JPH06186105A - Pressure sensor - Google Patents

Abstract

PURPOSE:To provide a pressure sensor wherein it corrects a vibration, a temperature fluctuation and a noise propagated to the whole of a fluid by means of a sensor structure, it executes an electric-signal processing operation, its S/N ratio is extremely high, its sensitivity is high and its reliability is high. CONSTITUTION:Two piezoelectric elements 2, 3 in which electrode layers have been formed on both faces of each filmlike piezoelectric body are arranged in such a way that faces in the same direction become the same polarizing polarity. Then, in a pressure sensor, a fluid pressure is introduced from two entrance nozzles respectively into a first face on the piezoelectric element on one side and into a second face on the piezoelectric element on the other side as well as into a first face on the piezoelectric element on the other side and into a second face on the piezoelectric element on one side, the faces, whose polarizing polarity is different from each other, on the piezoelectric elements 2, 3 are connected electrically, outputs from the piezoelectric elements are passed through a filter circuit 51 and the outputs are amplified.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used for a gas flow meter using a piezoelectric element, which can detect even minute pressure fluctuations and has a high ratio of noise due to vibration to an output signal (hereinafter referred to as SN ratio). And optimal pressure sensor.

[0002]

2. Description of the Related Art Conventional pressure sensors are
Various types are known, such as a digital camera, a Bourdon tube type, a capacitance type, a differential transformer type, and a semiconductor diaphragm type. These are generally used for detecting a large pressure, and are large in size, complicated in structure, and expensive.
Further, since the pressure to be detected is large, noise is not usually considered due to disturbance such as vibration. For this reason, it cannot be used in applications requiring detection of even minute pressures, or even if it is used, the proportion of noise becomes high, and high-precision detection becomes difficult.

As a preferable application that can detect even a minute pressure, for example, there is a fluidic flowmeter or a Karman vortex flowmeter which detects the flow rate of a fluid through pressure detection (for example, Japanese Patent Laid-Open No. 60-187814). 57-
54809). A pressure sensor using a film-shaped piezoelectric element is known as an optimal pressure sensor that can detect even minute pressure fluctuations using these flowmeters (for example, JP-A-57-54809). In the fluidic flowmeter, as disclosed in Japanese Patent Laid-Open No. 60-187814, in the flowmeter element portion, the main flow of the fluid ejected from the ejection nozzle is divided into a pair of partition walls by utilizing the Coanda effect. The pressure sensors are arranged alternately, and pressure fluctuations that occur when the flow changes are detected by a pressure sensor, and the flow rate of the fluid is detected at a frequency of pressure fluctuations corresponding to the flow rate. The Karman vortex flowmeter, as disclosed in Japanese Patent Laid-Open No. 57-54809, generates a Karman vortex intermittently in a flowing fluid by a vortex generator provided in a flowmeter element section, Is detected by a pressure sensor, and the flow rate of the fluid is detected by the number of Karman vortices generated according to the flow rate.

A pressure sensor used in such a flow meter is desired to have high sensitivity, that is, to detect even a weak pressure change and a pressure change in a wide frequency range. For that purpose, measures against disturbance such as vibration are taken, The signal-to-noise ratio must be fairly high. The pressure sensor disclosed in the above-mentioned Japanese Patent Laid-Open No. 57-54809 is known as a measure against vibration in order to increase the SN ratio. This pressure sensor uses a polymer piezoelectric material as a piezoelectric element, and two piezoelectric elements are attached like a bimorph type on both sides of a lever that branches for pressure detection and noise component correction, for noise component correction. The piezoelectric element covers the surrounding area to separate the pressure detecting piezoelectric element from the atmosphere. Since the vibration that is a noise component is applied to both the pressure detection piezoelectric element and the noise component correction piezoelectric element, it is theoretically possible to subtract the signal from the noise component correction piezoelectric element from the output signal from the pressure detection piezoelectric element. The noise due to vibration will be removed. This method is the most general method called the differential type that corrects noise components other than the detection target.

However, in the structure of the pressure sensor, since the noise component correcting piezoelectric element and the pressure detecting piezoelectric element are housed in different structures and atmospheres, respectively, vibration and temperature may be transmitted to both piezoelectric elements. However, there is a problem that the noise component cannot be removed accurately and sufficiently. In particular, since the piezoelectric element for correcting the noise component is isolated, it is difficult to remove the noise related to the temperature change of the fluid and the noise component transmitted to the entire fluid from the throttle valve and the like. Moreover, since the mechanically weak piezoelectric element is exposed in the fluid flow path, the problem that foreign matter in the fluid easily adheres to the piezoelectric element, and the manufacturing, handling,
There is a problem that the piezoelectric element is likely to be damaged or the sensitivity is lowered in the process of implementation.

[0006] In order to solve such a problem, the present applicant previously arranged two piezoelectric elements in parallel, and from two inlet nozzles to each surface of each piezoelectric element via a fluid pressure introduction path of the same volume. There has been proposed a pressure sensor in which each pressure is propagated and the pressure is simultaneously detected by each piezoelectric element in a reverse phase, and a gas flow meter using the pressure sensor (Japanese Patent Laid-Open No. HEI-2).
-268230). With this proposal, it is possible to accurately correct not only noise due to vibration but also noise such as temperature fluctuations and noise transmitted throughout the fluid, and the SN ratio is high.
A highly sensitive pressure sensor with a simple structure and high reliability that can detect even weak pressure fluctuations and a highly accurate gas flow meter using the pressure sensor were obtained, but the following problems were encountered. found. For example, in a fluidic flow meter, the frequency of fluid vibration is proportional to the flow rate, and its amplitude, that is, fluctuating pressure, is proportional to the square of the flow rate. When the flow rate increases, the pressure fluctuation (differential pressure) introduced into the piezoelectric element portion increases, but at the same time, the pressure fluctuation due to the noise component also increases. In JP-A-2-268230, since the output from the piezoelectric element is simply amplified to a measurable level, when the flow rate is large as described above, the output of the amplifier is changed from the relationship with a certain constant power supply voltage. As a saturated state (a state where the output does not reach a certain value or more is reached)
May become. At this time, if there is a large noise component, the noise component may be counted as the flow rate proportional frequency to be measured. That is, S
The N ratio decreases, and the measurement accuracy deteriorates.

Therefore, the applicant of the present invention further corrected the vibration, temperature fluctuation, noise propagated in the entire fluid by the sensor structure, and further performed electrical signal processing to obtain an extremely high SN ratio. , A highly sensitive and highly reliable pressure sensor is provided (Japanese Patent Application No. 3-274874).
issue).

