CN102307428A - Anti-electromagnetic interference piezoelectric interlayer - Google Patents

Abstract

The invention discloses an anti-electromagnetic interference piezoelectric interlayer, including a piezoelectric sensor, a flexible printed circuit board, and an SMA joint, wherein the piezoelectric sensor and the joint are connected to the flexible printed circuit board through welding and gluing, and the piezoelectric sensor senses a signal, and passes The wiring on the flexible printed circuit board goes to the header, and the header connects the cable to transmit the signal to the signal processing center. By reasonably setting the parallel wiring spacing of the flexible printed circuit board, the wiring method and the structure of the flexible printed circuit board, the performance of anti-crosstalk and radiation interference can be realized. The invention solves the problem that the piezoelectric interlayer is susceptible to electromagnetic interference during use, and improves the stability and reliability of the piezoelectric interlayer. This invention could facilitate the widespread use of piezoelectric interlayers in structural health monitoring.

Description

Anti-EMI piezo interlayer

technical field

The invention relates to an anti-electromagnetic interference piezoelectric interlayer, which belongs to the technical field of sensors and testing.

Background technique

At present, the structural health monitoring method based on piezoelectric sensors and Lamb wave active diagnosis technology is a typical and effective method to identify the health status of structures. In the research process of structural health monitoring systems, piezoelectric sensors are mostly pasted on the structure by hand. This method is difficult to ensure the reliability of the structure and the consistency of performance. It is inconvenient to set up the sensors when they are used in batches. The concept of "smart interlayer" solves this problem very well. Its design idea is to package the sensor in a flexible interlayer according to a certain process to form a sensor network, and use printed circuits instead of ordinary wires to connect, and finally output through a standard interface; The piezoelectric interlayer designed and manufactured according to the idea of intelligent interlayer is composed of a piezoelectric sensor, a flexible printed circuit board, and a joint. The piezoelectric sensor and the joint are welded on the flexible printed circuit board, and the piezoelectric sensor passes the signal it feels The wiring on the flexible printed circuit board goes to the header, and the header connects the cable to transmit the signal to the signal processing center. The piezoelectric interlayer made by this method can be directly arranged on the structure, which is convenient for application, and can ensure the consistency of the influence of the production process on the performance of the piezoelectric sensor, and the influence of the circuit on the structure is small. The piezoelectric interlayer can be used for different application objects. Design in 3D.

However, in practical engineering applications, there are the following problems: there are many parallel lines in the internal signal lines of the piezoelectric interlayer, and the excitation signals used in the structural health monitoring method based on piezoelectric sensors and active Lamb waves usually have a frequency range of 15KHz to 500KHz. The five-peak sinusoidal modulation signal is amplified by the power amplifier and becomes a high-frequency alternating current with a large amplitude. When it propagates on the excitation signal line, it is easy to couple to the nearby sensing signal line, which makes the collected sensing signal mixed. The crosstalk from the excitation signal line is changed, and the characteristics such as the amplitude and energy of the sensing signal are changed. The greater crosstalk may cause the charge amplifier in the subsequent processing circuit to saturate, and the useful signal will be lost; at the same time, the piezoelectric interlayer usually works under high-frequency radiation In the environment, the collected signal has large high-frequency interference, which makes the waveform characteristics of the time-domain signal blurred and the spectral lines in the frequency domain increase, which will affect the reliability of data processing and increase the difficulty of data processing.

Contents of the invention

The purpose of the present invention is to overcome the defect that the existing piezoelectric interlayer is easily subjected to electromagnetic interference and has low reliability during the application process. The invention provides an anti-electromagnetic interference piezoelectric interlayer, which reduces the influence of electromagnetic interference on the signal of a piezoelectric sensor and improves the application reliability of the piezoelectric interlayer by performing an optimized anti-electromagnetic interference design on a flexible printed circuit board.

In order to achieve the above object, the present invention adopts the following technical solutions:

(1) Principle of anti-electromagnetic interference and optimal design of flexible printed circuit board anti-electromagnetic interference

Crosstalk and radiated interference are two main EMI coupling ways of piezoelectric interlayer signals.

