Control System

What are feedback and feedforward control?

Feedback

What is an industrial process?

The major objective of an industrial plant is to create high-quality products. These products would be sold at a good price too, in order to sell these products at a good price it should have good quality. Several raw materials would be utilized to create a product and this operation is called a process. If the process is carried out in a good manner, then the quality of the end products would be really good. In order to control the industrial process, we would require a good control system.

What is a control system?

A Control system would be capable to control the industrial process, it can maintain the required condition in a physical system by adjusting several variables in a system. We can define the system as the combination of several devices and all these devices would act in an equal manner to achieve the requirements. In a control system, there would be a lot of components interconnected as a system and it would provide the required response.

What is the importance of the control system In industrial process?

•          Control system is an integral part of many manufacturing industries

•          Most of the industrial process is carried out by utilizing the automatic control system

•          We can give several commands to the industrial process via the control system

•          It would control the variations in parameters

•          Disturbance rejection

What Is a feedback control?

FEEDBACK

This method would utilize the values of the controlled variable to vary the values of the manipulated variable. The feedback loop would measure the process variable and this measurement would be sent to a controller to compare it with the set point. The system would take the necessary action to make the process variable equal to the set point or the required value.

The below image is an example of the feedback control, transmitter would measure the fluid temperature, and based on that it would open the hot steam valve to maintain the fluid temperature. This type of control would correct the problem after it has occurred, we can use feedback control to do the manual and automatic control. In this type of control, it would sense, compare, compute, and correct the process variable.

FEEDBACK
FEEDBACK CONTROL

What are the advantages of feedback control?

  • It won’t sense the changes in parameters
  • It won’t be able to determine the modeling errors
  • It doesn’t need to determine or require the measured value of the disturbance

What are the disadvantages of the feedback control?

  • It would only take action after the system is affected by the disturbance
  • It is not effective in case of a slow process
  • In the case of the closed loop response it can create instability

What is Feedforward control?

FEEDFORWARD

In feedback control, it would utilize the output of a process to make changes in the input of the system. This type of control would utilize the direct measurement of the disturbance variables to vary the values of the manipulated variable. This type of control is called predictive control in this control it would determine the load disturbance and this disturbance would be controlled before it affects the process.

We can describe a feedforward control as a control that takes place before it is affected by the disturbance. This type of control would anticipate the disturbance in the process variable and would take proper actions before it affects the process variable. The image which is shown below is an example of the feedforward control. In this, the flow transmitter would open or closes the hot steam valve according to the amount of cold fluid that passes through the flow sensor.

FEEDFORWARD CONTROL

What are the advantages of feedforward control?

  • It would act before the system is affected by the disturbance
  • It would be stable in case of the closed loop response
  • It would be effective for the slow system

What are the disadvantages the feedforward control?

  • It would require the measurement of disturbance for its operation
  • It won’t be able to handle the unmeasured disturbances
  • It would easily determine the variations of process parameters very easily
  • Proper knowledge of the process control is required

Major difference between Feedback and feedforward control?

Feedback vs Feedforward control

Feedback control systemFeedforward control system
It would only act in case the process is affected by the disturbanceActs before the disturbance affects the process
AccurateLess accurate because if there are any changes in the system
Response would be slowQuick response
Closed loop control systemOpen loop control system
Air conditioner, Electric IronExamples – Washing machines, Traffic lights
Complex systemNot as complex as feedback system
Costly due to its complexityEconomical
It can handle severe disturbancesIt won’t be able o handle the severe disturbance
It can handle variations in its environment, or in case of aging in certain componentsIt won’t be able to handle the variations in the process environment or any device.
It won’t be able to do any predictive controlPredictive control can be done
It cannot be used with the process that has large time delaysIt can be used with the process that has large time delays
Maintenance would be highLess maintenance
It won’t be easy to constructEasy to construct

What is feedforward plus feedback control?

In order to avoid the difficulty of anticipating the load disturbance in a feedforward system, feed back system can be used with it. So in this type of control system the combined input from the feedforward and feedback system would be given to the controller.

The below image is an example of feedforward plus feedback control. In this, the flow transmitter and the temperature transmitter would provide the details to control the hot steam valve.

FEEDFORWARD PLUS FEEDBACK
FEEDFORWARD PLUS FEEDBACK CONTROL

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