[0008]

In the pressure sensor previously proposed (for example, Japanese Patent Application Laid-Open No. 2-268230, Japanese Patent Application No. 3-274874, etc.), vibration and fluid pressure fluctuation are caused by two piezoelectric elements as in-phase noise. It has a high noise removal effect. However, as a result of further study, there was a large pressure fluctuation on the fluid supply side when it was actually attached to a fluidic flow meter and used, and the pressure that fluctuated simultaneously on both surfaces of the two piezoelectric elements was It has been found that there is a problem that the noise due to the so-called same pressure fluctuation may exceed the signal value at the flow rate normally measured.

An object of the present invention is to further reduce vibrations, temperature fluctuations, noises propagated to fluids, etc. by changing the connection method of the piezoelectric elements of the pressure sensor proposed above.
It is to provide a highly sensitive and reliable pressure sensor having an extremely high SN ratio.

[0010]

According to the pressure sensor of the present invention for this purpose, two piezoelectric elements each having an electrode layer provided on both sides of a film-shaped piezoelectric body are sandwiched by two pairs of holders from both sides. And the two piezoelectric elements are arranged so as to face in the same direction and have the same polarization polarity on the surfaces in the same direction, and the pressure of the fluid is introduced into the pressure detecting section. Two inlet nozzles are provided, one inlet nozzle communicates with the first surface of the one piezoelectric element via the first fluid pressure introducing path, and the other inlet nozzle via the fourth fluid pressure introducing path. The first surface of the one piezoelectric element is communicated with the second surface of the other piezoelectric element having a different polarization polarity, and the other inlet nozzle is connected to the second surface of the other piezoelectric element through the second fluid pressure introduction path. And the third fluid pressure introduction path Wherein a pressure sensor communicating with the first surface of the other piezoelectric element, the two mutually polarization polarities different surfaces of the piezoelectric element is electrically connected Te,
An output from the connection portion of one of the piezoelectric elements is connected to a filter circuit, and the filter circuit is connected to an amplifier circuit that amplifies the output from the filter circuit.

In order to remove noise from the output of the pressure sensor and extract a more accurate frequency corresponding to the flow rate, it is preferable to connect a voltage comparator for outputting a pulse of the frequency of the amplifier circuit to the amplifier circuit. . Further, in order to deform the piezoelectric element more sensitively to a pressure change to make a pressure sensor with higher sensitivity, it is preferable to stretch the piezoelectric element in a diaphragm shape or a spherical shape. In order to obtain the state, it is preferable that the holding surface of the holder that holds the two piezoelectric elements is formed as an inclined surface.

This pressure sensor is an optimum pressure sensor for use in a gas flow meter including a fluidic flow meter and a Karman vortex flow meter, and the two inlet nozzles of the pressure sensor are two of the same area. It is connected to a flow meter element section that generates a pressure fluctuation according to the flow rate of the fluid that is circulated, through a gasket having a pressure hole.

In the present invention, the piezoelectric body used for the piezoelectric element is a ceramic piezoelectric body represented by lead zirconate titanate (PZT), a polymeric piezoelectric body, or a polymeric P body.
It is composed of a composite piezoelectric material mixed with ZT powder. The polymeric piezoelectric body is made of, for example, a copolymer of vinylidene fluoride and ethylene trifluoride. The film thickness depends on sensitivity, strength,
It is preferable to select it in consideration of handleability. The film thickness can be easily and highly accurately controlled by a known film forming technique.

The electrode layers provided on both sides of the piezoelectric body are made of a material such as aluminum, copper, gold, nickel and palladium, and are formed on the surface of the piezoelectric body by a metallizing method such as vapor deposition, sputtering and plating. It is desirable that each fluid pressure introduction path has a bent structure or a throttle interposition structure so that the introduced fluid does not directly contact the piezoelectric element surface. Further, the polarization polarity of the piezoelectric element is given in the process of manufacturing the piezoelectric body, and a well-known method can be applied to this. The piezoelectric elements are arranged in the pressure detection unit so that both surfaces thereof face in the same direction, but they may be arranged in parallel or in series.

[0015]

In the pressure sensor as described above, the pressure of the fluid taken out from a certain pressure outlet is passed through one inlet nozzle, the first fluid pressure introducing path, and the fourth fluid pressure introducing path to the one side. Of the same fluid that is simultaneously introduced into the first surface of the piezoelectric element and the second surface of the other piezoelectric element and taken out from another pressure outlet,
Through the third fluid pressure introducing path and the second surface of the one piezoelectric element and the first piezoelectric element of the other piezoelectric element.
Are simultaneously introduced into the surface of the piezoelectric element, the amount of polarization changes due to the deformation of each piezoelectric element caused by the pressure difference between the two surfaces, and the electric charges on the both surfaces of the piezoelectric element generated thereby are output through the electrode layer. Then, it is connected to the filter circuit so that the sum of the charges of both piezoelectric elements is input.

In the detection by this pressure sensor,
(A) A structure in which fluid pressure is simultaneously introduced to both sides of the piezoelectric element and the differential pressure is detected, (b) Two piezoelectric elements are arranged side by side in the same atmosphere with substantially the same structure,
(C) Since the electrical signal processing is performed so that the SN ratio becomes high, the noise component is extremely accurately removed.

First, since the fluid pressure is simultaneously guided to both surfaces of one piezoelectric element, the pressure fluctuation of the entire fluid can be obtained by connecting two inlet nozzles to different measurement positions for the same fluid to be measured. The noise component) can be simultaneously introduced to both surfaces of the piezoelectric element, and the pressure fluctuation corresponding to the measurement position can be introduced from one path. Since the pressure fluctuation of the entire fluid is simultaneously transmitted to both surfaces of the two piezoelectric elements, the fluctuation component causes the two piezoelectric elements to undergo the same deformation and the piezoelectric element itself erases the potential difference due to the pressure fluctuation of the entire fluid. A signal having a frequency corresponding to the potential difference corresponding to the differential pressure component between the two surfaces and the number of times of differential pressure generation is output. The same applies to temperature fluctuations of the entire fluid, and temperature fluctuations are simultaneously transmitted to both surfaces of each piezoelectric element, so the two piezoelectric elements undergo the same deformation due to this fluctuation component, and the piezoelectric elements themselves are deformed. Noise components due to temperature fluctuations are automatically deleted. Therefore, by connecting this pressure sensor to the flowmeter element part of a fluidics flowmeter or a Karman vortex flowmeter, the fluctuation component of the entire fluid, which may cause noise, is automatically deleted, while the pressure fluctuation component due to fluid vibration or Karman Only the pressure fluctuation component due to the vortex can be detected with high accuracy.

