Does the future of power steering belong to fully electric systems because of their greater efficiency? The answer, for smaller vehicles, especially those in the A-, B-, and C-class range, is affirmative as they do not have a high power demand, and the compact size of the electric motors means they can be sited near the steering rack or column. In addition, torque overlay-the ability to add or subtract torque for add-ons like automatic parking control, split-mu steering assist, and lane departure support-can be added simply by updating software. Larger vehicles, however, won't follow the same path, says Dr. Dieter Fehlings, engineering director EPHS Steering Systems, TRW Automotive (Livonia, MI; www.trw.com), "because the more power you need to drive the electric motor, the more progressive your costs become as you increase the amount of copper in the system and move to more expensive rare-earth magnets." Therefore, electrically powered hydraulic systems must be substituted.
Electro-hydraulic assist systems don't use as much power as the electric motor isn't used to directly drive the steering rack. Its job is to take the place of the belt-driven hydraulic steering pump and provide boost only when it is required. On larger vehicles, especially light trucks and SUVs, the system can be scaled to counteract the bending forces at the wheel that arise from powering the steering rack as the vehicle climbs over obstacles like rocks, or downsized to handle the requirements of larger, heavier (i.e. D-segment and premium) vehicles that spend their time on-road. And while not as efficient as steering driven directly by an electric motor, Dr. Fehlings insists, "There's only a few percentage points in terms of overall efficiency between electrically powered hydraulic and fully electric power steering."
The efficiency gap between the two systems has closed to within five percentage points in recent years through improvements in electric motors, increases in gearing efficiency, and running higher pressures. Where 70 bar pressure-approximately 1,000 lb/in.2-was the upper end for most electro-hydraulic systems recently, it's not unusual to see pressures in the 120 to 135 bar (1,740 to 1,958 lb/in.2) range today. "The logical limit for the pressures we can run is the physical limit of the seals," says Fehlings. "As they improve, the pressures will go higher." Currently, TRW has an integrated motor pump assembly with on-board electronics capable of 1.0 kW of hydraulic output power that provides fast transient response, retains the familiar hydraulically assisted steering feel, can be used on hybrid vehicles, and comes in a pre-tested module that can be attached directly to the steering gear or remotely mounted. Software for the unit can be programmed to include driver-selected steering modes, and to speed the tuning process.
"The valve curve is critical to how a vehicle steers and responds," says Fehlings, "and we can do more with the system's software to dial-in the initial tuning of the vehicle and run iterations than was thought possible just a few years ago." With information on the vehicle's weight and its distribution, center of gravity, dimensions, and tire construction and compound, it's possible to determine the initial tune in the lab and greatly reduce the number of road tests. Of the 80 parameters that can be tuned, six are critical: the steering boost curve, its dynamic amplification, damping, weighting and speed progression, return-to-center characteristics, and stability. Says Fehlings, "You have to limit the possibilities in order to do what is needed or you'll find yourself tuning the steering system forever." That limitless potential also is why he thinks every vehicle with electrically boosted steering systems won't steer the same, even though it is theoretically possible. "There's just too much to play with, and too many things to optimize," he says with a smile.