Magnetic plucking applies the strategy of frequency up-conversion in inertial energy harvesting when the energy source, such as human motion, provides excitations with very low and irregular frequencies. In a typical implementation, a slower moving inertial mass magnetically plucks a piezoelectric cantilever beam which converts mechanical energy to electrical energy at a higher frequency. We categorize several feasible magnet configurations to achieve plucking. We classify these as either inplane (the beam is deflected in the plane of proof mass motion) or out-of-plane (the beam is deflected orthogonal to the plane of proof mass motion). Whereas in-plane plucking induces a clean ring down due to its inherent jump phenomenon, out-ofplane plucking enables the capability of fabricating multiple piezoelectric beams on a single substrate. This paper presents an analysis of three different out-of-plane plucking configurations along with the in-plane repulsive configuration based on a three-dimensional analytical cube permanent magnet model. We derive a magnetically plucked piezoelectric beam model to investigate the dynamic characteristic for different plucking configurations. After validating the model with experimental results we extend the simulation into a larger driving frequency domain to compare two types of magnet configurations in terms of power generation.

For piezoelectric energy harvesters, a large volume of piezoelectric material with a high figure of merit is essential to obtain a higher power density. The work describes the growth of highly (001) oriented sputtered lead zirconate titanate (PZT) films (f ≈ 0.99) exceeding 4 µm in thickness on both sides of an Ni foil to produce a bimorph structure. These films are incorporated in novel wrist-worn energy harvesters (<16 cm 2 ) in which piezoelectric beams are plucked magnetically using an eccentric rotor with embedded magnets to implement frequency up-conversion. The resulting devices successfully convert low-frequency vibration sources (i.e., from walking, rotating the wrist, and jogging) to higher frequency vibrations of the PZT beams (100-200 Hz). Measured at resonance, six beams producing an output of 1.2 mW is achieved at 0.15 G acceleration. For magnetic plucking of a wrist-worn nonresonant device, 40-50 µW is produced during mild activity.

Abstract. Energy harvesting from human motion addresses the growing need for battery-free health and wellness sensors in wearable applications. The major obstacles to harvesting energy in such applications are low and random frequencies due to the nature of human motion. This paper presents a generalized rotational harvester model in 3 dimensions to determine the upper bound of power output from real world measured data. Simulation results indicate much space for improvement on power generation comparing to existing devices. We have developed a rotational energy harvester for human motion that attempts to close the gap between theoretical possibility and demonstrated devices. Like previous work, it makes use of magnetically plucked piezoelectric beams. However, it more fully utilizes the space available and has many degrees of freedom available for optimization. Finally we present a prototype harvester based on the coupled harvester model with preliminary experimental validation. IntroductionEnergy harvesting has been a promising solution as the alternative to traditional systems powered by batteries when energy independence is required. As the power consumption becomes a major obstacle to the emerging market for wearable technologies, kinetic energy contained in human daily activities naturally appeals to the researchers in the field of energy harvesting. Due to the low and random frequencies of human motion, which usually occurs around 1 Hz [1], resonant harvesters cannot benefit from the peak dynamic magnification. The well-known Seiko Kinetic watch converts the kinetic energy of a rotor to electricity using an electromagnetic generator [2]. Such rotational designs with non-resonant configuration are the common approaches that researchers have adopted to overcome the limitation of conventional linear harvesters. While the Seiko Kinetic watch boosts its energy density by a sophisticated high-ratio gear train, another strategy, which is often referred to as frequency up-conversion, is to excite a higher-frequency resonance in a transducer via a slowerfrequency motion. Prior reported non-resonant harvesters include a piezoelectric harvester with inplane magnetic plucking based on a planar rotor model [3] and a magnetic spherical harvester [4]. The goal of this paper is to provide an estimate of the maximum power output from a rotational harvester regardless of the energy conversion mechanism using real-world measured data such as

Low-temperature effects on up-conversion emission of Er3+/Yb3+-co-doped Y2O3 V Lojpur, M G Nikoli, M D Dramianin et al. Abstract. Magnetic plucking applies the strategy of frequency up-conversion in inertial energy harvesting when the energy source, such as human motion, only provides excitations with very low and irregular frequencies. This paper presents an analysis of three different magnet configurations to achieve magnetic plucking based on a three-dimensional analytical cube permanent magnet model: direct repulsive configuration, orthogonal configuration and indirect repulsive configuration. Simulation and experimental results indicate that the indirect repulsive configuration generates the largest tip displacement given the pratical constraints in designing a wearable energy harvester. We have implemented this configuration in a wrist-worn rotational energy harvester to pluck multiple piezoelectric beams. Other configurations, however, can potentially be advantageous in applications with alternative constraints. IntroductionEnergy harvesting for wearable wellness sensors could provide the potential for continuous health monitoring by eliminating the need to replace or recharge batteries manually. The inherent limitation of utilizing human motion as the source for inertia energy harvesting is that it only provides excitations with very low and irregular frequencies. Frequency up-conversion is a commonly used strategy to tackle this issue by transforming the low-frequency input motion into high-frequency resonance of the transducer. Plucking is one technique which applies such a strategy. Eccentric rotorbased wearable energy harvesters have been demonstrated in previous endeavors either using magnets [1][2] or pins [3][4] to pluck piezoelectric beams. Magnetic coupling provides better reliability since it can be designed contact-free. Usually magnets are arranged to pluck the beam in the direction of magnet motion with either a direct repelling or an attractive configuration [5], i.e. in-plane plucking. While the in-plane plucking introduces a stronger jump phenomenon in the cantilever beam, the outof-plane deflection configuration provides the capability of fabricating multiple beams in the harvester on a single substrate. Furthermore, mechanical design complexity is reduced and space utilization is increased which ultimately improves power output. However, if the magnets are simply aligned, as in the direct repulsive configuration in Figure 1, the overall thickness of the harvester will grow. Additionally, in this configuration the opposing magnets can get stuck in a side by side orientation, i.e. there is a pull-in effect. Thus exploring alternative magnet configurations with the ability to trigger out-of-plane plucking as well is worthwhile.Among all the possible combination of orientations, we present two alternative magnet configurations: the orthogonal and the indirect repulsive configuration, which not only have demonstrated the capability of triggering plucking but also are free of the pull-in effe...

Energy harvesting from human motion addresses the growing need for self-powered wearable health monitoring systems which require 24/7 operation. Human motion is characterized by low and irregular frequencies, large amplitudes, and multi-axial motion, all of which limit the performance of conventional translational energy harvesters. An eccentric rotor-based rotational approach originally used in self-winding watches has been adopted to address the challenge. This paper presents a three-dimensional generalized rotational harvester model that considers both linear and rotational excitations. A hypothetical power upper bound for such architectures derived using this generalized model demonstrated the possibility for harvesting significantly more energy compared to existing devices. A wrist-worn piezoelectric rotational energy harvester was designed and fabricated attempting to narrow this gap between existing devices and the theoretical upper bound. The harvester utilizes sputtered bimorph PZT/nickel/PZT thinfilm beams to accommodate the need for both flexibility and high piezoelectric figure of merit in order to realize a multi-beam wearable harvester. The prototype was characterized using a benchtop swing arm setup to validate the system-level model, which provides many degrees of freedom for optimization.