Optical film thickness monitoring system based on FPGA

Abstract

The utility model discloses an optical film thickness monitoring system based on an FPGA (Field Programmable Gate Array), which belongs to the field of optical film coating and comprises an optical path and a circuit, the light path comprises a light source, a light chopper, a beam splitter, a monitoring piece and a monochromator, the circuit comprises a photoelectric conversion board and an FPGA development board, the light path part is responsible for generating and modulating light signals, the circuit part is responsible for signal conversion, amplification, demodulation, noise filtering and extreme point interpretation, on the FPGA development board, a digital signal processing technology is utilized, a reference signal is used as a benchmark, and the optical path part and the circuit part are integrated. The main signal is demodulated, in the demodulation process, noise and interference caused by factors such as environment and equipment can be eliminated, some noise and interference still exist in the demodulated main signal, the FPGA development board can further filter out the noise and improve the signal-to-noise ratio of the signal, and the extreme point of the main signal subjected to noise filtering processing corresponds to the change of the thickness of the film. The algorithm on the FPGA development board can accurately interpret the extreme points, so that the change information of the film thickness is obtained, the change of the film thickness can be more accurately identified, and the monitoring precision is improved.

Description

Technical Field

[0001] The present application belongs to the field of optical coating, and in particular to an optical film thickness monitoring system based on FPGA. Background Art

[0002] Film thickness monitoring technology is one of the key technologies in the field of optical coating. Film thickness has a decisive influence on the optical and mechanical properties of the film. Therefore, it is very important to be able to monitor the film thickness in real time during the production process and stop coating in time when the film reaches the set thickness.

[0003] At present, the most commonly used film thickness monitoring method is the optical monitoring method. This method uses the change in the optical thickness of the film on the monitoring piece during the coating process to reflect the change in the physical thickness of the film. The specific implementation method is to use a light source to emit a beam of light to the monitoring piece. As the thickness of the film increases, its transmittance (or reflectivity) will change accordingly, and when the film thickness increases by a certain amount, the corresponding transmittance (or reflectivity) will reach an extreme value, and the corresponding transmitted light intensity (or reflected light intensity) will also reach an extreme value. After converting the transmitted light (or reflected light) into an electrical signal, the corresponding film thickness can be obtained by reading the extreme point of the electrical signal. However, due to stray light, noise and some other interference, the converted electrical signal will be distorted, which will affect the accuracy of the monitoring system. At the same time, because the transmittance (or reflectivity) is extremely insensitive to the change of film thickness near the extreme point, it further increases the difficulty of our film thickness monitoring. Utility Model Content

[0004] The utility model provides an optical film thickness monitoring system based on FPGA, which is used to solve or alleviate the problem of affecting the accuracy of the film thickness monitoring system due to distortion of electrical signals converted by an optical monitoring method under stray light, noise and other interferences.

[0005] In order to achieve the above purpose, the utility model provides the following technical solutions:

[0006] An optical film thickness monitoring system based on FPGA, comprising an optical path and a circuit;

[0007] The optical path includes a light source, a chopper, a beam splitter, a monitoring plate and a monochromator, and the circuit includes a photoelectric conversion board and an FPGA development board;

[0008] The optical path is connected by optical fiber. The light source emits light and is modulated by a chopper. The modulated light is divided into two paths by a beam splitter. One path is sent to the monitoring chip as the main signal. The main signal passes through the monitoring chip and enters the monochromator to filter stray light and is sent to the photoelectric conversion board to be converted into an electrical signal. The other path is used as a reference signal and is directly sent to the photoelectric conversion board to be converted into an electrical signal for demodulation of the main signal.

[0009] The circuit part adopts electrical connection. The photoelectric conversion board can amplify the two electrical signals and convert them into digital signals and send them to the FPGA development board. The FPGA development board can use the reference signal to realize the demodulation, noise filtering and extreme point judgment of the main signal to improve the accuracy of film thickness monitoring.

