Ultrasonic wave transit time measuring method and system
Through instantaneous frequency calculation method and frequency transition detection technology, the problem of signal amplitude distortion in ultrasonic crossing time measurement is solved, and high-precision crossing time measurement is achieved.
Patent Information
- Application Number
- CN202510055896.1
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-01-14
- Publication Date
- 2025-05-13
AI Technical Summary
Among the existing ultrasonic transit time measurement methods, the envelope method and the pulse ratio method have low measurement accuracy in signal amplitude distortion scenarios and are complex in implementation.
The instantaneous frequency of the ultrasonic signal is detected by the instantaneous frequency calculation method, and the frequency transition detection threshold and the post-frequency transition detection threshold are used to judge the transition time and realize timing.
It effectively resists signal amplitude distortion, improves the accuracy of crossing time measurement, and has higher detection accuracy than envelope method and pulse ratio method.
Smart Images
Figure CN119986613A_ABST
Abstract
Description
Technical Field
[0001] The present application relates to the field of ultrasonic detection technology, and in particular to an ultrasonic transit time measurement method and system. Background Art
[0002] Ultrasonic transit time measurement technology is a key technology for various ultrasonic sensors. Ultrasonic waves are mechanical waves with a frequency higher than 20kHz. When an ultrasonic transducer emits an ultrasonic pulse, the pulse will propagate in a medium (such as gas, liquid, solid), and will be reflected after hitting the target object or the interface of the medium. Part of the reflected wave is captured by the receiving transducer. The transit time refers to the time it takes for an ultrasonic wave to be transmitted and received, which is closely related to the ultrasonic propagation distance and the speed of sound in the medium.
[0003] Ultrasonic waves are generated by exciting the transducer with a set of high-voltage pulse trains. The timing starts when the first high-voltage pulse excites the transducer, and stops when the receiving transducer outputs an electrical pulse. In this way, the ultrasonic transit time measurement can be completed. In fact, it is difficult to determine when to stop timing, and it is easy to introduce errors. There are usually two ways to stop timing, one is the envelope method, and the other is the pulse ratio method.
[0004] The envelope method is to detect the above signal waveform and then low-pass filter it. To end the timing of the envelope method, you first set a suitable threshold, which can be set to 40 (or other values). When the rising edge of the envelope reaches the threshold, the timing ends, thus completing the transit time measurement. As for the start time of the timing, it can be the first excitation electrical pulse emitted. The transit time obtained in this way has a certain deviation, which can be removed by the correction algorithm.
[0005] The pulse ratio method is to detect the amplitude value of each received pulse in real time, and calculate the ratio of the current pulse amplitude value to the previous pulse amplitude value. When the ratio is very close to the preset value, the timing is stopped. At the rising edge of the received pulse train, the ratio is monotonically decreasing with respect to the pulse sequence number. If the preset value is 1.5, the timing can be stopped when the fourth pulse is received.
[0006] The envelope method and the pulse ratio method each have their own advantages and disadvantages. The envelope method is easily affected by signal fading. In the scenario where the signal channel fading changes randomly, the transit time obtained by the envelope method will have a large error. The pulse ratio method uses the ratio of the pulses before and after the signal. As long as the channel fading does not change significantly in a short period of time, this method will not have a big problem. The pulse ratio method has higher measurement accuracy of transit time than the envelope method. The ratio used in the pulse ratio method comes from the pulse response of the transducer, and the pulse response of the transducer will change with temperature. In addition, the pulse response of the transducer is different due to individual differences. This determines that the preset of the pulse ratio method cannot be a fixed value. Different transducers and different temperatures require different values. In other words, the pulse ratio method is more complicated to implement. The pulse ratio method compares the received pulses one by one. The corresponding time points of these pulses are discrete. In other words, if the judgment is correct, there is basically no error in the transit time measurement. If the judgment is wrong, an error of a pulse cycle may be generated. In equipment related to wind speed or liquid flow rate, this is often called the slip problem.
