【作者】 李东升；

【导师】 隋青美；

【作者基本信息】 山东大学 ， 检测技术与自动化装置， 2008， 博士

【摘要】 与传统的机电传感器相比,光纤光栅传感器具有质量轻、体积小、频带宽、信号衰减弱、抗电磁干扰能力强、耐腐蚀性能好、寿命长、易实现分布式测量、不受潮湿环境影响等特点,自问世以来一直受到人们的关注。过去20年间,光纤光栅传感器得到迅速发展,其测量范围从机械量到电子量、化学量,几乎无所不包。但是,因为光纤光栅传感器结构复杂,价格高昂,而且可维护性差,所以虽然其性能十分完备,并且通常被认为具有优势,但应用范围却极为有限,只能用于非常特殊的场合。在传感器市场上,它只占有很小的市场份额,经济效益并不令人满意。在文献综述的基础上,本文指出光纤光栅传感器在封装形式、解调方案和网络结构上的缺陷是造成上述问题的主要原因,具体表现为:(1)结构上采用一体化设计方案,敏感光栅与传导光纤制作在同一根光纤上,重复性差,安装维护困难;(2)解调方法上完全依赖昂贵的光学器件进行频域分析,技术复杂,分辨率低,价格高昂;(3)复用系统中使用串联结构,光源与测量信号沿同一根光纤双向传输,容量有限,可靠性低,灵活性差,难于维护。为了解决以上问题,本文进行了以下创新:(1)为降低光纤光栅传感器的成本,满足工程测试领域对光纤光栅传感波长漂移高分辨率测量的需要,推动光纤光栅传感器在工程测试领域的应用,本文提出一种基于信号自相关原理的新型光纤光栅数字解调方法。该方法在可调谐滤波法的基础上,通过对光电探测器输出信号采用自相关分析,将对传感光栅反射谱波长漂移的测量转换为对光电探测器输出准周期信号时间漂移的测量,不仅能够准确解调,而且具有识别传感光栅反射谱形状的特点,能够有效克服各种加性噪声对信号的影响。仿真结果表明,即使在传感光栅具有相同波长变化范围的情况下,自相关数字解调方法也可以准确解调传感光栅波长漂移,但测量结果存在时间延迟,而且精度较低。(2)由于自相关数字解调方法对系统初始参数敏感,且需要通过数值方法求解非线性方程组以得到测量结果,所以不容易实现,解调精度也很难提高。为解决这一问题,本文进一步引入码分多址(CDMA:Code Division MultipleAccess)技术,通过对光电探测器输出信号进行编码简化了自相关数字解调方法的数值求解过程。码分多址技术是光纤通信主流信道复用技术之一。它能通过分配给每一个用户的唯一的地址码对用户发送信息进行扩频处理,并在数据接收端通过相关运算和特定门限判决技术从来自多个用户的叠加信号中恢复指定用户的原始信号,实现数据接收。研究表明,通过使用不同中心波长的光纤光栅对测点进行二进制编码,不同测点的传感器反射谱形状有了明显差异,使自相关数字解调方法对系统初始参数的敏感性明显降低。同时,自相关数字解调方法需要求解的非线性方程组被转换为线性方程组,测量精度明显提高。仿真结果表明,在传感光栅具有相同波长变化范围的情况下,采用码分多址技术实现的自相关数字解调方法也可以准确解调,而且不存在时间延迟,测量精度也远高于不采用码分多址技术的自相关数字解调方法。(3)为了增加复用系统的容量,提高系统的可靠性、可维护性和可扩展性,降低系统成本,本文在自相关数字解调方法的基础上,提出两种类似现场总线结构的光纤光栅传感系统:梯形光纤光栅传感网络和星型光纤光栅传感网络。在采用自相关数字解调方法构成的梯形光纤光栅传感网络中,光源与光纤光栅传感器并联在两根彼此独立的光纤之间,可靠性好,容易扩展;同时,通过对光电二极管输出信号的自相关分析,系统可以区分中心波长相同但谱线形状不同的传感器,不仅使系统容量大幅增加,而且能够保证测量精度不受系统规模影响。理论分析和实验研究表明,即使在传感光栅中心波长相同的情况下,采用自相关数字解调方法构成的光纤光栅传感网络也能够正确解调,从而使系统复用传感器的数目增加,而且可靠性高,易于维护和扩展,但测量灵敏度较理论值低。在采用码分多址技术实现的星型光纤光栅自相关数字传感网络中,每一个测点都用多个编码/传感光栅进行二进制编码,并通过星型网络连接到自相关数字解调仪上。测量时,宽带光源发出的光被编码/传感光栅反射后形成一系列离散谱片,其位置由外界被测量大小决定,经星型耦合器叠加后同时进入自相关数字解调仪进行数字解调,实现测量目的。理论分析表明,与现在使用的光纤光栅传感系统相比,采用码分多址技术实现的星型光纤光栅传感网络具有良好的灵活性,扩展方便,容易维护,可靠性高。而且由于允许为每一个测点提供一个独立的光源,所以容易获得高信噪比,更易于远程测量。总而言之,在光纤光栅自相关数字传感网络中,由于采用光纤光栅反射谱形状对测点进行寻址,系统容量明显增加;同时,由于采用相互独立的光纤分别传输能量信号和测量信号,系统的灵活性、可靠性和可维护性都得到增强。因此,基于自相关数字解调方法的光纤光栅传感系统能够有效降低光纤光栅传感器的安装成本和使用成本,充分发挥光纤光栅传感器复用能力强、易实现大规模远距离测量的优势。(4)最后,为了克服光纤光栅传感器重复性差、安装维护困难、安装质量无法保证的缺陷,方便工程技术人员在测试现场实现CDMA编码,本文给出一种分体插接式通用型光纤光栅应变传感预制结构——光纤光栅应变片。光纤光栅应变片采用分体式设计,把敏感光栅和传导光纤分别封装于光连接器两个相互独立的基体内部,并通过两个基体相互之间的插入实现光纤光栅与测量光路的机械联接。这种设计方法不但保留了光纤光栅良好的应变测试特性,减小了温度误差,而且在保证抗剪切能力的同时实现了机械式连接,便于一般工程技术人员使用,易于在测试现场进行CDMA编码,有利于光纤光栅传感技术在工程应用中的推广。理论分析和实验研究表明,光纤光栅应变片具有与光纤光栅相同的反射谱,测量线性度好,测量灵敏度高,且当试件材料与基底材料一致时,温度误差可以忽略。更多还原

【Abstract】 Compared with the traditional electromachanical sensors, Fiber Bragg Grating (FBG) sensor has many advantages, such as light weight, small size, wide frequency band, low signal attenuation, high anti-electromagnetic interference ability, excellent corrosion resistance properties, long service life, easy to form distributed measurement system, immune to humid environment, and so on. FBG sensors have received people’s attention continuously since its appearance, and have developed rapidly over the past 20 years, its measurement fields includes mechanical parameters, electronic parameters and chemical parameters, almost all-encompassing.However, the FBG sensor’s application scope is extremely limited.Because of its complex