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Warfighter Protection:
Mobile Interrogation of Densely Distributed Wearable Sensors
Sponsored by:
Jacob Rutheiser Aaron Gunn Maxwell Sun Matthew Williams
Kenneth Loh, Ph.D.
Associate Professor of
Structural Engineering &
Material Science
EIT Device
Executive Summary
Overview:
ARMOR Lab uses a process called electrical impedance tomography (EIT) to construct highly localized deformation and pressure maps of carbon nanotube (CNT) resistive films or “sensing skins” as seen in Fig 1. These sensing “skins” provide greater resolution than traditional piezoresistive sensors at a lower cost using less hardware. This makes them highly versatile and ideal for many applications, from non-invasive prosthesis monitoring to real time damage assessment of protective attire. Current EIT measurement techniques, however, depend on slow, bulky benchtop equipment. This project sought to develop a handheld, portable device capable of interrogating sensing skins of up to 32 nodes without sacrificing speed or resolution.
Method:
In order to interrogate a film, nodes are attached around the boundary of these sensing skins. One node is injected with a constant current, one node is grounded (either adjacent to or across from the node injected with current), and voltage measurements are taken at the remaining nodes. The current and ground nodes are then moved by one node and voltage measurements are taken again. This process is repeated until voltage measurements have been taken in all current-ground configurations, constituting one sample cycle. These boundary condition data are used in a convergent algorithm that provides a snapshot of the sensing skin’s change in conductivity relative to a baseline. This is consequently mapped to a change in temperature, pressure, and strain.
External
6061 Al enclosure with RF window
Completes Faraday cage around electronics
Durable and rigid
D-Sub connector
Robust and modular connection to sensors of varying geometries
OLED display + buttons
Intuitive user interface while testing
Internal
Battery Tray
Beaglebone Black Wireless
1 GHz, single-board embedded linux computer
Wireless communication + expandable storage
EIT Cape (Custom Printed Circuit Board)
Accurate and precise configuration, injection, and measurement of current, ground, and voltage for EIT data acquisition method
Power Management
2500 mAh 3.7 LiPo
3.9 hrs of battery life on a single charge (at full use)
Powerboost 1000C
Integrated charging and voltage regulation of electronics
12V Voltage Regulator
Voltage rail used by current injection source
Performance:
In action:
Objectives:
The objective of the project was to develop a portable EIT device that:
is portable, capable of operating off battery power for 30+ mins.
can inject current/ground, measure current, and measure voltage at any given node for films of 8 to 32 nodes.
can alternate nodal measurements automatically and measure with 16 bit precision.
can inject a 0 - 2mA DC current into the films within a 2% accuracy.
can stop locally on the device until retrieval.
Final Design:
Current Injection Stability:
Theoretical
Assumptions
Each error is normally distributed & independent
Result
Stability ±1.00% (400µA)
Measured
32 resistor network (5 kΩ - 8 kΩ)
Stability ±0.63%
ADC Repeatability:
Theoretical
Analog low-pass filter with 15 kHz cutoff (no high frequency noise)
Uniform quantization noise
Measured
No dominant frequencies
Quantization noise uniform across all frequencies
Full System Cycle Speed:
Theoretical
Assumptions
Sources of error: INL, DNL, Drift, and Offset (all normally distributed)
Result
Accuracy ±961µV, ±56µV (calibrated)
Measured
±126µV
<2 bit accuracy, ±0.5 bit precision
ADC Noise Coupling:
Theoretical
Assumptions
ADC slowest sampling speed (500kSPS)
Result
520 Hz (32 Node), 2.23 kHz (16 Node), 10.42 kHz (8 Node)
Measured
Inefficiencies in software implementation slow down cyclic sampling rate
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