Thread: Inductive Limit Switch problem
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03-04-2020 #31
I've got a crappy £5 board on my benchtop machine and it works well for me but I only have the estop connected no physical limit switches as yet.
Haven't figured out my spindle wiring either, still doing that manually
1 question from me is that,. If I have a NO estop I'll need NO limit switches yes? Or does it not matter if estop is NO and limits are NC?
I plan on getting an ess in the future and use one board running axis/spindle and one board running estop/limits.
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03-04-2020 #32
You think it works well because you have nothing to compare it against.! When you get the new controller with higher pulse freq etc you'll see the difference.
Regards the N/O question then provided you use separate inputs then you can use different switch types. Obviously if wiring in series and sharing the same input they must all be the same type.
Also if your thinking to buy Warp9 ESS then I'd look at others like UC300 because they are better than ESS and you are not stuck with mach3/4.
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03-04-2020 #33
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03-04-2020 #34
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03-04-2020 #35
Well this has been an interesting day! I think the moral has to be: If you've got the test gear, use it!!
I did some quick mods in line with previous suggestions but to no avail. I was soooooo confident as well.
I finally got the oscilloscope out and looked at the noise on the limit switch inputs to the board. Clean as a whistle until I enabled the motors. Then there are short bursts of a surprisingly sinusoidal 6MHz waveform. Same on the 5V power rail. Same on the 5V ground rail!! I thought all the earthing was OK, Could be better, but OK. Clearly the wiring to the sensors needs to be completely replaced. At the moment it's modded from what was there for the microswitches. Obviously not good enough.
Problem is at that frequency a 20cm length of earth wire is too long. I measured 400mV (peak-peak) across the two ends of a wire les than that length from the chassis to the 5V ground connection on the board. The real killer comes from switching on the mains to the VFD. There's a transient on the Z sensor input that goes from 4.5V to below zero. It only lasts about a tenth of a microsecond but that's long enough. Obviously I'm not switching the VFD mains on and off while the machine is in use but it indicates there's a problem in need of fixing.
I did take a squillion screen grabs of the waveforms to show off with but the fact is that the controller needs to be rebuilt with more care taken over the earthing and the sensor cabling needs to be replaced with a separate, unbroken screened cable from each sensor (existing unscreened cables cut short as advised earlier). I suspect some of the wiring and existing filtering attempts are ringing and I doubt that even the most expensive new control board could be relied upon to work faultlessly in this electrical environment, so I'll stick to my well used $20 unit for now, as well as the old computer and parallel interface. Perhaps a complete new machine will materialise once I've retired. Whenever that turns out be now that the world has turned upside down.
I now have to wait for RS to deliver the goodies required for the improvements so will have to try being patient for a few days. Maybe I should try designing that low cost DIY optical fibre interface that would allow the motor drivers to sit next to the motors and no long wires through the machine except power rails. Well we must keep the old brain busy during these long spells stuck at home. Does anyone know if the professionals have moved to fibre connections for motor controllers and/or limit switches? It would solve a lot of problems.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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03-04-2020 #36
The first time I attached a scope to my spindle encoder on the lathe my jaw hit the floor with the noise from the 3-phase motor, so I appreciate where you're coming from. I'm surprised you're seeing that behaviour on short cables though comms theory was never my strong point - I'll believe your experience over my memory any day. I still think using cheap (or expensive!) BoBs with opto-isolated inputs offers a lot of advantages, if only for the pathetic frequency response that will filter any amount of HF noise as well as providing much better noise immunity level than HC-series logic.
In my case (the spindle encoder) - the design of the 7i76e Mesa card is intelligent and offers a differential input for shaft encoders with 130R input, which allows easy connectivity to a RS485 driver to allow transmission over a balanced twisted pair - helped massively. I appreciate this doesn't work in your case.
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03-04-2020 #37
Of course, most machines are massive lumps of cast iron and steel cabinets which form a good hard ground and provide a lot of shielding into the bargain. In contrast, if you are constructing a relatively spaced out router(?) using extruded sections with long lengths (high stray inductance), you lose many of those benefits.
For my machines, I've used Lapp Olflex shielded drag cables, which have shielded bundles within a shielded outer braid. I noticed that the original wiring in my Shizuoka (Matchmaker CNC system, using Parker hannefin servos etc) used simple screened multicore cables for stuff like the encoders but simple unscreened wires for most of the rest (limit switches, solenoids etc). The spindle motor and brushed servos were wired through flexible, grounded steel conduits, which would have contained a lot of the HF noise. I reused that system when I swapped out the electronics and it's been very well behaved.
I have the Yaskawa VFD in the same cabinet as the rest of the electronics but I bought the pukka Yaskawa-Schaffner EMC filter to go with it. I've not had any issues with that either.
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03-04-2020 #38
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03-04-2020 #39
Thanks. There's eperate connections for estop and each axis limits. 2 switches will be wired in series per axis.
In regard to the controller in the u.k. I can easily get an ESS or a UC400eth but the 400 has limited ports.
I've found a UC300eth-5lpt in Ireland and that's about it.
I'll still end up using the cheap boards on them though but that shouldn't matter too much, should it.
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03-04-2020 #40
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