Where to put the PID windup guard?
Should I put a windup guard on each term of the PID, or just the I, or maybe the whole output?
pid-controller
|
show 2 more comments
Should I put a windup guard on each term of the PID, or just the I, or maybe the whole output?
pid-controller
1
Windup is mostly an issue of the integral term. If you are getting windups from other terms, these are probably not designed properly.
– Eugene Sh.
Dec 12 at 16:20
1
The I term only. Switch it's input to zero when it just enters saturation
– Chu
Dec 12 at 16:30
1
@Chu If you switch it's input to zero, it will never go down.
– Eugene Sh.
Dec 12 at 16:35
@EugeneSh. I'm not sure if "windup" is the correct official term, but in some circumstances plant states can get out of hand. In my experience it's been mechanical assemblies driven by torquer motors, putting the traveler into a combination of velocity and position such that even at maximum braking torque it'll whack into a stop.
– TimWescott
Dec 12 at 16:36
@TimWescott I would call it a general instability. Can be caused by integrator windup of course.
– Eugene Sh.
Dec 12 at 16:38
|
show 2 more comments
Should I put a windup guard on each term of the PID, or just the I, or maybe the whole output?
pid-controller
Should I put a windup guard on each term of the PID, or just the I, or maybe the whole output?
pid-controller
pid-controller
edited Dec 12 at 16:20
asked Dec 12 at 16:18
Dirk Bruere
5,36022755
5,36022755
1
Windup is mostly an issue of the integral term. If you are getting windups from other terms, these are probably not designed properly.
– Eugene Sh.
Dec 12 at 16:20
1
The I term only. Switch it's input to zero when it just enters saturation
– Chu
Dec 12 at 16:30
1
@Chu If you switch it's input to zero, it will never go down.
– Eugene Sh.
Dec 12 at 16:35
@EugeneSh. I'm not sure if "windup" is the correct official term, but in some circumstances plant states can get out of hand. In my experience it's been mechanical assemblies driven by torquer motors, putting the traveler into a combination of velocity and position such that even at maximum braking torque it'll whack into a stop.
– TimWescott
Dec 12 at 16:36
@TimWescott I would call it a general instability. Can be caused by integrator windup of course.
– Eugene Sh.
Dec 12 at 16:38
|
show 2 more comments
1
Windup is mostly an issue of the integral term. If you are getting windups from other terms, these are probably not designed properly.
– Eugene Sh.
Dec 12 at 16:20
1
The I term only. Switch it's input to zero when it just enters saturation
– Chu
Dec 12 at 16:30
1
@Chu If you switch it's input to zero, it will never go down.
– Eugene Sh.
Dec 12 at 16:35
@EugeneSh. I'm not sure if "windup" is the correct official term, but in some circumstances plant states can get out of hand. In my experience it's been mechanical assemblies driven by torquer motors, putting the traveler into a combination of velocity and position such that even at maximum braking torque it'll whack into a stop.
– TimWescott
Dec 12 at 16:36
@TimWescott I would call it a general instability. Can be caused by integrator windup of course.
– Eugene Sh.
Dec 12 at 16:38
1
1
Windup is mostly an issue of the integral term. If you are getting windups from other terms, these are probably not designed properly.
– Eugene Sh.
Dec 12 at 16:20
Windup is mostly an issue of the integral term. If you are getting windups from other terms, these are probably not designed properly.
– Eugene Sh.
Dec 12 at 16:20
1
1
The I term only. Switch it's input to zero when it just enters saturation
– Chu
Dec 12 at 16:30
The I term only. Switch it's input to zero when it just enters saturation
– Chu
Dec 12 at 16:30
1
1
@Chu If you switch it's input to zero, it will never go down.
– Eugene Sh.
Dec 12 at 16:35
@Chu If you switch it's input to zero, it will never go down.
– Eugene Sh.
Dec 12 at 16:35
@EugeneSh. I'm not sure if "windup" is the correct official term, but in some circumstances plant states can get out of hand. In my experience it's been mechanical assemblies driven by torquer motors, putting the traveler into a combination of velocity and position such that even at maximum braking torque it'll whack into a stop.
– TimWescott
Dec 12 at 16:36
@EugeneSh. I'm not sure if "windup" is the correct official term, but in some circumstances plant states can get out of hand. In my experience it's been mechanical assemblies driven by torquer motors, putting the traveler into a combination of velocity and position such that even at maximum braking torque it'll whack into a stop.
