Mechatronic and Robotic Systems Utilizing Pneumatic Artificial Muscles as Actuators

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The article presents two autonomous vehicles a four-legged walking robot and an active ankle-foot orthosis. The article is informative but generally innovative. The main issues are as follows:

1. In section 2.1.2, the article presents a schematic of the artificial muscle hardware system. The article does not mention the control strategy for the four-legged walking robot. Stable walking of the four-legged walking robots involves coordinated movements of multiple joints, and it is important to perform gait planning for the robots. The article provides a brief introduction to the system platform and it fails to present the operational performance of the robots and lacks experimental validation.
2. In section 2.4.1, several important parameters and conclusions regarding the active ankle-foot orthotic device actuated by PAM are not mentioned in this article. For instance, what is the total weight of the system? What types of rehabilitation exercises can be performed? What are the rehabilitation outcomes?

Author Response

1. We sincerely thank the reviewer for the insightful comment and fully agree with the observation. Indeed, the current version of the subsection focuses primarily on the hardware architecture of the quadruped robot actuated by pneumatic artificial muscles and lacks a detailed explanation of the control strategy and operational performance.

The robot is controlled by a program running on the onboard ATmega2560 microcontroller, without the use of feedback loops. Although the robot can be connected to a computer via USB for programming or debugging, the primary mode of communication in autonomous operation is wireless, using a Bluetooth module paired with a mobile device or computer. The robot is controlled through a mobile application that is available on the official Android store. Currently, the robot can operate autonomously for approximately 20 minutes on a full compressed air charge. It is capable of stable locomotion in all four movement directions (forward, backward, left, and right) at an average speed of 0.15 m/s. If the robot loses communication with the controller, it safely halts and remains stationary until the signal is re-established. To achieve stable locomotion, the gait was programmed to emulate a horse-like walking pattern, in which diagonal pairs of legs move synchronously while the other pair remains in contact with the ground. This required careful tuning of the inflation and deflation times of the pneumatic artificial muscles using flow control valves, to synchronize leg movement and ensure dynamic stability. We will revise the manuscript accordingly to provide a more complete description of the robot's gait planning, control strategy, and experimental validation.

Proposal to expand the existing subsection (additional text is included in the revised article):

To address the control strategy, the walking gait of the WRAPAM robot was inspired by the natural movement of quadrupeds, specifically horses. The robot uses a trot gait pattern, in which two diagonal legs move simultaneously while the other two remain grounded to maintain balance. This coordination ensures static stability during walking. Each pneumatic muscle is activated based on pre-programmed timing sequences, without closed-loop feedback, but with precisely adjusted actuation timing using flow control valves to modulate the muscle filling and venting durations. These timings were experimentally tuned to achieve consistent and repeatable gait cycles. The robot’s movement is governed by a program stored on the onboard ATmega2560 microcontroller. The control logic manages the sequence of leg movements and timing of muscle actuation to realize motion in all four directions. The system can be programmed and monitored via USB, but in autonomous mode, Bluetooth communication is used to interface with a mobile application, available on the internet, which serves as the user interface for motion commands. Operational testing demonstrated that the robot could walk continuously for approximately 20 minutes at a speed of 0.15 m/s. The step length was approximately 0.12 m with a gait cycle duration of 1.5 seconds. The gait was stable during forward and backward motion. Repeatability of the gait pattern was confirmed through multiple trials, with a small variation in step timing. The range of motion for each leg joint was approximately 20°, achieved via pneumatic muscle actuation. The robot's ability to recover from minor disturbances was also tested successfully. In the event of a communication failure, the robot automatically stops and remains stationary until the Bluetooth connection is restored. This behaviour enhances operational safety and reliability in mobile robotic tasks.

 

2. We sincerely thank the reviewer for their thoughtful and constructive comment. We fully agree that the original text does not sufficiently address several important parameters related to the active ankle-foot orthosis actuated by pneumatic artificial muscles (PAM), particularly regarding the system’s total weight and its role in rehabilitation.

The orthotic device is constructed primarily from aluminum sheets and ABS plastic, which was selected for its high toughness and strength-to-weight ratio. The metallic components were fabricated by sheet cutting and forming techniques, while the plastic parts were produced using 3D printing to reduce mass. The complete orthosis, including the actuator, valve, and electronics, weighs approximately 1.45 kg, ensuring portability and comfort during use.

The device is designed exclusively to assist patients with foot drop during ambulation and is not intended for use in rehabilitation exercises. One of the primary challenges in wearable pneumatic systems remains the limited capacity of portable compressed air supplies, which constrains long-term use. These details will be added to the revised manuscript for clarity and completeness.

