Scientists from the US Department of Energy’s Ames Laboratory have developed magnetic materials that may steer automotive companies toward the goal of lighter, more fuel-efficient vehicles.
Researchers David Jiles and Bill McCallum believe a 0.25in 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 vehicle manufacturers to eliminate the heavy, energy-draining hydraulic system currently used in power steering.
‘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,’ said Jiles, a senior physicist at the Ames facility.
Jiles and McCallum spent the last five years searching for an inexpensive sensor material that met industry specifications and only one viable option emerged: a composite consisting of cobalt ferrite and small amounts of nickel and silver to hold the material together.
Current power-steering systems use a hydraulic assist that requires continuous pressurisation in order to sense and respond to steering changes, but this produces a constant drain on the car’s power. 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 magnetisation 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, claim the researchers, consume minimal energy when the steering wheel was not being turned.
Key to the cobalt-ferrite composite’s success is a property dubbed magnetostriction, said Jiles, as magnetostrictive materials undergo slight length changes when magnetised.
Cobalt ferrite maintains its magnetostrictive abilities throughout the temperature range specified by the vehicle manufacturing industry, from minus 40oC to 150oC.
Jiles said that’s necessary because vehicle manufacturers 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 where excellent mechanical properties are needed.’
Jiles added that whilst other materials rank higher in terms of magnetostriction, they’re too costly to be used in wide-scale production.
‘If you normalise 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,’ concluded Jiles.