Vibrational Energy Converters

Summary

Inexhaustible, miniature electrical power supplies offer the means to power battery-free microelectronic systems, or at a minimum provide a recharging capability to reduce battery size and extend their lifetimes. In particular, the unattended operation of remote microelectronic sensors and communication systems stand to benefit from inexhaustible miniature power technologies.

Sandia researchers have learned to exploit the ambient vibration environment as an inexhaustible source of energy. In environments lacking sufficient light, thermal gradients, or chemical potential, vibrations may be the most viable energy resource. Sandia has taken a thorough approach to vibration energy harvesting, where our research has led to comprehensive models facilitating design optimization for both harmonic and random vibration environments. Our unique technologies offer two-fold improvements in power density over conventional configurations and the ability to operate at arbitrarily-low operating frequencies independent of device size.

Description

Sandia has created novel vibration-energy harvesting designs with improved efficiency using a modeling and optimization capability based on velocity-damped resonant generator (VDRG) principles. In a VDRG system, power is extracted from the vibration-induced motion of a sense mass with the same linear effect as viscous dampings.

In our primary vibration energy harvester configuration, a planform tapered piezoelectric bimorph serves as a spring support for a sense mass as well as the transduction mechanism for converting mechanical to electrical energy. In this case, there are two sets of interdigitated beams (red and green in fig. 1) with each segment tied together mechanically through a common sense mass. The planform taper of each beam allows for maximum transduction efficiency, due to stress equalization along the beam’s length, during vibration-induced motion of the sense mass (in/out of the page). Interdigitation of the tapered beams allows the maximum volumetric efficiency, thus the highest power density possible. Modeling shows that the planform-tapered interdigitated configuration has a two-fold power advantage over non-tapered configurations.

A schematic of a prototype piezoelectric transduction element (fig. 2), shows two sets of interdigitated beams with each consisting of two tapered beams. The fabrication is straightforward where a three-layer material stack up (piezo, shim, piezo) forms a plate in which a single zigzag through cut is made to define the interdigitated tapered beams. In this case, nickel electroding forms a parallel electrical connection between the two tapered beams of each beam set. An optimal resonance frequency, configuration and size can be designed for any vibration environment and application. As an example, a prototype device (fig. 3), has a packaged volume of 3.5 cc and employs two sets of interdigitated beams with a tungsten sense mass per beam set. This harvester has a resonance response at 70 Hz and generates 0.6 mW of alternating-current power at over 6 volts in response to a 0.4-g harmonic vibration source. For a random vibration environment with a power spectral density of 2×10^-3 g²/Hz near 70 Hz, this device outputs 0.24-mW of alternating current average power into a 70kW load. For additional details of the vibration energy harvester utilizing planform-tapered interdigitated beams see Patent # US 7,948,153.

In a second Sandia vibration energy harvester technology, a solution has been developed for achieving arbitrarily low operational resonance frequencies in VDRG systems independent of device size. In this concept, a “Vibrating Wire and Mass Configuration” employs a wire or ribbon with a mass at mid length, where the wire is held under tension and coupled to piezoelectric transduction elements. For such a device, the resonant frequency may be controlled independently of dimension through control of wire tension. MEMS technologies could take advantage of this effect, for operation in low-frequency vibration environments, given their inherently small size scales.

The vibration energy harvester technologies described above take advantage of piezoelectric materials for their high power densities for energy conversion. A limiting factor for piezoelectric materials such as Lead Zirconate Titanate (PZT), however, is a maximum operating temperature around 150 degrees C. To circumvent this limitation for higher temperature vibration environments, magnetic-induction based energy conversion using samarium-cobalt magnets (operationally good up to 550 degrees C) is most viable. With this in mind, Sandia has developed a magnetic-induction based miniature vibration energy harvester technology.

This technology incorporates a magnet that is movable through a gap in a ferromagnetic circuit, where a coil wound around a portion of the yoke forms an electromagnetic circuit for energy transduction (fig. 7). A flexible coupling is used to attach the magnet to a frame for providing alignment of the magnet as it moves or oscillates through the gap in the ferromagnetic circuit. The motion of the magnet can be constrained to occur within a substantially-linear range of the magnetostatic force developed by the magnet-pickup interaction to create a spring return force. In turn, the spring force and mass of the moving magnet combine to dictate the operational resonance frequency. As for the piezoelectric devices, velocity damped resonant principles provide a basis for modeling and performance prediction. A millimeter-scale magnetic induction based prototype (fig. 8) was fabricated using lithography-based molding and electroplated Permalloy to form the spring and magnetic components. Matching of the coefficient of thermal expansion of material forming the support frame and the magnet suspension was key to the structural stability for changing temperature conditions. For additional details of this vibration energy harvester utilizing magnetic induction for a range of temperature environments see Patent # US 7,498,681.

Benefits

• Converts mechanical energy into electrical energy
• Inexhaustible power supply
• Improved power densities
• High-temperature operation

Applications and Industries

• Persistent, unattended remote operation of microelectronics
• Trickle charge batteries
• Air, water and land-based vehicles
• Oil rigs
• Heavy machinery
• Bridges and other architectural structures subjected to vibrations