WO2019159395A1

WO2019159395A1 - Thermal excitation type sound wave generation device

Info

Publication number: WO2019159395A1
Application number: PCT/JP2018/030482
Authority: WO
Inventors: 晋一佐々木; 隆昭浅田
Original assignee: 株式会社村田製作所
Priority date: 2018-02-13
Filing date: 2018-08-17
Publication date: 2019-08-22
Also published as: JPWO2019159395A1

Abstract

This thermal excitation type sound wave generation device (1) is provided with a sound wave source (10), a switching element (23), a capacitor (21), and a limiting resistor (22). The sound wave source generates a sound wave by being heated by power supplied from a DC power supply. The limiting resistor is inserted between the DC power supply and the sound wave source and limits the power that is input to the sound wave source from the DC power supply. The switching element is connected in series with the sound wave source and is turned on/off to control the input/shutoff of the current flowing through the sound wave source. The capacitor is connected in parallel with the series circuit of the sound wave source and the switching element.

Description

Thermally excited sound generator

The present invention relates to a thermal excitation type sound wave generator that generates sound waves by heating air.

A sound source has been proposed that generates air pressure waves (ie, sound waves) by heating air using a heating element that generates heat when an electric current is applied.

Patent Document 1 discloses an ultrasonic generator that drives a sound wave source with a DC power source. The ultrasonic generator includes a pulse generation circuit, a DC power supply, a MOSFET, and a sound wave source. In the ultrasonic generator, the sound wave source is connected between the drain of the MOSFET and the DC power source. The source of the MOSFET is grounded. The pulse generation circuit applies a pulse voltage to the gate of the MOSFET. In the ultrasonic generator of Patent Document 1, a driving current is supplied from a DC power source to a sound wave source in accordance with a gate signal of the MOSFET.

International Publication No. 2012/020600

An object of the present invention is to provide a thermal excitation type sound wave generator capable of improving safety when sound waves are generated by heat generation according to current.

A thermal excitation type sound wave generator according to an aspect of the present invention includes a sound wave source, a switching element, a capacitor, and a limiting resistor. The sound wave source generates heat waves by generating heat due to the current supplied from the DC power supply. The limiting resistor is inserted between the DC power source and the sound wave source, and limits the current flowing from the DC power source to the sound wave source. The switching element is connected in series to the sound wave source, and controls on / off of current flowing through the sound wave source. The capacitor is connected in parallel to the series circuit of the sound wave source and the switching element.

A thermal excitation type sound wave generator according to another aspect of the present invention includes a sound wave source, a switching element, a capacitor, and a resistance element. The sound wave source generates heat by generating heat by passing an electric current. The switching element performs on / off control of a current flowing through the sound wave source. The capacitor is charged when the switching element is turned off, and supplies the current from the charging to the sound wave source when the switching element is turned on. The resistance element limits the current flowing to the sound wave source when the switching element is turned on.

According to the thermal excitation type sound wave generator according to the present invention, it is possible to improve safety when generating sound waves by heat generation according to current.

The figure which shows the thermal excitation type sound wave generator which concerns on Embodiment 1. FIG. Sectional drawing which shows the structure of the sound wave source in a sound wave generator The circuit diagram which shows the structural example of the drive part in a sound wave generator The figure which shows the equivalent circuit at the time of charge of a sound wave generator The figure which shows the equivalent circuit at the time of discharge of the sound wave generator Timing chart for explaining a sound wave generation method in a sound wave generator Waveform diagram showing the simulation result of the sound wave generator with resistance value R2 set to 50Ω Waveform diagram showing simulation results of a sound wave generator with a resistance value R2 set to 500Ω Waveform diagram showing simulation results of a sound wave generator with a resistance value R2 set to 5 kΩ Graph illustrating the relationship between the current I1 of the heating element and the resistance value R2 of the limiting resistor in the sound wave generator Graph illustrating the relationship between the current I2 of the limiting resistance and the resistance value R2 in the sound wave generator

Hereinafter, embodiments of a thermal excitation type sound wave generator according to the present invention will be described with reference to the accompanying drawings.

Each embodiment is an exemplification, and it is needless to say that a partial replacement or combination of configurations shown in different embodiments is possible. In the second and subsequent embodiments, description of matters common to the first embodiment is omitted, and only different points will be described. In particular, the same operational effects by the same configuration will not be sequentially described for each embodiment.

