Thread: Limit and Home Switch Choice
With the fault your not sending anything in just breaking or returning the signal.
Last edited by JAZZCNC; 29-11-2013 at 05:13 PM.
As always Dean a couple of very good points. I knew as soon as I said I was happy with the e-stop circuit that there would be something wrong with it , it's my super power.
As you suggested I've put in a relay and run 24V through the switches. Can I just check that my understanding of how this works is correct. I think this is a suitable relay. Since the voltage is 24V and the coil resistance is 2.8k will this portion of the circuit draw 23/2800 = 8.6mA? It can handle 2A across the contacts which must be about 500* what the PMDX will put through it.
I was using PNP sensors because I had a 24V supply already in the system. That's a really stupid reason to make life difficult for myself though so I've switched them over to using NPN. I assume 12V is sufficiently noise immune for this to not be a problem?
Is the two e-stop button arrangement a good / bad idea? The one at the top left will completely kill everything by removing power. I only envisage using that in "hand-being-eaten-by-the-machine" emergency and it will be on the control box (probably). The e-stop button connected to the PMDX is for "uh-oh-that's-not-good" type emergencies and will be mounted on the machine.
E-Stop are EMERGENCY stops, so should be wired for YOUR protection. When you've got your hand wrapped around a cutter, you won't have time to ponder on which E-Stop to press...
The "usual" way to setup E-Stops is using a 24VDC circuit with all E-Stops in series which then drop out a contactor (or proper safety relay if you have plenty of cash!). You latch the contactor via a reset button and N.O. auxiliary contact on the relay. This way, the circuit will be safe if any E-Stop is activated or a cable gets broken. I've put my axis limits in series with the E-Stops because a near-crash IS an emergency. As for your feedback, you simply use another spare aux contact on the relay to make or break the BOB com/5v link.
Last edited by birchy; 30-11-2013 at 12:26 AM.
Okuma Howa Mill AC2 Okuma 5 Pin Limit Multiple Switch 5 Pin Limit Okuma | eBay
Recommend this style of switch as they work almost as a scraper of the Dog as it travels over and very high position accuracy
Really it comes down to how you want to stop the machine if some thing goes wrong, that could be a serious situation or just because you see something like it's going to hit a clamp etc.? There's several ways to look at it.!
When it's Minor then you just want to Halt the machine movement so Feed hold is best, you don't always want the Spindle or Coolant, Vacuum etc to go Off. This could be the case with the Limits has it's only lost position why would you want the spindle off or Vacuum.? Turning the spindle off could damage the tool if it's still in the material, same goes for vacuum no need for it off.
When it's serious has in E-stop you want every thing OFF all power Killed.!
Last edited by JAZZCNC; 30-11-2013 at 10:38 AM.
Wobbly, I just noticed that revision 19 of the PMDX-126 manual was posted on the PMDX website a couple of days ago. Just thought i'd give you the heads-up.
I completely agree about the e-stop being for my protection which is why there is a button to cut the power. My thinking with the arrangement I had is essentially what Jazz has described. Hitting the limits is bad but I don't think it should be considered a life and limb emergency that calls for cutting the power as long as, and this is the important point, the machine can handle the situation safely.
Here's the plan then...
- Keep the e-stop that cuts the power and in fact add another button in series with it. One button will be on the control box and the other will be on the machine.
- Remove the e-stop button from the limits circuit.
- Add a machine hold button.
Great t-shirt story Dean, it reminded me of a near miss I had. I'd been doing some DIY around the house and I needed to clean up some debris so got the vacuum cleaner out. All the tools I'd been using were in the way so I picked them all up to clean around them. So there I am, both hands full of tools with the vacuum cleaner hose pinned under one arm and swinging about. I bend down to do some cleaning and the vacuum cleaner nozzle leaps up like a wild animal and starts sucking on my t-shirt. I practically jump out of my skin and try to bat it away with my other hand. Unfortunately I'd forgotten that hand had a Stanley knife in it and I managed to run the blade across my wrist. Luckily for me I'd been abusing the blade for the last hour and it was blunt so the cut wasn't very deep, if it had been a new blade I'd definitely have been in A&E trying to explain why it looked like I'd tried to kill myself.
Ok, slight problem with using the machine hold function of the PMDX: I only have one parallel port and it signals on port 2 pin 10. As I'm a glutton for punishment I was going try using LinuxCNC at first; if I was going the SmoothStepper route this wouldn't be an issue, sigh. I assume the only way I'm going to be able to use the machine hold function is fit a second parallel port so can anyone recommend a brand of card that works well?
I'm obviously not understanding something because I don't really see how the "K" input (assuming it's in machine hold mode) is different to the e-stop input on the BOB. Both inputs disable the step and direction signals and tell the computer the machine has stopped. The computer can tell the two situations apart because they come in on different pins but the machine is stopped either way so surely there's nothing the computer can do. Given that I currently only have one parallel port would an option be to put a machine hold switch in the limits circuit?
The PMDX-126 e-stop connector cuts the step and direction signals when the e-stop is triggered. My understanding is that this will probably stop the steppers faster than if you just cut the power and it doesn't rely on software (well, it probably relies on the firmware in the BOB but that's a lot better than going back to Mach / LinuxCNC to handle e-stop). My concern with having the limits cutting the power is that it will happen comparatively frequently compared to real emergencies and I've read in a few places it's not a good idea to just cut power to steppers that are in motion due to back emf. I wouldn't take just my word for it though. I assume if your bob can't cut the step and direction signals it would easy enough to stick in a relay to perform the same function.
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