Potential Energy Recovery System for Electric Heavy Forklift Based on Double Hydraulic Motor-Generators

: Heavy forklifts that are widely used in ports and stations have large gravitational potential energy at the lowering of the boom. As concerning the large rated power level, the engine is still the main power source for the heavy forklifts. With the increasingly stringent emissions regulations, the electric heavy forklift becomes an important choice. The structure and the working mode of an electric heavy forklift are introduced. Additionally the schematic of double hydraulic motor-generators is adopted to regenerate the potential energy when the boom is descending. The judge rule of the working mode and control strategy are analyzed. A test rig of a prototype electric heavy forklift is established. Control mode discrimination of potential energy regeneration, the control performance and the inﬂuence factors on regeneration e ﬃ ciency are tested based on the test rig. The experimental results show that the discrimination method of the working mode of the proposed double hydraulic motor-generators with a potential energy regeneration system for potential energy is feasible. The descending of the lifting cylinder is consistent with the handle. The forklift can obtain the good following ability. Although the lifting cylinder descends at low velocity, the velocity is stable and the ﬂuctuation of the rodless chamber pressure is within 0.1 MPa. With the increase of the load mass and descending velocity, the regeneration e ﬃ ciency increases accordingly. The maximum e ﬃ ciency is up to 74%. Hence, the proposed potential energy regeneration system is feasible and potential energy regeneration system does not a ﬀ ect the control performance of the boom.


Introduction
Non-road mobile construction machinery powered by a traditional internal combustion engine has problems such as low efficiency, bad emission and high pollution, which have a great impact on source utilization and global climate. Due to low noise, zero emission, pollution-free characteristics, pure electric drive construction machinery becomes a new development direction of construction machinery [1][2][3][4][5][6][7][8]. Electrification of the forklift is developed early and is relatively mature in the field of construction machinery. However, currently, the pure electric forklift on the market uses the low voltage system basically and the rated load is under 5-ton, which are called small type forklifts. There is a lot of gravitational potential energy that can be recovered for all the forklifts. Their lifting system frequently reciprocates up and down and converts the potential energy into heat energy, causing the high temperature of the hydraulic system and the short life of hydraulic components [9]. In view of

Working Mode
An operation roadmap of a forklift is shown in Figure 2. The purpose of the forklift is to carry the cargo from A to B safely and smoothly through the standard operation and to stack them neatly in a certain order. A complete operational cycle consists of the following steps.
Firstly, forklift lifts without a load for a short distance from the initial position C and then runs along the No. 1 route to the goods. It adjusts the lifting cylinder to make the height of the forks close to that of the cargo gap and then slowly runs to the place that is just under the cargo. Forks lift the cargo off the ground and make the cargo land over the forks safely and effectively.
Secondly, forklift carries the cargo along No.2 route and slowly reverses to the open space. Additionally, then the forklift moves to position B along No.3 route and slowly lowers the forks. When the goods are safely placed at position B, the forks drop to a safe position and the forklift moves back to position D along No.4 route.
Thirdly, forklift moves along No.5 route to position C. This is the complete operation cycle of the forklift. The distance L in Figure 2 is based on the actual situation. During the transport process, the forks must keep a certain angle and a certain distance from the ground. A typical work cycle of a forklift is shown in Figure 3, which mainly includes the following five parts: running from the initial position to the cargo position, lifting forks with cargo, moving and transporting, stacking the cargo and unloading. In this working cycle, there are eight main working conditions, including walking, steering, lifting without a load, lowering without a load, up-tilting of forks, down-tilting of forks, lifting with a load and lowering load. Among them, there is more recoverable potential energy in the lowering load condition. This is the main point of the following discussion. Commonly, the time for lowering of the boom about 10-30 s depending on the control strategy. Firstly, forklift lifts without a load for a short distance from the initial position C and then runs along the No. 1 route to the goods. It adjusts the lifting cylinder to make the height of the forks close to that of the cargo gap and then slowly runs to the place that is just under the cargo. Forks lift the cargo off the ground and make the cargo land over the forks safely and effectively.
Secondly, forklift carries the cargo along No.2 route and slowly reverses to the open space. Additionally, then the forklift moves to position B along No.3 route and slowly lowers the forks. When the goods are safely placed at position B, the forks drop to a safe position and the forklift moves back to position D along No.4 route.
Thirdly, forklift moves along No.5 route to position C. This is the complete operation cycle of the forklift. The distance L in Figure 2 is based on the actual situation. During the transport process, the forks must keep a certain angle and a certain distance from the ground.
A typical work cycle of a forklift is shown in Figure 3, which mainly includes the following five parts: running from the initial position to the cargo position, lifting forks with cargo, moving and transporting, stacking the cargo and unloading. In this working cycle, there are eight main working conditions, including walking, steering, lifting without a load, lowering without a load, up-tilting of forks, down-tilting of forks, lifting with a load and lowering load. Among them, there is more recoverable potential energy in the lowering load condition. This is the main point of the following discussion. Commonly, the time for lowering of the boom about 10-30 s depending on the control strategy. A typical work cycle of a forklift is shown in Figure 3, which mainly includes the following five parts: running from the initial position to the cargo position, lifting forks with cargo, moving and transporting, stacking the cargo and unloading. In this working cycle, there are eight main working conditions, including walking, steering, lifting without a load, lowering without a load, up-tilting of forks, down-tilting of forks, lifting with a load and lowering load. Among them, there is more recoverable potential energy in the lowering load condition. This is the main point of the following discussion. Commonly, the time for lowering of the boom about 10-30 s depending on the control strategy.