(Embodiment 1)
1. Configuration The configuration of the thermal excitation type sound wave generator according to Embodiment 1 will be described with reference to FIGS. FIG. 1 is a diagram showing a sound wave generator 1 according to the present embodiment. FIG. 2 is a cross-sectional view showing the configuration of the sound wave source 10 of the sound wave generator 1 corresponding to the AA ′ cross section of FIG.

The sound wave generator 1 according to this embodiment includes a sound wave source 10 and a drive unit 20 as shown in FIG. The sound wave generator 1 is a thermal excitation type sound wave generator that generates sound waves by heat generated in the sound wave source 10. For example, the sound wave generator 1 can generate a pulsed sound wave by changing the pressure of the surrounding air by instantaneous heat generation of the sound wave source 10. The sound wave generator 1 of this embodiment can be applied to uses such as a distance sensor or a proximity sensor of a TOF (time of flight measurement) system, for example.

In the sound wave generator 1, the drive unit 20 is a circuit that drives the sound wave source 10 by current drive, for example. The circuit configuration of the drive unit 20 will be described later.

The sound wave source 10 is an element that generates sound waves by heating air. As shown in FIG. 2, the sound wave source 10 includes a heating element 11, a substrate 12, a pair of electrodes 13, and a heat insulating layer 14. In the acoustic wave source 10, the heating element 11 and the heat insulating layer 14 are laminated on the substrate 12. Hereinafter, the main surface side of the substrate 12 on which the heating elements 11 and the like are stacked may be referred to as the upper side, and the other main surface side may be referred to as the lower side.

The heating element 11 is a resistor that generates heat when an electric current flows. The heating element 11 is provided so as to be in contact with the air on the upper side of the substrate 12, for example. The air around the heating element 11 expands or contracts due to the temperature change of the heating element 11. Thereby, an air pressure wave, that is, a sound wave is generated.

The heating element 11 is made of silver palladium, silver, gold, platinum, carbon nanotube, or the like. The heating element 11 has, for example, a specific heat of 500 [J / kg ° C.] or less and a thermal conductivity of 50 [W / mK] or more. The heating element 11 may be pattern-printed so as to have various shapes on the main surface of the substrate 12, for example.

The heat insulating layer 14 is provided between the heating element 11 and the substrate 12. The heat insulating layer 14 has, for example, a thermal conductivity smaller than that of the substrate 12. The heat insulating layer 14 suppresses heat conduction from the heating element 11 to the lower side. The heat insulating layer 14 is made of glass grace, porous silicon, silicon dioxide, or the like. The heat insulating layer 14 may be configured integrally with the heating element 11 as, for example, a thick film conductor including a glass component and a metal component.

The substrate 12 is made of an insulating material such as silicon or aluminum oxide. In the substrate 12, the heat conducted from the heating element 11 can be radiated without being completely blocked by the heat insulating layer 14. The substrate 12 may be a printed circuit board.

The pair of electrodes 13 are electrodes for flowing a current from the outside of the sound source 10 to the heating element 11. The pair of electrodes 13 are provided on both sides of the heating element 11. The drive unit 20 is connected to the sound wave source 10 via the electrode 13. Each electrode 13 is made of a metal material such as Cu, Au, or Al.

In the acoustic wave source 10 as described above, if the current for heating the heating element 11 continues to be supplied to the heating element 11 due to a failure or the like, an overheating state may occur. In response to such a problem, the sound wave generator 1 according to the present embodiment has a configuration in which the drive unit 20 can avoid an overheated state of the heating element 11 even at the time of the failure as described above. Hereinafter, details of the drive unit 20 of the present embodiment will be described.

1-1. About the drive part of a sound wave generator The circuit structure of the drive part 20 of the sound wave generator 1 in this embodiment is demonstrated using FIG. FIG. 3 is a circuit diagram illustrating a configuration example of the drive unit 20 in the sound wave generator 1.

In the following, the resistance value of the heating element 11 is R1. The resistance value R1 is, for example, 1Ω.

As shown in FIG. 3, the drive unit 20 includes a DC power source 24 that outputs a DC voltage E, and a limiting resistor 22 connected between the high-voltage side end of the DC power source 24 and the electrode 13b. The drive unit 20 further includes a MOSFET 23 connected between the electrode 13 a and the low-voltage side end of the DC power supply 24, a pulse drive circuit 25 that drives the MOSFET 23, and an electrode 13 b and the low-voltage side end of the DC power supply 24. And a capacitor 21 connected to.

