Design method of acoustic impedance matching device for gas flow measurement ultrasonic transducer

Abstract

The application discloses a kind of gas flow measurement ultrasonic transducer acoustic impedance matching device design methods, belong to ultrasonic transducer technical field.The gas flow measurement ultrasonic transducer acoustic impedance matching device design method is based on equivalent impedance theory, and design steps include: step 1: according to the relationship between equivalent input impedance, equivalent impedance, acoustic impedance and standing wave coefficient under equivalent impedance theory, the acoustic impedance of each acoustic impedance matching device in N acoustic impedance matching device is calculated;Step 2: according to the acoustic impedance value of each acoustic impedance matching device calculated in step 1, the appropriate material density and the acoustic velocity of the corresponding material are selected to match the acoustic impedance value of the corresponding acoustic impedance matching device;Step 3: according to the working frequency of ultrasonic transducer and the acoustic velocity selected in step 2, the thickness of each acoustic impedance matching device is determined, and the matching device design is completed.

Description

Technical Field

[0001] This invention relates to a design method for an acoustic impedance matching device for a gas flow metering ultrasonic transducer, belonging to the field of ultrasonic transducer technology. Background Technology

[0002] In ultrasonic flow meters, the ultrasonic transducer is a key component, often referred to as the instrument's "eyes and ears." A crucial indicator of an ultrasonic transducer is its transmission and reception efficiency. For ultrasonic transducers used in gas flow measurement, without an acoustic impedance matching device, the transmission rate of the emitted ultrasonic energy into the gas is negligible. Since the transmitting and receiving transducers are reciprocal devices, the receiving transducer receives the incoming wave energy with the same transmission rate (secondary transmission rate), further reducing the received ultrasonic energy. The fundamental reason for the low transmission rate is the large difference between the acoustic impedance of the piezoelectric element and the acoustic impedance of the gas, resulting in near total reflection. Therefore, an acoustic impedance matching device must be added to the piezoelectric element to improve the transmission rate. Acoustic impedance matching methods for ultrasonic transducers are mainly divided into single-section and multi-section acoustic impedance matching devices.

[0003] Currently, the mainstream ultrasonic transducers used in gas flow metering instruments employ single-section acoustic impedance matching devices. For ultrasonic transducers operating at 500kHz, the acoustic impedance matching device density is typically between 470kg / m³ and 500kg / m³, with a secondary transmittance of around 4.0%. Considering factors such as oscillation speed, the mechanical properties of the matching device material, and bandwidth, a reduction in transmittance is generally made to achieve good practical performance in other parameters. However, reducing transmittance negatively impacts the transducer's transmission and reception efficiency.

[0004] To improve transmittance and signal-to-noise ratio, acoustic impedance matching has become an important component of ultrasonic transducers for flow measurement, especially for transducers used for gas flow measurement. Acoustic impedance matching is indispensable, but currently there is no effective and relatively simple design method for acoustic impedance matching of ultrasonic transducers. Summary of the Invention

[0005] To address the aforementioned problems in the prior art, this invention provides an effective and relatively simple method for designing an acoustic impedance matching device for an ultrasonic transducer used for gas flow measurement.

