A Comparison Study of a Novel Self-Contained Electro-Hydraulic Cylinder Versus a Conventional Valve-Controlled Actuator—Part 2: Energy Eﬃciency

actuators

Article A Comparison Study of a Novel Self-Contained Electro-Hydraulic Cylinder versus a Conventional Valve-Controlled Actuator—Part 2: Energy Eﬃciency

Daniel Hagen * , Damiano Padovani and Martin Choux

Department of Engineering Sciences, University of Agder, 4879 Grimstad, Norway; [email protected] (D.P.); [email protected] (M.C.) * Correspondence: [email protected]

Received: 18 November 2019; Accepted: 3 December 2019; Published: 5 December 2019

Abstract: This research paper presents the second part of a comparative analysis of a novel self-contained electro-hydraulic cylinder with passive load-holding capability against a state of the art, valve-controlled hydraulic system that is typically used in load-carrying applications. After addressing the control design and motion performance in the first part of the study, the comparison is now focused on the systems’ energy efficiency. It is experimentally shown that the self-contained solution enables 62% energy savings in a representative working cycle due to its throttleless and power-on-demand nature. In the self-contained drive, up to 77% of the energy taken from the power supply can be used effectively if the recovered energy is reused, an option that is not possible in the state of the art hydraulic architecture. In fact, more than 20% of the consumed energy may be recovered in the self-contained system during the proposed working cycle. In summary, the novel self-contained option is experimentally proven to be a valid alternative to conventional hydraulics for applications where passive load-holding is required both in terms of dynamic response and energy consumption. Introducing such self-sufficient and completely sealed devices also reduces the risk of oil spill pollution, helping fluid power to become a cleaner technology.

Keywords: linear actuators; self-contained cylinders; electro-hydraulic systems; passive load-holding; proportional directional control valves; load-carrying applications; energy recovery; energy eﬃciency

1. Introduction

Due to the increasing focus on the environmental impact, such as CO2 emissions and oil spill pollution, inefficient state of the art hydraulic actuation tends to be replaced by electric drives in many industrial environments. This is the case, for instance, in offshore oil drilling [1]. However, hydraulic systems are still needed in load-carrying applications (e.g., knuckle-boom cranes or oil drilling equipment) because their force density is higher than that of their linear electro-mechanical counterparts and they do not present key issues related to reliability (e.g., strong impact forces) [2]. Consequently, compact and self-contained electro-hydraulic cylinders (SCCs) have received considerable attention in the last decade [3–10], showing the potential to replace both conventional hydraulics and linear electro-mechanical systems [11]. SCCs can in fact enhance energy efficiency, modular design, plug-and-play installation, and reduced maintenance [12]. Current commercial solutions of the SCC technology are limited and typically tailor-made, whereas the research emphasis is primarily on different electro-hydraulic configurations [13–16], energy-efficiency [17–21], thermal analysis [22–24], and low-power servo applications [25]. According to the survey presented in [10], compact and self-contained solutions comprising passive load-holding devices, able to operate in four quadrants, and suitable for power levels above 5 kW are missing, both on the market and

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four quadrants, and suitable for power levels above 5 kW are missing, both on the market and in the intechnical the technical literature. literature. Moreover, Moreover, the lack of the experime lack ofntal experimental comparisons comparisons in terms of energy in terms consumption of energy consumptionand efficiency and between efficiency SCCs between and conventional SCCs and conventional solutions for solutions load-carrying for load-carrying applications applications is evident. is evident.Only a Onlylimited a limited number number of simulation of simulation studies studies applying applying the the SCC SCC technology technology to load-carryingload-carrying applicationsapplications andand investigatinginvestigating thethe energy-savingenergy-saving potentialspotentials comparedcompared toto existingexisting hydraulichydraulic systemssystems werewere identifiedidentified [ 26[26,27].,27]. Hence,Hence, this this research research paper paper aims toaims experimentally to experimentally evaluate andevaluate compare and the compare energy consumption the energy andconsumption the energy and efficiency the energy of a self-contained efficiency electro-hydraulicof a self-contained cylinder electro-hydraulic versus a conventional cylinder approach.versus a Aconventional SCC concept approach. able to operate A SCC in concept four quadrants, able to oper includingate in four passive quadrants, load holding, including and passive suitable load for powerholding, levels and suitable above 5 for kW power was proposedlevels above in 5 [10 kW] and was implemented proposed in [10] on and a single-boom implemented crane on a in single- [12]. Aboom further crane analysis in [12]. of thisA further solution analysis is presented of this in solution the first partis presented of this research in the [first28], includingpart of this the research control design[28], including and the motion the control performance design comparison and the motion against performance the valve-controlled comparison cylinder against (VCC) the discussed valve- incontrolled [29]. Concerning cylinder (VCC) the paper discussed structure, in [29]. Section Concer2 presentsning the the paper two structure, considered Section actuation 2 presents systems the whiletwo considered Section3 the actuation theoretical systems background. while InSection Section 3 4the, the theoretical experimental background. results are In introduced Section 4, and the discussed.experimental The results comparison are introduced study will and show discussed. that the novelThe comparison self-contained study system will isshow a valid that alternative the novel toself-contained conventional system hydraulics is a alsovalid for alternative applications to whereconventional passive hydraulics load-holding also is requiredfor applications because where huge energypassive savings load-holding are enabled. is required because huge energy savings are enabled.

2.2. TheThe ConsideredConsidered ActuationActuation SystemsSystems TwoTwo actuation systems systems are are investigated investigated in this in thisresearch, research, namely namely a new aself-contained new self-contained electro- electro-hydraulichydraulic cylinder cylinder and state and of statethe art of valve-controll the art valve-controlleded architecture architecture that is typically that is typically used on usedload- oncarrying load-carrying applications. applications.

2.1.2.1. TheThe Self-ContainedSelf-Contained Electro-HydraulicElectro-Hydraulic CylinderCylinder TheThe combinationcombination ofof anan electricelectric drivedrive andand aa ﬁxed-displacementfixed-displacement axialaxial pistonpiston machinemachine drivesdrives thethe hydraulichydraulic cylindercylinder arrangedarranged inin aa closed-circuitclosed-circuit conﬁguration, configuration, as as illustrated illustrated in in Figure Figure1 .1.

FigureFigure 1.1. SchematicSchematic ofof thethe self-containedself-contained systemsystem addressedaddressed inin thisthis research.research.

TheThe auxiliaryauxiliary hydraulichydraulic componentscomponents areare implementedimplemented inin aa manifoldmanifold andand thethe bladder-typebladder-type accumulatoraccumulator representsrepresents thethe sealedsealed reservoir.reservoir. TheThe didiffﬀerentialerential ﬂow flow dictated dictated by by the the cylinder’s cylinder’s unequalunequal areasareas isis balancedbalanced byby the the two two pilot-operated pilot-operated check check valves valves FC FC1 1and and FC FC22,, thethe check check valves valves Ac Ac11 andand AcAc22,, andand the the check check valve valve CV CV1. The1. The pilot-operated pilot-operated check check valves valves LH1 andLH1 LH and2 are LH used2 are for used passive for passive load-holding load- purposesholding purposes by isolating by the isolating cylinder the when cylinder the 3 /when2 electro-valve the 3/2 electro-valve is not actuated. is not The actuated. electro-valve The mustelectro- be activatedvalve must to enablebe activated the actuator to enable motion, the actuator resulting motion, in transferring resulting the in highest transferring cylinder the pressure, highest selectedcylinder throughpressure, CV selected3 and CV through4, into CV the3 openingand CV4, pilot into linethe opening of the load-holding pilot line of valves. the load-holding Anti-cavitation valves. valves Anti- arecavitation installed valves on both are actuator installed sides, on both whereas actuator pressure-relief sides, whereas valves pressure-relief are present on valves the pump are present ports and on the pump ports and on the cylinder ports to protect from over-pressurizations. Finally, a cooler and a low-pressure filter complete the hydraulic system.

