Vex navigation with ultrasonic and encoders

[GlennGraham]

I'm a 3 year veteran FLL coach and I am starting to investigate what Vex has to offer "out of the box". I've been working with the Starter Kit and added tank treads, encoders, and the ultrasonic sensor. I'm using MPLAB for code development and the libraries installed with easyC. I'm trying to test the envelope of the encoders and sonic sensor to see what they can and can't do using the routines provided in the easyC libraries.

With the encoders hooked directly to the motors running at top speed, if start the encoders, start the motors, execute a while (count < 100) loop, stop the counting, stop the motors, I find the the ending counts are 104 and 106. At that rate, can the encoders achieve single count resolution? Could be skipped counts and I'm looking into whether I can improve things by adjusting the code.

With the ultrasonic sensor, I find that it works pretty well if it faces a smooth, flat, solid surface but gets pretty "iffy" with anything else (hand, leg, cat, etc.). Is this typical? I also noticed that the "cone" of sensitivity is pretty narrow. I was hoping for about 20-30 degrees but its more like 10-15. I suspect I would need to use several to cover a wide enough area to track a moving person/robot/cat.

If anyone has been spent time to characterize the limitations of the sensors I would really enjoy reading an overview of what they have learned. If not, I will be writing one later once I have more experience with them.