Characterization and tracking control of a nonlinear electrohydraulic valve-cylinder system

Abstract

It is well known that simple proportional, integral, and derivative control yields poor tracking performance due to the friction-and flow-related nonlinearities in electrohydraulic servo-systems. Nonlinear effects are more significant in proportional valve having deadband and non-matched ports with potential application in systems with complex ground friction in off-road vehicles or complex inertia loads in simulators meant for pilot training. A feedforwardbased controller has been designed here by performing a number of characterization experiments for achieving good tracking performance overcoming severe nonlinearities. An algebraic model of friction has been developed for including hysteresis beyond the static friction zone in a double-rod piston-cylinder arrangement. A proportional valve with nonmatched ports and large deadband has been characterized in terms of command signal to flow gains for each metered port and a leakage coefficient. Also, a dynamic model for the valve with embedded control has been constructed. All these models have been integrated together to predict the piston-motion dynamics. A simple theoretical analysis with a fixed command excitation revealed that following the initial transients a sustained oscillation over a constant mean piston velocity could exist for low pump pressure and valve damping due to the flow-motion coupling. The bandwidth and damping coefficient of the valve flow have been estimated through a comparison between the predicted and experimentally measured piston displacement variation with time. Besides evaluating the feedforward using the algebraic friction model along with assuming incompressible flow and negligible valve leakage, the predictions of the complete model were used to ascertain the proportional, integral, and derivative gains to be implemented in real-time control. Up to 0.5 Hz sinusoidal excitation, the proposed control revealed excellent tracking performance. Controls with only proportional, integral, and derivative and proportional, integral, and derivative together with feedforward exhibited noticeable phase shifts, respectively, from frequencies 0.0625 Hz and 0.6 Hz. Hence, the proposed controller can be useful in low-cost, low-power, precision applications up to 0.5 Hz input excitations.

Designing an optimized model-free controller for improved motion tracking by rugged electrohydraulic system

“…Within a range ± e 0 of the voltage variation, called the valve deadband, the leakage flow within the PV does not induce any perceptible piston motion. A previous study 29 on a similar system revealed the deadband as ±1.5 V. Beyond the deadband, the valve flow variation with the command voltage e was found as nonlinear, as highlighted in Figure 2(a). The figure additionally shows the associated pressure drop characteristics, where Δ P i ( i = 1 − 4 ) is the pressure drop across the proportional valve ports.…”

“…Figure 2 represents the system nonlinearities characterized earlier. 29 The flow and pressure drop of the proportional valve were determined experimentally, as shown in Figure 2(a) with Q r being the flow measured in the return line by a screw flow meter. Within a range ± e 0 of the voltage variation, called the valve deadband, the leakage flow within the PV does not induce any perceptible piston motion.…”

“…Figure 2(c) shows the friction characteristics where the static frictions were found 29 to be as high as F 0 − = − 814 N and F 0 + = 911 N . Compared to it, the cylinder used in another study with the same valve 22 had these two frictions as only ±33 N. 29 The figure also shows a significant hysteresis effect between the accelerating phase showing slip event S2 for small velocity zone and the decelerating phase showing stick zone as S1. Within these zones, the friction characteristics are also very uncertain.…”

Research and Development of Electro-hydraulic Control Valves Oriented to Industry 4.0: A Review

“…This can be calculated by measuring the pressure at both ends of the valve ports and the displacement of the valve spool. Das et al [13] and Borghi et al [14] used the polynomial equation with the flow rate as an independent variable to fit the pressure drop at both ends of the valve port for different valve openings. Åman et al [15] also used the pressure difference at both ends of the valve port to fit the flow rate through the valve port on the basis of considering the influence of the flow patterns.…”

Energy-efficient hydraulic actuator position tracking using hydraulic system operation modes

“…The prevalent design for position tracking purposes employs a proportional or servo directional control valve and a constant-pressure hydraulic source to control the position of the hydraulic actuator, similar to what is shown at the right-hand side of Figure 1. A rich body of research is available on improving the position tracking performance, through using control design methods such as robust control, 7,8 nonlinear control, 9,10 adaptive control, 11 sliding mode control, 12 and combinations of these techniques. 13 These efforts have been successful judging by the experimental results they provide.…”