JPS5832392B2 - piezoelectric acoustic device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a piezoelectric acoustic device used for various alarms, notifications, etc.

2. Description of the Related Art Piezoelectric acoustic devices that use a piezoelectric vibrator that applies an alternating current voltage to a piezoelectric element bonded to a plate-shaped body and utilizes its expansion and contraction to cause the plate-shaped body to bend and vibrate have conventionally existed.

That is, as shown in FIG. 1, a piezoelectric element 1 having electrodes on both sides is bonded to a diaphragm 2 with an adhesive to form a piezoelectric vibrator 3.
By applying an alternating current voltage of an appropriate frequency to this vibrator 3, the vibrator 3 is caused to bend and vibrate, thereby generating a sound wave.

Therefore, this type of piezoelectric acoustic device has advantages such as non-contact, high reliability, and low power consumption.

In order to increase the sound pressure generated by such a piezoelectric acoustic device as much as possible, it is necessary to increase the amplitude of the bending vibration of the piezoelectric vibrator.

For this purpose, the frequency of the voltage applied to the piezoelectric vibrator should be adjusted to approximately the resonant frequency of the piezoelectric vibrator, and the voltage should be made large, for example, 10 V or more, so that the generated sound pressure can be adjusted to suit the purpose of the alarm. I can't get it.

However, the relationship between the sound pressure generated by the piezoelectric vibrator and the frequency of one drive voltage is a curve as shown in FIG.

fr is the resonant frequency of the piezoelectric vibrator, and the generated sound pressure reaches a maximum at this frequency, and when the drive frequency deviates from the resonant frequency, the generated sound pressure decreases rapidly.

However, one of the characteristics of piezoelectric acoustic devices is low power consumption, so they often use a small-capacity power source such as a dry cell battery.

If a sufficiently large generated sound pressure can be obtained using a dry battery or the like with a power supply voltage of about 1.5 to 3 VDC, the applications of piezoelectric acoustic devices will greatly expand.

Conventionally, we have used a separately excited oscillation circuit that determines the frequency by itself and separately drives the piezoelectric vibrator, and by combining it with voltage step-up components such as transformers, it is possible to drive the piezoelectric vibrator from a low voltage power supply. There are devices that increase the applied voltage to counteract the increase in generated sound pressure.

However, such separately excited oscillation method has the following problems.

That is, for example, the resonance frequency of a disc-shaped piezoelectric vibrator is expressed by the following equation.

Here, t is the thickness of the piezoelectric vibrator, r is the radius of the piezoelectric vibrator, E is Young's modulus of the piezoelectric vibrator, ρ is the density of the piezoelectric vibrator,
σ is Poisson's ratio, and α is a constant determined by vibration mode, etc.

Therefore, if the dimensions, shapes, and materials of the piezoelectric element 1 and the diaphragm 20 vary, the resonant frequency fr varies, and since the temperature coefficient of the resonant frequency fr of the piezoelectric vibrator usually takes a negative value, the resonant frequency fr fluctuates due to temperature changes.

Furthermore, due to variations in constants and temperature characteristics of the oscillation circuit components, the oscillation drive frequency of the separately excited oscillation circuit also varies or fluctuates.

Therefore, as is clear from Fig. 3, no matter how much the drive voltage of the piezoelectric vibrator is increased by means such as a transformer, the sound pressure generated varies greatly and fluctuates depending on the temperature, making it unsuitable for alarms, notifications, etc. There are many cases where there is no.

In order to compensate for these shortcomings, an automatic oscillation circuit is used to extract a feedback signal from the piezoelectric vibrator itself and automatically drive the piezoelectric vibrator at the piezoelectric vibrator's resonant frequency fr, and a coil is also used. An example of increasing the sound pressure generated by
No. 62 (hereinafter referred to as the earlier application).

In this earlier application, a feedback electrode is provided on the surface of the piezoelectric element to extract a feedback signal, and the signal oscillates at the resonant frequency of the piezoelectric vibrator, and at the same time, a two-terminal coil is used to apply the feedback signal to the piezoelectric vibrator. This was an attempt to increase the voltage.

