Supersonic closed-loop flow control method based on high-speed linear array camera and schlieren system

Abstract

Disclosed is an optical system based on a high-speed linear array CCD camera and schlieren, comprising a light source, a slit, first and second reflecting spherical mirrors, a cutting edge, a beamsplitter, a high-speed linear array CCD camera, and a high-speed area array CCD camera. A method for operating the optical system based on a high-speed linear array CCD camera and schlieren is also provided. A closed-loop control system based on a high-speed linear array CCD camera and schlieren is also provided, comprising a pulsed arc plasma exciter, an optical system based on a high-speed linear array CCD camera and schlieren, a high-performance real-time controller with an embedded proportional-integral-differential (PID) control law, and a host computer. Also provided is a workflow for supersonic closed-loop flow control based on a high-speed linear array CCD camera and schlieren system. The present invention utilizes optical sensing methods to avoid the strong electromagnetic interference of pulsed arc discharge on closed-loop feedback signals, thereby achieving a transition from blind open-loop to closed-loop adaptive control of pulsed arc plasma excitation in supersonic flow control.

Description

Technical Field

[0001] The present invention relates to the field of active flow control, and in particular to a supersonic closed-loop flow control method based on a high-speed linear array camera and a schlieren system. Background Art

[0002] As a novel active flow control technology, pulsed arc plasma actuators (PAs) can rapidly inject thermal energy into a flow field, manipulating the flow field through perturbations such as density, vorticity, temperature, and thermally blocked virtual surfaces. These actuators offer advantages such as simple structure, high excitation intensity, and high frequency response, achieving excellent control effects on flow structures such as supersonic and hypersonic shock waves, boundary layers, and shear layers. However, current engineering practices still rely on traditional open-loop fixed-frequency excitation for PAP control, which presents two challenges. First, this fixed-frequency excitation struggles to couple with the strong nonlinearities of high-speed, complex flow fields, resulting in weak control effects. Second, due to the lack of a reliable sensor feedback mechanism, constant excitation parameters are difficult to adapt to complex and changing flight environments. To enhance the robustness of PAP control systems and enable flight-dependent adjustment of excitation parameters to maximize the energy gain of flow control, a closed-loop approach is essential for plasma flow control. For closed-loop intelligent flow control technology in wind tunnel experiments, it is first necessary to use a series of sensors to perceive the flow field environment. The controller then collects the sensor signals and uses them as input to perform control law calculations. After that, they are delivered to the excitation system to complete the closed-loop adjustment of the control instructions. However, the pulsed electromagnetic waves and rapidly changing voltage and current generated during the pulse arc discharge process can strongly interfere with the sensors themselves and the communication between the sensors, CPU, and excitation system, resulting in signal noise and distortion, system false triggering, and in severe cases, equipment damage. This strong electromagnetic interference problem has seriously restricted the development of closed-loop pulse arc plasma excitation. To date, researchers in this field have not successfully achieved closed-loop flow control for pulse arc plasma excitation. Summary of the Invention

[0003] In view of the problems existing in the prior art, the present invention proposes an optical path system based on a high-speed linear array CCD camera and schlieren, comprising a light source, a slit, first and second reflecting spherical mirrors, a cutting edge, a beam splitter, a high-speed linear array CCD camera, and a high-speed area array CCD camera; wherein

[0004] a light source that emits a converging beam of light;

[0005] Slit, the converging light beam emitted by the light source is imaged on the slit;

[0006] The first reflecting spherical mirror is located at the focal position of the first reflecting spherical mirror. Therefore, the convergent light beam diverges after passing through the slit and is transmitted to the first reflecting spherical mirror, generating an elliptical light spot larger than the first reflecting spherical mirror. The first reflecting spherical mirror is located at the center of the light spot. The first reflecting spherical mirror emits a parallel light beam, which passes through the flow field to be controlled.

[0007] The second reflecting spherical mirror receives the parallel light beam emitted by the first reflecting spherical mirror and passes through the flow field to be controlled, and generates a convergent light beam output through self-reflection;

[0008] The knife edge is located at the focus of the convergent light beam generated by the second reflector, and the knife edge is moved so that the knife edge cuts the center point of the light spot;

[0009] The 45° beam splitter is located behind the cutting edge and splits the optical signal of the flow field captured by the schlieren into two beams of light with equal intensity.

[0010] The high-speed area array CCD camera receives a beam of light emitted by the 45° beam splitter, and this beam of light is directly transmitted into the high-speed area array CCD camera;

[0011] The high-speed linear array CCD camera receives another beam of light emitted by the 45° beam splitter, which then enters the high-speed linear array CCD camera after being reflected;

[0012] In the optical path system based on a high-speed linear array CCD camera and schlieren, the light source, the slit, the first and second reflecting spherical mirrors, and the light-cutting edge constitute the schlieren system.

[0013] A method for operating an optical path system based on a high-speed linear array CCD camera and schlieren is also proposed. The method is based on the above optical path system based on a high-speed linear array CCD camera and schlieren. The method is specifically as follows:

[0014] Step 1: Turn on the light source, emit a convergent beam and align it with the optical axis. Move the light source system so that the light source image is formed on the slit.

[0015] Step 2: Move the slit to the focal position of the first reflecting spherical mirror;

[0016] Step 3: Fine-tune the slit angle; keep its position unchanged so that the divergent light is transmitted to the first reflecting spherical mirror, generating an elliptical light spot larger than the first reflecting spherical mirror, and make the first reflecting spherical mirror located at the center of the light spot;

[0017] Step 4: The parallel light beam emitted by the first reflecting spherical mirror passes through the flow field to be controlled and is received by the second reflecting spherical mirror. It is then reflected by the second reflecting spherical mirror to generate a convergent light beam.

