RU187120U1 - A device for detecting local defects in a conductive coating based on capacitance measurements using an induction electrode - Google Patents

Abstract

The invention relates to non-destructive testing devices and can be used to detect defects in the form of closed cracks on a conductive coating located on a dielectric material, under a layer of protective enamel. A device for detecting local defects of a conductive coating based on a capacitance measurement comprises a positioner 1 that moves the platform 2 with the flat measuring electrodes 3 located on it along the conductive coating 4. The flat measuring electrodes 3 form 4 conductors with a conductive coating, the capacitance of which is measured by the capacitance measuring unit 5. Data capacitors are transferred to the processing unit 6, which also controls the positioner 1. On the induction electrode 7 located around the perimeter from test electrodes 3, an alternating voltage is supplied from the reference generator. The invention provides increased sensitivity to the determination of defects of different sizes. 2 ill.

Description

The utility model relates to non-destructive testing devices and can be used to detect defects in the form of closed cracks on a conductive coating located on a dielectric insulating material, including under a layer of protective enamel, for example, in fuel tanks of aircraft.

Closed cracks in the conductive coating during the passage of gas can cause the accumulation of an electric charge with further detonation, therefore the safety of the aircraft depends on quality control.

A number of similar devices for defectoscopic inspection of various objects by capacitive methods are known, among which a device for monitoring the integrity of a conductive coating on a dielectric material (patent RU No. 2504730, IPC G01B 7/02 of 01/20/2014), containing a flat electrode installed with a gap above a controlled conductive surface , the electric drive is connected to the position recorder, while the flat electrode reciprocating moves parallel to the surface of the conductive coating, and the controlled product e moves perpendicular to the motion of the flat electrode.

The disadvantage of this device is the low accuracy due to the lack of control of the distance between the flat electrode and the conductive coating and, as a result, the inability to accurately position the electrode over the controlled coating.

A device for monitoring the integrity of a conductive coating on a dielectric material is known (patent for utility model RU No. 159780 U1, IPC G01N 27/24, published December 17, 2015), comprising a movable platform with a flat measuring electrode located above a controlled surface that is moved using a positioner, a sensor control the distance between the electrode and the surface being monitored, and the processing unit, the output of which is connected to the positioner, and the inputs to the measuring electrode and the distance sensor.

The disadvantage of this device is the low accuracy due to the lack of control over the inclination of the platform and the dielectric constant of the protective coating layer deposited on the conductive coating, as a result of which the capacitance of the measuring electrode may differ from the calculated one. In addition, the use of a separate rangefinder complicates the design of the device.

The prototype of the claimed utility model is a device for monitoring the integrity of a conductive coating deposited on a dielectric material (patent for utility model RU No. 175976 U1, IPC G01N 27/24, published December 25, 2017), containing a platform mounted to move over a controlled surface of the conductive coating with using a positioner, the input of which is connected to the output of the processing unit, while on the platform there is a matrix of flat measuring electrodes that are connected to the capacitance measuring unit, the output to connected to the processing unit.

A common disadvantage of the prototype and previous analogues is the low sensitivity of the device to defects in the conductive coating, the size of which exceeds the size of the electrode.

The problem to which the proposed utility model is directed is to increase the sensitivity of the sensor to defects of different sizes.

The problem is achieved in that in a device containing a platform with a matrix of flat measuring electrodes connected to a capacitance measuring unit, the output of which is connected to the processing unit, while the platform has the ability to move over a controlled surface of the conductive coating using a positioner whose input is connected to the output processing unit, according to a utility model, an induction electrode is located around the platform perimeter, surrounding the matrix of measuring electrodes and connected to the output o ornogo generator located in capacitance measuring unit.

The essence of the utility model is illustrated by drawings, where in FIG. 1 shows the design of the device, FIG. 2 - device configuration with one measuring electrode. The drawings show: a positioner - 1, a platform - 2, a matrix of flat measuring electrodes - 3, a layer of a controlled conductive coating (CCI) - 4, a unit for measuring capacitance - 5, a processing unit - 6, an induction electrode - 7, an isolated region of the CCI formed closed crack - 8, a layer of protective dielectric coating - 9, dielectric material - 10, metal wall of the fuel tank - 11.

