OVMD-ICA算法用于光纤电流传感器降噪 下载: 540次
The fiber current sensor based on the Faraday effect and Ampère's circuital law can measure the current accurately. It has many advantages, such as excellent insulation characteristics, simultaneous measurement of the alternating current (AC) and direct current (DC), flexible sensor diameter, and digital output. However, it can hardly measure the microcurrent because the magnetic field generated by the weak current is small, and the Verdet constant of the sensing fiber is tiny (about 1 μrad/A when the wavelength is 1300 nm). Therefore, the current resolution of the fiber current sensor is limited. The methods to improve the current resolution mainly include the following: improving the optical path structure, increasing the number of optical fiber loop turns, and improving the Verdet constant of the sensing fiber. However, these methods have the disadvantages of complex operations and high costs. The data processing method is a promising scheme to improve the current resolution. To meet the requirements of information sources for independent component analysis (ICA) and improve the performance of variational mode decomposition (VMD) to deal with impact noise, this paper proposes the co-clustering algorithms of ICA and VMD with the parameters optimized by the hunter-prey optimization (HPO) algorithm.
This paper proposes the co-clustering algorithms of ICA and VMD with the parameters optimized by the HPO algorithm. Firstly, the random Gaussian noise, shot noise, impact noise, and output signal are measured. The output signal and noise characteristics of the fiber current sensor are analyzed. Secondly, the key parameters of VMD are optimized by the HPO algorithm. With the energy spectrum entropy function as the fitness function, the modal parameter K and the quadratic penalty factor α are obtained by the HPO algorithm, and VMD is realized with the two parameters. Third, the virtual channels of ICA are constructed. The mode functions are classified by the setting of the threshold of the correlation coefficient to construct the virtual channels for ICA. In this way, the application conditions of ICA are satisfied. Finally, the FastICA algorithm is applied for denoising.