[0007] Therefore, it is urgent to design a solution to solve the above problems of the envelope method and the pulse ratio method in transit time measurement.
[0008] Application Contents The present application proposes an ultrasonic transit time measurement method and system, which ignores the signal amplitude and seeks a timing basis from the instantaneous frequency to complete the timing, can effectively resist the distortion of the signal amplitude, and has high detection accuracy.
[0009] The embodiment of the present application adopts the following technical solution: The present application provides an ultrasonic transit time measurement method, comprising the following steps: At time t1, the first transducer sends an ultrasonic signal to the second transducer, and the instantaneous frequency signal received by the second transducer is detected by an instantaneous frequency calculation method; wherein the instantaneous frequency signal includes an stimulated oscillation frequency signal determined by the first transducer and received by the second transducer, and a free oscillation frequency signal determined by the second transducer itself, and the frequency of the stimulated oscillation frequency signal is greater than the free oscillation frequency signal; Amplifying and filtering the instantaneous frequency signal to obtain an instantaneous frequency curve signal corresponding to the instantaneous frequency signal; Setting a frequency transition detection threshold from the stimulated oscillation frequency signal to the free oscillation frequency signal, and setting a post-frequency transition detection threshold for verifying the frequency transition detection threshold; wherein the occurrence time of the frequency transition detection threshold is defined as t2, the occurrence time of the post-frequency transition detection threshold is defined as tn, and n is a positive integer starting from 3; The interval time from the frequency transition detection threshold to the post-frequency transition detection threshold =tn-t2 is judged, when the interval time meets the judgment condition, the frequency transition detection threshold is qualified; when the interval time does not meet the judgment condition, continue to detect the post-frequency transition detection threshold that meets the judgment condition, and continue to cycle; If in The post-frequency transition detection threshold at time t meets the decision condition, then After the moment, a certain time is delayed until the frequency of the signal received by the second transducer reaches the neighborhood of the frequency value corresponding to the free oscillation frequency signal, and the definition The time period from the moment until the frequency value corresponding to the free oscillation frequency signal reaches is , then the transition time from the first transducer to the second transducer is T = + -t1.
[0010] Furthermore, the instantaneous frequency calculation method comprises the following specific steps: Receiving a signal input from the first transducer, the input signal first passes through an analog-to-digital converter ADC to convert the analog signal into a digital signal; The digital signal converted by the analog-to-digital converter ADC is subjected to narrow-band filtering, and then subjected to delay processing and Hilbert filtering respectively; After delay processing and Hilbert filtering, the signal is input to the rotation factor multiplier for algorithm processing, and then processed by difference and filtering, and finally the clock frequency is sampled to obtain the instantaneous frequency signal.
[0011] Furthermore, the decision condition is positively correlated with a response time constant of the filter, and the response time constant is the inverse of the 3dB bandwidth of the filter.
[0012] Furthermore, a frequency difference between the stimulated oscillation frequency signal and a frequency transition detection threshold is less than 10 KHz.
[0013] Furthermore, a frequency difference between the frequency transition detection threshold and the post-frequency transition detection threshold is less than 5 KHz.
[0014] Furthermore, a frequency difference between the post-frequency transition detection threshold and the free oscillation frequency signal is less than 5 KHz.
[0015] Furthermore, when the judgment condition is met, a certain time is delayed until the frequency of the signal received by the second transducer reaches the neighborhood of the frequency value corresponding to the free oscillation frequency signal, and the neighborhood range of the neighborhood is (- ,+ ), Less than 0.5KHz.
[0016] Furthermore, the corresponding frequencies of the stimulated oscillation frequency signal, the frequency transition detection threshold, the post-frequency transition detection threshold, the free oscillation frequency signal and the neighborhood of their frequency values are obtained using a phase-locked loop method, a discrete Fourier transform method or a wavelet transform method.
