Closed-Loop Control Systems are also known as Feedback Control Systems. The disadvantages of the open-loop control system are corrected through the closed-loop control system. The input transducer converts the form of the input to the form used by the controller. An output transducer, or sensor, measures the output response and converts it into the form used by the controller. For example, if the controller uses electrical signals to operate the valves of a temperature control system, the input position and the output temperature are converted to electrical signals.

The input position can be converted to a voltage by a potentiometer, and a variable resistor, and the output temperature can be converted to a voltage by a thermistor. A device whose electrical resistance changes with temperature. The first summing junction algebraically adds the signal from the input to the signal from the output, which arrives via the feedback path, the return path from the output to the summing junction. The output signal is subtracted from the input signal. The result is generally called the actuating signal.

Figure: Closed-Loop Control Systems (Feedback Control Systems)

The closed-loop system compensates for disturbances by measuring the output response, feeding that measurement back through a feedback path, and comparing that response to the input at the summing junction. If there is any difference between the two responses, the system drives the plant, via the actuating signal, to make a correction. If there is no difference, the system does not drive the plant, since the plant’s response is already the desired response. 

Closed-loop systems, then, have the obvious advantage of greater accuracy than open-loop systems. They are less sensitive to noise, disturbances, and changes in the environment. Closed-loop systems are more complex and expensive than open-loop systems.

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