Page 416 - Mechanical Engineers' Handbook (Volume 2)

6 Controller Hardware 407

Figure 26 Op-amp implementation of proportional control. 1

algorithm. This name results from the fact that historically it was difficult to implement
noninteractive PID control with mechanical or pneumatic devices.

6.3 Pneumatic Controllers
The nozzle–flapper introduced in Section 4 is a high-gain device that is difficult to use
without modification. The gain K is known only imprecisely and is sensitive to changes
ƒ
induced by temperature and other environmental factors. Also, the linear region over which
Eq. (14) applies is very small. However, the device can be made useful by compensating it
with feedback elements, as was illustrated with the electropneumatic valve positioner shown
in Fig. 19.

6.4 Hydraulic Controllers

The basic unit for synthesis of hydraulic controllers is the hydraulic servomotor. The nozzle–
5
flapper concept is also used in hydraulic controllers. A PI controller is shown in Fig. 32. It
can be modified for proportional action. Derivative action has not seen much use in hydraulic
controllers. This action supplies damping to the system, but hydraulic systems are usually

Figure 27 Op-amp adder circuit. 1 Figure 28 Op-amp implementation of PI con-
trol. 1