An Interdigitated Capacitive Proximity Sensor with Varied Space Electrode Structure

Abstract

Embodiments related to a capacitive proximity sensor with a variable spacing electrode structure, which is suited to a non-destructive testing operation, such as the detection of dielectric properties of the polymer materials with a thickness decreases gradually structure. The designed sensor includes a driving electrode, a sensing electrode, a substrate, a guarding electrode and a lead connector. The driving and sensing electrodes include several interdigitated fingers, which are arranged alternately in sequence, based on the characteristic of the thickness decreases gradually structure of the MUT, the width of the electrodes and spacing between two adjacent electrodes in each unit are optimized individually. Namely, under the condition of ensuring penetration depth, the electrode width is made as large as possible to achieve maximum signal strength and detection sensitivity. Compared with the traditional ES-IDE structure capacitive proximity sensor, the newly designed VS-IDE capacitive sensor increases the effective electrode area, which increases the signal strength and measurement sensitivity directly. Besides, the electric field lines of the designed sensor are confined within the thickness gradually changed materials under test mostly as expected simultaneously.

Claims

1. An interdigitated capacitive proximity sensor with a varied space electrode structure, the sensor comprising: a driving electrode, a sensing electrode, a substrate, a guarding electrode, and a lead connector, wherein: the driving electrode and the sensing electrode are deposited on a PMMA (polymethyl methacrylate) substrate, the guarding electrode is deposited on the opposite side of the substrate to protect the sensor out of the influence of surround, the lead connector is welded on the guarding plane to provide a reliable connect for a sensor and testing equipment, the driving and sensing electrodes including several interdigitated fingers, which are arranged alternately in sequence, and the width and metallization ratio for each interdigitated unit are determined by the local thickness of the test sample, the guarding electrode is deposited on the opposite side of the substrate, a “U” pattern slot is reserved on the guarding electrode 4 beside the connectors of driving electrode and sensing electrode, the guarding electrode 4 is deposited on the opposite side of the substrate, and the arrangement is suited to the driving electrode and sensing electrode, the middle pins of the SMB connectors are respectively connected to the driving electrode and the sensing electrode through the pre-reserved holes on the substrate, and the outside pins of the SMB connectors are connected to the guarding layer; and a VS-IDE structure, wherein design steps of the width and spacing of each interdigitated unit are as follows: the fundamental parameters of the sensor including a mainly unit width of each finger w, a length of the interdigitated electrode structure l, a spacing between two adjacent fingers g, a metallization ratio γ, a parameter C named width of a basic interdigitated unit, the metallization ratio γ is equal to the ratio of the finger width w to the basic interdigitated unit width C, which is γ=w/(w+g), the design steps including: Step 1: fabricating sensors such that basic unit width of the sensor is C and the metallization ratio is γ, the sensors including the driving electrode, the sensing electrode, the substrate and guarding electrode, and the electrode width is w=C*γ, and the spacing between two adjacent electrodes is g=C*(1−γ), and the guarding layer width is C. Step 2: measuring capacitance values in the case of different thickness MUT, a series of different thickness MUT are placed on surfaces of the sensors fabricated in step 1 respectively, and the relevant capacitance is recorded simultaneously. Step 3: computing the relative capacitance ratio to the constant value at each different MUT thickness, the evaluation of the penetration depth is according to the distribution of the relative capacitance ratio d %, which can be represented as $\begin{array}{cc}d.Math.\%=\frac{.Math.C_h-C_{h.fwdarw.\infty }.Math.}{C_{h.fwdarw.\infty }}\times 100.Math.\%,& \left(1\right)\end{array}$ wherein C.sub.h.fwdarw.∞ is a stable capacitance at the same metallization ratio, and the relative capacitance ratio curves are obtained. Step 4: computing the penetration depth curves of the sensors with single unit, based on Step 3, a horizontal line equals to 10% is drawn, and an effective penetration depth corresponds to the position where the relative capacitance equals 10%, Step 5: repeating the above steps 1 to 4 and the effective penetration depth curves for the sensors with a single unit, whose unit width and metallization ratio are C and γ respectively, Step 6: determining the width and spacing of each interdigitated to obtain the sensors, wherein according to the correspondence between the effective penetrative depth h, the length of each unit and the metallization ratio of the electrode, combined with the characteristic of the thickness decreases gradually structure of the MUT, the width of the electrodes and spacing between two adjacent electrodes in each unit are optimized individually, and by combining the different interdigitated units together according to the pre-defined rule, the novel VS-IDE structure capacitive sensors are designed, and Step 7: optimizing selection of the sensor by measuring performance of the sensors obtained based on the step 6 and selecting an optimal VS-IDE structure sensor according to the comparison of an electric field line distribution and signal strength between different combinations.

2. The sensor of claim 1, further comprising: a VS-IDE structure, wherein the substrate is a kind of insulation material with a certain strength and stiffness to support the electrode and guarding layer, a hole is drilled on substrate for the purpose to lead the driving electrode and sensing electrode to the reverse side of the substrate.

3. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is a schematic diagram of the invented VS-IDE capacitive sensor.

[0026] FIG. 2 is a schematic of experimental system for capacitance measurement

[0027] FIG. 3 shows electrode arrangement and the sign of the key parameters of VS-IDE capacitive sensor.

[0028] FIG. 4 is a cross section diagram of the capacitive proximity sensor with a single unit.

[0029] FIG. 5 shows capacitance value versus thickness of test samples for proximity interdigitated sensor with a single unit, whose metallization ratio is 0.5 and a vary unit width C.

[0030] FIG. 6 shows relative capacitance value versus thickness of test samples for proximity interdigitated sensor with a single unit, whose metallization ratio is 0.5 and a vary unit width C.

[0031] FIG. 7 shows penetration depth curves for capacitive proximity interdigitated sensors with different unit widths.

[0032] FIG. 8 shows the cross-sectional drawing of the specimen under test.

[0033] FIG. 9 shows installation diagram of the VS-IDE sensor and the thickness gradually changed MUT.

[0034] FIG. 10 shows electric field line distributions. 10a, electric field line distribution for ES-IDE sensors; 10b, electric field line distribution for VS-IDE sensor with metallization ratio 0.5; 10c, electric field line distribution for VS-IDE sensor with metallization ratio 0.6; 10d, Electric field line distribution for VS-IDE sensor with metallization ratio 0.7.

[0035] FIG. 11 shows Comparison of the signal strength between the VS-IDE sensors and ES-IDE sensor.

[0036] As shown in the figures above, the following numbers refer to one or more components, respectively: 1. A driving electrode; 2. a sensing electrode; 3. a substrate; 4. a guarding layer; 5. a lead connector; 6. a thickness gradually changed testing specimen; 7. an impedance analyzer; 8. a VS-IDE sensor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0037] The design of a VS-IDE capacitive proximity sensor is further described below with reference to FIGS. 1 to 11.

[0038] Based on the fringing effect of the electric field, a novel capacitive sensor with variable spacing electrode structure is designed and used to evaluate the dielectric properties of the polymer materials with thickness decreases gradually structure.

[0039] A novel capacitive proximity interdigitated sensor may include VS-IDE structure is shown in FIG. 1, which including driving electrode (1), sensing electrode (2), substrate (3), guarding electrode (4), and lead connector (5). The driving electrode (1) and the sensing electrode (2) are deposited on a PMMA (polymethyl methacrylate) substrate (3), while the guarding electrode (4) is deposited on the opposite side of the substrate to protect the sensor out of the influence of surround. The lead connector (5) is welded on the guarding plane to provide a reliable connect for the sensor and testing equipment.

[0040] As described in FIG. 1, the driving electrode 1 and the sensing electrode 2 were made of a highly conductive copper foil having a thickness of 0.1 mm. The driving electrode 1 and the sensing electrode 2 have a width of 40 mm and a total length of 38 mm respectively. Both the driving electrode 1 and the sensing electrode 2 may include three interdigitated fingers, which are arranged alternately in sequence.

[0041] The substrate 3 was made of a PMMA plate having a size of 60*50*2.5 mm, and two diameters of 2 mm holes were obtained at a distance of 10 mm from the left and right end faces respectively. A copper foil with a width of 2 mm was selected as the lead, the driving electrode 1 and the sensing electrode 2 are led to the back surface of the substrate by a lead wire, so as to be easily connected to the lead connector 5.

[0042] As shown in the FIG. 2, guarding electrode (4) is deposited on the opposite side of the substrate (3), the guarding layer is made of a highly conductive copper foil having a thickness of 0.1 mm, and the length and width are 60 mm and 50 mm respectively. The width of the U-shaped cutout on the guarding layer is 4 mm, and the length is 30 mm, the position of the shield is opposed to the driving electrode 1 and the sensing electrode 2.

[0043] As shown in the FIG. 2, the lead connector 5 is welded to the back of the substrate 3, the middle pins of the SMB connectors (5) are respectively connected to the leads of the driving electrodes 1 and the sensing electrodes 2, and the outer pins of the lead terminals 5 are connected to the shield layer 4.

[0044] The detailed procedures about how to decide each single interdigitated unit width and space of the above-mentioned VS-IDE sensors are presented as following.

[0045] Step 1: Fabrication of the capacitive proximity interdigitated sensor with a single unit, whose basic unit width C is 10 mm and the metallization ratio γ is 0.5. According to the sensor structure illustrated in FIG. 4, the capacitive proximity sensor is fabricated, whose electrode width is 5 mm, and spacing between two adjacent electrodes is 5 mm, and the length is 40 mm.

