How to select best sensor for robot

The below tutorials are very helpful to select sensor for a robot based on application in a economic way.

http://www.geocities.com/naveen_pn/circuitspage.html

http://www.owlnet.rice.edu/~elec201/Book/motors.html

http://www.seattlerobotics.org/encoder/200106/16csscnt.ht