Open loop vs closed Open loop vs closed looploop

By Norbert Benei ZI5A58By Norbert Benei ZI5A58

Open-loop Open-loop

An open-loop controller, also called a An open-loop controller, also called a non-feedback controller, is a type of non-feedback controller, is a type of controller which computes its input controller which computes its input into a system using only the current into a system using only the current state and its model of the system. state and its model of the system.

Open-loopOpen-loop A characteristic of the open-loop A characteristic of the open-loop

controller is that it does not use feedback controller is that it does not use feedback to determine if its output has achieved the to determine if its output has achieved the desired goal of the input. This means that desired goal of the input. This means that the system does not observe the output of the system does not observe the output of the processes that it is controlling. the processes that it is controlling. Consequently, a true open-loop system Consequently, a true open-loop system can not engage in machine learning and can not engage in machine learning and also cannot correct any errors that it also cannot correct any errors that it could make. It also may not compensate could make. It also may not compensate for disturbances in the system. for disturbances in the system.

Closed-loopClosed-loop Close loop control systems use the open Close loop control systems use the open

loop systems (as forward path) and loop systems (as forward path) and feedback from the output to decide the feedback from the output to decide the input level to the open loop system. And input level to the open loop system. And because the input is decided based on because the input is decided based on how much away the output is from the how much away the output is from the desired level, the non idealities in forward desired level, the non idealities in forward path do not degrade the system path do not degrade the system performance. The accuracy of the output performance. The accuracy of the output thus depends on the feedback path, which thus depends on the feedback path, which in general can be made very accurate. in general can be made very accurate.

DifferencesDifferences The terms open-loop control and closed-The terms open-loop control and closed-

loop control are often not clearly loop control are often not clearly distinguished. Therefore, the difference distinguished. Therefore, the difference between open-loop control and closed-between open-loop control and closed-loop control is demonstrated in the loop control is demonstrated in the following example of a room heating following example of a room heating system. In the case of system. In the case of open-loop controlopen-loop control of the room temperature according to of the room temperature according to Figure the outdoor temperature will be Figure the outdoor temperature will be measured by a temperature sensor and measured by a temperature sensor and fed into a control device. fed into a control device.

Open-loopOpen-loop

Closed-loopClosed-loop

In the case of In the case of closed-loop controlclosed-loop control of the of the room temperature as shown in Figure the room temperature as shown in Figure the room temperature is measured and room temperature is measured and compared with the set-point value. If the compared with the set-point value. If the room temperature deviates from the given room temperature deviates from the given set-point value, a controller (C) alters the set-point value, a controller (C) alters the heat flow. All changes of the room heat flow. All changes of the room temperature, e.g. caused by opening the temperature, e.g. caused by opening the window or by solar radiation, are detected window or by solar radiation, are detected by the controller and removed. by the controller and removed.

Closed-loopClosed-loop

The order of events to The order of events to organise a closed-loop control organise a closed-loop control

is characterised by the is characterised by the following steps: following steps:

Measurement of the controlled variable Measurement of the controlled variable Calculation of the control error Calculation of the control error

(comparison of the controlled variable with (comparison of the controlled variable with the set-point value ) the set-point value )

Processing of the control error such that by Processing of the control error such that by changing the manipulated variable the changing the manipulated variable the control error is reduced or removed control error is reduced or removed

Comparing open-Comparing open-loop control with loop control with

closed-loop closed-loop control the control the following following

differences are differences are seen: seen:

Closed-loop controlClosed-loop control

shows a closed-loop action (closed shows a closed-loop action (closed control loop); control loop);

can counteract against disturbances can counteract against disturbances (negative feedback); (negative feedback);

can become unstable, i.e. the can become unstable, i.e. the controlled variable does not fade controlled variable does not fade away, but grows (theoretically) to an away, but grows (theoretically) to an infinite value. infinite value.

Open-loop control Open-loop control

shows an open-loop action shows an open-loop action (controlled chain); (controlled chain);

can only counteract against can only counteract against disturbances, for which it has been disturbances, for which it has been designed; other disturbances cannot designed; other disturbances cannot be removed; be removed;

cannot become unstable - as long as cannot become unstable - as long as the controlled object is stable. the controlled object is stable.

Summarising these properties Summarising these properties we can define:we can define:

Systems in which the output quantity Systems in which the output quantity has no effect upon the process input has no effect upon the process input quantity are called open-loop control quantity are called open-loop control systems. systems.

Systems in which the output has an Systems in which the output has an effect upon the process input effect upon the process input quantity in such a manner as to quantity in such a manner as to maintain the desired output value are maintain the desired output value are called closed-loop control systems. called closed-loop control systems.

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