JP2011007764A - Ultrasonic level meter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an ultrasonic level meter having high accuracy of measurement and safety.SOLUTION: The ultrasonic level meter 1 includes an ultrasonic transducer 3 in which a composite piezoelectric element 4 to whose back surface a backing material 6 is joined is attached to a bottom of a container 2 through grease as a contact medium 5, and the composite piezoelectric element 4 is pressed to the bottom of the container 2 by a magnet 7 attached with an adhesive and a backing material cover 8 made of steel; and a controller controlling a detection operation of a liquid surface on the basis of time information from a time when an ultrasonic wave is transmitted by the ultrasonic transducer 3 to the time when the reflected wave is received.

Description

  The present invention relates to an ultrasonic transducer and an ultrasonic level meter, and more particularly, an ultrasonic transducer and an ultrasonic transducer for measuring a liquid level in a container using ultrasonic propagation time. The present invention relates to a sound level meter.

  An ultrasonic level meter measures the time until an ultrasonic wave is transmitted from an ultrasonic transducer and the reflected wave is received, and the distance to the object is calculated based on that time. Can be measured accurately and easily, and is used in various applications.

  When the liquid level inside the storage tank is at a high position (when it is far from the bottom of the storage tank), the wave is weaker than when the liquid level is close. However, it is necessary to increase the gain of the amplifier circuit in order to obtain sufficient echo. However, when the amplification circuit is set to a high gain, sufficient time is required until the drive reverberation is attenuated exponentially. That is, when trying to cope with a high liquid level, the drive reverberation time is prolonged, so that there is a problem that the liquid level cannot be captured when the liquid level is low.

  In order to solve this problem, a method is conceivable in which the receiving circuit is constituted by a variable gain amplifier circuit, and when the liquid level is low in the previous measurement result, the reflected echo is detected with a low gain. However, with this method, when the liquid level position changes suddenly or when the measurement interval is very long, it is impossible to follow the liquid level fluctuation, and stable measurement cannot be performed.

  On the other hand, in Patent Document 1, the dead zone time (distance) that cannot be detected due to reverberation is adjusted by controlling the ultrasonic transducer drive timing as a starting point so that the signal amplification factor increases. Therefore, a technique that can always realize stable reverberation characteristics and sensitivity characteristics is disclosed.

  However, in the case of the technique described in Patent Document 1, there is a limit in adjusting the dead zone time that cannot be detected by reverberation by controlling the signal amplification factor, and always achieving both stable reverberation characteristics and sensitivity characteristics. In addition, when the thickness of the storage tank is large, the dead zone time (distance) due to a certain amount of reverberation can be shortened by changing the signal amplification factor and changing the oscillation time of the pulse signal input to the ultrasonic transducer. In addition, there is a limit to the adjustment, and in consideration of temperature, changes over time, etc., there is a problem that the detection margin becomes a problem and the control of the timing is complicated and the power consumption is inevitably increased.

  Therefore, in Patent Document 2, there is little influence of reverberation characteristics when the liquid level is low, the liquid level is strong against changes in the liquid level such as liquid filling, and the liquid level is stable by simple control from low to high. It is described that an ultrasonic level meter that enables measurement can be obtained.

  JP 2004-257981 A   JP 2008-232801 A

Junichi Miyoshi, “Ultrasonic Technology Handbook”, Nikkan Kogyo Shimbun, December 1985, p722-723 Tadashi Shiogama, “Manufacture and Application of New Piezoelectric Materials”, CMC Corporation, December 1987, p99-109 Masaaki Kawamura, “Introduction to Electroacoustic Engineering”, Shosodo Co., Ltd., September 1998, p52-53 Ultrasonic Handbook Editorial Committee, “Ultrasonic Handbook”, Maruzen Co., Ltd., August 1999, p348-350

  However, even in Patent Document 2, since the acoustic impedances of the metal tank and the piezoelectric element are not so different, the ultrasonic vibration almost propagates to the metal tank, and the ultrasonic vibration propagating in the liquid is very much. Get smaller. For this reason, there is a problem that since the S / N ratio of the signal becomes small, measurement with high accuracy cannot be performed stably.

  In addition, since the mechanical quality factor of the piezoelectric element is large, reverberation continues for a long time, and there is a problem that measurement cannot be performed when the reflection time is short.