Since the pressures from the same path are introduced to the surfaces of the respective piezoelectric elements arranged so that the polarization polarities thereof are in the same direction, the deformation directions of both piezoelectric elements are different. The opposite phase is obtained, and by outputting as the sum of both charges, an output signal from which the noise component due to the fluctuation of the entire fluid is removed can be obtained.

As described above, both piezoelectric elements simultaneously detect the pressures to be measured in opposite phases, but when vibration is applied to both piezoelectric elements, the vibration reduces the detected pressure in one of the piezoelectric elements. If the deforming force is applied, the deforming force in the direction of increasing the detected pressure is always applied to the other piezoelectric element with the same magnitude. The output from the pressure detector is
It is taken out as the sum of the detected charges of opposite phases of both piezoelectric elements. That is, it is taken out as the sum of electric charges generated in both piezoelectric elements. (Hereinafter, this is referred to as "sum of outputs".)
Then, the in-phase noise component associated with the vibration is automatically erased. However, in reality, the pressure detecting portion including the piezoelectric element is carefully manufactured, but in some manufacturing steps, a minute sensitivity difference that causes a reduction in the SN ratio with respect to vibration occurs between the two piezoelectric elements. In some cases, when a large in-phase noise is added, the noise component may not be completely erased, and a minute charge may remain without being erased. That is, in-phase noise, which has a large noise component and is normally automatically canceled, may reach the so-called measurement signal level.

According to the piezoelectric element connection method provided by the present invention, the common-mode noise resistance is almost doubled as compared with the method of connecting piezoelectric elements in series proposed in Japanese Patent Application No. 3-274874. On the other hand, the measurement signal due to the original differential pressure is detected in the frequency range to be measured, and the output of the amplifier circuit is detected with substantially the same sensitivity as the previously proposed method of connecting the piezoelectric elements in series.

Further, in the pressure sensor of the present invention, even if the output of the pressure sensor is limited due to the relationship with the power supply voltage, that is, the output of the pressure detection unit is amplified. Even in such a case, electrical signal processing is performed so as to obtain a large SN ratio. As described above, the fluctuating pressure due to fluid vibration (that is, the differential pressure between the two surfaces of the piezoelectric element) is approximately proportional to the square of the flow rate. When the flow rate increases, the differential pressure increases and the pressure fluctuation due to the noise component also increases. . Therefore, when the output from the pressure detection unit is simply amplified and the amplified output is in a saturated state, if there is a large noise component close to the saturation value, the SN ratio decreases. In such a case, in the pressure sensor of the present invention, the pressure detection unit is connected to, for example, a filter circuit having a gain characteristic of a square of the output frequency from the pressure detection unit, and the filter circuit is connected to the filter circuit. By connecting to the amplifier circuit that amplifies the output from the circuit, the output from the pressure detection unit, which is proportional to the square of the flow rate, is multiplied by the gain characteristic of the frequency squared by the filter circuit, which is amplified. The output of the pressure sensor is amplified by the circuit and is controlled to an appropriate constant level, and the noise component is suppressed to a small level. As a result, only the flow rate proportional frequency that is the measurement target is efficiently counted, and the SN ratio can be greatly improved.

Such signal processing may be performed on a region having a frequency higher than a certain level, ie, a region having a flow rate higher than a certain level, in which the output from the pressure detection unit becomes large. When applied to a region of too low frequency, the output itself from the pressure detection unit is suppressed to a small value. Specifically, the gain characteristic of the frequency squared by the filter circuit is, for example, a characteristic of the frequency of the falling portion of the falling portion in which the gain decreases substantially in proportion to the increase in the frequency due to the low-pass filter, It is obtained by combining with the characteristic of the frequency division of the operational amplifier itself.

Further, which frequency range or more to which the gain characteristic of the square of the frequency is to be given can be easily changed by changing the resistance in the circuit as shown in the embodiments described later. A highly accurate pressure sensor having a high SN ratio can be obtained according to the specifications of the flow meter (the flow rate to be measured and the characteristics of the flow meter element).

In the present invention, the impedance of the piezoelectric element is lowered and the impedance of the filter circuit is also lowered by electrically connecting the surfaces of the two piezoelectric elements having different polarization polarities from each other, that is, in parallel connection. Due to the relationship between the operational amplifier and the impedance of the surroundings, the sensitivity is almost the same as the previously proposed two piezoelectric elements connected in series, while the same pressure caused by a large pressure fluctuation on the fluid supply side. The noise removal ratio due to fluctuations is improved. By the way, in the previously proposed invention, the removal ratio for the same pressure fluctuation is 80 dB or more, and 10 Hz, 100 Pa
0.01Pa _pp in a normal differential pressure measured under the same pressure fluctuations of _pp
The output is equivalent to. Generally, as described above, the electric signal processing circuit configuration except for the piezoelectric element is exactly the same, and the effect of removing in-phase noise is disclosed in Japanese Patent Application Laid-Open No. 2-268230 and Japanese Patent Application No. 3-274874. Although it is sufficient to connect two proposed piezoelectric elements in series, the present invention further improves the noise removal ratio by about 2 times.

[0025]

Embodiments of the present invention will be described below with reference to the drawings. 2 to 6 show a pressure sensor according to an embodiment of the present invention, and FIG. 7 shows an example in which the pressure sensor is used in a fluidic flow meter.

FIG. 1 shows an electric signal processing circuit of a pressure sensor according to an embodiment of the present invention. The present invention has a power supply conversion circuit for generating an intermediate voltage from a single DC power supply because it is designed to operate with only one battery 52, but in this embodiment, an operational amplifier 58 is used to connect a resistor R9 and a resistor R9. The circuit configuration is such that the resistor R10 outputs a voltage obtained by dividing the power supply voltage. In addition, a power supply conversion circuit is placed in each circuit part to enhance isolation so that noise between the analog circuit part such as the filter circuit and the amplifier circuit and the digital circuit part such as the comparator circuit do not affect each other. It's even better then. In the present invention, since the current consumption is reduced, the processing speed of the operational amplifier is slow. Therefore, in order to increase the slew rate, it is desirable to process the output of the final stage comparator by a C-MOS type Schmitt. At this time, the resistor R8 is added to the Schmitt output.

