A Parallel Robotic Antenna Design for Downlinking Leo Satellite Signal Subject to Wind Disturbance

Round 1

Reviewer 1 Report

Journal Name: Applied Science

Title: A PARALLEL ROBOTIC ANTENNA DESIGN FOR DOWNLINKING LEO SATELLITE SIGNAL SUBJECT TO WIND DISTURBANCE

In the current research article, the authors discussed robotic Gough-Stewart parallel platform maneuvering for satellite applications. The article looks interesting and suitable for Applied Science. But the article also needs to improve in a minor way. Therefore I recommend the current article is suitable for publication in Applied Science with the Minor Revision.

 

The article can be accepted after major corrections.

 

 

1. The abstract has to improve and it should be trimmed.  
2. The author should explain the key results in the abstract.  
3. Authors need to refer few earlier articles of Applied Science and explain why the article is suitable for Applied Science Journal
4. The authors need to improve the literature review further.
5. The quality of the figures should be improved
6. All equations should be cited properly
7. Recommend using single-color fonts in figures and text.  
8. The conclusion can be further improved
9. Please check the grammatical and syntax error

Author Response

=========================================================

Rebuttal Letter

• Manuscript: A parallel robotic antenna design for downlinking LEO satellite signal

subject to wind disturbance

• ID:           applsci-1908761
• Submitted to: Applied Sciences/MDPI
• Corresponding Author: Miguel de Jesus Cuauhtemoc Espinoza-Calderon
• Email: [email protected]

=========================================================

 

------     REVIEWER 1     ------

General comments: We appreciate your time and effort to provide this review. We have dedicated our utmost effort to entertain each comment.

Comment 1 and 2:

1 The abstract has to improve, and it should be trimmed.  

2 The author should explain the key results in the abstract.  

Our answers to comments 1 and 2:  We now address answers to both queries in the following text (in red in the revised manuscript), which stands for a modified and concise abstract.

Passive LEO (Low Earth Orbit) satellites stand nowadays for a telecom premier option. However, LEO satellites impose not only stringent specifications on resolution, precision and repeatability but also requiring advanced antenna technology for signal downlinking. To downlink efficiently LEO signals at each passage onto a given Earth region, there has been explored either large size passive Earth antenna, or an array of smaller size active Earth antennas to minimize trajectory loss. To guarantee design specifications, dynamics cannot be neglected given the size and inertia of base and antenna. In this paper, it is proposed the design, path planning and control of a 6 DoF robotic antenna maneuvering the antenna subject to aerodynamic wind disturbance. The system maneuvers to point at the LEO satellite over the whole envelope with the required precision to guarantee robust point-to-point tracking. Representative simulation results for three geolocations shows practical tracking with component of the shelf actuators, without requiring any knowledge of the dynamics while withstanding state-dependent persistent disturbances.

 

Comment 3: Authors need to refer few earlier articles of Applied Science and explain why the article is suitable for Applied Science Journal

Answer 3:  We apologize for missing referencing properly some previous articles of Applied Science. We found pertinent to refer to article “Adaptative Predictive Functional control of X-Y Pedestal for LEO Satellite Tracking Using Laguerre Functions” which propose a 2 DoF robot to point at LEO satellites. With respect to the second issue, we find quite convenient Applied Science journal given that it requires a formal and rigorous approach, substantiated in foundational knowledge, which is required to deal with the stringent functional specifications of the critical task: downlinking LEO signals of a passive satellite travelling hundred of miles per hour at 400 km above a georeferenced location. These specifications surpasses conventional engineering approach, thus requiring an applied science approach to blend knowledge on robotics, dynamical systems, control theory and stability analysis, in addition to numerical simulations of dynamics, and engineering design.

Comment 4: The authors need to improve the literature review further.

