Negative-Stiffness Structure Vibration-Isolation Design and Impedance Control for a Lower Limb Exoskeleton Robot
Round 1
Reviewer 1 Report
Dear authors! The paper is well-written and carefully organized. A new control eliminating low-frequency vibrations is proposed. The dynamics model of the proposed system was developed, the numerical simulation was carried out confirming the desired properties of the design.
I would recommend adding more technical information on your exoskeleton. Maybe, add some 3D models or technical drawings, making more easy to capture possible weak points of the system andconfirming that the proposed model is relevant. Otherwise, additional joints with actual stiffness (of course, large, but limited) should be taken into account. You can use simplifications given in many classical or recent works. I would recommend an analysis given in a paper "Estimating Natural Frequencies of Cartesian 3D Printer Based on Kinematic Scheme", which is easy to apply to your system.
Also, please, pay attention to possible nonlinearities such as backlash and saturation: are they possible in your NSS and exoskeleton design? It is also wery desired to explain how you avoid possible jamming of the mechanism, for example, when the angle beta is large. Some implementation details such as rail guides and limiters are desired to be given.
Why do you use the proposed NSS design? Please compare it with some existing designs such as one described in recent works, for example, the work "Design and analysis of novel negative stiffness structures with significant energy absorption". Please, add a bit more comparison with alternative NSS designs with argumentation why the proposed one is the most fitting.
One another question is why ball screws are chosen, since this type of mechanism is rather slow.
Also, in fig. 8, it is not shown where forces from motor and human are applied. Please clarify it adding corresponding details in the figure.
The above mentioned papers are welcome to be referenced to. Additional literature references are welcome as well.
Author Response
The authors are very grateful to the reviewers’ critical comments and thoughtful suggestions. Based on the comments and suggestions, the authors have made careful modification on the original manuscript. The following attachment includes the responses to the reviewers’ comments.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The paper is well written. The contribution is clearly presented in the introduction.
Please, could you confirm that the following equations are completed? There is not = symbol in those equations. Equations, (25), (26), (28) and (29)
The main issue that should be improved, in my opinion, are the simulation results. In this application, it should be interesting to show the robustness of the control to human force disturbance. Have you studied that? Depend of the user (human), the mobility can affect to some uncontrolled force than could destabilized the mechanism. Have you considered this in the stability proof.
Finally, more details about the tuning of the system must be provided. Have you chosen the optimal design parameters? Have you used some tuning criteria?
If the authors show/clarify that, the paper can be accepted.
Author Response
The authors are very grateful to the reviewers’ critical comments and thoughtful suggestions. Based on the comments and suggestions, the authors have made careful modification on the original manuscript. The following attachment includes the responses to the reviewers’ comments.
Author Response File: Author Response.pdf
Reviewer 3 Report
This paper presented the negative stiffness structure vibration isolation design and impedance control for a lower limb exoskeleton robot. The authors investigated a SEA structure with NSS and the response of the NSS structure isolation system in the time and frequency domains, analyzed the effect of different characteristic parameters on the isolation system. Generally, this paper was well organized and presented. It could be accepted to future publication if the authors can address some concerns as listed below:
(1) This paper mainly discussed the negative stiffness structure vibration isolation, please describe the value range of negative stiffness for the designed structure.
(2) Some aspects of this paper need to be further modified and improved, Figure 2 and Figure 5(b) need further confirmation.
(3) The amplitude of the knee joint position reaches about 300mm, please explain why it is so large.
(4) The author has carried out simulation research, and strongly suggests supplementing experimental research according to the content of this paper.
Author Response
The authors are very grateful to the reviewers’ critical comments and thoughtful suggestions. Based on the comments and suggestions, the authors have made careful modification on the original manuscript. The following attachment includes the responses to the reviewers’ comments.
Author Response File: Author Response.pdf
Round 2
Reviewer 3 Report
The author has responded according to the modification comments and is currently available for employment.