 

Chapter 2.4.1. was supplemented with the following text included in the manuscript:

The orthosis is lightweight (due to plastic and aluminium construction), easy to put on and remove, and provides effective assistance by partially lifting the foot during gait. The structure combines aluminum and ABS due to their strength, durability, and low weight. Plastic components were additively manufactured with internal cavities, while metal parts were shaped mechanically. The complete device, weighing 1.45 kg, includes all essential subsystems. Its low mass ensures user comfort and ease of handling. The system assists foot dorsiflexion during walking, improving gait stability for individuals with foot drop. It is intended solely as a mobility aid, not for rehabilitation. A notable limitation is its reliance on a backpack-mounted compressed air supply, which restricts usage time based on available pressure and power reserves.

Reviewer 2 Report

Comments and Suggestions for Authors

The article presents several mechatronic and robotic systems utilizing pneumatic artificial muscles as actuators, which is meaningful application values in ankle foot orthosis. However, it has to be improved as follows:

1. This article has almost no innovation in terms of theory and methods of soft muscle, which needs to be improved.
2. The introduction on the current research status needs to be improved in detail, for example, references 6-12.
3. The references need to updated, for example, reference 16.

Author Response

We sincerely thank the reviewer for their valuable comments and fully agree with the observations. We agree that the current version of the manuscript lacks sufficient theoretical innovation and an in-depth discussion of recent developments in soft muscle technologies, because the manuscript presents several self-made mechatronic and robotic systems, so we did not go into a deeper elaboration of the characteristics of individual systems.

In response to the first point, we acknowledge that the article primarily focuses on application-oriented implementations and experimental platforms. However, we will enhance the manuscript by including a more comprehensive discussion on control methodologies and modelling challenges associated with pneumatic artificial muscles (PAMs) and highlight areas of potential theoretical contribution within soft robotics.

Regarding the second point, we agree that the introduction needs to be expanded to better reflect the current state of research. We will revise the section to include a more detailed overview of recent advances in modelling, control, and application of PAMs, particularly focusing on the literature cited in references 6–12 and beyond.

Finally, we also agree with the reviewer's third point that some references, such as reference 16, are outdated. We will update the references to include more recent and relevant works that reflect the latest research trends in biologically inspired actuators and their integration in robotic systems.

Revised Introduction (in accordance with reviewer’s comments):

In the evolving field of mechatronics and robotics, there is a growing demand for actuators that combine adaptability, lightweight construction, and human-friendly interaction. Pneumatic artificial muscles (PAMs), originally inspired by the biological structure and function of human muscles, have emerged as a viable alternative to traditional actuators, offering inherent compliance, energy efficiency, and silent operation [1]. These soft, contractile actuators are especially advantageous in applications where safety and interaction with humans are critical, such as wearable assistive devices, robotic limbs, and collaborative robotic platforms [2-5]. Recent advancements in pneumatic artificial muscle (PAM) research have extended beyond basic actuation to encompass sophisticated modelling techniques and control strategies tailored to their highly nonlinear and time-varying dynamics. To address these complexities, researchers have developed adaptive backstepping-sliding mode control frameworks [6], dynamic models for bidirectional PAM actuators [7], and neural network-based control schemes [8] that enhance the system's adaptability and robustness under uncertainty [9]. Additionally, self-organizing fuzzy logic controllers [10], mathematical formulation using operator-based phenomenological scalar hysteresis modelling [11], and model predictive control algorithms [12] have further improved the controllability, responsiveness, and safety of PAM-driven systems. Despite their benefits—such as compliance, low weight, and bioinspired motion—PAMs remain challenging to control due to their inherent nonlinearities, sensitivity to external conditions, and variable performance under dynamic loads. These challenges have led to a growing interest in hybrid control strategies that combine model-based and data-driven methods, incorporating real-time feedback, learning, and prediction mechanisms. PAMs have found applications across a wide spectrum of fields, including orthotic devices, rehabilitation robotics, legged robots, and soft manipulators. Particularly in wearable robotics, such as active ankle-foot orthoses, PAMs offer the potential to deliver biomechanically inspired assistance that mimics natural human movement. Furthermore, their integration into fully soft robotic platforms is enabling the development of autonomous machines capable of adaptive locomotion over unstructured terrain. This article presents a set of experimental mechatronic and robotic systems that utilize PAMs in practical, application-oriented contexts. These include a quadruped walking robot, an active ankle-foot orthosis, and a variety of functional and educational robotic platforms. While the emphasis is placed on the application-level integration of PAMs, the article also discusses the associated design requirements, control architecture, and mechanical implementation aspects.

