The piston velocity curves with different switch control differential pressure ΔPsw are shown in Figure 9. When ΔPsw =0.05MPa, the piston velocity is comparatively smooth and steady. However, when ΔPsw =0.03MPa, the piston velocity fluctuates during the later stage and the stick-slip occurs. In this case the pressure P2 has approached the permissible maximum pressure P2max for
overcoming a certain load and the motion time is obviously prolonged.