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 a steering speed (Ns) 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 (Va) in accordance with said output signals (S 1 , S 2 , S 3 , S 4 ), wherein: said drive control means (30, 40) comprises: first determining means (125; 232) for determining a first provisional component (D(Ts)) of said motor drive signal (Va) in dependence on said output signal (S 1 , S 2 ) from said steering torque detecting means (32), said first provisional component (D(Ts)) being preset for the positive steering state of said steering system (50); second determining means (126; 230) for determining a second provisional component (D'(Ts)) of said motor drive signal (Va) in dependence on said output signal (S 1 , S 2 ) from said steering torque detecting means (32), said second provisional component (D'(Ts)) being preset for the steering wheel returning state of said steering system (50); third determining means (111) for determining a component (D(K·Nm)) of said motor drive signal (Va) that corresponds to said steering speed (Ns) of said steering wheel, depending on said output signal (S 3 , S 4 ) from said steering speed detecting means (36); steering wheel returning state detecting means (119, 124; 222, 223, 227, 228) for judging in which of the positive steering state and the steering wheel returning state said steering system (50) is put, said steering wheel returning state detecting means (119, 124; 222, 223, 227, 228) selecting from between said first provisional component (D(Ts)) and said second provisional component (D'(Ts)) that one which corresponds to the result of the judgment; and means (130, 140) for adding thus selected one of said provisional components (D(Ts)/D'(Ts)) to 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).