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  1. #21
    OK...so I've tried the motor on a different drive - no joy. Then tried a different motor on same drive and still have the problem. I only have 3 motors, all bought at the same time. Will try the 3rd motor tonight but am not holding much hope...

    I've played around with various settings (acceleration, max velocity, etc) but had no joy there, either. I then tried changing the gear ratio in software to 6:1 and had no problems as the motor runs faster even at low feed rates. The question is - should the motor be smooth throughout the range or will there be inherent "bad" spots due to the Chinese drives and motors? I'm convinced this is a stepping issue that causes resonance but only at low speeds. If I increase the step rate to 3200, the drive sounds much "cleaner" but I get a "realtime" error in LinuxCNC. What step rate is "normal" for this hardware?

  2. #22
    Slides 22 onwards in this document give a good explanation of how resonance affects machine frames. If you read that then what is happening here should be obvious. The other lecture notes in that series are well worth reading too.

    Quote Originally Posted by birchy View Post
    The question is - should the motor be smooth throughout the range or will there be inherent "bad" spots due to the Chinese drives and motors?
    You can't blame this on the Chinese (or in John S's case linux), since the Chinese also sell drivers with resonance damping functions (e.g. AM882). That doesn't mean you need to change drivers though. I'd try testing cutting something at the speeds where it sounds bad and see if it affects the finish noticeably. Also, thoroughly test the motor tuning, e.g. by running random G-code. If it doesn't stall, goes fast enough and gets an acceptable finish then you can safely ignore the 'problem'.

    At what step frequency do you start getting the real time error? Unless you're very unlucky with the computer it should be fine around 25kHz - quite likely more.
    Last edited by Jonathan; 19-11-2013 at 11:59 AM.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  3. #23
    Quote Originally Posted by Jonathan View Post



    You can't blame this on the Chinese (or in John S's case linux),
    Hey golden bollocks where did I say it was Linux ?
    John S -

  4. #24
    Jonathon, I understand the basics of machine structure and resonance and that I may need to add dampers at some point, however the motor itself is noticeably vibrating at very low speeds when it isn't even bolted to anything. The machine castings and framework are obviously amplifying the vibration but I think that will always be the case until the motor is made stable.

    Regarding the drives and motors, it wasn't a case of "I blame the Chinese", the question I was asking is could it be a problem due to the low cost setup? At the end of the day, you get what you pay for, etc, etc. I bought the motors, drives, BOB and PSU as a 3-axis kit from Zapp a few years ago. If I was buying again, I'd probably buy the parts individually via eBay or AliExpress but go for better quality. Having said that, there seems to be plenty of others using the same setup and not having any issues.

    Jonathon - I believe you use (or have used) LinuxCNC? Have you used it with these motors and drives? If so, maybe you could share a known working .ini file? The LinuxCNC latency/jitter test on my PC settles at 15600ns (with CPU at 100% load), so I have this set at 16000 in stepconf. I'm guessing this may be causing the RTAPI error but don't yet know enough about the system to understand what's going on.

  5. #25
    Quote Originally Posted by birchy View Post
    Jonathon, I understand the basics of machine structure and resonance and that I may need to add dampers at some point, however the motor itself is noticeably vibrating at very low speeds when it isn't even bolted to anything.
    The motor is entitled to resonate when not connected to anything. The motor is after all a mechanical system, which will have an associated resonant frequency.

    Quote Originally Posted by birchy View Post
    Regarding the drives and motors, it wasn't a case of "I blame the Chinese"
    I wasn't being entirely serious there!

    Quote Originally Posted by birchy View Post
    the question I was asking is could it be a problem due to the low cost setup? At the end of the day, you get what you pay for, etc, etc. I bought the motors, drives, BOB and PSU as a 3-axis kit from Zapp a few years ago. If I was buying again, I'd probably buy the parts individually via eBay or AliExpress but go for better quality. Having said that, there seems to be plenty of others using the same setup and not having any issues.
    Better drivers can help suppress the resonant effects induced by stepper motors, as I mentioned earlier. However that doesn't necessarily mean you need better drivers - if the system works, even with the motor vibrating, you might as well just use it.

