Nonlinear Load Force Compensation Method of Granular Materials Compaction Hydraulic Press Based on Cancellation Mechanism

When the elastoplastic deformation occurred during the granular materials compaction process, the load stiffness presented a complex nonlinear time-variant characteristic, so the control system was affected by a strongly nonlinear load force, which would increase the control difficulty. Firstly, the mathematical model of the electro-hydraulic servo system was established. Then, a nonlinear load force compensation method was proposed based on the cancellation mechanism. Lastly, the dynamic performance was analyzed with a 300 kN precision forming hydraulic press to compact a mixture material of laundry detergent and salt, where the PID (proportion integration differentiation) control algorithm and the PID+LOC (load observation compensator) nonlinear load force compensation method was taken respectively. The experiment results show that when taking the proposed PID+LOC as the control algorithm, the influence of nonlinear load force on the control system's performance is significantly reduced, and the motion control curve changes linearly during the compaction process. Due to the creep characteristics of the granular materials, it is easy to induce the fading oscillation phenomenon of the compaction force in the pressure-maintaining stage when taking PID as the control algorithm. However, this phenomenon of the compacting force is eliminated when the PID+LOC is taken. Furthermore, the tracking accuracy of the system is effectively improved, and the frequency bandwidth of the system is extended with the proposed PID+ LOC nonlinear load force compensation method.