SELF TUNING OF SELF TUNING OF CONTROLLERSCONTROLLERS What is a Controller

Controller is an active element or device that receives the information from the measurements and takes appropriate corrective action to adjust the values of manipulated variables of a process.

Different Types of Controllers

Proportional controllerProportional Integral ControllerProportional Derivative ControllerProportional Integral Derivative Controller

Tuning of a Controller

Tuning of a controller may be defined as process of identifying ideal set of values to a selected controller for a specific process control

We need to obtain the values of Kc, TI and TD for optimizing the process output.

Ziegler-Nichols Tuning Ziegler-Nichols Tuning (Ultimate Cycle (Ultimate Cycle method)method) Unlike process reaction curve method which uses data from the

open loop response of a system, this technique is a closed loop procedure. It goes through following steps

1. Bring the system to desired operation level

2. Using proportional control only & with the feedback loop closed, introduce a set point change and vary proportional gain until the system oscillates continuously. The frequency at continuous oscillation is known as cross over frequency ω co. Let M be the Amplitude Ratio of system response at ω co

3. Compute the following quantities

Ultimate Gain Ku = 1/M

Ultimate period of sustained cycling Pu = 2π/ ω co min/cycle

4. Using the values of Ku and M. Ziegler-Nichols have recommended the following settings for the feedback controls

The settings above reveal the following

1. For Proportional control alone use of gain margin is equal to 2

2. For PI control use lower Kc because the presence of Integral control mode introduces additional phase lag at all frequencies with destabilizing effects on the system. Therefore lower or approximately same is the value of Kc a

3. The presence of Derivative control introduces phase lead with strong stabilizing effects in the closed loop response. Thus Kc can be increased without threatening the stability of the system

Kc ŤI ŤD

Proportional Ku/2 --- ---

P I Ku/2.2 Pu/1.2 ---

P I D Ku/1.7 Pu/2 Pu/8

Definition of a Self Tuning controller

A Self tuning controller may be defined as a system capable of readjusting the controller tuning settings automatically to provide an optimal process output

Blocks of a Self Tuning Controller

System Identifier:

This Item estimates the parameters of the process

Controller Synthesizer:

This element synthesizes or calculates the controller parameters specified by control objective function

Controller Implementation Block:

This is the controller whose parameters are updated at periodical intervals by the controller parameter calculator

r: set point; c: controlled output; m: manipulated variable

BLOCK DIAGRAM OF SELF TUNING CONTROLLER

DESIGN CRITERION

IMPLEMENTATION BLOCK

Set Point

( r ) m c +

-

b = c

Fig: The Main components of Self Tuning Regulator

CONTROLSYNTHESIS

SYSTEMIDENTIFIER

CONTROLLER

PROCESS

Most self tuning controllers will self tune

-During Startup

-Retune on request

-Retune on naturally occurring upsets

-Continuously when excitation introduced into the process by controller

Advantages of Self Tuning Controllers

1. Facilitates controlling critical processes of systems;

2. Approaches optimum operation regimes;

3. Facilitates design unification of control systems;

4. Shortens the lead times of system testing and tuning;

5. Lowers the criticality of technological requirements on control systems by making the systems more robust;

6. Saves personnel time for system tuning.

Video on functioning of a Self Video on functioning of a Self Tuning Control (Real Time Tuning Control (Real Time

Application)Application)

THANK YOUTHANK YOU

Presentation byRavi Shankar 1 / 2 MTech(Industrial Process Instrumentation) AUCE