Thanks! I never realized how much academic interest this topic has. I'm going to watch the course, it seems fun.
There's been probably close to 50 years worth of research on the topic; plenty of stuff out there...
So far the best option seems to be the maxbotix sensors, even though they aren't cheap. All the other ones aren't close range enough. I have also decided I am not
Well, you can only get measure so close with an ultrasonic sensor anyhow, due to reflection and cross-talk issues.
going to use servos, and just keep the sensors stationary, as it simplifies everything a great deal. Maybe get 4 sensors and place them all facing to the sides or front of the robot, or maybe have an extra facing straight back.
If you look around at some of the older robots out there (check out Cybernetic Zoo, for instance: http://cyberneticzoo.com/
), you'll notice that more than a few from the 1980's used the Polaroid sensors, and would typically use 8-16 or more; you fire them off sequentially, and wait until you get the return from one before moving on to the next. That can get expensive, though, but if you really want to do SLAM with ultrasonics, you need as much "resolution" as you can get (and you can only use so many before the "cones" overlap; then again, I've often wondered if you couldn't do some form of interferometry using ultrasonics - I'm sure that's been studied, too).
I have also thought about using IR sensors. They seem to be cheaper, but larger and have less range. Are there any you would recommend, or should I focus on sonar?
Something to keep in mind with robotics, SLAM, etc: You can't rely on any one sensor; you need to take a range of readings from multiple sources, and integrate everything into a singular whole. In regards to them, the "standard" Sharp IR sensors are probably best; they come in a variety of "ranges", so you can pick the one that best suits your needs.
If you really wanted to get fancy, and money wasn't a huge issue, you could try to find a "broken/dead" Neato XV-11 on Ebay, then gut it for its built-in 2D LIDAR sensor (it's been hacked, so you can figure out how to use it fairly easily).