In Situ Structural Health Monitoring of Full-Scale Wind Turbine Blades in Operation Based on Stereo Digital Image Correlation

Round 1

Reviewer 1 Report

1- The idea and innovation of the study is not well presented in the abstract. 2- It is suggested to divide the introduction into 3 sub-sections: motivational section, review of past works and study innovations section.

3- It is necessary to determine the reference for figure (1) in the text.

4- The language of the article needs to be revised.

5- It is suggested to draw and present a comprehensive flowchart for your proposed model in section (2).

6- From which reference did you get the parameters of table (1)? It is necessary to provide reference for table (1).

7- Has a special software been used in this study to model and analyze the results?

8- It is suggested to add the future studies and challenges section at the end of the conclusion.

Moderate editing of English language required.

Author Response

(1) The idea and innovation of the study is not well presented in the abstract.

Response: Thank you for the valuable comment and suggestion. We realized that the innovation of the study was not presented in sufficient detail in the abstract, while the description of the experimental setup and results was overly specific. In the abstract, the limitations of the existing studies were added, and the description of the experimental part has been reduced. In addition, a brief description of the innovations of this study has been supplemented to the abstract.  Please see line 9~16 on Page 1.

(2) It is suggested to divide the introduction into 3 sub-sections: motivational section, review of past works and study innovations section.

Response: Thank you for the valuable comment and suggestion. In the original manuscript, the introduction was not formatted to be strictly divided into 3 sub-sections. In fact, it was divided into motivational section (paragraphs 1 and 2), review of past works (paragraphs 3, 4, and 5) and study innovations section (paragraphs 6). The introduction is divided into 5 subsections in the revised manuscript. Specifically, the 1st paragraph introduces the significance of wind energy for sustainability, the 2nd paragraph introduces the necessity of SHM of full-scale blades, the 3rd and 4th paragraphs introduce the existing contact and DIC-based measurement techniques and limitations, and the 5th paragraph briefly introduces the research and innovations of this manuscript.

(3) It is necessary to determine the reference for figure (1) in the text.

Response: Thank you for the valuable comment and suggestion. The stereo-DIC system shown in figure (1) of the original manuscript is a modified configuration of the conventional stereo-DIC system for SHM of full-scale blades. Since the system works in a non-cooperative environment, the modified configuration has new hardware (pan-tilt and inclinometer) to assist in camera attitude adjustment compared to the conventional stereo-DIC system.  In addition, the imaging system uses high-speed and high-resolution cameras. This description can be found in lines 112~116 on Page 3. Two references were cited when we first introduced stereo-DIC in the introduction, please see lines 65~68 on Page 2.

(4) The language of the article needs to be revised.

Response: Thank you for the suggestion. We asked several native English-speaking colleagues to check the English, and carefully revised the entire manuscript to avoid language errors. We believe that the language is now acceptable for the review process.

(5) It is suggested to draw and present a comprehensive flowchart for your proposed model in section (2).

Response: Thank you for the suggestion. We have added a comprehensive flowchart for the SHM and condition assessment of full-scale blades in the Section 2, as shown in Figure 1. Please see lines 101~106 on Page 3.

 

(6) From which reference did you get the parameters of table (1)? It is necessary to provide reference for table (1).

Response: Thank you for the valuable comment and suggestion. A common practice is to determine the camera intrinsic parameters using a chessboard with known geometries in laboratory in advance as these parameters are only related to the imaging unit as long as the focus length is constant. Therefore, the intrinsic parameters of the two cameras were pre-calibrated by using Zhang’s method in this study. The extrinsic parameters in Table 2 were determined according to the methodology presented in Section 2.2. The control points can be extracted and matched using the SIFT algorithm from the speckle patterns of the blades surface in the calibrated images. About 4000 pairs of control points are retained and used to calibrate the extrinsic parameters of the stereo camera. The detailed calculations have been added to the manuscript in response to your comments. Please see lines 297~300 on Page 9. In addition, a straightforward block diagram describing the steps to determine the parameters of a stereo-DIC is added in Section 3.2, please see lines 290~291 on Page 9.

(7) Has a special software been used in this study to model and analyze the results?

Response: Thank you for the valuable comment. In this study, all the algorithms are developed in C++ as the programming language. In addition, we used the open-source library OpenCV. We have added the note on lines 277~278 on page 8 of the manuscript.

(8) It is suggested to add the future studies and challenges section at the end of the conclusion.

Response: Thank you for the valuable comment and suggestion. In the manuscript, we have added future studies and challenges to the conclusions. Please see lines 460~464 on Page 15.

