In VEX, the field tiles are not really all that dark, but the white tape is much more reflective than the field tiles. If you’ve got black-on-grey, both with equal reflectivity, you’re going to have a more difficult time than if you’re trying to follow, say a nice black line on a white surface.
In any usage situation, the line that one is following must be sufficiently differentiated from the surfaces around it. The product info sheet suggests mounting the trackers near the center of the underside of a robot to shield it from any environmental infrared “light pollution,” such as from tungsten lighting. Dark-colored surfaces reflect less white-like the tape on the competition field-reflect more. In this case, there’s an infrared LED shining down, and a sensor that measures the rebounding light. Like the optical shaft encoders, these sensors include an infrared light and infrared sensor. The necessary 1/8″ from the floor may not play nice with a lot of scraping and banging. If the game has obstacles that your robot must drive over (such as In The Zone’s starting bar), give consideration to where you place these sensors on your robot and how you drive over the obstacle. Think about the game you’re building for.
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Putting the sensors close to the front of the robot will permit that series of actions to take place before your robot goes astray. There’s a slight delay in between reading the data, having your program figure out what’s what, issuing commands to the motors, and those commands taking effect in real life. I nstall the sensors as close to the front of the robot as possible. When teams are bolting these on to the bottom of the robot, be sure to make the attachment screws accessible (as opposed to we-can’t-reach-it-now) because there will probably be some adjusting needed to get them just the right distance from the floor (e.g., adding washers or spacers). Mounting them flat, even, and close to the ground is key to making use of the line tracker with the smallest amount of excess/compensating stuff in your programming. If one of them is slightly farther from the ground than the others, or the front of one is screwed in tighter than the back, or one of them is at a slight angle, then that one sensor will deliver different minimum and maximum values than its neighbors. I have had the opportunity to use these just once, and I’ll say that mounting them was a challenge for us (ours were positioned off the front of the robot, like the image at right). Let that sink in: 3 millimeters off the ground. Recommended distance is 1/8″ (3mm) off the floor, according to the VEX info sheet PDF. These babies need to be positioned REALLY close to the ground. Hopefully this article will provide some details to help with those purchasing decisions. So, $39.99 is not cheap I’d recommend buying a package only if you have a good chance of using it (as opposed to, “I’ll buy one just to have on hand.”). The line tracker comes in a 3-pack, which are used together as a single group. Each sensor gets plugged into an analog port, and each sensor returns a value your programming will combine the information from the 3 and decide how the robot should react. It’s definitely doable, and I hope that this post will help you learn a few less things “the hard way.” Table-O-Contents (this is a long one) As I describe below, I helped my daughter implement basic PID on a line-following squarebot for her 8th grade science fair project (after I learned it at 2am a few days in a row). Please don’t let this description make you think it’s too hard. This is a long post, with lots of information to understand & things to consider in using line trackers.
Those white lines of tape on the VEX competition field? They’re not just for show (or to help the field resetters), they create a path your robot can follow during autonomous. Next up in my journey through Sensor Land is the VEX Line Tracker.
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Photo credits above: VEX Robotics, RobotC wiki archive
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