Configurations of PERS
A 25 tons heavy forklift was carried out for the PERS research in this article. The mass of the fork rack was about 5 tons and the maximum load weight was 25 tons. That is to say, the load of the full load condition was 6 times of the no-load condition. Hence, driven by a single motor, the power level of the hydraulic motor-generator (HMG) was not only relatively high, but also difficult to ensure the HMG always work in an efficient range with different loads. Two HMGs were applied in the PERS to solve this problem. The schematic of the proposed system is shown in Figure 4. Two HMGs connected to the meter-out port of the hydraulic control valve were employed to convert the gravitational potential energy of the boom into electric energy, which could be used for the later pure electric drive system. The boom down mode has the traditional throttling-controlled mode and volume-controlled mode with PERS. The throttling-controlled mode uses the conventional multiway valve to realize the throttling control to adjust the boom velocity. In HMG controlled

Configurations of PERS
A 25 tons heavy forklift was carried out for the PERS research in this article. The mass of the fork rack was about 5 tons and the maximum load weight was 25 tons. That is to say, the load of the full load condition was 6 times of the no-load condition. Hence, driven by a single motor, the power level of the hydraulic motor-generator (HMG) was not only relatively high, but also difficult to ensure the HMG always work in an efficient range with different loads. Two HMGs were applied in the PERS to solve this problem. The schematic of the proposed system is shown in Figure 4. Two HMGs connected to the meter-out port of the hydraulic control valve were employed to convert the gravitational potential energy of the boom into electric energy, which could be used for the later pure electric drive system. The boom down mode has the traditional throttling-controlled mode and volume-controlled mode with PERS. The throttling-controlled mode uses the conventional multiway valve to realize the throttling control to adjust the boom velocity. In HMG controlled mode, the machine control unit (MCU) controls the two sets of HMGs to realize the lowering velocity control of the boom according to the handle signal, hydraulic signal and the state of batteries and the EMs according to the control strategy. In Figure 4, EGC is short for the electric generator controller.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 5 of 18 mode, the machine control unit (MCU) controls the two sets of HMGs to realize the lowering velocity control of the boom according to the handle signal, hydraulic signal and the state of batteries and the EMs according to the control strategy. In Figure 4, EGC is short for the electric generator controller.

Control Strategy
The flow chart of the lifting system based on double HMG PERS is shown in Figure 5. According to the handle signal Yp, rodless chamber pressure p3 of the lifting cylinder and the state of charge (SOC) of the battery, MCU determines the control mode of lifting cylinder descending. The judgment rule of working mode and the control strategies of different work modes were as follows. In Figure 5, Yp, Ymin and Ymax are the opening, minimum opening and maximum opening of the handle, respectively. Ys is the handle opening when the mode is switched from single HMG mode to double HMG mode. p3 is the rodless chamber pressure of the lifting cylinder. Cminp is the minimum pressure of the rodless chamber allowing descending with PERS. Smax is the upper limit value of battery SOC allowing descending with PERS. n1 and n2 are the speed request value of 1# HMG and 2# HMG, respectively. a1, b1 and c1 are the electromagnets shown in Figure 4.

