A dual-QCM difference frequency humidity sensor measurement method and device

Abstract

The present invention discloses a dual-QCM difference-frequency humidity sensor measurement method and device, which are applied to the field of electronics and circuit technology and address the problem of low humidity measurement accuracy in existing systems. The present invention utilizes the characteristic that dual QCMs output different frequencies in humidity environments to measure the output frequencies of the dual-QCM humidity sensors separately, perform difference frequency calculations, and reduce the influence of QCM output frequency drift. Frequency difference-humidity data fitting is performed using the frequency difference data. Subsequently, the frequency difference of the dual QCMs is converted into humidity data using a fitting relationship, thereby achieving humidity measurement. The present invention also introduces a dual-channel measurement scheme and a fitting algorithm to further reduce the error in humidity measurement results.

Description

Technical Field

[0001] The present invention belongs to the field of electronics and circuit technology, and in particular relates to a humidity measurement technology and device. Background Art

[0002] A QCM humidity sensor uses a humidity-sensitive film on the electrode surface of a quartz crystal resonator as the sensing element, and the quartz crystal resonator as the transducer element. It utilizes the inverse piezoelectric effect and surface mass sensitivity of the quartz crystal to achieve sensing measurements. Its characteristics are that when an electric field is applied to the quartz crystal, the crystal surface undergoes mechanical deformation. If the electric field is alternating, mechanical oscillations occur within the crystal lattice, causing the quartz crystal to vibrate at a specific frequency. When the frequency of the alternating voltage matches the crystal's natural frequency, resonance occurs, and the mechanical vibration reaches its maximum. This frequency is also called the resonant frequency. Under specific conditions, when a substance to be measured is adsorbed on the electrode surface of a quartz crystal, its natural frequency changes, and the amount of frequency change is related to the adsorbed mass. According to the Sauerbrey equation, since the fundamental frequency and electrode area of ​​a given quartz crystal are fixed, the change in the quartz crystal's frequency has a simple linear relationship with the change in the mass attached to the crystal surface electrodes.

[0003] Therefore, a thin film of humidity-sensitive material is plated on the electrode surface of a quartz crystal to make a QCM humidity sensor. The sensor is placed in a humid environment. When the humidity-sensitive film adsorbs water vapor molecules in the air, the surface quality of the sensor changes. The output frequency change of the QCM is measured, and the humidity of the environment to be measured can be obtained by processing and converting the resonant frequency.

[0004] Existing humidity measurement methods include gravimetric, electrolytic, capacitive, chilled mirror, and vibration frequency methods. While the gravimetric method offers high precision, it is cumbersome to operate and requires sufficient moisture absorption, making it particularly challenging for low-humidity gases. The electrolytic method offers good measurement accuracy and is inexpensive, but the electrolytic cell must be dried for a long time before use and has poor resistance to contamination. The capacitive method offers a very low measurement range and a fast response, but most suffer from significant aging and drift. While the chilled mirror method offers advantages such as a wide measurement range, low drift, and high precision, it responds slowly in low-humidity environments and is complex to operate. Furthermore, it places high demands on gas cleanliness and corrosiveness, making it generally used only as a humidity benchmark in laboratories. The QCM humidity sensor, a typical example of the vibration frequency method, offers potential advantages such as high sensitivity, excellent stability, high accuracy, a wide measurement range, low cost, and ease of on-site continuous monitoring. However, QCM humidity sensors require the selection of a suitable humidity-sensitive film to fabricate high-performance QCM humidity sensors for high-precision humidity measurement. However, frequency drift is a common problem with QCM humidity sensors, particularly in environments with high relative humidity (greater than 75% RH). The resonant frequency of a QCM is typically measured using an oscillation circuit method. This method employs a quartz crystal as a frequency-selective element connected to a positive feedback circuit to form an oscillator circuit. The circuit's oscillation frequency is the resonant frequency of the quartz crystal. The circuit's output is connected to a digital frequency meter to measure the signal, which provides the resonant frequency of the quartz crystal. This oscillation circuit method offers advantages such as simplicity and speed, making it a commonly used method for measuring the resonant frequency of quartz crystals. Summary of the Invention

[0005] To solve the above technical problems, the present invention proposes a dual-QCM difference-frequency humidity sensor measurement method and device. Taking advantage of the fact that dual QCMs output different frequencies in humidity environments, the output frequencies of the dual-QCM humidity sensors are measured separately, and the difference frequency is calculated. The frequency difference data is used to perform frequency difference-humidity data fitting. Then, the frequency difference of the dual-QCM is converted into humidity data through the fitting relationship to achieve humidity measurement.

