Ultrasonic sensor
An ultrasonic sensor includes an adhesion sensor including a variable capacitance and a fixed capacitance. The variable capacitance includes a sensor electrode provided on a first surface of a bottom having a microphone installed thereon and a capacitance value thereof changes in accordance with adhesion of foreign matter on the first surface. The fixed capacitance includes a first electrode connected to the sensor electrode, and a second electrode provided on a member different from a microphone housing and disposed facing the first electrode. The adhesion sensor further includes an adhesion detecting section that is connected to the sensor electrode via the fixed capacitance and detects adhesion of foreign matter to the first surface by supplying an alternating-current signal to the fixed capacitance and the variable capacitance.
1. An ultrasonic sensor comprising:
a microphone that is formed of a piezoelectric vibration element, the piezoelectric vibration element converting an electrical signal into vibration and converting vibration into an electrical signal;
a microphone housing that has a bottomed tubular shape and includes a bottom including a first surface and a side wall, and houses the microphone, the bottom constituting a vibration portion having the microphone installed thereon; and
an adhesion sensor that detects adhesion of foreign matter to the first surface, wherein
the adhesion sensor includes:
a variable capacitance that includes a sensor electrode provided on the first surface and whose capacitance value changes in accordance with adhesion of foreign matter on the first surface;
a fixed capacitance that includes a first electrode connected to the sensor electrode, and a second electrode provided on a member different from the microphone housing and disposed facing the first electrode; and
an adhesion detecting section that is connected to the sensor electrode via the fixed capacitance, and detects adhesion of foreign matter to the first surface by supplying an alternating-current signal to the fixed capacitance and the variable capacitance.
2. The ultrasonic sensor according to claim 1 , further comprising:
a circuit board that is provided with the adhesion detecting section;
a sensor case that includes:
a tubular portion that houses the microphone housing having the microphone installed therein, and
a housing portion including a hollow in which the circuit board is housed,
an interior of the tubular portion being in communication with the hollow of the housing portion; and
a cushion that is made from an elastic material and disposed between the microphone housing and the tubular portion, and serves as a buffer member that suppresses transmission of the vibration, wherein
the first electrode is disposed between the microphone housing and the cushion, and
the second electrode is disposed facing the first electrode, with the cushion interposed therebetween.
3. The ultrasonic sensor according to claim 1 , further comprising:
a circuit board that is provided with the adhesion detecting section;
a sensor case that includes:
a tubular portion that houses the microphone housing having the microphone installed therein, and
a housing portion including a hollow in which the circuit board is housed,
an interior of the tubular portion being in communication with the hollow of the housing portion; and
a cushion that is made from an elastic material and disposed between the microphone housing and the tubular portion, and serves as a buffer member that suppresses transmission of the vibration, wherein
the first electrode is disposed on the first surface of the bottom, and
the cushion further includes a distal end surface, the distal end surface being on a side of the bottom of the microphone housing, and the second electrode is disposed on the distal end surface of the cushion.
4. The ultrasonic sensor according to claim 1 , comprising:
a circuit board that is provided with the adhesion detecting section;
a sensor case that includes:
a tubular portion that houses the microphone housing having the microphone installed therein, and
a housing portion that includes a hollow in which the circuit board is housed,
an interior of the tubular portion being in communication with the hollow of the housing portion; and
a cushion that is made from an elastic material and disposed between the microphone housing and the tubular portion, and serves as a buffer member that suppresses transmission of the vibration, wherein
the first electrode and the second electrode are disposed separately from each other between the microphone housing and the cushion.
5. The ultrasonic sensor according to claim 1 , wherein
the fixed capacitance includes, in addition to a first fixed capacitance formed by the first electrode and the second electrode, a second fixed capacitance formed by a third electrode and a fourth electrode disposed facing the third electrode.
6. The ultrasonic sensor according to claim 1 , comprising:
a circuit board that is provided with the adhesion detecting section;
a sensor case that includes:
a tubular portion that houses the microphone housing having the microphone installed therein, and
a housing portion including a hollow in which the circuit board is housed,
an interior of the tubular portion being in communication with the hollow of the housing portion; and
a cushion that is made from an elastic material and disposed between the microphone housing and the tubular portion, and serves as a buffer member that suppresses transmission of the vibration, wherein
the fixed capacitance includes, in addition to a first fixed capacitance formed by the first electrode and the second electrode, a second fixed capacitance formed by a third electrode and a fourth electrode disposed facing the third electrode,
the third electrode is connected to the second electrode and disposed between the cushion and the tubular portion, and
the fourth electrode is disposed on the sensor case.
7. The ultrasonic sensor according to claim 1 , wherein
the side wall of the microphone housing includes a thin portion and a thick portion thicker than the thin portion, and
the first electrode is disposed at a position corresponding to the thick portion of the microphone housing.
8. The ultrasonic sensor according to claim 1 , wherein
the second electrode is provided on an auxiliary component for mounting the microphone and the microphone housing on an object.