PID tuning and accelereation
28 Sep 2014 03:55 #51618
by akb1212
Replied by akb1212 on topic PID tuning and accelereation
Ok, it was as I feared then.... its harder to implement acceleration changing during tuning than changing the PID parameters.
I guess there's a reason it's not in there already.....
I realize it's not a good thing to restart all the motion planning processes while working. But how about during calibration? That is after all a special case, and there should be no real implications of doing it. You really shouldn't go in to the calibration menu and start assigning new values if you don't need to. How difficult would it be to have the process restart the necessary processes in the calibration utility?
It is a little more than a "nice to have in there" I would say. So a better way to change it around during calibration would be great. A script to restart only the needed processes or something?
Are you saying you can alter the values of the user variables when starting LCNC in a terminal window, but acceleration isn't one of the parameters possible to change in there? I'm not in front of my Linux machine now, so I can't check for myself.
On a slightly related note. What is the best way to determine how high accel to use? I guess its all part of the black magic involved in PID tuning, but some guidelines from you experts would be very welcomed. It would be welcomed both by users like me who are retrofitting old quality iron with an updated controller and hoping for increased speed capabilities as well as users building new mills. Acceleration is one of the key elements here. In particular for 3D contouring and such. Increasing the acceleration could mean a big improvement there.
But the question is how much acceleration can be increased before you will get in to trouble.
One thought in that area is as follows:
I have seen torque meters and such on larger machines. These are to show you how much you are utilizing the machine, and is a good idea. My spindle drive has an analogue output for this, and I will implement this with an analogue input. A friend have this as an original feature on his Maho mill.
Anyway, the idea is to make a meter in the GUI that is showing the value of the f-error pin (possibly also one for each axis). This will continuously show you what accuracy you have at any given time, and you can add storing of max values and other features as wanted as well. My plan is to use the gmoccapy GUI, and as far as I understand there should be no problem to implement it there. It's in fact one of the things I hope to be able to implement myself....
The idea for this came when I was doing my PID tuning and saw the values of f-error I was able to get compared to the ones in JT's guide. I wondered if the values I was getting were in fact good or not. And to know that I needed to know what the scale is on the HAL scope. I noticed that I wasn't able to get anything as low values on f-error as JT had in his screenshots. But then I realized he is using inch as base units, right JT? That means that when I use mm I can have 25 times higher values of f-error and still be well off.
When tuning you need to set max f-error way high (as a side note this is also one of the things that would be a good idea to be able to change in the calibration feature... at least if it's easily accessible). But the idea is that your tuning should make you able to reduce the values of this down to reasonable values by tuning the axes. But I became quite scared when after tuning I did some quick jogging around and saw the values of f-error fly what seemed sky high compared to the values I got during tuning.
That was when I realized that my acceleration was too high. And reducing accel by half to a third of the first values I was able to reduce most of this error.
But this also made me realse, lowering the acceleration even further down would make it possible to increase the accuracy of my mill in general by reducing f-error values.
Then I came to think of one of my plans for the mill, I want to add a 4th axis and use the mill to sharpen HM tools with it (I have access to free worn out HM tools from work, for private use anyway....). I know that is possible, but this is one of the things that will require high accuracy. And a way to monitor this is a good idea. So if I have an indicator that shows what my accuracy is in real time I would be able to make trial runs and test out what accuracy I will be able to get, and possibly tune the g-code programs to reduce the error.
Grinding like this doesn't require high speed movements though, so there should be good possibility to get fairly high accuracy. But this will mean the PID tuning will have to be very good.
And I found that in particular FF1 are very sensitive, and changing it from 0.0092 to 0.0093 made the steady state error change surprisingly much. For this I found that the HAL scope was irreplaceable. It wouldn't even be possible to get a good tuning without it. So l found that learning how to use that was well spent time!
Back to acceleration..... reducing acceleration will increase accuracy (down to a certain value anyway), and some types of work will favor accuracy over acceleration (like my tool grinding example). Then again other kinds of work will benefit from higher acceleration and won't require high accuracy (like roughing out huge volumes of material as quick as possible, in particular if new HSM CAM software is used where circular acceleration is set as high as possible all the time).
What would be the best way to change between them? Would I need two completely different configurations of LCNC, one for each kind of work? Then how about if I need both in different parts of the code?
So come to think of it, changing acceleration is in fact something I'd like to be able to do while machining a part. It's in fact comparable to changing the values used with G65 throughout your parts as some operations require more accuracy compared to others.
The difference is that with changing the main acceleration value you will be able to controll the accuracy for all movements in a way the controller won't be able to by applying low values to G65. As long as acceleration is set high it will use relatively high acceleration even if it is asked to use higher accuracy. Is there a g-code to override acceleration (to lower it from standard value)?
