µTags : Passive microwave localization

µTags are low-cost passive microwave RF-transponders. Primarily used for tracking large numbers of tagged objects in 3-d, at short-range (3-100m), and in real-time. Unhindered localization of objects is desirable for many applications, including human computer interaction, product and asset tracking, and security. The proposed system will allow for selection, identification, tracking, and encoding of data from connected sensors to be returned to a host by these passive tags. The concept is illustrated to the left.

µTags utilize emerging technology developed for RFID and RADAR. A piezoelectric substrate of Lithium Niobate is patterned to create a Surface Acoustical Wave (SAW) device. SAW devices were pioneered in the 1970's, and are now used as passive filters, correlators, and oscillators in many telecommunication and RADAR applications. For this application, the SAW device is designed in a particular fashion to realize a passive transponder with encoding capabilities. A dynamic array of three or more base stations ping a network of µTags and cooperatively determine their location based on time-of-flight, angle of arrival, and phase-shift measurements. Triangulation calculations are performed on the resulting range and/or angle measurements from three or more base stations to localize a tag.

The SAW transponders in this application are only powered by the incident RF energy. A notational diagram of the patterning used for a simplified SAW transponder is shown in figure to the right. The inter-digital transducer (IDT) is a linear array of metal fingers that change incident electrical energy from an antenna into a surface acoustic wave on the piezo-electric substrate. This wave travels along at a slow velocity (in comparison to RF) until it reflects off of metallic strips. The reflected signals return to the IDT and are retransmitted via the antenna to the base-station. The gap between the IDT and the first reflector builds in a delay between the incident RF burst to the first reflector, which allows multi-path clutter to diminish before a rebroadcast, ensuring the base station receives a clear signal from the SAW transponder.

A SAW transponder reflects a short pulse of received RF energy back to the base-station as a phase-encoded pattern of data. Our SAW devices are designed to exploit spread spectrum RF interrogation and response signals. An arrangement of correlators and impedance modulated reflectors on the surface of the SAW transponder return properly encoded data patterns back to the antenna for retransmission to the base station. Several published techniques for selection, identification, and encoding of data with the tags will be applied. A standard one-step lift-off micro-fabrication process is used to make these passive SAW transponders. A microscope images of several initial test devices is shown at the left.

For more information, see the following documents:

Jason LaPenta -- Real-time 3-d Localization using Radar and Passive Surface Acoustic Wave Transponders (pdf) MS Thesis, MIT Media Lab, August 2007.

Last modified: Tue May 23 12:49 EST 2008

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