Ultrasonic transmitter / receiver

Abstract

To provide an airborne ultrasonic transducer (ultrasonic sensor) with high sound pressure and a wide bandwidth.SOLUTION: When ultrasonic waves are transmitted, an ultrasonic sensor places an elastic body 3 having a loss tangent tanδ, which is the ratio of storage modulus to loss modulus, of 0.001 or more and 0.6 or less in all or part of the area between a part 9 on the bottom surface of a bottomed cylindrical case 4 where the amplitude is maximum and the vibration node 10 on the bottom surface of the bottomed cylindrical case where the amplitude is minimum. This suppresses changes in vibration at the boundary between the two vibrations mentioned above due to the viscous effect of the elastic body, which is subjected to stress according to the deformation speed, and prevents a decrease in the radiation efficiency of sound waves even when the drive frequency and resonant frequency do not match.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an airborne ultrasonic sensor (hereinafter, an airborne ultrasonic transmitter / receiver will be referred to as an ultrasonic sensor) in which a piezoelectric element (2) is attached to a cylindrical case (1) with a bottom. [Background technology]

[0002] 2. Description of the Related Art A safety device in which a range finder unit using an ultrasonic sensor is attached to a vehicle and warns the driver of the risk of a collision when an object approaches the vehicle is widely used. In particular, a safety device that uses a distance meter unit to detect an object behind the vehicle when the vehicle is backing up and notifies the driver of the approaching object is often used (for example, Patent Document 1).

[0003] Specifically, safety devices have been invented that alert the driver with a buzzer when an object is approaching, and brake-linked safety devices that activate the brakes to prevent the vehicle from moving when there is an obstacle in front of the vehicle.

[0004] In the range finder unit using an ultrasonic sensor used here, a waterproof ultrasonic sensor is mainly used. A waterproof ultrasonic sensor is introduced in, for example, Patent Document 2. (In the document, the waterproof ultrasonic sensor is called a waterproof ultrasonic transmitter / receiver.) A waterproof ultrasonic sensor has a piezoelectric element with electrodes on both sides attached to the bottom of a cylindrical case with a bottom, and terminals electrically connected to each electrode of the piezoelectric element are taken out. The top of the piezoelectric element is covered with a sponge-like or felt-like sound-absorbing material, which is then sealed with an elastic filler such as silicone rubber. The back of the open side of the bottomed cylindrical case is completely covered with a filler such as silicone rubber, preventing liquid from seeping inside. Because of this structure, the electrodes of the piezoelectric element inside the ultrasonic sensor do not short-circuit, so it can be used outdoors where it may be exposed to liquid. Furthermore, a waterproof ultrasonic sensor has a piezoelectric element covered by a metal case, and therefore has a relatively strong structure.

[0005] Generally, the drive frequency of an ultrasonic sensor mounted on a vehicle is fixed. Ultrasonic sensors are designed to emit sound waves efficiently when the resonant frequency derived from the shape of the housing matches the externally input drive frequency, but if the difference between the resonant frequency and the drive frequency becomes large due to manufacturing variations in the product, sound waves may not be emitted efficiently and the sound pressure may decrease. If the resonant frequency of an ultrasonic sensor can be made wider, it will be possible to ensure a constant output regardless of product variations. It can also be used with multiple drive frequencies. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent Publication No. 2007-112297 [Patent Document 2] Patent Publication No. 2010-154059 [Patent Document 3] Patent Publication No. 2021-072589 Summary of the Invention [Problem to be solved by the invention]

[0007] Conventional ultrasonic sensors have a resonant frequency that is dependent on the shape of the housing, and if there is a difference between this resonant frequency and the drive frequency, the vibration efficiency decreases, preventing the sound waves from reaching farther, and thus making it impossible to detect objects.There are also ultrasonic sensors that operate at a drive frequency that is different from the resonant frequency by suppressing the vibration efficiency of the sensor (for example, Patent Document 3), but this has the problem of being unable to detect distant objects because the sound pressure is low and the sound waves can only reach a short distance. [Means for solving the problem]

