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

Abstract

The invention discloses a light path system based on a high-speed linear array CCD camera and schlieren. The light path system comprises a light source, a slit, a first reflecting spherical mirror, a second reflecting spherical mirror, a light cutting edge, a spectroscope, the high-speed linear array CCD camera and a high-speed area array CCD camera. The invention provides an optical path system working method based on a high-speed linear array CCD camera and schlieren. The invention provides a closed-loop control system based on a high-speed linear array CCD (Charge Coupled Device) camera and schlieren. The closed-loop control system comprises a pulsed arc plasma exciter, an optical path system based on the high-speed linear array CCD camera and the schlieren, a high-performance real-time controller embedded with a PID (Proportion Integration Differentiation) control law, and an upper computer. The invention further provides a working process of supersonic closed-loop flow control based on the high-speed linear array CCD camera and the schlieren system. On the basis of an optical sensing method, the problem of strong electromagnetic interference of pulsed arc discharge on closed-loop feedback signals is solved, and blind open-loop self-adaption to closed-loop self-adaption of pulsed arc plasma excitation in supersonic flow control is achieved.

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 new type of active flow control technology, pulsed arc plasma actuator can inject thermal energy into the flow field in a short time, and regulate the flow field through disturbance forms such as density, vorticity, temperature and thermal blocking virtual surface. It has the advantages of simple structure, high excitation intensity and high frequency response, and has achieved excellent control effects on flow structures such as ultrasonic shock waves, boundary layers and shear layers. However, in current engineering practice, the control method of pulsed arc plasma actuator still adopts traditional open-loop fixed-frequency excitation, which has two problems. On the one hand, this fixed-frequency excitation is difficult to couple with the strong nonlinearity of high-speed complex flow fields, and the control effect is weak; on the other hand, due to the lack of reliable sensor feedback adjustment mechanism, the unchanged excitation parameters are difficult to adapt to the complex and changeable flight environment. In order to improve the robustness of the pulsed arc plasma excitation control system and adjust the excitation parameters according to the flight state to maximize the energy gain of flow control, it is inevitable to move towards closed loop for plasma flow control. For the closed-loop intelligent flow control technology in wind tunnel experiments, it is first necessary to sense the flow field environment based on a series of sensors, and then the controller collects the sensor signals and uses them as input to perform control law calculations, and then delivers them 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 will strongly interfere with the sensor itself and the communication between the sensor, CPU, and excitation system, resulting in signal noise and distortion, system mis-triggering, and equipment damage in severe cases. This strong electromagnetic interference problem has seriously restricted the closed-loop development process of pulsed arc plasma excitation. So far, researchers in this field have not successfully achieved closed-loop flow control for pulsed 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, 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

[0004] a light source that emits a focused 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, since the slit is located at the focal position of the first reflecting spherical mirror, 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, and the first reflecting spherical mirror is located at the center of the light spot; the first reflecting spherical mirror emits a parallel light beam, and the parallel light beam passes through the flow field to be controlled;

[0007] A 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] A 45° beam splitter, located behind the cutting edge, splits the optical signal of the flow field captured by the Schlieren into two beams of light with the same intensity;

[0010] The high-speed area array CCD camera receives a beam of light emitted by the 45° beam splitter, and the 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 enters the high-speed linear array CCD camera after reflection;

[0012] In the optical path system based on high-speed linear array CCD camera and Schlieren, the light source, slit, first and second reflecting spherical mirrors, and 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 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 switch, shoot a convergent beam to align with the optical axis, and 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 let the first reflecting spherical mirror be 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, and is 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 to the center point of the light spot to generate 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 for overall recording of the 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 to support 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, the system comprising 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) PID control law, and a host computer; wherein

[0021] Select the compression corner shock wave / boundary layer interference flow field, install the arc plasma exciter in the compression corner model, install the compression corner model in the wind tunnel, and adjust the optical path of the schlieren system so that the parallel light emitted by the first reflecting spherical mirror completely covers 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 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 the closed-loop control law, the high-performance real-time controller outputs an excitation adjustment signal to the pulse arc plasma exciter.

[0026] A workflow of supersonic closed-loop flow control based on a high-speed linear array CCD camera and a schlieren system is also provided, which is based on the above-mentioned closed-loop control system based on a high-speed linear array CCD camera and schlieren, 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, and 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 this moment;

[0029] Step 3: The high-performance real-time controller reads the analog voltage signal output by the high-speed linear array CCD camera, identifies the characteristic flow state through threshold analysis and fitting algorithms, and calculates and outputs the control instructions according to the closed-loop control law output by the host computer;

[0030] Step 4: The high-performance real-time controller sends 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 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.

[0031] In one embodiment of the present invention, in the third step: a high-performance real-time controller is implemented by selecting 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 array CCD camera 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 a 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 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; otherwise, the control effect is not good.

