Explain the basics of Open loop and Closed loop control system


Open Loop Control System.

Key point for open loop system is there is no feedback.

That means controllers output doesn’t affect its subsequent input.

Open loop control system can take input from process as long as controller output doesn’t affect its subsequent input.

Basically the input is used for compensating process load and disturbances.

Example - Hot water temperature control by sensing temperature of supply water.

Here the hot water is produced by mixing steam in to a stream of cold water. Supply of steam is controlled by the controller according the temperature of incoming cool water supply.

Here the the process variable is hot water temperature. Load is cool water temperature. Compensator is control valve opening.

Limitations -

Not sensitive to disturbances in compensator’s side. Eg Steam supply
Not sensitive to unmeasured disturbance. Eg Cool Water flow
Set point and measured value cannot generally represent desired result.

Closed Loop

Closed loop control system uses feedback. It means controller’s output affect future controller’s input.

Here it takes temperature of hot water as controller input and controller calculate the output according to the hot water temperature.