling trough Simulink.

Project planning Project is realized during spring semester of 2019 at KTH, Sweden. In February and March, literature review is done on the topic and necessary knowledge about hydrodynamics, electromagnets and morphing structures is acquired. Once required knowledge is obtained the design solutions of each configuration are decided. Rapid prototypes are designed during April and tested for robustness. Final prototypes and optimization are done in May. The project is accomplished in June.

Risk assessment and action plan The electromagnetic actuator requires certain components to function. If the components allocate too much space inside of the mast it may lead to low flap area. There is a correlation between lever arm size, size of the electromagnets and the flap area. Since the flap requires to hold the requirement of 12 degree as a maximal flap angle it might decrease in size dramatically. Morphing structure relies heavily on choice of the flexible material. Too stiff material does not allow much bending and increases energy consumption meanwhile too loose material may be unrealistic for real application and require the magnets to continuously supply current for holding the flap straight. Also, practical construction of the structure can get complex.

13 A backup is that electromagnetic actuator can be placed upside in the hull, similar to a servomotor, allowing more movement freedom. A lower flap area is also acceptable, it of course lowers the odds for the electromagnetic actuator to become the optimal design. The study investigates how big the area can get and evaluate it.

1.5 Ethics A miniature sea vehicle with higher speed than competitors due to hydrofoil technique is used for scanning the area or for rescue missions. Two interested companies are Försvarsmakten, Swedish military force, and Sjöfartsverket, Swedish sea marine administration. Even if it is intended to be used for support and civil missions, there is no guarantee that the vehicles won’t be used to carry missiles or some other destructive units.

Finding the optimal configuration for the mast and actuator design pushes the development of miniaturized FoilCart forward. Author is partly responsible for what the vessel can do in the future. If it is capable to complete complex tasks, the demand of the technique will rapidly increase. This will lead to an unavoidable expansion of it, also meaning insecurity of how the product is used.

14 2 FRAME OF REFERENCE

The reference frame is a summary of the existing knowledge and former performed research on the subject. This chapter presents the theoretical reference frame that is necessary for the performed research.

2.1 Basic wing nomenclature Basic concepts of aerofoil terminology i.e. leading edge, trailing edge, chord length 푐 and angle of attack 훼 are illustrated in Figure 9.

푦

Fluid motion 푥 푧 푐 훼 Leading Trailing edge 푐 edge

Figure 9. Wing notation.

Angle of attack 훼 is the angle between incoming flow and the chord line. Chord 푐 is the straight line connecting leading and trailing edge. The edge that meets the fluid is referred to as leading edge.

2.2 Computational fluid dynamics

Computational fluid dynamics (CFD) analyses use numerical methods to solve the fluid flows problems. For those analyses information about geometry of wing profile, Reynolds number, Mach number and %N-crit value is required.

Geometry of standardised wing profiles are obtained from [11]. In [11], a profile generator for wing design is provided which includes most NACA1 airfoils. NACA developed a system to describe geometry of the profiles through digits. Most common are 4-digit and 5-digit airfoils [12]. The shape of the wing is important for its drag / lift (퐶퐷 / 퐶퐿) ratio. Symmetrical wings generate no lift at 훼 = 0 i.e. when the wing is flowing straight trough the medium. Asymmetrical wings generate lift even at 훼 = 0 [13]. A symmetrical wing is able to generate equal positive and negative lifts meanwhile asymmetrical wings are used to generate more of directed lift. %N-crit value (between 0 and 14) is coupled to ambient disturbance level in which foil operates. A standard value equals to 9 and corresponds to average wind tunnel [14].

Mach number 푀 is a dimensionless number related to speed of sound in the medium [15]. It is defined as 푢 푀 = (1) 퐶

1 National Advisory Committee for Aeronautics.

15 where 푢 is local flow velocity and 퐶 is speed of sound. For water 푐 = ~1500 m/s [16]. For Mach numbers < 0.2 a simplified incompressible flow equation can be used as the compressibility effects will be small [15]. Reynolds number is another dimensionless quantity that helps to analyze flow patterns by predicting transition between laminar and turbulent flow [17]. It is calculated from chord width 푐, traveling speed, density, velocity and viscosity of the fluid.