[0018] Step 5: Move the blade so that the convergent light spot generated by the second reflector is accurately focused on the blade, and then move the blade distance so that the blade cuts the center of the light spot to produce a Schlieren image of airflow changes;

[0019] Step 6: Install a 45° beam splitter behind the cutting edge to split the optical signal of the flow field captured by the schlieren into two beams of light with equal intensity. One beam of light is directly transmitted into the high-speed area array CCD camera to record the overall two-dimensional schlieren flow field. The other beam is reflected by the 45° beam splitter and enters the high-speed linear array CCD camera to detect characteristic flow structures such as shock waves and shear layers, supporting subsequent closed-loop control.

[0020] In addition, a closed-loop control system based on a high-speed linear array CCD camera and schlieren is proposed, which comprises a pulsed arc plasma actuator, an optical path system based on a high-speed linear array CCD camera and schlieren as claimed in claim 1, a high-performance real-time controller with an embedded proportional integral differential (PID) control law, and a host computer; wherein

[0021] The compression corner shock wave / boundary layer interference flow field was selected, an arc plasma exciter was installed in the compression corner model, and the compression corner model was installed in a wind tunnel. The optical path of the schlieren system was adjusted so that the parallel light emitted by the first reflecting spherical mirror completely covered the compression corner flow field.

[0022] After the knife-cut light, the light beam carrying the flow field density gradient is split into two beams of equal intensity by a beam splitter. One beam is reflected into a high-speed linear array CCD camera, and the other beam is transmitted into a high-speed area array CCD camera.

[0023] The high-speed linear array CCD camera converts the recorded flow field state into an analog voltage signal and transmits it to a high-performance real-time controller;

[0024] The high-speed area array CCD camera uploads the recorded flow field to the host computer, and the host computer outputs the initial closed-loop control law to the high-performance real-time controller;

[0025] According to the flow field state and closed-loop control law, the high-performance real-time controller outputs the excitation adjustment signal to the pulse arc plasma exciter.

[0026] A workflow for supersonic closed-loop flow control based on a high-speed linear array CCD camera and a schlieren system is also provided. The workflow is based on the closed-loop control system based on the high-speed linear array CCD camera and schlieren system, and is specifically as follows:

[0027] Step 1: Use the Schlieren system to convert the density gradient change of the flow field into a small deviation of the light along the propagation direction. Use a beam splitter to split the light beam signal containing the flow field information into transmitted light and reflected light and output them separately.

[0028] Step 2: After receiving the reflected light beam, the high-speed linear array CCD camera collects and converts the optical signal at a specific flow field position, and outputs an analog voltage signal corresponding to the intensity distribution of the reflected light beam at that moment;

[0029] Step 3: A high-performance real-time controller reads the analog voltage signal output by the high-speed linear CCD camera and identifies the characteristic flow state through algorithms such as threshold analysis and fitting. It then calculates and outputs control instructions based on the closed-loop control law output by the host computer.

[0030] Step 4: A high-performance real-time controller sends control instructions to the power module of the pulsed arc plasma exciter. By adjusting the on / off and strength of the excitation, the shock wave position is stabilized within the expected value. After stabilization, the control effect is evaluated by analyzing the state of the shock wave in the flow field continuously recorded by the high-speed area array CCD camera.

[0031] In one embodiment of the present invention, in the third step: a high-performance real-time controller is implemented using an FPGA chip, and the position change of the shock wave is used as a characteristic state of the flow field.

[0032] In another embodiment of the present invention, in the fourth step: the specific method of evaluating the control effect by analyzing the state of the shock wave in the flow field continuously recorded by the high-speed area array CCD camera is: the upper computer extracts the shock wave position in the schlieren frame by frame according to the time series, and converts the time domain signal of the position change into a frequency domain signal through fast Fourier transform, and the operator observes the change in the main frequency of the shock wave pulsation. If the low-frequency pulsation intensity of the shock wave is reduced compared with the open-loop control or uncontrolled flow field, or the shock wave position pulsation is converted into a high-frequency signal with very low intensity, then the closed-loop control obtains a better control effect; otherwise, the control effect is not good.

[0033] In another embodiment of the present invention, a high-performance real-time controller accepts a PID control law given by a host computer to control supersonic closed-loop flow. The control method is as follows:

[0034] (1) Extract the shock wave position change under the open-loop fixed-frequency pulse arc plasma excitation provided by the pulse arc plasma exciter:

[0035] Step 1: Debug the flow field to ensure the compression corner model is installed and the wind tunnel is operating normally;

[0036] Step 2: Complete the debugging of the Schlieren system and the high-speed linear array CCD camera so that the number of row pixels of the high-speed linear array CCD camera can cover the movement range of the shock wave in the flow field and calibrate the pixel distance. Combine the camera's own pixel clock and the number of row pixels to calculate the line frequency. The calculation formula is:

[0037]

[0038] The line frequency of the high-speed linear array CCD camera is greater than the dominant characteristic frequency of the flow field to be controlled;

[0039] Step 3: Use the pressure sensor inside the wind tunnel test section as a reference signal; when the signal reaches the normal operating pressure, trigger the high-speed linear array CCD camera and plasma actuator to operate;

[0040] Step 4: Connect the high-speed linear array CCD camera to the high-performance real-time controller via a high-speed data transmission interface. After the wind tunnel is running, the high-speed linear array CCD camera is used to capture the flow field under continuous plasma excitation.

[0041] Step 5: Identify the location with the largest grayscale value in the flow field image read by the high-speed linear array CCD camera as the location of the shock wave. For the compression corner flow field, the shock wave is located at the location with the largest density gradient in the flow field, which is reflected in the maximum grayscale value information in the flow field. A horizontal straight line at this location is extracted as the detection line of the grayscale value of the schlieren image. This line segment must completely cover the movement range of the shock wave in the flow direction.

