Lawrence Glaister wrote:
>
> Background:
> I am designing a microstepping stepper drive system for a home cnc router
> table (4 axis). The design involves a pic16f84 driving 2 Allegro a3955sb
> motor driver chips (one for each phase). The controller ic is only rated for
> 1.5A max phase current and I need 3-5amps for the motors I have. I would
> like to use a full bridge consisting of irf9530 and irf530 fets to act as
> external amplifiers for the controller. I contacted Allegro tech support to
> see if they had any application notes or other comments on using external
> bridges with the 3955 and their rather short response was "the 3955 is not
> suitable for driving capacitive loads". OK... if the chip cannot drive a
> 1600pf gate load of a couple of fets, I would even consider using discrete
> buffer transistors in the external bridge cct.
> Question:
> does anyone have any recommendations or suggestions on using external fets
> with any of the stepper controller IC's? Pitfalls? Example bridge ccts?
> Examples of bullet proof bridge ccts suitable for pwm at 25khz, 5a, 60v?
> Thanks

Hi Lawrence, I am working on one of these at
the moment. Exactly how many amps/phase do your
motors require? Have you done a motor test, ie
microstep angle/torque and worked out what
currents you need for exact angle and torque at
each microstep.

I have just done this for one of my CNC motors
and found that the one phase on current is 1A,
the two phase on currents are 0.57A for exact
torque and angle. So although the phase current
is 1A, I don't need a 2A driver to drive the
two phases, only 0.57A x2= 1.14A total per motor.

This is why it is important to do the motor lab
test first. Here is my results;

www.ezy.net.au/~fastvid/stepper8.gif
(the crosses are the final measured currents)

If you havent labbed a stepper motor yet I can
give you some tips, you will need two adjustable
power supplies and some cheap household junk.

I can help with info on building a higher power
h-bridge, but you may find it easiest to use
a LMD18201 or LMD18245 from National, they are
h-bridge chips good for 3A and 55v, and the
245 has built in chopper regulator. If you
REALLY want perfect motor performance you might
consider linear microsteping, these get hot but
are easier to build and give the best motor
high speed performance. I'm using bipolar
linear h-bridges in my design with discrete
transistors. :o)
-Roman

PS. I'm familiar with the 3955 chip, and it uses
the math model current values for torque
compensation, this is only so-so for performance.
You won't get exact sized or powered microsteps!
See my picture, the round dots are the math
model values, the real values for my motor
were as much as 30% off. Also, if you are using
23 frame hi-torque 3A motors yours will be
further off due to the hi-torque magnetic
factors.

PPS. The product I'm working on is a high
efficiency linear cc bipolar driver, they
are good for 5A and will be ready for sale in
about a month at similar price to the Geckodrive.

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