Ultralow-Frequency Biomechanical Energy Scavenging and Human Activity Recognition at Different Positions Using a Multifunctional Wearable Energy Harvester

As a variety of wearable electronics with different functions have emerged, multifunctional devices with self-powered nature are desired for solving both problems of the tighter on-body wearing space and energy budget. Herein, we propose an eccentric-rotor-based multifunctional wearable electromagnetic vibration energy harvester (EMVEH) to function as an ultralow-frequency biomechanical energy scavenging device and a self-powered motion sensor for human activity recognition (HAR) at different positions, respectively. Kinetic simulations based on Euler-Lagrange equations provide a detailed understanding of its vibration pick-up performance for effective energy scavenging and human motion sensing. Results of comprehensive bench tests further verify its adaptability to excitations with ultralow frequencies (<5 Hz) and different directions, and the capability of mechanical excitation perception. Kinetic energy in limb motions during walking and running has been scavenged by a fabricated prototype into electric energy up to 221.49 μW on average, whereas variations in multiple-feature-based profiles of output voltages in response to these human activities reflect repeatable and human-activity-related voltage patterns. On this basis, walking at 2–4 km/h and running at 4–8 km/h on a treadmill are recognized by these features and trained random forest classification models when the prototype is worn at the wrist, elbow, and ankle, respectively, all achieving high accuracies over 90%. Mutual constraints between dual design purposes of this multifunctional EMVEH are also fully discussed and released. Thus, the wearing-position-independent HAR performance combined with the efficient energy scavenging capability of our EMVEH expands the application range of wearable EMVEHs to more flexible and practical uses.