Ultrasonic transceiver

Abstract

This system provides a lower-cost distance measurement system that suppresses the decrease in sound pressure of ultrasonic sensors while stabilizing the reverberation characteristics against temperature changes. [Solution] The ultrasonic sensor is installed on the opposite side of the bonding surface between the bottomed cylindrical case 1 and the piezoelectric element 2. The area of ​​the polarized electrode 4 of the piezoelectric element is set to be in the range of 30% to 70% of the area of ​​the piezoelectric ceramic. The area of ​​the polarized electrode of the piezoelectric element installed on the bonding surface between the bottomed cylindrical case and the piezoelectric element is set to be equal to or greater than the area of ​​the aforementioned polarized electrode. This suppresses the decrease in sound pressure of the ultrasonic sensor while reducing capacitance and stabilizing the reverberation characteristics with respect to temperature changes.

Description

Technical Field

[0001] The present invention relates to an ultrasonic sensor for air in which a piezoelectric element is bonded to a bottomed cylindrical case.

Background Art

[0002] A safety device that attaches a distance measuring unit using an ultrasonic sensor to a vehicle and notifies a driver of a collision risk when an object approaches the vehicle is widely used. In particular, a safety device that detects an object behind when the vehicle is backing up with a distance measuring unit and notifies the driver of the approach of the object is often used. (For example, Patent Document 1)

[0003] Specifically, safety devices that notify the driver of the approach of an object with a buzzer and brake interlocking type safety devices that operate the brake so that the vehicle does not start when there is an obstacle in front of the vehicle have been invented.

[0004] [[ID=二十二]] In the distance measuring unit using an ultrasonic sensor used here, mainly a drip-proof ultrasonic sensor is used. The drip-proof ultrasonic sensor is introduced in, for example, Patent Document 2 (in the document, the drip-proof ultrasonic sensor is called a drip-proof ultrasonic transmitter / receiver). The drip-proof ultrasonic sensor has a piezoelectric element with electrodes on both sides adhered to the bottom surface of a bottomed cylindrical case, and terminals electrically connected to each electrode of the piezoelectric element are taken out to the outside. A sponge-like or felt-like sound absorbing material is placed on the upper part of the piezoelectric element, and then it is sealed with an elastic filler such as silicone rubber. There are also structures in which a foamed material is used instead of the sponge-like or felt-like sound absorbing material on the upper part of the element (for example, Patent Document 3). In addition, the opening on the side surface of the bottomed cylindrical case and the back surface on the opening side of the bottomed cylindrical case are completely covered with a filler such as silicone rubber so that liquid does not penetrate inside. Due to the above-described structure, the electrodes of the piezoelectric element do not short-circuit inside the ultrasonic sensor, so it can be used outdoors even when liquid splashes. Furthermore, because the piezoelectric element of a splash-proof ultrasonic sensor is covered by a metal case, it is an ultrasonic sensor with a relatively high-strength structure.

[0005] In ultrasonic sensors mounted on vehicles, the electrodes of the piezoelectric element are generally installed across the entire surface of the piezoelectric element parallel to the vibration plane. This is because areas where polarized electrodes are not installed do not function as piezoelectric materials, thus impairing vibration efficiency. If it is not possible to install electrodes across the entire surface due to the manufacturing process, the product is manufactured with the area where electrodes cannot be installed minimized as much as possible. Furthermore, in cases where one side of the piezoelectric element is obstructed by a wall or other obstacle and cannot make contact with the wire due to the structure of the ultrasonic sensor, an element with a folded electrode may be used (for example, Patent Document 4), but even in this case, the area where electrodes cannot be installed is minimized. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Japanese Patent Publication No. 2007-112297 [Patent Document 2] Japanese Patent Publication No. 2009-112248 [Patent Document 3] Japanese Patent Publication No. 2021-072589 [Patent Document 4] Japanese Patent Publication No. 2014-230109 [Overview of the project] [Problems that the invention aims to solve]

