An Analysis of a Flow Control System Including Nonlinearities

Abstract

In pneumatic flow control systems it occasionally occures sustained oscillation when some larger disturbance is added. This phenomenon is attributed to the effects of nonlinear elements in the system.
This article analizes a typical example of flow control system (Fig. 1) containing 3 effective nonlinearities as follows:
(1) Control valve speed saturation
(2) Control valve hysteresis
(3) Nonlinearity of flow system (from control valve position to pressure difference in oriffice) Of these three, (1)&(2) are put together to total describing function and (3) is treated as gain change of differential pressure converter output to control valve displacement (Fig. 4).
The amplitude and frequency of limit cycle can be estimated by plotting linear transfer function and nonlinear describing function on the s-plane.
The results of analysis are compared with measured values (Fig.15) and the effects of nonlinear elements are discussed.