[0010] The utility model of the optical film thickness monitoring system based on FPGA includes two parts, the optical path and the circuit. The optical path part is responsible for generating and modulating the optical signal, and the circuit part is responsible for signal conversion, amplification, demodulation, noise filtering and extreme point judgment. On the FPGA development board, the digital signal processing technology is used to demodulate the main signal with the reference signal as the benchmark. During the demodulation process, the noise and interference caused by factors such as the environment and equipment can be eliminated. The demodulated main signal may still have some noise and interference. The FPGA development board can further filter out these noises and improve the signal-to-noise ratio of the signal. The extreme points of the main signal after the noise filtering process correspond to the changes in the film thickness. The algorithm on the FPGA development board can accurately judge these extreme points, thereby obtaining the change information of the film thickness, so that the change of the film thickness can be more accurately identified and the monitoring accuracy can be improved.

[0011] In a preferred implementation, the FPGA is provided with a synchronous detection module to achieve demodulation of the main signal.

[0012] Synchronous detection uses a reference signal to demodulate the main signal, which can effectively suppress and eliminate the influence of interference factors such as stray light and noise on the main signal, enhance the stability of the system, and make the film thickness monitoring results more reliable.

[0013] In a preferred implementation, the FPGA is provided with a Kalman filter module for filtering out low-amplitude burr noise of the demodulated signal.

[0014] Although the demodulated film thickness monitoring signal (main signal) effectively suppresses noise interference to a certain extent, it still has low-amplitude burr noise. Low-amplitude burr noise may cause system misjudgment or false alarm, affecting the accuracy of the monitoring results. The Kalman filter module can reduce the false alarm rate of the system and improve the accuracy of the monitoring results by filtering out these noises.

[0015] In a preferred implementation, the FPGA is provided with an extreme point determination module, and the extreme point determination module uses a differential micro-threshold determination method to determine the signal extreme point.

[0016] The differential small threshold judgment method detects extreme points by calculating the difference of the signal (i.e. the difference between adjacent sampling points). Specifically, the signal changes slowly near the extreme point, so the difference between adjacent sampling points near the extreme point is smaller than that near the non-extreme point. Therefore, we can set a reasonable threshold. When the difference between adjacent sampling points is less than this threshold, we can determine that there is an extreme point, but if the extreme value is determined by only this one point, it is likely to cause misjudgment. Therefore, when multiple consecutive differences are less than the threshold we set, we believe that we have entered an extreme point area, that is, there is an extreme value. In the optical film thickness monitoring system, this helps to improve the accuracy of film thickness measurement, can effectively suppress the influence of noise and interference on the judgment results, enhance the system's anti-interference ability, and make the extreme point judgment more reliable.

[0017] In a preferred implementation, the photoelectric conversion board includes a photodiode that converts an optical signal into a weak electrical signal, a signal amplification module that amplifies the weak electrical signal, and an analog-to-digital converter that converts the amplified signal into a digital signal. The analog-to-digital converter is electrically connected to the FPGA development board, and the two use the SPI communication protocol.

[0018] In a preferred implementation, the analog-to-digital converter converts the analog signal into a digital signal using differential sampling to improve anti-interference capability.

[0019] In a preferred implementation, the signal amplification module includes a feedback resistor R1, a feedback capacitor C1, a feedback resistor R2, a feedback capacitor C2, a transimpedance amplifier and a single-pole double-throw switch, the output end of the photodiode is electrically connected to the input end of the transimpedance amplifier, and the output end of the transimpedance amplifier is electrically connected to the input end of the single-pole double-throw switch;

[0020] The feedback resistor R1 is connected in parallel with the feedback capacitor C1, one end of which is electrically connected to the reverse terminal of the transimpedance amplifier, and the other end is electrically connected to the single-pole double-throw switch. The feedback resistor R2 is connected in parallel with the feedback capacitor C2, one end of which is electrically connected to the reverse terminal of the transimpedance amplifier, and the other end is electrically connected to the single-pole double-throw switch. The output end of the single-pole double-throw switch is electrically connected to the input end of the differential sampling pre-circuit, and the output end of the differential sampling pre-circuit is electrically connected to the analog-to-digital converter.

