optimal controller settings

OPTIMAL CONTROLLER SETTINGS

IN 604 PROCESS CONTROL

Submitted By,

Muhammed Raees PCMunawar CKNikitha AgarwalNitheesh Jeevan Bose

OPTIMIZING THE OPERATION OF A PLANT Desirable condition for operation of a plant

Minimum Production Cost Maximum Profit

Achieved by Optimizing Control Strategy, which Identifies when the plant must be moved to a

new operating point in order to reduce the operating cost

Makes the appropriate set point changes to bring the plant to the new optimum operating point

DESIGNING SYSTEM

The Steady State input-output relationships

d1,d2 –two disturbances m1,m2 – two manipulated variables y1,y2 - two output variables

Let Operating cost = cost = F(y1,y2)

SIMPLE PROCESS WITH ITS CONTROL LOOPS

When disturbances have the values the minimum operating cost is achieved for

Where,

The Values are the current set point values for the two control loops.

But, when the disturbance change, say

The values of y1 and y2 that minimize the cost function also change.

SUPERVISORY IMPLEMENTATION OF OPTIMIZING CONTROL STRATEGY

Let new optimum values be Again they satisfy the following conditions for

optimality:

THE OPTIMIZING CONTROL SYSTEM MUST BE ABLE TO :- Identify that the current set points no

longer correspond to optimum operating conditions.

Find the new optimum operating conditions (i.e. the values )

Bring the process to the new optimum operation by changing the set point values of the two control loops.

IMPLEMENTATION OF DIGITAL COMPUTERS FOR PROCESS CONTROL:- Allowed the implementation of optimizing

control strategies in plants. Usual mode of implementation – Supervisory

Control. Computer calculates new optimum set point

values and communicates them to the two control loops.

Optimizing control- extensive use Beneficial for large plants – significant

economic savings.

SIMPLE PERFORMANCE CRITERIA Overshoot Rise time Settling time Decay ratio Frequency of Oscillation of the transient

DECAY RATIO Most popular C/A = ¼ One-Quarter Decay Ratio Criterion The ratio of the amounts above the ultimate

value of two successive peaks. Related to Damping factor

CHARACTERISTICS OF AN UNDER DAMPED SYSTEM

TIME INTEGRAL PERFORMANCE CRITERIA Integral of the Square Error – ISE

Integral of the absolute value of the error – IAE

Integral of the time-weighed absolute error – ITAE

Where

GENERAL GUIDELINES FOR DESIGNING BEST CONTROLLER Select the type of controller and the values of

its adjusted parameters in such a way as to minimize the IAE,ISE,ITAE of the system’s response. To Strongly suppress large errors – ISE For Suppression of small errors – IAE To suppress errors that persists for long times -

ITSE

When we tune the controller parameters using IAE,ISE, ITAE performance criteria- Remember the following points

1. Different criteria lead to different controller designs

2.For the same time integral criterion, different input changes lead to different designs

CLOSED LOOP RESPONSE USING VARIOUS TIME INTEGRAL CRITERIA

REFERENCES Chemical Process Control – George

Stephanopoulos Handbook of Instrumentation Engineers (Process Measurement and Analysis) - Liptak