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.

1. Passive Displacement Sensing using
Backscatter RFID with Multiple Loads
Jonathan Becker, Carnegie Mellon University
Joshua Griffin, Disney Research
Matthew Trotter, Disney Research

2. 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
2
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

3. 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 Z
Z
Z
⇔Γ
⇔Γ
⇔Γ
22
11
3
Displacement
Sensor
RFID
Reader

4. Why are 3-States Needed?
Γ1v = H Γ1 − Γv( )
Γ12 = H Γ1 − Γ2( )
Γv = Γ1 −
Γ1v
Γ12
Γ1 − Γ2( )
Known
Known Known
Unknown (Value desired)Unknown
4
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.

5. Sketches of Hand Mounted Displacement
Transducer
Straight
Bent
5

6. Demo of Hand Mounted Displacement Sensor
6

7. Fig. 4 from Paper:
Sensor in Test Setup
Covered Microstrip Displacement Sensor
for Precision Measurements
7
Sensor Parts Side by Side

8. Controlled Measurement Test Setup
Displacement
Sensor
RF Switches
Known Loads
Tag Antenna
Reader
Antenna
(not shown)
8

9. Results with Different Antenna Separations
9

10. 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)
10

11. Thank you!
Any questions?
11

12. Live Results: Hand Mounted Displacement
Transducer
12

13. 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
13

14. Why Use Three Loads? (Cont.)
Move
Antenna
Remove DC Offset
Change
Load
Third load removes ambiguity created by DC block
14

15. Prototype 5.8 GHz 2-State RFID Tags
15