Inductive Limit Switch problem

[Kitwn]

I had very similar problems until I did this.

https://www.youtube.com/watch?v=DJfiOqaeFDg

Simon

Simon,
Apologies for not getting back to you earlier, I got somewhat sidetracked away from the original reason for this thread.
That's a very interesting video and I especially like the improvised current transformer for investigating noise, I hadn't seen that trick before but it will be useful in future. As I've discovered the VFD is not to blame (yet!), it's enabling the motors that generates the noise I'm investigating at the moment. Once that's been quietened down I can then see if the VFD adds any problems. It wasn't an issue when I had NC microswitches for limit sensors so my fingers are crossed on that one.

[Kitwn]

Funny how the brain works. There I was, lying in bed reading a book when I suddenly realised what an idiot I was for suggesting screened audio cable for this application. Not enough wires!

[Voicecoil]

Funny how the brain works. There I was, lying in bed reading a book when I suddenly realised what an idiot I was for suggesting screened audio cable for this application. Not enough wires!

It depends on the polarity of your switches and how you're wiring them. NPN NO in parallel (possibly not the optimum choice) could work fine with twin screened, likewise PNP NC in series could work, as both have a commoned 0V - assuming your 24V ground is connected to chassis somewhere of course. Otherwise there's plenty of smallish 3 & 4 core screened cables available, I've a few bits of Mogami 2790 which have come in useful.

[Kitwn]

It depends on the polarity of your switches and how you're wiring them. NPN NO in parallel (possibly not the optimum choice) could work fine with twin screened, likewise PNP NC in series could work, as both have a commoned 0V - assuming your 24V ground is connected to chassis somewhere of course. Otherwise there's plenty of smallish 3 & 4 core screened cables available, I've a few bits of Mogami 2790 which have come in useful.

I'm really not happy with the idea of powering up electronic devices on the 'just in time' principle that series connection of these sensors involves. In practice the four I have are all independent anyway, X1, X2 , Y, Z.
The ones I have are NPN NO which seem to be the most common on eBay. I'm not sure if I have much 4-core screened left but have plenty of 8-core. At present I'm running the supplied 3-core cables to a junction box on the back of the Z axis where the Y & Z cables connect into an 8-core screened cable back the the controller.

[Neale]

I'm really not happy with the idea of powering up electronic devices on the 'just in time' principle that series connection of these sensors involves. In practice the four I have are all independent anyway, X1, X2 , Y, Z.
The ones I have are NPN NO which seem to be the most common on eBay. I'm not sure if I have much 4-core screened left but have plenty of 8-core. At present I'm running the supplied 3-core cables to a junction box on the back of the Z axis where the Y & Z cables connect into an 8-core screened cable back the the controller.

As per the previous post, combining these switches in series isn't a probem as you combine the switches at each end of the same axis - don't mix axes. As long as you have each axis (including A and X if it's a dual-axis machine) going to a separate input, that also lets you home more than one axis at a time. You need to do that anyway if you have a dual-axis machine and want proper gantry squaring on homing.

Personally I run spindle, motor, and switch cables all in the same cable chain; they are all CY and I use 24V signalling which helps with noise rejection. Don't have noise problems.I do have a solid earth wire running through each cable chain connecting the two end parts to avoid any need for continuity via the bearings. Haven't noticed the LEDs glowing. I did wonder if you had bad switches; I bought a cheap box of 10 from eBay when I built my machine and one was the wrong type (only had two wires coming out which was a bit of a giveaway), one was bad when I fitted it, and one failed shorty afterwards. Been OK since, though.

[Kitwn]

Neale,
X1 and X2 are for squaring the gantry. The only axis with two switches is X2. I've left the old mechanical microswitch at the other end of that one just in case the soft limits fail to stop a crash for some reason. Shouldn't ever be needed in practice.

I used NO switches to make interfacing 12V switches to a 5V BOB simple. Everything worked fine until I started the spindle. My circuit failed to work when I tried using 24V so a more complex interface will be required anyway. NC would have been better for the reason that broken wires show up immediately instead of causing a crash when you try to home the machine, but I'm not buying a new set now. I'm going to get the oscilloscope out and have a look at the noise, then some experiments will be needed to devise a reliable solution. I'll publish details once I have something useful to say. I might even make a video for everyone to watch while in lockdown!

[dazp1976]

When I come to do mine in the future I'll wire them in series for each axis and use solid core CAT7 cable.
Where I have to split it down from 8 wires into pairs as I get closer to the machine I'll heatshrink them.
CAT7 has each pair shielded already. It's just a case of carefully getting the outer sheath off without damaging the foil on each pair.

Atm I'm just using the soft limits.

[Kitwn]

When I come to do mine in the future I'll wire them in series for each axis and use solid core CAT7 cable.
Where I have to split it down from 8 wires into pairs as I get closer to the machine I'll heatshrink them.
CAT7 has each pair shielded already. It's just a case of carefully getting the outer sheath off without damaging the foil on each pair.

Atm I'm just using the soft limits.

Nice idea for fixed sensors but I don't think solid core cable will last long in a moving drag chain.

[dazp1976]

Nice idea for fixed sensors but I don't think solid core cable will last long in a moving drag chain.

I won't be going through the drag chain. The switches will be fixed to the machine casting so they won't be moving. The little angle plates they will read off will be axis attached. This way the cable won't need to be that flexible.

[cropwell]

If you are looking for 3 core shielded you might find an old USB cable which is shielded (some are, some are not).