Rotation detecting apparatus and rotation detecting system

A rotation detecting apparatus for detecting a rotational state of a direct-current motor that is rotatable by a power supplied from a direct-current power source, wherein the direct-current motor has at least one pair of brushes, to which a direct-current voltage is applied from the direct-current power source, wherein an impedance between the at least one pair of brushes is changeable periodically in accordance with rotation of the direct-current motor, the rotation detecting apparatus comprising:

• a driving device provided in an energization route in a circuit having the direct-current power source and the direct-current motor, wherein the driving device has at least one switching device that is configured to open and close the energization route to control energization therethrough;
• a control device that is configured to turn on and off the at least one switching device of the driving device;
• an energization detecting device that detects an electrical quantity of electric current that is supplied to the direct-current motor;
• an alternating-current component detecting device that is configured to detect change of an alternating current component of the electric current that is supplied to the direct-current motor based on the electrical quantity detected by the energization detecting device, wherein the change of the alternating-current component is caused by change of the impedance caused in accordance with the rotation of the direct-current motor;
• and
• a rotational state detecting device that is configured to detect at least one of a rotation angle, a rotational direction, and a rotational speed of the direct-current motor based on a detection result of the alternating-current component detecting device;
• wherein
• the control device executes a PWM control for turning on and off the at least one switching device of the driving device with a predetermined frequency and duty ratio in order to control the rotation of the direct-current motor;
• the PWM control executed by the control device causes fluctuation of the electric current supplied to the direct-current motor;
• the fluctuation of the electric current corresponds to the alternating-current component;
• the direct-current motor is configured to form a certain state during a capacitance element connection period in one rotation of the direct-current motor;
• in the certain state, a capacitance element having a predetermined capacitance value is formed between the at least one pair of brushes;
• a charge electric current is supplied to the capacitance element at timing of opening the energization route from the direct-current power source to the direct-current motor in the PWM control during the capacitance element connection period;
• a discharge electric current is discharged from the capacitance element at timing of closing the energization route in the PWM control during the capacitance element connection period;
• and
• at least one of the charge electric current and the discharge electric current corresponds to the alternating-current component.