Design and Implementation of Flexible Four-Bar-Mechanism-Based Long-Stroke Micro-Gripper
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsDear Authors,
Congratulations on your work, the following are my comments and suggestions aimed to contribute with the manuscript quality.
This paper indicates a design and implementation of a micro-gripper capable of achieving large-stroke displacement amplification (in the millimeter range) combined with submicron resolution, using a compliant four-bar mechanism driven by a piezoelectric actuator. The authors aim to overcome the traditional limitation of small strokes in micro-grippers while maintaining precision and compactness. The topic is relevant and addresses an important gap. Most existing micro-grippers struggle to combine large displacement amplification and high resolution in a compact form factor.
This work contributes to:
- A new configuration of a compliant four-bar amplification mechanism with a higher amplification ratio and compact size.
- A combination of theoretical modeling, finite element simulations, and experimental validation demonstrates consistent results across all stages.
- Demonstration of a dual-mode (force and displacement) control strategy.
Points to improve:
- The manuscript would benefit from clarifying the selection criteria and justification for specific geometric parameters of the four-bar mechanism (e.g., hinge positions and dimensions) beyond stating the values used. A sensitivity analysis of these parameters could strengthen the design validation.
- While the simulations include modal analysis, the manuscript should discuss how vibration modes might impact practical operation, especially under dynamic loads.
- The control strategy for switching between displacement and force modes could be elaborated (e.g., control loop parameters, bandwidth), as it is a key practical innovation.
- The gripping force experiments mostly used soft objects; additional tests with harder or fragile objects could illustrate broader applicability.
- Figures are generally clear and informative. However, Figures 3, 4, 5, and 6 could be improved by increasing font size in legend labels for better readability.
- In Section 2 (“Structural Design of the Micro-Gripper”), there is no introductory paragraph before Subsection 2.1. It would improve readability and clarity if the authors included a brief overview paragraph describing the purpose and organization of this section before presenting the detailed design steps. This helps guide the reader and provides context for the subsections. Also, rename Subsection 2.1 since it is the same as Section 2.
- Similarly, several sections were created without an introductory text before their subsections. Please, fix it for the entire document.
- minor typographical errors, such as spacing inconsistencies. Line 13, 117-118, 118, 142, 161, and some sections (e.g., 5.4.Gripping > 5.4. Gripping). Please fix it, looking from the top to the bottom.
- Throughout the manuscript, the citation style is inconsistent. Some references are indicated with superscript numbers, others with bracketed numbers, and in some cases, citations appear embedded in the text without clear formatting. It is recommended that all citations be standardized to a consistent format, such as [1], following the journal’s guidelines.
- Additionally, there are many occurrences where words are split across lines without hyphenation or with unnecessary hyphens. The authors should carefully review the text to correct these line breaks and ensure proper word wrapping and hyphenation throughout.
Line 221 and 224: Where are Fig. 5(a) and Fig. 5(b)? Should it be Figure 5(a) and Figure 5(b)?
Some questions to better discuss the work:
- What are the practical constraints in integrating this micro-gripper into a larger micro-assembly system or biomedical application?
- Could the proposed design be adapted to support multi-degree-of-freedom motion or only pure linear gripping?
- What are the advantages and disadvantages of using piezoelectric actuation compared to electrothermal or electromagnetic actuators in this context?
Best regards.