[0006] One of the technical solutions adopted by the present invention is: a dual-QCM difference frequency humidity sensor measurement method, comprising: respectively measuring the output frequencies of two humidity sensors with different sensitivities in a known humidity environment, and calculating the frequency difference in the known humidity environment; fitting the obtained frequency differences corresponding to multiple different known humidity environments with frequency difference-humidity data through a fitting algorithm to obtain a frequency-humidity relationship; then measuring the output frequencies of the two humidity sensors with different sensitivities in the humidity environment to be measured, and calculating the frequency difference in the humidity environment to be measured; using the fitted frequency-humidity relationship, converting the frequency difference in the humidity environment to be measured into humidity data, thereby realizing humidity measurement of the humidity environment to be measured.

[0007] The second technical solution adopted by the present invention is: a dual QCM difference frequency humidity sensor measuring device; comprising: two QCM humidity sensors with different humidity sensitivities, respectively named QS1 and QS2, and also comprising: an FPGA core board;

[0008] The FPGA module mainly includes two parts: a digital frequency counter and an embedded soft core;

[0009] The digital frequency meter includes: a multiplexer module, a clock signal generation module, a pulse counting module, and an operation module; the frequency measurement process of QS1 is:

[0010] The multiplexer first connects the clock frequency signal to be measured, clk_fx, to the input port of the digital frequency meter. The clock signal generation module generates a reference clock signal, CLK_FS, of equal frequency precision. Under the clock frequency signal to be measured, clk_fx, the pulse counting module uses its internal register to generate a gating signal for counting the signal to be measured. The rising and falling edges of the reference clock signal, CLK_FS, and the clock signal to be measured, clk_fx, are simultaneously captured to ensure that the counting is triggered and terminated within the gating signal. Within the gating signal, the pulse counting module counts the clock signal to be measured, clk_fx, and the reference clock signal, CLK_FS, respectively. Finally, the operation module converts the pulse counting results of the reference clock signal, CLK_FS, and the clock signal to be measured, clk_fx, to obtain the frequency measurement value of the clock signal to be measured, and stores the result in the first register, reg_1, in the operation module.

[0011] After completing the QS1 frequency measurement, the multiplexer switches the input signal of the digital frequency meter, connects the output frequency signal of QS2 to the digital frequency meter, and performs the same frequency measurement process as QS1 to obtain the frequency measurement value of QS2. The result is stored in the second register reg_2 in the operation module. Then, the input signal of the digital frequency meter is switched again through the multiplexer, and the output frequency of QS1 is connected to the digital frequency meter. This cycle is repeated to achieve the frequency measurement of the dual-channel QCM humidity sensor. At the same time, after each dual-channel QCM humidity sensor frequency measurement is completed, the operation module will also read the frequency measurement values ​​of QS1 and QS2 stored in its internal first register reg_1 and second register reg_2, perform difference frequency calculation, and obtain frequency difference data.

[0012] The embedded soft core includes: Nios II processor and Avalon bus module; Nios II processor reads the frequency difference data output by the operation module through the Avalon bus module; then Nios II processor fits the frequency difference-humidity data through the fitting algorithm to obtain a frequency-humidity relationship as shown in formula (1):

[0013] f(x)=ax 2+bx+c (1)

[0014] Where x is the frequency, f(x) is the humidity conversion result corresponding to the current frequency, and a, b, and c are correlation coefficients.

[0015] When measuring humidity, the Nios II processor converts the frequency difference to humidity using the frequency-humidity relationship.

[0016] The embedded soft core also includes a UART serial port module, which displays the frequency measurement results and the frequency-humidity relationship, as well as the impact of humidity measurement, on the PC side through the UART serial port module.

