Relay pneumatic servomechanism design

Relay servomechanisms were first designed in the 1940s [17, 71] and are still being widely used now in aerospace applications. Among their advantages over linear servomechanisms are simplicity, low cost, reliability, lower weight, and, as a rule, better dynamical characteristics [56]. The basic design of a pneumatic servomechanism is given schematically in Fig. 6.1.

The main components of a servomechanism are a source of the pressurized air, an air valve, a solenoid, an air cylinder, a piston position sensor, a comparison device, a compensating filter, and a relay (class D) amplifier. The servomechanism operates as follows. Pressurized air enters into either the left or the right chambers of the air cylinder, depending on the air valve position. Because of the pressure differential in the cylinder chambers, the piston moves in the appropriate direction. The position sensor measures the position of the cylinder and sends a signal proportional to the piston displacement from the neutral position to the comparison device, which provides an error signal equal to the difference between the reference input U inp and the sensor signal. If the error is positive, the relay energizes the solenoid, which moves the air valve (to the left, as in Fig. 6.1); if the error is negative, the solenoid is deenergized and the spring moves the air valve to the right. There are some other designs of the relay pneumatic servomechanism: with two solenoids, or the solenoid with two different windings, different types of air valves, etc.


Nonlinear Plant Comparison Device Compensator Design Equivalent Gain Magnitude Frequency Response 
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© Birkhäuser Boston, a part of Springer Science+Business Media, LLC 2009

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