With two-point control (also known as discrete or discontinuous control), there are two possible states, namely “on” and “off”.
The reference variable is switched on or off when the setpoint is reached by the actual value. A typical example of such a control is a conventional heating thermostat. The heating is switched off if the actual value exceeds the setpoint.
However, the actual value may fluctuate within the hysteresis range (see below) without the controller constantly switching over. Two-point control is used when precise compliance with the setpoint is not required.
The hysteresis represents the range around the setpoint value by which the actual value may fluctuate, so that constant switching on and off by the control is prevented.
If, for example, a heating thermostat is set to 20°C, the room is heated until a temperature of 21°C is reached. The heating is switched off and the room cools down. If the temperature falls below a value of e.g. 19°C, the heating is switched on again until the room reaches a temperature of 21°C again. The temperature value thus fluctuates by a few Kelvin around the set value of 20°C. Without this so-called hysteresis, the heating would be switched on and off unnecessarily often if the measured temperature fluctuates by 20°C.
The same principle applies to various simple systems, such as drainage.
This type of control can only be used if it is not important to achieve the setpoint value exactly.