[0008] The resonant frequency of an ultrasonic sensor is generally determined by the shape of the housing. This is because once the shape of the housing is determined, the distance between the vibration nodes is also determined, resulting in a single wavelength and resonant frequency. When the difference between the resonant frequency and the drive frequency is small, the vibration of the housing caused by the resonant frequency and the vibration of the element caused by the drive frequency are synchronized, and the vibration surface takes on a shape that is close to a mountain. Conversely, when the difference between the resonant frequency and the drive frequency is large, the vibration of the housing and the vibration of the element do not synchronize, so the vibration surface does not take on a mountain shape where the element is located, but becomes flat, and the overall shape takes on a shape with a flat peak. In this state, the amplitude of the vibration surface does not increase, so the sound pressure decreases. In the present invention, a unimorph vibrator is formed by bonding a piezoelectric element to the inside of the bottom surface of a cylindrical case with a bottom, and an ultrasonic transmitter / receiver for air that transmits and receives ultrasonic waves by the vibration of the unimorph vibrator is provided. By providing an elastic body having a loss tangent tanδ, which is the ratio of storage modulus to loss modulus, of 0.001 or more and 0.6 or less in the entire region (13) or a portion (14) of the bottomed cylindrical case where the distance (12) from the center of the bottomed cylindrical case to the inner wall is 50% to 90% of the distance (11) from the center of the bottomed cylindrical case, when the driving frequency is higher than the resonant frequency, a large force is applied to the edge of the upper base of the trapezoid, where the amount of change is large, due to the viscous effect of the elastic body, which applies stress according to the deformation speed, and the vibration surface can be made to be a convex shape. Furthermore, when the driving frequency is close to the resonant frequency, the vibration surface is originally a convex shape and the amount of change is small, so the effect of the viscous effect of the elastic body is small. Even in this case, the vibration waveform is similar to that when driven at the resonant frequency, and sound waves are efficiently emitted. This allows the ultrasonic sensor to vibrate efficiently over a wide frequency range. [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a cross-sectional view of an ultrasonic sensor according to an embodiment of the present invention; [Figure 2] Schematic diagram of an ultrasonic sensor of the present invention when driven. [Figure 3] 1 is a perspective view of an embodiment according to claim 1 of the present invention; [Figure 4]1 is a perspective view of an embodiment according to claim 2 of the present invention; [Figure 5] Cross-section of a conventional ultrasonic sensor [Figure 6] Relationship between sound pressure band and loss tangent tanδ of elastic material [Figure 7] Relationship between sound pressure, tanδ, and loss tangent of elastic material [Figure 8] Simulation of vibration displacement when the resonance frequency and driving frequency are the same [Figure 9] Simulation of vibration displacement when the resonance frequency and the drive frequency do not match in the invention of the embodiment related to claim 2 [Figure 10] Simulation of vibration displacement when the resonance frequency and driving frequency do not match in a conventional structure Embodiments relating to claim 1 of the present invention

[0010] Fig. 1 is an example of a structural diagram of an ultrasonic sensor according to an embodiment of the present invention. The ultrasonic sensor is configured such that a piezoelectric element (2) including a PZT ceramic and a folded electrode is bonded to the inner bottom surface of a cylindrical case (4) made of an aluminum alloy, and an elastic body (3) is placed next to the piezoelectric element (2). A molded sponge or foamed silicone (5) is placed on top of the piezoelectric element (2), and lead wires (6a) and (6b) are electrically connected by soldering to the electrodes of the piezoelectric element (2) and pin terminals (7a) and (7b). The open side of the inside of the cylindrical case (4) is sealed by filling it with an elastic body (8) made of silicone resin. As shown in Figure 4, the elastic body (3) is placed in an area (13) where the distance (12) from the center of the bottomed cylindrical case is 50% to 90% of the distance (11) from the center of the bottomed cylindrical case to the inner wall. Figure 5 shows the structure of a conventional ultrasonic sensor. The conventional structure does not have a part corresponding to the elastic body (3), and the other structures are the same. Figure 8 shows the results of a simulation of vibration displacement when the resonant frequency and drive frequency match in the structure of the present invention shown in Figure 1. In Figure 8, the resonant frequency and drive frequency match, and the vibration of the element and the vibration of the housing of the bottomed cylindrical case are synchronized, resulting in a mountain-like shape. Similar vibrations also occur in an ultrasonic sensor with a conventional structure, as shown in Figure 5. In the case of a conventional structure, if the resonant frequency differs from the drive frequency, the vibration of the element and the vibration of the bottomed cylindrical case do not synchronize, resulting in a flat vibration shape above the element, as shown in Figure 10. As a result, the amplitude is small and large sound waves cannot be emitted, and the sensor does not function properly. In contrast, in the structure of the present invention, as shown in Figure 1, if the resonant frequency does not match the drive frequency, the vibration of the element and the vibration of the bottomed cylindrical case do not synchronize, so the top of the element tends to become flat. However, due to the effect of the elastic body arranged around the element, stress is applied according to the deformation speed, suppressing sudden movements on the vibration surface, resulting in the simulation result of vibration displacement shown in Figure 9. Therefore, even in a situation where the resonant frequency does not match the drive frequency, the ultrasonic sensor has a wide bandwidth in which large sound pressure is generated. [Embodiment related to claim 2 of the present invention]

