Stepper motor drivers

[Ludis Langens]

I recently subscribed to this list (after reading _all_ the archives.)  I've
been hacking a GM C3 ECM, and will soon have some hardware questions related
to that.  However, let me start off with a simpler question:

I'm thinking of adapting a stepper motor driven device to monitor the
operation of an EFI motor.  The stepper motor is already from an automotive
application.  What sort of driver circuit should I use?  I expected this to
be a subject covered in the archives, considering that some IAC valves are
really stepper motors.

My motor has a single winding.  (It only runs in one direction.)  To step, it
needs to have about 12 volts applied in alternating polarities.  The steady
state current is about 50 ma.  Some testing showed that the fastest it can
step (full steps) is 50 to 60 Hz, depending on voltage.  I'll be stepping it
at a maximum of 20 Hz, but typically at 7 Hz.

I've only managed to find the following easily available single DIP solutions
that can source and sink the current levels needed:  A dual power audio
amplifier like the LM1877 used in a saturated output configuration.  Or a 556
timer with no timer components - the timer outputs can source and sink 200 ma.
Ideally, any solution would work across the automotive temperature range.

Another possibility is to unsolder the OEM driver chip (a 16 pin DIP) that
originally ran the stepper motor.  This of course would be an automotive
temperature grade part.

Further:  I need to divide an input clock by 15 to get the correct scaling
for the stepper motor drive rate.  Am I correct in assuming that an 8 cycle
positive, 7 cycle negative drive duty cycle might eventually degrade the
performance of the motor?  (Because of the continuous DC bias.)  My input
clock is not a square wave, so I can't do half cycle logic.

I'm planning on using an 8 positive, 8 negative, 7 positive, 7 negative
frequency division pattern.  Will the uneven step rate cause any mechanical
problems?  This is, however, a stepper motor running at less than the maximum
speed - so it will already stop/pause with each half step.  The output of the
motor is geared way down, so a human will not notice the uneven step rate.

PS, I thought I subscribed to DIY-ElectronicFuelInjection, not diy-insurance,
diy-radar/lidar, diy-mailloop, or diy-uuencode/base64.

                   unsigned long BinToBCD(unsigned long i) {unsigned long t;
Ludis Langens         return i ? (t = BinToBCD(i >> 1), (t << 1) + (i & 1) + 
ludis at netcom.com                 (t + 858993459 >> 2 & 572662306) * 3) : 0;}