[Sparks333]

Our team also used the MaxBotix sensor as well, and actually got a good auto mode working on it using four of them at once. They are extremely reliable, have terrific interfacing characteristics (PWM, Analog, RS232) and have a trigger mechanism that makes it easy to use several at once without reflection from its own sensors. The sensor is great, but, like Gdeaver, I'm dubious about any ultrasonic sensor's ability outdoors, save with lots of postprocessing.

IR is a slippery little beasie, though it does work rather well when used correctly. Some opaque materials are transparent to IR, and IR loves to spread and can be picked up very easily from almost any direction without proper shielding (try holding a TV remote behind your hand. The signal still gets there, by bouncing off everything in sight until it reaches the receiver. It''s magical). It also behaves like light, in that if it hits a reflective object at an angles worse than 90 degrees, you will have a substantially reduced return signal compared to something like ultrasonic. We tested this by bouncing both an Sharp IR beam and a MaxBotix ultrasonic wave off a spider leg cap, and slowly twisting the cap until the sensor could not see it anymore. The IR lost sight after a paltry 15-20 degrees, while the ultrasonic lost it when it was practically on edge.

If this is a project with a lot of resources and computing power, try LADAR - very expensive, but very accurate and 180 (or so) field of vision.

Hope this helps.