Lamb wave coding and decoding method based on two-state keying code division multiple access

Abstract

The invention discloses a two-state keying code division multiple access-based Lamb wave coding and decoding method, which comprises the following steps of: 1) determining corresponding signal parameters according to the arrangement position of a sensor node and the material characteristics and size parameters of a monitored object; 2) performing primary modulation on the spreading code sequence to generate a code division multiple access (CDMA) Lamb wave signal for defect monitoring; 3) exciting and collecting CDMA Lamb wave signals by the sensor nodes at different positions, evaluating signals on corresponding paths, and generating damage indexes; 4) coding the damage index, and modulating the spreading code sequence again to generate a CDMA Lamb wave signal carrying damage information; and 5) the sensing node transmits the CDMA Lamb wave signal carrying the information to a downstream sensor node, and the downstream sensor node decodes the CDMA Lamb wave signal to obtain transmitted sound data. According to the invention, Lamb wave signals are divided into defect monitoring signals and communication signals, and integration of signal communication and damage detection based on ultrasonic Lamb waves is realized.

Description

Technical Field

[0001] The Lamb wave encoding and decoding method based on two-state keying CDMA belongs to the field of ultrasonic guided wave structural health monitoring, especially for integral wall panel structures such as aircraft skin. The distributed wireless sensor network constructs ultrasonic Lamb wave signals containing coded information through this method to realize structural health monitoring and acoustic data communication of the integral wall panel structure of the aircraft skin. Background Art

[0002] Aerospace vehicles serve under high-speed variable load conditions and extreme environments for a long time. The large metal integral wall panel structures used in their wings, fuselages and fuel tanks, in addition to bearing fatigue loads, are also faced with high-speed impacts from external particles (such as hail, raindrops and dust), resulting in frequent defects such as skin impact damage and fatigue cracks, which seriously threaten the structural integrity and flight safety of the aircraft.

[0003] Distributed sensor monitoring networks have real-time sensing, collection and processing functions, and are widely used in aerospace and other fields for structural health monitoring. In practical applications, active Lamb wave and piezoelectric structural health monitoring networks can effectively evaluate the structural health of aircraft. Existing Lamb wave sensor networks still have the following shortcomings: (1) The sensor network is powered by long cables or batteries. A large number of power cables increase the weight of the structure, and it is difficult to perforate and wire for complex sealed aircraft structures; (2) Although electromagnetic waves can be used between nodes in the sensor network to achieve wireless data transmission, it may cause interference to the aircraft's onboard equipment and increase the complexity of the sensor network.

[0004] In view of the above shortcomings, the present invention discloses a Lamb wave encoding and decoding method based on two-state keying CDMA, which uses Lamb waves as a dual medium for structural health monitoring and communication transmission. The sine signal modulated by the Hanning window is used as the carrier signal, and a spread spectrum code sequence with orthogonal characteristics is used for modulation. The modulated signal is used as the excitation signal for structural health monitoring, and the damage information is encoded, transmitted and decoded, which can effectively solve the shortcomings of the existing Lamb wave sensing network. Summary of the invention

[0005] The purpose of the present invention is to solve the problem of using a spread spectrum code sequence to construct a Lamb wave excitation signal in a Lamb wave sensor network to achieve the following Figure 2 The structural health monitoring and wireless data transmission between sensor network nodes shown in the figure, in order to achieve the above purpose, the present invention implements a Lamb wave encoding and decoding method based on two-state keying CDMA, the method mainly comprises the following steps,

[0006] 1) According to the material parameters, dimensions and sensor layout information of the flat plate, the equation of the Lamb wave group velocity dispersion curve is calculated to obtain the signal parameters of each Lamb wave mode without aliasing.

