Do we really need microstepping for the Z-Axis?

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Do we really need microstepping for the Z-Axis?

Postby flurin » Mon Jul 22, 2013 9:21 am

The Z-Axis has a setting of 2560 steps/mm (M8); 2560 microsteps = 200 * 16 microsteps /1.25
For each microstep the Z-Axis moves 1.25/2560 = 0.0004882 mm!
But do we really need such an accuracy?
I dont think so, considering the stepper motor angle accuracy of +/- 5%
So why not setting the stepper motor driver to full step.
The resolution will be 1.25/200 = 0.00625 mm

What do you think about? pros and cons?
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Re: Do we really need microstepping for the Z-Axis?

Postby cvoinescu » Mon Jul 22, 2013 10:01 am

There's no accuracy benefit. You should consider half-step, though: it's less noisy. Which, by the way, is the advantage of microstepping: smoother, quieter motion at low speed, and less likely to excite mechanical resonances (which can be a problem with stepper motor systems and can lead to loss of steps, especially at low load).
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Re: Do we really need microstepping for the Z-Axis?

Postby orcinus » Tue Jul 23, 2013 2:53 pm

We don't need the accuracy, but we need the step to be as small as possible in order to "fit" the layer height to step height better.
The bigger the step, the bigger the error you'll get from not having the layer height an integer multiplication of steps.

There's also the possible issue of pole torque. If you're using full steps or half steps, the torque driving the stepper into a pole will be much higher than for driving it into a microstep. That will result in a stronger overshoot and more oscillation around the pole before settling.

Depending on the mechanics of your setup, that might cause the layer precision to go down (if the axis "sticks" in an overshot position and doesn't settle back).
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Re: Do we really need microstepping for the Z-Axis?

Postby flurin » Tue Jul 23, 2013 3:30 pm

Thanks for the replies.

My Printer is one of the first Hadron and I don't have any issues with the Z-Axis (1/16 microsteps). The approach was to reduce the steps/sec (steps/mm * feedrate mm/sec) in order to optimize the CPU time for the stepper management. This of course without any disadvantages. ;)
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Re: Do we really need microstepping for the Z-Axis?

Postby cvoinescu » Tue Jul 23, 2013 4:51 pm

orcinus wrote:Depending on the mechanics of your setup, [full stepping] might cause the layer precision to go down (if the axis "sticks" in an overshot position and doesn't settle back).

Conversely, if the axis "sticks" and needs more than one microstep to get moving, it will randomly be either in the correct position, or a small number of microsteps away, leading to the same type of error.

orcinus wrote:We don't need the accuracy, but we need the step to be as small as possible in order to "fit" the layer height to step height better.
The bigger the step, the bigger the error you'll get from not having the layer height an integer multiplication of steps.

It's still a good idea to have the layer height an integer multiple of the full step. Remember that, if you turn off the current in your Z motors when they're not moving, they will likely snap to one of the full step positions; and even if you don't, microstepping positioning is less accurate than full/half steps.

This goes without saying, but, with all slicing tools being metric, it's imperative to have metric leadscrews. Tr8x2 is ideal (0.01 mm per step); M6 and M8 are good (0.005 mm and 0.00625 mm, respectively -- the latter looks odd, but 4 steps are 0.025 mm and 8 steps are 0.05 mm); Tr8x1.5 is trickier because it's a multiple of 3 (0.0075 mm per step), and many common layer sizes aren't (e.g. 0.2 mm, 0.25 mm). Acme screws are bad for this application (although microstepping helps a lot), and there's no point at all in using ordinary SAE threads.

Anyway, I would not worry about microstepping too much, unless everything else is just perfect on your Z axis and you want even perfecter prints.
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Re: Do we really need microstepping for the Z-Axis?

Postby orcinus » Tue Jul 23, 2013 6:15 pm

cvoinescu wrote:Conversely, if the axis "sticks" and needs more than one microstep to get moving, it will randomly be either in the correct position, or a small number of microsteps away, leading to the same type of error.


If the current is too low, sure.
Which is why it's advisable to keep the current as high as your system can take :)

There isn't a "way out" of this for full stepping. Increasing the current only increases the overshoot and oscillation (or increases it's frequency).
Microstepping + maximum allowable current, however, solve the problem.

cvoinescu wrote:This goes without saying, but, with all slicing tools being metric, it's imperative to have metric leadscrews.


Nope. Mine aren't.
They work just fine. The numbers don't "look nice", but the error is minimal (not observable, i.e. gets drowned in extrusion variation noise).
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