The MOSFET 23 is an example of a switching element that performs on / off control of a current I1 that flows through the heating element 11. FIG. 3 shows a configuration example using an n-type MOSFET 23 as a switching element. The source of the MOSFET 23 is grounded, for example. The drain of the MOSFET 23 is connected to the electrode 13 a of the heating element 11.

The capacitor 21 is connected in parallel to a series circuit of the heating element 11 and the MOSFET 23. Capacitor 21 has a capacitance value C1. The capacitance value C1 of the capacitor 21 is, for example, 10 uF or more and 100 uF or less. The capacitor 21 is, for example, an electrolytic capacitor or a ceramic capacitor.

The limiting resistor 22 is an example of a resistance element that limits a current flowing from the DC power source 24 to the heating element 11. The limiting resistor 22 has a resistance value R2. The resistance value R2 of the limiting resistor 22 is larger than the resistance value R1 of the heating element 11, for example. The resistance value R2 is, for example, not less than 50Ω and not more than 5 kΩ.

The DC power supply 24 supplies a DC voltage E to the other end of the limiting resistor 22. The DC power supply 24 is composed of various power supply circuits and / or batteries. Various power supply circuits include, for example, an AC / DC converter, a DC / DC converter, a regulator, and a battery. The DC voltage E is, for example, 5V.

The pulse drive circuit 25 generates a pulse signal Sp indicating the on voltage Von or the off voltage Voff based on, for example, a preset cycle and duty ratio (see FIG. 6). The pulse drive circuit 25 performs on / off control of the MOSFET 23 using the pulse signal Sp. The on voltage Von of the pulse signal Sp is larger than the threshold voltage of the MOSFET 23. The off voltage Voff is smaller than the threshold voltage. The pulse drive circuit 25 is connected to the gate of the MOSFET 23. The pulse drive circuit 25 includes an oscillator and the like.

2. Operation The operation of the sound wave generator 1 according to this embodiment will be described below.
2-1. Operation of Drive Unit The operation of the drive unit 20 of the sound wave generator 1 according to this embodiment driving the sound wave source 10 will be described with reference to FIGS. Below, an example of the operation | movement by the drive part 20 of the structural example of FIG. 3 is demonstrated.

When the MOSFET 23 is off, no current flows through the heating element 11, so the heating element 11 does not generate heat. At this time, a DC voltage from the DC power supply 24 is applied to the capacitor 21 via the limiting resistor 22 and the capacitor 21 is charged. When the MOSFET 23 is turned on, the DC power source 24, the limiting resistor 22, the heating element 11, and the closed circuit of the MOSFET 23 are formed. However, since the resistance value R2 of the limiting resistor 22 is large, the DC power source 24 is connected to the heating element 11. Almost no current flows. At this time, the capacitor 21 is discharged, a current flows from the capacitor 21 to the heating element 11, and the heating element 11 generates heat. By continuously turning on and off the MOSFET 23, a state in which the air around the heating element 11 is heated and a state in which the air is not heated are continuously generated to generate expansion and contraction of the air. Thereby, a sound wave is generated.

In the drive unit 20 of the present embodiment, the pulse drive circuit 25 outputs the pulse signal Sp to the gate of the MOSFET 23 to drive the MOSFET 23 in a pulse manner. The MOSFET 23 is turned on (conductive) when the pulse signal Sp indicates the on voltage Von, and is turned off (non-conductive) when the pulse signal Sp indicates the off voltage Voff.

The drive unit 20 of the sound wave generator 1 charges the capacitor 21 when the MOSFET 23 is off. FIG. 4 shows an equivalent circuit of the sound wave generator 1 when the MOSFET 23 is in the OFF state. The DC power supply 24 supplies a DC voltage E to the limiting resistor 22. At this time, as the current I2 flows through the limiting resistor 22, the charging voltage V (t) of the capacitor 21 at time t increases. The charging voltage V (t) of the capacitor 21 reaches V (t) = E in a fully charged state where the current I2 of the limiting resistor 22 stops flowing.