[0006] This invention is achieved through the following technical solution: a design method for an acoustic impedance matching device for a gas flow metering ultrasonic transducer, wherein the source end of the acoustic impedance matching device is a piezoelectric element, the load end is the gas being measured, and N cascaded acoustic impedance matching devices are arranged between the source end and the load end. Its characteristic is that, based on the equivalent impedance theory, the equivalent input impedance, equivalent impedance, acoustic impedance, and standing wave ratio between adjacent acoustic impedance matching devices in the N cascaded devices satisfy the following relationship: The starting impedance of the acoustic impedance matching device in section k is both the equivalent input impedance and the maximum equivalent impedance of that section. The terminal impedance of the k-th acoustic impedance matching section is the minimum equivalent impedance of that section of the acoustic impedance matching section, i.e. ;and When k=1: , ; ; ; Where k is the section number where the acoustic impedance matching circuit is located. ; Let be the equivalent input impedance of the k-th acoustic impedance matching unit; Let be the standing wave ratio of the acoustic impedance matching device in section k; Let K be the acoustic impedance of the k-th acoustic impedance matching unit; is the maximum equivalent impedance of the k-th acoustic impedance matching unit; This is the equivalent impedance at the end of the k-th acoustic impedance matching unit; Let be the minimum equivalent impedance of the k-th acoustic impedance matching device; The acoustic impedance of the (k-1)th section acoustic impedance matching device; The standing wave ratio of the acoustic impedance matching device in section k-1; This is the equivalent input impedance of the acoustic impedance matching device in Section 1; The acoustic impedance of the piezoelectric element; The equivalent impedance at the end of the Nth acoustic impedance matching section; N is the number of sections of the acoustic impedance matching section. The load acoustic impedance; The steps involved in designing a matcher include: Step 1: Based on the relationship between the equivalent input impedance, equivalent impedance, acoustic impedance and standing wave coefficient mentioned above, calculate the acoustic impedance of each section of the N-section acoustic impedance matching circuit. Among them, when the number of acoustic impedance matching sections At this time, the standing wave ratio of the acoustic impedance matching device satisfies the following relationship: , This is the ratio of the standing wave coefficient of the acoustic impedance matching device at the end of the section to that at the beginning of the section. The standing wave ratio of the Nth acoustic impedance matching element; The standing wave ratio of the acoustic impedance matching device in Section 1; Step 2: Select appropriate material density based on the acoustic impedance values ​​of each section of the acoustic impedance matching device calculated in Step 1. and the speed of sound of the corresponding materials Make its acoustic impedance Match the acoustic impedance value of the corresponding section's acoustic impedance matching device; Step 3: Based on the operating frequency of the ultrasonic transducer and the sound velocity selected in Step 2, determine the thickness of each section of the acoustic impedance matching device to complete the matching device design. The thickness of the acoustic impedance matching element is determined by the following formula: ,in, Let the thickness of the k-th acoustic impedance matching section be denoted as . Let be the operating wavelength of the ultrasonic wave in the k-th section of the acoustic impedance matching circuit. .

[0007] Furthermore, the thickness of the acoustic impedance matching device is determined according to the following formula: .

[0008] Furthermore, the formula for calculating the acoustic impedance of a single-section acoustic impedance matching device is as follows: ; The formulas for calculating the acoustic impedance of each section of a two-section acoustic impedance matching system are as follows: ; In the formula, This represents the ratio of the standing wave ratio (SWR) of the acoustic impedance matching device in the last section to that in the first section. The range of values ​​for is: .

[0009] Furthermore, when the number of acoustic impedance matching sections... At that time, by making reasonable selection The values ​​are adjusted to change the acoustic impedance values ​​of each section of the acoustic impedance matching device, thereby allowing for the selection of suitable materials for each section. The range of values ​​for is: .

[0010] The beneficial effects of this invention are: the method of this invention makes the design of the acoustic impedance matching device for ultrasonic transducers simpler, and the acoustic impedance matching device for ultrasonic transducers can be quickly designed according to the acoustic impedance of the source end and the load end; the acoustic impedance matching device designed by this method has the selectability of matching device acoustic impedance and matching device material parameters, making it more practical; if the design parameters are selected, the transducer can transfer ultrasonic energy to the load to the maximum extent, and obtain the maximum transmittance; if the parameter deviation is appropriately selected, the desired transmittance, start-up speed and bandwidth can be obtained. Attached Figure Description

[0011] Figure 1 This is the equivalent impedance theoretical model of the N-section acoustic impedance matching device in this invention.

[0012] Figure 2 This is a design model for a single-section acoustic impedance matching device based on the equivalent impedance theory in a specific embodiment of the present invention.

[0013] Figure 3 This is a design model of a two-section acoustic impedance matching device based on the equivalent impedance theory in a specific embodiment of the present invention. Detailed Implementation

[0014] The invention will be further described below with reference to non-limiting embodiments and accompanying drawings.

