Parallel transmission, reception and demodulation method for ultrasonic tomography

Abstract

The present invention relates to a parallel transceiver and demodulator method for ultrasonic tomography. Assuming that a sensor array is composed of N ultrasonic sensors, the sequence length of an excitation signal is L, the ultrasonic sensor that transmits the signal is called a transmitting end, and the ultrasonic sensor that receives the signal is called a receiving end. The method comprises the following steps: using a Kasami sequence to generate N mutually orthogonal pseudo-random orthogonal sequences with a length of L; modulating a carrier signal; generating an excitation signal based on the modulated and filtered signal, simultaneously stimulating all transmitting ends, and each receiving end receives a superposition of sound source signals emitted by all other N-1 transmitting ends and local noise at the receiving end; demodulating the transit time; demodulating the signal attenuation; and calculating the average attenuation coefficient of the ultrasonic wave along the propagation path using the extracted attenuation spectrum based on the principle of ultrasonic tomography to achieve image reconstruction of the measured field.

Description

Technical Field

[0001] The present invention belongs to the technical field of ultrasonic tomography, and is a parallel receiving, transmitting and demodulating method for acquiring ultrasonic tomography projection data. Background Art

[0002] Ultrasonic process tomography uses an array of independent probes to transmit ultrasonic waves from multiple angles into the fluid within the measured section. The technique uses acoustic attenuation and propagation time within the ultrasonic propagation path (projection) to invert the fluid distribution. Compared to other tomography techniques, ultrasonic process tomography offers advantages such as a rich propagation pattern, independence from fluid salinity, and the absence of radiation. It can be mounted on the outer wall of a pipe using a "clamp-on" sensor to measure the fluid structure and distribution parameters such as phase holdup and flow velocity within a closed pipe, thus offering broad application prospects.

[0003] The super-tomography system primarily consists of three components: an ultrasonic sensor array; a signal excitation and data acquisition system; and image reconstruction software. The ultrasonic sensor array consists of multiple single-element probes. The ultrasonic sensor transmitter converts voltage signals into acoustic signals through the inverse piezoelectric effect, forming the excitation sound source signal. The ultrasonic sensor receiver converts the received acoustic signals into electrical signals through the piezoelectric effect. The signal excitation and data acquisition system controls the ultrasonic sensor excitation, recording, converting, and demodulating the voltage signals received by the sensor. Traditional ultrasonic tomography uses cyclic excitation of ultrasonic sensors at different locations through timing control. The image reconstruction software uses image reconstruction methods to estimate the medium distribution in the measured field using the transit time or signal attenuation obtained by demodulation of the received signal.

[0004] Traditional ultrasonic tomography methods use a one-shot, multiple-receiver, cyclic excitation scheme. Although it has advantages such as high precision and good directionality, the cyclic excitation measurement cycle is long, and the system's real-time performance still has room for improvement. It has certain limitations when meeting the requirements of higher-speed image reconstruction. Summary of the Invention

[0005] In order to improve the real-time performance of ultrasonic tomography systems, the present invention proposes a parallel transceiver and demodulation method for acquiring ultrasonic tomography projection data. This method adopts the idea of ​​"code division multiplexing" and uses the Kasami pseudo-random orthogonal sequence (Kasami sequence for short) to modulate the sound source signal. After all sensors are excited in parallel using the orthogonal modulated signal, the ultrasonic signal forms an aliased signal at the receiving end. By performing correlation demodulation on the aliased received signal, the multi-path transit time and signal attenuation information corresponding to each sound source signal component are extracted based on the power spectrum density estimation of the cross-correlation analysis method and the autocorrelation method. Compared with the cyclic excitation scheme originally used in ultrasonic tomography, this method speeds up the system's measurement of the multi-path signal transit time (TOF) and signal amplitude attenuation, and can obtain all the information for reconstructing a frame of cross-sectional image within one transceiver cycle, thereby improving the system's measurement speed and time resolution. The technical solution is as follows:

[0006] A method for parallel transmission, reception, and demodulation of ultrasonic tomography is disclosed. The ultrasonic tomography system employed utilizes an ultrasonic sensor array to perform tomographic imaging of a measured field. Assume that the sensor array consists of N ultrasonic sensors, the sequence length of the excitation signal is L, the ultrasonic sensor that transmits the signal is called a transmitter, and the ultrasonic sensor that receives the signal is called a receiver. The method comprises the following steps:

