Updated 1/29/05

 Implementing the Drop Off Sensor in PAAMI - 1

--- First Tests ---

Observers to the testing confirmed proper Operation.



 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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