A Positive Flow Control System for Electric Excavators Based on Variable Speed Control

applied sciences

Article A Positive Flow Control System for Electric Excavators Based on Variable Speed Control

Shengjie Fu, Zhongshen Li , Tianliang Lin *, Qihuai Chen and Haoling Ren College of Mechanical Engineering and Automation, Huaqiao University, Xiamen 361021, China; [email protected] (S.F.); [email protected] (Z.L.); [email protected] (Q.C.); [email protected] (H.R.) * Correspondence: [email protected]

Received: 11 June 2020; Accepted: 13 July 2020; Published: 14 July 2020

Abstract: Energy conservation and emission reduction of construction machinery are the focus of current research. The traditional excavator, whose hydraulic pump is driven by the engine, has high fuel consumption and emissions. Furthermore, it is difficult to match the working point of the engine to that of the hydraulic pump. Current pure electric drive technology has the advantages of zero pollution and low noise, and the motor used has the advantages of fast response and a wide speed range. Based on the characteristics of the pure electric drive technology, a positive flow system based on variable speed constant displacement instead of a variable displacement pump for pure electric construction machinery is put forward to realize the flow-matching of the whole machine. The basic structure and working principle were introduced. The control process was analyzed. The controllability and energy saving of the proposed system were tested through simulation and experimental analysis. The research results showed that the controllability of the proposed positive flow system was comparable to that of the traditional throttling speed-regulating control system. The energy-saving efficiency of the proposed positive flow system is increased by 35.2% compared to that of the tradition control system. To further exploit the strong overload capacity of electric motors of electric construction machinery and solve the insufficient power under sudden load, research on constant power control will be carried out in the future.

Keywords: construction machinery; energy saving; positive ﬂow system (PFS); throttling speed-regulating control system (TSCS); variable speed control

1. Introduction Construction machinery consumes much energy and has great significance to energy conservation and emissions reduction. Various energy-saving forms have been proposed, such as hybrid power technology, which can improve fuel efficiency to some extent, but still depend on the engine [1]. There are some problems with the engine, such as low energy conversion rate, large noise, high vibration, and bad pollution discharge. With the development of power electronics technology, frequency control technology, and battery-based energy storage technology, pure electric drive technology is now widely used. The application of pure electric drive technology in construction machinery can reduce the emissions and noise, which are the shortcomings of traditional diesel and hybrid motors. Pure electric drive technology has the following advantages: (1) Electric energy, which is clean and efficient, is used. It can truly achieve zero emissions and operate pollution-free, and can meet the goal of energy conservation, emissions reduction, and sustainable development [2]. (2) The motor is used as the power source, which can improve the energy conversion rate of the power source. (3) The motor has good speed regulation, which can help to realize the power matching of the system, reducing energy loss and improving the controllability and energy-saving [3,4]. (4) The motor has good short-term overload capacity and can be applied to large burst load conditions.

Appl. Sci. 2020, 10, 4826; doi:10.3390/app10144826 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 4826 2 of 13

To improve the energy utilization of the construction machinery, many research institutions and scholars have devoted themselves to studying the energy conservation of hydraulic systems. Some technologies, such as the negative flow control system [5,6], positive flow system (PFS) [7,8], load sensitive control system [9,10], and load port independent control system [11,12] have been successively proposed and developed. Their purpose is to reduce overflow loss and throttling loss of the system through flow matching to improve the energy utilization. PFS has no bypass oil return and no oil return loss. Furthermore, there is no pressure compensator, which can add throttling loss. Compared with other hydraulic energy saving technologies, PFS can reach the best energy saving effect. However, in a PFS, when the actuator needs to work, it takes the pump a long time to set up pressure to overcome the load [13]. Lin et al. studied the automatic idle control characteristics of a hydraulic excavator based on a PFS driven by pure electricity, but they did not discuss the characteristics of the PFS [14]. Bender et al. introduced a predictive operator modeling of a virtual prototype of a hydraulic excavator using a PFS and analyzed the influence of different driver factors on the working cycle time and capacity consumption, but they did not investigate the performance of the PFS [15]. Han put forward an energy-saving technology of a fully controlled positive flow excavator, and analyzed the energy saving property of the system. A variable pump was used in the proposed system [16]. At present, the research on the PFS of construction machinery is mainly focused on the variable pump driven by the engine, and there is less research on quantitative pumps driven by the variable speed control of the motor, which has excellent speed regulation performance. Using motor frequency control to substitute the engine is dependent on the performance of both the motor and quantitative pump. To achieve as high energy savings as possible, optimization of the hydraulic system and parameter matching between the pump and the motor should be carried out. The frequency control technology is adapted to the hydraulic excavator by adjusting the motor speed to achieve flow matching between the hydraulic pump and the load. A PFS based on variable speed control is proposed through optimizing the parameters of the hydraulic system and the motor. Simulation and experimental analysis are used to verify the feasibility and efficiency of the proposed PFS based on variable speed. The remainder of this paper is organized as followings: Section2 briefly introduces the structure scheme and working principle of the proposed PFS based on motor variable speed control. The control rules and control strategies of the PFS are introduced in Section3. The controllability and energy-saving performance of the proposed system are analyzed in Sections4 and5 by simulation and experiment. Conclusions are given in Section6.

