Microwave sensor for measuring liquid and measuring method thereof

Abstract

The invention discloses a microwave sensor for measuring liquid, which comprises a microwave circuit and a liquid container, and is characterized in that the microwave circuit comprises a dielectric substrate, and a top layer metal and a bottom layer metal which are arranged on two side surfaces of the dielectric substrate; the top layer metal comprises a 50-ohm microstrip line used for feeding and a resonator used for generating a transmission zero point and a transmission pole point, and the resonator and the 50-ohm microstrip line form a coupling structure. The invention further discloses a measuring method of the microwave sensor for measuring the liquid. According to the invention, the detection process of the microwave sensor is simpler and more efficient, the microwave sensor is convenient to carry and operate, and the manufacturing and measuring cost of the microwave sensor can be further reduced. Besides, when the sensor is used for testing, liquid quantitative control is facilitated, instrument calibration only needs to be carried out once, system errors can be reduced to a great extent in cooperation with a pseudo-differential scheme during data processing, and the sensor has high robustness.

Description

Technical Field

[0001] The invention belongs to the field of microwave sensors and relates to a microwave sensor for measuring liquid and a measuring method thereof. Background Art

[0002] Microwave sensors based on resonant circuit design have high sensitivity and can be applied to many fields of detection, such as water content of solid materials, liquid ion concentration, biological cell identification, and protein denaturation degree judgment. At the same time, compared with traditional sensors, microwave sensors also have the advantages of no labeling, low cost, real-time measurement, and non-invasiveness. They are favored by the industry and scientific research community and are widely used in the research and development and automated production of biochemical, food, medical, pharmaceutical and other products.

[0003] In recent years, microwave sensors designed based on different microwave resonant structures have developed rapidly. For example, microwave sensors based on differential microstrip lines, defective ground structures (DGS), step impedance resonators (SIR), split ring resonators (SRR), complementary split ring resonators (CSRR), etched groove lines, etc. These sensors have one or more transmission poles and transmission zeros. When the object to be tested is loaded, the equivalent dielectric constant of the detection area will change, resulting in a significant change in the frequency, amplitude, and quality factor of the transmission poles and zeros. Therefore, the characteristics of the object to be tested can be characterized by detecting the relevant characteristics of the resonance point. However, there are still some unresolved problems in the field of liquid detection of microwave sensors: 1) The manufacturing cost of microwave circuits and microfluidic channels is high; 2) Existing detection methods often cannot reuse circuits and devices, which not only increases costs, but also introduces unnecessary system errors when the device is disassembled and assembled multiple times, seriously affecting the detection accuracy; 3) There is still room for improvement in the detection range and sensitivity. Summary of the invention

[0004] In order to overcome the deficiencies of the prior art, the present invention provides a microwave sensor for measuring liquids and a measuring method thereof. The sensor proposed in the present invention is composed of a microwave circuit and a quantitative and recyclable testing device, and is mainly used to stably, accurately and cyclically measure the electrical properties of liquids, thereby achieving the measurement of liquids. The microwave sensor proposed in the present invention is simple to operate, portable, low-cost, highly sensitive, and has excellent robustness. At the same time, it also solves the problem that traditional microwave sensors have a narrow application range and cannot repeatedly measure liquids or reuse different types of liquids, providing a more convenient, efficient and low-cost means of implementation for non-invasive detection of automated production systems and biomedicine.

[0005] In order to achieve the above object, the present invention adopts the following technical solutions:

[0006] A microwave sensor for measuring liquid comprises a microwave circuit and a liquid container, wherein the microwave circuit comprises a dielectric substrate and a top metal and a bottom metal arranged on two side surfaces of the dielectric substrate, wherein the top metal comprises a 50-ohm microstrip line for feeding and a resonator for generating transmission zeros and transmission poles, wherein the resonator forms a coupling structure with the 50-ohm microstrip line, and the bottom metal is used for grounding, wherein one or a combination of the 50-ohm microstrip line and the resonator is provided with a via hole for connecting with the bottom metal; wherein a cavity for accommodating the liquid to be measured is provided in the liquid container, wherein an opening for adding a sample and a bottom small hole for draining the liquid are formed at the upper and lower ends of the cavity respectively, and flat square small holes for the microwave circuit to penetrate in the transverse direction are respectively provided on opposite sides of the liquid container so that the top metal is located in the cavity.