[0011] The elastic body (3) in Figure 1 can be driven in a narrow band by setting a high sound pressure, or in a wide band by setting a low sound pressure, depending on its physical properties. These physical properties are controlled by the loss tangent tanδ, which is the ratio of the storage modulus to the loss modulus. Figure 6 shows experimental values ​​showing the relationship between the loss tangent tanδ and the band in an embodiment related to claim 2 of the present invention, and the dotted line in Figure 6 represents the band when no elastic body is provided. The band here refers to the frequency band where the sound pressure is -3 dB from the maximum sound pressure, based on the frequency where the maximum sound pressure occurs. From these results, it is confirmed that the bandwidth is broadened when an elastic body is placed, and that the bandwidth is broadened when the value of tan δ is large. FIG. 7 shows experimental values ​​showing the relationship between loss tangent tanδ and sound pressure in the embodiment relating to claim 2 of the present invention, and the dotted line in FIG. 7 shows the sound pressure when no elastic body is provided. From this result, it can be seen that the smaller the loss tangent tanδ of the elastic body, the higher the sound pressure and the longer the sound waves can be transmitted. The results of Figures 6 and 7 show that when the loss tangent tanδ of the elastic body is small, the sound pressure is high and the frequency band is wide, and conversely, when the loss tangent tanδ of the elastic body is large, the sound pressure is low and the frequency band is wide. The required sound pressure differs depending on the application, but considering that a sound pressure of about 95 dB is necessary to operate the sensor, it is desirable to keep the loss tangent tanδ of the elastic body to 0.6 or less. [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 in which ultrasonic sensors are used. [Explanation of symbols]

[0013] 1 Inside the bottom of the cylindrical case 2 Piezoelectric element 3 Elastic bodies 4. Cylindrical case with bottom 5. Sponge or foam silicone 6a lead wire 6b lead wire 7a pin terminal 7b pin terminal 8 Elastic body made of non-porous silicone resin 9 The part of the bottom of the cylindrical case where the amplitude is maximum 10 Vibration node on the bottom of the cylindrical case 11 Distance between the center of the bottom of the cylindrical case and the vibration node on the bottom of the cylindrical case 12 Distance from the center of the bottom of a cylindrical case with a bottom 13 The entire area between the part where the amplitude is maximum on the bottom surface of the ultra-bottomed cylindrical case and the vibration node on the bottom surface of the bottomed cylindrical case where the amplitude is minimum 14 A part of the area between the part where the amplitude is maximum on the bottom surface of the ultra-bottomed cylindrical case and the vibration node on the bottom surface of the bottomed cylindrical case where the amplitude is minimum

Claims

1. An aerial ultrasonic transducer includes a piezoelectric element attached to the inside of the bottom of a cylindrical case to form a unimorph vibrator, and transmits and receives ultrasonic waves by the vibration of the unimorph vibrator. An ultrasonic transmitter / receiver for air, characterized in that an elastic body is provided in the entire area (13) or a part of the area (14) where the distance (12) from the center of the bottomed cylindrical case is 50% to 90% of the distance (11) from the center of the bottomed cylindrical case to the inner wall.

2. 2. The aerial ultrasonic transducer according to claim 1, wherein the elastic body has a loss tangent tan δ, which is the ratio of storage elastic modulus to loss elastic modulus, of 0.001 or more and 0.6 or less.

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