Limit swtich false trigger

[epineh]

I was having trouble with this sort of thing on my machine, so I decided to use 24Volt (my motor supply) for my limit switches and the problem vanished, same cabling.

I had to make a little circuit to optoisolate the motor supply from the 5V logic but it was very basic, and I breadboarded it, if you want details let me know.

The only time I get trips now is when I lean over the machine while watching it work and accidently bump a limit switch, I don't think they make a circuit to stop that from happening

Cheers.

Russell.

[Bubba]

I tend to agree with Russell on the voltage situation. The designer of my BOB and I have had discussions along this line. However to simply change the existing design on the working board is not quite that simple. First off, the resistors for the optos are of the sim type and quite small. (to small for this old man and his eyes) and would require other circuit changes (voltage supply is on board).

However, I still believe there is a coding problem as the anomaly gives a generic "a limit has tripped" message. This is not "A axis has tripped" as that is neither encoded nor is there a limit for the A axis. Somehow, the error routine is called giving this message from some other point. I could well be a valid trip for some reason, but it is not giving the proper message.

I also know there is now a problem with the voltage on the machine side of the BOB also. The 5V for this side is derived by use of a DC-DC converter and recently, I found I could not enable the 4 Gecko drives. After a quick trip to my handy dandy Fluke, I found the output voltage of the 5V converter was around 3V!!!! Well, doing some temp rigging with a 5V wall wort with a higher power rating, I found the problem seems to be one of the Geckos as can get 3 of the 4 to enable. The one that won't is an original version of a 340. So I suspect this may be a problem for some reason. Now I just have to dig in the control cabinet to replace it with a spare that I have. Of course as Murphy would have it, it is the one in the most congested space and being of the original type, does not have the removable terminal block:{(

[epineh]

Yeah I guess you have a point, simply hacking into a commercial BOB could be dangerous, my stuff is all homebrew so there is plenty of room for hammer and chisel

Russell.

[Bubba]

Russell,
My board "was" intended to be commercial, but I have the one and only prototype ever soldered up. You might say that I am the beta (alpha) tester. And other than this one problem which may or may not be because of the board, I am willing to live with it and worry about my next design. This one has some good ideas on it and I understand how it works. I have plans to cnc my lathe so will need another BOB and intended to design one based on the principles I have in this one as well as some from your threads and others

[HakBot]

my case this is an intermittent problem. I may go a couple of months before a trip may occur. I also note no particular pattern to this happening!

Mine occurs much more often. Maybe 3 times when milling a small 2"x3" part.

[cadmonkey]

I came across this and don't really have any answers or suggestions but in reading I wanted to make a comment about Stepper Monkey's suggestion.

Safe machine building practice suggests that controls intended to stop operation be logic LOW when in the tripped status. If your BOB doesn't support this then there are possibly some work arounds I'll mention at the end.

The underlying theory of the tripped state being a logic LOW state is that if a wire is cut, broken or shorts out to the machine (which 99.9% of the time is going to be ground - though whether referenced to logic ground or not making a difference could be debated with all the EMI generated by the movement of the machine but that's a larger and more complex debate) or if a power source separate from the controller fails - in my setup a charge pump switches power to everything other than the computer and the opto isolator dedicated to the charge pump so if that or anything after it fails there is no voltage at the limits, the drives, the spindle, anything - the control software - in my case EMC - see's a wholesale lack of "SAFE" signals since they are all expected to be HIGH under normal operating conditions.

Not trying to be a burr under anyones saddle, just trying to save a few fingers if I can.

The work arounds for the limits, if your BOB doesn't have the flexibility to let you run a switches as a high condition as the normal good to go condition, would be to utilize the PC 5V power supply (fused of course) and wire it through each switch using the normally closed terminals so that if the switch isn't activated the logic 5V of the PC is run back to the parallel port as the limit signal. If your software doesn't let you look for them as high being the go state then you're stuck with that limitation (and then I'd suggest looking at using EMC or Mach if that makes you more comfortable).

I hope I didn't offend anyone and everyone had the patience to read through this whole post, and that at least the concept makes sense. I would hate to see someone's part, or worst case scenario a finger, hand, etc. ruined because a wall wort died and there was no 5V signal to be seen so everything kept running because the software saw no 5V signal to tell it to stop doing what it was doing.

My .02 - and I hope your mileage doesn't vary If you have definitive reasons not to wire it as such I'd love to hear them, but in my years servicing, rebuilding and a few design tasks on a lot of equipment of many different types, any mechanical limit or interlock was HIGH in the safe to run state and LOW in the stop everything state.

HTH

[Stepper Monkey]

Very good point there cadmonkey.

I never looked at setting up the pin logic for the limit switches from a fail soft or fail safe perspective before. It is an excellent point.

I have always set my systems with the limits active high simply for convenience and interference reasons, but I never did give a thought to how that behaved for safety reasons. Kind of stupid of me now I look back on it. Especially using optocouplers on everything it is even more important as there is an extra potential point-of-failure added. Only sheer luck I won't have to go back and rewire now!

Makes a lot of sense. Best practice would definitely call for active high.