By showcasing real-world systems powered by PAMs, this work aims to bridge the gap between biologically inspired actuation technologies and their deployment in educational, medical, and service robotics, while acknowledging the need for continued advancement in modelling and control theory for soft actuators.

References 6-12 have been updated as requested by reviewers.

Reference 16 has been updated at the reviewer's request.

Reviewer 3 Report

Comments and Suggestions for Authors

The paper presents several innovative systems based on the deployment of pneumatic muscles. Overall, the novelty of this work is quite limited, and it simply reports the work of students in a research laboratory.
The authors may consider the following comments to improve the paper.
-    Line 72: there are two variants used in the paper: pneumatic artificial muscles (PAMs) and pneumatic muscle actuators (PMAs), respectively. For consistency purposes only one of the two variants should be used.
-    Section 2.2.1. The presentation of the four-legged walking robot is brief. It would be useful to complete this part of the paper with the results that were obtained, characteristic parameters of the motion, etc.
-    Section 2.1.2. Insert a reference to Figure 2 into the text of the paper.
-    Section 2.1.2. It would be of interest to provide information on the system compliance, the influence of hysteresis, the repeatability of reaching certain set positions.
-    Chapter 3 “Results” have no point in this form. The results were presented in Chapter 2. Chapters 3 and 4 should be merged.

Author Response

We sincerely thank the reviewer for his/her constructive and insightful comments. We appreciate the effort invested in reviewing our manuscript and for pointing out areas where improvements could be made. Below we provide detailed responses to each of the comments, along with the corresponding revisions in the manuscript.

Comment 1:

Line 72: there are two variants used in the paper: pneumatic artificial muscles (PAMs) and pneumatic muscle actuators (PMAs), respectively. For consistency purposes only one of the two variants should be used.

Response 1:

We agree with the reviewer that terminology consistency is essential. Throughout the manuscript, we have replaced the term "pneumatic muscle actuators (PMAs)" with "pneumatic artificial muscles (PAMs)" to maintain uniformity and so that this term would be identical to the term in the title of the manuscript. The acronym "PAMs" is now used exclusively in all sections of the paper.

 Comment 2:

Section 2.2.1. The presentation of the four-legged walking robot is brief. It would be useful to complete this part of the paper with the results that were obtained, characteristic parameters of the motion, etc.

Response 2:

Thank you for this suggestion. We have revised Section 2.2.1 to include additional results and characteristic parameters of the quadruped robot’s motion. Specifically, we now provide data on walking speed, duration of operation on a single charge, gait stability, as well as the range of joint motion. We also added information about the trot gait pattern and how it was tuned experimentally to ensure balance and repeatability.

Revised text excerpt (added to Section 2.2.1):

"Operational testing demonstrated that the robot could walk continuously for approximately 20 minutes at a speed of 0.15 m/s. The step length was approximately 0.12 m with a gait cycle duration of 1.5 seconds. The gait was stable during forward and backward motion. Repeatability of the gait pattern was confirmed through multiple trials, with a small variation in step timing. The range of motion for each leg joint was approximately 20°, achieved via pneumatic muscle actuation. The robot's ability to recover from minor disturbances was also tested successfully. In the event of a communication failure, the robot automatically stops and remains stationary until the Bluetooth connection is restored. This behaviour enhances operational safety and reliability in mobile robotic tasks."

Comment 3:

Section 2.1.2. Insert a reference to Figure 2 into the text of the paper.

Response 3:

Thank you for noting this. A reference to Figure 2 has now been added to the appropriate paragraph in Section 2.1.2. The sentence now reads:

"The antagonistic configuration of the PAM actuators, as shown in Figure 2, imitates a biceps-triceps system..."

Comment 4:

Section 2.1.2. It would be of interest to provide information on the system compliance, the influence of hysteresis, the repeatability of reaching certain set positions.

Response 4:

We appreciate this valuable comment. We have updated Section 2.1.2 to include a discussion on system compliance, hysteresis, and repeatability. These aspects are particularly important when using PAMs.

Revised text excerpt (added to Section 2.1.2):

" The system exhibits notable compliance due to the elastic properties of PAMs, which can contribute to safety in human interaction but introduces control challenges. Hysteresis was observed during repeated actuation cycles, with position lag of the total stroke, depending on the actuation pressure range. However, the repeatability of achieving the required joint angles when picking up workpieces was achieved by the feeder design, in which the manipulator arm activated a limiter that allowed the workpieces to fall exactly between the gripper fingers. To mitigate hysteresis effects, tuning of the proportional control valve and the use of feedback from the potentiometer sensor were essential. "

 Comment 5:

Chapter 3 “Results” have no point in this form. The results were presented in Chapter 2. Chapters 3 and 4 should be merged.

