Playing with GRBL 0.9g axis acceleration settings

Discussion about the arduino based g-code interpreter, grbl
cvoinescu
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Re: Playing with GRBL 0.9g axis acceleration settings

Post by cvoinescu » Thu Jul 31, 2014 11:38 pm

GRBL has second-order motion profiles (that is, it limits acceleration). That has not changed in 0.9x. The significant difference from 0.8x is that acceleration is independently configurable for each axis, which is a major improvement.

TinyG has third-order motion profiles: it limits the rate at which acceleration changes (a parameter named jerk). TinyG has always had independently configurable axes. So TinyG, with its constant-jerk motion profiles, is still better than GRBL 0.9x, which has constant-acceleration motion profiles.

If you graph speed vs. time, GRBL's graph is made of line segments (horizontal, and sloping up and down at a fixed rate), with sharp angles between them, whereas TinyG's is made mostly of curves (parabola arcs) and has no sharp bends at all. They're called S-curves, because the curved parts of the graph resemble the letter S.

The most striking difference between the two is that, in a fast machine, TinyG shakes the table much less, and its motions appear smoother and more natural. Because of this, TinyG can reach higher traverse speeds without causing missed steps. There's also the fact that TinyG has a much higher maximum pulse rate, which can become the limiting factor with GRBL for a fast Shapeoko (not terribly important for milling, but important for pick-and-place, for example).
Proud owner of ShapeOko #709, eShapeOko #0, and of store.amberspyglass.co.uk

Auarhau
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Joined: Tue Feb 25, 2014 8:46 pm

Re: Playing with GRBL 0.9g axis acceleration settings

Post by Auarhau » Fri Aug 01, 2014 7:14 am

From the top of my head I believe I have 150 acceleration on x and y and 100 on z (with the acme upgrade).
And max seek speed at 5000 for x and y and 2500 for z. I have also never suspected my machine to miss steps, so I'm either not very observant or I got the settings pretty right the first time:)

I did try to put both acceleration and seek speed to ridiculously high numbers. Like 800 acceleration and 30.000 mm/min. I could not get it to "crash" or mess up.. But I suspect I max the speed and acceleration at some point and that the numbers above that did nothing.? :roll:
ShapeOko 2. Nema 17 74 oz·in. GAUPS shield on Arduino Uno. DRV8825 Drivers x4 . Kress 1050 FME-1. Z Acme Screw. Threaded inserts table.

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