Actuators 2019, 8, 78 3 of 16 on the cylinder ports to protect from over-pressurizations. Finally, a cooler and a low-pressure filter complete the hydraulic system. The electric drive consists of the servo-motor and the servo-drive. The frequency converter is controlling the speed of the permanent magnet synchronous motor with field-oriented control whereas an outer closed-loop position controller is implemented on an embedded programmable logic controller (PLC) to supervise the motion of the hydraulic cylinder, sending the desired rotational speed (uSM) to the servo-drive. In addition, an external brake resistor is connected to the servo-drive to dissipate the regenerated power into heat. According to Ristic et al. [30], there exist solutions where this regenerated power is used profitably (e.g., power-sharing via common DC bus between multiple electric drives, return the electrical power to the grid, and energy storage on a battery or in a capacitor). . El Lastly, a power analyzer measures both the electric power consumed from the electrical grid (EGrid) . El and the electric power dissipated in the brake resistor (EBR). For more details about the self-contained system, Padovani et al. [12] describes its functionality, while the components used to implement this solution are presented in Table1.

Table 1. Components used to implement the self-contained system.

Symbol Component Manufacturer Model C Hydraulic cylinder PMC Cylinders 1 25CAL SM Servo-motor Bosch Rexroth 2 MSK071E-0300 P Axial piston machine Bosch Rexroth A10FZG SD Servo-drive Bosch Rexroth HCS02.1E-W0054 ACC Hydro-pneumatic accumulator Bosch Rexroth HAB10 CO Oil cooler Bosch Rexroth KOL3N F Hydraulic return line ﬁlter Bosch Rexroth 50LEN0100 3 LHV1,2 Pilot-operated check valves Sun Hydraulics CVEVXFN FC1,2 Pilot-operated check valves Sun Hydraulics CKEBXCN RV1–4 Pressure-relief valves Sun Hydraulics RDDA 4 CV1,3,4 Check valves Hawe Hydraulik RB2 CV2, Ac1,2 Check valves Hawe Hydraulik RK4 Ac3,4 Check valves Hawe Hydraulik RK2 EV 3/2 Directional valve Argo Hytos 5 SD1E-A3 p1–9 Pressure transducers Bosch Rexroth HM20 6 pp,r Pressure transducers Parker SCP-400 7 xC Cylinder position sensor Regal PS6300 PLC Embedded controller Bosch Rexroth XM22 BR Brake resistor Bosch Rexroth HLR01.1N-03K8 PA Power analyzer Hioki 8 PW6001 1 Sävsjö, Sweden; 2 Lohr, Germany; 3 Sarasota, USA; 4 München, Germany; 5 Zug, Switzerland; 6 Cleveland, USA; 7 Uppsala, Sweden; 8 Nagano, Japan.

A hydraulic cylinder with piston diameter of 65 mm, rod diameter of 35 mm, stroke length of 500 mm, and an integrated position sensor is common to both actuation systems (i.e., also to the . valve-controlled layout recalled in the sequel). The piston’s velocity (xC) is estimated by diﬀerentiating and lowpass ﬁltering the measured position (xC). Pressure transducers are installed directly on the piston-side (pp) and rod-side (pr) ports of the cylinder.

2.2. The Valve-Controlled System The valve-controlled system taken as a benchmark consists of a centralized hydraulic power unit (HPU) providing a constant supply pressure (pS) and a ﬁxed return pressure (pR) to the valve-controlled cylinder according to the schematic depicted in Figure2. Actuators 2019, 8, 78 4 of 16 Actuators 2019, 8, x FOR PEER REVIEW 4 of 16

𝑝 Control Valve

𝑄

𝑢 𝑝

𝑝

HPU 𝑝 C Electrical Grid 3~ P RV 𝑝 EM LHV

FigureFigure 2.2. SchematicsSchematics ofof thethe valve-controlledvalve-controlled systemsystem underunder investigation.investigation.

TheThe componentscomponents of of the the HPU HPU are are an an electric electric motor motor running running at theat the constant constant speed speed of 1500 of 1500rpm andrpm 3 aand variable-displacement a variable-displacement axial pistonaxial piston pump pump with a with maximum a maximum displacement displacement of 75 cm of/ 75rev cm. The/rev supply. The p = pressuresupply pressure is controlled is controlled by the absoluteby the absolute pressure pressure limiter limiter as S as180 𝑝 =bar, 180 while bar, a while pressure-relief a pressure-relief valve isvalve installed is installed for safety. for safety. The The motion motion of theof the hydraulic hydraulic cylinder cylinder is is controlled controlled by by a a state state ofof thethe art,art, pressure-compensatedpressure-compensated ﬂowflow controlcontrol valvevalve (i.e.,(i.e., aa proportionalproportional directionaldirectional controlcontrol valvevalve (PDCV))(PDCV)) thatthat receivesreceives the the control control input input (uV )( from𝑢 ) thefrom PLC. the The PLC. load-holding The load-holding valve is installed valve is to controlinstalled overrunning to control loadsoverrunning and for loads safety and purposes for safety during purposes standstill, during according standstill, to according regulations. to regulations. Finally, the Finally, system the is instrumentedsystem is instrumented with sensors with for sensors measuring for measuring the pressures the pressures labeled in labeled Figure 2in as Figure well as2 as the well rod-side as the

ﬂowrod-side rate flow (QR) rate and ( the𝑄) pistonand the position. piston position. For a more For detaileda more detailed description description of the functionalityof the functionality of the valve-controlledof the valve-controlled cylinder, cylinder, see for see instance for instance [29]. Details[29]. Details about about the components the components implemented implemented in the in valve-controlledthe valve-controlled system system arepresented are presented in Table in Table2. 2.

TableTable 2.2. ComponentsComponents usedused toto implementimplement thethe valve-controlledvalve-controlled system.system.

