A closed-loop control system is one in which control action is dependent on the output. If a radar operator, for example, could see the scanner, he could cancel the movement when it has moved the correct amount. This is now a closed loop, the operator completing the loop between output and input. He compares the desired effect and adjusts the control panel to reduce the error between them. He is what we call an error detector. The amount of error which he observes determine how he adjusts the input to produce the desired results. Modern systems incorporate automatic error detector to replace human interaction.
The essential features of a closed loop system are as follows:
- The feedback of information concerning the behaviour of the load.
- The comparison of this information with the behaviour demanded by the input
- The production of an error signal proportional to the difference between the desired
- behaviour and the actual behaviour.
- The amplification of the error signal to control the power into a servomotor.
- The movement of the load by the servomotor in such a direction as to reduce the error
- signal to zero.
- The error detector fulfills the requirements of sub para (b) and (c).
- The position feedback must be in anti-phase with the demand signal.
