Experimental study of multi-magnet excitation for enhancing micro-power generation in piezoelectric vibration energy harvester

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.

Dynamics of Cricket Sound Production

The clever designs of natural transducers are a great source of inspiration for man-made systems. At small length scales, there are many transducers in nature that we are now beginning to understand and learn from. Here, we present an example of such a transducer that is used by field crickets to produce their characteristic song. This transducer uses two distinct components—a file of discrete teeth and a plectrum that engages intermittently to produce a series of impulses forming the loading, and an approximately triangular membrane, called the harp, that acts as a resonator and vibrates in response to the impulse-train loading. The file-and-plectrum act as a frequency multiplier taking the low wing beat frequency as the input and converting it into an impulse-train of sufficiently high frequency close to the resonant frequency of the harp. The forced vibration response results in beats producing the characteristic sound of the cricket song. With careful measurements of the harp geometry and experimental measurements of its mechanical properties (Young's modulus determined from nanoindentation tests), we construct a finite element (FE) model of the harp and carry out modal analysis to determine its natural frequency. We fine tune the model with appropriate elastic boundary conditions to match the natural frequency of the harp of a particular species—Gryllus bimaculatus. We model impulsive loading based on a loading scheme reported in literature and predict the transient response of the harp. We show that the harp indeed produces beats and its frequency content matches closely that of the recorded song. Subsequently, we use our FE model to show that the natural design is quite robust to perturbations in the file. The characteristic song frequency produced is unaffected by variations in the spacing of file-teeth and even by larger gaps. Based on the understanding of how this natural transducer works, one can design and fabricate efficient microscale acoustic devices such as microelectromechanical systems (MEMS) loudspeakers.

Plasmonic Diodes THz Response to Impulse Train and Stochastic Optical Excitations

Terahertz (THz) plasma oscillation in n+nn+ InGaAs vertical diodes is studied by using a numerical approach based on the hydrodynamic (HD) equations. The 1D HD model is coupled to 1D Poisson equation. We simulate the diode response to the optical excitation of plasma waves at room temperature. Our results clearly show the presence of 3D plasma resonances in the THz frequency domain. The selection of appropriately shaped optical excitation pulses helps in the realization of the detected frequency. The investigation is completed by introducing stochastic perturbations according to the Langevin equation.

Wiener-Volterra Characterization of Neurons in Primary Auditory Cortex Using Poisson-Distributed Impulse Train Inputs

An extension of the Wiener-Volterra theory to a Poisson-distributed impulse train input was used to characterize the temporal response properties of neurons in primary auditory cortex (AI) of the ketamine-anesthetized cat. Both first- and second-order “Poisson-Wiener” (PW) models were tested on their predictions of temporal modulation transfer functions (tMTFs), which were derived from extracellular spike responses to periodic click trains with click repetition rates of 2–64 Hz. Second-order (i.e., nonlinear) PW fits to the measured tMTFs could be described as very good in a majority of cases (e.g., predictability ≥80%) and were almost always superior to first-order (i.e., linear) fits. In all sampled neurons, second-order PW kernels showed strong compressive nonlinearities (i.e., a depression of the impulse response) but never expansive nonlinearities (i.e., a facilitation of the impulse response). In neurons with low-pass tMTFs, the depression decayed exponentially with the interstimulus lag, whereas in neurons with band-pass tMTFs, the depression was typically double-peaked, and the second peak occurred at a lag that correlated with the neuron's best modulation frequency. It appears that modulation-tuning in AI arises in part from an interplay of two nonlinear processes with distinct time courses.