A MEMS capacitive wall shear stress measurement system

Abstract

The present application relates to the technical field of fluid wall shear stress measurement, and particularly relates to a MEMS capacitive wall shear stress measurement system, which comprises a capacitive wall shear stress sensor, a meter head circuit module, a carrier wave module, a rear-end circuit module and a host computer module, the capacitive wall shear stress sensor is used for converting a shear stress signal into a capacitive signal; the meter head circuit module is used for converting the capacitive signal into a voltage signal and performing differential amplification; the carrier wave module is used for generating a carrier wave signal and loading the carrier wave signal onto the capacitive wall shear stress sensor; the rear-end circuit module is used for performing power management on the system, amplifying, filtering, phase-sensitive demodulating, data sampling, arranging and sending the voltage signal output by the meter head circuit module; and the host computer module is used for decoding and calculating the data output by the rear-end circuit module to obtain a measured shear stress and supplying power for the system. The present application outputs intuitive shear stress measurement data and can realize accurate online measurement of a shear stress distribution.

Description

Technical Field

[0001] This invention relates to the field of fluid wall shear stress measurement technology, and specifically to a MEMS capacitive wall shear stress measurement system. Background Technology

[0002] The tangential stress generated when a fluid comes into contact with and slides relative to the surface of an object is called fluid wall shear stress. Fluid wall shear stress is a fundamental element in fluid dynamics for studying the boundary layer characteristics and evolution of the flow field around an aircraft or spacecraft. It is crucial for understanding flow mechanisms, verifying aerodynamic effects, and analyzing aerodynamic friction drag.

[0003] During aircraft flight, wall fluid shear stress accounts for up to 40% of the total drag. In high-speed flow environments, the accuracy of wall fluid shear stress predictions based on CFD (Computational Fluid Dynamics) simulations is low, and effective testing methods are lacking. Currently, drag prediction errors generally reach 10%–15%, severely impacting the aerodynamic performance of aircraft and its drag reduction optimization design. To accurately assess the wall fluid shear stress experienced by an aircraft during the cruise phase, and subsequently design and optimize the aircraft structure and aerodynamic performance to reduce flight drag, improve aircraft performance, and ensure structural safety, there is an urgent need for effective in-situ measurement of wall fluid shear stress.

[0004] The wall fluid shear stress of aircraft is characterized by rapid dynamic changes and large spatial distribution differences. Traditional methods such as oil film interferometry and friction balance have drawbacks such as poor dynamic performance, low sensitivity, large size, and difficulty in overall integration, which cannot meet the requirements for online testing of wall friction stress under flight conditions. MEMS (Micro-Electro-Mechanical System) based wall fluid shear stress sensors offer advantages such as small measurement points, fast response, high sensitivity, low temperature drift, high output linearity, and simple and robust structure, providing a new approach to wall fluid shear stress measurement.

[0005] With the continuous development of engineering applications, the requirements for sensors in shear stress measurement are also constantly increasing. As shear stress measurement increasingly demands high resolution, improving spatial resolution necessitates further reduction in sensor size. To address this need, a miniaturized design of capacitive sensors has been proposed, but this raises the following issues:

[0006] 1. Fluid wall shear stress is small in magnitude, highly dynamic, and has a small characteristic scale, making accurate measurement of fluid wall shear stress extremely difficult.

[0007] 2. The output capacitance of a capacitive wall shear stress sensor is typically in the picofarad range, while the change in sensing capacitance caused by the detected physical quantity is even smaller, generally in the femtofarad range. Furthermore, stray and parasitic capacitances existing in the system, between the sensor and its connecting wires, are much larger than the change in the measured capacitance. Therefore, cable stray capacitance affects the results of weak capacitance measurements. Moreover, factors such as structure, temperature, location, device selection, and internal / external electric field distribution all influence the magnitude of stray capacitance, thereby expanding the range of variation in the measured capacitance and reducing the reliability of the measurement data. Therefore, micro-capacitance measurement circuits must meet the requirements of good stray resistance, high sensitivity, low noise, and a large dynamic range.

[0008] Therefore, there is a need to provide a MEMS capacitive wall shear stress measurement system to solve the above problems. Summary of the Invention

[0009] This invention provides a MEMS capacitive wall shear stress measurement system to solve existing problems.

[0010] The MEMS capacitive wall shear stress measurement system of the present invention adopts the following technical solution, including:

[0011] Multiple capacitive wall shear stress sensors are used to acquire shear stress signals and convert them into capacitive signals;

[0012] The carrier module is used to generate a carrier signal and load the carrier signal onto the capacitive wall shear stress sensor, so that the capacitive wall shear stress sensor outputs a capacitive signal modulated onto the carrier.

[0013] The meter circuit module is used to convert the capacitor signal modulated onto the carrier wave into a voltage signal, and to perform differential operations and amplification on the voltage signal;

[0014] The back-end circuit module is used for power management of the system, for the first amplification and filtering of the voltage signal after differential operation and amplification, for demodulating the voltage signal after the first amplification and filtering using the carrier signal as a reference signal and outputting a DC voltage signal, for filtering the DC voltage signal, for analog-to-digital conversion of the filtered DC voltage signal and outputting a digital voltage signal, and for data processing of the digital voltage signal and outputting it.

[0015] The system also includes a host computer module, which decodes and calculates the digital voltage signal after data processing to obtain the measured shear stress and supplies power to the system.

[0016] Preferably, the meter head circuit module includes:

[0017] A charge amplifier is used to convert capacitive signals into voltage signals.

[0018] A differential operational amplifier is used to perform differential operations and amplify voltage signals.

[0019] The first interface is used to input the capacitance signal output by the capacitive wall shear stress sensor;

[0020] And a second interface for outputting the voltage signal after differential operational amplification;

[0021] The first interface, charge amplifier, differential amplifier, and second interface are connected in sequence.

