Open-loop controller: Difference between revisions

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{{Short description|Control system whose input is independent of output}}

{{redirect|Open loop}}

{{more footnotes|date=January 2015}}

In [[control theory]], an '''open-loop controller''', also called a '''non-feedback controller''', is a [[control loop]] part of a [[control system]] in which the control action from("input" to the controllersystem<ref>{{Cite journal |last1=Escudier |first1=Marcel |last2=Atkins |first2=Tony |date=2019 |title=A Dictionary of Mechanical Engineering |url=http://dx.doi.org/10.1093/acref/9780198832102.001.0001 |doi=10.1093/acref/9780198832102.001.0001|isbn=978-0-19-883210-2 |url-access=subscription }}</ref>) is independent of the "process output", which is the [[process variable]] that is being controlled.<ref name="auto">"Feedback and control systems" - JJ Di Steffano, AR Stubberud, IJ Williams. Schaums outline series, McGraw-Hill 1967</ref> It does not use [[feedback]] to determine if its output has achieved the desired goal of the input command or process [[Setpoint (control system)|setpoint]].

ItThere doesare not usemany open-[[feedbackControl loop|loop]] tocontrols, determinesuch ifas itson/off outputswitching hasof achievedvalves, themachinery, desiredlights, goalmotors ofor heaters, where the inputcontrol orresult processis "setknown point"to be approximately sufficient under normal conditions without the need for feedback. AnThe advantage of using open-loop systemcontrol cannotin engagethese cases is the reduction in [[machinecomponent learning]]count and alsocomplexity. However, an open-loop system cannot correct any errors that it couldmakes make.or It will not compensatecorrect for outside disturbances inunlike thea process[[closed-loop beingcontrol controlledsystem]].

== Open-loop and closed-loop (feedback) control==

{{excerpt|Control loop#Open-loop and closed-loop}}

[[File:Remote manipulators 003.jpg|thumb|upright|[[Remote manipulator]] arms for working with radioactive materials – an open-loop mechanism, controlled by hand controls.]]

== Applications ==

Fundamentally, there are two types of control loop: open loop control, and closed loop (feedback) control.

[[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 aan conventionalolder model domestic [[washingclothes machinedryer]], for which the length of machine wash time is entirely dependent on the judgment and estimationjudgement of the human operator. Generally, towith obtainno aautomatic more accurate or more adaptive control, it is necessary to feed the outputfeedback of the system back to the inputsdryness of the controller. This type of system is called a [[Closed-loop transfer function|closed-loop system]]clothes.

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.

In open loop control, the control action from the controller is independent of the "process output" (or "controlled process variable"). A good example of this is a central heating boiler controlled only by a timer, so that heat is applied for a constant time, regardless of the temperature of the building. The control action is the switching on/off of the boiler. The process output is the building temperature.

Another example is a [[Stepperstepper motor]]s are often used for open-loop control of position. ASending stepper motor rotates to one of a number of fixed positions, according to its internal construction. Sendingit 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-loop thecontrol. machine'sHowever, homeif there is a position) encoder, foror sensors to indicate the controlstart ofor simplefinish roboticpositions, machinesthen orthat evenis theclosed-loop commonplacecontrol, 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.

In closed loop control, the control action from the controller is dependent on the process output. In the case of the boiler analogy this would include a thermostat to monitor the building temperature, and thereby feed back a signal to ensure the controller maintains the building at the temperature set on the thermostat. A closed loop controller therefore has a feedback loop which ensures the controller exerts a control action to give a process output the same as the "reference input" or "set point". For this reason, closed loop controllers are also called feedback controllers.<ref name="auto"/>

OpenHowever, open-loop control is very useful and economic for well-defined systems where the relationship between input and the resultant state can be reliably 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.

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."<ref>{{cite book|title= The Origins of Feedback Control|last=Mayr|first= Otto| author-link= Otto Mayr| year= 1970

|publisher =The Colonial Press, Inc.|___location= Clinton, MA USA|isbn= |pages=}}</ref>

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>

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.

The advanced type of automation that revolutionized manufacturing, aircraft, communications and other industries, is feedback control, which is usually ''continuous'' and involves taking measurements using a [[sensor]] and making calculated adjustments to keep the measured variable within a set range.<ref>Bennett, Stuart (1992). A history of control engineering, 1930-1955. IET. p. p. 48. {{ISBN|978-0-86341-299-8}}.</ref> The theoretical basis of closed loop automation is the discipline of [[control theory]].

Thus there are many 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.

== Examples ==

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 conventional [[washing machine]], for which the length of machine wash time is entirely dependent on the judgment and estimation of the human operator. 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]].

==Combination with feedback control==

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.

A feed back control system, such as a [[PID controller]], can be improved by combining the [[feedback]] (or [[closed-loop control]]) of a PID controller with [[feed forward (control)|feed-forward]] (or open-loop) control. Knowledge about the system (such as the desired acceleration and inertia) can be fed forward and combined with the PID output to improve the overall system performance. The feed-forward value alone can often provide the major portion of the controller output. The PID controller primarily has to compensate whatever difference or ''error'' remains between the setpoint (SP) and the system response to the open-loop control. Since the feed-forward output is not affected by the process feedback, it can never cause the control system to oscillate, thus improving the system response without affecting stability. Feed forward can be based on the setpoint and on extra measured disturbances. Setpoint weighting is a simple form of feed forward.

[[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. Such motors are often used, together with a simple initial datum sensor (a switch that is activated at the machine's home position), for the control of simple robotic machines or even the commonplace [[inkjet printer]] head. 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.

For example, in most motion control systems, in order to accelerate a mechanical load under control, more force is required from the actuator. If a velocity loop PID controller is being used to control the speed of the load and command the force being applied by the actuator, then it is beneficial to take the desired instantaneous acceleration, scale that value appropriately and add it to the output of the PID velocity loop controller. This means that whenever the load is being accelerated or decelerated, a proportional amount of force is commanded from the actuator regardless of the feedback value. The PID loop in this situation uses the feedback information to change the combined output to reduce the remaining difference between the process setpoint and the feedback value. Working together, the combined open-loop feed-forward controller and closed-loop PID controller can provide a more responsive control system in some situations.

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 of open-loop control. If the load were not predictable, on the other hand, the motor's speed might vary as a function of the load as well as of 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.

==See also==

* [[Cataract (beam engine)|Cataract]], the open-loop speed controller of early [[beam engine]]s

* [[Control theory]]

* [[Controller (control theory)]]

* [[Feed forward (control)|Feed-forward]]

* [[PID controller]]

* [[Process control]]

* [[Open-loop transfer function]]

==References==

{{Reflist}}

==Further reading==

* Kuo, Benjamin C. (1991). ''Automatic Control Systems'' (6th ed.). New Jersey: Prentice Hall. {{ISBN|0-13-051046-7}}.

* Ziny Flikop (2004). "Bounded-Input Bounded-Predefined-Control Bounded-Output" (http://arXiv.org/pdf/cs/0411015)

* Basso, Christophe (2012). "Designing Control Loops for Linear and Switching Power Supplies: A Tutorial Guide". Artech House, {{ISBN|978-1608075577}}

{{Electric machines}}

[[Category:Control theory]]

{{Authority control}}

* [[ControllerCategory:Classical (control theory)]]