SOLUTION

Open-loop control system

• In any open-loop control system, the output is not compared with the reference input.
• Thus, to each reference input, there corresponds a fixed operating condition, as a result, the accuracy of the system depends on the calibration.
• In open loop system control action is independent of the output.
• In the presence of disturbances, an open-loop control system will not perform the desired task because when the output changes due to disturbances, it is not followed by changes in input to correct the output.
• In open-loop control systems, the changes in output are corrected by changing the input manually.
• Open-loop control systems can be used in practice only if the relationship between the input and the output is known and if there are neither internal nor external disturbances.
• In other words, the output is neither measured nor fed back for comparison with reference point.

For example, let us consider a washing machine in which soaking, washing and rinsing in the washer operates on the time basis. The machine does not measure the output signal, that is, cleanliness of clothes. Such open-loop control systems can be used in practice, if the relationship between input and output is known and there are no external or internal disturbances.

Control action is the quantity responsible for activating the system to produce the output.