handling limit switches

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handling limit switches

Postby mccafferty » Mon Dec 19, 2016 2:29 pm

I am upgrading an existing machine with UC300ETH-LPT5. I am currently developing the schematic and have a question about handling limit switches. I am using solid state limit switches and plan to bring each limit to its own input. I am wondering if I need to have an override switch for each limit, a master override switch, or....

If the machine hits a limit switch, it sets a reset which in my case will kill power to the drives. Do I have to have a physical bypass switch around each limit switch or can I have an input defined as "override" that would cause the software to ignore the limit logic so I can jog the machine off the limit. Or, is there some other clever way to handle this?

Thanks
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Re: handling limit switches

Postby beefy » Mon Dec 19, 2016 5:32 pm

If the machine hits a limit switch, it sets a reset which in my case will kill power to the drives.


If you are talking about your own circuit then you will need a bypass switch. UCcnc can't jog if YOUR circuit has killed power to the drives.


When you still have power to the drives after a limit condition, you have a big button on the main screen "Override Limits".
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Re: handling limit switches

Postby mccafferty » Mon Dec 19, 2016 9:29 pm

Ahhh, I see. When you hit a limit, it just stops motion..... Then the screen override button defeats this. No need to kill power to the drives....

Thanks
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Re: handling limit switches

Postby A_Camera » Tue Dec 20, 2016 7:52 am

mccafferty wrote:Ahhh, I see. When you hit a limit, it just stops motion..... Then the screen override button defeats this. No need to kill power to the drives....

Thanks

That is the normal procedure. Limit switch is not an emergency condition, so you don't need to cut power. UCCNC handles the limit switch, stops the movement and waits until the limit switch is inactive again. If you hit the Limit override button it will allow you to jog away from the limit.

If you want to kill power on hitting the limit switch, perhaps it is possible to switch it on again through a separate output connected to a relay. Maybe there is a function in UCCNC to set an output when "Limit Override" button is pressed. I don't know, have a look. If that is possible then you can go ahead the way you planned, but it is more complicated without any real benefits.

In my case the stepper drivers are disabled and that cuts the power to the steppers and stops them, which is what I want to do, because I want to release the torque in case a limit switch is hit. The disadvantage is that I have to reset the zeros for all axes after this. There is a different approach, some people don't want their drivers disabled in this situation so they have a manual disable/enable switch, and the benefit of that is that the steppers decelerate and don't lose zero position. Cutting the power to the drivers is a third approach which I have not seen before.

BTW, I would not use solid state switches as limit switch. The good old simple mechanical switches are in my opinion better and safer. I also don't know what the point is of connecting each limit switch to separate inputs. All my limit switches are connected in series, and unless you have a very advanced system, using up six (or eight with four axis) inputs for limit switches is waste of five (or seven) inputs. Is it important for you to indicate which limit switch is hit? Normally you have a look at the machine and you know which way to jog out of the limit.
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Re: handling limit switches

Postby charliem » Tue Jan 03, 2017 11:41 am

OK I have a question about this topic.

I would not use solid state switches as limit switch. The good old simple mechanical switches are in my opinion better and safer. I also don't know what the point is of connecting each limit switch to separate inputs. All my limit switches are connected in series, and unless you have a very advanced system, using up six (or eight with four axis) inputs for limit switches is waste of five (or seven) inputs. Is it important for you to indicate which limit switch is hit? Normally you have a look at the machine and you know which way to jog out of the limit.



Can you explain how a mechanical switch is better than a lets say a hall effect? Using mechanical switches I had to add some small caps to elimminate point bouce and had nothing but trouble out of them. It seems that a non contact switch would work better. Also My router has 2 motors on the Y axis, using one input how can the software square the axis?
Regards
Charlie M
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