A Two position controller is a device that has two operating conditions: Completely ON or completely OFF.
From the figure above, we can see the characteristic waveform for a two position controller that switches from its ‘’OFF’’ state to ‘’ON’’ state when the measured variable increases above the setpoint. Conversely, it switches from its ‘’ON’’ state to ‘’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.
Example of Application of Two Position Control
A process using a two position controller is shown in the figure above. The controlled process is the volume of water in the tank. The controlled variable is the level in the tank. The manipulated variable is the flow rate of the water. The level is measured by the level detector (Level transmitter) that sends the information to the controller. The output of the controller is sent to the final control element, which is the solenoid valve that controls the flow of water into the tank.
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As the water level decreases, 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 consequently 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 solenoid valve. This opening and closing of the final control element results in a cycling characteristic of the measured variable.
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