Answer 4: Thanks for pointing out at this issue. Unfortunately, we must clarify that we did browse several academic metasearch engines, such as Web of Science, IEEE xplorer, ASME, ScienceDirect, Springer Link, and others including open access such as MDPI, Hindawi. We could not find a single article that proposes a robotic system to track actively a satellite without occlusion. That is, that includes path tracking scheme to downlink LEO signals: a georeferenced motion planning scheme in closed loop with a dynamical 6 DoF robot handing a large payload -the antenna. Given the large payload -some of it produced by wind drag perturbation-, a high stiffness-to-load ratio robot was preferred, such as the parallel structure of the Gough-Stewart design. In addition, we did not find any application article using a Gough-Stewart for active tracking of LEO signals. In this sense, we decided to include only those references that are required.

 

 

Comment 5: The quality of the figures should be improved

Answer 5:  Vectorized high-quality EPS figures are included, for zooming if reader requires. In the revised manuscript, we split figures into two such that larger images are appreciated at normal viewing. 

 

 

Comment 6: All equations should be cited properly

Answer 6: The revised manuscript now presents as numerated equations only those that are cross referenced.

 

 

Comment 7: Recommend using single-color fonts in figures and text.  

Answer 7: The revised manuscript now follows this recommendation.

 

 

Comment 8: The conclusion can be further improved

Answer 8:  The revised manuscript now addresses succinctly the contribution with respect to the state of the art, as well as remarks on some further research directions derived from our proposal. The new Conclusion Section now includes 1300 characters, down from 2,515 characters of the original submission. The modified text now reds as follows:

A Gough-Stewart parallel robot handling a large antenna has been proposed for pointing space task to meet specifications of downlinking passive LEO satellites, including an online 3D georeferenced motion planning to update desired pose without structural occlusion. In contrast to previous schemes in the literature, our proposal (1) exploits redundancy to introduce a virtual attitude constraint without requiring home position (forward kinematics) to guarantee the existence of a unique solution for any configuration space trajectory without exceeding the physical limits of legs extensions; (2) it is considered realistic operational outdoor condition in which the large parallel antenna induces external aerodynamical disturbances when travelling all around the envelope; (3) consequently, dynamics are obtained so as to reliable industrial COTS are considered for a feasible design that instruments a the well-known equivalent PID scheme. Dynamic simulations with closed- and open-loop desired trajectories definitions fed to the close-loop controlled show that the system do in fact point at the LEO satellite. Notably, the open-loop case shows negligible errors, without requiring the burdensome forward kinematics. This study paves the way for an integral solution of a robotic antenna of the coming powerful passive LEO internet constellations, however newer challenges are in the horizon from this new technology, for instance bandwidth and gain antenna related to precision, resolution and repeatability of the proposed mechanical system, a matter of future research. For more challenging specifications, there can be explored nonlinear control schemes, however comparison to the PID/PI regulators are advised, in view of reliable COTS available in the market facilitates their instrumentation.

 

Comment 9: Please check the grammatical and syntax error

Answer 9: Revised manuscript has been revised and proof read by a professional service offered in our institute.

Author Response File: Author Response.docx

Reviewer 2 Report

I have read the manuscript number:applsci-1908761 with title:"

 A PARALLEL ROBOTIC ANTENNA DESIGN FOR DOWNLINKING LEO SATELLITE SIGNAL SUBJECT TO WIND DISTURBANCE", and I found that, authors have been make a Simulations for three worldwide locations are presented and discussed. I would like the authors consider the following points before I recommend publication

1-The abstract of manuscript should be written in compact form, authors should mention the main findings in the abstract.

2- In material and method section, the most parameters and quantities haven't a definition.

3- No comparison has been made with some earlier literature with your results.

 

 

Author Response

------     REVIEWER 2     ------

General comments:

General Answer: We appreciate your time and effort to provide this review. We have dedicated our utmost effort to entertain each comment.

