Coordinating the Redundant DOFs of Humanoid Robots
Chen Si
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThis article proposes a bio-inspired approach for coordinating the redundant DOFs of humanoid robots using the Passive Motion Paradigm , grounded in ideomotor theory.The proposed model integrates both attractive and repulsive torque fields,demonstrating adaptability and energetic efficiency.The article presents a novel alternative to traditional inverse kinematics and optimization-based methods.However, the paper also exhibits several issues related to experimental validation, comparison with existing methods, mathematical clarity, formatting, and reproducibility. These aspects require substantial revision before the manuscript can be considered for publication.
1.In the C-matrix adaptive module, the specific implementation details of the gradient descent algorithm (such as step size strategy, convergence conditions, and iteration limit) are not explained in detail. It is recommended to supplement mathematical derivation or algorithm pseudocode in Section 2.4.
- Maintain consistency in professional terminology.Mixing "DoFs" with "DOFs" (such as "DOFs" in the abstract and "DoFs" in the main text).
- Although Figures 3-6 show multiple sets of simulation results, they lack detailed explanations of the simulation conditions (initial attitude, velocity curve, control parameters, etc.), and some captions do not fully explain the meanings of colors or symbols. Suggest improving the caption information and adding comparative analysis results in the main text.
- The references are outdated and lack uniformity in their citation format. It is recommended to supplement recent relevant literature and adjust the citation formats to conform uniformly to journal requirements.
- The font size of the text is not uniform in Figure 1.
Author Response
Comment 1
In the C-matrix adaptive module, the specific implementation details of the gradient descent algorithm (such as step size strategy, convergence conditions, and iteration limit) are not explained in detail. It is recommended to supplement mathematical derivation or algorithm pseudocode in Section 2.4.
Answer 1
The requested clarification is reported in the appendix
Comment 2
.Maintain consistency in professional terminology. Mixing "DoFs" with "DOFs" (such as "DOFs" in the abstract and "DoFs" in the main text).
Answer 2
Thank for the nobservatinPerformed as suggested.
Comment 3
Although Figures 3-6 show multiple sets of simulation results, they lack detailed explanations of the simulation conditions (initial attitude, velocity curve, control parameters, etc.), and some captions do not fully explain the meanings of colors or symbols. Suggest improving the caption information and adding comparative analysis results in the main text.
Answer 3
The presentation of the results has been modified as suggested by the reviewer.
Comment 4
The references are outdated and lack uniformity in their citation format. It is recommended to supplement recent relevant literature and adjust the citation formats to conform uniformly to journal requirements.
Answer 4
The introduction is expanded in order to include recent motivantions and publications for humanoid robotics.
Comment 5
The font size of the text is not uniform in Figure 1.
Answer 5
A new copy of the figure is uploade
Reviewer 2 Report
Comments and Suggestions for AuthorsThis is an interesting kinematic concept for a robotic system with redundant degrees of freedom. Systems of this type have significant potential for use in modular, universal structures.
1. In the article's abstract, the author mentions that the presented concept includes dynamic constraints to improve the energetic efficiency of the generated actions. However, the article itself does not elaborate on the energy efficiency of the system, nor does it provide any arguments confirming the potential for increasing energetic efficiency in this way.
2. The article does not explain why studying the behaviour of a system with this degree of complexity (freedom) is important, nor does it compare its effectiveness to that of other systems.
3. There is also no analysis of how redundant parts of the arm affect the system's stiffness, resistance to vibrations, etc.
4. The overview of the current state of the art and the discussion contain references to outdated literature and do not compare with the latest research trends in the field.
5. There is a significant lack of experiments evaluating the correspondence of the kinematic model with a real cyber-physical system. For this reason, it seems that more specific conclusions regarding the possible use or continuation of research are also absent.
Author Response
Comment 1
In the article's abstract, the author mentions that the presented concept includes dynamic constraints to improve the energetic efficiency of the generated actions. However, the article itself does not elaborate on the energy efficiency of the system, nor does it provide any arguments confirming the potential for increasing energetic efficiency in this way.
