Inductive Limit Switch problem

[Kitwn]

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.

[Doddy]

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..

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.

[Kitwn]

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.

What gobsmacked me the most was the amount of noise before I ever put mains on to the VFD. It's enabling the motors that introduces all the trouble. And the fact that the same noise is on the ground of the BOB. I thought the screening and earthing was quite reasonable but noooooo!

One thing that all this does point out is how noise-resistant it is to use NC microswitches on the limits. Under normal operation all the limit inputs have a hard-wired, all metal connection short circuit on them. This is why I've never had any of this trouble before. Ironically I did briefly consider using miniature relays to act as the 12 or 24V to 5V interface but considered it unprofessional and was in fear of being laughed at by my forum fellows. Actually it would work a treat! I doubt if the variation in triggering delay would noticeably affect the accuracy of the homing and the low-pass filtering effect would be even better than your suggested opto-couplers!

Re the 'experience', which isn't really as great as you so kindly implied: When I was a trainee at the BBC, back when Boris Johnson had a poster of Prime Minister Thatcher on his school dormitory wall, one of the trainers gave a demonstration of how there's no such thing as an earth connection at radio frequencies. This was at the Daventry short wave station which is now long gone. We went out into the antenna field and he took a coin out of his pocket and dragged it at head height down the side of a steel support post which carried one of the feeders to an antenna. This post was about 20cm or so in diameter and solidly concreted into the ground. There were distinct, bright sparks between the coin and the post!

[Voicecoil]

What gobsmacked me the most was the amount of noise before I ever put mains on to the VFD. It's enabling the motors that introduces all the trouble. And the fact that the same noise is on the ground of the BOB. I thought the screening and earthing was quite reasonable but noooooo!

That doesn't really suprise me - I remember when I powered up my first PWM audio amplifier - which was switching roughly the same voltage and frequency as a decent microstepping drive - and the radio in the workshop promptly went off. Holding a 'scope probe 6" above the speaker leads gave a few volts of noise in thin air, I ended up using screened speaker cable :. Are the cables to your motors screened I wonder? As with the sensors when I extended my cables I cut them very short then extended with screened (2 pair) which might explain why I've not had any problems in a setup which seems somewhat similar to yours - I'm using basically the same type of sensor albeit in the M8 size. The motor cable screens I returned to the HT- pins on the drivers (always return your shit to where it's generated!) which then has a VERY SHORT ground lead down to the aluminium back plate of the enclosure which acts as a ground plane - being an RF man you'll appreciate the benefits of those.