Author Response
|
Comments 1: [The manuscript would benefit from clarifying the selection criteria and justification for specific geometric parameters of the four-bar mechanism (e.g., hinge positions and dimensions) beyond stating the values used. A sensitivity analysis of these parameters could strengthen the design validation..]
|
|
Response 1: Thank you for pointing this out. We agree with this comment. Therefore, we have briefly explained the selection criteria and justification for specific geometric parameters of the four-bar mechanism, which are marked in red in the revised manuscript . |
|
Comments 2: [While the simulations include modal analysis, the manuscript should discuss how vibration modes might impact practical operation, especially under dynamic loads.] |
|
Response 2: Agree. We have discussed how vibration modes might impact practical operation, which are marked in red in the revised manuscript.]
|
|
Comments 3: [The control strategy for switching between displacement and force modes could be elaborated (e.g., control loop parameters, bandwidth), as it is a key practical innovation.]
|
|
Response 3: Thank you for pointing this out.We agree with this comment. The force-displacement closed-loop control and switching control strategy will be discussed comprehensively in another paper. |
|
Comments 4: [The gripping force experiments mostly used soft objects; additional tests with harder or fragile objects could illustrate broader applicability.]
|
|
Response 4: Thank you for pointing this out.We agree with this comment. The same to the last response 3, more grasping experiments and control strategy will be discussed comprehensively in the next paper. |
|
Comments 5: [Figures are generally clear and informative. However, Figures 3, 4, 5, and 6 could be improved by increasing font size in legend labels for better readability.]
|
|
Response 5: Thank you for pointing this out. We agree with this comment. Therefore, we have replaced the mentioned figures by increasing font size in legend labels. |
|
Comments 6: [In Section 2 (“Structural Design of the Micro-Gripper”), there is no introductory paragraph before Subsection 2.1. It would improve readability and clarity if the authors included a brief overview paragraph describing the purpose and organization of this section before presenting the detailed design steps. This helps guide the reader and provides context for the subsections. Also, rename Subsection 2.1 since it is the same as Section 2.]
|
|
Response 6: Thank you for pointing this out. We agree with this comment. Therefore, we have added a brief overview paragraph describing the purpose and organization, and the title of Section 2.has been modified. |
|
Comments 7: [Similarly, several sections were created without an introductory text before their subsections. Please, fix it for the entire document.] |
|
Response 7: Thank you for pointing this out. We agree with this comment. Therefore, we have added brief overview paragraph for Section 5, shown in red in the revised manuscript. |
|
Comments 8: [minor typographical errors, such as spacing inconsistencies. Line 13, 117-118, 118, 142, 161, and some sections (e.g., 5.4.Gripping > 5.4. Gripping). Please fix it, looking from the top to the bottom.] |
|
Response 8: Thank you for pointing this out. We agree with this comment. Therefore, we have modified the typographical errors in the revised manuscript. |
|
Comments 9: [Throughout the manuscript, the citation style is inconsistent. Some references are indicated with superscript numbers, others with bracketed numbers, and in some cases, citations appear embedded in the text without clear formatting. It is recommended that all citations be standardized to a consistent format, such as [1], following the journal’s guidelines.]
|
|
Response 9: Thank you for pointing this out. We agree with this comment. Therefore, I/we have standardized the citation format throughout the manuscript in accordance with the reviewers' comments, adopting the style recommended by the journal guidelines, and have updated several outdated references. |
|
Comments 10: [Additionally, there are many occurrences where words are split across lines without hyphenation or with unnecessary hyphens. The authors should carefully review the text to correct these line breaks and ensure proper word wrapping and hyphenation throughout.]
|
|
Response 10: Thank you for pointing this out. We agree with this comment. Therefore, we have modified the hyphenation errors in the revised manuscript. |
|
Comments 11: [Line 221 and 224: Where are Fig. 5(a) and Fig. 5(b)? Should it be Figure 5(a) and Figure 5(b)?] |
|
Response 11: Thank you for pointing this out. Fig. 5(a) and Fig. 5(b)should it be Figure 5(a) and Figure 5(b), we have modified the these errors in the revised manuscript. |
|
Comments 12: [What are the practical constraints in integrating this micro-gripper into a larger micro-assembly system or biomedical application?] |
|
Response 12: Integrating this micro-gripper into real-world systems faces key challenges: (1) Precision-stability trade-offs in dynamic environments require advanced vibration isolation and thermal compensation. (2) Biomedical applications demand sterilizable materials and compliant force control for delicate tissues. (3) Industrial micro-assembly needs standardized interfaces for automation compatibility and parallel operation. (4) Piezoelectric actuator limitations (hysteresis, high-voltage needs) and sensor calibration complexity impact long-term reliability. Addressing these requires co-optimized hardware-control solutions tailored to specific use cases. |
|
Comments 13: [Could the proposed design be adapted to support multi-degree-of-freedom motion or only pure linear gripping?]