2. Structure and Principle To further improve the energy saving and controllability of hydraulic excavators, a PFS based on variable speed control for pure electric excavators is proposed, as shown in Figure1. The power of the motor is supplied by the power network, which minimally increases the installation cost, but can reduce the use cost to the user. In the proposed PFS, the output flow of the hydraulic pump is adjusted by changing the motor speed to meet the requirement of the load. Therefore, the energy saving of the hydraulic system can be realized by reducing the flow loss. The working process of the proposed system is as follows: when the electronic control handle receives a signal and leaves the middle position, it outputs the control signal to the motor frequency converter and to the pilot pressure-reducing valve through the controller. The multiway valve works at a certain opening according to the control signal and outputs a certain pressure and flow rate to the actuator. The sensor detects the pressure signal of the actuator and sends it to the controller. The controller outputs the control signal to the frequency converter to control the motor speed to make the output flow of the hydraulic pump match the requirement of the load. In this process, all the output flow of the hydraulic pump enters the actuator. Accordingly, the relief valve, in parallel with the main pump, which is quantitative, is used as a safety valve and there is no overflow loss. When the handle is returned to the middle position, the hydraulic pump outputs flow directly back to the tank through the multiway valve. The proposed system has the following advantages: (1) There is no Appl. Sci. 2020, 10, 4826 3 of 13 overflow loss and throttling loss; therefore, the efficiency is high. (2) Due to the fast response of the Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 13 motor, the pressure can be quickly established to overcome the load when the handle leaves the middle middleposition. position. (3) Since (3) the Since motor the hasmotor high has effi highciency efficien in acy wide in a speed wide range,speed range, it can makeit can themake main the pump main pumphave a have wide a range wide ofrange output of output and adapt and toadapt different to different working working conditions. conditions.

Figure 1. PrinciplePrinciple diagram diagram of of the the positive positive flow ﬂow system system (PFS) (PFS) of of an an electric electric excavator excavator based based on on variable variable speed control.

3. Control Control Strategy Strategy Figure2 2 is is the the control control structure structure diagram diagram of theof the PFS PFS based based on theon variablethe variable speed speed control control for a purefor a pureelectric electric excavator. excavator. It includes It includes the following the following three parts: three (1) parts: Control (1) unit,Control which unit, consists which of consists an electronic of an electroniccontrol handle control and ahandle controller. and (2)a controller. Volumetric speed(2) Volumetric regulation, speed which regulation, consists of awhich frequency consists converter, of a frequencya motor, and converter, a main pump. a motor, (3) and Throttling a main speed-regulating, pump. (3) Throttling which speed-regulating, includes a pilot pressure-reducingwhich includes a pilotvalve pressure-reducing and a multiway valve. valve The and controller a multiway receives valve. the The signals controller of the receives handle the and signals the sensors of the tohandle carry andout thethe logicsensors processing to carry and out flowthe logic forecast. processing According and to flow the forecast. logic operation, According the controllerto the logic sends operation, signals theto the controller pilot valve sends and signals inverter. to the Accordingly, pilot valve the and motor inverter. speed Accordingly, is controlled the to motor drive pumpspeed outputis controlled at the torequired drive pump flow, output and the at pilot the required valve outputs flow, and pressurized the pilot oilvalve to the outputs multiway pressurized valve at oil a certainto the multiway opening. valveTherefore, at a thecertain required opening. flow Therefore, enters the actuatorthe required and flow-matchingflow enters the is actuator realized. and flow-matching is realized.According to the control structure diagram shown in Figure2, the flow diagram of the control strategy is designed as shown in Figure3. The controller controls the output pressure of the pilot pressure-reducing valve according to the control signal of the electronic control handle. The spool displacement of the multiway valve is controlled by the output pressure of the pilot valve. Then, the flow entering the actuator is estimated. The estimated total flow required in the actuators is:

Qt = Q + Q + + Qn (1) 1 2 ··· where Qt is the total ﬂow required in the actuator in L/min. Qn is the required ﬂow rate of nth actuator working at the same time, n 1. ≥

Figure 2. Control structure diagram of a PFS based on variable speed. Appl. Sci. 2020, 10, x FOR PEER REVIEW 3 of 13 middle position. (3) Since the motor has high efficiency in a wide speed range, it can make the main pump have a wide range of output and adapt to different working conditions.

