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Author Topic: closing the loop in mach?  (Read 5363 times)

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closing the loop in mach?
« on: August 21, 2008, 01:17:55 PM »
My CNC is stepper based.   It happens to be a home built systems, so it does not work, all the time. I have added 3 encoder to my CNC looking to make my CNC closed loop. From what I can tell, this is not supported by mach.  It can be done with extra hardware, but only for fault states.  I am looking to do on the fly position correction. Is this possible?  Could it be done with a plugin?
-DingbatCA

Offline Jeff_Birt

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Re: closing the loop in mach?
« Reply #1 on: August 21, 2008, 01:23:52 PM »
Short answer, can't be done in Mach, need external hardware. Longer answer: This same question has been asked/answered about 142,000 times. A few minutes of searching (Search is towards the top of every screen) would have led you to several posts on the subject.

If your loosing steps it's because you have a tuning/mechanical issue. A closed loop system can still not prevent (all) step loss, most will just warn you that the error has exceeded some threshold.
Happy machining , Jeff Birt
 
Re: closing the loop in mach?
« Reply #2 on: August 21, 2008, 01:43:08 PM »
I think I can address the problem of missing/correcting steps with a controller board in the middle. Was just wondering if there was an easier way then building a controller board.
Re: closing the loop in mach?
« Reply #3 on: August 21, 2008, 01:50:44 PM »
Could this be done with smoothstepper?   It would take a mode to there firmware.
Need PoppaBear to reply to this! Re: closing the loop in mach?-
« Reply #4 on: August 22, 2008, 01:19:42 AM »
There is a new product/device called a CNC Brain that claims to be able to do exactly what you want.  It is still in R&D and there was talk of a plugin for Mach being developed with the help of PoppaBear/Scott but I have not seen/heard anything about this being completed yet.  Maybe Scott could comment on the status of the plugin development?

FYI...  http://www.safeguardrobotics.com/default.aspx?tab=home

HTH,
Sid
Re: closing the loop in mach?
« Reply #5 on: August 22, 2008, 10:14:27 AM »
I fail to see how closing the loop on a stepper-based system would be of any real value.  The *only* way a stepper will lose position is if you attempt to drive it beyond its capability - either accelerating too fast, or exceeding it's torque capability.  Once that's happened, how would you get it back to the correct position?  The speed will be down, so you need to accelerate, which will exacerbate the problem.  So, how are you going to "catch up"?  With steppers, you must make sure you *never* exceed its capabilities.  If you do that, then they never lose position.  Your real problem is either your motors are too small, or you're driving them too hard.

Regards,
Ray L.
Regards,
Ray L.
Re: closing the loop in mach?
« Reply #6 on: August 22, 2008, 10:46:02 AM »
That was what I was looking for!  Hummm, Mach support?
Re: closing the loop in mach?
« Reply #7 on: August 22, 2008, 10:52:59 AM »
Kibibble
The motors are strong enough, and I am not over driving them.  The problem for me is the controller, and signal.  Both of which are crappy.  I am looking at a smooth stepper to fix one of those problems.  Geckos to solve the rest.  But even then, I can still miss steps. 
DingbatCA

Offline ger21

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Re: closing the loop in mach?
« Reply #8 on: August 22, 2008, 11:20:52 AM »
But even then, I can still miss steps. 
DingbatCA

Yeah, you can still lose steps if your motors are too small, or you try to run them to fast, just as Kibble said. There are other reasons as well, but none of them can be fixed by a closed loop system. Get some good drives, and a properly designed motion system (motor, screws, gearing), and you should never lose steps if you run the machine as it was designed to run.

The CNC Brain seems to be promising the world, but until we can actually see what it can do, I'm still a bit skeptical.
Gerry

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Re: closing the loop in mach?
« Reply #9 on: August 22, 2008, 12:07:07 PM »
The brain can only do so much...  It should be able to fix single step problems, but any thing much bigger, i don't think so.  For that cost, I would rather up grade my drivers and get a smooth stepper. I think that would be a much better use of my money.