Automatic detection and analysis of complex ultrasonic fields based on ADLINK PCI-9846 high-speed digitizer

　　This research is in the field of design of complex medical ultrasound sensors, automatic measurement and analysis of ultrasound fields, and parameter modeling.

　　• challenge

　　In order to adapt to the characteristics of human tissue structure, the design of medical ultrasonic transducers is developing towards complex sound fields. The measurement and modeling of complex ultrasonic fields is a recognized problem in the industry. Traditional ultrasonic field measurement signal acquisition efficiency is low, and it cannot automatically collect and analyze signals, let alone meet the parameter evaluation and accurate modeling of complex ultrasonic fields, which restricts the design and application of complex medical ultrasonic transducers. In order to meet the requirements of complex ultrasonic transducer design and application, it is urgent to study an automatic detection and analysis system suitable for complex ultrasonic field signals to solve the many difficulties in the computational modeling and actual measurement of complex ultrasonic fields.

　　• solution

　　With ADLINK PCI-9846 high-speed digitizer as the core, combined with preamplifier and hydrophone, an efficient sound field signal acquisition system is developed using LabVIEW. Through the efficient data acquisition module, the sound pressure data of the three-dimensional sound field is displayed and saved in real time. Design and manufacture a four-axis precision industrial robot system driven by a stepper motor, develop automatic control and automatic measurement systems, achieve three-dimensional positioning of any part of the ultrasonic field, and coordinate the automatic positioning control and data acquisition of the robot measurement point. Develop a playback and multifunctional comprehensive analysis system for sound field measurement data to visualize the results.

  　1. Application background

　　Medical ultrasonic diagnostic and therapeutic equipment has become an indispensable part of the medical and health industry, especially playing an important role in the health of patients. Ultrasonic transducers are an important component in both ultrasonic diagnosis and treatment. Therefore, the accurate measurement of its acoustic field characteristics and frequency and other performances needs to be given enough attention by ultrasonic equipment researchers and transducer manufacturers. At present, comprehensive testing of acoustic transducer performance has not been popularized in China, especially compared with foreign products. Some manufacturers cannot provide reliable performance data for the transducers they produce. The gap in price, performance, and stability is not small, which has become a bottleneck in the development and production process of domestic ultrasonic transducer equipment [1].

　　Faced with complex medical clinical requirements, ultrasonic equipment has higher and higher requirements for transducer selection and design. During use, due to the characteristics of piezoelectric materials themselves and other reasons, such as large temperature changes, improper storage and operational errors, the performance of the transducer may be damaged. If it continues to be used without knowing it, it is easy to cause medical accidents and missed detection, and the reliability of its treatment and diagnosis effects is difficult to guarantee. The consequences and losses are also unimaginable. Therefore, it is urgent to design a system and solution to reasonably detect the acoustic field characteristics of ultrasonic transducers. The study of the physical properties of ultrasound is the basis for the study of ultrasonic bioeffects. With the wider application of ultrasonic technology, many research projects on the biological effects of ultrasonic radiation have been carried out at home and abroad, especially how transducer frequency, output power, radiation time, etc. interact with tissues. Many research results have been achieved in this regard, and the characteristics of ultrasonic radiation fields have also received much attention. Although various new technologies in ultrasonic engineering are constantly developing, visualization imaging technology and computer applications are still weak in medical ultrasonic engineering. Therefore, it is necessary to innovate independently on the basis of hardware equipment and software development to accelerate the research on measurement and modeling simulation of ultrasonic fields.

　　The research on the characteristics of ultrasonic radiation acoustic field in biomedical ultrasound engineering mainly includes two aspects: on the one hand, the development of simulation software based on computer-aided calculation, and on the other hand, the research of multifunctional systems for actual ultrasonic measurement. At present, there are few automatic detection systems for biomedical ultrasound. Ultrasonic automatic detection is mainly used in industrial flaw detection, such as the non-destructive testing process formulation expert system (CAPPNDT) developed by Zhejiang University [2], and the non-destructive testing special software NDTS developed by the Pressure Vessel Inspection Station of the Ministry of Metallurgy [3]. The mechatronics automatic control technology is applied to the acquisition of ultrasonic signals and the development of quantitative processing. Although there are few reports on theoretical research on ultrasonic measurement and simulation systems for medical ultrasound technology applications, some companies have developed related ultrasonic medical equipment, such as Fluke's Sonora ultrasonic sound field detection system.