Figure 1. Principle diagram of the positive flow system (PFS) of an electric excavator based on variable Appl. Sci.speed 2020 control., 10, x FOR PEER REVIEW 4 of 13

3. ControlAccording Strategy to the control structure diagram shown in Figure 2, the flow diagram of the control strategy is designed as shown in Figure 3. The controller controls the output pressure of the pilot Figure 2 is the control structure diagram of the PFS based on the variable speed control for a pressure-reducing valve according to the control signal of the electronic control handle. The spool pure electric excavator. It includes the following three parts: (1) Control unit, which consists of an displacement of the multiway valve is controlled by the output pressure of the pilot valve. Then, the electronic control handle and a controller. (2) Volumetric speed regulation, which consists of a flow entering the actuator is estimated. The estimated total flow required in the actuators is: frequency converter, a motor, and a main pump. (3) Throttling speed-regulating, which includes a =+++ pilot pressure-reducing valve and a multiwayQQQt12 valve. The controller Q n receives the signals of the handle(1) and the sensors to carry out the logic processing and flow forecast. According to the logic operation, thewhere controller Qt is the sends total signalsflow required to the pilot in the valve actuator and inverter.in L/min. Accordingly, Qn is the required the motor flow ratespeed of isnth controlled actuator toworking drive pump at the output same time, at the n required ≥ 1. flow, and the pilot valve outputs pressurized oil to the multiway According to the estimated flow rate, the motor speed can be calculated as: Appl.valve Sci. at2020 a certain, 10, 4826 opening. Therefore, the required flow enters the actuator and flow-matching4 of 13is realized. Q n = t t η (2) VCp ps where nt is the target motor speed in rpm. Vp is the displacement of the quantitative pump (main pump) in mL/r. ηp is the volumetric efficiency of quantitative pump (main pump). Cs is the coefficient of the estimated speed. It is used to ensure the pump output flow has a certain margin to compensate for oil leakage. The output power of the pump is:

nVrp p p P = (3) p 60000 where Pp is the pump output power in kW. nr is the real speed of the motor taken from the frequency Figure 2. Control structure diagram of a PFS based on variable speed. converter in rpm. pFigurep is the 2. pumpControl pressure structure in diagram MPa. of a PFS based on variable speed.

Figure 3. Flow chart of positive flow ﬂow control strategy based on variable speed control.

AccordingThe pump output to the estimated power is compared flow rate, thewith motor the co speednstant can power be calculated point. If the as: pump output power is less than the constant power point, a positive flow matching mode is adopted. Otherwise, the system switches to the constant power mode. Qt nt = (2) VpηpCs where nt is the target motor speed in rpm. Vp is the displacement of the quantitative pump (main pump) in mL/r. ηp is the volumetric efficiency of quantitative pump (main pump). Cs is the coefficient of the estimated speed. It is used to ensure the pump output flow has a certain margin to compensate for oil leakage. The output power of the pump is: nrVppp P = (3) p 60000 where Pp is the pump output power in kW. nr is the real speed of the motor taken from the frequency converter in rpm. pp is the pump pressure in MPa. Appl. Sci. 2020, 10, 4826 5 of 13

The pump output power is compared with the constant power point. If the pump output power is less than the constant power point, a positive ﬂow matching mode is adopted. Otherwise, the system switchesAppl. Sci. to2020 the, 10 constant, x FOR PEER power REVIEW mode. 5 of 13

4.4. Simulation Simulation Results Results ToTo verify verify the the performance performance of the of proposedthe proposed PFS basedPFS based on variable on variable speed speed for a pure for electrica pure excavator,electric theexcavator, simulation the model simulation is established model is established using AMESim, using AMESim, shown in shown Figure in4 .Figure The parameters4. The parameters of the of key the key components are set in accordance with the actual actuator of a 1.5 t hydraulic excavator test components are set in accordance with the actual actuator of a 1.5 t hydraulic excavator test rig. rig.

FigureFigure 4. 4.Simulation Simulation modelmodel of PFS based based on on variable variable speed speed control. control.

TheThe arm arm cylinder cylinder is is taken taken asas thethe researchresearch object.object. A A typical typical cycle cycle of of the the cylinder cylinder extension extension and and recoveryrecovery is studiedis studied to verifyto verify the controllabilitythe controllability and and energy-saving energy-saving of the of proposed the proposed PFS based PFS based on variable on speedvariable control speed for control an electric for an excavator. electric excavator. Table1 shows Table the 1 shows key parameters the key parameters used in the used simulation. in the simulation. Combined with the actual parameters, the safety pressure of the relief valve in the main Combined with the actual parameters, the safety pressure of the relief valve in the main oil circuit is oil circuit is set to 20 MPa and that of the pilot oil is set to 4 MPa. set to 20 MPa and that of the pilot oil is set to 4 MPa.

Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 13 Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 13

Appl. Sci. 2020Table, 10, 48261. Simulation parameters of positive flow system (PFS) based on variable speed control. 6 of 13 Table 1. Simulation parameters of positive flow system (PFS) based on variable speed control. Simulation Parameters Value Simulation Parameters Value Bore diameter 63 mm Table 1. Simulation parameters of positive flow systemBore diameter (PFS) based on variable 63 speed mm control. Arm cylinder Rod diameter 35 mm Arm cylinder Rod diameter 35 mm Simulation ParametersStroke Value 340 mm Stroke 340 mm Safety pressure of main oil Bore diameter 63 mm Safety pressure of main oil 20 MPa Armcircuit cylinder Rod diameter 35 mm20 MPa circuit Stroke 340 mm Safety pressure of pilot oil Safety pressure of pilot oil 4 MPa Safety pressure of maincircuit oil circuit 20 MPa 4 MPa Safety pressure of pilotcircuit oil circuit 4 MPa Pump displacement 25 mL/r Pump displacementPump displacement 25 mL/r 25 mL/r Motor rated speed 1800 rpm Motor ratedMotor speed rated speed 1800 rpm 1800 rpm 4.1. Controllability Analysis of Proposed PFS Based on Variable Speed 4.1.4.1. Controllability Controllability Analysis Analysis of of Proposed Proposed PFSPFS BasedBased on Variable Speed Speed Figure 5 shows the velocity and displacement curves of the arm cylinder during one cycle. It can FigureFigure5 shows 5 shows the the velocity velocity and and displacement displacement curvescurves ofof thethe arm cylinder cylinder during during one one cycle. cycle. It Itcan can be seen that the displacement increases to 340 mm at first and then starts to decrease, that is, the bebe seen seen that that the the displacement displacement increases increases to 340to 340 mm mm at firstat first and and then then starts starts to decrease, to decrease, that that is, the is, pistonthe piston rod first extends and is then retracted. In this process, the maximum velocity of the piston is piston rod first extends and is then retracted. In this process, the maximum velocity of the piston is rod0.36 first m/s extends when andit is extended is then retracted. and 0.33 Inm/s this when process, it is retracted. the maximum The piston velocity moves of the smoothly piston iswithout 0.36 m /s 0.36 m/s when it is extended and 0.33 m/s when it is retracted. The piston moves smoothly without whenstagnation. it is extended and 0.33 m/s when it is retracted. The piston moves smoothly without stagnation. stagnation.

Figure 5. Curves of velocity and displacement of the arm cylinder. FigureFigure 5. 5.Curves Curves ofof velocityvelocity and displacement of of the the arm arm cylinder. cylinder. FigureFigure6 shows 6 shows the the curves curves of motorof motor speed speed and and pilot pilot pressure. pressure. The The motor motor speed speed increases increases with with the Figure 6 shows the curves of motor speed and pilot pressure. The motor speed increases with the increase of the pilot pressure and is proportional to the pilot pressure, which conforms to the increasethe increase of the pilotof the pressure pilot pressure and is proportionaland is proporti toonal the pilotto the pressure, pilot pressure, which conformswhich conforms to the operating to the operating characteristics of the PFS based on variable speed control. characteristicsoperating characteristics of the PFS based of the onPFS variable based on speed variable control. speed control.

Figure 6. Curves of motor speed and pilot pressure. FigureFigure 6. 6.Curves Curves of motor speed and and pilot pilot pressure. pressure.

Figure7 shows the flow rate curves of the pump and the two chambers of the arm cylinder. It shows that the flow rate output from the hydraulic pump is consistent with the flow rate into the two chambers of the arm cylinder. This indicates that flow matching is realized between the hydraulic pump and the actuator. The results verify that the proposed PFS and control strategy have good controllability. Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 13

Appl. Sci.Figure 2020, 107 ,shows x FOR PEER the flowREVIEW rate curves of the pump and the two chambers of the arm cylinder.7 of 13It shows that the flow rate output from the hydraulic pump is consistent with the flow rate into the two Figure 7 shows the flow rate curves of the pump and the two chambers of the arm cylinder. It chambers of the arm cylinder. This indicates that flow matching is realized between the hydraulic shows that the flow rate output from the hydraulic pump is consistent with the flow rate into the two Appl. Sci.pump2020 and, 10, 4826the actuator. The results verify that the proposed PFS and control strategy have good7 of 13 chambers of the arm cylinder. This indicates that flow matching is realized between the hydraulic controllability. pump and the actuator. The results verify that the proposed PFS and control strategy have good controllability.