[0015] The key to ultrasonic transducer design lies in rationally selecting the number of sections in the acoustic impedance matching device based on the difference between the load acoustic impedance and the signal source acoustic impedance. This ensures that the transducer can transfer ultrasonic energy to the load to the maximum extent while achieving the required start-up speed and bandwidth.

[0016] This invention designs an acoustic impedance matching device for an ultrasonic transducer used for gas flow measurement based on the equivalent impedance theory.

[0017] I. Equivalent Impedance Theory of Multi-Section Acoustic Impedance Matching Devices As attached Figure 1 The figure shows the equivalent impedance theoretical model of an N-section acoustic impedance matching circuit. The source end is a piezoelectric element, and the acoustic impedance of the piezoelectric element is... The load is the gas being measured, and the load acoustic impedance is... N cascaded acoustic impedance matching sections are installed between the source and load ends, and the acoustic impedance of the k-th acoustic impedance matching section is... , The equivalent input impedance of the acoustic impedance matching device in section k is: The equivalent impedance of the acoustic impedance matching device in section k is The thickness of the k-th acoustic impedance matching section is The reflection coefficient when viewed from the end of the (k-1)th section of the acoustic impedance matching device towards the beginning of the kth section is: The reflection coefficient seen from the piezoelectric electrode into the first acoustic impedance matching section is: ,and Γ is the total reflection coefficient seen from the piezoelectric electrode towards the load. The standing wave ratio of the acoustic impedance matching circuit in section k is... For a lossless acoustic impedance matching device, the standing wave coefficients at all points on the k-th section of the acoustic impedance matching device are equal, that is, the standing wave coefficient at any point on this section is equal to... .

[0018] If the reflected wave from the end of the k-th acoustic impedance matching section returns to the beginning of the k-th acoustic impedance matching section and is out of phase with the reflected wave from the beginning of the k-th acoustic impedance matching section, their superposition can cancel or reduce the total reflected wave from the beginning of the k-th acoustic impedance matching section, thus achieving matching or approximate matching. The additional path length that the reflected wave from the beginning of the k-th section to the end of the k-th section and back to the beginning must travel compared to the reflected wave from the beginning is... To ensure that the reflected waves from the beginning and end of each segment are out of phase upon returning to the beginning, the phase difference between the two reflected waves must satisfy the following condition: And based on the propagation characteristics of waves, Therefore, the principle for selecting the thickness of the acoustic impedance matching device is as follows: ,in Let be the operating wavelength of the ultrasonic wave in the k-th section of the acoustic impedance matching circuit. Preferred , Let be the phase difference between the reflected wave from the beginning of section k and the reflected wave from the end of section k after they return to the beginning. Let be the phase constant of the k-th section.

[0019] In ultrasonic transducers commonly used for gas flow measurement, the acoustic impedance of the piezoelectric element... Much greater than the acoustic impedance of the load being measured (i.e., the gas) Then, the acoustic impedance distribution law of its multi-section acoustic impedance matching device is as follows. The acoustic impedance of the Nth section acoustic impedance matching device gradually increases as k decreases. Acoustic impedance of the load The closest, acoustic impedance of the acoustic impedance matching device in Section 1 Acoustic impedance of piezoelectric element The closest. Under the condition of satisfying the thickness selection principle of the acoustic impedance matching device, the equivalent input impedance of the k-th section of the acoustic impedance matching device is... ,when Then, I will be satisfied. This achieves acoustic impedance matching. Under the condition that the thickness of the acoustic impedance matching element is selected, the distribution law of the equivalent impedance is: the end of the k-th section of the acoustic impedance matching element is the point of minimum equivalent impedance for that section. And equal to the equivalent input impedance of the acoustic impedance matching device in section k+1, i.e. ,and That is, the point where the equivalent impedance of the (k+1)th acoustic impedance matching device is at its maximum. ,Right now ,and ,and The starting point of the acoustic impedance matching device in section k is the point where the equivalent impedance of that section is at its maximum. And equal to the equivalent input impedance of the k-th section. ,and This is also the point where the equivalent impedance of the acoustic impedance matching device in section k-1 is minimized. ,Right now ,and ; In summary, the equivalent impedance distribution law under the condition of satisfying the principle of selecting the thickness of the acoustic impedance matching device can be summarized as follows: During matching ; ; ; .

