Gyro in the Air: Tracking 3D Orientation of Batteryless Internet-of-Things; seminar @ polyu, Hong Kong, China
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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 Tagyro: RFID-based Orientation Tracking Basic idea Object RFID Reader Tag Array Standard phase-distance model in the RFID 2π ๐โ๐๐ ๐ 0 ๐ค๐๐ฃ๐๐๐๐๐๐กโ 2 ๐๐๐ ๐ก๐๐๐๐๐ก๐๐_๐ก๐_๐๐๐๐๐๐ Tagyro: RFID-based Orientation Tracking AoA model PDoA Tagyro: RFID-based Orientation Tracking Phase Difference of Arrival (PDoA) of a Tag Array θ= 0 θ> 00 θ< Tag2 Tag2 Tag2 Tag1Tag1 Tag1 RFID Antenna θ PDoA (Tag 1 and Tag 2) =0 ๐2 − ๐1 = 0 >0 ๐2 − ๐1 > 0 <0 ๐2 − ๐1 < 0 Tag Array Tracking algorithm Input: Array layout and phase Process: Compute intensity map that gauges the similarity between measured and theoretical PDoA Output: Orientation estimated orientation 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 Imperfect Tag Radiation Pattern Rotation of a single tag +Z +X +Y RFID Antenna Experiment Setup blind direction antenna polarity RSS Phase Imperfect Tag Radiation Pattern Rotation of a single tag blind direction Imperfect Tag Radiation Pattern Rotation of a single tag Linear polarized propagation electric field electric field +Z tag +X +Z +Y +Y +X Imperfect Tag Radiation Pattern Rotation of a tag array Tag1 Tag2 r RFID Antenna Experiment Setup Electromagnetic coupling deviates the measured PDoA Measured and Theoretical PDoA of A Two-Tag Array A Summary of Problems and Solutions Antenna polarity Solution: Polarity Alignment Polarity Aligned Not Aligined Electromagnetic coupling between tags Solution: Array Layout Sensing Blind direction Cannot track orientation in 3D Solution: Multi-array Tracking 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 Tag1 Geometry distance Tag2 6 cm 9 cm Effective distance Tag2’ Effective distance Geometry distance Array Layout Sensing Sense the effective layout of a tag array PDoA PDoA scaling t Randomly rotate tag array and measure the phase Track PDoA scaling, and map it to the effective distance Z Y X Compute the effective layout from distance matrix Tag 1 (ref.) … Tag j (ref.) … Tag 1 … Tag i … d11 … d1i … … … … … dj1 … dji … … … … … Construct a matrix of effective distance for each tag pair Toward 3-DoF Orientation Tracking Dual-array and dual-antenna setup Array 1 Array 2 Antenna 1 Y-axis, Z-axis Blind direction Antenna 2 Blind direction X-axis, Z-axis Reader antenna 2 Array 2 Z X Array 1 Algorithmic extension RSS Phase Orientation Tracking Combo Validator Determine which array is out of the blind direction Orientation Spectrum Synthesis Combine the redundant DoFs Orientation (๐๐ฅ , ๐๐ฆ , ๐๐ง ) Y Reader antenna 1 Implementation and Evaluation Setup Hardware RFID Antenna Impinj R420 RFID reader Circular-polarized antenna Impinj RFID Reader Tag Array Object Three types of tags Real-time implementation Evaluation setup RFID Tags Tagyro GUI Effectiveness of sensing effective layout Accuracy of selecting the valid combo Rotating two arrays together in 3D space, and inspect if Tagyro can select the correct tag array. Ground truth: Check if an array’s blind direction is within 40 degrees pointing to the reader antenna. Interrogating speed of tag array Evaluate the query speed by interrogating different sizes of tag arrays. Efficiency of tracking algorithm Examine the real-time processing speed of Tagyro Rely on RFID reader’s query speed Orientation Tracking Accuracy 3D orientation tracking accuracy (i) 1-DoF: Traking Z-axis rotation using single tag array and single-antenna reader; (ii) 2-DoF: Tracking Y and Z axis rotations using dual-array and singleantenna reader; (ii) 3-DoF: Tracking Y and Z axis rotations using dual-array and dualantenna reader; Accuracy over Distance Accuracy vs. DoF Orientation Tracking Accuracy Accuracy vs. DoF Accuracy over Distance Accuracy vs. Size of Tag Array Average tracking error ~ 5-degree Case Study 12-side dice 91.7% detection accuracy Smart home Fine-grained record 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 https://www.youtube.com/watch?v=sxT KrBZXP7k Case study 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 Imperfect Tag Radiation Pattern Rotation of a single tag Linear polarized Circular polarized propagation propagation electric field electric field +Z +X +Z tag +Y reader +Y +X