|
|
Response 13: The current design provide only pure linear gripping motion. |
|
Comments 14: [What are the advantages and disadvantages of using piezoelectric actuation compared to electrothermal or electromagnetic actuators in this context?] |
|
Response 14: Piezoelectric actuators offer superior precision (sub-50 nm) and fast response (kHz range), making them ideal for high-accuracy micro-gripping, but require high-voltage drivers and suffer from hysteresis. Electrothermal actuators provide larger strokes at low voltage but are slow and generate problematic heat, while electromagnetic systems are simpler but bulkier and less precise. The optimal choice depends on the specific needs for precision, speed, force, and stroke in the application. |
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsThe paper presents the design and implementation of a microgripper with a flexible four-stage mechanism based on a piezoelectric actuator. The work successfully combines theoretical analysis, numerical simulations and experimental verification of the microgripper design with significantly improved motion amplification parameters.
Main strengths:
• Systematic approach from theoretical design through simulations to experimental verification
• Achieving high amplification factor (40×) and nanometer accuracy (50 nm)
• Comprehensive characterization of mechanical properties using ANSYS simulations
• Practical implementation with experimental verification
Key shortcomings:
• Missing comparison with existing commercial solutions
• Insufficient discussion of long-term reliability and wear
• Limited analysis of thermal effects on system accuracy
DETAILED COMMENTS
The authors used an appropriate combination of analytical, numerical and experimental methods. The kinematic analysis of the four-stage mechanism is mathematically correct (equations 1-14). The ANSYS simulations are appropriately parameterized with realistic material properties (aluminium alloy 7075). The experimental setup with capacitive sensors and strain gauges allows for reliable measurement of the results.
Comments:
• The experimental setup (Figure 7) should include a schematic diagram of the measurement systems
The theoretical data agree well with the experimental results, which confirms the model's correctness. The achieved gripping range represents a significant improvement over traditional microgrippers.
Critical comments:
• There is no analysis of nonlinear effects at large deformations of flexible elements
• Discussion of the stability of force control is insufficient (page 11, Figure 11)
• The effect of temperature on positioning accuracy is not analyzed
The text is structured logically and clearly. The images are high-quality and informative, especially the deformation analyses (Figures 3-4) and experimental results (Figures 8-15).
Shortcomings in the presentation:
• Figure 2: the mechanism diagram could include dimensional dimensions
• A table comparing parameters with existing solutions would increase the value of the work
• Some references are older (most of them are less than 5 years old)
• The text on the page seems to be shifted to the right, and the last letters on the line are moved to a new line, which slightly impairs readability, but maybe that's my problem. If not, I don't know if the authors can solve something with it.
3. SPECIFIC COMMENTS
Page 4, equation (5): The condition -90° ≤ φ2 < 90° is not sufficiently justified regarding the mechanical limitations of flexible joints. What is this limitation given?
Page 6: The maximum stress of 143.39 MPa is close to the material's yield strength—the safety factor is not analysed.
Page 10, Figure 8: The graph of the deflection versus stress shows a slightly nonlinear behaviour (hysteresis?), which is not discussed in the text.
Page 11, lines 288-292: The claim about force regulation is not sufficiently supported by quantitative data on the accuracy of the regulation.
Page 13, conclusion: There is no discussion of the implementation's possible applications and economic aspects.
Author Response
Comments 1: [The experimental setup (Figure 7) should include a schematic diagram of the measurement systems.]
