This paper reviews the state-of-the art in vibration energy harvesting for wireless, self-powered microsystems. Vibration-powered generators are typically, although not exclusively, inertial spring and mass systems. The characteristic equations for inertial-based generators are presented, along with the specific damping equations that relate to the three main transduction mechanisms employed to extract energy from the system. These transduction mechanisms are: piezoelectric, electromagnetic and electrostatic. Piezoelectric generators employ active materials that generate a charge when mechanically stressed. A comprehensive review of existing piezoelectric generators is presented, including impact coupled, resonant and human-based devices. Electromagnetic generators employ electromagnetic induction arising from the relative motion between a magnetic flux gradient and a conductor. Electromagnetic generators presented in the literature are reviewed including large scale discrete devices and wafer-scale integrated versions. Electrostatic generators utilize the relative movement between electrically isolated charged capacitor plates to generate energy. The work done against the electrostatic force between the plates provides the harvested energy. Electrostatic-based generators are reviewed under the classifications of in-plane overlap varying, in-plane gap closing and out-of-plane gap closing; the Coulomb force parametric generator and electret-based generators are also covered. The coupling factor of each transduction mechanism is discussed and all the devices presented in the literature are summarized in tables classified by transduction type; conclusions are drawn as to the suitability of the various techniques.

/pdf/energy-harvesting-vibration-sources-for-microsystems-46t885pwdp.pdf

Energy harvesting vibration sources for microsystems applications

http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

An introduction to heat transfer

This paper presents a review of silicon micromachined accelerometers and gyroscopes. Following a brief introduction to their operating principles and specifications, various device structures, fabrication, technologies, device designs, packaging, and interface electronics issues, along with the present status in the commercialization of micromachined inertial sensors, are discussed. Inertial sensors have seen a steady improvement in their performance, and today, microaccelerometers can resolve accelerations in the micro-g range, while the performance of gyroscopes has improved by a factor of 10/spl times/ every two years during the past eight years. This impressive drive to higher performance, lower cost, greater functionality, higher levels of integration, and higher volume will continue as new fabrication, circuit, and packaging techniques are developed to meet the ever increasing demand for inertial sensors.

/pdf/micromachined-inertial-sensors-esf6197vdc.pdf

Micromachined inertial sensors

Enabling technologies for wireless sensor networks have gained considerable attention in research communities over the past few years. It is highly desirable, even necessary in certain situations, for wireless sensor nodes to be self-powered. With this goal in mind, a vibration based piezoelectric generator has been developed as an enabling technology for wireless sensor networks. The focus of this paper is to discuss the modeling, design, and optimization of a piezoelectric generator based on a two-layer bending element. An analytical model of the generator has been developed and validated. In addition to providing intuitive design insight, the model has been used as the basis for design optimization. Designs of 1 cm3 in size generated using the model have demonstrated a power output of 375 µW from a vibration source of 2.5 m s−2 at 120 Hz. Furthermore, a 1 cm3 generator has been used to power a custom designed 1.9 GHz radio transmitter from the same vibration source.

https://iopscience.iop.org/article/10.1088/0964-1726/13/5/018/pdf

A piezoelectric vibration based generator for wireless electronics

A lumped-parameter thermal model is described which provides both a steady-state and transient solution to the temperatures within an electrical machine of the TEFC design. The model is sufficiently complex to identify the temperatures at most locations in the machine, including the peak temperatures in the endwinding and the surface temperatures of the rotor. It is formulated out of purely dimensional information and constant thermal coefficients and is therefore easily adapted to a range of frame sizes. The thermal behaviour of the TEFC machine is accurately described by the solution of just eight linear differential equations. The model is therefore suitable for application to online temperature estimation for protection and duty-cycle evaluation. The application of the thermal model to a medium (75 kW) and two small (5.5 kW) induction motors is described in detail. The model performance is confirmed by experimental temperature data obtained from varying load tests on each of the three induction motors.

Lumped parameter thermal model for electrical machines of TEFC design

A design methodology for linear micro-generators is developed, and is applied to the design of a mm scale electromagnetic micro-generator. The fabrication of a prototype device is also described using generally available microfabrication techniques, and the results of testing the device on a variable amplitude vibration source, in air and vacuum, are presented. The experimental results confirm the design rules and indicate how the generation of useful power levels might be achieved.

Development of an electromagnetic micro-generator

The eddy-current loss in the permanent magnets of brushless AC machines is usually neglected, since the fundamental air-gap field usually rotates in synchronism with the rotor, and time harmonics in the current waveform and space harmonics in the winding distribution are generally small. However, an important category of brushless AC machine design is emerging in which the fundamental component of the stator MMF has fewer poles than the rotor, the torque being developed by a higher order MMF harmonic. The fundamental and lower order MMF harmonics can then give rise to significant rotor eddy currents. An analytical model is developed to predict rotor-induced eddy currents in such machines, and to quantify the effectiveness of circumferentially segmenting the permanent magnets in reducing the rotor loss.

Rotor loss in permanent-magnet brushless AC machines

It is common practice to represent a composite electrical winding as an equivalent lumped anisotropic material as this greatly simplifies a thermal model and reduces computation times. Existing techniques for estimating the bulk thermal properties of such composite materials use either analytical, numerical, or experimental approaches; however, these methods exhibit a number of drawbacks and limitations regarding their applicability. In this paper, a numerical thermal conductivity and analytical specific heat capacity estimation technique is proposed. The method is validated experimentally against three winding samples with differing configuration. A procedure is presented which enables bulk thermal properties to be estimated with a minimal need for experimental measurement, thereby accelerating the thermal modeling process. The proposed procedure is illustrated by the modeling of three coil exemplars with differing windings. Experimental thermal transients obtained by dc test of the coils show close agreement with a lumped-parameter thermal model utilizing estimated material data.

Estimation of Equivalent Thermal Parameters of Impregnated Electrical Windings

The eddy current loss in the permanent magnets of brushless AC machines is usually neglected, since the fundamental airgap field rotates in synchronism with the rotor and time harmonics in the current waveform and space harmonics in the winding distribution are generally small. However, machine designs are emerging for which the fundamental component of the stator MMF has fewer poles than the rotor, the torque being developed by a higher order MMF harmonic. The fundamental and lower order MMF harmonics can then give rise to significant rotor eddy currents. An analytical model is developed to predict rotor induced eddy currents in such machines, and to quantify the effectiveness of circumferentially segmenting the permanent magnets in reducing the rotor loss.

Phil Mellor

Papers

Lumped parameter thermal model for electrical machines of TEFC design

Development of an electromagnetic micro-generator

Rotor loss in permanent-magnet brushless AC machines

Estimation of Equivalent Thermal Parameters of Impregnated Electrical Windings

Rotor loss in permanent magnet brushless AC machines