– TimWescott
Dec 12 at 16:36
@TimWescott I would call it a general instability. Can be caused by integrator windup of course.
– Eugene Sh.
Dec 12 at 16:38
@TimWescott I would call it a general instability. Can be caused by integrator windup of course.
– Eugene Sh.
Dec 12 at 16:38
|
show 2 more comments
3 Answers
3
active
oldest
votes
Integrator anti-windup is a measure you need to take because of output saturation or other limits in the system, and such limits are nonlinear behavior. When you start doing nonlinear control, a lot of the nice, clear, procedural things that we're taught in undergraduate control theory classes don't entirely apply.
In general you should apply integrator anti-windup to just the integrator term, although you may also need to apply limiting to the output term before it's applied to a DAC (assuming you're doing the work in software). There are a lot of ways to do this. My preference is to either limit the integrator state to certain bounds by itself:
// (Calculate integrator_state)
if (integrator_state > integrator_max) {integrator_state = integrator_max;}
if (integrator_state < integrator_min) {integrator_state = integrator_min;}
Or to calculate a candidate output, then trim the integrator state:
output_candidate = integrator_state + error * prop_gain;
if (output_candidate > output_max) {
integrator_state = output_max - error * prop_gain;
} else if (output_candidate < output_min) {
integrator_state = output_min - error * prop_gain;
}
// Re-calculate the actual output, possibly with a D term
The method that @Chu mentions would work, if you remember to only apply it when the integrator is being pulled to excess, not pulled back (but my first method is equivalent). Another method that is used often is to hold the integrator term at zero when the error is large, then allow integrator action when the error gets below some threshold, or if you're doing a motion controller that knows when a "move" starts to set the integrator at zero at the start of a move and hold it there for some finite time. I'm not a big fan of either of those methods, but others are.
Because you're venturing into nonlinear control, even if you're so far in the shallow end of the pool you can lie down without drowning, there are options on options on options, and there's no one right way to do it. Moreover, you can't find an answer by analysis -- you have to either implement the real system and give it a whirl, or make a simulation and try your algorithm out on that.
I have opted for your first solution, and will try it out tomorrow. It's just a heater control with no safety critical features
– Dirk Bruere
Dec 12 at 16:54
2
You may find this article useful. Note that the model of a heating system in there is completely fake; you probably don't want to use it in your work. What you'll find useful is the code, and the discussion of integrator windup.
– TimWescott
Dec 12 at 16:58
Tim's article has been fundamental to my understanding of controls. It is well worth the short read and will help out immensely! I highly recommend it. Thank you @TimWescott!
– Drew Fowler
Dec 12 at 17:01
1
@DrewFowler: I thought I recognized that code.
– TimWescott
Dec 12 at 17:06
2
The young Padawan meets his master.
– Harry Svensson
Dec 12 at 17:10
add a comment |
Windup guard is typically referred to as the protector of the integral term in order that it will not accumulate continually. The controller can overshoot significantly and will continue to overshoot with the integral continuing to grow.
Therefore, the windup guard is just the integral term. As this term can "windup" and keep growing. But, the output of a PID can/should be limited on the output as well.
For example the following code calculates the integral term and then limits it according to a set value. It also limits the output of the controller, but according to a different limiting term.
void PID_Compute(PID *pid)
{
//Find all error variables
pid->lastError = pid->error;
pid->error = pid->setpoint - pid->input;
pid->derivative = pid->error - pid->lastError;
pid->integral += pid->Ki * pid->error;
//Anti-integral Windup
if(pid->integral > pid->IntegralLimit){
pid->integral = pid->IntegralLimit;
}
else if(pid->integral < -pid->IntegralLimit){
pid->integral = -pid->IntegralLimit;
}
//Calculate PID
pid->output = (pid->Kp*pid->error) + (pid->integral) + (pid->Kd * pid->derivative);
//Set limits
if(pid->output > pid->Outmax){
pid->output = pid->Outmax;
}
else if(pid->output < pid->Outmin){
pid->output = pid->Outmin;
}
}
For anything pertaining to writing or understanding a PID controller, refer to Tim Wescott's article.
add a comment |
As others have already said, windup is only a problem for the I
term.
Where I differ from almost everything else that I've seen, is that I don't like to arbitrarily limit my I
. I want it to be able to saturate the output no matter what it takes to do that, but there's no point in going beyond that. So my limits are floating, based on the final output.