[0033] In another embodiment of the present invention, 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:

[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 that the compression corner model is installed and the wind tunnel operates normally;

[0036] Step 2: Complete the debugging of the Schlieren system and the high-speed linear array CCD camera, so that the number of line 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 line 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 work;

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

[0041] Step 5: Identify the position 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 in the flow field is located at the position with the largest density gradient, which is reflected in the flow field as the maximum grayscale value information. Extract a horizontal straight line at this position 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 gray value of the input image and convert it into a two-dimensional matrix to form a two-dimensional gray 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 position, extracts the maximum value of the density gradient in the line segment with the gray value as the shock wave position, and converts the pixel position of the shock wave into the physical position according to the pixel distance calibrated by the test personnel before the test;

[0044] Step 8: Arrange the shock wave position in each frame of the schlieren image in time according to the time interval of each frame of the schlieren image, fit the curve based on these large amounts of shock wave position data, obtain the function r(t) of the shock wave position changing with time under continuous plasma excitation, and input r(t) into the controller as the expected value;

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

[0046] Step 1: Connect each system; the host computer communicates with the high-performance real-time controller through the network port or serial port communication link; the controller has a built-in digital-to-analog converter DAC, and the DAC output signal is connected to the power supply of the pulse arc plasma exciter, and the output of the power supply is directly controlled by the analog signal output by the high-performance real-time controller;

[0047] Step 2: Initial parameter setting: Preliminarily 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 much; set a longer integral time K i , in order to avoid integral saturation in the initial stage; differential control is not used temporarily in the initial stage; after the setting is completed, the PID control law is output to a 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: Run the wind tunnel and closed-loop control system equipment. The high-speed linear array CCD camera reads each frame of schlieren image into the high-performance real-time controller, which calculates the difference between the current shock wave position and the expected shock wave position, and adjusts the output of the exciter accordingly.

[0050] Step 5: The high-performance real-time controller calculates the shock wave position y(t) of the next frame of the schlieren image 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 has a tendency to be corrected to the range of r(t), the current output parameters of the actuator are maintained until the shock wave position is fully regulated to the range of r(t). On the contrary, if it is found that the shock wave position is over-regulated to the opposite direction and exceeds the range of r(t) again, the excitation frequency or excitation voltage is reduced to restore 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 array CCD camera, analyzes and observes the control effect under the current PID parameters, and finds the best 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 environmental 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 method based on optical sensing can avoid the strong electromagnetic interference problem of pulse arc discharge on the closed-loop feedback signal, and realize the transformation of blind open-loop to closed-loop adaptive pulse arc plasma excitation in supersonic flow control.

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

[0057] Traditional closed-loop control mostly 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 very 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 excited by pulsed arc plasma. 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 beam splitter 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 perform Schlieren photography 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 The optical path arrangement based on high-speed linear array CCD camera and Schlieren is shown;

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

[0061] Figure 3 The schematic 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 path system based on high-speed linear array CCD camera and Schlieren is shown. The system consists of 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.

[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, since the slit is located at the focal position of the first reflecting spherical mirror, 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, and the first reflecting spherical mirror is located at the center of the light spot; the first reflecting spherical mirror emits a parallel light beam, and the parallel light beam passes through the flow field to be controlled;

[0067] A 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 to the center point of the light spot, generating a Schlieren image of airflow changes.

[0069] The 45° beam splitter is located behind the light-cutting edge and is used to split the flow field optical signal captured by the schlieren into two beams of light with the same 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 optical path system based on high-speed linear array CCD camera and schlieren, the present invention provides a working method of the optical path system based on high-speed linear array CCD camera and schlieren, which is specifically as follows.

[0074] Step 1: Turn on the light source switch, shoot a convergent beam to align with the optical axis, and 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 let the first reflecting spherical mirror be 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, and is 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 point of the light spot to produce a Schlieren image of airflow changes.

[0079] Step 6: Different from the traditional Z-type Schlieren optical path, a 45° beam splitter is installed behind the cutting edge to split the optical signal of the flow field captured by the Schlieren into two beams of light with the same 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 to support subsequent closed-loop control.

[0080] Figure 2 The composition of the closed-loop control system based on high-speed linear array CCD camera and schlieren proposed in the present invention is shown. The system mainly includes a pulsed arc plasma actuator, an optical path system based on 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. The present invention selects the compression corner shock wave / boundary layer interference flow field as an embodiment, but its application scenario is not limited to this flow field. Other examples that can be implemented include supersonic cavity shear layer control, hypersonic boundary layer transition control, etc.

