Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: hemi43 on November 17, 2012, 03:18:19 PM
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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
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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?
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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
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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.
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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
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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.
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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
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Thanks for help. I will try those things mentioned and will post my results.
dan
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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.
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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.
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I chased false limit switch triggers for weeks with no success. My solution was to introduce a 12V DC relay - the six limit switches were wired in series and were in the 12V supply circuit to the relay coil. The normally closed circuit in the relay was wired to the limit input of my controller.
Works well. Not the best solution, but for the life of me I couldn't work out what was causing my limit switches to trip - and I tried to be very diagnostic.
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I chased false limit switch triggers for weeks with no success. My solution was to introduce a 12V DC relay - the six limit switches were wired in series and were in the 12V supply circuit to the relay coil. The normally closed circuit in the relay was wired to the limit input of my controller.
Works well. Not the best solution, but for the life of me I couldn't work out what was causing my limit switches to trip - and I tried to be very diagnostic.
That's a good idea, because I know that the cause for my limits to trip would only be a few miliseconds, but not long enough to activete the relay. I guess you could say that the relay is being used as a time delay. It's been a busy week, and hopefully will get time to mess with my mill later this week.
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You say its not the best solution, I disagree, it is :)
Its the way Industry does it, well normally 24v but same principal ie much greater difference between a low and a high signal so lot less chance of false triggering from noise.
Hood
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Another option for the noise in the supply line is a noise filter.
Cheap and easy to implement in any electric/electronic machine and easy to find in any good electronics supplier (RS/Farnell/Conrad etc).
I always have a couple of them in my parts bin.
It filters the noise in the power line into the machine and/or computer, most of the good PC power supplies have one.
Airborne RF/HF noise is a complete different story
Ric
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The problem is with the welding. Think of it as a radio transmitter. Remember when CB radios would bleed over into the tv and stero? I wonder if putting a grounded wire screen would trap the interferance?
Try a metal case for the computer or at least a metal box to help shield the eveything. Move the TIG away from the machine.
Hal
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Faraday cage any one ? ???
http://science.howstuffworks.com/faraday-cage.htm
Ric
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Well I still have not solve my problem, but here's where I'm at now.
It's not the limit switches causing the problem. When I activate my welder, the breakout board is what is messing up !! I've tried putting a capacitor across the 110v input to the B/O board but that did not help. I'm totally lost, and can't narrow down if the interference is coming from the wires going into the B/O board, or if the interference is air borne. ??? ???
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The noise is being picked up by the parallel cable going from the BOB to the PC...or possibly by the PC itself. We deal with HF noise all the time working with plasma machines. Industrial plasma power supplies use an HF arc start. No different than your HF TIG. Did you ground the PC chassis to the star ground? The other thing you can try is soldering a ground wire to the shield on the DB25 cable and grounding that to the star ground. You would only ground one end of the DB25, which is usually the BOB end.
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This morning I drove an 8ft ground rod right through the floor and into solid ground beside my mill and attached a ground wire to it. I brought the pedal from my TIG and placed ot the table of the mill so I could just activate it with my hand ( my welder is in a different room, but the pedal reaches). It tried grounding different areas in the control panel as well as the case of the computer and the cable. Still no luck !! I even ran an extension cord to power the computer tower so that the 120v feed for it came from a different area. It seems like the component that is causing problems is my breakout board ( Campbell all in one ) but there's no ground provisions on it. Like I said before, the Breakout board goes nuts when I activate my TIG and when it does it activates my VFD. I'm lost as to what else to try !! ??? ??? ??? ??? ???
Have a Merry Christmas !! :)
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Is the BOB in a metal box? A simple one could be enough ti ground the board.
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Is the BOB in a metal box? A simple one could be enough ti ground the board.
No, it's not in a box, but it is inside a metal cabinet along with the 3 servo drive boards, and 12v power supply.
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Did you ever try the caps on the limit switch inputs to your BOB?
Merry Christmas All,
John Champlain
www.picengrave.com
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The BOB grounds through the PC via the shield on the parallel cable. Did you ground the PC chassis or try soldering a ground to the DB25 shield?
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Is the cabinet grounded?