Method for determining the characteristic time reference wave of the acoustic signal of an ultrasonic flow meter

Abstract

A method for determining the characteristic time reference wave of an ultrasonic flow meter's acoustic signal, the ultrasonic flow meter comprising a first acoustic transceiver unit that transmits a first acoustic signal and receives a second acoustic signal, and a second acoustic transceiver unit that transmits the second acoustic signal and receives the first acoustic signal, the method comprising: (a) receiving a first waveform of the corresponding first acoustic signal and a second waveform of the corresponding second acoustic signal; (b) sampling multiple peaks of the first waveform and the second waveform; (c) setting a search range based on the multiple peaks; (d) setting the first peak within the search range as the characteristic peak; (e) recording a first time between the zero-crossing point and the zero point after the characteristic peak and a second time between the zero point and the zero-crossing point, and calculating the average time of the first time and the second time; and (f) calculating the total flight time based on the average time.

Description

Technical Field

[0001] This invention relates to a method for determining the acoustic signal of an ultrasonic flow meter, and more particularly to a method for determining the characteristic time reference wave of the acoustic signal of an ultrasonic flow meter. Background Technology

[0002] Flow meters are one of the most important instruments in industrial measurement, and they are closely related to various industrial applications and scientific research. The requirements for measurement accuracy are also getting higher and higher. Flow meters are widely used in various fields, such as semiconductor manufacturing processes: the manufacturing processes of coating equipment, etching equipment, cleaning equipment, and drying equipment all use flow meter measurement technology.

[0003] Ultrasonic technology was previously used in military and medical applications, but in recent years it has been developed into applications in many industries. Although ultrasonic flow meters are a relatively recent type of measuring instrument, their non-contact nature with the measuring body means they do not create resistance to the fluid, extend sensor life, and avoid contamination. As a result, they have gained attention and use from many industries in recent years.

[0004] Ultrasonic flow meters measure flow velocity in a pipe using an ultrasonic time-difference propagation algorithm, and then calculate the flow rate from the velocity. When ultrasonic waves move in the same direction as the fluid, the faster the flow velocity, the greater the time difference. Furthermore, the fluid state alters the speed of the ultrasonic waves, and the flow velocity and flow rate are estimated using the time difference obtained from the preceding and following sensors.

[0005] However, when the acoustic (ultrasonic) waveforms received by the corresponding front and rear sensors are not consistent, that is, when the acoustic characteristic time reference waves obtained by the front and rear sensors are inconsistent, it will cause a large error in the estimation of flow velocity and flow rate, and thus fail to achieve the function of precise measurement.

[0006] Therefore, how to design a characteristic time reference wave judgment method for the acoustic signal of an ultrasonic flow meter, perform pre-processing of the acoustic wave and judgment of the acoustic signal, so as to achieve precise measurement of the ultrasonic flow meter, is a major research topic that the inventors of this case intend to conduct. Summary of the Invention

[0007] One objective of this invention is to provide a method for determining the characteristic time reference wave of the acoustic signal of an ultrasonic flow meter, thereby solving the problems of the prior art.

[0008] To achieve the aforementioned objective, the present invention proposes a method for determining the characteristic time reference wave of an ultrasonic flowmeter's acoustic signal. The ultrasonic flowmeter includes a first acoustic transceiver unit that transmits a first acoustic signal and receives a second acoustic signal, and a second acoustic transceiver unit that transmits the second acoustic signal and receives the first acoustic signal. The method includes: (a) receiving a first waveform corresponding to the first acoustic signal and a second waveform corresponding to the second acoustic signal; (b) sampling multiple peaks of the first and second waveforms; (c) setting a search range based on the multiple peaks; (d) setting the first peak within the search range as a characteristic peak; (e) recording a first time between the zero-crossing point and the zero point after the characteristic peak, and a second time between the zero point and the zero-crossing point, and calculating the average time of the first and second times; and (f) calculating the total flight time based on the average time.

[0009] In one embodiment, before step (b), the method further includes: (f) setting a lower standard deviation threshold and an upper standard deviation threshold. Step (d) further includes: (d1) when the first peak value is greater than the upper standard deviation threshold, it is set as a characteristic peak value; (d2) when the first peak value is less than the lower standard deviation threshold, it is excluded as a characteristic peak value; and (d3) when it is determined that a subsequent peak value is greater than the upper standard deviation threshold, it is set as a characteristic peak value.

