Control System

What is a feedback control system and what are its types?

What is a feedback control?

The feedback is an electric signal which is transferred from the output to the controller, so the controller would be able to calculate how the output is different from the required value. The controller would know the past state of the system output with the help of this feedback signal. The controller would calculate the error and it would vary the system input to get the required output. The closed-loop system is also called a feedback system because it fed back the output into the input so that the errors can be cleared and the required output quality can be maintained. The feedback control is to remove the measured disturbances. The feedback controls are automated and it should do the sensing, calculating, and manipulating to be performed by the equipment, and that each element must communicate with the other elements in the control system.

What are the characteristics of feedback?

  • Good accuracy
  • It has a slight tendency towards oscillation (instability)
  • The non-linear effects are reduced
  • The effects of external disturbances or noise can be reduced
  • The effects of external disturbance and noise are reduced
  • Product quality can be increased
  • The response to the variation in the input is high

How does a feedback control work?

In a control system, a feedback loop is an important tool, the feedback loop will consider the system output and this will help the system to alter its operation in order to get the required output. In order to do this task, a system should have controllers, compensators, and feedback structures to the system. The feedback control system is composed of sensors, controllers, process systems, etc. The major components of a feedback control system are input, the process which is being controlled, output, sensing elements, controllers, and actuating devices. The feedback control would only need minimal knowledge of the process that is to be controlled.

The feedback control can be considered as the easy way to automate the control of a process. The sensors in the control system would measure the actual value of the controlled variable. So after measuring the value it will be transmitted to the feedback controller. The controller would compare the measured value to the desired value and the difference between these values is considered as error. So if there is an error then the controller would send a proportional output to the control value. The controller would send a signal to the plant and sets the process variable, according to the set point. So in case if there is a disturbance that affects the process variable of the plant then the sensor in the feedback section would detect this and sends a signal to the controller.

What are the objectives of the feedback control?

The major objective of feedback control is to ensure that the output value of the system must be similar to the required value. This is done by tracking the reference trajectories or by maintaining close to their setpoints. The feedback signal would resist the disturbance signal, which affects the output, and also it can improve the performance of the system. The feedback control can also stabilize the unstable plant.

What are the types of feedback system?

Positive feedback system

In this type of feedback system, the set-points and the output values are added together by the controller as the feedback in phase with the input. So in this feedback, it would increase the event that caused it, because of this an event that is influenced by the positive feedback would increase or decrease its output activation until it hits a limiting constraint.

Negative feedback

The negative feedback would take place, if the result of the process would affect the operation of the process in a way to reduce the changes. This type of feedback can create stability. This type of feedback is very stable, accurate, and responsive. In this type of feedback control, the output value will be subtracted from the input value (required value). The value that we get is an error and thus we could determine how far the output is varied from the required value. The major advantage of the negative feedback is that it has good accuracy, the system sensitivity to the disturbance is reduced, smoothing and filtering, and high bandwidth.

What are the advantages of feedback control?

  • Rejection of disturbances
  • The system sensitivity will be very less to the plant uncertainties
  • The steady-state error reduction of the system is increased
  • The transient response can be easily controlled
  • System stability is improved
  • The overall system gain is improved
  • High bandwidth
  • The effect of variation of the system parameters are reduced
  • There is no need to know what disturbance will affect the process
  • The relation between the process and the final control element is not a problem

What are the disadvantages of a feedback system?

  • If the system is not designed properly then there is a possibility for instability
  • The feedback inherently couples different art of a system
  • The whole feedback process with control system could be complex
  • The feedback control won’t take any action till there is a change in the controlled variable
  • The feedback control won’t be able to do the predictive control action in order to compensate for the effects of known or measurable disturbances.
  • In certain cases, the controlled variable can’t be measured online
  • The feedback system can’t handle large time constant or long time delays, in case if there is a large and frequent disturbance then the process would operate continuously at a transient state and it would never achieve the desired state

What is a feed-forward control?

The feed-forward control can anticipate the effect of disturbances that could affect the process, this type of control would sense and compensate these problems before it affects the process. This type of control has the limitation that it can’t completely compensate for the disturbance because of the variation and imperfection of the mathematical model and also because of the imperfections in the control action. So basically a feedforward control would measure the important disturbance variables and they would take proper actions before it affects the process. In order to prevent the disturbance in a system before it happens, this system would require to know the plant behavior or the plant model. So if the feedforward system gets a proper plant model then it can prevent the errors before it affects the process.

What are the advantages of the feedforward control?

  • It would take proper action in case there is any chance of deviation in the controlled variable so it would act before the error affects the process
  • It can act quickly according to the process model

What are the disadvantages of a feed forward control?

  • The measurement of the disturbance variable must be done online and for many applications it is not possible
  • If the feedforward control needs to operate properly a process model is needed and this is to determine how the control variables are affected by the disturbances. So the performance of the feedforward control is depended on the process model accuracy

What is the difference between feedback and feedforward control?

  • The major objective of the feedback system is to reduce error when it happens
  • In the feedforward control it would control or reduce the transient response
  • The feedback control can compensate for any disturbances that affect the control variable
  • In the case of feedforward control, it would compensate the disturbance before it affects the control variable
  • The feedback control won’t do anything until there is a deviation in the controller variable, while the feedforward control does take preventive actions.
  • The feedforward control takes predictive control action while the feedback control doesn’t
  • A feedforward control system has a feedback structure, while the feedback control system doesn’t have a feedforward structure
  • The major advantage of the feedback over the feedforward is that the feedback control can detect all the disturbances while the feedforward control can’t do this.
  • The feedforward control can’t be used for adaptable, non-linear, or time varied systems but there is no such problem for the feedback control.
  • The feedback control only requires minimal knowledge of the process that is to be controlled but the feedforward would need maximum details about the process.

Does the feedback and feed-forward can be used in a single process?

Mostly the feedback and the feedforward are used together in a process. So while combining the feedback and the feedforward system we would get the advantages of both. The feedforward components would provide quick response, and the feedback components do the rest of the response accurately, compensating for errors in the model.

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