[dorkbotsea-blabber] Looking for help with a motor controller circuit (or a cheap full-rotation servo)

[claude]

I am working on a sculpture in which 25 to 100 rotating robot heads will
watch passers by.  I have finished a preparatory sculpture with four heads
which watch TV, you can see it here (1 minute long):
http://www.youtube.com/watch?v=1cCMXl3k1sA
That one used hobby R/C servo motors and a phidget controller.
The problems I need to fix for the next sculpture are:
	(1) Hobby servos (generally) turn only up to about 180 degrees*.  I
need more than, say, one and a half complete revolutions, but preferably
arbitrarily many revolutions /without losing absolute positioning/**.
	(2) I need to control many, many more devices.

Buying something pre-built would be ideal - any suggestions?

Failing a pre-existing solution, I found a cheap (about $7) DC motor
w/gearhead and encoder***.  What would it take to build my own servo
controllers?  I figure I'd need:
	a) A reversible motor controller
	b) Something to keep track of the signals from the encoder
	c) Something else electromechanical to occasionally detect the
absolute orientation of the output shaft (a switch and an index on the
output shaft at "zero" degrees?)
	d) Some logic to coordinate those three functions and accept an
input for the desired position.

This seems complex to my level of electronics knowledge but it also seems
like the sort of standard circuit that must be designed and redesigned over
and over every week.  Can you point me at a ready-made circuit I can reuse,
or can you help me cobble something together?

Ultimately the whole shebang will probably be controlled by a PC or a PIC
microcontroller (so long as I get the hang of the demo board).

Any pointers appreciated.

Thanks in advance,



Claude

PS -- tips on locating multiple humans in a horizontal plane at distances of
6" to 10' also appreciated.


* I found sail winch servos from HiTec which rotate 3.5 times for about $50
which seems a bit spendy, or from GWS for $20 which will rotate once, only
once isn't really enough -- I don't want the heads to have to whip back
mid-turn while watching someone walk around the sculpture.  It seems obvious
to rearrange the sculpture so it doesn't need more than 180 degrees of
rotation but it is important that it be "in the round".
** gearing up the output from a standard servo is also a possibility but
between the price of the gears, shafts and housings and the labor to
assemble it all it seems like an expensive way to get a low resolution
limited rotation compromise.  tips in this direction would also be
appreciated, though.
*** 6v 20mA, 2-channel encoder 1 cycle per motor rotation, 141:1 gearhead on
output
http://maximumrobotics.com/store/index.php?main_page=product_info&products_i
d=98