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  1. #1
    Looking at my machine whilst wiring up the closed loop steppers and cursing the Chinese bods who don't use screened cable for the motor wires - WHY??? -there's 48V switching edges on those wires!!!!! - I suddenly thought why not put the driver modules on the machine next to the motors? That way all you have to run from the control box is low powered step/direction/enable lines which could easily live in a a bit of 26 AWG 6 core screened cable (about 4.5mm diameter) connected by M12 connectors and power which could be a few strands of 1mm2 wire twisted together*, or if you're flash some high power flexible coax like the Mogami 3082.

    * actually 2 x 2 x 0.5mm2 would be better on reflection.

  2. #2
    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.
    Denford never used shielded wiring to stepper motors, and it's never caused them any problems.

    Having the drivers at the motors is nothing new, but it's not something I'd personally want to do unless the step/dir were using differential signals.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  3. #3
    It's a cunning plan. There could be problems with dust and vibration for the drivers mounted on your gantry and don't forget that the power cables feeding the drivers, which will have to come from the control box alongside the control cables, will not have a smooth DC current flow, though you could add some filtering at each driver to reduce that.

    The biggest problem I can see is that you are replacing the nice short signal cables between your controller and the drivers by several metres of 'antenna' wiring which will have to run alongside the still noisy stepper driver power cabling and the VERY noisy spindle power cabling from your VFD. It might be case of frying pans and fires in spite of your best efforts at screening. But please don't stop thinking out loud!

    In my controller the break-out is mounted inside it's own screening box and has short screened cables running to the drivers leaving only about 20mm of exposed wiring. The output cables are deliberately routed away from the inputs to minimise coupling. I've cut the motor's own wires quite short and use screened cable to connect back to the drivers with all the cable screens connected only at the controller end. Limit switch inputs have 1uF capacitors across them to prevent noise-induced false triggering. I haven't had any problems since upgrading the controller to this design.

    Kit
    Last edited by Kitwn; 25-09-2019 at 11:25 PM.
    An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.

  4. #4
    Quote Originally Posted by Kitwn View Post
    It's a cunning plan. There could be problems with dust and vibration for the drivers mounted on your gantry and don't forget that the power cables feeding the drivers, which will have to come from the control box alongside the control cables, will not have a smooth DC current flow, though you could add some filtering at each driver to reduce that.
    Aye, I can see that dust etc. might be a problem and was thinking of a cover to minimise that.With a suitable cable (starquad or high current coaxial) I don't think the current flow in the power line would be too much of an issue.
    The biggest problem I can see is that you are replacing the nice short signal cables between your controller and the drivers by several metres of 'antenna' wiring which will have to run alongside the still noisy stepper driver power cabling and the VERY noisy spindle power cabling from your VFD. It might be case of frying pans and fires in spite of your best efforts at screening. But please don't stop thinking out loud!
    I was thinking of using a 3 x twisted pair screened cable running to the (Lichuan) drivers with have an iso-optalator input so are effectively floatting diff., so ought to be OK. But I'm not sure I can be arsed to undo all the "conventional" wiring I've already done
    In my controller the break-out is mounted inside it's own screening box and has short screened cables running to the drivers leaving only about 20mm of exposed wiring. The output cables are deliberately routed away from the inputs to minimise coupling. I've cut the motor's own wires quite short and use screened cable to connect back to the drivers with all the cable screens connected only at the controller end. Limit switch inputs have 1uF capacitors across them to prevent noise-induced false triggering. I haven't had any problems since upgrading the controller to this design.

    Kit
    Caps across the inputs are cool, been using them for years with great results.
    Last edited by Voicecoil; 26-09-2019 at 01:08 AM.

  5. #5
    Quote Originally Posted by Voicecoil View Post
    But I'm not sure I can be arsed to undo all the "conventional" wiring I've already done
    If it ain't broke, don't fix it!

    Kit
    An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.

  6. #6
    The machine design is all about balancing compromise. From an electrical power drive to the motor what you propose (locating the drivers close to the motors) is a good idea - you're removing parasitic resistance and impedance from supply lines to the motor - which will impact the performance of the motor at speed. You're having to balance this with driving the digital step/dir signals over a longer length of cabling, and these are more susceptible to noise, but it's not rocket science to switch these (non-linear) resistive loads of a few volts / milliamps at 200kHz - differential drive is nice but not necessary unless you have long cable runs and a high noise environment (and you're removing a good proportion of the emissive lines by removing the high-current AC signalling to the steppers). I'd invest in a heavier gauge power cabling to the driver.

    The other compromise is the mechanical aspects, and as others have said - vibration, dust etc. You are making life difficult for yourself here but this is just part of the overall design compromise.

  7. #7
    On my machine, and I suspect many others, the motor cables aren't nearly long enough anyway, so need extending. This being the case, my motor cables (i.e. the ones actually coming out of the motor itself) terminate in a little plastic junction box somewhere near the motor and therefore quite a long way from the "sensitive" cables (step/dir, etc) which are retained within the screened control box and use differential signalling anyway. Junction box to control box is via screened CY cable. My guess is that the combination of screening and, effectively, differential motor connection work to significantly reduce emitted interference. The long "signal" wires on my machine (limit switches and e-stops) use 24V for better noise rejection. The drivers also stay away from vibration and dust in the control box with a light waft of air from a couple of low-power fans to keep them cool.

    However, as Doddy says, engineering is all about compromise and there is a whole bunch of trade-offs to take into account. I went for the easy and most common configuration but that's not to say that in other cases, a different solution might not be better.

  8. #8
    Quote Originally Posted by m_c View Post
    Denford never used shielded wiring to stepper motors, and it's never caused them any problems.

    Having the drivers at the motors is nothing new, but it's not something I'd personally want to do unless the step/dir were using differential signals.
    I can see that as long as your limit switch wiring is well shielded then it shouldn't be a problem as far as the machine is concerned. However I'm conscious that I have some short lengths of the original unscreened cable of the limit switches in use (WHY don't they cable them with twin screened rather than the pissy miniature 3 core stuff???), and besides which I would like to be able to listen to the radio interference free whilst I'm working!

  9. #9
    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.
    Quote Originally Posted by Voicecoil View Post
    I can see that as long as your limit switch wiring is well shielded then it shouldn't be a problem as far as the machine is concerned. However I'm conscious that I have some short lengths of the original unscreened cable of the limit switches in use (WHY don't they cable them with twin screened rather than the pissy miniature 3 core stuff???), and besides which I would like to be able to listen to the radio interference free whilst I'm working!
    In my Triac, the only shielded wiring were the original spindle motor tacho wires, and one of the tool carousel sensors. Everything else is unshielded, and runs in the same cable tracks as power wiring.
    However, Denford always used 12 or 24V for control wiring, so noise should never be a problem.

    Given the amount of effort it takes to properly and reliably shield wiring, I'm surprised more people don't use higher voltage signal wiring.
    Avoiding the rubbish customer service from AluminiumWarehouse since July '13.

  10. #10
    Quote Originally Posted by Voicecoil View Post
    and besides which I would like to be able to listen to the radio interference free whilst I'm working!
    VHF-FM is likely to be more interference free than MW-AM and DAB better again. Having spent 21 years as a BBC Transmitter Engineer before moving down under I'd also like you to be able to listen to the radio interference free whilst you're working.

    Kit
    An optimist says the glass is half full, a pessimist says the glass is half empty, an engineer says you're using the wrong sized glass.

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