FigureFigure 7. Flow 7. Flow rate rate curves curves of of the the pump pump outlet an andd the the two two chambers chambers of the of thearm arm cylinder. cylinder. Figure 7. Flow rate curves of the pump outlet and the two chambers of the arm cylinder. The traditionalThe traditional hydraulic hydraulic excavator excavator is is aa fixed-speedfixed-speed fixed-displacement fixed-displacement system, system, and andthe actuator the actuator speed is controlled by the throttling control. Although the throttling control system has a large energy speed is controlledThe traditional by the hydraulic throttling excavator control. is a Althoughfixed-speed the fixed-displacement throttling control system, system and has the a actuator large energy loss, it has good controllability. Figures 8 and 9 show the comparison curves of the displacement and loss,speed it has is good controlled controllability. by the throttling Figures control.8 and Although9 show the the comparison throttling control curves system of the has displacement a large energy and velocity of the arm cylinder between the proposed PFS based on variable speed control and the velocityloss, of it thehas armgood cylinder controllability. between Figures the proposed 8 and 9 sh PFSow the based comparison on variable curves speed of the control displacement and the throttlingand throttling control system based on the constant speed constant displacement system. It can be seen velocity of the arm cylinder between the proposed PFS based on variable speed control and the controlfrom system the curves based that on thetheconstant displacement speed and constant velocity displacement of the two systems system. are It almost can be seenthe same from in the one curves throttling control system based on the constant speed constant displacement system. It can be seen that thecycle. displacement This indicates and that velocity the proposed of the two PFS systemscan achieve are almostthe same the controllability same in one as cycle. the throttling This indicates from the curves that the displacement and velocity of the two systems are almost the same in one that thespeed-regulating proposed PFS control can achieve system (TSC the sameS). In the controllability following figures, as the TSCS throttling is short speed-regulating for throttling speed- control cycle. This indicates that the proposed PFS can achieve the same controllability as the throttling systemregulating (TSCS). control In the system following and figures,PFS is short TSCS for ispositive short forflow throttling system. speed-regulating control system speed-regulating control system (TSCS). In the following figures, TSCS is short for throttling speed- and PFSregulating is short control for positive system and flow PFS system. is short for positive flow system.

Figure 8. Displacement curves of the arm cylinder of the two systems.

Appl. Sci. 2020, 10, xFigure FORFigure PEER 8. 8.Displacement REVIEW Displacement curvescurves of the the arm arm cylinder cylinder of ofthe the two two systems. systems. 8 of 13

FigureFigure 9. 9.Velocity Velocity curves curves ofof thethe arm cylinder cylinder of of the the two two systems. systems.

4.2. Energy Saving Analysis of Proposed PFS Based on Variable Speed The energy consumption of a hydraulic system is mainly reflected in the power consumption, and the hydraulic power is calculated by the flow and pressure. The following is the energy consumption comparison between the PFS based on variable speed and the TSCS based on constant speed. Figure 10 shows the pressure and flow rate curves of the main pump outlet of the two systems. As seen from the pressure curves, during the actuator’s movement, the changing trend of pump outlet pressure with the load in the two systems is basically the same. According to the flow curves, the pump output flow based on constant speed and constant displacement is almost constant, around 37 L/min. While the output flow of the PFS based on variable speed displacement is controlled by the opening of the electronic control handle, with the increasing of the handle movement amplitude, the output flow increases accordingly. Therefore, it can realize the flow matching between the pump and the load and can reduce the energy loss. This proves that the proposed PFS has good energy-saving ability.

(a) Pressure curves (b) Flow rate curves

Figure 10. Pressure and flow rate curves of the pump outlet of the two systems.

Figure 11 shows the energy consumption comparison curves of hydraulic pumps of the two systems in one working cycle. The energy consumption of PFS is lower than that of TSCS. In one working cycle, in which the arm cylinder extends and retracts, the energy consumed by TSCS is about 67.5 kJ and that of PFS is about 47.5 kJ. The energy-saving efficiency is up to 29.6%. Appl. Sci. 2020, 10, x FOR PEER REVIEW 8 of 13

Figure 9. Velocity curves of the arm cylinder of the two systems. Appl. Sci. 2020, 10, 4826 8 of 13 4.2. Energy Saving Analysis of Proposed PFS Based on Variable Speed