[0022] Preferably, the back-end circuit module includes:

[0023] The third interface is used to input the differentially amplified voltage signal output by the meter circuit module;

[0024] The system power management module is used for system power management;

[0025] The non-inverting amplifier circuit is used to amplify the voltage signal after differential amplification by the meter circuit module in the same phase.

[0026] A high-pass filter circuit is used to filter out low-frequency noise in the voltage signal after in-phase amplification.

[0027] The phase-shifting module is used to shift the phase of the carrier signal output by the carrier module and use it as a reference signal for demodulation.

[0028] The phase-sensitive demodulation module is used to demodulate the voltage signal processed by the high-pass filter circuit into a DC voltage signal that is proportional to the shear stress of the input wall fluid based on the reference signal.

[0029] The first low-pass filter circuit is used to filter out high-frequency noise in the demodulated DC voltage signal.

[0030] The main control circuit is used to convert the DC voltage signal processed by the first low-pass filter circuit into a digital voltage signal by analog-to-digital conversion, and to process the digital voltage signal and output it.

[0031] And a fourth interface, used to output the DC voltage signal after data processing;

[0032] The third interface, carrier module, in-phase amplifier circuit, high-pass filter circuit, phase-sensitive demodulation module, first low-pass filter circuit, main control circuit and fourth interface are connected in sequence.

[0033] Preferably, the system power management module includes:

[0034] The power protection module is used for high voltage pulse protection, reverse connection protection, and fuse overcurrent protection.

[0035] The primary voltage regulator module is used for common-mode filtering, differential-mode filtering, and clamping voltage regulation of the power supply.

[0036] A two-stage voltage regulator module is used in switching power supplies, linear power supplies, and current limiting.

[0037] And a voltage conversion module, used to convert the system voltage into the voltage required by each device.

[0038] Preferably, it further includes an external communication module, which is used to send the collected voltage signal to the host computer module via the fourth interface.

[0039] Preferably, the phase-shifting module includes:

[0040] An inverting amplifier circuit is used to amplify a sinusoidal carrier signal in reverse phase.

[0041] And a second low-pass filter circuit, used to filter out high-frequency noise in the inverted amplified sinusoidal carrier signal, and use the sinusoidal carrier signal processed by the second low-pass filter circuit as the demodulation reference signal.

[0042] Preferably, the host computer module includes a host computer or an integrated data acquisition and editing unit.

[0043] Preferably, the capacitive wall shear stress sensor, the meter circuit module, the back-end circuit module, and the host computer module are connected by an electrical connection component.

[0044] Preferably, it further includes: multiple shielding shell assemblies, wherein the meter head circuit module, the back end circuit module, the host computer module, and the electrical connection assembly are installed inside the corresponding shielding shell assembly.

[0045] The beneficial effects of this invention are:

[0046] The MEMS capacitive wall fluid shear stress measurement system of this invention provides a sinusoidal excitation source signal for a capacitive sensor, converts the output capacitance signal of the capacitive sensor into a voltage signal, and performs filtering, amplification, phase detection, and phase-shifting demodulation. The measurement resolution can reach the picofarad level, enabling precise measurement of wall fluid shear stress. It is small in size, highly adaptable, provides intuitive shear stress measurement data output, and is easy to use, allowing for accurate online measurement of shear stress distribution. It has advantages such as high sensitivity, fast response speed, and expandability to connect to different types and ranges of capacitive sensors for related measurements. Attached Figure Description

[0047] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 This is a system block diagram of a MEMS capacitive wall shear stress measurement system according to the present invention;

[0049] Figure 2 This is a schematic diagram of the structure of a MEMS capacitive wall shear stress measurement system according to the present invention;

[0050] Figure 3 This is a schematic diagram of the floating shear stress sensor structure of the present invention;

[0051] Figure 4 This is a schematic diagram of the carrier signal loading method of the present invention;

[0052] Figure 5 This is a circuit diagram of the charge amplification circuit in this embodiment;

[0053] Figure 6 This is a schematic diagram of the capacitance signal detection circuit in this embodiment;

[0054] Figure 7 This is a circuit diagram of the meter header circuit module in this embodiment;

[0055] Figure 8 This is a circuit diagram of the in-phase amplifier circuit in this embodiment;

[0056] Figure 9 This is a circuit diagram of the high-pass filter circuit in this embodiment;

[0057] Figure 10 This is a circuit diagram of the phase-sensitive demodulation module in this embodiment;

[0058] Figure 11 This is a schematic diagram of a second-order active low-pass filter in this embodiment;

[0059] Figure 12 This is a schematic diagram of the inverting amplifier circuit in this embodiment;

[0060] Figure 13 This is a schematic diagram of the power management module;

[0061] Figure 14 This is a schematic diagram of an electrical connector assembly. Detailed Implementation

[0062] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0063] An embodiment of the MEMS capacitive wall shear stress measurement system of the present invention, such as... Figure 1 and Figure 2 As shown, the system includes: two capacitive wall shear stress sensors, two meter circuit modules, a carrier module, a back-end circuit module, and a host computer module. The capacitive wall shear stress sensors acquire shear stress signals and convert them into capacitance signals. It should be noted that each capacitive wall shear stress sensor in the MEMS capacitive wall shear stress measurement system corresponds to one measurement channel. When the MEMS capacitive wall shear stress measurement system has multiple measurement channels, multiple capacitive wall shear stress sensors are correspondingly set. The carrier module generates a carrier signal and loads it onto the capacitive wall shear stress sensor, causing the sensor to output a capacitance signal with a carrier wave. The meter circuit module converts the capacitance signal with the carrier wave into an electrical signal. The system receives a voltage signal and differentially amplifies it. The back-end circuit module manages the system's power supply, performs initial amplification and filtering on the differentially amplified voltage signal, demodulates the first amplified and filtered voltage signal using the carrier signal as a reference, outputs a DC voltage signal, filters the DC voltage signal, performs analog-to-digital conversion on the filtered DC voltage signal to output a digital voltage signal, and processes the digital voltage signal before outputting it. The host computer module decodes the processed digital voltage signal to calculate the measured shear stress and supplies power to the system. Each of the two capacitive wall shear stress sensors is electrically connected to the back-end circuit module via a meter circuit module, and the output of the back-end circuit module is connected to the host computer module.