[0007] Furthermore, the resonator is a third-order I-shaped coupling line.

[0008] Furthermore, the 50-ohm microstrip lines are arranged relatively to each other, the third-order I-shaped coupling line is arranged between the two opposite ends of the two 50-ohm microstrip lines, and the two ends of the third-order I-shaped coupling line respectively form a third-order coupling structure with the two 50-ohm microstrip lines.

[0009] Furthermore, the via holes are respectively arranged at the extended ends of the third-order I-shaped coupling lines.

[0010] Furthermore, the via holes are respectively arranged at the extended ends of the third-order I-shaped coupling line and at two opposite ends of the 50-ohm microstrip line.

[0011] Furthermore, mechanical holes for installing SMA radio frequency interfaces are respectively provided at two lateral ends of the dielectric substrate.

[0012] Furthermore, the liquid container is made of resin material and 3D printing manufacturing process.

[0013] A method for measuring a microwave sensor for measuring liquid, comprising the following steps:

[0014] Step 1: Install microwave circuit and test device to form microwave sensor;

[0015] Step 2: Connect the vector network analyzer through the SMA RF interface to perform S parameter detection on the unloaded microwave sensor to obtain the resonant frequency f ref and amplitude A ref ;

[0016] Step 3: Add a sample of a set solution with a known dielectric constant to the unloaded test device, perform S parameter detection, and obtain the resonant frequency f x and amplitude A x;

[0017] Step 4: By resonant frequency f ref Subtract the resonant frequency f x Get the resonant frequency Δf x , through the amplitude A ref Subtract amplitude A x Get the amplitude ΔA x , fitting the dielectric constant and resonant frequency Δf of the solution sample x and amplitude ΔA x The mathematical expression of , generates the formula for the liquid dielectric constant;

[0018] Step 5: Add other solution samples with known dielectric constants to the unloaded test device, perform S parameter detection, and obtain the resonant frequency f x ' and amplitude A x ', minus the resonant frequency f ref and amplitude A ref To obtain the resonant frequency Δf x ' and amplitude ΔA x ', substitute it into the liquid dielectric constant formula in step 4 to verify.

[0019] Furthermore, in step 2, the S parameter detection is performed multiple times to obtain the average value and obtain the resonant frequency f ref and amplitude A ref .

[0020] Furthermore, the cutoff frequency of the SMA radio frequency interface is greater than or equal to 40 GHz.

[0021] In summary, the present invention is beneficial in that:

[0022] The present invention makes the detection process of microwave sensors simpler and more efficient, and is also convenient to carry and operate. At the same time, it can also further reduce the manufacturing and measurement costs of microwave sensors. In addition, the sensor is convenient for quantitative control of liquids during testing, and only requires one instrument calibration. The pseudo-differential scheme used in data processing can greatly reduce system errors, making the sensor more robust. The invention also has a broad application field: when the liquid storage tank is not loaded, the dielectric constant of solid plates can be detected; after loading a quantitative and cyclic testing device, it can be used for the detection and resolution of most liquids. Therefore, the invention solves the problem that traditional microwave sensors have a narrow application range and cannot repeatedly measure liquids or reuse different types of liquids. It can provide a more convenient, efficient and low-cost implementation method for non-invasive detection of automated production systems and biomedicine. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 It is a schematic diagram of the structure of the microwave circuit in the present invention.

[0024] Figure 2 for Figure 1 Circuit diagram of microwave circuit.

[0025] Figure 3 It is a schematic diagram of the structure of the testing device in the present invention.

[0026] Figure 4 for Figure 3 Schematic diagram of the test device.

[0027] Figure 5 It is a schematic diagram of the use status of the microwave circuit and the testing device in the present invention.

[0028] Figure 6 This is the simulation result diagram of the sensor return loss S11 changing with the dielectric constant.