Response 5:

We agree with the reviewer’s observation. Chapter 3 has been merged with Chapter 4 into a new section titled “3. Discussion and Results”. This new section integrates the outcome analysis with pedagogical and methodological discussions. Redundant text has been removed, and logical flow has been improved to align with the revised structure.

The rapid advancement of digital technologies, such as the Internet of Things, visual systems, 5G networks, and artificial intelligence, has renewed interest in pneumatic systems. Although often seen as traditional, pneumatic artificial muscles (PAMs) offer advantages like high power-to-weight ratio, mechanical compliance, and simple construction. However, challenges such as nonlinear behavior, response delays due to air compressibility, and reliance on external air sources still limit broader applications [28, 29]. The experimental systems presented in this study, including mobile robots, manipulators, balance systems, and orthotic devices, demonstrate the flexibility and feasibility of PAM-based actuation in both educational and research environments. A range of sensors and control strategies were employed, including PID control, proportional valves, and Matlab/Simulink-based software implementations. Systems such as the ball-on-beam and ball-on-plate highlight complex nonlinear dynamics and provide platforms for applying and comparing advanced control methods. In an educational context, PAM-driven systems support project-based learning by enabling students to connect theory with real-world applications. This hands-on approach promotes critical thinking and helps students understand system dynamics, control models, and the discrepancies between simulations and physical systems. Feedback highlights the value of practical engagement in understanding force, motion, and feedback control concepts.

Despite these benefits, practical issues remain, particularly for mobile applications such as maintaining a reliable air supply and ensuring stable operation under varying conditions. Nevertheless, the developed systems are modular, user-friendly, and easily upgradable, making them well-suited for continued research and development.

Modern educational methodologies increasingly emphasize hands-on learning and interdisciplinary approaches to enhance student engagement and understanding. Educators across the globe acknowledge the importance of the interaction of classical and modern control theory with practical applications and comparative analysis of various control techniques [30]. In the context of pneumatic muscle-powered systems, research has shown that incorporating project-based learning allows students to apply theoretical knowledge to real-world challenges, fostering critical thinking and problem-solving skills. Such studies can contribute to the development of educational kits that use pneumatic muscles to create robotic arms or assistive devices, which will allow students to explore the concepts of force, motion and control in an interactive way. Feedback from students underscores the advantages of laboratory-oriented teaching methodologies, which include a practical demonstration of control systems, activities with different electric and pneumatic components, understanding mathematical models of systems, and recognizing the discrepancies between real-world system operations and theoretical models which are commonly used in simulations. However, challenges remain in such educational processes, such as the complexity of control algorithms and the need for robust operation of highly nonlinear pneumatic systems. There is of course also the problem of ensuring enough compressed air in mobile systems with pneumatic drive. The developed innovative systems driven by pneumatic muscles in most cases contain user-friendly interfaces and allow modification and improvement of the system operation by choosing more advanced components. The integration of sensors and feedback mechanisms can provide students with system state data and a deeper understanding of system dynamics. The research work that precedes the practical implementation of the system includes an extensive review of the relevant literature, outlining the existing applications of pneumatic muscles and their educational impact. A detailed description of the design and implementation of the proposed systems is also necessary, including methodologies for testing and evaluation. Additionally, the analysis of the problem must suggest possible solutions and areas for future research, as well as a thorough investigation of the intersection between pneumatic technology and their actual industrial application.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The topic is not so novel, but the paper is well-written. The problems discussed in this paper are clear, but the content is relatively simple. For these reasons, I suggest accepting the paper with some revisions.

Verifying the effectiveness of the robotic system in its application  is essential. As a result, some experiments of active ankle foot orthosis are supposed to increase.

Author Response

Response to Reviewer’s Comment 1:

We thank the reviewer for this valuable and constructive comment. We agree that a clearer explanation of the control strategy, gait coordination, and experimental validation is essential to understanding the robot’s functionality and capabilities.

We have additionally supplemented subchapter 2.1.1 in accordance with the remarks of the reviewer from the first round of review. To address this concern, we have added a concise but informative extension to the end of subsection 2.1.1. This new paragraph elaborates on the biologically inspired trot gait used in the robot, outlines the open-loop timing-based control approach, and describes the methodology for gait planning and coordination. Furthermore, it provides a summary of preliminary testing conducted to verify basic locomotion functionality, stability during walking, and system robustness under controlled conditions.