[0021] In a preferred implementation, it also includes a power module, which supplies power to the photoelectric conversion board.

[0022] The above structure has the following beneficial effects:

[0023] The utility model discloses an optical film thickness monitoring system based on FPGA, which monitors the film thickness by optical monitoring method, reflects the physical thickness change of the film by monitoring the optical thickness change of the film on the chip, modulates the signal by a chopper, and then uses FPGA to realize synchronous demodulation of the signal, effectively suppresses the interference of noise, and uses Kalman filtering algorithm to further process the demodulated signal, filters out low-amplitude burr noise, and avoids signal distortion to the greatest extent, and finally adopts differential micro-threshold method to judge the extreme point, which ensures the correct reading of the extreme point and avoids misreading and missed reading. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The drawings described herein are used to provide a further understanding of the present invention and constitute a part of the present invention. The illustrative embodiments of the present invention and their descriptions are used to explain the present application and do not constitute an improper limitation on the present invention. In the drawings:

[0025] Figure 1 This is the structural diagram of the monitoring system;

[0026] Figure 2 This is the schematic diagram of the photoelectric conversion board;

[0027] Figure 3 This is the principle block diagram of digital phase-locked amplifier;

[0028] Figure 4 This is the flow chart of Kalman filter algorithm;

[0029] Description of labels:

[0030] 1-light source, 2-chopper, 3-beam splitter, 4-monitoring film, 5-monochromator, 6-photoelectric conversion board, 7-FPGA development board, 8-main signal, 9-reference signal, 10-photodiode, 11-feedback resistor R1, 12-feedback capacitor C1, 13-feedback resistor R2, 14-feedback capacitor C2, 15-transimpedance amplifier, 16-single-pole double-throw switch, 17-differential sampling pre-circuit, 18-analog-to-digital converter, 19-power module. DETAILED DESCRIPTION

[0031] In the following, only some exemplary embodiments are briefly described. As those skilled in the art will appreciate, the described embodiments may be modified in various ways without departing from the spirit and scope of the present invention. Therefore, the drawings and descriptions are considered to be exemplary and non-restrictive in nature.

[0032] In the description of the present invention, it is necessary to understand that the terms "center", "upper", "lower", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "axial", "radial", "circumferential" and the like indicate positions or positional relationships based on the positions or positional relationships shown in the drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. In the present invention, unless otherwise clearly specified and limited, a first feature being "upper" or "lower" than a second feature may be the first and second features directly contacting, or the first and second features indirectly contacting through an intermediate medium.

[0033] In the present utility model, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral one; it can be directly connected, or indirectly connected through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. However, if it is indicated as a direct connection, it means that the two connected bodies are not connected through an intermediate structure, but are connected to form a whole only through a connecting structure. For ordinary technicians in this field, the specific meanings of the above terms in the present utility model can be understood according to the specific circumstances.

[0034] In the present invention, the descriptions of "first", "second", etc. are only used for descriptive purposes and cannot be understood as indicating or implying their relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features defined as "first" or "second" may explicitly or implicitly include at least one of the features.

[0035] The utility model is described below in conjunction with the accompanying drawings.

[0036] The specific plans adopted are:

[0037] like Figure 1 As shown, in this embodiment, the optical film thickness monitoring system includes a light source 1, a chopper 2, a beam splitter 3, a monitoring plate 4, a monochromator 5, a photoelectric conversion board 6, and an FPGA development board 7.

[0038] The optical path is connected by optical fiber. The light source 1 can be set as a halogen lamp light source. After emitting light, it is modulated by the chopper 2. The modulated light is divided into two paths by the beam splitter 3. One path is sent to the monitoring plate 4 as the main signal 8 for monitoring the change of film thickness. The other path is sent directly to the photoelectric conversion board 6 as the reference signal 9 to be converted into an electrical signal for demodulation of the main signal. The main signal is sent to the monochromator 5 after passing through the monitoring plate. The monochromator 5 is used to filter out stray light and obtain light of a specific frequency. After passing through the monochromator 5, the main signal is sent to the photoelectric conversion board 6 to be converted into an electrical signal.

