Internal Model Control-Based Observer for the Sideslip Angle of an Unmanned Surface Vehicle
Round 1
Reviewer 1 Report
After carefully reading the manuscript, I can confirm that the topic of the paper is within the scope of the journal and the paper is potentially interesting in both theory and practice. The organization of the paper is good, the authors put some effort to present everything in a clear manner, but there are some sentences that should be improved in order to clarify what the authors wanted to say.
1/ Despite the motivating topic, the contribution and novelty of this paper were not enough; the proposed algorithm has already been conducted by several other researchers. Thus it is not clear what is the unique contribution. Also, the PI control used in the controller was extremely simple. The authors should explain justify the reason for proposing the PI controller.
2/ In the introduction part, I think that the authors could enrich the reference section by discussing the concept of control and guidance algorithm of marine vehicle systems, some new works related to sliding mode control and LOS guidance system, especially the dynamic sliding mode control methods, robust sliding mode method, multiple sliding mode methods and so on, should be included. To help the authors in this direction, I suggest the following reference: “Robust position control of an over-actuated underwater vehicle under model uncertainties and ocean current effects using dynamic sliding mode surface and optimal allocation control”, “Station-keeping control of a hovering over-actuated autonomous underwater vehicle under ocean current effects and model uncertainties in horizontal plane”, “Perturbation observer-based robust control using a multiple sliding surfaces for nonlinear systems with influences of matched and unmatched uncertainties”, “Design of a non-singular adaptive integral-type finite time tracking control for nonlinear systems with external disturbances”, and the introduction should be added to do a better job of explaining the existing methods and why they are or are not valuable. What research gap did you find from previous researchers in your field (it is still partially described, but needs to be expanded and made clearer)? Mention it in the Novelties section. It will improve the strength of the article.
3/ Write the organization of the paper in the introduction part.
4/ The presentation of the paper can be improved, and the quality of some figures should be enhanced. In all figures, axis names and units need to be revised, i.e, x/m should be x[m], t/s should be t[s]. Please correct all.
5/ In section 2, a sub-section “Assumptions” should be added to make the problem clearer. All assumptions and physical constraints should be provided. The author can refer to the “Assumptions” section of the following paper: https://doi.org/10.3390/s20051329
6/ If the Lyapunov functions are chosen via the viewpoint of practical application, the authors should give some effective suggestions. More discussions should be given to clearly demonstrate the limitations/validity of the obtained results.
7/ The comments about the speed controller and heading controller systems in Figure 3 are missing in the paper. This needs to be incorporated.
8/ In Section 4.2, the disturbances du=0.3sin(0.1*pi*t-0.2pi), dv=0.2cos(0.5*pi*t+0.1pi), and dr=0.3cos(0.2*pi*t+0.1pi) what is the principle of choosing these disturbances? Maybe these disturbances are too small compared to control inputs?
9/ In section 4.3, it is recommended to give an intuitive representation (e.g., a flow chart) of the whole technical procedure of the detailed implementation information. This section needs to be explained in more detail connecting to the steps of the algorithm presented in the section.
10/ Simulation and experimental results analysis are insufficient. The results are missing the most important results such as the control input, the torque output of motor T_u and T_r (in Figure 3)? Also, where are the results of velocities u and v or resultant velocity U with respect to time? These results need to be added to the paper. Moreover, more discussions should be given to clearly demonstrate the effectiveness of the obtained results.
11/ The Conclusion section is superficial, should include quantitative results, advantages and disadvantages, limitations, and recommendations for new implementations and future work.
12/ The English of the paper should be double-checked and revised since there are some mistakes in grammar and readability, for example, line 32 “USV don’t” should be “USV doesn’t” or “USVs don’t”, line 143, “up” should be “u_p”, “Eq. (13) and (14)” should be “Eqs. (13) and (14)”…
Author Response
Dear reviewers:
We are very grateful for your comments to improve the quality of the manuscript. We amended the relevant part in the version of our manuscript. We respond to every comment as following.
Reviewer 1
Comment 1: Despite the motivating topic, the contribution and novelty of this paper were not enough; the proposed algorithm has already been conducted by several other researchers. Thus it is not clear what is the unique contribution. Also, the PI control used in the controller was extremely simple. The authors should explain justify the reason for proposing the PI controller.
