Design of a Biomechatronic Device for Upright Mobility in People with SCI Using an Exoskeleton Like a Stabilization System

Round 1

Reviewer 1 Report

The paper presnts a design of an exoskeleton mobile for people with spinal cord injury. The paper need to be written in more scientific way with approporiate analysis. The Figure 2 for methodology should be removed and I don't think it is approporiate for journal paper. Table 4 is for security requirement while it is written user requirement  instead of security requirement. The paper is just a conceptual design of the proposed device without calculations, actuator sizing, and propoer mathematical modelling and analysis. Also, it would be better if the system was implemented and tested to evaulate its performance.

Author Response

Black font: Reviewer comment

Blue font: Authors response

Red font: Text added to the manuscript

 

Responses to the comments of the reviewer #1

1. The paper need to be written in more scientific way with appropriate analysis

Response: thank you for this observation, the paper was re-written following the scientific style.

2. The Figure 2 for methodology should be removed and I don’t think it is approporiate for journal paper.

Response: The figure 2 was removed following the reviewer suggestion and its content was better described at the methodology section L93-L98.

The mechanism was designed considering the state-of-the-art’ challenges and opportunities to establish the design requirements; the anthropometric dimensions, ISO standards, and standard materials dimensions lead to the mechanism CAD modeling. The mechanism kinetics and dynamics were obtained using mathematical modeling and simulation, and CNC machines performed manufacturing.

 

3. Table 4 is for security requirement while it is written user requirement instead of security requirement

Response: The reviewer is totally right, the table labels of table 4 (now table 3) were corrected.

 

4. The paper is just a conceptual design of the proposed device without calculations, actuator sizing, and propoer mathematical modelling and analysis.

Response: A new section was added in 3.6 Drive System Design, which explains in detail the calculations to obtain the ratio of DC electri motor and wheel tire, transmission and bearings (L258 to L306). In the results section, it is presented the characterization of the motor used was added (L307-L318) and finite element simulations for stress and deformation analysis (L319-L338).

The maximum walking speed for a healthy person is 5 kph, therefore, taking into account that 3530 rpm is the speed with the best efficiency of the DC motor, it is considered that at this speed the device operates at a speed of 5kph. To achieve this, the transmission is calculated and designed as shown below:

Ratio of DC electric motor and wheel tire: It has been determined to apply 8 rubber tires (open honeycomb type design), air free, weighing 580 grams each, diameter of 8.5" with the idea of not suffering punctures during the user's mobility, likewise, absorb road irregularities and simplify the ascent/descent of curbs due to its capacity of slight deformation. Four tires are applied on the left section and four on the right section, to maintain a continuous user-ground clearance of 10 cm on all types of surfaces and circumstances. The reduction factor is calculated from the angular velocity of the motor at its maximum efficiency which is 3530 rpm (369.66 rad/s) between the maximum angular velocity of displacement of a person (12.8721 rad/s) whose tangential displacement is 5kph (1.3880 mts/s) with a tire radius of 0.1079 mts. Resulting in a ratio of 28.71. With which it can be guaranteed that the device moves at 5kph using the motor at an angular velocity of 3530rpm where its maximum efficiency is located.

Transmission: The system of reduction and transmission of motion from the motor to the wheel uses a synchronous toothed belt or belt with toothed pulleys made of aluminum and ABS. Due to its low weight and low noise emission (55 dB) approximately, with respect to its counterparts sprockets and steel chains (65 dB). Due to the availability of commercial line components, a reduction of 27:1 is used, instead of the calculated 28.74:1 in (4), by means of three reduction stages, each one with a 3:1 ratio, as follows:

For the first reduction (3,530rpm to 1176.66 rpm) and second reduction (1176.66 rpm to 392.22 rpm) is used: a driven pulley Ø = 0.57" (14.5 mm), 16 teeth, GT3, 3mm Pitch, 9 mm width. The driven pulley Ø = 1.771 (45 mm), 48 teeth, GT3, 3 mm pitch, 9 mm wide. The elements are connected using a Toothed belt GT3, 9 mm wide, 075 teeth, 8.858" (225 mm) with neoprene-nylon cover. For the third reduction (292.22 rpm to 130.7 rpm) the following components  are used: one driven pulley Ø = 1.207" (30.68mm), 20 teeth, GT3, 5 mm pitch, 15 mm wide. One driven pulley Ø = 3.715" (94.36mm), 60 teeth, GT3, 5 mm Pitch, 15 mm width.