  Further, when viewed in detail, the piezoelectric element changes as shown in FIG. That is, in a stationary state when not in operation, as shown in A, it has a cylindrical shape. In the reduced state in the thickness direction, it extends in the radial direction and becomes the shape of a concave lens as shown in FIG. Further, in the stretched state in the thickness direction, it shrinks in the radial direction, and becomes a shape like a convex lens as shown by emphasizing C. As a result, as shown in D, only the ultrasonic vibration at the central part of the cylinder goes straight, and the ultrasonic vibration propagates in the direction in which the peripheral part extends with respect to the central axis. Therefore, the ultrasonic signal as a signal becomes small. The spread ultrasonic vibration has a problem that it is detected as an error.

The present invention has been made in view of the above circumstances, and a main object thereof is an ultrasonic transducer capable of measuring a liquid surface position with high accuracy from a low liquid level to a high liquid level, and an ultrasonic transducer including the ultrasonic transducer. It is to provide a sound level meter.
That means
1. Improve signal rise characteristics.
2. Improve reverberation characteristics.
3. Increase the directivity of ultrasound.
4). Increase signal resolution.
To achieve these,
・ Reduce the mechanical quality factor of piezoelectric elements.
・ Increase the number of piezoelectric elements to make the phase the same.
・ Increase the frequency of ultrasound.
More than can be considered. A composite piezoelectric element is most suitable for realizing these.

  In the present invention, ultrasonic waves are transmitted by an ultrasonic transducer, a reflected wave of the transmitted wave is received by the same or another ultrasonic transducer, and a level is measured by measuring a propagation time from transmission to reception of the ultrasonic wave. In the ultrasonic level meter to be measured, the ultrasonic transducer is a composite piezoelectric element.

  The present invention also provides an ultrasonic wave transmitted by an ultrasonic transducer, the reflected wave of the transmitted wave is received by the same or another ultrasonic transducer, and the propagation time from transmission to reception of the ultrasonic wave is measured. In an ultrasonic level meter for measuring the above, a composite piezoelectric element in which an ultrasonic transducer is joined to a metal diaphragm is used.

  The present invention also provides the diaphragm with a metallic diaphragm ring.

  The present invention also includes the ultrasonic transducer that is attached to the bottom outer wall surface of the container and that transmits ultrasonic waves toward the liquid surface of the liquid contained in the container and receives reflections from the liquid surface. This is an ultrasonic level meter.

  The present invention is also an ultrasonic level meter attached to the outer side wall surface of a container, having the ultrasonic transducer for transmitting an ultrasonic wave toward the container and receiving an echo.

  According to the ultrasonic level meter of the present invention, the liquid level of the liquid in the container can be measured with high accuracy.

It is sectional drawing which shows 1st Embodiment of the ultrasonic level meter by this invention. It is a top view of the compound piezoelectric element used for the ultrasonic level meter of a 1st embodiment. It is sectional drawing in the AA line of FIG. This is an apparatus for comparing ultrasonic propagation of a composite piezoelectric element and a piezoelectric element into water. It is a figure which shows the received wave which propagated the ultrasonic wave which propagated in water using the composite piezoelectric element for the apparatus of FIG. It is a figure which shows the received wave which propagated the ultrasonic wave which propagated in water using the piezoelectric element for the apparatus of FIG. It is the schematic of the measurement circuit used for an ultrasonic level meter. It is sectional drawing which shows 2nd Embodiment of the ultrasonic level meter by this invention. It is sectional drawing which shows 3rd Embodiment of the ultrasonic level meter by this invention. It is sectional drawing which shows 4th Embodiment of the ultrasonic level meter by this invention. It is sectional drawing which shows 5th Embodiment of the ultrasonic level meter by this invention. It is a displacement figure of the piezoelectric element used for the conventional ultrasonic level meter.

  The basic configuration of the first embodiment is shown in the sectional view of FIG.

  It is the ultrasonic level meter 1 attached to the stainless steel container 2 which accommodates the liquefied gas, kerosene, etc. which are shown in FIG. The ultrasonic level meter 1 includes a magnet 7 attached to the bottom surface of the container 2 with a composite piezoelectric element 4 having a backing material 6 bonded to the back surface via grease as a contact medium 5 and attached to the bottom surface of the container 2 with an adhesive. Based on the ultrasonic transducer 3 in which the composite piezoelectric element 4 is pressed by the steel backing material cover 8 and the time information from when the ultrasonic wave is transmitted by the ultrasonic transducer 3 to when the reflected wave is received, And a controller that controls the detection operation. Details of the contact medium are described in Non-Patent Document 1.