[0017] The beneficial effects of the present invention are as follows: the present invention uses the frequency difference value output by the dual QCM humidity sensor to perform frequency difference-humidity data fitting, and the determination coefficient of the obtained fitting relationship is higher; a fitting algorithm is introduced, and the correlation coefficients a, b, and c in the obtained frequency-humidity relationship are saved as double-precision floating-point types. When performing frequency-to-humidity data conversion operations, the correlation coefficient can be used with up to 16 decimal digits (15 decimal places) to participate in the operation, and the humidity value finally obtained is more accurate. At the same time, it also further simplifies the data calibration process before starting humidity measurement. The introduction of the dual-channel measurement scheme and the fitting algorithm further reduces the error of the humidity measurement results. The present invention has the following advantages:

[0018] (1) The present invention makes full use of the inverse piezoelectric effect and surface mass sensitivity effect of QCM. Humidity information can be obtained by measuring the output frequency change of the QCM humidity sensor. Compared with traditional humidity sensors, the structure is simpler and the measurement error is smaller.

[0019] (2) The present invention adopts a dual-channel measurement scheme, using two QCM humidity sensors, and performs frequency difference processing on the output frequencies of the dual QCM humidity sensors. The frequency difference between the two is used for frequency difference-humidity conversion. Experimental verification shows that the influence of the output frequency drift of the QCM humidity sensor is suppressed to a certain extent, further reducing the humidity measurement error.

[0020] (3) The present invention introduces a fitting algorithm to simplify the data calibration process. In the frequency difference-humidity conversion operation process, the correlation coefficient of the fitting relationship is directly calculated with double-precision floating-point numbers, avoiding the situation where the humidity conversion result has large errors or even errors due to insufficient coefficient precision;

[0021] (4) The digital frequency meter designed by the present invention uses FPGA as the core measurement device, which can accurately measure high-frequency signals. Compared with other microprocessors, it has more advantages in measuring high-frequency signals and other complex numerical calculations.

[0022] (5) The present invention designs a multiplexer that allows multiple QCM output frequency signals to be simultaneously connected to the digital frequency meter, so that the digital frequency meter can be effectively reused, promoting multi-channel measurement and sensor arrays with minimal logic resource overhead, and eliminating the need to call multiple digital frequency meters to consume additional resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a schematic diagram of the QCM humidity sensor measurement method provided by the present invention.

[0024] Figure 2 This is a structural block diagram of the QCM humidity sensor measurement device provided by the present invention.

[0025] Figure 3 This is a measurement timing diagram of a digital frequency meter in a measuring device according to an embodiment of the present invention.

[0026] Figure 4 This is a diagram of humidity measurement results achieved by an embodiment of the present invention. DETAILED DESCRIPTION

[0027] To facilitate those skilled in the art to understand the technical content of the present invention, the present invention is further explained below with reference to the accompanying drawings.

[0028] like Figure 1 As shown in FIG, the measurement method of the present invention, the specific steps are as follows:

[0029] (1) Before the measurement begins, two QCM humidity sensors with different humidity sensitivities are prepared and named QS1 and QS2, respectively. QS1 has a higher humidity-frequency response sensitivity. In this embodiment, the sensitivity is at least greater than 20 Hz / % RH, which is considered to be higher. QS2 has a lower humidity-frequency response sensitivity. In this embodiment, the sensitivity is less than 10 Hz / % RH, which is considered to be lower.

[0030] (2) Measure the RH of QS1 and QS2 respectively under a known humidity environment i The output frequency of QS1 and QS2 is calculated and the difference frequency is calculated. By setting the waiting time, the stable frequency difference data DF of QS1 and QS2 is obtained. i In this embodiment, the waiting time is set to 10s;

[0031] (3) Change the humidity environment, i.e. RH i+1 ≠RH i Repeat step (2) at least once to get the new frequency difference data DF i+1 ;

[0032] (4) Obtain frequency difference-humidity data, which are arrays DF[x] and RH[x], respectively, where x ≥ 2;

[0033] (5) Based on the frequency difference-humidity data obtained in step (4), the frequency difference-humidity data is fitted by a fitting algorithm to obtain a frequency-humidity relationship; the correlation coefficient of the fitting relationship is saved as a double-precision floating-point number type, which can be directly used in the frequency difference-humidity conversion operation later;

[0034] (6) Start measuring and place QS1 and QS2 in the humidity environment to be measured;

[0035] (7) By measuring the frequency difference between QS1 and QS2 and using the frequency-humidity relationship fitted in step (5), the frequency difference data of QS1 and QS2 are converted into humidity data to achieve humidity measurement.