More outstanding performance can be achieved in terms of the operation time, required iterations, and search for the globally optimal solution when the parameters of VMD are optimized by the HPO algorithm. The mode functions are classified by the setting of the threshold of the correlation coefficient to construct the virtual channels for ICA, and the FastICA algorithm is applied for denoising. The SNR of the output signal is enhanced, and the MSE is reduced by OVMD-ICA. By this algorithm, the SNR can be improved by at least 5 dB, and the resolution and measurement of 3 mA weak current can be realized.
1 引言
相比于传统的电磁式电流互感器,基于法拉第效应和安培环路定理的全光纤电流传感器(FOCS)[1]充分利用了光纤天然的绝缘性能,具有体积小、抗电磁干扰、动态范围广、能够同时测量交直流信息和安全环保等优点[2],在大电流测量领域中已经取得了重要进展[3-4],目前的研究集中于特殊应用背景下的微弱直流电流测量[5-7]。
通常采用电流分辨率来表征全光纤电流传感器的微弱电流测量能力。提升电流分辨率的途径包括改进光路结构、增加光纤环匝数、提升光纤的Verdet常数和改进数据处理算法。相对于偏振式光路结构,采用反射式Sagnac干涉结构[8]或者再入式光学结构[9]都能提升电流的响应能力,但提升幅度有限,通常不超过6倍[10]。增加的光纤环匝数受限于传感光纤的线性双折射效应[11]和成本,传感光纤的材质和半径确定时存在最优的光纤环匝数[12],此时再增加光纤环匝数,将降低系统的电流响应能力。通过掺杂金属离子等方式可以提升光纤的Verdet常数,但会导致光纤材料的温度稳定性下降,同时过高的损耗会导致该类光纤暂时无法应用于实际中[13]。与上述方法相比,采用改进数据处理算法来抑制系统噪声,可提升电流分辨能力,且具有不改变光路结构、节约成本和实现便捷等优点,是当前提升微弱电流传感能力的主要方法[14-16]。
相对于小波降噪[17]效果依赖于小波基的选取、卡尔曼滤波[18]对非平稳信号处理能力受限,以及神经网络算法[14-15]要求大量数据构建训练集、存在泛化能力弱和梯度消失等问题,独立成分分析(ICA)能够在无先验知识的情况下,自适应地分解出信号中的独立成分[19-20],但受限于信源数量,从而无法直接应用于全光纤电流传感器的信号处理之中。采用变分模态分解(VMD)的数据处理方案[21-23]能够将信号分解为一系列有限带宽的模态函数,且VMD算法的本质是一系列的维纳滤波器[24],对高斯白噪声具有较强的抑制作用,故可以根据模态函数的特征构建虚拟通道,来满足ICA算法对信源的要求。因此,本文采用VMD和ICA的组合算法,以能量谱熵作为目标函数,利用捕食者算法(HPO算法)确定VMD算法的模式参数K和二次惩罚因子α,再通过设置相关系数阈值实现对模态函数的分类,构建虚拟通道以满足ICA算法对信源数量的要求,最后通过固定点算法(FastICA算法)实现系统的降噪。
2 信号特征分析
2.1 传感信号分析
考虑系统准确度和稳定性等因素,采用反射式Sagnac型全光纤电流传感器[8]进行微弱电流的光纤传感测量。基本光路结构如
反射式Sagnac型全光纤电流传感器由光有源器件(光源、相位调制器和光电探测器)、光无源器件(耦合器、起偏器、波片、反射镜和光纤)和电子元器件(信号处理系统)组成,具体工作原理参见文献[8]。光电转换后将携带电流信息的干涉光强转换为电压信号,即
式中:
根据调制信号特征对数据解调以获取相应角度信息。微弱电流对应的法拉第旋转角较小,为避免“死区”对信号解调的影响[26],采用开环的数据处理算法,以正弦信号作为调制信号,通过解调获取的受测电流为
式中:
2.2 噪声特性分析
作为工作区域遍布于一次侧和二次侧的光电转换器件,全光纤电流传感器在工作过程中受温度、湿度、振动、光电器件老化、机械形变等众多因素和各种寄生效应影响,输出信号的噪声在来源上分为光学噪声、电子器件噪声和外界环境引入的噪声。在统计特征上,噪声可分为基于标准正态分布的白噪声、基于泊松分布的散粒噪声[28]和基于二项分布的冲击噪声。
符合标准正态分布的白噪声的概率密度函数为
式中:
基于泊松分布的散粒噪声的概率密度函数为
式中:
冲击噪声信号[29]可表示为
式中:
式中:
通过式(
式中:
3 降噪理论
由
3.1 OVMD算法及实现
3.1.1 VMD算法
VMD算法解决了经验模态分解(EMD)存在的端点效应和频谱混叠等问题[24],将信号分解为一系列具有中心频率的有效带宽的调幅、调频函数组合形式,实现了信号的时频域分解。设原始信号可分解为K个模态函数,则第
式中:
式中:f为原始信号;
式中:
式中:
式中:
模态参数K和二次惩罚因子
3.1.2 HPO算法
确定模态参数K和二次惩罚因子
HPO由Naruei等[38]近期提出,其基本思想是:在捕食猎物过程中,捕食者每次捕食距离自己最近的猎物,即获取局部最优解;猎物群在逃避捕杀的过程中远离捕食者,且猎物种群数量随着捕食者捕食的进程而逐步减少;当最后一个猎物被捕获时,即获取全局最优解。猎物或者捕食者的初始位置为
式中:
式中:
式中:
式中:
Z为自适应参数,其构成的自适应参数矩阵
式中:
式中:
Ppos为捕食者距离猎物种群平均位置的最大距离,其定义为
式中:
3.1.3 适应度函数
适应度函数即为优化算法的目标函数,决定了优化效果的质量。本文采用能量谱熵作为适应度函数。
