A novel mechanism called the vibration ring is being developed to enable energy conversion elements to be incorporated into the driveline of a helicopter or other rotating machines. Unwanted vibration is transduced into electrical energy, which provides a damping effect on the driveline. The generated electrical energy may also be used to power other devices (e.g., health monitoring sensors). PZT (‘piezoceramic’) and PMN-30%PT (‘single crystal’) stacks, as well as a Tb<sub>0.3</sub>Dy<sub>0.7</sub>Fe<sub>1.92</sub> (‘Terfenol-D’) rod with a bias magnet array and a pickup coil, were tested as alternative energy conversion elements to use within the vibration ring. They were tuned for broadband damping using shunt resistors, and dynamic compression testing was conducted in a high-speed load frame. Energy conversion was experimentally optimized at 750Hz by tuning the applied bias stress and resistance values. Dynamic testing was conducted up to 1000Hz to determine the effective compressive modulus, shunt loss factor, internal loss factor, and total loss factor. Some of the trends of modulus and internal loss factor versus frequency were unexplained. The single crystal device exhibited the greatest shunt loss factor
whereas the Terfenol-D device had the highest internal and total loss factors. Simulations revealed that internal losses in the Terfenol-D device were elevated by eddy current effects, and an improved magnetic circuit could enhance its shunt damping capabilities. Alternatively, the Terfenol-D device may be simplified to utilize only the eddy current dissipation mechanism (no pickup coil or shunt) to create broadband damping.
Iron-gallium alloys, known as Galfenol, are a class of magnetostrictive materials that convert mechanical energy to magnetic energy and vice versa. Galfenol devices especially unimorph consisting of a Galfenol beam bonded to a passive substrate, have great potential in energy harvesting applications, but advanced multiphysics models are lacking for these smart devices. This study presents a comprehensive finite element model for Galfenol unimorph harvesters which incorporates magnetic, mechanical, and electrical dynamics. Experiments considering impulsive tip excitations under purely resistive or resistive-capacitive electrical loads are conducted to validate the proposed model. The energy conversion efficiency and peak power density of a unimorph beam with a natural frequency of 139.5 Hz are analyzed experimentally. The maximum energy conversion efficiency is 5.93% when a 74 Ω resistor and a 2 μF capacitor are connected in parallel to the pickup coil in parallel. The maximum power density observed in experiments is 10.72 mW/cm<sup>3</sup> when load resistance is 74 Ω. This performance may be optimized in the future utilizing the proposed finite element model.
Vibrations generated by machine driveline components can cause excessive noise and structural dam- age. Magnetostrictive materials, including Galfenol (iron-gallium alloys) and Terfenol-D (terbium-iron- dysprosium alloys), are able to convert mechanical energy to magnetic energy. A magnetostrictive vibration ring is proposed, which generates electrical energy and dampens vibration, when installed in a machine driveline. A 2D axisymmetric finite element (FE) model incorporating magnetic, mechanical, and electrical dynamics is constructed in COMSOL Multiphysics. Based on the model, a parametric study considering magnetostrictive material geometry, pickup coil size, bias magnet strength, flux path design, and electrical load is conducted to maximize loss factor and average electrical output power. By connecting various resistive loads to the pickup coil, the maximum loss factors for Galfenol and Terfenol-D due to electrical energy loss are identified as 0.14 and 0.34, respectively. The maximum av- erage electrical output power for Galfenol and Terfenol-D is 0.21 W and 0.58 W, respectively. The loss factors for Galfenol and Terfenol-D are increased to 0.59 and 1.83, respectively, by using an L-C resonant circuit.
A novel and precise characterization of the constitutive behavior of solid and laminated research-grade, polycrystalline Galfenol (Fe<sub>81:6</sub><sub>Ga18:4</sub>) under under quasi-static (1 Hz) and dynamic (4 to 1000 Hz) stress loadings was recently conducted by the authors. This paper summarizes the characterization by focusing on the experimental design and the dynamic sensing response of the solid Galfenol specimen. Mechanical loads are applied using a high frequency load frame. The dynamic stress amplitude for minor and major loops is 2.88 and 31.4 MPa, respectively. Dynamic minor and major loops are measured for the bias condition resulting in maximum, quasi-static sensitivity. Three key sources of error in the dynamic measurements are accounted for: (1) electromagnetic noise in strain signals due to Galfenol's magnetic response, (2) error in load signals due to the inertial force of fixturing, and (3) time delays imposed by conditioning electronics. For dynamic characterization, strain error is kept below 1.2 % of full scale by wiring two collocated gauges in series (noise cancellation) and through lead wire weaving. Inertial force error is kept below 0.41 % by measuring the dynamic force in the specimen using a nearly collocated piezoelectric load washer. The phase response of all conditioning electronics is explicitly measured and corrected for. In general, as frequency increases, the sensing response becomes more linear due to an increase in eddy currents. The location of positive and negative saturation is the same at all frequencies. As frequency increases above about 100 Hz, the elbow in the strain versus stress response disappears as the active (soft) regime stiffens toward the passive (hard) regime.
Magnetostrictive iron-gallium alloys, known as Galfenol, are a recent class of smart materials with potential in energy harvesting applications. Unimorph energy harvesters consisting of a Galfenol beam bonded to a passive substrate are simple and effective, but advanced models are lacking for these smart devices. This study presents a finite element model for Galfenol unimorph harvester systems. Experiments considering various design parameters such as pick up coil size, load resistance, beam thickness ratio, and bias magnetic field strength are conducted to guide and validate the modeling effort. If the free length of the Galfenol unimorph beam is considered as the effective length, the maximum average power density, peak power density, and open-circuit voltage amplitude achieved in experiments are 13.97 mW/cm<sup>3</sup>, 35.51 mW/cm<sup>3</sup>, and 0.66 V, respectively. By only considering the length of Galfenol surrounded by the pickup coil, the maximum average power density and peak power density are 23.66 mW/cm<sup>3</sup> and 60.14 mW/cm<sup>3</sup>, respectively.
This paper focuses on the development of a three-dimensional (3D) hysteretic Galfenol model which is implemented using the finite element method (FEM) in COMSOL Multiphysics. The model describes Galfenol responses and those of passive components including flux return path, coils and surrounding air. A key contribution of this work is that it lifts the limitations of symmetric geometry utilized in the previous literature and demonstrates the implementation of the approach for more complex systems than before. Unlike anhysteretic FEM models, the proposed model can simulate minor loops which are essential for both Galfenol sensor and actuator design. A group of stress-flux density loops for different bias currents is used to verify the accuracy of the model in the quasi-static regime.