[0010] In one embodiment, before step (b), the method further includes: (f) setting a lower standard deviation threshold and an upper standard deviation threshold. Step (d) further includes: (d1) when the first peak value is greater than the upper standard deviation threshold, it is set as a characteristic peak value; (d2) when the first peak value is less than the lower standard deviation threshold, it is excluded as a characteristic peak value; and (d3) when it is determined that the subsequent peak value is greater than or equal to the lower standard deviation threshold and less than or equal to the upper standard deviation threshold, it is determined whether the subsequent peak value is greater than the previous peak value, and if so, it is set as a characteristic peak value.

[0011] In one embodiment, step (d3) includes: (d4) when the subsequent peak value is greater than the previous peak value, and the subsequent peak value is greater than the previous peak value by more than a change amount, then it is set as a characteristic peak value.

[0012] In one embodiment, the change is the ratio of the maximum slope change between any two adjacent peaks.

[0013] In one embodiment, the characteristic time reference wave determination method further includes: (e1) obtaining a first peak value and a second peak value of a first acoustic signal; (e2) obtaining a first peak value and a second peak value of a second acoustic signal; (e3) comparing the first peak value of the first acoustic signal and the second acoustic signal with the second peak value of the first acoustic signal and the second acoustic signal to determine whether the first acoustic signal and the second acoustic signal are aligned; and (e4) if the first acoustic signal and the second acoustic signal are not aligned, replacing the corresponding first peak value and the second peak value to align the first acoustic signal and the second acoustic signal.

[0014] In one embodiment, step (e4) includes: when the first sound wave signal leads the second sound wave signal, the second peak value of the first sound wave signal is replaced with the first peak value of the first sound wave signal; when the second sound wave signal leads the first sound wave signal, the second peak value of the second sound wave signal is replaced with the first peak value of the second sound wave signal.

[0015] In one embodiment, the upper limit threshold of the standard deviation is a signal-to-noise ratio of 10, and the lower limit threshold of the standard deviation is a signal-to-noise ratio of 5.

[0016] In one embodiment, peak values ​​outside the range of the first acoustic signal frequency and the second acoustic signal frequency are removed.

[0017] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments, but this is not intended to limit the present invention. Attached Figure Description

[0018] Figure 1 This is a schematic diagram illustrating the operation of the ultrasonic flow meter of the present invention.

[0019] Figure 2 : This is a schematic diagram of the acoustic signal waveform of the ultrasonic flowmeter of the present invention.

[0020] Figure 3 This is a schematic diagram of flow calculation for existing ultrasonic flow meters.

[0021] Figure 4 : This is a waveform diagram of the acoustic signal received by the acoustic transceiver unit of the present invention.

[0022] Figure 5 This is a waveform diagram illustrating the optimization of the characteristic time reference wave judgment of acoustic signals according to the present invention.

[0023] Figure 6 This is a waveform diagram illustrating another optimization of the characteristic time reference wave determination of acoustic signals according to the present invention.

[0024] Figure 7This is a flowchart of the method for determining the characteristic time reference wave of the acoustic signal of the ultrasonic flowmeter of the present invention.

[0025] In the attached figures, the following labels are used:

[0026] 100: Ultrasonic Flow Meter

[0027] 11: First acoustic transceiver unit

[0028] 12: Second acoustic transceiver unit

[0029] S1: First acoustic signal

[0030] S2: Second acoustic signal

[0031] S11~S16: Steps

[0032] Tup1, Tup2, Tdn1, Tdn2: Peak values Detailed Implementation

[0033] The structural and working principles of the present invention will be described in detail below with reference to the accompanying drawings:

[0034] like Figure 1 The diagram shown is a schematic representation of the operation of the ultrasonic flow meter of the present invention, illustrated in longitudinal section. The ultrasonic flow meter 100 of the present invention includes a first acoustic transceiver unit 11 and a second acoustic transceiver unit 12. In this embodiment, the first acoustic transceiver unit 11 and the second acoustic transceiver unit 12 are arranged in pairs at opposite positions on the outer surface of the flow pipe. However, the pairing of the two transceiver units does not necessarily imply... Figure 1 As shown, in other embodiments, the first acoustic transceiver unit 11 and the second acoustic transceiver unit 12 may be positioned on the same line on the outer surface of the flow pipe. The first acoustic transceiver unit 11 is used to transmit a first acoustic signal S1 and receive a second acoustic signal S2; the second acoustic transceiver unit 12 is used to transmit the second acoustic signal S2 and receive the first acoustic signal S1. The first acoustic signal S1 and the second acoustic signal S2 are ultrasonic signals.

