Modern Control Engineering

Section 4–3 / Pneumatic Systems 111

To atmosphere
Pa

Nozzle back pressure Pb

Air supply Ps

Pc

To pneumatic valve

(a) (b)

To atmosphere

Nozzle back pressure Pb

Air supply Pc Ps

To pneumatic valve

Pneumatic Relays. In practice, in a pneumatic controller, a nozzle–flapper

amplifier acts as the first-stage amplifier and a pneumatic relay as the second-

stage amplifier. The pneumatic relay is capable of handling a large quantity of

airflow.

A schematic diagram of a pneumatic relay is shown in Figure 4–6(a). As the nozzle

back pressure increases, the diaphragm valve moves downward. The opening to

the atmosphere decreases and the opening to the pneumatic valve increases, thereby

increasing the control pressure When the diaphragm valve closes the opening to

the atmosphere, the control pressure becomes equal to the supply pressure

When the nozzle back pressure decreases and the diaphragm valve moves upward

and shuts off the air supply, the control pressure drops to the ambient pressure

The control pressure can thus be made to vary from 0 psig to full supply pressure,

usually 20 psig.

The total movement of the diaphragm valve is very small. In all positions of the

valve, except at the position to shut off the air supply, air continues to bleed into the at-

mosphere, even after the equilibrium condition is attained between the nozzle back

pressure and the control pressure. Thus the relay shown in Figure 4–6(a) is called a

bleed-type relay.

There is another type of relay, the nonbleed type. In this one the air bleed stops

when the equilibrium condition is obtained and, therefore, there is no loss of pres-

surized air at steady-state operation. Note, however, that the nonbleed-type relay

must have an atmospheric relief to release the control pressure from the pneu-

matic actuating valve. A schematic diagram of a nonbleed-type relay is shown in Fig-

ure 4–6(b).

In either type of relay, the air supply is controlled by a valve, which is in turn

controlled by the nozzle back pressure. Thus, the nozzle back pressure is converted into

the control pressure with power amplification.

Since the control pressure changes almost instantaneously with changes in the

nozzle back pressure the time constant of the pneumatic relay is negligible

compared with the other larger time constants of the pneumatic controller and

the plant.

Pb ,

Pc

Pc Pa.

Pb

Pc Ps.

Pc.

Pb

Figure 4–6
(a) Schematic diagram of a bleed-type relay; (b) schematic diagram of a nonbleed-type relay.