The application of FBG sensors is a field that is in the ascendant and has a very broad development prospect. At present, the most important obstacle to limiting the practical application of FBG sensors is the demodulation of sensing signals. There are many methods for fiber grating sensing demodulation, but there are not many demodulation products that can be practically applied, and they are expensive. Therefore, researching and developing a demodulation system suitable for practical engineering applications and reducing the cost of the demodulation system is a key issue for enabling the fiber Bragg grating sensor to be popularized in practical engineering applications.
In view of this, in order to meet the needs of engineering applications, this paper proposes a fiber-optic grating demodulation technology based on single-chip microcomputer, which utilizes a very widely used MCU with low price and as a signal acquisition and processing MCU. Accurate, inexpensive, portable demodulator for fast measurement and easy access to the measured variable size. In order to solve the problem of slow speed of a single MCU, the system uses dual CPUs, one of which completes the signal demodulation algorithm, and the other MCU completes the logic control, the human-machine interface and communication with the host computer, through dual-port RAM Two-machine data sharing.
1. Demodulation system structure and principle
Mainly composed of three parts, Bragg grating (measuring grating), fiber grating demodulator, computer. The fiber Bragg grating demodulator can be subdivided into two parts, an analog circuit part and a digital circuit part. The function of the analog circuit part is to change the strain or temperature change of the Bragg grating (measurement grating) into a corresponding electrical signal, the digital part Converting an electrical signal into a digital signal that can be directly used by a host computer can be a wavelength value or a temperature or strain value, and the MCU that implements this function uses a single chip microcomputer.
The demodulation principle of the demodulation system is based on the principle of tunable Fabry-Perot cavity (FP demodulation). The fiber FP cavity filter used for Bragg grating sensor signal demodulation is actually a voltage-controlled optical band-pass filter, usually using piezoelectric ceramics as the driving element of the FP cavity length change. A scanning voltage is applied to the piezoelectric ceramic, and the piezoelectric ceramic expands and contracts to change the cavity length of the FP cavity to change the wavelength of light passing through the FP cavity. The transmitted light intensity is detected by the detector, and the voltage applied to the piezoelectric ceramic when the detector detects the maximum light intensity corresponds to the reflected wavelength of the FBG. This injects an optical signal into the Bragg fiber grating sensor, and the light reflected from the FBG sensor is applied to the input end of the fiber FP cavity filter, and a triangular scanning voltage is applied to the voltage control terminal of the fiber FP cavity filter. The output of the fiber FP cavity filter can obtain a time domain electrical signal corresponding to the input light spectrum. These time domain signals are shaped by the amplifying circuit and the comparison circuit to obtain a series of pulse signals. We add some standard pulse signals of fixed wavelength and position to these pulse signals, and then each pulse of these pulse signals is for standard pulse. The relative position of the FBG sensor reflects the spectral information of the light reflected by the FBG sensor. Figure 2 indicates this demodulation process. Finally, the obtained pulse is converted into a wavelength value by a circuit composed of a single chip microcomputer.
2. The composition and working method of the single chip demodulation system
The primary purpose of the microcontroller demodulation system is to process these pulse signals into corresponding wavelength values. Through the demodulation of the analog part, we obtain a pulse signal containing the relative position of the measurement grating and the standard grating in the scanning period. The standard grating corresponds to a fixed wavelength, and the position of its corresponding pulse signal is fixed in each scanning period. (The standard grating uses a constant temperature circuit to keep the wavelength constant), then if the relative position value of each pulse signal can be obtained, the wavelength value of the grating can be measured by an interpolation algorithm.
In this demodulation system, the fiber grating produced by Wuhan Science and Technology Co., Ltd. is used as the measurement grating. The wavelength selector based on the FP cavity principle is used as the demodulation cavity, and the measurement range can reach 30 nm. The period of the triangular wave scanning signal is 1 s, the measured frequency is 1 Hz. The rising edge of the triangular wave scanning signal is divided into a limited number of counting points that can reach the design precision, so that the position values ​​of the FBG1, FBG2, ... FBGn grating array and the standard grating pulse signal in the rising edge of the triangular wave can be read by the single chip microcomputer. The function of another microcontroller is to use these values ​​to calculate the wavelength and communicate with the computer. The circuit diagram is shown in the figure. Here, the single-chip microcomputer selects 89C52, and uses 4060 to generate a stable counting pulse. When the triangle wave starts, the No. 1 MCU counts. When a pulse arrives, the value of the counter is recorded and stored in the on-chip RAM; when the triangle wave reaches the highest point, the counter is cleared. Send the position value to the dual port RAM and wait for the next count. When the CPU 1 starts counting, the CPU 2 takes the data out of the dual-port RAM, calculates the wavelength value or temperature value corresponding to the pulse by interpolation or other algorithm, and communicates with the computer.