[0035] The first acoustic transceiver unit 11 transmits ultrasonic signals obliquely relative to the flow direction of the fluid in the flow pipe, and the signals are received by the second acoustic transceiver unit 12. At this time, the transmission and reception of the first acoustic transceiver unit 11 and the second acoustic transceiver unit 12 are switched. The second acoustic transceiver unit 12 transmits ultrasonic signals obliquely relative to the opposite flow direction of the fluid in the flow pipe, and the signals are received by the first acoustic transceiver unit 11. In this way, the flow rate can be measured from the time difference of ultrasonic signal transmission in the fluid, which will be described in detail later.

[0036] Please see Figure 2The diagram shows the acoustic signal waveform of the ultrasonic flowmeter of the present invention. The acoustic signal waveform includes a first acoustic signal S1 transmitted from the first acoustic transceiver unit 11 to the second acoustic transceiver unit 12 and a second acoustic signal S2 transmitted from the second acoustic transceiver unit 12 to the first acoustic transceiver unit 11. Although there is a time difference between the two acoustic signals, the two acoustic signals can be compared in parallel by setting the time offset to accurately determine the characteristic time reference wave position of the acoustic signal.

[0037] Please see Figure 3 The diagram shown illustrates the flow calculation process of a conventional ultrasonic flow meter.

[0038] …Formula (1)

[0039] …Formula (2)

[0040] …Formula (3)

[0041] …Formula (4)

[0042] in, For sound wave path, For the inner radius of the pipeline, For the incident angle of the ultrasonic wave, The speed of sound in a medium, The average velocity (flow rate) of the fluid. For the time required upstream, For the time required downstream, The time difference between the two acoustic transceiver units is given. Therefore, the flow rate in the flow pipeline can be calculated (estimated) according to equations (1) to (4).

[0043] The main technical feature of this invention is to perform pre-processing and judgment on the acoustic signals received by the acoustic transceiver unit (e.g., the first acoustic signal S1 received by the second acoustic transceiver unit 12 and the second acoustic signal S2 received by the first acoustic transceiver unit 11).

[0044] Please see Figure 4 The diagram shows the waveform of the acoustic signal received by the acoustic transceiver unit. To accurately determine (detect) the characteristic time reference wave of the ultrasonic flowmeter's acoustic signal (i.e., the first starting wave of the acoustic wave used for flow calculation), the technical means proposed in this invention are explained below.

[0045] Ideally, an ultrasonic signal has a complete envelope shape, but due to factors such as the measurement location or conditions, ultrasonic signals cannot necessarily present such a complete envelope shape. Therefore, the characteristic time reference wave determination method for acoustic signals proposed in this invention mainly includes the following key steps:

[0046] Step 1: Set the timing for sound wave reception. For example, due to the time delay between the first sound wave signal S1 emitted by the first sound wave transceiver unit 11 and the second sound wave transceiver unit 12, the timing for sound wave reception can be set by considering factors such as the speed of sound, the type of medium, etc., to turn on (enable) the receiver of the second sound wave transceiver unit 12 and begin receiving the required sound wave signal. Figure 4 The time t1 is shown.

[0047] Step 2: Calculate the standard deviation of the noise. Since the frequency of the transmitted sound wave is known (e.g., 1MHz), it can be determined whether the received sound wave signal is noise based on its frequency, and the standard deviation of the noise (magnitude) can be further calculated. Therefore, the threshold size for subsequent judgment can be dynamically designed based on a reasonable standard deviation of the noise magnitude.

[0048] Step 3: Establish the peak value of the sound wave. By sampling the peak value (including the trough value) of the received sound wave, the peak value of the sound wave is obtained (established), which is the waveform characteristic of the sound wave. In practice, since the frequency of the transmitted sound wave is known, unreasonable noise or glitching that may not belong to the correct waveform can be filtered out through secondary filtering, and regarded as the desired sound wave signal, that is, only the peak value at the desired frequency is left.

