Abstract:
Shoe-mounted wearable sensors can be used in applications, such as activity
monitoring, gait analysis, post-stroke rehabilitation, body weight measurements and energy
expenditure studies. Such wearable sensors typically require the modification or alteration
of the shoe, which is not typically feasible for large populations without the direct
involvement of shoe manufacturers. This article presents an insole-based wearable sensor
(SmartStep) that has its electronics fully embedded into a generic insole, which is usable
with a large variety of shoes and, thus, resolves the need for shoe modification. The
SmartStep is an always-on electronic device that comprises a 3D accelerometer, a 3D
gyroscope and resistive pressure sensors implemented around a CC2540 system-on-chip
with an 8051 processor core, Bluetooth low energy (BLE) connectivity and flash memory
buffer. The SmartStep is wirelessly interfaced to an Android smart phone application with
data logging and visualization capabilities. This article focuses on low-power implementation
methods and on the method developed for reliable data buffering, alleviating intermittent
connectivity resulting from the user leaving the vicinity of the smart phone. The conducted
tests illustrate the power consumption for several possible usage scenarios and the
reliability of the data retention method. The trade-off between the power consumption and
supported functionality is discussed, demonstrating that SmartStep can be worn for more
than two days between battery recharges. The results of the mechanical reliability test on
the SmartStep indicate that the pressure sensors in the SmartStep tolerated prolonged
human wear. The SmartStep system collected more than 98.5% of the sensor data, in real
usage scenarios, having intermittent connectivity with the smart phone.