Response: Thanks for the reviewer’s kind suggestion. The algorithm based on internal model control has been carried out by several other researchers, but its application to the design of a sideslip angle observer is still relatively novel. Our work focuses on designing an observer that can be applied to estimate large sideslip angles by introducing the principle of internal model control, whereas most existing observers are designed based on the premise of small sideslip angles. For the controller part, since our focus is only on designing a large-range sideslip angle observer and not for the controller, the perform simulations and experiments with the same PI parameters to verify the superiority of the designed observer.
Comment 2: In the introduction part, I think that the authors could enrich the reference section by discussing the concept of control and guidance algorithm of marine vehicle systems, some new works related to sliding mode control and LOS guidance system, especially the dynamic sliding mode control methods, robust sliding mode method, multiple sliding mode methods and so on, should be included. To help the authors in this direction, I suggest the following reference: “Robust position control of an over-actuated underwater vehicle under model uncertainties and ocean current effects using dynamic sliding mode surface and optimal allocation control”, “Station-keeping control of a hovering over-actuated autonomous underwater vehicle under ocean current effects and model uncertainties in horizontal plane”, “Perturbation observer-based robust control using a multiple sliding surfaces for nonlinear systems with influences of matched and unmatched uncertainties”, “Design of a non-singular adaptive integral-type finite time tracking control for nonlinear systems with external disturbances”, and the introduction should be added to do a better job of explaining the existing methods and why they are or are not valuable. What research gap did you find from previous researchers in your field (it is still partially described, but needs to be expanded and made clearer)? Mention it in the Novelties section. It will improve the strength of the article.
Response: The references suggested by the reviewer present the latest research results on sliding mode control and guidance systems. These papers are relatively inspiring and well worthy of reference, but we are considering observer design based on internal model control principles rather than sliding mode control. So, we do not cite these papers in the introduction.
Moreover, we revised the manuscript structure and added the organization of the paper in the last paragraph of the introduction, in Line 41-42, page 2, and Line 110-114, page 3.
Comment 3: Write the organization of the paper in the introduction part.
Response: We have added the organization of the paper in the last paragraph of the introduction. The revised details can be found in Line 110-114, page 3.
Comment 4: The presentation of the paper can be improved, and the quality of some figures should be enhanced. In all figures, axis names and units need to be revised, i.e, x/m should be x[m], t/s should be t[s]. Please correct all.
Response: We have fixed all errors in the figures and improved the quality of the figures.
Comment 5: In section 2, a sub-section “Assumptions” should be added to make the problem clearer. All assumptions and physical constraints should be provided. The author can refer to the “Assumptions” section of the following paper: https://doi.org/10.3390/s20051329
Response: It is necessary to provide all assumptions and physical constraints that will enhance the rigor of the paper. Based on the reviewer's suggestion, in the second section, some assumptions about the USV model are provided, and the structure of the section has been revised to make it clearer and more straightforward. The revised details can be found in Line 116-127, page 3.
Comment 6: If the Lyapunov functions are chosen via the viewpoint of practical application, the authors should give some effective suggestions. More discussions should be given to clearly demonstrate the limitations/validity of the obtained results.
Response: Lyapunov stability theory itself does not provide a general method of construction. We construct the Lyapunov function through practical experience. Since our focus is only on designing a large range sideslip angle observer, we did not design the controller, so we chose the PI controller which is most commonly used. At the same time, the results of the proof are based on the assumption that the controller part has been stable, and then the stability of the whole system closed loop is proved, so there are certain limitations. To more clearly illustrate the limitations/validity of the results obtained, some explanations are made in the paper. The revised details can be found in Line 247-250, page 8.
Comment 7: The comments about the speed controller and heading controller systems in Figure 3 are missing in the paper. This needs to be incorporated.
Response: The annotation of the speed controller and heading controller system in Figure 3 is not complete, and we have added to this point in the revised manuscript. The revised details can be found in Line 265-269, page 9.
Comment 8: In Section 4.2, the disturbances du=0.3sin(0.1*pi*t-0.2pi), dv=0.2cos(0.5*pi*t+0.1pi), and dr=0.3cos(0.2*pi*t+0.1pi) what is the principle of choosing these disturbances? Maybe these disturbances are too small compared to control inputs?