[0017] Furthermore, the occurrence time of the stimulated oscillation frequency signal, the frequency transition detection threshold, the post-frequency transition detection threshold, the free oscillation frequency signal and the neighborhood of their frequency values adopts a correlation detection method, specifically:
[0018] in, The measured signal is, As the reference signal, by finding The maximum value corresponds to Determine the time when the signal occurs.
[0019] The present application also provides a system for implementing the above-mentioned ultrasonic transit time measurement method, comprising: A signal generation and detection module, configured to cause the first transducer to send an ultrasonic signal to the second transducer at time t1, and detect an instantaneous frequency signal received by the second transducer by using an instantaneous frequency calculation method; wherein the instantaneous frequency signal includes an stimulated oscillation frequency signal determined by the first transducer and received by the second transducer, and a free oscillation frequency signal determined by the second transducer itself, and the frequency of the stimulated oscillation frequency signal is greater than the free oscillation frequency signal; A signal processing module amplifies and filters the instantaneous frequency signal to obtain an instantaneous frequency curve signal corresponding to the instantaneous frequency signal; A threshold setting module, which sets a frequency transition detection threshold from the stimulated oscillation frequency signal to the free oscillation frequency signal, and sets a post-frequency transition detection threshold for verifying the frequency transition detection threshold; wherein the occurrence time of the frequency transition detection threshold is defined as t2, and the occurrence time of the post-frequency transition detection threshold is defined as tn, where n is a positive integer starting from 3; The threshold decision module determines the interval time from the frequency transition detection threshold to the post-frequency transition detection threshold. =tn-t2 is judged, when the interval time meets the judgment condition, the frequency transition detection threshold is qualified; when the interval time does not meet the judgment condition, continue to detect the post-frequency transition detection threshold that meets the judgment condition, and continue to cycle; The crossing time calculation module, if The post-frequency transition detection threshold at time t meets the decision condition, then After the moment, a certain time is delayed until the frequency of the signal received by the second transducer reaches the neighborhood of the frequency value corresponding to the free oscillation frequency signal, and the definition The time period from the moment until the frequency value corresponding to the free oscillation frequency signal reaches is , then the transition time from the first transducer to the second transducer is T = + -t1.
[0020] At least one of the above technical solutions adopted in the embodiments of the present application can achieve the following beneficial effects: Compared with the envelope method and the pulse ratio method, the frequency transition method of the present application has little effect on the instantaneous frequency due to a slight distortion of the amplitude, can effectively resist the distortion of the signal amplitude, and has higher detection accuracy than the envelope method and the pulse ratio method. BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The drawings described herein are used to provide a further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute an improper limitation on the present application. In the drawings: Figure 1 A flow chart of an ultrasonic transit time measurement method of the present application; Figure 2 A flow chart of an instantaneous frequency calculation method of an ultrasonic transit time measurement method of the present application; Figure 3 This is a principle block diagram of an ultrasonic transit time measurement system of the present application; Figure 4 This is a waveform diagram showing the amplitude relationship of the signal pulse train received by the transducer of the present application; Figure 5 Waveform diagram of the types of signals received by the transducer of this application; Figure 6 This is a coordinate diagram of the experimental effect of an ultrasonic transit time measurement method using a frequency transition timing method in the present application. DETAILED DESCRIPTION
[0022] In order to make the purpose, technical solution and advantages of the present application clearer, the technical solution of the present application will be clearly and completely described below in combination with the specific embodiments of the present application and the corresponding drawings. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present application.
[0023] The technical solutions provided by various embodiments of the present application are described in detail below in conjunction with the accompanying drawings.
