Re: Stepper Motor Speed Problem
From: CFoley1064 (cfoley1064_at_aol.com)
Date: 10/25/04
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Date: 25 Oct 2004 14:49:08 GMT
>Subject: Stepper Motor Speed Problem
>From: sudheervemana@hotmail.com (sudheervemana)
>Date: 10/25/2004 7:06 AM Central Daylight Time
>Message-id: <972038f9.0410250406.2f6a93b9@posting.google.com>
>
>Hi all,
>
> I am developing an stepper motor position control board in the
>devicenet network.I want the motor to run in the microstepping (up to
>1/64) mode to an target speed of 2500RPM for an stepper motor with 200
>step resolution.
>
> In the case of Fullstep, my motor can have 200
>steps/revolution.Suppose if i want to rotate in the microstep
>mode(1/64)(the motor can have 12,800 steps/rev)to an target speed of
>2500 RPM,does the controller clock frequency 40Mhz is enough for my
>controlling or else i should get the controller with some more higher
>clock freq.Why because i am not able to achieve the desired speed in
>the 1/64 microstepping mode.
>
> Clear me regarding the speed of an stepper motor.Suppose
>if my motor is an 1.8 deg step motor where it can rotate up to 200
>steps /rev in full step mode,if i want to achieve the target speed of
>2500RPM with fullstep which is (2500*200*60) 3000000 pulses/sec,my
>timer freq which is 40Mhz is greater than the 3MHZ.Whereas in the case
>of microstepping(1/64) the stepper motor can rotate 12,800 steps
>/rev.now if want to reach an target speed of 2500RPM by using
>microstepping(1/64) the target velocity will be (2500*12800*60)1280Mhz
>which is less than my timer freq 40Mhz.Is this the problem for my
>speed,or else my interpretation is wrong.Please clear me regarding
>this speed of an stepper motor in the microstepping mode.
>
>With Regards,
>Sudheervemana.
>
Hi, Sudheervemana. Apart from doing a quick check on your math, you should be
aware that most all real world microstepping stepper controllers drop some and
then all of the microsteps as speed increases, and go to straight
half-stepping. There are dedicated controller ICs available which do this job
for you. And also, 1/64th microstepping is kind of unrealistic as far as
positional accuracy on even mil-spec stepper motors. Except at _very_ low
speeds, you won't get _any_ additional torque, either. And you haven't even
looked at driver efficiency losses with switching speeds that high.
As far as it goes, a controller of this type will generally get motion commands
and then internally calculate a motion profile prior to the move. This profile
can be ON-OFF, trapezoidal, or any number of other profiles. Commands are also
issued for minimum speed (to avoid mid-range resonance), and other things.
The Guru Don Lancaster once said:
"An hour on the net is worth a week in the lab."
You might want to do some reading before you spend a lot more time on this.
And you should seriously consider purchasing a one-chip solution for your
controller instead of rolling your own, unless you're making tens of thousands
of these. Of course, if it's a one-off, just buy a board. There are so many
good products out there at such reasonable prices that reinventing the wheel
just isn't worth it.
Good luck
Chris
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