[0036] The process of preparing two QCM humidity sensors with different humidity sensitivities in this embodiment is as follows:

[0037] Before film formation, the QCM sensor is cleaned and then placed in a clean container to dry at room temperature for future use. A graphene oxide solution of the required concentration is prepared using deionized water and graphene oxide mother liquor, and a humidity-sensitive film is deposited on the QCM electrode in a variety of ways, such as drop coating, spin coating, self-assembly, spray coating, etc. The drop coating method has the advantages of low cost, simple operation, high flexibility, and short preparation time. Therefore, in this embodiment, a drop coating method is used to form a layer of graphene oxide film on the QCM electrode. The specific operation is as follows: use a microsyringe to take 0.2μL of 2.5mg / mL graphene oxide solution and drop the solution on the surface of the QCM sensor electrode. After that, the graphene oxide film-modified QCM sensor is placed in a drying container and dried at room temperature for 6 hours. After the water is completely evaporated, the graphene oxide film-modified QCM humidity sensor (QS1) is prepared. Using the same method, 0.1μL of 0.5mg / mL graphene oxide film is deposited on the surface of another QCM sensor electrode to complete the preparation of QS2.

[0038] Graphene oxide, with its strong hydrophilicity, large surface area, excellent electrical properties, good stability, and ease of fabrication and processing, is widely used in QCM humidity sensors for humidity detection. The concentration of graphene oxide directly affects sensor sensitivity and hysteresis. Within a certain range, increasing graphene oxide concentration increases sensor sensitivity and decreases hysteresis. The coating weight directly affects the thickness of the sensor surface. A thicker humidity-sensitive layer may take longer to adsorb and desorb water molecules, thereby increasing the sensor's response and recovery times. Conversely, a thinner humidity-sensitive layer may exhibit faster response and recovery times.

[0039] When designing and manufacturing QCM humidity sensors, it is necessary to select appropriate humidity-sensitive material concentrations and coating amounts according to specific application requirements to obtain optimal sensor performance. At the same time, experimental verification and optimization are also required to ensure that the sensor can meet performance requirements in actual use. According to the results of current experimental tests, the concentration of graphene oxide film is in the range of 2mg / mL to 5mg / mL, and the coating amount is in the range of 0.1μL to 0.3μL. The QCM humidity sensor can show good performance, and humidity measurement can be achieved in combination with the dual-channel measurement solution of the present invention. The output frequencies of QS1 and QS2 are subtracted and frequency-reduced. In this embodiment, in the range of 0%RH-97%RH, the fitting determination coefficient R of QS1 is 2 =0.99, the fitting determination coefficient R of QS2 2 =0.97. Both QS1 and QS2 have good linearity. However, the sensitivities of QS1 and QS2 are different. The frequency difference also shows a gradually increasing trend. The frequency difference is also linearly related to humidity, which enables dual-channel humidity measurement. Since the output frequencies of the two sensors tend to decrease with increasing humidity, the frequency drift is reduced, thereby reducing the humidity measurement error.

[0040] For the reference sensor (QS2) in the dual-channel measurement, this embodiment attempted to use a sensor without a humidity-sensitive film as the reference sensor. However, this sensor exhibited minimal frequency shift across the entire 0% RH-97% RH range, and its output frequency exhibited irregularity, which could affect the humidity measurement results. The core concept of the present invention's measurement scheme is to downconvert the output frequency and compensate for frequency drift. If the reference sensor's frequency shift is excessive, or even close to that of the measurement sensor (i.e., the output frequency response curves of the reference and measurement sensors are nearly parallel), downconverting the frequency difference using dual-channel subtraction will result in a constant frequency difference, making subsequent frequency-humidity conversion impossible. The present invention prepared QS2 by coating the reference sensor with 0.1 μL of a 0.5 mg / mL graphene oxide solution. This resulted in a smaller frequency shift across the 0% RH-97% RH range, but with good linearity, and a pattern in which the output frequency gradually decreases with increasing humidity.

[0041] Explanation on the sensitivity of two QCM humidity sensors with different humidity sensitivities:

[0042] The sensitivity of a QCM humidity sensor is defined as the frequency change (Hz / %RH) per one percent change in relative humidity (%RH). Under this standard, a sensor that produces a large frequency change in response to humidity changes is considered to have high sensitivity. Generally speaking, a QCM humidity sensor with a sensitivity of tens of Hz / %RH or even higher is considered to have high sensitivity. In this paper, the sensitivity of QS1 is approximately 42.8 Hz / %RH, and the sensitivity of QS2 is approximately 6.2 Hz / %RH.

