RM-Act: A Novel Modular Harmonic Actuator

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The reviewed article introduces the proposal of a new Radial Modular Actuator (named RM-Act), which employs synchronous belt technology as a harmonic speed reducer. This actuator is a proposal for robotic systems, where low-cost modular transmission mechanisms play a crucial role in complex robotic systems. The Authors propose a more compact actuator with higher torque density, proper static and dynamic parameters. The advantages of the new actuator could be zero backlash, high single-stage reduction and torsional stiffness, while concurrently reducing the drive complexity and limited fatigue life. These established concepts led to the elaboration of a new electro-mechanical actuator for robotic use. The new device was tested to calculate static parameters and assess the effectiveness of the new concept.

The paper presents a comprehensive analysis of the scientific and technical problems related to the new concept, including the technical structure and mathematical analysis. The introduction provides a vital theoretical foundation for the scientific issue. In this part, there are novel concepts of the actuator, possible configurations of idle and central pulleys and mathematical analysis of the technical problem. At the end of this analysis, the CAD model and detailed description of the RM-Act device were presented. The Authors analyse the experimental results of testing the static and dynamic parameters of the new actuator. The time-historical and frequency basis characteristics are presented. In the conclusion, the Authors present a novel concept and evidence based on the theory and practice.

The article presents the effects of the theoretical, mathematical, and experimental research analyses to confirm the technical problem of the electro-mechanical actuator with proper static and dynamic parameters. The mathematical analysis effectively addresses research problems. The symbols (constants and variables) used in the mathematical model accurately describe the technical issue. However, the scientific soundness of the presented analysis is average. Additionally, the text includes some editorial errors which should be corrected to get a good-quality paper. Based on these comments, the paper requires a change to focus on the scientific problem regarding the mathematical model of the new device, its static and dynamic parameters. This article’s form needs to be corrected to get a high-quality scientific study.

The bibliography linked to the analysed scientific problem is relevant and sufficient. The choice of items is correct and sufficient in the field presented in the paper’s analysis. The references to the bibliography in the text are proper. All of them were appropriately described and presented. The article includes time and frequency characteristics, diagrams, tables, a case study, and conclusions, which were clearly presented.

The English language and style are fine.

All in all, the paper requires major revision. The technical aspect is correctly analysed and described, but it needs to improve the scientific level of the paper.

Author Response

Dear Reviewer,
The authors sincerely thank you for your thoughtful observations and valuable comments. We have submitted the answers to the comments in the attached PDF file.

Thank you for your time and consideration.

Kind regards,

Alok Ranjan

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The presented article dealing with a modular harmonic actuator presents a new concept made and assembled so far from 3D printed components as an initial test system. In the introduction, the authors analyze a professional literature review from various publications in a sufficient scope. The following chapters follow each other methodologically and thematically, which maintains the quality of the entire publication. In terms of graphics, the article is at a good level with appropriately selected images and created graphs.

The presented article is interesting and has potential, but several aspects need to be improved to increase its scientific contribution:

- Figure 2b shows the tooth peaks of the fixed pulley and the output pulley with different heights, and in Figure 2c, the tooth peaks are of the same height. Please comment on the correct version.

- Data such as eu, e1, ec, ... should be appropriately described for Table 1

- The texts of the article describe various dimensions of the components, while it would be appropriate to unify and state either radio or diameters.

- Given today's capabilities of various software and the fact that you have a complete 3D model of the entire assembly, it would be appropriate to perform simulations of the loaded actuator and compare them with the measured values to increase the quality of the article.

- In the conclusions, I would expect suggestions for possible specific applications of the new actuator and at the same time indicate in the planned sequels how the performance characteristics can be improved.

 

In conclusion, I state that the article can be published after minor modifications.

Author Response

Dear Reviewer,
The authors sincerely thank you for your thoughtful observations and valuable comments. We have submitted the answers to the comments in the attached PDF file.

Thank you for your time and consideration.

Kind regards,

Alok Ranjan

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper describes a Radial Modular Actuator that employs synchronous belts as a harmonic speed reducer. A prototype of the actuator is built and tested, and model identification is performed to obtain its characteristics.

The paper reports original experimental research performed by the authors and results are certainly interesting for the potential readers. However, clarity of the text and completeness of the information provided could be improved across different sections of the paper. Some comments are the following: 

1. How did the proposed actuator architecture address the key limitations of traditional Harmonic Drives ? 

2. Which drive configuration among those ones shown in Figure 3 was built ? How is the choice of the configuration justified by the design requirements of the actuator ?

3. The design in section 3 should start with a clear definition of the specifications/requirements for the considered application of the drive, e.g., nominal torque, maximum speed, etc., followed by the design steps followed to achieve them.

4. Details about the design of the two stages of the field-oriented controller (FOC), namely the proportional-integral-derivative (PID) controller of the exterior stage and the proportional-integral (PI) controller of the internal stage are missing.

