CN103454643B - Method for accurately measuring constant sound pressure FSK ultrasonic wave transition time - Google Patents

Abstract

The invention discloses a constant sound pressure FSK ultrasonic transit time accurate measurement method, which effectively overcomes the measurement error caused by the mechanical inertia and damping effect of the ultrasonic vibrator (stabilizes the mechanical inertia and damping effect at a fixed value) . It can effectively improve the accuracy of ultrasonic distance measurement. Experiments show that the measurement error of this method is stable on the order of 0.1mm. It is far superior to the technical level that can be achieved by similar technologies.

Description

Accurate measurement method of FSK ultrasonic transit time with constant sound pressure

technical field

The invention relates to a method for accurately measuring the transit time of ultrasonic waves, in particular to a method for accurately measuring the transit time of constant sound pressure FSK ultrasonic waves.

Background technique

The human ear can only perceive sound waves up to about 20kHz, and sound waves with a frequency above 20kHz are ultrasonic waves, which belong to the category of mechanical waves.

Ultrasound follows the general law of mechanical wave propagation in elastic media. For example, reflection and refraction will occur at the interface of the medium, and it will be absorbed by the medium after entering the medium and attenuate. The frequency of ultrasonic waves can be very high, reaching the megahertz level. Therefore, when ultrasonic waves propagate in the medium, the energy can be concentrated in a small range, with good beam forming and good directionality.

Theoretical research shows that, under the condition of the same amplitude, the vibration energy of an object is proportional to the square of the vibration frequency. When the ultrasonic wave propagates in the medium, the frequency of the particle vibration of the medium can be very high, so the energy is very large.

Utilizing the characteristic of large ultrasonic energy, it can produce mechanical effects, cavitation effects, thermal effects and chemical effects on the medium, and can perform ultrasonic welding, drilling, solid crushing, emulsification, atomization, degassing, dust removal, and boiler scale removal. , Cleaning, sterilization, promotion of chemical reactions and biological research, etc., have been widely used in various sectors such as industry, mining, agriculture, and medical treatment.

Another notable feature of ultrasound is that it propagates along a straight line and can be directional. The propagation speed of ultrasonic waves in the same medium is basically constant, so by measuring the time it takes for ultrasonic waves to pass a certain distance in the medium, non-contact distance measurement can be realized. For different media, the speed of ultrasonic propagation is different. Using this characteristic, the characteristics of the medium can also be measured, such as the concentration of alcohol, the temperature of the air in the boiler, and so on. If the ultrasonic propagation characteristics of different media are determined, the types of different substances can be determined, such as sucrose solution, alcohol solution, quality characteristics of various wines, etc.

In order to measure the slight change (difference) of ultrasonic velocity in different media, it is necessary to accurately measure the elapsed time (transit time) of ultrasonic waves in the medium. The precise measurement of transit time determines the accuracy of identifying the characteristics of the medium.

If the fishing boat is equipped with an underwater ultrasonic generator, it rotates and emits ultrasonic waves in all directions. The ultrasonic waves will be reflected back when they encounter fish schools. When the fishing boat detects the reflected waves, it will know the position of the fish schools. This instrument is called sonar, sonar It can also be used to detect reefs in the water, enemy submarines, and measure the depth of sea water. According to the same principle, ultrasonic waves can also be used to detect metals, ceramic concrete products, and even reservoir dams to check whether there are air bubbles, cavities and cracks inside to achieve non-destructive detection. The surface of each internal organ of the human body has different reflection capabilities to ultrasonic waves, and the reflection capabilities of healthy internal organs and diseased internal organs are also different. The so-called "B-ultrasound" is to perform contrast-enhanced images based on the ultrasound reflected by viscera to help doctors analyze lesions in the body.

With the continuous development of sensor and single-chip control technology, non-contact detection technology has been widely used in many fields. At present, typical non-contact ranging methods include ultrasonic ranging, CCD detection, radar ranging, laser ranging, etc. Among them, CCD detection has the characteristics of convenient use, no need for signal emission sources, and obtaining a large amount of scene information at the same time, but CCD ranging requires additional computing overhead. Radar ranging has the advantages of working around the clock and is suitable for short-distance and high-precision ranging in harsh environments, but it is easily interfered by electromagnetic waves. Laser ranging has the advantages of high directivity, high monochromaticity, high brightness, and fast measurement speed, especially has a certain penetration ability to rain and fog, and has strong anti-interference ability, but its cost is high and data processing is complicated.

