Adaptive Control
Adaptive control detects the changes in the
characteristics of the process and adjusts the controller parameters
automatically to compensate for the changing conditions of the process and in
turn to optimize the loop response.
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| Block Diagram of an Adaptive Control System |
An adaptive control system consists of a normal
feedback loop and a parameter adjustment loop. The normal loop includes the
controller and the process. The parameter adjustment loop consists of the
parameter adjustment mechanism and a controller with adjustable parameters. The
parameter adjustment loop is usually slower than the normal loop. The different
types of adaptive control differ only by the mechanism it used for parameter
adjustment.
The adaptive control differs from all other control
schemes. The adaptive control adjust the controller parameters whereas in other
strategies a process variable is adjusted.
Need for Adaptive Control
1. The key reason is that most of the processes are nonlinear. The control loops are
generally designed to maintain the controlled variable at its setpoint by
compensating for all disturbance occurring in the process. The controller
performance is optimum only for a particular range in which the process is linearized. Once the process
starts to operate beyond the linearized range, the controller fails to produce
desired performance. It is because of the fact that the parameters of the
controller is not suitable for the current operating conditions.
2. The changes in transfer function of process which
occurs due to parameter variations or variation in the coefficients or wear and
tear of important components.
3. The nature and magnitude of disturbances vary with
time. There may be an occurrence of an unpredictable and unknown disturbance in
the process.
4.There may be a change in nature of inputs to the
process and the properties of raw materials.
In all the above cases, a conventional controller
cannot perform at a satisfactory level. This demands the need for a special
type of controller that adapt in accordance with the uncertainties in the
process and in turn Adaptive Control
Types of Adaptive Control
There are two major approaches to determine the
controller parameters adaptation. They are,
1. Programmed or Gain Scheduled Adaptive Control
2. Self-Adaptive Control
The Programmed Adaptive Control is compared to feedforward compensation because it
adjust the controller parameters based on the measurement of an auxiliary
variable and the knowledge of operating conditions of process. Also, there is
no feedback to check the correctness of adaptation.
The Self-Adaptive Control is comparable to feedback compensation because the
adaptation of controller parameters is based on the measurement of closed loop
performance and aim to optimize it. Model Reference Adaptive Control (MRAC) and
Self Tuning Regulator (STR) are two mechanisms that come under self-adaptive
control.
Applications of Adaptive Control
1. Adaptive control is used in the robotic manipulators
of robotic systems which demands high positioning accuracy.
2. Adaptive control is used for altitude control of
satellites. The observation satellites should be operated at lower altitudes
where the air-drag makes the quick reorientation of satellite is necessary to
increase the observation time.
3. Adaptive control is used in the autopilot of air crafts and steering control of ships.
4. Adaptive control is used in the control of strip
temperature for the continuous annealing and processing in metallurgical
processes.
5. Adaptive control is used in distillation columns to
provide high product quality and a considerable reduction of thermal energy
usage.
6. Adaptive control is used to stabilize PID based pH
control system in chemical industries. Without adaptive control, the process
gain (Kp) increases as the pH value becomes neutral and it leads to change
in total loop gain and finally to instability of loop. The adaptive control
keeps the total loop gain at the desired value (usually 0.5) by lowering the
controller gain (Kc).
NEXT TOPIC: Programmed or Gain Scheduled Adaptive Control
Also read other control schemes
NEXT TOPIC: Programmed or Gain Scheduled Adaptive Control
Also read other control schemes
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