Page 53 - Instrumentation and Measurement

FIGURE 4.4 On-off controller with a dead band.

On-off control is not too bad at maintaining a constant value of the variable when the capacitance of
the system is very large, e.g. a central heating system heating a large air volume, and so the effect of
changes in, say, a heater output results in slow changes in the variable. It also involves simple devices
and so is fairly cheap. On-off control can be implemented by mechanical switches such as bimetallic
strips or relays, with more rapid switching being achieved with electronic circuits, e.g. thyristors or
transistors used to control the speed of a motor. On-off control is simple and inexpensive and is often
used where cycling can be reduced to an acceptable level. The normal domestic central heating
system has an on-off controller, though a modern one is more likely to be an electronic on-off sensor
rather than a bimetallic strip. Another example of on-off control is the control of a car radiator cooling
fan.

4.4 Proportional Control

With the on-off method of control, the controller output is either an on or an off signal and so the
output is not related to the size of the error. With proportional control the size of the controller output
is proportional to the size of the error (Figure 4.5), i.e. the controller input. Thus, we have: controller
output α controller input. We can write this as

controller output = Kp x controller input

FIGURE 4.5 Proportional control.

Page | 53