In this case, compared to the case of using a separately excited oscillation circuit, there are fewer variations and fluctuations in the generated sound pressure (the generated sound pressure is also large, but the piezoelectric element with a diameter of 35 mm and a thickness of 0.5 mm is When using a vibrator bonded to a .5 mm metal diaphragm and supporting the vibrator in a case with an optimal acoustic structure, the voltage applied to the piezoelectric vibrator is approximately 4 VDC for a power supply voltage of 1.5 VDC. It is about 6v, which is twice as much, and the sound pressure generated is 8 at 1m.
It was about 0 dB.

The size of the return electrode in the previous application is as long as it has an area that can obtain a sufficient signal to operate this oscillation circuit.
The sound pressure generated will increase if the area of the drive electrode is made as small as possible and the area of the drive electrode is made as large as possible.

In this case, the ratio R of the areas of the drive electrode and the return electrode is 8≦R≦
The inventors have confirmed that about 15 is optimal.

Furthermore, among conventional three-terminal vibrators for piezoelectric buzzers, no one has been found that has a return electrode with a larger area.

In this case, the method of supporting the vibrator is to support it near the node of free fundamental resonance as shown in Fig. 1, and the vibration state shown by broken lines 4 and 4' occurs, and the piezoelectric vibrator 3 at resonance. Since the center of gravity does not move, the vibrator 3 easily vibrates, and therefore the input impedance becomes low.

On the other hand, if the piezoelectric vibrator is supported by the outer edge of the same piezoelectric vibrator as in Figure 1 as shown in Figure 2, the vibration state will be as shown by the broken line 5°5', and the resonant frequency will be lower (easier to support). Although it is excellent in mass production, since the center of gravity of the piezoelectric vibrator 30 moves during resonance, the vibrator 3 is less likely to vibrate than when supported near the nodes described above, and therefore has a relatively high input impedance.

For this reason, it is difficult to operate a self-excited oscillation circuit (and the self-excited oscillation circuit of the earlier application does not oscillate when the piezoelectric vibrator is supported by its outer edge in this way, and cannot be used as a piezoelectric acoustic device. Ta.

An object of the present invention is to provide a piezoelectric acoustic device with a simple circuit configuration in which the sound pressure generated is large and has little variation even with a relatively low voltage power input, and to support a piezoelectric vibrator at its outer edge. The object of the present invention is to provide a piezoelectric acoustic device that can be driven to oscillate at the resonance frequency of the piezoelectric vibrator even in such a case.

Embodiments of the present invention will be described below based on the accompanying drawings.

As shown in FIG. 4 and FIG.
6 has divided electrodes 14 electrically insulated from each other on the other side.
, 15, a disk-shaped piezoelectric element 11 is provided with an electrode 1 on the entire surface.
A piezoelectric vibrator 13 is formed by bonding the piezoelectric vibrator 13 to the diaphragm 12 by means of adhesive or the like.

Here, the divided electrodes 14 and 15 are divided into concentric circles with respect to the piezoelectric element, and the electrode 15 is an inner circular electrode, and the electrode 14 is an outer ring-shaped electrode.
0 area is approximately equal 6 The piezoelectric vibrator 13 according to the present invention
An embodiment of the drive circuit is shown in FIG.

The coil 22 has a center tap 25 and is divided into a secondary coil 23 and a secondary coil 24.

One end 26 of the coil 23 is connected to the anode of the DC power supply B,
A center tap 25 of the coil 22 is connected to the collector of the transistor 20.

The base of the transistor 200 is connected to the anode of the DC power supply B through a bias resistor 28, the emitter of the transistor 20 is connected to the cathode of the DC power supply B, and a diode 21 is connected between the emitter and base of the transistor 20.

