Basic Control Action

By :-Vishal Thakur100104143B.Tech (EE) Sem VSection B

• A controller compares the actual value of

output with the reference input, determines the deviation, and produces a control signal that will reduce the deviation to zero or to a small value.

• The manner in which the controller produces the control signal is called the control action.

Introduction

Block Diagram of a Basic Industrial Control System

Classifications of Controllers.

• 1. Proportional controllers

• 2. Integral controllers

• 3. Proportional-plus-integral controllers

• 4. Derivative controllers

• 5. Proportional-plus-derivative controllers

• 6. Proportional-plus-integral-plus-derivative controllers

Proportional Control of Systems.

• Actuating signal is proportional to error signal

• Such a system always has a steady-state error in the step response. Such a steady-state error is called an offset.

Change in gain in P controller• Increase in gain:

Upgrade both steady-state and transient responses

Increases oscillations

Reduce steady-state

error

Reduce stability!

Integral Control of Systems.

Integral Controller

• Integral of error with a constant gain increase the system type by 1eliminate steady-state error for

a unit step input amplify overshoot and oscillations

Proportional-Plus-Integral Control

• To eliminate offset, the proportional controller may be replaced by a proportional-plus-integral controller.

• If integral control action is added to the controller, then, as long as there is an error signal, a signal is developed by the controller to reduce this error, provided the control system is a stable one.

Change in gain for PI controller

• Increase in gain:

Do not upgrade steady-state responses

Increase slightly

settling time

Reduces stability and bandwidth of system

Increase oscillations and overshoot!

Derivative Controller

• Differentiation of error with a constant gain

detect rapid change in output reduce overshoot and oscillation

do not affect the steady-state response

Proportional-Plus-Derivative Control

With derivative action, the controller output is proportional to the rate of change of the measurement or error. 

Effect of change for gain PD controller

• Increase in gain:

Upgrade transient

response

Decrease the peak and rise time

Improves stability and bandwidth of system

Increase overshoot

and settling time!

Proportional-Plus-Derivative -Plus-Integral Control

• A proportional–integral–derivative controller (PID controller) is a control loop feedback mechanism  widely used in industrial control systems – a PID is the most commonly used feedback controller.

• A PID controller calculates an "error" value as the difference between a measured process variable and a desired setpoint. The controller attempts to minimize the error by adjusting the process control inputs.

• The PID controller calculation involves three separate constant parameters, and is accordingly sometimes called three-term control: the proportional, the integral and derivative values, denoted P, I, and D.

• These values can be interpreted in terms of time: P depends on the present error, I on the accumulation of past errors, and D is a prediction of future errors, based on current rate of change.

Changes in gains for PID Controller