How a Control Loop Works


Electronics Process Control
Basic Theory of Control Loop

Figure one shows a basic block diagram of how a control loop works. The Set Point is determined by the operator, or user, of the circuit. The operator selects the process conditions by varying the Set Point. The Error Amplifier is an electronic circuit that determines the difference between the Set Point and the Process Variable. The Process Variable is a direct result of the Process Conditions as interpreted by the Process Sensor. The Output Acutator translates the voltage output of the Error Amplifier into a mechanical equivalent. The Output Acutator can change the conditions of the Process.

The Error Amplifier constantly compares the Set Point, which is the desired conditions of the Process, with the Process Variable, which reflects the actual conditions of the Process . If they are the same, the output of the Error Amplifier remains the unchanged and the Actuator will not change the conditons of the Process . If they are not the same, the output of the Error Amplifier will change to reflect the amount of change that the Output Actuator must provide to provide a correction to bring the process back to the Set Point conditions.

Example of a Control Loop


An example will help explain how the control loop works. Figure two shows the control loop as applied to the cruise control of a car. The driver sets the desired speed of the car by pushing the Set Speed button when the car is moving at the desired speed. This stores the desired speed into the Cruise Control Electronic circuit. This circuit compares the desired speed with the actual speed as interpreted by the Speed Sensor . The Speed Sensor translates the speed of the car, as determined by the rotation of the Driveshaft , into a voltage. The output of the Cruise Control Electronic circuit is fed into the Output Actuator which is a vacuum operated device that translates voltage from the Cruise Control Electronic circuit into an proportional activation of the car's Accelerator .

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This page, and all its contents, are Copyright (C) 1997 by David Charbonneau, Kamoops, B.C.