Implementing the Drop Off Sensor in PAAMI - 1 --- First Tests ---

Observers to the testing confirmed proper Operation.

 

      
Introduction

      
 In both the home and office environment,
      stairways present a fatal obstacle to the oblivious robot. When
      an over hang is encountered that exceeds a preset parameter,
      the robot must stop in time, and take evasive action to avoid
      harm. For most robots, which have two drive wheels and a third
      caster wheel on the rear, at least five sensors are used to detect
      drop offs. We are currently developing the sensor array and programming
      for PAAMI to function in a standard household environment, that
      includes an often encountered stairwell in its normal travel.
      This article will illustrate our solution to the problem, and
      prove the concept with movies of exhaustive testing. Four Sensors
      are installed at this point, and the rear caster sensor and possibly
      an additional frontal sensor is on order. In PAAMI's Subsumption
      architecture, the priority is 7, nearly at the top which is the
      same as the impact bumpers. Therefore a drop off is considered
      as serious as a collision. As a final test, the ultimate - the
      robot will be aimed directly at the stairs and well all watch
      and see what happens next...



      
 Left: Underside
      of Robot showing the location of the four drop off sensors, located
      one inch behind and in front of the wheels, and flush with the
      outside edge of the wheels. The rear castor sensor one is not
      installed yet.
 
 Right:
      Positions 1 (rear) and 2 (front) mark the locations for the sensors.
      While the frontal sensor could be mounted on the base plate,
      the rear one is on a flat tab to make it flush with the outside
      of the wheel.

      


 Left: Close
      up of the underside of the GP2D120 sensors from Sharp mounted
      on a tab. The wires are securely tied down as to not catch on
      anything, and a 1000uf capacitor MUST be installed across the
      power into the device (5vdc) to keep huge current spikes from
      resetting the microprocessors. The devices put out a non linear
      analog voltage from 0 to 3v corresponding to the distance, up
      to 30 inches or more. At PAAMI's height, the output is 1.8v and
      drops to 1v when a 2 inch drop is seen. The outputs of the Sharp
      sensors is connected to two PIC 12F675 micro controllers analog
      inputs, and is digitized to 10 bits. When the digitized threshold
      is exceeded, a signal is sent to the IMPACT microprocessor which
      also controls bumper, and stasis sensing and appropriate evasive
      action is taken. Being Level 7 in the Priority Arbitration Architecture,
      it overrides everything and will stop the bot right in its tracks
      if it tries to drive over an edge.
 
 Right:
      Side view showing location of Sharp GP2D120 drop off sensors.
      It is crucial that they see the path the wheel is about to take.
      What is missing here, is the one that will mount on and pivot
      with the rear caster wheel.

 

      
 Infrared
      Night Vision scope view of PAAMI in nearly total darkness, showing
      the spots projected onto the ground by the Sharp IR transmitters
      in the drop off sensors. The bright spray of light coming from
      the front of the bot is from the frontal IS417 IR proximity sensor
      array.
 

      

      
 Left, a still from
      a short and tiny movie (420K) showing a crucial test, driving
      on a table top elevated four inches off the floor. The robot
      did not drive off and avoided the edges fine, until its back
      wheel which has no sensor yet went over the edge while backing
      up and hung it up.

      
Click the image to the
      left to see the MPG movie with SOUND.



      
 Left: Stairwell test
      1 - Small movie. (400k) The robot is shown approaching and performing
      the escape maneuver on the edge. I think Id like to see a frontal
      sensor as well, it's pretty harrowing to see it hang over that
      much.

      
Click the image to the
      left to see the MPG movie with SOUND.



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