[0020] For lossy acoustic impedance matching, the standing wave ratio (SWR) varies at different points on each section of the matching device. The difference in SWR between the beginning and end is the greatest, and the degree of this difference depends on the attenuation of the impedance matching device. Based on transmission line theory, the load reflection coefficient... ,and ,for For acoustic impedance matching devices, From the end of the Nth section acoustic impedance matching device via Upon reaching the beginning of this section, the reflection coefficient at the beginning of the Nth section of the acoustic impedance matching circuit is... In the formula Let N be the attenuation constant of the acoustic impedance matching circuit in section N. Let be the phase constant of the Nth acoustic impedance matching unit. , The phase angle is the load reflection coefficient. The load reflection coefficient. Because the acoustic impedance matching circuit design meets the following conditions: ,and ,and and If all are real numbers, then , Standing wave coefficient , The equivalent input impedance of the acoustic impedance matching circuit in section N is... The reflection coefficient at the end of the N-1th section of the acoustic impedance matching device is And so on. , , , .

[0021] The standing wave coefficients at the beginning and end of the matched circuit in section k are as follows: , , .

[0022] The equivalent input impedance of the acoustic impedance matching circuit in section k is: The reflection coefficient at the end of the (k-1)th acoustic matching unit is The total reflection coefficient at the interface between the piezoelectric electrode and the first acoustic impedance matching unit is .

[0023] II. The steps involved in designing a matcher include: Step 1: Based on the relationship between the equivalent input impedance, equivalent impedance, acoustic impedance and standing wave coefficient mentioned above, calculate the acoustic impedance of each section of the N-section acoustic impedance matching circuit. Among them, when the number of acoustic impedance matching sections At this time, the standing wave ratio of the acoustic impedance matching device satisfies the following relationship: , This is the ratio of the standing wave coefficient of the acoustic impedance matching device at the end of the section to that at the beginning of the section. The standing wave ratio of the Nth acoustic impedance matching element; The standing wave ratio of the acoustic impedance matching device in Section 1; Step 2: Select appropriate material density based on the acoustic impedance values ​​of each section of the acoustic impedance matching device calculated in Step 1. and the speed of sound of the corresponding materials Make its acoustic impedance Match the acoustic impedance value of the corresponding section's acoustic impedance matching device; Step 3: Based on the operating frequency of the ultrasonic transducer and the sound velocity selected in Step 2, determine the thickness of each section of the acoustic impedance matching device to complete the matching device design. The thickness of the acoustic impedance matching element is determined by the following formula: ,in, Let the thickness of the k-th acoustic impedance matching section be denoted as . Let be the operating wavelength of the ultrasonic wave in the k-th section of the acoustic impedance matching circuit. .

[0024] Example 1 This embodiment is a design method for a single-section acoustic impedance matching device based on the equivalent impedance theory.

[0025] like Figure 2 The figure shows a design model of a single-section acoustic impedance matching device based on the equivalent impedance theory.

[0026] When using a single-section acoustic impedance matching device for matching, the optimal thickness of the acoustic impedance matching device is determined based on the equivalent impedance theory of multi-section acoustic impedance matching devices. ,or , This is the operating wavelength in the acoustic impedance matching circuit. Matching must satisfy... ,and From the above two equations, we can obtain .

[0027] For lossy acoustic impedance matching devices ,in , The attenuation constant of the acoustic impedance matching device is... , .

[0028] Design example of a single-section acoustic impedance matching circuit based on equivalent impedance theory: For ultrasonic transducers used for gas flow measurement, it is known that... , Substitute In the formula, the acoustic impedance of a single-section acoustic impedance matching device is obtained. Theoretically, it can be used The matcher can achieve a match or an approximate match.

[0029] Theoretical calculations are performed based on the above matching results: The theoretical value for achieving a match is ,but , ,visible Therefore, it is a matched state, and the total reflection coefficient is... transmittance Transmit and receive secondary transmittance .

