Designing with Galfenol

Pumps and motors all vibrate at various frequencies when they operate. Other structures such as buildings, ships, bridges, and roadways also exhibit extraneous vibrations during everyday use. These vibrations are generally regarded as a nuisance, but there has been considerable research into harvesting such vibration energy to power remote or wireless sensor networks or to run devices such as alarms or GPS locators.

One of the ways to harvest vibrational energy is with a magnetostrictive material, one that changes shape in response to a magnetic field. A new entry in this category is Galfenol. Conversely, Galfenol also responds to external stresses by changing its magnetic state. These changes in magnetic state can induce voltage in coil(s), which then gets converted to useful power.

Galfenol (Fe_100-xGa_x), an iron-gallium alloy, is a mechanically robust magnetostrictive smart material discovered in 1999 where x can vary to produce the desired magnetic and mechanical properties. Interest in Galfenol has risen over the past several years as actuators, sensors, and energy harvesters have begun to integrate it. This alloy system has several special advantages over other smart materials such as Terfenol-D and piezoceramics. These mature material systems lack mechanical robustness and their performance is more sensitive to fluctuations in temperature.

Galfenol offers a tensile strength 20 times that of typical piezoceramics, making it applicable for harsh and shockprone environments. Galfenol can be readily shaped using conventional machining techniques, formed using standard metalworking practices, forged, rolled, drawn, and welded to other ferrous materials while maintaining its magnetic performance.

Galfenol offers a tensile strength of approximately 350MPa and has elongation at fracture of about 1%, which means the material can operate magnetically while in tension. Therefore, there is no need to apply a compressive preload to Galfenol, a key differentiator between it and other smart material systems. It is even possible to machine threads into Galfenol and form it into complex shapes. The combination of these attributes makes Galfenol an attractive technology for next-generation transducers, sensors, and energy harvesters.

The simplified behavior of magnetostrictive materials can be represented by a set of linear, coupled magnetostrictive equations:

S = s^HT + d₃₃H (1) B=d₃₃ *T + μ^TH (2)

where S = strain; s^H = mechanical compliance (m²/N) at constant H; H = magnetic field (A/m); T = stress; d₃₃ = magnetostrictive coefficient or change in strain with H at a constant stress; B = magnetic flux density; d₃₃ * = inverse magnetostrictive coefficient or change in B with stress at a constant H; and μ^T = magnetic permeability at constant T. Equation (2) is of most interest for energy-harvesting applications with these materials because the change in magnetic-flux density with changing stress is the phenomenon used to harvest vibrational energy.

Other energy-harvesting methods
Off-the-shelf devices that harvest vibration energy are available. These energy harvesters work efficiently by matching their primary resonant frequency with the operating frequency of the vibrational source. In practice, the harvesters must be tuned to match the frequencies for each specific application. A small deviation, typically less than ±5Hz, from that resonant frequency cuts their power output by more than half. There are two basic technologies used in this type of energy harvester: piezoelectric materials and magnetic induction.

Piezoelectric materials respond to applied stress by generating an electrical charge. In use, they generally exhibit a small displacement under the influence of a large force. One typical arrangement for a piezoelectric energy harvester is as part of a resonant bending beam tuned to a specific operating frequency. This type of energy harvester has a limited bandwidth and tends to be fragile due to the brittleness of piezoelectric materials.

Energy harvesters that use magnetic induction tend to find use where there are low-frequency (tens of hertz) vibrations of large amplitude. They consist of a spring attached to magnets which move inside a coil. The resulting change in magnet flux induces voltage in the coil. These energy harvesters are resonant devices and also exhibit limited bandwidths largely because of the large time constant associated with the spring.

Unfortunately, many vibration-harvesting applications are characterized by a vibration-frequency spectrum that is unpredictable, especially during power-up, powerdown, and under variable loading conditions. The fragile nature of most commercially available energy harvesters also excludes them from use where large forces and accelerations are possible. In such cases, Galfenol-based vibrational energy harvesters can potentially overcome the bandwidth limitations of existing energy harvesters and stand up to impacts that are more severe.

The best way of configuring a Galfenol energy harvester depends entirely on the mechanical environment it will see. As with any vibration harvesting scheme, tailoring the setup to match the mechanical loads (forces and displacements) maximizes energy-harvesting efficiency. The vibrations induce changing stresses in the Galfenol, which cause the material to undergo changes in magnetic-flux density. This, in turn, induces a voltage in a coil.

