Electromagnetic servo device for electric power steering systems for vehicles

Claim 1

In an electromagnetic servo device (200) for an electric power steering system for vehicles having a steering wheel and a steered wheel, said servo device (200) including: an input shaft (1) adapted to be connected to the steering wheel; an output shaft (4) adapted to be operatively interconnected with the steered wheel; an electric motor (18) for providing said output shaft (4) with auxiliary torque; torque detecting means (11) for detecting steering torque (Ti) acting on said input shaft (1); vehicle speed detecting means (50) for detecting the vehicle speed; a driving control circuit (100) for generating a torque magnitude signal (Sa) and a torque direction signal (Sdr, Sdl), based on an output signal (VR, VL) from said torque detecting means (11) and an output signal (Vf) from said vehicle speed detecting means (50), to feed said electric motor (18) with an armature current (Io) of such a magnitude and in such a direction as desirous in accordance with both said torque magnitude signal (Sa) and said torque direction signal (Sdr, Sdl); and a dead zone (2K 1 or Dl, Dm, Dh) set for said torque magnitude signal (Sa), an improvement comprising: said dead zone (2K 1 or Dl, Dm, Dh) of said torque magnitude signal (Sa) having a band width thereof adapted to be widened in accordance with increase of the vehicle speed, in accordance with said output signal (Vf) of said vehicle speed detecting means (50).