Educate me about estop chains and latches

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20 May 2017 14:10 #93401 by rodw
When I google estops, the rules change around the world, but I did find a definition of 3 categories of estops, none of which required removal of all power from the machine.

Category 0 is an uncontrolled stop by immediately removing power to the machine actuators.

Category 1 is a controlled stop with power to the machine actuators available to achieve the stop then remove power when the stop is achieved.

Category 2 is a controlled stop with power left available to the machine actuators.


So to me it seems we only have to worry about stopping the machine actuators and in this machine they are the stepper motors running off a 48 volt power supply and there is no mains power outside of the control cabinet.

Don't forget that latch0 in this case is your classical estop as it cuts power to the actuator mains power supply. This is clearly a category 0 estop.This is where your classical estop switches should all live. But in my view, you need to add the mains switching as well so if you want additional estop switches, you need to route mains power out of the control box which in my view is not desirable from a safety perspective.

The estop pendant relay has some constraints that prevent it from being included in that chain and while you could add in a relay to drop mains power, this also adds in an additional failure point and my advice said remote switching was non-compliant.

Disabling the stepper drivers also immediately removes power from the actuators (eg the stepper motors) so there is no controlled stop (unless you are telling me an estop in LCNC means a controlled stop but I don't see that in the docs anywhere). So I think the pendant also qualifies as a Category 0 estop. It could be improved by adding a relay to drop mains power and maybe I will add that.

I know Tommy had some other advice about what to do on a torch breakaway sensor but to me, I thought it made sense to treat this as an estop condition as it is possible that a torch with a dangerous plasma jet is waving around the place so estop will drop the spindle on signal that turns the torch on. From what I can see, the torch breakaway is also a category 0 event even if its not technically an estop button and I think this config treats it as such.

I think by wiring in the driver disable signals, I've created a safer machine as previously without it the steppers were enabled on Linuxcnc start up and this is no longer the case.

So in summary, I think the only improvement I could make is to add a 240 volt relay so the pendant could also drop mains power. When I started I never envisaged the machine would have a wireless remote with an estop switch on it and was uncomfortable running mains power out of the cabinet to those extra estop switches so we decided we would only have one estop button.

So I think after working through this, I should add the extra relay to drop mains power to the actuators.

Sorry about the long winded reply. This thread went in a totally different direction and never really added any clarity on how to use multiple estop latches which was my original question that I solved per my previous post. However I have learnt a lot so I thank everybody for their input and the significant differences of opinion as they always force you to make your own decisions (and maybe add another difference of opinion).

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21 May 2017 01:57 #93419 by racedirector
There was a really big discussion on this on another forum with the guy working out various methods of handling estops, heres a link to the discussion and it all starts on page 6

www.mycncuk.com/threads/6565-Ready-Steady-Eddy/page6

I followed this generally when I built my machine and used his diagrams to implement my strategy's
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21 May 2017 02:41 #93420 by rodw
Thanks Bruce. An interesting read. I think I got it pretty right.

I forgot to mention something. (well actually I forgot about the feature totally as its been so long since I wired up the AC side of my machine).

My control box contains an AC power point for the future down draft exhaust fan so when the main estop is hit, power is dropped to that to ensure there is no power to anything on the table on an estop.

So now I've remembered the feature, I have to think about how the pendant estop should deal with that.

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23 May 2017 13:31 #93537 by bevins
I can see where maybe shutting power to all equipment on a machine like a plasma where not alot of forces involved.

Try running a gantry well over 1200lbs and removing power to the drives, then sit back and watch it slam into the stops or come off the rails. My Z head on the Y has no hard stop. thats 1200lbs flying ritght off the machine.

Plasma doesnt have a spindle, so you dont have to worry about the spindle spinning down because no power.

Me personally, I would never remove AC power on a e-stop, never, too dangerous, besides the es-stop is suppose to stop movement and removing AC power does not stop movement. Coasting to a stop is not what I call e-stop.

That's just my opinion. I guess maybe thats why they have different modes of e-stop. That write up is interesting.

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23 May 2017 20:13 #93567 by rodw
Bevins thanks, yes quite a difference given I only have about 90 pounds moving on the gantry.... It has always stopped dead in its tracks on estop. I think the idea when we set it up was to make sure there were no intermediate circuitry between disabling movement and pressing the button, just a mechanical switch. Switching 5 volts to the drive disable signal on the stepper controllers by the e-stop switch requires a working 5 volt power supply and using an output on the Mesa Card implies that the PC or the board has not hung.

The idea was always to learn how to make a machine so the next one will have a different estop switch that you can add as many switch mechanisms as you want to by clipping them to the back of the switch module.

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25 May 2017 22:08 #93639 by tommylight
Heavy machinery must have braking resistors. Even some smaller ones have them, some inside the servo drives some outside. Some have terminals for them.
All motors have physical brakes on those machines, all of them without exceptions.
I have some 200W ones with brakes on them.
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