LUGNET News : ~726/Alegomazer/LightBlock


Gus Jansson / Alegomazer / LightBlock

Back to Alegomazer main page

Light Sensor Block Assembly

The Light Sensor block is a key component of Alegomazer. Not only must it accurately track a 1/4” line, it also has to reliably detect perpendicular paths to the left and to the right, dead-ends, and the 6” diameter black finish circle.

Here you see the light sensor block partially disassembled. Note the use of the black tiles on the shrowding. I wanted to minimize the amount of light that came back into the sensor from the shrowding itself.

Angle of Light Sensor Block (and why Alegomazer was almost a no show)

On the night before the Robothon competition, there was a ‘hack session’ where the competitors could get together and talk about robots and to give people a chance to try out their robots on the official surfaces. After having demoed my robot on my own test maze with nearly 100% reliability, I finally got my chance to test it out on the official surface at approximatly 9pm that night. I assumed that all it would take was some careful callibration and all would be well. No such luck. The robot was totally unreliable. Despite many attempts to callibrate the sensors, nothing worked. The robot would only go past one or two intersections before it got lost. I was totally at a loss and I was running out of time. We had to be out of the room by 10pm.

Halfway to the car while walking through Seattle Center that night, it dawned on me. The glossy surface of the official maze was so reflective that it acted like a mirror. My light sensor assembly was designed so that it would get a gradual reading even if not exactly over the line. In other words, if one of the sensor areas was only partially covered by the line, then the light sensor reading would show a value half-way in between black and white. With the glossy surface, it was all or nothing. If the point of direct reflection of the red LED was on the black, the light sensor got a high raw reading and if it was off the black then the reading was low. My own test maze was rather matte in comparison.

My solution was to angle the light sensor block slightly so that the point of direct reflection would be blocked by the shrowding. That night starting around 11pm, I proceeded to take my robot apart to make this change. As all Lego builders know, positioning pieces at angles other then 90 degrees is usually not so easy. In this situation, I came upon a perfect construction. By stacking two sets of beams on top of each other and putting the black pegs at a diagonal, I was able to get an angle just over 10 degrees.

Primary content in this document is © Gus Jansson. All other text, images, or trademarks in this document are the intellectual property of their respective owners.