[0064] Specifically, the meter head circuit module includes: a charge amplifier, a differential operational amplifier, a first interface, and a second interface; the charge amplifier is used to convert the capacitance signal into a voltage signal; the differential amplifier is used to perform differential operations and amplify the voltage signal; the first interface is used to input the capacitance signal output by the capacitive wall shear stress sensor; the second interface is used to output the voltage signal after differential operation and amplification; wherein, the first interface, the charge amplifier, the differential amplifier, and the second interface are connected in sequence.

[0065] Specifically, the back-end circuit module includes: a third interface, a system power management module, a non-inverting amplifier circuit, a high-pass filter circuit, a phase-shifting module, a phase-sensitive demodulation module, a first low-pass filter circuit, a main control circuit, and a fourth interface. The third interface is used to input the differentially amplified voltage signal output from the meter circuit module; the system power management module is used to manage the system power to provide a stable power supply; the non-inverting amplifier circuit is used to amplify the differentially amplified voltage signal from the meter circuit module in the same phase; the high-pass filter circuit is used to filter out low-frequency noise in the non-inverting amplified voltage signal; and the phase-shifting module is used to phase-shift the carrier signal output from the carrier module as the demodulation signal. The system includes a reference signal; a phase-sensitive demodulation module demodulates the voltage signal processed by the high-pass filter circuit into a DC voltage signal proportional to the shear stress of the input wall fluid based on the reference signal; a first low-pass filter circuit filters out high-frequency noise in the DC voltage signal; an MCU main control circuit performs analog-to-digital conversion on the filtered DC voltage signal to output a digital voltage signal, processes the digital voltage signal, and outputs the processed DC voltage signal; a fourth interface outputs the processed DC voltage signal; wherein, the third interface, the in-phase amplifier circuit, the high-pass filter circuit, the phase-sensitive demodulation module, the first low-pass filter circuit, the main control circuit, and the fourth interface are connected sequentially. This embodiment also includes an external communication module, which sends the acquired voltage signal to the host computer module via the fourth interface.

[0066] The system power management module includes: a power protection module, a primary voltage regulator module, a secondary voltage regulator module, and a voltage conversion module. The power protection module is used for high voltage pulse protection, reverse connection protection, and fuse overcurrent protection. The primary voltage regulator module is used for common-mode filtering, differential-mode filtering, and clamping voltage regulation of the power supply. The secondary voltage regulator module is used for switching power supply, linear power supply, and current limiting. The voltage conversion module is used to convert the system voltage into the voltage required by each device.

[0067] The phase-shifting module includes an inverting amplifier circuit and a second low-pass filter circuit. The inverting amplifier circuit is used to amplify the sinusoidal carrier signal in reverse phase. The second low-pass filter circuit is used to filter out high-frequency noise in the sinusoidal carrier signal after inverting amplification, and uses the sinusoidal carrier signal processed by the second low-pass filter circuit as the carrier signal for demodulation processing by the phase-sensitive demodulation module.

[0068] Specifically, the host computer module includes a host computer or an integrated data acquisition and editing device. In this embodiment, the host computer module is a host computer.

[0069] Specifically, the capacitive wall shear stress sensor, the meter circuit module, the back-end circuit module, and the host computer module are connected via electrical connection components, such as... Figure 14As shown, in this embodiment, the electrical connection components include a sensor three-core connector, a five-core to ten-core electrolyzer, and a five-core electrical connector. One end of each of the sensor three-core connector, the five-core to ten-core electrolyzer, and the five-core electrical connector is provided with a plug, and the other end is provided with a socket. The sensor three-core connector is used to connect the capacitive wall shear stress sensor and the corresponding meter circuit module; the five-core to ten-core electrolyzer is used to connect the meter circuit module and the back-end circuit module; and the five-core electrical connector is used to connect the back-end circuit module and the host computer module.

[0070] Specifically, it also includes: multiple shielding shell assemblies, wherein the meter head circuit module, the back end circuit module, the host computer module, and the electrical connection components are installed inside the corresponding shielding shell assembly.

[0071] The following description, in conjunction with the accompanying drawings, illustrates this embodiment.

[0072] like Figure 3 As shown, the capacitive wall shear stress sensor in this embodiment is a floating capacitive friction shear stress sensor, which includes a suspended elastic beam, a floating unit, movable comb teeth, fixed comb teeth fixed to the substrate, and anchor points. The elastic beam supports and connects the suspended floating unit and the movable comb tooth structures on both sides. The movable comb teeth and the fixed comb teeth form multiple sets of opposing micro plate capacitors. The capacitance value of the micro plate capacitors is related to the size of the gap between adjacent comb teeth. The detection principle of the floating capacitive friction shear stress sensor is as follows: under the action of fluid wall friction stress, the floating unit of the floating capacitive friction shear stress sensor drives the movable comb teeth to produce a small displacement relative to the fixed comb teeth, causing the output capacitance value to change.

[0073] like Figure 4 As shown, this illustrates the carrier signal loading method. Since the MEMS capacitive wall shear stress sensor itself is structurally equivalent to two differential capacitors, a carrier signal line is connected to the floating unit on the MEMS capacitive wall shear stress sensor, and two capacitive signal lines are connected to the anchor points on the two stationary comb teeth. When the two differential comb teeth are displaced by frictional stress, a slight change in the capacitance between the comb tooth plates occurs, thus generating a capacitive signal. After the carrier signal passes through the comb tooth capacitance of the MEMS capacitive wall shear stress sensor, the capacitive signals are output from the anchor points of the two stationary comb teeth to the meter circuit module.

