Device and method for measuring velocity field of autoclave through hot-pressing wind speed

Abstract

The invention discloses a device and a method for measuring the velocity field of an autoclave through hot-pressing wind speed. The device comprises a thermosensitive probe, a Wheatstone bridge, a lock-in amplifier, an NI multi-channel data acquisition card and a computer, the thermosensitive probe is used for instantaneously detecting the temperature of a placement point position and generating resistance fluctuation based on sensing temperature change; the Wheatstone bridge is formed by connecting a fixed resistor, a variable resistor and a thermosensitive probe, and is used for forming a series circuit with the lock-in amplifier; the lock-in amplifier is used for supplying power to the Wheatstone bridge and providing co-frequency voltage at the same time, so that resistance data of a thermosensitive probe in the Wheatstone bridge is converted into voltage data to be output; the NI multi-channel data acquisition card is used for acquiring in-phase component voltage signals output by the lock-in amplifier; the computer is used for processing the voltage data to obtain the average speed and the pulsation speed of the measurement point. According to the invention, the problems of limited precision, insufficient sensitivity and poor adaptability to the internal environment of the autoclave in the conventional gas speed measurement method are solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of hot pressing tank, and particularly relates to a device for measuring the velocity field of hot pressing tank by hot pressing air speed. BACKGROUND

[0002] As the core equipment for producing aviation composite parts, the hot pressing tank realizes the curing forming of resin-based composite components by precisely timing the control of temperature, pressure, vacuum and other parameters, and is widely used in the manufacturing fields of main load-bearing structures such as aircraft cabin doors, wings and tail wings, and large parts such as satellites and high-speed train bodies. Its system covers the tank body, heating / pressurizing / cooling device and automatic control system. In order to improve the space utilization rate of the hot pressing tank, the heat source is concentrated at the tail of the tank body, and the hot air is driven by the fan to flow to ensure the temperature in the tank, which requires that the gas flow rate in the tank must be precisely controllable. The gas flow rate has a huge impact on the heating uniformity of the composite prepreg, the resin flow and the curing reaction process. If the gas flow rate is uneven, heat cannot be uniformly transferred to all parts of the prepreg, and local temperature differences will cause defects such as component deformation and residual stress, which seriously affect the quality and performance stability of the composite components. Especially in the high-end manufacturing fields such as aviation and high-speed rail, the quality requirements for components are extremely high, and small defects may cause serious consequences. Therefore, to realize the high-precision and high-sensitivity measurement of the gas flow rate in the hot pressing tank is the key to ensure the molding of high-quality composite materials and meet the stringent requirements of high-end manufacturing.

[0003] The existing gas flow rate measurement techniques such as hot-wire anemometry, pitot tube anemometry, laser Doppler anemometry and the like have obvious problems of limited accuracy and insufficient sensitivity when coping with the complex working conditions of the hot press. For the hot-wire anemometry, a thin metal wire (hot wire) heated by electricity is placed in the fluid, the fluid flow takes away the heat to change the temperature of the hot wire, and the change of the electric signal caused by the change of the resistance of the hot wire is measured to calculate the flow rate of the fluid. However, the thermal inertia of the hot wire will cause a delay in its response to the change of the flow rate. When the flow rate changes rapidly, the temperature of the hot wire cannot change in time, so that the measurement result cannot accurately reflect the actual change of the flow rate, and the problem of insufficient sensitivity is prominent. In addition, the change of the pressure in the hot press will also affect the heat dissipation characteristics of the hot wire, resulting in limited measurement accuracy. The pitot tube anemometry is based on Bernoulli equation, and the flow rate is calculated by measuring the difference between the total pressure and the static pressure of the fluid. It has high requirements for the incoming flow direction and uniformity of the airflow, and the measurement error is large in a non-uniform flow field. Moreover, the probe of the pitot tube may be deformed, corroded and the like due to the influence of the high temperature, high pressure and corrosive gas in the hot press, resulting in a decrease in measurement accuracy, and even damage to the normal work. The laser Doppler anemometry determines the velocity of the fluid particles by measuring the Doppler shift of the scattered light. However, the propagation and scattering characteristics of the laser are easily affected by the high temperature environment in the hot press, resulting in a decrease in the intensity and stability of the measurement signal. At the same time, the gas in the hot press may contain dust, particles and other impurities, which will scatter the laser and produce noise interference, affecting the measurement accuracy. Therefore, in order to meet the requirements of high precision, non-constant temperature and high sensitivity of the hot press forming process, a new hot press anemometry method needs to be developed. SUMMARY

