Transcript SELF TUNING OF CONTROLLERS - Instrumentation Engineer's Site

SELF TUNING OF CONTROLLERS

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 controller
Proportional Integral Controller
Proportional Derivative Controller
Proportional 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 (Ultimate Cycle 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
Kc
ŤI
ŤD
Proportional
Ku/2
---
---
PI
Ku/2.2
Pu/1.2
---
PID
Ku/1.7
Pu/2
Pu/8
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
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
SYSTEM
IDENTIFIER
CONTROL
SYNTHESIS
DESIGN
CRITERION
IMPLEMENTATION
BLOCK
Set Point
m
(r)
+
CONTROLLER
PROCESS
-
b=c
Fig: The Main components of Self Tuning Regulator
c
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 Tuning
Control (Real Time Application)
THANK YOU
Presentation by
Ravi Shankar
1 / 2 MTech(Industrial Process Instrumentation) AUCE