[0039] See also Figure 2 The photoelectric conversion board 6 needs to process the main signal 8 and the reference signal 9 respectively. The photoelectric conversion board includes a photodiode 10, a feedback resistor R111, a feedback capacitor C112, a feedback resistor R213, a feedback capacitor C214, a transimpedance amplifier 15, a single-pole double-throw switch 16, a differential sampling pre-circuit 17, an analog-to-digital converter 18, and a power module 19.

[0040] The output end of the photodiode 10 is electrically connected to the input end of the transimpedance amplifier 15, the output end of the transimpedance amplifier 15 is electrically connected to the input end of the single-pole double-throw switch 16, the feedback resistor R111 is connected in parallel with the feedback capacitor C112, one end of which is electrically connected to the reverse terminal of the transimpedance amplifier 15, and the other end is electrically connected to the single-pole double-throw switch 16, the feedback resistor R213 is connected in parallel with the feedback capacitor C214, one end of which is electrically connected to the reverse terminal of the transimpedance amplifier 15, and the other end is electrically connected to the single-pole double-throw switch 16, the output end of the single-pole double-throw switch 16 is electrically connected to the input end of the differential sampling pre-circuit 17, the output end of the differential sampling pre-circuit 17 is electrically connected to the analog-to-digital converter 18, and the power supply module 19 supplies power to the entire photoelectric conversion board.

[0041] Furthermore, the photodiode 10 is used to convert the optical signal into a weak electrical signal, the transimpedance amplifier 15 is used to amplify the weak electrical signal for subsequent processing, the feedback resistor determines the amplification factor of the transimpedance amplifier, the feedback capacitor determines the bandwidth of the amplifier circuit, the single-pole double-throw switch 16 is used to select the amplification factor, the differential sampling pre-circuit 17 is used to generate two input signals for differential sampling, and the analog-to-digital converter 18 is used to perform differential sampling on the amplified signal and convert the analog signal into a digital signal for subsequent processing using an FPGA, wherein differential sampling can reduce common-mode interference.

[0042] In this embodiment, the input of the FPGA development board 7 is electrically connected to the analog-to-digital converter 18, and the two use the SPI communication protocol.

[0043] Furthermore, the FPGA development board mainly sets up a synchronous detection module, a Kalman filtering module and an extreme point judgment module to complete the following three tasks: synchronous detection, Kalman filtering, and extreme point judgment.

[0044] In this embodiment, synchronous detection is used to demodulate the main signal. The principle is as follows: Figure 3 As shown, a digital phase-locked loop (DPLL) is used to lock the reference signal to generate two orthogonal digital sine wave signals of the same frequency, and then these two orthogonal digital sine wave signals are multiplied by the main signal to obtain two low-frequency demodulated signals, which are respectively proportional to the in-phase and orthogonal components of the input signal at the modulation frequency, and then filtered through a low-pass filter and processed. Figure 3 The demodulated signal can be obtained by the operation shown.

[0045] Furthermore, although the demodulated film thickness monitoring signal (main signal) effectively suppresses noise interference to a certain extent, it still has low-amplitude burr noise, so it needs to be further processed using Kalman filtering. Kalman filtering mainly has the following five formulas:

[0046]

[0047] The Kalman filter process is as follows Figure 4 First, initialize the initial state covariance matrix P0, the initial state x0, the process noise covariance matrix Q, and the measurement noise covariance matrix R, k = 1. Then perform a priori estimation, use the initial state x0 to estimate the state covariance matrix Pk at the next moment by formula (2), and estimate the system state variable value xk at the next moment by formula (1). Then perform a posteriori estimation, calculate the Kalman gain Kk by formula (3), and then correct the a priori estimate according to the current measurement value of the system, and get the optimal value by formula (4) That is the Kalman filter output value, and then the modified covariance matrix Pk is obtained by formula (5). At this time, one cycle of Kalman filtering has been completed, and then k=k+1 is set, and the value at the next moment is estimated a priori, and this cycle is repeated to realize the Kalman filtering of the input signal.

