Bridge type fractal capacitance sensor

Abstract

The invention relates to a bridge type fractal capacitance sensor, and belongs to the technical field of nondestructive testing. According to the structure of the capacitance bridge, tiny cracks of flat-bottomed holes with the diameter of 5mm * 0.5 mm * 1.5 mm of an aluminum test block and the diameter of 2mm of an epoxy resin test block can be detected; the capacitance bridge forms a loop, so that the capacitance value and the capacitance change rate are increased, and the signal is more obvious; cracks in all directions can be detected through single detection, and the recognition rate and the detection sensitivity are improved; the influence of the lift-off distance can be reduced, the signal stability can be maintained, and the ringing phenomenon can be suppressed.

Description

Technical Field

[0001] The invention relates to a bridge type fractal capacitance sensor, belonging to the technical field of nondestructive testing. Background Art

[0002] In order to ensure the integrity and safety of the structure and prevent structural failure or accidents, cracks are one of the common damages. Therefore, various sensors used for non-destructive testing are increasingly developing towards the characteristics of high signal-to-noise ratio, high sensitivity, good performance, safety and efficiency, and multi-functionality. Fractal capacitive sensors are an effective technology for detecting cracks. Therefore, it is necessary to explore the shape and structure of capacitive sensors.

[0003] The traditional capacitive sensor is a single-layer sensor composed of two rectangular, triangular or circular electrodes. The existing fractal capacitor structure for improving the signal-to-noise ratio is mainly a novel capacitive sensor with dual differential excitation described in the document "A novel differential excitation capacitive sensing for hydrogen pipeline inspection". The sensor has three electrode plates in the same plane, similar to a ring shape, with the electrodes on both sides as driving electrodes and the middle electrode as a sensing electrode. It reduces the impact of lift-off changes and subsequent imaging data degradation. The capacitor of this structure can detect cracks perpendicular to the electric field direction of the capacitor spacing, but it is insensitive to cracks parallel to the electric field direction of the capacitor spacing. The existing capacitive sensor has a single structure. When detecting the test block, its structure cannot generate the longitudinal and transverse electric field directions, so it is insensitive to the crack direction, and cannot detect cracks perpendicular to or parallel to the electric field direction of the capacitor spacing at a single time. The capacitance change rate is small, that is, the signal is not obvious, the sensitivity is low, and the missed detection rate is high. Summary of the invention

[0004] The technical problem solved by the present invention is: to overcome the shortcomings of the prior art, to propose a bridge-type fractal capacitance sensor, which is a bridge-type fractal capacitance sensor for precision measurement. The shape of the multi-bridge fractal capacitance sensor generates a multi-directional electric field, so it can improve the sensitivity, resolution, and signal-to-noise ratio of a single detection of cracks in different directions. At the same time, it can detect cracks that are perpendicular or parallel to the direction of the capacitor spacing electric field at a single time, reduce the missed detection rate, and improve performance.

[0005] The technical solution of the present invention is:

[0006] A bridge-type fractal capacitive sensor is used for metal and non-metal crack detection. The bridge-type fractal capacitive sensor includes a PCB board formed by two coplanar capacitors and a circular shielding layer, and there are pads on the PCB board;

[0007] Two coplanar capacitors are connected through vias;

[0008] The shield is located in the middle of two coplanar capacitors;

[0009] The diameter of the shielding layer is larger than the diameter of the coplanar capacitor but cannot exceed the via position;

[0010] The shield layer coincides with the center of the coplanar capacitor;

[0011] Each coplanar capacitor includes three metal (preferably copper) plates, one of which is located in the middle, i.e., the middle plate, and two of which are located on both sides of the middle plate, i.e., the side electrodes;

[0012] The three metal plates are not connected to each other;

[0013] The three metal plates are connected to the pads on the PCB through signal lines;

[0014] When the middle plate is the excitation plate, the electrodes on both sides are signal pickup plates;

[0015] The middle plate is the signal pickup plate, and the electrodes on both sides are the excitation plates;

[0016] The middle plate and any of the two side electrodes form a capacitor;

[0017] A method for preparing a coplanar capacitor in a bridge-type fractal capacitance sensor, the method comprising the following steps:

[0018] On the PCB board, a metal (preferably copper) circular plate with a radius of 7mm is processed according to the shape of the middle electrode. The outer contour of the middle electrode is then offset 1mm outward and the original shape is retained. The remaining circular part is processed into two equal parts as the two side electrodes.

