Ultrasonic sensor

1. An ultrasonic sensor comprising:

a first electrode that is provided in an ultrasonic microphone that includes a vibration element that provides a function for converting between mechanical vibrations and electrical signals;

a second electrode that is externally from the ultrasonic microphone such that an electrical characteristic between the second electrode and the first electrode changes based on an adhesion state of substances adhering to the ultrasonic microphone; and

a third electrode that is provided so as to suppress changes in the electrical characteristic between the first electrode and the second electrode caused by factors other than the adhesion of substances adhering to the ultrasonic microphone.

2. The ultrasonic sensor according to claim 1 , wherein:

the third electrode is provided so as to be interposed between the first electrode and the second electrode.

3. The ultrasonic sensor according to claim 1 , wherein:

the third electrode is provided so as to have a same potential as the first electrode or the second electrode.

4. The ultrasonic sensor according to claim 1 , wherein:

the second electrode or the third electric is provided in an auxiliary component that is a component that is used to mount the ultrasonic sensor to a mounting subject.

5. The ultrasonic sensor according to claim 1 , wherein:

the third electrode is laminated with the first electrode or the second electrode via an insulating layer.

6. The ultrasonic sensor according to claim 1 , wherein:

the ultrasonic microphone includes

microphone case that includes a diaphragm portion that is formed into a thin sheet so as to ultrasonically vibrate, and

diaphragm supporting portion that is formed into a cylindrical shape in which a directivity axis along a vibration direction of the diaphragm portion is a center axis and is provided so as to support the diaphragm portion in an outer edge portion of the diaphragm portion; and

the vibration element is supported by the diaphragm portion in a fixed manner.

7. The ultrasonic sensor according to claim 6 , wherein:

the microphone case is formed by a conductive material; and

the first electrode is integrated with the microphone case.

8. The ultrasonic sensor according to claim 6 , wherein:

the third electrode is provided so as to suppress the changes in the electrical characteristic between the first electrode and the second electrode that are caused by factors other than the adhesion of substances adhering to a detection surface that is an outer surface of the diaphragm portion and is exposed towards an outer side of a mounting subject in a state in which the ultrasonic sensor is mounted to an exterior shell of the mounting subject; and

the adhesion of substances adhering to the detection surface can be detected based on the electrical characteristic between the first electrode and the second electrode.

9. The ultrasonic sensor according to claim 8 , wherein:

the second electrode is provided along a back surface of the exterior shell.

10. The ultrasonic sensor according to claim 8 , wherein:

the third electrode is provided along the diaphragm supporting portion.

11. The ultrasonic sensor according to claim 8 , wherein:

the adhesion of substances adhering to the detection surface can be detected based on the electrical characteristic between the first electrode and the second electrode and an electrical characteristic between the second electrode and the third electrode.

12. The ultrasonic sensor according to claim 6 , wherein:

the diaphragm portion is formed in a shape that has a long direction and a short direction in a plane that is orthogonal to the directivity axis; and

a conductor portion that configures a power supply path to the second electrode or the third electrode is provided in a thick portion that is a portion along the long direction of the diaphragm supporting portion.

13. The ultrasonic sensor according to claim 1 , wherein:

the electrical characteristic is an electrostatic capacitance.