[0007] Step 1: Use the Kasami sequence to generate N mutually orthogonal pseudo-random orthogonal sequences of length L;

[0008] Step 2: modulate the carrier signal, and the modulated and filtered signal has autocorrelation;

[0009] Step 3: Generate an excitation signal based on the modulated and filtered signal. All transmitters are excited simultaneously. Each receiver receives the superposition of the sound source signals emitted by all other N-1 transmitters and the local noise at the receiver. The method is as follows:

[0010] From the sound source signals transmitted by N-1 transmitting ends, one signal is designated as the reference signal, recorded as x k (t), the sound source signal x emitted by different transmitting ends i (t) are mutually orthogonal, and the noise at the receiving end n j (t) and x i (t) is uncorrelated, the reference signal x k (t) and the received signal y j (t) cross-correlation function It is expressed by the following formula:

[0011]

[0012] Where l is the independent variable of the cross-correlation function, represents y j (t) and xk The cross-correlation function value of (t), α k,j (t) is x k (t) attenuation coefficient to receiving end j, Represents x k (t) and x k (l-Δt k,j )’s cross-correlation function value;

[0013] Determine the time corresponding to the maximum value of the cross-correlation and locate The time corresponding to the maximum value of the demodulation reference signal x k (t) the transit time Δt k,j ;

[0014] Step 4: Demodulate the transit time: From the sound source signals transmitted by N-1 transmitting ends, designate one signal as the reference signal. The sound source signals transmitted by different transmitting ends are mutually orthogonal. Based on the cross-correlation function between the reference signal and each received signal, demodulate the transit time of the reference signal.

[0015] Step 5: Demodulate the signal attenuation as follows:

[0016] Intercept y j (t) in the transit time Δt k,j The sequence with the length L is recorded as r k,j (t), r k,j (t) is determined by α k,j (t)*x k (t) and x k (t) Orthogonal sequence composition; obtain the reference signal x k Autocorrelation function of (t) and x k (t) and r k,j (t) cross-correlation function

[0017] calculate and Fourier transform of the reference signal x k Power spectral density of (t) It is expressed as follows:

[0018]

[0019] Where, represents the reference signal x k The power spectral density of (t), e (·) represents the power exponent with natural logarithm and (·) as base and exponent respectively;

[0020] x k (t) Received signal y at receiving end j k,jThe power spectral density of (t) is expressed as follows:

[0021]

[0022] Where, Indicates the received signal y j The power spectral density of (t), A k,j (f) is α k,j (t) the result after Fourier transform;

[0023] By calculation The attenuation spectrum of the signal from transmitter k to receiver j on the path is 10log (A k,j (f));

[0024] Step 6: Based on the principle of ultrasonic tomography, the average attenuation coefficient of the ultrasonic wave along the propagation path is calculated using the extracted attenuation spectrum to achieve image reconstruction of the measured field.

[0025] Furthermore, the method of step 2 is: using the N pseudo-random orthogonal sequences to modulate fixed frequency carriers respectively; using a bandpass filter to smooth the modulated carriers and control their bandwidth, and the modulated and filtered signals have autocorrelation.

[0026] To improve the real-time performance of ultrasonic tomography systems, the present invention proposes a parallel transceiver and demodulator method for ultrasonic tomography. A Kasami sequence is used to generate a sound source signal, and the autocorrelation maximum is calculated using the autocorrelation detection principle. The transit time of the signal from different transmitting ends to the receiving end is then determined based on the time node at which the maximum value occurs. Taking advantage of the fact that the power spectral density does not contain phase information of the signal, according to the Wiener-Schinchin theorem, the power spectral densities of the reference signal and the received signal are obtained by calculating the Fourier transform of the reference signal autocorrelation function and the cross-correlation function of the received signal and the reference signal, and then the attenuation spectrum of the signal is obtained by comparison. The proposed method significantly improves measurement speed and time resolution compared to the cyclic excitation scheme. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The following drawings illustrate selected embodiments of the present invention, which are exemplary and non-exhaustive and non-limiting.

