Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper focuses on the core pain points in trajectory tracking of quadcopter unmanned aerial vehicles, including underactuated strong coupling characteristics, external disturbances (such as gusts), and model uncertainty. The proposed SMC-IESO control method directly responds to actual flight requirements and can solve the problems of weak robustness of traditional PID and "flutter" of standard sliding mode control. It has practical application significance for civilian scenarios such as precision agriculture and logistics distribution, as well as military scenarios such as surveillance.
However, there are still some shortcomings in the paper, and the suggested revisions are as follows:
1.The introduction mentions the application scenarios of quadcopter UAVs (surveillance, aerial photography, etc.), but does not explain the practical problems caused by "insufficient control accuracy and vibration" in specific scenarios (such as pesticide spraying deviation in precision agriculture and landing point errors in logistics distribution). Suggest adding 1-2 scenario based problem descriptions to strengthen the engineering necessity of the research
2. The use of punctuation marks at the end of mathematical formulas in the paper is inconsistent. For example, Equations (1), (3), and (6) end with commas, while Equations (4), (7), and (11) end with periods. Such formatting issues are prevalent throughout the text, which compromises the standardization and rigor of the manuscript's presentation.
3. To avoid raising readers' concerns regarding the completeness of the model, would it be possible to relocate the explanation of the singularity issue—currently in lines 190 and 328—to immediately after the transformation matrix W(η) is defined in Equation (2)? This adjustment would clarify the model's limitations and corresponding solutions at the outset, thereby improving the logical flow of the presentation.
4. It should be noted that the control input u designed in Equation (37) still leads to chattering in the control input, as shown in Figure 9. Although the authors state in line 325 that the sign function has been replaced with the sat function to mitigate chattering, the simulation results demonstrate that this measure fails to achieve the intended effect. This outcome clearly contradicts the claim in the text that "the chattering phenomenon in the control input has been significantly suppressed," and the underlying reasons for this discrepancy warrant further investigation.
5. It should be noted that the ADRC structure typically consists of two components: the controller and the extended state observer (ESO). The authors explicitly state in line 269 that their improvement targets the "Improved Active Disturbance Rejection Sliding Mode Control Law," meaning the optimization is applied specifically to the controller. Furthermore, although the authors mention in line 154 that their designed IESO incorporates an additional parameter kz compared to the conventional ESO, a closer examination reveals that this parameter is introduced in Equation (14) and remains part of the controller optimization, rather than representing an enhancement to the observer structure. However, the subsequent repeated use of the abbreviation "IESO" in the text can easily be misinterpreted as "Improved Extended State Observer," implying modifications to the observer, which creates conceptual confusion.
6.Table 3 provides specific values for IESO gain and error compensation gain, but does not specify the basis for parameter selection - whether it is obtained through "bandwidth optimization method" or "robustness index calculation", or determined through trial and error method? Suggest supplementing the process of parameter design or explaining the experimental procedure for parameter tuning to avoid subjectivity in parameter selection and improve the reproducibility of the method.
7.The current simulation only applies periodic interference and does not cover the common "non periodic sudden interference" (such as instantaneous gusts and load changes) and "composite interference" (such as gusts+modeling errors+actuator delay superposition) in actual flight. Suggest adding simulation comparisons of such scenarios to further validate the robustness of SMC-IESO under extreme uncertainty conditions and avoid limitations in conclusions
8. To enhance the persuasiveness of the paper, it is recommended that the authors supplement the parameter settings of the PID controller and present its control performance in the corresponding figures. This would make the comparative analysis mentioned in line 393 more convincing.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This paper presents a robust trajectory-tracking control method for quadrotor UAVs that integrates Sliding Mode Control with an Improved Extended State Observer (SMC-IESO) in a hierarchical outer-position / inner-attitude architecture. The observer estimates composite disturbances in real time, and an observation-error compensation term inside the sliding law allows using smaller switching gains to mitigate chattering. Lyapunov-based analysis argues closed-loop stability, and MATLAB simulations on a 3D spiral trajectory with periodic force/torque disturbances show accurate tracking and smoother inputs compared with PID and conventional SMC. The paper is well-written and introduces an innovative approach, but the following points could enhance its clarity and impact:This paper presents a robust trajectory-tracking control method for quadrotor UAVs that integrates Sliding Mode Control with an Improved Extended State Observer (SMC-IESO) in a hierarchical outer-position / inner-attitude architecture. The observer estimates composite disturbances in real time, and an observation-error compensation term inside the sliding law allows using smaller switching gains to mitigate chattering. Lyapunov-based analysis argues closed-loop stability, and MATLAB simulations on a 3D spiral trajectory with periodic force/torque disturbances show accurate tracking and smoother inputs compared with PID and conventional SMC. The paper is well-written and introduces an innovative approach, but the following points could enhance its clarity and impact:

The paper does not discuss the computational complexity of the proposed controllers and observers (e.g., per-cycle cost on a representative flight controller, required sampling rates vs. ESO bandwidth), which is essential to assess scalability and real-time applicability.

