Exploration is a critical task for a micromouse. The maze must be explored quickly, thoroughly and without error. Any mistakes made during exploration are likely to completely ruin your chances in any of the contests where there are search time or mouse handling penalties. In the Japan event, you don’t get very long in the first place so wasting time by restarting the exploration is not good.
Ideally, the micromouse will not stop moving unless it enters a dead end. Any turns other than that need to be very smooth and accurate. Possibly the most difficult formation to explore is a series of 90 degree turns one immediately after another. Here the mouse cannot use the side or diagonal sensors to reliably correct any forward errors and there is only a few mm of travel to correct any angular or lateral errors. In fact, corrections started by the side/diagonal sensors may simply put the mouse off course as the next turn comes up.
Attempting to run the entire exploration run by dead reckoning is difficult as errors can accumulate. More than a few cells and the errors may soon become uncorrectable. That said, if there is no wall ahead and the mouse has to turn, then it must do so based only on its internal estimate of position.
If there is a wall ahead in the cell where the turn is to take place, corrections can be made from that. The sum of the readings from the two forward-looking sensors give the distance to the wall and the difference gives the angular error. Using these values to correct forward positional errors and as feedback for the rotation controller can be very effective in making sure that the turn happens exactly where it should. Naturally, the walls in any maze vary so the turning position may also vary by a few mm but, done right, these errors should average out.
Part of the preparation of Decimus 4B for the Taiwan 2014 contest is fine tuning the exploration. The video below shows Decimus 4B performing a search with one key difference. Instead of examining the maze to determine the best route to the goal, the mouse just makes a random turn. The maze here has only posts except for the bounding walls. Corrections can only be performed when a turn has a wall ahead of it. Otherwise, they are all done by dead-reckoning.