M. G. Prasad

A vibration energy harvesting device with bidirectional resonance frequency tunability

Vibration energy harvesting is an attractive technique for potential powering of wireless sensors and low power devices. While the technique can be employed to harvest energy from vibrations and vibrating structures, a general requirement independent of the energy transfer mechanism is that the vibration energy harvesting device operate in resonance at the excitation frequency. Most energy harvesting devices developed to date are single resonance frequency based, and while recent efforts have been made to broaden the frequency range of energy harvesting devices, what is lacking is a robust tunable energy harvesting technique. In this paper, the design and testing of a resonance frequency tunable energy harvesting device using a magnetic force technique is presented. This technique enabled resonance tuning to ±20% of the untuned resonant frequency. In particular, this magnetic-based approach enables either an increase or decrease in the tuned resonant frequency. A piezoelectric cantilever beam with a natural frequency of 26 Hz is used as the energy harvesting cantilever, which is successfully tuned over a frequency range of 22‐32 Hz to enable a continuous power output 240‐280 μW over the entire frequency range tested. A theoretical model using variable damping is presented, whose results agree closely with the experimental results. The magnetic force applied for resonance frequency tuning and its effect on damping and load resistance have been experimentally determined. (Some figures in this article are in colour only in the electronic version)

A coupled piezoelectric?electromagnetic energy harvesting technique for achieving increased power output through damping matching

Vibration energy harvesting is being pursued as a means to power wireless sensors and ultra-low power autonomous devices. From a design standpoint, matching the electrical damping induced by the energy harvesting mechanism to the mechanical damping in the system is necessary for maximum efficiency. In this work two independent energy harvesting techniques are coupled to provide higher electrical damping within the system. Here the coupled energy harvesting device consists of a primary piezoelectric energy harvesting device to which an electromagnetic component is added to better match the total electrical damping to the mechanical damping in the system. The first coupled device has a resonance frequency of 21.6 Hz and generates a peak power output of ~332 µW, compared to 257 and 244 µW obtained from the optimized, stand-alone piezoelectric and electromagnetic energy harvesting devices, respectively, resulting in a 30% increase in power output. A theoretical model has been developed which closely agrees with the experimental results. A second coupled device, which utilizes the d33 piezoelectric mode, shows a 65% increase in power output in comparison to the corresponding stand-alone, single harvesting mode devices. This work illustrates the design considerations and limitations that one must consider to enhance device performance through the coupling of multiple harvesting mechanisms within a single energy harvesting device.

Towards an autonomous self-tuning vibration energy harvesting device for wireless sensor network applications

Future deployment of wireless sensor networks will ultimately require a self-sustainable local power source for each sensor, and vibration energy harvesting is a promising approach for such applications. A requirement for efficient vibration energy harvesting is to match the device and source frequencies. While techniques to tune the resonance frequency of an energy harvesting device have recently been described, in many applications optimization of such systems will require the energy harvesting device to be able to autonomously tune its resonance frequency. In this work a vibration energy harvesting device with autonomous resonance frequency tunability utilizing a magnetic stiffness technique is presented. Here a piezoelectric cantilever beam array is employed with magnets attached to the free ends of cantilever beams to enable magnetic force resonance frequency tuning. The device is successfully tuned from �27% to +22% of its untuned resonance frequency while outputting a peak power of approximately 1 mW. Since the magnetic force tuning technique is semi-active, energy is only consumed during the tuning process. The developed prototype consumed maximum energies of 3.3 and 3.9 J to tune to the farthest source frequencies with respect to the untuned resonance frequency of the device. The time necessary for this prototype device to harvest the energy expended during its most energy-intensive (largest resonant frequency adjustment) tuning operation is 88 min in a low amplitude 0.1g vibration environment, which could be further optimized using higher efficiency piezoelectric materials and system components. (Some figures in this article are in colour only in the electronic version)

Acoustical source characterization studies on a multi-cylinder engine exhaust system

Abstract It is well known that the characterization of the acoustic source in an exhaust muffler system is of utmost importance in the proper evaluation of the acoustic performance of the muffler. However, in the literature, there are very few experimental studies on source characterization of a multi-cylinder internal combustion engine. This paper describes the use of a transfer function method (with a random excitation source) for measurement of the internal source impedance of an eight-cylinder engine under running conditions. The results obtained agree well with those obtained by the standing wave method by earlier investigators. The studies include the effect on the measured internal source impedance caused by variation of engine speed and load. The source impedance results obtained for the engine in operation are compared with those for the engine not in operation. The use of these results in the overall modeling of the exhaust system is described in an accompanying paper.