　　2. Problems

　　Although many studies have been conducted on the measurement of ultrasonic transducer acoustic field performance, the oscilloscope manual method is commonly used in the measurement process, which is inefficient, poorly mechanized, and has large human errors, seriously affecting the accuracy and credibility of the test results. With the increasing application of ultrasonic equipment in medical diagnosis and treatment, the functional requirements of ultrasonic transducers are becoming more diversified and precise.

　　In order to adapt to the characteristics of human tissue structure, the design of medical ultrasonic transducers is developing towards complex sound fields. The measurement and modeling of complex ultrasonic fields are recognized as a difficult problem in the industry. Especially for the design, test and evaluation of combined array transducers and complex frequency transducers, advanced ultrasonic field automatic detection technology can save detection time and money; in addition, since the medium for propagating ultrasound in medical applications is physiological materials, it has special characteristics such as inhomogeneity and anisotropy, which puts higher requirements on the scheme and parameter design of transducers used in ultrasonic diagnosis and treatment. Therefore, it is necessary to conduct in-depth research on the physical effects of ultrasonic transducers transmitting sound fields. The traditional ultrasonic field measurement signal acquisition efficiency is low, and it cannot perform automatic signal acquisition and analysis, let alone meet the parameter evaluation and accurate modeling of complex ultrasonic fields, which restricts the design and application of complex medical ultrasonic transducers. In order to meet the requirements of complex ultrasonic transducer design and application, it is urgent to study an automatic detection and analysis system suitable for complex ultrasonic field signals to solve the many difficulties in the computational modeling and actual measurement of complex ultrasonic fields.

　　3. Solution

　　This paper designs and develops an efficient automatic acquisition and analysis system for sound field signals using the PCI-9846 high-speed digitizer of ADLINK Technology as the information acquisition center, combined with a preamplifier and a detection sensor, and developed using LabVIEW. Through an efficient data acquisition module, the sound pressure data of the three-dimensional sound field is displayed and saved in real time. Design and manufacture a four-axis precision industrial robot system driven by a stepper motor, develop an automatic control and automatic measurement system, and achieve coordination between stereo positioning and data acquisition at any part of the ultrasonic field. Develop a playback and multifunctional comprehensive analysis system for sound field measurement data, and visualize the results. Realize rapid and accurate measurement of ultrasonic transducer performance indicators, and establish a modeling, simulation and analysis system for ultrasonic radiation fields to reduce the labor intensity of measurement personnel, shorten the working time of metrological verification, and improve the standardization and standardization of ultrasonic transducer design and use and the credibility of the results.

　　3.1 Ultrasonic signal acquisition and analysis

　　1) Signal acquisition unit: Ultrasonic signal acquisition is centered on a high-speed data acquisition card, combined with a preamplifier and a signal acquisition sensor, and then the signal acquisition software on a computer platform is used to realize signal acquisition.

　　In ultrasonic signal acquisition, a hydrophone with good broadband sensitivity is used to receive microvolt-level voltage signals, and then a bandpass filter is used to select the frequency range for acquisition. After amplification by a preamplifier, the signal is preprocessed and then converted by a high-speed data acquisition card A/D to a computer for storage and display. In the acquisition process, sampling frequency and bandwidth are important indicators. The bandwidth is generally taken as the -3dB bandwidth of the frequency spectrum, or the half-power point on the power spectrum as the signal bandwidth. The bandwidth of ultrasonic signal acquisition directly affects the overall resolution, sensitivity and signal-to-noise ratio of the entire device. A large bandwidth range can make the received signal spectrum rich, with less loss of high-frequency components and less waveform distortion. In medical ultrasonic equipment, the bandwidth of the ultrasonic transmitting and receiving transducers should be used as much as possible to improve resolution, while having higher sensitivity and signal-to-noise ratio, so that the bandwidth of the transmitting and acquisition circuits is greater than the bandwidth of the ultrasonic transducer [4]. The acquired signal spectrum is determined to be below 5M. The schematic diagram of the signal acquisition scheme is shown in Figure 3-1.

　　2) Selection of main equipment

　　The high-speed data acquisition card uses ADLINK's high-speed and high-resolution digitizer PCI-9846H, which has 4 channels, 16-bit high precision, 40MS/s sampling rate, low noise and high dynamic range performance, high signal acquisition accuracy and density, and can be widely used in intermediate frequency signals, radar applications, lidar applications, ultrasonic signals and non-destructive testing. This digitizer can fully meet application requirements.