[0043] Figure 2 This is a block diagram of the QCM humidity sensor measurement device, which implements the QCM humidity sensor measurement method of the present invention. This device can be used to measure humidity in various humidity environments. It primarily includes two QCM humidity sensors with different humidity sensitivities, designated QS1 and QS2. QS1 has a higher humidity-frequency response sensitivity, while QS2 has a lower humidity-frequency response sensitivity. It also includes an FPGA core board and an OLED display.

[0044] Frequency measurement, data fitting, and frequency difference-humidity data conversion are completed through FPGA. The FPGA module mainly consists of a digital frequency counter and an embedded soft core. The digital frequency counter designed based on the equal-precision measurement method processes the frequency signals from QS1 and QS2, measures the output frequencies of QS1 and QS2, calculates the difference frequency, sets a waiting time, and then performs frequency difference-humidity data fitting after the frequency difference data stabilizes. The embedded soft core processor can convert the output frequency difference of QS1 and QS2 into humidity data by fitting the relationship, and simultaneously drives the OLED display to display information.

[0045] The response time of the QCM humidity sensor refers to the time it takes for the output frequency to stabilize after the QCM humidity sensor is placed in a new humidity environment. The response time of the QCM humidity sensor used in this invention is approximately 5s (±1s). Therefore, after completing the frequency measurement and frequency difference calculation, the waiting time is theoretically set to greater than 5s to read the stable frequency difference value corresponding to the current humidity environment and perform frequency difference-humidity data fitting.

[0046] The FPGA module is mainly composed of a digital frequency counter and an embedded soft core; the digital frequency counter, as a coprocessor, mainly includes a multiplexer module, a clock signal generation module, a pulse counting module, and an operation module, among which the operation module is mainly composed of a multiplier and a divider; the embedded soft core mainly includes a Nios II processor, an Avalon bus module, a data fitting module, a humidity conversion module, a UART serial port module, and an SPI driver module.

[0047] In the frequency measurement phase, the digital frequency counter will measure the output frequencies of QS1 and QS2 respectively and perform the difference frequency calculation. First, the output frequency of QS1 is measured. After the measurement is completed, the input port signal of the digital frequency counter will be switched through the multiplexer. The output frequency of QS2 is measured using the same method. Then, the multiplexer will again connect the output frequency of QS1 to the digital frequency counter, and the cycle will be repeated to realize the dual-channel QCM frequency measurement. Nios II completes the frequency difference-data fitting through the fitting algorithm and completes the frequency difference-humidity conversion using the frequency-humidity relationship. Through the SPI driver module, the frequency measurement value and humidity measurement results during humidity measurement, as well as other information, are displayed on the OLED display.

[0048] The working principle of a digital frequency meter is:

[0049] The output frequencies of QS1 and QS2 are connected to the multiplexer at the same time. The multiplexer first connects the output frequency of QS1 to the input port of the digital frequency meter; the reference clock signal CLK_FS of equal precision frequency is generated through the clock signal generation module; the pulse counting module uses its internal register to generate a gating signal for counting the signal to be measured under the clock frequency clk_fx to be measured (i.e., the output frequency of QS1); the rising and falling edges of the reference clock signal CLK_FS and the clock signal to be measured clk_fx are captured at the same time to ensure that the counting is triggered and terminated within the gating signal; within the gating signal, the pulse counting module counts the clock signal to be measured clk_fx and the reference clock signal CLK_FS respectively; the frequency measurement value of the clock signal to be measured clk_fx (i.e., the output frequency of QS1) is obtained by conversion through the operation module, and The result is stored in the first register reg_1 in the operation module; at this time, the multiplexer switches the input signal of the digital frequency meter, connects the output frequency of QS2 to the digital frequency meter, uses the same measurement method to measure the frequency, obtains the output frequency of QS2, and stores the result in the second register reg_2 in the operation module; after each frequency measurement of the dual-channel QCM humidity sensor is completed, the operation module will also read the frequency measurement values ​​of QS1 and QS2 stored in its internal first register reg_1 and second register reg_2 to complete the difference frequency calculation, that is, calculate the difference between the output frequency of QS2 and the output frequency of QS1 to obtain the final frequency difference value; at the same time, the multiplexer again connects the output frequency of QS1 to the digital frequency meter, and the cycle is repeated to realize real-time measurement of the output frequency of the dual-channel QCM humidity sensor.

