MODELING, DESIGN AND CONTROL OF A MODERN ELECTROPNEUMATIC ACTUATOR

In the process industries there are many cases where substantial economies could be made, if valve actuators were available with response times shorter than those presently available, with traditional spring-opposed-diaphragm units, the performance of these actuators being inhibited by built-in loading, a low working pressure and flow passages which are restricted in relation to the swept volume of the actuator. The paper presents an analysis of pneumatic actuator system with response characteristics that are much improved compared with those available from conventional spring-opposed-diaphragm units. The equations are developed which characterise the behaviour of the pneumatic actuator, and comparisons of experimental (using a specially designed test rig) and simulated results show the accuracy of the model to be good. Linear approximate models of the pneumatic actuator system then form a basis for the application of two linear controller-design procedures. The design methods employed are the linear quadratic optimal control with extended cost functions and the Horowitz approach to sensitivity reduction. The resulting closed-loop designs are found to be robust and to achieve their required performance specifications under both emergency and normal actuator operations.