A four load method for evaluation of acoustical source impedance in a duct

A new and simple method to evaluate the acoustical source impedance in a duct is presented. In the method (termed the Four Load Method) the sound pressure level data measured downstream for four different length ducts, as acoustic loads, is used. The simplicity of the method is due to the fact that neither cross-spectra nor complex pressure measurements are required. Also, a second signal generating source is not needed. In addition, the sound pressure level measurements can be made either inside or outside the finite length duct. In order to verify the method, the insertion loss of a simple expansion chamber is predicted by using the source impedance evaluated by the four load method. Excellent agreement is observed between the predicted and measured insertion loss. Some applications of the four load method to duct acoustic studies, including the mean flow case, are discussed.

STUDIES OF ACOUSTICAL PERFORMANCE OF A MULTI-CYLINDER ENGINE EXHAUST MUFFLER SYSTEM

Abstract Among the various descriptors of a multi-cylinder engine exhaust muffler system performance, it is evident that insertion loss and radiated sound pressure are the most useful. Insertion loss depicts the effectiveness of a muffler, whereas radiated sound pressure level is the design criterion used to meet the noise regulations. The main difficulty in prediction of these descriptors is the characterization of the source. In addition, both flow and temperature gradient effects have to be included in the modeling. This paper describes a theoretical acoustical model to predict insertion loss and radiated sound pressure level, in which the source impedance is obtained from measurement (see the accompanying paper) and from which the source strength is estimated. Also, both flow and temperature gradient effects are included in the analysis. The implications due to the various assumptions for the source impedance in the modeling have been investigated. The studies are carried out on two exhaust system configurations. Good agreement is obtained provided the measured impedance is used and also if mean flow and temperature gradient effects are included in the theoretical model.

Insertion loss studies on models of automotive exhaust systems

Transmission loss, noise reduction, and insertion loss are the three main characteristics used to describe the performance of a muffler in an automotive exhaust system. Of these characteristics, insertion loss is the most useful. Unlike transmission loss and noise reduction, insertion loss is dependent on both source and radiation impedances. In this paper, source and radiation impedances were measured. The insertion loss of exhaust systems was predicted using theoretical values of radiation impedance and measured values of source impedance. Comparisons of predicted and measured insertion loss are given for various source and radiation conditions. So far the investigations are confined to an electroacoustic driver source and an expansion chamber muffler.

Evaluation of four‐pole parameters for a straight pipe with a mean flow and a linear temperature gradient

Four‐pole parameters for a straight pipe system in the presence of a uniform mean flow and a linear temperature gradient are presented. The system was analyzed using first‐order perturbation theory and a Green’s function approach. The general four‐pole matrix derived reduces exactly to several other simpler cases already in the literature. The use of the matrix in engine exhaust system predictions is also given.

A scheme to predict the sound pressure radiated from an automotive exhaust system

A general scheme to predict the sound pressure radiated from an automotive exhaust system is presented. The scheme is demonstrated with a step‐wise computational procedure on a model system comprised of an electroacoustic driver source, an expansion chamber muffler, and a tailpipe radiating to free space. The predicted sound pressure level spectra agree well with the corresponding measured spectra for various test cases with zero mean flow. Also, studies are made of the theoretical effects of variations of the source impedance on the sound pressure radiated from the model system.

On the Applications of the Boundary-Element Method to Acoustical Field Studies of Vibrating Structures

This paper describes the applications of the Boundary-Element Method (BEM) for studies on acoustical field of various vibrating structures. The studies emphasize the numerical aspects of the BEM. Both acoustical near and far fields of the vibrating structures are investigated in this work. The vibrating structures considered in this application studies are a circular piston in an infinite, rigid baffle and cantilever-type beams. In the case of piston in an infinite baffle, instead of using the method of images, the free-space Green’s function is used to evaluate boundary integral equation by including both piston and baffle surfaces. The influence of the stationary baffle in the case of piston is further investigated. The beams considered are of both rectangular and circular cross sections. The results obtained by BEM have compared well with the available results from classical methods. The studies indicate that in the application of BEM in such problems both the element size and the number of elements including stationary surface have significant effect on the results obtained. The studies have yielded that very good results are obtained when the largest dimension of an element is equal to 0.2 times the acoustic wavelength (in air) of the frequency of acoustic radiation.