4.2. EnergyThe energy Saving consumption Analysis of Proposed of a hydraulic PFS Based system on Variable is mainly Speed reflected in the power consumption, and the hydraulic power is calculated by the flow and pressure. The following is the energy The energy consumption of a hydraulic system is mainly reflected in the power consumption, consumption comparison between the PFS based on variable speed and the TSCS based on constant and the hydraulic power is calculated by the flow and pressure. The following is the energy consumption speed. comparison between the PFS based on variable speed and the TSCS based on constant speed. Figure 10 shows the pressure and flow rate curves of the main pump outlet of the two systems. Figure 10 shows the pressure and flow rate curves of the main pump outlet of the two systems. As seen from the pressure curves, during the actuator’s movement, the changing trend of pump As seen from the pressure curves, during the actuator’s movement, the changing trend of pump outlet outlet pressure with the load in the two systems is basically the same. According to the flow curves, pressure with the load in the two systems is basically the same. According to the flow curves, the pump the pump output flow based on constant speed and constant displacement is almost constant, around output flow based on constant speed and constant displacement is almost constant, around 37 L/min. 37 L/min. While the output flow of the PFS based on variable speed displacement is controlled by the While the output flow of the PFS based on variable speed displacement is controlled by the opening of opening of the electronic control handle, with the increasing of the handle movement amplitude, the the electronic control handle, with the increasing of the handle movement amplitude, the output flow output flow increases accordingly. Therefore, it can realize the flow matching between the pump and increases accordingly. Therefore, it can realize the flow matching between the pump and the load and the load and can reduce the energy loss. This proves that the proposed PFS has good energy-saving can reduce the energy loss. This proves that the proposed PFS has good energy-saving ability. ability.

(a) Pressure curves (b) Flow rate curves

FigureFigure 10.10. PressurePressure andand ﬂowflow raterate curvescurves ofof thethe pumppump outletoutlet ofof thethetwo twosystems. systems.

FigureFigure 1111 showsshows thethe energyenergy consumptionconsumption comparisoncomparison curvescurves ofof hydraulichydraulic pumpspumps ofof thethe twotwo systemssystems inin oneone workingworking cycle.cycle. TheThe energyenergy consumptionconsumption ofof PFSPFS isis lowerlower thanthan thatthat ofof TSCS.TSCS. InIn oneone workingworking cycle, cycle, in in whichwhich thethe armarm cylindercylinder extendsextends andand retracts,retracts, thethe energyenergy consumedconsumed by by TSCSTSCS isis aboutabout 67.5 kJ and that of PFS is about 47.5 kJ. The energy-saving eﬃciency is up to 29.6%. Appl.67.5 Sci. kJ 2020and, that10, x FORof PFS PEER is REVIEWabout 47.5 kJ. The energy-saving efficiency is up to 29.6%. 9 of 13

Figure 11. Energy consumption curves of th thee pump in the two systems.

5. Experimental Results and Discussion To furtherfurther verifyverify thethe controllabilitycontrollability and energy saving of the proposed PFS based on variable speed control, a 1.5 t excavatorexcavator test rig was built.built. The layout and key components of the test rig are shown in Figures 12 and 13. The test rig mainly includes the following parts: (1) The variable speed power system, including the motor, main pump, and pilot pump. The dual pump is driven by the motor. (2) The throttling speed-regulating system includes a pilot proportional pressure-reducing valve and multiway valve. According to the control signal, it controls the flow into the actuator to adjust the operating speed of the actuator. (3) The measurement and control system includes an industrial PC, acquisition board, frequency converter, electric control handle, various sensors, etc. The key parameters, such as displacement and pressures, are collected and processed. Moreover, the proportional relief valve is controlled to simulate the load with the measurement and control system. The arm cylinder is discussed in the following test. A typical working cycle consists of the piston extending and retracting.

Figure 12. A 1.5 t excavator test rig. Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 13

Figure 11. Energy consumption curves of the pump in the two systems.

5. Experimental Results and Discussion To further verify the controllability and energy saving of the proposed PFS based on variable speedAppl. Sci. control,2020, 10 a, 48261.5 t excavator test rig was built. The layout and key components of the test rig9 are of 13 shown in Figures 12 and 13. The test rig mainly includes the following parts: (1) The variable speed power system, including the motor, main pump, and pilot pump. The dual pump is driven by the motor.shown (2) in FiguresThe throttling 12 and speed-regulating13. The test rig mainly system includes includes the a followingpilot proporti parts:onal (1) pressure-reducing The variable speed valvepower and system, multiway including valve. the According motor, main to the pump, control and signal, pilot it pump. controls The the dual flow pump into isthe driven actuator by theto adjustmotor. the (2) operating The throttling speed speed-regulating of the actuator. system (3) The includes measurement a pilot proportional and contro pressure-reducingl system includes valve an industrialand multiway PC, acquisition valve. According board, to frequency the control converte signal, itr, controlselectric control the ﬂow handle, into the various actuator sensors, to adjust etc. the Theoperating key parameters, speed of thesuch actuator. as displacement (3) The measurement and pressures, and are control collected system and processed. includes an Moreover, industrial the PC, proportionalacquisition board, relief frequencyvalve is controlled converter, to electric simulate control the load handle, with various the measurement sensors, etc. and The control key parameters, system. suchThe as displacementarm cylinder is and discussed pressures, in the are following collected test. and A processed. typical working Moreover, cycle the consists proportional of the piston relief extendingvalve is controlled and retracting. to simulate the load with the measurement and control system.

