Inductive Limit Switch problem

[Doddy]

No scoffing from this direction.

I'm somewhat surprised with the Vce(sat) for the opto, but if that's what you've measured then that's what it is. Personally at that stage I'd be using change-over contacts for single-switched inputs and using NC=gnd, NO=5V just to yank that signal line to one rail or the other (nothing left floating).

For the nay-sayers, what Kitwn's post is providing is belted-and-braced robust and deterministic behaviour. There's too many problems in the world to save pennies on uncertainty on our machines.

Perhaps I'm feeling humbled after a day's worth of trying to get my control box switching automatically between two machines... and making some stupid assumptions... right, where's that damned scope gone...

[cropwell]

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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I am just rewiring my control panel and realised that my 48V to the drivers was also floating.

[Kitwn]

Muzzer, Neale,

My original issue with this was in regard to noise getting into the BoB supply which appeared to have been a major part of my problems. It seemed obvious at the time, but I take the points you both make. As long as the BoB supply and the motor supply are completely separated and the cable screens effectively grounded it should not be an issue. I have my machine working now and am keen to start using it to make things but I thing the next time I decide on a significant upgrade it will have to include a complete rebuild of a new controller in a better enclosure with a far more comprehensive approach to noise screening.

The clamp-on ferrites look like an excelent idea and I'll order a bagful.

Rob,
As you'll see from the above replies, you shouldn't be changing that anytime soon. My reasoning lacked the rigourous engineering logic it should have had.

[Kitwn]

My standrad reply is "I've spent 40 years working in high RF fields and it nev, nev, nev, never did me any harm!" On one site I worked at you couldn't turn the flourescent lights off in the transmitter building. They wouldn't strike on their own but once lit they wouldn't go out until the maintenance break . They flickered in time to the program so you could set your watch by the lights: the time pips have a very obvious pattern.

High power wireless has been around for a while now and the only known risk from such non-ionising radiation is the heating effect. VERY rare that anybody gets exposed to that kind of power, only antenna workers in practice. Contact with the conductors can give you an electric shock as well, but that isn't unique to RF.

Some people claim to have been harmed by mobile phone use. A coleague of mine had a tumour the size of a tenis ball taken out of his head some years ago and it was on the side that he had a phone clamped to for half of the day, but there's no evidence that was the cause.

Ionising radiation is the dangerous stuff but that's way above even the highest frequencies used for communications.

[cropwell]

Going back to the original topic, Kit, you don't have any transmitter masts near you ?

[Kitwn]

Going back to the original topic, Kit, you don't have any transmitter masts near you ?

Sorry, I missed this comment first time round:
Actually, yes. As far as I'm aware it's the largest wireless transmitting antenna in the southern hemisphere and is about 14Km away from the CNC machine.

[cropwell]

Sorry, I missed this comment first time round:
Actually, yes. As far as I'm aware it's the largest wireless transmitting antenna in the southern hemisphere and is about 14Km away from the CNC machine.

It transmits VLF signals for the US Navy Submarine Fleet. I thought this could be giving your sensor LEDs a buzz. The sensors themselves generate an ac em field, but I don't know the frequency, but there might be some heterodyning.

Just a thought, but it might have no merit.

[Kitwn]

Not measured on a specific opto, but it is the measured output voltage of the proximity sensor when triggered. Opto outputs will vary depending on device and load current but with only 0.6V to play with I'm not chancing it. I was surprised at the low trigger voltage, but it's what I measured on the board input and the chip spec says it can be as low as 0.7V
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This is likely to be a problem with any board that only uses 5V logic on it's inputs. I assume the more expensive boards are more sophisticated. They'd better be for the difference in price.
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I'm happy that the 10K pull-up on the board will work OK with the single pole relays. The relay will isolate the actual board input from any noise on the wiring from the sensors and the overall noise level is going to be significantly reduced by my improvements to the wiring overall.

[Voicecoil]

Not measured on a specific opto, but it is the measured output voltage of the proximity sensor when triggered. Opto outputs will vary depending on device and load current but with only 0.6V to play with I'm not chancing it. I was surprised at the low trigger voltage, but it's what I measured on the board input and the chip spec says it can be as low as 0.7V
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This is likely to be a problem with any board that only uses 5V logic on it's inputs. I assume the more expensive boards are more sophisticated. They'd better be for the difference in price.
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I'm happy that the 10K pull-up on the board will work OK with the single pole relays. The relay will isolate the actual board input from any noise on the wiring from the sensors and the overall noise level is going to be significantly reduced by my improvements to the wiring overall.

It's a shame they've used a 74HCT14 for the buffer on your BOB, the old (and now probably uncommon) 40106 hex Schmitt buffer was a better chip for such purposes IMHO, as the threshold voltages are more or less equispaced around 1/2 the supply voltage, something like 2.3V & 2.7V.

[Doddy]

I'll stick with my self-centred view of having BoBs and similar with opto-isolated inputs. I've never been a fan of distributing a high speed logic input signal into the big, bad world.

Now... where to buy a replacement for a badly crimped D25-26w IDC cable (Bob2->UC300) that's given me gip for the last day, on Easter Sunday?...