SymbolSymbol Component Component Manufacturer Manufacturer Model Model EMEM ElectricElectric motor ASEAASEA11 M225S60-4M225S60-4 P P AxialAxial piston piston variable pumppump BrueninghausBrueninghaus Hydraulik 22 A4V-S0-71A4V-S0-71 RVRV Pressure-reliefPressure-relief valvevalve Bosch Bosch Rexroth Rexroth DBDH6 DBDH6 V Flow control valve Sauer Danfoss 3 PVG32-9781 V Flow control valve Sauer Danfoss3 PVG32-9781 LHV Counterbalance valve Sun Hydraulics CWCA LHV pS,R,A CounterbalancePressure transducers valve Sun Hydraulics Parker CWCA SCP-400 Q𝒑r S,R,A PressureFlow ratetransducers meter Parker Parker SCP-400 SCQ-150 Qr 1 FlowVästerås, rate Sweden; meter2 Horb, Germany; 3 Nordborg, Parker Denmark. SCQ-150 1 Västerås, Sweden; 2 Horb, Germany; 3 Nordborg, Denmark. 3. Theoretical Background 3. Theoretical Background This section describes the analysis performed to process the experimental data collected from the two driveThis section systems. describes The objective the analysis is highlighting performed the to power process levels, the theexperimental energy consumption, data collected and from the etheﬃciency two drive of the systems. architectures. The objective is highlighting the power levels, the energy consumption, and the efficiency of the architectures. 3.1. Power and Energy Distribution 3.1. Power and Energy Distribution The diﬀerent power terms characteristic of both systems and highlighted in Figure3 are evaluated usingThe the equationsdifferent presentedpower terms below. characteristic For the sake of of bo clarity,th systems the input, and the highlighted transferred, in and Figure the output 3 are powersevaluated are using addressed the equations in separate presented subsections. below. For the sake of clarity, the input, the transferred, and the output powers are addressed in separate subsections.

Actuators 2019, 8, 78 5 of 16 Actuators 2019, 8, x FOR PEER REVIEW 5 of 16

𝐸 Hydraulic System 𝐸 𝐸 Electric 𝐸 Pump 𝐸 Aux. 𝐸 Hydraulic Drive Unit Valves Cylinder

(a) 𝐸 𝐸 Pressure 𝐸 𝐸 Hydraulic 𝐸 PDCV LHV Comp. Cylinder

(b)

Figure 3. TheThe different diﬀerent power terms: ( a)) Self-contained Self-contained system; ( b) valve-controlled system.

3.1.1. Input Power 3.1.1. Input Power The input power of the SCC corresponds to the electric power supplied by the electrical grid: The input power of the SCC corresponds to the electric power supplied by the electrical grid: 𝐸 .=𝑣El ⋅𝑖 +𝑣 ⋅𝑖 +𝑣 ⋅𝑖 , (1) E = va ia + v i + vc ic, (1) Grid · b · b · where 𝑣 and 𝑖 are the voltage and current of the i-th phase, respectively. Furthermore, when the whereexternalvi andloadii areis overrunning the voltage and (i.e., current when of the the icyli-th phase,nder is respectively. retracting), Furthermore,the SCC has when the potential the external to regenerateload is overrunning power, while (i.e., the when VCC the still cylinder consumes is retracting),energy. For the the SCCtest setup has the investigated potential toin regeneratethis study, power,the electric while power the VCC regenerated still consumes by the energy. SCC Foris diss theipated test setup in investigatedthe brake resistor in this study,and is theestimated electric poweraccording regenerated to: by the SCC is dissipated in the brake resistor and is estimated according to:

. El 𝐸 =𝑣 ⋅𝑖, (2) E = vBR iBR, (2) BR · where 𝑣 is the pulse width modulated voltage, and 𝑖 is the direct current, transferred from the whereservo-drivevBR is to the the pulse brake width resistor. modulated Hence, the voltage, combin anded iinputBR is theand direct output current, powers transferred of the electric from drive the defineservo-drive the SCC’s to the total brake electric resistor. power Hence, related the combinedto the power input supply: and output powers of the electric drive define the SCC’s total electric power related to the power supply: 𝐸 =𝐸 +𝐸. (3) . El . El . El The VCC’s input power is the hydraulicEPS = E Gridterm+ caEBRlculated. by involving the actuator’s flow (3) demand (𝑄) and the supply pressure of the HPU: The VCC’s input power is the hydraulic term calculated by involving the actuator’s flow demand 𝐸 =𝑝 ⋅ |𝑄|, (4) (QP) and the supply pressure of the HPU: where the demand depends on the valve command (𝑢 ), the measured flow rate (𝑄 ), and the areas . Hyd of the actuator according to the following Elogic: = p QP , (4) HPU S · | | 𝐴 where the demand depends on the valve⎧𝑄 command ⋅ ,𝑢 (uV),<0 the measured flow rate (Qr), and the areas of 𝐴 the actuator according to the following𝑄 = logic: . (5) ⎨ 0, 𝑢 =0 ⎩  𝑄,𝑢 >0  Ap Qr , uV < 0  · A  r 3.1.2. Transferred Power Losses QP =  . (5)  0, uV = 0   The mechanical power transferred between Qther, SCC’suV servo-motor> 0 and the axial piston machine: 𝐸 =𝑖 ⋅𝑘 ⋅𝜔 , (6) 3.1.2. Transferred Power Losses is described by the torque-producing current component (𝑖 ), the torque constant (𝑘 =2.05 Nm/A), The mechanical power transferred between the SCC’s servo-motor and the axial piston machine: and the measured rotational speed (𝜔) of the prime mover. The hydraulic power shared between the hydraulic machine and the manifold. Mechis defined by involving the effective flow supplied to the E = iq kt ω , (6) actuator and the pressure drop across the SMunit: · · SM i k = is described by the torque-producing𝐸 current=𝑥 ⋅ 𝐴 component ⋅(𝑝 −𝑝 ().q ), the torque constant ( t 2.05 Nm/(7)A ), and the measured rotational speed (ωSM) of the prime mover. The hydraulic power shared between Then, the hydraulic power distributed between the manifold and the cylinder results as: 𝐸 =𝑥 ⋅(𝐴 ⋅𝑝 − 𝐴 ⋅𝑝). (8)

Actuators 2019, 8, 78 6 of 16 the hydraulic machine and the manifold is defined by involving the effective flow supplied to the actuator and the pressure drop across the unit:

. Hyd . E = x Ap (p p ). (7) P C · · 1 − 2 Then, the hydraulic power distributed between the manifold and the cylinder results as:

. Hyd . E = x Ap pp Ar pr . (8) C C · · − · The power losses of the SCC take place in the electric drive (i.e., electric and mechanical losses), in the axial piston machine (i.e., mechanical-hydraulic and volumetric losses), in the auxiliary components (i.e., reduced throttling losses in the check valves), and in the hydraulic cylinder (i.e., friction losses and internal leakage). Concerning the VCC’s, the hydraulic power available downstream the pressure compensator is calculated based on the valve command as:   pp + p0 QP , uV < 0 .  · | | Hyd  EPC =  0, uV = 0, (9)   (pr + p ) QP , uV > 0 0 · | | including the ﬁxed pressure-drop across the control valve’s metering edge (p0). Further on, the hydraulic power delivered by the control valve is again function of uV:  pp QP, uV < 0 .  · Hyd  EPDCV =  0, uV = 0. (10)   pr QP, uV > 0 · Then, the hydraulic power related to the hydraulic cylinder results as described in Equation (8). The power losses taking place in the VCC are the throttling losses of the pressure compensator, the throttling losses of the control valve’s spools, the throttling losses in the counterbalance valve, and the friction losses and internal leakages of the hydraulic cylinder. Finally, the losses of the HPU are neglected in this experimental study to be conservative, since it is sized for delivering constant pressure to several applications available in the lab.

