Control System


Control systems

Open-Loop Control Systems.

Those systems in which the output has no effect on the control action are called open-loop control systems. In other words, in an open loop control system the output is neither measured nor fed back for comparison with the input. One practical example is a semi automatic washing machine. The machine does not measure the output signal, that is, the cleanliness of the clothes.

Open loop system are highly sensitive and stable if no disturbances are given

Closed-Loop Control Systems

Feedback control systems are often referred to as closed-loop control systems.. In a closed-loop control system the actuating error signal, which is the difference between the input signal and the feedback signal, is fed to the controller so as to reduce the error and bring the output of the system to a desired value. The term closed-loop control always implies the use of feedback control action in order to reduce system error. This system are more reliable but less stable.

The major advantages of open-loop control systems are as follows:

1. Simple construction and ease of maintenance.
2. Less expensive than a corresponding closed-loop system.
3. There is no stability problem.

The major disadvantages of open-loop control systems are as follows:

1. Disturbances and changes in calibration cause errors, and the output may be different from what is desired.
2. To maintain the required quality in the output, recalibration is necessary from time to time.

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