[{Image src='attach/PIDRugBumpChallenge/rug-bump.jpg' link='attach/KR01/on-the-bench.jpg' caption='The dreaded rug bump' width='300' align='right' class='imgFloatRight'}]

The __PID Rug Bump Challenge__ is the ability for a small robot (with 35mm radius wheels) to confront the edge of the thick Kilim rug in my lounge (17mm thick, about half of the wheel radius) with one wheel — the other remaining on the wood floor — without being deterred from its path, i.e., recovering from that bump whilst maintaining its original heading. 

The "ideal" behaviour would be follows, quoting David Anderson of the DPRG:
%%blockquote
Even at very low torque/low speed the robot rotates through maybe a 
degree before mounting the carpets, and then once mounted the PID 
controller over-corrects and that straightens it out onto the original 
path.   So a little figure 'S'.  At higher speeds it just goes straight 
over the elevated carpet, with no change in the path.
%%

Tuning the [PID Controllers] so that they respond in kind is the challenge.

''More on this as the story develops...''