CFD-problems are solved in 2D or 3D environment. 2D simulations assume that wing profile is endlessly long, in z-axis, not taking into account the effects that occur when the medium is passing on that side of the wing. 3D simulations run a full simulation but require a complex software and more computation power [18].

2.3 Electromagnetism

Due to Ampere’s law the current flowing through the wire generates a magnetic field [19]. The force of the magnetic field is greatly increased if a ferromagnetic core is placed inside of the coil [20]. Calculations of the magnetic field generated by an electromagnet is complex and involves finite element methods as both magnetic field and magnetic force are nonlinear functions of current [21]. A common simplification is to consider that magnetic field is constant at the core and is zero outside of it [22]. For continuous closed loop magnetic circuits (no air gap in winding) equation for magnetic field 퐵 is reduced to (2) [23]

푁퐼휇 퐵 = (2) 퐿 where 푁 is amount of turns around the core, 퐼 is current through the winding, 휇 is core’s relative permeability and L is its flux path. The electromagnetic force, 퐹 is expressed in (3)

퐵2퐴 퐹 = (3) 2휇0 where 퐴 is cross section of the core and 휇0 is permeability of free space expressed in (4).

−7 휇0 = 4휋(10 ) (4)

Substituting (2) in (3) give a current dependent expression of the electromagnetic force formulated in (5)

푁2퐼2휇2퐴 퐹 = (5) 2휇0퐿

Large cross section 퐴 is and short flux distance 퐿 will result in a greater magnetic force.

Magnetic permeability 휇 is the measure of the materials ability to magnetize i.e. how well can the metal support the formation of the magnetic field [24].

16 3 IMPLEMENTATION

In this chapter the working process is described.

3.1 Flap forces calculations

NACA0021 with chord length 푐 = 100 mm is chosen as wing profile in this study. A suffix 0021 means that the profiles maximal thickness is 21% of its chord length. NACA 0021 is chosen due to two main reasons. First, the mast should be able to generate lift on both sides, and therefore, the only valid wing profile, is a symmetrical one. Second, as thick wing as possible to eventually fit the electromagnetic actuator inside of it.

Figure 10. NACA 0021 from hydrofoil plotter of [11].

Maximal Reynolds number 푅푒 = 769 230 is calculated from maximal given speed of 10 m/s and water as traveling medium using (1). The flap is set at 70% of wings chord, similar to FoilCart vessel.

Torque coefficient in the flap 퐶퐻푚푎푠푡 is received tough 2D-simulations in Xflr5 software with “Ncrit” value set to 7 due to imperfections in masts manufacturing process. 3D printed parts, even after treatment, are not able to guarantee a complete laminar behavior in water. Figure 11 shows XFOIL2 simulation for NACA0021 with flap angle of 12 degree at Reynolds number of 769 000. Pressure distribution coefficient 퐶푝 across the chord is drawn in blue. Top line represents upper chamber and bottom - lower. The pressure on the mast is distributed according to green arrows. XFOIL results of torque coefficients, 퐶퐻푚푎푠푡, in the hinge of the flap are exported to Matlab to calculate flap torque 푀퐻.

2 XFOIL is the engine to run the CFD simulations and Xflr5 provides the graphical interface for it.

Figure 11. Pressure distribution of NACA0021 from Xflr5 software.

Maximal flap torque, 푀퐻푀퐴푋 = 13.5 Nmm, is calculated using (6) to decide the size of the flap actuator, i.e. electromagnet and servo.

푀퐻 = 푞 푐퐹푙푎푝 푆퐹푙푎푝 퐶퐻푚푎푠푡 (6) where 푐퐹푙푎푝 is chord length of the flap, 푆퐹푙푎푝 is the area and 퐶퐻푚푎푠푡 is torque coefficient from CFD-calculations. Dynamic pressure 푞 is expressed in (7)

1 (7) 푞 = 휌푣2 2

Matlab script for Reynold number calculations and flap torque can be found in Appendix A.

