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Backscatter radio frequency (RF) tags and radio frequency identification (RFID) tags can be used to create wireless sensors that do not require a battery by using the tag antenna's electrical load as a transducer. This paper presents such a backscatter sensor system operating in the 5.8 GHz unlicensed frequency band that wirelessly senses distance (i.e., displacement) between one and five millimeters. The system senses the variable impedance of a covered, open-circuit microstrip line and uses two fixed antenna load impedances to remove the effects of the radio channel. The primary contribution of this paper is a demonstration of the wireless sensing system in which the binary position of a human finger (e.g., “bent” or “straight”) is measured using a passive transducer for wireless human-computer-interaction (HCI) applications.
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Passive Displacement Sensing using Backscatter RFID with Multiple Loads
Jonathan Becker, Carnegie Mellon University Joshua Griffin, Disney Research
Matthew Trotter, Disney Research
Goal Wirelessly sense the bend or displacement of an object with a low cost passive device.
Prior Approaches • Piezoresis:ve materials • Fiber op:cs
Problem with Prior Approaches May require analog-‐to-‐digital converters and transmiAng circuitry to sense bend – rela:vely high power consump:on
Key Insight Use a displacement sensor aDached to hand to change scaDering proper:es of a backscaDer RFID tag as the hand bends and straightens. Applica8ons • Gestural interfaces • Virtual Reality • Mo4on capture
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For example, see: • Olson et. al., “Piezoresistive strain gauges for
use in wireless component monitoring systems” in 2008 IEEE Sensors Applications Symposium
• Jahed et. at., “Enhanced resolution fiber optic strain sensor based on mach-zehnder interferometer and displacement sensing principles” in IEEE ELECO 2009
Diagram of 3-State RFID Tag Test Setup
Want to determine this value
D = Antenna Separation
Can vary in time
H~Channel unknown a priori
vv ZZZ
⇔Γ
⇔Γ
⇔Γ
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3
Displacement Sensor
RFID Reader
Why are 3-States Needed?
Γ1v = H Γ1 − Γv( )
Γ12 = H Γ1 − Γ2( )
Γv = Γ1 −Γ1vΓ12
Γ1 − Γ2( )
Known
Known Known
Unknown (Value desired) Unknown
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Reference: S. Capdevila, L. Jofre, J. Romeu, and J. C. Bolomey, “Multi-Loaded Modulated Scatterer Technique for Sensing Applications,” IEEE Trans. Instrum. Meas., vol. 62, no. 4, pp. 794–805, 2013.
Sketches of Hand Mounted Displacement Transducer
Straight Bent
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Demo of Hand Mounted Displacement Sensor
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Fig. 4 from Paper: Sensor in Test Setup
Covered Microstrip Displacement Sensor for Precision Measurements
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Sensor Parts Side by Side
Controlled Measurement Test Setup
Displacement Sensor
RF Switches Known Loads
Tag Antenna
Reader Antenna
(not shown)
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Results with Different Antenna Separations
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Conclusions • Demonstrated a 5.8 GHz RFID tag with a
sensor that detects displacement passively • Need a total of three impedance states to
determine an unknown load state with an unknown channel – The first two impedance states contain channel
information • Future work
– Design a printed circuit board version of the tag – Integrate a charge pump into the circuit (passive
tag)
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Thank you! Any questions?
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Live Results: Hand Mounted Displacement Transducer
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Why Use Three Loads?
Removing DC offset creates ambiguity: Change in a load vs. moving antenna?
DC Offset Removed
Move Antenna
Change Load
Similar Rotation
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Why Use Three Loads? (Cont.)
Move Antenna
Remove DC Offset
Change Load
Third load removes ambiguity created by DC block
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Prototype 5.8 GHz 2-State RFID Tags
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