structure, expensive price and poor maintainability; the FBG sensor could only be used for very special situations and only occupies a very small market share in the sensor market, although it is generally considered an outstangding transducer and its performance is very perfect.Based on the review of its basic and applied researches, the reason that caused above questions was summed up in the shortcomings of the FBG sensor’s package, its demodulation method and multiplexing network in this dissertation. In details, (1) the FBG sensor’s package, which manufactured the sensitive grating and the conduction optic fibers on the same fiber, decease its reliability and maintainability; (2) the FBG sensor’s demodulator, in which the measurement results are obtained by frequency domain analysis entirely dependent on expensive optical devices, reduce its resolution and increase its price; and (3) the FBG sensor’s multiplexing network using series structure, which transmissit the lightsource signal and the measuring signal along opposite directions in same fiber, limits its capacity and makes it unextensible. Above reasons increased the FBG sensor’s installation costs and use costs, as a result, its application in engeering has been limited.To solve these problems, this dissertation proposed the following innovations:(1) A new digital demodulation method based on signal autocorrelation principle is proposed to detect the wavelength drift of FBG sensors used in engineering with high resolution and to reduce its expensive price.By autocorrelation analysis of photodiode output signal sequence, the digital demodulation method developed from the matched filtering method transform the wayelength measurement of FBG sensor’s reflect spectrum into the time measurement of photodiode output electronic signal, overcome the influence of noise while realize high precision detection of wavelength drift. And, this method also has quick scan speed and powerful multiplexing capability by recognize the reflection spectrum of FBG sensors.The numerical simulation results indicated that the digital demodulation method can detect the wavelength drift accurately when the FBG sensors have different range, and can be used when the FBG sensors have the same range. But the precision is low, and has time delay.(2)The CDMA (Code Division Mulitple Access) technology has been introduced into the autocorrelation numeral demodulation method, to reduce its sensitivity to the FBG sensor’s initial parameters and simplify its solving process of the nonlinear equations.CDMA is another technology of multiplexing and multi-access besides TDM and WDM, which supports multiple simultaneous transmissions in the same timeslot and the same frequency by coding in the optical domain. Especially, by autocorrelation analysis and threshold judgement, it can restore some user’s signal from multi-user’s superposition signal received at the same time using its unique address.By coding with Bragg grating, the sensitivity to the FBG’sensor’s initial parameters reduced significantly for the reflection spectrum from different measurement point has an obviously difference, and the precision improved significantly for the nonlinear equations have been converted to linear equations.The numerical simulation results indicated that the digital demodulation method realized by CDMA can detect the wavelength drift precisely when the FBG sensors have the same or different range, and has not time delay.