Patent ID: 7974754

Claim:
A vehicle steering control system comprising: an input shaft that is connected to a steering wheel so as not to be rotatable relative to the steering wheel; an output shaft that is connected to a steerable-wheel side; a motion conversion mechanism that converts rotary motion of the output shaft into a motion that changes a steering angle of steerable wheels; a variable transmission ratio device operable to rotate the output shaft relative to the input shaft so as to change a transmission ratio as a ratio of the amount of rotary motion of the output shaft to the amount of rotary motion of the input shaft; a locking device that is switched between a lock-on state in which the transmission ratio between the input shaft and the output shaft is inhibited from being changed, and a lock-off state in which the transmission ratio between the input shaft and the output shaft is allowed to be changed; and a control unit that controls a control current that is applied to the locking device so that the locking device is switched between the lock-on state and the lock-off state, wherein the variable transmission ratio device includes an electric motor and a speed reduction mechanism, and the electric motor includes a stator, and a rotor that is driven by rotor driving torque generated from interactions between the stator and the rotor and is rotated relative to the stator; the speed reduction mechanism includes a first ring gear member that rotates with the input shaft, a second ring gear member that rotates with the output shaft, an elliptic rotator that rotates with the rotor, and a band-like external-teeth gear member that is mounted around the rotator and is rotatable relative to the rotator, and the external-teeth gear member meshes with the first ring gear member and the second ring gear member at positions corresponding to the long diameter of an ellipse of the rotator, the number of teeth of the external-teeth gear member being equal to the number of teeth of one of the first ring gear member and the second ring gear member and different from the number of teeth of the other of the first ring gear member and the second ring gear member, the rotator being adapted to be driven by the electric motor and rotated relative to the input shaft so as to change the transmission ratio; the stator is fixed to a vehicle body; and the locking device inhibits the input shaft and the rotor from rotating relative to each other when placed in the lock-on state, and allows the input shaft and the rotor to rotate relative to each other when placed in the lock-off state.