So I use a bigger datatype to hold the output than what I actually need, so I can detect out-of-range and clamp it. Then as part of that clamp, I also unwind the I
. For one project, I actually back-calculated what it should have been to saturate exactly, but for another, I just switched on an exponential decay:
I_acc += Error;
//other code here
if (OUTPUT_OUT_OF_RANGE())
{
CLAMP_OUTPUT();
//anti_windup
I_acc -= (I_acc >> 2);
}
add a comment |
Your Answer
StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["\$", "\$"]]);
});
});
}, "mathjax-editing");
StackExchange.ifUsing("editor", function () {
return StackExchange.using("schematics", function () {
StackExchange.schematics.init();
});
}, "cicuitlab");
StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "135"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);
StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});
function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: false,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: null,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2felectronics.stackexchange.com%2fquestions%2f411868%2fwhere-to-put-the-pid-windup-guard%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
Integrator anti-windup is a measure you need to take because of output saturation or other limits in the system, and such limits are nonlinear behavior. When you start doing nonlinear control, a lot of the nice, clear, procedural things that we're taught in undergraduate control theory classes don't entirely apply.
In general you should apply integrator anti-windup to just the integrator term, although you may also need to apply limiting to the output term before it's applied to a DAC (assuming you're doing the work in software). There are a lot of ways to do this. My preference is to either limit the integrator state to certain bounds by itself:
// (Calculate integrator_state)
if (integrator_state > integrator_max) {integrator_state = integrator_max;}
if (integrator_state < integrator_min) {integrator_state = integrator_min;}
Or to calculate a candidate output, then trim the integrator state:
output_candidate = integrator_state + error * prop_gain;
if (output_candidate > output_max) {
integrator_state = output_max - error * prop_gain;
} else if (output_candidate < output_min) {
integrator_state = output_min - error * prop_gain;
}
// Re-calculate the actual output, possibly with a D term
The method that @Chu mentions would work, if you remember to only apply it when the integrator is being pulled to excess, not pulled back (but my first method is equivalent). Another method that is used often is to hold the integrator term at zero when the error is large, then allow integrator action when the error gets below some threshold, or if you're doing a motion controller that knows when a "move" starts to set the integrator at zero at the start of a move and hold it there for some finite time. I'm not a big fan of either of those methods, but others are.
Because you're venturing into nonlinear control, even if you're so far in the shallow end of the pool you can lie down without drowning, there are options on options on options, and there's no one right way to do it. Moreover, you can't find an answer by analysis -- you have to either implement the real system and give it a whirl, or make a simulation and try your algorithm out on that.
I have opted for your first solution, and will try it out tomorrow. It's just a heater control with no safety critical features
– Dirk Bruere
Dec 12 at 16:54
2
You may find this article useful. Note that the model of a heating system in there is completely fake; you probably don't want to use it in your work. What you'll find useful is the code, and the discussion of integrator windup.
– TimWescott
Dec 12 at 16:58
Tim's article has been fundamental to my understanding of controls. It is well worth the short read and will help out immensely! I highly recommend it. Thank you @TimWescott!
– Drew Fowler
Dec 12 at 17:01
1
@DrewFowler: I thought I recognized that code.
– TimWescott
Dec 12 at 17:06
2
The young Padawan meets his master.
– Harry Svensson
Dec 12 at 17:10
add a comment |
Integrator anti-windup is a measure you need to take because of output saturation or other limits in the system, and such limits are nonlinear behavior. When you start doing nonlinear control, a lot of the nice, clear, procedural things that we're taught in undergraduate control theory classes don't entirely apply.
In general you should apply integrator anti-windup to just the integrator term, although you may also need to apply limiting to the output term before it's applied to a DAC (assuming you're doing the work in software). There are a lot of ways to do this. My preference is to either limit the integrator state to certain bounds by itself:
// (Calculate integrator_state)
if (integrator_state > integrator_max) {integrator_state = integrator_max;}
if (integrator_state < integrator_min) {integrator_state = integrator_min;}
Or to calculate a candidate output, then trim the integrator state:
output_candidate = integrator_state + error * prop_gain;
if (output_candidate > output_max) {
integrator_state = output_max - error * prop_gain;
} else if (output_candidate < output_min) {
integrator_state = output_min - error * prop_gain;
}
// Re-calculate the actual output, possibly with a D term
The method that @Chu mentions would work, if you remember to only apply it when the integrator is being pulled to excess, not pulled back (but my first method is equivalent). Another method that is used often is to hold the integrator term at zero when the error is large, then allow integrator action when the error gets below some threshold, or if you're doing a motion controller that knows when a "move" starts to set the integrator at zero at the start of a move and hold it there for some finite time. I'm not a big fan of either of those methods, but others are.