(3)Based on the autocorrelation digital demodulating method, two kind of multiplying networks named ladder FBG sensing network and star FBG sensing network are proposed, which have a similar structure to field bus system and can increase its capacity and improve its reliability, maintainability and expansibility.In the ladder FBG sensing network, the light source and FBG sensors are parallel to two independent fibers, which decrease its fault rate and make it easy to expand. At the same time, sensors with same central wave length but different spectrum can be differentiated through the autocorrelation operation of photodiode output signal, which guarantee the measuring accuracy’s not be influenced by the measuring point quantity of system.The theoretical analysis and experiment study indicated that the ladder FBG sensing network has a lower fault rate, and the system’s measuring accuracy is not influenced by its scale, but lower than calculated value.In the star FBG sensing network using CDMA, every measurement point encoded by FBG has a unique binary code and connects to the autocorrelation digital demodulator by the star network. The lights emitted from a broadband source are reflected by encoding/sensing FBG, and generate a series of discrete spectrum films decided by the measurand.Through a star coupler, the spectrum film from different measurement points are superimposed and send to an autocorrelation digital demodulator, and obtained the measurands finally.The theoretical analysis indicated that the star FBG sensing network using CDMA has good reliability, maintainability and expansibility due to its easy access and flexible network structure, and is suitable for remote measurement because the SNR can be improved by an independent lightsource provided to the measurement point.In conclusion, either the ladder FBG sensing network or the star FBG sensing network can reduce the FBG sensor’s installation costs and use costs, and give full play to the FBG sensor’s multiplexing ability and its remote measurement ability.(4) Finally, a new kind of general prefabricated strain sensor called FBG strain gauge is designed in order to improve the FBG sensor’s reliability, maintainability and meet the engineer’s requirements for CDMA coding in engineering. The FBG strain gauge has a separated plug structure. In this structure, the sensitive grating and the transmit optical fiber are designed separately in independent matrixes, and can be plugged in each other to realize the mechanical connection.Theoretical analyses indicated that the FBG strain gauge has the same reflection spectrum with FBG grating, and successive experiments indicated that it has good linearity and high sensitivity, whereas the temperature error changed with the specimen’s changing. When the specimen and the strain gauge’s matrix are made by the same material, the temperature error can be ignored.更多还原