Further, the drive unit 20 discharges the capacitor 21 and causes the current I1 to flow through the heating element 11 of the acoustic wave source 10 when the MOSFET 23 is on. FIG. 5 shows an equivalent circuit of the sound wave generator 1 when the MOSFET 23 is on. In the equivalent circuit of FIG. 5, the current I2 of FIG. 3 is ignored based on the setting of the resistance value R2 of the limiting resistor 22.

As shown in FIG. 5, during the period in which the MOSFET 23 is on, the charging voltage V (t) of the capacitor 21 is applied to the heating element 11 and a current I1 flows through the heating element 11. The heating element 11 generates heat according to the magnitude of the current I1. In the capacitor 21, the charging voltage V (t) is reduced by supplying current to the heating element 11.

According to the operation of the drive unit 20 described above, the sound wave source 10 can be driven by causing the current I1 that generates heat from the heating element 11 to flow by charging / discharging of the capacitor 21 that is discharged when the MOSFET 23 is turned on. Here, in the present embodiment, the limiting resistor 22 is set to a resistance value R2 that is so large that the current I2 from the DC power supply 24 can be ignored with respect to the current I1 that causes the heating element 11 to generate heat. Thereby, for example, even when the MOSFET 23 is always turned on due to a failure, the heating element 11 can be prevented from being overheated, and the safety of the sound wave generator 1 can be improved.

Note that the resistance value R2 of the limiting resistor 22 does not necessarily have to be so large that the current I2 can be ignored. For example, the resistance value R2 of the limiting resistor 22 can be set on the basis of the resistance value R1 of the heating element 11, and may be larger than the resistance value R1.

2-2. About the generation method of a sound wave The method in which the sound wave generator 1 generates a sound wave by operation | movement of the above drive part 20 is demonstrated using FIG.

FIG. 6 is a timing chart for explaining a sound wave generation method in the sound wave generator 1. FIG. 6A shows the input timing of the pulse signal Sp in the MOSFET 23. FIG. 6B shows the output timing of sound waves from the sound source 10.

The pulse signal Sp input to the gate of the MOSFET 23 indicates the off voltage Voff before time t1, as shown in FIG. 6 (a). At this time, the capacitor 21 is in a state near full charge (see FIG. 4).

At time t1, the pulse signal Sp rises to the on voltage Von (FIG. 6 (a)), and the MOSFET 23 is turned on. Then, the capacitor 21 starts discharging, and causes the current I1 to flow through the heating element 11 (see FIG. 5). At this time, the temperature of the heating element 11 rises, and the heating element 11 heats the surrounding air. As a result, the air in the vicinity of the sound wave source 10 is thermally expanded, and the pressure of the air (that is, the sound pressure) increases from the steady value P0 as shown in FIG. 6B.

During the period Ton during which the MOSFET 23 is on (hereinafter sometimes referred to as “pulse width Ton”), the discharge of the capacitor 21 and the supply of the current I1 to the heating element 11 are continued. During the passage of the period Ton, the temperature change of the air near the sound source 10 is stabilized, and the sound pressure returns to the steady value P0 (FIG. 6B).

At time t2 after the pulse width Ton from time t1, the pulse signal Sp falls to the off voltage Voff (FIG. 6A), and the MOSFET 23 is turned off. Then, the capacitor 21 stops discharging, and the supply of the current I1 to the heating element 11 is stopped. At this time, the heating element 11 does not generate heat and cools the air as the temperature decreases. As a result, the air in the vicinity of the sound wave source 10 contracts, and the sound pressure decreases from the steady value P0 as shown in FIG. Thereafter, the sound pressure returns to the steady value P0.

As described above, when the sound pressure decreases and increases according to the period of the pulse width Ton of the pulse signal Sp, a single-pulse sound wave having the period Ton as one cycle is formed (FIG. 6 ( b)).

Further, the capacitor 21 is charged during the period Toff (hereinafter sometimes referred to as “pulse interval Toff”) in which the MOSFET 23 is off. At time t3, which is after the pulse width Toff from time t2, the pulse signal Sp rises again (FIG. 6 (a)). As a result, a single-pulse sound wave similar to the above is repeatedly formed with a pulse interval Toff.

As described above, according to the sound wave generator 1 of the present embodiment, a single-pulse sound wave is converted into a pulse period Tp () as shown in FIG. 6B according to the pulse width Ton and the pulse interval Toff of the pulse signal Sp. = Toff + Ton).