[0007] 2) According to the number of sensor nodes in the sensor network, the number of shift register levels of the spread spectrum code sequence and the number of cycles of the code chip sequence, the spread spectrum code sequences such as Gold sequence, Kasami sequence and Walsh sequence that meet the parameter requirements are calculated and determined, the sinusoidal signal modulated by the Hanning window is used as the carrier signal, and the spread spectrum code sequence is modulated once by phase modulation, and the signal combination with the smallest mutual correlation coefficient is selected as the CDMA signal.

[0008] 3) The CDMA signal is distributed to different sensor nodes. Different sensor nodes are stimulated in turn. The sensor receiving node collects the CDMA Lamb wave signal, uses the wave number filtering method to extract the Lamb wave single mode signal, and performs dispersion compensation according to the following formula.

[0009]

[0010] Where D is the propagation distance of the Lamb wave signal; y(t,D) is the Lamb wave signal with a propagation distance of D; W W {·} is the warp frequency transform; F{·} is the Fourier transform; K is the normalization parameter. w(f) is the warp map; It refers to the capacity conservation operator based on the first-order differential of the bending map;

[0011] 4) By performing cross-correlation calculation on the compensated Lamb wave signal and the CDMA signal, the emission source of the signal can be identified, the structural state on the corresponding path can be evaluated and a damage index can be generated. Then, the CDMA signal is modulated again to obtain a Lamb wave communication signal carrying damage information.

[0012] 5) After the downstream sensor node has completely collected the CDMA signal modulated twice in the sensor node excitation step (4), it first aligns the signal in the time domain and uses the CDMA signal for matched filtering to obtain a binary data string, and finally demodulates it through the threshold judgment method to recover and extract the transmitted information.

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] Currently, the transmission of Lamb wave data is mainly carried out through cables. Due to the limitations of the working environment, the installation of cables in aerospace will destroy the structural integrity and increase the difficulty of installing the sensor network. In addition, the amplitude of the ultrasonic signal will also pose a threat to the safety of the aircraft. Compared with the traditional Lamb wave defect monitoring signal, this method has the characteristics of small excitation amplitude, strong anti-interference ability and high sensitivity. It can realize structural state monitoring, multi-node identification and acoustic data transmission, and is suitable for application in engineering practice. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 This is a schematic diagram of the CDMA excitation signal modulation process.

[0016] Figure 2 Schematic diagram of sensor network nodes in metal siding.

[0017] Figure 3 Schematic diagram of the layout of sensor network nodes in the tablet.

[0018] Figure 4 This is a schematic diagram of code division multiple access excitation signal modulation.

[0019] Figure 5 This is a schematic diagram of the demodulation process of the code division multiple access excitation signal.

[0020] Figure 6 Schematic diagram of constructing Lamb wave excitation signals for sensor network spread spectrum code sequences. DETAILED DESCRIPTION

[0021] The Lamb wave encoding and decoding method based on two-state keying CDMA provided by the present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0022] The purpose of the present invention is to solve the problem of using a spread spectrum code sequence to construct a Lamb wave excitation signal in a Lamb wave sensor network to achieve the following Figure 2 The structural health monitoring and wireless data transmission between sensor network nodes shown in the figure, to achieve the above-mentioned purpose, the present invention implements a Lamb wave encoding and decoding method based on two-state keying CDMA.

[0023] like Figure 3 As shown, according to the plate material parameters, size and Figure 3 The sensor layout information shown in (a) is calculated Figure 3 (b) The Lamb wave group velocity dispersion curve equation shows that the signal duration of each Lamb wave mode without aliasing is 0.1ms.

[0024] like Figure 4 As shown, according to the number of sensor nodes in the sensor network, the number of shift register stages of the spread spectrum code sequence and the number of cycles of the chip sequence, we can calculate and obtain Figure 4 (a) and Figure 4 (b) is the spreading code sequence, Figure 4 The sine signal modulated by the Hanning window of (c) is used as the carrier signal, and the spread spectrum code sequence is modulated once by phase modulation to obtain the following: Figure 4 (d) shows the CDMA signal.