[0046] Step 2: Measuring the capacitance values in the case of different thickness MUT. A 1 mm, thick high-temperature vulcanized silicone rubber slice, is placed above the capacitive proximity sensor fabricated in step 1, and the capacitance value measured at that time was recorded. Then, a silicone rubber sheet having a thickness of 1 mm was superimposed on each layer, and the capacitance value measured after increasing the thickness of 1 mm was recorded. Finally, the capacitance value varies with the thickness of the silicone rubber curve shown in FIG. 5 can be obtained. It can be seen from FIG. 5 that the measured capacitance tends to steady state when the silicon rubber thick is 14 mm.

[0047] Step 3: Computing the relative capacitance ratio to the constant value at each different MUT thickness. The capacitance values at different silicone rubber thicknesses obtained in step 2 are calculated according to formula (1), and the rate of change of the capacitance value relative to the stable value under the thickness h of the sample to be measured is shown in FIG. 6.

[0048] Step 4: Computing the penetration depth curves of the interdigitated capacitive proximity sensors with a single unit. Based on step (3), a 10% is selected as the difference %, and a horizontal line is drawn as shown in the dashed line in FIG. 6. The h-value corresponding to the intersection of the dashed line and the change rate curve of the capacitance value with respect to the stable value at different thickness of the sample to be tested is 6.03 mm, which is the effective penetration of the sensor with the unit width C is 10 mm and the metallization ratio is 0.5.

[0049] Step 5: Repeating steps (1) to (4), the different single unit capacitive sensors are fabricated. The unit width C is 4 mm\5 mm\6 mm\7 mm\8 mm\9 mm\10 mm, and the metallization ratio are 0.1˜0.9. Measure and calculate the effective penetration depth of the sensor under different combinations of parameters, plotted as shown in FIG. 7.

[0050] Step 6: Determining the width and spacing of each interdigitated unit, and combine to obtain a novel structure interdigitated capacitive proximity sensor with a variable spacing electrode. Analysis of the characteristics of the geometric dimensions of the thickness decrease specimen gradually to be measured as shown FIG. 8. As shown in the FIG. 8, the thickest part to be tested is 6.27 mm, and the thinnest thickness is 2 mm, the angle between the inclined surface and the horizontal direction is about 5 degree, the horizontal length is 46 mm. The first pair of finger units is arranged at a distance of 5 mm from the left end of the specimen to be measured, where the maximum thickness of the test piece to be tested is 5.89 mm. It is determined that when the metallization ratios are 0.5/0.6/0.7, and the cell width C is 10 mm, the penetration depth of the sensor can meet the requirements, and three different combinations of variable pitch interdigitated sensors are obtained. FIG. 9 shows the relative position of the VS-IDE sensor, and the thickness gradually changed specimen to be tested. When the metallization ratio is 0.5, select C1=10 mm, C2=8 mm, C3=6 mm, C4=5 mm and C5=4 mm are selected. When the metallization ratio is 0.6, C1=10 mm, C2=8 mm, C3=6 mm, C4=5 mm and C5=4 mm are selected. When the metallization ratio is 0.7, C1=10 mm, C2=8 mm, C3=7 mm, C4=6 mm and C5=5 mm are selected. Finally, the detailed parameters of each designed VS-IDE sensors are shown in Table 1, Table 2 and Table 3 respectively, and the sensors are fabricated as the structure shown in FIG. 1.

[0051] Step 7: Optimizing selection of the proximity interdigitated sensors with variable spacing electrode structure. FIG. 10a-10d show the distribution of the electric field lines of different types of sensors. The distribution of the electric field lines of the VS-IDE sensors is better than that of the traditional ES-IDE sensor. The experimental system is constructed as shown in the FIG. 2, which includes gradually changed thickness specimen 6, an impedance analyzer 7 and a VS-IDE sensor 8. The impedance analyzer 7 is connected to the interdigitated capacitive sensor 8, the signal strength of the sensor of the three variable pitch combinations measured by the impedance analyzer 7 is shown in FIG. 11. Combination 1 has a metallization ratio of 0.5, combination 2 has a metallization ratio of 0.6, and combination 3 has a metallization ratio of 0.7. Compare the three combinations; the maximum signal strength is 11.49 pF when the metallization is 0.7. And its signal strength is 3.7 times than ES-IDE structure sensor. Finally, the sensor-3 with metallization ratio 0.7 was selected due to the maximum signal strength while satisfying the requirement of penetration depth.

[0052] The above is a typical application of the present disclosure, and the application of the present disclosure is not limited thereto.

TABLE-US-00001 TABLE 1 Interdigitated 5 mm 5 mm 4 mm   3 mm 2.5 mm 2 mm width Spacing 5 mm 4 mm 3 mm 2.5 mm   2 mm

TABLE-US-00002 TABLE 2 Interdigitated 6 mm   5 mm 4.8 mm 3.6 mm   3 mm 2.4 mm width Spacing 4 mm 3.2 mm 2.4 mm   2 mm 1.6 mm

TABLE-US-00003 TABLE 3 Interdigitated 7 mm 5.6 mm 4.9 mm 4.2 mm 3.5 mm 2 mm width Spacing 3 mm 2.4 mm 2.1 mm 1.8 mm 1.5 mm