Control Strategy
The flow chart of the lifting system based on double HMG PERS is shown in Figure 5. According to the handle signal Y p , rodless chamber pressure p 3 of the lifting cylinder and the state of charge (SOC) of the battery, MCU determines the control mode of lifting cylinder descending. The judgment rule of working mode and the control strategies of different work modes were as follows. In Figure 5, Y p , Y min and Y max are the opening, minimum opening and maximum opening of the handle, respectively. Y s is the handle opening when the mode is switched from single HMG mode to double HMG mode. p 3 is the rodless chamber pressure of the lifting cylinder. C minp is the minimum pressure of the rodless chamber allowing descending with PERS. S max is the upper limit value of battery SOC allowing descending with PERS. n 1 and n 2 are the speed request value of 1# HMG and 2# HMG, respectively. a1, b1 and c1 are the electromagnets shown in Figure 4.

Judge Rule of the Working Mode of the Boom
The forklift is a speed-controlled system, and the handle opening represents the target velocity of the boom. When the boom descends, there is a certain energy loss of the energy regeneration unit composed of HMG and lithium battery. When the energy recovered by the external load is less than the loss generated by PERS itself, using the HMG controlled mode to control the descending velocity of the lifting cylinder will generate additional energy loss, which will reduce the energy regeneration efficiency of the whole machine. Therefore, there is a minimum load requirement for the external load under HMG controlled mode to recover the potential energy. The load is measured by detecting the rodless chamber pressure of the lifting cylinder. To prevent being overcharged by the lithium batteries, there is a requirement for the maximum allowable charging of SOC. Accordingly, the main factors influencing the decision of the control mode of lifting cylinder descending are the handle opening, the minimum rodless chamber pressure and the maximum SOC of the battery. Therefore, the judge rules for the descending mode of the lifting cylinder can be shown in Table 1.

Judge Rule of the Working Mode of the Boom
The forklift is a speed-controlled system, and the handle opening represents the target velocity of the boom. When the boom descends, there is a certain energy loss of the energy regeneration unit composed of HMG and lithium battery. When the energy recovered by the external load is less than the loss generated by PERS itself, using the HMG controlled mode to control the descending velocity of the lifting cylinder will generate additional energy loss, which will reduce the energy regeneration efficiency of the whole machine. Therefore, there is a minimum load requirement for the external load under HMG controlled mode to recover the potential energy. The load is measured by detecting the rodless chamber pressure of the lifting cylinder. To prevent being overcharged by the lithium batteries, there is a requirement for the maximum allowable charging of SOC. Accordingly, the main factors influencing the decision of the control mode of lifting cylinder descending are the handle opening, the minimum rodless chamber pressure and the maximum SOC of the battery. Therefore, the judge rules for the descending mode of the lifting cylinder can be (1) When the rodless chamber pressure satisfies p 3 ≥ C minp , and the residual charge of lithium battery satisfies SOC < S max and the opening of the handle with controller area network (CAN) communication satisfies Y min ≤ Y p ≤ Y s , the boom down system enters the single HMG mode.
(2) When the rodless chamber pressure satisfies p 3 ≥ C minp , and the residual charge of lithium battery satisfies SOC < S max and the opening of the handle with CAN communication satisfies Y s < Y p ≤ Y max , the boom down system enters the double HMG mode. (3) When the rodless chamber pressure satisfies p 3 < C minp or the residual charge of the lithium battery satisfies SOC ≥ S max and the opening of the handle with CAN communication satisfies Y min ≤ Y p ≤ Y max , the boom down system enters in the traditional throttling-controlled mode.

Control Strategy
In the PERS, there is a certain proportional relationship between the control signal of the hydraulic motor and the descending velocity of the lifting cylinder. Therefore, the generator is controlled by the speed mode to improve the control performance when the lifting cylinder descends.
The potential energy regeneration control strategy consists of stop, single HMG and double HMGs, respectively, shown in Figure 6. Point A is the minimum signal of handle corresponding to the switch from the stop mode to the single HMG mode and the minimum recovery speed of 1#HMG. Point B is the handle signal corresponding to the switch from the single HMG mode to the double HMG mode and the speed of 1#HMG. Point C is the handle signal corresponding to the switch from the single HMG mode to the double HMG mode and the minimum recovery speed of 2 #HMG. Point D is the handle signal corresponding to the double HMG mode and the maximum recovery speed of 1#HMG and 2 #HMG.
Ys < Yp ≤ Ymax, the boom down system enters the double HMG mode.
(3) When the rodless chamber pressure satisfies p3 < Cminp or the residual charge of the lithium battery satisfies SOC ≥ Smax and the opening of the handle with CAN communication satisfies Ymin ≤ Yp ≤ Ymax, the boom down system enters in the traditional throttling-controlled mode.

