JPH06294694A - Pressure sensor and flowmeter employing the same - Google Patents

Abstract

PURPOSE:To provide a highly accurate and inexpensive pressure sensor in which the performance can be sustained for a long term. CONSTITUTION:The pressure sensor comprises a cantilever beam 18 of piezoelectric ceramic, a body 14 for forming pressure chambers 15, 16 being fed with fluid on the opposite sides of the cantilever beam 18, two inlet nozzles 15a, 16a for introducing the fluid individually into the pressure chambers 15, 16, and an amplifier 21 for amplifying an electric output induced by the distortion of the cantilever beam 18.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a pressure sensor for measuring the pressure of gas or liquid, and a flow meter for measuring the flow rate of gas or liquid using this pressure sensor.

[0002]

2. Description of the Related Art As a flow meter capable of detecting a flow rate of a fluid, particularly a gas with high accuracy, a flow meter which detects a pressure fluctuation by a fluid element to measure the flow rate, for example, a Fruidek flow meter or a Karman vortex flow meter is known. There is. These utilize a pressure fluctuation phenomenon or the like according to the flow rate generated in the gas flowing through the fluid element, and the flow rate is measured by accurately detecting the pressure fluctuation (fluctuation frequency). Therefore, as a pressure sensor used in such a flow meter, it is desirable that it has high sensitivity, that is, it can detect even weak pressure changes and pressure changes in a wide frequency range, and
It is desirable that the structure be simple, inexpensive, and easy to manufacture. Conventionally, as a typical pressure sensor used in this type of flow meter, a pressure sensor using a silicon diaphragm disclosed in JP-A-57-110918 and JP-A-2-268228. A pressure sensor using a piezoelectric polymer film as shown as a diaphragm is known.

[0003]

However, the pressure sensor using the silicon diaphragm has the following problems. That is, since the silicon chip itself is not a piezoelectric body, it does not have a pressure detection function itself and must detect a pressure change as a change in resistance value due to its distortion, and a resistance change detection circuit such as a bridge circuit is required. To do. Also,
The silicon chip is close to a rigid body, and when it is processed by etching there is a limit to thin processing, it is difficult to detect weak pressure changes, the response speed is slow, and the frequency range that can follow pressure changes Is narrow. For these reasons, a conventional pressure sensor using a silicon diaphragm is usually 0.1 mmH ₂ O even if it is of a high sensitivity type.
Smaller pressure fluctuations could not be detected and the frequency that could be followed was less than a few hertz.

Further, it is difficult to control the thickness of the silicon diaphragm in manufacturing, and there is a problem that the sensitivity of the pressure sensor for detecting a weak pressure fluctuation is large. Furthermore, since this method detects the resistance value that is greatly affected by temperature changes, a temperature compensation circuit is indispensable. Normally, this compensation circuit is also integrated and formed on the same chip, so complicated technology is required for manufacturing. , Large-scale manufacturing equipment is required.

Further, as described above, the manufacturing difficulty, the necessity of the detection and compensation circuits usually make the silicon diaphragm type pressure sensor considerably expensive, especially the highly sensitive one. ing.

In this respect, in the pressure sensor using the polymer piezoelectric film, the potential difference generated by the polymer piezoelectric film itself can be directly output as a voltage signal, and the thin film can be easily manufactured with high accuracy. Therefore, the pressure sensor is inexpensive and has high accuracy.

Here, a flow meter using such a pressure sensor must generally guarantee its performance over a long period of time in an environment of -20 ° C to 60 ° C. However, since the polymer piezoelectric film and the copolymer of polyvinylidene fluoride and ethylene trifluoride as disclosed in JP-A-2-268228 are organic compounds, secular change or the like is observed. Therefore, in the detection of weak pressure, the piezoelectric polymer film is insufficient in terms of long-term reliability. Further, the temperature characteristic of the output of the piezoelectric film is a function of the temperature characteristic of the piezoelectric characteristic (the ability to generate an electric charge in response to stress) and the temperature characteristic of the elastic compliance (softness of the material). Both temperature characteristics do not show linearity, and since it is a polymer material in particular, the change in elastic compliance due to temperature change is large, and furthermore, there is an inflection point within the operating temperature range, and the output is not constant.

