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Author Topic: Limit switch false triggering in mach3 potential interference issue?  (Read 223 times)

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Hello I have a home built CNC plasma table and I am having issues with false triggering of limit switch messages. I have changed my debounce settings uo to as high as 10,000 takes a little longer but still get false trip error. all of my limits are daisy chained to one input. All motors are running shielded wire and are grounded at one end. Is there anything else I can add to help this issue. I have read some use a capacitor is that the case and if so how would it be wired and what value? The table runs fine when reset but I am sure it would probably false trip on a larger job. I am using Mach 3 with XP version 67. Any help mush appreciated.
Re: Limit switch false triggering in mach3 potential interference issue?
« Reply #1 on: November 29, 2018, 06:15:45 PM »
Hi,
yes you can use a capacitor, it behaves in a broadly similar manner to debounce.

If you connect a 0.1uF (approx....any value between 0.01uF and 1uF will work) capacitor between the limit switch input of your
BoB and 0V will help with noise suppression.

Many users of plasmas have reported this issue, especially if your plasma uses HF start.

You may have to consider using home switches to reference your machine and use soft limits thereafter. Occasional noise glitches
on your home switch circuits will do no harm. The plasma will not be operating at the time of homing and therefore the only time when
the machine is vulnerable to noise impulses the 'noisy' bit is not operating.

Craig
My wife left with my best friend...
     and I miss him!
Re: Limit switch false triggering in mach3 potential interference issue?
« Reply #2 on: November 29, 2018, 06:54:09 PM »
Thanks Craig yes I do have high frequency start. So I will go ahead and try your suggestion. Thanks very much for your time!!

Offline benha

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Re: Limit switch false triggering in mach3 potential interference issue?
« Reply #3 on: December 03, 2018, 07:50:59 PM »
You may have to consider using home switches to reference your machine and use soft limits thereafter. Occasional noise glitches
on your home switch circuits will do no harm. The plasma will not be operating at the time of homing and therefore the only time when
the machine is vulnerable to noise impulses the 'noisy' bit is not operating.

This is a really interesting concept that, for some reason, never occurred to me.  I have phantom limit switch issues from time to time and I'd love to do this.  Is this something that's easy to configure in Mach3's parameters?  How would I go about employing this approach?

Thanks,
-Ben
Re: Limit switch false triggering in mach3 potential interference issue?
« Reply #4 on: December 04, 2018, 02:03:59 AM »
Hi,
that is the absolute standard way to enact soft limits.

Soft limits REQUIRES that Mach knows where it is, usually at the begining of a session, called homing or referencing.
Note this is NOT THE SAME as zeroing your axes.

I will use an analogy.

Imagine you have an automatic lawn mower and you wish it to mow a rectangular paddock. You could put bump sensors
on the mower and allow it free rein, if it gets to a boundary it bumps into it and stops. So long as it doesn't hit to hard
that works. Its a bit like limits in Mach, if you bump into a limit Mach stops. The problem with a plasma table is that the plasma
is electrically noisy and if it triggers a limit mistakenly then Mach, which cant tell it is a mistake so it has to respect the signal and stop,
bugger!

Back to our lawmmower example. Imagine instead that there rock somewhere in the paddock, of course the rock doesn't shift. If we happen
to know that the Northern boundary is 50m away, the Southern boundary is 20m away, the East 55m and the west boundary is 20m away.
When we first start the mower we take it to the rock and tell Mach 'you are at the rock'. As the mower progresses it calculates where it is
relative to its starting position( the rock). Mach would also know how far away each boundary is and can keep a running update.
Thus if the mower is 10m North of the rock and 10m East of the rock and the mowing program says 'now go 50m North and 50m East'
but Mach would know that if it did so it would end up 60m North and 60m East of the rock and therefore out of bounds. The Soft Limits
would issue an error and refuse to execute the move.

Home switches are the equivalent of the rock. Note that it is common for the home switches to be and the end or near the end of an axis but
they don't have to be. Your 'rock' could be at the centre of the table (paddock), it doesn't matter...all that is required is that you know where
the machine boundaries are relative to that reference or home. Note also that the home switch circuits are only accessed when homing, at the beginning
of the session. A noise glitch when homing is happening will screw Mach up bigtime, it might think its at the center of the table (rock) when in fact
its way off towards one boundary. When homing the plasma will not be operating and therefore the chances of Mach getting screwed up by noise
is minimized. Once homing is complete Mach ignores the home switches....after all it can keep a running update of where it is, it doesn't need to
keep redoing it. Therefore if electrical noise afflicts the home switches later in the session, who cares?

I used el-cheapo limit switches on my mill and when I had an upset some of them got wiped out...bugger. What I did was put three good quality roller pluger
microswitches as home switches and use Soft Limits as I have described. I don't have ANY limit switches fitted on my machine at the moment, I rely on
Soft Limits and have done for three years, works good for me. The good quality switches, still only $15US each, allow me repeatability of 0.02mm.
I can have a job in the vice, and get sick of it and end the session. I can come back tomorrow, re-home (reference) and start from where I left off
without touching the job or resetting it, real handy.

So just to be clear...HOME switches allow Mach to accurately and repeatedly find its REFERENCE position where it sets its MACHINE COORDINATES to
zero. When you ZERO your machine you are manipulating the WORK OFFSET so the WORK COORDINATES are zeroed. This may be done multiple times
in a session whereas machine coordinates are zeroed (referenced) just once. Work coordinates are convenient for us but Mach cares about machine
coordinates because that's how it knows where the boundaries are.

Craig
My wife left with my best friend...
     and I miss him!
Re: Limit switch false triggering in mach3 potential interference issue?
« Reply #5 on: December 04, 2018, 09:40:57 AM »
Thank you very much for your excellent easily understood reply. I did follow the previous suggestion and installed the capacitor and have had no false trips since.

Offline benha

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Re: Limit switch false triggering in mach3 potential interference issue?
« Reply #6 on: December 04, 2018, 12:12:06 PM »
Ok interesting.  I'm using a single set of switches for home and limit, but I think what I'm hearing is that I can just configure Mach so that it's not aware it HAS limit switches, at which point it will ignore the switches unless it's in the process of homing.  Then I just config soft limits.

Coooooool...
Re: Limit switch false triggering in mach3 potential interference issue?
« Reply #7 on: December 06, 2018, 05:58:18 AM »
just go into the ports and pins menu and disable them ^