Inductive Limit Switch problem

[Kitwn]

Voicecoil,
I thought I'd been quite careful with the wiring: the BOB is in it's own aluminium box, the outputs to the drivers are 4 core screened, the cables from the drivers to the motors are screened, the PSU for the BOB (it's the original ATX supply in the PC case that is my control box) is a separate unit from the motor PSUs, the VFD is separately mounted on the wall, the spindle power cable is proper screened cable designed for the job.

The weak point is the sensor wiring which was not all screened cable for the old microswitches and has not been totally replaced yet, but will be soon. All the grounding of every component in the control box will need to be looked at again as well. No going down to the pub this evening for me!

[Voicecoil]

Voicecoil,
I thought I'd been quite careful with the wiring: the BOB is in it's own aluminium box, the outputs to the drivers are 4 core screened, the cables from the drivers to the motors are screened, the PSU for the BOB (it's the original ATX supply in the PC case that is my control box) is a separate unit from the motor PSUs, the VFD is separately mounted on the wall, the spindle power cable is proper screened cable designed for the job.

The weak point is the sensor wiring which was not all screened cable for the old microswitches and has not been totally replaced yet, but will be soon. All the grounding of every component in the control box will need to be looked at again as well. No going down to the pub this evening for me!

Well, you've done a thorough job then, strange that you're seeing so much induced noise - unless it's induced pickup of magnetic field radiation?. It will be interesting to see what you come up with as a fix.

[Kitwn]

Well, you've done a thorough job then, strange that you're seeing so much induced noise - unless it's induced pickup of magnetic field radiation?. It will be interesting to see what you come up with as a fix.

Hadn't considered magnetic induction, probably not the cause at such a high frequency, but I'm willing to be proved wrong. It wouldn't be the first time!

I've still got some investigation I want to do. The controller is connected to the machine by screened multicore cables which have plugs on the end which fit into sockets on the machine itself. The two X axis sensors and the Z&Y sensors have one 8-way for each pair. I want to disconnect the cables and see if that makes a difference. This will help to decide if the noise is being picked up in the wiring within the machine itself where the Z and Y sensor cables run through the same drag chain as the motor wiring or whether the problem is within the controller itself. Both will be dealt with anyway but knowledge is never wasted. I'm also tempted to use the technique I learned from the video that Cube3 linked to to look at the noise on the motor cables with my laptop oscilloscope. But 6MHz???
While I wait for RS to deliver the bits required I'm doing some necessary tidying up. There isn't much room in my workshop with the machine itself pulled off the wall to get at all the wiring (note for new builders: If you have to put your machine up against a wall, do NOT put the drag chains and all the gantry wiring on the 'wall' side of the machine in order to keep it conveniently out of the way). The only place to put the trolley with the PC on is in the doorway to the rest of the shed (sorry, that should be 'studio' if my wife is listening) if I want to leave it all connected so the picture below had to be taken through the window!

[Voicecoil]

Hadn't considered magnetic induction, probably not the cause at such a high frequency, but I'm willing to be proved wrong. It wouldn't be the first time!

The weirdest things can happen with EMC issues, at 6Mz a couple of bits of wire running parallel for a few m probably makes a half decent transformer

Voicecoil,
No going down to the pub this evening for me!

No chance of that for me either - 'coz of the virus they've been shut here for over 2 weeks

[Kitwn]

The weirdest things can happen with EMC issues, at 6Mz a couple of bits of wire running parallel for a few m probably makes a half decent transformer

This picture shows the output coupling transformer of a Marconi BD272 AM short wave radio transmitter rated at 250KW (that's a whole Megawatt peak output). I cut my teeth on these things in the 80's. All that power is being coupled through this single turn, air spaced transformer. These are the coils for 15MHz I think, but the 6MHz ones are not much bigger. The peak voltage across the primary turn is about 40KV even though it looks remarkably like a short circuit to the uninitiated.

No chance of that for me either - 'coz of the virus they've been shut here for over 2 weeks

Same here. The local microbreweries are doing canned takeaways though, so I can still enjoy my favourite local brew at much less than the in-a-glass price they charge if you sit on an old wooden chair in their gravel yard getting swamped by midgies and other people's children. You have to look on the bright side!

