Theoretical and experimental investigation of a pulse-width modulated digital hydraulic position control system

In this study, the open loop and closed loop behaviours of a pulse-width modulated hydraulic system are investigated both theoretically and experimentally. First a mathematical model is formulated for the system and methods for obtaining system responses to pulse-width modulated inputs are developed. Using these methods, the responses of the servo valve, the open loop as well as closed loop hydraulic systems are calculated for pulse-width modulated step and sinusoidal inputs. Effect of pulse frequency on the sinusoidal response is investigated. An experimental set-up, which consists of a PC as the controller, a pulse-width modulator, a servo valve driven hydraulic system and feedback elements, was constructed. Experiments show that precise position control can be achieved by using pulse-width modulated inputs in electro-hydraulic control systems. The results obtained for sinusoidal reference inputs show that the pulse frequency should be more than 50 times the reference input frequency for a satisfactory performance of the tested system, if the reference frequency is less than 10 Hz. (C) 2002 Elsevier Science Ltd. All rights reserved.