Well, for a 100% success rate, the robot must have sensor to determine whether
it's current direction is going to cause trouble, sensors to detect the extent
of it's environment, and it must have a memory to determine where it has failed
before.

ie, assumeing the robot can be place anywhere on the one side, drive straight
toward the target.  During this time, scan the area around you to get a general
idea of where you are in the box, and whether you are going to go over the right
or left portion of the tiles.  If you run into an obstacle, back up to a
predetermined point (perhaps where you started) since you know which tile you
went over, you can aim for the other tile.

If one could place a ultrasonic sensor on a rotating turret on the robot, one
could determine the position it is in inside the box.  It would also be able to
tell the robot of obstructions in its path.  Using wire senors around the front
of the robot, pointing downward, it could detect a drop off.

The trapdoor is could be detected with sonar pointing down.  Cracks in the
tile(which would indicate a trapdoor) are 'visible' in sonar signals.
Unfortunately, the robot could succeed, where the human might not notice such a
setup.

I'm a firm believer that there is a sensor for everything...  The real question
is is how to use them effectively and efficiently.  The more you add, the more
power they require... etc....

-Adam

Anne Ogborn wrote:
>
> I propose, as a thought experiment, a robot contest.
>
>   A series of typical small mobile robots are constructed
> to a standard design - perhaps wheeled "turtles".
>
>   One of these robots is equipped with a remote control unit.
> The others are distributed to the contestants. The contestants
> equip their robots with an autonomous control system and a set of
> sensors. Contestants are forbidden to modify the mobility mechanism
> (no fair making the robot fly, for example).
>
>   A bare room is constructed with a solid floor and walls, except
> for a "ditch", say 4' wide and 4' deep, which bisects the room
> and which is not navigable by the robots.
> Robots start on one side of the ditch, and navigate towards a
> well_marked goal on the other.
> The ditch is spanned by two replacable tiles, creating a binary
> choice - to reach the goal by crossing tile A, or by crossing tile B.
> Each tile covers half the ditch's length, so there is no question of missing
> the tile entirely.
>
> An assortment of these tiles, of varying construction, are available.
>
>   A tile is placed across the ditch, the remote control robot is then placed i
n the room and
> driven to the goal by a human driver. Those tiles which the robot
> crosses are called "crossable" and the others "uncrossable".
>
> The two tiles are selected at random from an assortment, always one from
> the crossable tiles and one from the uncrossable.
>
> Each robot is repeatedly released into the room to cross the ditch, with
> different sets of tiles.  The one which successfully reaches the goal the
> largest percentage of times wins.
>
> Now, my question is, how shall we design a robot to win this contest?
> We have no prior knowledge of the nature of the tiles.
> One might be bisected by a wall, another simply an open frame that
> will drop the robot into the ditch, another might have a hole only
> in one portion.
>
> It seems that many robot navigation problems (and I freely admit my
> robot building experience is limited) revolve around winning my little
> contest.
>
> And clearly there are some problems for which there is no solution.
> Suppose you are designated to drive the remote control robot, and you
> are confronted with two tiles which appear identical, both "boxes" that
> form a barrier across their whole width, but with an entry and exit door.
> What isn't known is that the floor inside one box is solid, the other
> a trapdoor.
>
> So, I invite you all, how does one construct such a robot?
>
> --
> Anniepoo
> Need loco motors?
> http://www.idiom.com/~anniepoo/depot/motors.html