2-3. Circuit Constants In the sound wave generator 1 that generates sound waves as described above, the pulse width Ton, the pulse interval Toff, and the like can be set to various values by appropriately adjusting various circuit constants (FIG. 3) of the drive unit 20. It can be set. Hereinafter, the relationship between the pulse width Ton and the pulse interval Toff and the circuit constant will be described.

The relationship between the pulse width Ton and the circuit constant is that when the resistance value R2 of the limiting resistor 22 is sufficiently large (see FIG. 5), the charging voltage V (t1) at the start of discharging the capacitor 21 and the charging at the stop of discharging. Based on the voltage V (t2) (FIG. 6), it is expressed as the following equation.
V (t2) = V (t1) × exp [−Ton / (C1 × R1)] (1)

According to the sound wave generator 1 of the present embodiment, a desired pulse width Ton can be used by adjusting the circuit constants C1 and R1 as appropriate according to various specifications.

For example, consider a case in which 90% or more of the charging voltage V (t1) at the start of discharging is secured as the charging voltage V (t2) at the time when discharging of the capacitor 21 is stopped. In this case, from V (t2) ≧ 0.9V (t1), the pulse width Ton can be set within the range of the following equation.
Ton ≦ −C1 × R1 × ln (0.9) ≈0.1 × C1 × R1 (11)

In the case of the above equation (11), for example, the amount of change in current I1 before and after time t2 (see FIG. 6) when the current I1 flowing through the heating element 11 is stopped is 90% or more of the amount of change in current I1 before and after time t1. Can be secured.

Further, for example, from the viewpoint of using the capacitor 21 in a discharge that is significantly larger than that in the case of the above formula (11), the charging voltage V (t2) at the time of stopping discharging is set to 50% of the charging voltage V (t1) at the time of starting discharging. Consider the case above. In this case, the pulse width Ton can be set within the range of the following equation from the equation (1) as described above.
Ton ≦ −C1 × R1 × ln (0.5) ≈0.7 × C1 × R1 (12)

Specifically, when the circuit constant of the drive unit 20 is set to C1 = 30 uF and R1 = 1Ω, the pulse width Ton is 3 microseconds or less according to the equation (11). Further, according to the equation (12), the pulse width Ton is 21 microseconds or less.

Further, the relationship between the pulse interval Toff and the circuit constant is that current I2 = (EV (t2)) / R2 flows through the limiting resistor 22 at time t2 when charging of the capacitor 21 starts ( Based on the current I2 at time t3 after the pulse interval Toff (see FIG. 4), it is expressed as the following equation.
I2 = (EV (t2)) / R2 × exp [−Toff / (C1 × R2)] (2)

According to the above equation (2), the pulse interval Toff has a correlation with the resistance value R2 of the limiting resistor 22. According to this embodiment, a desired pulse interval Toff can be used by adjusting the circuit constants R2 and C1.

For example, consider a case where the capacitor 21 is charged so that the current I2 when charging ends at time t3 reaches 10% or less of the charging start time (t2). In this case, the pulse interval Toff can be set within the range of the following equation.
Toff ≧ −C1 × R2 × ln (0.1) ≈2.3 × C1 × R2 (21)

In the case of the above equation (21), for example, the capacitor 21 can be charged to near full charge during the pulse interval Toff from time t2 to time t3, and the capacitor 21 can be used efficiently.

According to the above equation (21), when the circuit constant of the drive unit 20 is set to R2 = 50Ω and C1 = 30 uF, the pulse interval Toff is 3.45 milliseconds or more. Further, when the resistance value R2 of the limiting resistor 22 is increased to 500Ω, the pulse interval Toff becomes 34.5 milliseconds or more. Further, when the resistance value R2 is increased to 5 kΩ, the pulse interval Toff becomes 345 milliseconds or more.

2-3-1. Simulation Results The simulation results of the sound wave generator 1 when the resistance value R2 of the limiting resistor 22 is changed as described above will be described with reference to FIGS.

FIG. 7 is a waveform diagram showing a simulation result of the sound wave generator 1 in which the resistance value R2 of the limiting resistor 22 is set to 50Ω.