　　The hydrophone used is the Haiying ZS-500 needle hydrophone with a frequency response range of 100K-5M. Common ultrasonic signal acquisition sensors include PVDF film type and needle hydrophone [5]. Due to the low spatial resolution, edge effect and temperature limitation of the film type hydrophone, the measurement method of this study is a high-density point-by-point automatic scanning method. Therefore, the needle hydrophone is selected as the signal acquisition sensor. The diameter is less than 1mm and it has the characteristics of high sensitivity. The preamplifier uses the Pengxiang Technology PXPA Ⅳ acoustic signal acquisition amplifier. The amplifier has a bandwidth range of 15k-2M and a low noise gain of 40dB, which can fully meet the preamplification requirements of ultrasonic signal acquisition.

　　3) Ultrasonic signal analysis

　　When performing spectrum analysis on the time domain signal of ultrasonic transient, ensure that no distortion occurs in signal processing. In order to reduce the "leakage" phenomenon caused by ultrasonic signals with limited sampling length, the method of adding a time window function can effectively prevent spectrum aliasing, suppress noise, and improve frequency recognition ability. Adjust the distance between the ultrasonic transmitting transducer and the hydrophone to keep the transducer axis and the echo sound beam coaxial; adjust the sampling frequency and sampling points of the surface echo signal, and perform FFT conversion on the sampling points after analog digital data discrete processing; according to the measured waveform amplitude data, after processing, draw the frequency response curve of the load; calculate the frequency characteristic parameters of the ultrasonic transducer, such as the center frequency. The key parameters for measuring the sound field include sound pressure, sound intensity, and sound focal range, etc. The corresponding basic forms of describing the sound field mainly include axial sound pressure curve, focal plane radial sound pressure curve, and focal plane sound field. Massive data will be generated in the sound field measurement, and visualization technology needs to be used. This technology uses concrete and vivid graphics to represent complex calculation and simulation results, which deepens the understanding of data and regularity analysis, improves processing efficiency, and can analyze changes in the test process. LabVIEW visualization technology provides a powerful tool for the analysis and design of complex ultrasonic transducers [6].

　　3.2 Automatic measurement and positioning of ultrasonic field

　　The control of the entire measurement process and the positioning of the measurement points are completed by a Chengdu Haiwei Technology Cartesian coordinate robot, which organically links the motion control and data acquisition modules. On the one hand, it controls the robot arm to drive the hydrophone to perform automatic scanning movements, and on the other hand, it controls the signal acquisition module to collect signals and perform post-processing and visualization of the collected data. The entire automatic control platform is developed using the LabVIEW system, combining the control and measurement hardware, establishing a human-computer interaction interface, and completing the control of the hardware, data analysis and display. The structure of the automatic measurement control platform is shown in Figure 3-2. [page]

　　According to the needs of ultrasonic radiation field measurement and analysis, it is divided into measurement module and analysis module. The measurement function module includes the host computer control platform and the real-time data processing platform. In the comprehensive analysis platform, according to the needs of sound field description, the collected electrical signals are converted into sound pressure or sound intensity values, and the converted values ​​are projected to the corresponding collection area, and the change law of sound field distribution is intuitively displayed using visualization technology.

　　The most important part of automatic measurement is the motion control module and the data acquisition control module. The function of the motion control module and the acquisition module is to communicate with the lower computer DMC2410 four-axis motion card and the signal acquisition unit, send or extract the required information to the motion control card and the digital oscilloscope through the dynamic link library DLL provided by the hardware component, drive the X, Y, and Z axis stepper motors to move, and achieve the purpose of fast and accurate measurement. Therefore, one end of the module is connected to the DLL of the dynamic link library provided by the hardware, and can obtain the information required by the software system from the hardware system, such as the position information of the ultrasonic receiving sensor, that is, the position coordinates of the robot's X, Y, and Z axes, and the collected ultrasonic information, and transmit the information to the display output module for display and output after basic processing. Its principle block diagram is shown in Figure 3-3 below. In the design of data structure and software framework, it is necessary to consider the friendly human-machine interface, the hardware control and error correction functions should be perfect, and it also includes data display module, data visualization module, etc.

　　4. System function realization

　　The system functions are fully realized according to the above design scheme, and are realized in two steps according to automatic acquisition positioning control and signal acquisition analysis. According to the actual measurement needs, it is divided into four parts according to different functions: hydrophone positioning, single-axis scanning, three-dimensional scanning and spectrum analysis control platform.