[0081] An arc plasma exciter is installed in the compression corner model (the installation position and method of the exciter are well known to technical personnel in this field), 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 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, and the host computer 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 according to the response of the power supply, including the proportional coefficient, integral time and differential time;

[0085] According to the flow field state and the closed-loop control law, the high-performance real-time controller outputs an 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 change of the flow field into a small deviation of the light along the propagation direction (this technology is well known to technicians in this field), and 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.

[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 this 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 taking the position change of the shock wave as the characteristic state of the flow field). The specific implementation method here is given by the closed-loop control principle and method below. And 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, linear genetic programming, etc. These control laws / algorithms are well known to those skilled in the art and will not be repeated. 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 command to the power module of the pulse 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 technicians in this field), and the operator observes the change of 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. On the contrary, it is a bad control effect.

[0091] Figure 3 It is shown that 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 that the compression corner model is installed and the wind tunnel operates 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), and then calculate the row frequency by combining the camera's own pixel clock and the number of row pixels. The calculation formula is:

[0095]

[0096] To ensure that the main flow field pulsation characteristics of the high-speed flow field 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 the compression corner shock wave as an example, this frequency is the pulsation frequency of the separation shock wave.

[0097] 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 exciter to work (in a specific embodiment of the present invention, in order to suppress the low-frequency pulsation of the shock wave, the maximum excitation frequency is set to 10kHz and the excitation voltage is set to 20kV);

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

[0099] Step 5: Identify the position with the largest grayscale value in the flow field image read by the high-speed linear array CCD camera as the position of the shock wave (for the compression corner flow field, the shock wave in the flow field is located at the position with the largest density gradient, which is reflected in the flow field as the maximum grayscale value information), and extract a horizontal straight line at the position as the detection line of the grayscale value of the Schlieren image. The 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 gray value of the input image and convert it into a two-dimensional matrix to form a two-dimensional gray 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 position, extracts the maximum value of the density gradient in the line segment with the gray value as the shock wave position, and then converts the pixel position of the shock wave into a physical position according to the pixel distance calibrated by the test personnel before the test (this technology is well known to those skilled in the art);

[0102] Step 8: Arrange the shock wave position in each frame of the schlieren image in time according to the time interval of each frame of the schlieren image, fit the curve based on these large amounts of shock wave position data, obtain the function r(t) of the shock wave position changing with time under continuous plasma excitation, and input r(t) as the expected value into the controller. The curve fitting method is well known to those skilled in the art and will not be described again.

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

[0104] Step 1: Connect each system. The host computer communicates with the high-performance real-time controller through the 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 pulse arc plasma exciter, and the output of the power supply 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, a moderate proportional coefficient K is initially set for the PID control law. p , so that the system has a certain response to the deviation, but not too much; 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 temporarily 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 by the power supply have a slow response speed or continuous fluctuations, appropriately increase the proportional coefficient K. p ; If the voltage and frequency of the power supply output fluctuate due to the interference of environmental noise in the flow field, the integration time should be appropriately increased. In 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, which then 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 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.

[0108] Step 5: The high-performance real-time controller calculates the shock wave position y(t) of the next frame of the schlieren image 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 has a tendency to be corrected to the range of r(t), the current output parameters of the actuator can be maintained until the shock wave position is fully regulated to the range of r(t). On the contrary, if it is found that the shock wave position is over-regulated to the opposite direction and exceeds the range of r(t) again, the excitation frequency or excitation voltage is reduced to restore it to the range of r(t).

[0109] Step 6: During the closed-loop control process, the host computer reads the complete Schlieren flow field data recorded by the high-speed array CCD camera, and analyzes and observes the control effect under the current PID parameters. In order to maximize the closed-loop control benefits, the best PID parameter combination can be found by relying on heuristic parameter optimization algorithms, such as genetic algorithms, particle swarms, and simulated annealing. This method is known to those skilled in the art and will not be repeated here.

[0110] For general compression corner flow fields, the number of pixels in the collected field of view is generally hundreds of pixels. Here, 100 pixels / row is used as an example for calculation, 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 6.8 Gbps. 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 supersonic flow closed-loop control method based on a high-speed linear array CCD camera and schlieren. Compared with the traditional closed-loop flow control implementation scheme, 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 by optical means, avoiding the problem that various types of 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 the supersonic flow field. At the same time, the delay time for real-time image acquisition in the entire closed-loop control process is in 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 spectroscope 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 shoot the schlieren of the flow field and evaluate the flow control effect. In the control process, the control strategy can be adjusted in real time based on the evaluation results. It can be applicable to the closed-loop control requirements of various complex flow fields, with good versatility and broad application prospects.

