Wearable sensors will one day instantly provide physicians with updates on their patients’ health conditions, even their up-to-the-moment whereabouts. In support of that goal, a team of students at Ohio University developed a BAN with heart-rate monitor (HRM) and other sensors, such as a fall detector and temperature monitor, integrated into a chest strap. The chest strap includes a low-cost microcontroller to collect and organize the data from the difference sensors also mounted within the chest strap.
The HRM, temperature sensor, and fall detector use a low-frequency data link at 5.5 kHz to communicate with the microcontroller, which in turn makes the collected data available to a smartphone or other monitoring device by means of 2.4-GHz Bluetooth low energy (BLE) communications. The choice of low-frequency 5.5 kHz data link results in less signal attenuation through and around the body of the wearer, using an inter-integrated-circuit (I2C) bus to the microcontroller as the HRM interface (HRMI).
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The students designed the system and specified different sensors and components for a prototype system as part of an undergraduate project, a competition in the 2015 IEEE Antennas and Propagation Society Student Design Contest. Since the wearable electronics must be portable and battery-powered, the student designers were constrained in their choice of sensor components, such as the fall detector sensor. It is based on MEMS technology and powered by a +3.3-V dc source and includes a three-axis accelerometer and three-axis gyroscope. This sensor is capable of monitoring changes in acceleration in three axes, taking constant samples to determine when a wearer has experienced a fall by the rapid changes in acceleration.
One of the challenges in completing the BAN design was the development of a compact BLE antenna that could be mounted in the chest strap and radiate outward from the body with sufficient gain. After trying several antenna configurations, an inset-fed patch antenna was developed that was somewhat larger than an earlier planar-inverted-F antenna (PIFA) design, but with good gain and radiation characteristics. Antenna testing was performed at the university’s own shielded anechoic chamber and the total cost of parts and labor was tallied at just under $600.
See “Prize-Winning Ohio University Students Present Their Work on an Antenna for Body Area Networks,” IEEE Antennas & Propagation Magazine, Vol. 59, No. 1, February 2017, p. 116.