Liquid contraband inspection instrument based on quasi-static capacitance tomography technology

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, making liquid contraband detection widely concerned.

The principles of the more common liquid contraband detectors currently include: static X-ray tomography technology, microwave radiation technology, Raman spectroscopy, gas chromatography, etc. Although these methods can detect liquid contraband well, 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, etc.

The liquid contraband inspection device based on quasi-static capacitance chromatography technology is expected to solve the above problems. This method uses the difference in dielectric constant between flammable and explosive liquid contraband and water and other daily liquids. As the basis for judgment, pure electric field measurement is used, without other potential dangerous factors such as microwaves, rays, and radioactive sources; it can achieve rapid detection; it can eliminate the influence of container shape and thickness on the measurement results, without opening the bottle; its pure electronic measurement principle enables low cost and portable measurement.

1 Measurement principle

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, the dielectric constant can be measured using quasi-static capacitance tomography technology to determine whether the measured liquid is a liquid contraband.

The dielectric constant of the liquid electrolyte is calculated based on the capacitance characteristics of the liquid electrolyte.
It can be seen that when the distance d between the capacitor plates and the effective area S are determined, the dielectric constant is proportional to the capacitance, so the dielectric constant can be calculated by measuring the capacitance value. Therefore, the core of the instrument is to detect the capacitance between two fixed electrodes. However, in order to overcome the influence of the container shape and thickness on the detection results, it is necessary to adopt a multi-electrode scanning detection method to solve this problem by solving the distribution of the electric field under the multi-electrode. Capacitance scanning measurement is to quickly measure multiple groups of capacitance, so the CDC (capacitance digital conversion) circuit can be selected to simplify the design process.

2 Electrode design

Generally, parallel plate capacitors are easily affected by edge effects and destroy the measurement accuracy. The capacitive sensor design used for liquid detection in this article makes full use of the edge effect and uses a planar plate capacitor. Any one of the multiple electrodes is provided with an AC voltage as an excitation electrode, and the remaining electrodes are used as receiving or detection electrodes, thereby forming a stable electric field between the multiple electrodes. Different liquids in the container have different dielectric constants, which will cause changes in the inter-electrode capacitance value. The change in inter-electrode capacitance and the principle of capacitance edge effect are shown in Figure 1.

The design of sensor electrodes needs to be considered from three aspects: the more sensor measurement electrodes there are, the more information the sensor obtains, and its measurement sensitivity is relatively higher; the sensor electrode geometry and electrode distribution also determine whether sufficient measurement information can be obtained to identify different liquids in the container; considering the different shapes of containers, wavy bubble gaps will be generated on the container wall, forming the so-called air gap. Due to the existence of air gaps, especially when the number of measurement electrodes is small and the electrode distribution is sparse, the measurement data information is not enough to judge the impact of air gaps on the measurement, which will cause different degrees of tolerance, resulting in measurement errors and misjudgment of liquids.

Based on the above considerations, we need to first determine the influence of electrode shape. Two schemes, the flat plate type and the capacitance tomography (ECT) type, need to be compared. The schematic diagrams of the two electrodes are shown in Figure 2.

However, due to the variety of container shapes, it is difficult to overcome the influence of container shape by selecting capacitive tomography. Therefore, consider selecting a flat plate sensing electrode.
Now it is necessary to further determine the sensing electrode from the aspects of electrode shape and number of electrodes. In order to reduce the influence of container shape and wall thickness, the selected electrode size should be as small as possible, so that S and d can be regarded as approximately constant values. However, small-sized electrodes can only provide tiny and difficult-to-measure capacitance values, so it is necessary to reduce the distance between electrodes to increase the capacitance value. In summary, selecting a small-sized linear electrode monomer array for measurement can meet the design requirements.

The final electrode is the improved comb electrode shown in Figure 3. Since the capacitance of the capacitive sensor based on the edge effect is very small, it is necessary to strengthen the shielding to reduce the interference caused by the small electric field changes. Therefore, a large shielding electrode is added on the back of the sensor. In order to prevent oxidation, the substrate material needs to be covered on the surface of the excitation electrode and the receiving electrode, and the thickness and material of the substrate material on the surface of the shielding electrode must be the same to eliminate the influence of the substrate material.

3 Software and hardware design of capacitance detection circuit based on CDC

Using CDC capacitance digital conversion device for capacitance measurement can simplify the entire design process. The basic working block diagram of the entire system is shown in Figure 4.

The AD7143 chip produced by ADI has an operating voltage of 2.6 to 3.6 V and is equipped with an I2C serial interface with an operating voltage of 1.65 to 3.6 V, which can basically meet the operating voltage range of common MCUs. In order to save power, the chip will enter low-power mode when idle, and the working power consumption is only 50μA. Its internal structure mainly includes ∑-△ converter, reference voltage, excitation source, capacitance digital converter (CDC), temperature compensator and an I2C interface, through which the microcontroller can control the internal registers and conversion results of AD7143.

ATmega16 is selected as the core MCU of the entire system to complete the control of peripheral devices and the final data processing. ATmega16 is a low-power 8-bit CMOS microcontroller based on the enhanced AVR reduced instruction set structure, with an advanced instruction set and single clock cycle instruction execution time.

The data throughput of ATmega16 is as high as 1 MIPS/MHz, which effectively alleviates the contradiction between system power consumption and processing speed. Its core has a rich instruction set and 32 general working registers. All registers are directly connected to the arithmetic logic unit (ALU), so that one instruction can access two independent registers simultaneously in one clock cycle, greatly improving code efficiency. The

ATmega16 microcontroller collects the capacitance value measured by the capacitance sensor through serial communication to determine whether the measured liquid is a prohibited item. Each pin of the microcontroller is connected to the peripheral module to control the operation of the entire system and realize the corresponding system functions, including system initialization, capacitance sensor initialization, capacitance value acquisition and processing, and measurement result display. Therefore, the ATmega 16 microcontroller plays a very important role in the entire system. The system connection diagram is shown in Figure 5.

In Figure 5, MOSI and MISO pins are used for serial communication; AJ1 and AJ2 are used for key input; PA0~PA7 are used to connect LCD1602 liquid crystal screen to display measurement data; RXD and TXD are used to control the sending and receiving status of MAX3232 chip; LED1 and LED2 are used to control the status indicator light; C3, C6 and Y1 form a clock circuit with a frequency of 16 MHz, which can provide a stable and high-precision clock signal for ATmega16 microcontroller.

The software flow chart is shown in Figure 6. The entire design was completed by programming the microcontroller according to the flow chart.

4 Experimental results

According to the formula for the dielectric constant of the 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 obtain 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, a set of experiments were conducted on alcohols of different components, and the test results are shown in Figure 7(a). At the same time, the shape and thickness of the container were changed to test different liquids, and the test results are shown in Figure 7(b).

It can be seen that during the detection process, 50% alcohol is significantly distinguished from water, proving that the system has sufficient discrimination. And for containers with a wall thickness of less than 4 mm, common liquids can also be distinguished.

5 Conclusion

The liquid detector using quasi-static capacitance tomography technology can distinguish between liquid contraband and ordinary liquids, and is small in size and low in power consumption, achieving fast, safe, and radiation-free detection. There is no need to open the container during the detection process. The single detection time is within 2 s, which is convenient and fast, meeting the needs of daily security inspections.

Since the measured capacitance value is very small, in order to enhance the detection accuracy, the method of increasing the frequency and amplitude of the excitation signal can be considered. In addition, in order to improve the design reliability and ensure the detection accuracy, in addition to considering the shielding of the electrode, the external shielding of the entire instrument is also very important.