JP5600431B2 - Obstacle ultrasonic detection device - Google Patents

Description

  The present invention relates to a device that detects an obstacle by ultrasonic waves reflected from the obstacle.

  In recent years, small ultrasonic sensors have attracted attention for obstacle detection and crime prevention intruder detection sensors for passenger cars and autonomous mobile robots, or behavior assistance devices for visually impaired persons. Since ultrasonic waves have a slower propagation speed than light, distance measurement is easy. By arraying ultrasonic sensors, three-dimensional measurement of an object is possible.

  In order to detect an obstacle in a three-dimensional space, it is necessary to measure the distance to the reflection point at each azimuth and elevation using an ultrasonic sensor as a phased array. In order to obtain angle information without using a mechanical movable part, it is necessary to electronically scan the angle by delay-adding the received waveform. At this time, it is assumed that the frequency of the output signal of each element of the array matches.

  Patent Document 1 (Japanese Patent Laid-Open No. 2005-039720) describes that an ultrasonic sensor is manufactured using a thin-film hollow piezoelectric body. Patent Document 2 (Japanese Patent Laid-Open No. 2005-167820) describes that a phased array is produced by combining a large number of ultrasonic sensors and applied to three-dimensional measurement.

  According to Non-Patent Document 1, only one surface side of the Z plate is subjected to polarization inversion by proton exchange of one surface of the Z plate of lithium tantalate single crystal and then heat treatment, and the other surface side Is left unpolarized. Further, such a Z plate is vibrated in thickness, and the resonance response of thickness vibration is measured. This indicates that a lithium tantalate Z plate is used as the piezoelectric resonator.

  Patent Document 3 (Japanese Patent Laid-Open No. 2007-228320) describes that a ferroelectric single crystal Z-plate can be used as a resonator or an oscillator by vibrating the thickness.

JP2005-039720 JP2005-167820 JP2007-228320

The Institute of Electronics, Information and Communication Engineers, Ultrasound Study Group, US87-37 (1987) pp. 17-22 “Polarization inversion of LiTaO3 plate using proton exchange”

  In general, an ultrasonic wave emitted from an ultrasonic wave transmission source propagates in the air, is reflected by an obstacle, and enters an ultrasonic sensor array. Since noise sound also exists in the air, it is necessary to provide a threshold value for the detection signal in order to separate it. When the detection signal is larger than the threshold value, it is determined that an obstacle exists, and when the detection signal does not exceed the threshold value, it is determined that there is no obstacle.

  For example, in an in-vehicle sensor, a walking assist sensor, and a robot sensor, an obstacle must be detected with certainty. If detection of an obstacle fails or if a detection signal is issued when no obstacle exists, the possibility of causing an accident cannot be denied.

  However, in the ultrasonic sensor array as described in Patent Document 2, there is a tendency that the accuracy rate decreases for a small object or an object far from the sensor. That is, there are many cases where an obstacle cannot be detected even though the obstacle actually exists. In order to prevent this inability to detect, the present inventor has studied to improve the sensitivity by reducing the threshold value at the time of determination. However, in practice, if the threshold value is decreased, the influence of noise noise increases, and there is an increasing number of signals indicating that an obstacle has been detected even though no obstacle exists.

  In Non-Patent Document 1, a lithium tantalate thin plate whose polarization has been inverted is vibrated in thickness to operate as a filter. However, even if such a piezoelectric vibrator is arranged to produce a phase array, the ultrasonic wave reception sensitivity is extremely low, and reliable detection of an obstacle cannot be considered.

  The piezoelectric thin film device described in Patent Document 3 is intended for an FBAR such as an oscillator, a trap, a filter, and a duplexer. For this reason, the piezoelectric thin film device is resonated using the thickness vibration of the piezoelectric single crystal thin film. For this reason, even if such piezoelectric vibrators are arranged side by side to produce a phase array, the ultrasonic wave reception sensitivity is extremely low, and use as an ultrasonic sensor is not considered.

  An object of the present invention is to make it possible to reliably detect an obstacle in an ultrasonic sensor array device that detects an obstacle by ultrasonic waves reflected from the obstacle.