[0074] Existing open-loop detection circuits for picofarad-level capacitance signal detection mainly include resonant circuits, switched capacitor detection circuits, or charge amplifier circuits. While all three circuits can detect minute capacitance signals, resonant circuits exhibit significant output nonlinearity and poor dynamic performance due to gate time limitations, making them unsuitable for measuring dynamic signals. Switched capacitor detection circuits may generate high-frequency spikes during capacitor charging and discharging, essentially applying a momentary overload to the MEMS capacitive wall shear stress sensor, potentially damaging it. Therefore, in this embodiment, a charge amplifier is chosen to convert the capacitance signal into a voltage signal.

[0075] like Figure 5 The diagram shown illustrates the schematic of the charge amplifier circuit in a charge amplifier. The charge amplifier circuit exhibits good output linearity and reduces the influence of parasitic capacitance. The input-output relationship of the charge amplifier circuit is expressed as follows:

[0076]

[0077] in, It is a carrier signal. The imaginary unit, Angular frequency, The capacitance signal of the sensor. For feedback resistor, For feedback capacitor, This is the output voltage signal. Based on this, it can be deduced that:

[0078]

[0079] Because the carrier signal has a high frequency, and The value is usually chosen to be relatively large, therefore There are far greater than Therefore, the relationship between voltage input and output can be simplified as follows:

[0080]

[0081] From the above equation, it can be concluded that after passing through the charge amplifier, the comb-tooth capacitance signal is proportional to the output voltage of the charge amplifier, and the capacitance signal of the capacitive wall shear stress sensor has been modulated to the amplitude of the sinusoidal carrier signal. After the two capacitance signals are output by the charge amplifier, differential operation between them is still required, which is achieved through a differential amplifier.

[0082] It should be noted that, as Figure 6 As shown, Figure 6This document presents the overall circuit design for the capacitance signal detection of a MEMS wall fluid shear stress sensor. A high-frequency sinusoidal signal is generated by a DDS chip and used as a carrier signal, which is then applied to the MEMS wall fluid shear stress sensor. Through carrier modulation, the capacitance change due to the sensitive shear stress is modulated onto the amplitude of the high-frequency sinusoidal signal. Since the output modulated signal undergoes a phase shift after passing through the front-end MEMS capacitive wall shear stress sensor and charge amplifier, the reference signal needs to pass through a phase-shifting circuit before being input to the phase-sensitive demodulation module. Therefore, the phase-shifting circuit requires an inverting amplifier circuit and a low-pass filter circuit for phase shifting before being output to the phase-sensitive demodulation module. The carrier signal of the path shift direction is used as a reference signal, and the voltage signal modulated onto the carrier from the high-pass filter circuit is demodulated by the phase-sensitive demodulation module to restore it into a DC voltage signal proportional to the input shear stress. Then, a low-pass filter circuit is used to filter the DC voltage signal, and an analog-to-digital converter module is used to convert the filtered DC voltage signal from the low-pass filter circuit to a digital voltage signal. After data processing, the digital voltage signal is output to the host computer module through the RS422 serial communication standard. The host computer module decodes and calculates to obtain the measured shear stress signal.

[0083] like Figure 7 As shown, Figure 7 The diagram illustrates the circuit schematic of the meter head circuit module. This module is responsible for converting minute capacitive signals into voltage signals to reduce the impact of parasitic capacitance on the output capacitive signal of the MEMS capacitive wall shear stress sensor and to facilitate long-distance transmission of the measurement signal. Figure 7As shown, in this embodiment, the meter circuit module includes two charge amplifier circuits, feedback capacitors (first capacitor and tenth capacitor), and feedback resistors (first resistor and third resistor). The charge amplifier circuits convert the capacitor signal into a voltage signal through the feedback resistors and feedback capacitors. The capacitor signal is proportional to the voltage signal output by the charge amplifier circuit. The capacitor signal of the capacitive wall shear stress sensor has been modulated to the amplitude of a sinusoidal carrier signal. After the two capacitor signals are converted into voltage signals by the corresponding charge amplifier circuits, differential operation between the two voltage signals is required. The differential operation is implemented by a differential amplifier. Specifically, the first resistor and the first capacitor are connected in parallel to the inverting terminal and the output terminal of the first operational amplifier chip, and the third resistor and the tenth capacitor are connected in parallel to the inverting terminal and the output terminal of the second operational amplifier. The second resistor is connected in parallel to the gain control terminal of the third amplifier. At the same time, the modulation signals on the upper and lower plates of the comb capacitor are respectively connected to the inverting terminals of the first and second operational amplifiers, and the non-inverting terminals of the first and second operational amplifiers are both grounded. The second and third capacitors are connected in parallel to bypass the positive power supply pin of the first operational amplifier chip. The sixth and seventh capacitors are connected in parallel to bypass the negative power supply pin of the second operational amplifier chip. The fourth and fifth capacitors are connected in parallel to bypass the positive power supply pin of the third operational amplifier chip, and the sixth and seventh capacitors are connected in parallel to bypass the negative power supply pin of the third operational amplifier chip. The second, third, fourth, fifth, sixth, seventh, eighth, and ninth capacitors are all power supply filter capacitors. The meter circuit module must be placed not too far from the sensor to minimize signal interference and for mechanical installation considerations; otherwise, parasitic and stray capacitance signals will annihilate the capacitance signal being measured.

