PID CONTROLLER TUNING

BY MITESH KUMARROLL NO-10300513026

Applied Electronics & Instrumentation Engg.Haldia Institute Of Technology

CONTENTSCONTENTS

-WHAT IS PID CONTROLLER?-WHAT DO YOU NEED TO FORM A PID CONTROLLER?-HOW DO YOU TUNE YOUR PID PARAMETERS TO THE

OPTIMAL RESPONSE?-WHAT PERFORMANCE CRITERION SHOULD BE USE FOR THE SELECTION AND THE TUNING OF THE

CONTROLLER?-PID TUNING BY( ZIEGLER NICHOLUS METHOD) :OPEN LOOP METHOD : CLOSED LOOP METHOD : TRIAL AND ERROR METHOD-PID ARCHITECTURE

--What is PID ControllerWhat is PID Controller??

-It stands for proportional , integral and derivative controller.

-it’s a mathematical description of the way of think.-PID helps you automatically achieve your goal, exactly the

same way you used to do it manually .-This diagram shows a general structure for a PID controller.

SP

PV

Controller outpute(t)

Process diagram of PID controllerProcess diagram of PID controller

--What do you need to form a PID Controller?What do you need to form a PID Controller?-you need the following six basic elements:-Error: it is the difference between your command and the output the output of the controller.-Proportional term P: it is a constant directly related to the amount

of the error .if you have large error ,the term gives you a large output. and if you have a small error ,it will give you a small

output the simple! The p term affects the speed to reach your target.

-Integral term: it is constant related to the integration (summation) of errors over time. If your error is increasing, this term give you a

large output. however if the error is decreasing ,the I term give you a small output.thu its used to the fine tune your results ,i.e,

when u almost reach your goal ,the p term canot serve you any more

I term is the one you can count on to drive you error signal to zero.

How to tune PID Controller?How to tune PID Controller?

-Derivative term D:it is constant related to the rate of change (derivative) of errors with time .

-what does this mean? it means that if your error signal changes rapidly , i.e., you have a

highly dynamic system like a multi copter , the D term will give you higher output to catch up with the changes .on the other hand if your error changes slowly like in the room temperature example ,

the D term won’t find amplify . Thus it ‘ll look for your noise signal(which usually has a high frequency ) and amplify it to make your life miserable! -the D term is a very dangerous controller if it’s not tuned perfectly!Limits: you need to limit the output of each of the previous controller!-Finally , you need your system of course unless you are satisfied withcontroller

-How do you tune your PID parameters to the -How do you tune your PID parameters to the optimal response?optimal response?

-Most often tuning is an art more than a science.- Observe the system and use your intuitive guess and logicalReasoning.Here are seven golden rules for general PID tuning:1.After nulling all the parameters ,increase the P term so that theoutput reaches the target in the shortest possible time.2.If your output starts oscillating ,it means you have too much P.lower your P term until the oscillation disappears. you will end upSlightly higher or lower than your target. don’t worry; we will fix that in the next step.3.Now increase I term slightly until your errors goes away. note that usual I values are very small(in the order of one thousandth for

example)

-PID TUNING-PID TUNING-and they are dependent on the update rate of your PID loop. The I term is very useful when you have outside error signal affecting your system (e.g. wind in a multi copter).-It drives your error to zero whenever possible.4.If you feel your output is oscillating and it was not before you

adjusted your I term , lower I slightly.5.For many slow dynamic system ,your job is almost done! You justhave to jump to the last step . when dealing with highly dynamicsystem however you need to adjust the D term .If you feel your output “lagging” behind the error variation and trying hard but falling to catch them ,increase this term slightly.6.f your system starts to oscillate with high frequency and smalltransition, you probably have to much D term which is amplifyingYour noise. Decrease D appropriately. If your system ,however has tomuch noise it is better to keep this parameter to zero.

PID TUNINGPID TUNING7.At the last watch your limit ! If you were changing the previous parameters without any noticeable change in the output ,Remember that limits cut down your output signal. Increase themProbably but be careful not to burn or saturate your system.

Simulation figure of PID Controller

-What performance criterion should be use for the selection and the tuning of the controller?

