The Bristol Wearables group have been developing an Ultrasonic Object Tracker, initially as a prototype for use in the Equator Curious Home project. The tracker can be used to monitor the positions of objects of interest as they are used in the home, over extended periods of time. It differs from the earlier ultrasonic positioning systems developed in Bristol by swapping the arrangement of ultrasonic transmitters and receivers. An infrastructure using several fixed receivers within a room is used to monitor small transmitters fixed to objects, rather than objects receiving signals and carrying hardware to compute their own positions independently. This is better suited to an application where position data needs to be collected centrally. The transmission unit uses a PIC chip to drive an ultrasonic transmitter, which transmits short 40kHz ultrasonic pulses. The unit is a small device (42x18x10mm) with a battery life of up to several years. At the infrastructure level, six ultrasonic receivers are placed around the room and linked to a receiver unit which monitors incoming ultrasonic pulses. This is linked via a serial cable to a computer, where software interprets incoming distance data and computes object positions. A transmitter and receiver unit are shown in the images below. 
Position tracking is achieved by measuring the relative time of arrival of a transmitted signal at each of the fixed receivers, and then solving for the unknown position co-ordinates. The positions of the fixed receivers must be known in order to find a solution, and we have developed auto-calibration algorithms to determine these in advance. As the system will attempt to find the origin of any ultrasound source, we also need a unique transmission signature for each object that is to be tracked. This is achieved by transmitting three pulses in quick succession, using a unique timing pattern for each distinct transmitter. While the tracker has a specific purpose in the Curious Home project, it is more widely applicable. It was used in the Lighthouse project, where it tracked museum visitors and supported mixed reality museum visits involving web, VR and physical museum visitors sharing their visit together. The accuracy of the trackers allowed visitors in a gallery in the Lighthouse architecture museum to be tracked and displayed on a portable map of the gallery. It is most suited to applications that require low frequency position updates for a relatively large number of objects, over the area of an average 25-50m2 room. Computed positions have a RMS error of 1.7cm, or 2.3cm when using auto-calibration, so it is practical to track even quite small objects. |