[0019] The shape of the fractal capacitive sensor plate is not single, such as the moiré shape.

[0020] Beneficial Effects

[0021] (1) The present invention is a capacitive bridge circuit that detects cracks based on whether the bridge is balanced;

[0022] (2) A coplanar sensor is placed on each of the first and third layers of the PCB board to construct a four-capacitor bridge structure to form differential excitation or differential measurement. The structure of the sensor increases the recognition rate of cracks perpendicular to and parallel to the electric field direction of the capacitor spacing in a single detection, and the sensitivity is improved. Compared with a single-layer capacitive sensor, the sensor of the present invention can detect smaller cracks and has a more stable signal.

[0023] (3) The structure of the capacitor bridge of the present invention can detect tiny cracks in the flat-bottom holes of the aluminum test block with a diameter of 5mm×0.5mm×1.5mm and the epoxy resin test block with a diameter of 2mm; the capacitor bridge forms a loop, which increases the capacitance value and the capacitance change rate, and the signal is more obvious; a single detection can detect cracks in all directions and improve the recognition rate and detection sensitivity; it can reduce the influence of the lifting distance, help maintain the stability of the signal, and suppress the ringing phenomenon. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a schematic diagram of the structure of the present invention;

[0025] Figure 2 It is a schematic diagram of the circular bridge type fractal capacitance sensor of the present invention;

[0026] Figure 3 It is a schematic diagram of the M6K3 bridge-type fractal capacitance sensor of the present invention;

[0027] Figure 4 Schematic diagram of the Koch bridge fractal capacitance sensor of the present invention;

[0028] Figure 5 It is a structural block diagram of the experimental system of the present invention. DETAILED DESCRIPTION

[0029] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0030] Example

[0031] A bridge-type fractal capacitive sensor is used for metal and non-metal crack detection. The bridge-type fractal capacitive sensor includes two coplanar capacitors and a shielding layer formed by a circular PCB board, and the PCB board has a solder pad;

[0032] Two coplanar capacitors are connected through vias;

[0033] The shield is located in the middle of two coplanar capacitors;

[0034] The diameter of the shielding layer is larger than the diameter of the coplanar capacitor but cannot exceed the via position;

[0035] The shield layer coincides with the center of the coplanar capacitor;

[0036] Each coplanar capacitor includes three metal (preferably copper) plates, one of which is located in the middle, i.e., the middle plate, and two of which are located on both sides of the middle plate, i.e., the side electrodes;

[0037] The three metal plates are not connected to each other;

[0038] The three metal plates are connected to the pads on the PCB through signal lines;

[0039] When the middle plate is the excitation plate, the electrodes on both sides are signal pickup plates;

[0040] The middle plate is the signal pickup plate, and the electrodes on both sides are the excitation plates;

[0041] The middle plate and any of the two side electrodes form a capacitor;

[0042] A method for preparing a coplanar capacitor in a bridge-type fractal capacitance sensor, the method comprising the following steps:

[0043] On the PCB board, a metal (preferably copper) circular plate with a radius of 7mm is processed according to the shape of the middle electrode. The outer contour of the middle electrode is then offset 1mm outward and the original shape is retained. The remaining circular part is processed into two equal parts as the two side electrodes.

[0044] The fractal capacitive sensor uses a four-capacitor bridge structure to form differential measurement or differential excitation, so that the bridge-type fractal sensor can scan vertically and parallel to the direction of cracks on the surface of the measurement test block and can detect tiny cracks of 5mm×0.5mm×1.5mm in the aluminum test block and through-hole cracks with a diameter of 2mm in the epoxy resin test block.

