EJ 301: BASIC CONTROL SYSTEM

CHAPTER 4: CONTROLLER PRINCIPLE

BASIC PRINCIPLES OF CONTROLLER

Function of Controller

To stabilize the system where the system will hold the set point and not swinging around.

TERM DEFINITIONERROR - Difference between a set point and the process value.

VARIABLE RANGE - Range of values within which the control is required to be maintained.

CONTROLLER OUTPUT RANGE

-Range of the output from the controller.-The controller output expressed as percentage.

CONTROL LAG -The time for the process-control loop to make necessary adjustment to the final control element

DEAD ZONE -The amount of time it takes for the process variable to start changing after changing output as a control valve, variable frequency drive etc.

Properties of Controller

CLASSIFICATION OF CONTROLLER

General Classification of Controller

Figure 4.1: Flow chart of classification of controller

Definition.• Discontinuous control system are nonlinear

system.• The input and output properties are based on

the approximate describing function method.

Discontinuous Controller Mode

Two-position mode• compares the value of an analog or variable input

with instructions and generates a digital output.

• Also known as ON-OFF Controller.

• The output changes its values as the input crosses these limit values.

• Applications:- temperature control, relative humidity, liquid level.

Figure 4.2: Example of Two-position mode

Multi-position controller.• Manage multiple loads.

• Controller works as the selector.

• Applications: Fans, dampers, heater.

Figure 4.3: Example of Multiple-position mode

Definition.• The system which are not sufficient to use

discontinuous control mode.• Classified as proportional controllers, integral

controllers and derivative controllers.• Example: controlling steam flow system.

Continuous Controller Mode

Types of Continuous control TYPESITEMS

PROPORTIONAL CONTROL MODE

INTEGRAL CONTROL MODE

DERIVATIVE CONTROL MODE

CONTINUOUS CONTROLLER MODES

COMPOSITE CONTROL MODES

DEFINITION• The system that involves the measurement of the

offset or error over time.

• This error is integrated and a adjustment is made to the output signal from the proportional part of this model.

PROPORTIONAL + INTEGRAL MODE (PI CONTROL MODE)

PROPORTIONAL + DERIVATIVE MODE (PD CONTROL MODE)

Definition• A proportional–integral–derivative controller (PID

controller) is a generic control loop feedback mechanism (controller) widely used in industrial control systems.

• A PID controller calculates an "error" value as the difference between a measured process variable and a desired set point.

• The controller attempts to minimize the error by adjusting the process control inputs.

PROPORTIONAL + INTEGRAL + DERIVATIVE MODE (PID CONTROL MODE)

ADVANCE CONTROL TECHNIQUES

Definition• Uses the output of the primary controller to manipulate the

set point of the secondary controller.

Characteristics

CASCADE CONTROL

Definition.• Feed-forward is a term describing a kind of system which

reacts to changes in its environment, usually to maintain some desired state of the system.

FEED FORWARD CONTROL

Definition.• Ratio control is used to ensure that two or more flows are

kept at the same ratio even if the flows are changing.

RATIO CONTROL

Definition.• Adaptive Control is the control method used by a controller

which must adapt to a controlled system with parameters which vary, or are initially uncertain.

• For example, as an aircraft flies, its mass will slowly decrease as a result of fuel consumption; a control law is needed that adapts itself to such changing conditions.

ADAPTIVE CONTROL