Reviewer 2 Report

The manuscript entitled "In-situ structural health monitoring of full-scale wind turbine blades in operation based on stereo-DIC" deals with evaluating the of blades wind turbine operation by applying a structural health monitoring SHM with deep management of the operational condition.  All these assessing processes were carried out to prevent faulty blades from occurring. This was achieved using stereo digital image correlation (stereo-DIC). This manuscript includes novel analysis and results and it is required minor revisions as shown in the following points:

1.  The introduction section needed to describe more information about the main design features that can be carried out to prevent faulty blades from occurring.

2.  Please it is required more details on the main shortage in the previous evaluation methods cited in the introduction in comparison with the applied approach in the present work.

3.  On page 4, more details are required to show the importance of the design configuration of the stereo-DIC and correlation matching algorithm that was shown in Figures 2 and 3. Explain their influence on the evaluation system.

4.  If possible show the comparison between the results of the predicted analysis system of the present work with that of many previous works in cited references.

5.  On page 9 (3.2. Parameters of the stereo camera), please more information needed to describe how the authors determined the parameters of the stereo camera.

 

6.  More discussions are needed regarding the results shown in Figure 12 about the 3D movement (displacement trajectories of the POIs).

Author Response

Review 2

The manuscript entitled "In-situ structural health monitoring of full-scale wind turbine blades in operation based on stereo-DIC" deals with evaluating the of blades wind turbine operation by applying a structural health monitoring SHM with deep management of the operational condition.  All these assessing processes were carried out to prevent faulty blades from occurring. This was achieved using stereo digital image correlation (stereo-DIC). This manuscript includes novel analysis and results and it is required minor revisions as shown in the following points:

1. The introduction section needed to describe more information about the main design features that can be carried out to prevent faulty blades from occurring.

Response: Thank you for the valuable comment and suggestion. The impact of sand and gravel particles, strong ultraviolet radiation and ice accretion on blades can cause damage to the blade. As the main load-bearing component, blade faults can manifest in its 3D trajectory, and many faults only occur during blade rotation. Therefore, it is significant to prevent the deterioration of blade micro-defects by periodically performing in-situ SHM of the blade to obtain the 3D motion and deformation of the blade. We have added the note on lines 68~72 on page 2 of the manuscript.

2. Please it is required more details on the main shortage in the previous evaluation methods cited in the introduction in comparison with the applied approach in the present work.

Response: Thank you for the valuable comment and suggestion. In the manuscript, the major shortages that existing studies have in comparison to this study have been added to the back of its brief description. Please see lines 46~49, lines 57~61, line 76, lines 80~81 and lines 83~85 on Pages 2.

3. On page 4, more details are required to show the importance of the design configuration of the stereo-DIC and correlation matching algorithm that was shown in Figures 2 and 3. Explain their influence on the evaluation system.

Response: Thank you for the valuable comment and suggestion. The factors affecting the measurement accuracy mainly include image resolution, FOV size, FOV angle, system parameters, etc., in which the first three factors are closely related to the configuration of the stereo-DIC system. To accommodate the size of the blades, the high-speed CMOS camera with high resolution was used to ensure the clarity of the captured images. For an ideal FOV angle, the two cameras were arranged at a distance of 63.9 m apart, which facilitates the 3D reconstruction of the POIs. In addition, for convenience in field environments, each camera was fitted with an inclinometer and a pan-tilt to assist in adjusting the camera's attitude. We have added the note on lines 401~408 on page 14 of the manuscript. The gray gradient of the subset determines the performance of the correlation matching algorithm. In the case of a mismatch, a non-negligible error is introduced in the 3D reconstruction of the POIs. We have added the note on lines 140~142 on page 4 of the manuscript.

4. If possible show the comparison between the results of the predicted analysis system of the present work with that of many previous works in cited references.

Response: Thank you for the valuable comment and suggestion. Figure 15(e) shows the displacement spectrum of the faulty blade presented in reference 42, which is significantly different from the displacement spectrum of the healthy blade in this study. It is described in detail in lines 384-392 on page 13 of the manuscript.

5. On page 9 (3.2. Parameters of the stereo camera), please more information needed to describe how the authors determined the parameters of the stereo camera.

Response: Thank you for the valuable comment and suggestion. A common practice is to determine the camera intrinsic parameters using a chessboard with known geometries in laboratory in advance as these parameters are only related to the imaging unit as long as the focus length is constant. Therefore, the intrinsic parameters of the two cameras were pre-calibrated by using Zhang’s method in this study. The extrinsic parameters in Table 2 were determined according to the methodology presented in Section 2.2. The control points can be extracted and matched using the SIFT algorithm from the speckle patterns of the blades surface in the calibrated images. About 4000 pairs of control points are retained and used to calibrate the extrinsic parameters of the stereo camera. The detailed calculations have been added to the manuscript in response to your comments. Please see lines 297~300 on Page 9. In addition, a straightforward block diagram describing the steps to determine the parameters of a stereo-DIC is added in Section 3.2, please see lines 290~291 on Page 9.