　　The data acquisition module is the core of the system. To develop this module, you must first download the LabVIEW support function library related to PCI-9846H provided by ADLINK Technology and load it into the LabVIEW tool library (see Figure 4-1). Then you can easily compile the acquisition program like the original tool. A high-speed acquisition module for 4-channel data is implemented, and the module's built-in online observation preprocessing functions are implemented, such as multi-parameter filters, power spectrum, spectrum analysis, and measurement of parameters such as amplitude and frequency (see Figure 4-2). At the same time, the module also implements a variety of analysis methods such as time domain, high-order spectrum, short-time Fourier transform and wavelet. The collected signals can be preprocessed as needed, and synchronized in three dimensions, and then transmitted to the subsequent three-dimensional sound field automatic analysis module for modeling and analysis. This module can be used as a separate 4-channel ultrasonic signal acquisition and analysis, and the data can be stored in text and data file formats, or the stored data can be replayed for analysis. Use this module as a LabVIEW ultrasonic signal acquisition class and implant it into the corresponding data node for subsequent comprehensive analysis.

　　It realizes linear scanning motion control, plane scanning motion control, and three-axis stereoscopic scanning space motion control, and collects data point by point in a one-step-one-stop motion mode. The sensor can be quickly moved to the area where it needs to be collected, and then the data is read from the data node of the above signal acquisition function module, the change of the electrical signal at the location of the sensor is observed, and the sound pressure value at the point is quantitatively determined. The parameter control panel of the sound field automatic positioning function module is shown in Figure 4-3.

　　Single-axis scanning, three-dimensional scanning and spectrum analysis software are special software for measuring the spatial distribution and time-frequency characteristics of the sound field. Single-axis scanning can display the distribution curve of the sound pressure or sound intensity along the straight line direction of each measurement point in the X, Y, and Z directions. By comparing with the theoretical value of the sound field distribution in each axis of the design, the performance of the ultrasonic transducer can be quickly evaluated.

　　The 3D scanning software can depict in detail the distribution of the sound field in the entire 3D space and the XY, XZ, and YZ planes. Due to its high spatial resolution, it can display tiny changes on the plane. Through the correspondence between different sound pressures or sound intensities and colors, the changing laws of the sound field in space can be intuitively seen, providing researchers and engineers with a reliable and accurate analysis method. The application interface is shown in Figures 4-5 and 6.

　　Spectral analysis uses discrete Fourier transform to process the collected acoustic field electrical signals. The spectrum analysis method can be used to study the frequency domain distribution of high-frequency ultrasonic signals (see Figure 4-6), to fully understand the ultrasonic field, and is also an important indicator for measuring transducer performance. [page]

　　The system integrates precision robots, hardware and various software subsystems to form an automatic sound field measurement system. In actual measurement, its high spatial resolution and real-time accuracy can be used to accurately measure the sound field characteristics and frequency characteristics of ultrasonic transducers, and provide effective data for the establishment of theoretical models.
 

　　5. Verification of system functions

　　In order to verify the main functions of the system, a standard concave spherical shell self-focusing ultrasonic transducer was selected and driven by a continuous wave ultrasonic power source to conduct a verification test of the sound pressure distribution parameters of the ultrasonic field. The geometric parameters of the transducer design are: the outlet radius of the transducer radiation surface r=30mm, the radius of curvature of the spherical shell R=90mm, the radiation center frequency f=1.3MHz, and the sound speed of the water medium is 1500m/s. The key parameters for measuring the sound field include sound pressure, sound intensity and sound focal range, etc. The basic forms of describing the sound field are mainly the axial sound pressure curve, the focal plane radial sound pressure curve, the focal plane sound field surface diagram, and the focal spot three-dimensional stereogram.

　　After testing, the sound pressure curve of the transducer axis shows the law of the change of the sound pressure amplitude on the axis with the distance. The initial position of the hydrophone is 40mm from the center axis of the sound beam to the transducer, and it ends at 140mm after scanning point by point. The spacing between each point is 0.5mm. The results of the verification of the measured value and the theoretical value are shown in Figure 5-1:

　　Figure 5-1 Theoretical and measured values ​​of axial sound pressure distribution; (a) Actual measured value of the system in this paper (b) Comparison of simulation value and measured value of the theoretical model of focused ultrasound field

　　Figure 5-1 (a) is the actual measured data value of the system developed in this paper. This data point is obtained by testing with a robot single-axis scanning method. Figure 5-1 (b) is the result of comparing the theoretical value of sound pressure calculated by modeling the axial sound pressure distribution model of the classical sound field with the measured value. The fluctuating curve is the actual measured value, and the smooth solid line is the theoretical model value. From the model, it can be seen that the sound pressure distribution outside the focus shows low-energy oscillation and gradually decays, and the attenuation amplitude in the near field and far field is not symmetrical. The test results show that the measured value and the theoretical value fit well at the main peak, but at the side lobe, the measured sound pressure value is too large, mainly because the high-frequency ultrasonic transducer generates complex harmonics under the excitation of continuous pulses, the hydrophone and ADLINK 9846H digitizer have a wide response frequency range, and the energy generated by the superposition of multiple harmonic frequency components detected, while the theoretical model and manual oscilloscope detection are fixed at a core frequency, making the information incomplete.