3.1.3. Output Power When actuating the considered single-boom crane depicted in Figure4a, the mechanical output power that is applicable to both systems, is deﬁned as:

. Mech d E = (EK + EP), (11) SBC dt · where EK and EP are the kinetic and potential energy, obtained according to Equations (12) and (13). The diagram shown in Figure4b identiﬁes the parameters used in the calculation of the total mechanical energy. Relevant numerical values describing the kinematics are listed in Table3. Actuators 2019, 8, x FOR PEER REVIEW 6 of 16

The power losses of the SCC take place in the electric drive (i.e., electric and mechanical losses), in the axial piston machine (i.e., mechanical-hydraulic and volumetric losses), in the auxiliary components (i.e., reduced throttling losses in the check valves), and in the hydraulic cylinder (i.e., friction losses and internal leakage). Concerning the VCC’s, the hydraulic power available downstream the pressure compensator is calculated based on the valve command as:

(𝑝 +𝑝)⋅|𝑄|,𝑢 <0 𝐸 = 0, 𝑢 =0, (9) (𝑝 +𝑝)⋅|𝑄|,𝑢 >0 including the fixed pressure-drop across the control valve’s metering edge (𝑝 ). Further on, the hydraulic power delivered by the control valve is again function of 𝑢:

𝑝 ⋅𝑄,𝑢 <0 𝐸 = 0, 𝑢 =0. (10) 𝑝 ⋅𝑄,𝑢 >0 Then, the hydraulic power related to the hydraulic cylinder results as described in Equation (8). The power losses taking place in the VCC are the throttling losses of the pressure compensator, the throttling losses of the control valve’s spools, the throttling losses in the counterbalance valve, and the friction losses and internal leakages of the hydraulic cylinder. Finally, the losses of the HPU are neglected in this experimental study to be conservative, since it is sized for delivering constant pressure to several applications available in the lab.

3.1.3. Output Power When actuating the considered single-boom crane depicted in Figure 4a, the mechanical output power that is applicable to both systems, is defined as: 𝑑 𝐸 = ⋅ (𝐸 +𝐸 ), (11) 𝑑𝑡 where 𝐸 and 𝐸 are the kinetic and potential energy, obtained according to Equations (12) and Actuators 2019, 8, 78 7 of 16 (13).

𝐿 G 𝐿

𝛼 𝑚 ⋅𝑔

𝐿 𝛼 𝜃(𝑥 ) A 𝑦 C 𝐿 𝛼 𝐿 (𝑥 ) 𝑥 𝐿

B 𝐿 (a) (b)

Figure 4. TheThe considered considered application: application: (a) ( aThe) The hydraulic hydraulic actuator actuator conne connectedcted to the to thecrane-boom; crane-boom; (b) (simplifiedb) simpliﬁed schematic schematic diagram diagram of the of thecrane’s crane’s kinematics. kinematics.

Table 3. Kinematic parameters of the single-boom crane with full payload. The diagram shown in Figure 4b identifies the parameters used in the calculation of the total mechanical energy.Parameter Relevant numerical Value values describing Parameter the kinematics are Value listed in Table 3.

mcm 415.08 (kg) LAB 1136 (mm) 2 Jcm 159.63 (kg m ) α0 0.232 (rad) · LAGx 3139 (mm) α1 0.020 (rad) LAGy 64 (mm) α2 1.192 (rad) 2 LAC 565 (mm) g 9.81 (m/s )

. The kinetic energy and the potential energy, as a function of the angular speed (θ) and angular position (θ) of the boom, are deﬁned according to the following equations:

. . 2 1 2 2 EK θ = mcm L + mcm L + Jcm θ , (12) 2 · · AGx · AGy · EP(θ) = mcm g L cos(θ + α ) + L sin(θ + α ) , (13) · · AGy · 1 AGx · 1 where the angular position of the boom and the eﬀective length of the hydraulic cylinder (LC), including the initial length (LC,0 = 772 mm), are given below as a function of the piston position:   L2 + L2 L2 (x ) 1 AC AB C C  θ(xC) = cos−  −  α2 + α0, (14)  2 L L  − · AC · AB

LC(xC) = xC + LC,0. (15)

3.2. Eﬃciency of the Systems

Starting from the SCC, the overall efficiency (ηOverall), the efficiency of the electric drive (ηED), and the efficiency of the hydraulic sub-system (ηHS) are given in Equations (16)–(18). Due to the dual behavior of the system being regenerative while retracting the cylinder (i.e., the load is overrunning when the crane boom is lowered), and consuming when extending the cylinder (i.e., the load is resistant when the crane boom is lifted), two alternatives are introduced in the following definitions:

.  El . El  EPS ( )  . Mech , EPS > 0 resistant load SCC  ESBC η = . , (16) Overall  Mech . El  ESBC ( )  . El , EPS 0 overrunning load EPS ≤ Actuators 2019, 8, 78 8 of 16

.  El . El  EPS ( )  . Mech , EPS > 0 resistant load SCC  ESM η = . , (17) ED  Mech . El  ESM ( )  . El , EPS 0 overrunning load EPS ≤ .  Mech . Mech  ESM ( )  . Hyd , ESM > 0 resistant load SCC  EC η = . . (18) HS  Hyd . Mech  EC ( )  . Mech , ESM 0 overrunning load ESM ≤

Similar terms are also proposed for the VCC, where the eﬃciency of the control valve (ηV) and of the remaining hydraulic system, namely the load-holding valve (ηLHV), are given as follows:

Actuators 2019, 8, x FOR PEER REVIEW  . Hyd 8 of 16  E . Hyd  HPU , E > 0 (resistant load)  . Mech HPU VCC =  ηOverall 𝐸 ESBC , (19)  ,𝐸 > 0 (resistant load)  . Hyd 𝜂 =𝐸 , (19) 0, EHPU 0 (overrunning load) ≤ 0, 𝐸 ≤ 0 (overrunning load)  . Hyd   EHPU . Hyd 𝐸 , E > 0 (resistant load)  . Hyd,𝐸 >HPU 0 (resistant load) ηVCC =  , (20) 𝜂 V =𝐸EPDCV , (20)   . Hyd  0,0, 𝐸 E≤ 0 (overrunning0 (overrunning load) load) HPU ≤ . 𝐸 Hyd . Hyd ⎧EPDCV  . ,𝐸, >E 0 (resistant> 0 (resistant load) load) ⎪ Hyd PDCV VCC 𝐸 EC η = . Hyd . (21) 𝜂LHV=  . Hyd . (21) ⎨𝐸 EC  . Hyd , EPDCV 0 (overrunning load) ⎪ ,𝐸 ≤ 0 (overrunning load) EPDCV ≤ ⎩𝐸 4. Experimental Results and Discussion 4. Experimental Results and Discussion After mentioning the experimental test-beds, the considered working cycle is addressed. Then, theAfter power mentioning levels andthe experimental efficiencies oftest-beds, the diff erentthe considered sub-systems working are separately cycle is addressed. evaluated Then, while liftingthe power and lowering levels and the efficiencies single-boom of crane.the different Finally, sub-systems the overall are energy separately consumption evaluated and while efficiency lifting are assessed.and lowering The experimental the single-boom data werecrane. collected Finally, the by usingoverall the energy test setups consumption illustrated and in efficiency Figure5. are assessed. The experimental data were collected by using the test setups illustrated in Figure 5.

(a) (b)

FigureFigure 5. 5.The The experimental experimental setups: setups: ( a()a The) The self-contained self-contained electro-hydraulic electro-hydraulic system; system; ( b(b)) a a portion portion of of the valve-controlledthe valve-controlled system. system.