[0030] (1) If an acoustic impedance matching device close to the theoretical matching value is selected For example, select , ,but The ultrasonic transducer operates at a frequency of 500kHz, and the thickness of the acoustic matching element is selected as follows: . The result is 3.5% different from the theoretical matching value.

[0031] Theoretical calculations are performed based on the above matching results: If it is a lossless acoustic impedance matching device, then , , , , Therefore, it is an approximate matching state.

[0032] If it is a lossy acoustic impedance matching device, the attenuation constant of the low-loss acoustic impedance matching device, based on comprehensive measurements, is... Calculate its transmittance: , ,Right now , , , , The calculated results differ from those of the lossless acoustic impedance matching device by 8.5%. These results also indicate that when the actual selected acoustic impedance is not significantly different from the theoretical design value, the transmittance of the lossless and low-loss acoustic impedance matching devices is not significantly different.

[0033] (2) In contrast, the density of the acoustic impedance matching device of the 500kHz ultrasonic transducer currently used for gas flow measurement is mostly 470~500kg / m^3.

[0034] Cash , ,but The thickness of the acoustic matching device is selected. . The difference from the theoretical matching value is 753%.

[0035] because The values ​​taken differ significantly from the theoretical matching values, and acoustic impedance matching devices always have some loss. Therefore, a comprehensive measurement was performed. , , ,Right now , , , , The transmit transmittance is 20.5%, and due to the reciprocity of transmission and reception, the receive transmittance is also 20.5%. Therefore, the combined transmit and receive transmittance (secondary transmittance) is 4.2%, which is relatively low. Special note: When the actual acoustic impedance differs significantly from the theoretical design value, even with low power consumption, it must be treated as lossy impedance.

[0036] Example 2

[0037] This embodiment is a design method for a two-section acoustic impedance matching device based on the equivalent impedance theory.

[0038] like Figure 3 The figure shows a design model of a two-section acoustic impedance matching device based on the equivalent impedance theory.

[0039] When a two-section acoustic impedance matching device is used for matching, according to the equivalent impedance theory of the aforementioned multi-section acoustic impedance matching device, the equivalent input impedance of the two-section acoustic impedance matching device is: Matching should include... ,Right now ; ,Right now ;and ,Right now From the above relation, we can obtain ,and To facilitate calculation without affecting the overall matching performance, we can take... From the above relationship, we can obtain... The relationship between each section's matcher is obtained as follows: , .

[0040] When performing acoustic impedance matching with a matching device: To facilitate the selection of suitable materials for each section of the sound matching unit, appropriate selection can be made. The values ​​are considered comprehensively for optimal selection. .

[0041] If take ,Right now The acoustic impedance relationship of each section of the matching unit is: , .

[0042] If take , The acoustic impedance relationship of each section of the matching unit is: , .

[0043] For lossy two-section acoustic impedance matching circuits , , , .

[0044] Design example of a two-section acoustic impedance matching circuit based on equivalent impedance theory: For gas transducers, it is known , .

[0045] For a gas ultrasonic transducer with an additional double-section acoustic impedance matching device, if we take ,but , Calculated , Theoretically, a suitable material density and corresponding sound velocity can be selected to adjust its acoustic impedance. achieve and The value is used to determine the thickness of the acoustic impedance matching device. or and or Theoretically, this completes the design of the two-section acoustic impedance matching device, but due to If the impedance is too small, it will be impossible to obtain acoustic impedance matching material suitable for practical environments; therefore, it is necessary to increase the impedance. The value of .

[0046] If take ,but , Calculated , The acoustic impedance value obtained at this point can be used to select a matching material that meets the requirements. Generally, as the acoustic impedance increases, the hardness and strength of the corresponding material also increase, which is beneficial for obtaining an acoustic matching material that meets practical requirements.

[0047] If the strength and hardness of a material are difficult to meet the requirements of a practical environment, then for comprehensive consideration, it is usually not necessary to pursue a perfect or near-perfect match, but rather to sacrifice some transmittance in exchange for suitable transducer performance.