[0049] Step 4: Establish a search window. Once the acoustic signal, noise, and sampled peak values ​​are determined, a search window can be opened before approaching the estimated characteristic time reference wave position. By establishing a search window, only the data within the search window is calculated, saving computation time and reducing the amount of data to be processed. For example, typically, over 4000 data points are acquired (sampled). Calculating all 4000+ data points would create a computational burden. The scope of the search window will be designed to cover the characteristic time reference wave data; for example, within... Figure 4 The search window opens at time t2 and closes at time t3.

[0050] Step 5: Obtain the first peak within the search window. Based on the search window and the multiple sampled peaks, the first peak within the search window can be obtained. For example, in... Figure 4The peak value is shown at time t2. Furthermore, the search window determines whether the first peak value is a characteristic time reference wave of the sound wave; if not, it proceeds to the next peak value. The method for determining whether a wave is a characteristic time reference wave is as follows:

[0051] (1) Use a signal-to-noise ratio (SNR) of less than 5 as a criterion: If the peak value is less than SNR=5, then the peak value is directly judged to be a non-characteristic time reference wave.

[0052] (2) Using a signal-to-noise ratio (SNR) greater than 10 as a criterion: If the peak value is greater than SNR=10, then the peak value is directly determined to be the characteristic time reference wave, such as... Figure 4 The peak value shown at time t4 is greater than SNR=10, so it is determined that the peak value is the characteristic time reference wave.

[0053] (3) If the signal-to-noise ratio (SNR) is between 5 and 10, further judgment is made to determine whether the peak value is a characteristic time reference wave, as explained below.

[0054] The signal-to-noise ratio (SNR) value is not limited to the aforementioned values ​​(SNR=5, SNR=10). The purpose is to illustrate that the relationship between these two values ​​can be used to directly determine whether the peak value is a characteristic time reference wave, or to make a more detailed judgment when it is between the two values.

[0055] Step 6: Calculate the zero-crossing time: When the signal-to-noise ratio of the peak value is greater than 10, it is determined to be a characteristic time reference wave (e.g., Figure 4 If the time is t4), then the arithmetic mean of the time difference between the time before and the time at the zero point and the time difference between the zero point and the time after the zero point is calculated can overcome the error caused by the offset of the sound wave waveform.

[0056] Step 7: Calculate the total time of flight (TOF). The total flight time is calculated based on the average time, thus enabling the estimation of traffic flow.

[0057] Among the methods for determining the characteristic time reference wave, (3) is particularly noteworthy if the signal-to-noise ratio (SNR) is between 5 and 10. When the sampled peak value is near the SNR=5 or SNR=10, it is easy to misjudge whether it is a characteristic time reference wave. The following explanation is provided with reference to the attached figures.

[0058] Please see Figure 5As shown, this is a waveform diagram illustrating the optimization of the characteristic time reference wave judgment of the acoustic signal according to the present invention. The following explanation assumes that steps 3 (establishing the peak value of the acoustic wave) and 4 (establishing the search window) have already been completed; that is, the peak value described below refers to the peak value obtained in the search window. Figure 5 As shown, at time t1, a possible characteristic time reference wave is obtained (because this peak value is between the signal-to-noise ratio of 5 and 10). Therefore, it is then determined whether the sampled peak value is greater than the peak value obtained at time t1. If so, it is determined that the peak value obtained at time t1 is not the characteristic time reference wave. Furthermore, it is re-evaluated whether the new peak value is the characteristic time reference wave until the correct characteristic time reference wave is found.

[0059] like Figure 5 As shown, after (assuming) 90 sampling points, another peak is found at time t2. At this point, it is determined whether the second peak (Tup2) is greater than the first peak (Tup1). Furthermore, if the second peak (Tup2) is greater than the first peak (Tup1), it is determined whether the second peak (Tup2) is sufficiently greater than the first peak (Tup1). This determination can be based on the maximum slope of two adjacent peaks. For example, if the maximum slope of the two peaks is Smax, and the difference between the second peak (Tup2) and the first peak (Tup1) is greater than 20% of the maximum slope (i.e., 0.2 * Smax), but this is not a limitation, then it can be determined that the second peak (Tup2) is greater than the first peak (Tup1). Therefore, this second peak (Tup2) is identified (set) as a new (possible) characteristic time reference wave. Furthermore, by performing the same determination on subsequent sampling peaks, the true characteristic time reference wave when the peak value is between a signal-to-noise ratio (SNR) of 5 and 10 can be identified.

