Electronic Design

Electronic Torque Sensor May Improve Power Steering Systems

A group of researchers at Ames Laboratory, Ames, Iowa, is now developing a material that's lightweight and cost-effective enough to rival conventional hydraulic power-steering systems. With just a 0.25-in. thick ring of the material, an electronic torque sensor could be developed to regulate the steering power provided to a car's wheels by an electric motor.

According to David Jiles, a senior physicist at Ames Lab and a professor of materials science and engineering at Iowa State University in Ames, "Replacing the hydraulic power-steering system with an electrical system that uses this type of sensor should im-prove the fuel efficiency of a car by about 5%. Lighter, more energy-efficient vehicles would use less gasoline, conserving fossil fuels and reducing transportation costs."

The key to this new automotive sensor is a material basically composed of cobalt oxide and iron oxide. Small amounts of nickel and silver are added to hold the material together. Together, these elements form a cobalt-ferrite composite that complies with the auto industry's specifications. It is relatively low-priced, making wide-scale production possible.

The cobalt-ferrite ceramic-metallic composite also meets the strength and corrosion-resistance standards for a sensor material. It is similar in concept to materials used in high-strength tool bits that require high-performance mechanical properties. Classified as a high-class rust, it's extremely difficult to corrode during operation.

Researchers fully evaluated the composite, deeming it the optimal choice. They then set out to devise and prototype an electronic-based automotive sensor (see the figure). What they came up with is a sensor that uses a small ring of the cobalt-ferrite material strategically placed on a car's steering column. As the wheel is turned, the magnetization of the cobalt-ferrite ring varies in proportion to the force applied by the driver. This change is detected by a nearby field sensor. It interprets the amount of energy necessary to turn the wheels and relays the information to an electrical power-assist motor.

What makes this concept so ingenious is its reliance on the magnetostriction property of the cobalt-ferrite composite. Magnetostrictive materials undergo slight length changes when magnetized. Rather than capitalize on this fact, researchers opted to take advantage of the property in reverse. In their proposed sensor, the turning of the wheel applies stress to the cobalt-ferrite ring. This action then causes a change in the magnetic field emitted.

Interestingly, the cobalt-ferrite composite maintains its magnetostrictive abilities throughout a temperature range of ­40°C to 150°C—the auto industry's standard gauge. As Jiles explains, this is a crucial point considering most "automakers don't 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."

The proposed electronic torque sensor offers a number of benefits over today's power-steering systems. Conventional systems use a hydraulic assist. Through this method, a continuous circulation of hydraulic oil is used to sense and respond to steering changes. This process creates a constant drain on the car's engine, even when the steering wheel isn't being turned. Since the hydraulic system is pressurized, the car tends to use a lot of energy.

In contrast, Ames' proposed electrical system would consume minimal energy when the steering wheel was not being turned. Couple this fact with the significantly lower weight of the system. What you end up with is the potential for a much more fuel-efficient vehicle. An electronic torque sensor would enable steering systems to be fine-tuned through the addition of software and other controls. Hydraulic power-steering systems do not allow for this possibility.

Researchers have already applied for a patent on the cobalt-ferrite compound. They plan to continue working with automotive manufacturers who seek to use the material in an electronic torque sensor. Another idea is to use the sensor in smart cars that are capable of adapting to an individual's driving style. As Jiles points out, "You may eventually have something like a neural network in your car that learns about your driving characteristics as you drive."

For more information on this research, check out the Ames Laboratory web site at www.ameslab.gov.

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