JP2005057555A - Inspection method of piezoelectric diaphragm - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inspection method of a piezoelectric diaphragm, with which a piezoelectric diaphragm having a crack is clearly selected, and occurrence of the crack is prevented in the normal piezoelectric diaphragm. <P>SOLUTION: A plurality of piezoelectric ceramic layers 2 and 3 are laminated by providing an inner electrode 4 in between so as to form a laminated body. Outer electrodes 5 and 6 are arranged on main faces on a front surface and a rear face of the laminated body, and a prescribed frequency signal is applied between the outer electrode and the inner electrode. Thus, a range from a use frequency of the piezoelectric board 1 to a high frequency range where a response of resonance and antiresonance is not present is swept at least once by applying a rated voltage in the piezoelectric diaphragm 1 where the laminated body is bent and vibrated. Impedance in the high frequency range is measured and the crack in the piezoelectric diaphragm 1 is detected by a measured impedance value. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to an inspection method for detecting cracks in a piezoelectric diaphragm used in piezoelectric acoustic parts such as a piezoelectric receiver and a piezoelectric sounder.

Conventionally, a bimorph piezoelectric diaphragm having a laminated structure excellent in piezoelectric acoustic effect is known. FIG. 1 shows an example of such a piezoelectric diaphragm. The vibration plate 1 is a laminate in which two or more piezoelectric ceramic layers 2 and 3 are laminated with an internal electrode 4 interposed therebetween, and external electrodes 5 and 6 are provided on the front and back main surfaces thereof. , 6 are electrically connected to each other through the side electrode 7. The internal electrode 4 is led out to a lead electrode 9 on the front and back surfaces via a side electrode 8 formed on a side surface opposite to the side electrode 7. The two piezoelectric ceramic layers 2 and 3 are polarized in the same direction as indicated by arrows in the drawing. Resin protective films 10 and 11 are formed on almost the entire surface of the front and back surfaces of the diaphragm 1 in order to increase the strength against drop impact. At both ends of the protective film 10, notches 10 a and 10 b are formed in which a part of the external electrode 5 and the extraction electrode 9 are exposed, and a part of the external electrode 5 and the extraction electrode 9 are exposed from the notches 10 a and 10 b. doing. When an AC signal is applied to the external electrode 5 and the extraction electrode 9 exposed from the notches 10a and 10b, an electric field is applied between the external electrodes 5 and 6 and the internal electrode 4, so that the laminate is bent as a whole. Can be vibrated. A large sound pressure can be obtained as compared with the unimorph type by vibrating the ceramic layers 2 and 3 arranged in order in the thickness direction in opposite directions.

FIG. 2 shows an example of a piezoelectric acoustic component A using the piezoelectric diaphragm 1. Reference numeral 20 denotes a case, and reference numeral 30 denotes a cover. Both ends of the piezoelectric diaphragm 1 are supported by a support portion 21 provided on the case 20, and the piezoelectric diaphragm 1 is attached to the case 20 by a sealing adhesive 22 such as silicone rubber. At the same time, the entire circumference is sealed. Before bonding with the sealing adhesive 22, the external electrode 5 and the extraction electrode 9 exposed from the notches 10 a and 10 b of the piezoelectric diaphragm 1 are connected to terminals 24 and 25 provided on the case 20 by the conductive adhesive 23. Connected. Note that an elastic body 26 such as urethane rubber is applied under the position where the conductive adhesive 23 is applied, so that curing shrinkage stress due to the conductive adhesive 23 is prevented from acting on the piezoelectric diaphragm 1. The cover 30 is bonded to the opening of the case 20 with an adhesive 31. A sound emitting hole 32 is formed in the cover 30, and sound generated by the diaphragm 1 is emitted from the sound emitting hole 32.

