Abstract
A four-way valve used in a peneumatic servo system may have uneven-underlaps or different port widths. A cylinder is often used as a load for the valve, and generally it has Coulomb friction as well as viscous friction.
The frequency characteristics of such a valve-controlled-cylinder system show a nonlinearity, and a simple linearization of the system may lead to a large error.
Two methods to find out such frequency characteristics are described here. One is to approximate a system partially with 2nd order lag. The other is to use the describing function of a system. The formulae used in these methods are very easy to calculate and will give results with small error, and may be suitable for the analysis and the design of a peneumatic servo system.
