P.A.A.M.I. as of 9/24/04

 

      
Here we describe the stasis sensor
      and encoder used in our latest robot, PAAMI.


      
Every robot encounters
      a situation in which it may be stuck, and the sensors do not
      detect the condition. When this happens, a robot will continue
      to spin its wheels, thinking its moving forward, with no impacts
      from either the bumpers or any IR sensors. Unless something is
      done, the robot will simply run down its batteries and stop functioning
      after a while. An example of this condition, is when a robot
      gets hung up on a rock, or piece of furniture and flails back
      and forth trapped. Since no forward motion is occurring, the
      addition of a Stasis Sensor will enable the robot to detect its
      stuck or trapped condition, and take actions to escape accordingly.
      Here is how our stasis sensor works, and perhaps you can modify
      it for your own uses.

      
Theory of Operation

      
Most robots use two drive
      wheels mounted centrally, each independently driven to achieve
      forward movement and turning rotations. At a point exactly in
      between them lies an interesting point. This is the ONLY location
      where a robot can get stuck, with wheels skidding or rocking
      back and forth trapped and a detection be made with some confidence.
      A freely rolling wheel is placed in this location, with an encoder
      disk secured to its side face. An optical detector such as a
      photo interrupter or reflective sensor looks at this sensor,
      and sends the data to the processor for analysis. When the robot
      is moving, the wheel is turning and the processor counts the
      pulses per interval of time, say for 5 seconds. Normally, the
      counts will exceed a set value, say 10 counts per 5 seconds or
      more. When the count is lower, anywhere from 0 to 9, the robot
      is either stuck, slipping or simply not moving. Thats the theory
      behind the Stasis Sensor.

      
Implementation

      
We first made a wheel out
      of lexan sheet stock on a lathe and put a nice v groove in its
      edge. A rubber O ring fits in the groove and acts like a dandy
      non slip wheel. we next drew up the encoder pattern in Autocad,
      and printed it out full size. Attaching it to the side of the
      wheel was easy, we used a glue stick. The wheel was mounted on
      an axle and two arms that are hinged to the bottom of the body.
      This way the wheel rides on the surface between the drive wheels
      and can move up and down on an irregular surface. The optical
      reflective encoder is a Fairchild QRB1114 sensor, available from
      Digikey. Since our bot has priority arbitration architecture,
      we elected to use a PIC12629 microcontroller which cost less
      than a buck for the stasis sensor controller. The controller
      has one function in life - count pulses for 5 seconds, and if
      its less than 2, then the output line goes high. Thats it. I
      do also have a 10 second delay before the program starts in the
      PIC to allow the robot to power up and get going.

      
Caveats

      
So what level of priority
      to we put the Stasis sensor? It cant be at the top, or we would
      never be able to stop the bot for sleep, or picking up a toy.
      It must be subsumealbe by other more important levels. We tried
      it in the lowest module, and that didnt work either, since everytime
      a higher level stopped to avoid an impact, and then returned
      control to the lower levels it would always jump into an stasis
      escape mode. What we finally deceided upon was to treat it like
      an IR impact, and put it at that level, about half way up the
      priorities.

      
You must be careful not
      to allow the Stasis sensor to control the bot when it tries to
      sleep as well ! So careful placement in the control hierarchy
      will give great benefits, and make your robot a true escape artist.

Click on Thumbnails for large view

 

      
 With the
      robot tilted on its side, you can see the wheel, and armature
      assembly that keeps the wheel alway on the ground.

      
 Ground view
      from the front showing the stasis wheel riding well exactly between
      the two drive wheels. It must be large enough to clear the obstacles
      it is expected to see. ( like socks, cat toys, and yes, the drive
      through the moist cat food dish)

      
 Rear underside
      view of wheel touching floor doing its job. Note the Fairchild
      sensor also rides up and down with its wiring with the wheel.
 

      
MPG Movie
(441k)
 Be
      patient with my attempt to make the robot think its slipping.
      I grab it by the rear, and the wheels are slipping away. At some
      point, the stasis sensor kicks in, and it goes into a peculiar
      escape maneuver - rocks back and forth in reverse in an attempt
      to free it self from the offending obstacle. Then it rotates
      a random angle, and goes onward.


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