Improvement of Backdrivability of a Force-Controlled EHA by Introducing Bypass Flow Control

The dynamic response of an electro-hydrostatic actuator (EHA) is limited due to rotational inertia of its servomotor and pump. Thus, when an external disturbance with a high frequency is input to the force-controlled EHA, the pump speed may not be fast enough to attenuate the force control errors. In order to expedite the oil transfer between the cylinder chambers, a bypass valve was employed in this study. The bypass valve connecting both the cylinder chambers can then decompress the high-pressure chamber quickly by bypassing the oil to the low-pressure chamber to compensate for force control errors. To control the bypass flow without affecting the system stability, while the rotational velocity of pump was continuously varied by the force controller, a sliding mode control technique was applied. The sliding mode controller showed satisfactory control performance with respect to stability and robustness, in spite of highly nonlinear properties in the proposed system. Experimentally, where sinusoidal or stepwise velocity disturbance was exerted on the proposed EHA system with the force reference values set at 0 N or 200 N, the force control errors could be reduced markedly, compared to a conventional force-controlled EHA without bypass flow control. The performance improvement was more obvious when the sinusoidal excitation frequency increased from 1 to 3 Hz.