[0060] Please see Figure 6 The diagram shows a waveform illustration illustrating another optimization of the characteristic time reference wave judgment of the acoustic signal according to the present invention. As mentioned above, since the ultrasonic flow meter includes a first acoustic transceiver unit 11 that transmits a first acoustic signal S1 and receives a second acoustic signal S2, and a second acoustic transceiver unit 12 that transmits a second acoustic signal S2 and receives a first acoustic signal S1, the acoustic signals will be two similar sets. Therefore, the aforementioned judgment steps are the first peak value (Tup1) and the second peak value (Tup2) for the first acoustic transceiver unit 11, and the first peak value (Tdn1) and the second peak value (Tdn2) for the second acoustic transceiver unit 12. The same judgment steps are executed twice, that is, once for the first acoustic signal S1 and once for the second acoustic signal S2.

[0061] More optimally, the sound waves transmitted by the upstream transceiver unit and the sound waves transmitted by the downstream transceiver unit are compared. First, according to the aforementioned steps, the first peak value (Tup1) and the second peak value (Tup2) of the first sound wave transceiver unit 11 and the first peak value (Tdn1) and the second peak value (Tdn2) of the second sound wave transceiver unit 12 are obtained, and they are compared pairwise to see if they are "similar in size". That is, Tup1 is compared with Tdn1 (by (Tdn1-Tup1) / Tup1), Tup1 is compared with Tdn2 (by (Tdn2-Tup1) / Tup1), Tup2 is compared with Tdn1 (by (Tdn1-Tup2) / Tup2), and Tup2 is compared with Tdn2 (by (Tdn2-Tup2) / Tup2), respectively, to obtain the first comparison value Cmp1, the second comparison value Cmp2, the third comparison value Cmp3, and the fourth comparison value Cmp4.

[0062] When Tup1 is close to Tdn1, and Tup2 is also close to Tdn2, then the first sound wave signal S1 and the second sound wave signal S2 are considered to be aligned. Conversely, when Tup2 is relatively close to Tdn1, it indicates that the first sound wave signal S1 and the second sound wave signal S2 are not aligned, and the first sound wave signal S1 precedes the second sound wave signal S2. Similarly, when Tup1 is relatively close to Tdn2, it indicates that the first sound wave signal S1 and the second sound wave signal S2 are not aligned, and the second sound wave signal S2 precedes the first sound wave signal S1.

[0063] Once a misalignment between two sound waves is identified, the order of their peak values ​​is adjusted. That is, when the first sound wave signal S1 precedes the second sound wave signal S2, Tup2 is replaced by Tup1; conversely, when the second sound wave signal S2 precedes the first sound wave signal S1, Tdn2 is replaced by Tdn1. This ensures that the calculations of Tup1 and Tdn1, and Tup2 and Tdn2, are consistent (matched), meaning that Tup1 is calculated corresponding to Tdn1, and Tup2 is calculated corresponding to Tdn2. By adjusting the alignment signals (i.e., replacing the characteristic time reference wave with the correct position), the situation where the signal disappears (enters noise) during transmission, causing a shift in the characteristic time reference wave, can be eliminated (canceled).

[0064] To further optimize the process and avoid misjudging the characteristic time reference wave due to minor waveform variations, peak substitution can be used to ensure that the established characteristic time reference wave is not altered. For example, if Tup1 or Tdn1, which has been identified as the characteristic time reference wave, does not change significantly, then Tup1 and Tdn1 are maintained as the characteristic time reference waves, thereby reducing frequent changes in the characteristic time reference wave. Unless, of course, Tup1 or Tdn1 changes significantly, a new characteristic time reference wave is used to replace the original one.

