Problem with Y axis, need help!!


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Thread: Problem with Y axis, need help!!

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    Default Problem with Y axis, need help!!

    Hi Guys, I was working on my mill today and just after starting a job i thought i heard the Y axis make a bit of a rumbling noise but couldn't say for sure as i had ear plugs in. Everything seemed fine again and then a few minutes later after drilling and tapping about 30 holes, it toolchanged to a different tap and proceeded to try and tap a hole nearly 1/4" off center. So it broke the tap and messed up the part and obviously now i have some kind of feedback problem.

    After this, I made some rapid moves in the Y axis, moving a few inches and then back to zero again and it seems fine at low rapids, but when you increase speed it sounds rough and loosing position slightly. I didnt do too much testing here as i was very fearful of a runaway. So initially i'm thinking this sounds like a loose encoder coupling or bad encoder, but I've checked coupling and its perfect. I have some spare encoders so i tried a whole new encoder and same problem. I've checked all the connections on the encoder cable, and traced the full length of the cable and everything is perfect. The Y axis cable is stationary and completely free of coolant too so unlikely to cause too much trouble anyway.

    So starting to get a little nervous now. What else could I look for? Could there be an issue with how the pulses are read by Kanalog? Im guessing this is very unlikely. Is it worth my while to try moving the connections to a different encoder input? I do have one spare set of inputs i think. Anything else i can try? Thanks!

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    Default Re: Problem with Y axis, need help!!

    Hi Mark,

    If the machine moves to the wrong place and doesn't disable with a following error then the encoder's Position must be incorrect (assuming a reasonably small Max Following Error setting).

    You might use the Step Response Screen at a speed that fails and plot the result. That may provide some clues.

    You might measure the encoder A+ A- B+ B- signals at Kanalog to make sure they are all switching to valid voltage levels and that none of the 4 are floating.



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    Default Re: Problem with Y axis, need help!!

    Sounds more mechanical than electrical.
    Have you checked all the mechanics?
    If it's driven via belt, a stripped belt could cause those symptoms. Even just a loose coupler may cause similar problems, although couplers are more likely just to slip continually.



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    Default Re: Problem with Y axis, need help!!

    A little update guys, got some good news and bad, I powered up the machine again last night and everything is back to normal. Still have the spare encoder on there. So everything is working fine now. But the bad news is I have no idea what caused it and i'm terrified it could happen again any time. I ran a test program again last night with repeated Y axis moves and everything was fine, sounded great at all speeds. And i went ahead and ran 2 more parts with the machine and they cut fine. I did keep the rapids dialed back. The only thing i did different since was look over every inch of the cable again for damage (looks perfect) and I wiggled the green terminal block on the kanalog board, pulled it half way off and pushed back on. Could there possibly be a bad connection between this terminal block and the pins on the board? Anybody ever have any issues with these? Guessing this is very unlikely as well, but i cant think of anything else? Should i replace the entire cable as a precaution? This is a fairly large machine, a runaway at a bad time could cause a lot of damage.

    Tom, yes following error is set at 1500 counts or about 0.012" and it didnt disable so I must have lost position somehow. I will try the step response screen next time, thats a good idea. I measured the voltage across each a/b pair and im getting 3.59 from my X axis, 3.35 from my Y and 3.65 from Z. Interesting that the Y is lower then the others. This is with the replacement spare encoder I have on there. Not sure what the original was. Also noted my spindle encoder at 3.1v and this reads fine. What should i be seeing here? does this sound ok? Do I also need to check to ground?

    Moray, the servos are direct drive with fairly heavy duty couplers between servo and ballscrew. I had a little backlash here before so i went a little overkill and added a second setscrew and loctited everything. They are keyed as well so even if loose I should only be seeing a little extra backlash instead of something like this. Also checked encoder couplings and it is solid.



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    Default Re: Problem with Y axis, need help!!

    Hi Mark,

    I assume you measured the encoder voltages across the + and - pins? And those were sometimes positive and sometimes negative? I normally check each pin relative to Kanalog GND that they switch between ~0.4V and ~3.4V providing like 3V differential. The threshold is only like 0.4V.

    I haven't heard of any issues with the connector pins.

    Sounds like most likely the cable. You might put the machine in the air moving Y back and forth and wiggle and pull on the cable to see if you can cause a problem.

    Maybe tighen up the Max Following Error to have a better chance of disabling if it starts to "rumble" again.



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    Default Re: Problem with Y axis, need help!!

