A sonar-ring is one of the most popular sensor for indoor mobile robots,
because it is simple and gives omni-directional distance information directly.
However, it is difficult to measure accurate bearing of reflecting points
by a conventional sonar-ring sensor.
And also, it is slow to get omni-directional information
due to sequential driving of transducers for avoiding interference of ultrasound.
In this paper, we propose a new sonar-ring sensor system for a mobile robot
which can measure the accurate bearing angles of reflecting objects.
The proposed system employs both
simultaneous transmission/receive of all directions and
accurate bearing angle measurement using differences of time-of-flight,
and consequently archived fast and accurate measurement.
We implemented a prototype of the proposed sonar-ring on a mobile robot, and
experimental data show the effectiveness of the proposed method.