. Control Strategy
In the PERS, there is a certain proportional relationship between the control signal of the hydraulic motor and the descending velocity of the lifting cylinder. Therefore, the generator is controlled by the speed mode to improve the control performance when the lifting cylinder descends.
The potential energy regeneration control strategy consists of stop, single HMG and double HMGs, respectively, shown in Figure 6. Point A is the minimum signal of handle corresponding to the switch from the stop mode to the single HMG mode and the minimum recovery speed of 1#HMG. Point B is the handle signal corresponding to the switch from the single HMG mode to the double HMG mode and the speed of 1#HMG. Point C is the handle signal corresponding to the switch from the single HMG mode to the double HMG mode and the minimum recovery speed of 2 #HMG. Point D is the handle signal corresponding to the double HMG mode and the maximum recovery speed of 1#HMG and 2 #HMG.  Suppose the coordinates of the four points of A, B, C and D are A = (Y pA , n min ), B = (Y pB , n max ), C = (Y pC , 0.5n max ) and D = (Y pD , n max ), respectively. Where, Y pA , Y pB , Y pC and Y pD are the handle signals of points of A, B, C and D, respectively. n min is the minimum speed when using HMG unit for single HMG mode. n max is the maximum efficient speed of 1#HMG or 2 #HMG for the double HMGs mode.
To ensure the linear relationship between the descending velocity of the lifting cylinder and the handle signal and no abrupt changing of the boom velocity, the increment of the sum of the target speed is positively correlated with the increment of the handle signal no matter whether it is in the single HMG mode or in the double HMGs mode. That is to say, the curves are proportional.
where, k t is the proportional relationship between the total target speed and the handle signal. k 1 is the proportional relationship between the target speed controlled by 1#HMG and the handle signal. k 2 is the proportional relationship between the target speed controlled by 2#HMG and the handle signal. In the stage of "Stop", namely, Y p ∈ (0,Y pA ), both 1#HMG and 2#HMG do not work, namely In the stage of "single HMG", namely, Y p ∈ (Y pA , Y pB ), the relationship between speed of 1#HMG and 2#HMG and the handle signal can be given by Equation (3) or Equation (4) according to the temperature of generators. Additionally, 1#HMG takes precedence over 2#HMG. Hence, if the temperature of the generator 1 is below the maximum allowable operating temperature, 1#HMG works and 2#HMG does not work. Otherwise, 2#HMG works and 1#HMG does not work.
where, k AB is the line slope from point A to point B. n min is the minimum working speed of 1#HMG and 2#HMG. n min can be deduced by the efficiency map of the hydraulic motor and generator.
In the stage of "double HMGs", namely, Y p ∈ (Y pB , Y max ), 1#HMG and 2#HMG are exactly the same. Hence, the speed of the 1#HMG and 2#HMG is the same. Additionally, the relationship between the speed of 1#HMG and 2#HMG and the handle signal can be calculated as: where, k CD is the line slope from point C to point D. n max is the maximum high efficient speed of 1#HMG and 2#HMG. n max can also be deduced by the efficiency map of the hydraulic motor and generator. The coefficient satisfies the following formula as follows:

Test Rig
Experiments to test the proposed PERS on a 25-ton EHF using two separate HMG units were carried out, as shown in Figure 7. The system consisted of the power battery, two identical HMGs and their controllers EGC, one electric motor-pump and its controller EMC, a MCU, a CAN bus handle, hydraulic control units and other components, shown in Figure 8. The communication and data acquisition during the test were realized through the CAN bus, the baud rate of which was 250 K. The detailed parameters of the key component are shown in Table 2. The hydraulic motor was driven by the generator in coaxial, the speed ranges of which was from 0 to 3000 r/min and could be controlled by a frequency convertor, which is called EMC in Figure 7. The working mode and control performance of the boom were tested to analyze and compare the features of the systems. Additionally, the energy generation efficiency characteristics were compared. Figure 9 shows the curve of the handle signal and the actual speeds of 1#HMG and 2#HMG during the boom descending. The whole test time was 52 s. The signal of the MCU increased firstly and then decreased according to the handle signal. According to the proposed double HMGs with PERS, the HMG works in five stages: stop, single HMG, double HMGs, single HMG and stop.