[0042] Step 6: The high-speed linear array CCD camera extracts the real-time schlieren image of the flow field frame by frame and transmits it to the high-performance real-time controller. The high-performance real-time controller has a built-in image recognition algorithm to extract the grayscale value of the input image and convert it into a two-dimensional matrix to form a two-dimensional grayscale value matrix of the image;

[0043] Step 7: The image recognition algorithm in the real-time controller uses the corner point of the model as the flow zero point and the grayscale value to extract the maximum density gradient in the line segment as the shock wave position. The pixel position of the shock wave is converted into a physical position based on the pixel distance calibrated by the experimenter before the test.

[0044] Step 8: Arrange the shock wave positions in each frame of the schlieren image in time according to the time interval of each frame of the schlieren image. Fit a curve based on this large amount of shock wave position data to obtain the function r(t) of the shock wave position changing with time under continuous plasma excitation. Input r(t) as the expected value into the controller.

[0045] (2) PID-based closed-loop control process:

[0046] Step 1: Connect the various systems. The host computer communicates with the high-performance real-time controller via a network port or serial communication link. The controller has a built-in digital-to-analog converter (DAC). The DAC output signal is connected to the power supply of the pulsed arc plasma exciter, and the analog signal output by the high-performance real-time controller directly controls the output of the power supply.

[0047] Step 2: Initial parameter setting: Initially set a moderate proportional coefficient K for the PID control law in the host computer. p, so that the system has a certain response to the deviation, but not too extreme; set a longer integral time K i , in order to avoid integral saturation in the initial stage; differential control is temporarily not used in the initial stage; after the setting is completed, the PID control law is output to the high-performance real-time controller;

[0048] Step 3: Parameter adjustment; first adjust the proportional coefficient of the proportional control link in the PID control law, observe the response of the power supply, and then gradually adjust the integral time and differential time of the integral control link and differential control link in the PID control law;

[0049] Step 4: Operate the wind tunnel and closed-loop control system. The high-speed linear array CCD camera reads each schlieren image frame into the high-performance real-time controller, which then calculates the difference between the current shock wave position and the desired shock wave position and adjusts the output of the actuator accordingly.

[0050] Step 5: The high-performance real-time controller calculates the shock wave position y(t) of the next schlieren image frame read by the high-speed linear array CCD camera and uses it as a feedback signal. When the high-performance real-time controller finds through calculation that the shock wave position is tending to be corrected within the range of r(t), the current exciter output parameters are maintained until the shock wave position is fully regulated to within the range of r(t). Conversely, if the shock wave position is found to be over-regulated in the opposite direction and exceeds the range of r(t) again, the excitation frequency or excitation voltage is reduced to return it to the range of r(t).

[0051] Step 6: During the closed-loop control process, the host computer reads the complete Schlieren flow field data recorded by the high-speed area array CCD camera, analyzes and observes the control effect under the current PID parameters, and finds the optimal PID parameter combination to maximize the closed-loop control benefit.

[0052] In another embodiment of the present invention, in the third step of step (2), the parameter adjustment is specifically as follows: if the voltage and frequency output by the power supply have a slow response speed or continuous fluctuations, appropriately increase the proportional coefficient K p If the voltage and frequency output by the power supply fluctuate due to interference from ambient noise in the flow field, increase the integration time appropriately.

[0053] In another embodiment of the present invention, in the fourth step of step (2), the high-performance real-time controller adjusts the output of the exciter according to the difference between the current shock wave position and the expected shock wave position. The specific method is as follows: when the shock wave position is outside the r(t) range, the plasma excitation is started, and the excitation voltage and excitation frequency are adjusted according to the size of the excess range. The larger the excess range, the higher the excitation voltage and excitation frequency; on the contrary, if the shock wave position is within the r(t) range, the pulse arc plasma exciter is stopped.

[0054] In a specific embodiment of the present invention, in the sixth step, in order to maximize the closed-loop control benefit, a heuristic parameter optimization algorithm is used to find the best PID parameter combination.

[0055] The present invention proposes a supersonic closed-loop flow control method based on a high-speed linear array camera and a schlieren system. The optical sensing-based method can avoid the strong electromagnetic interference of pulsed arc discharge on the closed-loop feedback signal, and realize the transformation of pulsed arc plasma excitation from blind open-loop to closed-loop adaptive in supersonic flow control.

[0056] The advantages of the present invention are as follows:

[0057] Traditional closed-loop control often uses wall-mounted dynamic pressure sensors as a means of flow field sensing. Since the effective signal of the sensor itself is in the mV range, it is extremely susceptible to electromagnetic interference during the voltage amplification process and the communication process with the microcontroller, causing signal distortion and making it difficult to apply to closed-loop flow control of pulsed arc plasma excitation. The closed-loop control method proposed in the present invention uses a high-speed linear array CCD camera to monitor the flow field structure based on optical measurement, which can effectively avoid electromagnetic interference and achieve reliable closed-loop control of the supersonic flow field. At the same time, a spectrometer is added to the traditional Z-type schlieren optical path. While completing the closed-loop control based on the high-speed linear array CCD camera, the traditional area array CCD camera can still be used to shoot the schlieren of the flow field and evaluate the flow control effect. During the control process, the evaluation results can be used to adjust the control strategy in real time.