We can simultaneously input more measurement pulses through the other I/O ports of the microcontroller. By improving the optical path and the analog circuit part, a 2-channel, 4-channel fiber-optic grating demodulator can be fabricated to provide more measurement points, and the digital circuit does not need to be changed at all, and only the software part can be adjusted.
3. System analysis and data processing
When the MCU wants to record each pulse completely and correctly, its counting and transmitting instructions should be completed within the pulse width of each pulse. If the pulse width is only 1 counting unit, the counting and transmitting instructions need to be about 10 microseconds. In the time of completion, AMTEL's 89C52 maximum operating frequency can reach 24MHz, then its clock cycle is 0.5 microseconds, then as long as the counting and transmitting instruction cycle does not exceed 20 clock cycles, it can meet the requirements, reasonable read and write procedures Obviously it is able to meet this requirement. Usually, the width of the pulse is generally much larger than one count unit, so the pulse change can be recorded in real time. At the same time, the No. 2 MCU has 1 s time to take the data out of the RAM, calculate the median value of the pulse, and then perform the interpolation calculation, and the time is also sufficient. If the algorithm is too complex, such as using the Lagrangian algorithm, etc., the position value can also be transferred to the computer for data processing.
There may be errors in the data transfer from the microcontroller to the computer. Error correction processing must be added to the communication program. Parity can be used, such as single-byte check or multiple-byte check. At the same time, in order to prevent occasional abrupt changes in the raster position value, it is necessary to smooth the position value. Through the above processing methods, the computer can obtain a correct and stable set of data. In order to reduce the drift of the FP cavity and the influence of the system nonlinearity on the position value, we use a standard grating to compare and calculate with the measurement grating, which can be calculated by a linear algorithm. However, in practical applications, it is found that the closer the measured grating is to the standard grating, the more accurate the measured value is; the farther is the larger the error. In order to further improve the accuracy, you can use the 2 standard, 5 standard or comb filter to perform piecewise linear interpolation calculation, which can greatly improve the measurement accuracy. Of course, you can also use the Lagrangian algorithm or the multi-item formula. A complicated method to perform wavelength calculations. In our instrument, the Lagrangian algorithm of 5 standard gratings is used to calculate the wavelength, and the temperature measurement accuracy can reach ±1 °C.
4 Conclusion
The relationship between the relative position value of the pulse and the wavelength cannot be derived from the theoretical knowledge at present, but it is possible to find the change law by mathematical statistics method to find the correspondence between them. Using this correspondence, data processing is performed in the single-chip microcomputer to obtain the measured temperature or stress. At present, we use the Lagrangian algorithm and use some suitable data processing and calibration methods. According to the current working condition of the demodulator, the effect is still ok, the measurement accuracy can reach ±5pm, and the maximum repeatability error is 8pm. . In order to improve the operating frequency of the demodulator and improve the applicability of the demodulator, a demodulation circuit based on DSP or DSP+ARM can also be used, but the cost is relatively high.
FBG gratings have broad application prospects and can play an important role in communication, construction, machinery, medical, aerospace, navigation, and mining. Theoretical research on FBG gratings has achieved great success so far. The use of suitable demodulation technology to reduce the cost of fiber gratings can promote the widespread application of fiber Bragg grating sensors in practical engineering.
The author of this paper innovates: The fiber-optic grating demodulator based on single-chip microcomputer is a demodulation system suitable for practical engineering applications, which greatly reduces the cost of the fiber grating demodulation system and is convenient for use in industrial sites, so that the fiber grating sensor can be practical. It has been rapidly promoted in engineering applications.
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