The paper reports on numerical-experimental analysis of magnetic plucking, which is commonly applied for contactless mechanical frequency up-conversion in vibration energy harvesters with the aim of enhancing power output and efficiency under low-frequency (non)periodic excitations. In-plane magnetic plucking is examined thoroughly by treating it as a specific type of transient excitation that is governed by temporal characteristics of complex-shaped magnetic force impulses. The study focuses on systematic analysis of the measured dynamic and electric characteristics of a magnetically plucked cantilever-type piezoelectric energy harvester, which operates in quasi-static, transitory and dynamic modes. Experimental findings are supported by results of magnetostatic/dynamic finite element analysis, enabling to establish key criteria for attaining sharp impulsive excitation, which is a prerequisite for the highly effective frequency up-conversion. The criteria are defined in terms of dimensionless governing parameters that relate relevant temporal characteristics of magnetic force impulses (duration, ramping times) to the dynamic characteristics of the transducer (natural period, rise time). A phenomenon of transient resonance is demonstrated to deliver peak power and energy outputs. The reported findings are applicable to different magnetic plucking configurations and transducers of arbitrary natural frequency. The results serve as a guideline for the rational design of a wide variety of frequency up-converting devices by providing valuable insight into magnetic plucking dynamics, which is conducive to the development of high-performance biomechanical energy harvesters intended for self-powered wearable electronics.

The reported work experimentally investigates a method of more effective contactless mechanical frequency up-conversion that is based on multi-magnet plucking of a piezoelectric vibration energy harvester. Several moving excitation magnets are used to produce a periodic impulse train, which during a single plucking event consecutively deflects and then releases the cantilevered transducer to freely oscillate, thereby enabling enhanced micro-power generation performance. It was established that the proposed method is effective if a couple conditions are met. First, the transducer must be impulsively excited to produce resonant transient responses, which occurs when the ramping time of the magnetic impulse is close to the transducer rise time (defined as a quarter of the natural period). Second, the gap between the moving excitation magnets must be tuned to ensure that the impulse train period is as close to the natural period as possible. Measurements indicate that, in comparison to the conventional single-magnet plucking case, the consecutive excitation with three moving magnets leads to nearly six-fold (seven-fold) increase in average power output and total generated energy during the in-plane (out-of-plane) plucking regime.

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