Weight: According to the User-Design and Operational requirements, derived from the anthropometric study 50 percentile (male and female), the user's weight is 176.37lb (80Kg), the weight of the device with the mechanical components, electronics and batteries is 114.64lb (52Kg) giving a total weight of 319.67lb (132Kg).

Bearings: a) Loads: Considering specifically the drive shafts N1, N2 and N3 \ref{DriveSystems}, which rotate by means of pulleys and synchronous belts, they are subjected to loads of different magnitude and direction as described below.

Axial load is not present in rotating shafts N1 and N2. There is a momentary and reduced axial load on N3 axis when is performed the mobility of the device.

Radial load is fixed and permanent type by traction of synchronous belts on rotary axes N1, N2 and N3.  The N3 rotating shaft must also support a permanent type load, corresponding to the carrying of a User of 784 N, and including the mobility device of 509.95N = 1,294N (132Kg).

For N1 and N2 shaft a single row deep groove ball bearing with sliding sealing plates is used. d=8, D=22 and B=7, with a Dynamic load capacity of 3,450 N, Static of 1,370 N, Fatigue limit load 57 N and a Nominal Speed with lithium grease lubrication of 75,000 r/min.

For shaft N3, a single row deep groove ball bearing with sliding sealing plates is used. d=10, D=26 and B=8, it has a dynamic load capacity of 4,750 N, a static load capacity of 1,960 N, a fatigue limit load of 83 N and a nominal speed with lithium grease lubrication of 67,000 r/min.

Motor Characterization: The motors were characterized using a Prony Break absorption type dynamometer, it was used a reversible DC electric motor with brushes, Yaegoo brand, model JK-0228 of 24 volts and a power of 150 W at 3800 rpm, with a weight of 3 lb and ball bearings, shown in Figure \ref{figMCa} where A) Dynamometers, B) Digital laser tachometer, C) Power supply, D) Leather belt ½" wide and 1/8" thick, E) Stainless steel cross support with longitudinal holes, F) Milling machine with manual operation, G) CD motor with aluminum pulley.

Seventeen tests were performed varying the force exerted from 0.2lb to 2.74lb on the pulley (moving the z axis of the milling machine (F) where are connected the 2 dynamometers and the leather belt) . The results are shown in Table \ref{tab:MotorCharacterization} obtaining a maximum efficiency in 79.5\% at 3530 rpm and with a power of 106.5 W.

Finite Element Analysis: A finite element study was performed to calculate the stresses and strains of two parts considered critical in the device.  The analysis were carried out using Nastran software in Autodesk Inventor.

A static analysis was performed using the finite element model of the figures \ref{Fem1} and \ref{Fem2}.  The model consists of a total of 16660 nodes and 8545 elements. An isotropic material carbon steel was used and a PSS linear solver was used. The first finite element analysis was performed on the left section part of the drive system structure. A motion constraint was placed on the lower ends of the part. A load of -185 lbf was applied to each of the 2 shafts supporting the 2 parallel bars that support the entire weight of the exoskeleton and the person. It can be seen in the figure \ref{Fem1} that the maximum stress obtained is 652 psi which indicates that it is below the maximum allowable stress of carbon steel. The maximum displacement obtained was 0.008 in.

The second analysis was performed on the parallel support part which is responsible for carrying the weight of the exoskeleton and the user. A motion constraint was placed on each of the lateral ends. Due to the fact that there are 2 parallel bars in the design, only a load of -185lbf (corresponding to half the weight of the exoskeleton and the user) was applied.  It can be seen in the figure \ref{Fem2} that the maximum stress obtained is 3508psi. The maximum displacement is 0.037 in.  This indicates that the part will be operating in the elastic zone of the material and will support the load provided.

Reviewer 2 Report

The paper presents a novel biomechatronic device that resolves mobility necessities for people with spinal cord trauma (SCI) and disability.

The manuscript requires some major modifications.

11. Introduction: The Introduction section is too long.

I propose to reorganize of Introduction. First, you should describe the issues related to the article's topic and then form an article purpose. And you can also add the article structure. This will be useful for the reader to get an idea of ​​the article layout.