Compared with the previous ranging methods, ultrasonic ranging can directly measure short-distance targets, with high vertical resolution, wide application range, strong directionality, and is not affected by light, smoke, electromagnetic interference and other factors, and has a wider coverage. Great advantages. At present, ultrasonic ranging has been widely used in the fields of liquid level measurement, mobile robot positioning and obstacle avoidance, and has broad application prospects.

In medicine, if the measurement accuracy of B-mode ultrasound echo time is increased by an order of magnitude, the clarity of B-mode ultrasound images can be greatly improved and the probability of misjudgment by doctors can be reduced.

In recent years, ultrasonic testing technology has clearly shown the following trends:

1. From qualitatively judging the existence of defects to quantitatively judging the position, size, shape and nature of defects, and using various imaging technologies to directly display two-dimensional and three-dimensional images of defects;

2. To the intelligent development of online automatic detection and instruments, among which non-contact ultrasonic testing technology has made breakthrough progress:

3. The combination of ultrasonic testing technology and physical property evaluation of materials has led to rapid development of material design, processing and engineering applications.

As shown in Figure 1, the basic principle of ultrasonic ranging is that the ultrasonic sensor is excited by a pulse signal to emit ultrasonic waves, which are transmitted to the measured object through the sound transmission medium to form reflected waves; the reflected waves then return to the receiving sensor through the sound transmission medium, and the sensor sends The acoustic signal is converted into an electrical signal, and the system measures the time elapsed (transit time) from the emission to the reception of the ultrasonic wave, using the formula:

The distance between the ultrasonic sensor and the measured object can be obtained.

in: is the ultrasonic transmission speed, , the unit is , is the ambient temperature;

It is the time elapsed from the transmission of the ultrasonic wave to the reception, in seconds.

Since the range and accuracy of ultrasonic ranging are affected by many factors (such as the performance of the ultrasonic transmitter and receiver itself, signal power, signal-to-noise ratio, ambient temperature, humidity, timer timing accuracy, etc.), the measurement accuracy and range are different. has been greatly restricted. After the excitation signal acts on the transmitter, the ultrasonic wave emitted by the transmitter will show a process of increasing until the amplitude is stable; The electrical signal also has an amplification process. When the transmission power is small or the measurement distance is long, the receiver may not feel the excitation of the first transmitted wave. Figure 2 shows the process of this augmentation. That is, after the attenuation of the propagation process, due to The amplitude of the ultrasonic waves at the moment is very small, The ultrasonic echo at that time may not necessarily be detected. In other words, the first echo leading edge detected by the receiver is not necessarily the moment the leading edge of the emitted ultrasonic waves, which may be moments, even Ultrasonic frontier emitted at all times. But when The time has already added the excitation signal to the ultrasonic transmitter, but the timer has already started timing, which will bring inestimable errors to the measurement. In the case of the same transmission power of the ultrasonic transmitter, the farther the measured distance is, the smaller the amplitude of the ultrasonic wave reflected by the measured target. When the amplitude of the reflected ultrasonic wave is small to a certain extent, it cannot be detected or is not easy to be detected, so that the measurement range is limited. Due to the limitation of the transmitting power of the ultrasonic transmitter, the effective test distance is generally short. In terms of measurement accuracy, on the one hand, the accuracy of the ultrasonic echo frontier detection and the counting frequency of the counter will directly affect the measurement accuracy. If the ultrasonic transmitter and receiver with a resonant frequency of 40kHz are selected, the measurement error caused by the difference of one pulse in the echo front detection is 8.5 mm (the wave speed of the ultrasonic wave is calculated according to 340m/s). For the measurement of mm-level precision, the error cannot be ignored. On the other hand, every 1 μs difference in the measured value of the transit time will also bring a ranging error of 0.34 mm.

