Author Topic: What would cause my limit switches to trip?  (Read 9301 times)

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Offline hemi43

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What would cause my limit switches to trip?
« on: November 17, 2012, 03:18:19 PM »
A bit about my machine;
It's a converted Hurco KMB1 that I did a few years ago but rarely use it, so I'm still finding the odd bug.
My limit switches are 24v proximity switches, so there are no moving parts. Since I built this machine, everytime I would TIG weld near it, the machine would shut down and display a "limit switch trigger" fault. So I don't TIG wels anymore when the machine is running. The machine used to trip also when relays would activate, but I fixed that problem by putting capacitors across the pull down coils.
In the past couple of days, the "limit switch trigger" fault shut my machine down 3 or 4 times right in the middle of a program for no absolute reason !! I had a 1/16 cutter carving wood, so there was no load on the machine. I tried to watch the Diagnostics screen to see if I could see a light flicker if it happened again, but of course it ran great afterwards.
Any ideas? Has anyone experience similar problems?
Thank-you, Dan

Offline Fastest1

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Re: What would cause my limit switches to trip?
« Reply #1 on: November 17, 2012, 04:29:08 PM »
Yep. What kind of grounding technique are you using? Do you use a star ground? If not the problem could be there. Does the V- of your power supply tie down to the chassis ground too? Also do run any debounce on the general config page of Mach3?
« Last Edit: November 17, 2012, 04:31:07 PM by Fastest1 »
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)

Offline hemi43

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Re: What would cause my limit switches to trip?
« Reply #2 on: November 17, 2012, 04:48:49 PM »
The only ground I have is the ground from the input power plug. Not sure what you mean about "star" ground. The negative of my power supply does not go to the chassis ground, unless it's done inside the power supply.
I've never heard of debounce, so I will look into that

Offline Fastest1

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Re: What would cause my limit switches to trip?
« Reply #3 on: November 17, 2012, 07:01:29 PM »
Ok so a little clarification. A star ground is a central point to where all the ground wires (chassis ground), limit switch shields and the V- of your power supply tie into. This is usually a screw or bolt on the chassis of the controller. This is to minimize ground loops. The debounce setting in Mach 3 (there are 2, one is for the spindle the other for the limit switches). This setting is a time that a signal has to be present before Mach 3 sees it. In this case we are talking about the debounce interval. It is in microseconds (40us per unit). A small number of anything less than 500 is usually acceptable. The machine will continue to travel in the same direction for (500 x 40 x .0000001seconds or .002 of a second) til Mach recognizes the signal. How far will your machine travel in that time? This is a software work around for noise. If your V- is only powering your controller or drives and is not tied into the chassis ground, I would lay odds that is the issue. I would tie it into the same place as the chassis ground anyway. It is an accepted practice. try the debounce at 500, if it works, try again at 250. Keep cutting it in half til it reappears. When it does reappear, double the debounce. Personally I fought this issue for years and tried caps, resistors, shields separating stepper and limit switch wires. Nothing helped until I connected that V- to the chassis ground. Now I can remove all the other fixes at the same time and still not get a false trip. Are you a member on CncZone? Check out their electronics section. this question comes up all of the time.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)

Offline hemi43

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Re: What would cause my limit switches to trip?
« Reply #4 on: November 17, 2012, 10:59:12 PM »
Thanks for that reply fastest1;
I infact do have all the ground wires going to one point (bolt) including the shields from all my limit switches. The negative (-) of my power supply does not go to this ground, because I thought this may cause problems. I checked the "debounce" setting on Mach3, and both are set to "0" . I am a member of CNC zone, but the info you gave me here seems to address a lot of my issues. I will try some of the tips you gave me this week with the help from a friend. I will get him to trigger the TIG welder as I make changes to my mill, and hopefully this will get rid of faults when the machine runs during a program.
Thanks for your help !! It is greatly appreciated.
Dan

Offline Fastest1

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What would cause my limit switches to trip?
« Reply #5 on: November 17, 2012, 11:24:57 PM »
The fear of that V- being tied into that same bolt is what caused my delay of the repair too. Finally after being frustrated and not completely understanding the advice given me, I blindly jumpered it thinking maybe I would pull the jumper off real fast if the magic smoke started coming out. Right, fortunately that didn't happen and it has worked great ever since.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)

Offline Picengraver

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Re: What would cause my limit switches to trip?
« Reply #6 on: November 18, 2012, 06:59:37 AM »
Dan,
Try placing a 0.1 ceramic cap between the limit switch input and ground connections.  This works very well to filter noise triggering the limits, and better than Mach's debounce setting in my opinion.

Regards,
John Champlain
www.picengrave.com

Offline hemi43

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Re: What would cause my limit switches to trip?
« Reply #7 on: November 18, 2012, 09:31:53 AM »
Thanks for help. I will try those things mentioned and will post my results.
dan

Offline rrc1962

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Re: What would cause my limit switches to trip?
« Reply #8 on: November 18, 2012, 03:38:19 PM »
The TIG produces a lot of HF noise.  Your issue is probably not in the machine but in the PC.  Sometimes grounding the chassis of the PC to your star ground can help.  Desktop PC's were never designed to be used in harsh, electrically noisy environments.  If the parallel cable connecting the PC to the controller is picking up noise, adding caps will not help.  It's a good and inexpensive thing to try, but don't be surprised if the problem is still there.   Dealing with HF noise is like chasing a ghost.

Offline Fastest1

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What would cause my limit switches to trip?
« Reply #9 on: November 19, 2012, 08:48:12 AM »
Without the V- tied in the potentials will be different still allowing the problem to exist in my understanding. The caps didn't help mine but they were a cheap solution or preventative. You could also use a ferrite on the PP cable.
I want to die in my sleep like my grandfather, not like the passengers in the car! :-)