Add-on 5kHz Inductive Link for Mobile Phones Using Audio Port

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Information about Add-on 5kHz Inductive Link for Mobile Phones Using Audio Port

Published on February 18, 2014

Author: sammakko1



Today's mobile phones provide several wireless interfaces, both for local and for cellular connectivity. What has been lacking, however, is connectivity to the de facto standard wearable heart beat sensors, usual accessories for fitness and wellness applications, using a 5 kHz inductive link. We have developed, for mobile phones, a simple add-on device (hardware) that can be plugged to the audio port (TRS) of a mobile phone, and simple add-on software based on GStreamer audio library for the Nokia N900 Maemo Linux phone together capable of reading the heart beat signal. The solution is not restricted to heart beat sensor applications.

Add-on 5kHz Inductive Link for Mobile Phones Using Audio Port I. Jantunen1, S. Siikavirta1, V. Nässi1, T. Korhonen1, J. Kaasinen2 1Aalto University, Department of Communications and Networking 2Sports Tracking Technologies oy

Contents • • • • • • UBI-SERV project Body area networks (BAN) and mobile phones Wireless BAN sensor architecture 5 kHz receiver hardware implementation Signal processing software implementation Results and discussion – conclusions

UBI-SERV – Targeting • There are needs and there are services & technologies but they often do not match in practice! UBI-SERV • Aalto University and Peking University cooperation • Investigate services such that they will be truly applicable • Research & develop service and technology solutions for health/well-being and public safety systems

UBI-SERV project structure

History of wireless sensor networks • 5 kHz solution by Prof. Säynäjäkangas in the late 1970’s • Infrared links (IrDA and earlier solutions) from 1990’s • Emergence of (Classic) Bluetooth since 1998 – Many uses found, ranging from synchronizing phone data to wireless sensors – Bluetooth 4.0 (including BT Low Energy) in 2010 • Proprietary digital solutions to provide lower power use in the 2000’s – 27 MHz (CB) proprietary radios in many devices (e.g. wireless mice) – Nordic Semiconductor’s 2.4 GHz ANT radio (used by Suunto, Logitech) • Emergence of ZigBee • RFID technologies for passive sensors – NFC included in commercial mobile phones in early 2007

BANs and mobile phones • Bluetooth widely available – Crippled in US market to be used only with headsets due to teleoperator business reasoning • hinders sales of e.g. Bluetooth keyboards worldwide – Bluetooth not high bandwidth enough for some use cases – Bluetooth not low energy enough for some use cases – Bluetooth 4.0 solves some of this, but still more expensive circuitry for simple sensing than e.g. 5 kHz analog inductive link • Other body area networks not supported – adding more radio transceivers to circuit board not simple • Add-on 5 kHz receiver could solve this

Add-on 5 kHz link • 5 kHz receiver is basically just a tuned coil • 5 kHz is on audio range (20 Hz – 20 kHz) • 5 kHz signal, as analog electrical signal, can thus be received and processed with phone audio processing unit • Just an amplifier circuit needed to input the signal received with the coil to the phone audio processing • Software for further signal processing, peak detection, protocol reading

Hardware implementation • 6.8 mH coil • LM324N amplifier – Powered by a 9 V battery in our proof-of-concept setup • TRRS/TRS plug to connect to phone/computer audio jack • ADC chip within the phone/computer audio processing unit • Tested with Polar WearLink 31 heart beat sensor and Polar G1 GPS speedometer.

Hardware implementation – 1

Hardware implementation – 2

Software implementation • Based on Linux computer and Maemo Linux mobile phones (Nokia N900) • Using Gstreamer library • Raw audio data – no preprocessing – Preprocessing would remove our data as noise • • • • Effectively sampling 441 samples/second Pulse detection, ignorance window after a pulse Protocol module decodes the data Different protocol module for different pulse data codes – E.g., heart beat, other sensor

Software implementation – 1

Software implementation – 2

Discussion • Input sensitivity of the sound cards differs notably – Signal normalization needed (software issue) • Signal receive could be done also with an existing coil within the mobile phone, e.g., in speaker – Requires hardware modifications in audio circuit, thus needs to be taken care of in manufacturing phase. • Our solution could be sold as an add-on to any phone with programming interfaces available (smart phone) – Actually not limited to phones, but any computer with audio input • Phones control input devices, not any microphone input is enough

Conclusions • Our add-on hardware and software solution for receiving 5 kHz heart rate signals to mobile phone is demonstrated to work on a computer • Phone implementation needs to get the phone accept the input device – a commercial issue • Not limited to heart rate sensors, but any low-bandwidth body area sensor networks

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