In FIG. 2, reference numeral 1 denotes the entire pressure sensor according to the present invention. The pressure sensor 1 includes a pressure detecting section 4 having two piezoelectric elements 2 and 3, and the above-mentioned electrical signal processing circuit. 5 (FIG. 1). Two piezoelectric elements 2, 3
Is sandwiched by two pairs of holders 12, 13 and 14, 15 from both sides, and the central part is stretched so as to be deformable. As shown in the enlarged view of FIG. 3, the piezoelectric elements 2 and 3 are formed on both surfaces of film-shaped piezoelectric bodies 6 and 7 made of a piezoelectric material, and are made of a metal such as aluminum, copper, gold, nickel, or palladium having good corrosion resistance. Electrode layers 8, 9 and 10, 11 consisting of
It is formed by plating or the like. The planar shape of the two piezoelectric elements 2 and 3 is circular in the present embodiment, but is not limited to this. Although various piezoelectric materials can be applied as the material of the piezoelectric bodies 6 and 7, in this embodiment, a polymer piezoelectric material made of a copolymer of vinylidene fluoride and ethylene trifluoride was used. The thickness of the piezoelectric polymer film is determined considering sensitivity, strength, and ease of handling.
In this embodiment, the thickness is 15 μm. However, the pressure sensor according to the present invention includes, of course, a liquid sensor, and the thickness of the polymer piezoelectric film and the structure such as an electrode portion described later may be appropriately selected according to the liquid to be measured. Although not shown, depending on the fluid to be measured and the magnitude of pressure fluctuation, it is possible to attach a polymer piezoelectric film to a thin metal or rubber diaphragm and treat it as a reinforced polymer piezoelectric film as a whole. .

The peripheral portions of the piezoelectric elements 2 and 3 are held from both sides by holders 12, 13 and 1 via electrode layers 8, 9 and 10, 11.
Held by 4, 15. The piezoelectric elements 2 and 3 held by the respective holders are arranged in parallel so that both surfaces thereof face in the same direction, and the surfaces in the same direction have the same polarization polarity. Holder 12, 1
3, 14, and 15 are metal such as brass, plastic with good water resistance and oil resistance, which is an insulator such as PBT (polybutylene terephthalate), mixed with conductive material, plated, etc. , The whole or the surface of which is made of a conductor, and when the temperature dependence of the sensitivity is close to that of the piezoelectric element, it is preferable to use a material having a linear expansion coefficient close to that of the piezoelectric element. 8, 9, 10, 11
To function as output terminals of each piezoelectric element. Between the holders 12 and 13, 14 and 15, the piezoelectric body 6,
Insulated by 7.

This pair of two holders 12, 13 and 14, 15
Then, the piezoelectric elements 2 and 3 are sandwiched and stretched as shown in FIGS. 4 and 5. Piezoelectric elements 2 and 3 having a circular planar shape
Is sandwiched by the holder at its peripheral portion, and the piezoelectric element sandwiching surfaces 12a 1, 13a and 14a 1, 15a of the holder are formed as inclined surfaces inclined in the same direction at the same angle θ. The inclination angle θ was 7 ° at which the maximum output was obtained from the test results of the inclination angle θ and the output of the pressure sensor shown in FIG.
The angle of is adopted. However, this inclination angle may be appropriately determined depending on the material, thickness, rigidity, etc. of the piezoelectric elements 2 and 3 so that the output of the pressure sensor is maximized. Piezoelectric element 2,
Before being assembled, that is, before being sandwiched between both holders 12, 13 or 14, 15, 3 is formed as a simple flat film, as shown in FIG. And slopes 12a, 13a and
When sandwiched between the holders 12, 13 and 14 and 15 having 14a and 15a, the piezoelectric elements 2 and 3 are forcibly forced along the inclined surfaces 12a, 13a and 14a and 15a as shown in FIG. Since the piezoelectric elements 2 and 3 are deformed, the entire piezoelectric elements 2 and 3 have a diaphragm shape (the shape shown) or a spherical shape, and are stretched in that state.

By using a piezoelectric element previously formed in a diaphragm shape or a spherical shape and carefully holding it between both holders, the shape is maintained and the sensitivity is maintained.

On both sides of the piezoelectric elements 2 and 3, pressure chambers 16 and
17, 18, 19 are formed. Each pressure chamber 16, 17, 18, 19
Respectively communicate with the corresponding front chambers 20, 21, 22, 23.

The piezoelectric elements 2 and 3 are pressed and sandwiched as shown in FIG. Although the illustrated example shows the piezoelectric element 2, the piezoelectric element 3 has a similar structure. The piezoelectric element 2 is sandwiched by the holders 12 and 13 from both sides, the lower holder 12 is supported via the spacer 24 and the electrode plate 25, and the electrode plate 25 has a square head (rotated). It is fixed via screws 26. Upper holder
13 is urged by a spring 28 via an electrode plate 27, and the urging force of the spring 28 is used to cause the piezoelectric element 2 to move to both holders 1
It is clamped by pressure between 2 and 13. The electrode plate 27 is fixed in position by a screw 29 having a square head, while having the degree of freedom in the pressure contact direction. The screw 29 is fixed to the lid 33 of the substrate 32 via an O-ring 49, a washer 30, and a nut 31 for preventing gas from leaking from the pressure detecting portion 4. A circuit board 35 is fixed to each screw 29 via a nut 34, and the electrical signal processing circuit 5 is mounted on the circuit board 35.
Is built in. Reference numeral 36 and 37 in FIG.
The power input terminal of the circuit 5 and the output terminal 38 of the circuit 5 are shown. The input / output terminals 36, 37, 38 are hermetically sealed to the lid 46 of the shield case to prevent gas from leaking from the terminals. In addition, one electrode plate 25 and the other electrode plate 27 of the piezoelectric elements 2 and 3 are connected to each other via screws 26 and 29 and a short-circuit plate 86, respectively, and one of the connecting portions via the screw 29. Is connected to the electrical signal processing circuit 5 as an input to the circuit 5, and the other is connected to a pole (to be described later) having an intermediate potential among the potentials supplied from the power source.

The pressure detector 4 includes holders 12, 13 and 14,
A substrate 32 for accommodating and holding 15 and a second fluid pressure introduction path 43
Of the substrate and a part of the fourth fluid pressure introduction path 45 are surrounded by a substrate lid 33 and a shield case 41 in which two inlet nozzles 39, 40 for introducing the fluid pressure are formed. . The shield case 41 and the substrate 32, the substrate 32 and the lid 33 thereof, and the shield case 41 and the lid 46 thereof are joined to each other via a deformed O-ring so as not to leak from the inside. The substrate 32 and its lid 33 may be made of engineering plastic such as epoxy resin or polyacetal resin having good insulation and high mechanical strength. In this embodiment, it is made of polybutylene terephthalate resin (PBT resin). ing.

