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Likewise, a "'Feedback Control System' is a system which tends to maintain a prescribed relationship of one system variable to another by comparing functions of these variables and using the difference as a means of control."<ref>{{cite book|title= The Origins of Feedback Control|last=Mayr|first= Otto| author-link= Otto Mayr| year= 1969|publisher =The Colonial Press, Inc.|___location= Clinton, MA USA|isbn= |pages=}}</ref>
== ExamplesApplications ==
[[File:Electric Clothes dryer.jpg|thumb|Electric clothes dryer, which is open loop controlled by running the dryer for a set time, regardless of clothes dryness.]]
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 dyerdryer]], 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. Generally, to obtain a more accurate or more adaptive control, it is necessary to feed the output of the system back to the inputs of the controller. This type of system is called a [[Closed-loop transfer function|closed-loop system]].
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.
[[Stepper motor]]s are often used for open-loop control of position. A stepper motor rotates to one of a number of fixed positions, according to its internal construction. Sending a stream of electrical pulses to it causes it to rotate by exactly that many steps, hence the name. SuchIf motorsthe aremotor oftenwas used,always togetherassumed withto aperform simpleeach initialmovement datumcorrectly, sensorwithout (apositional switchfeedback, thatit iswould activatedbe atopen theloop machine'scontrol. home position)However, forthere theif controlthere ofis simplea roboticposition machinesencoder or evensensors to indicate the commonplace"start" or finish positions , then that is closed-loop control, such as in many [[inkjet printer]] heads. 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 [[servomotor]]s rather than stepper motors, which incorporate [[rotary encoder|encoder]]s and [[closed-loop controller]]s.
OpenHowever, open-loop control is useful for well-defined systems where the relationship between input and the resultant state can be modeled by a mathematical formula. For example, determining the [[voltage]] to be fed to an [[electric motor]] that drives a constant load, in order to achieve a desired [[speed]] would be a good application. ofBut open-loop control. Ifif the load were not predictable, onand thebecame other handexcessive, the motor's speed might vary as a function of the load asnot well as ofjust the voltage, and an open-loop controller would therefore be insufficient to ensure repeatable control of the velocity.
An example of this is a conveyor system that is required to travel at a constant speed. For a constant voltage, the conveyor will move at a different speed depending on the load on the motor (represented here by the weight of objects on the conveyor). In order for the conveyor to run at a constant speed, the voltage of the motor must be adjusted depending on the load. In this case, a closed-loop control system would be necessary.
Thus there are a large number of open loop controls, such as switching valves, lights, motors or heaters on and off, where the result is known to be approximately sufficient without the need for feedback.
==See also==
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