Browsing by Author "Liu, Mingyi"

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    Energy Harvesting from the Human Body for Wearable and Mobile Devices
    Liu, Mingyi (Virginia Tech, 2020-07-08)
    Wearable and mobile devices are an important part of our daily life. Most of those devices are powered by batteries. The limited life span of batteries constitutes a limitation, especially in a multiple-day expedition, where electrical power can not access conveniently. At the same time, there is a huge amount of energy stored in the human body. While walking, there is a large amount of power dissipated in the human body as negative muscle work and the energy loss by impact. By sourcing locally and using locally, human body energy harvesting is a promising solution. This dissertation focuses on harvesting energy from the human body to power wearable and mobile devices while poses a minimum burden on the human body. Three topics related to the human body energy harvesting are explored, i.e, energy harvesting backpack, negative muscle work harvester, and energy harvesting tile/paver. The energy harvesting backpack was invented in 2006. Extensive work was done to improve the performance of backpack energy harvester. The backpack is modeled as a spring-mass-damper system. Mechanical Motion Rectifier was added to the spring-mass-damper system to increase the frequency bandwidth. A spring is added to the spring-mass-damper system, between the harvester and the backpack mass, and a inerter-based 2DOF (degree-of-freedom) backpack is created. The inerter-based 2DOF backpack improves the power output, frequency bandwidth, and power stroke ratio performance. MMR was added to the inerter-based 2DOF backpack to reduce the peak stroke. Compared with the conventional spring-mass-damper backpack, the MMR and inerter-based 2DOF backpack can harvest more power with large bandwidth at a small sacrifice of stroke. The electric damping was also tuned to increase the power output and bandwidth for the energy harvesting backpack. The negative work harvester mounts on the human ankle and harvests energy in the terminal stance phase in human walking, when the calf muscle is doing negative muscle work. This harvester is an analogy to regenerative brake in vehicles. The energy harvesting paver/tile harvests energy when the heel contacts with ground and energy are dissipated by impact.
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    Evaluation of a Simple Model for the Acoustics of Bat Swarms
    Liu, Mingyi (Virginia Tech, 2016-09-15)
    Bats using their biosonar while flying in dense swarms may face significant bioacoustic challenges, in particular mutual sonar jamming. While possible solutions to the jamming problem have been investigated multiple times in literature, the severity of this problem has received far less attention. To characterize the acoustics of bat swarms, a simple model of the acoustically relevant properties of a bat swarm has been set up and evaluated. The model contains only four parameters: bat spacial density, biosonar beamwidth, duty cycle, and a scalar measure for the smoothness of the flight trajectories. In addition, a threshold to define substantial jamming was set relative to the emission level. The simulations results show that all four model parameters can have a major impact on jamming probability. Depending on the combination of parameter values, situations with or without substantial jamming probabilities could be produced within reasonable ranges of all model parameters. Hence, the model suggests that not every bat swarm does necessarily impose grave jamming problem. A fitting process was introduced to describe the relationship between the four parameters and jamming probability, hence produce a function with jamming probability as output and four parameters as input. Since the model parameters should be comparatively easy to estimate for actual bat swarms, the simulation results could give researchers a way to assess the acoustic environment of actual bat swarms and determine cases where a study of biosonar jamming could be worthwhile.