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Author Topic: How to make limit only trigger in one direction?  (Read 461 times)

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How to make limit only trigger in one direction?
« on: February 09, 2019, 06:28:27 AM »
I have the z-- limit set to pin 13 and the z++ limit is NOT set to that pin; the switch on pin 13 will trigger when moving in the negative direction as it should, but then after I hit reset and try to move the opposite way the limit will trigger once more as the switch is opening.

So how do I make the z-- limit switch only work when the Z axis is moving in the negative direction?

Thanks!
Limit triggering twice? & Mach not knowing which direction triggered limit?
« Reply #1 on: February 09, 2019, 06:35:33 AM »
So when I trigger the limit I have the auto limit override set so all I have to do is hit reset, but then as I try to move off the switch the limit will trigger again because the button is still technically depressed. I must reset a second time before I can move off the switch.

I'm sure there is a way to fix this, since this seems like it would be a very common issue. If someone could enlighten me I would greatly appreciate it.

But also... shouldn't there be a way that mach knows which direction it was moving when it triggered the limit, and then allow you to move in the opposite direction without triggering again, since most people have both directions wired to the same switch?

Thanks!

Offline MN300

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Re: How to make limit only trigger in one direction?
« Reply #2 on: February 09, 2019, 07:46:44 AM »
Limit switches mark the ends of the safe range of travel, in other words, the two limits. You setup is doing exactly what you have asked of it, allowing travel between the turn on and turn off points of the switch. If you only have one switch make it the home switch and use soft limits. Having a limit switch at each end is safer but many people successfully run with only soft limits.
Re: How to make limit only trigger in one direction?
« Reply #3 on: February 11, 2019, 04:14:16 PM »
I specifically made two separate posts on purpose because they are different questions. Merging them for me just makes it so much more confusing. Asking multiple questions in a forum post is always a bad idea, and usually resulting in only one being answered. And of course thats exactly what happened and now the last question didnt even get addressed.

Anyway- why does mach have a separate line for z++ and z-- if they both do the same thing then?
« Last Edit: February 11, 2019, 04:31:01 PM by cds333 »
Re: How to make limit only trigger in one direction?
« Reply #4 on: February 11, 2019, 04:59:18 PM »
Fine, everyone please ignore the first "reply", that was actually supposed to be a separate post. Ill re-post that one.

Regarding the original question that MN300 replied to- so youre telling me that it is normal for every time a limit triggers that you must hit reset twice, as the limit is supposed to trigger again while the physical switch contacts are opening?
« Last Edit: February 11, 2019, 05:11:43 PM by cds333 »

Offline MN300

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Re: How to make limit only trigger in one direction?
« Reply #5 on: February 11, 2019, 05:33:31 PM »
From the Mach4 Operation manual,
https://www.machsupport.com/wp-content/uploads/2014/05/Mach4%20Operation%20Manual.pdf

5.1 Limit Switches
Limit switches are physical switches that mark the end of a machine’s range of motion. Sometimes one or more of these will double as a home switch. The hard limit switches are basically the last line of defense before major damage occurs. Without these switches the machine could slam into a hard stop at the end of travel or even send the table right off the end of the rails, depending on the configuration. Running a machine without limit switches is risking damage to ball screws and nuts, bearings, castings, people, floors and walls.

Z++ and Z-- do not do the same thing. They should be used with two different switches. There are three switches designated for each axis, two limits and home.
The limit switch at the positive end of travel connects to the ++switch input, the limit switch at the negative end of travel connects to the --switch input. You seem to have configured both inputs to the same switch. The home switch is used to establish a known position on the axis. As said above, one of the limit switches can do double duty and serve as a home switch too.

If you have one switch at the end of travel it can be both a limit and home switch. Soft limits would then be used to sense the other end of travel.

Alternately your single switch could be the home switch and it would be mounted anywhere within the range of safe travel. Then soft limits would be used instead of physical limit switches.

Offline ger21

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Re: How to make limit only trigger in one direction?
« Reply #6 on: February 11, 2019, 06:51:23 PM »
Quote
so youre telling me that it is normal for every time a limit triggers that you must hit reset twice, as the limit is supposed to trigger again while the physical switch contacts are opening?

No, it's not normal. With limit override, you should be able to back off the switch without triggering the limit again?
Are you using a chinese motion controller by chance? If so, that may be the issue.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: How to make limit only trigger in one direction?
« Reply #7 on: February 12, 2019, 10:09:54 AM »
Thank you for the detailed answer!

It is a C10 controller card - https://cnc4pc.com/c10-bi-directional-parallel-port-interface-card.html, perhaps this is the culprit?

So why then, if I only have *one* limit switch set up (either z++ or z--) does the machine halt when I depress the switch regardless of what direction it is traveling? If the head is jogging (--) and I hit the switch it stops; if the head is jogging (++) and I hit the switch it stops. Why does it do that?
« Last Edit: February 12, 2019, 10:12:12 AM by cds333 »

Offline ger21

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Re: How to make limit only trigger in one direction?
« Reply #8 on: February 12, 2019, 10:37:31 AM »
If you are using the parallel port, then the board is not the issue.

Mach3 does not know which way you are moving when the switch is hit. (Or it doesn't care). Doesn't matter which switch it is either.

Your issue may be noise or switch bounce.
Try setting the debounce in General Config to 5000 and see if it helps. If it does, lower it until the problem returns, then add a bit more.
Gerry

2010 Screenset
http://www.thecncwoodworker.com/2010.html

JointCAM Dovetail and Box Joint software
http://www.g-forcecnc.com/jointcam.html
Re: How to make limit only trigger in one direction?
« Reply #9 on: February 13, 2019, 09:00:59 PM »
Then why does mach have the ability to put both switches on different pins if it doesnt know or care which way it was going before triggering?

Why not just have one pin for (+/-) and one for home?