18 3.2 Structural studies 3.2.1 Hinge-type

First of two structures to evaluate in the study consists of simple mechanical structure for mast and flap. Figure 12 shows a part of the mast, in black, and a flap, in blue. A stiff joint, in green, is fixated to the actuator and is responsible for position of the flap. Flap is designed as two parts which clamps around the axle.

Figure 12. KeyShot rendering of hinge-typed mast design.

From the left to the right, in Figure 12, renderings show mast with the flap at its neutral position, mast and one side of the flap for clear view of rotational rod (green) and bearing (red), mast and a flap at 12°. The colours are for visualisation purpose only.

Dynamical friction coefficient in bearing is varying from 0.08 to 0.15 휇 when no grease is used which is considered very low and is not considered in this study.

19 3.2.2 Morphing-type

Conceptual design of morphing mast structure was presented in Figure 5. The idea is to fill the gaps with a flexible material, depicted in red in Figure 13, to allow the displacement of trail edge of the mast, i.e. the flap.

Figure 13. KeyShot rendering of morphing mast design.

Figure 13 presents the rendering of the morphing mast design. Mast and a flap are connected at the tiny triangular area forming a natural pivot point for the rotation. The flexible material, in red is holding the flap straight and expands / shrinks during actuation allowing rotation of the flap.

The material planned to be used for the mould, PMC-770 got it maximal tensile strength at 750 psi, equivalent to ~ 5 MPa and 100% modulus at 250 psi, equivalent to 1.7 MPa.

20 3.3 Actuator studies 3.3.1 Servo

Servo actuator is proposed as a traditional actuator for this study. From the flap forces calculations, we know the maximal torque to withstand during actuation is 13,5 Nmm. All the off the shelf servos available for hobby and professional work generates required torque with good margin. Micro servo that is used for this study is REELY S-8246. It is chosen due to its light weight (11.5g), compact design and high actuation speed of 0.09 s (at 6V) from 0 to 60 °.

Datasheet, presented in table 1, of REELY S-8246 does not include any information about idle power consumption. Neither there are any studies for micro servos nor datasheet information from other manufacturers. Therefore, power consumption at standstill is considered to be zero.

Figure 15. Servo actuator installed on hinge-typed mechanical structure.

Table 1 presents the available information about REELY S-8246 micro servo.

Table 1. Datasheet: REELY S-8246.

Servo technology Analogue servo Gear box Plastic Torque at 4.8V / 6.0 V 12 / 15 Ncm Actuation time at 4.8 / 6.0 V 0.11 sec / 0.09 sec (60 deg) Weight 11.5 g Connector system JR Length / Width / Height 23.5 / 12.2 / 26.4 mm

21 3.3.2 Electromagnetic actuator

Electromagnets available as off the shelf components are too huge for application type. Smallest electromagnet for custom usage found is from [25] (art. nr. 503656 Feb 2019) with dimensions 11 x 18 mm (length, diameter). Considering that NACA0021, with 100 mm chord length, is only 21 mm wide, at its thickest region, placement of an electromagnetic actuator of those dimensions inside of the mast is not possible. Applying knowledge from chapter 2 a custom-made electromagnet was developed. Datasheet for both coil and the bobbin is in Appendix B. The core is made of M33 material, which is an alloy based on MnZn metals, with relative magnetic permeability 휇푟 = 750. Relative magnetic 휇 permeability is defined as 휇푟 = . Outer dimensions of electromagnet are 3,5 x 9 mm (length, 휇0 diameter). Applying (3) attraction force is calculated in respect to amount of turns of the winding, 푁, and current, 퐼. Figure 16 depicts the electromagnetic force calculations for custom- made electromagnet in this study.

Figure 16. Electromagnetic force in respect to amount of turns and current change.

The design of the electromagnetic actuator is presented in Figure 17 that shows the rudder (from left to the right) in its the neutral position, at positive 10 ° and negative 10 °. Electromagnets (orange colour) are partly placed inside of the NACA0021 profile to allow more rudder (red) movement space.

Figure 17. Electromagnetic actuator installed inside of the morphing structure.

22 4 RESULTS

In the results chapter the results that are obtained with the process/methods described in the previous chapter are compiled, and analyzed and compared with the existing knowledge and/or theory presented in the frame of reference chapter.

