JTEKT Corporation, working jointly with Toyota Motor Corporation, has developed a new electric power steering system for installation on Toyota’s Lexus LS that represents world-leading steering technology. This system, comprising an electronically controlled variable gear ratio (E-VGR) mechanism integrated in a one-piece structure with a rack direct-drive type electric power system (RD-EPS) having the world’s highest-class output that JTEKT succeeded in developing, contributes significantly to energy conservation and driving safety.

  Demands for improved automobile environmental friendliness and safety have intensified in recent years. Regarding environmental friendliness, while hydraulic power steering derives its power from the vehicle’s engine, electric power steering (EPS) derives its power from an electric motor, enabling vehicles equipped with EPS to achieve three to five percent better fuel efficiency than those using hydraulic power steering. This is bringing about a rapid shift among automakers toward EPS; however, insufficient output and noise problems have hindered its adoption on larger vehicles.

  Regarding the RD-EPS newly developed this time, motor winding efficiency was improved by adopting a highly efficient built-in brushless DC motor and multi-divided teeth. Furthermore, the electronic control unit and DCDC converter was enabled to handle higher voltage, resulting in output 70 percent higher than the conventional product. Noise was reduced 23 percent in comparison with the conventional product despite the increase in output by adopting a super-high-precision ball screw and nut, improving the precision of parts, optimizing clearances by adjusting part configurations, and improving assembly precision.

  In regard to safety, E-VGR is able to change the steering gear ratio in accordance with vehicle speed so that a relatively small amount of steering wheel turning causes the tires to turn much at slow speeds or in parked condition and the tires to turn very little in relation to steering wheel turning at high vehicle speeds, which improves driving stability. The IFS (intelligent front steering) system created by combining this E-VGR with RD-EPS contributes to VDIM systems (TRC, VSC, etc.) *1 and pre-crash safety systems.

  In addition, by integrating the E-VGR mechanism with RD-EPS to form a one-piece structure, the spiral cable has been eliminated, and overall length was shortened 48 percent by making the reduction gear and motor coaxial with the torque sensor, making it easier to package this system in the engine compartment.

  This system received the Automotive Component Award at the fourth Monozukuri Component Awards Ceremony sponsored by the Japanese newspaper Nikkan Kogyo Shimbun and supported by the Ministry of Economy, Trade and Industry and The Japan Chamber of Commerce and Industry.