One of the inlet nozzles 39 has a front chamber 20 and a pressure chamber 16
Via the communication passage 47, the front chamber 23, and the pressure chamber 19 to the surface of the other piezoelectric element 3 having a different polarization polarity (the second surface). ing. Therefore, the path from the front chamber 20 to the pressure chamber 16 constitutes the first fluid pressure introduction path 42 in the present invention, and the path from the communication passage 47 to the pressure chamber 19 is the fourth fluid pressure introduction path.
It comprises 45. The other inlet nozzle 40 communicates with the other surface (first surface) of the piezoelectric element 3 through the front chamber 22 and the pressure chamber 18, and also through the communication passage 48, the front chamber 21, and the pressure chamber 17. 2 communicates with the other surface (second surface). Therefore, the path from the front chamber 22 to the pressure chamber 18 constitutes the third fluid pressure introduction path 44, and the communication passage 48 to the pressure chamber
The path to 17 constitutes the second fluid pressure introduction path 43.

Further, in this embodiment, two piezoelectric elements 2,
3, the arrangement of the inlet nozzles 39, 40, the fluid pressure introduction paths 42, 44, and the fluid pressure introduction paths 43, 45 and the internal structure of the pressure detection unit 4 are configured substantially symmetrically.

The pressure detector 4 and the electric signal processing circuit 5
Are housed in a case 41 which also serves as a shield made of metal, which is an electrically good conductor, so that they are not subjected to electromagnetic induction noise from a commercial power source or the like. The negative electrode of the power source in the circuit and the case 41 are connected via screws, nuts and washers, which are not shown. Also,
In order to prevent noise generated due to movement of wiring materials and electronic parts, the case 41 and the pressure sensor 1 are fixed to each other with epoxy-based filling material.

Next, the electrical signal processing circuit 5 incorporated on the circuit board 35 will be described with reference to FIG. The piezoelectric elements 2 and 3 connect surfaces having different polarization polarities, and one of the connecting portions is connected to the filter circuit 51 as an input to the filter circuit 51, and the other is a power source 52 (in this embodiment, one Battery power source) is connected to the intermediate potential pole.

The outputs from the piezoelectric elements 2 and 3 are input to the filter circuit 51 and the AC amplification circuit 53. The filter circuit 51 constitutes a buffer circuit that obtains an output of low impedance from an extremely high impedance one (from the outputs of the piezoelectric elements 2 and 3).
With respect to the outputs from the piezoelectric elements 2 and 3, it has a gain characteristic of a frequency squared with respect to a frequency higher than a certain value.
A resistor R1 connected in series between the piezoelectric element and the positive pole of the operational amplifier 54 and a capacitor C1 connected in parallel between the circuit and the pole of the intermediate potential constitute a so-called low-pass filter, It has an integral characteristic that the gain decreases for a certain frequency and above. That is, as shown in FIG. 11, there is a falling portion in which the gain decreases with an increase in frequency above a certain value, and this falling portion has
It has a gain characteristic of. Above which frequency is 1 / Frequency
It is possible to change whether or not to have the characteristic of by changing the resistance value of the resistor R1. The resistor R2 is a piezoelectric element 2,
A so-called high-pass filter is configured by a combination with the electrostatic capacity of 3 itself. The arrows in the piezoelectric elements 2 and 3 in FIG. 1 indicate the polarization directions.

A resistor R is connected to the inverting input terminal of the operational amplifier 54.
A circuit for sending a feedback current through 3 and a circuit from the intermediate potential in which a resistor R4 and a capacitor C2 are connected in series are connected. These circuits form an AC amplifier circuit. In particular, by incorporating the capacitor C2, only the AC component flows, and regardless of the offset of the reference potential of the operational amplifier 54 (specific to the operational amplifier),
You can raise the gain to the required level. That is, the offset voltage becomes independent of the amplification factor. This AC amplifier circuit has an original amplifying function and also has a gain characteristic that becomes 1 / frequency above a certain frequency. In other words, an operational amplifier that is 1 / frequency above a certain frequency
54 The gain characteristic of itself (the characteristic shown in Fig. 12) appears.

1 / frequency divided by the low-pass filter
And the gain characteristic of 1 / frequency possessed by the operational amplifier 54 itself are integrated to obtain a gain characteristic of 1 / square of the frequency which is the target of the present invention. Therefore, the circuit that constitutes the low-pass filter, the operational amplifier 54, and the feedback circuit thereof have the filter circuit 51 having the gain characteristic of the square of the frequency, which is referred to in the present invention.
The operational amplifier 54, its feedback resistor R3, and the resistor R
4. The capacitor C2 constitutes an amplifier circuit which amplifies the output from the filter circuit according to the present invention.

In this embodiment, the output from the AC amplifier circuit 53 is a comparator (voltage comparator) 55 having hysteresis.
And processed into a pulse signal, which is output from the output terminal 56. The circuit to the hysteresis comparator 55 is provided with a capacitor C3 so as to send only an AC component, and is adjusted to an appropriate potential by a resistor R5. The capacitor C3 is connected to the inverting input terminal of the operational amplifier 57 in the comparator 55. With this configuration, the output through the capacitor C3 swings around a certain midpoint. Resistors R6, R
Although the amplification factor is set by 7, the output reaches the saturation point with a small input by increasing the amplification factor. In other words, since a pulse output is to be obtained, a clear output signal reaching the saturation point is preferable at this output stage. The amplified signal is output from the output terminal 56 so that an excessive voltage is not applied to the output side and an excessive current does not flow through the resistor R8.

In the present embodiment, the potential of one battery power source 52 is divided, and the intermediate potential is used for signal processing from the piezoelectric element. By doing this, one battery is enough,
Since the space for accommodating the power supply may be small, it is possible to downsize the entire flow meter to which the pressure sensor is attached.

The voltage of the power supply 52 is divided by the resistors R9 and R10, and the intermediate potential pole is formed therebetween. In this embodiment, the resistors R9 and R10 have the same resistance value and are exactly divided into two. The capacitor C5 is for removing noise. The pole of the intermediate potential is connected to the non-inverting input terminal of the operational amplifier 58 having a high output impedance so as to obtain the AC intermediate potential. As shown in FIG. 1, a resistor R14 and a capacitor C9 are connected to the positive power source of the operational amplifier 54.
By providing an integrating circuit according to
And stabilizes the power supply of the amplifier circuit configured by the AC amplification circuit 53, thereby reducing the fluctuation of the output due to the imbalance of the impedance against the fluctuation of the power supply. If the power supply is safe or the power supply stability of the operational amplifier 54 is good, the integrating circuit formed by the resistor R14 and the capacitor C9 is unnecessary.