[0084] It's important to note that the PCB layout is crucial during the design of the meter's circuit module. Improper routing can negatively impact the sensor's output signal. Since the carrier signal of the friction stress sensor is a high-frequency signal, overly close traces on the PCB can easily cause crosstalk. Therefore, during routing, the input carrier signal should be kept as far away as possible from the output signal at the charge amplifier's rear end to avoid interference. Furthermore, the meter's circuit module requires a reliable interface. An unreliable interface will result in signal errors during meter operation.

[0085] like Figure 8 As shown, Figure 8This is a schematic diagram of a non-inverting amplifier circuit. In the signal processing circuit integrated into the back-end circuit module, an amplifier circuit is needed to process the output signal of the front-end capacitive wall shear stress sensor. In this paper, an analog circuit is used to amplify the signal, specifically a non-inverting amplifier circuit. The non-inverting amplifier circuit includes: a first amplifier, a fourth resistor, a fifth resistor, a sixth resistor, and a potentiometer. The fourth resistor is a feedback resistor, and the sixth resistor is an input resistor, with one end connected to the input signal and the other end connected to the non-inverting input of the fourth operational amplifier. The fourth operational amplifier is grounded through the fifth resistor. The fourth resistor and the potentiometer are connected in series, with one end connected to the non-inverting input of the fourth operational amplifier and the other end connected to the output, thus amplifying the signal. The differential signal output from the meter circuit is input to the non-inverting input of the fourth operational amplifier via the sixth resistor.

[0086] like Figure 9 As shown, when designing amplifier circuits, the measured amplification factor generally deviates from the simple theoretical design value. Therefore, a precision adjustable resistor is needed during signal processing to fine-tune the amplification factor. Furthermore, in analog circuits, noise interference is introduced into the signal along with the amplifier circuit; therefore, a filter circuit is connected after the non-inverting amplifier circuit. It should also be noted that analog amplifier circuits have a certain degree of zero bias, which will have a certain impact on the output signal. Specifically… Figure 9 This is a schematic diagram of a high-pass filter circuit connected to the back end of a non-inverting amplifier circuit. The signal output from the second interface of the meter circuit module contains crosstalk from the 50Hz AC power supply and its harmonics introduced by the charge amplifier. A high-pass filter circuit is needed to filter out these interference signals. A second-order active high-pass filter is selected, consisting of a seventh resistor, an eighth resistor, an eleventh capacitor, a twelfth capacitor, and a fifth operational amplifier. One end of the eleventh capacitor is the input interface, and the other end is connected in series with the twelfth capacitor to the non-inverting input of the fifth operational amplifier. One end of the seventh resistor is connected to the non-inverting input of the fifth operational amplifier, and the other end is grounded. One end of the eighth resistor is connected between the eleventh and twelfth capacitors, and the other end is connected to the output of the fifth operational amplifier. The return signal, after being amplified by the non-inverting amplifier, is input to the high-pass filter circuit via the eleventh capacitor. The gain of the second-order active high-pass filter is set to 1, the eighth resistor is the feedback resistor, and the eleventh capacitor is the input impedance capacitor.

[0087] Since the overall scheme of the micro-capacitance signal detection circuit proposes to modulate the micro-capacitance signal of the wall fluid shear stress sensor onto the amplitude of a sinusoidal signal, the signal processing circuit of the back-end circuit module needs to convert the amplitude signal into a DC voltage signal output through demodulation, ultimately realizing the conversion of the capacitance signal into a voltage signal. Therefore, this embodiment proposes as follows: Figure 10The circuit diagram of the phase-sensitive demodulation module shown in this embodiment employs switching phase-sensitive demodulation. This method has the advantages that the demodulation effect is only related to the phase of the input reference signal and that the output waveform distortion is minimal. The AD630ARZ is selected as the demodulation chip. As a high-precision balanced modulator-demodulator, the AD630ARZ can be used in the following signal processing fields: balanced modulation and demodulation, synchronization detection, phase detection, phase-sensitive detection, quadrature detection, and phase-locked loop amplification. During demodulation, the original carrier signal is required as a reference signal. Ideally, there should be no phase difference between the carrier signal and the front-end output signal. However, after passing through the front-end MEMS capacitive wall shear stress sensor and charge amplifier, the phase of the output signal shifts. Therefore, the reference signal needs to pass through a phase-shifting circuit before being input into the AD630ARZ. The phase-sensitive demodulation module includes: an AD630ARZ demodulation chip, a ninth resistor, a thirteenth capacitor, a fourteenth capacitor, a fifteenth capacitor, and a sixteenth capacitor. Among these, the thirteenth, fourteenth, fifteenth, and sixteenth capacitors are used for power supply filtering. The negative terminal of the system power supply is connected to the -VS pin of the AD630ARZ demodulation chip. One end of each of the fifteenth and sixteenth capacitors is grounded, and the other end is connected between the negative terminal of the system power supply and the -VS pin of the AD630ARZ demodulation chip. The positive terminal of the system power supply is connected to the +VS pin of the AD630ARZ demodulation chip. One end of each of the thirteenth and fourteenth capacitors is grounded, and the other end is... The system power supply is connected between the positive terminal and the +VS pin of the AD630ARZ demodulation chip. The high-frequency carrier signal generated by the carrier module is adjusted by the phase-shifting module and connected to the SELB pin of the AD630ARZ demodulation chip. The carrier signal serves as a reference signal. The SELA, RB, and RINA pins of the AD630ARZ demodulation chip are all grounded. The RINB and RA pins of the AD630ARZ demodulation chip are connected to the output of the high-pass filter circuit (i.e., the output of the second amplifier in the high-pass filter circuit). The VOUT pin of the AD630ARZ demodulation chip is connected to the input of the low-pass filter circuit through the ninth resistor. The thirteenth and fourteenth capacitors are connected in parallel to bypass the positive power supply pin of the demodulation chip, and the fifteenth and sixteenth capacitors are connected in parallel to bypass the negative power supply pin of the demodulation chip.

