A two position controller is a device that has two operating conditions: completely on or completely off.

Figure 12 shows the input to output, characteristic waveform for a two position controller that switches from its "OFF" state to its "ON" state when the measured variable increases above the setpoint. Conversely, it switches from its "ON" state to its "OFF" state when the measured variable decreases below the setpoint. This device provides an output determined by whether the error signal is above or below the setpoint. The magnitude of the error signal is above or below the setpoint. The magnitude of the error signal past that point is of no concern to the controller.

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Figure 12 Two Position Controller Input/Output Relationship

Example of Two Position Control

A system using a two position controller is shown in Figure 13.

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Figure 13 Two Position Control System

The controlled process is the volume of water in the tank. The controlled variable is the level in the tank. It is measured by a level detector that sends information to the controller. The output of the controller is sent to the final control element, which is a solenoid valve, that controls the flow of water into the tank.

As the water level decreases initially, a point is reached where the measured variable drops below the setpoint. This creates a positive error signal. The controller opens the final control element fully. Water is subsequently injected into the tank, and the water level rises. As soon as the water level rises above the setpoint, a negative error signal is developed. The negative error signal causes the controller to shut the final control element. This opening and closing of the final control element results in a cycling characteristic of the measured variable.