Author Topic: Determining rotational accuracy of a drive unit.  (Read 4415 times)

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Offline Doctorandus_P

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Re: Determining rotational accuracy of a drive unit.
« Reply #25 on: May 21, 2021, 03:47:34 am »
Stepper motors don't have "25000 steps/rev".

Stepper motors usually have 200 steps/rev, but 400 steps/rev also exist, and it can be as low as 24 steps/rev.

With microstepping you can increase resolution a bit, maybe by a factor or two or four, but if you try to go further, then you get into the magnetic elasticity too far and position is too much load dependent.
 

Online Siwastaja

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Re: Determining rotational accuracy of a drive unit.
« Reply #26 on: May 21, 2021, 03:09:27 pm »
Yes I don't believe the OP has a "25000 steps/rev" motor, it would be very special.

Larger and larger error towards one end sounds like you have just some slightly wrong number somewhere. For example, if you expect your reduction gear is 1:20 but in reality it is 1:21, you would have linearly increasing error ending up at 5%.

Recheck all numbers.

Regarding microstepping, the point is not to magically generate ridiculously high positional accuracy, the point is to drive the motor with sinusoidal waveform instead of square wave and minimize stutter / vibration. The more microsteps, the better it approximates sinusoidal drive. You can't have too much really, use as much as you can implement.

Regarding missed steps, you'll notice if that happens from the jerk and sound it makes. If it is sitting on your lab table and operating smoothly without you noticing any weird, you are not missing steps.

Backlash compensation can be implemented by always stopping the motion in one certain direction. If you need to go to the opposite direction, go half a degree over the destination then come back half a degree.

Steppers are OK for many applications if you know and accept the limitations.
« Last Edit: May 21, 2021, 03:11:36 pm by Siwastaja »
 

Offline mikerj

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Re: Determining rotational accuracy of a drive unit.
« Reply #27 on: May 21, 2021, 05:17:38 pm »
Yes I don't believe the OP has a "25000 steps/rev" motor, it would be very special.

I wonder if the 25,000 steps/rev is after the 20:1 reduction, that seems more feasible when combined with microstepping.
 

Offline Puffie40Topic starter

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Re: Determining rotational accuracy of a drive unit.
« Reply #28 on: May 21, 2021, 05:36:36 pm »
Stepper motors don't have "25000 steps/rev".

Stepper motors usually have 200 steps/rev, but 400 steps/rev also exist, and it can be as low as 24 steps/rev.

With microstepping you can increase resolution a bit, maybe by a factor or two or four, but if you try to go further, then you get into the magnetic elasticity too far and position is too much load dependent.

That is rather curious.  It took me a bit to determine the stepper motor model, but I found it here: https://www.moonsindustries.com/series/stm23r-series-integrated-stepper-motors-a01020402
It has an integrated driver and claims to have a microstepping resolution of up to 25,000 steps.
 

Offline Benta

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Re: Determining rotational accuracy of a drive unit.
« Reply #29 on: May 21, 2021, 06:05:12 pm »
Note that resolution is not the same as accuracy (which the data sheet does not specify).


 

Offline james_s

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Re: Determining rotational accuracy of a drive unit.
« Reply #30 on: May 21, 2021, 06:46:06 pm »
The accuracy should be fairly constant though, a stepper motor is always going to move in discrete steps which are directly tied to the mechanical construction of the motor. It's not going to randomly take a different number of steps to make a full revolution.
 

Offline Benta

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Re: Determining rotational accuracy of a drive unit.
« Reply #31 on: May 21, 2021, 07:53:41 pm »
The accuracy should be fairly constant though, a stepper motor is always going to move in discrete steps which are directly tied to the mechanical construction of the motor. It's not going to randomly take a different number of steps to make a full revolution.

Quite, no dicussion on this.
But to take a (possible) resolution of 1/25000 and translate that to an angle accuracy of 0.0144 degrees (or whichever unit) is a very far leap.
The magnetic poles in the motor are not even machined to tolerances that would allow that kind of accuracy.

And then adding a worm mechanism on top of that for even higher resolution (not accuracy) is asking for trouble.


« Last Edit: May 21, 2021, 07:59:53 pm by Benta »
 

Offline nctnico

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Re: Determining rotational accuracy of a drive unit.
« Reply #32 on: May 21, 2021, 08:56:17 pm »
It could be the stepper motor has an internal reduction which then adds to the error as well.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline Puffie40Topic starter

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Re: Determining rotational accuracy of a drive unit.
« Reply #33 on: May 21, 2021, 09:26:16 pm »
It could be the stepper motor has an internal reduction which then adds to the error as well.
No, the stepper motor has no other reduction- it is a direct drive.  It's full step size appears to be 1.8 degrees, since its lowest resolution is 200steps/rev.
« Last Edit: May 21, 2021, 09:53:37 pm by Puffie40 »
 

Offline james_s

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Re: Determining rotational accuracy of a drive unit.
« Reply #34 on: May 21, 2021, 11:09:54 pm »
Quite, no dicussion on this.
But to take a (possible) resolution of 1/25000 and translate that to an angle accuracy of 0.0144 degrees (or whichever unit) is a very far leap.
The magnetic poles in the motor are not even machined to tolerances that would allow that kind of accuracy.

And then adding a worm mechanism on top of that for even higher resolution (not accuracy) is asking for trouble.

Yeah 25,000 steps/rev is totally unrealistic I think, that just sounds like marketing wank to me. Microstepping can be useful to make a stepper operate more smoothly, but I would still not expect it to stop accurately at a point in between physical steps of the motor.
 

Offline nctnico

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Re: Determining rotational accuracy of a drive unit.
« Reply #35 on: May 21, 2021, 11:39:30 pm »
It could be the stepper motor has an internal reduction which then adds to the error as well.
No, the stepper motor has no other reduction- it is a direct drive.  It's full step size appears to be 1.8 degrees, since its lowest resolution is 200steps/rev.
But that is not what it says in your original post; there you start with a stepper which has 25000 steps/revolution and then goes through a 1:20 reduction.
There are small lies, big lies and then there is what is on the screen of your oscilloscope.
 

Offline james_s

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Re: Determining rotational accuracy of a drive unit.
« Reply #36 on: May 21, 2021, 11:47:15 pm »
But that is not what it says in your original post; there you start with a stepper which has 25000 steps/revolution and then goes through a 1:20 reduction.

The motor is direct drive with no integral gearbox, it drives a separate mechanism which results in a 1:20 reduction, at least that's how I read it.
 


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