To establish a basis for discussion agreement must be reached on the meaning of the terms used. A glossary of terms is provided by BS 1523: Part 1: 1967 which augments those given below.

1. Controlled variable. The quantity or physical property measured and controlled, e. g. room air temperature.

2. Desired value. The value of the controlled variable which it is desired the control system should maintain.

3. Set point. The value of the controlled variable set on the scale of the controller, e. g. 21°C set on a thermostat scale.

4. Control point. The value of the controlled variable that the controller is trying to maintain. This is a function of the mode of control, e. g. with proportional control and a set point of 21° ± 2°C, the control point will be 23°C at full cooling load, 21°C at 50 per cent load and 19°C at zero load.

5. Deviation. The difference between the set point and the measured value of the controlled variable at any instant, e. g. with a set point of 21°C and an instantaneous measured value of 22°C the deviation is +1°C.

6. Offset. A sustained deviation caused by an inherent characteristic of the control system, e. g. with a set point of 21° ± 2°C the offset is + 2°C at a continuous full cooling load, 23°C being maintained.

7. Primary element (also termed a sensor). That part of a controller which responds to the


Fig. 13.2 Two-position control.

Value of the controlled variable in order to give a measurement, e. g. a bimetallic strip in a thermostat.

8. Final control element. The mechanism altering the plant capacity in response to a signal initiated at the primary element, e. g. a motorised valve.

9. Automatic controller. A device which compares a signal from the primary element with the set point and initiates corrective action to counter the deviation, e. g. a room thermostat.

10. Differential (also Differential gap). This refers to two-position control and is the smallest range of values through which the controlled variable must pass for the final control element to move between its two possible extreme positions, e. g. if a two-position controller has a set point of 21° ± 2°C the differential gap is 4°C. See Figure 13.2.

11. Proportional band. Also known as throttling range, this refers to proportional control and is the range of values of the controlled variable corresponding to the movement of the final control element between its extreme positions, e. g. a proportional controller with a set point of 21° ± 2°C has a proportional band of 4°C. See Figure 13.3.

12. Cycling. Also known as hunting this is a persistent, self-induced, periodic change in the value of the controlled variable.

13. Open loop system. A control system lacking feedback.

14. Closed loop system. A control system with feedback so that the deviation is used to


Fully 50% open 100% open

Closed valve position

Fig. 13.3 Proportional control for a cooling application.

Control the action of the final control element in such a way as to tend to reduce the deviation.

15. Deadtime. The time between a signal change and the initiation of perceptible response to the change.

Posted in Engineering Fifth Edition