In addition to the manuscript revision, we have provided a supplementary video demonstrating the walking behavior of the quadrupedal robot. This video visually illustrates the gait pattern and operational characteristics described in the text, offering further insight into the robot’s locomotion capabilities.

The added text clarifies the operational behavior of the robot and serves as an initial validation of the hardware-software integration. This clarification also sets the stage for future development of closed-loop control and sensor feedback integration.

We hope that these revisions and the supplementary material adequately address the reviewer’s concerns and strengthen the manuscript.

The following paragraph was added at the end of subsection 2.1.1 in the revised manuscript:

“Although the WRAPAM system was introduced primarily as a hardware demonstration platform, particular emphasis was placed on achieving coordinated locomotion using a biologically inspired gait. The selected trot pattern ensures that diagonal leg pairs move simultaneously, which allows the robot to maintain static stability during walking. This approach simplifies the control requirements while preserving a realistic and repeatable gait sequence. The gait cycle was designed through iterative testing, in which activation timings for each pneumatic muscle were manually tuned to achieve smooth transitions between stance and swing phases. Even though the control is open-loop and does not include real-time feedback, the timing parameters were adjusted to synchronize joint motions across all four legs, ensuring that the robot can move in a stable and balanced manner. Initial tests on flat indoor surfaces demonstrated that the robot could maintain continuous locomotion without loss of balance or irregular gait patterns. The robot's ability to recover from small disturbances and to stop safely in case of communication loss further supports the robustness of the implemented strategy. These results serve as a preliminary validation of both the mechanical design and the control approach, and form the basis for future work focused on integrating sensors and developing adaptive, closed-loop gait control.”

Response to Reviewer’s Comment 2:

We thank the reviewer for this important observation. To address this concern, we have expanded subsection 2.4.1 by including a concise description of preliminary experiments performed with healthy volunteers simulating foot drop gait patterns. These tests focused on evaluating the device’s responsiveness to gait phases, timing accuracy of pneumatic muscle activation, and its impact on walking stability and foot clearance during the swing phase. Qualitative results indicated improved dorsiflexion support, contributing to a smoother and more stable gait.

Although the scope of these initial trials was limited, they provide a first validation of the orthosis’s functionality and its potential to assist users effectively. This addition clarifies the current experimental status of the device and strengthens the discussion of its practical applicability.

We hope this revision satisfactorily addresses the reviewer’s comment.

The following text was added at the end of subsection 2.4.1 in the revised manuscript:

“To further confirm the effectiveness of the active ankle-foot orthosis, preliminary experiments were conducted with healthy volunteers simulating foot drop gait patterns. These tests evaluated the device’s responsiveness during different gait phases, focusing on the timing accuracy of pneumatic muscle actuation and its effect on walking stability and foot clearance during the swing phase. Qualitative observations indicated improved dorsiflexion support, contributing to a smoother and more stable gait cycle. Although these initial trials were limited in scope, they demonstrated the orthosis’s ability to assist dorsiflexion effectively and enhance gait stability. This preliminary validation supports the practical functionality of the device in aiding foot drop gait.”

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have made the revisions according to the previous comments, thus this manuscript almost meets the requirements for publication.

Comments on the Quality of English Language

It can be improved to more clearly if possible.

Author Response

Response to Reviewer’s Comment:

We thank the reviewer for the positive feedback and for acknowledging the revisions made according to the previous comments. In response to the suggestion regarding the quality of the English language, we have thoroughly reviewed and significantly improved the grammar and clarity throughout the manuscript. We carefully revised the text to ensure it is more readable and precise.

Additionally, to further clarify the operation of the presented systems, we have provided supplementary video materials demonstrating their functioning. These videos offer a visual explanation that complements the written descriptions in the manuscript.

We hope that these improvements and the supplementary materials meet the journal’s standards and that the manuscript is now acceptable for publication.

Reviewer 3 Report

Comments and Suggestions for Authors

Almost all of my comments were taken into consideration.

Manuscript can be published in its current form.

Author Response

Response to Reviewer’s Comment:

We greatly appreciate the reviewer’s positive and supportive comments, and we are pleased that our revisions have satisfactorily addressed nearly all of your suggestions.

Additionally, we have provided supplementary video materials with the manuscript to further illustrate and explain the operation of the presented systems.

We hope that the manuscript, along with these supporting materials, is now suitable for publication in the journal.

Round 3

Reviewer 1 Report

Comments and Suggestions for Authors

The problems discussed in this paper are clear, I suggest accepting the paper.