More About Odometry

A few weeks after I had the odometry working, I found that the robot started veering off course, and it wouldn't follow waypoints as accurately. I was adding sonar sensors at the time and was worried I was getting noise from all the additional wiring. So I researched a little more and experimented with a pull-up resistor instead of a pull-down.

The old wiring:

With the pull-down resistor, the analog reading on pin A0 ranged from about 30 (black stripe) to about 150 (white stripe), enough to distinguish, but not high enough to use a digital input.

Odometry

I wanted my robot to be able to do dead reckoning, so I needed wheel encoders. There are some encoders on the market that can work with the Dagu yellow motors, as in the Magician chassis, or with the Solarbotics gear motors. And Pololu makes an encoder for their micro metal gear motors that detects wheel motion and direction by detecting white reflective tabs built-in to the wheels.

But I had already decided to use continuous rotation servos, which don't have a second shaft on the non-drive side to mount an encoder disk. Instead, I decided to mount a paper ring of encoder stripes on the inside of a Solarbotics servo wheel. I needed some way to detect the changes between black and white as the wheel rotated.

I found a set of components from Optek on mouser.com that seemed promising. They were a combination of an LED and a phototransistor, packaged together in a trapezoidal case with a screw hole.