Because you're venturing into nonlinear control, even if you're so far in the shallow end of the pool you can lie down without drowning, there are options on options on options, and there's no one right way to do it. Moreover, you can't find an answer by analysis -- you have to either implement the real system and give it a whirl, or make a simulation and try your algorithm out on that.
I have opted for your first solution, and will try it out tomorrow. It's just a heater control with no safety critical features
– Dirk Bruere
Dec 12 at 16:54
2
You may find this article useful. Note that the model of a heating system in there is completely fake; you probably don't want to use it in your work. What you'll find useful is the code, and the discussion of integrator windup.
– TimWescott
Dec 12 at 16:58
Tim's article has been fundamental to my understanding of controls. It is well worth the short read and will help out immensely! I highly recommend it. Thank you @TimWescott!
– Drew Fowler
Dec 12 at 17:01
1
@DrewFowler: I thought I recognized that code.
– TimWescott
Dec 12 at 17:06
2
The young Padawan meets his master.
– Harry Svensson
Dec 12 at 17:10
add a comment |
Integrator anti-windup is a measure you need to take because of output saturation or other limits in the system, and such limits are nonlinear behavior. When you start doing nonlinear control, a lot of the nice, clear, procedural things that we're taught in undergraduate control theory classes don't entirely apply.
In general you should apply integrator anti-windup to just the integrator term, although you may also need to apply limiting to the output term before it's applied to a DAC (assuming you're doing the work in software). There are a lot of ways to do this. My preference is to either limit the integrator state to certain bounds by itself:
// (Calculate integrator_state)
if (integrator_state > integrator_max) {integrator_state = integrator_max;}
if (integrator_state < integrator_min) {integrator_state = integrator_min;}
Or to calculate a candidate output, then trim the integrator state:
output_candidate = integrator_state + error * prop_gain;
if (output_candidate > output_max) {
integrator_state = output_max - error * prop_gain;
} else if (output_candidate < output_min) {
integrator_state = output_min - error * prop_gain;
}
// Re-calculate the actual output, possibly with a D term
The method that @Chu mentions would work, if you remember to only apply it when the integrator is being pulled to excess, not pulled back (but my first method is equivalent). Another method that is used often is to hold the integrator term at zero when the error is large, then allow integrator action when the error gets below some threshold, or if you're doing a motion controller that knows when a "move" starts to set the integrator at zero at the start of a move and hold it there for some finite time. I'm not a big fan of either of those methods, but others are.
Because you're venturing into nonlinear control, even if you're so far in the shallow end of the pool you can lie down without drowning, there are options on options on options, and there's no one right way to do it. Moreover, you can't find an answer by analysis -- you have to either implement the real system and give it a whirl, or make a simulation and try your algorithm out on that.
Integrator anti-windup is a measure you need to take because of output saturation or other limits in the system, and such limits are nonlinear behavior. When you start doing nonlinear control, a lot of the nice, clear, procedural things that we're taught in undergraduate control theory classes don't entirely apply.
In general you should apply integrator anti-windup to just the integrator term, although you may also need to apply limiting to the output term before it's applied to a DAC (assuming you're doing the work in software). There are a lot of ways to do this. My preference is to either limit the integrator state to certain bounds by itself:
// (Calculate integrator_state)
if (integrator_state > integrator_max) {integrator_state = integrator_max;}
if (integrator_state < integrator_min) {integrator_state = integrator_min;}
Or to calculate a candidate output, then trim the integrator state:
output_candidate = integrator_state + error * prop_gain;
if (output_candidate > output_max) {
integrator_state = output_max - error * prop_gain;
} else if (output_candidate < output_min) {
integrator_state = output_min - error * prop_gain;
}
// Re-calculate the actual output, possibly with a D term
The method that @Chu mentions would work, if you remember to only apply it when the integrator is being pulled to excess, not pulled back (but my first method is equivalent). Another method that is used often is to hold the integrator term at zero when the error is large, then allow integrator action when the error gets below some threshold, or if you're doing a motion controller that knows when a "move" starts to set the integrator at zero at the start of a move and hold it there for some finite time. I'm not a big fan of either of those methods, but others are.
