Design Method of Parametric Array Test System Based on LabVIEW

introduction

The Parametric Acoustic Array is an acoustic emission device that utilizes the nonlinear characteristics of the medium and uses two high-frequency initial waves propagating in the same direction to obtain difference frequency, sum frequency, and multiple frequency in the far field. According to the principle of sound absorption in the medium, absorption is proportional to the square of the signal frequency. During the propagation of sound waves, high-frequency signals such as sum frequency and multiple frequency decay quickly. After a certain distance, only the low-frequency difference frequency signal remains. Compared with conventional transducers, firstly, the difference frequency signal has better directivity; secondly, the difference frequency signal has almost no side lobes, avoiding interference and complexity of signal processing caused by uneven boundaries during shallow sea bottom or sediment detection; thirdly, the difference frequency signal has a bandwidth greater than 10kHz, high spatial resolution, anti-reverberation, and can obtain higher signal processing gain.

Based on the above advantages, parametric arrays have broad application prospects in underwater detection, underwater communications and other fields. For example, abroad, the SES-96 and SES-2000 series parametric array depth sounders/shallow bottom profilers produced by the German INN0-MAR company are currently widely used in shallow sea underwater detection. The beam angle of the SES-96 low frequency is ±1.8°, and the maximum penetration depth is 50m; in China, the parametric array "levee hidden danger monitoring sonar" successfully developed by the East China Sea Research Station of the Chinese Academy of Sciences can detect and identify the bottom of rivers, lakes and seabed sediments or detect and evaluate the degree of levee damage. In addition, the patented parametric speaker product developed by the American technology company, the Hypersonic Sound System (HSS), realizes the directional propagation of sound in the air.

However, the current parametric array technology is not mature, and no unified international standard or industry specification has been formed. This paper aims to make some preliminary exploration and research on the application of acoustic parametric array in air, in preparation for the application of acoustic parametric array technology in underwater acoustic detection.

1 Acoustic parametric array theory and transducer array design

1.1 Acoustic Parametric Array Theory

Assuming that the frequencies of the two high-frequency initial sound wave signals are ω1 and ω2 respectively (assuming that ω1>ω2), the signal forms a difference frequency signal (ω1-ω2), a sum frequency signal (ω1+ω2), a multiplied frequency signal (2ω1 and 2ω2) and the original signal (ω1 and ω2) due to the nonlinear effect of the medium during propagation, which can be expressed as follows:

Where: ei (i=1, 2,…, 6) is a dimensionless parameter.

Since the high-frequency initial acoustic wave signals ω1 and ω2 can be made very close, the frequency of the difference frequency signal (ω1-ω2) is very low. The difference frequency signal has a strong sediment penetration ability and can be used to detect the shallow bottom structure of the seabed, while the reflected main frequency signal can be used for accurate water depth measurement. In addition, the original wave frequency is high, and the transducer can be made very small, which can not only reduce the volume of the transmitter, but also detect smaller objects. The intensity of the generated difference frequency signal is slightly higher than that of the original wave, decays slowly, and is very close to the beam angle at high frequency, and has no side lobes, so its beam directivity is good and has a higher resolution. At the same time, the controllable difference frequency acoustic wave signal can carry more sediment layer information, so as to classify and identify targets buried in the sediment layer.

1.2 Transducer array design

The transducer here refers to an electroacoustic transducer, that is, a device used to realize the mutual conversion of energy between electrical energy and acoustic energy. Since the directivity of a single transducer is poor (or even has no directivity), and the transmission power of a single transducer is not large. Therefore, the method of using an array is considered, that is, a number of transducers are arranged in an array according to a certain pattern. This not only increases the transmission power, but also reduces the directional side lobes of the beam formed by the array, and the directivity is greatly improved, thereby greatly improving the positioning, orientation and speed measurement of the target. At the same time, with the increase of transmission power, the spatial processing gain and the signal-to-noise ratio at the input end of the receiving array are improved, and the system's operating range is increased, and the requirements for the directivity of a single transducer are also reduced, making it easier to implement.

When designing the transducer array, a variety of arrangements and combinations can be used, such as rectangular array, hexagonal array, circular array, etc. This system uses 9 circular piezoelectric ceramic transducers to form a 3×3 rectangular array to transmit ultrasonic signals, and uses 4 microphones to receive echoes. As shown in Figure 1, transducers 1, 3, 7 and 9 form one channel, and the remaining 5 transducers form another channel.

1.3 Transmission method of parametric array

There are two types of parametric array transmission methods: single-channel transmission method and dual-channel transmission method. The single-channel transmission method refers to two original wave frequency signals, which are linearly added and power amplified to simultaneously excite all array elements in the transducer array; while the dual-channel transmission method refers to two original wave signals, which are power amplified and then each passes through a channel in the transducer array to excite the corresponding array element.

In comparison, the single-channel transmission mode has a simpler structure and is easy to implement, but high-power output is more difficult; while the dual-channel transmission mode has a larger output power, but the transducer array element combination is more complicated. In this system, the 3×3 rectangular array composed of 9 circular piezoelectric ceramic transducers adopts a single-channel transmission mode, that is, the carrier modulation signal is simultaneously connected to the two channels of the transducer array. [page]

2. Composition and structure design of acoustic parametric array test system

The block diagram of the system structure designed in this paper is shown in Figure 2, which mainly includes PC (LabVIEW signal processing platform), power amplifier circuit, transducer transmitting and receiving array, signal receiving circuit and data acquisition card. Among them, the PC mainly completes the generation of initial signal and high-frequency carrier, signal distortion preprocessing and subsequent processing of received signal (including real-time display of signal, spectrum analysis, etc.) through LabVIEW software; the transducer transmitting and receiving array realizes the transmission of carrier modulated signal and the reception of echo signal respectively; the power amplifier circuit is used to increase the transmission power of carrier modulated signal; and the signal receiving circuit processes the echo signal received by the microphone, including several processing links such as front-end amplification, bandpass filtering and final amplification.