[0088] like Figure 11 As shown, the first low-pass filter circuit and the second low-pass filter circuit operate on the same principle, both employing... Figure 11The schematic diagram of the low-pass filter circuit is shown. The output signal after processing by the phase-sensitive demodulation module contains not only the required DC voltage signal but also high-frequency signal components. A low-pass filter circuit needs to be connected after the phase-sensitive demodulation module to filter out the high-frequency components mixed in with the signal. Both the first and second low-pass filter circuits are second-order active low-pass filters with a gain set to 1. The cutoff frequency of the low-pass filter needs to be determined based on the operating bandwidth of the MEMS capacitive wall shear stress sensor. Specifically, the second-order active high-pass filter includes: a tenth resistor, an eleventh resistor, a seventeenth capacitor, an eighteenth capacitor, and a sixth operational amplifier. One end of the tenth resistor is the input interface, and the other end of the tenth resistor is connected in series with the eleventh resistor to the non-inverting input of the sixth operational amplifier. One end of the seventeenth capacitor is connected to the non-inverting input of the fifth operational amplifier, and the other end of the seventeenth capacitor is grounded. One end of the eighteenth capacitor is connected between the tenth and eleventh resistors, and the other end of the eighteenth capacitor is connected to the output of the fifth operational amplifier. The return signal is amplified by the sixth operational amplifier and then input to the high-pass filter circuit through the tenth resistor. The gain of the second-order active low-pass filter is set to 1. The cutoff frequency of the second-order active low-pass filter needs to be determined according to the operating bandwidth of the MEMS capacitive wall shear stress sensor. The seventeenth capacitor is the feedback capacitor and the tenth resistor is the input resistor.

[0089] Specifically, such as Figure 6 As shown, in this embodiment, the carrier signal of the capacitive wall shear stress sensor detection circuit is a high-frequency sine wave. Using a sine wave does not place high demands on the bandwidth of the operational amplifier, and the sine wave has low distortion during signal transmission. Since the sine wave generated by the DDS (Direct Digital Synthesis) chip has high amplitude, frequency, and phase stability, it is well-suited as a carrier for small capacitor signals. Therefore, the carrier module uses a DDS chip. This paper selects the AD9831 DDS chip to generate the sine wave signal. The AD9831 has a maximum clock frequency of 25MHz, supports power supply under 3.3V or 5V conditions, and a maximum power consumption of 120mW. It has advantages such as low operating voltage, economy, and built-in DAC. After configuring the frequency of the output sine wave signal, its output amplitude also needs to be considered. A corresponding resistor needs to be configured at the IOUT pin of the DDS chip to obtain the required voltage signal amplitude. Since the sine wave signal is output after analog-to-digital conversion, it is recommended to add a resistor at the back end... Figure 11 and Figure 6 The second low-pass filter of the phase-shifting module shown in the diagram filters out high-frequency noise interference. That is, the carrier module uses a carrier generator (DDS chip), and the sinusoidal carrier signal generated by the carrier generator is used as the carrier signal.

[0090] like Figure 6As shown, the phase-shifting module includes: an inverting amplifier circuit and a second low-pass filter circuit, wherein, as... Figure 12 As shown, Figure 12 This is a schematic diagram of an inverting amplifier circuit. It amplifies the amplitude of the sinusoidal signal generated by the carrier generator to the output. Simultaneously, a feedback loop controls the amplifier's gain and frequency response, preventing distortion of the sinusoidal wave generated by the carrier generator. Specifically, the inverting amplifier circuit includes: a fourth amplifier, a twelfth resistor, a thirteenth resistor, a fourteenth resistor, a nineteenth capacitor, and a potentiometer. The twelfth resistor is the feedback resistor, and the nineteenth capacitor is the input capacitor. One end of the nineteenth capacitor is connected to the input signal, and the other end is connected in series with the thirteenth resistor to the non-inverting input of the seventh operational amplifier. The seventh operational amplifier is grounded through the fourteenth resistor. One end of the twelfth resistor, connected in series with the potentiometer, is connected to the non-inverting input of the seventh operational amplifier, and the other end is connected to the output of the seventh operational amplifier, thus amplifying the signal. The differential signal output from the meter circuit module is input to the non-inverting input of the seventh operational amplifier via the nineteenth capacitor.

[0091] Among them, such as Figure 6 and Figure 12 As shown, the output of the carrier generator is connected to the negative input of the fourth amplifier through the nineteenth capacitor and the thirteenth resistor. One end of the twelfth resistor is connected between the thirteenth resistor and the negative input of the fourth amplifier, and the other end of the twelfth resistor is connected to the output of the fourth amplifier through a potentiometer. One end of the fourteenth resistor is grounded, and the other end is connected to the positive input of the fourth amplifier. The twelfth resistor is the feedback resistor, and the nineteenth capacitor is the input impedance capacitor.

[0092] The design of the above circuits and modules completes the design of the back-end circuit module of the MEMS wall fluid shear stress sensor, and completes the conversion of weak capacitance signals into voltage signals. Subsequently, only the DC voltage signal needs to be sampled to plot the relationship curve between the voltage signal and the front-end shear stress signal. Specifically, an MCU main control circuit and an external communication module are set up after the first low-pass filter circuit. The MCU main control circuit is used to collect the DC voltage signal processed by the low-pass filter circuit; the external communication module is used to send the collected voltage signal to the host computer module via the fourth interface. In this embodiment, the MCU main control circuit uses a high-speed, low-power, and highly interference-resistant STM32F103C8T6 microprocessor as the main control chip. A 12-bit ADC interface is used to sample the voltage signal output by the demodulation circuit, and the collected data is organized into the required data format and packaged within the main control chip. The collected current voltage signal is then sent to the host computer via the serial port through the external communication module to achieve cyclic measurement and timely data transmission of capacitance data.

