Why is it so difficult for a robot to pass through a doorway using ultrasonic sensors?

Complex tasks are usually described as high-level goals, leaving out the details on how to achieve them. However, to control a robot, details must be provided. Having the robot move itself to and through an unknown, and possibly narrow, doorway is an example of such a task. The authors illustrate the difficulty of such a task using actual data from a real robot. The authors show how the transformation from high-level goals to primitive commands can be performed at execution time and they propose an architecture based on reconfigurable objects that contain domain knowledge and knowledge about the sensors and actuators available. The authors then show how their approach is used in solving the illustrated task