    Quote Originally Posted by birchy View Post
    Jonathon - I believe you use (or have used) LinuxCNC? Have you used it with these motors and drives? If so, maybe you could share a known working .ini file? The LinuxCNC latency/jitter test on my PC settles at 15600ns (with CPU at 100% load), so I have this set at 16000 in stepconf. I'm guessing this may be causing the RTAPI error but don't yet know enough about the system to understand what's going on.
    Yes I use LinuxCNC. The motors on my CNC router are the same and the drivers are PM752 (like you, I bought them from Zapp a long time ago). I'm currently not at home, so can't check what settings I use, however that shouldn't be necessary. The value you've set to 16000ns in stepconf is the one which determines the kernel frequency and thus determines if you get the real time error. I'd find out what the minimum you can set this number to is without the real time error occurring, then add a bit just to be sure.
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

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  7. #26
    Thanks Jonathon. I would of gone for the 70V setup if I was buying again, but this 42V setup *should* be OK. I'm going to re-run the latency test as I have a wireless USB keyboard connected now (which I didn't have previously), so that may be causing issues. Attached are screenshots of my stepconf. Do these numbers look OK to you? I'm not sure about the timing settings of 5000 as I can't find any actual values in the drive manual. There are a few "542" type drives on this page, but they all have drastically different timings: LinuxCNC Documentation Wiki: Stepper Drive Timing. Again, I don't have enough info to get these set correctly...

    Out of interest, what step rate do you guys normally set your 542/752 drives at? 800, 1600 or some higher value? Don't forget I'm a newbie, so am just wondering what range of settings are considered "normal".

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  8. #27
    Quote Originally Posted by birchy View Post
    Do these numbers look OK to you? I'm not sure about the timing settings of 5000 as I can't find any actual values in the drive manual.
    Yes, 5000 works for me. You should be able to use much higher acceleration than 50mm/s^2. Around 500mm/s^2 is probably a good starting point.

    Generally 'guys' normally don't use any finer than 1/8th microstepping, so 1600 step/rev. Search 'microstepping' on the forum and you'll find plenty of explanation...
    Old router build log here. New router build log here. Lathe build log here.
    Electric motorbike project here.

  9. #28
    Hi Birchy,

    Not tried it myself as don't have the noise problem you are experiencing but others have tried adding inertia to the stepper via flywheels etc. and achieved a much smoother low speed running. If you have a shaft from the rear of the stepper you can add one there, or otherwise bolt it to the front face of the pulley (tap holes in pulley face).

    Solving Stepper Motor Resonance Problem on CNC Lathe - YouTube

    I would suggest adding the lowest inertia possible to just remove the problem, otherwise acceleration of the stepper will suffer too much. The one in the video looks a bit big - you could do some calcs, or just try something to at least see if that would help before refining the flywheel design iteratively.
    Last edited by routercnc; 19-11-2013 at 10:08 PM.
    Building a CNC machine to make a better one since 2010 . . .
    MK1 (1st photo), MK2, MK3, MK4

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  11. #29
    What about maximum velocity, Jonathon? Is 50 too high? Too low? Or somewhere near?

    Interesting use of the counterweight there. I was thinking that I need to add a wheel of some sort so I can manually turn the motor if one of the E-Stop limit switches is activated. Two birds, one stone...

  12. #30
    This is probably nothing to do with it but I'll mention it anyway.
    When I first powered up my system, 4 x AM882 and CNC4YOU Nema23 4Nm motors, it sounded like a load of whales singing from one particular driver and the motor was noisy when running. I thought it was goosed so I tried another motor on it with the same result. There is a utility with the AM882's called Protune and I had been playing with this connected to one of the drivers via a special cable (connections available on Leadshine website to make one). I therefore connected the noisy driver to Protune and ran the Auto Current Tuning section, immediately no more whale song and motor nice and quite when running. Without looking at the manual for your drivers I don't know if this utility is relevant but it shows that something on the current side was having a huge effect on motor noise.

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