[0028] Figure 1 This is a schematic diagram of the projection line network of the measured cross section of the 16-probe ultrasonic tomography system;

[0029] Figure 2 A flow chart of the method for demodulating transit time and signal amplitude attenuation according to the present invention;

[0030] Figure 3 (a) is the Kasami sequence used for demodulation, Figure 3 (b) is the sequence after modulation and filtering;

[0031] Figure 4 is the autocorrelation function value of signal 1;

[0032] Figure 5 Schematic diagram of the cross-correlation of all sound source signals;

[0033] Figure 6 The transit time for demodulating the signal transmitted by transmitter No. 1 using the received signals from receivers No. 3, 5, 7, and 9;

[0034] Figure 7 The original receiving signals of receiving terminals 3, 5, 7, and 9;

[0035] Figure 8 are the received signals of receiving terminals 3, 5, 7, and 9 after preprocessing;

[0036] Figure 9 is the power spectrum density of the signal transmitted by transmitter No. 1;

[0037] Figure 10 is the power spectrum density of the signals received by receiving terminals 3, 5, 7, and 9;

[0038] Figure 11 The attenuation spectra of the signal transmitted from transmitter 1 to receivers 3, 5, 7, and 9 respectively. DETAILED DESCRIPTION

[0039] The present invention will be further described with reference to the accompanying drawings and embodiments:

[0040] The present invention is a parallel transceiver and demodulator method for acquiring ultrasonic tomography projection data. In an embodiment, the method proposed by the present invention is used to control the ultrasonic tomography system to excite in parallel and simultaneously receive and demodulate the transit time and signal amplitude attenuation.

[0041] An ultrasonic tomography system was constructed, with ultrasonic sensors distributed around the same cross-section of the measured pipe. Tomography was performed within a field bounded by the circular pipe. The sensor array consisted of 16 ultrasonic sensors, and the excitation signal sequence length was 4000 sampling points. The ultrasonic sensor that transmits the signal is called the transmitter, and the ultrasonic sensor that receives the signal is called the receiver. Ultrasonic sensor numbered i is referred to as transmitter or receiver i. Because the sensors are symmetrically distributed across the measured cross-section, the signal transmitted by ultrasonic sensor 1 is used as the reference signal, and the signals received by ultrasonic sensors 3, 5, 7, and 9 are used as the received signals for demodulation. The effectiveness of the proposed method in demodulating transit time and signal amplitude attenuation is tested. Figure 1The positions of ultrasonic sensors 1 to 16 on the measured section are marked. The following examples are intended to describe embodiments of the present invention and are not the only form that can be manufactured or used. Other embodiments that can achieve the same function are also included in the scope of the present invention. Figure 2 A flowchart of demodulating the transit time and signal amplitude attenuation using the method of the present invention is described, and the specific implementation method is as follows:

[0042] Step 1: Generate a pseudo-random orthogonal sequence.

[0043] Use as Figure 3 The Kasami sequence shown in (a) generates 16 pseudo-random orthogonal sequences with a length of 4000 sampling points.

[0044] Step 2: Modulate the carrier signal

[0045] Since the spectrum of the Kasami sequence is arbitrarily wide and has sharp edges and discontinuities in the time domain, although ultrasonic sensors can transmit and receive ultrasonic signals with limited bandwidth, it is actually difficult to use ultrasonic sensors to generate and transmit Kasami sequences. Therefore, before transmitting the signal, it is necessary to modulate the Kasami sequence onto a sinusoidal carrier with a frequency of 1MHz. Then, a bandpass filter is used to smooth the modulated signal and control its bandwidth to 0.8MHz to 1.20MHz. The sampling frequency is 10MHz, generating the following: Figure 3 (b) The modulated filtered signal has good autocorrelation, and each signal has a Figure 4 The obvious autocorrelation peak is not correlated with other signals or ultrasonic sensor noise. Figure 5 As shown in the correlation relationship, each sound source signal has good autocorrelation, while the sound source signals transmitted by different transmitting ends are uncorrelated with each other.

[0046] Step 3: All ultrasonic sensors transmit and receive ultrasonic signals simultaneously.