As described above, the piezoelectric diaphragm having a laminated structure has excellent characteristics, but on the other hand, it has a disadvantage that its mechanical strength is lower than that of a piezoelectric diaphragm having a unimorph structure in which piezoelectric ceramics are bonded to a metal plate. For this reason, micro-cracks are generated in the piezoelectric ceramics during the manufacturing process such as when the laminate is polarized or when the piezoelectric diaphragm 1 is electrically connected to the terminals 24 and 25 of the case 20 with the conductive adhesive 23. There is.
Since microcracks degrade the performance as a piezoelectric diaphragm, it is necessary to inspect for the presence of microcracks. Conventionally, visual appearance selection has been carried out, but there are problems such as the fact that it is not possible to reliably determine the presence or absence of cracks by visual fuzzy judgment, the work time increases due to visual selection, and costs increase. It was.

In Patent Document 1, the frequency characteristic of the impedance of a piezoelectric element is measured, a curve pattern indicating the frequency characteristic is compared with a curve pattern of a reference element, and if both curve patterns are different, a microcrack is formed in the piezoelectric element. Is disclosed.
Since this method detects cracks based on the electrical characteristics of the piezoelectric element, there is an advantage that the reliability is increased as compared with the appearance selection. However, this method has a drawback that it takes a long time for one inspection because it can be compared with the reference pattern only after continuous frequency characteristics are measured. Further, since the impedance characteristic is measured by applying a signal in the vicinity of the operating frequency, there is a problem that when the crack is very small, it cannot always be clearly recognized as a defect.

Patent Document 2 proposes a method of detecting a defect of a piezoelectric element by sweeping a piezoelectric element continuously and multiple times (frequency sweep) from a low frequency range to a high frequency range.
However, this method is a method of destroying and removing a cracked piezoelectric element, and since the applied voltage is high, damage to a normal piezoelectric element cannot be ignored. In particular, in the case of a piezoelectric diaphragm having a laminated structure, its mechanical strength is low. Therefore, if sweeping is performed while applying a high voltage, cracks may occur even in a normal piezoelectric diaphragm.
JP-A-6-3305 JP 2003-88137 A

Accordingly, an object of the present invention is to provide a method for inspecting a piezoelectric diaphragm that can clearly select a piezoelectric diaphragm having cracks and does not cause cracks in a normal piezoelectric diaphragm.

The object is achieved by the invention described in claim 1.
That is, the invention according to claim 1 forms a laminate by laminating a plurality of piezoelectric ceramic layers with an internal electrode interposed therebetween, and external electrodes are provided on the front and back main surfaces of the laminate. In the piezoelectric diaphragm in which the laminated body is flexibly vibrated by applying a predetermined frequency signal between and the above, from the use frequency range of the piezoelectric diaphragm to a high frequency range where there is no response of resonance or anti-resonance A method for inspecting a piezoelectric diaphragm, comprising: sweeping at least once by applying a rated voltage, measuring impedance in the high frequency range, and detecting cracks in the piezoelectric diaphragm based on the measured impedance value.

FIG. 3 is an equivalent circuit of the piezoelectric diaphragm. In the figure, Rd1 and Rd2 are resistances between the conductive adhesive 23 and the diaphragm 1, and C1, R1 and C2, R2 are capacitances and internal resistances constituting an equivalent circuit inside the diaphragm 1. Rin is the resistance of the input side electrode (for example, the external electrode 5), and Rout is the resistance of the output side electrode (for example, the extraction electrode 9). When the vibration plate 1 is cracked, the input-side electrode or the output-side electrode is partially disconnected (indicated by x).
In the present invention, at least one sweep is experienced from the use frequency range to the high frequency range for the piezoelectric diaphragm, and the diaphragm is greatly displaced at the resonance frequency to grow a crack. Since the impedances of the capacitance components C1 and C2 included in the equivalent circuit of the diaphragm 1 are reduced as the frequency is increased, the impedance as the diaphragm 1 is reduced. However, when cracks grow due to the sweep, a disconnection portion is generated in the input side electrode or the output side electrode, so that the series resistances Rin and Rout increase. Therefore, the overall impedance does not decrease even if the frequency increases. Therefore, the occurrence of a crack can be easily detected by measuring the impedance value in the high frequency range.