4.1 Theoretical results To decide the optimal configuration parameters such as energy usage, response time, maximal rudder force and volume of the solution are considered. Table 2 summarises the outcome of respective configurations.

Table 2. Theoretical results.

Evaluation Parameter Configuration Unit

hinge + servo + hinge morphservo + el + hinge morphel + Energy usage (idle) 0 0 1000 0 mA Energy usage (actuation) 200 250 5000 5000 mA Response time 3 3 0 0 ms Maximal rudder force 15 15 7.5 7.5 Ncm Volume of the solution 12 631 16 450 7 828 11 648 mm^3

Due to gears in servo and natural stiffness of the morphing structure, idle energy usage for all configurations except hinge-typed structure in combination with electromagnetic actuator is zero. Power consumption of the later is controlled by the user but requires continuous current on both electromagnets to keep a flap straight.

For micro servo actuator energy usage under actuation is calculated using Simulink model that is presented in Appendix C. Since hinge-typed mechanical construction is expected to have less stiffness, lower current is also expected. Electromagnets used as actuator consist of 9 turns of 0.8 mm thick wire which allows to draw continuous 5A through the magnets. Consider that max torque value is given for a single electromagnet.

Response time of each configuration depends on the actuator type. For the chosen micro servos no information from the developers is available in the datasheet. Researching similar products information about BMS-410C is found and used as reference for the study. Both servos are analogue, have plastic transmission, JR connector system and are powered by same voltage. Datasheet for BMS-410C can be found in Appendix D. Since the actuation begins as fast as the current is supplied to the electromagnets the response time is considered 0.

Lowest volume by far is achieved by configuration hinge-typed mechanical structure in combination with electromagnetic actuator. The volume is almost the same in both morphing mechanical structure with electromagnetic actuator and hinge-typed mechanical structure together with servo as actuator. Combination morphing structure in combination with servo as actuator has highest volume of proposed solutions. It doubles the volume of the most compact solution and outranges others by 33%. Volume of the solutions is calculated from CAD assembly

23 of each mechanical structure with corresponding actuator type. Control units as microcontrollers and wires are not considered during the calculations. Exact part list considered in each calculation and respective components volume is presented in Appendix E.

4.2 Verification To verify the theoretical results, physical prototypes are built. The first one is the hinge-typed structure with servo as actuator, second is the morphing type structure and finally an electromagnetic actuator. Figure 18 presents the design of hinge-type structure with servo as actuator (left) and the morphing structure (right).

Figure 18. Physical prototypes for validation of theoretical results.

Prototypes assist in verifying conceptual design, stiffness of the morphing structure, energy usage and electromagnetic forces study. The validated results of the presented parameters in Table 2 are given in Table 3.

Table 3. Verification results.

Evaluation Parameter Configuration Unit

hinge + servo + hinge morphservo + el + hinge morphel + Energy usage (idle) 6 6 1000 0 mA Energy usage (actuation) 40-60 250 5000 5000 mA Response time 30 30 0 0 ms Maximal rudder force 15 15 7.5 7.5 Ncm Volume of the solution 12 631 16 450 7 828 11 648 mm^3

24 Energy usage is measured by setting an accurate multimeter (in this case FLUKE75) in series with the actuator and noting the DC value. Energy usage at actuation, for servo actuator, is measured by continuously turning the flap between ±15° in the air (dry tests without hydrodynamic load). For the electromagnetic actuator the values are measured for continuous actuation between ±10° as the actuator design does not allow higher actuation range.

The prototypes prove that the design is reasonable and the volumes of the configurations are validated to be the same as theoretical one.

The strength of the electromagnetic actuator is verified by measuring its magnetic field. Figure 19 shows linear behaviour as the magnetic field is proportional to the current, see (2), but the strength of the magnetic field is way lower than expected. Respective datapoints are received through varying the current, in steps of 0.1, from 0.5 to 1A. Actuator is powered by KENWOOD power supply and magnetic field is measured by SDL900 magnetic meter.

Figure 19. Measured magnetic field generated by electromagnet in respect to current change.

The electromagnet used for measurements of the magnetic field is own coiled by the author and presented in Figure 20.