[0024] Example 1 Common ultrasonic sensors include ultrasonic wind sensors, ultrasonic liquid flow rate sensors, ultrasonic liquid level sensors, etc. The through-beam wind speed and direction sensor has four ultrasonic transducers. After the equipment is installed, the four transducers are respectively in the four directions of east, west, south and north. During measurement, the transit time from north to south is measured respectively, recorded as , and then measure the transit time from south to north, recorded as The transmission distance is determined by the structure and is a known number, represented by D here. The speed of sound is an unknown parameter represented by c. The wind speed from north to south is what we want, set , then according to the above test, the following two equations can be listed: Equation 1 Equation 2 Combining the above two equations, it is easy to calculate the north-to-south component wind speed: Equation 3 The component wind speed from east to west can be obtained in the same way. The next step is to use the component wind speed to synthesize the wind speed and direction.
[0025] The high-voltage pulse train used for transmission is generally a 50% duty cycle equal-amplitude square wave, and the number of pulses is generally within 10. This is related to the transducer frequency, propagation medium, detection method, etc. In any case, the transmission time of each pulse in the transmitted pulse train can be said to be known, and it is very clear, while the received electrical pulses are no longer of equal amplitude, and the number is far greater than the number of transmitted excitation pulses. Figure 4 Figure 1 is a schematic diagram of the received signal waveform. Figure 4 It can be seen that the received pulse amplitude first increases from small to large, and then begins to decrease from large to small. It is difficult to say here that the first pulse received is the first pulse of excitation, because the ultrasonic signal generated after the first pulse excitation may be very small, and after a certain propagation distance, the received amplitude may be too small to be detected normally.
[0026] like Figure 1 As shown, in order to solve the above problems, the present application provides an ultrasonic transit time measurement method, comprising the following steps: S1, at time t1, the first transducer sends an ultrasonic signal to the second transducer, and the instantaneous frequency signal received by the second transducer is detected by the instantaneous frequency calculation method; wherein the instantaneous frequency signal includes the stimulated oscillation frequency signal determined by the first transducer and received by the second transducer, and the free oscillation frequency signal determined by the second transducer itself, and the frequency of the stimulated oscillation frequency signal is greater than the free oscillation frequency signal, such as Figure 5 As shown; S2, amplifying and filtering the instantaneous frequency signal to obtain an instantaneous frequency curve signal corresponding to the instantaneous frequency signal; S3, setting a frequency transition detection threshold from the stimulated oscillation frequency signal to the free oscillation frequency signal, and setting a post-frequency transition detection threshold for verifying the frequency transition detection threshold; wherein, the occurrence time of the frequency transition detection threshold is defined as t2, the occurrence time of the post-frequency transition detection threshold is defined as tn, and n is a positive integer starting from 3; S4, the interval time from the frequency transition detection threshold to the post-frequency transition detection threshold =tn-t2 is used for judgment. When the interval time meets the judgment condition, the frequency transition detection threshold is qualified; when the interval time does not meet the judgment condition, the post-frequency transition detection threshold that meets the judgment condition is continuously detected, and the cycle continues; S5. If The post-frequency transition detection threshold at time t meets the decision condition, then Delay a certain time after the moment until reaching the neighborhood of the frequency value corresponding to the free oscillation frequency signal, define The time period from the moment until the frequency value corresponding to the free oscillation frequency signal arrives is , then the transition time from the first transducer to the second transducer is T = + -t1.
[0027] like Figure 2 As shown in the figure, the instantaneous frequency calculation method has the following specific steps: Receiving a signal input from the first transducer, the input signal first passes through an analog-to-digital converter ADC to convert the analog signal into a digital signal; The digital signal converted by the analog-to-digital converter ADC is subjected to narrow-band filtering, and then subjected to delay processing and Hilbert filtering respectively; After delay processing and Hilbert filtering, the signal is input to the rotation factor multiplier and processed by an algorithm (such as the Codic algorithm), and then processed by differentiation and filtering, and finally the clock frequency is sampled to obtain an instantaneous frequency signal.
[0028] The judgment condition is positively correlated with the response time constant of the filter. The response time constant is the inverse of the filter's 3dB bandwidth. Generally, the step response ends when the time constant is 3 times. For example, if the step response time from 240K to 235K needs to be calculated, the range is 12~18us. This period is part of the entire frequency change process, so it is related to the filter parameters.
