Robust identification and control of mobile hydraulic systems using a decentralized valve structure

The control of mobile hydraulic systems presents several challenges: valve characteristics and position-dependent system behavior are sources of nonlinearity. In addition, position and velocity sensors are not common in mobile machines, although the primary objective is piston velocity control. The particular type of hydraulic system considered in this article uses four decentralized valves to control the inflow and outflow of the two cylinder chambers. In contrast to conventional controller synthesis by tuning PID-type controllers for different operating points, a systematic approach for robust identification and model-based control is presented. It benefits from the possibility of bypassing the cylinder chambers for the identification step. Here, the ranges of physical parameters are estimated to obtain a parametrization of all possible system realizations. To reduce the valve-dependent nonlinearity, electro-hydraulic pressure compensation is applied to all valves. Based on the identified model, a nominal linear quadratic Gaussian controller and a robust $\mu$-synthesis controller are designed, tested, and compared to a state of the art PID controller with active damping The identification and control are partially demonstrated on a hydraulic test bed and in simulation.