Figure 20. Electromagnet used for magnetic field measurements.

Measurement of response time of the actuators is done by filming. In the video, it is clear to see when the current is supplied and a stopwatch with resolutions of milliseconds allows us to tell how long the actuation time is. Setup used for electromagnetic actuator measurements is presented in Figure 21, and for servo, in Figure 22.

Figure 21. Response time measurement of electromagnetic actuator.

The response time of the electromagnetic actuator is accurate according to theory i.e. instant.

Figure 22. Response time measurement of servo actuator.

On the contrary, response time of the servo is longer than the theoretical one.

26 5 DISCUSSION AND CONCLUSIONS

A discussion of the results and the conclusions that the authors have drawn during the Master of Science thesis are presented in this chapter. The conclusions are based from the analysis with the intention to answer the formulation of questions that is presented in Chapter 1.

5.1 Discussion

Theory studies imply that servo energy usage at idle is 0 but in fact the measured power consumption for servo is 6 mA. El-actuator in combination with hinge-typed structure is drawing power to keep the flap straight as this configuration got no internal stiffness. Servo can hold the position at standby by exchanging the gears but combination of internal signals and thickness of servo wires (becoming a small resistance) results in constant current leak. Power consumption seems to be low, but it is important for more exact calculation of battery / weight ratio of the vessel. Theoretical results for micro servo energy usage under actuation are affected by model design. Since servo motor block in Simulink works to keep fixated torque reference, in our case 15 Ncm, maximal rated torque of REELY S-8246. This ensures that simulation has a good margin for the testes under load. Actual results show that actuation in air does not require more than 60 mA continuous current. The response time is measured with accuracy of milliseconds. The electromagnetic actuator behaves as expected and attracts the rod as fast as the current is supplied. Servo actuator is on the other hand showing a delay of 30 milliseconds. Important to consider delays for control signal to the servo. During measurements Arduinos internal library servo.h is used. It is written in Arduino code and is well-knows for not being the fastest. Also the servo needs a small delay to actually reach the specified position before handling next signal. Maximal torque value for the micro servo is stated in its datasheet and are reliable. In case of other applications that require higher torque, the value can be increased by temporary violating maximal current ratings. A single electromagnet supplies 7.5 Ncm attracting the rotational rod. Using the smart control algorithm results in a greater torque due to one electromagnet repelling and one getting stronger as the rod moves towards it. Due to complexity about how magnetic field changes by distance from the coil, lowest value is presented in this study. In practice, the actuator could show better performance. It is important to consider how the mechanical design affects volume calculations. In hinge- typed structure, servo is mounted directly to the rotational rod and in morphing type case, on the flap. This design does not require extra joints or shafts for mounting between servo and the structure. Servo connected right on the mast meaning that this part of the mast will need to become wider and is negative in some applications. Therefore, the servo is, in many cases, located higher in the hull which requires a longer rotational rod or maybe even a joint or some other mechanical solution meaning that calculated volume values can become way larger depending on application scenario. Configuration hinge-typed mechanical structure in combination with morphing actuator seem very promising regarding the space-efficiency but the drawback of these designs is the energy usage. Servo actuator and mold that forms morphing structures are stiff enough to keep the flap straight when no actuation is required, meanwhile this configuration can’t. For both electromagnets, a continuous current is needed to generate equal magnetic attraction force and to keep the flap straight. Presented electromagnetic actuator does not have a position feedback. By

27 using the feedback controller the configuration could draw way less current by some adjustments rather than continuously being ready to withstand maximal disturbance. Actuation range is not considered as evaluation parameter but still is an important parameter to keep in mind once choosing the design. The range allowed by the servo motor is superior over electromagnetic actuator as the attraction force decreases rapidly for each 푚푚 to the actuation rod. For some applications where greater angles are desired, servomotor is a great choice meanwhile in marine applications the wing / rudder reaches cavitation point before all range of the servo is utilized. Measured magnetic field of the electromagnet shows that theoretical values are higher than the real ones. This depends on two main factors. First, magnetic permeability of the core material has a very low impact at small inductance values, i.e. weak magnetic fields. Second, the theory is a common simplification but in fact, magnetic field generated by electromagnets is a nonlinear function of the current which requires finite element methods to solve. And the dynamics seem to have a large input on small electromagnets.