Working Mode
Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 18 control performance of the boom were tested to analyze and compare the features of the systems. Additionally, the energy generation efficiency characteristics were compared.    Appl. Sci. 2020, 10, x FOR PEER REVIEW 9 of 18 control performance of the boom were tested to analyze and compare the features of the systems. Additionally, the energy generation efficiency characteristics were compared.     system was in the stage of "Stop". At time 3.3 s, the handle signal was increased to 100 mV, and 1#HMG started to work. The system entered the stage of "single HMG". When the handle signal increased to 455 mV at time 6.65 s, the speed of the 1#HMG approached −1600 r/min. Then, the system entered the stage of "double HMGs", and the 2#HMG started to work. Request values of the two HMGs are the same and the two curves were almost identical, which could be seen from Figure  9. When the handle slowly backed from the maximum opening to the median position, the handle signal decreased and the target speed gradually reduced accordingly. At time 16.5 s, the handle signal decreased to 455 mV and 2#HMG stopped working. Speed of 2#HMG decreased to zero with inertia and the speed of 1#HMG increased to the target speed. Then, the system entered the stage of "single HMG" again. At time 48.7 s, the handle signal was less than 100 mV and the system entered the stage of "Stop". Speed of 1#HMG decreased to zero with inertia and the lifting cylinder was no longer descending. Based on the above test, the discrimination method of the working mode of the proposed double HMGs with PERS for potential energy was feasible. The whole process included the stop mode, single HMG and double HMGs. Additionally, each mode could realize a response to the handle signal with the control strategy.  Seen from Figure 9, at time 2.9 s, the handle signal started to increase from 0 mV, and the system was in the stage of "Stop". At time 3.3 s, the handle signal was increased to 100 mV, and 1#HMG started to work. The system entered the stage of "single HMG". When the handle signal increased to 455 mV at time 6.65 s, the speed of the 1#HMG approached −1600 r/min. Then, the system entered the stage of "double HMGs", and the 2#HMG started to work. Request values of the two HMGs are the same and the two curves were almost identical, which could be seen from Figure 9. When the handle slowly backed from the maximum opening to the median position, the handle signal decreased and the target speed gradually reduced accordingly. At time 16.5 s, the handle signal decreased to 455 mV and 2#HMG stopped working. Speed of 2#HMG decreased to zero with inertia and the speed of 1#HMG increased to the target speed. Then, the system entered the stage of "single HMG" again. At time 48.7 s, the handle signal was less than 100 mV and the system entered the stage of "Stop". Speed of 1#HMG decreased to zero with inertia and the lifting cylinder was no longer descending.

Control Performance
Based on the above test, the discrimination method of the working mode of the proposed double HMGs with PERS for potential energy was feasible. The whole process included the stop mode, single HMG and double HMGs. Additionally, each mode could realize a response to the handle signal with the control strategy.

