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Figure 2-15.
A slightly dangerous program
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A serious problem occurs if the same bumper is quickly hit twice. Suppose the bumper on input 1 is hit once. It begins executing its sensor watcher code by reversing the direction of the motors.
The robot travels backwards for half a second, then output A reverses direction and the robot spins in place. Suppose, now, that the bumper on input 1 is triggered again, before output C's direction
is reversed again. The sensor watcher routine will begin again, reversing the direction of both the motors. Hank, therefore, will begin spinning the other direction instead of moving backwards.
Then output A's direction reverses, and the robot moves forward. Finally, output C's direction reverses, and the robot spins in place again, instead of moving forward.
There are two solutions to this problem. First, you can be more explicit about controlling outputs. Instead of just reversing the output directions in the sensor watcher routines, you could
specifically set the directions and turn on the motors. This technique is shown in Hank's first program, in Figure 2-6. It doesn't matter if the sensor watchers are interrupted before they finish,
because the directions of the motors are always set explicitly. The other solution is to structure your program differently. If your sensor watchers don't have any delays built into them, for example,
they will be much less likely to be interrupted.
Online Resources
The Art of LEGO Design
ftp://cherupakha.media.mit.edu/pub/people/fredm/artoflego.pdf
This is an outstanding paper about building with LEGO parts. It includes helpful tips on making strong structures and using gears. The paper is written by Fred Martin, one of the people at the
MIT Media Lab whose programmable brick work formed the basis of the RCX. I highly recommend this paper, especially if you are having trouble getting things to fit together.
Fred's 6.270 Home Page
http://lcs. www.media.mit.edu/people/fredm/projects/6270/
For a deeper treatment of many aspects of small mobile robotics, read the course guide for MIT's famous 6.270 class. In this class, students build robots from the ground up. The 6.270 Robot
Builder's Guide was written by Fred Martin; it is a real bonanza of information and advice.
Doug's LEGO Technic Tri-Star Wheel ATV and Robotics page
http://www.net-info.com/~dcarlson/
Doug Carlson's fascinating page is full of pictures of his implementations of the tri-star design, synchro drive, and the Killough platform. For sheer mechanical finesse, this page is hard to beat.