5. Section 5.1, the sentence: "The static tests were performed by applying a linear input torque. For that, feed-forward torque values ranging from 0.1Nm are given for a time interval until the position value converges." Was a constant torque of 0.1 Nm applied?

6. In table 3, the torque τavg is much higher than τtheo. Explanation provided does not appear to be convincing.

7. What results indicate improvements with respect to the state of the art and why ? The proposed actuator is not compared against a traditional one.

8. In section 6, define the state variables x1, x2, x3, x4 and explain how the model in equation (7) was derived.

9. Comparison between EM-Act and RM-Act reported in table 4 is unclear because EM-Act was not previously introduced in a satisfactory manner. What are the conclusions from this comparison?

10. Conclusions are very short and did not satisfactorily explain what improvements were achieved with respect to the state-of-the-art.

11. Conclusions could elaborate more on how improvements of the performance characteristics (i.e., the specific power of the actuator) could be achieved. Also how could the possibility of a single belt actuator be explored? How would the authors demonstrate the utilization of modular actuators in different platforms ? 

Author Response

Dear Reviewer,
The authors sincerely thank you for your thoughtful observations and valuable comments. We have submitted the answers to the comments in the attached PDF file.

Thank you for your time and consideration.

Kind regards,

Alok Ranjan

Author Response File: Author Response.pdf

Reviewer 4 Report

Comments and Suggestions for Authors

Title : RM-Act: A NovelModular Harmonic Actuator
Authors : Ramesh krishnan Muttathil Gopanunni, Alok Ranjan, LorenzoMartignetti, Franco Angelini and Manolo Garabini
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In this paper, the authors present the RM-Act, a novel RadialModular Actuator that employs synchronous belts as a harmonic speed reducer. It retains the advantages of the harmonic principle, making it a promising candidate for robotic actuation.

Strengths:
. The paper includes a thorough explanation of the design process, component selection, and parameter trade-offs.
. The design is compact, modular, and low-cost, using commercially available components, which enhances real-world applicability.
. Experimental validation is carefully conducted, including static, dynamic, and model identification tests.
. Torque density is significantly improved compared to existing actuators, with a meaningful quantitative comparison.
. The paper is very well illustrated with good figures.

Weaknesses:
. The novelty could be better highlighted relative to existing alternatives (strain wave gearing improvements, series elastic actuators).
. The writing quality is uneven and requires light polishing to improve clarity and flow. But this point can be addressed quite easily.
. Most of the parts are 3D printed for prototyping; practical robustness in real-world environments is not demonstrated.
. Conclusion section is too short (10 lines), and should be extended in the final version of the paper (with a limitations discussion, and perspectives for example).

References :
. 38 research references, out of which only 1 self-reference, giving a good self-reference ratio of 2.6%. 
. There are 8 non-research references ([7], [8], [21], [33-36], [38]). 
. Avoid citing groups of references: [1-3], [7-9], [13-15], [16-19], [22-24], [25-27]. Or if you do it anyway, please comment more any of these references. 
. The references are all cited in the text, but each cited reference should be adequately described and presented with regard to the addressed subject.  
. The bibliography of this paper is mainly composed of recent references: 8 of them are more than 10 years old, and 30 of them are less than 10 years old.
. Please avoid the formulation 'et al.' in the references section: [1], [32]. The complete list of authors deserve to be cited in this section. 

Originality:
. This work addresses a known challenge in actuator design, but proposes an original approach using synchronous belts to emulate harmonic drives.
. The concept of replacing flexspline-based harmonic mechanisms with belt-driven modular designs is interesting and relatively novel.

Scientific Soundness, Significance and Interest:
. The study is well designed and technically sound.
. Experimental procedures are clearly described.
. The proposed actuator is validated both numerically and experimentally.
. The manuscript presents an actuator with high relevance for applications requiring lightweight, cost-effective, and customizable actuation.
. I think there is an overall benefit to publish this work, after minor revisions to improve clarity and strengthen the comparative novelty.
. The English language quality is globally appropriate and acceptable. Some stylistic revisions would improve readability.

Overall evaluation:
. The English language quality and style of this paper are globally appropriate and understandable.
. I think there is an overall benefit to publish this work, after having answered to the suggestions I propose in this review. 
. In my opinion, methods and conclusions of this paper seem to be interesting for the readership of the journal. 

As a conclusion, my suggestion to the editor is to propose to the authors to perform minor revisions on their paper before publication in Actuators.

Author Response

Dear Reviewer,
The authors sincerely thank you for your thoughtful observations and valuable comments. We have submitted the answers to the comments in the attached PDF file.

Thank you for your time and consideration.

Kind regards,

Alok Ranjan

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

The comments provided to the first version of the paper were addressed in the revised version.