The diaphragm electrode 12 of the piezoelectric vibrator 13 is connected to the emitter of the transistor 20, the drive electrode 14 is connected to one end 27 of the coil 24, and the feedback electrode 15 is connected to the base of the transistor 20.

Next, the operation of this circuit will be explained.

First, immediately after the DC power is turned on, the transistor 20 is self-biased by the resistor 28, so it immediately turns on.

Therefore, a voltage is generated across the coil 23 and a voltage is induced across the coil 240 as well.

At this time, if the number of turns of the coil 240 is greater than the number of turns of the coil 230, the voltage across the coil 24 will be a voltage that is higher than the voltage across the coil 23.

Since a voltage is applied between the drive electrode 14 and the diaphragm electrode 12 of the piezoelectric vibrator 13, the piezoelectric vibrator 13 is excited by the electrostrictive effect of the piezoelectric element 11.

Due to the piezoelectric effect caused by the bending of the vibrator 13, a negative potential feedback signal is generated at the feedback electrode 15, lowering the base potential of the transistor 200, so that the transistor 20 is turned off.

This change generates a back electromotive force across the coil 230,
A boosted back electromotive voltage is also generated at both ends of the coil 240, and a voltage in the opposite direction to the previous one is applied between the drive electrode 14 and the diaphragm electrode 12 of the piezoelectric vibrator 13, so the piezoelectric vibrator 13 is excited in the opposite direction to the previous one. be done.

Due to this bending of the vibrator 13 in the opposite direction, a feedback signal with a positive potential is generated at the feedback electrode 15, and the transistor 200
In order to raise the base potential, the transistor 20 is turned on again and returns to its initial state.

The plate layer stably and repeatedly performs the above operation at the same frequency as the resonant frequency of the vibrator 13.

Here, the diode 21 not only protects the transistor 20 when a reverse voltage is applied between the base and emitter of the transistor 200, but also serves to increase the generated sound pressure.

In this way, according to the present invention, the voltage applied to the piezoelectric vibrator 13 can be 10 times or more the DC power supply voltage depending on the turns ratio of the coil 23 and the coil 24, and its frequency is the natural resonance frequency of the vibrator 13. Therefore, even if the DC power supply voltage is low, the amplitude of the vibrator 13 becomes large, and the sound pressure generated by the vibrator 130 can be increased.

A vibrator in which a piezoelectric element with a diameter of 35 mm and a thickness of 0.5 mm as described above is bonded to a metal diaphragm with a diameter of 501 nm and a thickness of 0.5 mm is supported in a case having the same optimal acoustic structure as above. In this case, according to the above embodiment, the power supply voltage 1.
The voltage applied to the piezoelectric vibrator was 18V, which is about 12 times higher than 5VDC, and the generated sound pressure was 92 dB at 1rrL.

Next, FIG. 7 shows the relationship between the sound pressure generated by the drive circuit and the ratio R of the area of the drive electrode 14 and the return electrode 150.

From FIG. 7, when the area of the feedback electrode 150 is too small, that is, when the ratio R of the drive electrode 14 and the area of the return electrode 150 is 4 or more, the amount of feedback signal obtained from the feedback electrode 15 is too small, and the generated sound pressure suddenly increases. It reaches the point where the oscillation stops.

Also, when this ratio R is less than h, a sufficient amount of feedback signal can be obtained, but the area of the drive electrode 140 is too small, and the generated sound pressure is small, and even if the drive voltage is boosted by the coil, the generated sound pressure is Under the same conditions, with a 1.5VDC input, it would be less than 80 dB at 1m.

The sound pressure generated is largest when the area ratio R of the drive electrode 14 and the return electrode is in the range of %≦R≦2.

Also, when the outer wire of the piezoelectric vibrator 13 shown in FIGS. 4 and 5 is supported and the vibrator 13 is driven by the circuit shown in FIG. 6, the same operation as above is performed and the piezoelectric Since the vibrator 13 is driven once with a voltage that is 10 times or more the power supply voltage at the resonance frequency when the vibrator 13 is supported by an external line, the generated sound pressure is thicker and has less variation than the conventional one.