[0008]

According to a first aspect of the present invention, there is provided a cantilever made of piezoelectric ceramics, and pressure chamber forming bodies for forming pressure chambers into which fluid is introduced on both sides of the cantilever, respectively. Introduce fluid individually into the pressure chamber 2
Each inlet nozzle and an amplifier for amplifying and outputting an electric output generated by the distortion of the cantilever are provided.

According to the invention of claim 2, the cantilever in the invention of claim 1 has a bimorph structure.

According to the invention of claim 3, the cantilever in the invention of claim 1 has a unimorph structure.

According to the invention of claim 4, the cantilever in the invention of claim 1 has a monomorph structure.

In a fifth aspect of the present invention, in a flowmeter for measuring a flow rate by detecting a pressure fluctuation due to a fluid element with a pressure sensor, the pressure sensor is the pressure sensor according to the first, second, third or fourth aspect. did.

[0013]

According to the invention described in claim 1, when the fluid is introduced into each pressure chamber through the inlet nozzle, the cantilever is deformed due to the difference in pressure introduced into each pressure chamber. That is, when the pressure of the fluid introduced into each pressure chamber fluctuates, the fluctuation components common to the pressure chambers among the fluctuations cancel each other out, and the cantilever is deformed only by the non-common fluctuation components. . When the cantilever is deformed in this way, since the cantilever is made of piezoelectric ceramics, a voltage signal according to the deformation is output. That is, since a signal is generated from the piezoelectric ceramics themselves, there is no need for a detection circuit such as a resistance change detection circuit unlike a pressure sensor using a silicon diaphragm. In addition, since piezoelectric ceramics can be easily manufactured thinly and accurately with well-known sheet manufacturing techniques such as tape casting method, if they are thinly formed by these manufacturing techniques, they are flexible and lightweight, and even if a slight pressure change causes distortion, they are high. The frequency is tracked well, and the variation in sensitivity between individuals is small.

Further, since the piezoelectric ceramic is an inorganic substance, its aging is small. Furthermore, the elastic compliance is almost constant in the operating temperature range, and the temperature characteristic of the output becomes a function of only the temperature characteristic of the piezoelectric characteristic. For the temperature characteristic of the piezoelectric characteristic, addition of additives and selecting an appropriate composition with a high Curie point are also selected. As a result, the temperature characteristic of the output becomes constant in the operating temperature range.

According to the second aspect of the present invention, the cantilever has a bimorph structure, so that the output is increased.

According to the third aspect of the invention, since the cantilever has a unimorph structure, the durability is improved and the output drift is reduced.

According to the invention described in claim 4, since the cantilever has a monomorph structure, the durability is improved and the output drift is reduced.

According to the fifth aspect of the present invention, there is provided a pressure sensor for detecting a pressure fluctuation caused by the fluid element.
Alternatively, since the pressure sensor described in 4 is used, minute pressure fluctuations are accurately detected by this pressure sensor, and fluctuations at high frequencies are also accurately detected. Therefore, the flow rate can be detected accurately in a wide range.

[0019]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of the present invention will be described with reference to FIGS. This embodiment is applied to a fluidic flow meter. As shown in FIG. 2, this fluidic flow meter is provided with a set ring space 3, a flow channel contracting unit 4, a jet nozzle 5, and a flow channel expanding unit 6 in this order on a flow channel connecting the inflow pipe 1 and the discharge pipe 2, and , The flow path expanding portion 6
Inside thereof, an exciter 7 is provided so as to face the jet nozzle 5, and an end block 8 is provided behind it to form a fluid element 9. A discharge space 10 is provided behind the end block 8. Further, in this fluidic flow meter, pressure outlets 11 and 1 for guiding pressures at two points with the center line of the jet nozzle 5 as an axis of symmetry to the outside are provided.
2 is provided near the downstream of the jet nozzle 5 at right angles to the flow.