FIG. 4 shows the frequency characteristics of impedance of a piezoelectric diaphragm (non-defective product) without cracks and the frequency characteristics of impedance of a piezoelectric diaphragm (cracked product) having cracks.
As is apparent from the figure, there is no difference in impedance up to around 10 kHz, which is the operating frequency range, but as the frequency becomes higher than that, the impedance of non-defective products continues to decrease, whereas the impedance of defective products decreases slowly. It has become. In the example of FIG. 4, the defective product is almost constant at around 80Ω when the frequency exceeds 100 kHz. Therefore, it can be seen that the impedance values of the non-defective product and the defective product are clearly different when compared in a high frequency range (for example, 1 MHz) having no response of resonance and antiresonance.

According to the present invention, it is not necessary to compare the curve pattern indicating the frequency characteristic of the impedance with the reference curve pattern, it is only necessary to measure the impedance value at one point in the high frequency range, and the measurement does not require time.
Further, it is sufficient to apply a voltage while maintaining a rated voltage (for example, 1 Vrms) during the sweep, and it is not necessary to apply a high voltage. Therefore, there is no fear of generating cracks in a normal diaphragm.
The sweep of the present invention is preferably performed in a room temperature environment. Although sweeping may be performed at a high temperature, it is not desirable to apply more stress than necessary to the piezoelectric diaphragm because it promotes the generation of cracks.
Generally, the operating frequency range in a piezoelectric acoustic component is several kHz. In the case of such a piezoelectric diaphragm, the high frequency region in which the response of resonance / antiresonance disappears is 1 MHz to 10 MHz.

According to another aspect of the present invention, the sweep may be repeated a plurality of times, and a crack in the piezoelectric diaphragm may be detected based on the magnitude of the measured impedance value fluctuation.
If the crack is relatively large, the impedance value becomes larger than the reference value in one sweep, so that the crack can be easily identified. However, in the case of very small cracks, it may not be possible to determine an abnormality with only one sweep. In that case, the sweep is repeated a plurality of times, and the abnormality is determined based on the magnitude of the fluctuation of the measured impedance value. That is, if the measured impedance value varies greatly, it means that the disconnected portion generated in the input side electrode or the output side electrode repeats conduction / separation, and therefore, it may be selected as a defective product.

As in claim 3, the external electrodes on the front and back main surfaces are preferably made of a metal material having a resistivity of 30 μΩ · cm or more.
The external electrodes of the piezoelectric diaphragm include Ni-Cu alloy and Ag. However, in the case of a good conductor such as Ag, even if there is a partial disconnection of the electrode due to a small crack, the resistance value is almost Since it does not appear, it may be difficult to distinguish.
On the other hand, in the case of an electrode having a relatively large resistance value with a resistivity of 30 μΩ · cm or more, even if the electrode is partially broken due to a small crack, the resistance value changes greatly. Clearly distinguishable.
As described above, the material having a resistivity of 30 μΩ · cm or more includes a Ni—Cu alloy and a Ni—Cr alloy. Ni—Cu alloys and Ni—Cr alloys are preferable because migration can be prevented.

According to the first aspect of the present invention, the piezoelectric diaphragm is caused to undergo at least one sweep from the use frequency range to the high frequency range, and the vibration plate is greatly displaced at the resonance frequency so as to grow a crack. Therefore, the impedance of the diaphragm having cracks does not decrease even when the frequency is increased as compared with a normal piezoelectric diaphragm. Therefore, the occurrence of a crack can be easily detected by measuring the impedance value in the high frequency range.
In the present invention, it is not necessary to compare the frequency characteristics of the impedance with the reference pattern, and it is only necessary to measure the impedance value at one point in the high frequency range, so that the measurement time can be shortened. In addition, since it is sufficient to apply a rated voltage during sweeping, it is not necessary to apply a voltage higher than necessary, and there is no possibility of causing cracks in a normal diaphragm.

Embodiments of the present invention will be described below with reference to examples.

