Patent ID: 7711463

Claim:
An electric power steering system including a torque control system for generating a steering assist torque based on a detected steering torque, the system further comprising a filter portion for improving a phase characteristic or a gain characteristic of the torque control system, wherein the filter portion is constituted as a multiple-step filter including a first filter having a transfer function G 1 (s) for suppressing resonance and a second filter having a transfer function G 2 (s), wherein the transfer function G 1 (s) is represented by the following expression: G 1 ( s )=( s 2 +2ζ 11 ω 1 +ω 1 2 )/( s 2 +2ζ 12 ω 1 +ω 1 2 ), where s is a Laplace operator, ζ 11 is a damping coefficient, ζ 12 is a damping coefficient, and ω 1 is an angular frequency, wherein the transfer function G 2 (s) is represented by the following expression: G 2 ( s )=( s 2 +2ζ 21 ω 2 +ω 2 2 )/( s 2 +2ζ 22 ω 2 +ω 2 2 ), where s is a Laplace operator, ζ 21 damping coefficient, ζ 22 is a damping coefficient, and ω 2 is an angular frequency, and wherein the damping coefficients ζ 21 , ζ 22 satisfy the following expression: ζ 21 ≧ζ 22 ≧1.