The Redesigned Serpens, a Low-Cost, Highly Compliant Snake Robot^†

Round 1

Reviewer 1 Report

The authors have presented a modified design of a low-cost, highly compliant snake robot. They did three additions in their previous design. They clearly discussed the benefits of these additions. The paper lacks to show results that back their design improvements. I have the following comments:

1) Can you please explain how the IMU sensor provides information about the surface on which the robot is moving?

2) It is good to see machine learning algorithms can be applied to this robot. But can you please give more information about what type of data you gather and also the possible input and output parameters?

3) From figure 6, please explain what type of data and what type of commands are used as input.

4) What exactly do you want to signify from the results shown in figure 8?

5) Please show the benefits of your design improvements through experiments or simulated results.

Author Response

Please refer to the attached PDF document.

Author Response File: Author Response.pdf

Reviewer 2 Report

The article deals with current issues of snake-like robots. It is a highly flexible system with many degrees of freedom, so it is able to move in confined spaces and overcome various obstacles. The authors strive for a low-cost and open source solution with high modularity and series-elastic actuators.

These snake-like robots are inspired by wildlife and also have similar properties to the biological pattern of a snake. The introduction presents the application possibilities of such robots and the motivation for creating this work. Relevant references are also provided. A detailed review of related work is also provided.

A detailed description of the proposed robot is described. The designed robot is based on the requirements to be inexpensive, modular, easy customizable, fast to realize. The principle of screw less assembly design is interesting and innovative. The article also deals with the control system and the possibilities of the machine learning approaches application. Experimental results and simulations are also a useful part of the article.The considered elastic actuator is simulated in Gazebo environment.

Comments:

The article has potential and explores an interesting principle of the robot, but I miss a better reworking of this issue. In this state, it reminds me of some master thesis.

The article lacks a mathematical description of the proposed robot solution. A scientific article in this journal should include a kinematic model of such a robot as well as a simulation using this model and then confront the results with real requirements.

Although the chapter is entitled "Experimental results and simulations", no results of real experiments are given here. Only one simulation of one module with an actuator is presented. This is not enough for the robot to assess behavior. It is necessary to implement a simulation with several modules as a redundant mechanism.

The conclusion of the article is very brief and does not state everything necessary. Please specify the basic resulting parameters of the proposed robot model (maximum number of modules used, maximum dimensions, achievable speed and load capacity of the robot, energy balance). There is also a lack of a more detailed discussion of the results of the article. Please specify in detail your own contribution and novelty of your work compared to existing solutions of other snake robots.

I recommend the article for major revision.

Author Response

Please refer to the attached PDF document.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors have addressed the comments in the revised manuscript. It is ready to be published.

Author Response

List of changes in journal paper

The authors would like to thank the reviewers for constructive feedback on our journal paper "The redesigned Serpens, a low-cost, highly compliant snake robot" by Askan Duivon, Pino Kirsch, Boris Mauboussin, Gabriel Mougard, Jakub Woszczyk and Filippo Sanfilippo, submitted to the MDPI Robotics Journal.

Reviewer 1

Comment from reviewer:

The authors have addressed the comments in the revised manuscript. It is ready to be published.

Our response:

Thank you for the constructive comment and for all the previous suggestions.

Reviewer 2 Report

This version of the article is significantly better and redesigned. The missing mathematical model of the robot is added. However, serious results of simulations and experiments are still missing from the article.Serious results are expected in this scientific journal.If there is no real robot yet, then it is necessary to state at least the results of simulations of at least the kinematic model of the robot.

No kinematic model of the robot is presented here. There are several suitable mathematical and simulation methods for solving such redundant kinematic structures. So I don't know why the authors didn't use them.

Based on the results of these simulations, it is then possible to implement mechanical design. The article still does not have new knowledge that would have scientific added value and novelty. I recommend supplementing these missing parts and the article would gain the required scientific contribution.

Author Response

List of changes in journal paper

The authors would like to thank the reviewers for constructive feedback on our journal paper "The redesigned Serpens, a low-cost, highly compliant snake robot" by Askan Duivon, Pino Kirsch, Boris Mauboussin, Gabriel Mougard, Jakub Woszczyk and Filippo Sanfilippo, submitted to the MDPI Robotics Journal. As described in details below, we have made changes to the paper in accordance with the suggestions from each reviewer.

Reviewer 1

Comment from reviewer:

The authors have addressed the comments in the revised manuscript. It is ready to be published.

Our response:

Thank you for the constructive comment and for all the previous suggestions.

Reviewer 2

Comment from reviewer:

This version of the article is significantly better and redesigned. The missing mathematical model of the robot is added. However, serious results of simulations and experiments are still missing from the article.Serious results are expected in this scientific journal.If there is no real robot yet, then it is necessary to state at least the results of simulations of at least the kinematic model of the robot.

No kinematic model of the robot is presented here. There are several suitable mathematical and simulation methods for solving such redundant kinematic structures. So I don't know why the authors didn't use them.

Based on the results of these simulations, it is then possible to implement mechanical design. The article still does not have new knowledge that would have scientific added value and novelty. I recommend supplementing these missing parts and the article would gain the required scientific contribution.

Our response and resulting changes in the paper:

Thank you for giving us the opportunity to clarify this point. As clearly stated in the revised manuscript, the following three contributions are outlined in this work as a whole: the remodelling of the elastic joint with the addition of a damper element; a refreshed design for the screw-less assembly mechanism that can now withstand higher transverse forces; the re-design of the joint module with an improved reorganisation of the internal hardware components to facilitate heat dissipation and to accommodate a larger battery with easier access. The Robot Operating System (ROS) serves as the foundation for the software architecture. The possibility of applying machine learning approaches is considered. The results of preliminary simulations are provided.

In this work, we focus specifically on the design, prototype and simulation of a single module of the snake robot. The mathematical model of the proposed elastic actuator is present in the revised Section 4. Corresponding simulations results are presented in Section 6. 

Regarding the kinematics, as suggested by the reviewer, we have added the following new subsection.

“4.2 Kinematic Model of the Whole Robot

In the literature there are examples of kinematic models developed for snake robots. Noticeable studies are presented in [26, 27]. However, to the best of the authors knowledge, only limited studies exist regarding the kinematic model of a snake robot with series elastic actuators. In [28], the authors present the equations of motion of a modular 2D snake robot with SEAs moving in a vertical plane. The kinematics of such 2D modular snake robot is presented in an efficient matrix form and Euler–Lagrange equations are constructed to model the robot. Furthermore, external contact forces, which are required for simulating pedal wave motion (undulation in the vertical plane), are taken into account using a spring-damper contact model, which, unlike previous approaches, can be utilised to describe the impact of multiple contact points. The pedal wave motion of the robot is simulated using such a contact model, and the torque signal measured by the elastic element from the simulation and experiments is utilised to demonstrate the model's validity. Furthermore, pedal wave locomotion of such robot on uneven terrain is also described, and an adaptive controller based on torque feedback in gait parameter space with optimised control gain is presented. The suggested controller's efficacy is demonstrated in simulation and experimental settings, as the robot successfully climb over a stair-type obstacle without any previous knowledge of its position at a speed of at least 24.8% faster than non-adaptive motion.

Our work focus specifically on the design, prototype and simulation of a single module of the snake robot. The kinematic model of the whole snake is beyond the scope of this paper. We leave this as a future work.”