The simulation in FIG. 7 was performed under the following simulation conditions. That is, the circuit constants R2 = 50Ω, C1 = 30 uF, R1 = 1Ω and the DC voltage E = 5V were set. Furthermore, the pulse width Ton was set to 10 microseconds, and the pulse period Tp was set to 100 milliseconds. At this time, the pulse interval Toff (approximately 100 milliseconds) is within the range of the conditional expression (21) described above.

FIG. 7A shows the charging voltage V (t) of the capacitor 21 in the simulation of R2 = 50Ω. FIG. 7B shows the current I2 of the limiting resistor 22. FIG. 7C shows the current I1 of the heating element 11.

According to the simulation of FIG. 7, the charging voltage V (t) of the capacitor 21 reaches full charge V (t) = 5 V during the pulse period Tp as shown in FIG. 7A. Accordingly, as shown in FIG. 7B, the current I2 of the limiting resistor 22 has a peak value of 28 mA at the timing of the pulse width Ton (that is, during discharge). At this time, the peak value of the current I1 of the heating element 11 is 5A as shown in FIG.

FIGS. 8 and 9 show the simulation results when the resistance value R2 of the limiting resistor 22 is increased from the above simulation conditions of FIG.

FIG. 8 is a waveform diagram showing a simulation result of the sound wave generator 1 in which the resistance value R2 of the limiting resistor 22 is set to 500Ω. Note that the conditional expression (21) of the pulse interval Toff is also satisfied in this case.

FIGS. 8A, 8B, and 8C show the charging voltage V (t) of the capacitor 21, the current I2 of the limiting resistor 22, and the current I1 of the heating element 11 in the simulation of R2 = 500Ω, respectively. .

According to the simulation of FIG. 8, the charging voltage V (t) of the capacitor 21 reaches V (t) = 5 V as in FIG. 7A (FIG. 8A). On the other hand, the peak value of the current I2 of the limiting resistor 22 is reduced to 2.8 mA from the case of FIG. 7B (FIG. 8B). For the current I1 of the heating element 11, the same peak value 5A as in FIG. 7C is obtained (FIG. 8C).

FIG. 9 is a waveform diagram showing a simulation result of the sound wave generator 1 in which the resistance value R2 of the limiting resistor 22 is set to 5 kΩ. In this case, the conditional expression (21) of the pulse interval Ton is not satisfied.

9 (a), 9 (b), and 9 (c) show the charging voltage V (t) of the capacitor 21, the current I2 of the limiting resistor 22, and the current I1 of the heating element 11 in the simulation of R2 = 5 kΩ, respectively. .

According to the simulation of FIG. 9, the charging voltage V (t) of the capacitor 21 is less than 3.9V as shown in FIG. 9 (a). Further, as shown in FIG. 9B, the peak value of the current I2 of the limiting resistor 22 is reduced to 450 uA from the cases of FIGS. 7B and 8B. Further, as shown in FIG. 9C, the peak value of the current I1 of the heating element 11 is reduced to less than 4.0 A from the cases of FIGS. 7C and 8C.

According to the simulation results of FIGS. 7 to 9, it is confirmed that the peak value of the current I2 of the limiting resistor 22 is sufficiently smaller than the value of the heating element 11 that generates heat, such as the current I1, when R2 ≧ 50Ω. (Fig. 7 (b), Fig. 8 (b), Fig. 9 (b)). Therefore, according to the sound wave generator 1 of the present embodiment, it was confirmed that the influence of the overcurrent on the heating element 11 can be reduced by the limiting resistor 22 even if there is a malfunction in which the MOSFET 23 is always turned on.

On the other hand, in the simulation of FIG. 9 at R2 = 5 kΩ, since the pulse interval Toff used for the simulation does not satisfy the above-described equation (21), the discharge starts before the capacitor 21 is sufficiently charged. (FIG. 9A). Even in the case of R2 = 5 kΩ, an operation for sufficiently charging the capacitor 21 can be realized by appropriately setting the pulse interval Toff (see Expression (21)).

FIG. 10 is a graph illustrating the relationship between the current I1 of the heating element 11 and the resistance value R2 of the limiting resistor 22 in the sound wave generator 1.

FIG. 10 shows the result of numerical calculation of the peak value of the current I1 when the resistance value R2 of the limiting resistor 22 is changed under the same simulation conditions as in FIGS. The peak value of the current I1 corresponds to the current I1 that flows through the heating element 11 when the capacitor 21 is discharged.