Western Australia has now closed it's borders to all non-essential travellers and travelling between different regions within WA without a good reason can cost you $50,000.

[Voicecoil]

This picture shows the output coupling transformer of a Marconi BD272 AM short wave radio transmitter rated at 250KW (that's a whole Megawatt peak output). I cut my teeth on these things in the 80's. All that power is being coupled through this single turn, air spaced transformer. These are the coils for 15MHz I think, but the 6MHz ones are not much bigger. The peak voltage across the primary turn is about 40KV even though it looks remarkably like a short circuit to the uninitiated.

Thanks for posting that - awesome bit of kit even if it looks completely illogical to the uninitiated!

[Kitwn]

Thanks for posting that - awesome bit of kit even if it looks completely illogical to the uninitiated!

This machine is 1960's technology. Some of the later, higher power designs, have water cooled the coils. A key difficulty with those was how to clamp the plastic water pipes onto the copper coils. Metal jubilee clips glowed white hot as soon as we switched the bloody thing on and plastic clips went brittle in about a week. We ended up using waxed cotton whipping cord, similar to to an old fashioned cricket bat handle. It's surprising the skills I had to learn as a BBC Transmitter Engineer back in the days when it was still fun.

[dazp1976]

I get noise when my stepper motors enable. It affects my FM radio.
Now.. I have no switches and all circuits etc in a metal control box. The noise is from my estop (un-screened cable in use).

[Kitwn]

I get noise when my stepper motors enable. It affects my FM radio.
Now.. I have no switches and all circuits etc in a metal control box. The noise is from my estop (un-screened cable in use).

Have a look at that video Cube3 posted on page 1. Ferrite rings should help you there. You can wind the cable through the ring to increase it's effect.

[Kitwn]

Embarrassing but I must own up: I realised during my tests that for all these years I've left the 36V supply for my stepper motors floating. Neither rail was grounded so the whole supply was bouncing up and down by 8 Volts at several KHz. It's a miracle the machine worked at all! Any way that's now fixed and some of the bits from RS have arrived, though not the screened cable for re-wiring the sensors yet.
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Fortunately I have access to the highly sophisticated electronics test laboratory shown below and have been able to make some very revealing, noise-free measurements on the spare cheap Chinese breakout board. Earlier reported voltage measurements were made in the presence of noise and are not reliable.
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With an input fed to the board header rather than the screw terminals, a falling voltage must drop below 0.6V to set the output of the board. A rising voltage must go above 1.7V to reset it. This gap is the hysteresis we expect from a Scmitt trigger input chip and provides some noise immunity.
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When using normally closed (NC) microswitches as sensors the inputs are a good solid short circuit for most of the time and only open briefly during homing. Excellent noise immunity is obtained in this configuration as proved by the fact that my machine has worked reliably for a few years!
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Using proximity sensors presents a problem, even with opto-isolators or other electronic interfacing, since the saturated collector-emitter voltage at the output of the proximity detector or from an isolator when triggered is about 0.6V. No guarantee of reliable triggering here even without noise. The use of a potential divider circuit as described earlier is an option but I've decided to use a more direct and low-impedance option... reed relays. These will be fed by an NPN emitter-follower transistor which serves to boost the current available from the proximity sensor.
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Some will scoff no doubt, but it actually makes sense for a few reasons:
I know from past experience that a solid short circuit on the inputs works reliably, even in the presence of high noise.
Low-pass filtering is excellent and requires no additional components.
When wired as I plan to have them, with the 12V from the sensor holding the relay contacts closed as the normal state (this mimics the previously known-good NC microswitches), the circuit is proof against wiring faults as any breaks will trip the limit input.
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Regarding positional accuracy, a key point of using proximity sensors is improved repeatability over microswitches. The reed relays I'm going to use have a specified response time of 0.5mS. Allowing for, say, 20% variation that gives a repeatability of within of 0.1mS. I can't remember exactly what my second approach speed is set at for homing in LinuxCNC but if it's even as high as 1m/min that equates to a variation of around 2 microns. Which equals nothing.

The 4 relays are $20 the lot from RS, the transistors and a wee bit of veroboard are existing stock worth much less. The whole circuit has been tested on the bench ( I have 2 relays in stock already) and works a treat. So I might just have a fully working machine again well before Mark McGowan let's me back in the pub!