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  1. #91
    Quote Originally Posted by JAZZCNC View Post
    Sounds more like it's following errors to me, I'd be looking at PID tuning the motor to drives again rather than Steps per, if the servo/Drive tuning isn't right none of that matters.
    AAh Ok, this sounds much more likely to be the issue then. I am fairly happy with my knowledge on setting up with regards to CAM and posts/coding so was fairly happy that was right.

    but with the tuning I literally had/have no idea about what I'm doing! I'm away at the moment but when I get back at the weekend I will grab some screen shots of the tuning software and my settings for you guys to have a look at if you wouldn't mind?

  2. #92
    Quote Originally Posted by Muzzer View Post
    Tuning a PID is a bit of an art. Here's how I do it:
    Start by turning I and D down to zero, then increase P until it's unstable and back off a bit. Then dial in some D until it's unstable and back it off a bit. Then fiddle with both until it feels about as good as you can get. At this point you will have a pretty snappy response - but without the I term, you may end up with a steady state error. So now dial in some I until it's unstable again and back off a bit. You may now need to back off the P and D a little more. Finally, you should check to see what it actually looks like.

    Naturally, cncdrive show a slightly different method http://www.cncdrive.com/downloads/help.pdf. It's not a perfect art but you get the general idea.
    Sounds good, I will give this a go a the weekend. I did have a bit of vibration develop on one of the axis when it was trying to hold it's position, so that's probably a pretty good indication something isn't right in the settings!!

  3. #93
    Most likely too much P or D. Start off with I and D at zero and go from there. The steady state error is taken care of by the I term, so if PID tuning is indeed the issue, that's most likely where your solution will be found.

  4. #94
    I finally got another chance to play with the mill, with no success at all so far!

    The more I mess with tuning the more I realise I have no idea what I'm doing!

    When you say unstable, I assume you are referring to the visual display on the servoconfig software displaying any line that is not a straight one?

    I am a bit unsure of the actual shape of the plot I should be looking for as well, from what I've researched on the subject it should be a square up and back down kind of thing?

    I have attached a screenshot of the settings I have currently, which I believe are not correct at all but this is the only combination I got to that doesn't result in the motor oscillating back and forward.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	X axis Servo Config.PNG 
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ID:	28883  

  5. #95
    I always note the settings when i do stuff like this. Having said that, I also find no trace of said records when I need them. It might be helpful to be able to show you my final values but I can't find jack shit when I look.

    The response doesn't look too daft. Ideally you wouldn't get any overshoot or oscillation on this kind of step change. You aren't far off, although the numbers do seem a bit odd. The P value seems very low and the D seems high. The I seems way OTT. You also seem to have a bit of an offset left when the system has settled down. The I would take care of this normally but perhaps your very low P is an issue.

    Did you start with P, then dial in some D, then finally some I? It will start to wobble, oscillate, tremble etc if you overdo it and it can get a bit exciting on occasion. Back off about 20% or so to give yourself some margin before trying to dial in the next term. It's a bit of a black art!

  6. #96
    I started with everything at zero ( Ap, Ad, Ai, Li) then started with the Ap but anything past 200 made it oscillate like mad. Whereas on the other two motors I think they were around 1000-1500 figure.

    I ended up taking the motor off the machine in the end, I got a bit carried away with the settings at one stage and the machine got a bit of a jig on. Hopefully it didn't do any lasting mechanical damage! Having it off the machine shouldn't effect setting it up too much should it?


    I guess I will just have to keep at it, try increasing P and backing off the D and I?

    When you say it has a bit of an offset, I should be aiming to get the green line back down to zero (red line) and have as little oscillation before that as possible? I think I was getting confused by what I should be looking for. Researching PID tuning for the 100th time seems to be making it a little bit clearer now

  7. #97
    m_c's Avatar
    Lives in East Lothian, United Kingdom. Last Activity: 3 Days Ago Forum Superstar, has done so much to help others, they deserve a medal. Has been a member for 9-10 years. Has a total post count of 2,908. Received thanks 360 times, giving thanks to others 8 times.
    Tuning really needs to be done with the motor attached to the machine, as the weight/inertia of the machine essentially acts as a big damper.

    What Muzzer said about the basic PID tuning is the basic process.
    Increase P until things go unstable, and back it off a bit.
    Add some D which should then allow you to increase P.
    Then once you find the limits of P and D, if needed you then add some I to reduce position error, but it will make everything unstable, so you then probably have to reduce P and D to get things stable again.

    I'm not sure what other settings CNCdrive gives you, but once you've done the above on some controllers, you can then apply Feed Forward / filters to help further reduce any following errors.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  8. #98
    Ok nice one, I might get it somewhere near off the machine so it doesn't shake itself to bits, then mount it up again and start adjusting the tuning again.

    Thanks for the info on the tuning Muzzer and m_c, I watched a few videos on the process earlier as well and I now at least have a better understanding of what each value does to the tuning and how they relate to each other.

    I think the screenshot I previously attached of the settings is the only values you can change for the drives. There may be some other settings in UCCNC control software but I haven't delved into that too far yet.

  9. #99
    Odd that one motor goes unstable with Ap = 200 or so while the others manage 1000 or so. Perhaps you could swap a couple of them over to see if the problem stays with the motor.

    Unless you are shifting a massive (large mass and / or large force) load on one of them, they shouldn't be significantly different. Have you got similar encoder PPRs in the drive and similar steps per pulse in the controller? It's possible that could change the system gain enough to cause this issue - not sure how that would work without actually thinking it through.

    There are also a few parameters in there that I don't recall or understand immediately. At the very least you should ensure they are consistent for all the drives if you want to eliminate this strange behaviour. Perhaps a parameter reset would help.

  10. #100
    I've been playing with this again today with the motor off the machine still and got what looks to be a pretty decent (not perfect) tune.

    So I loaded up UCCNC just to jog the motor and see what it was like. When I jog the motor it will happily rotate in one direction but when I try the other way it is fighting itself and just judders a bit and doesn't really rotate at all....

    Does this indicate it may be a hardware issue rather than the tune?

    I will try the motor on another drive tomorrow and see what results I get to start ruling some stuff out.

    But could this be down to an issue with the motor or encoder? or does it still seem likely to be my lack of skills with the tuning software?

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