[0007] In conventional distance measuring units using ultrasonic sensors, it is crucial to shorten and stabilize the reverberation characteristics of the ultrasonic sensor in response to temperature changes in the external environment in order to reliably detect objects at close range. In this regard, a method is used to minimize reverberation by adjusting the inductance of a variable step-up transformer installed in the ultrasonic sensor's drive circuit and matching the capacitance of the ultrasonic sensor with the drive circuit. However, this method is costly, so in recent years, methods have been adopted in which the inductance of the step-up transformer is fixed or in which the ultrasonic sensor is driven without using a transformer. In this case, variations in the capacitance of the ultrasonic sensor due to temperature changes affect the deterioration of the reverberation characteristics. One possible solution is to reduce the capacitance of the ultrasonic sensor. The relationship between the electrode distance, electrode area, and capacitance in a piezoelectric element is expressed as C = ε·S / D. Here, C is capacitance, ε is dielectric constant, S is electrode area, and D is the distance between electrodes, i.e., the thickness of the piezoelectric element. To reduce capacitance, one can either reduce the electrode area of ​​the piezoelectric element or increase its thickness. However, in that case, the sound pressure and sensitivity of the ultrasonic sensor will decrease. Therefore, although lowering the capacitance is beneficial for stabilizing the system's reverberation characteristics, the area of ​​the piezoelectric ceramic corresponding to the electrode area is also small, which presents a problem in that it is difficult to increase the output power of the ultrasonic sensor. [Means for solving the problem]

[0008] In an airborne ultrasonic transducer that transmits and receives ultrasonic waves by vibrating a unimorph transducer, which is formed by bonding piezoelectric elements to the inside of the bottom surface of a bottomed cylindrical case, the capacitance of the piezoelectric element is determined by the distance between electrodes, the area of ​​the electrodes, and the dielectric constant, as described above. Therefore, if the distance between electrodes is the same, it does not depend on the size of the piezoelectric ceramic sandwiched between them. When the area of ​​the piezoelectric ceramic sandwiched between the electrodes is set to 100% and the area of ​​the polarized electrode is set to 70% or less, leaving a margin, it was confirmed that the capacitance of the ultrasonic sensor remains the same, but the amplitude of the piezoelectric element increases, resulting in higher output, compared to when both the area of ​​the piezoelectric ceramic and the area of ​​the polarized electrode are the same at 70%. This is because, in ultrasonic sensors, one side of the piezoelectric element is fixed to a bottomed cylindrical case. When the ends of the piezoelectric ceramic and the polarized electrode are in the same position, stress concentration occurs at the end of the piezoelectric element, resulting in energy loss. However, when there is a margin, the ends of the piezoelectric ceramic and the polarized electrode are different, so the stress is dispersed, reducing energy loss. This increases the displacement of the unimorph oscillator for the same power, thus suppressing the decrease in output. Here, if the displacement loss of the unimorph transducer is kept within -3dB at a level that does not impair the practicality of the ultrasonic sensor, as shown in Figure 5, this effect is achieved when the polarization electrode area ratio to the piezoelectric ceramic is between 30% and 70%, assuming the size of the piezoelectric ceramic is 100%. [Effects of the Invention]

[0009] This invention makes it possible to provide a lower-cost distance measurement system that suppresses the decrease in sound pressure of an ultrasonic sensor while stabilizing the reverberation characteristics against temperature changes. [Brief explanation of the drawing]

[0010] [Figure 1] Schematic diagram of an ultrasonic sensor according to claim 1 of the present invention. [Figure 2] Structural diagram of an ultrasonic sensor according to an embodiment of claim 1 of the present invention. [Figure 3] Structural diagram of a conventional ultrasonic sensor [Figure 4] The polarization electrode area ratio S to the area of ​​the piezoelectric ceramic in an embodiment of the present invention and the vibration level of the ultrasonic sensor. [Figure 5]The polarization electrode area ratio S to the area of ​​the piezoelectric ceramic and the sound pressure level of the ultrasonic sensor in an embodiment of the present invention. [Figure 6] Polarized electrode area ratio S to area of ​​piezoelectric ceramics and reverberation temperature characteristics of ultrasonic sensor in embodiments of the present invention [Figure 7] Vibration mode of the bottom surface of a closed cylindrical case in an ultrasonic sensor according to an embodiment of the present invention [Figure 8] The vibration modes of the bottom surface of a closed-bottom cylindrical case when the area of ​​the piezoelectric ceramic is the same as the polarized electrode area in Figure 6, and the polarized electrode area is also the same. [Modes for carrying out the invention] Embodiment according to claim 1 of the present invention