Figure 12. A 1.5 t excavator test rig. Appl. Sci. 2020, 10, x FOR PEER REVIEW Figure 12. A 1.5 t excavator test rig. 10 of 13

Figure 13. Key components of the test rig.

5.1. ControllabilityThe arm cylinder Analysis is discussed in the following test. A typical working cycle consists of the piston extendingFigure and 14 shows retracting. the pilot pressure signal curves corresponding to the two chambers of the arm 5.1.cylinder. Controllability Under the Analysis control signal of the electronic control handle, the corresponding pilot pressure- reducing valve works and outputs the pilot pressure, while the pilot pressure corresponding to the otherFigure chamber 14 remainsshows the zero. pilot Within pressure 0–3.2 signal s, the curvespilot pressure-reducing corresponding to valve the twocorresponding chambers ofto the armrodless cylinder. chamber Under works, the and control the pilot signal pressure of the increa electronicses from control zero to handle, 3.2 MPa. the The corresponding cylinder completes pilot pressure-reducingthe extension movement. valve works Within and 3.5–6.5 outputs s, the the pilot pilot pressure, pressure while corresponding the pilot pressure to the corresponding rod chamber toincreases the other from chamber zero to remains 3.2 MPa, zero. at which Within time 0–3.2 the s,cylinder the pilot completes pressure-reducing the retraction valve movement. corresponding

Figure 14. Pilot pressure curves of the two chambers of the arm cylinder.

Figure 15 shows the curves of the pilot pressure and motor speed. The motor speed and the pilot pressure are both controlled by the electronic control handle signals at the same time. When the pilot pressure increases, the motor speed increases accordingly, which is conforms with the operation characteristics of the PFS based on variable speed. The motor speed rises rapidly, and there is no speed drop, except the motor speed fluctuates slightly at the maximum speed. This indicates that the output flow of the hydraulic pump can meet the requirements of the actuator for rapid movement. It also verifies that the proposed PFS based on variable speed has good controllability. Appl. Sci. 2020, 10, x FOR PEER REVIEW 10 of 13

Figure 13. Key components of the test rig.

5.1. Controllability Analysis Figure 14 shows the pilot pressure signal curves corresponding to the two chambers of the arm

Appl.cylinder. Sci. 2020 Under, 10, 4826 the control signal of the electronic control handle, the corresponding pilot pressure-10 of 13 reducing valve works and outputs the pilot pressure, while the pilot pressure corresponding to the other chamber remains zero. Within 0–3.2 s, the pilot pressure-reducing valve corresponding to the torodless the rodless chamber chamber works, works,and the and pilot the pressure pilot pressure increases increases from zero from to 3.2 zero MPa. to The 3.2 MPa.cylinder The completes cylinder completesthe extension the movement. extension movement. Within 3.5–6.5 Within s, the 3.5–6.5 pilot s, pressure the pilot corresponding pressure corresponding to the rod to chamber the rod chamberincreases increases from zero from to 3.2 zero MPa, to 3.2 at MPa,which at time which the time cylinder the cylinder completes completes the retraction the retraction movement. movement.

Figure 14. Pilot pressure curves of the two chambers of the arm cylinder.

Figure 15 showsshows the the curves curves of of the the pilot pilot pressure pressure and and motor motor speed. speed. The Themotor motor speed speed and the and pilot the pilotpressure pressure are both are bothcontrolled controlled by the by electronic the electronic contro controll handle handle signals signals at the at same the same time. time. When When the pilot the pilotpressure pressure increases, increases, the themotor motor speed speed increases increases acco accordingly,rdingly, which which is is conforms conforms with with the operation characteristicscharacteristics ofof the the PFS PFS based based on on variable variable speed. speed. The The motor motor speed speed rises rises rapidly, rapidly, and there and isthere nospeed is no drop,speedexcept drop, except the motor the speedmotor fluctuatesspeed fluctuates slightly slight at thely maximum at the maximum speed. Thisspeed. indicates This indicates that the that output the flowoutput of flow the hydraulic of the hydraulic pump canpump meet can the meet requirements the requirements of the of actuator the actuator for rapid for rapid movement. movement. It also It verifiesAppl.also Sci.verifies 2020 that, 10 thethat, x proposedFOR the PEER proposed REVIEW PFS basedPFS based on variable on variable speed speed has good has good controllability. controllability. 11 of 13

Figure 15. PilotPilot pressure pressure and motor speed curves of the PFS based on variable speed control.