It is useful to review a few general examples of configuring Galfenol for energy harvesting. The first takes advantage of Galfenol’s ability to be machined and welded. Here, a boltlike configuration operates under forces from tens to hundreds of Newtons and displacements on the order of tens of micrometers. Force gets applied directly to the energy harvester, so only the input energy limits the bandwidth. This particular hardware setup has been demonstrated from 10 to 500ƒHz. It could potentially replace or augment existing fasteners with versions that harvest energy.

A second type of Galfenol energy harvester, developed by Dr. Toshiyuki Ueno at Kanazawa University in Japan, utilizes a bending beam like that of piezoelectric harvesters and has resonant qualities that are similar as well. However, the Galfenol energy harvester handles more abuse and has been demonstrated on beams shorter than 10 mm as well as those exceeding 100-mm long. This design has the potential to scale up for harvesting power beyond the milliwatt and watt range typically reported. It depends on the volume of the Galfenol material. While this design resembles a piezoelectric beam, its scale-up potential far exceeds that of the piezo approach.

A third configuration targets pump or motor-mount applications. Springlike elements are used to match the harvester stiffness with that of motor mounts. The spring elements also serve to amplify the forces transferred to the Galfenol active material. This type of energy harvester responds directly to forces and displacements of the pump or motor and is not bandwidth limited. This kind of setup has been demonstrated from 10 to 200 Hz and under 100ƒ‹m of displacement at low frequencies.

All Galfenol energy-harvester designs also have magnetic considerations. Galfenol needs a permanent magnet to supply magnetic bias and, in so doing, maximize changes in flux density. Other components which make up the magnetic circuit include high-flux-density magnetic return paths and air gaps. The goal is to maximize the changing magnetic flux in the Galfenol, so designers should minimize air gaps and pay particular attention to the overall efficiency of the magnetic circuit.

The changing magnetic flux is converted to useful electrical energy via a wire-wound coil. Voltage induced in a coil from a changing flux density is given by Faraday’s law:

V = –NA dB/dt (3)

where V = induced voltage, N = number of turns in a coil, A = crosssectional area of the Galfenol, and dB/dt = change in magnetic-flux density with time. Equation 2 can be used to estimate the changing flux density with a changing stress by taking a time derivative, resulting in an expression for dB/dt. Assuming that the magnetic field, H, is constant and substituting for dB/dt in Equation 3, the open-circuit voltage in a Galfenol energy harvester can be estimated as:

V = –NAd₃₃ * dT/dt (4)

 We can then estimate the maximum power output from a particular energy-harvester configuration by assuming a purely resistive load equalling the magnitude of the coil impedance, R, shown in Equation 5. This value provides a good estimate of energy harvesting potential.

P_max = V²/4R (5)

Once a voltage is generated in the coil, the harvester’s resulting power output must be conditioned to provide a useful form of energy. The coil is a critical aspect of power conditioning. Coil design is a balance between matching the incoming energy with the requirements of the powerconditioning electronics. Adding turns to the coil boosts voltage. However, there is a corresponding rise in coil resistance and inductance. Impedance matching between the coil and power-conditioning electronics is important for maximizing the power transferred. In addition, space constraints may limit the size of the coil and there is a weight penalty associated with larger wire gauges and additional turns.

Power-conditioning electronics for Galfenol harvesters typically consist of two parts: rectification to convert the raw-voltage output response to a dc signal, and voltage regulation to produce the level of dc power needed for the downstream electronics. One example of a complete energy-harvesting system provides a 3.3-Vdc signal to the downstream components. Also visible in the accompanying schematic are the rectification and regulation circuitry along with the power-management and energy-storage components. Operation of breadboard hardware resembling the schematic shown consumed 106­μ€W and was 60% efficient with respect to supplying power to the load.

Applications
Galfenol energy harvesters are particularly suitable for applications with broadband input energy or with varying operating frequencies. The bolt-type energy harvester described earlier generated power exceeding 60­mW with force inputs on the order of 100­N in a package occupying about 30­cm3. Instantaneous peak powers of 2­W have been demonstrated using resonant-beam-type energy harvesters that are about 250-mm long. Predictions are that motormount- type energy harvesters will produce on the order of tens of milliwatts with the vibration levels typical of vacuum pumps and motors. However, this capability is still in development.

Laboratory demonstrations have shown that different configurations of Galfenol energy harvesters can provide “free” energy to operate remote sensors and radios. The most-promising applications include fasteners, mounts, and mounting bolts for large industrial pumps and motors and large-scale energy harvesters where significant forces and/or displacements are present.

© 2013 Penton Media, Inc.