Precision moves with magnetostriction

Typical actuator cross section

A cross-sectional diagram of a typical Terfenol-D actuator reveals the Terfenol-D rod, permanent magnets, and coil driver. Magnets bias the Terfenol-D magnetic domains for ac use. Versions biased for dc use would substitute other coils for the magnets. The Terfenol-D rod pushes against a spring-loaded output rod to generate displacements in the range of hundreds of micrometers.


Typical operating range for magnetostrictive applications

Terfenol-D has a higher energy density than piezoceramic materials often used for magnetostrictive actuation.


Bryon Dudley

Jon Snodgrass
Etrema Products Inc.
Ames, Iowa

In the 1970s, the U.S. Navy wanted to improve its sonar technology. It needed a material able to transmit powerful acoustic signals to detect enemy submarines. Engineers at the Naval Ordinance Laboratory in Washington, D.C., solved the problem by developing a giant-magnetostrictive material called Terfenol-D.

Magnetostriction is the property that causes ferromagnetic materials to change shape in a magnetic field. It is the source of the 60-cycle buzz you hear from electrical transformers. Terfenol-D is an alloy of terbium, dysprosium, and iron. It is said to produce "giant" magnetostriction, strain much greater than any of the other magnetostrictive materials.

Simply, this gets accomplished by energizing an electrical coil that surrounds a piece of Terfenol-D. Electrical current flowing through the coil induces a magnetic field which acts to realign the magnetic domains in the Terfenol-D. As the domains rotate, they distort the atomic structure, causing the material to grow and contract as the current is applied and removed, efficiently converting electricity into motion. The result is proportional, positive and repeatable expansion in microseconds.

Terfenol-D has a Curie temperature (380°C) which lets it provide this magnetostrictive performance from room temperature to around 200°C. While the material will still function between 200°C and Curie, the performance does drop as a function of temperature and the losses above 200° offset the advantages of the technology. There is also a lower operating temperature limit of 15°C. Lower temperatures can be reached by adjusting the stoichiometry of the alloy. This has been done to enable fuel-injector applications which require operation down to 40°C. Larger chemical modifications can extend the operating range down to cryogenic temperatures, but at the loss of other properties of the material.

What's interesting about Terfenol-D is that it can be more than just a means of pinging the ocean. At lower frequencies, say 10 to 100 Hz, Terfenol-D actuators can provide repeatable displacements in the range of hundreds of micrometers or even greater. This makes them candidates for high precision motion necessary to realize various state-oftheart manufacturing processes. The material can also respond at very high frequencies, in excess of 20 kHz, while still producing a large amount of force.

The forces available from Terfenol-D allow for substantial energy output in both acoustical and mechanical terms. Sonar systems employing Terfenol-D transducers can typically produce in excess of 210 dB per transducer. In mechanical terms, a 2.5-in.-diameter rod of Terfenol-D is capable of generating over 50,000 lb of dynamic force. The trick is harnessing that force.

MAKING COMPARISONS
There are other Smart Materials available for designers than have similar properties to Terfenol-D. The most common of these are piezoceramics, typically comprising materials around the lead-titanate-zirconate (PZT) system. PZT materials operate by changing shape in response to an applied voltage, in a mechanism similar to Terfenol-D's response to a magnetic field. But there are differences between these Smart materials technologies. PZT materials typically have a lower energy density, the amount of power produced in a given size, than Terfenol-D. And because PZT is a ceramic, it must be specially designed to withstand harsh environments. But the key weakness is that the ordering imparted to the material to create the smart response (called poling) is artificially imposed on the material. Over time, this poling wears off and the material loses the ability to work. The phenomenon is referred to as aging. The rate of aging depends on how hard the material is driven, temperature and, of course, time.

Terfenol-D's ability to be smart is inherently tied to its crystal structure, and so does not fade over time or with usage. If you heat the material above Curie, it will lose its magnetostriction, but the property will come back once you cool down below Curie. In PZTs, if you exceed Curie, the poling is lost, and upon cooling the stack will remain dead.

The other material sometimes applied in magnetostriction is nickel. Nickel served as a sonar transducer in World War I as the allies hunted German U-boats. Although limited by its power density and strain capabilities, nickel is still used today in cleaning baths at ultrasonic frequencies.

DESIGN CONSIDERATIONS
There are of course nuances to designing applications based on Terfenol-D. Technical solutions capitalize on Terfenol-D's force, displacement, and frequency capabilities. Terfenol-D drive systems consist of the Terfenol-D rod, coil, bias magnets, and housing. They generally are provided in the form of either a fully packaged actuator or drive motor.

