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Re: [APML]: Byers drive and the MototrakIV
> From: Steve Bell <sb635@delphi.com>
> The simple JMI DC motors have just two wires, one for plus and one for
> minus.
If it only has two leads it's not a DC stepper motor, just a plain
old DC motor.
> I'm pretty sure just putting a 110V AC to 12V DC
> transformer between it and the MotoTrak will give you that, although I
> don't know if it will have enough power to turn a whole scope. What I
> didn't know was what goes on the the Hz coming out of the MotoTrak was
> increased and decreased, but my experiment shows that works too. Or at
> least it does speed up and slow down a simple DC motor.
You may get a speed change, but I question the speed precision and
rate stability of a simple DC motor for astrophotography. AC synchro
and DC steppers don't care if the voltage fluctuates a bit over time.
. .it's the rate of the pulses/voltage swings that determine their
shaft rate. . .and that's crystal controlled to a pretty high
precision. Simple DC motors are driven by voltage. . .and any
voltage changes (or even any rough/higher drag spots on their
bearings, or on the rest of the drive train) will change the motor's
rate.
My simple DC motor for declination adjustment has its rate controlled
by (I think) pulse width modulation. In other words for low
speeds an oscillator/timer allows the full 12V to get to the motor -
but in a narrow/short duration, fast repeating pulse. In effect the
circuit is "on" only a fraction of the time - also called the duty
cycle. For higher speed you make this "on" pulse duration longer
without changing the pulse frequency (duty cycle increases.) For max
speed the circuit is on all the time and the duty cycle is 100%.
Pulse width modulation allows good DC motor torque at low speeds -
better than just pumping a lower voltage through the motor. You get
less tendency of the motor stalling and moving in a jerky fashion.
But I don't think a simple DC motor with pulse width modulation is
stable enough for a right ascension drive. You need an AC synchro or
DC stepper with a crystal controlled frequency base.
Tom Krajci
PS. You can use pulse width modulation with DC steppers. . .that
allows "micro stepping" with digital (a laptop computer)
controlling electronics instead of resorting to analog electronics.
With micro stepping you can take a 200 step/revolution motor and
divide that into about 10 micro steps per whole step - and get 2000
microsteps per revolution. Helps smooth the motor motion and may
give you a little better positional accuracy of the stepper shaft. A
great application of that for computer controlled telescopes is at:
http://www.efn.org/~mbartels/tindex.html schematics, text, photos,
software! Some day I'll have a monster like that!
Capt Tom Krajci
B-52 Intelligence Officer
"Military Intelligence - an oxymoron!"
http://spur.barksdale.af.mil