    Quote Originally Posted by TomKerekes View Post
    Hi Mark,

    I assume you measured the encoder voltages across the + and - pins? And those were sometimes positive and sometimes negative? I normally check each pin relative to Kanalog GND that they switch between ~0.4V and ~3.4V providing like 3V differential. The threshold is only like 0.4V.
    Hi Tom, Yes that was across positive and negative, i guess some were negative, i didnt really take notice. I just checked voltages to ground and im getting:

    X 3.77, 0.17, 0.17, 3.77

    Y 3.65, 0.27, 0.27, 3.65

    Z 3.79, 0.13, 3.79, 0.13

    So you say the threshold is 0.4 volts? So does this mean im kinda close on the Y axis here? If this is questionable maybe i should switch encoders again and see what the origional was? Possible I have 2 boarderline encoders?

    I haven't heard of any issues with the connector pins.

    Sounds like most likely the cable. You might put the machine in the air moving Y back and forth and wiggle and pull on the cable to see if you can cause a problem.

    Maybe tighen up the Max Following Error to have a better chance of disabling if it starts to "rumble" again.
    Ok good to hear about connectors. I did a repeating program of Y moving and wiggled and pulled at the connectors and all over the cable and nothing showed up. Was starting to gain a little confidence again and was setting up another job and before running it I ran my test program again and "think" i heard the noise again for a second Maybe it was just my mind this time, not exactly sure.

    I will reduce following error, thats a good idea. I wonder how low i can go without nuisance trips? Servo gain is P 2. I 0.025 and max output is 1000. So i guess max output would be at 500 counts ignoring I gain. Would this be a good place to start? Or even less? Thanks.

    Edit: I tried following error of 100, planning to work upwards and it actually runs like this! Machine ran fine for last hour or so. Amazing, thats an error of less then a thou and its able to do rapids at 100% speed and fairly heavy cuts without giving a following error. Hopefully this will catch an error the next time it happens.


    Mark

    Last edited by mmurray70; 10-28-2018 at 09:19 PM.


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    Default Re: Problem with Y axis, need help!!

    Hi Mark,

    X 3.77, 0.17, 0.17, 3.77

    Y 3.65, 0.27, 0.27, 3.65

    Z 3.79, 0.13, 3.79, 0.13

    So you say the threshold is 0.4 volts? So does this mean im kinda close on the Y axis here? If this is questionable maybe i should switch encoders again and see what the origional was? Possible I have 2 boarderline encoders?
    That all looks perfectly correct. The difference between the + and - voltages must exceed either +0.2V or -0.2V to switch the output state. In your case of 3.65 - 0.27 = 3.38 far exceeds the necessary +0.2V

    I will reduce following error, thats a good idea. I wonder how low i can go without nuisance trips? Servo gain is P 2. I 0.025 and max output is 1000. So i guess max output would be at 500 counts ignoring I gain. Would this be a good place to start? Or even less?
    You can't really determine the following errors from the gains. The best way is to use the step response screen and do worst case moves and observe the maximum errors, then set a limit a bit higher.

    Regards



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    Default Re: Problem with Y axis, need help!!

    Quote Originally Posted by TomKerekes View Post
    Hi Mark,

    That all looks perfectly correct. The difference between the + and - voltages must exceed either +0.2V or -0.2V to switch the output state. In your case of 3.65 - 0.27 = 3.38 far exceeds the necessary +0.2V
    Ok I understand what your saying now. Great this looks good.

    You can't really determine the following errors from the gains. The best way is to use the step response screen and do worst case moves and observe the maximum errors, then set a limit a bit higher.

    Regards
    See edit in the last post. I tried setting it really low, planning to work my way up to the point where it would run, and its actually running with a following error of just 100 counts or 0.0008". Hopefully this will catch it if it messes up again.

    Also FYI, i tried setting it at 10 just to be sure it would actually disable and it does. But gives me a G-code error message when it happens? I thought back in a previous version it actually said following error exceeded. Not a big deal but just thought i would mention it.



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    Default Re: Problem with Y axis, need help!!

    Hi Mark,

    Also FYI, i tried setting it at 10 just to be sure it would actually disable and it does. But gives me a G-code error message when it happens? I thought back in a previous version it actually said following error exceeded. Not a big deal but just thought i would mention it.
    You should see this:

    Problem with Y axis, need help!!-axisdisabled-png

    Do you see something different?



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    Default Re: Problem with Y axis, need help!!

    Yes thats what I got. I guess thats the way its meant to be. Sorry I sort of thought that at some point in the past I got an error that actually said following error exceeded but i guess not.

    Ill keep you guys updated if i have any more trouble with this. Thanks again for the help!



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Problem with Y axis, need help!!

Problem with Y axis, need help!!