-There are a variety of performance criteria we could use ,such as:

-Keep the maximum deviation (error) as small as possible .-Achieve short settling time .-Minimize the integral of the errors until the process has

settled to its desired set point, and so on-The most often quoted simple performance criteria are: -overshoot(A/B) -Rise time -Settling time - Decay ratio(C/A)

Performance criteriaPerformance criteria

PID TuningPID Tuning - - OPEN LOOP METHOD(ZIEGLER NICHOLUS METHOD)OPEN LOOP METHOD(ZIEGLER NICHOLUS METHOD)

-It is done in manual mode-It is way of relating the process parameters( i.e delay

time ,process gain and time constant) to the controller parameters

(i.e .controller gain and reset time)-It has been developed for use on delay-followed -by-first-

order-lag processes.

-Process parameter (delay time, process -Process parameter (delay time, process gain and the time constant ) from the gain and the time constant ) from the graphgraph

-Once the value of process parameter are -Once the value of process parameter are obtained the PID parameter can be obtained the PID parameter can be calculated from the table.calculated from the table.

-Closed-Loop method ( Closed-Loop method ( Ziegler Nichols tuning method Ziegler Nichols tuning method )PID TuningPID Tuning

- In controller automatic mode ( operating condition ), PV approximate to set-point. change the %PB of controller to maximum, Integral time maximum and Derivative time

minimum, then decrease %PB and take load step ( change set-point or change process loads ) for monitor PV responding

until PV occur slight oscillation. Record %PB of oscillate condition ( Ultimate controller gain, Kcu ) and Band width

( ultimate period, Pu ).

Set-point

Pu

PV

%PB osc = 1/Kcu x %PB osc = 1/Kcu x 100(%)100(%)

- Closed-Loop method ( Ziegler Nichols tuning method )PID Tuning

- In case cannot find out the point of PV oscillation, another one method is alternative. Adjust %PB and Ti

until PV become to Decay ratio form ( B/A = 1/4 ). Then estimate Pu and Kcu by following equations.

Set-point

Pq

A

BPu = 0.9 PqPu = 0.9 Pq

Kcu = 1.67 KpqKcu = 1.67 Kpq%PB decay ratio = 1/Kpq x %PB decay ratio = 1/Kpq x

100(%) 100(%)

- Closed-Loop method ( Ziegler Nichols tuning method )PID Tuning

Roughly Tuning by initial value as table below.

Where:Where:Kc – Controller gainKc – Controller gain

%PB = 1/Kc x 100(%)%PB = 1/Kc x 100(%)

%PB - % Proportional band%PB - % Proportional band

Ti – Integral time or reset Ti – Integral time or reset timetime

( Sec./ Repeat )( Sec./ Repeat )

Td – Derivative time or rateTd – Derivative time or rate ( Sec.)( Sec.)

- Trial & Error method ( Ziegler Nichols tuning method )

PID Tuning

- In controller automatic mode ( operating condition ), PV approximate to set-point. change the %PB of

controller to maximum, Integral time maximum and Derivative time minimum, then decrease %PB and take load step ( change set-point or change process loads )

for monitor PV responding until PV occur slight oscillation.

Set-point

PV Period

PV

- Trial & Error method ( Ziegler Nichols tuning method )PID Tuning

- Adjust initial value of the controller as following equations.

Ti = 0.67 PV Period%PB = 1.33 %PBOSC

- Fine adjust the controller until PV response as Decay ratio form.

Set-point

PV Period

A

B

Kc vs. Ti Chart

- Trial & Error method ( Ziegler Nichols tuning method )PID Tuning

- Adjust initial derivative value of the controller by Td = 0.1 Ti ( Ti @ decay ratio ).

- Fine tuning, Derivative time ( Td ) should vary around 0.1Ti – 0.25Ti incase PV responding is not target however after Td increasing Kc maybe increased to 1.25

times and Ti maybe decreased to 2/3 times of the previously, Finally Decry ratio form is the best practice

for tuning criteria after PV damping ( overshoot ) stay in tolerable.

- P Controller mitigates error but initiates offset.

- I Controller mitigates offset but initiates overshoot.

- D Controller mitigates overshoot for optimization.

PID Tuning Architecture

Error P Controller I Controller D

ControllerOptimiz

e

Offset

Error

Overshoot

Offset