Improving the Steady-State Response of Full-Digital Pneumatic Motor Speed Control Using Two-Step Current Control Scheme

In this paper, a full-digital closed-loop pneumatic motor speed control system with reduced steady-state error is developed and realized. A significant feature of the proposed structure is the utilization of a proportional full-digital control valve (FDCV), which consists of 12 parallel-connected 2/2 pneumatic switching valves. Compared to the PWM fast-switching proportional flow control scheme, the new FDCV possesses several advantages like medium operating noise, long life, ease of control and low cost. The conventional PNM coding system is chosen for this study. The major fault of the FDCV, however, is the nonlinear saw-toothed flow-rate characteristic which generally results in limit-cycle oscillation and the undesirable steady-state error in the steady-state response. Therefore, a novel technique to reduce the amplitude of steady-state error is developed in this paper. The basic idea is to reduce the opening areas of 12 switching valves in the FDCV simultaneously by applying two-step current inputs to the valve coils. Consequently, the steady-state error of the pneumatic motor speed control can be successfully reduced without any hardware modification. Finally, from the experimental results, it is proved that the steady-state error is significantly reduced by using the proposed novel two-step current control scheme as compared to the utilization of a conventional PNM (Pulse Number Modulation, PNM) coding controller.