Limit switch problem

[paulb]

Hi folks,

My home-built CNC router has been running for about a year. It's a 3-axis stepper motor trapezoid (metric ACME) thread machine driven by Mach3.

It's been working fine, but recently I've been making heavier cuts in MDF and pine timber and have seen an issue with the limit switches.

When I run a g-code program the machine starts OK, but as the cutter load increases Mach3 reports a limit switch hit - but the machine is nowhere near any of the limits. The machine will stop in exactly the same place if I re-run the job (the machine will air-cut until it hits the point it stopped at before). If I turn off the limit switch sensing (obviously I don't like doing this) the job will run without a hitch.

The limit switches are microswitches (low operating force in fact - I had a batch lying around) wired N/C to 0V with a 1K pull-up to 5V. The switch lines are all unshielded (this is probably a bad idea).

Seems there are two possiblities:
1) The cutter load is vibrating the machine and the vibration is triggering a uswitch (is this even possible?).
2) The load causes more current demand through the mains (240v) cable to my router - noise from the lead is breaking through to the limit switch wiring.

I'm sure it must be (2) - I guess it's time to re-wire with shielded cable?

Also, is there any way in Mach3 to tell which limit switch fired?

Cheers,
Paul.

[analogman]

Its probably #2 - noise. You will probably need to shield your limit switch wires. In Mach3 go to to diagnostic page. You will see little indicators in the lower RH corner which show which limit switch was tripped.

[Juka]

Put little capasitor between input pin and ground...

Hope that helps!

[paulb]

Thanks for the filter capacitor idea Juka - that would save some re-wiring if it works. I'll give it a try.

The glitch is so short, I don't see any of the input indicators flash (perhaps my eyes are too slow!). It's a shame Mach3 doesn't have a more complete internal knowledge of the limits. Oh well.

[martinw]

Dear paulb,

I would put money on diagnosis #2.

Get down to Maplin and buy a few metres of twisted pair screened cable. For a limit switch, it can be as thin as you like.

1) Wire up the two conductors as before

2) Wire the screen at one end only to 0 volts. Which end to do this at is a matter of some discussion, LOL.

Good luck,

Best wishes,

Martin

[pminmo]

Set a debounce time in MACH3 is the easiest. I seriously doubt that it's an electrical noise issue with NC switches. It's not impossible, but unlikely. I'd suspect your low opearing force switches tripping because of mechanical noise before electrical noise. Either way a debounce value will help.