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 paper reports on a nonlinear frequency up-converting vibration energy harvester with enhanced broadband performance below 50 Hz, which is attained by leveraging vibro-impact interaction between two high-frequency piezoelectric generators and two low-frequency resonators with commensurate natural frequencies in the 2:1 ratio. Finite element model was implemented to assess the proposed concept by treating the harvester as a vibro-impact coupled piezoelectric-circuit system. The model accommodates multiple mechanical contact pairs defined by nonlinear dissipative Hunt–Crossley contact force formulation. Effective design principles were applied to maximize power output including near-optimal positioning of the neutral plane in the generators and rational adjustment of response settling time. The fabricated proof-of-concept device was tested in different clearance configurations and under varying harmonic excitation conditions to reveal rich nonlinear dynamics behavior. The measured hardening-type power frequency responses were examined to gain better insight into nonlinear resonance characteristics (instability jumps, frequency hysteresis, etc), which determine effectiveness of micro-power generation. It was demonstrated that the proposed dual-resonator energy harvester favorably harnesses nonlinear resonance amplification to deliver larger average power output (up to 37 μ W) and useable bandwidth (up to 9.5 Hz) in comparison to the conventional single-resonator counterparts (delivering up to 26 μ W and up to 7 Hz, respectively). The maximum normalized power density of the device is 41 μ W cm ⁻³ g ⁻² , while volume figure of merit and 3 dB or 1 dB bandwidth-based figures of merit reach 0.18%, 11 μ W cm ⁻³ g ⁻² and 1.5%, respectively. The latter value indicates superior broadband performance of the device. Adoption of mean performance index and coefficient of variation as complementary figures of merit proved to be valuable in identifying device configuration that yields the most effective and stable broadband operation in a targeted frequency range.

This work studies the influence of high-frequency excitation of a cutting tool during end milling of workpieces made of difficult-to-cut metallic alloys. It is demonstrated that high-frequency vibrations superimposed onto the continuous movement of the tool lead to milling process stabilization with superior surface finish in comparison to conventional machining. A finite element model of the vibration milling tool was built and verified experimentally. The model treats the tool as an elastic pre-twisted structure (mill cutter) characterised by its natural vibration modes. The resonance frequencies of the axial vibration mode of cutters of two different lengths were predicted numerically and subsequently used for excitation of the vibration milling tool during cutting experiments. Qualitative and quantitative characterization of the surface quality of the machined stainless steel and titanium alloys was performed. Measurement results have confirmed that excitation of a specific tool mode is a prerequisite for achieving maximal efficiency of the vibration milling process. Statistical analysis of the collected roughness measurement data identified factors that most significantly contribute to the improved surface finish of the workpieces