Inductive Limit Switch problem

[Kitwn]

I've just upgraded from using microswitches as limit/homing switches to LJ12A3-4-Z/BX type inductive proximity sensors. I'm getting some spurious trips from the Z-axis sensor due to interference from the spindle, a Huan Yang 2.2kW water cooled unit fed from a VFD. The spindle and sensor wiring go through the same drag chain for part of the run and I'd like to avoid having to separate them if possible.

One of the symptoms is that the red LEDs on the relevant sensors glow visibly, but very dimly, when the spindle is running. A ferrite ring on the sensor cable makes no difference, neither does a 1uF capacitor between ground and either the +V supply or sensor output.

Has anyone else seen this faint glow before and, if so, how did you fix it?

All help gratefully received.

Kit

[Muzzer]

Are the wires from the VFD output to the motor shielded in any way?

[JAZZCNC]

Has anyone else seen this faint glow before and, if so, how did you fix it?

All help gratefully received.

Kit

I've seen this before when the voltage is close to the minimum 6v. I've also seen it when wired wrong but can't imagine you've done that. Another thing that often happens is the switch can be marked up wrong ie: NPN when should be PNP or NC when NO.

What voltage and breakout board are you using.?

[Voicecoil]

Are you using the cable attached to the sensors? When I was wiring up my machine I noticed that the cable wasn't errrm exactly up to the standards Auntie Beeb would have liked, so extended with screened twisted pair: not had any problems at all, even running the spindle up to 900Hz (12.5KHz carrier)

[Kitwn]

I've seen this before when the voltage is close to the minimum 6v. I've also seen it when wired wrong but can't imagine you've done that. Another thing that often happens is the switch can be marked up wrong ie: NPN when should be PNP or NC when NO.

What voltage and breakout board are you using.?

The VFD cable is screened, earthed at the VFD and connected to the spindle body. Just writing that makes me wonder if there's an earth-loop because of that. Must try removing the spindle from it's bracket.

It all worked fine before I started the spindle! I'm using a basic cheapo Chinese parallel BO with diodes to isolate the 5V on it's inputs from the higher sensor voltages. I originally installed the sensors with 12V and have tried 24V but still get the same slight glow on the LEDs.

I have known issues with the BO inputs not being pulled down far enough by the sensors. I can fix that, it's the slight glow and my inability to fix it with filtering that worries me most at the moment. I was wondering if it was a known issue. If somebody is getting the same symptoms but still has the sensors working reliably I'll be happy.

[JAZZCNC]

The VFD cable is screened, earthed at the VFD and connected to the spindle body. Just writing that makes me wonder if there's an earth-loop because of that. Must try removing the spindle from it's bracket.

That would be my first move. It's a much debated thing but I never ground both ends and I never have issues with these cheap sensors. Thou like voice coil mentions I cut the shitty cable off close to the sensor and use screened back to the board and ground it.

[Cube3]

I had very similar problems until I did this.

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

Simon

[Voicecoil]

It all worked fine before I started the spindle! I'm using a basic cheapo Chinese parallel BO with diodes to isolate the 5V on it's inputs from the higher sensor voltages. I originally installed the sensors with 12V and have tried 24V but still get the same slight glow on the LEDs.

.

Mmmm, just had a thought..... any chance you could post a little pic of the exact arrangement please? The way I'm reading the above, you're using only the bottom 5V of the voltage swing, and hence will only get the noise immunity of a 5V system - having a series resistor or resistive divider would be better. However I may be reading it wrongly!

[Kitwn]

That would be my first move. It's a much debated thing but I never ground both ends and I never have issues with these cheap sensors. Thou like voice coil mentions I cut the shitty cable off close to the sensor and use screened back to the board and ground it.

That will be test one. How many layers of cling-wrap do you think I'd need round the spindle to insulate it from it's bracket if that works?

Interestingly I get the same amount of glow from two sensors. The Y one which goes to a junction box and onto screened cable before joining the VFD cable in the drag chain, and the Z which has the original cable running in a common length of drag chain before being connected into the same (8way) screened cable as the Y.

Voicecoil,
I do take the point about cutting off the original cable short and going screened, I shall probably do that anyway. Possibly with some balanced audio cable if I have enough, it's thinner than the other stuff I have in stock and Auntie would surely approve.

Simon,
I'll have a look at the full video tomorrow, but I've tried winding a few tuns of the cable onto a ferrite ring at the sensor end and adding capacitors as described earlier at the joint into the multicore. Much to my disappointment it made no difference. Next will be to hook up my oscilloscope and look at the noise level with different solutions.

I'm between 12 hour shifts tonight (and very glad to be still working) so it will be Wednesday before much more gets done on this. Thanks for all the input everyone.

PS I have this cunning design for a low cost DIY optical fibre interface that would solve all these problems but I really would like to get the machine upgrades finished right now and actually make something with it!!!

[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!