[0093] like Figure 13 As shown, Figure 13 This is a schematic diagram of the system power management module. The system power supply is supplied with 15V, but the main control unit (processor STM32F103C8T6) requires 3.3V. The RS422 chip and carrier generator of the external communication module require 5V. The operational amplifier requires dual power supplies with negative voltages of -15V and +15V, so power adaptation is necessary for each component. An LDO chip is selected to reduce and stabilize the input voltage at around 5V. The main control power supply and sensor module power supply use a low dropout converter circuit. The selected LDO chip can output a relatively stable voltage around 3.3V. Digital and analog power supply isolation traces are used because integrated circuits have different power quality requirements, and the microcontroller's power pins have both analog and digital power ports.

[0094] The RS422 chip uses differential mode transmission, offering good robustness and long transmission distance. It can operate in full-duplex mode via two pairs of twisted-pair cables, with no interference between transmitting and receiving signals, making it widely used in engineering projects. Because the RS422 chip's voltage level differs from the microcontroller's TTL level, direct communication is not possible. A conversion circuit is required before connecting it to the microcontroller's serial port for data transmission. USART_RX is the receiver for microcontroller serial communication data, connected to the RS422 chip's output. USART_TX is the receiver for microcontroller serial communication data, connected to the RS422 chip's input. TX+ is the positive terminal for RSS422 signal transmission, and TX- is the negative terminal. The RS422 transceiver integrates a 5 kV rms isolated DC / DC power supply, eliminating the need for an external DC / DC isolation module. Furthermore, the RS422 chip provides complete isolation between the power supply and signal grounds, ensuring high-quality signal transmission. In addition, it has current limiting and thermal shutdown features, which can prevent output short circuits and excessive power consumption caused by bus contention, thus ensuring the safety of the signal transmission system.

[0095] like Figure 14 As shown, Figure 14 This is a schematic diagram of the electrical connector assembly of the system. Figure 14 In this context, 'a' represents a three-core circular connector. Figure 14 In this context, 'b' represents a 5-pin to 10-pin connector. Figure 14The 'c' in the diagram represents a five-pin connector. A three-pin circular connector consists of a three-pin circular plug, a three-pin circular socket, and a three-pin cable, used to connect the sensor's signal output lines and the meter's signal processing circuit. The three-pin cable includes two capacitance signal output lines (C+ and C-, representing the capacitance values ​​formed between the two fixed plates and the floating plate of the capacitive sensor), and one carrier input line. A five-pin to ten-pin connector includes a five-pin plug, a five-pin socket, a ten-pin plug, a ten-pin socket, and a ten-pin cable. It is used to connect the back-end circuitry to two meter circuit modules. The ten-pin cable includes two ±15V power lines, two power ground lines, two carrier signal lines, and two voltage difference signal lines output by the meter circuitry. A five-pin connector includes a five-pin plug, a five-pin socket, and a five-pin cable. It is used to connect the back-end circuitry to the integrated data acquisition unit / host computer. The five-pin cable includes a +15V power line, a power ground line, an RS422 differential transmission twisted pair cable, and an RS422 signal reference ground line.

[0096] In this embodiment, the shielding shell assembly is a metal shielding shell assembly, which includes a metal shielding shell for the meter head circuit and a metal shielding shell for the rear circuit. The meter head circuit metal shielding box includes a housing, a cover plate, fastening screws, and studs. The metal shielding shell assembly is used to mount the PCB carrying the meter head circuit module, and to fix the metal shielding shell assembly to the external carrier, shielding against external electromagnetic interference. Its front end is fixed to the socket of a three-pin circular connector through studs and fixing holes, and its rear end is fixed to the socket of a five-pin connector through studs and fixing holes. A capacitive sensor is connected through a three-pin connector. To reduce the influence of parasitic and stray capacitance on the sensor's small signal, it is placed not far from the rear end of the capacitive wall shear stress sensor. The rear circuit module is also encapsulated using a shielding shell assembly, which is a metal shielding box, including a housing, a cover plate, fastening screws, and studs. It is used to mount the PCB carrying the rear circuit module, and to fix the metal shielding box to the external carrier, shielding against external electromagnetic interference. Its front end is fixed to the socket of the 10-pin connector through studs and fixing holes, and its rear end is fixed to the socket of the 5-pin connector through studs and fixing holes.

[0097] Because the capacitance signal output from the front end of the micro-shear stress sensor is relatively weak and subject to significant interference, the following measures are required to improve the reliability of the circuit:

[0098] 1. Select an appropriate operating bandwidth: When a capacitive wall shear stress sensor is working, since the useful signal only exists in a certain frequency range, the passband bandwidth of the filter should be reduced as much as possible without affecting the normal operation of the signal. In addition, the noise interference mixed in the output signal can be reduced by adding a filter.

[0099] 2. Reasonable circuit routing: When analog and digital signals coexist on the same circuit board, the digital circuit can easily interfere with the ground line, which in turn reduces the measurement accuracy of the analog circuit. Therefore, in order to reduce the mutual interference between analog and digital circuits, the analog and digital parts should be routed separately as much as possible when routing the PCB, and the power lines should be made as thick as possible. When routing, try to minimize the loop area in the circuit to reduce the induced noise of the circuit.

[0100] 3. Pay attention to signal filtering: Since there are some high-frequency ripples in the power supply, in order to reduce the interference of these high-frequency ripples on the output signal, it is necessary to add filter capacitors in the circuit to filter them out; and try to add decoupling capacitors connected to ground near the power input pins of each circuit chip.

[0101] 4. PCB layout should facilitate signal flow: Arrange the positions of various components on the PCB board in a reasonable manner according to the signal flow sequence. While ensuring that the components do not interfere with each other, try to keep the signal flow direction consistent in the modules composed of various components. Each functional module needs to be laid out with the core component as the center and arranged neatly and compactly on the PCB board.