  One of the most characteristic features of the present invention is that the piezoelectric element of the ultrasonic transducer is a composite piezoelectric element.

  A composite piezoelectric element is a material in which a piezoelectric body such as a piezoelectric ceramic and a resin are structurally combined, and has characteristics not found in piezoelectric ceramics and piezoelectric polymers. Although described in detail in Non-Patent Document 1, a composite piezoelectric vibrator has been developed to obtain characteristics that cannot be realized with a single piezoelectric element, and is currently expensive for medical use. Mainly used. Composite piezoelectric vibrators are classified into ceramic and polymer composites based on how many dimensions (in which direction) each ceramic and polymer are self-bonded. That is, an mn composite is represented by a dimension number m in which the piezoelectric ceramic as a piezoelectric active component is continuous in the composite and a dimension n in which a polymer as a piezoelectric inactive component is continuous. Among the composite piezoelectric vibrators, it is the most practical 1-3 composite and was used in the present invention.

  2 and 3 show details of the composite piezoelectric element 4. Each piezoelectric element 9 has a diameter of 20 mm and a thickness of 3 mm, a 2 mm square and a length of 3 mm, and the width of the epoxy resin 10 joining the piezoelectric element 9 and the piezoelectric element 9 is 1 mm. The piezoelectric element 9 is a lead-based piezoelectric ceramic, and is a so-called Low Q material.

  In addition, by using a composite piezoelectric vibrator having a large mechanical loss compared to a normal piezoelectric element, the rise of the signal waveform is improved, so that the measurement resolution is improved and the measurement accuracy is improved.

Further, when the individual piezoelectric elements of the composite piezoelectric vibrator are considered as point ultrasonic wave generation sources, the directivity increases as the number of piezoelectric elements increases. This is described in detail in Non-Patent Document 3.
In order to improve the directivity of ultrasonic waves and improve the accuracy of the level meter, which is the object of the present invention, the number of piezoelectric elements is preferably nine or more.
In addition, in order to reduce the acoustic impedance of the adhesive that joins the individual piezoelectric elements in order to make the individual piezoelectric elements of the composite piezoelectric vibrator act as a point ultrasonic wave generation source, a filler having a lower density than the adhesive is used. It is desirable to mix.

For reference, the physical properties of the materials used or compared here are shown.
The density of the lead-based piezoelectric ceramic is about 7.8 × 10 ³ kg / m ³ , the speed of sound is 4100 m / s, and the acoustic impedance is about 3.2 × 10 ⁷ kg / m ² s.
The density of the epoxy resin is about 1.1 × 10 ³ kg / m ³ , the sound speed is 2480 m / s, and the acoustic impedance is about 2.7 × 10 ⁶ kg / m ² s.

  In the present invention, the piezoelectric element constituting the composite piezoelectric element includes a piezoelectric single crystal as well as a piezoelectric ceramic. Piezoelectric ceramics include lead zirconate titanate, barium titanate, lead titanate, lead niobate and the like. Examples of the piezoelectric single crystal include lead zirconate titanate single crystal, lithium niobate, and quartz.

  In the present invention, a composite piezoelectric element using a polycrystalline piezoelectric element is used as the piezoelectric element. However, when higher performance is required, a composite piezoelectric element using a piezoelectric single crystal as a piezoelectric element is also conceivable.

  Here, in order to compare the performance of the ultrasonic transducer 3 using the composite piezoelectric element 4 of the present invention with that of the ultrasonic transducer 3 using the conventional piezoelectric element 9, a propagation characteristic measuring apparatus 11 shown in the sectional view of FIG. Use. In order to compare the basic performance, a stainless steel disk having a diameter of 50 mm and a thickness of 1.9 mm is joined to both sides of a stainless steel measuring tube 16 having an outer diameter of 50 mm, an inner diameter of 40 mm and a length of 100 mm. The composite piezoelectric vibrators 4a and 4b shown in FIG. 4 are bonded to the outer central portion with grease. FIG. 5 shows the result of observing the reception voltage at the opposite composite piezoelectric vibrators 4a and 4b by applying six burst waves of 492 KHz at about 98 Vp-p to the composite piezoelectric vibrators 4a and 4b. The propagation sound speed is about 1470 m / s, which is almost the same as the propagation speed of ultrasonic water. Therefore, you can see that the waves are moving straight through the water. The decay time of the received signal is about 230 μs.

