  We propose a novel use of the conventional energy storage component, i.e.,
capacitor, in kinetic-powered wearable IoTs as a sensor to detect human
activities. Since different activities accumulate energies in the capacitor at
different rates, these activities can be detected directly by observing the
charging rate of the capacitor. The key advantage of the proposed capacitor
based activity sensing mechanism, called CapSense, is that it obviates the need
for sampling the motion signal during the activity detection period thus
significantly saving power consumption of the wearable device. A challenge we
face is that capacitors are inherently non-linear energy accumulators, which,
even for the same activity, leads to significant variations in charging rates
at different times depending on the current charge level of the capacitor. We
solve this problem by jointly configuring the parameters of the capacitor and
the associated energy harvesting circuits, which allows us to operate on
charging cycles that are approximately linear. We design and implement a
kinetic-powered shoe sole and conduct experiments with 10 subjects. Our results
show that CapSense can classify five different daily activities with 95%
accuracy while consuming 73% less system power compared to conventional motion
signal based activity detection.
