Proximity switch "contact bounce"

**njhussey** · 09-01-2017

I was of the understanding (seem to remember Dean saying it) that you couldn't use the same proxy as limit and home with the IPM?

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**Robin Hewitt** · 09-01-2017

Wild guess, the switch output is an open collector and your pull up resistor isn't connecting.

**Neale** · 09-01-2017

Thanks for the comments, guys.

Don't think that it's pull-up problem as the IP/M has a bit more sophisticated input arrangement that does not need these. In any case, the indicator led on the switch itself flickers which suggests a problem upstream of the IP/M.

Grounding - I'll come back to that.

Don't remember anything in the docs that says that IP/M does not support combined home/limit switching; that bit seems to work and it's just the unreliable home switch trip (again, you can see this at the led without considering the motion controller) that does not work. In other words, the IP/M seems to work as per standard Mach3 functionality.

Now - grounding. For testing the control box is sat on the floor in front of the machine. So the wire from the star point in the box is not connected to the mounting bolt as intended. I shall sort this out, but I seem to remember noticing that there was not a clear switching point when testing using a bench power supply, so I'm starting to suspect that this is not noise-related.

Around 4 am I woke up thinking about hysteresis. When I was reading up on these switches, I remembered seeing something about this so I have just gone back to check. Typically, these things should have a hysteresis range of 10-20% of the switching distance, so about 0.4-0.8mm in my case. Switch-on and switch-off points are separate, specifically to avoid this kind of switching uncertainty and the possibility of false switching when things vibrate. Just what I'm seeing. I think that some of my switches are better than others so I shall go back and test more carefully. Anyone know a source of good examples of these switches? My ebay source doesn't seem to be there any more. I wonder why?

Please keep comments coming. The silliest problems are always the ones you can't see yourself

**Clive S** · 09-01-2017

Neile How about setting the home position to say about 2 -3 mm away from the switch so that the limit won't see it any more. Just a thought!

**Robin Hewitt** · 09-01-2017

I might be completely misunderstanding what you are doing, but...
An NPN output usually requires a separate pull up resistor
If you want to gang them up on one line you get NO not NC

**Neale** · 09-01-2017

Home position - problem here is that the same switch is home and limit. During homing, Mach3 disables limit function to allow homing to take place (l believe). It homes, and then immediately re-enables limit function and the next "flicker" (random switch operation) trips the limit. I think! Sometimes the z homing does work, and Mach3 starts the XY homing (I home both together). I suspect that gantry vibration then trips the hair-trigger Z limit switch again and it all stops. I could, I suppose, move z away from the home position once it has homed (if it does without tripping a limit) so that subsequent vibration does not cause false triggers, but that's not really fixing the underlying problem.

Pull-ups - IP/M doesn't work that way. A lot of BOBs use some kind of buffer input so you use a pull-up/pull-down resistor to one rail and switch to the other rail. The IP/M input looks rather like an opto-isolator input (with some clever current limiting, etc, to handle a range of input voltages and with both ends brought out to a terminal block) so you take one end to one of the rails and switch to the other. In my case, with open-collector NPN switches, I take one side of each input to the 24V rail and switch the other side to ground. My X and Y axes both have two switches wired in cascade (effectively in series as they are NC); this seemed to work fine testing on the bench and seems to work OK on the machine for limit switch purposes. In any case, the problematic axis at present is Z where there is only one switch anyway!

Sorry if I might come over as teaching anyone to suck eggs but I wanted to explain some of my reasoning to people with less knowledge; sometimes those with less knowledge make fewer assumptions and can see wood for trees

Personally, I still suspect those switches...

**Boyan Silyavski** · 09-01-2017

Hi Neale,
how exactly you have connected that limit switches? If you connected them separately to 24VDC power source / the 2 wires only/ do they flicker or not?

**Neale** · 09-01-2017

I bought a box of ten a couple of years ago (this machine has taken longer to finish than I expected!) I have fitted 6, so I have gone through the rest of the box testing each of the other four. I bolted each one in turn to the machine so I could trigger it by turning the ballscrew by hand and connected it to a bench power supply. So, completely independent of the control box, motion controller, etc.

One does not work at all. One turned out to be an "EX" version which only had two wires instead of a three-wire AX. The other two both have this sharp switch-on/switch-off point with a microscopic movement of the ballscrew needed to go from on to off with a flickering stage between. As I understand it, this is not what a proximity switch should do - there should be a difference between on and off trigger points to avoid exactly this situation. I think I have a bad batch of switches.

**Boyan Silyavski** · 09-01-2017

You must have been unlucky with that batch/ seller. Yes, there is not a flickering stage normally. Its either on or off. Hysteresis means once you are ON and the back to OFF, like you noted there is a ~0.4mm distance / at least on my switches.