Re: Mark IV control software

[Tom Droege <droege@FNAL.GOV>]

OK, I guess I should write a little about how the stepping motor
drives work on the Mark IV.

At the moment there are 9 stepping motor drives build into the 
Mark IV design.  Eight are general purpose, one is special for
the RA drive.

The general purpose stepper drives are run by a two bit register
and an enable bit.  The enable bit turns on the power.  The two 
bits are the phase drive.  To drive a motor, one turns it on with
the enable bit, then loads a sequence of two bit codes into the 
register at intervals that the stepper motor can stand.  The 
motors I use will go to about 200 steps a second from a dead 
start.  I was able to get them to 2000 steps a second with some
fancy home built hardware.  

At the moment, stamp code receives a command like "step forward 
300 steps", and does it at a fixed rate.  There is also a command
of step until you reach a limit.  In either case, the rate is 
determined by code in the stamp.  It would not be too hard to 
program in a ramp up.  Ramp down is no problem.  Eventually one
might run out of room in the stamp with too fancy a step program.

I envision commands like "step to forward limit","step n steps 
forward", "step n steps back", "step to backward limit".  These
should solve most moves.  If you keep track in the PC of where 
you are, then this should be enough for any move.  One should
only have to move to a limit on turn on.  

The RA stepper drive is different.  It is step time critical.  It
will be driven by a precision VCO.  We will measure the temperature
at the VCO so that it will be possible to temperature compensate 
it.  The VCO drives through a 7 stage binary count down, with 
selection at each stage by a register bit.  There is a bit that
controls the direction, and a bit to turn on the power.  There is
a DAC that will provide a minor trim, and a POT that provides a 
major trim.  The binary count down will be used to select to
within a factor of two of the correct sideral rate.  The pot 
will be used to set the rate, and the DAC will be used to trim
it withing 0.01% or so.  Any temperature compensation will be 
done by program control.  There is also a direct path where the
Stamp can pulse the RA drive to slew it, and to home it to a limit
switch.

I envision stamp commands like "start sideral drive", "stop 
sideral drive", "backup n steps", "forward n steps", etc..
Again there will be limit switches to detect end points.

I have found the limit switches to be good to 0.001".  This is 
about 10" of arc for the RA drive.  We will half step the RA 
drive for about 5" of arc per step.  This means that the normal
(sideral rate) for the stepper is 3 steps per second.  If we 
can only slew the motor at 200 steps per second, then that is
a 3/200 forward to back ratio.  If we track for an hour, it will
take 54 seconds to get back to the starting point.  This is not
so bad since it takes 40 seconds to read out an exposure.  So 
most rewinds can take place while the CCD is being read out.

For the declination drive, I have only picked part of the 
gear train.  It will probably go a few minutes per step.  This 
is negotiable.  Can anyone think of a reason to step more
accurately?  The slower one steps the slower one slews. 

So Chris, I think everything is built in that you might want.
The only question is whether or not there is enough room in the
stamp to do the ramp up and ramp down schemes.  I am using 
cheap motors, so they could be improved.  The Hurst SLS which
you can find in the Allied Catalog.  I will think about where 
I could put a header, but don't see why everything you want
to do can't be done now.  The last code used about half the 
space in the stamp, and that was with a 4 bit bus which caused
a lot of program problems.  The new 8 bit bus is more efficient.

Tom Droege