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Thread: Kit's Machine

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  1. #21
    Thanks John,,

    All cables, including the short fruns from the BOB to the steppers, the cables out to the motors and the returns for the proximity sensors are screened and grounded at a common point. The false trips I get are for a limit switch, the same problem I got when I first switched over from microswitches to proximity sensors. I ended up fixing this by using relays as isolators so that there was a dead short to ground for the BOB inputs at all times except when the limit is triggered. The BOB itself sits inside it's own screened box. I haven't been able to check if it's the BOB circuits that trigger or the noise is going direct into the parallel cable and upsetting the controlling PC.

    Investigations with an osilloscope make me sure the problem is leakage from the VFD mains wiring. The scope shows very spikey noise all over the earthed case of the controller, the PSUs inside it, the BOB supply rails and all the terminals on it as well when the VFD is running. Changing the timescale shows each pulse to be a burst of several cycles of about 6MHz. This could be a fast edge triggering ringing somewhere but I can't delve that deep. Using a more remote mains socket for the VFD fixes the fault, though I have not measured the change in noise inside the controller. Further investigation of the cause would be an interesting project but I don't have time for that, I just wan't the machine working! Fingers crossed the mains filter, possibly with some filtering on the earth lead as well, will sort it out.

    Have you been following the thread 'Why Star Ground?' at all? There's been quite a lot of discussion on there over recent days.

    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.

  2. #22
    When I first built my machine, using proximity switches, I had similar problems and immediately worried about noise issues. Turned out to be a completely different issue - lack of hysteresis in the switches. All the reading I had done suggested that there was significant hysteresis - having approached its trigger, the switch would operate but it would then need to be moved a measurable distance away before releasing. Definitely not the case with these cheap Chinese switches - hair-trigger stuff around the switching point. So, Z axis homes, Mach3 sets it back from homing to limit switch mode, and Y starts moving. The vibration was enough to trigger the Z (now limit) switch and everything stopped. Sometimes homing would work and the machine would trip when I started moving away from home position at the start of a cut. The fix was simple - the Mach3 setting that moves the axis just off the home position once homing is complete.

    I'm just glad that I didn't need to go hunting around with a 'scope, which I suspect throws up more worrying measurements than you really want to deal with!

  3. #23
    I've always moved all the axes away from the limit switches by around 10-15mm. This is fundamental to LinuxCNC so pulling all the axes away from the switches was a bit of a no-brainer before I really knew what I was doing. I'm sure we all have memories of things that had us confused for ages in the early days and are obvious now. This applies to any technical subject you are learning about for the first time. "How could I have ever NOT been able to ride a bicycle!?"

    Oscilloscopes and spectrum analysers were my day-to-day screwdrivers and spanners for many years so at the first suspicion of spurious signals appearing where they don't belong they are the tools I'm going to reach for. We each have our different specialist knowledge and experience and it is the variety of skills, experience and knowledge among the members of a forum like this that make it so valuable.

    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. #24
    I've fitted the mains filter and installed it in the machine. I can now power the whole beast including the controlling PC running LinuxCNC, controller with BOB, stepper controllers and motor supplies, water pump and VFD from a single mains socket. I haven't yet got the scope out to see how much noise is still visible on the controller wiring but at least it works!

    The filter is a standard, two stage filter from RS which I linked to earlier. I've also added a ferrite ring to the earth wire since this is a probable route for high frequency noise to escape.

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    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.

  5. #25
    So after all that the wretched thing tripped again after 5 minutes of behaving itself! Plugging the VFD into a different mains socket fixed it again... I hope.

    Maybe I'll try putting the filter on the controller and/or LinuxCNC PC mains input. Or maybe I'll take up sculpting portraits in clay instead where none of the tools need electrickery. Or maybe I'll make do with it as it is for now and, if this hobby ever does look like it may turn into a bit of a money-spinner, build a complete new reliable controller with an AXBB-E and learn UCCNC. I've got it running in demo mode on my laptop already. Looks OK to me.
    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. #26
    Quote Originally Posted by Kitwn View Post
    5V for the BoB and 12V for fans is provided by the original PC power supply.

