Thought I would share my limit switch noise solution. I searched here looking for answers a couple years ago when my PM25 CNC conversion kept getting false triggers on the limit switches. I wired in some resistors which helped, but never totally fixed it. I tried shielded wires too but that didn't work.
I kept getting "Limit Switch Triggered" in Mach 3 while cutting, which would freeze the machine. I have a 48V system two large Nema 23's and a Nema 34, all running at 5A's.
Anyways, I researched it and decided to make a better solution.
My limit switch filter board puts a load across the limit switches to reduce voltage spikes, an RC circuit to filter that signal, then a Schmitt trigger to remove jitter and add hysteresis, a level shifter to convert the signal to 5V, and current limiting resistors to protect the output wires from short circuits.
It's been on the machine for the last couple months, and has finally fixed my noise problem!
Pics of the final board and a screenshot of the signals!
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The only thing with filters, they may cure the problem but the cause is still there, I have lost track of the amount of post That have had apparent spurious L.S triggering where I have advised on earth bonding all supply commons and possible equipotential earth bonding of the machine and it has cured the problem.
Unfortunately many of these legacy posts are no longer there to quote as they disappear after a certain age due to forum 'cleaning'
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
The reason you had noise that caused false triggers is because of bad Grounding somewhere in your system and no Mains Power Filter the filter you made is just a band-aid, the noise is still there which will affect other parts of your electronics over time
This is one of the most common problems with Hobby builds and easy to fix without the filter you made, this type of filter you made normally will slow down the reaction time of the limit switches
Are you using a VFD Drive for your Spindle control / Drive if so this is where most of the EMI noise comes from
Mactec54
I appreciate your willingness to diagnose possible underlying problems with my machine. The one thing you are correct about, is that the filter board slows down the reaction time. That is an engineered value. You can see that in action in my first post with the logic analyzer screenshot.
I do have a VFD. I've also tested your theory as that was mentioned on the forums here. It ran the program without the spindle/without cutting, it would still kick out due to limit switch errors.
I scoped the switches, there was high frequency noise coming from the steppers under high acceleration moves (my machine runs a heavy vice, heavy spindle/headstock) and those high current moves were inducing noise into the limit switch wires.
My motion control board sends the weak 12v signal from the switches through a resistor, then straight to the CPU pin. Bad design, but I didn't design that. The board should have had it's own engineered filter for the inputs. My filter board solved that problem. You can call it band-aid 20 more times, but I have had zero issues since installing this board, and am very happy to be able to make parts without issue.
So you new where the noise was coming from so why did you not fix that source, before you made the board, there is nothing wrong with you making the filter board, just not needed if you had fixed the problem
Sounds like your motion control board is not great as well which one did you use is good to post about control boards that could cause problems, so others don't make the same buy
Mactec54
Yes, I had posted previous post on this, but after a certain period of time, the old legacy posts get deleted when they reach a certain age it seems.
I will see If I can post a revision.
I generally make a point of earth grounding all power supply commons to the star point, your PC PS low voltage is already at GND via the MB common plane screws
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
Been running the filter board for over a year now. Got a few hundred hours on the machine and zero issues!
Do you think the filter is needed, or is it a bandaid for some underlying issue? If it works, it works, no issues on my end there. I am just curious because I don't use any filters on my proximity switches and have never had an issue. I run my switches off a dedicated 24v supply, I suppose that could help.
Even with correct wiring practice's in place, filters should be throughout the electrical cabinet for the different devices, being used, noise is noise and it will always be there, filters in the right places take care of most noise problems
Even your 24v supply will emit noise so nobody is immune to noise of some kind
Mactec54
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.
My motion control board should have filtering on the input pins, but it's a cheap chineese board and it doesn't. It just has pull up resistors.
If I had higher quality board, there would be no issue because it either has hardware filtering, or software filtering, or both. It has none so this board fixed it. I have them on ebay now and sold a few.