ultrasonic sensor for wall following

I would think that one of the Sharp IR distance sensors would be a good
choice.  The Mark II store has several versions.  Some put out an
analog signal for the distance.  Others simply trigger if it detects
within a certain range.  Note that is not simply a light sensor.  It
actually puts out a spot of light, and focuses the image of it
reflecting from the surface onto its linear sensor.  So, parallax is
the method used.  The LED output is modulated, so it has good noise
rejection as well.

There are several models with different ranges and outputs. Here is
one, the  Sharp GP2Y0A02YK.  It has a 150 cm range with an analog
output http://www.junun.org/MarkIII/Info.jsp?item 

I just noticed they have a minature
version...http://www.junun.org/MarkIII/Info.jsp?item † but it may not
have enough range for you.

Joe Dunfee

Oops,  that link didn't' work right. The minature one is
http://www.junun.org/MarkIII/Info.jsp?iteme

 Joe Dunfee

If you don't mind, can you explain why you think IR sensors will work
better than sonar in this application?  Don't IR sensors provide
different readings depending on the reflectiveness of the surface?

There are simpler types of distance sensors that work on
reflectiveness, but the particular Sharp sensors I referenced were of a
different category.  They work on a parallax method.  A LED outputs a
modulated light focused through a lens.  A second lens focuses a view
of the world onto a sensor which essentially detects where the dot is
seen (not really a linear array, but functioning like one).

So, it works on a parallax principal, not reflectivity.  It has proven
to be a very robust sensor from the standpoint of rejecting ambient
light, and varying colors of surfaces.  Of course, there are problems
with reading highly reflective surfaces or things like glass.

Joe Dunfee

You might get some interesting interference from the rotor blades whizzing
about if you're planning on using sonar. How about a laser rangefinder?
Regards,
John Ebbinghaus

Know of any that are light in weight (<20 grams) and still reliable?