An improved method combined SMC and MLESO for impedance control of legged robots' electro-hydraulic servo system

The electro-hydraulic servo system (EHSS) drives the hydraulic quadruped robot, which has the advantages such as high load capacity, fast response velocity, and powerful motion ability. EHSS of single leg consists of three sets of hydraulic drive unit (HDU), which is the joint driver. As a result, HDU control is the fundamental control of the hydraulic quadruped robot, and it controls the robot's motion performance directly. In order to improve the control accuracy and adaptability to different working conditions of impedance control for HDU, a composite control method combining sliding mode control (SMC) and model-based linear extended state observer (MLESO), which is called SMC-MLESO, is designed in this paper. Firstly, the chattering problem of SMC is improved by designing a novel composite reaching law and adding total disturbance to sliding mode control law. Secondly, the parameters of sliding mode surface are calculated by the optimal control. The parameters of MLESO are calculated by the bandwidth of the controller. And the known model of the system is added to observer to reduce the influence of sensor noise. Finally, comparative experiments show that SMC-MLESO has a good control effect. The maximum error of using SMC-MLESO is 0.101 mm and the biggest change of the maximum error is 36.5% under different working conditions, which is better than PI+G_cp and PI+G_cp+G_fp(The two controllers were designed by the author's previous research, which was published by Journal of the Franklin Institute).