Introduction

In mechatronic engineering, there exist three common actuators used in automation system, that is, servo motor, electro-hydraulic, and pneumatic. The servo motor system has high mechanical e?ciency and control accuracy than the other two actuators. However, its power is restricted by the element size and structure of motor and driver. Pneumatic actuator is an environmentally friendly actuator, which has no obvious pollution to environment. But the pneumatic actuator has very low execution e?ciency in engineering. Many available energy dissipates as heat. Although the mechanical e?ciency of the electro-hydraulic actuator (EHA) is not higher than that of the servo motor actuator, electro-hydraulic servo systems (EHSs) are nowadays more widely used in mechatronic control engineering due to their higher load-bearing and small size-to-power ratio [40] than the general servo motor actuator. When the supply pressure is large, the dynamic response may also be faster than the servo motor under some large external load and other unknown disturbance. It was found that EHSs are beginning to be commonly applied for large power equipment such as wheel loaders [13], load simulators [63], insulator fatigue test devices [66], and exoskeletons [22]. EHA is a favorable control executive part to realize some multi-DOF manipulator, crane, space manipulator, and lower limb of BigDog due to the large dynamic external load.