Let's Make Robots!

4WD robot chassis

I'm looking at a Hitari Tumbler radio-controlled car chassis.  It has four wheel drive, with front and rear wheels on each side geared together, and to the motor.  There are, of course, two motors, for left and right side drives.  It's old, and missing the bodywork, battery and radio transmitter -- but we won't be needing any radio!

Next step is to get some kind of sensor on it for odometry (measuring how far it has travelled, by sensing wheel rotation).  But that's not going to be easy, if I want high resolution.  I need something like 200 sensor pulses per wheel revolution, so that I can get good control of the motor(s) at low speed.  So far, I have a really neat way to get about 20 pulses per wheel rev., whcih isn't really enough.

 

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The problem with using wheel rotation for odometry on a 4wd skid steering device would be that the wheels will slide sideways when turning. different wheels might rotate differently depending on type of turn etc.

-or am I too pesimistic?

Too pessimistic? I don't know -- but once it's all working, we'll find out!  I plan to fit two sensors, of course, one for each side of the robot (that is, one for each motor).  There's a little section in "Mobile Robots: Inspiration to Implementation" (Jones and Flynn) that shows how to make a dual PID controller for a differential-steering robot.  I'll code it in that way once I have the hardware built.
I knew of someone who used a Turbo Rebound like mine and is similar to your Tumbler chassis. They carefull drilled into the side of each gearbox, and used photo-interupters (LED-detector pairs) on the earlier gears (close to the motors). It would probably be possible to add a code wheel and use a photo-reflector too.