According to FIG. 10, the peak value of the current I1 flowing through the heating element 11 is not reduced until the resistance value R2 of the limiting resistor 22 reaches 1 kΩ. From this, it was confirmed that the limiting resistor 22 can be applied without affecting the output of the sound wave from the sound wave source 10.

FIG. 11 is a graph illustrating the relationship between the current I2 of the limiting resistor 22 and the resistance value R2 in the sound wave generator 1.

FIG. 11 shows the calculation result of the peak value of the current I2 flowing through the limiting resistor 22 when the capacitor 21 is discharged in a simulation in which the resistance value R2 of the limiting resistor 22 is changed under the same simulation conditions as in FIGS. . FIG. 11 shows a calculation result when the capacitance value C1 of the capacitor 21 is changed to 10 uF and 100 uF in addition to the case of C1 = 30 uF.

In FIG. 11, the current I2 of the limiting resistor 22 at the same resistance value R2 decreases as the capacitance value C1 of the capacitor 21 increases. Further, as the resistance value R2 of the limiting resistor 22 is increased, the current I2 is decreased, and the difference in the current I2 between the different capacitance values C1 is converged.

For example, when the internal resistance of the DC power supply 24 is 0.1Ω, the current I2 flowing from the DC power supply 24 is 5% or less of the case where the limiting resistor 22 is not used (R2 = 0.1Ω in the figure). think of. According to FIG. 11, the condition can be achieved by setting the resistance value R2 to 100Ω or more. Therefore, the resistance value R2 of the limiting resistor 22 may be 1000 times or more the internal resistance of the DC power supply 24.

3. Summary As described above, the sound wave generator 1 according to this embodiment includes the sound wave source 10, the MOSFET 23, the capacitor 21, and the limiting resistor 22. The sound wave source 10 generates heat by generating heat due to the current supplied from the DC power supply 24. The limiting resistor 22 is inserted between the DC power supply 24 and the sound wave source 10 and limits the current flowing from the DC power supply 24 to the sound wave source 10. The MOSFET 23 is connected in series with the sound wave source 10 and is turned on / off to control inflow / cutoff of a current flowing through the sound wave source 10. The capacitor 21 is connected in parallel to the series circuit of the sound wave source 10 and the MOSFET 23.

Further, the sound wave generator 1 according to the present embodiment includes the sound wave source 10, the MOSFET 23, the capacitor 21, and the limiting resistor 22. The sound wave source 10 generates heat by generating heat by the current I1 supplied from the DC power supply 24. The MOSFET 23 controls ON / OFF of the current I1 flowing into the sound wave source 10 by turning on / off. The capacitor 21 is charged by the DC power supply 24 when the MOSFET 23 is turned off, and supplies the current I1 due to charging to the sound wave source 10 when the MOSFET 23 is turned on. The limiting resistor 22 limits the current flowing through the sound wave source 10 when the MOSFET 23 is turned on.

According to the sound wave generator 1 described above, even if the MOSFET 23 remains on, the current flowing through the sound wave source 10 is limited by the limiting resistor 22. Thereby, in the sound wave generator 1, the safety | security at the time of generating a sound wave by the heat_generation | fever according to an electric current can be improved.

For example, when the MOSFET 23 fails and the drain and the source are short-circuited, the limiting resistor 22 limits the current flowing through the sound source 10, thereby suppressing excessive heat generation of the sound source 10. Further, the limiting resistor 22 can limit the current consumption of the sound wave generator 1 even when the MOSFET 23 is short-circuited. For this reason, an inexpensive power source having a small current capacity can be used as the DC power source 24. According to the present embodiment, it is possible to provide a sound wave generator with limited current consumption.

Further, in the sound wave generator 1 according to the present embodiment, the resistance value R2 of the limiting resistor 22 is larger than the resistance value R1 of the sound wave source 10. Thereby, the safety | security at the time of generating a sound wave by the heat_generation | fever according to an electric current can be improved more. The resistance value R2 of the limiting resistor 22 may be, for example, 50 times or more than the resistance value R1 of the

sound source

10, or 100 times or more.

In the sound wave generator 1 according to this embodiment, the sound wave source 10 is connected to the MOSFET 23 and the capacitor 21. One end of the limiting resistor 22 is connected between the sound wave source 10 and the capacitor 21. A DC voltage E is supplied from the DC power source 24 to the other end of the limiting resistor 22.