Claims

1. An optical system based on high-speed linear array CCD camera and Schlieren. It is 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 focused beam of light; Slit, the converging light beam emitted by the light source is imaged on the slit; The first reflecting spherical mirror, since the slit is located at the focal position of the first reflecting spherical mirror, 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, and the first reflecting spherical mirror is located at the center of the light spot; the first reflecting spherical mirror emits a parallel light beam, and the parallel light beam passes through the flow field to be controlled; A 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; A 45° beam splitter, located behind the cutting edge, splits the optical signal of the flow field captured by the Schlieren into two beams of light with the same intensity; The high-speed area array CCD camera receives a beam of light emitted by the 45° beam splitter, and the 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 enters the high-speed linear array CCD camera after reflection; In the optical path system based on high-speed linear array CCD camera and Schlieren, the light source, slit, first and second reflecting spherical mirrors, and 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, It is characterized in that The method is as follows: Step 1: Turn on the light source switch, shoot a convergent beam to align with the optical axis, and 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, and is 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 to the center point of the light spot to generate 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 for overall recording of the 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 to support subsequent closed-loop control.

3. A closed-loop control system based on high-speed linear CCD camera and Schlieren. It is characterized in that The system comprises a pulse 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 Select the compression corner shock wave / boundary layer interference flow field, install the arc plasma exciter in the compression corner model, install the compression corner model in the wind tunnel, and adjust the optical path of the schlieren system so that the parallel light emitted by the first reflecting spherical mirror completely covers 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 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 the closed-loop control law, the high-performance real-time controller outputs an excitation adjustment signal to the pulse arc plasma exciter.

4. A workflow of 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 a Schlieren system as claimed in claim 3, It is 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, and 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 this moment; Step 3: The high-performance real-time controller reads the analog voltage signal output by the high-speed linear array CCD camera, identifies the characteristic flow state through threshold analysis and fitting algorithms, and calculates and outputs the control instructions according to the closed-loop control law output by the host computer; Step 4: The high-performance real-time controller sends 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 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.

5. The workflow of supersonic closed-loop flow control based on a high-speed linear array CCD camera and a Schlieren system as claimed in claim 4, It is 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 as claimed in claim 4, It is 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 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 the frequency domain signal through the fast Fourier transform. The operator observes the change of 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 as claimed in claim 4, It is 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 that the compression corner model is installed and the wind tunnel operates normally; Step 2: Complete the debugging of the Schlieren system and the high-speed linear array CCD camera, so that the number of line 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 line 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 work; Step 4: Connect the high-speed linear array CCD camera to the high-performance real-time controller through a high-speed data transmission interface. After the wind tunnel is running, the high-speed linear array CCD camera is used to collect the flow field under continuous plasma excitation. Step 5: Identify the position 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 in the flow field is located at the position with the largest density gradient, which is reflected in the flow field as the maximum grayscale value information. Extract a horizontal straight line at this position 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 gray value of the input image and convert it into a two-dimensional matrix to form a two-dimensional gray 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 position, extracts the maximum value of the density gradient in the line segment with the gray value as the shock wave position, and converts the pixel position of the shock wave into the physical position according to the pixel distance calibrated by the test personnel before the test; Step 8: Arrange the shock wave position in each frame of the schlieren image in time according to the time interval of each frame of the schlieren image, fit the curve based on these large amounts of shock wave position data, obtain the function r(t) of the shock wave position changing with time under continuous plasma excitation, and input r(t) into the controller as the expected value; (2) Closed-loop control process based on PID: Step 1: Connect each system; the host computer communicates with the high-performance real-time controller through the network port or serial port communication link; the controller has a built-in digital-to-analog converter DAC, and the DAC output signal is connected to the power supply of the pulse arc plasma exciter, and the output of the power supply is directly controlled by the analog signal output by the high-performance real-time controller; Step 2: Initial parameter setting: Preliminarily 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 much; set a longer integral time K i , in order to avoid integral saturation in the initial stage; differential control is not used temporarily in the initial stage; after the setting is completed, the PID control law is output to a 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: Run the wind tunnel and closed-loop control system equipment. The high-speed linear array CCD camera reads each frame of schlieren image into the high-performance real-time controller, which calculates the difference between the current shock wave position and the expected shock wave position, and adjusts the output of the exciter accordingly. Step 5: The high-performance real-time controller calculates the shock wave position y(t) of the next frame of the schlieren image 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 has a tendency to be corrected to the range of r(t), the current output parameters of the actuator are maintained until the shock wave position is fully regulated to the range of r(t). On the contrary, if it is found that the shock wave position is over-regulated to the opposite direction and exceeds the range of r(t) again, the excitation frequency or excitation voltage is reduced to restore 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 array CCD camera, analyzes and observes the control effect under the current PID parameters, and finds the best 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 as claimed in claim 7, It is characterized in that In the third step of step (2), the parameter adjustment is as follows: if the voltage and frequency of the power supply output 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 environmental 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 as claimed in claim 7, It is 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 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 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 as claimed in claim 7, It is 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.

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