    Attachment 28709
    Kit, you're one person on here that I really wouldn't want to teach how to suck an egg. But that picture of your BoB - firstly addresses on solution that crossed my mind - you're trying to smooth the supply to the BoB with that cap hanging off the PSU (I would suggest a low esr cap in parallel with the floating electrolytic - but there'll be enough of that on the board anyway)... so that's all good. But, the one thing that does worry me is I can't see any isolation on the BoB (in the form of opto-isolators). It could be a different package that I'm used to - but the geographic positioning of devices on the board doesn't appear to support that. You say you're using relays on the inputs - I can't see the detail of this in the figure, but assume you're using a NC or NO contact?, hopefully you have it that in the non-triggered state the input is hard-tied to ground? (i.e. not floating?). Have you the connectivity to use a C/O contact between 0V and 5V, so the inputs are hard-tied in both states? Also, I've experienced so much crap with proxy diagnosis of noise on BoB inputs that I wouldn't now entertain any without opto-isolated inputs. Not for the galvanic isolation, but simply it's good to have a device that needs proper current flowing to switch, rather than the usual HCT high-impendance inputs. From the UK I'd be throwing 6 quid at China for a cheap replacement BoB to trial.

    Again - I'm confident you know what you're doing, and you sound like you're addressing this top-down with removing noise from source, just worth conisidering going bottom-up and looking at the noise susceptibility on the signalling to the PC.

    UCCNC, btw, is the way forward :)

  7. #27
    Just for clarity, when you say "it trips," what exactly is happening? Limit switch trigger, e-stop trigger, or what? Always keen to learn from other's experiences!

    My own background started off in electronics - building amateur radio stuff as a teenager - but after university and a year as apprentice with Marconi, I switched to IT for the rest of my career. Just glad I've never needed to take an oscilloscope to my machine... Did once take a look at the ripple on the linear PSU output but out of curiosity rather than need!

    Good luck!

  8. #28
    Doddy, Neale,

    Thanks for the words of encouragement and I'm open to any advice anyone wants to offer! More decoupling of the BOB power supply is worth looking at but the VFD noise is all over everything so there's something more fundamental at work here.

    The problems I've always had with this machine show up as a limit switch trip on the Z axis. It all started when I changed from microswitches to proximity sensors. Oscilloscope measurements showed lots of spikey noise on the limit inputs of the cheapo Chinese BOB but I ruled out the individual proxy sensor early on. There's a whole thread about this from last year. http://www.mycncuk.com/threads/13519...-problem/page2 post #20 explains what I discovered about the input circuits to the BOB.

    The solution then was to use electro-mechanical isolators. Sounds much better to young ears than 'relay' but that's what I used. Miniature relays hold the inputs solidly to ground except when the proxy sensors trigger. Excelent noise supression and low-pass filtering in one simple package. The relays and driver transistors are mounted on a board which sits just above the BOB with only a few mm of wiring between them.

    There is only any noise present when the VFD starts and the noise is all over everything. The earthed box of the controller and all it's contents including the BOB power supply rails and the earthed cases of the motor PSUs as well as the BOB inputs/outputs show it. I even tried shorting the scope probe to it's own earth screen clip and you can still see the noise when the earthed probe touches anything metal! It's a PC scope and I use it on my laptop running on batteries so the scope is entirely isolated from ther mains.

    last year I rebuilt the controller with the BOB in it's own ally box with screened cables for all signals in and out and individual screened cables for each proxy sensor and each stepper motor. I really thought I'd done evrything right and it was working OK until the recent rebuild. The noise is very short pulses of about 1uS each. Closer inspections shows each pulse to be something ringing at about 6MHz for that short period. It's not just the VFD output waveform to the spindle windings and I'd love to find out what it is but I'd much rather be making stuff with the machine than fault-finding it again.

    I really thought the mains filter would fix it, especially after plugging the VFD into a different mains socket fixed everything but it seems the filter doesn't work as reliably as a few metres of 3-core wiring. The only other possibility is that particular input of the parallel port into the PC is dodgy but that seems a long shot, though I'm convinced it is not spurious triggering of the solidly earthed limit switch input so something else is triggering that specific port input to the PC. It's always the same one.

    Sorry it's ended up such a long rant. Today the LinuxCNC PC decided to throw a fit as well, so that now has a new system battery and a re-seated CPU. I think it may be telling me something. I think it might be yearning for a one way trip to the dustbin. It's working again at the moment but I'm not going to put too much more time into this current controller if it plays up again.
    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.

  9. #29
    I have also found these type of bobs as solid as a rock. They do need 5V and 12-24 volt attaching to them


    From AliExpress they can be got a lot cheaper
    The more you know, The better you know, How little you know

  10. #30
    Kit, your last response does concern me - if you remove the "electromechanical galvanic isolation module" from the Z-limit input to the bob, and simply connect this input to the local 0V, is it still noisy? If so you're down to BoB/Cable/PC.

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