## 4 Experiment and Results Analysis  
### 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 8: Prototype EHF and its key components for the test.  
Figure 7: Schematic of the test rig.  
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.  
Figure 11: Curves of rodless chamber pressure at 400 r/min.  
Figure 10: Curve of the forklift descending velocity and handle signal.  
(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.  
Figure 14: Relationship between the rodless chamber pressure and handle signal.  
Figure 12: Curves of HMG speeds and handle signal.  
Figure 13: Curves of the rodless pressure and speed.  
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.