[0058] The present invention has strong versatility. Within the described framework, the flow field type, closed-loop control method, and actuator type can be changed to meet the flow control needs of different flow fields to be controlled. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] Figure 1 Shows the optical path arrangement based on a high-speed linear array CCD camera and schlieren;

[0060] Figure 2 Shows the composition of the closed-loop control system based on a high-speed linear array CCD camera and Schlieren;

[0061] Figure 3 The principle block diagram of closed-loop supersonic flow control based on PID is shown. DETAILED DESCRIPTION

[0062] The present invention will be described in detail below with reference to the accompanying drawings.

[0063] Figure 1 The optical system based on a high-speed linear array CCD camera and schlieren is shown. The system consists of a light source, a slit, first and second reflecting spherical mirrors, a cutting edge, a beam splitter, a high-speed linear array CCD camera, and a high-speed area array CCD camera.

[0064] A light source that emits a focused beam of light.

[0065] Slit, the converging light beam emitted by the light source is imaged on the slit.

[0066] The first reflecting spherical mirror is located at the focal position of the first reflecting spherical mirror. Therefore, the convergent light beam diverges after passing through the slit and is transmitted to the first reflecting spherical mirror, generating an elliptical light spot larger than the first reflecting spherical mirror. The first reflecting spherical mirror is located at the center of the light spot. The first reflecting spherical mirror emits a parallel light beam, which passes through the flow field to be controlled.

[0067] The second reflecting spherical mirror receives the parallel light beam emitted by the first reflecting spherical mirror and passes through the flow field to be controlled, and generates a convergent light beam output through self-reflection;

[0068] The knife edge is located at the focus of the convergent light beam generated by the second reflector. The knife edge is moved appropriately so that the knife edge cuts the center point of the light spot, generating a Schlieren image of airflow changes.

[0069] The 45° beam splitter is located behind the cutting edge and is used to split the flow field optical signal captured by the schlieren into two beams of light with equal intensity.

[0070] The high-speed area array CCD camera receives a beam of light emitted by the 45° beam splitter. This beam of light is directly transmitted into the high-speed area array CCD camera to perform an overall record of the two-dimensional schlieren flow field.

[0071] The high-speed linear array CCD camera receives another beam of light emitted by the 45° beam splitter. After reflection, this beam of light enters the high-speed linear array CCD camera and is used to detect characteristic flow structures such as shock waves and shear layers to support subsequent closed-loop control.

[0072] In the optical path system based on the high-speed linear array CCD camera and schlieren, the light source, the slit, the first and second reflecting spherical mirrors, and the light-cutting edge constitute the schlieren system. Therefore, the optical path system based on the high-speed linear array CCD camera and schlieren is composed of the schlieren system, the beam splitter, the high-speed linear array CCD camera, and the high-speed area array CCD camera.

[0073] Based on the above-mentioned optical path system based on a high-speed linear array CCD camera and schlieren, the present invention provides a working method of the optical path system based on a high-speed linear array CCD camera and schlieren, which is specifically as follows.

[0074] Step 1: Turn on the light source switch, shoot out a convergent beam and align it with the optical axis, move the light source system so that the light source is on the slit;

[0075] Step 2: Move the slit to the focal position of the first reflecting spherical mirror;

[0076] Step 3: Fine-tune the slit angle. Keep its position unchanged so that the divergent light is transmitted to the first reflecting spherical mirror, generating an elliptical light spot larger than the first reflecting spherical mirror, and make the first reflecting spherical mirror located at the center of the light spot;

[0077] Step 4: The parallel light beam emitted by the first reflecting spherical mirror passes through the flow field to be controlled and is received by the second reflecting spherical mirror. It is then reflected by the second reflecting spherical mirror to generate a convergent light beam.

[0078] Step 5: Move the blade so that the convergent light spot generated by the second reflector is accurately focused on the blade, and then move the blade distance appropriately so that the blade cuts to the center of the light spot to produce a Schlieren image of airflow changes.

[0079] Step 6: Unlike the traditional Z-shaped schlieren optical path, a 45° beamsplitter is installed behind the cutting edge to split the optical signal of the flow field captured by the schlieren into two beams of equal intensity. One beam is transmitted directly to a high-speed area array CCD camera to record the entire 2D schlieren flow field; the other beam is reflected by the 45° beamsplitter and then enters a high-speed linear array CCD camera to detect characteristic flow structures such as shock waves and shear layers, supporting subsequent closed-loop control.

[0080] Figure 2 The components of the closed-loop control system based on a high-speed linear array CCD camera and schlieren are shown. The system primarily comprises a pulsed arc plasma actuator, an optical system based on a high-speed linear array CCD camera and schlieren, a high-performance real-time controller with an embedded proportional-integral-derivative (PID) control law, and a host computer. While the present invention uses the compression corner shock wave / boundary layer interaction flow field as an example, its application scenarios are not limited to this flow field. Other potential implementations include supersonic cavity shear layer control and hypersonic boundary layer transition control.

[0081] An arc plasma exciter is installed in the compression corner model (the installation position and method of the exciter are well known to those skilled in the art), and the compression corner model is installed in a wind tunnel. The optical path of the above-mentioned schlieren system is adjusted so that the parallel light emitted by the first reflecting spherical mirror completely covers the compression corner flow field. The debugging process and position relationship of the schlieren are as described above and will not be repeated here.

[0082] After the knife-cut light, the light beam carrying the flow field density gradient is split into two beams of equal intensity by a beam splitter. One beam is reflected into a high-speed linear array CCD camera, and the other beam is transmitted into a high-speed area array CCD camera.

[0083] The high-speed linear array CCD camera converts the recorded flow field state into an analog voltage signal and transmits it to a high-performance real-time controller.

[0084] The high-speed area array CCD camera uploads the recorded flow field to the host computer, which outputs the initial closed-loop control law to the high-performance real-time controller. Subsequently, the closed-loop control law in the high-performance real-time controller will adjust parameters based on the power supply response, including the proportional coefficient, integral time, and differential time.