Is there something to these thoughts, or am I way off? I would think there is something to gain from looking in to it.
Anders
I guess there's a reason it's not in there already.....
I realize it's not a good thing to restart all the motion planning processes while working. But how about during calibration? That is after all a special case, and there should be no real implications of doing it. You really shouldn't go in to the calibration menu and start assigning new values if you don't need to. How difficult would it be to have the process restart the necessary processes in the calibration utility?
It is a little more than a "nice to have in there" I would say. So a better way to change it around during calibration would be great. A script to restart only the needed processes or something?
Are you saying you can alter the values of the user variables when starting LCNC in a terminal window, but acceleration isn't one of the parameters possible to change in there? I'm not in front of my Linux machine now, so I can't check for myself.
On a slightly related note. What is the best way to determine how high accel to use? I guess its all part of the black magic involved in PID tuning, but some guidelines from you experts would be very welcomed. It would be welcomed both by users like me who are retrofitting old quality iron with an updated controller and hoping for increased speed capabilities as well as users building new mills. Acceleration is one of the key elements here. In particular for 3D contouring and such. Increasing the acceleration could mean a big improvement there.
But the question is how much acceleration can be increased before you will get in to trouble.
One thought in that area is as follows:
I have seen torque meters and such on larger machines. These are to show you how much you are utilizing the machine, and is a good idea. My spindle drive has an analogue output for this, and I will implement this with an analogue input. A friend have this as an original feature on his Maho mill.
Anyway, the idea is to make a meter in the GUI that is showing the value of the f-error pin (possibly also one for each axis). This will continuously show you what accuracy you have at any given time, and you can add storing of max values and other features as wanted as well. My plan is to use the gmoccapy GUI, and as far as I understand there should be no problem to implement it there. It's in fact one of the things I hope to be able to implement myself....
The idea for this came when I was doing my PID tuning and saw the values of f-error I was able to get compared to the ones in JT's guide. I wondered if the values I was getting were in fact good or not. And to know that I needed to know what the scale is on the HAL scope. I noticed that I wasn't able to get anything as low values on f-error as JT had in his screenshots. But then I realized he is using inch as base units, right JT? That means that when I use mm I can have 25 times higher values of f-error and still be well off.
When tuning you need to set max f-error way high (as a side note this is also one of the things that would be a good idea to be able to change in the calibration feature... at least if it's easily accessible). But the idea is that your tuning should make you able to reduce the values of this down to reasonable values by tuning the axes. But I became quite scared when after tuning I did some quick jogging around and saw the values of f-error fly what seemed sky high compared to the values I got during tuning.
That was when I realized that my acceleration was too high. And reducing accel by half to a third of the first values I was able to reduce most of this error.
But this also made me realse, lowering the acceleration even further down would make it possible to increase the accuracy of my mill in general by reducing f-error values.
Then I came to think of one of my plans for the mill, I want to add a 4th axis and use the mill to sharpen HM tools with it (I have access to free worn out HM tools from work, for private use anyway....). I know that is possible, but this is one of the things that will require high accuracy. And a way to monitor this is a good idea. So if I have an indicator that shows what my accuracy is in real time I would be able to make trial runs and test out what accuracy I will be able to get, and possibly tune the g-code programs to reduce the error.
Grinding like this doesn't require high speed movements though, so there should be good possibility to get fairly high accuracy. But this will mean the PID tuning will have to be very good.
And I found that in particular FF1 are very sensitive, and changing it from 0.0092 to 0.0093 made the steady state error change surprisingly much. For this I found that the HAL scope was irreplaceable. It wouldn't even be possible to get a good tuning without it. So l found that learning how to use that was well spent time!
Back to acceleration..... reducing acceleration will increase accuracy (down to a certain value anyway), and some types of work will favor accuracy over acceleration (like my tool grinding example). Then again other kinds of work will benefit from higher acceleration and won't require high accuracy (like roughing out huge volumes of material as quick as possible, in particular if new HSM CAM software is used where circular acceleration is set as high as possible all the time).
What would be the best way to change between them? Would I need two completely different configurations of LCNC, one for each kind of work? Then how about if I need both in different parts of the code?
So come to think of it, changing acceleration is in fact something I'd like to be able to do while machining a part. It's in fact comparable to changing the values used with G65 throughout your parts as some operations require more accuracy compared to others.
The difference is that with changing the main acceleration value you will be able to controll the accuracy for all movements in a way the controller won't be able to by applying low values to G65. As long as acceleration is set high it will use relatively high acceleration even if it is asked to use higher accuracy. Is there a g-code to override acceleration (to lower it from standard value)?
Is there something to these thoughts, or am I way off? I would think there is something to gain from looking in to it.
Anders
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29 Sep 2014 22:49 - 29 Sep 2014 22:52 #51663
by dgarrett
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