Patent ID: 8521368

Claim:
An electric power steering device for generating a steering assist force by a motor, comprising: a battery for supplying the motor with electric power; an auxiliary power source that supplies the motor with electric power; and a control circuit that controls a power supply to the motor, wherein the control circuit adjusts the power supply to the motor from the auxiliary power source in accordance with an amount of energy remaining in the auxiliary power source so as to reduce the steering assist force in the event of a breakdown of the battery; calculates an amount of initial charged energy of the auxiliary power source based on a voltage of the auxiliary power source at a time when a backup is initiated by the auxiliary power source; calculates the amount of remaining energy by subtracting an amount of consumed energy of the auxiliary power source from the amount of initial charged energy; and multiplies a reference assist value, that is acquired when the battery is unbroken, by a gain that is determined by dividing the amount of remaining energy by the amount of initial charged energy, to reduce the steering assist force.