need some help with homing switch config

[jcizek]

Hi all,
I feel like I'm oh-so-close to having my first CNC machine actually do something, but I am stuck and need some assistance...

I cannot figure out what I should be using (and what I have wrong) for my limit switch config. I have read the (somewhat limited it seems) documenatation on homing in the EMC docs but am still confused.

Basically, my machine has limit switches placed about 1/2" from the hard stops. They are wired in series and are normally closed switches. What I'd like to have happen is to have "home" .5" from the limit switch.
Not sure if I should choose "both limit" or "home+both limit" or what in the config... I assume that if I want the home to be .5 inside the switch limit, I just enter .5 in the "home" box in the stepconf setup. Is that correct? I have tried inverting the input, changing combinations, you name it. Kind of frustrated.. I keep getting errors of "joint 0 error- limit switch" or "can't unhome while joint0 moving" errors depending on the setup.

Any guidance would be *greatly* appreciated!! Thanks in advance.
-James

[jcizek]

well folks, I have solved part of my problem. The documents were wrong about the pinouts on my break out board. I now have my machine homing properly.

I still have a question though, I am a little unclear about the difference between "Home" and "Home+Limit"

When I use "Home", the machine homes properly and sits at the home position waiting like I'd expect. But if I use "Home+limit", the machine homes properly, then about 5 seconds after homing is completed, it throws a joint 0 limit error.

So my first question is, is it safe to use HOME only? I assume if things went awry and the machine reached the switched during normal operation, it wouldn't stop it, correct? Isn't that what the LIMIT part of HOME+LIMIT is for?
Second, if I want limit protection, do I have to leave the HOME position at 0,0 so the machine sits on the switch the whole time it's homed?

Thanks!! -James

[chester88]

I'll try to tell no lies here...homing can be confusing

using HOME only will not give you limits protection at that end of the axis.
Yes that is what the HOME + LIMIT is for.
HOME POSITION is a location that the machine will go to AFTER homing is complete.
HOME SWITCH location is a reference point for EMC to figure out where everything is.

You don't mention what you entered for home switch location or table travel.

Chris M

[jcizek]

Hi Chris,

Thanks for your reply. You are right! Homing is confusing. Being brand new to CNC, I have spent literally months trying to read as much as I could about everything. I think I have a pretty good grasp on things, but homing is defintely causing me some lost hair :-)
I have my home position set to 0.1 on the X axis, and 1.0 on the Y axis. Both of those locations are far enough away that they un-trip the switch. Currently I have the table travel set "0 to 24" in both X and
Y. I wondered about this some, but after reading the manual, and seeing
where axis starts up, it seemed like 0,0,0 would be in the corner of the table and X and Y would be postive from there and Z would be negative as it traveled towards the table.

So, I wonder if it's possible to have a HOME anywhere BUT on the switch trip location if you use HOME+LIMIT?

Thanks. -James

[chester88]

hmm you still haven't clearly told me what your home switch location is set at.
i will assume it's set to zero.
It's easy to confuse home switch location and home location - they are not the same thing.

Your right you don't want home location on a limit switch.
but home switch location can be on a limit switch.
( Well technically thats not true either - the stepconf program will make sure the limit and home are just slightly different by about .001 if i remember right)

The other thing to note is all these settings are defining the MACHINE COORDINATE system. When machining you set a user coordinate system (G54 etc) where the Origin (zero point) can be set anywhere inside the machine envelope.

[jcizek]

sorry Chris, I thought I had explained that in the first post. I have only one set of switches on each axis, and they are about 1/2" from the hard stop of the machine. I have the "home switch location" set to 0 in the config, although "HOME" is set to a bit inside of each of the switches. I don't know why I'm having such a hard time understanding this.... the docs are just so vague as to what each setting means exactly, and what requirements each has...
After much screwing around, i still can't get anything to work right except choosing "home" only. As soon as I add limit in any way, I get errors.

So I wonder... is my problem that i have listed 'home switch location' to 0?
Thanks! I appreciate your patience as I struggle to understand!!
-James

[chester88]

hmm off hand I can't think whats wrong with this.
Can you post your configuration files including the stepconf file?

Chris M

[jcizek]

Well Chris, I think I have figured it out. I went back out there today, set it up the way I thought was correct... and it worked... well, ok, it worked for about 2 minutes ;-)

I setup with both home+limit, the machine homed properly and I just let it set... doing nothing, not touching anything. About 2 minutes later, I get a joint 0 limit error pop up. I start experimenting, and the axis' sometime home successfully and sometimes not, Z is the worst, Y second and X never has a problem.

So, I'm thinking (as I have read in other posts) that I have some type of noise problem on the switch lines. I used shielded cable, and tied the shield to gnd on the controller end (and left the ground unconnected at the machine end, as is usually best practice for a shield line) but still have troubles. My break out board is supposed to have pull up resistors to pull the line up when the switch opens, but I'll have to ohm that out and verify. If all is well, looks like i'll have to build a buffer circuit to buffer the limit lines.

I'll work on my false tripping problem and report back if that solved the problem.
Thanks again Chris!! I got anxious after all the days of screwing around with this and disabled home and limit switches, manually homed it, and carved my first piece today. It looks beautiful! I appreciate your help, it's really nice to have a forum like this with great people like yourself to bounce things off of and receive help from. Thanks again!!

-James

[chester88]

Hey James.

Glad ur figuring it out.
You did basically what was the next thing I was going to ask.
could be the switches themselves too.

I find trying to help people helps me understand EMC better.
You are very welcome.
There is also an EMC forum too at LinuxCNC.org - Home

Chris M