 

Comment 1: The abstract of manuscript should be written in compact form, authors should mention the main findings in the abstract

Answer 1:  We have modified the abstract to the following text (in red in the revised manuscript):

Passive LEO (Low Earth Orbit) satellites stand nowadays for a telecom premier option. However, LEO satellites impose not only stringent specifications on resolution, precision and repeatability but also requiring advanced antenna technology for signal downlinking. To downlink efficiently LEO signals at each passage onto a given Earth region, there has been explored either large size passive Earth antenna, or an array of smaller size active Earth antennas to minimize trajectory loss. To guarantee design specifications, dynamics cannot be neglected given the size and inertia of base and antenna. In this paper, it is proposed the design, path planning and control of a 6 DoF robotic antenna maneuvering the antenna subject to aerodynamic wind disturbance. The system maneuvers to point at the LEO satellite over the whole envelope with the required precision to guarantee robust point-to-point tracking. Representative simulation results for three geolocations shows practical tracking with component of the shelf actuators, without requiring any knowledge of the dynamics while withstanding state-dependent persistent disturbances.

 

Comment 2: In material and method section, the most parameters and quantities haven't a definition.

Answer 2: The revised manuscript now includes parameters definitions and unit quantities of wind velocity (m/s) and altitude (msnm), see Table 1.

 

 

Comment 3: No comparison has been made with some earlier literature with your results

Answer 3:  Thanks for pointing out at this issue. Unfortunately, we must clarify that we did browse several academic peer-review metasearch engines, such as Web of Science, IEEE xplorer, ASME, ScienceDirect, Springer Link, MDPI, Hindawi, and others. We could not find a single article that proposes a dynamical robotic system to track actively a satellite without occlusion. That is, that includes path tracking scheme to downlink LEO signals: a georeferenced motion planning scheme in closed loop with a dynamical 6 DoF robot handing a large payload -the antenna. Given the large payload -some of it produced by wind drag perturbation-, a high stiffness-to-load ratio robot was preferred, such as the parallel structure of the Gough-Stewart design. In addition, we did not find any application article using a Gough-Stewart for active tracking of LEO signals such that comparison may not be fair. The work [Tehrani,2021], mentioned in the article did mention the difficulty to find operational space solutions near the zenith pointing direction, which is a phenomenon not appearing in our proposal after a proper task trajectory definition, instead of using pseudoinverse matrix of the velocity kinematic operator. This subtle but important detail has been included in the revised manuscript under the following text in Section 2.5 Inverse Task Velocity Kinematics:

Notice that with this formulation the product B(y) ^wJω_a(y) = I₂, thus operator ^wJω_a(y) qualifies as the right pseudo-inverse of B(y), with the additional advantage of being well-conditioned for the zenith-pointing attitude (β= π/2), in contrast to B⁺(y)= B^T(y) [B(y) B^T(y)]^-1. This problem was also reported in the proposed solution on  [Tehrani,2021].

[Tehrani,2021] ]Dadkhah Tehrani, R.; Givi, H.; Crunteanu, D.-E.; Cican, G. Adaptive Predictive Functional Control of X-Y Pedestal for LEO Satellite Tracking Using Laguerre Functions. Appl. Sci. 2021, 11, 9794.

Author Response File: Author Response.docx

Reviewer 3 Report

In this manuscript, the authors have proposed a new approach to realize the control of a novel design of a parallel robotic antenna with a closed-loop driveline structure that can track the signal of a LEO satellite with higher efficiency even under wind disturbance. It is shown that a simple adaptive PI controller can be used with this structure without losing the signal even under bad time climate.

 

In reviewer's opinion, the paper can be recommended for publication in Journal of applied science with the current form with addressing few remarks as follows:

- The Gough-Stewart platform used in this research uses six prismatic actuators. In similar references authors used three prismatic actuators which makes the structure very easy to control. What is the advantage of using six prismatic actuators compared to a structure using just 3 actuators.

- In the formulation and equations authors does not explained if the six actuators are considered as a flexible joins or as dumpers.

- In the block diagram in Figure 6, aerodynamic forces and wind disturbances are considered together, but we can consider that aerodynamic forces are generally a reaction against the wind, which amounts to the same thing.