Because you're venturing into nonlinear control, even if you're so far in the shallow end of the pool you can lie down without drowning, there are options on options on options, and there's no one right way to do it. Moreover, you can't find an answer by analysis -- you have to either implement the real system and give it a whirl, or make a simulation and try your algorithm out on that.
edited Dec 12 at 16:53
Harry Svensson
6,40032346
6,40032346
answered Dec 12 at 16:50
TimWescott
2,779210
2,779210
I have opted for your first solution, and will try it out tomorrow. It's just a heater control with no safety critical features
– Dirk Bruere
Dec 12 at 16:54
2
You may find this article useful. Note that the model of a heating system in there is completely fake; you probably don't want to use it in your work. What you'll find useful is the code, and the discussion of integrator windup.
– TimWescott
Dec 12 at 16:58
Tim's article has been fundamental to my understanding of controls. It is well worth the short read and will help out immensely! I highly recommend it. Thank you @TimWescott!
– Drew Fowler
Dec 12 at 17:01
1
@DrewFowler: I thought I recognized that code.
– TimWescott
Dec 12 at 17:06
2
The young Padawan meets his master.
– Harry Svensson
Dec 12 at 17:10
add a comment |
I have opted for your first solution, and will try it out tomorrow. It's just a heater control with no safety critical features
– Dirk Bruere
Dec 12 at 16:54
2
You may find this article useful. Note that the model of a heating system in there is completely fake; you probably don't want to use it in your work. What you'll find useful is the code, and the discussion of integrator windup.
– TimWescott
Dec 12 at 16:58
Tim's article has been fundamental to my understanding of controls. It is well worth the short read and will help out immensely! I highly recommend it. Thank you @TimWescott!
– Drew Fowler
Dec 12 at 17:01
1
@DrewFowler: I thought I recognized that code.
– TimWescott
Dec 12 at 17:06
2
The young Padawan meets his master.
– Harry Svensson
Dec 12 at 17:10
I have opted for your first solution, and will try it out tomorrow. It's just a heater control with no safety critical features
– Dirk Bruere
Dec 12 at 16:54
I have opted for your first solution, and will try it out tomorrow. It's just a heater control with no safety critical features
– Dirk Bruere
Dec 12 at 16:54
2
2
You may find this article useful. Note that the model of a heating system in there is completely fake; you probably don't want to use it in your work. What you'll find useful is the code, and the discussion of integrator windup.
– TimWescott
Dec 12 at 16:58
You may find this article useful. Note that the model of a heating system in there is completely fake; you probably don't want to use it in your work. What you'll find useful is the code, and the discussion of integrator windup.
– TimWescott
Dec 12 at 16:58
Tim's article has been fundamental to my understanding of controls. It is well worth the short read and will help out immensely! I highly recommend it. Thank you @TimWescott!
– Drew Fowler
Dec 12 at 17:01
Tim's article has been fundamental to my understanding of controls. It is well worth the short read and will help out immensely! I highly recommend it. Thank you @TimWescott!
– Drew Fowler
Dec 12 at 17:01
1
1
@DrewFowler: I thought I recognized that code.
– TimWescott
Dec 12 at 17:06
@DrewFowler: I thought I recognized that code.
– TimWescott
Dec 12 at 17:06
2
2
The young Padawan meets his master.
– Harry Svensson
Dec 12 at 17:10
The young Padawan meets his master.
– Harry Svensson
Dec 12 at 17:10
add a comment |
Windup guard is typically referred to as the protector of the integral term in order that it will not accumulate continually. The controller can overshoot significantly and will continue to overshoot with the integral continuing to grow.
Therefore, the windup guard is just the integral term. As this term can "windup" and keep growing. But, the output of a PID can/should be limited on the output as well.
For example the following code calculates the integral term and then limits it according to a set value. It also limits the output of the controller, but according to a different limiting term.
void PID_Compute(PID *pid)
{
//Find all error variables
pid->lastError = pid->error;
pid->error = pid->setpoint - pid->input;
pid->derivative = pid->error - pid->lastError;
pid->integral += pid->Ki * pid->error;
//Anti-integral Windup
if(pid->integral > pid->IntegralLimit){
pid->integral = pid->IntegralLimit;
}
else if(pid->integral < -pid->IntegralLimit){
pid->integral = -pid->IntegralLimit;
}
//Calculate PID
pid->output = (pid->Kp*pid->error) + (pid->integral) + (pid->Kd * pid->derivative);
//Set limits
if(pid->output > pid->Outmax){
pid->output = pid->Outmax;
}
else if(pid->output < pid->Outmin){
pid->output = pid->Outmin;
}
}
For anything pertaining to writing or understanding a PID controller, refer to Tim Wescott's article.
add a comment |
Windup guard is typically referred to as the protector of the integral term in order that it will not accumulate continually. The controller can overshoot significantly and will continue to overshoot with the integral continuing to grow.
