RM-Act 2.0: A Modular Harmonic Actuator Towards Improved Torque Density

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors present an improved design featuring a single synchronous belt in a helical radial modular drive (designated RM-Act 2.0) and assert that this design eliminates the need for precision machining, rendering it suitable for mass production. Furthermore, they say that their approach reduces transmission slip and improves the torque density associated with harmonic drive systems while providing a more modular alternative with a wider range of gear ratios. A functional prototype has been manufactured and a performance model validated, with its characteristics compared to those of a previous similar design with two belts. The comparison prioritises compactness and torque density. The actuator has an original design consisting only of spur gears with external teeth. This makes it more technologically advanced and cheaper than classic helical gears.

I have the following remarks to the authors:

1. In my opinion, the introduction describes a small number of similar devices. Besides the classic helical gear design, could the authors point out other original solutions of competing devices?
2. The proposed design does indeed have the listed advantages over the classic version. However, in my opinion, the idle gears increase its dimensions, which may cause it to lose its compactness.  What constructive solutions could be adopted in this regard so that the dimensions are not increased in the new design?
3. Although torque tests are important, the timing belt introduces elasticity that will likely affect positioning accuracy. Has any research been conducted into the relationship between positioning accuracy and torque magnitude?
4. In my opinion, Figure 2 would be clearer if the timing belt were depicted with its teeth. Currently, the belt's smooth appearance creates the initial impression that it is a friction gear.

 

Author Response

Please refer to the attached PDF file.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This work reports introduces RM-Act 2.0, a novel, compact, and modular harmonic speed reducer designed to achieve a broader range of speed reduction ratios in a single stage. The manuscript is comprehensive and its content is abundant. Some specific suggestions are as follows.

1. There are some parts of the manuscript that can be simplified, such as the abstract. The current level of detail is more like a work report than a research paper.Figure 1 is also unnecessary.
2. In Table 3. Specifications of RM-Act 2.0 compared with RM-Act, it is suggested to include quantitative data results of the relative changes.
3. The manuscript includes numerous photos of actual objects. It is suggested that necessary annotations be added to these photos as well.
4. “the development of the Radial Modular Actuator RM-Act 2.0using commercially available components, thereby minimizing machining requirements ” It is suggested that appropriate and quantitative explanations be included in the manuscript.
5. In Figure 7, the input of the position controller should explain why it is the position, velocityand torque signals. Generally, only the target position signal needs to be input here.

Author Response

Please refer to the attached PDF file.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper presents an impressive contribution to the field of modular harmonic actuation, introducing RM-Act 2.0, a compact, modular harmonic actuator designed to enhance torque density while maintaining manufacturability and cost-efficiency. From a professional standpoint, the work demonstrates a high degree of mechanical innovation and engineering practicality. The authors successfully address key challenges inherent to traditional harmonic drives—namely complex machining, high cost, limited modularity, and inefficiencies at lower reduction ratios—by proposing a single-belt, synchronous-drive mechanism that functions as a harmonic speed reducer. The systematic approach, from conceptual development to prototype realization and experimental validation, reflects solid design methodology and well-executed engineering research.

The paper is well structured, with clear logical flow across design concept, CAD modeling, experimental validation, and performance benchmarking. The integration of commercially available components, coupled with 3D-printed structural elements, aligns with current trends in rapid prototyping and low-cost robotic actuation. The detailed static and dynamic testing provides convincing empirical support for the claimed improvements in stiffness, torque capacity, and volumetric efficiency. Quantitative analysis—such as stiffness of 360.3 Nm/rad and 30% performance improvement over the earlier RM-Act version--demonstrates substantial progress toward achieving higher torque density in a compact footprint.

The model identification section, incorporating Bode plot fitting and parameter estimation, further enhances the scientific rigor of the study. The paper provides limited discussion of long-term durability or fatigue behavior of the belt-based transmission under cyclic loading. The authors are suggested to include a reliability assessment or preliminary life-cycle analysis would enhance the engineering completeness of the work. The dynamic characterization could be extended to include frequency response bandwidth, control stability margins, and thermal performance during continuous operation. Several related works can be considered, optimal pole-transition current distribution for constant torque maintenance and stator copper loss minimization in variable-pole machines, model-free current predictive control for pmsms with ultralocal model employing fixed-time observer and extremum-seeking method, robust model predictive control of position sensorless-driven ipmsm based on cascaded ekf-leso. The conclusion part could more clearly articulate future research directions, such as integrating torque sensing, optimizing material selection, etc.

Overall, this manuscript presents a strong balance between theoretical insight and experimental realization, advancing the design philosophy of harmonic actuation systems toward modular, accessible, and high-performance solutions.

Author Response

Please refer to the attached PDF file.

Author Response File: Author Response.pdf