[0029] Figure 7 This is the simulation result diagram of the sensor insertion loss S21 changing with the dielectric constant.

[0030] Figure 8 This is the microwave circuit layout in via embodiment 1.

[0031] Fig. 9 This is a diagram of the S11 simulation results in via embodiment 1.

[0032] Fig.10 This is a diagram of the S21 simulation results in via embodiment 1.

[0033] Fig.11 This is the microwave circuit layout in via embodiment 2.

[0034] Fig.12 This is a diagram of the S21 simulation results in via embodiment 2. DETAILED DESCRIPTION

[0035] The following describes the embodiments of the present invention by specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict.

[0036] It should be noted that the illustrations provided in the following embodiments are only schematic illustrations of the basic concept of the present invention, and thus the drawings only show components related to the present invention rather than being drawn according to the number, shape and size of components in actual implementation. In actual implementation, the type, quantity and proportion of each component may be changed arbitrarily, and the component layout may also be more complicated.

[0037] All directional indications in the embodiments of the present invention (such as up, down, left, right, front, back, horizontal, vertical...) are only used to explain the relative position relationship, movement status, etc. between the components in a certain specific posture. If the specific posture changes, the directional indication will also change accordingly.

[0038] The present invention provides a microwave sensor for measuring liquid, comprising a microwave circuit and a quantitative and cyclic testing device, wherein the microwave circuit is used to convert the dielectric constant of liquid or solid into a change in transmission characteristics, and the testing device is used to hold a liquid sample to be tested.

[0039] Reference Figure 1 and Figure 2 As shown, the microwave circuit includes a dielectric substrate, a top metal, a bottom metal, and a via, wherein the dielectric substrate can be made of materials commonly used in various microwave devices, such as a Rogers substrate, a ceramic material, etc., and a planar structure circuit can be designed on the top and bottom thereof.

[0040] A pair of mechanical holes are respectively reserved on both sides of the dielectric substrate for installing the SMA RF interface during detection.

[0041] The top metal layer is designed with metal copper and is arranged on the front side of the dielectric substrate, including a 50-ohm microstrip line and a resonator. The 50-ohm microstrip line is used for feeding, and the resonator is preferably a third-order I-shaped coupling line, which is used to generate transmission zeros and poles and is the main structure for providing sensing function. The 50-ohm microstrip line is arranged in a symmetrical structure, and the third-order I-shaped coupling line is arranged between the opposite ends of the two 50-ohm microstrip lines. The left and right sides of the I-shaped coupling line respectively form a third-order coupling structure with the two 50-ohm microstrip lines. The above-mentioned top metal design is only one embodiment of the present invention. In actual use, the third-order I-shaped coupling line can also be replaced by other suitable microwave resonators, including but not limited to open-circuit short stubs, split ring resonators, etc.

[0042] The bottom metal is designed with metallic copper, which covers the back side of the dielectric substrate and provides a ground terminal for the entire microwave circuit.

[0043] The via hole is embedded in the dielectric substrate, and the two ends of the via hole are connected to the top metal and the bottom metal, providing a grounding effect for some ports of the third-order I-shaped coupling line.

[0044] The location of the via hole can be flexibly arranged according to the actual test sample type and test frequency band, and any point in the microstrip line or resonator can be grounded.

[0045] Reference Figure 3 and Figure 4As shown, the testing device includes a liquid container and a bottom hole, wherein the liquid container is a hollow cubic structure, a cavity for accommodating the liquid to be tested is arranged in the liquid container, the top of the cavity is an opening, the bottom hole is arranged at the bottom of the liquid container, the bottom hole is connected to the cavity for discharging the liquid in the cavity, and the bottom hole can be closed by a rubber stopper.

[0046] Flat square holes are respectively provided on opposite sides of the liquid container to allow the dielectric substrate to pass through horizontally. Figure 5 As shown, when the dielectric substrate is set, the position of the third-order I-shaped coupling line is in the cavity, so that when the liquid to be tested is poured into the cavity, the third-order I-shaped coupling line can be immersed in the liquid to be tested, and the mechanical holes on both sides of the dielectric substrate are respectively located outside the flat square holes on both sides.

