Volumetric Tag Tracking

Kai-yuh Hsiao (under Joe Paradiso)


A Swept-Frequency Tag Reader

  • Detects proximity of magnetically resonant tagged objects in field.
  • Sweeps through 60kHz - 300kHz frequency band at 30 Hz.
  • Detects all magnetic tags resonating within sweep frequencies with each sweep.
  • Enables real-time CHI interaction (faster than multi-chip RFID) with simple physical objects.
  • Motivation: wanted faster tag tracking response, willing to sacrifice detect range.
  • Inexpensive reader electronics and simple, easily-crafted tag structure permit quick adaptation to CHI, sensing projects.

How it works

  • Uses dynamic imbalance of inductor bridge to detect response from resonant tags.
  • Sweeps continuously through frequencies, transmits out through coil.
  • Tags couple inductively to coil, imbalance bridge.
  • Bridge imbalance is filtered, translated to simple voltage output in time (picture below).

Single-coil system

  • Simplicity, safety, and tag structure very well suited to many applications.
  • Tag resonant frequency can be made to vary with physical parameters (e.g., force, position, etc.).
  • Limitations
    • Field lines are somewhat oddly shaped.
    • Only one measurement possible: detects proximity coupled with orientation.
    • Multi-tag objects produce reasonable but limited results.
  • Musical/graphical demo written, harmonic, melodic, graphical changes reflect tag presence.

Helmholtz arrangement

  • Two parallel coils shape field lines, create better-behaved field.
  • Tags between coils produce signal on each coil proportional to distance to coil.
  • Tags between coils produce sum signal proportional to cosine of orientation angle.
  • Still limited to one axis.


Plot of axial field vs. z


Multi-coil system

  • Work in progress - Helmholtz-coil arrangement of six coils in cube formation for full three-dimensional tracking.
  • Powered by six normal tag reader boards hac^H^H^Hstacked together.
  • Multiplexing of transmit and receive circuits allows sampling of each axis alternately.

Applications/Future Directions

  • Improvements:
    • Increased range (probably as far as 2 meters) - increase power, optimize frequencies/resonances.
    • Multi-axis tags or bent tags to ensure no loss of tags when rotated out of axis.
    • Fewer boards (most components on six boards currently unused).
    • Mathematical fit algorithms needed to find precise locations.
  • Controller interfaces:
    • Musical controller - Multi-axis single-hand continuous parameter control.
    • Computer interface - VR glove, simple, wireless, batteryless.
    • Tabletop/countertop interfaces.
    • Original motivation: Tumor tracking for radiation therapy. (proof of concept)

    updated 2000/04/26 by khsiao@mit.edu