References should be more up-to-date. For example, there are several recent articles that you can find:

" Lee, L.-W.; Li, I.-H.; Lu, L.-Y.; Hsu, Y.-B.; Chiou, S.-J.; Su, T.-J. Hardware Development and Safety Control Strategy Design for a Mobile Rehabilitation Robot. Appl. Sci. 2022, 12, 5979. https://doi.org/10.3390/app12125979",

" Yepes, J.C.; Rúa, S.; Osorio, M.; Pérez, V.Z.; Moreno, J.A.; Al-Jumaily, A.; Betancur, M.J. Human-Robot Interaction Torque Estimation Methods for a Lower Limb Rehabilitation Robotic System with Uncertainties. Appl. Sci. 2022, 12, 5529. https://doi.org/10.3390/app12115529",

" Catalan, J.M.; Blanco, A.; Bertomeu-Motos, A.; Garcia-Perez, J.V.; Almonacid, M.; Puerto, R.; Garcia-Aracil, N. A Modular Mobile Robotic Platform to Assist People with Different Degrees of Disability. Appl. Sci. 2021, 11, 7130. https://doi.org/10.3390/app11157130",

”Kumar, S.; Wöhrle, H.; Trampler, M.; Simnofske, M.; Peters, H.; Mallwitz, M.; Kirchner, E.A.; Kirchner, F. Modular Design and Decentralized Control of the Recupera Exoskeleton for Stroke Rehabilitation. Appl. Sci. 2019, 9, 626. https://doi.org/10.3390/app9040626",

and other review articles published in 2022.

The contributions and importance of the paper should be highlighted. What are the main advantages of this article concerning other similar works?

Also, I recommend moving table 2 and figure 1 to another section, Discussions.

22.      Materials and Methods:

First, Figure 2 must be moved and renumbered 1 after the paragraph where it was mentioned.

Then, you must describe your design system idea and maybe something about the control system or the sensory system if it exists. Next, you need to improve the article structure. I recommend that subsections 3.1. up to 3.3. including, be moved to section 2 Methods ..., and renumbered starting with 2.3, 2.4 ..., and subsection 3.4 to become 3.1.

33.   Results

Line 182, element 5J, is not shown in any figure.

In this chapter, I would recommend the Introduction of some simulations of the results presented in section 3.4., Especially since there is also a mathematical model, even a comparison between the simulated results for moving on the straight and inclined ground.

Also, figure formatting should be followed according to MDPI format requirements.

 

44.     Discussions.

To improve the article structure, you can use the adequate traditional structure of papers for the MDPI journal and use it in many other scientific journals.

The results need to be discussed and considered in other studies. Please refer to these studies.

It would be best if you described the limitations of your study

55.    Conclusion:

What are the future works?

What are the main challenges of the current work?

Comments for author File: Comments.docx

Author Response

Black font: Reviewer comment

Blue font: Authors response

Red font: Text added to the manuscript

 

Introduction

1. The Introduction section is too long.

Response: Thank you for the observation, the introduction section was reduced as reviewer suggest.

2. I propose to reorganize of Introduction. First, you should describe the issues related to the article’s topic and then form an article purpose. And you can also add the article structure.

Response: The authors appreciate the reviewer suggestion, the introduction was reorganized following the reviewers’ comments, the article structure was also included L83-L88

The article is structured as follows; first, the user, security, and operative requirements are presented; then, using those requirements, the system design is introduced where sitting-standing, sagittal stability, and coronal stability mechanism are detailed and described; finally, the mathematical modeling, the drive system design. The results of the DC motor characterization and finite element analysis are presented. And finally a comparison of the advantages of this device respect to the prior state-of-the-art is included in this section. 

3. References should be more up-to-date. For example, there are several recent articles that you can find:

Response: Thank you for your kind observation, the references were updated as the reviewer suggest, and new references were added.

