Patent ID: 8531068

Claim:
A rotation sensor in combination with a direct current motor, the combination comprising: an armature coil including at least three phase coils; a commutator having a plurality of commutator segments connected to the armature coil; a pair of brushes in slidable contact with the commutator to supply power to the phase coils through the commutator; a rotation shaft; a ring varistor coaxially fixed to the rotation shaft and including a ring-shaped varistor body and a plurality of electrodes that are circumferentially arranged on a surface of the varistor body; a power source configured to generate an alternating superimposed voltage by superimposing a direct current voltage on an alternating current voltage and apply the alternating superimposed voltage between the pair of brushes; an energization detector configured to detect at least one of a motor current and a path voltage, the motor current flowing to the motor through the pair of brushes, the path voltage being a voltage between predetermined two points on a path in which the motor current flows; and a rotation state detector configured to detect a rotation state of the motor based on an alternating current component of the detected motor current or the detected path voltage, the rotation state including at least one of a rotation angle, a rotation direction, and a rotation speed of the motor, wherein each of the plurality of commutator segments of the commutator is connected to a corresponding one of the plurality of electrodes of the ring varistor, the plurality of electrodes of the ring varistor is paired to form a plurality of electrode pairs, each electrode pair comprising any two of the plurality of electrodes, a first one of the plurality of electrode pairs has a first capacitance therebetween, a second one of the plurality of electrode pairs has a second capacitance therebetween, and the first capacitance is different from the second capacitance.