Stepping motor controller - is this chip suitable?

[Peter]

I am hoping to build a controller board for this motor

http://www.faulhaber-group.com/uploadpk/EN_AM2224_PCS.pdf

which says is needs 0.125A (12V version), using the A3987 chip

http://www.allegromicro.com/en/Products/Part_Numbers/3987/3987.pdf

What I find staggering is that the above chip comes in a miniscule
package, yet it is rated at 1.5A and should thus easily cope with this
motor - by about a factor of 10.

Admittedly the PCB design rules for the A3987 are pretty specific, to
get the heat out of it, but in this application the heat generated
(I2R where R is the Rds - about 0.6 ohm max) should be miniscule - of
the order of 20mW.

I am really puzzled how such a motor could draw so little current. Is
that 0.125A figure meaningful? The coil resistance is 75 ohms so it
can't draw that much!

What concerns me is that I may be misunderstanding something basic
about the motor spec. But with the coil resistance apparently being 75
ohms, the most the motor could possibly draw (excluding transients
caused by the parallel capacitance of the coil) is 12/75 = 0.16A.

The price of the motor is interesting - about US$500!!

This one

http://www.allegromicro.com/en/Products/Part_Numbers/3986/3986.pdf

uses external MOSFETs and should be a lot more robust... but is it
necessary?

There is a vast difference - about 10x - between the torque of a
stepper like the one above and a brushless motor e.g.

http://www.faulhaber-group.com/uploadpk/EN_2444B_MIN.pdf

(roughly 10mNm v. 100mNm) which probably explains the different
currents involved.

I am new to stepping motors and just haven't come across a motor of
that size drawing so little current. But maybe stepping motors are
like that...

 

[Stef]

In comp.arch.embedded,

I am hoping to build a controller board for this motor

http://www.faulhaber-group.com/uploadpk/EN_AM2224_PCS.pdf

which says is needs 0.125A (12V version), using the A3987 chip

http://www.allegromicro.com/en/Products/Part_Numbers/3987/3987.pdf

What I find staggering is that the above chip comes in a miniscule
package, yet it is rated at 1.5A and should thus easily cope with this
motor - by about a factor of 10.

Why do you find that staggering? The chip uses switch mode current
control and there should be no other losses than the switching and
RdsON losses. There are SOT23 FETs rated at 5A.

[...]

I am new to stepping motors and just haven't come across a motor of
that size drawing so little current. But maybe stepping motors are
like that...

Start off by reading the classic stepper tutorial:
http://www.cs.uiowa.edu/~jones/step/

 

[Peter]

[email protected] wrote

I am hoping to build a controller board for this motor

http://www.faulhaber-group.com/uploadpk/EN_AM2224_PCS.pdf

which says is needs 0.125A (12V version), using the A3987 chip
[snip]

There is a vast difference - about 10x - between the torque of a
stepper like the one above and a brushless motor e.g.

http://www.faulhaber-group.com/uploadpk/EN_2444B_MIN.pdf

(roughly 10mNm v. 100mNm) which probably explains the different
currents involved.

Steppers usually win on grunt, servos on speed. That stepper is
26mNM, versus the brushless DC-servo's 11.8mNM. Maybe you're thinking
about the torque of the servo motor /after/ gearing down?

Actually, on a closer look, the stepper does about 10mNm and the
brushless is similar. I was looking at the stall torque on the
brushless one.

 

[Peter]

Why do you find that staggering? The chip uses switch mode current
control and there should be no other losses than the switching and
RdsON losses. There are SOT23 FETs rated at 5A.

Yes, I have seen them, but would *you* design a product which had to
be highly reliable, over a wide temperature range, which used SOT23
FETs carrying 5A?

With a stepper motor driver one might need to adjust things like the
current and various pulse timings, and if anything goes slightly out
of perfection, a device like that will go up in a puff of smoke in
about 1 second.

The one thing I have sussed out when looking at these stepper
controllers is that most new ones work in "current mode" and one
chooses a stepper motor which is specced at a small fraction of the
supply voltage e.g. a 3V motor with a 30V supply. The resulting
performance is much better, over simply switching a constant voltage
onto the motor coils which is what all the older stepper chips do.

 

[Stef]

In comp.arch.embedded,

Yes, I have seen them, but would *you* design a product which had to
be highly reliable, over a wide temperature range, which used SOT23
FETs carrying 5A?

That was not the point, I just mentioned them because of your amazement
by the rated current of a certain size chip. Just compare the rated
currents.

But to answer your question: Probably not. I'd feel more comforable if
there was at least a 2:1 safety margin. But I would have to know all
operating conditions and read the entire datasheet for a definitive
answer.

But you seem to be on the virge of dicarding a 10:1 overrated chip
just because of it's physical size, that does not make sense to me.

Study the specs, and make sure it is overrated by a marging you are
comfortable with under all expected conditions. And if the chip still
fits the application, just be happy with the small size.

With a stepper motor driver one might need to adjust things like the
current and various pulse timings, and if anything goes slightly out
of perfection, a device like that will go up in a puff of smoke in
about 1 second.

In chips like these (IIRC you were considering the A3987), current is
set by a resistor, switch timing is not adjustable and it has built in
protections. Adding external components increases the possible points
of failure and you must make sure your upper and lower switch times
don't cause overlap. And if you use standard mosfets for your external
switches, you lose the benefit of thermal shutdown.

The one thing I have sussed out when looking at these stepper
controllers is that most new ones work in "current mode" and one
chooses a stepper motor which is specced at a small fraction of the
supply voltage e.g. a 3V motor with a 30V supply. The resulting
performance is much better, over simply switching a constant voltage
onto the motor coils which is what all the older stepper chips do.

These current mode drivers have been around for a while now, so I
would not call them 'new'. ;-)
But you can still drive your 12V stepper from 12V with one of these.
Have you read the Jones tutorial? I think it has a section on the
pro's and cons of current drivers (it has been a while since I read
it myself).

--
Stef (remove caps, dashes and .invalid from e-mail address to reply by mail)

Life is like a bowl of soup with hairs floating on it. You have to
eat it nevertheless.
-- Flaubert