4. Tiboni, M.; Borboni, A.; Vérité, F.; Bregoli, C.; Amici, C. Sensors and Actuation Technologies in Exoskeletons: A Review. Sensors 2022, 22, 884.
5. Lee, L.W.; Li, I.H.; Lu, L.Y.; Hsu, Y.B.; Chiou, S.J.; Su, T.J. Hardware Development and Safety Control Strategy Design for a Mobile Rehabilitation Robot. Applied Sciences 2022, 12, 5979. 301
6. Tang, X.; Wang, X.; Ji, X.; Zhou, Y.; Yang, J.; Wei, Y.; Zhang, W. A Wearable Lower Limb Exoskeleton: Reducing the Energy Cost of Human Movement. Micromachines 2022, 13, 900

I recommend moving table 2 and figure 1 to another section

Response: Following the reviewer observation, the table 2 was moved to the results section, in addition, the main advantages of the proposed system were included in the aforementioned table transforming this table into a comparison of this work with the state of art. Figure 1 was modified to better explain the authors intention.

Denomination	Configuration	classification	Main contribution
Exoskeleton like (This work)	Standing / sitting mobility	Reduce mobility and elderly assistance	Sagittal / coronal stability, standing sitting mechanism, solved for inclines
EXO [10]	Active hip and knee joints. Passive in the ankle	Reduced mobility. Assistance and rehab.	15 Kg weight, FRF when changing CG when stationary/walking without manual control.
EXO [11]	Knee-ankle-foot active orthosis made of carbon fiber with interconnected steel segments.	Reduced mobility and elderly assistance.	3 Kg weight, automatic ascent/descent handlebars for sitting/standing.
HAL-5 [12]	Segmented exoskeleton with hip, knee, and ankle joints.	Adults with total or partial paraplegia.	15 kg including batteries and control processor. No manual control is required for activation by applying PD-controlled healthy walking patterns.

 

Materials and Methods

5. First, Figure 2 must be moved and renumbered 1 after the paragraph where it was mentioned.

Response: The figure 2 was removed and its content was better described at the methodology section L93-L98.

The mechanism was designed considering the state-of-the-art’ challenges and opportunities to establish the design requirements; the anthropometric dimensions, ISO standards, and standard materials dimensions lead to the mechanism CAD modeling. The mechanism kinetics and dynamics were obtained using mathematical modeling and simulation, and CNC machines performed manufacturing.

6. You must describe your design system idea and maybe something about the control system or the sensory system if it exists. Next, you need to improve the article structure. I recommend that subsections 3.1. up to 3.3. including, be moved to section 2 Methods ..., and renumbered starting with 2.3, 2.4 ..., and subsection 3.4 to become 3.1

Response: The structure of the article was modified.  The information was organized into:

1) Introduction

2) Materials and methods (Design requirements, anthropometry).

3) System design (Mechatronic system design, standing mechanism, mobility motor system, mobility in an urban environment, mathematical model, drive system design,

4) Results (motor characterization, finite element analysis)

5) Discussion

6) Conclusions

 

Results

8. In this chapter, I would recommend the Introduction of some simulations of the results presented in section 3.4., Especially since there is also a mathematical model, even a comparison between the simulated results for moving on the straight and inclined ground.

Response:  A new section was added in 3.6 Drive System Design, which explains in detail the calculations to obtain the ratio of DC electric motor and wheel tire, transmission and bearings (L255 to L305). In the results section, it is presented the characterization of the motor used was added (L306-L315) and finite element simulations for stress and deformation analysis (L316-L335).

The maximum walking speed for a healthy person is 5 kph, therefore, taking into account that 3530 rpm is the speed with the best efficiency of the DC motor, it is considered that at this speed the device operates at a speed of 5kph. To achieve this, the transmission is calculated and designed as shown below:

Ratio of DC electric motor and wheel tire: It has been determined to apply 8 rubber tires (open honeycomb type design), air free, weighing 580 grams each, diameter of 8.5" with the idea of not suffering punctures during the user's mobility, likewise, absorb road irregularities and simplify the ascent/descent of curbs due to its capacity of slight deformation. Four tires are applied on the left section and four on the right section, to maintain a continuous user-ground clearance of 10 cm on all types of surfaces and circumstances. The reduction factor is calculated from the angular velocity of the motor at its maximum efficiency which is 3530 rpm (369.66 rad/s) between the maximum angular velocity of displacement of a person (12.8721 rad/s) whose tangential displacement is 5kph (1.3880 mts/s) with a tire radius of 0.1079 mts. Resulting in a ratio of 28.71. With which it can be guaranteed that the device moves at 5kph using the motor at an angular velocity of 3530rpm where its maximum efficiency is located.

