Electric power steering system for vehicles

Claim 1

An electric power steering system (50) for vehicles having an electromagnetic servo device (1) including an input shaft (5) operatively connected to a steering wheel, an output shaft (6) operatively connected to a steered wheel, an electric motor (14) for operatively supplying auxiliary torque to said output shaft (6), steering torque detecting means (32) for detecting steering torque (Ts) acting on said input shaft (5), steering speed detecting means (36) for detecting the steering speed of said steering wheel, and drive control means (30, 40) which receives an output signal (S 1 , S 2 ) from said steering torque detecting means (32) and an output signal (S 3 , S 4 ) from said steering speed detecting means (36) and feeds said electric motor (14) with a motor drive signal in accordance with said output signals (S 1 , S 2 , S 3 , S 4 ), wherein: said drive control means (30, 40) comprises: means (109, 110; 200) for determining a component (D(L)) of said motor drive signal (Va) that corresponds to the road surface side load and a component (D(F)) of said motor drive signal (Va) that corresponds to the load due to friction elements of said electromagnetic servo device (1), depending on said output signal (S 1 , S 2 ) from said steering torque detecting means (32); means (121) for determining a component (D(K·Nm)) of said motor drive signal (Va) that corresponds to the steering speed, depending on said output signal (S 3 , S 4 ) from said steering speed detecting means (36); and means (111, 124, 40; 200, 124, 40) for summing said road surface side load component (D(L)), said frictional load component (D(F)), and said steering speed component (D(K·Nm)) to thereby determine the magnitude of said motor drive signal (Va) and outputting said motor drive signal (Va) to said electric motor (14).