Why do closed-loop control systems use a local power source rather than power from the input signal?

Closed-loop control systems utilize a local power source rather than drawing power from the input signal for two primary reasons:

• Power Availability: A local power source provides a large amount of energy, which is necessary to effect the required variations in the controlled quantity.1
• Signal Integrity: By using a separate power source, the system ensures that the operations required to adjust the controlled quantity do not load or distort the input signals upon which the accuracy of the control depends.1

In these systems, a motor or similar actuating device draws power from this local source, while the rate of energy delivery is determined by a function of the difference between the actual and desired values of the controlled quantity. This design is a fundamental characteristic of closed-loop control systems, also known as servomechanisms.1