The __SR04__ robot was designed and built by David P. Anderson. 
It was David's fourth robot, hence the "04", a bit curious what the "SR" stands for (small robot?). 

From David's description:
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__SR04__ is a small mobile robot suitable for exploring human habitats unattended. 
It is controlled by a Motorola HC6811 microprocessor running in an M.I.T. 6.270 CPU card, similar to the commercially available "Handy Board." Two 12-volt DC gear-head motors maneuver the robot in a dual-differential drive configuration, balanced by a non-driven tail wheel caster and powered by a 12 volt 2.2 amp-hour sealed lead acid battery. Sensory input is provided by (in order of priority): front bumper switches, IR collision avoidance, stereo sonar ranging, photo detectors, passive IR motion detection, and shaft-encoder odometry. 
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!! Design

%%small
~[The following text and images are from David Anderson's web site, used with permission.]
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%%hx Chassis %%
[{Image src='attach/SR04/sr04-front-49-thumb.jpg' link='attach/SR04/sr04-front-49.jpg' caption='SR04 Front View (image used with permission)' align='right' class='imgFloatRight' }]

The design which has emerged is a small dual-differential drive platform with the geometry of an 11" circle. The drive wheels and tail caster sit on the perimeter of this circle, and thus it can rotate in its own space. This greatly simplifies software for maneuvering and collision avoidance.

%%hx CPU %%

The robot controller is a Motorola HC6811 microprocessor running in an M.I.T. 6.270 board. This card was developed for the introductory robotics course taught at M.I.T. It provides the HC6811 processor with 32k of battery-backed RAM, 32 eight-bit A/D channels, hardware 40khz IR , 6 one-amp H-Bridge (L293) motor driver channels, an LCD readout, and some digital I/O ports for timer and output compare functions useful to robot builder-types.
 


!! Links

* [SR04 Mobile Robot|http://www.geology.smu.edu/~dpa-www/robots/sr04/sr04.html], project description by David P. Anderson ([PDF version|http://www.cs.cmu.edu/~motionplanning/papers/sbp_papers/integrated2/anderson_sr04.pdf])
* [SR04 Mobile Robot|http://www.seattlerobotics.org/encoder/199806/sr04.htm] from the ''Newsletter of the Seattle Robotics Society''
* ''[Subsumption for hte SR04 and JBot Robots|http://www.dprg.org/articles/2007-03a/]'' \\
   David P. Anderson, 26 March 2007