Therefore, the windup guard is just the integral term. As this term can "windup" and keep growing. But, the output of a PID can/should be limited on the output as well.
For example the following code calculates the integral term and then limits it according to a set value. It also limits the output of the controller, but according to a different limiting term.
void PID_Compute(PID *pid)
{
//Find all error variables
pid->lastError = pid->error;
pid->error = pid->setpoint - pid->input;
pid->derivative = pid->error - pid->lastError;
pid->integral += pid->Ki * pid->error;
//Anti-integral Windup
if(pid->integral > pid->IntegralLimit){
pid->integral = pid->IntegralLimit;
}
else if(pid->integral < -pid->IntegralLimit){
pid->integral = -pid->IntegralLimit;
}
//Calculate PID
pid->output = (pid->Kp*pid->error) + (pid->integral) + (pid->Kd * pid->derivative);
//Set limits
if(pid->output > pid->Outmax){
pid->output = pid->Outmax;
}
else if(pid->output < pid->Outmin){
pid->output = pid->Outmin;
}
}
For anything pertaining to writing or understanding a PID controller, refer to Tim Wescott's article.
add a comment |
Windup guard is typically referred to as the protector of the integral term in order that it will not accumulate continually. The controller can overshoot significantly and will continue to overshoot with the integral continuing to grow.
Therefore, the windup guard is just the integral term. As this term can "windup" and keep growing. But, the output of a PID can/should be limited on the output as well.
For example the following code calculates the integral term and then limits it according to a set value. It also limits the output of the controller, but according to a different limiting term.
void PID_Compute(PID *pid)
{
//Find all error variables
pid->lastError = pid->error;
pid->error = pid->setpoint - pid->input;
pid->derivative = pid->error - pid->lastError;
pid->integral += pid->Ki * pid->error;
//Anti-integral Windup
if(pid->integral > pid->IntegralLimit){
pid->integral = pid->IntegralLimit;
}
else if(pid->integral < -pid->IntegralLimit){
pid->integral = -pid->IntegralLimit;
}
//Calculate PID
pid->output = (pid->Kp*pid->error) + (pid->integral) + (pid->Kd * pid->derivative);
//Set limits
if(pid->output > pid->Outmax){
pid->output = pid->Outmax;
}
else if(pid->output < pid->Outmin){
pid->output = pid->Outmin;
}
}
For anything pertaining to writing or understanding a PID controller, refer to Tim Wescott's article.
Windup guard is typically referred to as the protector of the integral term in order that it will not accumulate continually. The controller can overshoot significantly and will continue to overshoot with the integral continuing to grow.
Therefore, the windup guard is just the integral term. As this term can "windup" and keep growing. But, the output of a PID can/should be limited on the output as well.
For example the following code calculates the integral term and then limits it according to a set value. It also limits the output of the controller, but according to a different limiting term.
void PID_Compute(PID *pid)
{
//Find all error variables
pid->lastError = pid->error;
pid->error = pid->setpoint - pid->input;
pid->derivative = pid->error - pid->lastError;
pid->integral += pid->Ki * pid->error;
//Anti-integral Windup
if(pid->integral > pid->IntegralLimit){
pid->integral = pid->IntegralLimit;
}
else if(pid->integral < -pid->IntegralLimit){
pid->integral = -pid->IntegralLimit;
}
//Calculate PID
pid->output = (pid->Kp*pid->error) + (pid->integral) + (pid->Kd * pid->derivative);
//Set limits
if(pid->output > pid->Outmax){
pid->output = pid->Outmax;
}
else if(pid->output < pid->Outmin){
pid->output = pid->Outmin;
}
}
For anything pertaining to writing or understanding a PID controller, refer to Tim Wescott's article.
edited Dec 12 at 17:46
answered Dec 12 at 16:52
Drew Fowler
13910
13910
add a comment |
add a comment |
As others have already said, windup is only a problem for the I
term.