It is possible to obtain system components separately, without the housing.But projects taking this route usually get design help from supplier personnel because some aspects of component placement can be tricky.

The technology can work across a broad band of frequencies, and enables applications in many diverse industries. Maximum displacements available from Terfenol-D actuators range from 50 to 250 microns. Custom-application actuators are also routinely created to meet the specific requirements of a specific application. Displacement, though, is not determined solely by the applied current, but is also a function of the load the actuator works against and the frequency of operation. Consequently, the design process can be complex. It involves modeling the load or developing a physical prototype and then examining the load versus displacement qualities of the overall system.

One factor designs must consider is the resonant frequency of the Terfenol-D actuator. The breadth of the resonance peak is called the Q, or mechanical quality factor. One of the key factors of Terfenol-D is that it is inherently a low-Q material as compared to other smart materials like PZT.

An actuator is most efficient when operating at resonance. So it is often a goal to design the actuator such that the actuator/load combination can operate at a frequency close to actuator resonance. Changes from the load or environment shift the natural frequency of the system, which can critically reduce the operation of high Q, narrow-band transducer but has little impact to a low-Q, broadband design.

APPLICATIONS
Technologists of all kinds are incorporating Terfenol-D into a variety of products. Through the first seven months of 2004, 81 patents involving Terfenol-D have been granted or are pending, and another 43 were granted in 2003. Among these are file-cabinet-sized Naval sonar arrays that once would have filled a pick-up truck; acoustically stimulated oil production; and the machining of efficient and emission-friendly pistons running at 6,000 rpm instead of the 1,200 rpm is handled by traditional technology. Passersby on New York's famed Fifth Avenue cannot only view movies playing on new 63-in. plasma televisions in store windows, but also hear them as well, thanks to Terfenol-D actuators that turn store windows into two-sided speakers. Home theater buffs can completely immerse themselves in the sound of their systems without any externally visible wires or speakers because their walls, driven from behind by Terfenol-D actuators, become speakers.

Looking forward, efforts are underway to help the food industry with separating, emulsifying, defoaming, degassing, disintegrating, and homogenizing. Similar techniques are being evaluated in the pharmaceutical, cosmetic, and beauty markets. Smart material technologies are also helping to reduce costs while boosting throughput in various biofuel processes. Additionally, most attention has focused on the active aspect of the material (electrical to mechanical). But several different applications have recently begun to exploit the reverse phenomenon, commonly called to Villari Effect.

New material developments are also underway. Researchers are now trying to improve the strain and efficiency of Terfenol-D. They are also working on new material compositions that will allow for an ever-expanding list of possible uses.

Galfenol is the latest magnetostrictive material to be invented, and is currently under development. Originally devised in 1999 by the Magnetic Materials Group at NSWC-CD, Galfenol is slowly gaining a commercial foothold. Its magnetostriction is only a third to a quarter that of Terfenol-D, but can operate at significantly lower drive fields. Recent research has also shown that Galfenol can operate in tension and compression, something no other high frequency smart material can do. Galfenol's transition to production, and therefore commercial availability is expected in the next 24 months.

GENEALOGY OF A SMART MATERIAL

The name Terfenol-D comes from the metallic elements terbium (TER), iron (FE), Naval Ordnance Labs (NOL), and Dysprosium (-D). NOL developed and named the material. The research group from NOL is now located at Naval Surface Warfare Center Carderock Div. (NSWC-CD) in Bethesda, Md. Etrema Products Inc. holds patents and licenses to many Terfenol-D applications as well as the exclusive worldwide licenses to manufacture all types of Terfenol-D materials. This patent protection also includes the composition of matter for Terfenol-D. In addition, Etrema has made several innovations for the low-cost manufacture of Terfenol-D, which the company retains as trade secrets.

In 1987, Edge Technologies was founded to transfer promising technologies from Iowa State University into commercial markets. The Iowa State University Research Foundation also had a significant role by licensing key technologies to Edge. The first license was for Terfenol-D which, in 1990, led to the launch of Edge's first subsidiary company, Etrema Products. Etrema's mission was to further develop the manufacture of the Terfenol-D raw material and the numerous applications that could benefit from the material.

Substantial changes have taken place since the invention of the material. In the 1980s, as the technology was emerging from the laboratory, drivers of 8-mm diameter at most were state of the art. Only one rod could be made at a time, and this was done completely by hand. In 1995 came the first true automatedproduction system for the growth of Terfenol-D drivers. This system can produce single drivers as large as 65 mm in diameter or, with a change of the mold configuration, multiple small diameter rods (15 to 30 mm) simultaneously.

Edited by Leland Teschler

MAKE CONTACT
Etrema Products Inc., (515) 296-8030, etrema.com