首先,对VMD算法得到的各模态函数分别进行希尔伯特变换,即
式中:
式中:
3.2 独立成分分析算法及实现
3.2.1 独立成分分析算法
ICA算法假定接收信号是由多个彼此独立的非高斯信号分量组成,是实现将多元信号分离为加性分量的计算方法[19]。经典ICA算法的数学模型为
式中:
式中:
常用的ICA算法包括Informax法、基于梯度的Informax法和固定点算法(FastICA算法)等[19]。本文采用基于负熵最大的FastICA算法,实现盲信号处理。其算法流程如下。
步骤一,进行信号预处理,完成信号去均值和白化处理:去均值后信号的均值为零;白化处理的目的是去除各观测信号间的相关性,简化独立分量的提取过程。
步骤二,进行初始化。初始化估计的分量个数m、迭代次数p=1和随机权矢量
步骤三,令
步骤四,判断
3.2.2 改进独立成分分析算法
FastICA算法通常采用二阶收敛的标准牛顿迭代法,具有收敛速度快、稳健性好、并行分布、计算简单和内存要求低等优点,但该算法对初始权值比较敏感。为提升该迭代算法对初始权值的鲁棒性,应用阻尼牛顿法来降低算法对初始值的敏感性。因此,
式中:αp为步长因子,是以
3.2.3 多源信号构建
ICA算法要求
1)计算相关系数
分别计算VMD后的模式函数与原信号的相关系数
式中:
2)设定阈值
根据信号特征,设置阈值分别为T1和T2,且
3)循环移位
由噪声特性分析可知,噪声的分布与时间无关,故对仅含有噪声的成分
4)模式重构
将信号成分
根据
3.3 OVMD-ICA算法流程
OVMD-ICA算法主要包括VMD参数自适应选取、信源重构和FastICA处理等步骤,算法流程图如
1)数据预处理,剔除数据中的野值,并对数据重采样以降低数据处理负担;
2)以能量谱熵为适应度函数,采用HPO优化算法获取全局最优的模式参数K和二次惩罚因子α;
3)采用优化算法获取的K和α实现VMD处理,获取模态函数
4)根据
5)对噪声信号进行循环移位操作,并根据
6)采用改进FastICA算法实现盲信号处理,并根据
4 信号降噪实验分析
4.1 搭建实验系统
光纤微弱电流测量系统如
表 1. 主要器件性能指标
Table 1. Performance indexes of main components
|
对直流稳压电源进行软件编程生成调制电流信号作为待测量的电流值。信号发生器与相位调制器和锁相放大器相连,用于生成调制信号和参考信号。数字信号处理系统利用锁相放大器实现信号的解调后,利用计算机软件实现信号的降噪处理,输出测量的电流信息。光无源器件中的耦合器、起偏器、镀膜反射镜,以及单模光纤和保偏光纤均采用商用化产品,而1/4波片由自研得到,制作方法参照文献[40]。
4.2 结果分析及讨论
通过编程实现直流稳压电源输出,设置电流起始值为0,终止值为51 mA,电流的步进值为3 mA,持续时间为5 s。
4.2.1 优化算法对比分析
以能量谱熵作为适应度函数,各优化算法性能对比如
表 2. 优化算法性能对比
Table 2. Comparison of optimization algorithms
|
由
4.2.2 降噪方法对比分析
采用OVMD-ICA算法与现有的应用于全光纤电流传感器的主流算法进行对比,结果如
图 4. 降噪算法性能对比。(a)Wavelet(sym10);(b)Kalman;(c)VMD-wavelet;(d)EMD-ICA;(e)OVMD-ICA
Fig. 4. Performance comparison of denoising algorithms. (a) Wavelet (sym10); (b) Kalman; (c) VMD-wavelet; (d) EMD-ICA; (e) OVMD-ICA
将信噪比、均方误差和相关系数作为评价参数,以确定最优降噪算法。信噪比的定义为
式中:
均方误差的定义为
相关系数的定义如
表 3. 各种滤波算法性能对比
Table 3. Performance comparison of various filtering algorithms
|
各数据处理算法以参考文献所提函数为基准进行降噪处理,即假定各参考文献的结论是正确的。根据电子式电流互感器应用标准[41],电流信号的信噪比需大于30 dB,故由
同时,通过
5 结论
利用HPO算法实现VMD参数优化,相对于当前应用于VMD参数优化的其他优化算法,在运算时间、所需迭代次数和搜寻全局最优解等方面具有更加突出的寻优能力。通过设定相关系数阈值对VMD各模态函数进行分类并构建虚拟通道,应用FastICA算法完成全光纤微弱电流信号降噪处理的OVMD-ICA算法,有效提升了输出信号的信噪比、降低了均方误差,更能反映信号的特征。采用该算法能够将信号信噪比提升至少5 dB,相对于现有算法提升至少1 dB,且能够实现3 mA微弱电流的分辨和测量。
由于所提算法采用了全局最优算法,增加了算法的时间成本,故不适用于实时数据处理系统,可以作为事后数据处理方法或应用于对数据处理实时性要求不高的场景中。
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Article Outline
吴健华, 张晓锋, 陈亮. OVMD-ICA算法用于光纤电流传感器降噪[J]. 光学学报, 2023, 43(2): 0207001. Jianhua Wu, Xiaofeng Zhang, Liang Chen. Denoising Method Based on OVMD-ICA for Fiber Current Sensor[J]. Acta Optica Sinica, 2023, 43(2): 0207001.