Coordination control for parallel actuation of hydraulic power assisted ball screw drive system

Hydraulic actuators are commonly incorporated into the vertical feed drive system of heavy duty machine tools to achieve higher load capacity and stiffness by balancing the moving load. In the linear motion control system with ball screw and cylinder driving in parallel, interactions between two drive sets are directly related with machining accuracy. On one hand, motion synchronization should be dealt with in term of master and slave scheme with hydraulic cylinder following the speed generated by screw nut real-timely. On the other hand, temperature rise due to heat generation of moving parts causes not only thermal expansion of the screw shaft but also property change of hydraulic oil, finally exerting a significant influence on the positioning accuracy. In this paper, the coupled model of this combination is built up to analyze the dynamic performance under different operating conditions. Based on the electromechanical system model with hydraulic dynamics and thermal dynamics, the problem is formulated as coordinating the velocity of two actuation systems with coupling. The hydraulic actuator is modeled as a combination of an ideal velocity source and a spring for modeling the compressibility effects of the fluid medium to address the coordination control issue. It is shown in the simulation results that the proposed control strategy can achieve desired coordination of different drive systems with acceptable tracking errors.