Transmission: The system of reduction and transmission of motion from the motor to the wheel uses a synchronous toothed belt or belt with toothed pulleys made of aluminum and ABS. Due to its low weight and low noise emission (55 dB) approximately, with respect to its counterparts sprockets and steel chains (65 dB). Due to the availability of commercial line components, a reduction of 27:1 is used, instead of the calculated 28.74:1 in (4), by means of three reduction stages, each one with a 3:1 ratio, as follows:

For the first reduction (3,530rpm to 1176.66 rpm) and second reduction (1176.66 rpm to 392.22 rpm) is used: a driven pulley Ø = 0.57" (14.5 mm), 16 teeth, GT3, 3mm Pitch, 9 mm width. The driven pulley Ø = 1.771 (45 mm), 48 teeth, GT3, 3 mm pitch, 9 mm wide. The elements are connected using a Toothed belt GT3, 9 mm wide, 075 teeth, 8.858" (225 mm) with neoprene-nylon cover. For the third reduction (292.22 rpm to 130.7 rpm) the following components  are used: one driven pulley Ø = 1.207" (30.68mm), 20 teeth, GT3, 5 mm pitch, 15 mm wide. One driven pulley Ø = 3.715" (94.36mm), 60 teeth, GT3, 5 mm Pitch, 15 mm width.

Weight: According to the User-Design and Operational requirements, derived from the anthropometric study 50 percentile (male and female), the user's weight is 176.37lb (80Kg), the weight of the device with the mechanical components, electronics and batteries is 114.64lb (52Kg) giving a total weight of 319.67lb (132Kg).

Bearings: a) Loads: Considering specifically the drive shafts N1, N2 and N3 \ref{DriveSystems}, which rotate by means of pulleys and synchronous belts, they are subjected to loads of different magnitude and direction as described below.

Axial load is not present in rotating shafts N1 and N2. There is a momentary and reduced axial load on N3 axis when is performed the mobility of the device.

Radial load is fixed and permanent type by traction of synchronous belts on rotary axes N1, N2 and N3.  The N3 rotating shaft must also support a permanent type load, corresponding to the carrying of a User of 784 N, and including the mobility device of 509.95N = 1,294N (132Kg).

 

For N1 and N2 shaft a single row deep groove ball bearing with sliding sealing plates is used. d=8, D=22 and B=7, with a Dynamic load capacity of 3,450 N, Static of 1,370 N, Fatigue limit load 57 N and a Nominal Speed with lithium grease lubrication of 75,000 r/min.

For shaft N3, a single row deep groove ball bearing with sliding sealing plates is used. d=10, D=26 and B=8, it has a dynamic load capacity of 4,750 N, a static load capacity of 1,960 N, a fatigue limit load of 83 N and a nominal speed with lithium grease lubrication of 67,000 r/min.

Motor Characterization: The motors were characterized using a Prony Break absorption type dynamometer, it was used a reversible DC electric motor with brushes, Yaegoo brand, model JK-0228 of 24 volts and a power of 150 W at 3800 rpm, with a weight of 3 lb and ball bearings, shown in Figure \ref{figMCa} where A) Dynamometers, B) Digital laser tachometer, C) Power supply, D) Leather belt ½" wide and 1/8" thick, E) Stainless steel cross support with longitudinal holes, F) Milling machine with manual operation, G) CD motor with aluminum pulley.

Seventeen tests were performed varying the force exerted from 0.2lb to 2.74lb on the pulley (moving the z axis of the milling machine (F) where are connected the 2 dynamometers and the leather belt) . The results are shown in Table \ref{tab:MotorCharacterization} obtaining a maximum efficiency in 79.5\% at 3530 rpm and with a power of 106.5 W.

Finite Element Analysis: A finite element study was performed to calculate the stresses and strains of two parts considered critical in the device.  The analysis were carried out using Nastran software in Autodesk Inventor.

