Ford’s 2011 F-150 is the industry’s first high-volume full-size pickup truck to use the vehicle’s 12-V power supply for Electric Power Assist Steering (EPAS). The EPAS system provides steering assist from an electric motor that is geared directly to the steering rack, according to Garry Smith, lead EPAS engineer at Ford Motor Co.
Explained Smith, “The on-demand electric assist provides up to 4% fuel-economy improvement by eliminating the constant energy draw of an engine-mounted hydraulic assist pump that runs whether you are steering or not.”
The self-contained power steering unit includes conventional rack-and-pinion mechanicals; a Hall-effect noncontact torque sensor in the pinion tower to measure driver input steering torque; internal communications from the torque sensor to the ECU (electronic control unit) to calculate precise steering assist based on driver torque, vehicle speed, and straight ahead truck motion; a 12-V brushless dc electric motor to provide steering assist; and a cogged belt/ball screw reduction drive to translate motor assist torque to rack assist force.
All software is within the ECU, including speed variable assist, pull-drift compensation, and active nibble control.
“Ford’s exclusive active nibble control is similar in concept to noise-canceling headphones but acts on tactile vibrations, sensing small amplitude, high-frequency torque disturbances from rough road surfaces and providing corresponding steering assist to prevent torque disturbances at the steering wheel,” explained Smith.
Pull-drift compensation is provided by the EPAS system’s adaptive controls.
“Above a certain speed and when driving straight ahead, a steady-state torque is calculated. A corresponding assist torque is then provided by the EPAS motor, relieving the driver of that [steering] effort,” explained Smith.
Other 2011 vehicles, including the Ford Fiesta, Mustang, Fusion, Explorer, and Escape as well as the Lincoln MKS and MKT, are equipped with EPAS. Several technology developments were needed, however, to provide steering assist sufficient for full-size pickup trucks within the limits of a 12-V power supply.
“One was the improvement in electrical efficiency of the ECU and motor. Another was the reduction of motor inertia, and similarly the reduction of inertia, friction, and lash in the torque reduction drive system, all of which allowed a higher numerical ratio of motor torque to rack force. Finally, since steering full-size pickups requires high EPAS motor current, Ford developed a smart vehicle charging system to ensure maximum electric power is always available for steering assist while other electric systems are cycled to level the demand,” said Smith.
The EPAS on the F-150 is the largest in terms of rack load, and the truck is the heaviest vehicle yet to use the system with a 12-V power supply, according to Ted Seeger, Global Chief Engineer for Electric Power Steering at Nexteer, the supplier that partnered with Ford to help design and prove-out the F-150 EPAS application.
Noted Seeger, “EPAS has been done on a very limited number of mostly hybrid vehicles at 42 V. Nexteer has prototyped 42-V systems and has the capability to implement these systems in production [applications] using our scaleable electrical architecture. The higher voltage will provide the ability to steer even higher loads when OEMs choose to move to 42-V systems.”