Intermittently-powered energy harvesting step counter for fitness tracking

Over the past decade, there has been a rapid increase in the popularity of wearable and portable devices, such as step counters, to monitor fitness performance. However, these devices are battery-powered, meaning that their lifetimes are restricted by battery capacity. Ideally, wearable devices could be powered by energy harvested from human motion. Energy harvesting systems traditionally incorporate energy storage to cope with source variability. However, energy storage takes time to charge and increases the size and cost of systems. This paper proposes an intermittently-powered energy harvesting step counter for integrated wearable applications, which aims to remove the energy storage element. The proposed step counter sustains its operation by harvesting energy from footsteps using a ferroelectret insole, which also works as an event detection sensor, i.e. the system is powered by the parameter that is being sensed. Designing this required the characterization of the insole to evaluate the amount of energy provided, and analysis of the energy needed by the overall system. Finally, the system was implemented and experimentally validated. The proposed step counter has an error of less than 4% when walking, which is lower than the error in conventional smartphone applications.