[0004] In view of the deficiencies in the prior art, the purpose of the present application is to provide a device and method for measuring the velocity field of a hot press by measuring the wind speed of a hot press. A high-precision, high-sensitivity and fast-response thermistor probe is used to measure the gas flow rate in the hot press. Based on the cross-correlation and temperature / wind speed coupling algorithm, the probe temperature and the gas flow rate are correlated and calculated to solve the problems of limited accuracy, insufficient sensitivity and poor adaptability to the internal environment of the hot press existing in the conventional gas flow rate measurement method, and to expand the application occasions of the hot press anemometry technology. In order to achieve the above-mentioned purposes and other advantages according to the present application, a device for measuring the velocity field of a hot press by measuring the wind speed of a hot press is provided, comprising: a thermistor probe, a Wheatstone bridge, a lock-in amplifier, an NI multi-channel data acquisition card and a computer; The thermistor is used to instantaneously detect the temperature at the placement point, generating resistance fluctuations based on the sensed temperature changes. The Wheatstone bridge consists of a fixed resistor, a variable resistor, and the thermistor connected in series, forming a series circuit with the lock-in amplifier. The lock-in amplifier supplies power to the Wheatstone bridge and provides a voltage of the same frequency, thereby converting the resistance data of the thermistor in the Wheatstone bridge into voltage data output. The NI multi-channel data acquisition card is used to acquire the in-phase component voltage signal output by the lock-in amplifier. The computer processes the voltage data to obtain the average velocity and pulsation velocity of the measurement point.

[0005] This invention places N×4 thermistors at N fixed points inside an autoclave, with four thermistors at each point arranged vertically. These probes have been calibrated to correlate their resistance with temperature, and their resistance fluctuates with the temperature of the flowing air. A sinusoidal voltage set by a lock-in amplifier is input to a Wheatstone bridge as the total input voltage. This Wheatstone bridge consists of two fixed resistors with known and identical resistances, a variable resistor with known and adjustable resistance, and the thermistors. With the total input voltage known, the instantaneous resistance of the thermistors can be converted into an AC voltage divider signal for output. Since this voltage signal is generated by the lock-in amplifier, its frequency is the same as the lock-in amplifier voltage. After modulation and demodulation, this voltage signal is converted into a high-precision DC signal and output to an NI multi-channel data acquisition card. The four DC signals acquired were then input into the computer. At the same time, normalized cross-correlation calculations were performed on the DC signals of every two consecutive probes. The displacement offset obtained by cross-correlation was fitted into two linear curves. These fitted curves were expanded by Taylor expansion using the two-dimensional cross-correlation coefficient to finally obtain the average velocity and pulsation velocity at the current fixed point.

[0006] The Wheatstone bridge is self-made, and the resistance error of the fixed resistor due to voltage and temperature fluctuations is up to 5ppm, while the resistance error of the variable resistor does not exceed 1%.

[0007] The calibrated temperature accuracy of the thermal probe is 0.01K, the temperature measurement range is -60 to 300℃, and the response time is 30ms.

[0008] The lock-in amplifier can output a data frequency of up to 512Hz.

[0009] The NI multi-channel data acquisition card can acquire data at a maximum frequency of 2MHz.

[0010] The computer data processing is based on cross-correlation and temperature / wind speed coupling algorithms, which can display and output the average and instantaneous speed results of fixed points in real time.

[0011] Compared with the prior art, the advantages and positive effects of the present invention are: 1. Thermistor probes are used for measuring wind speed in autoclaves. Within the probe's allowable temperature range, there are no additional requirements for ambient temperature, flow field turbulence, dust content, or pH. The probes can withstand high temperature and high pressure, making them ideal for measuring the velocity field inside autoclaves. The spatial layout of multiple thermistor probes, after calibration, combines high accuracy and sensitivity with fast response time. The accuracy reaches the order of 0.001K, and the response time reaches the order of milliseconds.

[0012] 2. Data acquisition and noise reduction are performed by combining a Wheatstone bridge, a lock-in amplifier, and an NI multi-channel data acquisition card. Compared with directly using a data acquisition card, the current combination significantly reduces data redundancy and converts AC voltage signals into DC voltage signals, greatly improving data accuracy.

[0013] 3. During data processing, the displacement offset obtained by normalized cross-correlation is fitted into two linear curves. At the same time, the velocity field is calculated by Taylor expansion of the two-dimensional cross-correlation coefficient. This takes into account the dual effects of displacement offset and time offset, making up for the shortcomings of conventional cross-correlation calculation that only considers time offset, and greatly improving the accuracy and reliability of velocity measurement results. Attached Figure Description

[0014] Figure 1 A schematic diagram of the device for measuring the velocity field of an autoclave according to the present invention; Figure 2 This is a flowchart of a method for measuring the velocity field of an autoclave according to the present invention.