The transmission speed of ultrasonic waves It can be seen that the ambient temperature for the ultrasonic propagation velocity is about 0.6m/°C, and the influence of temperature on the ultrasonic propagation velocity can be compensated by measuring the ambient temperature, but the measurement error of the ambient temperature and the inconsistency of the ambient temperature in the propagation path will lead to temperature compensation. Technology falls short of intended compensation purpose. Therefore, the benchmarking method came into being.

The benchmarking method is to set a marker pole in front of the ultrasonic transmitter, because the distance between the marker pole and the transmitter is known , so it is only necessary to measure the transit time between the ultrasonic wave passing through the marker pole and the transmitter ,according to , the sound velocity value in the actual measurement environment can be obtained. The benchmark measurement method does not need to consider the influence of the ambient temperature on the sound velocity, and avoids the secondary error caused by temperature compensation of the sound velocity. However, the benchmarking method cannot solve the problem of accurate detection of the echo front.

The "ultrasonic distance measuring device with automatic sensitivity gain control function" described in CN03207478.6 discloses that a digital potentiometer is used to automatically adjust the magnification of the received signal, so that in the whole process of distance measurement, the signal processing circuit can obtain Relatively stable and balanced echo signal. The device includes a microprocessor, an ultrasonic transmitting circuit, an ultrasonic receiving circuit, an echo signal amplifying circuit, an echo signal detecting circuit and a display circuit. The ultrasonic distance measuring device also includes a digital potentiometer, which is connected with the microprocessor and the operational amplifier of the echo signal amplifying circuit, and the microprocessor controls the digital potentiometer so that the digital potentiometer automatically changes its output resistance. value to adjust the magnification of the echo signal, so that a relatively balanced echo signal is obtained during the entire distance measurement process, and effectively shields the generation of false display signals such as geodesy without affecting the short distance measurement distance. The magnification of the echo signal can be greatly improved during long-distance measurement, so the effect of long-distance distance measurement is effectively improved.

However, according to the attenuation law of the sound pressure and sound intensity of the ultrasonic wave during its propagation:

in: , : Sound pressure and sound intensity at distance x from the sound source

x : the distance between the measurement point and the sound source

: Attenuation coefficient, unit is Np/cm (Nepel/cm)

It shows that as the propagation distance increases, the sound pressure and sound intensity at the receiving point decay exponentially and decrease rapidly. When measuring at a long distance, the energy of the ultrasonic wave will be continuously lost during the transmission process, and the signal at the receiving end will It will be submerged in the noise of the medium and cannot be identified. At this time, no matter how you adjust the magnification, it will not help. At the same time, this patent cannot solve the problem of inaccurate measurement of the echo front.

CN200710071684.4 "Ultrasonic Echo Frontier Detection Method Based on Modulation Domain Measurement" describes that by transmitting two ultrasonic frequencies , , the measurement distance is calculated by measuring the transit time of the frequency switching point from the transmitter to the receiver, thus overcoming the inaccurate measurement of the echo front in the traditional ultrasonic ranging method.

As shown in Figure 3, Time is the frequency jump point of the transmitted ultrasonic wave, at Before the moment, the period of the transmitted ultrasound is , with a frequency of ;exist After time, the emitted ultrasonic period is , with a frequency of . At the same time, the timer starts from Time starts counting. time for The moment when the emitted ultrasonic wave reaches the ultrasonic receiver through the medium transmission, that is, the moment when the echo frequency jumps. and The difference is the transit time of ultrasonic waves in the medium.

when the frequency of the ultrasound and The larger the difference, the easier it is to detect the frequency conversion point during the measurement. In fact, due to the limited bandwidth of the ultrasonic transmitter and receiver, and It is very close, which brings certain difficulties to the detection of the frequency conversion point.

