Unit – 2

Control System

CONTROLLERS

• Controller is a device introduced in the system to modify the error and produce a control signal. The manner in which controller produces control signal are called control action.
• Two position or ON-OFF controllers
• Proportional Control controllers (P)
• Integral Controllers (I)
• Proportional plus integral controllers (PI)
• Proportional plus derivative controllers (PD)
• Proportional plus integral plus derivative controllers (PID)

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COMPONENTS OF AUTOMATIC CONTROL SYSTEM

• Error Detector
• Amplifier and Controller
• Actuator
• Plant and sensor/ Feed back system

ON-OFF/ Two position Controller

• It has only two fixed position, they either on or off. They use electromagnetic relay to perform this operation.
• Its simple and less expensive.

Steady-state error is defined as the difference between the input (command) and the output of a system in the limit as time goes to infinity 

PROPORTIONAL CONTROLLER

• The proportional controller is a device that produces a control signal, u(t) proportional to the error signal, e(t)
• Proportional controller amplifies the error signal by an amount Kp.
• Improve in gain improves the steady state tracking accuracy, disturbance signal rejection and relative stability and makes system less sensitive to parameter variations.
• Drawback is it leads to constant stead state error.

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INTEGRAL CONTROLLER

• The integral controller is a device that produces a control signal, u(t) which is proportional to integral of the input error signal, e(t).
• Integral signal removes or reduces the steady state error without the need for the manual reset.
• It may lead to oscillatory response or decreasing amplitude which is undesirable and system may become unstable

PROPORTIONAL PLUS INTEGRAL CONTROLLER (PI)

• The proportional plus integral controller (PI controller) produces an output signal consisting of two terms one proportional and the other proportional to the integral of error signal.
• Advantage of PI controller is increases loop gain and makes system less sensitive to variation of system parameter. This also eliminates steady state error.

PROPORTIONAL PLUS DERIVATIVE CONTROLLER (PD)

• The PD controller produces an output signal consisting of two terms one proportional to error signal and other proportional to derivative of error signal.
• The derivative control acts on rate of change of error and not on error signal, derivative control action is effective only during transient periods and so it does not produce corrective measures for any constant error. So derivative is not used separately.
• PI controllers initiates an early corrective action and tends to increases stability of the system.
• Disadvantage is it amplifies noise signals and may cause a saturation effect in the actuators

PROPORTIONAL PLUS INTEGRAL PLUS DERIVATIVE CONTROLLER (PID CONTROLLER)

• PID Controller produces an output signal consisting of three terms: one proportional to error signal, another one proportional to integral of error signal and third one proportional to derivative of error signal.
• P controller stabilizes the gain but produces a steady state error, I controller reduces steady state error, D controller reduces the rate of error.