Abstract
In order to realize a flow control system with quick response, characteristics of the instruments of which the system is composed were analysed, and it was found that velocity saturation exists in the system because there is an upper limit to air flow which passes through a pilot relay. The system composed of a pilot relay and a control valve being analysed by the method of describing function, the following conclusion was obtained:
1) Pilot relay has a narrow linear zone and when input exceeds it, velocity saturation is born.
2) The system which includes velocity saturation is approximated pretty well to the first order lag, in which break frequency is shifted lower.
3) Analysis results obtained by the author nearly coincide with those by Takahashi, Ziegler and Nichols.
4) In an ordinary flow control systems loop gain is not so high that velocity saturation can not cause limit cycle.
