Studies on a Constant Speed Drive of a Pneumatic Cylinder

In many industrial applications, open-loop control which uses on-off valves and restrictors are widely used for constant speed drive of pneumatic cylinders. This paper describes the conditions and characteristics of constant speed drive of pneumatic cylinders. First, we derived the conditions for the constant speed drive based on the equation of motion and equations of pressure change in cylinder chambers. It was shown that pressure in both chambers must be constant for constant speed drive. In case where air is exhausted with a choke velocity, the outlet restriction determines piston speed uniquely as well as the conventional meter-out circuit. Steadystate pressure of the forcing side is determined by the inlet restriction and this affects the velocity response when motion starts. In the conventional meter-out circuit, the inlet restriction is given only by the inlet valve and large overshoot occurs at the start of motion. To get a fast response with a small overshoot, the inlet restriction should be adjusted properly. The size of optimum inlet restrictions was calculated for various reference speeds and loads by using basic equations and a method to estimate the size of inlet restriction by using friction forces was discussed. Experiments were carried out and the effectiveness of inlet restriction for desirable motion was confirmed.