Design and Gait Control of an Active Lower Limb Exoskeleton for Walking Assistance^†

Round 1

Reviewer 1 Report

It is an interesting study on lower body exoskeleton control and design for walking. Here are some suggestions. 

1. In Figure 3, how are the gain kp and kd obtained? In this control loop, how to decide the optimal exoskeleton assistances?

2. In Eq. (9), how to obtain the ground reaction forces? Directly measured from a treadmill?

3.  What's purpose of this exoskeleton? Reduce metabolic energy or rehabilitation? Can authors quantitatively demonstrate its applications?

4. Only three muscles are not enough for lower body motion analysis. Especially, the tibia muscles are not included. The exoskeletons can reduce some muscle activities but can even increase other muscles' activities on the same time. 

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

The paper presents the architectural design and control of a lower limb exoskeleton for active walking assistance.

Considering the importance of exoskeletons in assistive walking, the paper addresses a very interesting and discussed topic. The presented draft is well structured, the information is delivered in an understandable manner and the study is supported by significant results.

However, there are some aspects that should be addressed in order to properly transmit the information to the audience.

1. I suggest replacing the terms “DOF” and “Freedom Type” from Table 1 with terms that could describe the column better. Even though I can understand what the authors wanted to say with DOF for flexion/ extension, the term is not used with its appropriate meaning. In the same way, Freedom type associated with passive sounds like a contradiction.

2. Figures 4 and 5 should be enlarged for a better understanding. I would also recommend making the continuous line larger than the discontinued one. In Figure 5. (e) and (f), the authors should use a more contrasted palette of colors and figures 6(b) and 6(d) should be placed before subsection 5.2.

I would also recommend to all authors reread the entire paper in order to correct the minor mistakes related to the English language.  

There are some minor mistakes that could be easily corrected. I would recommend a rereading of the paper. 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

This paper introduces the ALEXO walking aid robot. The robot developed by the research group has been published elsewhere. The reviewer needs to find a strong reason to suggest acceptance of this paper. The impact of the contribution presented in this paper is not sufficiently supported by the description of the context and rationale behind developing an innovative exoskeletal robot for walking assistance. The authors must emphasize this contribution's relevance more because otherwise the paper is reduced to a mere technical report of the functioning of an undoubtedly exciting system. 

I, therefore, suggest greater revisions of the contribution before considering acceptance of the work. The reviewer makes the following suggestions:

- the rationale is not strong enough. It is not evident from the literature review that there is a real need to develop in-house exoskeleton systems when several commercial overground and treadmill-based solutions exist. As the contribution is mainly focused on the presentation of design and control, I suggest the authors focus the analysis of the state of the art on these aspects and better highlight what limitations in the literature the ALEXO system manages to overcome in terms of design and control.

- In the state of the art, I recommend the authors cite recent papers about the tailoring of robot-aided rehabilitation also for upper limb and/or the treatment of orthopaedic patients according to their status. Indeed, robot-aided rehabilitation research has to propose control laws that take into account as many patients' parameters to better encode their status and accordingly respond.

- In line 78 the 'mechanical structure' of the robot is defined. I suggest that the authors consider the definition of the mechanical structure in the next section as this is also part of the hardware component of the system. 

- line 120 introduces \tau_M which, from the looks of it, is not the same as \tau_{mot}. Define better what \tau_M is or correct if it is a typo.

- Fig 5 presents the trend of trajectories in joint space as the applied external disturbance varies. The reviewer cannot appreciate any difference when the disturbance contribution varies. An increase in the error of about 0.1 deg is shown in the text. The authors should better discuss this result. Furthermore, in line 123 it is defined that random disturbances can vary between 0-10 N while it is then mentioned that random disturbances are increased from 10 to 30 and 50 N in line 127. The authors need to adjust this.

- Line 161 refers to Fig 5.2. The ref pointing to the section instead of the figure should be wrong...

- In the reviewer's opinion, the analysis of the EMG signal is completely out of context. The authors want to support the use of exo by showing that muscle activation decreases. However, this result is already widely disused in the literature and seems out of the scope of a paper that wants to propose the design and control law for an exoskeleton robot. If the aim of the paper had been to demonstrate that the implemented control strategy contributes to reducing EMG activation then it would be more justified... I suggest the authors think about how to resolve this inconsistency between the objective and content of the paper.

- line 170 defines the threshold for a difference to be considered significant, but neither in the text nor in the figure is any significant difference (evidenced by the p-values). I, therefore, suggest reporting this information.

The English quality is satisfactory.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

It is acceptable now, thanks. 

Reviewer 3 Report

The authors implemented all the suggested revisions. The paper is therefore suitable for publication on Machines journal.