Distance measurement while tracking a line

Tracking a line has advanced to the next stage, as the distance travelled can be measured in mm.

A movie of line tracking with distance measuring is given below, where first the light sensor is calibrated to the environment and the line (crack in this case) followed by the MER following the line (with untuned PID-control) for a couple of loop-iterations until it stops and writes the travelled distance to the LCD display of the Lego brick. It has travelled 613 mm in this case.

The MER transmission is adapted to a pleasing cruising speed when following a line, as well as a nice gear ratio for the rotation sensors. The transfer ratio from the wheels to the encoder is 50/3, while the circumference of the wheel is (about) 257.6 mm. This results in a resolution of around the 1 mm/encoder count  (0.966 mm/count to be precise).
In this way, the distance travelled can be given to a millimeter, as is required in the line tracking competition.

In the movie above, we used two motors, one light sensor and one encoder.
At this moment, we are planning to use only one Lego brick for the line tracking competition. In this way, we make everything as simple as possible, but not simpler.
We are programming to use two motors to drive the wheels, two light sensors and one encoder to do the job. One light sensor should take care of the straight line PID tracking, while the other one (in combination with the first) should detect the beginning, ending and possible interuptions of the line without problems. The encoder is used to measure the length of the line to a millimeter.