The "ultrasonic echo frontier detection method based on modulation domain measurement" described in CN200710071684.4 ignores an important characteristic of the ultrasonic transmitter and receiver during design, that is, whether it is a transmitter or a receiver, its working principle is Piezoelectric effect, the transmitter converts electrical signals into mechanical waves, and the receiver converts mechanical waves into electrical signals, while mechanical vibration has mechanical inertia and damping effects, when the vibrator is converted from one vibration frequency to another, its frequency switching The process cannot be instantaneous, and the switching time must be different if the excitation power is different. In this sense, the "ultrasonic echo front detection method based on modulation domain measurement" described in CN200710071684.4 will no longer be effective, or the effect will no longer be as remarkable as it says.

Theory shows that mechanical vibration from a vibration frequency switch to another vibrational frequency When , its initial frequency change will be chaotic and disorderly, and then slowly move towards Closer, and finally stabilized at Above, the time required for the entire switching process is related to the excitation voltage for the transmitter, and related to the sound pressure and sound intensity at the receiving point for the receiver.

Experiments have shown that when using When the electrical signal excites the transmitter, its switching time is about 4 oscillation cycles.

Contents of the invention

In order to solve the above-mentioned technical problems existing in the prior art, the present invention provides a kind of constant sound pressure FSK ultrasonic transit time accurate measurement method, comprises the following steps:

(1) The carrier pulse generator generates pulse signals of two frequencies , , when the logic level of the variable-frequency identification pulse changes, the output frequency of the carrier pulse generator will change. When the variable-frequency identification pulse is logic "1", the frequency of the output pulse signal of the carrier pulse generator is , when the variable frequency identification pulse is logic "0", the frequency of the carrier pulse generator output pulse signal is , the pulse signal output by the carrier pulse generator is sent to the programmable pulse amplifier Ⅰ one way to amplify the pulse signal in phase, and the other way is inverted and then sent to the programmable pulse amplifier Ⅱ to amplify the pulse signal in reverse phase. The dynamic power signal drives the transmitter to output ultrasonic waves;

(2) The ultrasonic receiver converts the ultrasonic wave into an electrical signal, which is divided into two circuits after being amplified by the amplifying circuit. One circuit outputs a pulse signal for the counter to measure the frequency of the ultrasonic wave after passing through the zero trigger, and is used to detect the frequency conversion point; the other circuit passes through the precision detection The circuit converts the received signal strength into a DC voltage, and the double-limit comparator screens the magnitude of the DC voltage. When the signal strength is greater than the set upper limit of the received signal level, the comparator's output pin up_H=1, down_L= 1; When the signal strength is less than the set lower limit of the received signal level, the output pin of the comparator is up_H=0, down_L=0; when the signal strength is between the set upper and lower limits of the received signal level, the comparator The output pin up_H=0, down_L=1; the working state of the reversible counter is controlled by the logic level of up_H and down_L: when up_H=1, down_L=1, the reversible counter counts down; when up_H=0, down_L=0 , the reversible counter counts up; when up_H=0, down_L=1, the reversible counter maintains the original state;

(3) The amplification factor of the programmable pulse amplifier is proportional to the count value of the reversible counter. When the signal level received by the ultrasonic receiver is small, the reversible counter will perform counting operation, and the output signal of the programmable pulse amplifier will be will increase, and the signal level received by the ultrasonic receiver will also increase. When this level exceeds the set upper limit level (up_H=1, down_L=1), the reversible counter will count down, and the receiving level will decrease. Small, at this time up_H=0, down_L=1, the reversible counter stops counting, and the received signal level is stable at this level;

If the measurement distance increases at this time, the signal level received by the ultrasonic receiver will decrease accordingly. The level increases, at this time up_H=0, down_L=1, the reversible counter stops counting, and the received signal level is stable at this level;

(4) When the logic level of the SCM controls the variable frequency marking pulse to change periodically, the output frequency of the carrier pulse generator will also periodically and When the logic level of the variable frequency identification pulse changes, the time measurement circuit is started to measure the period of the pulse signal output by the received pulse signal terminal, and to judge the frequency conversion point of the signal and the logic level of the frequency conversion identification pulse The time between the change and the frequency conversion point of the pulse signal output from the received pulse signal terminal is the transit time of the ultrasonic wave.

Further, the greater the count value of the reversible counter, the greater the amplitude of the pulse signal output by the programmable pulse amplifier.

