Rory, our roboticist's helper

      
Our newest addition
      to the strictly outdoor roaming Geobot, is a slope tilt sensor
      which allows the robot to act on uphill slopes that are too steep
      to climb, and downhill slopes that if traversed at full speed
      would cause a possible crash with obstacles at an unacceptable
      speed. Our solution to this was to add a damped pendulum style
      tilt sensor as described below in greater detail to sense these
      conditions an allow the programming architecture to deal with
      such issues.

      
We decided that
      the reaction to over and under tilt in the priority arbitration
      architecture would be considered an impact, since running into
      a steep hill is no better for forward motion to the goal than
      hitting a big rock. Consequently, exceeding the limits of the
      tilt sensor which we set for +/- 12 degrees causes a top priority
      impact like behavior to be initiated.

      
The Math

      
With a 5.0v regulated
      voltage on the ends of the pot, the center tap was read with
      a DVM. The center of the pots range was set for a vertical pendulum
      and of course was 2.50vdc. With our 10 bit A/D converters in
      the 16F877, and using the 5v as the reference, the following
      equation describes the ADU value for a given voltage at the pots
      wiper:

      
Value = (Vo (1023))/5.00

      
Therefore for midrange
      we get around 512, and the values go up as the tilt went down
      and vice versa. 



      


      
The tilt sensor
      is marked also in 22.5 degree increments, however we found a
      12 degree tilt was right for most dirt and gravel surfaces.
Here
      is the basics for the pendulum style sensor. A six inch aluminum
      bar is attached at the top to a 10K linear taper potentiometer,
      with a heavy brass knob at the bottom acting like a pendulum
      weight. The grease on the pot damps the movements but does not
      alter the swing rest position significantly. As the robot goes
      up or downhill, the pendulum swings and causes the potentiometer
      to vary its output voltage into the analog input of the microcontroller
      and thus the tilt angle can be detected.

      


      
Here is the schematic
      for the sensor. We put a 5v signal on the pot, with a 1uf for
      decoupling. On the right is the input of the micro controller.
      Using a 10uF for additional damping, we achieve about a 2 second
      time constant, allowing the robot to rock and roll over obstacles
      without reacting to an over tilt condition.


 It
      is important to understand the reason for the time constant for
      the swing time. When released here at a 22 degree angle, the
      time to swing to center was about half a second, and then it
      stops without swinging back and forth. For an outdoor robot,
      the rocks, bumps and side to side tilts must not set off the
      tilt behavior unless it continues for at least 2 seconds or more.

 
 End
      on view, the entire unit stands bout six inches high, and the
      wires run neatly down the side rail. The golden 10k potentiometer
      is on the right side.
 
 Two
      shots of the sensor reacting to an uphill on the left, and downhill
      on the right. The pendulum always pointing to the ground. About
      half a volt each way from the 2.5v center position is seen for
      a 22 degree tilt.

 

      

Uphill Movie
 This
      first mpg movie clip (400k) shows the robot encountering a large
      pile of dirt, with a steep uphill slope. The robot first hits
      a weed patch and backs off, but then goes up the hill. At the
      point where the tilt sensor indicates over tilt, the robot stops
      for a second, beeps then backs down hill for 1 time constant
      - 2 seconds. Then a 90d turn to go parallel to the hill to escape
      it.


      

Downhill Movie
 The
      second clip (400k) shows the reaction to a steep downhill slope.
      The idea is that since the robot will probably not be able to
      back up, and is generally heading toward its goal anyway, we
      cut the speed in half to avoid impacting on rocks and trees so
      hard as it flies down hill as to cause extreme damage to the
      front of the robot. So as the robot senses the down hill over
      tilt limit has been exceeded, it cuts speed to a roughly 50 percent
      PWM on both motors, and as seen near the end of the movie, when
      the surface becomes level again, it resumes full speed toward
      its goal.

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