Abstract: During the motion process of the hydraulic driven legged robot, the outer loop impedance control method, based on the hydraulic control inner loop, is applied in the hydraulic drive unit(HDU) which drives the robotic joints. For this control method, the hydraulic control inner loop can be selected from two different control loops, position closed loop and force closed loop. When the position closed loop is adopted, the impedance control performance of outer loop is directly decided by the position control performance of the inner control loop. Therefore, it is significant to design a position control method with high accuracy for HDU. Thus, the paper is organized as follows according to the design idea. First, the six order mathematical model of HDU position control system is simplified and the corresponding transfer functions are figured out. Then, the mathematical expressions which describe the input feedforward compensation controller of position control are derived out. This controller has the natural nonlinear characteristics and load characteristics of the hydraulic system. At last, the position control performance is researched through experiments, where the multiple typical input signals and trotting gait input signals are applied in the control system, on the HDU performance test platform. Then, the experimental results are analyzed quantitatively. It is found that the application of the feedforward compensation controller designed in this paper can improve the position control performance greatly under different input signal. Besides, this controller shows good robustness. Combined with the corresponding anti-disturbance strategy, the research results can provide the valuable reference and experimental foundation for the hydraulic control inner loop of position-based impedance control.

Key words: feedforward compensation controller, hydraulic drive legged robot, hydraulic drive unit(HDU), position control system