The device operates as follows. Positioner 1 moves the platform 2 with the matrix of flat measuring electrodes 3 located on it along the conductive coating layer 4. The flat measuring electrodes 3 form capacitors with a conductive coating 4, the capacitance of which is measured by the capacitance measuring unit 5. The capacitance data is transmitted to the processing unit 6, which also controls the positioner 1. An alternating voltage from the reference generator is supplied to the induction electrode 7 located along the perimeter from the measuring electrodes 3, stvuyuschem any capacitance meter, which increases sensitivity to the definition of major defects. To explain the principle of operation, we use the simplest configuration of the sensor, consisting of one measuring electrode 3, surrounded around the perimeter by an inducing electrode 7 (Fig. 2). It is known that in block 5 the electric current I through the ammeter A is proportional to the measured capacitance C and is connected by the ratio:

where U is the voltage of the reference generator;

ω is the angular frequency of the reference generator.

We assume that the edge effects of the capacitors formed by the electrodes are small and their capacitance is proportional to the area of the electrodes. Assume that the measuring electrode is located above the undamaged portion of the conductive coating. Then the current through the ammeter will be maximum and will be:

where Cs is the capacitance formed by the measuring electrode 3 and the CCI;

- реактивное сопротивление получаемого конденсатора.

- reactance of the resulting capacitor.

When the sensor is completely above the damaged TIP section, the current through the ammeter will be less and in the absence of an inducing electrode 7 will be:

where Z _S is the reactance of the capacitor formed by the wall of the fuel tank 11 and the defective section of the CCI 8. It follows from expression (3) that with a large defect size and, accordingly, its small resistance Z _d << Z _{S, the} current difference between the damaged one (3 ) and the intact (2) section of the CCI will be insignificant, which makes it difficult to fix such defects.

In the case of the introduction of the induction electrode 7, also located above the defect of the CCI, a capacitive resistance Z _i appears in the circuit, connected in parallel with the resistance Z _S. Using the laws of electrical engineering, you can get an expression for the current in the resulting circuit:

From (4) it follows that in the case Zs >> Zi, when located above the defective area, a significant change in the current occurs, which increases the sensitivity.

For a more accurate assessment of the metrological characteristics of the resulting device, we introduce a coefficient n equal to the ratio of the area of the inducing electrode to the area of the measuring electrode. The capacitances will then be related by inverse values:

Substituting (5) into (4) we obtain the current in through the ammeter:

On the other hand, if we make a single measuring electrode with the same total area as the inducing electrode together with the measuring electrode, that is, with an area n + 1 times larger, then taking into account (3) we get:

Expressions (6) and (7) differ only in the static coefficient n + 1, which means that in both cases, when the entire sensor moves from the whole to the damaged section of the CCI, the relative change in capacitance will be the same. Therefore, for control, the design of one small measuring electrode together with the inducing electrode is equivalent to the same area as a single measuring electrode, only the entire signal in the first case will be less than the amplitude by an factor of n + 1. Moreover, the proposed sensor with an inducing electrode is more preferable than a single measuring electrode with a larger area for several reasons:

1. The measuring electrode of a large area loses sensitivity to small defects of the CCI. From the calculations it follows that the maximum sensitivity is provided for defects of the CCI, equal in area to the measuring electrode. With an increase in the area of the measuring electrode, the defect does not fall entirely under the plate, and entire sections of the CCI form an additional capacitance with the electrode, reducing the change in current and thereby reducing the sensitivity of the sensor.

2. With a small area of the measuring electrode, it is easier to find the coordinates of the defect. Changes in capacity during the movement of the platform are more dynamic.

3. When using a matrix of measuring electrodes, adjacent measuring electrodes also begin to play for each other the role of inducing, since they are connected to the same reference generator. An external induction electrode 7 located along the perimeter, on the one hand, equalizes the conditions for all electrodes, on the other hand, provides an increase in the sensitivity to defects of the CCI of a large area.

The device allows you to identify defects in the conductive coating, including under a layer of protective dielectric (paint) coating 9, under which it is impossible to visually determine the defect. Maximum sensitivity is provided for defects ranging from the size of a single measuring electrode to the total area of all electrodes, including the inducing one.

Claims (1)

A device for detecting local defects in a conductive coating based on a capacitance measurement, comprising a platform with a matrix of flat measuring electrodes connected to a capacitance measuring unit, the output of which is connected to a processing unit, while the platform has the ability to move over a controlled surface of the conductive coating using a positioner whose input is connected to the output of the processing unit, characterized in that along the perimeter of the platform there is an induction electrode surrounding the measuring matrix electrodes and connected to the output of the reference generator located in the unit for measuring capacitance.

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