Microsteps ?

[rmachinist]

Hi,
How do I know what microstep setting to use on pm25mv mill?

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[ger21]

Whatever runs the smoothest. Most people use 1/8-1/10, as if you go much higher you may be limited by the pulse rate.

[rmachinist]

The driver I have doesn't have fractions

[CL_MotoTech]

What does it have then? What's your driver?

[ger21]

1/8 = 1600, (200 x 8)

1/10 = 2000 (200 x 10)

[rmachinist]

My driver is HBS57 it has pulse/rev, smallest number is 800 and the biggest is 40000 it just have numbers

[dazp1976]

I've got mine at 2000.
5mm pitch screw is 400 rev per unit.

[peteeng]

Hi RM - As others have said the smallest that is smooth. If you are using commercial grade steppers then 4000 is a good number. A commercial 200step/rev motor usually has a 5% mechanical accuracy. This means the mechanical accuracy is 1.8*0.05= +/-0.09deg. If you use 4000usteps/rev this gives you 360/4000=0.09deg so once you use over 4000usteps you can't achieve any better accuracy then the mechanical accuracy. So I usually start at 4000. I sometimes use 400step/rev motors and then I use 8000 or 10000 ustep settings. Cheers Peter

There are 2% motors and scientific grade motors are even better but I expect them to be uber $$$$

to put the tolerance into perspective:
1) say you have a 1610 ballscrew. 1 rev = 10mm move so the mechanical tolerance is 1rev = 360deg = 10mm = 200steps. 1.8x0.05 = tolerance = +/-0.09deg so 10/360x0.09= +/-0.0025mm thats the best accuracy you can get from the motor.
2) 4000usteps gives the same 10/4000
3) 8000usteps gives 10/8000= +/-0.00125mm but the motor cannot achieve this
4) 10/2000 = +/-0.005mm which is inside the mechanical tolerance so all good.

Just because the motor may achieve this tolerance does not mean the whole system can achieve this. Everything else has to be up to it, ballscrew accuracy ,backlash, workshop temp, machine stiffness etc etc....

[dazp1976]

Hi RM - As others have said the smallest that is smooth. If you are using commercial grade steppers then 4000 is a good number. A commercial 200step/rev motor usually has a 5% mechanical accuracy. This means the mechanical accuracy is 1.8*0.05= +/-0.09deg. If you use 4000usteps/rev this gives you 360/4000=0.09deg so once you use over 4000usteps you can't achieve any better accuracy then the mechanical accuracy. So I usually start at 4000. I sometimes use 400step/rev motors and then I use 8000 or 10000 ustep settings. Cheers Peter

Also depends on weather he's using p/port controller or not.
The higher the steps you go then the lower your max velocity is, unless you change the pulse rate to more than 25000.
It's a balancing act.
My 2000/400 setting gives me a resolution of 0.0025mm per step on a 5mm pitch screw. That's plenty enough for a mill.

[phomann]

Hi ,
Keep in mind the difference between resolution and accuracy.
If you set your drive to say, 10 microsteps (2000 steps/rev), your resolution is now 0.18 degrees, but that is not your accuracy.
If you issue a command to move an axis by 1 micro step, the axis will probably not even move as there is not enough force to overcome the various drags on the axis.
Basically the rotor of the stepper is held in place by the magnetic forces in the two coils olive the permanent magnet providing the detent force at the full step position.
When you issue a single step on a 10 micro step drive, it provides a sin(10) and a sin(80) force on the two coils. One of the coils is providing very little force , not enough to overcome the detent force so the axis doesn’t even move.
When quoting the the accuracy of an axis, you should use the full step distance the axis moves +/- any errors in the drive train.
In a pinch you could rely on the 1/2 step position as both coils should have equal force on them.

Think of the coils in a stepper as variable force springs trying to hold the rotor in position. If there is enough force, the rotor will be pushed off its desired position.

Remember that micro stepping was introduced to smooth out the motion at low speeds. It was first used by telescope tracking systems to allow them to smoothly track objects across the sky.

Once a stepper motors speed increases above about 2rps microstepping is of no use. In fact it limits the top rpm of the motor. It’s the reason why Geckodrive stepper drives morph from microstepping to full stepping as the stepper speed increases.

Cheers

Peter.


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[phomann]

My 2000/400 setting gives me a resolution of 0.0025mm per step on a 5mm pitch screw. That's plenty enough for a mill.

That may be your resolution, but your repeatable accuracy for a 5mm pitch ballscrew and a 200 steps/rev motor will be 5mm/200 steps =0.025mm regardless of the microstepping.

Cheers

Peter.



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[peteeng]

Hi PeterH and others - Yes I should not have used "accuracy" as this has a technical definition. Resolution is the correct term and as explained by PeterH it does get fuzzy as things speed up an slow down. cheers PeterS

terms to investigate
accuracy
resolution
precision for the interested

[rmachinist]

I have 4mm pitch for X and Y axis and 5mm pitch screw for Z axis

[dazp1976]

That may be your resolution, but your repeatable accuracy for a 5mm pitch ballscrew and a 200 steps/rev motor will be 5mm/200 steps =0.025mm regardless of the microstepping.

Cheers

Peter.

Still near enough for me
Set a cam to run it back and forward for a while and the fitted up dial indicator stayed within a 0.01mm range.

[dazp1976]

I have 4mm pitch for X and Y axis and 5mm pitch screw for Z axis

I'd start at 3200 / 800 on the 4mm
and 4000 / 800 on the 5mm myself.