5.2. Energy Energy Saving Analysis Figure 16 shows the pump power comparison of the PFS based on variable speed and the TSCS based on fix fix speed fixed fixed displacement. It is obviou obviouss that the PFS consumes less energy than the TSCS during the working process. The maximum instantaneous power reduction is up to 3 kW. Figure 17 shows the comparison of the pump energy consumption of the two systems. The consumed energy of the FPS is lower than that of the TSCS. During one working cycle of arm cylinder extension and retraction, the consumed energy by TSCS is about 73 kJ, while that of PFS is 54 kJ. The energy-saving efficiency of the proposed PFS can reach 35.2%. The reported energy-saving efficiency of the PFS is about 20% [17]. The experimental results prove that the proposed PFS based on variable speed has good energy-saving ability.

Figure 16. Pump output power curves of the two systems.

Figure 17 shows the comparison of the pump energy consumption of the two systems. The consumed energy of the FPS is lower than that of the TSCS. During one working cycle of arm cylinder extension and retraction, the consumed energy by TSCS is about 73 kJ, while that of PFS is 54 kJ. The energy-saving efficiency of the proposed PFS can reach 35.2%. The reported energy-saving efficiency of the PFS is about 20% [17]. The experimental results prove that the proposed PFS based on variable speed has good energy-saving ability.

Figure 17. Consumed energy curves of the two systems. Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 13

Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 13

Figure 15. Pilot pressure and motor speed curves of the PFS based on variable speed control.

5.2. Energy Saving Analysis Figure 16 shows the pump power comparison of the PFS based on variable speed and the TSCS based on fix speed fixed displacement. It is obvious that the PFS consumes less energy than the TSCS during the working process. The maximum instantaneous power reduction is up to 3 kW. Figure 15. Pilot pressure and motor speed curves of the PFS based on variable speed control.

5.2. Energy Saving Analysis Figure 16 shows the pump power comparison of the PFS based on variable speed and the TSCS basedAppl. Sci. on2020 fix, 10speed, 4826 fixed displacement. It is obvious that the PFS consumes less energy than the11 TSCS of 13 during the working process. The maximum instantaneous power reduction is up to 3 kW.

Figure 16. Pump output power curves of the two systems.

Figure 17. ConsumedConsumed energy energy curves of the two systems.

6. Conclusions Pure electric construction machinery uses a motor to drive a hydraulic pump. Considering the good speed regulation performance and quick response of the motor, a PFS based on variable speed constant displacement for pure electric construction machinery is put forward to improve energy utilization and is verified by simulation and experiment. The results show that the proposed PFS based Figure 17. Consumed energy curves of the two systems. on variable speed can achieve the same controllability as a TSCS. The proposed PFS can use up to 35.2% less energy and has good energy savings compared to that of the traditional system. In addition, the cost of the hydraulic system is reduced and the volumetric efficiency of the hydraulic pump is improved because a quantitative pump is adopted. The use cost is also reduced because of the cheaper price of electric energy than diesel. The overall energy utilization rate is improved to some extent. The proposed system is not only suitable for hydraulic excavators, but also for other construction machinery such as loaders and forklifts. To fully take advantage of the strong overload capacity of electric motors and solve the issue of insufficient power under sudden load, constant power control research based on PFS with variable speed will be carried out in the future.

Author Contributions: S.F. proposed the idea of positive flow system based on variable speed control. Z.L. developed the structure and working mode. T.L. wrote the paper. Q.C. checked and edited the paper. H.R. analyzed the data. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by National Natural Science Foundation of China (No. 51875218 & 51905180), Excellent Outstanding Youth Foundation of Fujian Province (No. 2018J06014), Industry Cooperation of Major Science and Technology Project of Fujian Province (No. 2019H6015), Natural Science Foundation of Fujian Province (No. 2018J01068 & 2019J01060), and STS project of Fujian Province (No. 2018T3015). This work also has been supported by Fujian Southchina Heavy Machinery Manufacture Co., Ltd. Conflicts of Interest: The authors declare no conflict of interest. Appl. Sci. 2020, 10, 4826 12 of 13

Nomenclature

Cs coefficient of the estimated speed nc target motor speed nr real speed of the motor taken from the frequency converter pp pump pressure Pp pump output power Qn required flow rate of nth actuator working at the same time Qt total flow required in the actuator Vp displacement of the quantitative pump (main pump) ηp volumetric efficiency of quantitative pump (main pump)

Abbreviations

PFS positive ﬂow system TSCS throttling speed-regulating control system

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