FIG. 5 shows a first embodiment of an inspection apparatus for carrying out the method for inspecting a piezoelectric diaphragm according to the present invention.
This inspection apparatus includes an impedance analyzer 40 that measures the impedance of the object A, and a controller 50 that determines the quality of the piezoelectric diaphragm 1 based on the measured value. Here, the object A is a piezoelectric acoustic component in a completed state shown in FIG. 2 including the piezoelectric diaphragm 1, the case 20, and the cover 30.
As is well known, the impedance analyzer 40 measures the impedance of the piezoelectric diaphragm 1 while sweeping (frequency sweeping) from a low frequency to a high frequency. Here, sweeping is performed by applying a rated voltage from the use frequency range (for example, 100 Hz) of the piezoelectric diaphragm 1 to a high frequency range (for example, 1 MHz) where there is no response of resonance and anti-resonance. Then, the impedance in the high frequency range is measured, and the measured value is sent to the controller 50. The controller 50 determines the presence or absence of cracks by comparing the measured value with the impedance value in the high frequency range obtained from the normal (non-cracked) piezoelectric diaphragm 1 or a reference value obtained by adding an allowable value to the impedance value. To do.
Furthermore, the controller 50 not only compares the measured values by one sweep, but also repeats sweeps a plurality of times (here, three times), and the presence or absence of cracks is also determined from the magnitude of the fluctuation of the impedance value in the high frequency range. Is judged.

If sweeping is performed on the piezoelectric acoustic component A in the finished product state as described above, a defect that cannot be found by the piezoelectric diaphragm alone, that is, the occurrence of cracks in the piezoelectric diaphragm 1 due to incorporation into the case 20 is also found. Can do. For example, when the piezoelectric diaphragm 1 is electrically connected to the case 20 with the conductive adhesive 23, cracks may occur in the piezoelectric diaphragm 1 due to the curing shrinkage stress of the conductive adhesive 23. In particular, in the case of the piezoelectric diaphragm 1 in which the protective films 10 and 11 (see FIG. 1) are formed on the front and back surfaces, the electrode portions exposed from the notches 10a and 10b are not reinforced at all. Cracks are likely to occur at the periphery. Such a crack can be detected by performing a sweep on the piezoelectric acoustic component A in a finished product state.

FIG. 6 is a flowchart of an example of an inspection method performed using the inspection apparatus.
First, when the inspection is started, a sweep is performed by applying a rated voltage from the use frequency range to a high frequency range where there is no resonance or anti-resonance response (step S1). Specifically, frequency sweep is performed from about 100 Hz to about 1 MHz. Then, the impedance in the high frequency region (near 1 MHz) is measured (step S2). Next, the measured impedance value is compared with a reference value (step S3). This reference value is determined based on the impedance value when sweeping is performed on a normal piezoelectric diaphragm without cracks. If it is determined in step S3 that the measured impedance value is higher than the reference value, it is determined that a crack has occurred (step S4). On the other hand, if it is determined in step S3 that the impedance value is lower than the reference value, it is subsequently determined whether or not the number of sweeps has been performed n times (for example, 3 times) or more (step S5). Steps S1 to S3 are repeated. If n times or more, then the variation of the impedance value measured in step S2 is detected (step S6). For example, the difference between the maximum value and the minimum value of each impedance value for the first to nth times is obtained. Next, the impedance value variation is compared with the set value (step S7). If the variation is equal to or greater than the set value, it is determined that a crack has occurred (step S4). It is determined that it is a non-defective product (step S8).
As described above, when the presence / absence of cracks was determined not only by comparing impedance values by one sweep but also by taking into account variations in impedance values by multiple sweeps, it was determined to be a good product without cracks. The product is a highly reliable product.

In addition, although FIG. 6 demonstrated the example which determined the presence or absence of a crack in consideration of not only the comparison of the impedance value by one sweep but also the variation of the impedance value by a plurality of sweeps, the broken line in FIG. The control shown, i.e., crack detection by multiple sweeps, may be performed as necessary. That is, crack detection may be performed by simply comparing the measured impedance with the reference value.