[0050] Setting the gate signal time in a digital frequency meter too short can affect measurement accuracy, while setting it too long can affect measurement efficiency. Therefore, it is important to set an appropriate gate signal time to ensure accurate and efficient frequency measurement. In this invention, the gate signal time is set to 1,000,000 cycles of the signal to be measured. The frequency clk_fx of the clock signal to be measured is approximately 16 MHz, meaning that the time required to complete a frequency measurement is approximately 0.063 seconds, ensuring that the digital frequency meter achieves accurate and efficient measurement.

[0051] The working principle of the embedded soft core is:

[0052] The Nios II processor reads the frequency difference output from QS1 and QS2 via the Avalon bus module. The Nios II then uses a data fitting module to fit the frequency difference to humidity data, generating a frequency-humidity equation. The frequency measurement results and the frequency-humidity equation corresponding to the fitting process are displayed on a PC via the UART serial port module. When measuring humidity, the Nios II processor also includes a humidity conversion module, which converts the frequency difference to humidity using the frequency-humidity equation. The frequency and humidity results, along with other information, are displayed on an OLED display via the SPI driver module.

[0053] Figure 3 This is a measurement timing diagram of a digital frequency meter based on the equal-precision measurement principle in an embodiment. The equal-precision measurement method is used to design a digital frequency meter. The most significant feature of this method is that the timing of the gating signal is related to the clock signal to be measured and is an integer multiple of the clock signal to be measured. By setting an appropriate counting time, the pulses of the reference clock signal and the clock signal to be measured are counted separately, and the frequency signal to be measured is obtained through mathematical conversion. The gating signal is controlled by the clock signal to be measured, so that the gating signal is an integer multiple of the clock signal to be measured. While counting the pulses of the reference clock signal will produce a ±1 clock error, because the frequency of the reference clock signal is much greater than that of the clock signal to be measured, the resulting error is minimal.

[0054] The gating signal time needs to be set in advance. In the clock domain of the signal to be measured, the gating signal is generated by counting the pulses of the signal to be measured, ensuring that the gating signal is controlled by the clock signal to be measured and is an integer multiple of the signal to be measured. During the gating time, the pulse counting module counts the pulses of the clock signal to be measured and the reference clock signal in different clock domains respectively. The multiplier and divider in the operation module complete the mathematical operation of formula (2) based on the pulse counting results to obtain the frequency measurement value of the clock signal to be measured.

[0055]

[0056] Where clk_fx is the measured value of the clock frequency to be measured, fx_cnt is the pulse count of the clock signal to be measured, CLK_FS is the reference clock signal frequency, and fs_cnt is the pulse count of the reference clock signal.

[0057] Using the above example, a QCM humidity sensor can be used for humidity measurement. Two QCM humidity sensors with different humidity sensitivities, named QS1 and QS2, are used, each with a fundamental frequency of 16 MHz. 0.2 μL of a 2.5 mg / mL graphene oxide solution is coated on QS1, and 0.1 μL of a 0.5 mg / mL graphene oxide solution is coated on QS2. This results in a higher humidity-frequency response sensitivity for QS1 and a lower humidity-frequency response sensitivity for QS2. Saturated salt solutions are then added to a sealed container to simulate a sealed humidity environment. Seven salt solutions are used, simulating humidity environments of 0% RH, 11% RH, 33% RH, 54% RH, 75% RH, 84% RH, and 97% RH. Data calibration is performed before humidity measurement. The measurement device is set to data calibration mode. QS1 and QS2 are placed in seven different relative humidity environments. The measurement device automatically completes frequency measurement and data fitting. After completing the data calibration, switch the measuring device to the humidity measurement mode, start the humidity measurement, place QS1 and QS2 in the humidity environment to be measured, and finally display the humidity measurement results on the OLED display. Repeat the humidity measurement experiment several times to obtain the following Figure 4 The humidity measurement results shown are Figure 4 The reference line in the diagram is a straight line with a slope of 1 passing through the origin, indicating the degree to which each measurement result deviates from the correct value. Experimental data demonstrates that the method and apparatus proposed in this invention can achieve humidity measurement with a maximum error of less than ±3% RH.