All data, besides the electric power, is processed through a real-time interface (i.e., 1 ms sample rate) between the PLC and MATLAB-Simulink®. The measured electric power is collected with a 50 ms sample rate and further processed in the MATLAB-Simulink® environment. Mineral oil ISO VG 46 is used as the hydraulic fluid in both actuation systems. All tests were carried out for a working cycle with maximum payload (i.e., with a load mass equal to 304 kg). The motion profile generator, presented in [27], provides reference signals for the desired piston position and velocity, as illustrated

in Figure 6. The cylinder’s piston extends and retracts between the start position 𝑥, =50 mm and the desired position 𝑥, = 450 mm, at a desired maximum velocity 𝑣, = 120 mm/s.

Actuators 2019, 8, 78 9 of 16

All data, besides the electric power, is processed through a real-time interface (i.e., 1 ms sample rate) between the PLC and MATLAB-Simulink®. The measured electric power is collected with a 50 ms sample rate and further processed in the MATLAB-Simulink® environment. Mineral oil ISO VG 46 is used as the hydraulic ﬂuid in both actuation systems. All tests were carried out for a working cycle with maximum payload (i.e., with a load mass equal to 304 kg). The motion proﬁle generator, presented in [27], provides reference signals for the desired piston position and velocity, as illustrated in Figure6. The cylinder’s piston extends and retracts between the start position xC,0 = 50 mm and the desired position xC,re f = 450 mm, at a desired maximum velocity vC,max = 120 mm/s. Actuators 2019, 8, x FOR PEER REVIEW 9 of 16

(a) (b)

Figure 6. GeneratedGenerated motion motion profile: proﬁle: ( (aa)) Desired Desired piston piston position; position; ( (b)) desired desired piston piston velocity.

The SCC is operated in closed-loop, including position feedback control, velocity feedforward, The SCC is operated in closed-loop, including position feedback control, velocity feedforward, pressure feedback, and passive load-holding, according to the control strategy and the control algorithm pressure feedback, and passive load-holding, according to the control strategy and the control presented in the first part of this comparison study [28]. In passive load-holding mode, the load-holding algorithm presented in the first part of this comparison study [28]. In passive load-holding mode, the valves are closed by deactivating the electro-valve and switching off the signal enabling power to the load-holding valves are closed by deactivating the electro-valve and switching off the signal enabling prime mover when motion is not desired. power to the prime mover when motion is not desired. 4.1. Power Levels 4.1. Power Levels The power levels of the different terms mentioned in Section 3.1 are shown in Figure7. The power levels of the different terms mentioned in Section 3.1 are shown in Figure 7.

(a) (b)

Figure 7. Power measurements: ( aa)) Self-contained Self-contained system; system; ( (b)) valve-controlled valve-controlled system.

When the values in Figure 7 7 areare positive,positive, thenthen thethe powerpower isis transferredtransferred toto thethe actuator.actuator. ViceVice versaversa (i.e., when the trends are negative),negative), there is potentialpotential to recover energy. During the crane’s lowering phase, thethe SCCSCC outputsoutputs electricalelectrical power,power, while while the the VCC VCC dissipates dissipates hydraulic hydraulic energy energy in in the the valves. valves. It isIt worthis worth mentioning mentioning that, that, during during steady-state steady-state operations, operations, the VCCthe VCC requires requires a maximum a maximum power power of about of about7.6 kW 7.6 while kW thewhile SCC the demands SCC demands only 4.8 only kW 4.8 (37% kW less). (37% less).

4.2. Systems Efficiency Efficiency The overalloverall eefficiencyfficiency ofof both both systems systems is is plotted plotted in in Figure Figure8 together 8 together with with the the terms terms related related to the to thediff erentdifferent sub-systems. sub-systems.

(a) (b)

Figure 8. Efficiency of the systems: (a) Self-contained system; (b) valve-controlled system.

Actuators 2019, 8, x FOR PEER REVIEW 9 of 16

(a) (b)

Figure 6. Generated motion profile: (a) Desired piston position; (b) desired piston velocity.

The SCC is operated in closed-loop, including position feedback control, velocity feedforward, pressure feedback, and passive load-holding, according to the control strategy and the control algorithm presented in the first part of this comparison study [28]. In passive load-holding mode, the load-holding valves are closed by deactivating the electro-valve and switching off the signal enabling power to the prime mover when motion is not desired.

4.1. Power Levels The power levels of the different terms mentioned in Section 3.1 are shown in Figure 7.

(a) (b)

Figure 7. Power measurements: (a) Self-contained system; (b) valve-controlled system.

When the values in Figure 7 are positive, then the power is transferred to the actuator. Vice versa (i.e., when the trends are negative), there is potential to recover energy. During the crane’s lowering phase, the SCC outputs electrical power, while the VCC dissipates hydraulic energy in the valves. It is worth mentioning that, during steady-state operations, the VCC requires a maximum power of about 7.6 kW while the SCC demands only 4.8 kW (37% less).

4.2. Systems Efficiency

ActuatorsThe2019 overall, 8, 78 efficiency of both systems is plotted in Figure 8 together with the terms related10 of to 16 the different sub-systems.

Actuators 2019, 8, x FOR PEER( aREVIEW) (b) 10 of 16 Figure 8. EfficiencyEfficiency of the systems: ( (aa)) Self-contained Self-contained system; system; ( (bb)) valve-controlled valve-controlled system. system. The results in Figure 8a show that the SCC’s efficiency, when the cylinder is lifting the crane at steady-stateThe results (i.e., in at Figure 4 s) is8 abouta show 60 that% for the the SCC’s overall e ffi system,ciency, when93% for the the cylinder electric is drive, lifting and the crane65% for at steady-statethe hydraulics. (i.e., The at 4overall s) is about efficiency 60% forwhen the lowering overall system, the crane 93 %boom,for the assuming electric full drive, recovery and 65 of% thefor theregenerated hydraulics. power, The varies overall at e 51–65fficiency%. It when can be lowering observed the in craneFigure boom, 8a that assuming there is a full1.2 s recovery delay between of the regeneratedwhen the SCC power, starts varies to lower at 51–65 the %crane. It can boom, be observed and the inelectric Figure drive8a that starts there to is regenerate a 1.2 s delay power. between For whenthe benchmark the SCC starts system to lower(Figure the 8b), crane the boom,overall and efficiency the electric is 36 drive% when starts the to cylinder regenerate is lifting power. the For crane the benchmarkboom at steady-state system (Figure (i.e., at8 4b), s). the Furthermore, overall e ffi almostciency 50 is% 36 of% thewhen VCC’s the cylinderinput power is lifting is dissipated the crane in boomthe control at steady-state valve (the (i.e., most at in 4 the s). Furthermore,pressure-compensat almostor). 50% Theof theefficiency VCC’s inputof the powercounterbalance is dissipated valve, in theduring control lifting valve (i.e., (the the most flow inis thebypassed pressure-compensator). through the LHV’s The check effi ciencyvalve), ofis the74% counterbalance at the lowest, and valve, at duringthe maximum lifting (i.e., 51% the through flow is the bypassed lowering through phase the (i.e., LHV’s the LHV’s check poppet valve), introduces is 74% at the a lowest,desired andpressure- at the maximumdrop). 51% through the lowering phase (i.e., the LHV’s poppet introduces a desired pressure-drop).