[0085] According to the flow field state and closed-loop control law, the high-performance real-time controller outputs the excitation adjustment signal to the pulse arc plasma exciter.

[0086] The workflow of supersonic closed-loop flow control based on a high-speed linear array CCD camera and a schlieren system is given below.

[0087] The first step is to use the Schlieren system to convert the density gradient changes of the flow field into a small deviation of the light along the propagation direction (this technology is well known to those skilled in the art), and use a spectrometer to split the light beam signal containing the flow field information into transmitted light and reflected light and output them separately.

[0088] Step 2: After receiving the reflected light beam, the high-speed linear array CCD camera collects and converts the optical signal at a specific flow field position, and outputs an analog voltage signal corresponding to the intensity distribution of the reflected light beam at that moment.

[0089] Step 3: A high-performance real-time controller (preferably an FPGA chip) reads the analog voltage signal output by the high-speed linear array CCD camera, and identifies the characteristic flow state through threshold analysis and fitting algorithms (preferably the position change of the shock wave is used as the characteristic state of the flow field). The specific implementation method here is given by the closed-loop control principle and method below. The control instructions are calculated and output according to the closed-loop control law output by the host computer. The closed-loop control law can be a classic PID control law, or it can be an intelligent control algorithm such as reinforcement learning and linear genetic programming. These control laws / algorithms are well known to those skilled in the art and will not be repeated here. The present invention focuses on execution speed, so PID is preferably used as the control law.

[0090] Step 4: The high-performance real-time controller sends the control instructions to the power module of the pulsed arc plasma exciter, and stabilizes the shock wave position within the expected value by adjusting the opening and closing and the strength of the excitation. After stabilization, the control effect is evaluated by analyzing the state of the shock wave in the flow field continuously recorded by the high-speed array CCD camera. The specific method is: the host computer extracts the shock wave position in the schlieren frame by frame according to the time series and converts the time domain signal of the position change into a frequency domain signal through fast Fourier transform (this technology is well known to those skilled in the art). The operator observes the change in the main frequency of the shock wave pulsation. If the low-frequency pulsation intensity of the shock wave is reduced compared to the open-loop control or uncontrolled flow field, or the shock wave position pulsation is converted into a high-frequency signal with very low intensity, then the closed-loop control obtains a better control effect. On the contrary, it is a poor control effect.

[0091] Figure 3 The high-performance real-time controller accepts the PID control law given by the host computer to control the supersonic closed-loop flow in the present invention. The control principle and method are described in detail as follows:

[0092] (1) Extract the shock wave position change under the open-loop fixed-frequency pulse arc plasma excitation provided by the pulse arc plasma exciter:

[0093] Step 1: Debug the flow field to ensure the compression corner model is installed and the wind tunnel is operating normally;

[0094] Step 2: Complete the debugging of the Schlieren system and the high-speed linear array CCD camera so that the number of row pixels of the high-speed linear array CCD camera can cover the movement range of the shock wave in the flow field and calibrate the pixel distance (the specific implementation is known to those skilled in the art). Then, combine the camera's own pixel clock and the number of row pixels to calculate the line frequency. The calculation formula is:

[0095]

[0096] To ensure that the main flow field pulsation characteristics of high-speed flow fields can be captured, the line frequency of the high-speed linear array CCD camera should be greater than the dominant characteristic frequency of the flow field to be controlled. Taking the boundary layer interference of compression corner shock waves as an example, this frequency is the pulsation frequency of the separation shock wave.

[0097] Step 3: The pressure sensor inside the wind tunnel test section serves as a reference signal. When this signal reaches the normal operating pressure, it triggers the high-speed linear array CCD camera and plasma actuator (in one embodiment of the present invention, to suppress the low-frequency pulsation of the shock wave, the maximum excitation frequency is set to 10 kHz and the excitation voltage is set to 20 kV).

[0098] Step 4: Connect the high-speed linear array CCD camera to the high-performance real-time controller via a high-speed data transmission interface. After the wind tunnel is running, the high-speed linear array CCD camera is used to capture the flow field under continuous plasma excitation.

[0099] Step 5: Identify the location with the largest grayscale value in the flow field image read by the high-speed linear CCD camera as the location of the shock wave (for the compression corner flow field, the shock wave is located at the location with the largest density gradient, which is reflected in the maximum grayscale value information in the flow field). A horizontal straight line at this location is extracted as the detection line of the grayscale value of the schlieren image. This line segment must completely cover the movement range of the shock wave in the flow direction.

[0100] Step 6: The high-speed linear array CCD camera extracts the real-time schlieren image of the flow field frame by frame and transmits it to the high-performance real-time controller. The high-performance real-time controller has a built-in image recognition algorithm to extract the grayscale value of the input image and convert it into a two-dimensional matrix to form a two-dimensional grayscale value matrix of the image;

[0101] Step 7: The image recognition algorithm in the real-time controller uses the corner point of the model as the flow zero point and extracts the maximum density gradient in the line segment as the shock wave position based on the grayscale value. The pixel position of the shock wave is then converted to a physical position based on the pixel distance calibrated by the experimenter before the test (this technology is well known to those skilled in the art).

[0102] Step 8: Arrange the shock wave positions in each schlieren image frame by time based on the time interval between each frame. A curve is fitted to this large amount of shock wave position data to obtain a function r(t) representing the temporal variation of the shock wave position under continuous plasma excitation. This function r(t) is then input into the controller as the desired value. Curve fitting methods are well known to those skilled in the art and will not be further described.

[0103] (2) PID-based closed-loop control process:

[0104] Step 1: Connect the systems. The host computer communicates with the high-performance real-time controller via a network or serial communication link. The controller has a built-in digital-to-analog converter (DAC). The DAC output signal is connected to the pulsed arc plasma exciter's power supply, which is directly controlled by the analog signal output by the high-performance real-time controller.