[0102] 5. Reliable cables must be used for both the meter head circuit and the back-end circuit: The transmission cable is a crucial component of the MEMS capacitive wall shear stress sensor system. Not only does the back-end circuit need to send the carrier signal to the meter head via the cable, but the meter head circuit's output signal also needs to travel through the transmission cable to the back-end processing signal. The selection of the transmission cable must consider the reliability of long-distance transmission and the stability of transmission in the presence of external electromagnetic interference. The transmission cable can be protected using shielding tape or shielding tubing. For applications in harsh environments, protection against significant temperature drops and underwater conditions must also be considered.

[0103] 6. The lead between the anchor point of the MEMS capacitive wall shear stress sensor and the back-end charge amplification circuit should be as short as possible: because the total capacitance of the MEMS capacitive wall shear stress sensor itself is only a few picofarads, excessively long leads will introduce large parasitic capacitance, thereby affecting the sensor's output signal.

[0104] 7. Minimize circuit power consumption: Without affecting the normal function of the circuit, minimize power consumption. This can effectively reduce the operating temperature and reduce the interference of thermal noise on the output signal of the MEMS capacitive wall shear stress sensor.

[0105] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A MEMS capacitive wall shear stress measurement system, characterized in that, include: Multiple capacitive wall shear stress sensors are used to acquire shear stress signals and convert them into capacitive signals; The carrier module is used to generate a carrier signal and load the carrier signal onto the capacitive wall shear stress sensor, so that the capacitive wall shear stress sensor outputs a capacitive signal modulated onto the carrier. The meter head circuit module is used to convert the capacitance signal modulated onto the carrier wave into a voltage signal, and to perform differential operation and amplification on the voltage signal. The meter head circuit module includes: a charge amplifier, a differential operational amplifier, a first interface, and a second interface. The charge amplifier is used to convert the capacitance signal into a voltage signal; the differential operational amplifier is used to perform differential operation and amplification on the voltage signal; the first interface is used to input the capacitance signal output from the capacitive wall shear stress sensor; the second interface is used to output the differentially amplified voltage signal; wherein, the first interface, the charge amplifier, the differential amplifier, and the second interface are connected in sequence. The back-end circuit module is used for system power management, performing initial amplification and filtering on the differentially amplified voltage signal, demodulating the first amplified and filtered voltage signal using the carrier signal as a reference signal to output a DC voltage signal, filtering the DC voltage signal, performing analog-to-digital conversion on the filtered DC voltage signal to output a digital voltage signal, and processing the digital voltage signal before outputting it. The back-end circuit module includes: a third interface, a system power management module, a non-inverting amplifier circuit, a high-pass filter circuit, a phase-shifting module, a phase-sensitive demodulation module, a first low-pass filter circuit, a main control circuit, and a fourth interface. The third interface is used to input the differentially amplified voltage signal output from the meter circuit module; the system power management module is used for system power management; the non-inverting amplifier circuit is used to process the differentially amplified voltage signal from the meter circuit module. The system comprises the following components: a non-inverting amplifier; a high-pass filter circuit to remove low-frequency noise from the amplified voltage signal; a phase-shifting module to phase-shift the carrier signal output from the carrier module and use it as a reference signal for demodulation; a phase-sensitive demodulation module to demodulate the voltage signal processed by the high-pass filter circuit into a DC voltage signal proportional to the shear stress of the input wall fluid based on the reference signal; a first low-pass filter circuit to remove high-frequency noise from the demodulated DC voltage signal; a main control circuit to perform analog-to-digital conversion on the DC voltage signal processed by the first low-pass filter circuit to output a digital voltage signal, and to process the digital voltage signal before outputting it; and a fourth interface to output the processed DC voltage signal. The third interface, carrier module, non-inverting amplifier circuit, high-pass filter circuit, phase-sensitive demodulation module, first low-pass filter circuit, main control circuit, and fourth interface are connected sequentially. The system also includes a host computer module, which decodes and calculates the digital voltage signal after data processing to obtain the measured shear stress and supplies power to the system.

2. The MEMS capacitive wall shear stress measurement system according to claim 1, characterized in that, The system power management module includes: The power protection module is used for high voltage pulse protection, reverse connection protection, and fuse overcurrent protection. The primary voltage regulator module is used for common-mode filtering, differential-mode filtering, and clamping voltage regulation of the power supply. A two-stage voltage regulator module is used in switching power supplies, linear power supplies, and current limiting. And a voltage conversion module, used to convert the system voltage into the voltage required by each device.

3. The MEMS capacitive wall shear stress measurement system according to claim 1, characterized in that, Also includes: An external communication module is used to send the collected voltage signal to the host computer module via the fourth interface.

4. The MEMS capacitive wall shear stress measurement system according to claim 1, characterized in that, The phase-shifting circuit includes: An inverting amplifier circuit is used to amplify a sinusoidal carrier signal in reverse phase. And a second low-pass filter circuit, used to filter out high-frequency noise in the inverted amplified sinusoidal carrier signal, and use the sinusoidal carrier signal processed by the second low-pass filter circuit as the demodulation reference signal.

5. A MEMS capacitive wall shear stress measurement system according to claim 1, characterized in that, The host computer module includes a host computer or a comprehensive data acquisition and editing device.

6. The MEMS capacitive wall shear stress measurement system according to claim 1, characterized in that, The capacitive wall shear stress sensor, the meter circuit module, the back-end circuit module, and the host computer module are connected by an electrical connection component.

7. A MEMS capacitive wall shear stress measurement system according to claim 1, characterized in that, Also includes: Multiple shielding shell assemblies, wherein the meter head circuit module, the back end circuit module, the host computer module, and the electrical connection components are installed inside the corresponding shielding shell assembly.

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