A static analysis was performed using the finite element model of the figures \ref{Fem1} and \ref{Fem2}.  The model consists of a total of 16660 nodes and 8545 elements. An isotropic material carbon steel was used and a PSS linear solver was used. The first finite element analysis was performed on the left section part of the drive system structure. A motion constraint was placed on the lower ends of the part. A load of -185 lbf was applied to each of the 2 shafts supporting the 2 parallel bars that support the entire weight of the exoskeleton and the person. It can be seen in the figure \ref{Fem1} that the maximum stress obtained is 652 psi which indicates that it is below the maximum allowable stress of carbon steel. The maximum displacement obtained was 0.008 in.

 

The second analysis was performed on the parallel support part which is responsible for carrying the weight of the exoskeleton and the user. A motion constraint was placed on each of the lateral ends. Due to the fact that there are 2 parallel bars in the design, only a load of -185lbf (corresponding to half the weight of the exoskeleton and the user) was applied.  It can be seen in the figure \ref{Fem2} that the maximum stress obtained is 3508psi. The maximum displacement is 0.037 in.  This indicates that the part will be operating in the elastic zone of the material and will support the load provided.

 

9. The results need to be discussed and considered in other studies. Please refer to these studies

Response: It was added a new section called “5. Discussions” (L339) where is presented a comparison table between this work and other studies.

This research has achieved a technological development based on a thorough study of the real and important mobility needs of people with SCI. Simplicity in the design was one of the great challenges and goals of the project. The generation of a simple mechanical model makes it possible to develop technology that is within the reach of most people. Making the system easier to manufacture and replicate. Upright mobility presents several difficulties when the device is used on the streets, where slopes, edges, potholes and other obstacles can generate a stability problem. Therefore, a mechanism was designed in which the user can always maintain an upright position, without tilting, making the rest of the mechanism adapt to maintain the user's posture.

 

Table \ref{tab:state_of_the_art} presents a comparison of the main contributions of some exosqueletons available in the state-of-the-art of different researches where technological developments related to the issue of upright mobility in people with lower limb disabilities have been carried out.

 

Conclusion

11. What are the future works?

Response:  (L364-L367) comments related to future work were added

As future work, we consider evaluating the system under an experimental protocol with people who have SCI, endorsed by a bioethics committee and medical specialists, which will allow us to obtain relevant information about the performance of the device.

 

Reviewer 3 Report

This paper proposes a mechanical device design for spinal cord trauma patients. The design is guided by specific requirements established by the user, security, and operative approaches.

 

I consider it necessary to include some corrections, declarations, and additions to the current paper version to improve the impact of this research work.

 

1. The introduction has taken 5 out of 19 pages. This introduction is taken too much space from the complete paper (26%). This rate could be improved through a better explanation of the design and results sections.

 

2. In Figure 2. "windows of opportunities" should be replaced by State of the art challenges and opportunities. ("windows opportunities" does not have the same meaning as the authors try to refer to in Spanish)

 

3. The title on Table 4 should be "Security Requirements" instead of "User requirements." 

 

4. Section 3 has the Title of "Results."

This section shows the system design, and the design section is forgotten.

 

I advise you to call section 3 "System Design"  and create a new section (4) called "Results"  In this new section, the authors should focus on the analysis of the results taking into account the requirements named on the tables 3, 4, and 5

 

5. An study and analysis of the mechanical effort should be included across the complete body of the designed system.

 

6. Another study could be the stability analysis on perturbation forces to establish the range of the perturbation allowed with stability control reached.

     

Author Response

Black font: Reviewer comment

Blue font: Authors response

Red font: Text added to the manuscript

1. The introduction has taken 5 out of 19 pages. This introduction is taken too much space from the complete paper (26%). This rate could be improved through a better explanation of the design and results sections.

Response: The reviewer observation is highly appreciated, the introduction was changed accordingly and reduced, the article structure was included at this section.

2. In Figure 2. “windows of opportunities” should be replaced by State of the art challenges and opportunities. (“windows opportunities” does not have the same meaning as the authors try to refer to in Spanish)

Response: The figure 2 was removed and its content was better described at the methodology section L91-L96.

The mechanism was designed considering the state-of-the-art’ challenges and opportunities to establish the design requirements; the anthropometric dimensions, ISO standards, and standard materials dimensions lead to the mechanism CAD modeling. The mechanism kinetics and dynamics were obtained using mathematical modeling and simulation, and CNC machines performed manufacturing.

