Maximum Acceleration Calculator
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@genewild said in Maximum Acceleration Calculator:
Hi! Thanks for making this tool, it is very handy.
I'm currently trying to grasp the limitation of accelleration on stepper motor drives. I've used your calculator on a few motors, and i noticed that generally people report running 2-4 times higher accelleration on their printers before running into issues than your calculator says is possible.
I am not sure these printers actually run at the requested speed, but certainly there is some difference between running at the speed your calculator produces and a vastly higher speed, i can see that on my own printer which accellerates more violently.
So i am wondering, what gives? Shouldn't the stepper skip steps if you ask it to go TOO fast? The stepper driver has no feedback loop to tell. I can't notice lost steps below 20.000 mm/s² accelleration using really small NEMA 17 motors with only 1.68 A capability and a rated torque of only 36Ncm, inertia of 54 g/cm² driving a rather heavy gantry at 800gr.
I would really apprechiate if someone could explain this discrepancy to me, i am not very familiar with stepper drives.
You raise some good questions - and unfortunately I'm not the one that can appropriately answer them.
I too see a lot of discussions of folks using acceleration values ~10x what I would expect. A related reason why this and the inverse is true (why the calculator is 10% of what some other folks are using) is because of the below fact:
https://forum.duet3d.com/topic/6/stepper-motors-for-corexy
From: @dc42If you want the motion lag to be no more than one 1/16 microstep during acceleration, then you need to multiply by 9.8%. So the available torque is 65 * 0,85 * 0.71 * 0.098 = 3.8Ncm.
In the past the general consensus was to multiply by the available torque for a given stepper by 9.8% as a sort of factor-of-safety. I follow this factor-of-safety approach.
I recently came across this super cool spreadsheet - however the equations/macros omits the factor-of-safety for available torque, while using it when calculating what the required torque is - the opposite of what I would expect. Here is an example screen grab of a few motors using the equations (without the factor-of-safety):
The chart shows the expected speeds at which motor torque begins to drop off - which match the Duet calculations for max speeds here:
https://duet3d.dozuki.com/Wiki/Choosing_and_connecting_stepper_motorsThe issue, to me, is that the red dashed line for torque required is of an extraordinarily low value relative to the stepper curve maximums. A (in my eyes, appropriately allocating the factor-of-safety) corrected chart is below - note the Y-axis changes:
Anyways, more to dig in there. Perhaps @dc42 can go into more detail regarding the motion lag considerations at 1/16th of a microstep. Maybe nowadays that consideration isn't too big of a deal.Side note - but most folks I see using more than, say, 20,000 mm/s^2 acceleration values (on motor/machine setups I would calculate having issues) typically run Klipper. Perhaps issues they may see at such accelerations are reduced by that input-shaping implementation.