[0011] Figure 1 shows a bottomed cylindrical case (1) and a piezoelectric element (2) of an ultrasonic sensor according to claim 1 of the present invention. With respect to the polarized electrode (4) of the piezoelectric element (2), its area ratio is in the range of 30% to 70% of the area of ​​the piezoelectric ceramic (3), and the area of ​​the polarized electrode (5) on the bonding surface between the bottomed cylindrical case (1) and the piezoelectric element (2) is set to be larger than the area of ​​the aforementioned polarized electrode (4). Figure 2 is an example of a structural diagram of an ultrasonic sensor according to an embodiment of claim 1 of the present invention. The ultrasonic sensor is configured such that a piezoelectric element (2) including piezoelectric ceramic (3), a polarized electrode (4), and a polarized folded electrode (5) is bonded to the inner bottom surface of a bottomed cylindrical case (1) made of aluminum alloy, and a molded sponge or foamed silicone (6) is placed on top of the piezoelectric element (2). The lead wires (7a) and (7b) are electrically connected by soldering to the electrodes of the piezoelectric element (2) and the terminals of the pin terminals (8a) and (8b), and the opening inside the bottomed cylindrical case (1) is sealed by filling it with an elastic body (9) made of silicone resin. Alternatively, a normal polarized electrode may be used instead of the polarized folded electrode (5), and the Gnd-side lead wires and terminals may be connected directly to the bottomed cylindrical case (1). Figure 3 is a structural diagram of a conventional ultrasonic sensor. In the conventional structure, the areas of the polarized electrodes (4) and (5) of the piezoelectric element are equal, and other structures are the same. Figure 4 is a graph showing the area ratio S of the polarized electrode (4) to the area of the piezoelectric ceramics (3) and the bottom surface vibration level of the bottomed cylindrical case (1) of the ultrasonic sensor according to an embodiment of the present invention. Figure 5 is also a graph showing the relationship between the area ratio S of the polarized electrode (4) to the area of the piezoelectric ceramics (3) and the sound pressure level of the ultrasonic sensor according to an embodiment of the present invention. When the allowable range of the sound pressure level is set to -3 dB from the value of 100% of the area ratio S, the optimal value is taken when the value of S is from 30% to 100%. When the area ratio S is 100%, it corresponds to the characteristic value of the conventional ultrasonic sensor. Figure 6 is a graph showing the area ratio S of the polarized electrode (4) to the area of the piezoelectric ceramics (3) and the reverberation characteristics of the ultrasonic sensor with respect to temperature change in the ultrasonic sensor according to an embodiment of the present invention. At this time, the boost transformer of the circuit for driving the sensor has a fixed inductance value. From this graph, it can be said that the change amount of the reverberation temperature characteristics is small, and the good range is when the value of S is 70% or less. Figure 7 is an analysis of the vibration mode of the bottom surface of the bottomed cylindrical case in the ultrasonic sensor according to an embodiment of the present invention by simulation. Also, Figure 8 is an analysis of the vibration mode of the bottom surface of the bottomed cylindrical case when the area of the piezoelectric ceramics (3) is the same as the polarized electrode area (4) in Figure 6 and the area of the polarized electrode (4) is also the same as that by simulation. From the comparison between Figure 7 and Figure 8, it can be confirmed that if the area of the polarized electrode (4) is the same, the bottom surface vibration level of the bottomed cylindrical case (1) becomes larger and the output is increased when the area of the piezoelectric ceramics is larger than that. As described above, in the present invention, by setting the area ratio S of the polarized electrode (4) to the area of the piezoelectric ceramic (3) to be from 30% to 70%, it is possible to minimize the sound pressure drop of the ultrasonic sensor and improve the temperature characteristics of the reverberation. Therefore, it becomes possible to provide a distance measuring unit with low cost and stable detection performance.

Industrial Applicability

[0012] The present invention can be applied not only to back sensors and corner sensors for vehicles and automatic parking systems, but also to various fields where ultrasonic sensors are used.

Explanation of Reference Numerals

[0013] 1 Bottomed cylindrical case 2 Piezoelectric element 3 Piezoelectric ceramic 4 Polarized electrode 5 Polarized folded electrode 6 Sponge or foamed silicone 7a Lead wire 7b Lead wire 8a Pin terminal 8b Pin terminal 9 Elastic body made of silicone resin

Claims

[Claim 1] An ultrasonic transducer for air use that transmits and receives ultrasonic waves by vibration of a unimorph transducer formed by bonding a piezoelectric element to the inside of the bottom surface of a bottomed cylindrical case, characterized in that the polarized electrode area of ​​the piezoelectric element installed on the opposite side of the bonding surface between the bottomed cylindrical case and the piezoelectric element is in the range of 30% to 70% of the area of ​​the piezoelectric ceramic, and the polarized electrode area of ​​the piezoelectric element installed on the bonding surface between the bottomed cylindrical case and the piezoelectric element is larger than the polarized electrode area of ​​the piezoelectric element installed on the opposite side of the bonding surface between the bottomed cylindrical case and the piezoelectric element.

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