Hydraulic system has high power density and excellent dynamic response, so it is widely applied to position, speed, force and torque control of various machinery. Existing hydraulic systems control the movement of loads through valves of variable displacement pump control. Although these systems can obtain high power, they require large equipment and hydraulic pipes compared to an electric motors, and has problems such as oil leakage, performance change due to temperature change, and low power transmission efficiency. To solve this problem, EHA (Electro Hydrostatic Actuator) system is integrated with servo motor, two-way hydraulic pump, cylinder, hydraulic valve and piping, so that it can be miniaturized compared to the existing hydraulic system and minimizes piping. It can reduce the leakage of oil. In addition, since the position and pressure control of the hydraulic cylinder is performed through the hydraulic pump flow control directly connected to the servomotor without using the control valve, it is possible to improve the energy transfer efficiency problem. The EHA system is mainly applied to aircrafts, but recently, research and development are being conducted to apply industrial actuators to presses, injection molding machines, and construction machinery in general industrial sites.
However, the EHA system is relatively less responsive than conventional hydraulic systems due to the rotational inertia of the pump and motor. It also has the nonlinearity of the system and uncertain system parameters such as, changes in viscosity and volume modulus with the temperature and pressure of the fluid and frictional characteristics of the cylinder. Uncertainty due to these operating environments and nonlinearity of the system are factors influencing control performance in controlling the EHA system. Despite this, the EHA system has to be robust and precise in order to be applied to various industrial sites.
In this study, we designed an EHA system to implement the rotational motion of a load, construct a nonlinear simulation model based on a mathematical model, and verify the validity of the model by comparing the experimental and simulation results. Sliding mode controller (SMC) and PID controller are designed for rotation load angle control using EHA and control performance is verified by nonlinear simulation and experiment.