Control Performance
As concerning about that EHF is a large inertial system, the request for a dynamic response of speed is not high. The focuses are mainly on the speed following ability, speed stability during mode switching and the smoothness at low speed.
(1) Following ability The relationship between the forklifts descending velocity and the handle signal is given in Figure 10. The handle signal gradually increased from the minimum opening degree to the maximum opening degree and then back to the minimum opening degree. Additionally, it was proven that the actual descending velocity of the forklifts was very close to the theoretical descending velocity and was consistent with the trend of the handle signal. The deviation of the descending velocity was small and the following ability was good.
As concerning about that EHF is a large inertial system, the request for a dynamic response of speed is not high. The focuses are mainly on the speed following ability, speed stability during mode switching and the smoothness at low speed.
(1) Following ability The relationship between the forklifts descending velocity and the handle signal is given in Figure 10. The handle signal gradually increased from the minimum opening degree to the maximum opening degree and then back to the minimum opening degree. Additionally, it was proven that the actual descending velocity of the forklifts was very close to the theoretical descending velocity and was consistent with the trend of the handle signal. The deviation of the descending velocity was small and the following ability was good. (2) Stability of low descending velocity The stability of the descending velocity is an important performance indicator for the forklifts, especially for EHFs. The minimum operating speed of the HMG was 300 r/min, but the handle signal was at the turning point between the "Stop" stage and "Single HMG" stage. Then 400 r/min was taken as the low speed performance test speed. Seen from Figure 11, the rodless chamber pressure had a small fluctuation when switching from the "Stop" stage to the "Single HMG" stage. On the one hand, it was caused by the fluctuation of the solenoid directional valve. On the other hand, it was caused by the higher target speed due to the larger initial handle signal. When HMG speed stabilized around 400 r/min, the rodless chamber pressure was around 13.9 MPa, the fluctuation was within 0.1 MPa. The overall pressure fluctuation was small. Seen from Figure 12, the actual HMG speed was very close to the target speed 400 r/min, and the stability of the speed control was good. (2) Stability of low descending velocity The stability of the descending velocity is an important performance indicator for the forklifts, especially for EHFs. The minimum operating speed of the HMG was 300 r/min, but the handle signal was at the turning point between the "Stop" stage and "Single HMG" stage. Then 400 r/min was taken as the low speed performance test speed. Seen from Figure 11, the rodless chamber pressure had a small fluctuation when switching from the "Stop" stage to the "Single HMG" stage. On the one hand, it was caused by the fluctuation of the solenoid directional valve. On the other hand, it was caused by the higher target speed due to the larger initial handle signal. When HMG speed stabilized around 400 r/min, the rodless chamber pressure was around 13.9 MPa, the fluctuation was within 0.1 MPa. The overall pressure fluctuation was small. Seen from Figure 12, the actual HMG speed was very close to the target speed 400 r/min, and the stability of the speed control was good.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 18 As concerning about that EHF is a large inertial system, the request for a dynamic response of speed is not high. The focuses are mainly on the speed following ability, speed stability during mode switching and the smoothness at low speed.
(1) Following ability The relationship between the forklifts descending velocity and the handle signal is given in Figure 10. The handle signal gradually increased from the minimum opening degree to the maximum opening degree and then back to the minimum opening degree. Additionally, it was proven that the actual descending velocity of the forklifts was very close to the theoretical descending velocity and was consistent with the trend of the handle signal. The deviation of the descending velocity was small and the following ability was good. (2) Stability of low descending velocity The stability of the descending velocity is an important performance indicator for the forklifts, especially for EHFs. The minimum operating speed of the HMG was 300 r/min, but the handle signal was at the turning point between the "Stop" stage and "Single HMG" stage. Then 400 r/min was taken as the low speed performance test speed. Seen from Figure 11, the rodless chamber pressure had a small fluctuation when switching from the "Stop" stage to the "Single HMG" stage. On the one hand, it was caused by the fluctuation of the solenoid directional valve. On the other hand, it was caused by the higher target speed due to the larger initial handle signal. When HMG speed stabilized around 400 r/min, the rodless chamber pressure was around 13.9 MPa, the fluctuation was within 0.1 MPa. The overall pressure fluctuation was small. Seen from Figure 12, the actual HMG speed was very close to the target speed 400 r/min, and the stability of the speed control was good.  (3) Working mode switching characteristics Research on working mode switching characteristics focuses on the speed following and pressure fluctuation of the two switching points. At time 6.65 s, the system switches from the "single HMG" stage to "double HMGs" stage. As shown in Figures 13 and 14, though the rodless chamber pressure fluctuated about 0.5 MPa, the fluctuation range was small. Seen from Figures 15 and 16, the actual speed of 1#HMG and 2#HMG were slightly different from their target speed, but the fluctuation range was also small.
At time 16.5 s, the system switched from the "double HMGs" stage to "single HMG" stage, and the pressure fluctuation range was also small. The actual speed of 1#HMG slightly fluctuated but the fluctuation range was small. Additionally, 2#HMG entered the "Stop" stage and had no effect on the descending process of the forklifts. Both the speed and pressure fluctuations were small and had little influence on the control performance of the EHF.  (3) Working mode switching characteristics Research on working mode switching characteristics focuses on the speed following and pressure fluctuation of the two switching points. At time 6.65 s, the system switches from the "single HMG" stage to "double HMGs" stage. As shown in Figures 13 and 14, though the rodless chamber pressure fluctuated about 0.5 MPa, the fluctuation range was small. Seen from Figures 15 and 16, the actual speed of 1#HMG and 2#HMG were slightly different from their target speed, but the fluctuation range was also small.  (3) Working mode switching characteristics Research on working mode switching characteristics focuses on the speed following and pressure fluctuation of the two switching points. At time 6.65 s, the system switches from the "single HMG" stage to "double HMGs" stage. As shown in Figures 13 and 14, though the rodless chamber pressure fluctuated about 0.5 MPa, the fluctuation range was small. Seen from Figures 15 and 16, the actual speed of 1#HMG and 2#HMG were slightly different from their target speed, but the fluctuation range was also small.
At time 16.5 s, the system switched from the "double HMGs" stage to "single HMG" stage, and the pressure fluctuation range was also small. The actual speed of 1#HMG slightly fluctuated but the fluctuation range was small. Additionally, 2#HMG entered the "Stop" stage and had no effect on the descending process of the forklifts. Both the speed and pressure fluctuations were small and had little influence on the control performance of the EHF.