The operation of the pressure sensor 1 configured as described above will be described below together with the configuration and operation when the pressure sensor 1 is used in a fluidic gas flow meter.

FIG. 7 shows a configuration in which the pressure sensor 1 is applied to a fluidic flow meter, and 60 in the figure shows a flow meter element portion in the fluidic flow meter.
Gas to be measured is circulated through the flowmeter element unit 60 in the direction of the arrow. Reference numeral 61 is a flow path contracting portion, and a jet flow having an increased flow velocity is jetted from a jet nozzle 62 at the tip thereof. Due to the Coanda effect, the main flow of the jet flow is a pair of partition walls that form the flow passage expanding portion 63.
It flows along one of the wall surfaces of either 64 or 65, but the wall surface flow is redirected along the reflecting vane 69 and directed so as to be orthogonal to the jet from the jet nozzle 62 from the control nozzle 66 (or 67). Then, a force acts so that the jet flow from the jet nozzle 62 flows along the other partition wall. Since this is repeated, the gas flows alternately along the partition walls 64 and 65, and a pressure fluctuation occurs in the flowmeter element unit 60 in response to this flow change. Since the frequency of the flow change is proportional to the gas flow rate, the flow rate can be measured by detecting the frequency of pressure fluctuation. In the figure, 68 is a target for efficiently directing the jet flow to one of the partition walls, and 70 is a guide plate for guiding the flow.

Internal pressure outlets 71 and 72 are provided in the flow meter element portion 60 as described above, and the internal pressure outlets 71 and 72 are gaskets.
Pressure sensor 1 through 73 (preferably rubber with chemical resistance)
Are connected to the inlet nozzles 39 and 40, respectively.

Inside the flowmeter element 60, the flow is the internal pressure outlet
When switching to the 71 side, the fluid pressure in the vicinity of the internal pressure outlet 71 becomes lower than in the vicinity of the internal pressure outlet 72, and conversely the flow becomes the internal pressure outlet.
When switched to the 72 side, the fluid pressure near the internal pressure outlet 72 becomes lower. The pressure (P1) at the internal pressure outlet 71 is the gasket 73, the inlet nozzle 39, and the first fluid pressure introduction path 42.
And the piezoelectric element 2 via the fourth fluid pressure introduction path 45.
The pressure (P2) at the internal pressure outlet 72, which is introduced to the first surface of the piezoelectric element 3 and the second surface of the piezoelectric element 3, the gasket 73, the inlet nozzle 40, the second fluid pressure introduction path 43, and the third fluid pressure. It is introduced into the other surface (second surface) of the piezoelectric element 2 and the other surface (first surface) of the piezoelectric element 3 through the introduction path 44. The pressures P1 and P2 are alternately applied to the two piezoelectric elements 2 and 3, and the amount of polarization changes due to the deformation of the piezoelectric element due to the pressure variation,
As a result, a potential difference is generated on both sides of the piezoelectric body itself. The sum of the outputs of both piezoelectric elements 2 and 3 is sent to the electric circuit 5, processed, and taken out from the output terminal 56 as a pressure fluctuation detection signal.

In the above detection, the fluid pressure is simultaneously introduced to both surfaces of the piezoelectric element, and the differential pressure is detected by each piezoelectric element. Therefore, the noise component due to the pressure variation and temperature variation generated in the entire fluid is Since they are added to both sides of the piezoelectric element at the same time, the piezoelectric element undergoes the same deformation due to the noise component, and the noise component is automatically erased in the piezoelectric element itself. Then, the detection output of the pressure to be measured from which the noise has been eliminated is taken out as the sum of the outputs from both piezoelectric elements 2 and 3, and extremely highly accurate detection is possible.

When vibration is applied as a disturbance, the exciting force acts on both piezoelectric elements 2 and 3 simultaneously as a force in the same direction. Although the deformation directions of the piezoelectric elements 2 and 3 due to the introduction of the fluid pressure are opposite to each other, the deformation margins of the two piezoelectric elements due to the above-mentioned vibration force are the same in the directions of increasing and decreasing the deformation. Become. Therefore, the increase / decrease in the output due to this noise component is automatically erased at the stage of the sum of the outputs of the piezoelectric elements 2 and 3.

Regarding the noise due to the sound pressure, the piezoelectric elements 2 and 3 are simultaneously deformed by the same amount in the thickness direction due to the propagated sound pressure. Therefore, the increase / decrease in the output due to this noise component is automatically erased at the stage of adding the outputs of both piezoelectric elements 2 and 3. As a result, the noise due to the sound pressure is also automatically deleted, and the pressure fluctuation of the measurement target is detected with extremely high accuracy.

Further, in this embodiment, in order to further enhance the noise removing effect, the arrangement of the inlet nozzles 39 and 40, the fluid pressure introducing paths, and the structure of the pressure detecting portion for the two piezoelectric elements 2 and 3. Are configured to have almost the same shape and symmetry.

Next, the electric signal processing will be described. The basic principle of the pressure sensor of the present invention is to measure the frequency of pressure fluctuations that occur in proportion to the flow rate, and to use it in a gas flow meter for highly accurate flow rate measurement. As shown in FIG. 9, the differential pressure (differential pressure detected on both sides of the piezoelectric element) due to the pressure fluctuation transmitted to the inlet nozzles 39, 40 of the pressure sensor 1 is approximately 2 of the frequency (flow rate). Proportional to the square. The output obtained by multiplying the output from the piezoelectric elements 2 and 3 corresponding to this differential pressure by the gain of the signal processing circuit is the output as the pressure sensor 1. However, the output of the frequency corresponding to the differential pressure is The output of the amplifier circuit may be saturated in the large flow rate region by simply amplifying or by simply amplifying with an amplifier circuit having an amplifier having a gain characteristic of 1 / frequency. In such a state, the output of the frequency to be measured is not amplified above the saturation value, which is in the peak state, and eventually the noise component is unnecessarily amplified and the SN ratio deteriorates. Therefore, this SN
In order to prevent the ratio from deteriorating, it is necessary to reduce the gain when amplifying the outputs from the piezoelectric elements 2 and 3 in a region above a certain frequency (region above a certain flow rate). In the present invention, as described above, as one suitable method, the filter circuit 51 is configured so as to have the gain characteristic of the square of the frequency above a certain frequency, and therefore, FIG.
As shown in, a gain characteristic having a characteristic line A of a square of the frequency above a certain frequency is obtained. By passing this filter circuit 51, the output from the piezoelectric elements 2 and 3 proportional to the square of the frequency is multiplied by the gain characteristic of the square of the frequency, and as a result, shown in FIG. As described above, the characteristic B that provides a substantially constant output at a certain frequency or higher is obtained. Since the output characteristic B is less than the maximum output K (output corresponding to the saturation point) of the operational amplifier 54 which is determined by the power supply voltage, the pressure sensor is used as a pressure sensor even when the frequency becomes large, that is, in the large flow rate region. The output is not saturated, the SN ratio is kept large, and high measurement accuracy is secured. The characteristic line C in FIG.
The gain characteristic of 1 / frequency, that is, the gain characteristic of the operational amplifier 54 itself is shown.

