I spend 2 - 2.5 hours each day commuting to and from work, and I have plenty of time to think about things.
One thing I have been thinking about is that I have never seen anyone use liquid as a way to keep a robot level. I can't imagine that I am the only one to think of this; people must have looked into it and discarded it as a solution, or we would have three of these on our workbenches right now. Or am I onto something really, truly original?
My thought was why not have two little u-shaped pieces of clear tubing at 90 degree angles to each other, with a slightly viscous, but not too viscous liquid which would be colored black, so no light or very little light could get through. Where the lines meet across the U of the tubing will indicate that the sensor is flat. Use a total of four light detecting resistors and four leds (could use IR as well, but would need to research a "clear" and opaque materials to IR) placed at the tubing right where it would be perfectly level. It would take some fiddling with the positioning, some resistors on the LDRs, (maybe a voltage regulator chip or some such to ensure constant voltage from supply to get rid of bounce) to get about 50% on pwm or analog value. As the bot tips one way or another, the ldr values would change and your angle to the ground could be pretty accurately calculated.
I think water would be too "watery" or allow too much bounce as the bot moved, but something a bit thicker might work kind of like what is in well, a construction level. Although, the thicker the liquid, the slower the reaction. It would be something fiddly to get this all working no doubt.
One might be able to use lasers as well using the same principles.
Anyways, has anyone tried this or know a good reason why this wouldn't work? Ideas and suggestions? This might be fun to try sometime.