1. Teng Wei and Xinyu Zhang
Gyro in the Air: Tracking 3D Orientation of Batteryless Internet-of-Things
Teng Wei and Xinyu Zhang
Department of Electrical and Computer Engineering University of Wisconsin - Madison

2. Batteryless Orientation Tracking
Orientation is an essential input
Mobile gaming
Virtual reality
Indoor navigation
Battery-powered
Many applications require passive orientation sensing
Ad-hoc setup
Least maintenance
Low cost
Gaming
Smart Home
Supply Chain
Production Line

3. Technique Review Image-based orientation detection
Sensitive to background variation
Require good ambient light condition
Fail during the occlusion
Privacy concern

4. Tagyro: RFID-based Orientation Tracking
Basic idea
RFID Reader
Tag Array
Object
Standard phase-distance model in the RFID
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑎𝑔_𝑡𝑜_𝑟𝑒𝑎𝑑𝑒𝑟
𝑃ℎ𝑎𝑠𝑒 2π
𝑤𝑎𝑣𝑒𝑙𝑒𝑛𝑔𝑡ℎ 2

5. estimated orientation
Tagyro: RFID-based Orientation Tracking
Phase Difference of Arrival (PDoA) of a Tag Array
Tag Array
Tag1
Tag2
θ = 0
θ > 0
θ < 0 θ
PDoA (Tag 1 and Tag 2)
= 0
𝜑 2 − 𝜑 1 = 0
> 0
𝜑 2 − 𝜑 1 > 0
< 0
𝜑 2 − 𝜑 1 < 0
Tag2
Tag2
RFID Antenna
Tag1
Tag1
Tracking algorithm
estimated orientation
Input: Array layout and phase
Process: Compute intensity map that gauges the similarity between measured and theoretical PDoA
Output: Orientation

6. Conditions Underlying the Phase-based Tracking
Tag backscatters like an isotropic point source
RFID Antenna
Phase changes linearly with tag-to-reader distance
Layout of the tag array is known

7. Imperfect Tag Radiation Pattern
signal +X +Y +Z
RFID Antenna
Rotation of a single tag
Experiment Setup
RSS
Phase
blind direction
antenna polarity

8. Measured and Theoretical PDoA of A Two-Tag Array
Imperfect Tag Radiation Pattern
Rotation of a tag array
signal
Tag2 r
Tag1
RFID Antenna
Experiment Setup
Measured and Theoretical PDoA of A Two-Tag Array
Electromagnetic coupling deviates the measured PDoA

9. A Summary of Problems and Solutions
Antenna polarity
Solution: Polarity Alignment
Not Aligined
Aligned
Polarity
Electromagnetic coupling between tags
Solution: Array Layout Sensing
Blind direction
Cannot track orientation in 3D
Solution: Multi-array Tracking

10. Dealing With the Coupling
Key observation
Tag coupling scales PDoA by a constant factor
scaling
Scaling factor depends on effective tag-to-tag distance
Effective distance
Geometry distance
Tag1
Tag2
6 cm
Geometry distance
Effective distance
Effective distance
Tag2’
9 cm

11. Array Layout Sensing Sense the effective layout of a tag array
Randomly rotate tag array and measure the phase t
PDoA
PDoA scaling
Track PDoA scaling, and map it to the effective distance X Y Z
Tag 1 …
Tag i
Tag 1 (ref.)
d11
d1i
Tag j (ref.)
dj1
dji
Compute the effective layout from distance matrix
Construct a matrix of effective distance for each tag pair

12. Toward 3-DoF Orientation Tracking
Dual-array and dual-antenna setup
Reader antenna 2
Array 1
Array 2
Antenna 1
Y-axis, Z-axis
Blind direction
Antenna 2
X-axis, Z-axis
Array 2
Reader
antenna 1
Array 1 X Y Z
Algorithmic extension
Combo
Validator
OrientationTracking
Orientation Spectrum Synthesis
Phase
RSS
Orientation (𝜃 𝑥 , 𝜃 𝑦 , 𝜃 𝑧 )
Determine which array is out of the blind direction
Combine the redundant DoFs

13. Calibrating Reader Frequency-Hopping
Random frequency hopping in RFID reader
902~928 MHz; 50 channels; stay at each channel for 200 ms
Observation: Different frequencies induce different initial phase-offsets at the reader
Frequency-hopping calibration algorithm
Objective: Calibrate the phase so that it looks like output from a single frequency
Key: cancel the initial phase-offset and calibrate phase shift of different frequencies

14. Implementation and Evaluation Setup
Hardware
Impinj RFID Reader
RFID Antenna
Tag Array
RFID Tags
Object
Impinj R420 RFID reader
Circular-polarized antenna
Three types of tags
Evaluation setup
Tagyro GUI
Real-time implementation

15. Orientation Tracking Accuracy
Accuracy vs. DoF
Accuracy over Distance
Accuracy vs. Size of Tag Array
Average tracking error ~ 5-degree

16. Case Study 12-side dice Smart home 91.7% detection accuracy
Fine-grained record

17. Conclusion
Tagyro: 3D passive orientation sensing by using RFID tags, based on phase-distance model
Solve key challenges that break the ideal phase-distance model
Blind direction
Antenna polarity
Electromagnetic coupling
Extensive evaluation using COTS RFID reader, antenna and tag
Demonstrate accurate passive orientation tracking

18. Questions?
Thank you