[0048] For example: to further improve The value is chosen to facilitate the selection of a suitable acoustic impedance matching device. ,at this time , .

[0049] Actual , , , The ultrasonic transducer operates at a frequency of 500kHz, therefore: , , , Actual amount taken It is close to the theoretical value, differing by only 3%. However, considering the mechanical properties of the acoustic impedance matching device, its density cannot be too low. The difference from the theoretical matching value is 424%.

[0050] Theoretical calculations are performed based on the above matching results: because The actual value differs greatly from the theoretical value, so its transmittance must be calculated using the theory of lossy acoustic impedance matching.

[0051] Because a two-section acoustic impedance matching device has an additional adhesive layer compared to a single-section acoustic impedance matching device, and due to the difference in density between the two types of matching devices, the corresponding loss will increase. This was confirmed by actual measurements. , , , , , , , , , , , , To achieve good mechanical properties, the acoustic impedance of the second acoustic impedance matching device was increased, which reduced the transmittance. It can be seen that suitable transducer performance can be obtained by sacrificing some transmittance.

[0052] It should be noted that in the above formulas and calculations concerning the standing wave ratio in lossy acoustic impedance matching, the length of the matching device is taken as the average value. Calculated, or can be done according to design requirements. , Calculations are then performed.

[0053] III. Test Results of Single-Section and Double-Section Acoustic Impedance Matching Transistors Based on Equivalent Impedance Theory The transmittance of an ultrasonic transducer depends on factors such as the electromechanical coupling coefficient of the piezoelectric element, the reflection and refraction of ultrasonic waves, the multipath effect of ultrasonic waves, the loss of ultrasonic waves in the adhesive layer and acoustic impedance matching device, and the attenuation of ultrasonic waves in the gas. Accurate calculation of the transmittance is difficult. Furthermore, due to the strong directionality of the acoustic energy distribution in an ultrasonic transducer, accurate measurement of its transmittance is even more challenging. To demonstrate the validity of this invention, directional tests were conducted at a fixed position along the principal direction of the ultrasonic transducer, and the test results are presented in the form of relative transmittance, relative start-up time, and relative bandwidth.

[0054] Using a gas metering ultrasonic transducer currently operating at 500kHz as a reference, a relative comparison is made between the single-section acoustic matching transducer, which focuses solely on improving transmittance, and the two-section acoustic matching transducer, which considers overall performance, based on three parameters: transmittance, start-up speed, and bandwidth. This comparison verifies the correctness of the design method of this invention.

[0055] Currently, mainstream single-section matched transducers take into account factors such as start-up speed, mechanical properties of matched transducer materials, and bandwidth, sacrificing wave transmission rate to achieve good practical performance of the above parameters.

[0056] The matching impedance of mainstream single-section acoustic impedance matching transducers operating at 500kHz is mostly [missing information]. ,Pick Assume its secondary transmittance for both transmission and reception is 100%. The bandwidth is BW0 and the oscillation time is t0.

[0057] This invention presents theoretical calculations and comparative tests on one example of the aforementioned mainstream single-section acoustic impedance matching transducer (single section 1), another example of a single-section acoustic impedance matching transducer (single section 2), and one example of a double-section acoustic impedance matching transducer (double section). Taking the mainstream single-section acoustic impedance matching transducer as a reference, the relative transmittance, relative bandwidth, and relative start-up velocity are given. The specific test data are listed in Table 1.

[0058] Table 1:

[0059] As shown in Table 1, the measured secondary transmittance matches the theoretical value. For single-section 2, the deviation between the measured and theoretical values ​​is 6.8% compared to single-section 1; for double-section 2, the deviation is 4.9% compared to single-section 1. The deviation between the measured and theoretical values ​​is mainly due to the attenuation of the acoustic impedance matching circuit and the dispersion of the resonant frequency in the tested sample. Therefore, the design method of this invention is reliable.

[0060] As shown in Table 1, if high transmittance is not a priority, a single section 1 is sufficient; if high transmittance is the only priority, a single section 2 is sufficient; and if better overall parameters are desired, a double section is the best option.

