Machsupport Forum
Mach Discussion => General Mach Discussion => Topic started by: kb18951452 on February 09, 2011, 03:11:35 PM
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So, i have a very weird problem. I can't figure out how to look for this on the forums, though i looked around best i could. Thought someone might have some ideas.
So, If i jog at a high speed, like 500ipm, this is what happens.
on the speed readout in mach:
From 0-250 ish, the motors accelerate
from 250-500 ish, the motors decelrate.
If i run at 400 ipm, it actually moves as a low speed.
I am sure i am losing steps. I get the idea that the kernel speed might be a factor, but i really don't know. Honestly, i cant even spell kernel :)
Has anyone seen this kind of thing before? Maybe i don't know to determine the max speed. , I honestly dont know. Would love to get back.
Thanks,
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Not really sure what you are meaning, is that the numbers you actually see in the Units/Min DRO as you jog?
What are your steps per unit?
Is it steppers?
Do you hear the motors sort of squeal when the deceleration is happening?
What do you have the kernel set to
Hood
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Could it be possible that the accel is set at a very high point.
Check on the Motor Tuning under config menu and verify the accel slider and send it on the other end.
Just a tough
Jeff
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Where would i go to see what the kernel speed is set to? (I havn't changed it, don't even know how :), It might be the acceleration, but i changed that, And the problem was happening even when the motor was dismounted (900oz steppers).
Hood,
Yes, when Mach is showing a running speed of , say, 0-100, the motor will, Accelerate from 0-50, then decelerate from 50-100. I don't know the exact numbers, it seems we can get it to going 250 (as the computer reads) before going faster will cause it to decelerate. If youll let me know where to check those settings, or maybe i can load a file that shows all my settings
This machine is electronically identicle to another machine i have. Save the computer. This computer has much less RAM, that my other one.
Secondly, On my machine at home, when i drag the Velocity slider all the way over, it stops at 1184. I have thhis machine (the one with the problems) set at 1000IPM. Same number of Steps per in., mechanicals are identicle drive systems. Steppers are identicle.
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Do you have a high kernel frequency set, if so then it could be your problem. If the machine is identical to the other one copy over the xml from the good one and that will put the same settings in.
Hood
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Hood, Thanks for the reply.
Where might i go to see/set the kernel speed? And secondly, what would be a reasonable speed, or how might i go about determining that?
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Reasonable speed is impossible to say as it varies from machine to machine depending on many things such as motors. drives, power supply, weight of axis etc.
Kernel speed is seen from Config menu then Ports and Piins.
Hood
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Kernel speed set to 25.
We did some precise testing, and find that the "turning point" appears to be just above 500ipm Whats interesting is that this is about exactly half of the so called Max Velocity (According to Mach motor tuning). I know from experience that i almost never go over 200 (Not while machining, and the higher speed just helps with rapids. Any other Mach settings? Otherwise, seems my next stop is the BOB manufacturer.
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Can you attach your xml please,
Also what type of drives and BOB do you have?
Hood
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Gotta get the XML file later, but we are using the cnc4pc C-11 with 4 Gecko 201x Drivers.
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What is the pitch of screws and have you any gearing between motor and screw?
Reason I am asking is I am just wondering what RPM your steppers will actually be running when you are attempting 500IPM.
Hood
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~1275 steps per inch. I don't know all the gearing details, but i do remember that the steps per is ~1275 (maybe 1272.24)
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That doesnt sound much, 200 steps per rev of motor x 10uS of drive is 2000 per rev of motor so your axis moves over 1.5 inches per motor rev?
Hood
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that sounds about right.
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So what kind of screws do you have? or is it a rack and pinion?
Hood
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R&P and i don't know the pulley diameters (Only needed to know them when calculating things at the beginning.)
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I would guess the problem may well be the Breakout board. I suspect you're seeing the upper limit of the step pulse frequency it will pass through, and beyond that point the step pulses are "aliasing", so the frequency coming out of the BOB is lower than the frequency going in. CNC4PC uses some strange pulse-shaping circuitry on their boards that I could see causing that kind of problem if you've got a defective chip or component on the board. It would be instructive to try temporarily wiring up a different breakout board, or even a completely passive one, just on a single drive, and see what happens.
Regards,
Ray L.
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I actually replaced the ICs on this board after i conencted 12v to the 5v terminal.....
Do you recommend a BOB? I'm familiar w/ cnc4pc, and would probably just get he updated version of this board.
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So. It appears to have been the BOB. Replaced it.
We are having a diff problem now, however.
One side of the gantry seems to be running faster than the other.
I am wondering: We slaved the A to the X, and since then have adhjusted the steps per inch rate for the motors. Is there a chance that the steps per inch isnt getting properly applied to one of the motors? Maybe we need to unslave change the settings, then reslave the motors?