Further, the fluidic flowmeter is provided with a pressure sensor 13 for detecting a pressure fluctuation generated in the flow passage expanding portion 6 connected to the pressure outlets 11 and 12. As shown in FIG. 1, the pressure sensor 13 is first provided with a cubic pressure chamber forming shell 14 (shown by an imaginary line in the drawing) which is a pressure chamber forming body. This pressure chamber forming shell 1
4 is provided with a partition plate 17 that divides the interior into two and forms pressure chambers 15 and 16 on both sides thereof, and a cantilever 18 is provided along the partition plate 17. The partition plate 1
7 is a through hole 17a for communicating the pressure chambers 15 and 16 with each other.
Are formed along the shape of the free end of the cantilever 18. The pressure chambers 15 and 16 are provided with inlet nozzles 15a and 16a facing the free ends of the cantilever 18, respectively.
The pressure outlets 11 and 12 are connected via a. Here, the cantilever 18 has a length L = 5 m.
m, plate thickness t = 100 μ, density ρ = 7.9 g / cm ³ , elastic constant s ₁₁ = 16 × 10 to ¹² m ² / N, piezoelectric constant d ₃₁ = 300
A strip-shaped piezoelectric ceramics plate 19 of pC / N has a bimorph structure in which the polarization directions indicated by arrows are different from each other as shown in FIG. The relationship between the displacement δ and the generated electric field V and the resonance frequency f in this structure are as follows: δ = (3/4) d ₃₁ (L ² / (2t) ² ) V f = 0.158 (2t / L ² ). It is shown by √ (1 / ρs ₁₁ ).

The piezoelectric ceramic plate 19 is made of a material such as lead zirconate titanate (PZT) or lead lanthanum zirconate titanate (PLZT), or a complex oxide such as barium titanate. As the material, ceramics derived from these materials, lithium niobate, tantalum niobate, or the like can be applied, but in this embodiment, PZT is used.
The system is applied. The piezoelectric ceramic plate 19
The electrode layers 20 are formed on both surfaces of the.

The pressure sensor 13 is integrally provided with an amplifier 21 as shown in FIG. The amplifier 21 amplifies and outputs a potential difference generated between both electrode layers 20 of the piezoelectric ceramic plate 19.

In such a construction, first, the tubular flow from the upstream side of the flow path is rectified into a two-dimensional flow in the set ring space 3 and further rectified by the flow path contracting section 4 to smoothly jet the nozzle 5. Head to. Then, the jet flow rectified by the jet nozzle 5 is divided into left and right (upper and lower in FIG. 2) by hitting the exciter 7, but in the space of the flow path expanding portion 6 up to the end block 8,
Above a certain flow rate, due to the instability of the vortex formed behind the exciter 7, a flow that is biased to the left or right is formed. Therefore, the flow colliding with the end block 8 reaches the outlet of the jet nozzle 5 along the front surface of the end block 8 and then along the inner wall 6a of the flow path expanding portion 6, and collides with the jet flow at a right angle. Therefore, the direction of the jet flow is biased in the direction opposite to the initial drift by the flow returning from that side. As a result, the same thing happens again on the opposite side, and as a result, the flow exiting the jet nozzle 5 is alternately biased to the left and right. Since the frequency of this drift increases linearly with the increase in the flow rate, the flow rate can be measured by measuring this frequency.

Here, in this fluidic flow meter, the vibration of the uneven flow is introduced into the pressure chambers 15 and 16 of the pressure sensor 13 as alternating pressure fluctuations near the pressure outlets 11 and 12. Therefore, the cantilever 18 vibrates due to the pressure difference between the pressure chambers 15 and 16, and a potential difference corresponding to the strain is generated between both electrode layers 20 of the piezoelectric ceramic plate 19 and output. Then, this output is amplified by the amplifier 21 and then output.
This output fluctuates according to the vibration of the nonuniform flow. That is, the flow rate can be obtained by obtaining the frequency of the nonuniform flow from the fluctuation of the output. Further, in this fluidic flow meter, fluctuations in the pressure of the fluid as a noise component are caused by the pressure chambers 15 and 16 respectively.
Since they have the same value, they cancel each other and are automatically canceled. Therefore, only the pressure fluctuation due to the frequency of the uneven flow is detected, and the flow rate can be accurately obtained.

