Hydraulic drive mode enables legged robots to have excellent characteristics, such as greater power-to-weight ratios, higher load\ncapacities, and faster response speeds than other robots. Nowadays, highly integrated valve-controlled cylinder, called the\nhydraulic drive unit (HDU), is employed to drive the joints of these robots. However, there are some common problems in the\nHDU resulted from hydraulic systems, such as strong nonlinearity, asymmetry dynamic characteristics between positive and\nnegativemoving directions of the piston rod, and time-varying parameters. It is difficult to obtain the desired control performance\nby just using classical control methods such as the traditional PID control. In this paper, a position controller that combines fuzzy\nterminal sliding mode control (FTSMC) and time delay estimation (TDE) is proposed, in which FTSMC adopts a compound\nreaching law which combines the tangent function and the exponential reaching law.Moreover, the fuzzy control is introduced to\nadjust the parameters of the reaching law in real time to improve the adaptability of FTSMC. Based on FTSMC, the external\nuncertain disturbance of the HDU position control system is estimated by TDE, which ensures the simplicity of system modeling\nand the normal application of FTSMC. Finally, the control effects of the controller combining FTSMC and TDE are verified on the\nHDU performance test platform and the load simulation experiment platform. The experimental results show that the proposed\ncontroller greatly improves the system position control performance and has a strong disturbance rejection ability and a good\nadaptability under different working conditions. The above research results can provide an important reference and experimental\nbasis for the inner loop of compliance control of legged robots.
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