5.2 Conclusions The presented results in this study show that most optimal configuration for rudder-flap construction and actuator, based on energy usage, is the hinge-typed mechanical structure in combination with servo actuator. Considering volume of the solution the hinge-typed structure in combination with electromagnetic actuator are the most optimal. Servo actuator is superior over electromagnetic actuator in maximal rudder force. However, the electromagnetic actuator is the optimal choice when considering fastest response time. Evaluation parameters don’t take the hydrodynamic losses, due to the structural gaps, into account. Considering that servo actuator with morphing structure configuration, has no hydrodynamic losses, can supply maximal rudder force and has compact volume, author would therefore recommend this configuration for design of rudder and flap for miniaturized FoilCart vessel. The conclusion is valid for miniaturized hydrofoil vessel of similar design as FoilCart. If implemented, this can be first successful step to adapt morphing structures into marine applications. The theoretical results is a good basis for future projects to decide the design for their specific application. Electromagnetic verifications serve as a good basis for small-scaled electromagnets and electromagnetic actuators. Electromagnetic actuator performance is weaker than expected but a lack of torque can be compensated by setting several of them along the mast. In this way a torque value can increase greatly and the huge advantages of being able to keep the actuator exposed to water and place it in hard accessible areas will prevail. The electromagnetic actuator can still be a very solid choice of actuator in hydrofoil applications depending on use case.

6 RECOMMENDATIONS AND FUTURE WORK

In this chapter, recommendations on more detailed solutions and future work in this field are presented.

6.1 Recommendations Actuating the wings by a servomotor imply more complicated physical connections since they are located far away from the hull. This is to ensure that fuselage stays as small as possible for better hydrodynamic performance. There are several mechanical solutions that can be implemented so the complexity and volume would vary a lot. Wires of the electromagnets allow flexibility in design that is an advantage in wing actuation. Those aspects should be studied carefully before choosing design of the wing and flap of a hydrofoil vessel. Molding the morphing structure seem easy at first but not as trivial as one could think. It is important to design some kind of support construction that will allow smooth inflow to the channels and fixate it so the mold would not leak away. Degassing the mold is important as in such a small constructions the bubbles will heavy affect the solidity.

6.2 Future work The CFD analysis that is carried out during the study is a 2D analysis. For a more exact prediction of hydrodynamic behavior a 3D analysis should be done. To evaluate the performance of the constructions in real application wind tunnel tests followed by tests in water are mandatory. This study had no focus on material choice of the morphing structure which itself could be a master thesis in material design. A good material for the application is the one which is exactly stiff enough to keep the flap straight but as easy for actuator to bend as possible. The mold attachment should hold for long time as it will be the part that is most exposed to hydrodynamic pressure. Since this study focused on the mast and rudder construction, an obvious choice was the symmetrical wing profile. For the elevator and lifting wings, the profile should rather be asymmetrical to generate lift even at 훼 = 0°. The finite element method calculations of the magnetic field for electromagnetic actuator is an interesting topic. It is of high value to compare the FEM calculations with simplified ones and also with the actual results presented in the study. A position feedback for the actuator, no matter the choice, will affect the control system of the vessel and will be a closed loop control which is more energy efficient and precise.