The method seems designed for single-UAV trajectory tracking with bounded disturbances and moderate attitudes. A discussion on adaptability to other scenarios, such as fault conditions, communication delays, or multi-UAV coordination, would be valuable.

Evaluate robustness to parameter uncertainty and implementation constraints: vary mass/inertia by +/-20%, include actuator saturation and rate limits, and model sensor/actuator delays to show performance margins.

Add a practical tuning workflow: link ESO pole placement and switching gains to sampling rate and disturbance bandwidth; include recommended ranges for boundary-layer width in the sat() function and step-by-step tuning guidance.

Detail sensor and estimator assumptions: specify required IMU/GNSS/vision specs, noise models, and how observer initialization is handled during takeoff; discuss drift mitigation and time-sync requirements.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript titled “Attitude Control of a Quadcopter UAV Using Sliding Mode Control with an Improved Extended State Observer” presents a control scheme combining Sliding Mode Control (SMC) with an Improved Extended State Observer (IESO).

Although the topic is relevant within the field of Systems and Control Engineering, the paper suffers from serious conceptual and methodological flaws that prevent publication in its current form.

1. Lack of Novelty:
   The proposed SMC–IESO structure closely reproduces existing Active Disturbance Rejection Control (ADRC) and SMC hybrids already published in the literature (Han, 2009; Gao, 2003; Li et al., 2022). The so-called “improved ESO” merely adds a proportional correction term without any rigorous theoretical justification or comparative analysis.

2. Weak Theoretical Analysis:
   The Lyapunov proof is incomplete and does not guarantee asymptotic convergence. Several key assumptions (e.g., bound $\epsilon$ in Eq. 27) are introduced without derivation. The results demonstrate only boundedness, not stability.

3. Ambiguous System Architecture:
   Figure 2 and the related text include undefined blocks (\textit{PlayDisposeControl1}, \textit{PlayDisposeControl2}) and duplicated observer loops. The interaction between position and attitude subsystems is not mathematically defined, resulting in unclear control logic.

4. Simulation Design Issues:
   Simulations use idealized conditions with perfect parameter knowledge, no noise, and no actuator limits. Reported RMSE values are inconsistent and in some cases contradict the claimed performance improvements. No experimental or hardware-in-the-loop validation is provided.

5. Redundant Observer Introduction:
   Since SMC inherently compensates matched disturbances, introducing an additional observer subsystem is unnecessary and duplicates functionality. The justification for combining both mechanisms is not convincing.

6. Literature Context and Citations:
   The introduction and related work sections omit several highly relevant recent ADRC and nonlinear observer studies. Consequently, the originality of this contribution is overstated.

In conclusion, the paper repeats known control structures with insufficient novelty, incomplete stability proofs, and unrealistic simulation results. The work does not meet the publication standards of Electronics.

Recommendation: Reject (major conceptual and methodological flaws; novelty and rigor insufficient for publication).

Comments for author File: Comments.pdf

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The reviewer is generally satisfied with the author's response and believes that it can be published in the current version.

Reviewer 3 Report

Comments and Suggestions for Authors

After carefully reviewing the revised version of the manuscript titled “Attitude Control of a Quadcopter UAV Using Sliding Mode Control with an Improved Extended State Observer”, I confirm that the authors have satisfactorily addressed all major concerns raised during the previous review round.

The theoretical framework has been strengthened, the Lyapunov stability analysis is now complete and consistent, and the simulation results are clearer and more coherent with the claimed improvements. The figures and explanations were adequately revised to eliminate previous ambiguities, and the new references appropriately situate the contribution within the state of the art.

In my opinion, the paper has achieved a solid and publishable form. I recommend acceptance, provided the editor agrees, as the manuscript now meets the scientific and editorial standards of Electronics (MDPI).