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.