Design and Verification of a Portable Scanner for Body-Worn Wireless Resistive Analog Passive (WRAP) Sensors

Abstract

Wireless resistive analog passive (WRAP) sensors are low cost, battery less, body-worn physiological sensors. Portable Scanner is a hardware device capable of acquiring physiological signals from multiple types of WRAP sensors using amplitude modulation (AM) technique. The signal acquisition relies on inductive loading principal of signal due to impedance mismatch between the LC resonator circuits of scanner and the wireless passive sensor. The physiological signals of the subject affect the impedance that results in the mismatch, which eventually modulates the amplitude of scanner-generated carrier wave at 6.15 MHz. The DDS along with RF power amplifier of the scanner generates the carrier wave. The AM wave's top envelope is picked up by an envelope demodulator $(\mathbf{f} >> 1\ \mathbf{KHz})$ from the AM signal. The signal is processed in analog domain by utilizing a 2nd order Butterworth low pass filter (1.5 KHz), a variable gain (Av) amplifier $(\mathbf{Av}_{\mathbf{min}}=1, \mathbf{Av}_{\mathbf{max}}=9)$, then signal is acquired in digital domain via 12 bit ADC and finally is transmitted to a smartphone application via a Class 2 Bluetooth wireless link at a baud rate of 115.2 kbps. The developed scanner device uses a 4-layer PCB $(14\ \mathbf{cm} \times 5\ \mathbf{cm}\times 1.25\ \mathbf{cm})$, and contained a 32-bit low power flash microcontroller (STM32L476) based on ARM Cortex - M processor. The device is powered by a 2000 mAh Li-Poly battery. The circuit consumes 0.69 mA during scanning burst, which can sustain continuous scanning of more than 2028 hours (for consumption rate of 0.7).