Capacitive sensor design for liquid contraband detection

0 Introduction

The invention and use of liquid explosives and combustibles have a long history, but because they are not as powerful as solid explosives, they are not widely used in military and other fields. However, in recent years, terrorists and criminals have used their cheap raw materials, easy access, simple manufacturing steps, easy disguise as ordinary beverages or daily necessities, and easy detonation as tools for various types of terrorist attacks or disturbances of public order, which seriously threaten public safety and social stability. At present, the more common liquid contraband detectors are generally based on the following technologies: static X-ray tomography technology, microwave radiation technology, Raman spectroscopy, gas chromatography, etc. These methods can effectively complete the detection or early warning of contraband, but they also have different disadvantages: such as radiation hazards, inability to overcome the influence of container shape and wall thickness on the detection results, long analysis time, unsuitable for rapid security inspection environment, large size, high price, high misjudgment rate, etc.

1 Overview

Quasi-static capacitance tomography can be used to detect whether the liquid in a non-metallic container is a prohibited item without opening the outer packaging. The core of this technology is to measure the dielectric constant of the liquid. The dielectric constant can reflect the properties of the liquid. The dielectric constant of liquid dangerous goods is very different from that of ordinary liquids such as water, as shown in Table 1. Therefore, by measuring the dielectric constant using quasi-static capacitance tomography technology, it can be determined whether the measured liquid is a liquid contraband. Because it is a purely electronic measurement, there is no radiation hazard and it is easy to miniaturize. This technology has the same basic principle as ECT (capacitance tomography) used in the fields of petroleum processing, food, papermaking, etc. From C=εS／(4πkd), it can be seen that when the distance d of the capacitor plates is determined by the effective area S, the dielectric constant is proportional to the capacitance, so the dielectric constant can be inferred by measuring the capacitance value. The liquid contraband inspection device based on quasi-static capacitance tomography technology is based on this principle.

However, in order to overcome the influence of containers of different shapes, the traditional parallel plate capacitor with fixed pole spacing and the cylindrical sensor that can be used to detect the shape of a specific object to be measured, which is currently widely used in ECT (capacitive tomography), can no longer be used. A flat plate sensor based on the edge effect is more suitable for this study, as shown in Figure 1.

2 Sensor Design

According to the basic theory of capacitance, the standard mathematical model is simplified. If the object to be measured is an insulator, under the influence of the edge effect, the electric lines between the two electrode plates are shown in Figure 1.

When the length of the plate is L, the width is a, and the plate spacing is b, within the small area increment (△S=I△x) on the small incremental unit △x in the x direction, the electric field lines can be regarded as semicircular arcs with a length of d, so the approximate:

Where: εx is a parameter proportional to the dielectric constant, which is defined as the effective dielectric constant. Integrating the above formula from 0 to a/2 yields:

If the number of electrode pairs is increased, the measured capacitance value can be increased and the measurement sensitivity can be improved. Taking into account the influence of substrate material and air gap, the total capacitance is expressed as:

CE=Kn(Cω+Cb)      (2)

In the formula: Kn represents the number of electrode pairs; Cb represents the substrate capacitance. Therefore, the dielectric constant of the substance to be tested can be obtained by simply measuring the change in capacitance.

In order to reduce costs and manufacturing processes, the sensor uses a double-sided copper-clad PCB. The material of the substrate is a general epoxy resin-glass fiber cloth-based copper-clad board (commonly known as FR-4), and its nominal dielectric constant is 4.7. The shielding electrode is covered with copper on the entire surface, and the copper surface insulation layer is made of epoxy resin. In order to reduce the influence of the insulation layer on the measurement, the thickness of the insulation layer should be much smaller than the electrode spacing b. On the side with the electrodes, an insulation layer of the same material and thickness is laid.

[page] Parallel plate electrodes (see Figure 2(a)) and comb electrodes (as shown in Figure 2(b)) are used.

The excitation electrode of the parallel plate electrode (see Figure 2(a)) is E1, and the receiving electrodes are E2 to E5. The number of electrodes can also be increased as needed. The more electrode plates there are, the more information can be measured, which can fully reflect the liquid level height information of the liquid level, but it puts forward higher requirements for measurement accuracy. The dressing electrode has only two electrodes and reflects an average information. Its characteristics are that each electrode has a large contact surface and an increased effective area, so the test sensitivity can be increased. Because when detecting liquid contraband, the liquid is allowed to cover the entire electrode surface by tilting the bottle body, so the dressing electrode is also one of the options. The detection sensitivity of the two electrodes will be verified through further experiments.

Here, a correction is made to the combing electrode, as shown in Figure 3. This is equivalent to connecting several capacitors in parallel between E1 and E2. If the capacitance between E1 and E2 of the lateral monolithic chip is equivalent to the capacitance between E1 and E2 in Figure 2(a), it is equivalent to increasing the capacitance several times. In theory, the detection sensitivity can be effectively improved.

The size of the distance between the electrode plates affects the penetration depth of the electric field lines. When the shape and area of ​​the plates are constant, reducing the distance between the plates will reduce the penetration depth of the electric field lines, but at the same time it will also increase the strength of the sensor measurement signal; conversely, increasing the distance between the plates can increase the penetration depth of the electric field lines, but will also correspondingly reduce the strength of the sensor measurement signal. Figure 4 shows the direct relationship between the penetration depth and the distance between the plates.