FIG. 7 is a distribution diagram of impedance values by sweeping. (A) of FIG. 7 shows the impedance distribution of the good product which does not have a crack, (b) shows the impedance distribution of the product in which the crack generate | occur | produced. The number of inspected non-defective products is 100, the number of inspected cracked products is 53, and the impedance value is a value at 1 MHz.
As is apparent from FIG. 7, the impedance value is 15Ω or less for the non-defective product, whereas it is 30Ω or more for the cracked product. Therefore, in the case of this product, the impedance reference value for determining pass / fail is preferably set in the vicinity of 20 to 25Ω.

FIG. 8 is a graph comparing impedance value variations due to three sweeps. (A) of FIG. 8 shows the distribution of impedance fluctuation of a non-defective product having no crack, and (b) shows the distribution of impedance fluctuation of a product in which a crack has occurred. The number of inspected non-defective products is 100, the number of inspected cracked products is 53, and the impedance value is a value at 1 MHz.
As can be seen from FIG. 8, the fluctuation range of the impedance value is 4Ω or less in the case of the non-defective product, whereas it is 8Ω or more in the case of the cracked product. Therefore, the reference value of the impedance fluctuation range for determining pass / fail is preferably set around 6Ω.

The piezoelectric diaphragm targeted by the present invention is not limited to the two-layer structure as shown in FIG. 1, but may be a three-layer structure having an intermediate layer or four or more layers.
Moreover, although the protective film was formed in the front and back of a laminated body, you may abbreviate | omit this protective film as needed.
Furthermore, although the example which connected the piezoelectric diaphragm to the terminal of the case using the conductive adhesive was shown, it is also possible to connect using a solder or a lead wire.
In the inspection method of the present invention, it is necessary to sweep from at least the usage frequency range of the piezoelectric diaphragm to a high frequency range where there is no resonance or anti-resonance response. Sweep is started from a lower region (for example, 100 Hz). The reason is that the piezoelectric diaphragm is vibrated in a wider frequency range, thereby promoting the growth of minute cracks and increasing the detection sensitivity.
Of course, the inspection method of the present invention is not limited to the case of the piezoelectric acoustic component A in a finished product state as in the above-described embodiment, but may be performed in the state of the piezoelectric diaphragm 1 alone.

It is a perspective view of an example of a piezoelectric diaphragm concerning the present invention. It is a disassembled perspective view of an example of the piezoelectric acoustic component using the piezoelectric diaphragm shown in FIG. It is an equivalent circuit diagram of a piezoelectric diaphragm. It is a frequency characteristic figure of the impedance of a piezoelectric diaphragm without a crack and a piezoelectric diaphragm with a crack. It is a schematic block diagram of an example of the test | inspection apparatus concerning this invention. It is a flowchart figure of an example of the inspection method implemented using an inspection apparatus. It is a distribution map of the impedance value by the sweep of a good product and a crack product. It is the figure which compared the dispersion | variation in the impedance value by a multiple times sweep with a good product and a crack product.

Explanation of symbols

A Piezoelectric acoustic component 1 Piezoelectric diaphragm 2, 3 Piezoelectric ceramic layer 4 Internal electrode 5, 6 External electrode

Claims (3)

A plurality of piezoelectric ceramic layers are laminated with internal electrodes in between to form a laminate, external electrodes are provided on the front and back main surfaces of this laminate, and a predetermined frequency signal is applied between the external electrodes and the internal electrodes. In the piezoelectric diaphragm configured to flexurally vibrate the laminate,
Sweep at least once by applying the rated voltage from the frequency range of use of the piezoelectric diaphragm to the high frequency range where there is no resonance or anti-resonance response, measure the impedance in the high frequency range, and measure the piezoelectric vibration based on the measured impedance value. A method for inspecting a piezoelectric diaphragm, comprising detecting a crack in the plate. 2. The method for inspecting a piezoelectric diaphragm according to claim 1, wherein the sweep is repeated a plurality of times, and cracks of the piezoelectric diaphragm are detected based on the magnitude of the fluctuation of the measured impedance value. 3. The method for inspecting a piezoelectric diaphragm according to claim 1, wherein the external electrodes on the front and back main surfaces are made of a metal material having a resistivity of 30 [mu] [Omega] .cm or more.

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