4.3. Energy Consumption As described in in the the first ﬁrst part part of ofthis this study study [28] [28, the], theSCC SCC can canbe operated be operated with withtwo different two diﬀ erentload- load-holdingholding strategies, strategies, namely namely passive passive load-holding load-holding (PLH) (PLH) and and active active load-holding load-holding (ALH). (ALH). The The electric powerpower andand energyenergy consumptionconsumption whenwhen utilizingutilizing thesethese twotwo strategiesstrategies areare comparedcompared inin FigureFigure9 .9.

(a) (b)

Figure 9.9. ComparisonComparison between between passive passive and and active active load-holding load-holding for thefor self-contained the self-contained cylinder: cylinder: (a) Input (a) power;Input power; (b) energy (b) energy consumption. consumption.

When thethe SCC SCC maintains maintains the the desired desired piston piston position position between between 6.5 and 6.5 16 sand (Figure 16 s9 a),(Figure the measured 9a), the electricmeasured input electric power input varies power at 5–7varies W at with 5–7 passiveW with load-holding,passive load-holding, and at 187–312and at 187–312 W with W active with load-holding.active load-holding. Active Active load-holding load-holding results inresults an energy in an consumptionenergy consumption of 5.32 Wh of 5.32 for the Wh entire for the working entire cycleworking considered cycle considered here (Figure here9b), (Figure compared 9b), tocompared 4.34 Wh withto 4.34 passive Wh with load-holding passive load-holding (i.e., 18.4% less). (i.e., However,18.4% less). performing However, performing active load-holding active load-holding for a reduced for amounta reduced of amount time is stillof time acceptable is still acceptable since the powersince the consumption power consumption remains relatively remains relatively low. low. Moving toto the the valve-controlled valve-controlled layout, layout, load-sensing load-sensing pumps pumps are used are insteadused instead of constant-pressure of constant- suppliespressure tosupplies increase to the increase overall the eﬃ overallciency inefficiency many applications. in many applications. Two scenarios Two are scenarios therefore are addressed; therefore theaddressed; ﬁrst one the (Figure first one10a) (Figure focuses 10a) on measurements focuses on me fromasurements the test from setup. the The test scenario setup. The #2 (Figure scenario 10 #2b) considers(Figure 10b) an HPUconsiders equipped an HPU with equipped a load-sensing with a pump load-sensing and driven pump by anand induction driven by motor an induction running atmotor 1500 running rpm (the at losses 1500 ofrpm the (the HPU losses are simulated of the HP basedU are onsimulated the model based presented on the inmodel [27]). presented Full energy in [27]). Full energy recovery is then assumed for the SCC in this second situation (i.e., the energy dissipated in the brake resistor when the SCC is lowering the crane boom is now reused).

(a) (b)

Figure 10. Energy consumption comparison: (a) The scenario #1; (b) the scenario #2.

Actuators 2019, 8, x FOR PEER REVIEW 10 of 16

The results in Figure 8a show that the SCC’s efficiency, when the cylinder is lifting the crane at steady-state (i.e., at 4 s) is about 60% for the overall system, 93% for the electric drive, and 65% for the hydraulics. The overall efficiency when lowering the crane boom, assuming full recovery of the regenerated power, varies at 51–65%. It can be observed in Figure 8a that there is a 1.2 s delay between when the SCC starts to lower the crane boom, and the electric drive starts to regenerate power. For the benchmark system (Figure 8b), the overall efficiency is 36% when the cylinder is lifting the crane boom at steady-state (i.e., at 4 s). Furthermore, almost 50% of the VCC’s input power is dissipated in the control valve (the most in the pressure-compensator). The efficiency of the counterbalance valve, during lifting (i.e., the flow is bypassed through the LHV’s check valve), is 74% at the lowest, and at the maximum 51% through the lowering phase (i.e., the LHV’s poppet introduces a desired pressure- drop).

4.3. Energy Consumption As described in the first part of this study [28], the SCC can be operated with two different load- holding strategies, namely passive load-holding (PLH) and active load-holding (ALH). The electric power and energy consumption when utilizing these two strategies are compared in Figure 9.

(a) (b)

Figure 9. Comparison between passive and active load-holding for the self-contained cylinder: (a) Input power; (b) energy consumption.

When the SCC maintains the desired piston position between 6.5 and 16 s (Figure 9a), the measured electric input power varies at 5–7 W with passive load-holding, and at 187–312 W with active load-holding. Active load-holding results in an energy consumption of 5.32 Wh for the entire working cycle considered here (Figure 9b), compared to 4.34 Wh with passive load-holding (i.e., 18.4% less). However, performing active load-holding for a reduced amount of time is still acceptable since the power consumption remains relatively low. Moving to the valve-controlled layout, load-sensing pumps are used instead of constant- pressure supplies to increase the overall efficiency in many applications. Two scenarios are therefore addressed; the first one (Figure 10a) focuses on measurements from the test setup. The scenario #2 Actuators 2019, 8, 78 11 of 16 (Figure 10b) considers an HPU equipped with a load-sensing pump and driven by an induction motor running at 1500 rpm (the losses of the HPU are simulated based on the model presented in recovery[27]). Full is energy then assumed recovery for is the then SCC assumed in this second for the situation SCC in (i.e.,this thesecond energy situation dissipated (i.e., inthe the energy brake resistordissipated when in the the brake SCC isresistor lowering when the the crane SCC boom is lowering is now reused).the crane boom is now reused).

(a) (b)

Figure 10. Energy consumption comparison: (a) The scenario #1; (b) the scenario #2. Actuators 2019, 8Figure, x FOR 10.PEER Energy REVIEW consumption comparison: (a) The scenario #1; (b) the scenario #2. 11 of 16 The results in Figure 10a show that the SCC does not consume any energy when lowering the The results in Figure 10a show that the SCC does not consume any energy when lowering the load, resulting in 62.1% less consumption compared to the 11.44 Wh of the VCC. This improvement is load, resulting in 62.1% less consumption compared to the 11.44 Wh of the VCC. This improvement expected since the VCC needs to build up pressure to enable ﬂow through the counterbalance valve is expected since the VCC needs to build up pressure to enable flow through the counterbalance valve that introduces functional losses, while the SCC uses the highest actuator pressure to fully open the two that introduces functional losses, while the SCC uses the highest actuator pressure to fully open the load-holding valves. For scenario #2 (Figure 10b), the VCC consumes energy also when the cylinder two load-holding valves. For scenario #2 (Figure 10b), the VCC consumes energy also when the is not moving because the electric-motor is continuously running; due to the load-sensing unit, then cylinder is not moving because the electric-motor is continuously running; due to the load-sensing 19.6% less energy is taken compared to the VCC in scenario #1. When comparing the two systems in unit, then 19.6% less energy is taken compared to the VCC in scenario #1. When comparing the two scenario #2, the SCC still consumes less energy than the VCC (62.4% less); this behavior favorable to systems in scenario #2, the SCC still consumes less energy than the VCC (62.4% less); this behavior the SCC is also the case when there is no useful usage of the recovered energy. favorable to the SCC is also the case when there is no useful usage of the recovered energy. 4.4. Energy Distribution 4.4. Energy Distribution The energy distribution between the main components of the two systems, initially monitored The energy distribution between the main components of the two systems, initially monitored when the cylinder is lifting the crane boom, is illustrated in Figure 11. when the cylinder is lifting the crane boom, is illustrated in Figure 11.