　　Since the transmitting transducer is a concave spherical shell, the propagating wavefront is also a spherical surface. The sound pressure curve of a diameter in the focal plane can be measured. As a characteristic of the ultrasonic field distribution, it reflects the change law of the radial sound pressure amplitude with distance. The scanning interval between each point is 0.05mm. The experimental results are shown in Figure 5-2.

　　Figure 5-2 Comparison between the theoretical and measured values ​​of the radial sound pressure distribution on the focal plane; (a) Actual measured values ​​of the measurement system (b) Comparison between the simulation and measured values ​​of the theoretical model of the focused ultrasound field

　　Figure 5-2 (a) shows the actual measured data points of the system in this paper, which are measured by a single-axis scanning method. Figure 5-2 (b) shows the comparison between the values ​​calculated by classical sound pressure theory modeling and the actual values ​​in the radial direction of the focal plane. The fluctuating curve is the actual measured value, and the smooth solid line is the theoretical simulation value. It can be seen that the sound pressure distribution in the focal plane is oscillating and symmetrically distributed along the center point. From the comparison between the theoretical value and the actual measured value in Figure 5-2 (b), it can be seen that the main peak is well fitted, but at the side lobe, the actual measured value is rich. The reason is still that the ultrasonic transducer has complex harmonics, and the hydrophone and ADLINK 9846H digitizer have a wide frequency response, which can collect harmonic energy. The theoretical model is only a model established for the core frequency.

　　On the focal plane, the sound pressure value of each point in the sound field is measured sequentially according to the specified scanning path, and the scanning step is 0.5mm. The experimental results and the theoretical model of the focal plane are shown in Figure 5-3 below:

　　Figure 5-3 Actual measured value (a) of the sound pressure distribution in the focal plane of the test transducer and the simulated value of the sound field theoretical model

　　Figure 5-3 (a) is the focal plane data point based on the actual measurement of the precision robot measurement and analysis system. The data point is tested and saved by the robot sound field scanning motion software and processed by the ultrasonic sound field analysis software. Figure (b) is the theoretical value of sound pressure calculated by modeling on the focal plane. From the two-dimensional model of the focal area, it can be seen that the energy of the sound pressure on the focal plane is relatively concentrated, the sound pressure is oscillating and attenuated along the radial direction, the energy distribution in the non-focal area is very low, and the sound field of the single-frequency concave spherical shell is symmetrically distributed along the main axis of the ultrasonic transducer in the focal plane. Comparing the actual value with the theoretical value, it can be seen that the fitting is very good at the main peak in the focal area, and the actual measurement value is richer in the non-focal area, showing the good broadband frequency response of ADLINK PCI-9846H, making the collected data more in line with the actual situation than the single-frequency theoretical model, which is of great significance to the transducer design parameters and manufacturing process and product quality evaluation and safe use.

　　6 Conclusion

　　Through the verification test, it can be seen that the ultrasonic signal acquisition and analysis system centered on the ADLINK 9846 high-performance digitizer can meet the application of automatic measurement and modeling of ultrasonic fields, making the accuracy, speed, and parameter measurement of signal acquisition much better than before. This paper takes the ADLINK PCI-9846 high-speed digitizer as the center, combines the preamplifier and the hydrophone, and develops an efficient sound field signal acquisition system. Through the efficient data acquisition module, the sound pressure data of the three-dimensional sound field is displayed and saved in real time. Through the motor-driven four-axis precision industrial robot system, the automatic control and automatic signal acquisition and measurement system are realized, and the coordination between the stereo positioning of any part of the ultrasonic field and the automatic positioning control of the measurement point and data acquisition is realized. The playback of sound field measurement data and the multi-functional comprehensive analysis system and the visual analysis and display of the three-dimensional sound field are realized.

　　The ultrasonic field automatic measurement system accurately and reliably measures the distribution parameters and frequency characteristics of the sound field. The measurement results show that the measurement points can achieve a high spatial resolution, can accurately and quickly complete the sound field detection, and make the collected ultrasonic field signal spectrum wider. The accuracy of the measurement function has been verified through verification tests. Through the visual graphic display, the data analysis is more intuitive, comprehensive and accurate, providing effective parameters for the performance evaluation of ultrasonic transducers and the modeling of the biological effects of ultrasonic fields on the human body.