Open-loop controller: Difference between revisions

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

{{Other uses ofredirect|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 "set[[Setpoint point"(control system)|setpoint]].

There are many open-[[Control loop|loop]] controls, such as on/off switching of valves, machinery, lights, motors or heaters, where the control result is known to be approximately sufficient under normal conditions without the need for feedback. The advantage of using open-loop control in these cases is the reduction in component count and complexity. However, an open-loop system cannot correct any errors that it makes or correct for outside disturbances, andunlike cannot engage ina [[machineclosed-loop learningcontrol system]].

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

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

[[File:Electromechanicaltimer.JPG|thumb|right|An electromechanical timer, normally used for open-loop control based purely on a timing sequence, with no feedback from the process.]]

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

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, but the controlled variable should be the building temperature, but is not as this is open-loop control of the boiler, which does not give closed-loop control of the temperature.

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"/>

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}}</ref>

== Applications ==

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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.

==FeedbackCombination with feedback control==

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.

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.

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* [[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)