Ultrasonic sensor
An ultrasonic sensor includes an adhesion sensor detecting adhesion of a foreign substance to a first surface of a bottom of a microphone housing. The adhesion sensor includes: a plurality of variable capacitances defined between a plurality of sensor electrodes disposed on the first surface of the bottom, each of the variable capacitances having a capacitance value which changes in response to adhesion of a foreign substance to the first surface; and an adhesion detection unit configured to measure an individual capacitance value which is a capacitance value of each of the variable capacitances and a total capacitance value which is a total of capacitance values of all the variable capacitances to determine, based on the individual capacitance value and the total capacitance value, whether a foreign substance adheres to the first surface, and, when it is determined that a foreign substance adheres, specify a type of the foreign substance.
1. An ultrasonic sensor comprising:
a microphone that is comprised of a piezoelectric vibration element converting an electrical signal into a vibration and converting a vibration into an electrical signal;
a microphone housing that accommodates the microphone, the microphone housing being formed in a bottomed tubular shape having a bottom constituting a vibration portion to which the microphone is attached and a side wall; and
an adhesion sensor that detects adhesion of a foreign substance to a first surface of the bottom, wherein
the adhesion sensor includes:
a plurality of variable capacitances defined between a plurality of sensor electrodes disposed on the first surface of the bottom, each of the plurality of variable capacitances having a capacitance value which changes in response to adhesion of a foreign substance to the first surface; and
an adhesion detection unit configured to measure an individual capacitance value which is a capacitance value of each of the plurality of variable capacitances and a total capacitance value which is a total of the capacitance values of all the plurality of variable capacitances to determine, based on the individual capacitance value and the total capacitance value, whether a foreign substance adheres to the first surface, and, when it is determined that a foreign substance adheres, specify a type of the foreign substance.
2. The ultrasonic sensor according to claim 1 , wherein the adhesion detection unit is configured to apply a different potential to each of the sensor electrodes that constitute each of the plurality of variable capacitances to thereby supply a sensing current to each of the plurality of variable capacitances for measurement of the individual capacitance value.
3. The ultrasonic sensor according to claim 1 , wherein
the adhesion detection unit is configured to include a plurality of property measurement units having a number corresponding to the plurality of variable capacitances, and
the total capacitance value is measured by simultaneously measuring the individual capacitance values of the plurality of variable capacitances by the plurality of property measurement units.
4. The ultrasonic sensor according to claim 1 , wherein
the adhesion detection unit is configured to include a single property measurement unit, and
the total capacitance value is measured by measuring a capacitance value of each of the plurality of variable capacitances one by one by the property measurement unit and adding up all the capacitance values of the plurality of variable capacitances.
5. The ultrasonic sensor according to claim 1 , wherein the plurality of sensor electrodes are arrayed in a first direction on the first surface.
6. The ultrasonic sensor according to claim 1 , wherein the plurality of sensor electrodes are arrayed in a first direction on the first surface and in a second direction perpendicular to the first direction on the first surface.
7. The ultrasonic sensor according to claim 1 , wherein
the plurality of sensor electrodes include a first sensor electrode and a second sensor electrode each having a straight shape, and
at least three of the sensor electrodes comprised of the first sensor electrode and the second sensor electrode are alternately arranged and laid out in a comb-like pattern.
8. The ultrasonic sensor according to claim 1 , wherein
the plurality of sensor electrodes include a first sensor electrode and a second sensor electrode each having a straight shape, and
the first sensor electrode and the second sensor electrode are alternately arranged at a predetermined angular interval in a circumferential direction on the first surface in a radial layout.
9. The ultrasonic sensor according to claim 1 , wherein
the plurality of sensor electrodes include:
a plurality of first sensor electrodes each having a straight shape, the first sensor electrodes being arranged at an equal interval to form a stripe pattern; and
a plurality of second sensor electrodes each having a straight shape, the second sensor electrodes being arranged at an equal interval to form a stripe pattern in a direction intersecting with the first sensor electrodes, and
the first sensor electrodes are disposed on the second sensor electrodes with an insulating layer interposed therebetween, whereby the first sensor electrodes and the second sensor electrodes are arranged in a mesh pattern.
10. The ultrasonic sensor according to claim 1 , wherein
the side wall of the microphone housing includes a thin wall portion and a thick wall portion having a thickness larger than that of the thin wall portion, and
the plurality of sensor electrodes are disposed only at positions corresponding to the thick wall portion of the side wall.
11. The ultrasonic sensor according to claim 1 , wherein
the microphone housing is made of a conductor and connected to a ground potential point, and
the microphone housing constitutes a part of the plurality of sensor electrodes.
12. The ultrasonic sensor according to claim 11 , wherein a guard electrode is provided between the bottom of the microphone housing and a particular sensor electrode-disposed on the bottom among the plurality of sensor electrodes, the guard electrode being set to a same potential as that of the particular sensor electrode disposed on the bottom among the plurality of sensor electrodes.