Looking for a miniature low-power range detector for a robot
I think you might have to compromise on at least one of those requirements. The normal way of doing this sort of thing on small robots is ultrasound, e.g. http://dx.com/p/ultrasonic-sensor-distance-measuring-module-138563

That fails your "directionality" and probably "size" requirement, but is cheap and effective. There are smaller, narrower ones: https://www.sparkfun.com/products/639

If directionality is really important, I don't think there's any substitute for doing it with a laser. There's a discussion here: Any published designs for time-of-flight laser distance measurement , but the engineering involved is not simple and I can't find any modules implementing it. The "time of flight resonance" technique looks best to me, although the round-trip time for 15cm is 1ns => 1GHz signal. That's going to make for tricky PCB design and isn't feasible for a beginner.