Ultrasonic Location System

Introduction

Obviously, such systems must gather information about their current context from sensor systems distributed throughout the environment (in the above example, the location and orientation of the user and cameras). Active Badges were used for our first experiments with context-aware systems, but many applications require fine-grained 3D location and orientation information which Active Badges cannot supply.

Our experiments required a low-power, wireless and relatively inexpensive 3D position sensor. No device which met these requirements was commercially available, so we have developed an ultrasonic location system (known within ORL as the "Active Bat" system) which has the desired characteristics.

Theory

A short pulse of ultrasound is emitted from a transmitter attached to the object to be located, and we measure the times-of-flight of the pulse to receivers mounted at known points on the ceiling. The speed of sound in air is known, so we can calculate the distances from the transmitter to each receiver - given three or more such distances, we have enough information to determine the 3D position of the transmitter (and hence that of the object on which it is mounted).

By finding the relative positions of two or more ultrasonic transmitters attached to an object, we can calculate its orientation. Furthermore, we can deduce some information about the direction in which a person is facing, even if they carry only a single transmitter, by analysis of the pattern of receivers that detected ultrasonic signals from that transmitter.

Hardware

Transmitters measure 5.5cm x 3cm x 2.4cm, weigh 30g, and are powered by a single 3.6V Lithium Thionyl Chloride cell, which has a lifetime of up to three months. A 418MHz radio link synchronises the ultrasonic transmitters with the fixed receivers, and each transmitter has a unique 16-bit address, allowing different location qualities-of-service to be assigned to up to 65536 objects.

The receivers used to detect the ultrasonic signals consist of a sensor head, which is mounted in a tile of a suspended ceiling (commonly found in office buildings), and a sensor body, which rests above the tile. Receivers are placed in a square grid, 1.2m apart, and are connected by a high-speed serial network in daisy-chain fashion. The serial network is terminated by a PC, which collects results from the receivers and uses them to calculate transmitter positions.

As can be seen from the above photograph of the ceiling of our main laboratory, the installed receivers are unobtrusive (sensor positions are indicated by the orange arrows).

Results

We are currently extending the system to cover larger areas of our building, and are investigating minaturisation of the ultrasonic transmitter, as well as ways of increasing the system accuracy and update rate.

Papers

• Andy Ward, Alan Jones, Andy Hopper. A New Location Technique for the Active Office. IEEE Personal Communications, Vol. 4, No. 5, October 1997, pp. 42-47. (download)

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