Energy Harvesting and Assistive Wearables Based on Lower Limb Dynamics


Student thesis: Doctoral Thesis

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Awarding Institution
  • Zhengbao YANG (External person) (External Co-Supervisor)
  • Ji-jung KAI (Supervisor)
  • Xiaofang CHENG (External person) (External Supervisor)
Award date17 Oct 2023


Human locomotion has evolved for millions of years to maximize energy efficiency, creating a significant challenge for further energy efficiency improvements of locomotion by developing assistive and energy-harvesting devices. Specifically, the following three challenges hinder the practicability of assistive and energy-harvesting wearables. 1: The limited power output of existing energy harvesters cannot meet the power requirements of most wearable electronics; 2: Wearing assistive devices interfere with the normal gait; 3: The monofunctional design of existing devices decreases the overall acceptability in real-world application. To address these challenges, we systematically investigated mechanisms to enhance power output based on the gait dynamics of human locomotion and explored the versatility of the prototype while minimizing any negative effects on normal gait.

Firstly, the dissertation reports a heel pad-based assistance (HPA) device for walking that not only optimizes the energetic economy of walking and prevents plantar fasciitis but also harvests energy from heel impact. Our footwear-embedded device improves the energy efficiency of walking by offering shock absorption and gait assistance while simultaneously providing energy-harvesting functions. This thesis demonstrates that the use of our device reduces the activation of the gastrocnemius and soleus muscles during the foot strike by 5.8 ± 1.0% and 4.1 ± 0.6%, respectively. The collisional energy conserved from the impact at the touchdown is transformed into 3.8 ± 0.3 watts of electrical power (mean ± SEM). Compared with walking in normal shoes, the energy savings with the device imply that walking endurance could be increased by as much as 10% without extra effort from the wearer.

To improve the power output and the wear comfort under the ultra-low frequency and limited vertical displacement of footstep motion, the dissertation proposes a leaf-spring-based motion converter that converts low-frequency linear motion into high-speed rotation and uses electromagnetic induction to generate electricity. A moderate stiffness leaf spring coupled with a pair of bearings is employed to transfer the vertical displacement of the heel into the horizontal deformation, which is also used to absorb the shock at the footstep touchdown moment. A ratchet clutch is utilized to switch the working modes of the generator between the stance and swing phases. This research characterizes the dynamic response of the motion converter and develops an analytical model to predict the power output of the system. Furthermore, a lightweight and compact prototype was fabricated and tested under pseudo and natural walking conditions. The prototype achieves a displacement amplification ratio of up to 2.2 and reduces acceleration amplitude at touchdown by 10.7% compared with walking without the device. At a stride frequency of 1 Hz, the prototype outputs an open-circuit voltage of 20 V, short-circuit current of 0.4 A, a peak power of 1.88 W, and a power density of 15.2 mW/cm3, higher than the previously reported footwear energy harvester.

Lastly, this dissertation develops an energy-harvesting exoskeleton, Knee Booster, that overcomes the limitations of the reliance on rapid sensory feedback for human-device coordination, and replaces the traditional active control strategy with an energetically efficient mechanical coupling to generate self-engaging and disengaging transition. A unique cam-spring coupled network engages rapidly at the terminal swing phase of the gait while simultaneously driving a three-phase generator for power generation, and then enables a rapid disengagement state upon the heel touchdown, assisting the knee flexion with the elastic energy of the spring. Experiments indicate that the Knee Booster 1) reduces the knee joint pressure by 15.8 ± 1.6 % (mean ± SEM) of body weight without notable changes in gait patterns, 2) decreases the hamstring and tibialis anterior activity by 6.7 ± 0.8 %, 9.6 ± 1.4 % respectively, while 3) converting the braking energy into 1.6 ± 0.2 (mean ± SEM) watts of electrical power. The Knee Booster achieves an augmentation factor of 0.36 W/kg at a walking speed of 1.3 m/s, exceeding the capabilities of state-of-the-art assistive exoskeletons for the lower limb. Those findings suggest that the passive energy-harvesting exoskeleton has a significant positive effect on improving real-world mobility and the quality of life for people with lower limb impairments.

In summary, this thesis introduces a novel approach to the development of wearable devices for the lower limb that aims to improve the energy efficiency of human walking. Unlike conventional biomechanical energy harvesters that focus on the power output, and ignore the human-device coordination affected by the asymmetric kinematics of the human motion in forward and return trips, the developed technology considers the asymmetric driving force between the lower extremity and the energy harvester with effective switch mechanisms to achieve natural gait pattern. Such a paradigm shift from a single energy harvester to a fused solution of versatile wearables paves a new way for the exploration of next-generation assistive wearables. The framework of this study extends the functionality of future wearable devices and provides insight into gait dynamics-based wearables, along with guidance for designing and optimizing biomechanical energy harvesters. The proposed design method and experimental study may contribute to a better understanding of these wearables and their potential applications in the real world.

    Research areas

  • Energy harvesting, Wearable devices, Human walking, Mechanical design, Mechatronics