Where I differ from almost everything else that I've seen, is that I don't like to arbitrarily limit my I
. I want it to be able to saturate the output no matter what it takes to do that, but there's no point in going beyond that. So my limits are floating, based on the final output.
So I use a bigger datatype to hold the output than what I actually need, so I can detect out-of-range and clamp it. Then as part of that clamp, I also unwind the I
. For one project, I actually back-calculated what it should have been to saturate exactly, but for another, I just switched on an exponential decay:
I_acc += Error;
//other code here
if (OUTPUT_OUT_OF_RANGE())
{
CLAMP_OUTPUT();
//anti_windup
I_acc -= (I_acc >> 2);
}
add a comment |
As others have already said, windup is only a problem for the I
term.
Where I differ from almost everything else that I've seen, is that I don't like to arbitrarily limit my I
. I want it to be able to saturate the output no matter what it takes to do that, but there's no point in going beyond that. So my limits are floating, based on the final output.
So I use a bigger datatype to hold the output than what I actually need, so I can detect out-of-range and clamp it. Then as part of that clamp, I also unwind the I
. For one project, I actually back-calculated what it should have been to saturate exactly, but for another, I just switched on an exponential decay:
I_acc += Error;
//other code here
if (OUTPUT_OUT_OF_RANGE())
{
CLAMP_OUTPUT();
//anti_windup
I_acc -= (I_acc >> 2);
}
add a comment |
As others have already said, windup is only a problem for the I
term.
Where I differ from almost everything else that I've seen, is that I don't like to arbitrarily limit my I
. I want it to be able to saturate the output no matter what it takes to do that, but there's no point in going beyond that. So my limits are floating, based on the final output.
So I use a bigger datatype to hold the output than what I actually need, so I can detect out-of-range and clamp it. Then as part of that clamp, I also unwind the I
. For one project, I actually back-calculated what it should have been to saturate exactly, but for another, I just switched on an exponential decay:
I_acc += Error;
//other code here
if (OUTPUT_OUT_OF_RANGE())
{
CLAMP_OUTPUT();
//anti_windup
I_acc -= (I_acc >> 2);
}
As others have already said, windup is only a problem for the I
term.
Where I differ from almost everything else that I've seen, is that I don't like to arbitrarily limit my I
. I want it to be able to saturate the output no matter what it takes to do that, but there's no point in going beyond that. So my limits are floating, based on the final output.
So I use a bigger datatype to hold the output than what I actually need, so I can detect out-of-range and clamp it. Then as part of that clamp, I also unwind the I
. For one project, I actually back-calculated what it should have been to saturate exactly, but for another, I just switched on an exponential decay:
I_acc += Error;
//other code here
if (OUTPUT_OUT_OF_RANGE())
{
CLAMP_OUTPUT();
//anti_windup
I_acc -= (I_acc >> 2);
}
answered Dec 13 at 3:58
AaronD
4,076528
4,076528
add a comment |
add a comment |
Thanks for contributing an answer to Electrical Engineering Stack Exchange!
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
Use MathJax to format equations. MathJax reference.
To learn more, see our tips on writing great answers.
Some of your past answers have not been well-received, and you're in danger of being blocked from answering.
Please pay close attention to the following guidance:
- Please be sure to answer the question. Provide details and share your research!
But avoid …
- Asking for help, clarification, or responding to other answers.
- Making statements based on opinion; back them up with references or personal experience.
To learn more, see our tips on writing great answers.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2felectronics.stackexchange.com%2fquestions%2f411868%2fwhere-to-put-the-pid-windup-guard%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
Required, but never shown
1
Windup is mostly an issue of the integral term. If you are getting windups from other terms, these are probably not designed properly.
– Eugene Sh.
Dec 12 at 16:20
1
The I term only. Switch it's input to zero when it just enters saturation
– Chu
Dec 12 at 16:30
1
@Chu If you switch it's input to zero, it will never go down.
– Eugene Sh.
Dec 12 at 16:35
@EugeneSh. I'm not sure if "windup" is the correct official term, but in some circumstances plant states can get out of hand. In my experience it's been mechanical assemblies driven by torquer motors, putting the traveler into a combination of velocity and position such that even at maximum braking torque it'll whack into a stop.
– TimWescott
Dec 12 at 16:36
@TimWescott I would call it a general instability. Can be caused by integrator windup of course.
– Eugene Sh.
Dec 12 at 16:38