 

3. The title on Table 4 should be “Security Requirements” instead of “User requirements.”

Response: The authors apologize for the mistake; the table title was corrected.

4. Section 3 has the Title of “Results.” This section shows the system design, and the design section is forgotten.

Response: The authors apologize for the mistake; the title of this section was corrected and named the “System design”.

 

5. I advise you to call section 3 “System Design” and create a new section (4) called “Results”  In this new section, the authors should focus on the analysis of the results taking into account the requirements named on the tables 3, 4, and 5

Response: The authors thank the reviewer advise, the section 3 was named “System design” and a new section called “Results” was created

3. System Design 

3.1. Design of the Mechatronic System

4. Results

An study and analysis of the mechanical effort should be included across the complete body of the designed system.

Response: A new section was added in 3.6 Drive System Design, which explains in detail the calculations to obtain the ratio of DC electric motor and wheel tire, transmission and bearings (L255 to L303). In the results section, it is presented the characterization of the motor used was added (L304-L315) and finite element simulations for stress and deformation analysis (L316-L335).

The maximum walking speed for a healthy person is 5 kph, therefore, taking into account that 3530 rpm is the speed with the best efficiency of the DC motor, it is considered that at this speed the device operates at a speed of 5kph. To achieve this, the transmission is calculated and designed as shown below:

Ratio of DC electric motor and wheel tire: It has been determined to apply 8 rubber tires (open honeycomb type design), air free, weighing 580 grams each, diameter of 8.5" with the idea of not suffering punctures during the user's mobility, likewise, absorb road irregularities and simplify the ascent/descent of curbs due to its capacity of slight deformation. Four tires are applied on the left section and four on the right section, to maintain a continuous user-ground clearance of 10 cm on all types of surfaces and circumstances. The reduction factor is calculated from the angular velocity of the motor at its maximum efficiency which is 3530 rpm (369.66 rad/s) between the maximum angular velocity of displacement of a person (12.8721 rad/s) whose tangential displacement is 5kph (1.3880 mts/s) with a tire radius of 0.1079 mts. Resulting in a ratio of 28.71. With which it can be guaranteed that the device moves at 5kph using the motor at an angular velocity of 3530rpm where its maximum efficiency is located.

Transmission: The system of reduction and transmission of motion from the motor to the wheel uses a synchronous toothed belt or belt with toothed pulleys made of aluminum and ABS. Due to its low weight and low noise emission (55 dB) approximately, with respect to its counterparts sprockets and steel chains (65 dB). Due to the availability of commercial line components, a reduction of 27:1 is used, instead of the calculated 28.74:1 in (4), by means of three reduction stages, each one with a 3:1 ratio, as follows:

For the first reduction (3,530rpm to 1176.66 rpm) and second reduction (1176.66 rpm to 392.22 rpm) is used: a driven pulley Ø = 0.57" (14.5 mm), 16 teeth, GT3, 3mm Pitch, 9 mm width. The driven pulley Ø = 1.771 (45 mm), 48 teeth, GT3, 3 mm pitch, 9 mm wide. The elements are connected using a Toothed belt GT3, 9 mm wide, 075 teeth, 8.858" (225 mm) with neoprene-nylon cover. For the third reduction (292.22 rpm to 130.7 rpm) the following components  are used: one driven pulley Ø = 1.207" (30.68mm), 20 teeth, GT3, 5 mm pitch, 15 mm wide. One driven pulley Ø = 3.715" (94.36mm), 60 teeth, GT3, 5 mm Pitch, 15 mm width.

Weight: According to the User-Design and Operational requirements, derived from the anthropometric study 50 percentile (male and female), the user's weight is 176.37lb (80Kg), the weight of the device with the mechanical components, electronics and batteries is 114.64lb (52Kg) giving a total weight of 319.67lb (132Kg).

Bearings: a) Loads: Considering specifically the drive shafts N1, N2 and N3 \ref{DriveSystems}, which rotate by means of pulleys and synchronous belts, they are subjected to loads of different magnitude and direction as described below.

Axial load is not present in rotating shafts N1 and N2. There is a momentary and reduced axial load on N3 axis when is performed the mobility of the device.

