Tech Briefs
A lightweight alternative to hydraulic power-steering systems

The configuration of the ferrite sensor ring on the driveshaft.

A material developed at the U.S. Department of Energy's (DOE) Ames Laboratory may provide another means for automotive companies to achieve their goal of lighter, more fuel-efficient vehicles. Researchers say a 6.35-mm (0.25-in) 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 pumps currently used in power-steering systems.

"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%," said David Jiles, a Senior Physicist at Ames Lab and a Professor of Materials Science and Engineering at Iowa State University (ISU).

Jiles and Bill McCallum, a Senior Ames Lab Materials Scientist and an ISU Adjunct Professor of Materials Science and Engineering, looked at a number of options during the past five years for an inexpensive sensor material that met the specifications of the auto industry. They said 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 said. "The fact that it's also a relatively low-priced material makes it very attractive."

Current power-steering systems use a hydraulic assist that requires the continuous circulation of hydraulic oil to sense and respond to steering changes. This produces a constant drain on the car's engine, even when the steering wheel is not being turned. "The hydraulic system has to be pressurized to work and the car uses up energy to do that," Jiles said. "Also, the hydraulic system weighs a lot, so there's a significant weight reduction if you can replace it with an electrical system."

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. A nearby field sensor would detect 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. Magnetostrictive materials undergo slight length changes when magnetized, a phenomenon first discovered in nickel in 1842 by J.P. Joule. Subsequently cobalt, iron, and alloys of these materials were found to show a significant magnetostrictive effect. 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 -40° to +150°C (-40° to +302°F). Jiles said that is necessary because automakers do not 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 added 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 for which excellent mechanical properties are needed," he said. "And cobalt ferrite is basically high-class rust, so it's hard to corrode any further."

Jiles said the composite is also a cost-effective choice. While other materials may rank higher in terms of magnetostriction, they are 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, exploring the compound's phase equilibria and single-crystal growth anisotropy. It possesses a much higher degree of magnetostriction, especially as a single crystal, but can cost up to 100 times more than the cobalt-ferrite composite.

Electronic torque sensors would allow steering systems to be fine-tuned with the addition of software and other controls. "With a hydraulic power-steering assist, there's not much that you can do," Jiles said. The sensors may also be part of the development of "smart" cars that are capable of adapting to an individual's driving style. "You may eventually have something like a neural network in your car that learns about your driving characteristics as you drive."

Much of the research on the sensor material was funded through an $820,000 grant Ames Lab received in 1996 from the DOE'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.

Jean L. Broge