Sensor makes electronic steering safer

Solihull, England--With power-assisted steering being fitted to more cars, even compact models, electronic power assisted steering (EPAS) systems are attracting the attention of vehicle designers. More versatile than hydraulic power assisted steering (HYPAS), EPAS integrates easily into vehicle control and safety systems.

Of the EPAS systems built to date, most employ inductive torque sensors to measure steering-wheel torque. One major concern, however, involves the electromagnetic compatibility (EMC) of this type of sensor. To avoid potential problems with EMC, Lucas Electrical Systems developed and integrated an opto-electronic torque sensor and integrated it into their electronic power assisted steering system.

The new sensor operates on the principle of LEDs shining through aligned and non-aligned holes in disks attached to the steering column. The disks rotate relative to each other under the action of the torque applied to a torsion bar within the column; light intensity correlates to torque.

Product Director Brian Edwards explains that, "although there are some Japanese cars with EPAS, the North American and European markets have legislative requirements different from those in Japan. These affect the design and life of the torque sensor and the fail-safe aspects of the software."

Consequently, the Lucas EPAS system comprises the opto-electronic torque sensor, a vehicle speed sensor, brushless electric motor, and an electronic controller. The motor and torque sensor mount on the steering column inside the passenger cabin. Fitted to a small or medium-sized car, the motor typically requires 2A to 3A average current from the vehicle's 12V electrical system during urban driving. Dual-lane sensor operation, incorporating parallel electronics systems, enhances safety.

Edwards cites several advantages of EPAS relative to hydraulic systems. These include: a 5% reduction in fuel consumption because the system needs no pump; lower installation costs and underhood space requirements; and less vehicle servicing because there are no fluid levels to check or belts to tighten. One of the most promising features, he says, arises from the ease of modifying the steering feel via software changes and external sensors.

Additional details, U.S....Contact Derek Savage, Lucas Automotive Inc., 5500 New King St., Box 7002, Troy, MI 48007-7002, (810) 641-2500, FAX: (810) 641-9330.