[0015] The components are: 1-autoclave, 2-calibration board, 3-thermal probe, 4-Wheatstone bridge, 5-thermal probe resistance input terminal, 6-AC voltage input terminal, 7 & 8-thermal probe voltage divider output terminals, 9-locked-in amplifier, 10 & 11-thermal probe voltage divider input terminals, 12-AC voltage output terminal, 13-DC voltage output terminal, 14-NI multi-channel data acquisition card, and 15-computer. (Detailed implementation details follow.) 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.

[0016] Reference Figure 1 A device for measuring the velocity field of an autoclave using hot-pressing air velocity measurement, comprising: The speed measurement structure includes a thermal probe 3, a Wheatstone bridge 4, a lock-in amplifier 9, an NI multi-channel data acquisition card 14, and a computer 15.

[0017] After the spatial position of the thermal probe 3 is determined by the calibration plate 2, the lock-in amplifier 9 inputs an AC voltage with a fixed frequency to the AC voltage input terminal 6 via the AC voltage output terminal 12, thereby connecting the Wheatstone bridge 4 circuit. In practical applications, the number and position of measurement points, the number of thermal probes, and the amplitude and frequency of the AC voltage are not fixed and can be adjusted as needed. It should be noted that the number of probes at each point should not be less than 4. The temperature accuracy of the calibrated thermal probe is 0.01K, the temperature measurement range is -60-300℃, and the response time is 30ms. The thermal probe 3 is used to instantaneously detect the temperature of the placement point, based on the resistance fluctuation generated by the sensing temperature change. The Wheatstone bridge 4 consists of a fixed resistor, a variable resistor, and the thermal probe connected together, forming a series circuit with the lock-in amplifier. The resistance error of the fixed resistor due to voltage and temperature fluctuations is at most 5ppm, and the resistance error of the variable resistor does not exceed 1%.

[0018] The resistance of the thermal probe 3 fluctuates due to the temperature change of the flowing air. This resistance fluctuation is transmitted to the resistance input terminal 5 of the thermal probe, which causes the voltage divider output terminals 7 and 8 of the thermal probe to change due to the resistance fluctuation. This voltage divider is input to the voltage divider input terminal 10 of the first thermal probe and the voltage divider input terminal 11 of the second thermal probe.

[0019] The lock-in amplifier 9 modulates the AC voltage at the voltage divider input terminal 10 of the first thermistor and the voltage divider input terminal 11 of the second thermistor, and inputs the demodulated DC voltage signal to the NI multi-channel data acquisition card 14 via the DC voltage output terminal 13. In practical applications, the sampling frequency of the lock-in amplifier and the data acquisition card is not fixed and can be adjusted as needed.

[0020] The NI multi-channel data acquisition card 14 inputs the DC voltage signal to the computer 15 for data processing. After data processing, the computer 15 synchronously displays and outputs the average and instantaneous velocity results at fixed points.

[0021] By selecting a high-precision, high-sensitivity, and fast-response thermal probe 3 for gas velocity measurement inside the autoclave, and based on the cross-correlation and temperature / wind speed coupling algorithm, the temperature of the thermal probe 3 and the gas flow rate are correlated and calculated, which solves the problems of limited accuracy, insufficient sensitivity, and poor adaptability to the internal environment of the autoclave that exist in conventional gas velocity measurement methods.

[0022] like Figure 2 As shown, a method for measuring the velocity field of an autoclave using hot-pressing air velocity measurement is applied to an apparatus for measuring the velocity field of an autoclave, specifically including the following steps: Step 1: Determine the flow velocity measurement point, and set up a calibration plate at a distance s in front of the measurement point to determine the spatial position x of the thermal probe; Step 2: Turn on the autoclave fan, lock-in amplifier, NI multi-channel data acquisition card, and computer; the temperature change (R, t) sensed by the thermistor is converted into an AC voltage divider signal (Q, t) via a Wheatstone bridge and input to the lock-in amplifier; the lock-in amplifier modulates and demodulates the voltage divider signal (Q, t) into a DC signal (T, t) and outputs it to the NI multi-channel data acquisition card; the NI multi-channel data acquisition card synchronously outputs the acquired data to the computer; Step 3: Based on the cross-correlation and temperature / wind speed coupling algorithm, calculate the average velocity U and instantaneous velocity V at the measurement point, and display and output them synchronously on the computer.

[0023] In step 1, the calibration plate 2 is placed in front of the thermistor 3. Before the Wheatstone bridge 4 is connected, the correspondence between the calibrated probe resistance and temperature and the spatial position of its placement point should be obtained.

[0024] In step 2, after the autoclave fan 16 is turned on, the resistance signal (R, t) of the thermistor 3 is converted into a high-precision, low-redundancy DC voltage signal (T, t) through the Wheatstone bridge 4, the lock-in amplifier 9, and the NI multi-channel data acquisition card 14, and finally input to the computer 15. In practical applications, the voltage division accuracy of the thermistor 3 can be further improved by reducing the line load and reducing the error between fixed and variable resistors.

