<P> The definition of a closed loop control system according to the British Standard Institution is "a control system possessing monitoring feedback, the deviation signal formed as a result of this feedback being used to control the action of a final control element in such a way as to tend to reduce the deviation to zero ." </P> <P> An open - loop controller is often used in simple processes because of its simplicity and low cost, especially in systems where feedback is not critical . A typical example would be a domestic clothes dryer, for which the length of time is entirely dependent on the judgement of the human operator, with no automatic feedback of the dryness of the clothes . </P> <P> For example, an irrigation sprinkler system, programmed to turn on at set times could be an example of an open - loop system if it does not measure soil moisture as a form of feedback . Even if rain is pouring down on the lawn, the sprinkler system would activate on schedule, wasting water . </P> <P> Another example is a Stepper motors used for control of position . Sending it a stream of electrical pulses causes it to rotate by exactly that many steps, hence the name . If the motor was always assumed to perform each movement correctly, without positional feedback, it would be open loop control . However, if there is a position encoder, or sensors to indicate the "start" or finish positions, then that is closed - loop control, such as in many inkjet printers . The drawback of open - loop control of steppers is that if the machine load is too high, or the motor attempts to move too quickly, then steps may be skipped . The controller has no means of detecting this and so the machine continues to run slightly out of adjustment until reset . For this reason, more complex robots and machine tools instead use servomotors rather than stepper motors, which incorporate encoders and closed - loop controllers . </P>

Open loop and closed loop control system wikipedia