  The ultrasonic transducer 3 using a normal piezoelectric element is evaluated using the propagation characteristic measuring apparatus 11 described above. A laminated piezoelectric element in which three conventional piezoelectric elements 9 having a diameter of 20 mm and a thickness of 1 mm are laminated is prepared. A description will be given of an ultrasonic transducer 3 using the laminated piezoelectric element. A stainless steel disc having a diameter of 50 mm and a thickness of 1.9 mm is joined to both sides of a stainless steel tube having an outer diameter of 50 mm, an inner diameter of 40 mm, and a length of 100 mm, and a laminated piezoelectric element is adhered to the center of the outer side of the disc with grease. . FIG. 6 shows the result of observing the reception voltage at the opposite laminated piezoelectric element by applying six burst waves of 94 KHz at approximately 101 Vp-p to this laminated piezoelectric element. The reason why the laminated piezoelectric element is used is to reduce the driving voltage.

  Furthermore, in order to measure the attenuation characteristic, it was confirmed that the wave continued for 400 μs or more by extending the time axis of the oscilloscope. The propagation sound velocity is about 1140 m / s, and it can be seen that the wave does not travel straight through the water.

  As described above, it has been clarified that the directivity of ultrasonic waves can be improved by using the composite piezoelectric vibrator as compared with the conventional piezoelectric element. Moreover, even if the same shape uses a composite piezoelectric vibrator, a high frequency in the thickness direction is used, so that the measurement resolution can be increased. Since the laminated piezoelectric element has a Qm of about 60 and the composite piezoelectric vibrator has a Qm of about 20, the rising characteristic of the reception signal of the composite piezoelectric vibrator is excellent.

Here, even with the same shape of the composite piezoelectric transducer is directed to increasing the number of piezoelectric elements is increased, the cross-sectional area of the piezoelectric element is too small, anxiety mechanical strength less be 0.01 mm ² or more since they produce A cross-sectional area is required. Further, in order to excite vibration in the thickness direction, a cross-sectional area of 25 mm ² or less is required.

  As shown in FIG. 5, the apparatus using the ultrasonic transducer of the present invention receives only the wave propagating in the water, and the rising characteristic of the wave is good. Therefore, the liquid level can be accurately measured.

  As shown in FIG. 6, the conventional ultrasonic transducer uses a relatively low drive frequency, so the resolution is small and the rise characteristic is not good. Furthermore, the straightness of ultrasonic waves is not good. Therefore, the liquid level cannot be measured accurately and with high resolution. In addition, since various waves continue for a long time, the measurement interval becomes long, and a rapid change in the liquid level cannot be followed. One reason for this is considered to be that the Qm of the ultrasonic transducer of the present invention is about 24, and the Qm of the conventional ultrasonic transducer is about 300.

  Therefore, when a piezoelectric element having a Qm of about 100 is joined to a metal having a high mechanical quality factor Qm, the ultrasonic transducer has a Qm of about 300, which adversely affects the measurement accuracy of the ultrasonic level meter. In contrast, when a composite piezoelectric element with a Qm of about 20 is bonded to a metal, which is a material with a high mechanical quality factor Qm, the ultrasonic transducer has a Qm of about 20, improving the measurement accuracy of the ultrasonic level meter. Let

  The operation of the ultrasonic level meter having the above configuration will be described below with reference to FIG.

  A measurement circuit similar to the measurement circuit of the ultrasonic level meter 1 shown in Non-Patent Document 4 is used. The ultrasonic transducer is used for both transmission and reception. The amplifier gain controller is a function for increasing the gain with time, and for removing unnecessary waves. The integrating circuit is for avoiding disturbance. The output is a current output (4 to 20 mADC).

  About 100 Vp-p, about 500 KHz, and six tone bust waves are applied to the composite piezoelectric element to the ultrasonic transducer. Switch the transmission and reception to receive. The propagation time of the ultrasonic wave 6 propagated along the same path is measured, and the level of the liquid level is calculated based on the measured value.

  With the above measurement, it is possible to measure the position of the liquid surface with higher accuracy than in the past.

  The basic configuration of the second embodiment is shown in the sectional view of FIG.

