Libing Bai

An Improved MPPT Controller for Photovoltaic System Under Partial Shading Condition

Maximum power point tracking (MPPT) is an integral part of a system of energy conversion using photovoltaic (PV) arrays. The power-voltage characteristic of PV arrays operating under partial shading conditions exhibits multiple local maximum power points (LMPPs). In this paper, a new method has been presented to track the global maximum power point (GMPP) of PV. Compared with the past proposed global MPPT techniques, the method proposed in this paper has the advantages of determining whether partial shading is present, calculating the number of peaks on P-V curves, and predicting the locations of GMPP and LMPP. The new method can quickly find GMPP, and avoid much energy loss due to blind scan. The experimental results verify that the proposed method guarantees convergence to the global MPP under partial shading conditions.

Automatic Defect Identification of Eddy Current Pulsed Thermography Using Single Channel Blind Source Separation

Eddy current pulsed thermography (ECPT) is an emerging nondestructive testing and evaluation (NDT&E) technique and has been applied for a wide range of conductive materials. In this paper, a single-channel blind source separation is proposed to process the ECPT image sequences. The proposed method enables: 1) automatically extract valuable spatial and time patterns according to the whole transient response behavior without any training knowledge, 2) automatically identify defect patterns and quantify the defects, and 3) to provide guidelines of choosing the optimal contrast functions that can improve the separation results. In this paper, both mathematical and physical models are discussed and linked. The basis of the selection of separated spatial and time patterns is also presented. In addition, an artificial slot and a thermal fatigue natural crack are applied to validate the proposed method.

Thermography pattern analysis and separation

Analysis of thermography spatial-transient patterns has considerable potential to enable automatic identification and quantification of defects in non-destructive testing and evaluation. This Letter proposes a non-negative pattern separation model for eddy current pulsed thermography to automatically extract important spatial and time patterns according to the transient thermal sequences without any pre-training or prior knowledge. In particular, the method is scale-invariant, such that large differences in surface emissivity, hot spots, and cool areas with dynamic range of thermal contrast can be extracted. Finally, an artificial slot in a steel sample with shining, black strip on the surface is tested to validate the proposed method.

Spatial and Time Patterns Extraction of Eddy Current Pulsed Thermography Using Blind Source Separation

Eddy current pulsed thermography (ECPT), a new emerging nondestructive testing and evaluation (NDT&E) technique, has been applied for a wide range of conductive materials. The acquired image sequences contain valuable information in both spatial and time domain. ECPT techniques mainly use a specific frame to detect and quantify the defects. However, selection of specific frame from transient thermal image video to maximize the contrast of thermal variation and defect pattern from complex geometrical samples remain a challenge. In order to accurately find anomalous patterns from the transient thermal pattern for defect detection and further quantitative NDE, this paper employs a single channel blind source separation algorithm. This method enables spatial and time patterns to be extracted according to the whole transient response behavior without any training knowledge. In this paper, both mathematical and physical models are discussed, and the basis of the proper selection of contrast image is given. In addition, the artificial slot and thermal fatigue natural crack are applied to validate the proposed method.

Smooth Nonnegative Matrix Factorization for Defect Detection Using Microwave Nondestructive Testing and Evaluation

This paper addresses the interpolation issue of current spectral estimation methods in microwave-based nondestructive testing and evaluation. We developed a spatial-frequency feature extraction algorithm for defect detection with an open-ended waveguide system using smooth Itakura-Saito nonnegative matrix factorization. In addition, the mathematical models of spatial-frequency characteristics for both defects and nondefects areas are derived. The newly developed algorithm has two prominent characteristics, which benefit the detection system. First, it is scale-invariant in the sense that spatial-frequency features that are characterized by large dynamic range of energy can be extracted more efficiently. Second, it imposes smoothness constraint on the solution to enhance the spatial resolution of defect detection. To evaluate the proposed technique, we demonstrate the efficacy of the proposed method by performing extensive experiments on four samples: four defects in an aluminum plate with different depths, a steel plate with 15-mm coating thickness, one tiny defect on steel and one natural defect. Experimental results have unanimously demonstrated the capabilities of the proposed technique in accurately detecting defects, especially for shallow and coated samples with high resolution.