[0047] When the system starts working, all transmitters are excited at the same time. Each receiver receives the superposition of the sound source signals emitted by all other 15 transmitters and the local noise of the receiver. The receiving signal y of the jth receiver is j (t) is expressed as follows:

[0048]

[0049] In the above formula, y j (t) is the received signal at the receiving end j, x i (t) is the sound source signal emitted by transmitter i, α i,j (t) is x i (t) attenuation coefficient to receiving end j, Δti,j is x i (t) Transit time to receiver j, n j (t) is the receiving noise of receiving end j, x i (t-Δt i,j ) is x i (t) is delayed by Δt in time i,j The result of (·)*(·) means the convolution of (·) and (·). represents the sum of (·) over i from 1 to 16 and i≠j.

[0050] Step 4: Demodulate the time of flight

[0051] First, calculate the cross-correlation value between the reference signal and the received signal. Select one signal from the 15 sound source signals transmitted by the transmitter as the reference signal, and record it as x k (t). Reference signal x k (t) and the received signal y j The cross-correlation function expression of (t) is as follows:

[0052]

[0053] In the above formula, l is the independent variable of the cross-correlation function, x k (t) is the reference signal, represents y j (t) and x k (t) the cross-correlation function value, Represents x i (t-Δt i,j ) and x k (t) the cross-correlation function value, Represents x k (t-Δt k,j ) and x k (t) the cross-correlation function value, Indicates n j (t) and x k (t) the cross-correlation function value, represents the sum of (·) over t from -∞ to +∞, represents the sum of (·) over i from 1 to 16 with i≠j,i≠k.

[0054] Since the sound source signal is generated by pseudo-random orthogonal sequence modulation, there is And the receiving end noise n j (t) and the sound source signal x i (t) is not relevant, so there is Interference and noise terms The values ​​of are all 0. The reference signal x k (t) and the received signal y j (t) cross-correlation function It can be simplified as follows:

[0055]

[0056] Determine the time corresponding to the maximum value of the cross-correlation. The cross-correlation function of the reference signal and the received signal At l = Δt k,j Get the maximum value by finding The time corresponding to the maximum value of the reference signal x is determined k (t) the transit time Δt k,j The transit time of the signal transmitted by transmitter No. 1 is demodulated using the received signals from receivers No. 3, 5, 7, and 9. The result is as follows: Figure 6 shown.

[0057] Step 5: Demodulate signal attenuation.

[0058] The demodulated signal amplitude attenuation is estimated using the Power Spectra Density (PSD) method. According to the Wiener-Hinchin theorem, the power spectral density of a signal is defined as the Fourier transform of the signal's autocorrelation function.

[0059] Since the power spectrum density estimation of the autocorrelation method requires the signal to be a wide stationary signal, the received signal y j (t) does not meet this condition, so the received signal needs to be preprocessed before power spectrum density estimation. j (t) in Δt k,j The sequence with a length of 4000 sampling points is recorded as r k,j (t), r k,j (t) by a k,j (t)*x k (t) and x k (t) The orthogonal sequence composition meets the requirements of wide stationary signal. The original receiving signals of receiving terminals 3, 5, 7, and 9 are as follows Figure 7 As shown, the received signals of receiving terminals 3, 5, 7, and 9 after preprocessing are as follows Figure 8 shown.

[0060] Then obtain the reference signal x k Autocorrelation function of (t) and x k (t) and r k,j (t) cross-correlation function and The expression is as follows:

[0061]

[0062] calculate and The Fourier transform of the reference signal x k Power spectral density of (t) It is expressed as follows:

[0063]

[0064] In the above formula, represents the reference signal x k The power spectral density of (t), lg(·) represents the logarithm of (·) with a base of 10, represents the sum of (·) over m from -∞ to +∞, e (·) represents the power exponent with natural logarithm and (·) as base and exponent, respectively.

[0065] x k (t) The received signal y at receiver j k,j The power spectral density of (t) is expressed as follows:

[0066]

[0067] In the above formula, Indicates the received signal y j The power spectral density of (t), A k,j (f) is α k,j (t) The result after Fourier transform.

[0068] By calculation The attenuation spectrum 10log (A k,j (f)). Use the received signals from receivers 3, 5, 7, and 9 to demodulate the attenuation of the transmitted signal from transmitter 1. Figure 8 is the PSD of the signal transmitted by transmitter No. 1, Figure 9 is the PSD of the signals received by receiving terminals 3, 5, 7, and 9, Figure 10 This is the attenuation information of the signal transmitted by transmitter No. 1 to receivers No. 3, 5, 7, and 9 respectively.

