what is reset windup in pid controler?

A

Thread Starter

amar

hi friend,

what is reset windup and how we can eleminate it?
what is anti reset windup?
what is effect of this windup?

thank u.
 
Wind up occurs when your PID system has constant error. In other words, if the difference between the set point and the process variable never gets to zero, the integral term can grow to a very large number. Anti windup must be some system to stop the I term from growing without bound. You might be able to stop the reset by setting the reset interval to a large number.
 
Please consult a textbook on control engineering to answer such basic questions such as this.

The "I" or Integral or "Reset" term of the PID algorithm provides the "memory" to allow a controller to continue to generate output when the PV (process variable) does not equal the setpoint. To do this, the integral term accumulates the signed error remaining after each control cycle to use in the next control cycle of a digital controller as the error at t-1. When the output of the controller (a control valve for example) becomes limited because process conditions cause it to be fully open or fully closed, and the PV is still not at the setpoint value, the reset remainder term continues to increase by the remaining error. When the process conditions change to allow the control valve to once again do its work, the reset remainder term is so large that even when the sign of the error changes, the output may not respond until all of the reset remainder term is "used up." The normal solution (anti reset windup) is to stop accumulating reset remainder when the output is limit-stopped. Other solutions cause the controller to go into Manual then reinitialize when the limit-stop conditions change.

Another similar condition is called open-loop windup, and happens when some condition causes a PID controller to not be in control of the output device. This happens on boilers when an output selector is used to select between perhaps two control schemes based on either excess oxygen or carbon monoxide in the flue gas. The control scheme not dynamically in control of the air damper position will wind up. The solution or antiwindup for this is called external reset feedback that forces the integral term to recalculate error based on the actual output value of the other controller.

Dick Caro
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F

Friedrich Haase

Moin amar, moin all,

On June 18, 2003, amar wrote:
> what is reset windup and how we can eleminate it?
> what is anti reset windup?
> what is effect of this windup?


Let's look at a typical example.

There is a plant and a PID controller and the set-point is changed by a rather big (but reachable) amount. The PID controller will calculate at very big control action, too large for the system. So the controller output will be limited and only a smaller partion goes to the plant. As a consequence the real plant reaction differs from what the PID controller expected - it's too small.

The integral part of the PID controller sums the difference from setpoint and process variable (controlled variable) and will use this sum for an even larger control action. As if the controller says "well, if the plant does not do what I want, I will use more steam." Again, the control action gets limited and so on for a while.

Sometimes this might continue for a long time and the integral part control action can reach a very large value compared to the biggest possible control action. This is called integrator wind-up.

Finally, when the plant output reaches the desired set-point but the integral part is still much too high and will continue to calculate large control actions. The plant output overshoots and the difference from setpoint and process variable now reduces the integral sum. And it will take some more time until the sum reaches zero.

In bad cases (quite often!) the controller will issue much too small control actions and the situation reverses. And this can continue forever, i.e. the closed loop gets unstable. Please don't confuse this with "ringing," another real-word problem with controllers.

The main reason, as you can see, is the non-linear behaviour when the control action is too large or too small and gets limited.

One solution (not the very best :) is to limit the integral part what is sometimes (and incorrectly) called reset wind-up. It means the integral sum is "reset" to a smaller limiting value (NOT to zero!) after calculating the differential or difference equation.

Hacking the equation for a PID controller is a students exercise. Constructing a PID controller is an art :)

Best regards
Friedrich Haase

Ing.-Büro Dr. Friedrich Haase
Consulting - Automatisierungstechnik
Meliesallee 42
D 40597 Düsseldorf
Germany
phone +49 (211) 716026
fax +49 (211) 7182045
email Friedrich_Haase<AT>compuserve.com
 
R

Ranjan Acharya

Get the book "PID Controllers: Theory, Design and Tuning" by Karl ?str?m and Tore Hägglund. ISBN 1-55617-516-7. It is in its second edition. A good description of wind-up (for example) starts in Section 3.5.

R
 
Anti reset windup was primerialy used in older pneumatic controllers but also applies to some electronic controllers.

Simply the increasing output generated by a PID controller using Integral action (Also called reset)when there is a deviation between setpoint and PV causes the controller output to become saturated as the integral action (Reset) will continue to increase the output in order to match the PV and setpoint. In older pneumatic controllers this was a problem as it could take quite some time for the output to become unsaturated if the PV and setpoint went the oppossite way. Anti reset windup was added to controllers to prevent this happening and reduce or eliminate the delay time to become unsaturated and regain control. It normaly is only associated with the extreme ends of the controller output ie <3 Psi and > 15 PSI. For instance if the controller air supply is 20 PSI then the output of the controller will eventualy reach 20 PSI due to integral action (Reset) this leaves 3 PSI to vent befor we come back to the controlling area of say 3-15 PSI.

Hope this helps

Regards

Mike
 
Wind up actually occurs when your actuator is a real one, meaning that he has a saturation value.
A valve can not be more than fully open, or less than fully closed.

A PID without anti-wind-up may ask for >100% of the actuator, and at this moment, your integral term is skyrocketing, because the actuator can not follow, and your PID doesn't know that.

Back-calculating the integral term based on actuator saturation is a way to teach your PID that the actuator has saturation values.
 
> what is reset windup and how we can eleminate it?
> what is anti reset windup?
> what is effect of this windup?

I have a different take on reset windup

Suppose for example you have a pump filling a tank through a control valve with a level controller. Water is flowing out of the tank to consumers.

Now suppose the pump trips, the level controller will soon see a deviation (level dropping) so the proportional action will go up but the integral action also keeps going up trying to get process up to setpoint, eventually the valve will reach 100%, that's windup.

To employ Anti Reset windup you would trigger the PID with a discrete (pump not running)signal to halt the reset action.

When the operator re-starts the pump the valve will go to a new position determined by the deviation and proportional gain but the integral portion will start just where it left off rather than 100% which may cause the tank to overflow.
 
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