Crosstalk is an electromagnetic phenomenon of parallel lines inside the piezoelectric interlayer. The crosstalk between parallel lines is generated by capacitive and inductive coupling. Figure 1 shows the model and equivalent circuit of crosstalk coupling between parallel lines.

As shown in Figure 1(a) and (b), the capacitive coupling model and schematic diagram, the energy in one wire is coupled to the other wire through parasitic capacitance, and the coupling coefficient is:

$\mathrm{<span\; class="notranslate">\; CC</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; jw</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 12</span>}}\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="R"\; filenames="HDA0000084190700000012.png"\; state="\{\{state\}\}">R</figure-callout></span>}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; jw</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="12"\; label="C"\; filenames="HDA0000084190700000021.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 12</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>\mathrm{<span\; class="notranslate">\; R</span>}}<span\; class="notranslate">\; \&ap;</span>\frac{{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 12</span>}}}{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="12"\; label="C"\; filenames="HDA0000084190700000021.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 12</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; )</span>$

In the formula, U _N is the coupling voltage, C ₁₂ is the parasitic capacitance, C ₂ is the ground impedance of the wire, and R is the subsequent circuit impedance of the sensitive circuit. Since the piezoelectric interlayer is in the application process, the subsequent circuit of the sensitive source is a charge amplifier. So R is high impedance.

As shown in Figure 1 (c), (d) is the inductive coupling model and schematic diagram, the change of the current on the wire causes the surrounding magnetic field to change, and the electromotive force is induced by the adjacent wire, so that the signal on one wire will be coupled to another wire. inductive voltage:

In the formula, M is the mutual inductance coefficient per unit length, and l is the length of the coupling section.

In formulas (1) and (2), the mutual capacitance and mutual inductance values per unit length of parallel wires with a diameter of d and a spacing of D are:

C＝0.0885π× _εr /(arccos(D/d))pF/cm (3)

M＝0.001(ln(1+(2h/D) ² )mH/cm (4)

From the above analysis, it can be concluded that the impact of crosstalk on the piezoelectric interlayer signal is proportional to the length, mutual capacitance, and mutual inductance of parallel wires.

According to the formulas (3) and (4), the accompanying drawing 2 shows the influence of the spacing on the mutual capacitance and mutual inductance of the parallel wires, taking d=0.5mm, ε _r =3.4 (polyimide). It can be seen from the figure that when the wire spacing is within 1-6mm, the mutual capacitance and mutual inductance values decrease significantly with the increase of the distance, and when the spacing is greater than 10mm, the mutual capacitance and mutual inductance values decrease slowly. The spacing between parallel wires is designed to be 10mm. At this time, the mutual capacitance value is less than 0.25pF/cm, and the mutual inductance value is less than 0.1nH/cm.

In order to reduce capacitive coupling, in addition to reducing the length of parallel wires and setting the spacing between parallel wires reasonably, shunting can also be considered, that is, a grounded conductor is added between the interference and sensitive lines. The coupling coefficient expression is shown in formula (5):

$\mathrm{<span\; class="notranslate">\; CC</span>}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; N</span>}}}{{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; S</span>}}}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; jw</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="Z"\; filenames="HDA0000084190700000012.png,HDA0000084190700000013.png"\; state="\{\{state\}\}">Z</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; jw</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="C"\; filenames="HDA0000084190700000012.png,HDA0000084190700000013.png"\; state="\{\{state\}\}">C</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}<span\; class="notranslate">\; \&times;</span>\frac{\mathrm{<span\; class="notranslate">\; jw</span>}{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="1"\; label="Z\; j"\; filenames="HDA0000084190700000012.png"\; state="\{\{state\}\}">Z</figure-callout></span><figure-callout\; id="1"\; label="Z\; j"\; filenames="HDA0000084190700000012.png"\; state="\{\{state\}\}">\; </figure-callout>}}_{\mathrm{<span\; class="notranslate">\; </span>}}}{\mathrm{<span\; class="notranslate">\; 1</span>}<span\; class="notranslate">\; +</span>\mathrm{<span\; class="notranslate">\; jw</span>}<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; j</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; Z</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; )</span>{\mathrm{<span\; class="notranslate">\; C</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}<span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; 5</span>}<span\; class="notranslate">\; )</span>$