In reality, due to the design of the flow meter element, etc.,
The relationship between frequency and differential pressure (characteristic at a certain flow rate)
The characteristics shown in 14 and the characteristics shown in FIG. 15 are obtained. In the characteristic as shown in FIG. 14, the peak frequency P, which is the measurement target frequency, can be clearly distinguished from the noise components in other frequency regions, so that the gain characteristic A of the square of the frequency as described above is used. All you have to do is not to cause any problems.

However, in the case of the characteristics shown in FIG. 15,
Although the noise component increases as the flow rate increases, it is possible to clearly distinguish the noise component N in the low frequency region from the frequency component P to be measured at the output stage (that is, the differential pressure × gain). Must be low enough. Therefore, in such a case, it is rather preferable that the output increases as the frequency increases, that is, the output of the measurement target frequency on the high and low frequency side becomes larger. For that purpose, for example, the frequency region where the gain characteristic of 1 / square of the frequency is set to a region higher than that for the characteristic shown in FIG. It suffices that the gain characteristic of 1 is obtained and the gain characteristic of the square of the frequency is obtained in a relatively high frequency region. That is, it is necessary to change from which frequency the gain characteristic of the square of the frequency is obtained. This change is achieved by changing the resistance value of the resistor R1 and changing the gain characteristic of the low pass filter. By changing the resistance R1, the characteristics D, E, and F in FIG. 10 can be changed, and as a result,
The characteristics in FIG. 13 are adjusted like the characteristics G, H, and I. As a result of this adjustment, an optimum pressure sensor that can clearly distinguish the measurement target frequency from the noise component frequency according to the characteristics of the flowmeter is obtained.

One feature of the AC amplifier circuit 53 of this embodiment is that the offset voltage of the operational amplifier 54 is constant regardless of the amplification factor. In order to obtain a value suitable for measurement when amplifying a weak signal from a sensor with high impedance such as a polymer piezoelectric film, 1
A high amplification factor of about 00 to 10,000 times is required, but since the offset voltage is irrelevant to the amplification factor, such a high amplification factor can be easily set.

Further, in the pressure sensor 1 of this embodiment, the output from the amplifier circuit 53 is output as a pulse signal by passing through the hysteresis comparator 55.
The comparator 55 has an input / output waveform as shown in FIG. 16 and has a hysteresis characteristic as shown in FIG. 17, but since there are threshold levels SH1 and SH2, noise components below this level can be almost completely removed. . In addition, the input is one threshold level SH1.
When the level reaches the threshold level of the other S
Since it changes to H2, the amount of overdrive instantaneously increases. As a result, the output rises faster. Therefore, a stable pulse can be obtained accurately even if some noise is superimposed. As a result, the pulse signal can be extracted more accurately. In order to increase the slew rate in order to reduce the current consumption, there is a method of connecting the output of the hysteresis comparator 55 to a C-MOS type Schmitt. This further has an advantage that when connected to a MOS type digital IC for signal processing, the power consumption is small because the rise is fast.

Although the final output is a pulse in this embodiment, the pressure sensor final output may be a voltage or current output proportional to the frequency. Also, the output can be displayed as a numerical value or the like, or can be a circuit that converts the intensity of light or sound, blinking of light, or pitch of sound.

Further, in this embodiment, in order to operate the pressure sensor 1 with only one battery, a circuit for converting a single DC voltage into a power supply having an intermediate voltage is provided. The reason why the operational amplifier 58 is used in this circuit is that the intermediate potential becomes very unstable depending on the load only with resistance division, and in many cases, the power consumption in resistance division is higher than that with an amplifier. Because it is quite large. In the embodiment, it is about 10 times. In this embodiment, an intermediate point is provided by dividing the resistors R9 and R10.

A battery may be provided in the pressure sensor in order to cope with the case where it is necessary to keep the pressure sensor from being affected by the quality of the power source or when it is difficult to supply the necessary power source. is there. In the above embodiment, the power consumption is 10
Since it is about μA, it can be used for a long time without replacing the battery.

FIG. 18 shows an example in which the pressure sensor according to the present invention is used in a Karman vortex flowmeter. FIG.
In, the reference numeral 80 indicates the flow meter element portion of the Karman vortex flow meter, and the reference numeral 81 indicates the vortex generator. The fluid flowing in the direction of the arrow in the figure hits the vortex generator 81 to intermittently generate Karman vortices 82 in the subsequent flow. One internal pressure outlet 83 is provided at the passage of the Karman vortex 82, and the other internal pressure outlet 84 is provided on the inner wall surface portion of the flowmeter element portion 80. Both internal pressure outlets 83, 84 are gaskets 85
Is connected to the inlet nozzles 39, 40 of the pressure sensor 1. Other configurations are the same as those shown in the above-mentioned embodiment.

In such a Karman vortex flowmeter,
The number of Karman vortices generated per unit time changes in proportion to the flow rate of the fluid, and the flow rate is detected by measuring the number of Karman vortex passages at the internal pressure outlet 83. The pressure fluctuation when passing through the Karman vortex is introduced to one side of both piezoelectric elements 2 and 3 through the internal pressure outlet 83, the gasket 85, and the inlet nozzle 39, and the internal pressure outlet 84, the gasket 85, and the inlet nozzle 40. The pressure of the whole passing fluid and the pressure fluctuation are both piezoelectric elements 2,
3 is introduced on the other side. As the sum of the outputs of both piezoelectric elements 2 and 3, only the pressure fluctuation due to the Karman vortex passage is accurately detected, and highly sensitive and highly accurate flow rate measurement becomes possible. Other functions are similar to those of the fluidic flowmeter described above.

Although the above description has been made on the fluidic flowmeter and the Karman vortex flowmeter, the pressure sensor according to the present invention can be applied to other flowmeters as long as pressure fluctuation detection is essential. is there.

