Thursday, December 8, 2011

Some old notes

I was browsing around some of my files on the clock and figured I'd post some of the notes I took on the drive components.  From my notes on 3/16/2011:


Servos/motors:
I thought about using stepper motors to drive the hands, but gave up because the Arduino motor shield can only drive 3 steppers.  I need 5.  This is the model of sail winch servo I used:  GWS Sail Winch Servo S125-1T.  Most places have them for $20-30 each.


After the fact, I think that steppers would be possible with the Arduino using some custom mux/demux circuits to select one of multiple steppers to drive and an external driver board like this one:  http://www.sparkfun.com/products/10267


The mux/demux circuit could use this chip:  http://search.digikey.com/us/en/products/HEF4051BP,652/568-1689-5-ND/763293




Gears:
I used these two types of gears:  A 1T 2-Y32048 from https://sdp-si.com/index.asp

drive gear on servo:  48 pitch, 96 teeth, 2" pitch dia
hand shaft gear:  48 pitch, 72 teeth, 1.5" pitch dia

For the concentric hand shaft, the brass tubes nest nicely, but to interface with the 1/4" gear hole I will have to increase the diameter of the smaller shafts to fit.  I can cut small pieces of the larger shafts and glue them together.  The tight fit of the nest shafts should work nicely.




Servo positioning:
The Arduino servo class allows controls of 0 to 180 degrees.  Starting with 90 at the 12 o'clock position, 0 and 180 are approx 3 and 9 o'clock positions anti-clockwise and clockwise, respectively.

10 degrees in the Servo library equates to 30 degrees on the clock, or one out of 12 positions.  The full 360 degree range on the clock translates to 30-150 degrees in the servo library.  It's pretty accurate, but may need some fine tuning per servo.