POWER CONSIDERATIONS

The amount of electrical power required for EPS has limited the system's usage primarily to smaller vehicles. Part of the issue with a 12 V system is getting enough instantaneous power to have a full electric power steering for a large car. “As of today, I think you'll find them on smaller cars and not so much on large ones,” said Phil Headley, chief enginee, Advanced Technology, Continental Automotive Systems.

TRW's electrically assisted steering (EAS) specifically targets the small car segment. The integrated EPS system is a column unit that uses a low inertia permanent magnet ac motor. A worm gear transmits the assist torque from the motor to the steering column. The system provides a maximum assist torque of 70 Nm (rack assist force 8500 N) with peak output power of 470 W. This requires peak battery current of 70 A at 13.5 V. EPS systems can easily exceed 100 A using a standard 12 V battery supply.

Toyota's Prius has a high-voltage EPS system that operates around 40 V but it is similar to the 12 V system. It is Toyota's oldest EPS system and the first EPS system the company offered in the United States. The system provides power steering when the engine is off. ...............The Highlander and the Camry hybrids have a system that works on the same principles as the Prius with a torque sensor in the steering column, but the assist motor's location changes from the steering column to into the steering rack. As shown in Figure 3, the higher-performance motor runs on 42 V in the Highlander with a buck dc-dc converter reducing the vehicle's 288 V battery voltage to 42 V. The Camry runs between 27 V and 34 V.

Electronic Power Steering Systems - control, sensing and power aspects of EPS Electronic Design