[0065] Therefore, through the aforementioned basic steps (steps 1 to 7), the position of the characteristic time reference wave can be roughly determined, and the total flight time can be calculated, thereby achieving the calculation (estimation) of the flow rate. Furthermore, through optimized judgment and adjustment, the position of the identified characteristic time reference wave can be made more accurate, thereby improving the measurement results of the ultrasonic flow meter and realizing the technical efficacy of the present invention.

[0066] Please see Figure 7 The diagram shows a flowchart of the characteristic time reference wave determination method for the acoustic signal of the ultrasonic flowmeter of the present invention. See also: Figure 1 As shown, the ultrasonic flow meter includes a first acoustic transceiver unit 11 that transmits a first acoustic signal S1 and receives a second acoustic signal S2, and a second acoustic transceiver unit 12 that transmits the second acoustic signal S2 and receives the first acoustic signal S1. The method includes: first, receiving a first waveform corresponding to the first acoustic signal and a second waveform corresponding to the second acoustic signal (S11). Then, sampling multiple peaks of the first and second waveforms (S12). Then, setting a search window based on the multiple peaks (S13). Then, setting the first peak within the search window as the characteristic peak (S14). Then, recording a first time between the zero-crossing point and the zero point after the characteristic peak, and a second time between the zero point and the zero-crossing point, and calculating the average time of the first and second times (S15). Finally, calculating the total time of flight (TOF) based on the average time (S16).

[0067] Before step (S12), the method further includes setting a lower standard deviation threshold and an upper standard deviation threshold, wherein the lower standard deviation threshold and the upper standard deviation threshold are signal-to-noise ratios (SNR).

[0068] Step (S14) further includes: when the first peak value is greater than the upper limit threshold of the standard deviation (e.g., SNR=10), it is set as the characteristic peak value. When the first peak value is less than the lower limit threshold of the standard deviation (e.g., SNR=5), it is excluded as the characteristic peak value. Alternatively, when it is determined that a subsequent peak value is greater than the upper limit threshold of the standard deviation, it is set as the characteristic peak value.

[0069] Step (S14) further includes: when the first peak value is greater than the upper limit threshold of the standard deviation, it is set as the characteristic peak value. When the first peak value is less than the lower limit threshold of the standard deviation, it is excluded as the characteristic peak value. Alternatively, when it is determined that a subsequent peak value is greater than or equal to the lower limit threshold of the standard deviation and less than or equal to the upper limit threshold of the standard deviation, it is determined whether the subsequent peak value is greater than the previous peak value; if so, it is set as the characteristic peak value. Specifically, when the subsequent peak value is greater than the previous peak value, and the subsequent peak value is greater than the previous peak value by a change amount, it is set as the characteristic peak value. In one embodiment, the change amount is the ratio of the maximum slope change amount between any two adjacent peak values.

[0070] The characteristic time reference wave determination method further includes: obtaining the first peak and second peak of a first sound wave signal; obtaining the first peak and second peak of a second sound wave signal; comparing the plurality of first peaks and the plurality of second peaks to determine whether the first sound wave signal and the second sound wave signal are aligned; and, if the first sound wave signal and the second sound wave signal are not aligned, replacing the corresponding first peak and second peak to align the first sound wave signal and the second sound wave signal. Specifically, when the first sound wave signal leads the second sound wave signal, the second peak of the first sound wave signal is replaced with the first peak of the first sound wave signal. When the second sound wave signal leads the first sound wave signal, the second peak of the second sound wave signal is replaced with the first peak of the second sound wave signal.

[0071] Therefore, through the characteristic time reference wave determination method of the ultrasonic flowmeter provided by this invention, the position of the characteristic time reference wave can be roughly determined through basic steps (steps 1 to 7), and the total flight time can be calculated, thereby achieving flow rate calculation (estimation). Furthermore, through optimized judgment and adjustment, the identified characteristic time reference wave position can be made more accurate, thereby improving the measurement results of the ultrasonic flowmeter and realizing the technical efficacy of this invention.

[0072] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the appended claims.