Potential Energy Regeneration Efficiency
The potential energy recovery efficiency is defined as the percentage of the charged electric quantity of the battery and the potential energy of the load and door rack in one descending process, seen in Equation (7).

Potential Energy Regeneration Efficiency
The potential energy recovery efficiency is defined as the percentage of the charged electric quantity of the battery and the potential energy of the load and door rack in one descending process, seen in Equation (7). At time 16.5 s, the system switched from the "double HMGs" stage to "single HMG" stage, and the pressure fluctuation range was also small. The actual speed of 1#HMG slightly fluctuated but the fluctuation range was small. Additionally, 2#HMG entered the "Stop" stage and had no effect on the descending process of the forklifts. Both the speed and pressure fluctuations were small and had little influence on the control performance of the EHF.

Potential Energy Regeneration Efficiency
The potential energy recovery efficiency is defined as the percentage of the charged electric quantity of the battery and the potential energy of the load and door rack in one descending process, seen in Equation (7).
where, η s is the regeneration efficiency of the potential energy of the system. E Q is the regenerated electric quantity of the lithium battery, J. E dL is the potential energy released by the descending of the door rack system and the load, J. The charged electric quantity of the lithium battery can be described as: where, U b is the busbar voltage of the lithium battery, V. I b is the busbar current of the lithium battery, A. If I b is negative, it is generating. The generated electric energy by the motor when boom descending with a load can be calculated by Equation (9).
where, M e is the effective mass of the load, forks, fork rack and inner door rack, kg. M is the load mass, kg. M f is the forks mass, kg. M fr is the mass of the fork rack, kg. M dr is the mass of the inner door rack, kg. h is the effective descending height, m. g is the gravitational acceleration, m/s 2 . Seen from Equation (9), the regeneration efficiency of the potential energy is related to the load mass and the effective descending height. The effects of load mass, effective decreasing height and descending velocity on the regeneration efficiency of the potential energy were analyzed when the lifting cylinder descended.
(1) Potential energy regeneration efficiency corresponding to a different load mass To study the influence of different load mass on the potential energy regeneration efficiency, using a single variable principle, the specific requirements were as follows. Before the test, the load was lifted to the same height. The load was descended to the same height with the same velocity no matter how heavy the load was. The busbar voltage and busbar current of the lithium battery were collected. The potential energy regeneration efficiency was calculated according to Equation (7) by the ratio of the charging quantity of the lithium battery and the released potential energy when the load and the door rack descending. The charging current and potential energy regeneration efficiency under 3 tons, 5 tons, 9 tons, 16 tons and 25 tons were discussed.
In the test rig, the capacity of a lithium battery was 404 Ah, the rated voltage was around 540 V. Additionally, the recoverable energy when the load descending one time was small and the voltage rises caused by it was not obvious. Therefore, the voltage curve would not be discussed here. As shown in Figure 17, the generated current was not a steady-state value but fluctuated within a certain range when the load descending. The generated currents with loads of 3 tons, 5 tons, 9 tons, 16 tons and 25 tons were about −7.0 A, −9.2 A, −14.9 A, −25.7 A and −38.3 A, respectively. The effective value of the generated current was positively correlated with the load mass. That is, the larger the load mass is, the larger the generated current is and the faster the charging speed can achieve. At the same height and the same descending velocity, the energy loss of the system increased with the increase of the load mass seen from Figure 18. This is because with the increase of the load, the inlet pressure of the hydraulic motor increased, which led to the increase of the leakage of the hydraulic system. Therefore, the volume efficiency of the hydraulic system decreased and the system loss increased. However, the proportion of energy loss in the recoverable potential energy decreased gradually and the regeneration efficiency increased with the increase of load. Due to various damping consumption of the system, the regeneration efficiency of the system would not At the same height and the same descending velocity, the energy loss of the system increased with the increase of the load mass seen from Figure 18. This is because with the increase of the load, the inlet pressure of the hydraulic motor increased, which led to the increase of the leakage of the hydraulic system. Therefore, the volume efficiency of the hydraulic system decreased and the system loss increased. However, the proportion of energy loss in the recoverable potential energy decreased gradually and the regeneration efficiency increased with the increase of load. Due to various damping consumption of the system, the regeneration efficiency of the system would not increase indefinitely.
At the same height and the same descending velocity, the energy loss of the system increased with the increase of the load mass seen from Figure 18. This is because with the increase of the load, the inlet pressure of the hydraulic motor increased, which led to the increase of the leakage of the hydraulic system. Therefore, the volume efficiency of the hydraulic system decreased and the system loss increased. However, the proportion of energy loss in the recoverable potential energy decreased gradually and the regeneration efficiency increased with the increase of load. Due to various damping consumption of the system, the regeneration efficiency of the system would not increase indefinitely.
In addition, as can be seen from Figure 18, when the load mass was 25 tons, the regeneration efficiency of the system was close to 74%. While when the load was 3 tons, the regeneration efficiency of the system was still about 50%. This indicates that the proposed double HMGs with PERS could work in the efficient range even if the load mass changed greatly. (2) Potential energy regeneration efficiency corresponding to a different descending velocity Seen from Figure 19, when the forks descending velocity was less than 83.6 mm/s, the system was at the stage of "Single HMG". With the increase of the descending velocity, the HMG speed increased, the motor efficiency increased and the volume efficiency of the hydraulic motor decreased. Therefore, the energy loss of the system decreased gradually and the regeneration efficiency, which could be up to 74.38% increased accordingly. When the system entered the stage of "Double HMGs", with the increase of the descending velocity, HMG speed and two motors' efficiency increase. While the volume efficiency of the hydraulic motor decreased. Since there were In addition, as can be seen from Figure 18, when the load mass was 25 tons, the regeneration efficiency of the system was close to 74%. While when the load was 3 tons, the regeneration efficiency of the system was still about 50%. This indicates that the proposed double HMGs with PERS could work in the efficient range even if the load mass changed greatly.
(2) Potential energy regeneration efficiency corresponding to a different descending velocity Seen from Figure 19, when the forks descending velocity was less than 83.6 mm/s, the system was at the stage of "Single HMG". With the increase of the descending velocity, the HMG speed increased, the motor efficiency increased and the volume efficiency of the hydraulic motor decreased. Therefore, the energy loss of the system decreased gradually and the regeneration efficiency, which could be up to 74.38% increased accordingly. When the system entered the stage of "Double HMGs", with the increase of the descending velocity, HMG speed and two motors' efficiency increase. While the volume efficiency of the hydraulic motor decreased. Since there were two sets of HMGs, the energy loss of the system increased and the regeneration efficiency began to gradually decrease. The overall regeneration efficiency remained above 60%.
Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 18 two sets of HMGs, the energy loss of the system increased and the regeneration efficiency began to gradually decrease. The overall regeneration efficiency remained above 60%.