[0105] Step 2: Initial parameter setting. In the host computer, set a moderate proportional coefficient K for the PID control law. p , so that the system has a certain response to the deviation, but not too extreme; set a longer integral time K i , in order to avoid integral saturation in the initial stage; since differential control is sensitive to noise, differential control is not used in the initial stage; after the setting is completed, the PID control law is output to a high-performance real-time controller (here differential control is a link in the PID control law);

[0106] Step 3: Parameter adjustment. First adjust the proportional coefficient of the proportional control link in the PID control law, observe the response of the power supply, and then gradually adjust the integral time and differential time of the integral control link and differential control link in the PID control law. Specifically: If the voltage and frequency output of the power supply respond slowly or fluctuate continuously, increase the proportional coefficient K appropriately. p If the voltage and frequency of the power supply output fluctuate due to interference from ambient noise in the flow field, increase the integration time appropriately. During this adjustment process, both the proportional coefficient and the integration time will affect the response speed, so a certain amount of debugging time is required.

[0107] Step 4: Run the wind tunnel and closed-loop control system equipment. The high-speed linear array CCD camera reads each frame of the schlieren image into the high-performance real-time controller. The high-performance real-time controller calculates the difference between the current shock wave position and the expected shock wave position, and adjusts the output of the exciter accordingly. Specifically, when the shock wave position is outside the r(t) range, the plasma excitation is started, and the excitation voltage and excitation frequency are adjusted according to the extent of the excess range. The larger the excess range, the higher the excitation voltage and excitation frequency. Conversely, if the shock wave position is within the r(t) range, the pulsed arc plasma exciter is stopped.

[0108] Step 5: The high-performance real-time controller calculates the shock wave position y(t) of the next schlieren image frame read by the high-speed linear array CCD camera and uses it as a feedback signal. When the high-performance real-time controller finds through calculation that the shock wave position is tending to be corrected within the range of r(t), the current actuator output parameters can be maintained until the shock wave position is fully regulated to within the range of r(t). Conversely, if the shock wave position is found to be over-regulated in the opposite direction and exceeds the range of r(t) again, the excitation frequency or excitation voltage is reduced to return it to the range of r(t).

[0109] Step 6: During closed-loop control, the host computer reads the complete Schlieren flow field data recorded by the high-speed area array CCD camera and analyzes the control effect under the current PID parameters. To maximize the closed-loop control benefits, heuristic parameter optimization algorithms such as genetic algorithms, particle swarm optimization, and simulated annealing can be used to find the optimal PID parameter combination. These methods are well known to those skilled in the art and will not be described here.

[0110] For a general compression corner flow field, the number of pixels in the captured field of view is generally hundreds. Here, we take 100 pixels / row as an example, and the pixel depth is 8 bits / pixel in grayscale mode. Data volume = 100 (pixels) × 8 (bits) = 800 bits / row. The theoretical transmission speed under CameraLink configuration (CameraLink is a transmission interface for high-speed linear CCD cameras) is 6.8Gbps. Therefore, the theoretical delay time for real-time image acquisition is 800 / (6.8×10 9 )=0.127μs. This delay time has little effect on the supersonic flow field. Within the allowable error range, it can be considered that the system has the ability to collect data in real time and can meet the control requirements of the supersonic flow field.

[0111] The present invention proposes a closed-loop control method for supersonic flow based on a high-speed linear array CCD camera and schlieren. Compared with traditional closed-loop flow control implementation schemes, this method uses a high-speed linear array CCD camera to monitor the evolution of the flow structure of the flow field in real time through optical means, avoiding the problem that various micro-electromechanical sensors in traditional closed-loop control are susceptible to electromagnetic interference, and provides a solution for plasma excitation to carry out closed-loop control in supersonic flow fields. At the same time, the delay time for real-time image acquisition during the entire closed-loop control process is on the order of several microseconds, which can fully meet the flow control requirements of the supersonic flow field. In addition, the present invention adds a spectrometer to the traditional Z-type schlieren optical path. While completing the closed-loop control based on the high-speed linear array CCD camera, the traditional high-speed area array CCD camera can still be used to capture the schlieren of the flow field and evaluate the flow control effect. During the control process, the control strategy can be adjusted in real time based on the evaluation results. It can be applied to the closed-loop control requirements of various complex flow fields, has good versatility, and broad application prospects.

Claims

1. An optical system based on a high-speed linear array CCD camera and Schlieren, characterized in that: It includes a light source, a slit, a first and a second reflecting spherical mirror, a light-cutting edge, a beam splitter, a high-speed linear array CCD camera, and a high-speed area array CCD camera; wherein a light source that emits a converging beam of light; Slit, the converging light beam emitted by the light source is imaged on the slit; The first reflecting spherical mirror is located at the focal position of the first reflecting spherical mirror. Therefore, the convergent light beam diverges after passing through the slit and is transmitted to the first reflecting spherical mirror, generating an elliptical light spot larger than the first reflecting spherical mirror. The first reflecting spherical mirror is located at the center of the light spot. The first reflecting spherical mirror emits a parallel light beam, which passes through the flow field to be controlled. The second reflecting spherical mirror receives the parallel light beam emitted by the first reflecting spherical mirror and passes through the flow field to be controlled, and generates a convergent light beam output through self-reflection; The knife edge is located at the focus of the convergent light beam generated by the second reflector, and the knife edge is moved so that the knife edge cuts the center point of the light spot; The 45° beam splitter is located behind the cutting edge and splits the optical signal of the flow field captured by the schlieren into two beams of light with equal intensity. The high-speed area array CCD camera receives a beam of light emitted by the 45° beam splitter, and this beam of light is directly transmitted into the high-speed area array CCD camera; The high-speed linear array CCD camera receives another beam of light emitted by the 45° beam splitter, which then enters the high-speed linear array CCD camera after being reflected; In the optical path system based on a high-speed linear array CCD camera and schlieren, the light source, the slit, the first and second reflecting spherical mirrors, and the light-cutting edge constitute the schlieren system.