The present invention is a device for detecting an obstacle by ultrasonic waves reflected from the obstacle,
Support substrate,
A ferroelectric single crystal plate including a plurality of flexural vibration parts that receive and vibrate ultrasonic waves;
Legs that are bonded to the back side of the support substrate and the ferroelectric single crystal plate, and form a plurality of gaps between the support substrate and the ferroelectric single crystal plate,
A plurality of first electrodes formed on the back surface side of the ferroelectric single crystal plate, the first electrodes respectively provided on the flexural vibration part, and formed on the front surface side of the ferroelectric single crystal plate A plurality of second electrodes that are formed on the flexural vibration portion corresponding to the first electrode, and the ferroelectric single crystal plate is provided on the surface side. Are divided into an upper part and a lower part on the back side, the polarization direction of the upper part is opposite to the polarization direction of the lower part, and a flexural vibration part is formed on each gap, and reflection from an obstacle Obtaining the distance and angle to the obstacle based on each electric signal excited between each first electrode and each second electrode by each bending vibration of each bending vibration part by the ultrasonic wave Features.

  The present inventor has examined the reason why the accuracy of detecting an obstacle has decreased for an ultrasonic sensor as described in Patent Document 2. As a result, since the piezoelectric film was formed by the sol-gel method, it was found that the film thickness and density of each piezoelectric film had fine unevenness. Since there was a description that such unevenness was solved in Patent Documents 1 and 2, it was an unexpected result. Such fine film thickness and uneven film quality cause a slight shift in the resonance frequency of each piezoelectric film, resulting in attenuation of the ultrasonic wave of the wavelength carrying the obstacle position information and an increase in the noise level. It is thought that.

  Based on this hypothesis, the present inventor has a structure in which the piezoelectric single crystal thin plate is resonated without being restricted on the gap. The thickness of the piezoelectric single crystal plate can be made constant by polishing, and a uniform single crystal can be provided. Each piezoelectric single crystal plate is flexibly vibrated without being mechanically regulated in the gap, thereby forming each element and arranging a plurality of elements as a phased array for one piezoelectric single crystal plate. did. As a result, the ultrasonic wave carrying the obstacle position information was successfully supplemented with a high signal / noise ratio, and the present invention was achieved.

1 is a plan view schematically showing an ultrasonic sensor array device according to an embodiment of the present invention. It is typical sectional drawing which cut the device of FIG. 1 along the II line. (A), (b) is typical sectional drawing which shows the vibration of one bending vibration part of the device of FIG. It is a graph which shows the relationship between the setting value of radiation angle, and a measured value in case a sensor part and a support substrate are the same kind of materials. It is a graph which shows the relationship between the setting value of radiation angle, and a measured value in case a sensor part and a support substrate are different materials.

  1 and 2 are diagrams showing a device 1 according to an embodiment of the present invention. FIG. 1 is a schematic plan view of the device as viewed from above, and FIG. 2 is a schematic cross-sectional view taken along the line II of FIG.

  The piezoelectric single crystal plate 2 is bonded to the support substrate 9 via the legs 8 at a constant interval. The legs 8 are formed with gaps 10A, 10B, and 10C as shown in FIG. 1 when viewed in plan, and the gaps are sandwiched between the support substrate 9 and the piezoelectric single crystal plate 2, respectively. In this example, the piezoelectric single crystal plate is divided into an upper part 2a and a lower part 2b, and the polarization direction of the upper part 2a is opposite to the polarization direction of the lower part 2b. That is, one of the upper part 2a and the lower part 2b is spontaneously polarized, and the other is polarization-inverted.

  The piezoelectric single crystal plate 2 is so thin that it cannot withstand its own weight by itself, and is thus supported by the support substrate 9 and the legs 8. As a result, on the gaps 10A, 10B, and 10C that do not contact the leg portion 8 of the piezoelectric single crystal plate 2, bending vibration portions 3A, 3B, and 3C that receive the ultrasonic waves and cause the bending vibration are formed. .

  First electrodes 7A, 7B, and 7C are formed on the back surface 2d side of the air gap side of each of the flexural vibration portions 3A, 3B, and 3C of the piezoelectric single crystal plate 2, and the second electrodes 4A, 4B and 4C are formed. Thereby, each bending vibration part functions as an ultrasonic sensor that senses and resonates with ultrasonic waves.

  The piezoelectric single crystal plate 2 is obtained by removing and processing the piezoelectric single crystal material. Specifically, it can be obtained by thinning a material having a thickness that can withstand its own weight (for example, 50 μm or more) to a thickness that cannot withstand its own weight. This thickness is preferably 10 μm or less, more preferably 5 to 1 μm.

  Further, the form and dimensions of the flexural vibration part are appropriately changed according to the frequency of the measurement ultrasonic wave and the application. Typically, it may be circular or polygonal.

