High Speed Measurement of Normal Direction of Walls by Ultrasonic Sensor.
Akihisa Ohya, Yoshiaki Nagashima, Shin’ichi Yuta · Journal of the Robotics Society of Japan · 1995
A novel ultrasonic sensing method which can measure the normal direction of walls is proposed .The inclination of the wall is calculated from the difference of the round-trip time-of-flight plural receivers .This sensor system has four receivers for the error correction using inconsistency check of the propagation times.