Response 1
The quotation by Liu and Ballard (2021) addresses the energetic efficiency of human actions, whose main feature is global smoothness. The quotation by Ackerman (2015) related to industrial robotics explains that smoothness of robot motion induces significant energetic improvements. The proposed model of synergy formation is bio-inspired and based on a global smoothness of the generated synergies.
Comment 2
The article does not explain why studying the behaviour of a system with this degree of complexity (freedom) is important, nor does it compare its effectiveness to that of other systems.
Response 2
The article focuses on the principled coordination of the redundant DOFs of humanoid robots, i.e. on the synergy formation process, not on the specific control that is a function of the actuators. A comment is added in the conclusion. As a matter of fact, the current growing interest on humanoid robotics in the general field of robotic research is motivated by the belief that in general application areas that require flexibility and adaptability, including human-robot cooperation ability, humanoid robots the default paradigm.
Comment 3
There is also no analysis of how redundant parts of the arm affect the system's stiffness, resistance to vibrations, etc.There is also no analysis of how redundant parts of the arm affect the system's stiffness, resistance to vibrations, etc.
Response 3
We suggest that the answer to the previous objection applies also in this case.
Comment 4
The overview of the current state of the art and the discussion contain references to outdated literature and do not compare with the latest research trends in the field.
Response 4
The introduction is expanded in order to include recent motivantions and publications for humanoid robotics.
Comment 5
There is a significant lack of experiments evaluating the correspondence of the kinematic model with a real cyber-physical system. For this reason, it seems that more specific conclusions regarding the possible use or continuation of research are also absent.
Response 5
The concluisions briefly address this point.
Round 2
Reviewer 2 Report
Comments and Suggestions for AuthorsDear Author,
Thank you for revising your work, which has clarified some of the points. Please try to provide more detailed answers to the second and third points in my first review, as this would help to clarify your research.
Author Response
Suggestion by the reviewer. Thank you for revising your work, which has clarified some of the points. Please try to provide more detailed answers to the second and third points in my first review, as this would help to clarify your research.
Comment 2. The article does not explain why studying the behaviour of a system with this degree of complexity (freedom) is important, nor does it compare its effectiveness to that of other systems.
Response 2. The importance of kinematic redundancy in humanoid robotics is motivated by the need of flexibility required by the variety and unpredictability of environmental conditions. The proposed, bio-inspired model of redundancy resolution, in comparison with traditional methods in industrial robotics which typically are based on complex mathematical optimization procedures, is motivated by the following winning features: 1) mathematical simplicity; 2) computational robustness; 3) energetic frugality. The first two features are related to the force-field formulation of goals and constraints which allows the integration of a variety of constraints by the simple addition of the specific force fields. Energetic frugality is the consequence of the intrinsic smoothness of the generated kinematic patterns, in agreement with general characteristics of biological motion.
Comment 3. There is also no analysis of how redundant parts of the arm affect the system's stiffness, resistance to vibrations, etc.
Response 3.
The proposed model of coordination of redundant humanoid robots has no direct effects on the global system stiffness as well as the resistance to vibrations and mechanical perturbations, despite the crucial role of the compliance vector for the shaping of kinematic patterns that distribute the action to all the DoFs of the body model. The elements of such vector express how much a DoF resists to an “ideomotor perturbation”, i.e. the intention to move in a given direction with a given speed profile. The system stiffness is a function of the microcrontrollers which activate the actuators of each DoF, including variable stiffness actuators (Wolf et al., 2016). Moreover, it is possible to integrate the motor cognitive network described in this paper with the motor control mechatronics in such a way to anticipate the physical interaction of the end-effector with the environment or an object/tool to be manipulated with a specific stiffness ellipsoid (Morasso, 2025).
Morasso, P. (2025) A computational model of hybrid trunk-like robots for synergy formation in anticipation of physical interaction. Biomimetics, 10, 21. DOI:10.3390/biomimetics10010021.
Wolf, S. et al. (2016). Variable Stiffness Actuators: Review on Design and Components.IEEE/ASME Trans. on Mechatronics. 21(5), 2418-2430. DOI: 10.1109/TMECH.2015.2501019.
