Control System Instrumentation

What is an adaptive control?

Adaptive control can be described as an auto-correcting form of control, it is composed of both feedback and optimal control. An adaptive control system would measure the output process variable and this will be used to control the speed. So basically an adaptive control system can adjust its parameters automatically in a way to compensate for the variations in the characteristics of the process it controls. It would take certain feedback from the process variables and also it would act like an optimal system where the overall measurement of the performance is used. The adaptive control system is specially designed to operate in the changing environment. So it can check the performance of the process and also can make changes to get the proper result.

What is the need for adaptive control?

This type of control system can be used to vary a particular condition and it can also make a process compatible with a particular environment. So we can adjust the process according to the environmental conditions. The adaptive controller is a fixed controller which has adjustable parameters and it has the ability to automatically adjust these parameters. The adaptive control system uses a microprocessor as the controller, so this type of control system can vary or modify its parameters according to the change in its circumstances. The adaptive controller is a type of controller which would use the information it gathers online to change itself and also to improve their performance.

What are the functions of adaptive control?

  • It can identify the current performance by taking measurements of the input and output
  • So according to the measurements, a decision procedure will be carried out to know the required changes for the improvement of systems performance.
  • So according to the above-mentioned factors, the system will be modified

Why use adaptive control and where should it be used?

  • It can be used in systems with time-varying dynamics
  • It would give proper output
  • The adaptive control system can be used in the chemical process because most of the chemical process is non-linear
  • So it can be used in systems where the desired steady-state operation of a process changes, then the best values of the controller parameter changes
  • Most of the chemical process is non-stationary
  • Linear controllers
  • The control system would perform better
  • It can be used in a system where there is a change in plant transfer function because of the changes in the environment, properties of raw materials, and also due to damages of certain components
  • It can be used in a system where the chances of stochastic disturbances are high
  • These controllers will be useful if there is a change in the nature of the input
  • Appreciable dead-time
  • It can be used when there are unknown parameters, while the control system is commissioned for a new process
  • It can be used when there is a propagation of disturbances along the process

How does an adaptive controller work?

The adaptive controller is a type of controller which will modify its behavior according to the variation in the process dynamics and also according to the disturbance that acts on the process. An adaptive controller has a control loop that can adjust the parameter value over a longer time scale. This type of controller has an adjustable mechanism so that the parameters can be varied or adjusted. The adjustment of the parameters will be done to compensate for the changes in dynamics of the plant and also for the disturbances which act on the plant.  This has two loops first one is composed of feedback from the process and controller and the next one is the parameter adjustment one. The parameter adjustment loop is not quick as the normal feedback loop.

Adaptive control does its operation in three steps and they are

  • Identification function
  • Decision function
  • Modification function

Identification function

In this the controller would identify the current performance of the system, this function will determine the current value by using the feedback data from the process. The identification must proceed over time and less continuously because the environment will change over time and due to this the system performance will vary according to this. In order to identify the system, several possible measurements should be done.

Decision function

So after the determination of the system performance, the next step is to decide how to modify the control mechanism to improve the performance of the process. This procedure is done according to the preprogrammed logic which is given by the designer. So according to the logic, the decision will be taken to change one or more of the controlled processes.

Modification function

This function is to implement the decision which is a logic function, so the implementation of the design will involve the physical or mechanical changes in the system. So this function can be considered as a hardware function and not as a software function. So the modification is done by changing the system parameters or variables and thus the process can be modified towards an optimal state. So the adaptive controller would determine the current performance by measuring the input and output and according to this performance, certain procedures are done to improve the performance of the system. The modification function will do the actual changes in the system. 

What is the indirect and direct adaptive control?

Indirect adaptive control

The indirect adaptive control can be used to control the dynamic plants with unknown parameters. So in this type, it would determine the plant parameters dynamically with the help of the input and output. The control parameters are designed in a way to improve the system performance. So the indirect adaptive control will vary the control parameters continuously based on the difference between the plant output and the reference model output. The indirect adaptive controllers would assume the plant parameters and then it would compute the plant parameters. So this type of control is completely dependent upon the estimated control parameters. The indirect adaptive control can be used in a system where the disturbance cannot be measured. This type of adaptive control is mostly used in chemical reactors, distillation columns, and other mass transfer operations.

Direct adaptive control

In this type, there is no prediction of the plant parameter and it would predict only the controller parameters. In this type of adaptive control, the predicted parameters are directly used in the adaptive control. This type of control is also called implicit adaptive control.

What are the advantages of adaptive control?

  • Adaptation to parametric uncertainties is possible
  • Able to handle structural and environmental uncertainties
  • Stability and tracking of the process is possible
  • Self-tuning of the controller parameters
  • Real-time control is possible
  • This type of control is really effective for faults and also for non-smooth nonlinearities
  • This type of controller can also handle challenging issues
  • Production rate can be increased
  • Tool life can be extended
  • Operator intervention will be less

What are the applications of adaptive control?

  • Power plant
  • Cement industry
  • Process industry
  • Flight control
  • Paper industry
  • Chemical industry
  • Electrical drives
  • Steel and metallurgical drives

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