Ultrasonic sensor, electronic apparatus, and method of driving ultrasonic sensor

A method of driving an ultrasonic sensor which includes a transmission unit that is disposed on a first axis which is inclined with respect to a normal line of a surface of an object sheet, the object sheet traveling along an object feed direction by first and second feed roller pairs, and a reception unit that is provided on a side opposite to the transmission unit of the object sheet, on the first axis, and in which the transmission unit includes a plurality of transmission elements that transmit ultrasonic waves, the ultrasonic waves are transmitted from a transmission surface of the transmission unit, the plurality of transmission elements are arranged in a first direction that intersects the first axis in a plane including the normal line and the first axis, the transmission surface of the transmission unit are substantially parallel to the surface of the object sheet, a reception surface of the reception unit receives the ultrasonic waves, a normal line of the reception surface is substantially the same as the first axis, and the reception surface is non-parallel to the surface of the object sheet, the method comprising:

• detecting a voltage of a received signal that is output when the ultrasonic waves are received by the reception unit by setting m as a natural number greater than or equal to 1, and by delaying a periodic drive voltage of a predetermined cycle by an integer multiple of a 1/m cycle of the periodic drive voltage to input the plurality of transmission elements so that a direction of wave surfaces of the ultrasonic waves transmitted from the transmission unit is aligned with the first axis;
• setting a cycle of the periodic drive voltage, based on the voltage of the received signal that is detected in the detecting the voltage;
• and
• detecting a state of the object sheet, based on the voltage of the received signal that is received by the reception unit, by delaying the periodic drive voltages of the cycle which is set in the setting of the cycle by the integer multiple of the 1/m cycle of the periodic drive voltage to input the plurality of transmission elements and to drive the plurality of transmission elements,
• wherein in the detecting the voltage, the voltage of the received signal for each cycle is detected by changing the cycle of the periodic drive voltage,
• wherein in the setting the cycle, a cycle is selected when the voltage of the received signal reaches a maximum value and the cycle is set as a cycle that is used in the detecting the state,
• a line along the first axis that connects a center of the transmission surface and a center of the reception surface intersects the object sheet at a crossing point,
• the normal line passing through the object sheet at the crossing point is aligned on the transmission surface and is outwardly shifted from a periphery of the reception surface, and
• the ultrasonic sensor is provided between the first feed roller pair and the second feed roller pair along the object feed direction.