Author Response

=========================================================

Rebuttal Letter

• Manuscript: A parallel robotic antenna design for downlinking LEO satellite signal

subject to wind disturbance

• ID:           applsci-1908761
• Submitted to: Applied Sciences/MDPI
• Corresponding Author: Miguel de Jesus Cuauhtemoc Espinoza-Calderon
• Email: [email protected]

=========================================================

 

 ------     REVIEWER 3     ------

General comments:

General Answer: We appreciate your time and effort to provide this review. We have dedicated our utmost effort to entertain each comment.

 

 

Comment 1. The Gough-Stewart platform used in this research uses six prismatic actuators. In similar references authors used three prismatic actuators which makes the structure very easy to control. What is the advantage of using six prismatic actuators compared to a structure using just 3 actuators.

Answer 1. We did an exhaustive search in the literature concerning robots used for pointing antennas. Indeed, some authors have proposed using the so-called Delta structure employing 3 actuators, which is suitable for position tasks only, not for attitude control. Henceforth, since tracking satellites in the whole range of the visible sky demands position-attitude, the six degrees of freedom Gough-Stewart platform is required to cover such workspace over arbitrary georeferenced locations.  

 

Comment 2. In the formulation and equations authors does not explain if the six actuators are considered as a flexible joins or as dumpers.

Answer 2. Thanks to point at this issue that gives us the opportunity to clarify. The Gough-Stewart robotic structure is a well-known platform with a high stiffness-to-load ratio provided that mechanical components are rigid. Nowadays, there exists commercially available components (components-of-the-shelf, or COTS) that meet such specifications, including rigid prismatic actuators for these applications. Therefore, a good engineering design provides such rigid specifications such that it leads us to assume reasonably that no flexibility appears at joints neither any damper-like behavior. Under this rationale, we did not assume flexible joint or flexible link behaviour. In the revised manuscript, a new sentence has been added along this clarification in Section 2.

We assume a structural design with a high stiffness-to-load ratio, a requirement easy to meet nowadays using commercially available components (components-of-the-shelf, or COTS), henceforth  we did not assume flexible joint or flexible link behaviour.

 

Comment 3. In the block diagram in Figure 6, aerodynamic forces and wind disturbances are considered together, but we can consider that aerodynamic forces are generally a reaction against the wind, which amounts to the same thing.

Answer 3. Thanks again to point at this issue, because you are right that it requires precision our argument. Firstly, block diagram shown in Figure 6 depicts together aerodynamic forces and wind disturbances, however wind aerodynamic forces stand indeed as wind disturbance (exogenous) forces that may provoke reaction (endogenous) forces. In this block diagram, wind block stands for the wind velocity arising from climatic conditions at ground station, which together with the antenna configuration, it yields the input operator to map wind disturbances as aerodynamics forces. To eliminate any further confusion legend of Figure 6 was modified to:

Block Diagram of the full dynamic system.  The wind block stands for the wind velocity value considered at each ground station.

[Tehrani,2021] ]Dadkhah Tehrani, R.; Givi, H.; Crunteanu, D.-E.; Cican, G. Adaptive Predictive Functional Control of X-Y Pedestal for LEO Satellite Tracking Using Laguerre Functions. Appl. Sci. 2021, 11, 9794.

 

 

Author Response File: Author Response.docx

Reviewer 4 Report

The proposed work in its present form is accepted for publication in the subject journal. 

Author Response

=========================================================

Rebuttal Letter

• Manuscript: A parallel robotic antenna design for downlinking LEO satellite signal

subject to wind disturbance

• ID:           applsci-1908761
• Submitted to: Applied Sciences/MDPI
• Corresponding Author: Miguel de Jesus Cuauhtemoc Espinoza-Calderon
• Email: [email protected]

=========================================================

 

------     REVIEWER 4     ------

Comment: The proposed work in its present form is accepted for publication in the subject journal. 

Answer: We authors appreciate and acknowlegde the time and effort to convey this assessment. We are doing our utmost effort to comply to Editor and Reviewers comments and queries.

Author Response File: Author Response.docx