Response: Since the USV used in the experiments is similar in size and quality to that used in the paper "Finite-time PLOS-based integral sliding-mode adaptive neural path following for unmanned surface vessels with unknown dynamics and disturbances", we used the same order of magnitude of disturbance as in that paper. At the same time, we have made additional explanations in the paper. The revised details can be found in Line 283-284, page 10.
Comment 9: In section 4.3, it is recommended to give an intuitive representation (e.g., a flow chart) of the whole technical procedure of the detailed implementation information. This section needs to be explained in more detail connecting to the steps of the algorithm presented in the section.
Response: The flow chart about the realization of the algorithm in the navigation controller and the control process of the USV system is added, i.e., Figure 8, and the related explanation is given. The revised details can be found in Line 324-325, page 13.
Comment 10: Simulation and experimental results analysis are insufficient. The results are missing the most important results such as the control input, the torque output of motor T_u and T_r (in Figure 3)? Also, where are the results of velocities u and v or resultant velocity U with respect to time? These results need to be added to the paper. Moreover, more discussions should be given to clearly demonstrate the effectiveness of the obtained results.
Response: Because our focus is only on the design of a large range of sideslip angle observer. The design of the controller part is not carried out, and the PI controller is directly used to control the speed and heading angle. In the experiment part, our purpose is to analyze the observation ability of the designed sideslip angle observer to a large range of sideslip angle and the influence of more accurate sideslip angle on tracking accuracy through simulation and experiment. However, the torque output of the motor Tu and Tr as well as the curves of the speed u and v cannot explain the problem, so we did not choose to give relevant graphs and discussions in this paper.
Comment 11: The Conclusion section is superficial, should include quantitative results, advantages and disadvantages, limitations, and recommendations for new implementations and future work.
Response: Thanks for the reviewer’s kind suggestion. Based on the reviewer's suggestions, we have added the conclusion section by adding a quantitative analysis of the experimental results. The strengths and weaknesses, as well as limitations of the design process, are also summarized, and suggestions for future work are provided. The revised details can be found in Line 369-374, page 16.
Comment 12: The English of the paper should be double-checked and revised since there are some mistakes in grammar and readability, for example, line 32 “USV don’t” should be “USV doesn’t” or “USVs don’t”, line 143, “up” should be “u_p”, “Eq. (13) and (14)” should be “Eqs. (13) and (14)”…
Response: We have modified some sentences with some errors in grammar and readability.
Reviewer 2 Report
Abstract: This reads like an introduction rather than an overview!! Include, results, the novelty of the research, contributions.
Long introduction, I would split this into two sections. In the introduction, introduce the topic and the challenges that you intended to address. In the second section 'related works' I would put the background reading.
I think is very important for the structure of your paper as the introduction ends quite abruptly and the reader is left considering, what exactly are the challenges of this research?
Section 2 is rather consulted, I would consider a more precise structure to this section. Do you really need all this? could you make it more readable?
Section 3 is much better in terms of format and presentation, you should do this for section 2.
Section 4 is also a little difficult to follow, I think it would be beneficial to put some of these equations in tables rather than in the text. It does nothing for the readability of the paper. I would even go as far as saying some of these calculations could be put in an appendix to make the body of the paper easier and clearer for the reader.
In the experiment, you simulate sideslip, was this done under different variables that may more accurately represent real-world conditions>? this is a little difficult to ascertain in the text.
I guess if not, then it is a limitation of your study and needs to be addressed in the discussion. If you did emulate these conditions, you need to make this clearer in the text.
Conclusion is very dry, use this space to re-address the novelty, contributions and key findings of this study.
Paper would benefit from a future works section - i.e more advanced real world testing
Author Response
Dear reviewers:
We are very grateful for your comments to improve the quality of the manuscript. We amended the relevant part in the version of our manuscript. We respond to every comment as following.
Reviewer 2
Comment 1: Abstract: This reads like an introduction rather than an overview!! Include, results, the novelty of the research, contributions.
Response: Based on the reviewer's suggestions, we have revised and added to the abstract section by adding a section on the novelty of the research we have done, and a section on contributions to existing work. The revised details can be found in Line 12-16, page 1.
