Wheel encoders
      - 2nd version

      
Left: After removing
      the home brew version with the rotating cylinder and opto interupter
      type sensor, which became useless when clogged with the fine
      cat litter dust in the bathroom area, I went to a commercial
      shaft encoder. Made of the more expensive optical type, The model
      I chose was the Grayhill 63R256, which I bought from Digikey
      for about $80. Totally sealed it is impervious to dust and produces
      beautiful wave forms which are very uniform due to the internal
      drivers and Schmidt triggers. Counting this type of pulse is
      easy for a processor, the individual pulses - 256 per revolution
      - are about 5mS wide when the robot is moving full speed.



      
In this photo,
      you can see the red encoder on the right wheel, mounted on an
      aluminum bracket. The pulleys which are on both the wheel and
      encoder were from ACE hardware and are Arcadia door track rollers.
      The belt is butyl rubber for minimal stretch. Again, because
      of the fact that the robot cannot be guaranteed to move in a
      straight line on a home floor with tiles, litter granules or
      slippery spots beyond about 6 feet, you only really need the
      encoder on one wheel and simply select the motors to run similar
      in speed by buying precision motors. Extensive experiments as
      you have seen showed that by driving both wheels the exact same
      speed did not in any way make the robot move straight for any
      distance in the home. Straight line navigation will be dealt
      with later with other sensors.

      
Movies of the robots
      current capabilities.

      
Movie
      1: Precision rotation on a selected home surface

      
Here we see the
      robot making two 90 degree turns on a piece of the exact floor
      tile - parquet wood that the robot is to be used on. Other surfaces
      yielded completely different rotations because of the amount
      of slippage varies from say tile to carpet.

      
Movie
      2: Navigation on actual intended floor surface

      
Here the robot
      goes out about 10 feet, makes a precision 90 degree turn and
      returns within inches of the starting place. You can see the
      robot actually wandering back and forth a bit on its travels,
      in response to the irregularities of the wood floor surface.
      This is life - this is the surface it must work on and we must
      be able to work around the limitations of in home navigation
      like this so the robot can successfully move about accurately
      in the home doing its task. (I think that will be transporting
      of the bathroom waste basket to the big bag in the kitchen...)

      
Next on the list
      - Hall effect sensors on a stasis wheel.

      
Update 2/27/11
Update 1/2/11
Update 12/27/10
Update 11/25/10


HOME