2.1 Signal Processing

Signal processing is one of the key parts of this system, which mainly completes the distortion preprocessing of the input signal and the amplitude modulation of the ultrasonic carrier. The basic theory of the signal processing part is Berktay far-field solution.

2.1.1 Distortion Preprocessing

The purpose of distortion preprocessing is to enhance the strength of the signal, reduce distortion, enhance low frequencies, etc. In 1965, Berktay used the concept of envelope in modulation to propose a more precise and complete explanation for the parametric array, believing that the final demodulated signal will be determined by this envelope, that is, the demodulated signal P2(t) of the parametric array is proportional to the double differential of the square of the envelope E(t) with respect to time. According to Berktay's far-field solution, there are three main existing preprocessing methods:

The first and initial preprocessing method assumes that the envelope is E(t)=1+mg(t), where m is the modulation factor and g(t) is the audio signal. Then:

According to formula (1), it can be seen that under the action of nonlinearity, the self-demodulation of the signal can demodulate the modulation signal Ps(t) which is proportional to the envelope signal E(t); however, the self-demodulation process will be accompanied by the generation of the second harmonic distortion signal Pd(t).

A closer look at equation (1) shows that the distorted signal Pd(t) is proportional to m2, that is, reducing m can reduce distortion, but the demodulated signal Ps(t) will also decrease, resulting in reduced conversion efficiency. Therefore, there is a second preprocessing method, which is to integrate the envelope twice and then take the square root, that is:

Obviously, the single-sideband preprocessing method corresponds to generating a pure audio signal without distortion, that is, no other frequency components are generated.

2.1.2 Carrier modulation

The function of carrier modulation is to amplitude modulate the preprocessed signal with the ultrasonic carrier signal to generate an ultrasonic carrier modulation signal. Carrier modulation can be divided into double sideband (DSB) modulation and single sideband (SSB) modulation. In DSB modulation, the spectrum of the output signal is composed of upper and lower sidebands located on the left and right sides of the carrier frequency, and the modulation signal information carried by the upper and lower sidebands of the signal is exactly the same; SSB modulation is to select a sideband in DSB modulation for transmission, thereby saving half of the transmission power. Assuming the carrier frequency is 85kHz and the audio signal frequency is 5kHz, the schematic diagram of DSB and SSB modulation is shown in Figure 3.

After the input signal passes through the operational amplifier PA85, the power is increased, but the output current is small. In order to obtain a higher output current, a complementary symmetrical amplifier composed of Q1, Q2, Q3 and Q4 is connected to the output end of PA85 to increase the output current of the operational amplifier PA85. In addition, the protection circuit composed of diodes D1 and D2 can not only limit the input differential voltage of PA85 to be lower than the reverse breakdown voltage of the input transistor base-emitter, but also play a role in limiting the input instantaneous current. [page]

2.3 Signal receiving circuit design

The signal receiving circuit mainly includes a front-end amplifier circuit, a bandpass filter circuit and a final amplifier circuit, and provides power for the four microphones in the transducer receiving array, as shown in FIG5 .

The front-end amplifier circuit uses the LM324 integrated operational amplifier with the advantages of low power consumption, high gain and high reliability. The circuit realizes the addition and amplification of the four-way echo receiving signal. The bandpass filter is composed of a high-impedance operational amplifier (TL082) and RC resistors and capacitors. It not only plays the role of a bandpass filter, but also has the function of amplification. The final amplifier circuit adopts the structure of a typical inverting amplifier circuit, and changes the gain of the circuit by adjusting the potentiometer, so that the output amplitude of the receiving circuit meets the input requirements of the data acquisition card.

3. System LabVIEW software design

The front panel of the software system based on LabVIEW development tools is shown in Figure 6. It can instantly display the input signal, SSB output signal, and received signal, and save the data for further signal processing, such as spectrum analysis.

Things to note when programming:

After the signal is transmitted, it will be reflected back when it hits an obstacle. Therefore, the duration of each signal transmission cannot be too long, otherwise the received signal and the transmitted signal will overlap and interfere with each other. The specific duration can be determined according to the distance between the transducer transmitting array and the obstacle.
During the experiment, the power of the transmitted ultrasound is relatively large. The experimental process cannot last too long, otherwise it will affect the human body. Therefore, the data received each time is best saved in the form of a file for subsequent processing, such as spectrum analysis.

4 Conclusion

The designed system uses LabVIEW software as a platform. Compared with the traditional system, the system circuit is greatly simplified, and the input and carrier signals are adjustable, which improves the system's efficiency and can test the acoustic parametric array more comprehensively. During the experiment, when the input signal is 5kHz and the carrier frequency is 85kHz, the sound can be heard at the obstacle, and the system also receives the echo signal. In other words, the carrier modulation signal emitted by the system can be self-demodulated in the air to form a difference frequency signal, and the system can also receive the echo signal, which proves that the system design is feasible.

However, the system still has the disadvantages of low conversion efficiency of parametric transducers and short system range. Therefore, the next step will be to start from the basic theory of parametric arrays, optimize circuits, improve transducer arrays and signal distortion preprocessing algorithms, and explore effective methods to improve conversion efficiency and increase the system range, so that it can be better applied to underwater acoustic detection and other fields.