Okay, the basic sensors that FIRSTers use:
Limit switches: Mechanical switch.
USE: Robot has an extending ladder. Mount limit switch and the bottom, and when the switch is pressed, don't let the motor retract the arm (in software). < $4 per.
(Also, reed switches (magnetic) and optical sensors can be used similarly.)
Potentiometers: Variable resistor. Think of a volume control knob. Plugs into the Analog input; reads 0-1023 in code. You'll probably want LINEAR taper, not Audio taper, so one "tick" is the same # of degrees rotation. 1 turn, 5 turn and 10 turn are the most common.
USE: Robot has an articulated arm. Mount a pot to the joints to see what angle the arm is at. (Good for autonomous control, or making a mini-arm for the OI to control it by position.) < $4 per.
Gyro: Senses the rate of rotation in one plane. You can integrate the data to get the degrees of rotation. (Position from rate of change.)
Kevin has code for this. (I think, anyway, you can find what direction you're facing...)
USE: Rotate accurately autonomously. Or interesting driver control of a robot with omni wheels. (Push the joystick forward and the robot goes away from you no matter what, you'd need a gyro for that.) ~$50 per. Kevin's site has suggestions on where to buy and which.
Encoders: Count gear teeth that go by. You can tell how far you've gone or the position of an arm.
USE: Driving accurate distances autonomously, for one. In that use, coupled with a gyro, you can tell where you are on the field at all times. (You'll need sin and cos functions, if you'd like a table look up sin approximation function in software, PM me. I still have to comment it, though.)
I don't know which types of encoders are best (optical? hall effect? etc.), where to buy them, or how much they cost. Could someone reply with that information please?
Dials/dipswitches: Many switches --> multiple combinations.
USE: Set to a position, do a different autonomous mode based on the switch position. If you have 4 switches, you could have 16 different modes.
You can also get encoded dials (easier to solder and attach PWM cables). (I can't find the link to the one we bought on jameco.com).
One set of dipswitches (8): < $1
Encoded dial w/ 16 positions: ~$3
(Our encoded dial was really small and you had to set it with a flathead screwdriver. Has anyone found a bigger one that can be set manually? Thanks.)
Other sensors possibly used: Camera, Infrared, Light sensors (for line following in 2004).
Hope this helped, not really sure what you were looking for.
~Stephanie
Team 1351 Controls Leader and Leader Programmer 2005-2006