Development and applications of an innovative wearable system based on time-of-flight technology for the measurement of the human movement

The analysis of the human movement is the subject of many research projects. Recently, thanks to the advancement in the development of high performance and low-power electronic components, wearable sensors have given rise to devices and techniques which allow an objective evaluation of different human movement quantities both inside and outside the laboratory setting (e.g. during activities of daily living). The objectives of the research conducted and reported in this Ph.D. thesis regard the devise, development, validation and applications of an innovative wearable system, named SWING, for the human movement monitoring and analysis. The SWING system is the result of a design aimed at providing a wireless system-on-board processing capabilities integrating a magneto-inertial measurement unit and a Bluetooth module (main board) and up to three infrared time-of-flight distance sensors (satellite boards). It was specifically devised to take advantage of the positive points of magneto-inertial measurement units, which are capable of measuring the human movement for a long period of time and also during daily life activities with a good level of accuracy, but also to overcome some of their limitations (e.g. drift, ferromagnetic interferences, etc.). Moreover, the SWING system allow the direct measurement of quantities, such as the inter-foot distance and the step width, that magneto-inertial measurement units can only obtained indirectly (high estimation errors). The advantages of using the infrared time-of-flight technology over other technologies, such as ultrasound and light intensity infrared which have been already investigated in the literature, are that the same or higher performance can be obtained with a simpler experimental setup (only one foot instrumented) and with a higher robustness to the changing of the experimental/environmental conditions (e.g. colour of the shoe, ambient light). First, a thorough testing protocol for evaluating the infrared time-of-flight distance sensor performances was performed under experimental conditions resembling those encountered during gait. Second, the SWING system was validated for the inter-foot distance estimation and step detection during walk on sixteen healthy subjects. Third, the SWING system was tested and validated on a group of subjects characterised by highly abnormal gait patterns (e.g. low speed walks, foot dragging walks, use of walking aids) while performing a six-minute walking test. Finally, by exploiting the Bluetooth low energy as an alternative solution for indoor-localisation and proximity sensing, a thorough characterisation of the received signal strength indicator and distance relationship under controlled conditions was provided. The findings of this Ph.D. thesis lead to the conclusion that the SWING system and the proposed methods could be reliably applied to both normal and abnormal gaits obtaining a high level of accuracy while maintaining a very simple experimental setup (only one lower limb instrumented). Indeed, the mean absolute errors obtained for the measurement of the inter-foot distance on healthy subjects were in the range of 9.3–12.4 mm. The results of the validation of the SWING system, as step counter, showed an accuracy of 100 % on healthy and between 94.6 % and 98 % on pathological subjects (i.e. multiple sclerosis). Lastly, the findings of the characterisation of the Bluetooth low energy technology for the inter-distance estimation showed an average percentage error of 25.7 % (0.4 m). Therefore, Bluetooth low energy can be a solution for indoor positioning applications, but cannot be used for proximity sensing applications which require very high accuracy (resolution down to 0.1 m).