## 5 The Verification Experiment of VAPP  
According to the controlling method of variable mechanism in Figure 2, the principle of characteristic test of VAPP variable displacement is shown in Figure 14. The variable displacement system of asymmetric pump controls the angle and direction of the swash plate by controlling the flow direction of oil and the opening area of servo-proportional valve. Thus, it can achieve accurate control of asymmetric pump displacement through setting the initial state of the swash plate at meso-position,  
Figure 12: Ten megapascals load in Slots B and T, respectively at 600 rpm: (**a**) Slot B output pressure; (**b**) Slot T output pressure; (**c**) Slots B output flow; (**d**) Slots T output flow; and (**e**) instantaneous cylinder pressure. Figure 13: Ten megapascals load in Slots B and T, respectively at 1200 rpm: (**a**) Port B output pressure; and (**b**) Port T output pressure. or setting the swash plate angle at 0\({}^{\circ}\). Pump 4 supplies oil for the left chamber when Valve 3 is in the left position. The swash plate articulated with the slider in the variable cylinder rotates clockwise in this process, as chamber sucks the oil but Port B and Port T discharge oil. When Valve 3 is in a meso-position, the two chambers of variable cylinders do not suck oil, and when the swash plate is in meso-position and A, B and T chambers do not discharge oil. When Valve 3 is in the right position, Pump 4 supplies oil to the right chamber, but this time, the swash plate rotates anticlockwise and Chambers B and T suck the oil, while Chamber A discharges the oil. Among them, _dSpace_ can collect the angle signal of the swash plate and send accurate control signals to the servo-proportion valve. The loading on discharging Port B and Port T is realized by the relief valves. Figure 15 is the image of the VAPP that is used for the experiment, the physical photo of VAPP and test scene of VAPP can be found in Supplementary Materials. The angle displacement sensor, whose mode is MIDORI PRECISIONS CP-2 UK-R260, is used for measuring the swash plate angle. The maximum input voltage of the sensor is 8 V, and it can detect the angle within \(\pm\) 30 degrees. The servo proportion valve, model Rexroth 4 WRPEH6C4B12L-20/G24K0/A1M, is used for controlling the angle of the swash plate. The maximum input flow of the servo proportion valve is 12 L. Further, the hydraulic flow meter, whose model is PARKERSCLV-PTQ-300, is used for measuring the flow and pressure of Port B and Port T. The maximum flow and pressure which can be measured by the hydraulic flow meter is 300 L/min and 42 MPa separately. The directly operating relief valve, model HUADE DBD56P10B/315, is used for loading on Port B and Port T. The maximum set pressure of directly operating relief is 31.5 MPa. Asymmetric pump speed is set at 600 r/min and swash plate angle at 18\({}^{\circ}\). Port B and Port T take 10 MPa and 20 MPa at two levels of pressure to load, respectively. The loading condition of the test includes two kinds: loading on Port B alone and loading on both Port B and Port T. Collecting  
Figure 14: The characteristic test principle of VAPP. Figure 15: The VAPP prototype image. the pressure signals of Ports B and T, respectively, in the experiment. The pressure of Port B under two different pressure levels is depicted in Figure 16a, when the pressure is loaded on Port B alone. As shown in the image, the results got from the simulation process are consistent with the results obtained from the experiment. The pressure pulsation of Port B increases along with the increase of pressure loaded on Port B. The total pulsation amplitude is still small, thus the system performance is good. The pressure of Ports B and T under four pressure levels is also shown in Figure 16b,c, in which the pressure is loaded on Ports B and T at the same time. As detailed in the figure, the results obtained from the simulation process are consistent with the results attained from the experiment.