6. More discussions are needed regarding the results shown in Figure 12 about the 3D movement (displacement trajectories of the POIs).

Response: Thank you for the valuable comment and suggestion. Further discussions of the 3D motion trajectories of the POIs have been added on lines 359~362 of page 12 in the manuscript.

 

Reviewer 3 Report

This interesting study used stereo digital image correlation (stereo-DIC) to perform SHM and analyze the condition of operational full-scale blades. Some important points are as follows:

·         The study shows that the method can accurately estimate stereo-DIC parameters for large FOV applications. It would be helpful to have a straightforward block diagram that illustrates the experimental procedure.

·         What contribution does this method make to the literature? It may be mentioned more clearly in the introduction and conclusion.

·         In terms of the frequency spectrum of 3D motion, it is important to note that it can only determine whether the blade operation is orderly or disordered. However, it does not possess the capability to identify the specific cause of the disordered operation. Could you give some more explanation for the readers to understand?

 

·         Is there any meaning to the numerical value of 11 in the term "global 11 warming" as mentioned on the 30th line?

The English of the manuscript is adequate.

Author Response

Review 3

This interesting study used stereo digital image correlation (stereo-DIC) to perform SHM and analyze the condition of operational full-scale blades. Some important points are as follows:

(1) The study shows that the method can accurately estimate stereo-DIC parameters for large FOV applications. It would be helpful to have a straightforward block diagram that illustrates the experimental procedure.

Response: Thank you for the valuable comment and suggestion. A straightforward block diagram describing the steps to determine the parameters of a stereo-DIC is added in Section 3.2, please see lines 290~291 on Page 9.

(2) What contribution does this method make to the literature? It may be mentioned more clearly in the introduction and conclusion.

Response: Thank you for the valuable comment and suggestion. As the main load-bearing component, blade faults can manifest in its 3D trajectory, and many faults only occur during blade rotation. Therefore, it is significant to prevent the deterioration of blade micro-defects by periodically performing in-situ SHM of the blade to obtain the 3D motion and deformation of the blade. The experimental results show that the proposed method is a convenient and nondestructive technique that enables online assessment of the operational condition of full-scale wind turbine blades. We have added the note on lines 57~61 on page 2 and lines 455~457 on page 15 of the manuscript.

(3) In terms of the frequency spectrum of 3D motion, it is important to note that it can only determine whether the blade operation is orderly or disordered. However, it does not possess the capability to identify the specific cause of the disordered operation. Could you give some more explanation for the readers to understand?

Response: Thank you for the valuable comment and suggestion. By analyzing the 3D displacement and frequency spectrum shown in Figure 14, it can be clearly identified that the measured blade is in good operating condition. According to Figure 15, it is known that the introduced technique can diagnose the presence of damage in the blade. In fact, different types of blade damage correspond to different 3D motion trajectories, which were not investigated in more depth in this study. In the future, we plan to incorporate the blade's 3D motion data into a deep learning model to accurately identify and locate blade faults. We have added the note on lines 432~435 on Page 14 and lines 460~464 on Page 15 of the manuscript.

(4) Is there any meaning to the numerical value of 11 in the term "global 11 warming" as mentioned on the 30th line?

Response: Thank you for the valuable comment. The “global 11 warming” was a writing mistake on my part, and it has been corrected in the manuscript.

Round 2

Reviewer 1 Report

The revised version of the article has made significant progress and has a chance to be accepted.

Minor editing of English language required.

Author Response

Dear Reviewer,

Thanks very much for taking the time to review this manuscript. I really appreciate all your comments and suggestions! Please find my response below and my revised version in the re-submitted files.

Thank you for the valuable comment and suggestion. For the first response, we asked a native English-speaking colleague to check the English of the manuscript. However, because we did not carefully check the revised manuscript, the quality of the English language was still insufficient to meet the journal's requirements. We are very sorry about this. Now, this manuscript has been edited for appropriate English language, grammar, punctuation, spelling and overall style by a highly qualified native English-speaking editor. And, we have proofread the revised content one by one to avoid remaining language errors. During this revision process, dozens of inappropriate languages in the manuscript were corrected, and we believe that the English language of the manuscript now meets the requirements for publication. 

Sincerely yours,

Dr. Weiwu Feng