Servo - April 2017

As an obstacle detector, this sensor has range issues with black or dark objects, so it would be wise to have a companion sensor; either ultrasonic or Sharp IR sensors, which are less color sensitive. Personally, I see more potential in using it as a long-range Scotchlite reflector sensor.

My outdoor tests to sense a Scotchlite retroreflective sticker in sunny conditions showed a working range of over 60 feet in my wiggly handheld tests. Probably more if the sensor and reflector are firmly fastened down. You could easily make a long distance break-beam sensor system, either a perimeter alarm, or (as my drone-racing buddy Jon suggested) a finish line timing sensor. There are many possible applications.

In the toy business, when a product has a quirk, we often say “if you can’t fix it, feature it.” This sensor’s color sensitivity may work against it as an obstacle detector, but in the right situation it brings something new to the party. My first order of business was to try it on a line following robot.

Using just one sensor seeing black or white, it would have to be an edge detector. I built the small tricycle robot shown in the opening shot and Figure 6. The front drive wheel is steered via servo and the sensor is mounted overhead to steer along with the wheel. It is used without the lens and looks downward at the line about an inch ahead of the wheel. The tiny PICAXE program in Figure 7 follows the right edge of a wide black line quite simply by zigging left when white is seen and zagging right when black is seen. My video demonstration at https://www.youtube.com/watch?v= Q5tBp2_Ya0Y shows the line follower steering system is in constant oscillation; somewhat like the old “galloping ghost” control system used in the earliest days of radio control: https://www.youtube.com/watch?v= IBxXsRwRn1M. That wiggling is the price you pay for such a minimalist one-sensor edge following system. If we could add more sensors, it would help reduce oscillation. Unfortunately, these laser sensors interfere if they overlap each other’s field of view, so we’re stuck with using one sensor — unless you want to multiplex several sensors so only one laser is on at a time. I did that on an S6986 based laser tracker project you can see at https://www.youtube.com/ watch?v=5IvkHokhZsQ.

Back to the single laser sensor, vehicle dynamics could be improved by adding a second servo and decoupling the sensor from the wheel. In this system, the extra servo would oscillate/scan the sensor across the line edge while the steering servo holds a more stable position.

There are much faster line following robots out there; my “waddling duck” (as someone named it in the PICAXE I’ve taught students in college and I know it’s easier to engage whole classes when projects are simple and seemingly obvious. When people say “even I could build that,” I’ve done my job.

In similar fashion, I wanted to make a simple optical scanner using this laser sensor. Its sensitivity to color makes it a low resolution digitizer of sorts. There’s no electronic sensitivity or contrast adjustment for what’s detected as black versus white; it triggers at a preset value of reflected laser signal.

That signal will vary based on the color and reflectivity of the object seen, as well as the distance to the object. I settled for scanning a simple B/W image of our