A data-driven model-free adaptive sliding mode control (MFASMC) method is proposed to tackle the problems of inaccurate model and time-varying parameters of a micro electro-magnetic linear actuator system, considering the dependence of existing model-based control methods on the system dynamics model and the impact of unmodeled dynamics on the control performance. Firstly, the pseudo-gradient (PG) concept in the model-free adaptive control (MFAC) framework is used to transform the electromagnetic linear actuator dynamics model, which is difficult to obtain parameters accurately, into a full-format dynamic linearized data model, the dependence of the controller on the electromagnetic linear actuator model is reduced. To compensate for the effects of unknown perturbations, an improved discrete sliding mode exponential convergence law is introduced to derive a new composite control algorithm based on the pseudo partial derivative estimator, which improves the robustness of the control system. Then, the convergence of the control error and the stability of the system are demonstrated by theoretical analysis. The results show that the proposed MFASMC reduces the 10mm step response time of the electromagnetic linear actuator by 46.3% and 25.3% compared with PID and MFAC. The phase lag time and root-mean-square error of MFASMC under sinusoidal conditions are 0.8ms, 0.178mm respectively, and the steady-state error does not exceed 0.05mm. The experimental and simulation results keep the error within 5%, the proposed control algorithm has good trajectory tracking response speed and control accuracy, and has good robustness to system uncertainty and external force disturbance.

The hydrostatic actuation system based on linear actuators improves the complex piston force and long transmission path of the traditional electro-hydrostatic actuator (EHA). However, new nonlinear factors in the linear actuator and direct-driven piston are introduced into the system, which present challenges to system modeling and control. To improve the accuracy of system performance prediction, this paper analyzed the working characteristics of an electromagnetic direct-drive hydrostatic actuation system (EDHAS). A dynamic model of the electromagnetic linear actuator including the LuGre friction model was established. The high-pressure internal leakage of the direct-drive pump was described by an inclined eccentric leakage model. The Karnopp friction model was applied to solve the problem of switching between viscous and sliding friction in a cylinder. The hydraulic components model was established based on AMESim, and the electromagnetic linear actuator model and the system controller model were established in Matlab/Simulink, to establish a refined electromechanical–hydraulic co-simulation model of the EDHAS with electromagnetic, mechanical, hydraulic, and control coupling. A system performance test platform was built. The simulation results of the direct-drive piston displacement, the system pressure, the system flow rate, and the cylinder displacement match well with experimental results, which verifies the validity and accuracy of the refined modeling method.