Nonlinear modeling and validation of solenoid-controlled pilot-operated servovalves

Solenoid-controlled pilot-operated servovalves are flow control devices widely used to drive high-flow hydraulic actuators in heavy-duty off-highway machinery. In these applications, a spool-operated pilot stage is required to account for large flow force in the main hydraulic valve. Design optimization, performance improvement, and fault diagnosis of such complex two-stage servovalves often require sophisticated analytical models with accurate physical parameters. The paper presents a step-by-step methodology for nonlinear modeling, parameter determination, model validation, and performance evaluation of solenoid-controlled pilot-operated spool valves. The proposed model takes into account such nonlinearities as pilot spool dead band, spool friction, flow coefficient variability, and leakage. This new model is complete in the sense that all nonlinear interactions between various electromechanical elements have been incorporated in a compact yet comprehensive model. The simulation model accepts a command voltage to the solenoids as input and gives the second stage spool displacement as output. To obtain the physical parameters used in the valve model, a systematic approach is proposed based on state measurement and curve-fitting techniques. The identified parameters of an example valve were used in simulations both to validate the nonlinear valve model and also to assess the valve's performance when certain valve parameters change.