[Fwd: ATM steppers & microcontrollers]

[Mel Bartels <mbartels@efn.org>]

Jack Brindle wrote:

> Gerry;
>
> I saw a mention of my name in here, so I thought I would reply also...
>
> You picked out several of the interesting problems with Mel's
> approach.
> The current inflow to the motor is not linear. It is instead
> exponential.
> By adjusting the timing, Mel is getting a good guess as to the proper
> timings for each step. It is not perfect, but appears to be "good
> enough."
> There is a limit to how fine a step can be taken with this approach,
> although the mechanics of the motor problem determine this more than
> the electronics.
>
> >>
> >> You might want to get in contact with Jack Brindle and his group
> that is
> >> porting my computerized dob system to a Mac.
> >
> >What is Jack Brindle's email address?
>
> That's easy. jackb@ricochet.net
>
> >> The issue to watch out for is with altazimuth mounts: the drive
> rates
> >> must vary constantly in order that the scope proscribe an arc
> across the
> >        <snip>
> >> degree drift in 15 minutes of tracking, so this is bad news.  So,
> you
> >> are faced with either needing to communicate constantly with the
> >> microcontrollers so that you can correct for this, or give the
> >> microcontroller enough cpu power to do the altaz drive rate
> calculations
> >> itself, or screw the microcontroller and let the laptop or PC talk
> >> directly to the motors, the approach I chose.
> >
> >At present I myself am just interested in controlling an equitorial
> >mount, which as you point out is a much simpler problem.  For altaz
> >mounts, the only 2 approaches which make sense to me are either
> having
> >the microcontroller(s) do the high level number crunching, or use
> your
> >approach of using _only_ the PC.
>
> Ok, let me describe a bit of things. Our system, and that of Dan Gray
> (see
> Mel's web page for Dan's URL), use bipolar steppers with
> current-controlled
> drivers to get very good control over stepping. There are several new
> stepper drivers out now at relatively cheap prices that allow very
> good
> control of a stepper. Add a few components, and in my case a small
> microcontroller, and you have super control of the scope. The big
> reason
> for the microcontroller is to take over functions that are better done
>
> outside the PC. The best example here is step timing. A PC (or Mac),
> not
> being a real-time system, has better things to do than low-level
> timing for
> individual functions. Rather than burdon (sp?) the system with these
> tasks,
> I added a controller whose purpose is to watch over the stepper,
> issuing
> step commands at appropriate intervals as determined by the host
> computer.
> This means the host can pre-compute a series of steps, sending them to
> the controller's command queue so that it will execute the steps in
> real time.
> The host can then go do other things, like display star charts, go to
> sleep,
> or do anything else the user wants.
>
> As to the specifics of the stepper control, the stepper driver chip
> contains
> an H-bridge driver, comparators and chopper timer to make the control
> task
> easier. I feed it with an small 8-bit DAC which sets the current level
> for
> the stepper winding. The controller feeds the appropriate currents
> into the
> stepper windings, sensing the flow to match the desired levels. I
> simply set
> the DAC to a value from a sine/cosine lookup table to get the desired
> microstep setting. There are two drivers per stepper, one for each
> winding,
> corresponding to the sine and cosine values. The only thing I then
> have to
> worry about is the time until the next step needs to be taken.
>
> There is really not much more to the system. The two big pieces are
> control of
> the stepper driver and communications with the host controller. This
> makes
> for a simple system, one that is expandable, and one that is
> inexpensive...
>
> I hope this helps. Let me know if you have any more questions...
>
> - Jack Brindle
> ham radio: wa4fib/6
> internet: jackb@ricochet.net



--
Clear skies, Mel Bartels    Programmer/Analyst, amateur astronomer
Eugene, Oregon, USA         homepage: http://www.efn.org/~mbartels
mailto:mbartels@efn.org     atm, atm-digest list-owner
Motorize A Dob: http://zebu.uoregon.edu/~mbartels/altaz/altaz.html