In the formula, C ₁ is the parasitic capacitance of the interference source wire and the shunt conductor, C ₂ is the parasitic capacitance of the conductor and the sensitive line, and Z _j is the ground impedance of the conductor. It can be seen from this that if the conductor has good conductivity and is well grounded, the Z _j value is very small, and the second term of the formula is less than 1, that is, the coupling voltage value is greatly reduced due to the shunt effect of the intermediate conductor.

Radiation coupling is the transmission of electromagnetic energy from an interfering source to a sensitive source through space in the form of an electromagnetic field. When electromagnetic waves irradiate the transmission line, the distributed small voltage sources induced along the way can be divided into common mode and differential mode components. In the piezoelectric interlayer application environment, it is generally the far-field coupling mode, and the length of the wire is l>λ/4, then the voltage induced on the wire is:

Among them, E is the incident electric field, l is the length of the wire, θ is the angle between the incident electric field vector and the wire, λ is the wavelength, h is the distance from the wire to the ground, α is the angle between the incident direction of the electromagnetic field and the plane where the circuit is located, b is the spacing between parallel wires, b, h□λ.

From the above analysis, it can be concluded that under the premise that the relative position of the radiation field and the piezoelectric interlayer is fixed, the size of the radiation interference is proportional to the length, spacing and distance to the ground of the parallel wires. Reduce radiation interference by reasonably determining these parameters. In order to reduce radiation interference, the spacing and length of parallel wires should be reduced, so that the wires are as close to the ground as possible, and the loop area is reduced. These are the same conditions as reducing crosstalk. At the same time, the most effective way to suppress radiation interference in the far area is the electromagnetic shielding of good conductors. When the magnetic field passes through a good conductor, eddy currents are generated. The reverse magnetic field generated by the eddy currents cancels the original magnetic field, and at the same time strengthens the magnetic field on the side of the conductor, so that the interference magnetic field lines are in the conductor. By bypassing the sides, the purpose of shielding the high-frequency magnetic field is achieved. The size of the eddy current directly affects the shielding effect, which shows that conductive materials are suitable for shielding high-frequency magnetic fields. The shielding of high-frequency magnetic field (greater than 100KHz) radiation coupling usually uses low-resistivity good conductor materials such as copper and aluminum.

(2) Combination process of flexible printed circuit board, piezoelectric sensor and joint

The electrical and physical connection between the piezoelectric sensor and the flexible printed circuit board can be realized by using high-reliability conductive silver glue; the electrical and physical connection can also be realized by welding. At the same time, in order to ensure reliability and aesthetics, the The piezoelectric sensor is welded on the flexible printed circuit board, and after the welding is completed, glue is evenly covered around the piezoelectric sensor.

The headers are soldered to the flexible printed circuit board.

Beneficial effect

The present invention has the following advantages: (1) An optimized design for anti-electromagnetic interference is provided, which greatly reduces the influence of the electromagnetic environment on the piezoelectric interlayer during application, reduces the difficulty of data processing, and has a wider application range; The crosstalk of the electrical interlayer is greatly reduced, which solves the problem of data failure that may occur. (2) The leads are simplified. (3) The high reliability of the piezoelectric interlayer is realized through an appropriate connection process.