Further, when the circuit is working normally, the signal level received by the ultrasonic receiver is always between the upper and lower limits of the set level. If the upper and lower limits of the set level are set very close, the ultrasonic receiving The signal level received by the receiver is constant.

The constant sound pressure FSK ultrasonic transit time accurate measurement method of the present invention effectively overcomes the measurement error caused by the mechanical inertia and damping effect of the ultrasonic vibrator (stabilizes the mechanical inertia and damping effect at a fixed value). It can effectively improve the accuracy of ultrasonic distance measurement. Experiments show that the measurement error of this method is stable on the order of 0.1mm. It is far superior to the technical level that can be achieved by similar technologies.

Description of drawings

Figure 1 is a schematic diagram of ultrasonic ranging;

Fig. 2 is the corresponding relationship diagram of the actual launch wave and echo on the time axis;

Figure 3 is a schematic diagram of the waveforms of the transmitted wave and the echo modulated by FSK;

Fig. 4 is a block diagram of a method for accurate measurement of constant sound pressure FSK ultrasonic wave time.

Detailed ways

The present invention will be further described below in conjunction with accompanying drawing.

As shown in Figure 4, the constant sound pressure FSK ultrasonic transit time accurate measurement method includes the following implementation steps:

The carrier pulse generator generates pulse signals of two frequencies , , when the logic level of the variable-frequency identification pulse changes, the output frequency of the carrier pulse generator will change. When the variable-frequency identification pulse is logic "1", the frequency of the output pulse signal of the carrier pulse generator is , when the variable frequency identification pulse is logic "0", the frequency of the carrier pulse generator output pulse signal is . The pulse signal output by the carrier pulse generator is sent to the programmable pulse amplifier Ⅰ one way to amplify the pulse signal in phase, and the other way is inverted and then sent to the programmable pulse amplifier Ⅱ to amplify the pulse signal in reverse phase. The power signal drives the transmitter to output ultrasonic waves.

The ultrasonic receiver converts the ultrasonic waves into electrical signals, which are amplified by the amplifying circuit and then divided into two paths. One path passes through the zero trigger and then outputs pulse signals for the counter to measure the frequency of the ultrasonic wave, which is used to detect frequency conversion points; the other path passes through the precision detection circuit to receive The received signal strength becomes a DC voltage, and the double-limit comparator screens the magnitude of the DC voltage. When the signal strength is greater than the set upper limit of the received signal level, the output pin of the comparator is up_H=1, down_L=1; when When the signal strength is less than the set lower limit of the received signal level, the output pin of the comparator is up_H=0, down_L=0; when the signal strength is between the upper and lower limits of the set received signal level, the output pin of the comparator up_H=0, down_L=1.

The working state of the reversible counter is controlled by the logic levels of up_H and down_L. When up_H=1, down_L=1, the reversible counter counts down; when up_H=0, down_L=0, the reversible counter counts up; when up_H=0, down_L=1, the reversible counter maintains the original state.

The amplification factor of the programmable pulse amplifier is proportional to the count value of the reversible counter, that is to say, the larger the count value of the reversible counter, the larger the output pulse signal amplitude of the programmable pulse amplifier. Therefore, when the signal level received by the ultrasonic receiver is small, the reversible counter will count up, the output signal of the programmable pulse amplifier will increase, and the signal level received by the ultrasonic receiver will also increase. When the level exceeds the set upper limit level (up_H=1, down_L=1), the reversible counter counts down, and the receiving level decreases. At this time, up_H=0, down_L=1, the reversible counter stops counting and receives The signal level is stabilized at this level.

If the measurement distance increases at this time, the signal level received by the ultrasonic receiver will decrease accordingly. At this time, up_H=0, down_L=1, the reversible counter stops counting, and the received signal level is stable at this level.

It can be seen that when the circuit is working normally, the signal level received by the ultrasonic receiver is always between the upper and lower limits of the set level. If we set the upper and lower limits of the set level very close, Then the signal level received by the ultrasonic receiver will be constant.