29 7 REFERENCES

[1] T. E. o. E. Britannica, “Encyclopædia Britannica,” 09 Juni 2017. [Online]. Available: https://www.britannica.com/technology/hydrofoil. [2] O. M. Faltinsen, “Knovel,” 2005. [Online]. Available: https://app.knovel.com/hotlink/pdf/id:kt008MM7FA/hydrodynamics-high-speed/2d-flow. [3] A. B. AvLiang Yun, High Performance Marine Vessels, 2012. [4] J. a. K. S. Urde, “Hydrofoiling Europe-Dinghy,” 2017. [Online]. Available: https://lup.lub.lu.se/student-papers/search/publication/8938139. [Accessed 2017]. [5] D. K. a. M. P. Brady Doepke, “Design and demonstration of a flexible matrix composite morphing control surface for air gap control in a Fowler flap,” Journal of Intelligent Material Systems, vol. 28, no. 20, pp. 3139-3151, 2017. [6] Q. G. Y. L. a. J. L. Jian Sun, “Morphing aircraft based on smart materials and structures: A state-of-the-art review,” Journal of Intelligent Material Systems and Structures, 2016. [7] V. K. P. K. a. R. M. P. Jaenker, “PIEZO ACTIVE VIBRATION AND NOISE CONTROL IN,” in 26TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES, 2008. [8] O. B. a. M. I. F. B. K. S. Woods, “Wind Tunnel Testing of the Fish Bone Active Camber Morphing,” in 23 rd International Conference on Adaptive Structures and Technologies, Nanjing, China, 2012. [9] I. Yukikazu, Case Studies in Superconducting Magnets: Design and Operational Issues: Second Edition, 2009. [10] “Advanced Textbooks in Control and Signal Processing,” in Actuators and sensors. 2009., 2009, pp. 191-231. [11] “airfoiltools,” [Online]. Available: airfoiltools.com . [12] J. Moran, An introduction to theoretical and computational aerodynamics, 2003. [13] NASA, “Shape Effects on Lift,” 05 April 2018. [Online]. Available: https://www.grc.nasa.gov/www/k-12/airplane/shape.html. [14] M. D. a. H. Youngren, “XFOIL documentation,” 30 November 2001. [Online]. Available: http://web.mit.edu/drela/Public/web/xfoil/xfoil_doc.txt. [15] D. Young, A Brief Introduction to Fluid Mechanics, 1996. [16] NE, “Ljudhastighet,” 11 07 20019. [Online]. Available: http://www.ne.se/uppslagsverk/encyklopedi/enkel/ljudhastighet. [17] A. M. Robert Fox, Introduction to Fluid Mechanics, 1973. [18] J. Bergström, “CFD for mixing efficiency in comercial and industrial advanced air oxidation,” 2018. [19] J. Maxwell, A Treatise on Electricity and Magnetism, 1873. [20] BBC, “bbc,” 2019. [Online]. Available: https://www.bbc.co.uk/bitesize/guides/z3g8d2p/revision/4. [21] D. S. K. Skubov, Non-Linear Electromechanics, Springer Berlin Heidelberg, 2008. [22] B. Cullity, Introduction to magnetic materials, second edition, Somerset: Wiley, 2009. [23] P. L. H. S. Lorrain, Magento-Fluid Dynamics: Fundamentals and Case Studies of Natural Phenomena, Springer New York, 2006. [24] D. Koshal, Manufacturing Engineer's Reference Book, Elsevier, 1993. [25] “Conrad,” [Online]. Available: https://www.conrad.se.

Appendix A: Reynolds number calculations code

Code 1: Reynolds number calculation for speeds ranging from 7 to 10 m/s.

Code 2: Torque in the hinge calculations.

31 Appendix B: Datasheet for M33 core and bobbin

32 Appendix C: Simulink model for energy usage estimation

33 Appendix D: BMS-410C datasheet

34 Appendix E: Volume calculations for the design solutions

Table 1. Exact volume of designed configuration for hinge-typed mechanical structure with servo as actuator. Component Amount Volume [mm^3] Bearing 2 175,929 Rotational Rod 1 5230,752 Micro Servo 1 7048,127

Total volume 12630,737

Table 2. Exact volume of designed configuration for morphing mechanical structure with servo as actuator. Component Amount Volume [mm^3] Morphing mold 2 4701,092 Micro Servo 1 7048,127

Total volume 16450,311

Table 3. Exact volume of designed configuration for hinge-typed mechanical structure with el-actuator. Component Amount Volume [mm^3] Bobin + Core 2 222,66 Rotational Rod 1 5230,752 Attraction Rod 1 1800 Bearing 2 175,929

Total volume 7827,93

Table 4. Exact volume of designed configuration for morphing mechanical structure with el-actuator. Component Amount Volume [mm^3] Bobin + Core 2 222,66 Morphing Mold 2 4701,092 Attraction Rod 1 1800

Total volume 11647,504

35 TRITA ITM-EX 2019:587