Overview

Pneumatic systems offer a number of advantages: typically they are fast, rugged, simple to maintain and low cost. To move a part from one fixed position to another, pneumatic cylinders with fixed stops and switching valves are a natural choice. This situation changes if a stop in between the end-positions is required. This situation arises for instance in material handling when fault parts have to be ejected to another container than OK parts. This task requires either a multi-position cylinder or a closed-loop positioning system.

Due to fact that electric DC motors with ballscrew drives can be very easily controlled they have been used in the past. Today there are more electric alternatives: controlled AC motors are available as off-the-shelf products as are electric linear drives. Due to the compressibility of air, adverse friction characteristics and low damping, pneumatic drives require sophisticated controllers. But they also offer advantages: the compressibility of air provides a "cushioning" effect which is important in applications like blow moulding or glass forming (Han and Alleyne 2000). They can also be used in a vacuum without generating heat; a situation that arises in semiconductor production (Kagawa et al. 2000b). The reported accuracy in that application is 0.2 m.

First theoretical and experimental results of closed-loop, position controlled pneumatic systems were presented in 1954 by Shearer. He did a theoretical analysis of a pneumatic cylinder, a proportional directional control valve and a mechanical proportional controller. To stabilise the system, he used gas tanks which had...