  FIG. 3 shows the flexural vibration. On each of the gaps 10A, 10B, and 10C, the piezoelectric single crystal plate 2 can be easily vibrated. When an AC electrical signal is applied between the first electrodes 7A, 7B, and 7C and the second electrodes 4A, 4B, and 4C sandwiching each flexural vibration portion, the piezoelectric lateral effect is applied. Thus, expansion and contraction occurs in the in-plane direction of the piezoelectric single crystal. Since the expansion and contraction directions are opposite between the upper and lower parts having opposite directions, flexural vibrations A and B are generated in each vibration part, and ultrasonic waves are oscillated. Reference numerals 5A to 5C and 6A to 6C are terminals for the respective electrodes.

  Further, as shown in FIG. 2, when ultrasonic waves propagate from the obstacle 20 to the ultrasonic sensor array device, each flexural vibration portion is substantially perpendicular to the surface of the piezoelectric single crystal plate as indicated by arrows A and B. (FIG. 3). An electrical signal excited between the pair of electrodes by this vibration is transmitted to a predetermined arithmetic processing unit. All electric signals transmitted from a plurality of flexural vibrators are calculated, and the distance and angle to the obstacle are calculated.

In order to form a flexural vibration part on a piezoelectric single crystal plate and convert the flexural vibration into an electric signal, it is necessary to form portions 2a and 2b having different polarization directions. This method includes the following.
(1) A piezoelectric single crystal material is bonded to the support substrate through the legs 8. Next, the lower portion 2b is obtained by thinning the piezoelectric single crystal material. Next, another piezoelectric single crystal material having the opposite polarization direction is joined to the lower portion 2b and thinned to form the upper portion 2a.
(2) After thinning a single piezoelectric single crystal material, the upper part 2a and the lower part 2b are obtained by polarization inversion processing by heat treatment.

  Piezoelectric single crystal plates include quartz (SiO2), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), lithium tetraborate (Li2B4O7), zinc oxide (ZnO), potassium niobate (KNbO3) and langasite (La3Ga3SiO14) ) And the like.

  As the crystal orientation in the piezoelectric single crystal plate, a crystal orientation having desired piezoelectric characteristics is selected. Here, if the crystal orientation in the flexural vibration part is a crystal orientation in which the temperature characteristics of the resonance frequency and the anti-resonance frequency are good, preferably a crystal orientation in which the frequency temperature coefficient is “0”, the temperature characteristics are good. A piezoelectric ultrasonic sensor can be realized.

  The removal processing of the piezoelectric single crystal material is performed by mechanical processing such as cutting, grinding and polishing, and chemical processing such as etching. Here, by combining a plurality of removal processing methods, the piezoelectric material can be removed while switching the removal processing method step by step from a removal processing method with a high processing speed to a removal processing method with a small process alteration that occurs in the processing target. For example, after performing grinding in which a piezoelectric substrate is brought into contact with fixed abrasive grains and polishing in which a piezoelectric substrate is brought into contact with loose abrasive grains in order, a work-affected layer generated on the piezoelectric substrate by the polishing is finished and polished. Can be removed.

  The first electrode and the second electrode are conductor thin films obtained by depositing a conductive material. The film thickness of each electrode is determined in consideration of adhesion to the flexural vibration portion, electrical resistance, power resistance, and the like.

  The material of each electrode is not particularly limited, but aluminum (Al), silver (Ag), copper (Cu), platinum (Pt), gold (Au), chromium (Cr), nickel (Ni), molybdenum (Mo) It is desirable to select from metals such as tungsten (W), and it is particularly desirable to select aluminum having excellent stability. Moreover, you may use these alloys as an electrode material.

  The material of the leg 8 is not particularly limited as long as it has insulation and mechanical strength, but silicon dioxide (SiO2), alumina (Al2O3), ditantalum pentoxide (Ta2O5), trisilicon tetranitride (Si3N4), titanium dioxide (TiO2) is preferred.

  The method of bonding the piezoelectric single crystal plate to the leg portion 8 and the support substrate 9 is preferably an organic adhesive. This is more preferably an epoxy adhesive (thermosetting epoxy resin) and an acrylic adhesive (photocuring and thermosetting) that have a filling effect and exhibit sufficient adhesion even if the object to be bonded is not completely flat Acrylic resin) can be exemplified.

  As another bonding method, bonding can be performed by a method that does not use an adhesive such as surface activated bonding, thermocompression bonding, anodic bonding, or eutectic bonding. In this case, since there is no adhesive on the joint surface, such as vibration attenuation, adverse effects on ultrasonic transmission and reception characteristics, sensor degradation such as adhesive material degradation at high temperatures, and temperature limitations in the manufacturing process, etc. The problem can be solved.