[0048] In this embodiment, the extreme point judgment adopts the differential small threshold judgment method, using the first-order differential value Δn,n-1 of the main signal sampling value, and setting a small reference threshold δ according to the characteristics of the signal change and the accuracy requirements.

[0049] Δ n,n-1 ≤δ Formula (6)

[0050] At this point, it can be determined that the signal has entered the extreme point area and there is an extreme point.

[0051] The parts not described in the present invention can be realized by adopting or drawing on the existing technology.

[0052] The above are only specific implementations of the utility model, but the protection scope of the utility model is not limited thereto. Any technician familiar with the technical field can easily think of various changes or substitutions within the technical scope disclosed by the utility model, which should be included in the protection scope of the utility model. Therefore, the protection scope of the utility model should be based on the protection scope of the claims.

Claims

1. An optical film thickness monitoring system based on FPGA, characterized in that: Including optical path and circuit; The optical path includes a light source, a chopper, a beam splitter, a monitoring plate and a monochromator, and the circuit includes a photoelectric conversion board and an FPGA development board; The optical path is connected by optical fiber. The light source emits light and is modulated by a chopper. The modulated light is divided into two paths by a beam splitter. One path is sent to the monitoring chip as the main signal. The main signal passes through the monitoring chip and enters the monochromator to filter stray light and is sent to the photoelectric conversion board to be converted into an electrical signal. The other path is used as a reference signal and is directly sent to the photoelectric conversion board to be converted into an electrical signal for demodulation of the main signal. The circuit part adopts electrical connection. The photoelectric conversion board can amplify the two electrical signals and convert them into digital signals and send them to the FPGA development board. The FPGA development board can use the reference signal to realize the demodulation, noise filtering and extreme point judgment of the main signal to improve the accuracy of film thickness monitoring.

2. The FPGA-based optical film thickness monitoring system according to claim 1, characterized in that: The FPGA is provided with a synchronous detection module to realize the demodulation of the main signal.

3. The FPGA-based optical film thickness monitoring system according to claim 1, characterized in that: The FPGA is provided with a Kalman filter module for filtering out low-amplitude burr noise of the demodulated signal.

4. The FPGA-based optical film thickness monitoring system according to claim 1, characterized in that: The FPGA is provided with an extreme point determination module, and the extreme point determination module uses a differential micro-threshold determination method to determine the signal extreme point.

5. The FPGA-based optical film thickness monitoring system according to claim 1, characterized in that: The photoelectric conversion board includes a photodiode that converts an optical signal into a weak electrical signal, a signal amplification module that amplifies the weak electrical signal, and an analog-to-digital converter that converts the amplified signal into a digital signal. The analog-to-digital converter is electrically connected to the FPGA development board, and the two use the SPI communication protocol.

6. The FPGA-based optical film thickness monitoring system according to claim 5, characterized in that: The analog-to-digital converter converts the analog signal into a digital signal using differential sampling to improve the anti-interference capability.

7. The FPGA-based optical film thickness monitoring system according to claim 6, characterized in that: The signal amplification module includes a feedback resistor R1, a feedback capacitor C1, a feedback resistor R2, a feedback capacitor C2, a transimpedance amplifier and a single-pole double-throw switch, the output end of the photodiode is electrically connected to the input end of the transimpedance amplifier, and the output end of the transimpedance amplifier is electrically connected to the input end of the single-pole double-throw switch; The feedback resistor R1 is connected in parallel with the feedback capacitor C1, one end of which is electrically connected to the reverse terminal of the transimpedance amplifier, and the other end is electrically connected to the single-pole double-throw switch. The feedback resistor R2 is connected in parallel with the feedback capacitor C2, one end of which is electrically connected to the reverse terminal of the transimpedance amplifier, and the other end is electrically connected to the single-pole double-throw switch. The output end of the single-pole double-throw switch is electrically connected to the input end of the differential sampling pre-circuit, and the output end of the differential sampling pre-circuit is electrically connected to the analog-to-digital converter.

8. The FPGA-based optical film thickness monitoring system according to claim 1, characterized in that: It also includes a power module, which supplies power to the photoelectric conversion board.

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