Simple two-position control
There are only two values of the manipulated variable: maximum and zero. The sensing element switches on full capacity when the temperature falls to, say, the lower value of the differential and switches the capacity to zero when the upper value of the differential is reached. Figure 13.2 illustrates the way in which the controlled variable alters with respect to time when an air heater battery, say, is switched on and off. Overshoot and undershoot occur because of the total lag. It can be seen that the range of values of the controlled condition exceeds the differential gap and that if the lag is large, poor control will result. This is not to denigrate the method; it is an excellent, simple and cheap form of control for the correct application.
Posted in Engineering Fifth Edition