[0058] Those skilled in the art will appreciate that the embodiments described herein are intended to aid the reader in understanding the principles of the present invention, and it should be understood that the scope of the present invention is not limited to such specific descriptions and embodiments. Various modifications and variations are readily apparent to those skilled in the art. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention are intended to be included within the scope of the claims.

Claims

1. A dual-QCM difference frequency humidity sensor measuring device, characterized in that: include: Two QCM humidity sensors with different humidity sensitivities, named QS1 and QS2, also include: FPGA core board; The FPGA core board contains two parts: a digital frequency counter and an embedded soft core; The digital frequency meter includes: a multiplexer module, a clock signal generation module, a pulse counting module, and an operation module; the frequency measurement process of QS1 is: The multiplexer first transmits the clock frequency signal to be measured clk_fx Connect to the input port of the digital frequency meter; generate a reference clock signal of equal precision frequency through the clock signal generation module CLK_FS ; The pulse counting module is at the clock frequency signal to be measured clk_fx In this case, the internal register is used to generate a gate signal to count the signal to be measured; at the same time, the reference clock signal is captured. CLK_ FS and the clock signal to be measured clk_fx The rising and falling edges of the pulse counting module ensure that the counting is triggered and terminated within the gate signal; within the gate signal, the pulse counting module respectively clk_fx and reference clock signal CLK_FS Counting; finally, the operation module counts according to the reference clock signal CLK_FS and the clock signal to be measured clk_fx The pulse counting result is converted into the clock signal to be measured clk_fx The frequency measurement value is stored in the first register reg_1 in the operation module; After completing the QS1 frequency measurement, the multiplexer switches the input signal of the digital frequency meter, connects the output frequency signal of QS2 to the digital frequency meter, and performs the same frequency measurement process as QS1 to obtain the frequency measurement value of QS2. The result is stored in the second register reg_2 in the operation module. Then, the input signal of the digital frequency meter is switched again through the multiplexer, and the output frequency of QS1 is connected to the digital frequency meter. This cycle is repeated to achieve the frequency measurement of the dual-channel QCM humidity sensor. At the same time, after each dual-channel QCM humidity sensor frequency measurement is completed, the operation module will also read the frequency measurement values ​​of QS1 and QS2 stored in its internal first register reg_1 and second register reg_2, perform difference frequency calculation, and obtain frequency difference data. The embedded soft core includes a Nios II processor and an Avalon bus module. The Nios II processor reads the frequency difference data output by the calculation module through the Avalon bus module. The Nios II processor then uses a fitting algorithm to fit the frequency difference-humidity data to obtain a frequency-humidity relationship. When performing humidity measurement, the Nios II processor converts the frequency difference into humidity using the frequency-humidity relationship.

2. A dual-QCM difference frequency humidity sensor measuring device according to claim 1, characterized in that: The frequency-humidity relationship obtained is: f ( x )= ax 2 + bx + c in, x is the frequency, f ( x ) is the humidity conversion result corresponding to the current frequency, a, b, c is the correlation coefficient.

3. A dual-QCM difference frequency humidity sensor measuring device according to claim 2, characterized in that: Correlation coefficient a, b, c Stored as double-precision floating point number type.

4. A dual-QCM difference frequency humidity sensor measuring device according to claim 3, characterized in that: The humidity-sensitive material used in QS1 and QS2 is graphene oxide.

5. A dual-QCM difference frequency humidity sensor measuring device according to claim 4, characterized in that: QS1 and QS2 deposit graphene oxide films on the surface of the QCM sensor electrode by drop coating.

6. A dual-QCM difference frequency humidity sensor measuring device according to claim 5, characterized in that: The concentration of graphene oxide films drop-coated on QS1 and QS2 was 2 mg / mL~5 mg / mL, and the coating amount was 0.1 μL~0.3 μL.

7. A dual-QCM difference frequency humidity sensor measuring device according to claim 6, characterized in that: The embedded soft core also includes a UART serial port module, which displays the corresponding frequency measurement value and frequency-humidity relationship during fitting, as well as the frequency measurement value and humidity measurement result during humidity measurement on the PC side through the UART serial port module.

8. The dual-QCM difference frequency humidity sensor measuring device according to claim 7, characterized in that: The embedded soft core also includes: an SPI driver module and an OLED display screen; the Nios II processor displays the frequency measurement value and the humidity measurement result during humidity measurement on the OLED display screen through the SPI driver module.

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