Here, the piezoelectric ceramic plate 19 is
It has a Curie temperature of 200 ° C. or higher, and spontaneous polarization below the Curie temperature shows a good flat characteristic particularly in the operating temperature range. Therefore, the fluidic flow meter can accurately measure the flow rate without being affected by the ambient temperature. Further, since the piezoelectric ceramic plate 19 is an inorganic substance, its change over time is small, and therefore, this fluidic flowmeter can maintain a constant performance for a long period of time.

Further, as described above, the piezoelectric ceramic plate 19 of the fluidic flowmeter can be easily manufactured as a thin film by a well-known technique with high precision, and thus can be a flexible and lightweight film. It is possible to detect a weak pressure with high accuracy and follow a high frequency well. More specifically, even if the conventional pressure sensor using a silicon diaphragm is a high-sensitivity type, it can detect only a pressure fluctuation of about 0.1 mmH ₂ O, whereas the pressure sensor 13 of the present embodiment. It is possible to recognize the generation of a sufficient alternating electric field even with a minute pressure fluctuation of about 0.001 mmH ₂ O. Further, while the follow-up frequency of the pressure sensor using the conventional silicon diaphragm is several Hz or less, in the pressure sensor 13 of this embodiment, the resonance frequency of the cantilever 18 is 3. It will be around 6 kHz. In other words, the resonance frequency can be accurately tracked, which is significantly improved as compared with the conventional pressure sensor using the silicon diaphragm. That is, the fluidic flowmeter of the present embodiment can measure a wide range from low flow velocity to high flow velocity.

FIG. 4 shows an output waveform when a fluid is flowed at 200 liters / hour in the fluidic flowmeter of this embodiment. As can be seen from the figure, a clear waveform is formed and accurate measurement can be performed.

Further, since the piezoelectric ceramic plate 19 of this fluidic flowmeter directly outputs a voltage signal due to strain, a resistance change detecting circuit and a temperature compensating circuit are unnecessary like those for detecting a change in resistance due to strain. Since it can be easily manufactured as described above, the flow meter can be made inexpensive.

The piezoelectric ceramic plate 19 of this embodiment is used.
In the above, the polarization directions were different, but the polarization directions may be the same as shown in FIG. With such a structure, the relationship between the displacement δ and the generated electric field V is expressed by the following equation δ = (3/2) d ₃₁ (L ² / (2t) ² ) V.

Further, in the case where the piezoelectric ceramic plate 19 alone is not sufficient, as shown in FIG. 6, the piezoelectric plate 19 is attached to the surface of the elastic plate 22 as a base material to form the cantilever 18. Is also good.

Next, a second embodiment of the present invention will be described with reference to FIG. The same parts as those shown in the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted. (The same applies to the following examples). The flow meter of the present embodiment is structurally similar to the above embodiments, but the cantilever 18 of the pressure sensor 13 has a unimorph structure as shown in FIG. The material used is a Z-cut lithium niobate single crystal that is heat-treated at around 600 ° C. and arranged so that the polarization direction is different from the film thickness direction. A diamond cutter was used for processing.

In such a structure, the pressure sensor 13 of the flow meter of this embodiment has a sensitivity of 0.01 mmH ₂ O, which is slightly lower than that of the pressure sensor of the above embodiment, but the piezoelectric ceramic plate 19 Since there is no joining, the process can be simplified and highly reliable.

Next, a third embodiment of the present invention will be described with reference to FIG. The flowmeter of this embodiment is structurally similar to that of the first embodiment, but the cantilever 18 of the pressure sensor 13 has a monomorph structure as shown in FIG. With this monomorph structure, when a semiconductor and a metal are brought into contact with each other, a Schottky-type barrier is formed in the electron energy band structure.When a voltage is applied in this state, the barrier becomes high and uneven at either junction. , And is bent by strain proportional to the electric field. That is, an electric field corresponding to the strain is generated, and the voltage generated due to the barrier becomes non-uniform,
A potential difference is generated between both electrodes 20. In this embodiment, the material is PLZT ceramic Pb _0.97 La.
A material obtained by adding Bi to _0.09 (Zr _0.52 Ti _0.45 ) _0.9925 O ₃ to form a semiconductor is applied. Further, in the monomorph structure, an extra current flows through the Schottky barrier, so that the sensitivity is slightly reduced as compared with the unimorph structure. Therefore, in this embodiment, the length L of the cantilever is set to 25 mm and the plate thickness t is set to 200 μ.

