Gantrykins Questions

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28 Sep 2012 10:24 #24686 by andypugh
Replied by andypugh on topic Gantrykins Questions
PortlandGTS wrote:

At first, I was getting following errors. I found this thread which helped a lot:

http://www.linuxcnc.org/index.php/english/component/kunena/?func=view&catid=10&id=22514&limit=6


Have you tried different values for DEFAULT_ACCELLERATION?
It needs to be a little less than the STEPGEN_MAXACCEL

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28 Sep 2012 14:50 #24692 by PCW
Replied by PCW on topic Gantrykins Questions
Yes, for the hardware stepgen its important to set stepgen
maxaccel to about 20% more than max_acceleration and default_acceleration

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28 Sep 2012 18:20 #24698 by PortlandGTS
Replied by PortlandGTS on topic Gantrykins Questions
PCW wrote:

Yes, for the hardware stepgen its important to set stepgen
maxaccel to about 20% more than max_acceleration and default_acceleration


That sounds like the problem. I have max_acceleration higher than maxaccel.

I'll give it a try and report back.

thanks,
Tom

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28 Sep 2012 20:02 - 29 Oct 2012 00:21 #24701 by andypugh
Replied by andypugh on topic Gantrykins Questions
PortlandGTS wrote:

That sounds like the problem. I have max_acceleration higher than maxaccel.


That fits the behaviour seen, with an overshoot and backup.

Imagine that the motion planner is the rabbit on the side of a greyhound track. Further imagine that you are driving a sleigh drawn by reindeer, and job is to exactly follow the rabbit.
(you are the stepgen, by the way).
So, the rabbit sets off, and you crack the whip and set off in pursuit. The rabbit is a bit faster (being electric, after all) but you have a higher top speed, so eventually, after an initial lag, you catch up and pace the rabbit for a lap.
Then the rabbit stops. Your lack of acceleration is now more obvious, as no matter how loudly you shout "Whoah Prancer! Whoah! Dancer" the sleigh over shoots the rabbit, and you then have to come to a halt and persuade the reindeer to back up.

That's your machine, that is.
Last edit: 29 Oct 2012 00:21 by andypugh.

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29 Sep 2012 06:39 #24711 by dab77
Replied by dab77 on topic Gantrykins Questions
I've always believed there were rabbits inside my pc....

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02 Oct 2012 05:49 #24849 by PortlandGTS
Replied by PortlandGTS on topic Gantrykins Questions
Ok, I fixed the acceleration parameters, and now everything runs fine. I was confused about all the different acceleration parameters, but now I think I understand.

One last question: Since I switched to the 5i25, I frequently hear a little "chirp' from the motor when a joint stops moving. Is this anything to worry about?

Thanks to everyone. Without this forum, i would be lost!
Tom

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02 Oct 2012 09:59 #24855 by andypugh
Replied by andypugh on topic Gantrykins Questions
PortlandGTS wrote:

One last question: Since I switched to the 5i25, I frequently hear a little "chirp' from the motor when a joint stops moving. Is this anything to worry about?


It might be, is there any indication that the motor is slipping a few steps at that point? it may be that your DEFAULT_ACCELLRATION limit is still a little high.

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02 Oct 2012 10:45 #24860 by BigJohnT
Replied by BigJohnT on topic Gantrykins Questions
PortlandGTS wrote:

One last question: Since I switched to the 5i25, I frequently hear a little "chirp' from the motor when a joint stops moving. Is this anything to worry about?


Tom,

Run the stepper test from the manual to check for missing steps at direction change.

John

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06 Oct 2012 01:26 #25045 by PortlandGTS
Replied by PortlandGTS on topic Gantrykins Questions
I ran the stepper test. I even bumped it up to 1000 repetitions and let it run for an hour or so. After that long run, position was only off by 1/32". Even if that small discrepancy was caused by lost steps, that would be 6 steps missed out of 400,000. So I don't think I have a lost step problem. So I'm going to assume that it's just a noise the steppers make when they switch direction.

That's a nice little program in the back of the manual. I hadn't noticed it.

Thanks,
Tom

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06 Oct 2012 10:39 #25055 by BigJohnT
Replied by BigJohnT on topic Gantrykins Questions
If you missed any steps at all something is wrong. The chirp at direction change may be you need to increase the direction change times.

John

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