Investigating the Symmetric Control of a Hydraulic System Based on Status Feedback

Valve-controlled hydraulic systems are widely used in various hydraulic equipment, but their asymmetric characteristics are the most critical factor restricting further improvements in system performance. This paper takes large asymmetric complex hydraulic equipment as the industrial background, and proposes a state feedback-based symmetric switching control method to address the complex control strategy and difficult control accuracy caused by input–output asymmetry and the inconsistent response of asymmetric valve-controlled hydraulic systems. A system state space model is established, and the parameterized expression that satisfies the state space switching-based symmetric control law is solved. Feedback and feedforward links based on state space symmetric switching are designed to transform the asymmetric system into a state space symmetric system. And the research results will be applied to the experimental setup of the 300 MN forging hydraulic press control system. Through simulation verification, under asymmetric PID control conditions, due to the influence of the asymmetric characteristics of the system structure, load, and their coupling relationship, the forward response time is shorter than the unloaded response time, and the overshoot is larger than the unloaded response time. The reverse response time is longer than the unloaded response time, and the overshoot is smaller than the unloaded response time. After symmetric control, the forward and reverse dynamic system characteristic curves completely overlap, proving that the system has achieved symmetric transformation; through experimental verification, under asymmetric PID control conditions, when the proportional valve opening remains constant, changes in the load pressure will cause changes in the load speed. For every 1 MPa increase in the load pressure, the load speed will slow down by about 0.0033 m/s. The load speed of the system after symmetrical control replacement will be much less affected by changes in the load pressure. The simulation and experimental results have shown that this method is expected to solve the key problem of inconsistent dynamic characteristics of complex equipment hydraulic systems in both the forward and reverse directions due to asymmetry, and the inability to ensure control accuracy in both directions using symmetric control strategies. This paper has developed a set of control theories and methods applicable to hydraulic systems with complex asymmetry.