Robot Mechanics

My build for the Sparkfun Autonomous Vehicle Competition continues.  Slowly.  I intended to be busy with system integration by now, but I haven’t even built the physical platform.

Mechanical design is difficult because there are many inter-related factors to consider.  For example, the motor selection partially determines weight which in turn partially determines motor selection.  And, motor selection determines battery selection which in turn determines weight which influences motor selection.  Both motor selection and battery selection affect physical size necessary to hold both components.  Never mind the matter of finding components with the desired specifications at a reasonable price …

The easiest solution is to buy an existing robot, like the fancy new Stingray, or repurpose a radio-controlled (RC) vehicle.  I’ve elected to build my own because I enjoy DIY, believe I’ll learn more, and need to control costs.

So, here are my requirements for the physical platform:

  • Carry the GPS receiver, magnetic compass, MRM micro-controller, motors, motor drivers, LCD, and batteries
  • Travel on paved terrain with sufficient clearance for speed bumps and pebbles at least 1/4″ tall
  • Have a chance of matching last year’s winning ground time of 1:32

The Society of Robots has an excellent introduction to robot dynamics and discussion of topics like torque, force, velocity, and acceleration.  After many hours with this page, pencil and paper, a scale, and a spreadsheet, I’ve decided on the following design.

  • Two Polulu Micro-Metal Gear Motors:  Last year’s winning ground entry was a retro-fitted RC race car that, by my calculation, must have moved pretty fast.  I estimate it moved at an average speed of about 7 mph.  Coming anywhere close to this speed is difficult with commonly available gear head DC motors and wheels.  A custom gear train is beyond my time, skill, and ambition at the moment.  I think the Pololu motors slightly over-volted will give adequate performance with a maximum speed only slightly lower (estimated at 6.8 mph) than last year’s average winning speed.  The weight of the robot will have to be low since the torque from these motors is low.  I’ll be happy if I can just finish the race, which is a significant challenge by itself.
  • Two Model Aircraft Wheels (3.25″ diameter):  I wandered around the local hobby store looking for fairly large diameter wheels (to increase speed) made of fairly firm rubber (because the robot is running on pavement).  These wheels seemed like the best choice, and I think the Pololu hubs will attach easily.
  • Three rectangular decks made from expanded PVC:  This material is light, inexpensive, and sturdy.  Plus, I already own enough.  The robot will grow up via additional decks rather than out due to limited size of the material I own, ease of transporting a small robot to Colorado, and reduced turning radius.  Plus, I like the look of multiple decks more than a single large deck.  I estimate that I’ll need three decks, each roughly 17cm square with sawed off corners, to hold the parts.

I haven’t solved weather resistance.  As is, my robot will have to stay home during inclement weather.  Based on photos from last year’s competition, I suspect many builders just hoped for good weather, too.  I might consider a plastic housing for the expensive electronics, just in case.

I think I’m ready to make a full size template of the decks and start cutting.