Numerical Simulation of Depth Tracking Control of an Underwater Towed System Coupled with Wave–Ship Interference
Round 1
Reviewer 1 Report
This paper investigates the performance of the depth tracking control system of an underwater towed system using numerical simulation. The authors carried out the numerical simulation in detail. A basic mechanism of the depth tracking control system is not clear for me. The vertical displacement of the towed system can be determined by the balance among the tension of the rope, the weight, and the drag of the towed vehicle. I do not understand well how the authors adjust the weight of the towed system, controlling the lift by shifting the weight in the longitudinal direction. The results of numerical simulation are validated by the experimental results regarding the relationship between towing speed and a submerged depth. The authors should also validate the control system, which is a major part of this study. The other comments are as follows.
- The diameter of the towing cable is 0.005 m in Table 1. But I think it is very thin. Is it possible to tow an underwater vehicle by this thin rope?
- I do not understand well the overlapping grid technique. Is the domain around the vehicle solved by both the coarse and fine meshes? In case of the nested grid system, fine and coarse meshes are used for near and far area from the vehicle without overlapping, respectively.
- What is the reason why the velocity of the shifting weight is controlled? The position and the acceleration are also the candidates for control parameter.
- The pitching angle of a towed vehicle increasing with increasing depth. However, the effects of wave on the motion of the underwater vehicle must be reduced with increasing depth. Please explain the reason.
- The tension does not change much in Fig. 16. However, the tension must increase when the decrease in lift resulting in lower position of the towed vehicle. Please explain the reason.
Author Response
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Author Response.pdf
Reviewer 2 Report
This paper presented the depth tracking control of an underwater towed system integrated with wave-ship system using a simple PID controller. However, I am concerned about the application and practicability of this theory for marine vehicles. Although it is related to the marine system control strategy, the selection of parameters is not discussed in detail, the explanation of the procedure and contents of the paper are ambiguous. Therefore, I would suggest major revision before it can be published in JMSE. Several suggestions can be given:
1/ Despite the motivating topic, the novelty of the approach of the manuscript does not seem significant since it seems all the methods are existing and the experimental contribution does not consider. This paper should have more theoretical contributions.
2/ The contributions need to be more highlighted in the abstract. The novelty of the proposed control system should be emphasized. Also, the limitations of the existing method and new ideas for improvement should be explained more.
3/ Introduction is tiny and the novelty of the proposal is not clearly defined. Moreover, a literature overview that should bring an insight into the current state of affairs in the field under study is insufficient. Approaches related to the one presented should be addressed and comparisons between them should be made. The References should be extended to include the latest publications on some types of underwater vehicles and some multi-marine vehicles that refer to the discussed subject. The author can refer to the following newest references:
https://doi.org/10.1016/j.oceaneng.2017.02.035, https://doi.org/10.3390/s21030747
https://doi.org/10.3390/s20092633, DOI: 10.1109/ACCESS.2020.3048706, https://ojs.imeti.org/index.php/PETI/article/view/2892
And the introduction should be added to do a better job of explaining the existing methods and why they are or are not valuable.
4/ Write the organization of the paper in the introduction part.
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/ Some parameters in Figure 1 should be explained. Such as: phi_s, phi_p, G_1, F_s…,
7/ In section 2.2, the analysis is insufficient. A detailed design and explanation of proposed systems need to be discussed. Where is the hydrodynamic forces acting on the systems?
8/ What are the boundary conditions at 2 connected points of cable applied to your method?
9/ In equation 6, What are c, a, and y? Please explain.
10/ In section 2.4, the authors have to show the manner of implementation in professional details which will be beneficial to the readers. A detailed explanation of proposed method needs to be explained. A flowchart needs to be included to explain the sequence of steps of the proposed method. Some characteristics of meshing method should be mentioned.
11/ In section 4, why did authors conduct the test on another towed vehicle? Some Figures related to cable interaction (tension forces of cable in the test) should be added and explained.
12/ In section 5, the parameters for simulation such as hydrodynamic coefficients of the system and controller gains PID) should be given in the paper.
13/ The explanations and analysis of simulation results should be enriched to show the validity of the data.
14/ Finally, from the simulations with satisfactory results, the system performance is expected in actual experiments with your proposed method. If a maritime test is not available, a comparative study could act as an alternative. This paper now is difficult to prove the advantages of the proposed algorithm. The paper would benefit greatly if it were expanded with a comparison of the proposed method with other algorithms known in the literature. Results are not compared with other approaches and it is difficult to assess whether the proposed approach is better than the existing ones.
15/ The reviewers recommend that more future work should be added on Conclusion Section.
16/ The presentation of the paper can be improved and the quality of some figures should be enhanced.
17/ The manuscript writing can be further polished with professional English. The manuscript can be thoroughly revised for grammar check. Some typo errors, Figure 8 and 9 should be “Figures 8 and 9” and so on. Please check all!
Author Response
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Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The manuscript was revised well according to the review’s comments. I have some additional comments as follows.
Regarding the experiment for validation, please show the trim of the vehicle in the experiment. I thought that the authors can validate the numerical simulation more if the experiments are carried out by shifting the weights. Also, the authors can calculate the lift from the weight and the vertical component of the tension obtained by the experiment. This estimated lift can be compared with the lift estimated by numerical simulation.
I do not understand well the following description “the angle between the towing cable and the vertical direction is smaller, and the disturbance at the towing point Ps will be more directly transmitted to the connected point Pt of the towed vehicle”. Do the authors mean that the vertical displacement of the Ps is more transmitted to the connected point Pt if the angle between the towing cable and the vertical direction is smaller?
Overall, the authors show the results of the position of the vehicle, lift, and tension, individually. The drag and the difference between weight and lift of the vehicle are simply balanced with the horizontal and vertical components of the tension, which can be determined by the angle of the rope. Hence, please show the relationship among the vertical position, weight, lift, and drag of the vehicle, and the tension of the rope in a summarized way.
Author Response
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Author Response File: Author Response.pdf
Reviewer 2 Report
I appreciate your efforts to revise the manuscript in light of the comments addressed in the previous review. Most of the recommendations I made were addressed by the authors, therefore I believe that the paper is now ready for publication.
Other comments:
In ref [8], the author´s name is a little wrong. Please re-arrange as below:
Vu, M.T.; Choi, H.S.; Kang, J.; Ji, D.H.; Jeong, S.K. A study on hovering motion of the underwater vehicle with umbilical cable. Ocean Eng. 2017, 135, 137-157, doi:10.1016/j.oceaneng.2017.02.035
In addition, the format of references needs to be unified with the aim to satisfy the requirement of the journal, and the DOI number of some new references needs to be added in this paper. Please check carefully.
Author Response
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Author Response File: Author Response.pdf