In such a structure, the resonance frequency of the cantilever 18 of the pressure sensor 13 of the flowmeter of this embodiment is 25.
0Hz, sensitivity is 0.001 compared to L = 5mm
mmH ₂ O improved. Further, since the piezoelectric ceramic plate 19 is not joined, the process can be simplified and the reliability can be improved.

[0035]

According to the invention described in claim 1, since the cantilever is deformed by the pressure difference between the pressure chambers, when the pressure of the fluid introduced into each pressure chamber fluctuates. , The fluctuation component common to each pressure chamber can be canceled and only the non-common fluctuation component can be detected. Furthermore, since this pressure sensor has a cantilever formed of piezoelectric ceramics, it detects resistance change. In addition, there is no need for a circuit or a temperature compensating circuit, and since piezoelectric ceramics can be manufactured easily by known technology, that is, at low cost, they can be constructed at low cost. In addition to having constant characteristics, a thin film can be manufactured with high precision by well-known technology, and since it is an inorganic substance, its change over time is small. Also can be detected accurately and sensitively pressure weak pressure without, moreover have an effect and the like can be maintained over the performance long.

According to the second aspect of the invention, since the cantilever has a bimorph structure, the output can be increased, and if the output is constant, the size can be reduced.

According to the invention of claim 3, since the cantilever has a unimorph structure, the bonding step can be omitted as compared with the bimorph type, and further, the drift of the amount of electric field generated can be reduced. Therefore, durability can be improved.

According to the invention as set forth in claim 4, since the cantilever has a monomorph structure, the bonding step can be omitted as compared with the bimorph type, and further, the drift of the amount of electric field generated can be reduced. Therefore, durability can be improved.

According to a fifth aspect of the present invention, in the flowmeter for measuring the flow rate by detecting the pressure fluctuation due to the fluid element by the pressure sensor, the pressure sensor is provided in the first, second, and third aspects.
Since the pressure sensor described in 4 is used, accurate flow rate measurement can be performed without being affected by environmental temperature, constant performance can be maintained over a long period, and a wide range from low flow rate to high flow rate can be maintained. The flow rate can be measured with high accuracy, and the cost can be reduced.

[Brief description of drawings]

FIG. 1 is a perspective view showing a pressure sensor according to a first embodiment of the present invention.

FIG. 2 is a plan view showing a fluidic flow meter.

FIG. 3 is a side view showing a bimorph-shaped cantilever.

FIG. 4 is a graph showing an output waveform.

FIG. 5 is a side view showing a modified example.

FIG. 6 is a side view showing a modified example.

FIG. 7 is a side view showing a second embodiment of the present invention.

FIG. 8 is a side view showing a third embodiment of the present invention.

[Explanation of symbols]

 9 Fluid Element 14 Pressure Chamber Forming Body 15, 16 Pressure Chamber 15a, 16a Inlet Nozzle 18 Cantilever 19 Piezoelectric Ceramics 21 Amplifier

Claims (5)

[Claims] 1. A cantilever made of piezoelectric ceramics,
Pressure chamber forming bodies that form pressure chambers into which fluid is introduced on both sides of the cantilever, two inlet nozzles that individually introduce fluid into the pressure chambers, and electricity generated by distortion of the cantilever. A pressure sensor comprising: an amplifier that amplifies and outputs an output. 2. The pressure sensor according to claim 1, wherein the cantilever has a bimorph structure. 3. The pressure sensor according to claim 1, wherein the cantilever has a unimorph structure. 4. The pressure sensor according to claim 1, wherein the cantilever has a monomorph structure. 5. A flow meter for measuring a flow rate by detecting a pressure fluctuation due to a fluid element with a pressure sensor, wherein the pressure sensor is the pressure sensor according to claim 1, 2, 3 or 4. Flowmeter.