[0063]

As described above, when the pressure sensor of the present invention is used, one of the two piezoelectric elements arranged so that the surfaces in the same direction have the same polarization polarity is connected to one of the inlet nozzles. The pressure from the other inlet nozzle is applied to the other surface at the same time, and the noise component due to pressure fluctuation and temperature fluctuation is automatically eliminated by taking the differential pressure between the two surfaces, and at the same time, The detection accuracy is doubled by taking the sum of the outputs, and two piezoelectric elements are positively juxtaposed under the same condition. The two piezoelectric elements have different polarization polarities electrically. By connecting, the noise due to the vibration that is automatically erased is further reduced as compared with the previously proposed pressure sensor. Furthermore, since the signal from the piezoelectric element is passed through the filter circuit and the output from the filter circuit is obtained through the amplifier circuit to obtain the output as the pressure sensor, the noise component can be removed electrically and the measurement target Only the pressure fluctuation and the frequency to be measured can be detected with extremely high accuracy. Further, in the present invention, a capacitor is arranged at the connection with the power supply, and further, an integrating circuit is added between the positive power supply of the operational amplifier forming the filter circuit and the AC amplification circuit and the intermediate voltage power supply, and the pulse is added. By taking measures to reduce the influence of noise on the power source, it is possible to detect the target pressure with high accuracy even in an environment where the quality of the power source is poor.

The minimum detectable pressure fluctuation range is 0.0
It becomes very small, from 01 to 1 Pa, and can be made even smaller. The SN ratio is 0.001 Pa and 20d.
BV is very high at 60 dBV at 1 Pa. For this reason, it has become possible to detect minute pressure fluctuations of gas or liquid that could not be detected conventionally without being affected by ambient noise. With the technology up to the present, a minute pressure fluctuation of about 1 Pa is quite large, or measurement cannot be performed unless expensive equipment is used. However, the product of the present invention can be configured as an extremely small sensor, and can be easily configured by a fluidic flow meter or Kalman. It can be used not only in the vortex flowmeter but also in other instruments and a part of the machine, so that it can be expected to be used in a wide range of fields. In addition, since the mechanically weak piezoelectric element is not exposed, foreign matter in the fluid does not adhere to the piezoelectric element, and it does not cause damage or decrease in sensitivity during the manufacturing, handling, and implementation processes, and thus has a long life. Furthermore, the structure is simple, and there are no parts that require high technology for manufacturing, so that it can be manufactured at low cost.

[Brief description of drawings]

FIG. 1 is an electrical signal processing circuit diagram of a pressure sensor according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view of a pressure sensor according to an embodiment of the present invention.

FIG. 3 is an enlarged partial sectional view of a piezoelectric element portion of the pressure sensor of FIG.

FIG. 4 is an enlarged partial exploded cross-sectional view of the pressure sensor of FIG. 2 before attachment of a piezoelectric element.

5 is a vertical cross-sectional view after the piezoelectric element of FIG. 4 is attached.

6 is an enlarged exploded vertical cross-sectional view of a piezoelectric element holding portion in the pressure sensor of FIG.

7 is a schematic configuration diagram when the pressure sensor of FIG. 2 is used in a fluidic flow meter.

FIG. 8 is a relationship diagram between a holder inclination angle and a pressure sensor output.

FIG. 9 is a relationship diagram between a frequency in a pressure sensor and a differential pressure due to pressure fluctuation.

FIG. 10 is a relationship diagram between frequency and gain in the filter circuit.

FIG. 11 is a relationship diagram between frequency and gain in a low pass filter.

FIG. 12 is a relationship diagram between a frequency and a gain of the operational amplifier itself.

FIG. 13 is a relationship diagram between a pressure sensor output and frequency.

FIG. 14 is a diagram showing the relationship between the frequency and the differential pressure due to pressure fluctuation in a certain flow meter.

FIG. 15 is a relationship diagram between the frequency and the differential pressure due to pressure fluctuation in another flow meter.

FIG. 16 is an input / output characteristic diagram of a voltage comparator that outputs a pulse.

FIG. 17 is a characteristic diagram showing hysteresis in the voltage comparator.

18 is a schematic configuration diagram when the pressure sensor of FIG. 2 is used in a Karman vortex flowmeter.

[Explanation of symbols]

 1 Pressure Sensor 2, 3 Piezoelectric Element 4 Pressure Detection Section 5 Electric Signal Processing Circuit 6, 7 Piezoelectric Body 8, 9, 10, 11 Electrode Layer 12, 13, 14, 15 Holder 16, 17, 18, 19 Pressure Chamber 20 , 21, 22, 23 Front chamber 32 Substrate 33 Lid 35 Circuit board 36, 37 Input terminal 38 Output terminal 39, 40 Inlet nozzle 41 Shield case 42 First fluid pressure introduction path 43 Second fluid pressure introduction path 44 Third Fluid pressure introduction path 45 Fourth fluid pressure introduction path 46 Lid 47, 48 Communication path 49 O-ring 51 Filter circuit 52 Power supply 53 Amplification circuit 54, 57, 58 Operational amplifier 55 Voltage comparator 56 Output terminal 60 Flow meter element part 61 Flow reduction part 62 Jet nozzle 63 Flow expansion part 64, 65 Partition 66, 67 Control nozzle 68 Target 69 Reflector 70 Reflector 70 Guide plate 71, 72 Internal pressure outlet 73 Gasket 80 Karman vortex flowmeter element part 81 Vortex generator 82 Karman vortex 83, 84 Internal pressure outlet 85 Gasket 86 Short-circuit plate

Claims (2)

[Claims] 1. A pressure detecting portion, comprising two piezoelectric elements each having an electrode layer provided on both sides of a film-shaped piezoelectric body, sandwiched between two pairs of holders from both sides, and provided with tension. The elements are arranged so as to face in the same direction and have the same polarization polarity in the same direction, and the pressure detecting portion is provided with two inlet nozzles for introducing the pressure of the fluid. The polarization polarity is different from that of the first surface of the one piezoelectric element communicated with the first surface of the one piezoelectric element via the first fluid pressure introduction path and the fourth surface of the one piezoelectric element via the fourth fluid pressure introduction path. The second surface of the other piezoelectric element is communicated with, and the other inlet nozzle is communicated with the second surface of the one piezoelectric element via the second fluid pressure introducing path, and the third fluid pressure introducing path is also provided. Via a pressure sensor connected to the first surface of the other piezoelectric element via And electrically connecting surfaces of the two piezoelectric elements having different polarization polarities,
A pressure sensor characterized in that an output from one of the piezoelectric element connecting portions is connected to a filter circuit, and the filter circuit is connected to an amplifier circuit for amplifying an output from the filter circuit. 2. The pressure sensor according to claim 1, wherein the holding surface of the holder for holding the two piezoelectric elements is formed as an inclined surface.