[0029] The frequency difference between the stimulated oscillation frequency signal and the frequency transition detection threshold is less than 10KHz, the frequency difference between the frequency transition detection threshold and the post-frequency transition detection threshold is less than 5KHz, and the frequency difference between the post-frequency transition detection threshold and the free oscillation frequency signal is less than 5KHz. After a certain time delay, it reaches the frequency value neighborhood of the free oscillation frequency signal. The frequency value neighborhood of the free oscillation frequency signal is (- ,+ ), Less than 0.5KHz.
[0030] In addition, the corresponding frequencies of the stimulated oscillation frequency signal, the frequency transition detection threshold, the post-frequency transition detection threshold, the free oscillation frequency signal and the neighborhood of their frequency values are obtained using a phase-locked loop method, a discrete Fourier transform method or a wavelet transform method.
[0031] The occurrence time of the stimulated oscillation frequency signal, the frequency transition detection threshold, the post-frequency transition detection threshold, the free oscillation frequency signal and the neighborhood of its frequency value adopts a correlation detection method, specifically:
[0032] in, The measured signal is, As the reference signal, by finding The maximum value corresponds to Determine the time when the signal occurs.
[0033] like Figure 3 As shown, the present application also provides a system for implementing the above-mentioned ultrasonic transit time measurement method, comprising: The signal generation and detection module 101 is used for transmitting an ultrasonic signal from the first transducer to the second transducer at time t1, and detecting an instantaneous frequency signal received by the second transducer by using an instantaneous frequency calculation method; wherein the instantaneous frequency signal includes an stimulated oscillation frequency signal determined by the first transducer and received by the second transducer, and a free oscillation frequency signal determined by the second transducer itself, and the frequency of the stimulated oscillation frequency signal is greater than the free oscillation frequency signal; The signal processing module 102 amplifies and filters the instantaneous frequency signal to obtain an instantaneous frequency curve signal corresponding to the instantaneous frequency signal; The threshold setting module 103 sets the frequency transition detection threshold from the stimulated oscillation frequency signal to the free oscillation frequency signal, and sets the post-frequency transition detection threshold for verifying the frequency transition detection threshold; wherein the frequency transition detection threshold is defined as t2, the post-frequency transition detection threshold is defined as tn, and n is a positive integer starting from 3; The threshold decision module 104 determines the interval time from the frequency transition detection threshold to the post-frequency transition detection threshold. =tn-t2 is used for judgment. When the interval time meets the judgment condition, the frequency transition detection threshold is qualified; when the interval time does not meet the judgment condition, the post-frequency transition detection threshold that meets the judgment condition is continuously detected, and the cycle continues; The crossing time calculation module 105, if The post-frequency transition detection threshold at time t meets the decision condition, then Delay a certain time after the moment until reaching the neighborhood of the frequency value corresponding to the free oscillation frequency signal, define The time period from the moment until the frequency value corresponding to the free oscillation frequency signal arrives is , then the transition time from the first transducer to the second transducer is T = + -t1.
[0034] Example 2 The first transducer sends an ultrasonic signal to the second transducer. The ultrasonic pulse signal received by the second transducer can be divided into two parts. The first part is equivalent to the response of the transducer when the excitation pulse exists, which is called stimulated oscillation, and the latter part is the free oscillation waveform of the transducer, such as Figure 5 shown.