  The material of the support substrate 9 is preferably quartz glass, borosilicate glass, zirconia, or silicon single crystal.

  Particularly preferably, the material of the support substrate is the same as the material of the piezoelectric single crystal plate. This means that the main component and crystal system of the material are the same, and the trace component, the doped component, or the inevitable impurities may be different. These materials include quartz (SiO2), lithium niobate (LiNbO3), lithium tantalate (LiTaO3), lithium tetraborate (Li2B4O7), zinc oxide (ZnO), potassium niobate (KNbO3) and langasite (La3Ga3SiO14) Can be illustrated.

  When the piezoelectric single crystal plate and the support substrate are made of different materials, sound wave reflection occurs at the interface between the sensor unit and the substrate, so that the sound waves between the sensor units constituting the array are coupled to each other, so-called crosstalk occurs. May occur. Then, independent control of each element necessary for obtaining a desired radiation angle becomes impossible. As a result, a difference between the set angle and the actual radiation angle occurs. Further, since this crosstalk is affected by differences in sensor portion thickness, bonding state with the substrate, etc., the radiation angle varies from element to element. This causes a problem in the location of the obstacle and becomes a practical problem. By making the material of the support substrate the same as the material of the piezoelectric single crystal plate, it is possible to solve the problems caused by the crosstalk between the elements (between the vibrating parts).

  By using the ultrasonic sensor array device of the present invention, the distance to the obstacle and the angle to the sensor can be measured. This method is known per se and is described, for example, in Patent Document 2.

  That is, by electrically delay-adding each output signal from each flexural vibration unit, a phased array can be formed, and directivity can be scanned without mechanically moving the sensor element itself. In order to realize this, it is necessary that the resonance frequencies of the ultrasonic sensor elements coincide with each other.

[Experiment 1]
(Device manufacturing)
An ultrasonic sensor array device having a configuration as shown in FIGS. 1 to 3 was manufactured.
However, in this embodiment, the piezoelectric single crystal plate 2 and the support substrate 9 are formed of lithium niobate single crystal, the first electrode and the second electrode are formed of aluminum, and the legs 8 are formed of silicon dioxide. .

  First, a circular niobate single crystal wafer (140 ° Y plate) having a thickness of 0.5 mm and a diameter of 3 inches was prepared as a piezoelectric single crystal and a support substrate 9. An aluminum film having a thickness of 1000 angstroms was formed on the entire surface of one main surface of the piezoelectric single crystal material by sputtering, and the first electrodes 7A, 7B, and 7C were patterned by etching using a photolithography process. Next, a silicon dioxide film 8 having a thickness of 1 μm was formed on the main surface of the piezoelectric single crystal material on the first electrode side by sputtering. Then, the gaps 10A to 10C were patterned by wet etching using hydrofluoric acid.

  Next, the piezoelectric single crystal plate 2 and the legs 8 were bonded to the support substrate 9 by surface activation bonding. Each void (cavity) had a circular shape and a diameter of 2 mm. The gap pitch was 3 mm.

  Next, the support substrate 9 was bonded and fixed to a polishing jig made of silicon carbide, and the piezoelectric single crystal material side was ground by a fixed abrasive grinder to reduce the thickness of the material to 50 μm. Furthermore, the grinding surface of the piezoelectric single crystal material was polished with diamond abrasive grains, and the thickness was reduced to 5 μm. Finally, in order to remove the work-affected layer generated on the substrate by polishing with diamond abrasive grains, the substrate is subjected to final polishing using loose abrasive grains and a non-woven cloth polishing pad, and the thickness is 4.00 μm ± 0. A .01 μm piezoelectric single crystal plate 2 was formed.

  Furthermore, the upper layer 2a and the lower layer 2b were refined by heat-treating the piezoelectric single crystal plate 2 at a predetermined temperature and subjecting it to polarization inversion. The lower layer 2b is a spontaneous polarization layer, and the upper layer 2a is a polarization inversion layer. Next, a part of the piezoelectric single crystal plate 2 was etched to provide a through hole and a lower electrode extraction hole. Further, the polished surface of the piezoelectric single crystal plate is washed with an organic solvent, an aluminum film having a thickness of 1000 angstroms is formed on the entire polished surface by sputtering, and the second electrode 4A, by etching using a photolithography process, 4B and 4C were formed.

(Detection of obstacles)
As an obstacle, a concrete sphere having a predetermined diameter was provided and installed at a position based on the following experimental conditions. The following table shows the obstacle installation conditions used in the experiment.

