🚀 go-pugleaf

A couple ideas for detecting inclination immediately pop to mind.  The first
(and easiest) method would be similar to your "optical shaft encoder" idea.
Take a variable resistor (the rotating kind) attached to the pendulum.  A 
voltage is applied through the resistor and then fed into an Analog to
Digital Converter.

For instance, using a 100K resistor (set so that with the pendulum vertically
downward resistance = 50K increasing counterclockwise) and an opposing
resister (at 50K, increasing clockwise), connect the leads from the
voltage source to one resister.  From the other end of the resister connect
both to the second resistor and the ADC.  Finally, connect the free end of
the second resistor to ground.

What we have is a voltage divider, which passes (given a 12-volt supply)
12 volts when the device is rotated 180 degrees forward and 0 volts when
rotated 180 degrees backward.  Once fed through (say) an 8-bit ADC you get
out values from 0 to 255, with 128 being horizontal.

ADC * 360 / 256 - 180 = angle robot is inclined FORWARD.

<<I'm not an electronics person, so I realize that this explanation is
rather greek.  Forgive me.>>
--
     __
    /..\      In quest of knowledge....
 --mm--mm--         Mike Gobbi