[0069] Step 6: Image reconstruction

[0070] Based on the principle of ultrasonic reflection tomography, the extracted transit time is used to reconstruct the boundary contour of the medium in the measured field. Based on the principle of ultrasonic tomography, the extracted attenuation information is used to calculate the average attenuation coefficient of the ultrasonic wave along the propagation path, realizing the visual reconstruction of the measured field.

[0071] The above embodiments are simulation examples of the present invention, and the present invention is not limited to the contents disclosed in the embodiments and drawings. Any equivalent or modification completed without departing from the spirit disclosed in the present invention is within the scope of protection of the present invention.

Claims

1. A method for parallel transmission, reception, and demodulation of ultrasonic tomography. The ultrasonic tomography system employed utilizes an ultrasonic sensor array to perform tomographic imaging of a measured field. Assume the sensor array consists of N ultrasonic sensors, the sequence length of the excitation signal is L, the ultrasonic sensor that transmits the signal is called the transmitting end, and the ultrasonic sensor that receives the signal is called the receiving end. The method comprises the following steps: Step 1: Use the Kasami sequence to generate N mutually orthogonal pseudo-random orthogonal sequences of length L; Step 2: modulate the carrier signal, and the modulated and filtered signal has autocorrelation; Step 3: Generate an excitation signal based on the modulated and filtered signal. All transmitters are excited simultaneously. Each receiver receives the superposition of the sound source signals emitted by all other N-1 transmitters and the local noise at the receiver. The method is as follows: From the sound source signals transmitted by N-1 transmitting ends, one signal is designated as the reference signal, recorded as x k (t), the sound source signal x emitted by different transmitting ends i (t) are mutually orthogonal, and the noise at the receiving end n j (t) and x i (t) is uncorrelated, the reference signal x k (t) and the received signal y j (t) cross-correlation function It is expressed by the following formula: Where l is the independent variable of the cross-correlation function, represents y j (t) and x k The cross-correlation function value of (t), α k,j (t) is x k (t) attenuation coefficient to receiving end j, Represents x k (t) and x k (l-Δt k,j )’s cross-correlation function value; Determine the time corresponding to the maximum value of the cross-correlation and locate The time corresponding to the maximum value of the demodulation reference signal x k (t) the transit time Δt k,j ; Step 4: Demodulate the transit time: From the sound source signals transmitted by N-1 transmitting ends, designate one signal as the reference signal. The sound source signals transmitted by different transmitting ends are mutually orthogonal. Based on the cross-correlation function between the reference signal and each received signal, demodulate the transit time of the reference signal. Step 5: Demodulate the signal attenuation as follows: Intercept y j (t) in the transit time Δt k,j The sequence with the length L is recorded as r k,j (t), r k,j (t) is determined by α k,j (t)*x k (t) and x k (t) Orthogonal sequence composition; obtain the reference signal x k Autocorrelation function of (t) and x k (t) and r k,j (t) cross-correlation function calculate and Fourier transform of the reference signal x k Power spectral density of (t) It is expressed as follows: Where, represents the reference signal x k The power spectral density of (t), e (·) represents the power exponent with natural logarithm and (·) as base and exponent respectively; x k (t) Received signal y at receiving end j k,j The power spectral density of (t) is expressed as follows: Where, Indicates the received signal y j The power spectral density of (t), A k,j (f) is α k,j (t) the result after Fourier transform; By calculation The attenuation spectrum of the signal from transmitter k to receiver j on the path is 10log (A k,j (f)); Step 6: Based on the principle of ultrasonic tomography, the average attenuation coefficient of the ultrasonic wave along the propagation path is calculated using the extracted attenuation spectrum to achieve image reconstruction of the measured field.

2. The ultrasonic tomography parallel transmission, reception and demodulation method according to claim 1, characterized in that: The method of step 2 is: using the N pseudo-random orthogonal sequences to modulate fixed frequency carriers respectively; using a bandpass filter to smooth the modulated carriers and control their bandwidth, and the modulated and filtered signals have autocorrelation.

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