Electronic Design

Sensor Guides Electric Power Steering Into Mainstream

A new torque-and-position sensor will help drive electric power-steering systems out of the laboratory and onto the automotive assembly line. BI Technologies of Fullerton, Calif., a manufacturer of automotive sensors, developed the device using a blend of known potentiometric sensing technology and innovative packaging. The module is considered the key element in an all-electric alternative to traditional hydraulic-based power steering.

The sensor module, along with its related equipment, has been incorporated into one automaker's production vehicles, which are being sold in Europe. Before this, electric power-steering systems have found limited applications. Systems based on alternative sensors such as inductive or optical types have been hampered by issues of cost and electromagnetic compatibility (EMC).

Power-steering systems now rely on a hydraulic assist that continuously circulates hydraulic oil to sense and respond to steering changes. Maintaining the hydraulic pressure draws energy from the car's engine even when the steering wheel is idle. Additional energy is expended in supporting the weight of the hydraulics.

In contrast, electric power-steering systems are relatively light and consume power only when the steering wheel is turned. One architecture uses sensing devices mounted on the steering column, control electronics that interpret sensor outputs, and motor assemblies that provide the steering assist. Reducing the weight and energy associated with power-steering equipment will allow car makers to increase fuel efficiency by about 5% over hydraulic designs. Automakers also will be able to introduce power steering into smaller-engine vehicles for the first time. (For a more detailed discussion of energy consumption in electric versus hydraulic power-steering systems, go to www.hev.doe.gov/components/steering.html.)

The BI Technologies module was designed specifically for the latter goal. It allows power steering to be added to cars that lack the power, space, and pricing to support a hydraulic system.

A number of technologies have been considered for the steering-wheel sensor, including inductive and optical torque sensors. In building this steering sensor, however, BI Technologies elected to go with tried-and-true potentiometric-sensing technology. That approach assures a level of reliability—critical in automotive applications—as the company's resistive film technology has an established track record. And unlike inductive sensing, potentiometric sensing avoids hard-to-fix EMC problems.

The module consists of two potentiometric sensors. One, a round-style sensor, determines steering-wheel positions over 360°. The other is an angular sensor that detects how much torque is being applied to the wheel. This allows the power-assist motor to vary the steering boost ac-cording to driving conditions. For example, when the car is at a standstill, greater power assist is needed.

To fabricate the sensor module, existing materials such as ceramic substrates, high-quality thermoplastic housings, and the company's thick-film inks were used. But while the substances were conventional, the mechanical design of the sensor could not be. Allen Hilton, general manager for Automotive Products at BI Technologies, says the challenge lay in fitting the two sensor elements into the steering-column's confined space, while solving tricky mounting problems. The resulting sensor measures just 3 in. in diameter and 0.75 in. in thickness (see the figure).

For more information, contact BI Technologies' Sales and Marketing Department at (714) 447-2345, or visit www.bitechnologies.com.

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