Research On FPGA Based Data Acquisition And Demodulation Control Methods For Fiber Bragg Grating Sensing System

In the recent years, fiber Bragg grating (FBG) sensing is a type of rapidly developing sensing technology. The research of quasi-distributed FBG sensing system has been focused on how to expand the capacity and improve the accuracy and speed of the system. With the rapid development of embedded system and computer technology, making a data acquisition and demodulation control system with high speed and high capacity, in order to improve its practicability in large scale complex structure detection is the demand of the future. On one hand, how to effectively acquire and handle the sensing data is a serious consideration in the industrial application of FBG sensing. In order to raise the speed and capacity of the sensing system while maintaining its accuary and stability, the data acquisition scheme used in this kind of application must be fast and precise. Only through this way, the scale of the system, as well as the controlling and calculation speed are ensured. On the other hand, the demodulation of quasi-distributed sensing system has always been a difficult problem. Among the various presented methods, the tunable F-P filter demodulation has become the most developed and applicable approach. But because of the F-P filter has to be driven by a tiny shift driver, its accuracy is confined by the precision of this driver. Thus, a proper method for demodulation algorithm and control becomes significant for this kind of scheme.This thesis first realizes a quasi-distributed FBG sensing system based on the tunable F-P filter demodulation method. Then we focus on the FPGA based data acquisition and demodulation control system, which integrates the data collection, calculation and the automatic control of demodulation into one embedded system. Through this approach, the precision and speed, as well as the capacity and the potencial expansion capability are improved, which makes the FBG sensing sytem more practical in industrial application. We first analyses the theoriotical model and sensing principle of FBG, then briefly introduces the categorization and features of FBG sensor. Based on these, we elaborates the multiplexing and demodulation methods for quasi-distributed FBG sensing system and realize one such system with the F-P filter demodulation approach. Among the sensing system, we focus on the data acquisition and demodulation control module, which serves as the control core of the whole system. According to the data acquisition module, we first analyses the requirement for data collection and characteristics of different types of data acquisition methods, then design and realize a high speed multi-channel data acquisition module based on FPGA and USB 2.0. PIN is used for optical power detector and a Spartan-3 XCS 400 chip is used as the FPGA core. The EZ-USB FX2 from Cypress Corp. is used as the USB control chip while LTC1740 is used as the A/D translator. We introduce the programming scheme from both the FPGA programming aspect and the firmware aspect after presenting the interface design in detail. As for the demodulation control module, we use the same FPGA chip along with LTC1597 and LTC1450 as the D/A translator. Based on stating the hardware interface design, peak detection algorithm and real-time adjustment algorithm in detail, we integrate the data acquisition and demodulation control into one embedded system. At last, we carry out the operation of the system and tests its performance. While analyzing the experimental result, we summarize the work of this thesis and put forward the future potential improvement for this system.