Page 54 - Instrumentation and Measurement

where Kp is a constant called the gain. This means the correction element of the control system will
have an input of a signal which is proportional to the size of the correction required. Thus, for
example, if we have a heater being controlled to give a set temperature then as the temperature
approaches the set point the power supplied to the heater will be reduced and so slow down the
heating so that it will not result in the temperature overshooting the set value and end up oscillating
about the set point.
The float method of controlling the level of water in a cistern (Figure 4.6) is an example of the use
of a proportional controller, the control mode being determined by the lever.
The output is proportional to the error, the gain being x/y. The error signal is the input to the ball end
of the lever, the output is the movement of the other end of the lever. Thus, we have output movement
= (x/y) x the error.

FIGURE 4.6 The float-lever proportional controller.
Another example of a proportional mode controller is an amplifier which gives an output which is
proportional to the size of the input. Figure 4.7 illustrates, for the control of temperature of the
outflow of liquid from a tank, the use of a differential amplifier as a comparison element and another
amplifier as supplying the proportional control mode.

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