Summary and Conclusions
Concepts of the PERS based on an EHF were studied. Additionally, as shown in this analysis, it was possible to increase the efficiency of the EHF. It was evident that double HMGs with PERS Figure 19. Curve of regeneration efficiency under a different load velocity.

Summary and Conclusions
Concepts of the PERS based on an EHF were studied. Additionally, as shown in this analysis, it was possible to increase the efficiency of the EHF. It was evident that double HMGs with PERS could regenerate a large amount of gravitational potential energy and could guarantee the control performance of the boom. Some useful conclusions were achieved as follows: (1) According to the special working style, the structure of double HMGs with PERS was presented.
The discrimination method of the working mode of the proposed double HMGs with PERS for EHF was feasible. (2) The actual descending velocity of the forks was consistent with the trend of the handle signal.
The deviation of the descending velocity was small and the following ability was good. The overall pressure fluctuation was small and the stability of the speed control was good when the boom descending velocity of EHF was low. (3) When the system switches between different working modes, both the speed and pressure fluctuations were small and had little influence on the performance of EHF. (4) With the increase of the load mass and descending velocity, the regeneration efficiency increased.
The maximum efficiency was up to 74%.
Though the test results show that the proposed structure could realize the energy regeneration, the volume efficiency of the hydraulic motor affected the regeneration efficiency. The optimization of the HMG is worth further studying.

Conflicts of Interest:
The authors declare no conflict of interest. proportional relationship between the target speed controlled by 1#HMG and the handle signal k 2 proportional relationship between the target speed controlled by 2#HMG and the handle signal k t proportional relationship between the total target speed and the handle signal k AB line slope from point A to point B