[0025] In step 3, the DC voltage signal is cross-correlated using normalized methods to obtain the cross-correlation coefficient C(z,τ) plot between two consecutive probes. This plot allows calculation of the displacement offset z and time offset τ as the wind flows through these two probes, resulting in a linear curve plotted with displacement offset z and time offset τ as coordinates. The slope obtained from fitting this curve is then subjected to a two-dimensional Taylor expansion of the cross-correlation coefficient to calculate the average and instantaneous velocities at the measurement points. Taylor expansion often retains terms up to the second order for ease of calculation. Obviously, selecting higher-order terms for expansion will yield more accurate velocity results, but will reduce computational efficiency. In practical applications, this can be adjusted as needed.

[0026] The number of devices and processing scale described herein are for simplification of the invention. Applications, modifications, and variations of this invention will be readily apparent to those skilled in the art. Although embodiments of the invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. It can be applied to various fields suitable for this invention, and further modifications can be readily implemented by those skilled in the art. Therefore, without departing from the general concept defined by the claims and their equivalents, this invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A device for measuring the velocity field of an autoclave using hot-pressing air velocity measurement, characterized in that, include: The autoclave (1), the calibration plate (2) disposed inside the autoclave (1), the thermal probe (3) disposed above the calibration plate (2), the Wheatstone bridge (4) connected to the thermal probe (3), the lock-in amplifier (9) connected to the Wheatstone bridge (4), the NI multi-channel data acquisition card (14) connected to the lock-in amplifier (9), and the computer (15) connected to the NI multi-channel data acquisition card (14). The thermal probe (3) is used to instantaneously detect the temperature at the placement point and generates resistance fluctuations based on the changes in the sensed temperature. The Wheatstone bridge (4) and the lock-in amplifier (9) form a series circuit; The lock-in amplifier (9) is used to power the Wheatstone bridge (4) and provide a voltage of the same frequency, thereby converting the resistance data of the thermistor (3) in the Wheatstone bridge (4) into voltage data output. The NI multi-channel data acquisition card (14) is used to acquire the in-phase component voltage signal output by the lock-in amplifier (9); The computer (15) is used to process voltage data to obtain the average velocity and pulsation velocity of the measurement points.

2. The device for measuring the velocity field of a hot-pressed autoclave as described in claim 1, characterized in that, The Wheatstone bridge (4) includes a fixed resistor, a variable resistor and a thermistor (3). The Wheatstone bridge (4) is self-made. The resistance error of the fixed resistor caused by voltage and temperature fluctuations is up to 5 ppm, and the resistance error of the variable resistor is no more than 1%.

3. The device for measuring the velocity field of a hot-pressed autoclave as described in claim 1, characterized in that, The temperature accuracy of the calibrated thermistor (3) is 0.01K, the temperature measurement range is -60-300℃, and the response time is 30ms.

4. The device for measuring the velocity field of a hot-pressed autoclave as described in claim 1, characterized in that, The lock-in amplifier (9) has a maximum data output frequency of 512 Hz.

5. The device for measuring the velocity field of a hot-pressed autoclave as described in claim 1, characterized in that, The NI multi-channel data acquisition card (14) has a data acquisition frequency of up to 2MHz.

6. The device for measuring the velocity field of a hot-pressed autoclave as described in claim 1, characterized in that, The computer (15) processes data based on cross-correlation and temperature / wind speed coupling algorithms, and can display and output the average and instantaneous speed results of fixed points in real time.

7. A method for measuring the velocity field of an autoclave using hot-pressing air velocity, the method being applied to an apparatus for measuring the velocity field of an autoclave as described in any one of claims 1-6, characterized in that, Specifically, it includes: S1. Determine the flow velocity measurement point and set up a calibration plate at a distance s in front of the measurement point (2). Determine the spatial position x of the thermal probe (3). S2, start the autoclave fan, lock-in amplifier (9), NI multi-channel data acquisition card (14), computer (15); the temperature change (R, t) sensed by the thermal probe (3) is converted into an AC voltage divider signal (Q, t) by the Wheatstone bridge (4) and input to the lock-in amplifier (9); the lock-in amplifier (9) converts the voltage divider signal (Q, t) into a DC signal (T, t) through modulation and demodulation and outputs it to the NI multi-channel data acquisition card (14); the NI multi-channel data acquisition card (14) synchronously outputs the acquired data to the computer (15). S3. Based on the cross-correlation and temperature / wind speed coupling algorithm, the average velocity U and instantaneous velocity V at the measurement point are calculated and displayed and output synchronously on the computer (15).

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