When the logic level of the SCM controls the variable frequency marker pulse to change periodically, the output frequency of the carrier pulse generator will also periodically and change back and forth between. When the logic level of the variable-frequency identification pulse changes, the time measurement circuit is started to measure the period of the output pulse signal at the received pulse signal end, and judge the frequency conversion point of the signal. The time between the frequency conversion points of the output pulse signal at the signal terminal is the transit time of the ultrasonic wave. This time includes the delay caused by the mechanical inertia and damping effect of the transmitter and receiver. Since the signal level loaded on the transmitter and the ultrasonic sound pressure and sound intensity at the receiving point are kept constant, this delay time is a fixed value and can be Eliminate it by calibration.

Claims (3)

1. a constant sound pressure FSK ultrasonic wave transit time accurate measurement method is characterized in that: comprise the steps: (1) The carrier pulse generator generates pulse signals of two frequencies , , when the logic level of the variable-frequency identification pulse changes, the output frequency of the carrier pulse generator will change. When the variable-frequency identification pulse is logic "1", the frequency of the output pulse signal of the carrier pulse generator is , when the variable frequency identification pulse is logic "0", the frequency of the carrier pulse generator output pulse signal is , the pulse signal output by the carrier pulse generator is sent to the programmable pulse amplifier Ⅰ, and the pulse signal is amplified in the same phase. The dynamic power signal drives the transmitter to output ultrasonic waves; (2) The ultrasonic receiver converts the ultrasonic wave into an electrical signal, which is divided into two circuits after being amplified by the amplifying circuit. One circuit outputs a pulse signal for the counter to measure the frequency of the ultrasonic wave after passing through the zero trigger, and is used to detect the frequency conversion point; the other circuit passes through the precision detection The circuit converts the received signal strength into a DC voltage, and the double-limit comparator screens the magnitude of the DC voltage. When the signal strength is greater than the set upper limit of the received signal level, the comparator's output pin up_H=1, down_L= 1; When the signal strength is less than the set lower limit of the received signal level, the output pin of the comparator is up_H=0, down_L=0; when the signal strength is between the set upper and lower limits of the received signal level, the comparator The output pin up_H=0, down_L=1; the working state of the reversible counter is controlled by the logic level of up_H and down_L: when up_H=1, down_L=1, the reversible counter counts down; when up_H=0, down_L=0 , the reversible counter counts up; when up_H=0, down_L=1, the reversible counter maintains the original state; (3) The amplification factor of the programmable pulse amplifier is proportional to the count value of the reversible counter. When the signal level received by the ultrasonic receiver is small, the reversible counter will perform counting operation, and the output signal of the programmable pulse amplifier will be It will increase, and the signal level received by the ultrasonic receiver will also increase. When this level exceeds the set upper limit level up_H=1, down_L=1, the reversible counter will count down, and the receiving level will decrease. , at this time up_H=0, down_L=1, the reversible counter stops counting, and the received signal level is stabilized at this level; If the measurement distance increases at this time, the signal level received by the ultrasonic receiver decreases accordingly. When the level is lower than the set lower limit level up_H=0, down_L=0, the reversible counter counts up, and the receiving level Increase, at this time up_H=0, down_L=1, the reversible counter stops counting, and the received signal level is stable at this level; (4) When the logic level of the SCM controls the variable frequency marking pulse to change periodically, the output frequency of the carrier pulse generator will also periodically and When the logic level of the variable frequency identification pulse changes, the time measurement circuit is started to measure the period of the pulse signal output by the received pulse signal terminal, and to judge the frequency conversion point of the signal and the logic level of the frequency conversion identification pulse The time between the change and the frequency conversion point of the pulse signal output from the received pulse signal terminal is the transit time of the ultrasonic wave. 2. The method for accurately measuring the transit time of constant sound pressure FSK ultrasonic waves as claimed in claim 1, characterized in that: the larger the count value of the reversible counter, the larger the amplitude of the programmable pulse amplifier output pulse signal. 3. constant sound pressure FSK ultrasonic wave time-of-flight accurate measurement method as claimed in claim 1, is characterized in that: when circuit works normally, the signal level that ultrasonic receiver receives is always between the upper and lower of setting level , lower limit, if the upper and lower limits of the set level are set very close, the signal level received by the ultrasonic receiver is constant.