[0047] Preferably, the testing device is made of resin material and manufactured by 3D printing.

[0048] When performing liquid detection, insert the microwave circuit into the liquid container and install SMA RF interfaces at both ends of the microwave circuit to transmit the microwave signal to the 50 ohm microstrip line. The microwave signal will generate transmission zeros and poles through the third-order I-shaped coupling line, and produce signal amplitude and phase mutations at some frequency points. After different liquids are injected into the liquid container, different electromagnetic field distributions will appear above the third-order I-shaped coupling line, resulting in regular changes in transmission zeros and poles, and the changes can be obtained through S parameters. After software simulation and a large number of sample tests, the relationship curve between the S parameters and the liquid to be tested can be obtained, thereby realizing liquid sensing.

[0049] In addition, the microwave circuit can also directly sense solid materials. By placing the solid material close to the top of the third-order I-shaped coupling line, the dielectric constant of the solid material can be measured.

[0050] The present invention also provides a microwave sensor measurement method for measuring liquids. The method adopts a pseudo-differential approach and uses a test device to limit the liquid to be measured above the detection area to measure the scattering parameters. The measurement of the liquid dielectric constant and the distinction between different liquids are achieved through the changes in the transmission pole frequency and amplitude. The measurement method is applicable to sensors made of any microwave / millimeter wave planar structure resonator, and can also be applied to the above-mentioned microwave sensors.

[0051] When the liquid to be tested is a general solution, the determination method comprises the following steps:

[0052] Step 1: Sensor design, simulation and manufacturing. Obtain the structural parameter design of the microwave circuit and test device, set the size parameters of the microwave circuit and test device, and set the position of the vias through the simulation software. After the design is completed, the microwave circuit is manufactured on the Rogers5880 substrate, and the test device is manufactured through 3D printing.

[0053] Step 2: After step 1, install the microwave sensor. Insert the microwave circuit into the test device and install the SMA RF interface through the mechanical holes on both sides of the microwave sensor. The SMA RF interface preferably has a cutoff frequency greater than or equal to 40 GHz. Then use a small amount of vaseline to seal the gap between the microwave sensor and the flat square hole to prevent leakage, and insert a rubber plug of the corresponding size into the bottom hole to seal it.

[0054] Step 3: After step 2, calibrate the sensor. Connect the vector network analyzer to the SMA RF interface to perform S parameter detection of the unloaded microwave sensor. When the vector network analyzer shows a stable S parameter image, save the data and record the frequency and amplitude of the resonant point in the S parameter curve. In order to ensure the accuracy and reliability of the data, multiple measurements are taken to obtain the average value. The final processed resonant frequency and amplitude are recorded as f ref and A ref , as the basis for subsequent differential calculations.

[0055] Step 4: After step 3, test the liquid with known dielectric constant. Inject the solution with known dielectric constant from the top of the liquid container with a syringe and perform S parameter detection. After recording the resonant frequency and amplitude, open the bottom rubber plug, discharge the waste liquid that has been measured from the bottom hole, and inject deionized water from the top several times to clean the inside of the liquid container and the surface of the microwave sensor. Then prepare other liquids with known dielectric constants, repeat the experiment using the same test method for liquids with known dielectric constants, and record the resonant frequency and amplitude obtained from each test as f x and A x In order to ensure the accuracy of the data, this step also recommends taking multiple measurements and taking the average value.

[0056] Step 5: After step 4, perform pseudo-difference fitting of the complex dielectric constant function of the liquid. x and A x Subtract f respectively ref and A ref , we get the resonant frequency and amplitude after subtraction, which are denoted as Δf x and ΔA xThese data are the values ​​after eliminating the error of the test device and the influence of the environment, and can be used to characterize the dielectric constant of the liquid. Combined with the known dielectric constant of the liquid, a rational function or matrix can be used to fit the dielectric constant and Δf x and ΔA x The mathematical expression of , generates the formula for the dielectric constant of the liquid.