2. A method for operating an optical path system based on a high-speed linear array CCD camera and Schlieren, which is based on the optical path system based on a high-speed linear array CCD camera and Schlieren as claimed in claim 1, characterized in that: The method is as follows: Step 1: Turn on the light source, emit a convergent beam and align it with the optical axis. Move the light source system so that the light source image is formed on the slit. Step 2: Move the slit to the focal position of the first reflecting spherical mirror; Step 3: Fine-tune the slit angle; Keeping its position unchanged, the divergent light is transmitted to the first reflecting spherical mirror, generating an elliptical light spot larger than the first reflecting spherical mirror, and the first reflecting spherical mirror is located at the center of the light spot; Step 4: The parallel light beam emitted by the first reflecting spherical mirror passes through the flow field to be controlled and is received by the second reflecting spherical mirror. It is then reflected by the second reflecting spherical mirror to generate a convergent light beam. Step 5: Move the blade so that the convergent light spot generated by the second reflector is accurately focused on the blade, and then move the blade distance so that the blade cuts the center of the light spot to produce a Schlieren image of airflow changes; Step 6: Install a 45° beam splitter behind the cutting edge to split the optical signal of the flow field captured by the schlieren into two beams of light with equal intensity. One beam of light is directly transmitted into the high-speed area array CCD camera to record the overall two-dimensional schlieren flow field. The other beam is reflected by the 45° beam splitter and enters the high-speed linear array CCD camera to detect characteristic flow structures such as shock waves and shear layers, supporting subsequent closed-loop control.

3. A closed-loop control system based on a high-speed linear array CCD camera and Schlieren, characterized in that: The system comprises a pulsed arc plasma actuator, an optical path system based on a high-speed linear array CCD camera and schlieren as claimed in claim 1, a high-performance real-time controller with an embedded proportional integral differential (PID) control law, and a host computer; wherein The compression corner shock wave / boundary layer interference flow field was selected, an arc plasma exciter was installed in the compression corner model, and the compression corner model was installed in a wind tunnel. The optical path of the schlieren system was adjusted so that the parallel light emitted by the first reflecting spherical mirror completely covered the compression corner flow field. After the knife-cut light, the light beam carrying the flow field density gradient is split into two beams of equal intensity by a beam splitter. One beam is reflected into a high-speed linear array CCD camera, and the other beam is transmitted into a high-speed area array CCD camera. The high-speed linear array CCD camera converts the recorded flow field state into an analog voltage signal and transmits it to a high-performance real-time controller; The high-speed area array CCD camera uploads the recorded flow field to the host computer, and the host computer outputs the initial closed-loop control law to the high-performance real-time controller; According to the flow field state and closed-loop control law, the high-performance real-time controller outputs the excitation adjustment signal to the pulse arc plasma exciter.

4. A workflow for supersonic closed-loop flow control based on a high-speed linear array CCD camera and a Schlieren system, which is based on the closed-loop control system based on a high-speed linear array CCD camera and Schlieren as claimed in claim 3, characterized in that: The details are as follows: Step 1: Use the Schlieren system to convert the density gradient change of the flow field into a small deviation of the light along the propagation direction. Use a beam splitter to split the light beam signal containing the flow field information into transmitted light and reflected light and output them separately. Step 2: After receiving the reflected light beam, the high-speed linear array CCD camera collects and converts the optical signal at a specific flow field position, and outputs an analog voltage signal corresponding to the intensity distribution of the reflected light beam at that moment; Step 3: A high-performance real-time controller reads the analog voltage signal output by the high-speed linear CCD camera and identifies the characteristic flow state through algorithms such as threshold analysis and fitting. It then calculates and outputs control instructions based on the closed-loop control law output by the host computer. Step 4: A high-performance real-time controller sends control instructions to the power module of the pulsed arc plasma exciter. By adjusting the on / off and strength of the excitation, the shock wave position is stabilized within the expected value. After stabilization, the control effect is evaluated by analyzing the state of the shock wave in the flow field continuously recorded by the high-speed area array CCD camera.

5. The workflow of supersonic closed-loop flow control based on a high-speed linear array CCD camera and a Schlieren system according to claim 4, characterized in that: In the third step: a high-performance real-time controller is implemented using an FPGA chip, and the position change of the shock wave is used as the characteristic state of the flow field.

6. The workflow of supersonic closed-loop flow control based on a high-speed linear array CCD camera and a Schlieren system according to claim 4, characterized in that: In the fourth step: the specific method of evaluating the control effect by analyzing the state of the shock wave in the flow field continuously recorded by the high-speed area array CCD camera is: the upper computer extracts the shock wave position in the schlieren frame by frame according to the time series, and converts the time domain signal of the position change into a frequency domain signal through fast Fourier transform. The operator observes the change in the main frequency of the shock wave pulsation. If the low-frequency pulsation intensity of the shock wave is reduced compared with the open-loop control or uncontrolled flow field, or the shock wave position pulsation is converted into a high-frequency signal with very low intensity, then the closed-loop control obtains a better control effect; otherwise, it is a poor control effect.