  The “distance from the ultrasonic sensor” is the shortest distance from the vibrating surface of the ultrasonic sensor array device to the obstacle. “An angle from the ultrasonic sensor” indicates an angle formed by a line connecting the device and an obstacle with a direction perpendicular to the surface of the ultrasonic sensor.

Obstacle detection experiments were performed under the respective obstacle installation conditions shown in Table 1. Specifically, the following was detected.
・ If there is an obstacle, can you judge that there is an obstacle? ・ If there is no obstacle, you can judge that there is no obstacle.

  In addition to the standard threshold level, the threshold level was set to a low sensitivity level of 1/5 when there was a problem with the correct answer rate. Using the ultrasonic sensor array described in Patent Document 2 as a comparative example, the correct answer rate under each experimental condition is shown.

  Under conditions 1 and 2, a correct answer rate of about 90% is obtained, and although there are some problems, practical application to some extent is possible. However, it was found that under the more severe conditions 3 and 4, the correct answer rate when there was an obstacle was significantly reduced. This is mainly because the incident intensity of the ultrasonic wave reflected from the obstacle is lowered because the obstacle is small or the obstacle is placed in the distance. Therefore, an experiment was conducted when the threshold level was set to a high sensitivity level. Indicates the correct answer rate.

  Under conditions 3 and 4, the correct answer rate with an obstacle improved, but this time the correct answer rate with no obstacle decreased. Since the threshold level has been lowered, sound waves emitted from other than the sensor that have been discarded as noise sounds, and sound waves from the sensor that have been reflected by multiple reflections other than the target obstacle from the obstacles. This is thought to be due to misrecognition of the reflection. Therefore, the change in the threshold level setting did not improve the correct answer rate.

  Subsequently, the correct answer rate in the device of the said Example is shown.

  As described above, a good accuracy rate is obtained. In some cases, misjudgment has occurred, but by combining with methods such as repeated measurement and error correction, obstacle detection with no problem in practice is possible.

[Experiment 2]
Using the device of the example of Experiment 1, the relationship between the radiation angle setting value based on each array driving condition and the actual radiation angle was evaluated. The result is shown in FIG. The radiation angle set value and the actual radiation angle corresponded well, and normal operation was performed.

  Next, in the element of the example of Experiment 1, the material of the support substrate was changed to a silicon single crystal. Three elements were manufactured, and the relationship between the radiation angle setting value based on each array driving condition and the actual radiation angle was evaluated. The result is shown in FIG.

  As a result, it can be seen that in a region where the set value of the radiation angle is large, the actual radiation angle is smaller than the set value, and a difference for each element is also seen. This causes a problem in the location of the obstacle and becomes a practical problem.

(Explanation of symbols)
DESCRIPTION OF SYMBOLS 1 Obstacle detection device 2 Ferroelectric single crystal board 2a Upper layer 2b Lower layer 3A, 3B, 3C Flexural vibration part 4A, 4B, 4C 2nd electrode 7A, 7B, 7C 1st electrode 8 Leg part 9 Support substrate 10A, 10C, 10C gap 20 gap

Claims (3)

A device for detecting an obstacle by ultrasonic waves reflected from the obstacle,
Support substrate,
A ferroelectric single crystal plate including a plurality of flexural vibration parts that receive and vibrate the ultrasonic waves;
Legs that are bonded to the back side of the support substrate and the ferroelectric single crystal plate, and that form a plurality of voids between the support substrate and the ferroelectric single crystal plate,
A plurality of first electrodes formed on the back surface side of the ferroelectric single crystal plate, each of the first electrodes provided in the flexural vibration portion, and the ferroelectric single crystal plate a plurality of second electrodes formed on the surface side provided with a second electrode that are formed on the deflection vibration unit in response to the first electrode, the ferroelectric single The crystal plate is divided into an upper part on the front side and a lower part on the back side, the polarization direction of the upper part is opposite to the polarization direction of the lower part, and the flexural vibration part is formed on each gap. And based on each electrical signal excited between each first electrode and each second electrode by each flexural vibration of each flexural vibration part due to ultrasonic waves reflected from the obstacle Obstacles characterized by obtaining the distance and angle to the obstacle Ultrasonic detection device for harmful substances.   The device according to claim 1, wherein the ferroelectric single crystal plate and the support substrate are made of the same kind of material. Wherein the thickness of said ferroelectric single crystal plate is 10μm or less, according to claim 1 or 2 wherein the device.

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