[0057] Step 6: After step 5, conduct an experiment to verify the accuracy of the liquid dielectric constant formula. Use the test method for the known dielectric constant liquid in step 4, and prepare other liquids with known dielectric constants to measure the corresponding Δf x ' and ΔA x '. Substitute the liquid dielectric constant formula in step 5 for verification. If the calculated dielectric constant is close to the actual situation, it can be proved that the sensor has the ability to accurately detect liquids.

[0058] Furthermore, the microwave sensor is simulated and verified using three-dimensional electromagnetic simulation software. After model building and full-wave simulation, the Figure 6 and Figure 7 As shown in the S parameter waveform, in the 2-10GHz frequency band, both the return loss S11 and the insertion loss S21 will produce a peak. When the dielectric constant changes, the frequency and amplitude of the peak will also change accordingly. This proves that the transmission characteristics of the microwave sensor are sensitive to the dielectric constant and can be used for liquid resolution and dielectric constant detection.

[0059] Furthermore, the simulation data was fitted using the above method. According to the simulation results of return loss S11, f ref and A ref 10.84Ghz and 2.76dB respectively. Figure 6 The data in can be used to calculate the liquid dielectric constant and Δf x and ΔA x The quadratic rational function of is:

[0060]

[0061] Similarly, according to the insertion loss S21 simulation results, f ref and A ref 10.88Ghz and 11.86dB respectively. Figure 7 The data in can be used to calculate the liquid dielectric constant and Δf x and ΔA x The quadratic rational function of is:

[0062]

[0063] In actual application, you can choose the one that is more conducive to testing from the return loss and insertion loss for testing.

[0064] Furthermore, the microwave circuit proposed by the present invention can flexibly design the position of the via hole according to the actual situation, and the test device and the measurement method are also applicable to the modified microwave circuit. The change of the via hole position will cause the change of the transmission characteristics and resonance characteristics of the microwave circuit. With this change, a sensor suitable for different dielectric constant ranges and multiple test frequency ranges can be designed, and finally the multifunctionality of the sensor can be realized.

[0065] The via embodiment 1:

[0066] Reference Figure 8 The topological structure shown in the figure, the vias are arranged at the positions of the four extended ends of the third-order I-shaped coupling line. Based on the microwave sensor in this embodiment, the above-mentioned determination method is used to distinguish the liquid and detect the dielectric constant, and the three-dimensional electromagnetic simulation software is used for simulation verification to obtain Fig. 9 and Fig.10 As shown in the S parameter waveform, in the 9-13 GHz frequency band, the return loss S11 and the insertion loss S21 of this embodiment will both produce a peak (transmission pole). The greater the dielectric constant of the liquid to be measured, the lower the frequency of the peak.

[0067] Fig. 9 and Fig.10 It shows that the transmission characteristics of this via design are similar to those of the original design, so the fitting scheme described above can be used for fitting. However, the resonant frequency of this scheme is significantly higher than Figure 6 and Figure 7 Therefore, if the test instrument is convenient for high-frequency testing, this via design solution can be used.

[0068] The via embodiment 2:

[0069] Reference Fig.11 In the topological structure shown in FIG. 1 , vias are arranged at the four extended ends of the third-order I-shaped coupling line and at the two opposite ends of the two 50-ohm microstrip lines. Based on the microwave sensor in this embodiment, the above-mentioned determination method is used to distinguish the liquid and detect the dielectric constant, and the three-dimensional electromagnetic simulation software is used for simulation verification to obtain Fig.12 The S parameter waveform shown in the figure shows that this embodiment will form a high-pass transmission characteristic. The larger the dielectric constant of the liquid to be measured, the lower the cut-off frequency.

[0070] Since the transmission characteristics of this scheme are different from those of the original design and Example 1, a new fitting scheme can be used: fitting by the lower cutoff frequency, or fitting by the insertion loss at a single frequency point.