Comment 2: Long introduction, I would split this into two sections. In the introduction, introduce the topic and the challenges that you intended to address. In the second section 'related works' I would put the background reading. I think is very important for the structure of your paper as the introduction ends quite abruptly and the reader is left considering, what exactly are the challenges of this research?
Response: We have optimized and adjusted the structure of the introduction section. The adjusted content can be more clearly divided into two parts. The first part introduces the problem that we want to solve, i.e., how to achieve more accurate sideslip angle compensation, and the second half describes the current status of the research and what we have done. To make the end of the introduction not seem abrupt, we added the organization of the paper in the last part of the introduction to make the structure seem more complete. The revised details can be found in Line 41-42, page 2, and Line 110-114, page 3.
Comment 3: Section 2 is rather consulted, I would consider a more precise structure to this section. Do you really need all this? could you make it more readable? Section 3 is much better in terms of format and presentation, you should do this for section 2.
Response: We have added an "assumption" part to make the question clearer. Since the USV model needs to be used in the simulation and experiment section, we think it is necessary to retain the content of section 2.
Comment 4: Section 4 is also a little difficult to follow, I think it would be beneficial to put some of these equations in tables rather than in the text. It does nothing for the readability of the paper. I would even go as far as saying some of these calculations could be put in an appendix to make the body of the paper easier and clearer for the reader.
Response: Section 4 is the simulation and experiment part. According to the suggestions of reviewers, we put some USV parameters used in the experiment in the table instead of the text, to increase the readability of the article. In addition, in the experimental part, we added the flow chart of algorithm realization in the navigation controller, i.e., Figure 8. To more intuitively represent the whole technical program. There are fewer equations in Section 4, so we chose not to place them in the appendix section.
Comment 5: In the experiment, you simulate sideslip, was this done under different variables that may more accurately represent real-world conditions? this is a little difficult to ascertain in the text.
I guess if not, then it is a limitation of your study and needs to be addressed in the discussion. If you did emulate these conditions, you need to make this clearer in the text.
Response: As USV is likely to produce sideslip angle due to the large curvature change of the path and external interference when it is following the path, we set a path with large curvature in the simulation and experiment, and artificially added interference amount in the simulation part. On this path, USV is prone to a large sideslip angle. The effect of the designed observer can be known by comparing the estimated value with the actual value. To make this part clearer, we have modified and supplemented this paper. The revised details can be found in Line 274-277, page 10.
Comment 6: Conclusion is very dry, use this space to re-address the novelty, contributions and key findings of this study. Paper would benefit from a future works section - i.e more advanced real world testing.
Response: We revised and added the conclusion section by adding a quantitative analysis of the experimental results. We also summarize the advantages, disadvantages, and limitations of the design process of the sideslip angle observer, and make several suggestions for future work. The revised details can be found in Line 369-374, page 16.
Author Response File: 
Author Response.pdf
Round 2
Reviewer 1 Report
The authors have addressed the reviewer's concerns. It can be recommended for publication.
Author Response
No problem. thanks
Reviewer 2 Report
Thank you for addressing my comments.
Although I think this research is interesting, I still believe there are issues in the design and formatting of the paper.
The introduction is still too long, and the abstract needs completely re-working per my previous comment rather than adding a small section at the end.
However, these are minor issues and could be easily solved by looking at previous studies in this area and emulating their format.
As these issues are superficial and not directly linked with the content of the paper but more about the readability, I think these are minor corrections that you should make.
Author Response
Comment: The introduction is still too long, and the abstract needs completely re-working per my previous comment rather than adding a small section at the end. However, these are minor issues and could be easily solved by looking at previous studies in this area and emulating their format. As these issues are superficial and not directly linked with the content of the paper but more about the readability, I think these are minor corrections that you should make.
Response: Thanks for the reviewer’s kind suggestion. To address the problem of a long introduction, we deleted and revised some complicated sentences. At the same time, some unimportant content has been reduced. The abstract is rewritten. To increase the readability of the paper, we have revised and adjusted part of the article to make it easier to understand and clearer in structure. The revised details can be found in Line 1-16, page 1, Line 28, page 1, Line 40-48, page 2, Line 272-290, page 11, and Line 293-294, page 12.