Radial load is fixed and permanent type by traction of synchronous belts on rotary axes N1, N2 and N3.  The N3 rotating shaft must also support a permanent type load, corresponding to the carrying of a User of 784 N, and including the mobility device of 509.95N = 1,294N (132Kg).

 

For N1 and N2 shaft a single row deep groove ball bearing with sliding sealing plates is used. d=8, D=22 and B=7, with a Dynamic load capacity of 3,450 N, Static of 1,370 N, Fatigue limit load 57 N and a Nominal Speed with lithium grease lubrication of 75,000 r/min.

For shaft N3, a single row deep groove ball bearing with sliding sealing plates is used. d=10, D=26 and B=8, it has a dynamic load capacity of 4,750 N, a static load capacity of 1,960 N, a fatigue limit load of 83 N and a nominal speed with lithium grease lubrication of 67,000 r/min.

Motor Characterization: The motors were characterized using a Prony Break absorption type dynamometer, it was used a reversible DC electric motor with brushes, Yaegoo brand, model JK-0228 of 24 volts and a power of 150 W at 3800 rpm, with a weight of 3 lb and ball bearings, shown in Figure \ref{figMCa} where A) Dynamometers, B) Digital laser tachometer, C) Power supply, D) Leather belt ½" wide and 1/8" thick, E) Stainless steel cross support with longitudinal holes, F) Milling machine with manual operation, G) CD motor with aluminum pulley.

Seventeen tests were performed varying the force exerted from 0.2lb to 2.74lb on the pulley (moving the z axis of the milling machine (F) where are connected the 2 dynamometers and the leather belt) . The results are shown in Table \ref{tab:MotorCharacterization} obtaining a maximum efficiency in 79.5\% at 3530 rpm and with a power of 106.5 W.

Finite Element Analysis: A finite element study was performed to calculate the stresses and strains of two parts considered critical in the device.  The analysis were carried out using Nastran software in Autodesk Inventor.

A static analysis was performed using the finite element model of the figures \ref{Fem1} and \ref{Fem2}.  The model consists of a total of 16660 nodes and 8545 elements. An isotropic material carbon steel was used and a PSS linear solver was used. The first finite element analysis was performed on the left section part of the drive system structure. A motion constraint was placed on the lower ends of the part. A load of -185 lbf was applied to each of the 2 shafts supporting the 2 parallel bars that support the entire weight of the exoskeleton and the person. It can be seen in the figure \ref{Fem1} that the maximum stress obtained is 652 psi which indicates that it is below the maximum allowable stress of carbon steel. The maximum displacement obtained was 0.008 in.

The second analysis was performed on the parallel support part which is responsible for carrying the weight of the exoskeleton and the user. A motion constraint was placed on each of the lateral ends. Due to the fact that there are 2 parallel bars in the design, only a load of -185lbf (corresponding to half the weight of the exoskeleton and the user) was applied.  It can be seen in the figure \ref{Fem2} that the maximum stress obtained is 3508psi. The maximum displacement is 0.037 in.  This indicates that the part will be operating in the elastic zone of the material and will support the load provided.

 

Round 2

Reviewer 2 Report

The article has been clearly improved.

I have only one observation to make, namely: in Section 5 Discussions, enter at the end of it some things about the Limitations of This Study.

Author Response

Response: The authors thank the reviewer’s advice; this work's limitations have been included; future work was also included in the revised text. 

(Line 350-356) Limitations of this study. At this time, the main components have been tested separately, and the tests have been limited to stress analysis and spatial stability; however, users have not tested the system, nor has the scalability in the manufacturing processes been verified. The feasibility of manufacturing the proposed system must be verified according to the industrial design. The duration and wear of the parts susceptible to wear must be checked. Users should test the system in a suitable environment close to activities of daily living.  

Reviewer 3 Report

Thank you for considering the comments and making the required additions and corrections.

Author Response

work's limitations have been included; future work was also included in the revised text. 

(Line 350-356) Limitations of this study. At this time, the main components have been tested separately, and the tests have been limited to stress analysis and spatial stability; however, users have not tested the system, nor has the scalability in the manufacturing processes been verified. The feasibility of manufacturing the proposed system must be verified according to the industrial design. The duration and wear of the parts susceptible to wear must be checked. Users should test the system in a suitable environment close to activities of daily living.