Question L4-C4

The question is about the choice of algorithm for local avoidance of obstacles in a particular case where the obstacle has a U shape with the bottom turned toward the goal, to have a kind of local minima that the robot need to solve to reach the goal. On the right there is drawing of the obstacle. The obstacle is printed on the ground, allowing the robot to see in all moments the goal. The question is: "To test local navigation algorithms, we place the Thymio robot on a paper surface where a local obstacle has been printed (see image on the right). Thymio can detect it with the ground sensors, can drive on its borders to do obstacle following, for instance, but should not pass this obstacle. 
A light with a strong IR component is placed as a goal and Thymio has to reach it. Thymio can detect the light with the two ground sensors (the only sensors on Thymio that have an ambient light detector) and can therefore orient toward the light. 
Which local avoidance strategy will allow to pass the obstacle?". Three possibilities are proposed:

Answer A. "Potential field". This answer is wrong, as this will trap the robot in the obstacle. The U generates a local minima for the potential field, having the robot repulsed by obstacles in all direction while being attracted by the goal.
In the explanation of the student we would like to see that they understand that this configuration, combined with this algorithm, will generate a local minima where the robot will be trapped, nver reaching the goal.

Answer B. "Pledge algorithm". This answer is correct, as this algorithm makes wall following but by remembering the angle and going to the goal only when back to the original angle.
In the explanation of the student we would like to see that they understand that this algorithm has a particular property to go toward the goal only after having avoided the obstacle. One can refer to the slide of the course for a detailed explanation.

Answer C. "Moving toward the goal when no obstacle, turn left when obstacle" This answer is wrong, as this will trap the robot in the obstacle because of its shape, making infinite loops.
In the explanation of the student we would like to see that they understand that this is a similar case than the potential field, trapping the robot in a situation that cannot be solved to reach the goal.