According to the above configuration, the capacitor 21 is charged when the MOSFET 23 is turned off. The capacitor 21 supplies a current due to charging to the sound wave source 10 when the MOSFET 23 is turned on. At the same time, the limiting resistor 22 limits the current flowing from the DC power supply 24 to the sound wave source 10 when the MOSFET 23 is turned on. Thereby, the safety | security of the sound wave generator 1 can be improved.

The sound wave generator 1 according to this embodiment further includes a pulse driving circuit 25 that drives the MOSFET 23 in pulses. Thereby, for example, a single-pulse sound wave can be formed. Various sound waves can be generated by changing the pulse width and the pulse period.

In the sound wave generator 1 according to the present embodiment, the sound wave source 10 includes a heating element 11, a heat insulating layer 14, and a substrate 12 on which the heating element 11 and the heat insulating layer 14 are laminated. The heating element 11 generates heat when a current flows. The heat insulating layer 14 insulates heat from the heating element 11. Thereby, the sound wave generator 1 can efficiently conduct the heat from the heating element 11 to the air on the side opposite to the substrate 12.

(Other embodiments)
In the first embodiment, the configuration example of FIG. 3 has been described for the drive unit 20 of the sound wave generator 1. The drive unit 20 of the sound wave generator 1 is not limited to the configuration example of FIG. 3 and may have various circuit configurations. For example, the switching element of the drive unit 20 is not limited to the n-type MOSFET 23 but may be a p-type MOSFET. In this case, the high-pressure side and the low-pressure side of the connection relationship in the drive unit 20 are appropriately reversed.

Further, the switching element of the drive unit 20 is not limited to the MOSFET, but may be, for example, an IGBT (insulated gate bipolar transistor). In the configuration example of FIG. 3, an example in which each part such as the MOSFET 23 in the drive unit 20 is grounded has been described. However, the configuration is not limited thereto, and each part of the drive unit 20 may be connected to various potentials as appropriate.

Moreover, although the sound wave generator 1 of each said embodiment was provided with the DC power supply 24 and the pulse drive circuit 25, it is not restricted to this, The sound wave generator 1 is provided with the DC power supply 24 and / or the pulse drive circuit 25. It does not have to be. In this case, the DC voltage E and / or the pulse signal Sp is supplied to the sound wave generator 1 from the outside.

In each of the above embodiments, the example in which the cycle of the pulse signal Sp is set in advance has been described. The pulse width of the pulse signal Sp and the like may be set from a control circuit outside the sound wave generator 1.

In each of the embodiments described above, a ceramic capacitor is used as an example of the capacitor 21 in the driving unit 20 of the sound wave generator 1. However, a ceramic capacitor in which noise reduction measures are taken may be used.

In the above embodiments, specific values of the circuit constants of the sound wave generator 1 are exemplified, but values of other circuit constants may be used as appropriate.

Claims

A sound source that generates heat waves by generating heat from a current supplied from a DC power source;
A resistance element that is inserted between the direct current power source and the sound wave source and restricts a current flowing from the direct current power source to the sound wave source;
A switching element connected in series to the acoustic wave source and controlling on / off of current flowing through the acoustic wave source by turning on / off;
A capacitor connected in parallel to a series circuit of the acoustic wave source and the switching element;
A thermal excitation type sound wave generator comprising:
The thermal excitation type sound wave generator according to claim 1, wherein a resistance value of the resistance element is larger than a resistance value of the sound wave source.
The thermal excitation type sound wave generator according to claim 1, further comprising a direct current power source connected to the resistance element.
The thermal excitation type sound wave generator according to any one of claims 1 to 3, further comprising a pulse driving circuit for driving the switching element in pulses.
The sound wave source includes: a heating element that generates heat when an electric current flows; a heat insulating layer that insulates heat from the heating element; and a substrate on which the heating element and the heat insulating layer are stacked. The thermal excitation type sound wave generator according to any one of the preceding claims.
A sound source that generates heat waves by generating heat from a current supplied from a DC power source;
A switching element for controlling on / off of current flowing into the sound wave source by turning on / off;
A capacitor that is charged by the DC power supply when the switching element is turned off, and that supplies a current from the charging to the sound wave source when the switching element is turned on;
A thermal excitation type sound wave generator comprising: a resistance element that limits a current flowing through the sound wave source when the switching element is turned on.