Mechanical
Chassis
The chassis is a simple U-shaped piece of 0.100" aluminum, cut on an NC mill. Holes were drilled to provide a mounting point for the cheesy Hsiang Neng HN-GH12-1634T motors I bought from JameCo. I broke a basic rule by using the motor shaft as the wheel shaft. (Generally, motor shafts are suspended in bushings or bearings not capable of bearing the radial loads imposed by the weight of the vehicle). Since I didn’t want to deal with gears or belts, I skipped this step. The next revision will incorporate such changes, along with better brushless motors.
Built into the chassis and wheel hub system are optical quadrature encoders for each wheel. Rather than attempt to make my own quadrature encoder (a relatively easy project if you can tolerate low resolution), I thought I'd get stock encoders for the motors. Unfortunately, as you can see in the image above, there is no room to add encoders. However, I came across US Digital, a company that makes linear and rotary encoders, and also sells the individual parts the make up their encoders. For about $50 I was able to get 400 cpr disks and two encoders that I could easily adapt to my needs.
You can see how the system fits together in the next picture. (The sparkly black mass on top of the encoder module is a bit of anti-static foam protecting the module’s pins.)
Wheels
Following David Anderon’s lead, I built wheels out of Du-Bro 4.00 T.L. tires and custom-machined hubs. Special thanks to Syl Heuman for lending me his machine shop and expertise in order to turn the hubs. He did all the work while I stood watching, fascinated by the process. That was many moons ago, and I’m sure he’s eager to see some results. Bear with me, Syl…I’m trying!
By carefully studying the mechanical drawings for the encoder module and disk, I designed a wheel hub that would hold the disk and mount to the keyed motor shaft. The hub is turned from a piece of solid round aluminum stock. An section protrudes inboard from the main hub and has a small stub on which the plastic encoder disk is press-fit. An aluminum retaining ring fits over that stub and holds the disk in place. A set screw in the protruding section holds the entire hub assembly on the shaft.