[0025] The CDMA signal is distributed to different sensor nodes, and different sensor nodes are stimulated in turn. The sensor receiving node Figure 5The CDMA Lamb wave signal shown in (a) is collected, and the Lamb wave single mode signal is extracted using the wave number filtering method, and the following is obtained: Figure 5 (b) shows the compensated CDMA Lamb wave signal.

[0026] By performing cross-correlation calculation on the compensated Lamb wave signal and the CDMA signal, the signal's transmitting source can be identified, the structural state on the corresponding path can be evaluated and a damage index can be generated. The CDMA signal is then modulated again to obtain a Lamb wave communication signal carrying damage information.

[0027] The sensor node excites the CDMA signal modulated twice in step (4). After the downstream sensor node has completely collected the signal, it first aligns the signal in the time domain and uses Figure 5 The CDMA signal shown in (c) is matched filtered to obtain a binary data string, and finally Figure 5 The signal shown in (d) is demodulated by threshold judgment, and the signal is recovered and extracted. Figure 5 (e) Information Transmitted.

Claims

1. A Lamb wave encoding and decoding method based on binary keying code division multiple access, characterized in that: The implementation process of this method is as follows: Step 1) Determine the corresponding signal parameters according to the arrangement position of the sensor node, the material characteristics and size parameters of the monitored object; Step 2) Modulate the spread spectrum code sequence once to generate a code division multiple access CDMA Lamb wave signal for defect monitoring; Step 3) Sensor nodes at different positions stimulate and collect CDMA Lamb wave signals, evaluate the signals on the corresponding paths, and generate damage indicators; Step 4) Encode the damage indicators and modulate the spread spectrum code sequence again to generate a CDMA Lamb wave signal carrying damage information; Step 5) The sensor node transmits the CDMA Lamb wave signal carrying the information to the downstream sensor node, and the downstream sensor node decodes to obtain the transmitted sound data.

2. The Lamb wave encoding and decoding method based on binary keying code division multiple access according to claim 1 is characterized in that: In step 1), the Lamb wave group velocity dispersion curve equation is calculated based on the plate material parameters, dimensions and sensor layout information to obtain the signal parameters of each Lamb wave mode without aliasing.

3. The Lamb wave encoding and decoding method based on binary keying code division multiple access according to claim 1 is characterized in that: In step 2), according to the number of sensor nodes in the sensor network, the number of shift register levels of the spread spectrum code sequence and the number of cycles of the code chip sequence, the Gold sequence, Kasami sequence and Walsh sequence spread spectrum code sequences that meet the parameter requirements are calculated and determined, the sinusoidal signal modulated by the Hanning window is used as the carrier signal, and the spread spectrum code sequence is modulated once by phase modulation, and the signal combination with the smallest mutual correlation coefficient is selected as the CDMA signal.

4. The Lamb wave encoding and decoding method based on binary keying code division multiple access according to claim 1 is characterized in that: In step 3), the CDMA signal is distributed to different sensor nodes, and the different sensor nodes are stimulated in turn. The sensor receiving end node collects the CDMA Lamb wave signal, uses the wave number filtering method to extract the Lamb wave single mode signal, and performs dispersion compensation.

5. The Lamb wave encoding and decoding method based on binary keying code division multiple access according to claim 1 is characterized in that: In step 4), the Lamb wave signal after dispersion compensation and the CDMA signal are cross-correlated to identify the signal transmission source, evaluate the structural state on the corresponding path and generate a damage index, and then the CDMA signal is modulated again to obtain a Lamb wave communication signal carrying damage information.

6. The Lamb wave encoding and decoding method based on binary keying code division multiple access according to claim 1 is characterized in that: In step 5), the sensor node stimulates the CDMA signal modulated twice in step 4). After the downstream sensor node fully collects the signal, it aligns the signal in the time domain and uses the CDMA signal for matched filtering to obtain a binary data string. Finally, it demodulates the signal through the threshold judgment method to recover and extract the transmitted information.

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