[0035] The frequency of the stimulated oscillation part is mainly determined by the frequency of the excitation pulse train, while the frequency of the free oscillation part is determined by the free oscillation frequency of the transducer. If the free oscillation frequency of the transducer is 230KHz, and the transducer is excited by a 250KHz pulse train, the frequency of the front part is 250KHz, and the back part is 230KHz. The instantaneous frequency of the received signal is detected in real time through the instantaneous frequency calculation method. When the frequency jumps from 250KHz to 230KHz, the timing ends and the transit time measurement is completed. This is the frequency transition timing method. Figure 2 As shown, the instantaneous frequency calculation method has the following specific steps: receiving the signal input of the first transducer, the input signal first passes through the analog-to-digital converter ADC to convert the analog signal into a digital signal; performing narrow-band filtering on the digital signal converted by the analog-to-digital converter ADC, and then performing delay processing and Hilbert filtering respectively; after the delay processing and Hilbert filtering, the signal is input to the rotation factor multiplier for algorithm processing, and then undergoes differential and filtering processing, and finally samples the clock frequency to obtain the instantaneous frequency signal. The algorithm has no feedback loop and has good robustness.
[0036] like Figure 6As shown, from left to right in the figure are data of four directions (send from north to south, receive from south to north, send from east to west, and send from west to east). The envelope with a smaller internal peak-to-peak value in the upper half of the curve of each direction is the signal received in each direction, and the envelope with a larger external peak-to-peak value is the signal after saturation amplification and filtering. The lower half is the calculated instantaneous frequency curve. The instantaneous frequency will rise to about 250KHz when the excitation pulse exists. When the excitation pulse stops and the transducer switches to free oscillation, the frequency will quickly drop to around 230KHz. A threshold is set on the falling edge. If it is 240KHz (corresponding to the frequency transition detection threshold of Example 1), when the frequency starts to drop from 250KHz, when the frequency reaches 240KHz, the timing is stopped, so that the transit time measurement can be completed. It can be seen from the figure that some interference will also reach the threshold of 240KHz, which can be avoided by adding some auxiliary conditions. Commonly used additional conditions are: the transition time length from 250KHz to 230KHz needs to be within a reasonable range. The solution is that when the instantaneous frequency drops to 240KHz, the step-over time is recorded, and txx is used to represent the step-over time. At the same time, another timing is started. When the frequency further drops to 235KHz (corresponding to the post-frequency transition detection threshold of Example 1), another timing ends, and the timing time is recorded, represented by tw. When the time interval from txx to tw is between 12~18us, txx is considered to be a reasonable step-over time, otherwise the measurement process continues. The 12~18us time interval here is related to the filter parameters in the algorithm. The 12~18us time is the filter response time constant. The response time constant is the inverse of the filter 3dB bandwidth. Generally, when it reaches 3 times the time constant, the step response is basically over. When the filter parameters change, the value will also change accordingly. For example, for other filter parameters, it becomes 15~20us. Finally, after the step response is completed, the post-trigger frequency needs to be maintained near 230KHz for a reasonable time (corresponding to the neighborhood of the frequency value corresponding to the free oscillation frequency signal of Example 1), and finally the transition time from the first transducer to the second transducer can be calculated.
[0037] Compared with the envelope method and the pulse ratio method, the frequency transition method of the present application can effectively resist the distortion of the signal amplitude. Because the frequency transition detects the frequency information, a slight distortion of the amplitude has little effect on the instantaneous frequency, so it can resist the amplitude distortion relatively well; the envelope method and the pulse ratio method use the amplitude information, and the distortion of the amplitude will affect the accuracy of these two detection algorithms.
[0038] The above are only embodiments of the present application and are not intended to limit the present application. For those skilled in the art, the present application may have various changes and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included within the scope of the claims of the present application.
Claims
1. A method for measuring ultrasonic transit time, characterized in that: The following steps are involved: The first transducer sends an ultrasonic signal to the second transducer, and detects the instantaneous frequency signal received by the second transducer by using an instantaneous frequency calculation method; wherein the instantaneous frequency signal includes a stimulated oscillation frequency signal and a free oscillation frequency signal; Amplifying and filtering the instantaneous frequency signal to obtain an instantaneous frequency curve signal corresponding to the instantaneous frequency signal; Setting a frequency transition detection threshold from the stimulated oscillation frequency signal to the free oscillation frequency signal, and setting a post-frequency transition detection threshold for verifying the frequency transition detection threshold; Determining the interval time from the frequency transition detection threshold to the post-frequency transition detection threshold until a decision condition is met; When the judgment condition is met, a certain time is delayed until the frequency of the signal received by the second transducer reaches the neighborhood of the frequency value corresponding to the free oscillation frequency signal, and the crossing time from the first transducer to the second transducer is calculated.