Common liquid containers are generally more than 20 cm high. Considering that the container is generally not fully filled, the total length of the electrode is controlled within 15 cm.

[page]In order to facilitate the test, the evaluation board of MC34940 of FREESCALE was selected for the experiment. As shown in Figure 5, the evaluation board is equivalent to performing envelope detection on the voltage formed on the electrode capacitance and then calculating its average value. The larger the capacitance, the greater the filtering effect and the smaller the voltage, that is, the voltage is inversely proportional to the dielectric constant. MC34940 can connect 9 electrodes at the same time, and only 5 of them are needed here.

MC34940 has a high sensitivity in the range of 10 to 70 pF, and this factor must be taken into account when designing the electrode. According to Table 1, in order to determine the electrode parameters, 1≤εx≤7 (×8.85 pF/m) is taken as the strictly prohibited item demarcation point. In formula 1, the following conditions must be met:

From equations (3) and (4), it can be obtained that when a/b is about 0.5 times, the above requirements can be met. Based on the previous derivation, 5 groups of electrodes are designed, using the improved comb electrode and flat panel capacitor models respectively. The specific parameters are as follows:

Electrode A uses an improved comb electrode, and the dimensions between the two pairs of electrodes are: 16 mm × 4 mm × 2 mm (L × a × b)

Electrode B uses the parallel plate electrode shown in Figure 3(a), and the electrode size is: 16 mm×6 mm×10 mm (L×a×b)

The C electrode adopts the parallel plate electrode shown in Figure 3(a), and the electrode size is: 16 mm×6 mm×12.5 mm (L×a×b)

The D electrode adopts the parallel plate electrode shown in Figure 3(a), and the electrode size is: 12 mm×6 mm×12.5 mm (L×a×b)

The E electrode adopts the parallel plate electrode structure shown in Figure 3(a), and the electrode size is: 12 mm×6 mm×10 mm (L×a×b)

3 Experimental process and results

Use a glass container with a wall thickness of 1 mm and a height greater than the height of all electrodes. Fill it with gasoline, acetone, alcohol and water respectively and measure with different electrodes.

If the channel selection terminal is set to channel 1, then channel 1 will be used as the excitation electrode, and the other electrodes will be the receiving electrodes. Read the test results and use a line graph to represent them. You may choose the No. 2 electrode in each group of electrodes for comparison, as shown in Figure 6.

Comparing the two groups of electrodes B and C, the distance between the two plates in the B electrode is small, and the measured value is significantly smaller than that in C, that is, the measured capacitance value is significantly larger than that in C, which is consistent with the aforementioned penetration depth and plate distance theory. Comparing the two groups of electrodes B and E, we can see the influence of electrode width on the measurement results. The wider the electrode, the larger the measured capacitance, that is, the higher the sensitivity.

Comparing the two groups of electrodes B and A, the measurement result change rate of electrode A is higher when the dielectric constant of the material changes, which is reflected in the figure as the higher slope of the broken line between the two groups of materials. Therefore, the distinction between objects is most obvious and the sensitivity is the highest.

According to the formula of dielectric constant of mixture: ε=ε1×P1+ε2×P2 (P1 and P2 are the solute volume fractions, ε1 and ε2 are the dielectric constants of the corresponding solutes), it is easy to get that the dielectric constant of 50% alcohol is 51.4, which is between dangerous goods and safe liquids in Table 1. It is more stringent to use it as a dividing point. If the electrode can accurately distinguish 50% alcohol and water, the sensitivity of the electrode has reached a high level. Therefore, we conducted a set of experiments on alcohols of different components. The container was replaced with a glass bottle.

Comparing the data in Figure 7, we can see that the sensitivity of the two groups of electrodes C and D is poor, and they cannot distinguish between 50% and 70% alcohol, while the other electrodes can effectively distinguish them, and the A electrode has the best effect.

After completing the design of the capacitor plate, the performance parameters of the capacitor sensor plate were analyzed, including the maximum wall thickness of the container detected by the sensor, the influence of different capacities, and the air gap on the measurement results. The experiment was mainly carried out under the conditions of 80 mL and 150 mL capacity, 1-4 mm wall thickness, and a maximum air gap of 5 mm. The measurement results are shown in Figure 8.

Experiments have shown that the capacitance sensor plate of this size can meet the measurement requirements under the conditions of a minimum capacity of 80 mL and a maximum container wall thickness of 4.06 mm. Its maximum detection distance is about 5 mm, which basically meets the design requirements.

4 Conclusion

The improved comb electrode can better meet the measurement requirements than the flat plate electrode. However, during the experiment, due to the limitations of the experimental conditions, we can only use a 240 kHz sine wave to excite the plate. According to Zc=1/jωC, if the frequency of the excitation signal is increased, the capacitive reactance becomes smaller. In the schematic diagram shown in Figure 5, the final detected voltage will increase, which can further improve the test sensitivity. If the frequency can be increased to twice the original, the detection sensitivity will be higher. If you want to expand the detection range, you should reduce the detection frequency.

On the other hand, for planar plate electrodes, if the sampling method is changed to detect the changes in sampling values ​​between every two adjacent plates, and the sampling accuracy of the A/D converter is increased, this type of electrode can also have better sensitivity and can fully meet the detection needs.