Reducing the Effect of Surface Emissivity Variation in Eddy Current Pulsed Thermography

Manually selected frames have been used for both defect detection and quantification in Eddy current pulsed thermography. Defects are indicated by high/low temperatures within frames. However, variation of surface emissivity sometimes introduces illusory temperature inhomogeneity and results in detection false alarms. To improve the probability of detection, this paper proposes a two heat balance states-based method. For each pixel, a surface emissivity coefficient is extracted from the two heat balance states and is used to correct the transient response. The temperature and emissivity is thus separated. A verified experiment was carried out on a steel sample having both a polished and black strip painted surface, and the image quality was improved significantly.

Enhanced Laser-Based Magneto-Optic Imaging System for Nondestructive Evaluation Applications

An enhanced and innovative laser-based magneto-optic (MO) imaging (LMOI) system is presented in this paper to detect buried subsurface flaws in metallic structures for structural nondestructive evaluation. Several key improvements for the new imaging device have been discussed, including the following: the optimization of the MO sensor, the design of the magnetic excitation device, and the development of the image processing approaches, which result in the enhanced MO image quality comparing to the first generation of LMOI. Experimental data have also been obtained that clearly demonstrated the improvement in imaging results.

A comparative study of principal component analysis and independent component analysis in eddy current pulsed thermography data processing

Eddy Current Pulsed Thermography (ECPT), an emerging Non-Destructive Testing and Evaluation technique, has been applied for a wide range of materials. The lateral heat diffusion leads to decreasing of temperature contrast between defect and defect-free area. To enhance the flaw contrast, different statistical methods, such as Principal Component Analysis and Independent Component Analysis, have been proposed for thermography image sequences processing in recent years. However, there is lack of direct and detailed independent comparisons in both algorithm implementations. The aim of this article is to compare the two methods and to determine the optimized technique for flaw contrast enhancement in ECPT data. Verification experiments are conducted on artificial and thermal fatigue nature crack detection.

Successive approximation method for the measurement of thickness using pulsed eddy current

In pulsed eddy-current testing (PECT), material thickness and defect depth are indicated by features such as time to peak and peak value. Before testing, the relationship between thickness/depth and features must be established in advance. This requires multiple reference samples with known thickness/depth. On the other hand, the parameters of each material, such as electrical conductivity and magnetic permeability should be known in advance when calculating by computer. It is time consuming and expensive. In this paper, a new processing scheme is proposed to separate the geometric and conductivity parameters. The thickness/depth of the specimen is quantified into a series of formal unit thickness. There is an excitation frequency which corresponds to every unit thickness level. Using the rich frequency components of the pulsed eddy current response signal, they can be used to scan the thickness of the specimen step by step. When the scanned thickness is close to the thickness, the current frequency is very different with the reference frequency. So this frequency is used to calculate the thickness. In this method, the initial values of the thickness and frequency are detected first to calculate the thickness. It is help to separate the geometric and conductivity parameters. Thus this scheme only requires one reference sample and one known thickness/depth specimen for each material, which significantly reduces the cost of testing.

Improvement of the grid-connect current quality using novel proportional-integral controller for photovoltaic inverters

Precise control of the grid-connected current is a challenge in photovoltaic inverter research. Traditional Proportional-Integral (PI) control technology cannot eliminate steady-state error when tracking the sinusoidal signal from the grid, which results in a very high total harmonic distortion in the grid-connected current. A novel PI controller has been developed in this paper, in which the sinusoidal wave is discretized into an N-step input signal that is decided by the control frequency to eliminate the steady state error of the system. The effect of periodical error caused by the dead zone of the power switch and conduction voltage drop can be avoided; the current tracking accuracy and current harmonic content can also be improved. Based on the proposed PI controller, a 700 W photovoltaic grid-connected inverter is developed and validated. The improvement has been demonstrated through experimental results.