퐸푆퐵퐶 2.48 6.6% 퐸 푆퐵퐶 퐸 퐸 2.48 퐻푃 퐺푟𝑖푑 6.80 57.7% 4. 0 100% 100%

퐸 퐸𝐸 퐸푃 퐸 𝑉 퐶 10.8% 28.5% 0.9% 2.1%

퐸푃퐶 퐸푃 퐶𝑉 퐸퐿퐻𝑉 퐸퐶 48.6% 4.1% 9.4% 1. %

(a) (b)

FigureFigure 11. 11. TheThe energy energy distribution distribution when when lifting lifting the crane the boom: crane (a boom:) Self-contained (a) Self-contained system; (b) system; valve- controlled(b) valve-controlled system. system.

TheThe complete complete energy energy conversion conversion from from the the source source (i.e., (i.e., the the electric electric grid grid for for the the SCC SCC and and the hydraulichydraulic power power supply supply for for the the VCC) VCC) to tothe the end end user user (i.e., (i.e., the thepayload payload of the of crane) the crane) is handled. is handled. The totalThe totalamount amount of energy of energy being being lost in lost the in SCC the SCCis 42.3%, is 42.3%, while while it increases it increases up toup 63.4% to 63.4% in the in VCC the VCC(this is(this a conservative is a conservative estimation estimation because because the losses the lossesof the HPU of the are HPU neglected). are neglected). It should It be should noted be that noted all lossesthat all in losses the SCC in theare parasitic, SCC are parasitic,with the pump with thebeing pump the predominant being the predominant source followed source by followed the electric by motor.the electric This motor.aspect This does aspect not hold does true not for hold the true VCC for since the VCC the compensatorsince the compensator introduces introduces a relevant a dissipationrelevant dissipation that is functional that is functional (this term (this could term be could reduced be reduced for higher for higherloads, or loads, when or using when a using load a- sensing power supply). Moreover, only the SCC has the potential to recover energy; Figure 12 illustrates the energy distribution in the SCC when the crane boom is lowered.

퐸퐵푅 퐸푆퐵퐶 0.89 5.9% 2.48 100%

퐸퐶 퐸 𝑉 퐸푃 퐸𝐸 2.5% 4.7% 5.5% 21.4%

Figure 12. The energy distribution of the self-contained system when lowering the crane boom.

A significant portion (35.9%) of the energy delivered by the load that is lowered can be outputted by the electric drive; it is worth noticing that this quantity is completely lost in the valve-controlled architecture. In particular, the total energy being dissipated in the SCC during piston retraction (64.1% of the input) is mainly due to the losses in the axial piston machine and in the electric drive.

Actuators 2019, 8, x FOR PEER REVIEW 11 of 16

The results in Figure 10a show that the SCC does not consume any energy when lowering the load, resulting in 62.1% less consumption compared to the 11.44 Wh of the VCC. This improvement is expected since the VCC needs to build up pressure to enable flow through the counterbalance valve that introduces functional losses, while the SCC uses the highest actuator pressure to fully open the two load-holding valves. For scenario #2 (Figure 10b), the VCC consumes energy also when the cylinder is not moving because the electric-motor is continuously running; due to the load-sensing unit, then 19.6% less energy is taken compared to the VCC in scenario #1. When comparing the two systems in scenario #2, the SCC still consumes less energy than the VCC (62.4% less); this behavior favorable to the SCC is also the case when there is no useful usage of the recovered energy.

4.4. Energy Distribution The energy distribution between the main components of the two systems, initially monitored when the cylinder is lifting the crane boom, is illustrated in Figure 11.

퐸푆퐵퐶 2.48 6.6% 퐸 푆퐵퐶 퐸 퐸 2.48 퐻푃 퐺푟𝑖푑 6.80 57.7% 4. 0 100% 100%

퐸 퐸𝐸 퐸푃 퐸 𝑉 퐶 10.8% 28.5% 0.9% 2.1%

퐸푃퐶 퐸푃 퐶𝑉 퐸퐿퐻𝑉 퐸퐶 48.6% 4.1% 9.4% 1. %

(a) (b)

Figure 11. The energy distribution when lifting the crane boom: (a) Self-contained system; (b) valve- controlled system.

The complete energy conversion from the source (i.e., the electric grid for the SCC and the hydraulic power supply for the VCC) to the end user (i.e., the payload of the crane) is handled. The total amount of energy being lost in the SCC is 42.3%, while it increases up to 63.4% in the VCC (this is a conservative estimation because the losses of the HPU are neglected). It should be noted that all losses in the SCC are parasitic, with the pump being the predominant source followed by the electric Actuatorsmotor. 2019 This, 8 ,aspect 78 does not hold true for the VCC since the compensator introduces a relevant12 of 16 dissipation that is functional (this term could be reduced for higher loads, or when using a load- sensing power supply). Moreover, only the SCC has the potential to recover energy; Figure 12 load-sensing power supply). Moreover, only the SCC has the potential to recover energy; Figure 12 illustrates the energy distribution in the SCC when the crane boom is lowered. illustrates the energy distribution in the SCC when the crane boom is lowered.

퐸퐵푅 퐸푆퐵퐶 0.89 5.9% 2.48 100%

퐸퐶 퐸 𝑉 퐸푃 퐸𝐸 2.5% 4.7% 5.5% 21.4%

Figure 12. The energy distribution of the self-contained system when lowering the crane boom. Figure 12. The energy distribution of the self-contained system when lowering the crane boom. A signiﬁcant portion (35.9%) of the energy delivered by the load that is lowered can be outputted A significant portion (35.9%) of the energy delivered by the load that is lowered can be outputted by the electric drive; it is worth noticing that this quantity is completely lost in the valve-controlled by the electric drive; it is worth noticing that this quantity is completely lost in the valve-controlled architecture. In particular, the total energy being dissipated in the SCC during piston retraction (64.1% Actuatorsarchitecture. 2019, 8, x In FOR particular, PEER REVIEW the total energy being dissipated in the SCC during piston retraction12 of 16 of the input) is mainly due to the losses in the axial piston machine and in the electric drive. (64.1% of the input) is mainly due to the losses in the axial piston machine and in the electric drive. Finally, the energy distribution during a complete working cycle is depicted in Figure 13 for both Finally, the energy distribution during a complete working cycle is depicted in Figure 13 for the SCC and the VCC, where the energy taken from the grid is now considered also for the VCC (i.e., both the SCC and the VCC, where the energy taken from the grid is now considered also for the VCC scenario #2 with the power supply equipped with a load-sensing pump). (i.e., scenario #2 with the power supply equipped with a load-sensing pump).

퐸푆퐵퐶 2.48 27%

20.4%

퐸푆퐵퐶 퐸퐺푟𝑖푑 퐸퐺푟𝑖푑 2.48 9.20 4. 4 0.89 57.1% 100% 100%

퐸 퐸𝐸 퐸푃 퐸 𝑉 퐶 11.5% 28.5% 0.8% 2.1%

퐸퐻푃 퐸𝑉 퐸퐿퐻𝑉 퐸퐶 2.9% 2.1% 7.0% 1.0%

(a) (b)

Figure 13. The energy distribution during a complete working cycle: ( (aa)) Novel Novel self self-contained-contained system; system; ((b)) valve-controlledvalve-controlled systemsystem whenwhen simulatingsimulating aa load-sensingload-sensing pumppump andand hydraulichydraulic powerpower unitunit losses.losses.