8, 9, and 10 show other embodiments of the circuit according to the present invention, and are examples of circuits in which the connection points of the coil 22 and the piezoelectric vibrator are changed from the circuit diagram of FIG. 6, respectively. , both perform the same operation as in FIG. 6, and therefore obtain similar effects.

In the above explanation, the inner circular electrode of two electrodes divided by almost concentric circles is the return electrode and the outer ring-shaped electrode is the drive electrode. Almost the same result is obtained when the outer ring-shaped electrode is a return electrode.

Furthermore, the above explanation has been made for the case where the return electrode and the drive electrode are divided into almost concentric circles, but since this type of piezoelectric vibrator uses bending vibration caused by expansion and contraction in the radial direction of the piezoelectric element, one drive electrode is It is preferable that the piezoelectric vibrator, that is, the piezoelectric element, be divided in a shape that is close to center symmetry, that is, a shape that is close to concentric circles.

Further, in the above description, a piezoelectric vibrator consisting of a disc-shaped piezoelectric element and a diaphragm was explained, but the same effect can be obtained even if the piezoelectric vibrator is formed into a rectangular plate shape or a shape similar to the rectangular plate shape.

As described above, according to the present invention, the voltage applied to the piezoelectric vibrator is increased by a simple circuit to more than 10 times the power supply voltage without using a booster circuit for the power supply voltage, but the resonance of the piezoelectric vibrator is Since the piezoelectric buzzer is driven at a frequency of 1, it is possible to obtain a piezoelectric buzzer that generates an extremely large sound pressure compared to conventional ones even with a low voltage power supply, and has less variation and variation.

At the same time, even a piezoelectric acoustic device that supports the outer edge of a piezoelectric vibrator can always be driven at its resonant frequency, which has the advantage of eliminating large variations in generated sound pressure.

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing the structure, operation, and support method of a general piezoelectric vibrator, Figure 2 is a cross-sectional view showing the structure, operation, and other support methods of the same piezoelectric vibrator, and Figure 3 is a piezoelectric vibration FIG. 4 is a plan view showing an embodiment of the piezoelectric vibrator according to the present invention, FIG. 5 is a cross-sectional view of the same, and FIGS. 10 is a drive circuit diagram of a piezoelectric acoustic device showing each embodiment of the present invention, and FIG. 7 is a diagram showing the generated sound pressure with respect to the area ratio of the drive electrode and the return electrode of the piezoelectric vibrator according to the present invention. It is a figure showing a relationship.

Claims (1)

[Claims] 1. A piezoelectric vibrator formed by bonding a piezoelectric element to a diaphragm, in which the area ratio R of the drive electrode and the return electrode satisfies %≦R≦2;
A transistor, a base bias resistor of the transistor, a diode connected between the emitter and the base of the transistor, and a coil divided into a primary winding and a secondary winding by a center tap, the primary winding of the coil are connected in series to the collector and emitter of the transistor, and the feedback electrode is connected to the base of the transistor. 2. The drive electrode is connected to the terminal of the secondary winding other than the center tap, the diaphragm electrode of the piezoelectric element is connected to one pole of a DC power supply, and the center tap of the primary winding is 2. The piezoelectric acoustic device according to claim 1, wherein the piezoelectric acoustic device is connected to the collector of the transistor, and the other terminal is connected to the other pole of a DC power source. 3. The drive electrode is connected to the terminal of the secondary winding other than the center tap, the diaphragm electrode of the piezoelectric vibrator is connected to one pole of a DC power supply, and the center tap of the primary winding is connected to the terminal of the secondary winding that is not the center tap. 2. The piezoelectric acoustic device according to claim 1, wherein the piezoelectric acoustic device is connected to the other pole of a power source, and the other terminal is connected to the emitter of the transistor. 4. The piezoelectric acoustic device according to claim 1, wherein the drive electrode and the return electrode are divided approximately concentrically with respect to the piezoelectric element.