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Author Topic: microsteps should not be included in resolution calculations  (Read 6424 times)

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Offline stirling

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Re: microsteps should not be included in resolution calculations
« Reply #10 on: April 07, 2008, 08:32:05 AM »
Thanks John

For the sake of fun... (will I ever learn?)

Say you're using morphing capable drives and you're cutting a diagonal straight line in XY and suppose the angle or gradiant of the line is such that the X axis is moving at a rate above where the drive morphs and the Y axis is moving at a rate below where the drive morphs.

Are you going to get the X axis moving in full step increments and the Y axis moving in microstep increments and therefore the detail of the path may be not quite what you'd expect? i.e. is the "staircase" going to be coarser than you'd have otherwise expected from the microstep resolution of the system? FWIW I think the answer's - no.

Answers on a postcard please - cos I don't know  ;D

And no I don't have too much time on my hands - it's lunch time!
« Last Edit: April 07, 2008, 08:38:48 AM by stirling »
Re: microsteps should not be included in resolution calculations
« Reply #11 on: April 07, 2008, 05:25:27 PM »
Stirling,

For a definitive answer to this, ask the same question in the Yahoo Gecko group. You will probably get an reply by THE expert, Mariss Freimanis, he is very good at explaining such problems.

As only half an expert, I think you are probably right in theory, but other factors come into the equation. Say you have a stepper with 200 full steps and a driver set at 10 microsteps and this setup yields 1000 microsteps per mm i.e. each microstep is 0.001 mm and a full step is 0.01 mm. Then if you were running in full step mode on two axes, you would generate a staircase of 0.01 steps, as you suggested. However, this would only occur on a G0 move, as at cutting speeds, the stepper will probably be in microstep mode, so your staircase steps would be too small to make any practical difference. . Mechanical inertia, cutting tool deflection and any latency in the driver and software will smooth this out. If you need a perfectly smooth finish with a stepper, then you are going to need a setup with more steps per mm.

As I understand it, a well set up servo will plot a mean path and so will not suffer the same problems.

Ian (who is still climbing the learning curve. :))

Offline stirling

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Re: microsteps should not be included in resolution calculations
« Reply #12 on: April 08, 2008, 07:37:18 AM »
For a definitive answer to this, ask the same question in the Yahoo Gecko group. You will probably get an reply by THE expert, Mariss Freimanis, he is very good at explaining such problems.

Agreed - What little I know about steppers and drivers has mainly come from reading Mariss's posts and papers on one site or another.

As only half an expert, I think you are probably right in theory, but other factors come into the equation. Say you have a stepper with 200 full steps and a driver set at 10 microsteps and this setup yields 1000 microsteps per mm i.e. each microstep is 0.001 mm and a full step is 0.01 mm. Then if you were running in full step mode on two axes, you would generate a staircase of 0.01 steps, as you suggested. However, this would only occur on a G0 move, as at cutting speeds, the stepper will probably be in microstep mode, so your staircase steps would be too small to make any practical difference. . Mechanical inertia, cutting tool deflection and any latency in the driver and software will smooth this out. If you need a perfectly smooth finish with a stepper, then you are going to need a setup with more steps per mm.

Agreed again - as you say, except at really slow speeds, due to the momentum of the rotor (in an unmounted motor) plus the system (for a mounted motor) a stepper is rotating smoothly and not actually "stepping" at all.

Ian (who is still climbing the learning curve. :))

Likewise - may your learning curve be a smooth one. ;D

Cheers

Ian