Depending upon the process to be controlled and technical and economic considerations, either an open-loop or closed loop design may be preferable. However, a feedback control system is generally considered superior to an open-loop system.
The following advantages are the fundamental reasons for using feedback.
- Many unnecessary disturbances and noise signals from outside the system can be rejected.
- The change in the performance of the system due to parameter variations is reduced.
- The steady-state error of the system can be relatively small.
- The transient behavior of the process can be easily manipulated.
- The feedback is compared with the desired state in order to take corrective measures.
On the other hand, using feedback can have several disadvantages:
- The system is complicated by the increased number of components, such as sensors and error detectors.
- The overall gain of the system is reduced and must be compensated for in the design.
- The system may not be stable (it may oscillate or depart greatly from the desired output), even though the comparable open-loop system is stable.
- The error detector is necessary in order to compare two states.
- If there is a change in an Output, it will affect the system input.
Because the advantages of feedback exceed its disadvantages in most cases, feedback has become the major concept in the design of control system.