7. The workflow of supersonic closed-loop flow control based on a high-speed linear array CCD camera and a Schlieren system according to claim 4, characterized in that: The high-performance real-time controller accepts the PID control law given by the host computer to control the supersonic closed-loop flow. The control method is as follows: (1) Extract the shock wave position change under the open-loop fixed-frequency pulse arc plasma excitation provided by the pulse arc plasma exciter: Step 1: Debug the flow field to ensure the compression corner model is installed and the wind tunnel is operating normally; Step 2: Complete the debugging of the Schlieren system and the high-speed linear array CCD camera so that the number of row pixels of the high-speed linear array CCD camera can cover the movement range of the shock wave in the flow field and calibrate the pixel distance. Combine the camera's own pixel clock and the number of row pixels to calculate the line frequency. The calculation formula is: The line frequency of the high-speed linear array CCD camera is greater than the dominant characteristic frequency of the flow field to be controlled; Step 3: Use the pressure sensor inside the wind tunnel test section as a reference signal; when the signal reaches the normal operating pressure, trigger the high-speed linear array CCD camera and plasma actuator to operate; Step 4: Connect the high-speed linear array CCD camera to the high-performance real-time controller via a high-speed data transmission interface. After the wind tunnel is running, the high-speed linear array CCD camera is used to capture the flow field under continuous plasma excitation. Step 5: Identify the location with the largest grayscale value in the flow field image read by the high-speed linear array CCD camera as the location of the shock wave. For the compression corner flow field, the shock wave is located at the location with the largest density gradient in the flow field, which is reflected in the maximum grayscale value information in the flow field. A horizontal straight line at this location is extracted as the detection line of the grayscale value of the schlieren image. This line segment must completely cover the movement range of the shock wave in the flow direction. Step 6: The high-speed linear array CCD camera extracts the real-time schlieren image of the flow field frame by frame and transmits it to the high-performance real-time controller. The high-performance real-time controller has a built-in image recognition algorithm to extract the grayscale value of the input image and convert it into a two-dimensional matrix to form a two-dimensional grayscale value matrix of the image; Step 7: The image recognition algorithm in the real-time controller uses the corner point of the model as the flow zero point and the grayscale value to extract the maximum density gradient in the line segment as the shock wave position. The pixel position of the shock wave is converted into a physical position based on the pixel distance calibrated by the experimenter before the test. Step 8: Arrange the shock wave positions in each frame of the schlieren image in time according to the time interval of each frame of the schlieren image. Fit a curve based on this large amount of shock wave position data to obtain the function r(t) of the shock wave position changing with time under continuous plasma excitation. Input r(t) as the expected value into the controller. (2) PID-based closed-loop control process: Step 1: Connect the various systems. The host computer communicates with the high-performance real-time controller via a network port or serial communication link. The controller has a built-in digital-to-analog converter (DAC). The DAC output signal is connected to the power supply of the pulsed arc plasma exciter, and the analog signal output by the high-performance real-time controller directly controls the output of the power supply. Step 2: Initial parameter setting: Initially set a moderate proportional coefficient K for the PID control law in the host computer. p , so that the system has a certain response to the deviation, but not too extreme; set a longer integral time K i , in order to avoid integral saturation in the initial stage; differential control is temporarily not used in the initial stage; after the setting is completed, the PID control law is output to the high-performance real-time controller; Step 3: Parameter adjustment; first adjust the proportional coefficient of the proportional control link in the PID control law, observe the response of the power supply, and then gradually adjust the integral time and differential time of the integral control link and differential control link in the PID control law; Step 4: Operate the wind tunnel and closed-loop control system. The high-speed linear array CCD camera reads each schlieren image frame into the high-performance real-time controller, which then calculates the difference between the current shock wave position and the desired shock wave position and adjusts the output of the actuator accordingly. Step 5: The high-performance real-time controller calculates the shock wave position y(t) of the next schlieren image frame read by the high-speed linear array CCD camera and uses it as a feedback signal. When the high-performance real-time controller finds through calculation that the shock wave position is tending to be corrected within the range of r(t), the current exciter output parameters are maintained until the shock wave position is fully regulated to within the range of r(t). Conversely, if the shock wave position is found to be over-regulated in the opposite direction and exceeds the range of r(t) again, the excitation frequency or excitation voltage is reduced to return it to the range of r(t). Step 6: During the closed-loop control process, the host computer reads the complete Schlieren flow field data recorded by the high-speed area array CCD camera, analyzes and observes the control effect under the current PID parameters, and finds the optimal PID parameter combination to maximize the closed-loop control benefit.

8. The workflow of supersonic closed-loop flow control based on a high-speed linear array CCD camera and a Schlieren system according to claim 7, characterized in that: In the third step (2), the parameter adjustment is as follows: if the voltage and frequency of the power supply output respond slowly or fluctuate continuously, increase the proportional coefficient K appropriately. p If the voltage and frequency output by the power supply fluctuate due to interference from ambient noise in the flow field, increase the integration time appropriately.

9. The workflow of supersonic closed-loop flow control based on a high-speed linear array CCD camera and a Schlieren system according to claim 7, characterized in that: In the fourth step of step (2), the high-performance real-time controller adjusts the output of the exciter according to the difference between the current shock wave position and the expected shock wave position. The specific method is as follows: when the shock wave position is outside the r(t) range, the plasma excitation is started, and the excitation voltage and excitation frequency are adjusted according to the size of the range exceeded. The larger the range exceeded, the higher the excitation voltage and excitation frequency; on the contrary, if the shock wave position is within the r(t) range, the pulse arc plasma exciter is stopped.

10. The workflow of supersonic closed-loop flow control based on a high-speed linear array CCD camera and a Schlieren system according to claim 7, characterized in that: In the sixth step, in order to maximize the closed-loop control benefit, a heuristic parameter optimization algorithm is used to find the best PID parameter combination.

Images