🚀 go-pugleaf

If you are operating in a fixed environment you might consider the
following methods.  These are methods which are used in an experimental
platform based on a mobile robot build by a company called Denning. Last
I heard, the robot(s) lived in the EE department at the University of 
Washington.

     1.  Ultrasonic sensors which can be used to identify known
         objects such as walls, doors, etc.  These sensors are
         easily fooled by reflections from hard surfaces.  For
         instance, to ultrasonics, a hardwood door looks like a 
         mirror.  The Denning Mobile robot has 24 Polaroid sensors.

     2.  Lasers can be used to scan walls for reflective tape.  Using
         several pieces of tape, you can create bar-codes which can
         be read by the laser.  The angular width of the scan can
         be used to get a rough estimate of distance.

     3.  Beacons.  These work kind of like a lighthouse.  Each beacon
         hangs on a wall and emits a unique code.  These can be implemented 
         using LED's.  The robot would determine its position by locating 
         and identifying two or more beacons.  Since the absolute position
         of each beacon is known, the position of the robot can be determined 
         from the angular positions of the beacons relative to the robot.