The results show that 57.1% of the input energy is transferred to the load for the SCC and 27% for the VCC. Figure 1313aa reveals thatthat the SCC may recover 20.4% of the electrical energy taken from the gridgrid duringduring aa completecomplete workingworking cycle.cycle. Assuming a realistic 94% conversion efficiency efficiency to return the recovered energy to the grid, then 76.5% of the total energy taken from the source can be used eeffectively.ffectively. Further Further on, on, when when considering considering a VCC a VCC supplied supplied by bya load a load-sensing-sensing pump pump (Figure (Figure 13b), 13 theb), E theefficiency efficiency increases increases due due to theto the major major reduction reduction of of the the functional functional losses losses in in the the control control valve valve (퐸𝑉V)).. However, includingincluding also the ineinefficienciesfficiencies of the HPUHPU (i.e., the losses of the electric motor and of the pump), the overall eefficiencyfficiency is only increased by 5.3% compared to the VCC supplied with constant pressure supplysupply (scenario(scenario #1)#1) thatthat isis characterizedcharacterized byby anan overalloverall energyenergy eefficiencyfficiency ofof 21.7%.21.7%.

5. Concl Conclusionsusions This research paper experimentally compares a novelnovel self-containedself-contained electro-hydraulicelectro-hydraulic cylinder and a hydraulic,hydraulic, valve-controlled,valve-controlled, linear linear actuator actuator representative representative of of the the state state of of the the art art in in many many ﬁelds fields of of industry. A single-boom crane requiring passive load-holding is taken as the reference application, while the emphasis of the study is placed on the energy efficiency of the drives. The results show that the electro-hydraulic layout reduces the energy consumption significantly (up to 62% less) due to its throttleless nature and power-on-demand functioning. More specifically, the following aspects emerge from the study:  The power demand to the prime mover during steady-state operations reduces to 4.8 kW against the 7.6 kW in the valve-controlled system (37% less) throughout the investigated working cycle;  Utilizing passive load-holding results in an energy saving of 18.4% compared to actively controlling the SCC’s desired position using the prime mover during load holding phases;  The system’s overall efficiency of the self-contained drive, being approximately 57% during actuation, turns out to be highly satisfactory compared to the 22% efficiency of the valve- controlled system with a constant-pressure supply;  A significant amount of energy (i.e., 20% of the consumed energy) is recovered by the self- contained solution during the proposed working cycle. Hence, when assuming a realistic 94% conversion efficiency to return the recovered energy to the grid, then 77% of the total energy taken from the source can be used effectively. In contrast, this efficient operation is not possible

Actuators 2019, 8, 78 13 of 16 industry. A single-boom crane requiring passive load-holding is taken as the reference application, while the emphasis of the study is placed on the energy efficiency of the drives. The results show that the electro-hydraulic layout reduces the energy consumption significantly (up to 62% less) due to its throttleless nature and power-on-demand functioning. More specifically, the following aspects emerge from the study:

The power demand to the prime mover during steady-state operations reduces to 4.8 kW against • the 7.6 kW in the valve-controlled system (37% less) throughout the investigated working cycle; Utilizing passive load-holding results in an energy saving of 18.4% compared to actively controlling • the SCC’s desired position using the prime mover during load holding phases; The system’s overall efficiency of the self-contained drive, being approximately 57% during • actuation, turns out to be highly satisfactory compared to the 22% efficiency of the valve-controlled system with a constant-pressure supply; A significant amount of energy (i.e., 20% of the consumed energy) is recovered by the self-contained • solution during the proposed working cycle. Hence, when assuming a realistic 94% conversion efficiency to return the recovered energy to the grid, then 77% of the total energy taken from the source can be used effectively. In contrast, this efficient operation is not possible in the valve-controlled system where, rather than being recovered, the available energy is dissipated when the load acting on the actuator is overrunning; An alternative scenario based on the load-sensing concept is also considered for the valve-controlled • system; its energy consumption reduces from 11.44 Wh to 9.20 Wh but remains inefficient with respect to the electro-hydraulic actuator (i.e., total consumption 4.34 Wh).

Recalling the positive outcomes about motion control obtained in the first part of the research and disseminated in a different article, it is therefore concluded that the novel energy-efficient, self-contained drive represents a valid alternative to conventional hydraulic systems for load-carrying applications. Introducing a self-sufficient and completely sealed device also reduces the risk of oil spill pollution, helping fluid power to become a cleaner technology. Concerning future developments about the self-contained system, effort will be placed on both optimizing the sizing procedure to maximize energy efficiency as well as on designing and testing advanced solutions that reuse the recovered energy.

Author Contributions: Conceptualization, D.H. and D.P.; methodology, D.H.; software, D.H.; validation, D.H.; formal analysis, D.H.; investigation, D.H.; data curation, D.H.; writing—original draft preparation, D.H.; writing—review and editing, D.H., D.P., and M.C.; visualization, D.H.; supervision, D.P. and M.C. Funding: This research was funded by the Norwegian Research Council, SFI Offshore Mechatronics, project 237896. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

Abbreviations AC Alternating current ACC Accumulator Ac Anti-cavitation valve ALH Active load-holding AV Auxiliary valves BR Brake resistor C Hydraulic cylinder CO Oil cooler CV Check valve DC Direct current ED Electric drive EM Electric motor EV Electro-valve Actuators 2019, 8, 78 14 of 16

F Low pressure oil filter FC Flow compensation valve HPU Hydraulic power unit HS Hydraulic system LHV Load-holding valve P Axial piston machine (pump) PDCV Proportional directional control valve PLC Programmable logic controller RV Pressure-relief valve SBC Single-boom crane SCC Self-contained electro-hydraulic cylinder SD Servo-drive SM Servo-motor V Control valve VCC Valve-controlled cylinder Symbols Ap Cylinder area on the piston-side Ar Cylinder area on the rod-side EK Kinetic energy EP Potential energy E Energy . E Power ia,ib,ic Three-phase current of the electric power supply iBR Direct current of the brake resistor iq Torque-producing current component Jcm Inertia at center of mass kt Torque constant LAB Length between joint A and B LAC Length between joint A and C LAGx Length between joint A and G in x-direction LAGy Length between joint A and G in y-direction LC,0 Length of the hydraulic cylinder when fully retracted LC Effective total length of the hydraulic cylinder mcm Mass at center of mass ωSM Rotational speed of the servo-motor in radians per seconds QP Actuator’s flow demand Qr Rod-side flow rate p0 Fixed pressure-drop across the proportional directional control valve p1 Pressure at the pump/motor port on the piston-side of the actuator p2 Pressure at the pump/motor port on the rod-side of the actuator pp Actuator’s piston chamber pressure pr Actuator’s rod chamber pressure pR Return pressure pS Supply pressure uSM Commanded servo-motor speed uV Commanded opening of the control valve’s spool position va, vb, vc Three-phase voltage of the electric power supply vBR Direct current voltage of the brake resistor xC Actuator’s piston position . xC Actuator’s piston velocity Greek Symbols αi Arctangent of the xy-length between the i-th joints ηi Efficiency of the i-th system θ Angular position of the boom Actuators 2019, 8, 78 15 of 16

References

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