Claims

1. A design method for an acoustic impedance matching device for a gas flow metering ultrasonic transducer, wherein the source end of the acoustic impedance matching device is a piezoelectric element, the load end is the gas being measured, and N cascaded acoustic impedance matching devices are arranged between the source end and the load end, characterized in that: Based on the equivalent impedance theory, the equivalent input impedance, equivalent impedance, acoustic impedance, and standing wave coefficient between the N-section acoustic impedance matching circuit and its adjacent sections satisfy the following relationship: The starting impedance of the acoustic impedance matching device in section k is both the equivalent input impedance and the maximum equivalent impedance of that section. The terminal impedance of the k-th acoustic impedance matching section is the minimum equivalent impedance of that section of the acoustic impedance matching section, i.e. ;and When k=1: , ; ; ; Where k is the section number where the acoustic impedance matching circuit is located. ; Let be the equivalent input impedance of the k-th acoustic impedance matching unit; Let be the standing wave ratio of the acoustic impedance matching device in section k; Let K be the acoustic impedance of the k-th acoustic impedance matching unit; is the maximum equivalent impedance of the k-th acoustic impedance matching unit; This is the equivalent impedance at the end of the k-th acoustic impedance matching unit; Let be the minimum equivalent impedance of the k-th acoustic impedance matching device; The acoustic impedance of the (k-1)th section acoustic impedance matching device; The standing wave ratio of the acoustic impedance matching device in section k-1; This is the equivalent input impedance of the acoustic impedance matching device in Section 1; The acoustic impedance of the piezoelectric element; The equivalent impedance at the end of the Nth acoustic impedance matching section; N is the number of sections of the acoustic impedance matching section. The load acoustic impedance; The steps involved in designing a matcher include: Step 1: Based on the relationship between the equivalent input impedance, equivalent impedance, acoustic impedance and standing wave coefficient mentioned above, calculate the acoustic impedance of each section of the N-section acoustic impedance matching circuit. Among them, when the number of acoustic impedance matching sections At this time, the standing wave ratio of the acoustic impedance matching device satisfies the following relationship: This is the ratio of the standing wave coefficient of the acoustic impedance matching device at the end of the section to that at the beginning of the section. The standing wave ratio of the Nth acoustic impedance matching unit; The standing wave ratio of the acoustic impedance matching device in Section 1; Step 2: Select appropriate material density based on the acoustic impedance values ​​of each section of the acoustic impedance matching device calculated in Step 1. and the speed of sound of the corresponding materials Make its acoustic impedance Match the acoustic impedance value of the corresponding section's acoustic impedance matching device; Step 3: Based on the operating frequency of the ultrasonic transducer and the sound velocity selected in Step 2, determine the thickness of each section of the acoustic impedance matching device to complete the matching device design. The thickness of the acoustic impedance matching element is determined by the following formula: ,in, Let the thickness of the k-th acoustic impedance matching section be denoted as . Let be the operating wavelength of the ultrasonic wave in the k-th section of the acoustic impedance matching circuit. .

2. The acoustic impedance matching device design method for the gas flow metering ultrasonic transducer according to claim 1, characterized in that: The thickness of the acoustic impedance matching element is determined by the following formula: .

3. The acoustic impedance matching device design method for the gas flow metering ultrasonic transducer according to claim 1 or 2, characterized in that: The formula for calculating the acoustic impedance of a single-section acoustic impedance matching device is: ; The formulas for calculating the acoustic impedance of each section of a two-section acoustic impedance matching system are as follows: ; In the formula, This represents the ratio of the standing wave ratio (SWR) of the acoustic impedance matching device in the last section to that in the first section. The range of values ​​for is: .

4. The acoustic impedance matching device design method for the gas flow metering ultrasonic transducer according to claim 3, characterized in that: When the number of acoustic impedance matching sections At that time, by making reasonable selection The values ​​are adjusted to change the acoustic impedance values ​​of each section of the acoustic impedance matching device, thereby allowing for the selection of suitable materials for each section. The range of values ​​for is: .

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