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Author Topic: Closed loop question  (Read 55 times)
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Mooselake
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« on: November 12, 2017, 09:25:47 AM »

Another off the wall kinematic s question.  I have a maslow CNC router that uses brushed DC motors with a worm drive gearbox and an (around) 8K PPR encoder.  The current firmware doesn't have any accelerat ion or planning but it's being discussed on their very active forum.  With the current motors it's limited to somewhere in the 40 ipm range by hardware.  There's two motors off the upper corners and a polarbot style 2 chain drive.

I can see where decelerat ion and path planning could be useful, but what about accelerat ion (first or higher order)?  Does it fall out of the closed loop drive, just start up fill tilt and let the encoder feedback sort out, and continue to where it's time to start slowing down.

It sounds (to me) like it's essential ly a servo.  How did the big iron handle it?

Kirk
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steve323
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« Reply #1 on: November 15, 2017, 01:16:14 PM »

I suppose it depends on the controlle r if it will eventuall y catch up to the exact position or if the motors will just position to the closest full shaft rotation. 

It also could depend on the machine.  My CNC router has 2 X motors.  If one motor skips, the gantry will bind and there is no recovery.

Steve
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ArtF
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« Reply #2 on: November 15, 2017, 03:09:34 PM »

Generally, you have to take into account accelerat ion in order to do planning,
otherwise your accuracy will depend on the motor and its current direction al load. Not a good way to move anything. In any motion firmware or software you need at least first order bang-bang accelerat ion and to take it into account in positiona l calculati ons.
   I prefer higher if heavier in order to get speed. But allowing the encoder to sort things out would be very bad, the X and Y axis wouldn't be equal in step
count during load, and lines would become ellipses. .

  May not be noticed depending on speed or power, but it'd be there. (If Im understan ding the question. .)

Art
 
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Thanks, have fun,
Art
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