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Limit Switch Problems
« on: December 18, 2008, 09:25:03 AM »
I just finished a new router table and am having problems with the limit switches. Upon activating them in Mach, I am getting intermittent estops and the status indicates a limit switch opened. It does this even when there is no file running, so the switch is not actually opening.
I currently have two hobby tables and both have the same problem. Both machines use a Xylotex driver and Nema steppers. The switches are wired in series and the homing feature works. There is shielded cable on all the wiring which is grounded to a central ground. 
The driver is only a pass thru for the limit switches and does nothing as far as functionality goes. With the driver powered down I can manually operate a switch and get an led on the diagnostics page in Mach and cause an estop. That said, it never trips an estop with the driver board powered down. To me this eliminates the ac source and the ground as the cause or I would be getting estops with the driver powered down. As soon as the driver is powered up the problem begins randomly.
I have had both machines on at the same time to see if the hits were occurring simultaneously and they did not. Both took hits, but at different times. Mach is configured the same in both machines with the exception of the table size and motor tuning. To me, a noise spike large enough to trip an estop with the debounce set at 20,000 would have tripped both machines but it dont.
I initially thought there may have been a low voltage condition coming from the computer that might have cause Mach to think the circuit had opened, but if that were the case it would happen with the driver powered down since there is always continuity and it dont.
I am totally out of things to try and really need to get the limits working on the new machine. I would appreciate any and all suggestions.  HELP!!!

Thanks Guys,
Richard

Offline simpson36

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Re: Limit Switch Problems
« Reply #1 on: December 18, 2008, 10:07:54 AM »
From your post, it seems you are getting power from the computer. I had two problems with limit switches; one was when metal chips were getting on the exposed switch terminals . . easily remedied.

The other was a host of random spooky occurences . .  false e-stops, motors randomely bumping a step here and there. Occational motor stalls for no apparent reason . . etc. I was drawing power from a laptop USB port. All of those issues went away when I set up a separate clean regulated source (and old PC power supply) for the 5v and 12v power to the electronics, and moved the DC variable speed drive into its own metal box away from the other electronics and the steppers. 

Re: Limit Switch Problems
« Reply #2 on: December 18, 2008, 01:50:47 PM »
I didn't mention it, but I am using desktops and a parallel port cards. Now that you mention it, I remember someone saying they had  placed their driver in a metal box to isolate it from outside interference. Maybe that is worth a try.
Thanks for the info.

Offline simpson36

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Re: Limit Switch Problems
« Reply #3 on: December 19, 2008, 07:42:27 AM »
Just FYI, the info I came across said that the drive is the source of the noise.

The metal box is to keep the drive from bothering the other electronics. It seems to work . . although I made several changes at the same time so I can;t say which was the effective cure. I can say that it is a plus for convenience and safety to have the spindle control in a box on the bench right next to me (and the computer) rather than on the mill head where it originally was mounted.

I have Mach turning the spindle on and off, but I still control the speed manually because I find I need to vary the speed during a run sometimes to eliminate chatter. Having the knob nearby is extremely helpful in this regard.
Re: Limit Switch Problems
« Reply #4 on: December 19, 2008, 07:57:13 AM »
There is also the "debounce" (from memory) setting in the basic config...

When I first wired my limit switches (wired being a fancy word for spaghetti) I had random "limit switch" events, I swapped out power leads for RFI suppression ones and all sorts, before actually RTFM and realising that it was a simple software config issue, which once altered has never returned to this day.

should make the point, this was with totally unshielded single conductor wire for limit switches, which runs past and around spindle induction motor and the axis steppers etc etc etc. No problems.
Re: Limit Switch Problems
« Reply #5 on: December 19, 2008, 09:54:51 AM »
I have the debounce set up to 20,000 and still get hits.
Question, If I turn the limit option on in Mach and watch the diagnostics page I see the limit and homing lights flicker when it takes hits. The homing uses the same circuit, but with the the limit swith option turned off and only the homing on the homing lights never flicker, no hits. If there is interference feeding into the circuit wouldnt it affect both functions?
Re: Limit Switch Problems
« Reply #6 on: December 19, 2008, 11:39:09 AM »
If you'll take a look at the picture below (thumbnail link) you can see the limit switch wiring on my Z axis.

The X and Y are basically wired the same way, to the same "standard" and using the same wire.

There is a crap load of other stuff, including compressors and variable speed motor inverters, all sharing the same spur mains feed from the main house... of course the "workshop PC" shares this same power source.

The workshop PC itself is an ex corporate dell. It doesn't just run mach3, but is also burdened with some CAD software, CAM software, and is used to browse the net or simply play mp3s when I'm not doing CNC stuff.

The CNC electronics hardware itself is however half decent quality kit, originally Chinese made to be sure, but half decent nonetheless.

I'm posting this because it is worth making the point that despite what should be a pretty atrocious and problem ridden installation, I get basically zero problems, even when starting and stopping or varying the speed on the lathe motor, even when the compressors kick in, blah blah blah.

Clearly I'm running the same copy of Mach3 as everyone else, I don't have gold plated artsoft bytes, and bill gates didn't send me a personally made version of XP.

And yet is is as stable as can be.

Clearly stability is not something that can only be achieved by the use of properly screened and grounded limit cables, etc etc etc.

So the variations between machines, why is mine stable and yours not? Logically must revolve around the stuff that is different.

The stuff that is different is going to be the actual workshop computer components / hardware / psu etc, and the stuff that is different is going to be the actual stepper circuits and even steppers themselves.

I used an old corporate PC and half decent stepper hardware, and the whole thing seems rock solid DESPITE my efforts to the contrary.

Whereas there doesn't appear to be any shortage of people having problems using high end laptops and heinz 57 flavours of stepper drivers and even USB interfaces between control computer and stepper electronics.

Clearly, logic dictates that installations like mine are proof that there is actually nothing whatsoever "wrong" with Mach3, or WindowsXP, or old corporate computers and parallel ports...

The alternative is that I am just ********* lucky and mach3 would run on an old casio watch provided it was within 5 metres of me, which isn't borne out by anything else around me.

HTH etc



From CNC Mill Conversion
Re: Limit Switch Problems
« Reply #7 on: December 19, 2008, 12:06:38 PM »
I'm not trying to imply that there is anything wrong with Mach. With exception of this problem it has work flawless for me. I'm not the only one experiencing this problem. I have found others on this forum with random estops as well. I think we are all looking for some input and direction as to where to look and what could cause it.
Re: Limit Switch Problems
« Reply #8 on: December 19, 2008, 12:31:24 PM »
I'm not trying to imply that there is anything wrong with Mach. With exception of this problem it has work flawless for me. I'm not the only one experiencing this problem. I have found others on this forum with random estops as well. I think we are all looking for some input and direction as to where to look and what could cause it.

OK, so it ain't mach3 we agree? and it ain't the actual limit switch wiring quality or proximity to anything else we agree? It aint Windows we agree?

So what is different?

What is your PC hardware spec?

What is your stepper control hardware spec?
Re: Limit Switch Problems
« Reply #9 on: December 19, 2008, 12:46:09 PM »
Computers are nothing fancy. One is a refurb Dell and the other is a Compaq both used. Both have 1Ghz clock to comply with the latest Mach version. The driver is from Xylotex  and I am using Nema 23 269oz steppers.