2. The ultrasonic transit time measurement method according to claim 1, characterized in that: The instantaneous frequency calculation method comprises the following specific steps: Receiving a signal input from the first transducer, the input signal first passes through an analog-to-digital converter ADC to convert the analog signal into a digital signal; The digital signal converted by the analog-to-digital converter ADC is subjected to narrow-band filtering, and then subjected to delay processing and Hilbert filtering respectively; After delay processing and Hilbert filtering, the signal is input to the rotation factor multiplier for algorithm processing, and then processed by difference and filtering, and finally the clock frequency is sampled to obtain the instantaneous frequency signal.
3. The ultrasonic transit time measurement method according to claim 1, characterized in that: The decision condition is positively correlated with the response time constant of the filter, and the response time constant is the inverse of the 3dB bandwidth of the filter.
4. The ultrasonic transit time measurement method according to claim 1, characterized in that: The frequency difference between the stimulated oscillation frequency signal and the frequency transition detection threshold is less than 10 KHz.
5. The ultrasonic transit time measurement method according to claim 1, characterized in that: The frequency difference between the frequency transition detection threshold and the post-frequency transition detection threshold is less than 5 KHz.
6. The ultrasonic transit time measurement method according to claim 1, characterized in that: The frequency difference between the post-frequency transition detection threshold and the free oscillation frequency signal is less than 5 KHz.
7. The ultrasonic transit time measurement method according to claim 1, characterized in that: When the judgment condition is met, a certain time is delayed until the frequency of the signal received by the second transducer reaches the neighborhood of the frequency value corresponding to the free oscillation frequency signal, and the neighborhood range of the neighborhood is (- ,+ ), Less than 0.5KHz.
8. The ultrasonic transit time measurement method according to claim 1, characterized in that: The corresponding frequencies of the stimulated oscillation frequency signal, the frequency transition detection threshold, the post-frequency transition detection threshold, the free oscillation frequency signal and the neighborhood of their frequency values are obtained by using a phase-locked loop method, a discrete Fourier transform method or a wavelet transform method.
9. The ultrasonic transit time measurement method according to claim 8, characterized in that: The occurrence time of the stimulated oscillation frequency signal, the frequency transition detection threshold, the post-frequency transition detection threshold, the free oscillation frequency signal and the neighborhood of their frequency values adopts a correlation detection method, specifically: in, The measured signal is, As the reference signal, by finding The maximum value corresponds to Determine the time when the signal occurs.
10. A system for implementing the ultrasonic transit time measurement method according to any one of claims 1 to 9, characterized in that: include: A signal generation and detection module, wherein the first transducer sends an ultrasonic signal to the second transducer, and detects the instantaneous frequency signal received by the second transducer by using an instantaneous frequency calculation method; wherein the instantaneous frequency signal includes a stimulated oscillation frequency signal and a free oscillation frequency signal; A signal processing module amplifies and filters the instantaneous frequency signal to obtain an instantaneous frequency curve signal corresponding to the instantaneous frequency signal; A threshold setting module, which sets a frequency transition detection threshold from the stimulated oscillation frequency signal to the free oscillation frequency signal, and sets a post-frequency transition detection threshold for verifying the frequency transition detection threshold; A threshold decision module, which determines the interval time from the frequency transition detection threshold to the post-frequency transition detection threshold until a decision condition is met; The crossing time calculation module, when the judgment condition is met, delays for a certain time until the frequency of the signal received by the second transducer reaches the neighborhood of the frequency value corresponding to the free oscillation frequency signal, and calculates the crossing time from the first transducer to the second transducer.