[0045] like Figure 1 As shown, the circuit consists of two coplanar fractal capacitive sensors C1, C2, C3, C4 and a measurement signal output. The test block to be tested is scanned by a bridge-type fractal capacitive sensor. If the dielectric constant of the test block to be tested does not change, the bridge balance output voltage is 0, indicating that the test block to be tested has no cracks; if the dielectric constant of the test block to be tested changes, the electric field is distorted, and the bridge unbalanced output voltage is not 0, then there is a crack.

[0046] like Figure 2 , 3 , 4 are schematic diagrams of the sensor of the present invention. According to the bridge structure, the same sensors are placed on the first and third layers to form a four-capacitor bridge structure. The sensor material is a copper sheet. Each copper sheet electrode leads to a patch pad placed on the bottom layer for external connection of excitation and signal pickup instruments. A circular metal shielding layer is placed on the second layer to shield the first and third layer sensors to reduce mutual influence. The coplanar capacitors C1 and C2 are connected to the coplanar capacitors C3 and C4 of the third layer through two vias. After the connection, the patch pad is placed on the bottom lead-out port through the via as excitation, and the two middle electrodes are placed on the bottom lead-out port through the via as differential excitation voltage signal pickup; or vice versa, the middle electrodes of the two layers are used as excitation, and the other two ports are used for differential measurement voltage signal pickup.

[0047] The defect imaging experimental system of the present invention is constructed by a function generator, a phase-locked amplifier, a power amplifier, a preamplifier, a stepper motor scanner, an oscilloscope, and a host computer. Figure 5 shown.

[0048] The experiment uses a function generator to provide sinusoidal excitation for the capacitive drive electrode, and the power amplifier amplifies its signal. At the same time, the function generator provides a synchronous reference signal for the lock-in amplifier.

[0049] During the scanning and data collection process, the host computer program controls the stepper motor scanner to move the sensor with a set step length to perform a C scan on the test block. Since the signal collected by the sensor is weak, it needs to be amplified by a preamplifier (AD8421) and then sent to the phase-locked amplifier. The phase-locked amplifier outputs the digitized data, which can be displayed by an oscilloscope and read by the host computer.

[0050] In summary, the above are only preferred embodiments of the present invention and are not intended to limit the protection scope of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. Bridge type fractal capacitance sensor, characterized by: The bridge-type fractal capacitive sensor includes a PCB board formed by two coplanar capacitors and a circular shielding layer, and the PCB board has a solder pad; Two coplanar capacitors are connected through vias; The shield is located in the middle of two coplanar capacitors; The diameter of the shielding layer is larger than the diameter of the coplanar capacitor but cannot exceed the via position; The shield coincides with the center of the coplanar capacitor.

2. The bridge-type fractal capacitance sensor according to claim 1, characterized in that: Each coplanar capacitor includes three metal plates, one of which is located in the middle, namely the middle plate, and the other two are located on both sides of the middle plate, namely the side electrodes; The three metal plates are not connected to each other; The three metal plates are connected to the pads on the PCB through signal lines; When the middle plate is the excitation plate, the electrodes on both sides are signal pickup plates; When the middle plate is the signal pickup plate, the electrodes on both sides are the excitation plates; The middle plate and any of the two side electrodes form a capacitor.

3. The bridge-type fractal capacitance sensor according to claim 2, characterized in that: The material of the metal plate is copper.

4. The bridge-type fractal capacitance sensor according to claim 2, characterized in that: A method for preparing a coplanar capacitor in a bridge-type fractal capacitance sensor, comprising the following steps: On the PCB board, a metal circular plate with a radius of 7mm is processed into the middle electrode according to the shape of the middle electrode. Then the outer contour of the middle electrode is offset outward by 1mm and the original shape is retained. The remaining circular part is processed into two parts of equal size as the two side electrodes.

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