[0071] If the test instrument supports multi-band testing, the lower cutoff frequency f can be selected. L Characterize the dielectric constant of the liquid to be tested. Fig.12 For the data in , a cubic rational function fitting is performed with a lower cutoff frequency of -3dB:

[0072]

[0073] If the test instrument only supports single-frequency testing, you can use the insertion loss fitting of a single frequency point. Fig.12 The data in the figure are used to fit the cubic rational function using the insertion loss S21 at 5GHz:

[0074] ε 待测液体 =0.0024(S21) 3 -0.1278(S21) 2 +0.8891(S21)+19.155

[0075] The above-mentioned Embodiment 1 and Embodiment 2 prove that the microwave sensor can perform personalized liquid detection by flexibly arranging the positions of the via holes.

[0076] Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

Claims

1. A microwave sensor for measuring liquid, characterized in that: The invention comprises a microwave circuit and a liquid container, wherein the microwave circuit comprises a dielectric substrate and a top metal and a bottom metal arranged on two side surfaces of the dielectric substrate, the top metal comprises a 50-ohm microstrip line for feeding and a resonator for generating transmission zeros and transmission poles, the resonator and the 50-ohm microstrip line form a coupling structure, the bottom metal is used for grounding, and one or a combination of the 50-ohm microstrip line and the resonator is provided with a via hole for connecting with the bottom metal; a cavity for accommodating a liquid to be tested is provided in the liquid container, an opening for adding a sample and a bottom hole for draining the liquid are formed at the upper and lower ends of the cavity respectively, and flat square holes for the microwave circuit to penetrate in the transverse direction are respectively provided on opposite sides of the liquid container so that the top metal is located in the cavity.

2. A microwave sensor for measuring liquid according to claim 1, characterized in that: The resonator is a third-order I-shaped coupling line.

3. A microwave sensor for measuring liquid according to claim 2, characterized in that: The 50-ohm microstrip lines are arranged opposite to each other, the third-order I-shaped coupling line is arranged between the two opposite ends of the two 50-ohm microstrip lines, and the two ends of the third-order I-shaped coupling line respectively form a third-order coupling structure with the two 50-ohm microstrip lines.

4. A microwave sensor for measuring liquid according to claim 3, characterized in that: The via holes are respectively arranged at the extended ends of the third-order I-shaped coupling lines.

5. A microwave sensor for measuring liquid according to claim 3, characterized in that: The via holes are respectively arranged at the extended end of the third-order I-shaped coupling line and at two opposite ends of the 50-ohm microstrip line.

6. A microwave sensor for measuring liquid according to claim 1, characterized in that: Mechanical holes for installing SMA radio frequency interfaces are respectively provided at two lateral ends of the dielectric substrate.

7. A microwave sensor for measuring liquid according to claim 1, characterized in that: The liquid container is made of resin material and 3D printing manufacturing process.

8. A method for measuring liquid using a microwave sensor, characterized in that: The following steps are involved: Step 1: Install a microwave circuit and a liquid container to form a microwave sensor; Step 2: Connect the vector network analyzer through the SMA RF interface to perform S parameter detection on the unloaded microwave sensor to obtain the resonant frequency f ref and amplitude A ref ; Step 3: Add a sample of a set solution with a known dielectric constant to the unloaded test device, perform S parameter detection, and obtain the resonant frequency f x and amplitude A x ; Step 4: By resonant frequency f ref Subtract the resonant frequency f x Get the resonant frequency Δf x , through the amplitude A ref Subtract the amplitude A x Get the amplitude ΔA x , fitting the dielectric constant and resonant frequency Δf of the solution sample x and amplitude ΔA x The mathematical expression of , generates the formula for the liquid dielectric constant; Step 5: Add other solution samples with known dielectric constants to the unloaded test device, perform S parameter detection, and obtain the resonant frequency f x ' and amplitude A x ', minus the resonant frequency f ref and amplitude A ref To obtain the resonant frequency Δf x ' and amplitude ΔA x ', substitute it into the liquid dielectric constant formula in step 4 to verify.

9. The microwave sensor measuring method for measuring liquid according to claim 7, characterized in that: In step 2, the S parameter detection is performed multiple times to obtain the average value and obtain the resonant frequency f ref and amplitude A ref .

10. The microwave sensor measuring method for measuring liquid according to claim 7, characterized in that: The cutoff frequency of the SMA RF interface is greater than or equal to 40 GHz.

Images