Vibration Energy Harvesting by Magnetostrictive Material for Powering Wireless Sensors

Abstract

Wireless Sensor Networks (WSN) have been increasingly applied to Structural Health Monitoring (SHM). For WSN to achieve full potential, self-powering these sensor nodes needs to be developed. A promising approach is to seamlessly integrate energy harvesting techniques from ambient vibrations with the sensor to form a self-powered node. The objective of this study is to develop a new magnetostrictive material (MsM) vibration energy harvester for powering WISP (Wireless Intelligent Sensor Platform) developed by North Carolina State University. Apart from piezoelectric materials which currently dominate in low frequency vibration harvesting, this new method provides an alternate scheme which overcomes the major drawbacks of piezoelectric vibration energy harvesters and can operate at a higher frequency range.

A new class of vibration energy harvester based on MsM, Metglas 2605SC, is deigned, developed, and tested. Compared to piezoelectric materials, Metglas 2605SC offers advantages including ultra-high energy conversion efficiency, high power density, longer life cycles, lack of depolarization, and high flexibility to survive in strong ambient vibrations. To enhance the energy conversion efficiency and shrink the size of the harvester, Metglas ribbons are transversely annealed by a strong magnetic field along its width direction to eliminate the need of bias magnetic field.

Governing equations of motion for the MsM harvesting device are derived by Hamilton's Principle in conjunction with normal mode superposition method based on Euler-Bernoulli beam theory. This approach indicates the MsM laminate wound with a pick-up coil can be modeled as an electro-mechanical gyrator in series with an inductor. Then a generalized electrical-mechanical circuit mode is obtained. Such formulation is valid in a wide frequency range, not limited to below the fundamental natural frequency. In addition, the proposed model can be readily extended to a more practical case of a cantilever beam element with a tip mass. The model resulting in achievable output performances of the harvester powering a resistive load and charging a capacitive energy storage device, respectively, is quantitatively derived.

An energy harvesting circuit, which interfaces with a wireless sensor, accumulates the harvested energy into an ultracapacitor, is designed on a printed circuit board (PCB) with plane dimension 25mm*35mm. It mainly consists of a voltage quadrupler, a 3F ultracapacitor, and a smart regulator. The output DC voltage from the PCB can be adjusted within 2.0˜5.5V which is compatible with most wireless sensor electronics. In experiments, a bimetallic cantilever beam method is developed to determine the piezomagnetic constant d from the measured Lambda-H curve. The maximum output power and power density on the resistor can reach 200 uW and 900 uW⁄cm3, respectively. For a working prototype, the average power and power density during charging the ultracapacitor can achieve 576 uW and 606 uW⁄cm3 respectively, which compete favorably with the piezoelectric vibration energy harvesters.