Description of drawings

Figure 1 is the model and equivalent circuit of crosstalk and radiation interference coupling between parallel lines; (a) capacitive coupling model, (b) equivalent circuit, (c) inductive coupling model, (d) equivalent circuit;

Figure 2 is the relationship between the mutual capacitance, mutual inductance and spacing of parallel wires;

Figure 3 is the wiring diagram of the anti-electromagnetic interference piezoelectric interlayer flexible printed circuit board

Fig. 4 is a top view and a structural sectional view of the anti-electromagnetic interference piezoelectric interlayer;

In the figure, 1 is a flexible printed circuit board, 2, 3, and 4 are three piezoelectric sensor pads, 5 is the positive electrode of one of the piezoelectric sensor pads, and 6, 7, and 8 are three signal input/output Connector 9 is the ground level of one of the piezoelectric sensor pads, 10, 11, and 12 are three piezoelectric sensors, and 13, 14, and 15 are three SMA connectors.

Detailed ways

Below in conjunction with accompanying drawing, the technical scheme of invention is described in detail:

The piezoelectric interlayer is composed of piezoelectric sensors, flexible printed circuit boards, and joints. The piezoelectric sensors and joints are welded on the flexible printed circuit board. The piezoelectric sensor transmits the sensed signal to the joints through the lines on the flexible printed circuit board. , the connector connects the cable to transmit the signal to the signal processing center. Figure 3 shows the wiring diagram of the anti-electromagnetic interference piezoelectric interlayer flexible printed circuit board. The size of the flexible printed circuit board size 1 is 336×30mm, and three circular piezoelectric sensor pads 2 are arranged at equal intervals (150mm) on it. , 3, 4, wherein the semicircular pad 5 is the positive pole, which is respectively connected to the signal input/output connectors 6, 7, 8 arranged at one end of the flexible printed circuit board at equal intervals, so as to minimize the length of the parallel wires and increase the Large spacing, the spacing between adjacent parallel traces is 10mm; the other pad 9 is grounded. As shown in Figure 4, it is a top view and a structural cross-sectional view of the anti-electromagnetic interference piezoelectric interlayer. The piezoelectric sensors 10, 11, and 12 are respectively welded on the pads 2, 3, and 4, and the SMA connectors 13, 14, and 15 are respectively welded on the pads 6, 15. 7, 8 on. The piezoelectric interlayer flexible printed circuit board has a double-sided structure, the top layer is a shielding layer, and the shielding material is thin-film copper with a thickness of 35.56 μm, which plays an electromagnetic shielding role; the bottom layer is a signal layer, and the signal layer is covered with copper and grounded to maximize The distance from the wire to the ground and the loop area are reduced.

The piezoelectric sensor is welded on the flexible printed circuit board through solder paste, and after the welding is completed, epoxy glue is uniformly covered around the piezoelectric sensor. The headers are soldered to the flexible printed circuit board.

The flexible printed circuit board material is polyimide material with good flexibility.

The piezoelectric sensor is PSN33 with a diameter of 8 mm and a thickness of 0.48 mm.

The coaxial cable connector is an SMA connector.

The solder paste for soldering is equipped with a certain proportion of silver paste.

The adhesive is 353ND.

Claims (3)

1. An anti-electromagnetic interference piezoelectric interlayer is characterized in that it is made up of three parts: a piezoelectric sensor, a flexible printed circuit board, and a joint, wherein the piezoelectric sensor is connected to the flexible printed circuit board by welding, gluing or a combination of two methods The electrical and physical connection of the connector is welded on the flexible printed circuit board, and the piezoelectric sensor senses the signal, which is transmitted to the connector through the circuit on the flexible printed circuit board, and the connector is connected to the coaxial cable to transmit the signal to the signal processing center; the piezoelectric interlayer is flexible The printed circuit board is a double-sided structure, the top layer is a shielding layer, and the shielding material is thin-film copper; the bottom layer is a signal layer, and the signal layer is surrounded by copper and grounded, and the distance between parallel wires is in the range of [6mm, 20mm]. 2. The anti-electromagnetic interference piezoelectric interlayer according to claim 1, characterized in that the piezoelectric sensor is made of PZT material with piezoelectric properties. 3. The anti-electromagnetic interference piezoelectric interlayer according to claim 1, characterized in that the joint is a coaxial cable joint.

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