  An ultrasonic level meter 1 attached to a stainless steel container 2 containing liquefied gas or kerosene is shown in FIG. The ultrasonic transducer 3 of the ultrasonic level meter 1 is obtained by bonding a composite piezoelectric element 4 to the bottom surface of a container 2 using an epoxy resin. When it is not necessary to remove the ultrasonic transducer 3 from the container 2, it is better to join the composite piezoelectric element 4 to the bottom surface of the container 2 using epoxy resin than to contact the composite piezoelectric element 4 to the container using a contact medium. Excellent signal magnitude, signal rise characteristics, and attenuation characteristics. Therefore, the configuration shown in FIG. 8 is suitable.

  A basic configuration according to the third embodiment is shown in a sectional view of FIG.

  FIG. 9 shows an ultrasonic level meter 1 attached to a steel container 2 that contains liquefied gas, kerosene, and the like. When it is necessary to remove the ultrasonic transducer 3 and the container 2 is made of a magnetic material, a configuration as shown in FIG. 9 is desirable. A ring-shaped magnet 7 is joined to a steel diaphragm 12 using an epoxy resin. The composite piezoelectric element 4 is bonded to the center portion of the diaphragm 12 using an epoxy resin. Then, grease is applied to the joint surface between the container 2 and the diaphragm 12 and pressed against the container 2 by the magnetic force of the magnet 7.

  A basic configuration according to the fourth embodiment is shown in a sectional view of FIG.

  An ultrasonic level meter 1 attached to a stainless steel container 2 containing liquefied gas, kerosene, etc. is shown in FIG. When the ultrasonic transducer 3 cannot be installed on the bottom surface, it is attached to the upper part of the container. In that case, the ultrasonic wave is propagated in the air through the stainless steel and reflected by the liquid surface, the ultrasonic wave is propagated again in the air, and the stainless steel is further propagated and received by the composite piezoelectric element 4. In such a configuration, the composite piezoelectric element is superior to a normal piezoelectric element.

  A basic configuration of the fifth embodiment is shown in a sectional view of FIG.

  FIG. 11 shows an ultrasonic level meter 1 attached to a stainless steel container 2 containing liquefied gas, kerosene and the like. When the ultrasonic transducers 3a and 3b are installed on the side surface of the container 2, the ultrasonic transducers 3a and 3b may be moved relative to the side surface of the container. The ultrasonic transducers 3 a and 3 b are composite piezoelectric elements 4 a and 4 b in which carbon fiber composite materials 20 a and 20 b are arranged on the surface that contacts the side surface of the container 2.

DESCRIPTION OF SYMBOLS 1 Ultrasonic level meter 2 Container 3 Ultrasonic transducer 4 Composite piezoelectric element 5 Contact medium 6 Backing material 7 Magnet 8 Backing material cover 9 Piezoelectric element 10 Propagation characteristic measuring device 12 Diaphragm 13 Spring 14 Introducing pipe 15 Discharge pipe 16 Measurement Tube 17 Detection waveform when using composite piezoelectric element 18 Detection waveform when using piezoelectric element 19 Ultrasonic level meter measurement circuit 20 Carbon fiber composite material

Claims (5)

  Ultrasound is transmitted by an ultrasonic transducer, the reflected wave of the transmitted wave is received by the same or another ultrasonic transducer, and the level is measured by measuring the propagation time from transmission to reception of the ultrasonic wave In the level meter, the ultrasonic transducer is a composite piezoelectric element.   Ultrasound is transmitted by an ultrasonic transducer, the reflected wave of the transmitted wave is received by the same or another ultrasonic transducer, and the level is measured by measuring the propagation time from transmission to reception of the ultrasonic wave In the level meter, the ultrasonic transducer is a composite piezoelectric element bonded to a metal diaphragm.   The ultrasonic level meter according to claim 2, wherein the diaphragm has a metal diaphragm ring.   The ultrasonic transducer is attached to the outer wall surface of the bottom of the container, and transmits ultrasonic waves toward the liquid surface of the liquid contained in the container, and receives reflections from the liquid surface. The ultrasonic level meter according to any one of 3 above.   The ultrasonic level meter according to claim 1, wherein the ultrasonic level meter is an ultrasonic transducer that is attached to an outer side wall surface of a container, transmits an ultrasonic wave toward the container, and receives an echo.

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