By Susan Dieterle
You might say that David Jiles and Bill McCallum share a mutual attraction to magnetism.
For much of their careers, the Ames Laboratory scientists have developed magnetic materials that can be used in innovative, energy-efficient technologies.
Their latest development is a material that may steer automotive companies toward their goal of lighter, more fuel-efficient vehicles. The researchers say a 1/4-inch-thick ring of the material could be used in an electronic torque sensor to regulate the steering power provided to a car's wheels by an electric motor. This would enable automakers to eliminate the heavy, energy-draining hydraulic system currently used in power-steering assists.
"Replacing the hydraulic power-steering system with an electrical system that uses this type of sensor should improve the fuel efficiency of a car by about 5 percent," says Jiles, senior physicist. Lighter, more energy-efficient vehicles would use less gasoline, conserving fossil fuels and reducing transportation costs, he adds.
Jiles and McCallum, senior materials scientist, looked at a number of possibilities during the past five years as they searched for an inexpensive sensor material that met the specifications of the auto industry. They say only one viable option emerged: a composite consisting of cobalt ferrite (a compound of cobalt oxide and iron oxide) and small amounts of nickel and silver to hold the material together.
"I think we've looked into all of the possibilities, and it's difficult to conceive of a better material at this time," Jiles says. "The fact that it's also a relatively low-priced material makes it very attractive."
He says current power-steering systems use a hydraulic assist that requires continuous pressurization in order to sense and respond to steering changes. This produces a constant drain on the car's power, even when the steering wheel isn't being turned. "Also, the hydraulic system weighs a lot, so there's a significant weight reduction if you can replace it with an electrical system," he adds.
A sensor using a small ring of the cobalt-ferrite composite would be strategically placed on the steering column. As a driver turned the wheel, the magnetization of the cobalt-ferrite ring would change in proportion to the amount of force applied by the driver. The change would be detected by a nearby field sensor that would interpret how much force should be applied to turn the wheels and then relay the information to an electrical power-assist motor. Unlike the hydraulic system, the electrical system would consume minimal energy when the steering wheel was not being turned.
What makes the cobalt-ferrite composite ideal for this application is a property known as magnetostriction, Jiles says. Magnetostrictive materials undergo slight length changes when magnetized. Jiles and McCallum take advantage of that property, but in reverse. In their approach, the turn of the steering wheel would apply stress to the cobalt-ferrite ring, producing a change in the magnetic field it emits.
Cobalt ferrite maintains its magnetostrictive abilities throughout the temperature range specified by the auto industry, from minus 40 C (minus 40 F) to 150 C (302 F). Jiles says that's necessary because automakers donŐt agree on the best location on the steering column for the torque sensor. Some want it in the passenger compartment while others want it in the engine compartment, where it would be subjected to engine heat as well as winter conditions.
McCallum adds that cobalt ferrite also meets the strength and corrosion-resistance requirements for the sensor material. "This ceramic-metallic composite is similar in concept to materials used in high-strength tool bits where excellent mechanical properties are needed," he says. "And cobalt ferrite is basically high-class rust, so it's hard to corrode any further."
Jiles says the composite is also a cost-effective choice. While other materials may rank higher in terms of magnetostriction, they're too costly to be used in wide-scale production. For example, Terfenol-D is a rare-earth, magnetostrictive compound that Ames Lab helped develop in the 1980s. It possesses a much higher degree of magnetostriction, but can cost up to 100 times more than the cobalt-ferrite composite.
"If you normalize the measurements based on the cost of the different materials, you can see that our cobalt-ferrite material is far and away the best performer," Jiles says.
The five-year effort to develop an inexpensive sensor material that met the auto industry's needs had its share of frustrations and setbacks. Jiles and McCallum hit several roadblocks that sent them back to the drawing board. As part of the development process, they devised their own software and a unique test bed to measure the performance of the materials they were studying.
"We'd test some of the materials, and they wouldn't respond in the way that we had anticipated, so we would go back to understand why," Jiles recalls. "But now we have a practical material, and we also have computer simulations that can tell you how it will perform in engineering applications."
Much of the research on the sensor material was funded through a three-year, $820,000 grant they received in 1996 from the Department of Energy's Advanced Energy Projects Division. One of the DOE's primary missions is to engage in research that leads to the development of materials that improve the efficiency, economy, environmental acceptability and safety of energy sources.
Jiles and McCallum have applied for a patent on the cobalt-ferrite composite and plan to continue working with automotive manufacturers interested in using the material in an electronic torque sensor.
"When we began this project, we knew it was possible that there might not be a material that would meet the specifications," Jiles says. "It's satisfying that we've gone through this long search and come up with a material that works."
For more information:
David Jiles, (515) 294-9685
gauss@ameslab.gov
Research funded by:
DOE Office of Basic Energy Sciences
Last revision: 9/15/00 sd
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