Claims

1. A method of determining a characteristic time reference wave of an acoustic signal of an ultrasonic flow meter, characterized in that The ultrasonic flow meter comprises a first ultrasonic transceiver unit for transmitting a first ultrasonic signal and receiving a second ultrasonic signal, and a second ultrasonic transceiver unit for transmitting the second ultrasonic signal and receiving the first ultrasonic signal, the method comprises: (a) receiving a first waveform corresponding to the first ultrasonic signal and a second waveform corresponding to the second ultrasonic signal; (b) setting a lower threshold of standard deviation and an upper threshold of standard deviation, and sampling a plurality of peak values of the first waveform and the second waveform; (c) setting a search range according to the plurality of peak values; (d) setting a first peak value in the search range as a characteristic peak value; (e) recording a first time between a zero-crossing point before the characteristic peak value and a zero point, a second time between the zero point and a zero-crossing point after the characteristic peak value, and calculating an average time of the first time and the second time; and (f) calculating a total flight time according to the average time. The method further comprises: (e1) obtaining a first peak value and a second peak value of the first ultrasonic signal; (e2) obtaining a first peak value and a second peak value of the second ultrasonic signal; (e3) comparing the first peak values of the first ultrasonic signal and the second ultrasonic signal with the second peak values of the first ultrasonic signal and the second ultrasonic signal to determine whether the first ultrasonic signal and the second ultrasonic signal are aligned; and (e4) when the first ultrasonic signal is ahead of the second ultrasonic signal, replacing the second peak value of the first ultrasonic signal with the first peak value of the first ultrasonic signal; when the second ultrasonic signal is ahead of the first ultrasonic signal, replacing the second peak value of the second ultrasonic signal with the first peak value of the second ultrasonic signal; so that the first ultrasonic signal and the second ultrasonic signal are aligned.

2. The method of claim 1, wherein the characteristic time reference wave is determined by the steps of: (a) determining a first time reference wave; (b) determining a second time reference wave; and (c) determining the characteristic time reference wave as the average of the first and second time reference waves. In step (d), further comprising: (d1) when the first peak value is greater than the upper threshold of standard deviation, setting it as the characteristic peak value; (d2) when the first peak value is less than the lower threshold of standard deviation, excluding it as the characteristic peak value; and (d3) when it is determined that the subsequent peak value is greater than the upper threshold of standard deviation, setting it as the characteristic peak value. In step (d), further comprising:

3. The method of claim 1, wherein the characteristic time reference wave is a wave having a time interval between a wave peak and a wave trough of 0.5 to 2.0 μsec. (d1) when the first peak value is greater than the upper threshold of standard deviation, setting it as the characteristic peak value; (d2) when the first peak value is less than the lower threshold of standard deviation, excluding it as the characteristic peak value; and (d3) among the plurality of peak values in the search range, when the first peak value and the subsequent second peak value are greater than or equal to the lower threshold of standard deviation and less than or equal to the upper threshold of standard deviation, determining whether the subsequent peak value is greater than the previous peak value, and if so, setting the subsequent peak value as the characteristic peak value. In step (d3), comprising: (d4) when the subsequent peak value is greater than the previous peak value, and the subsequent peak value is greater than the previous peak value by a change amount, setting the subsequent peak value as the characteristic peak value.

4. The method of claim 3, wherein the characteristic time reference wave is a wave having a time interval between the wave and the wave immediately preceding the wave that is equal to a time interval between the wave and the wave immediately following the wave. 5 The change amount is a proportion of a maximum slope change amount of any two adjacent peak values. The upper threshold of standard deviation is equal to 10 of signal signal-to-noise ratio, and the lower threshold of standard deviation is equal to 5 of signal signal-to-noise ratio.

5. The method of claim 4, wherein the characteristic time reference wave is determined by the steps of: determining a first time reference wave; determining a second time reference wave; and determining the characteristic time reference wave as the average of the first and second time reference waves. ​ 6. The method of claim 2 or 3, wherein the characteristic time reference wave is a wave having a time interval of 0.5 to 2.0 times a time interval of a wave having a maximum amplitude among the waves of the reflected wave signal. ​ 7. The method of claim 1, wherein the characteristic time reference wave is a wave having a time period of 1 / (f0 + Δf), where f0 is a center frequency of the ultrasonic wave signal and Δf is a frequency deviation of the ultrasonic wave signal. In step (b), peaks outside the vicinity of the first acoustic signal frequency and the second acoustic signal frequency are removed. In step (b), peaks outside the vicinity of the first acoustic signal frequency and the second acoustic signal frequency are removed.

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