Response 1: [Figure 5(a) shows the sensor setup for force-displacement tests. To keep the paper concise, we didn't include a separate measurement system diagram. Instead, we added explanations in the text and marked them in red.
Comments 2: [The theoretical data agree well with the experimental results, which confirms the model's correctness. The achieved gripping range represents a significant improvement over traditional microgrippers. There is no analysis of nonlinear effects at large deformations of flexible elements.]
Response 2: Although near-millimeter-scale deformations are described, the simulated equivalent stress of the flexure hinges confirms their linear elastic behavior, justifying the omission of nonlinear effects.
Comments 3: [Discussion of the stability of force control is insufficient (page 11, Figure 11)]
Response 3: Thank you for pointing this out. We agree with this comment. Figure 11(b) presents the gripping force variation over a 4-second duration, demonstrating an error margin of 2 mN. We contend that this 4-second interval represents a typical operational timeframe for micro-gripping processes, and the achieved 2 mN precision and stability meet operational requirements under specified working conditions. Additional discussions on system stability have been incorporated in the manuscript.
Comments 4: [.The effect of temperature on positioning accuracy is not analyzed. ]
Response 4: Thank you for pointing this out. We agree with this comment. Micro/nano manipulation is typically performed under constant temperature conditions, therefore the influence of temperature variations on positioning accuracy was not analyzed. Furthermore, since micro/nano operations are generally conducted at low frequencies, the heat generated by the micro-gripping system has minimal impact on positioning accuracy. Additionally, the closed-loop control system further mitigates thermal effects on precision. We have supplemented the manuscript with explanations regarding these aspects, highlighted in red text.
Comments 5: [Figure 2: the mechanism diagram could include dimensional dimensions. ]
Response 5: Thank you for pointing this out. We agree with this comment. Therefore, Some basic dimensions have been added, while more detailed dimensioning has not been included as this is a schematic diagram.
Comments 6: [A table comparing parameters with existing solutions would increase the value of the work ]
Response 6: Thank you for pointing this out. We agree with this comment. Therefore,we have added a table comparing parameters with existing solutions.
Comments 7: [Some references are older (most of them are less than 5 years old)]
Response 7: Thank you for pointing this out. We agree with this comment. Therefore,we have conducted a comprehensive review of the cited references in this manuscript. Although these seminal works were published in earlier periods, they remain fundamentally instructive for the current research. Furthermore, we have incorporated citations to several cutting-edge studies to demonstrate recent advancements in the field.
Comments 8: [The text on the page seems to be shifted to the right, and the last letters on the line are moved to a new line, which slightly impairs readability, but maybe that's my problem. If not, I don't know if the authors can solve something with it.]
Response 8: Thank you for pointing this out. We agree with this comment. Therefore, we have modified the hyphenation errors in the revised manuscript.
Comments 9: [Page 4, equation (5): The condition -90° ≤ φ2 < 90° is not sufficiently justified regarding the mechanical limitations of flexible joints. What is this limitation given?]
Response 9: Thank you for pointing this out. I/We agree with this comment. Based on the schematic diagram of the flexible crank-slider mechanism in Figure 2, the range of φ2 has been constrained to -90° ≤ φ2 < 90° to ensure the compactness of the overall gripper structure. Relevant explanations have been supplemented in the text and highlighted in red font for emphasis.
Comments 10: [Page 6: The maximum stress of 143.39 MPa is close to the material's yield strength—the safety factor is not analysed.]
Response 10: [ The yield strength of aluminum alloy 7075 is at least 480 MPa, while the maximum stress observed in our study (143.39 MPa) is significantly lower than this threshold. Consequently, the safety factor was not explicitly discussed. We have supplemented the manuscript with additional explanations regarding this aspect.
Comments 11: [ Figure 8: The graph of the deflection versus stress shows a slightly nonlinear behaviour (hysteresis?), which is not discussed in the text.]
Response 11: Figure 8 illustrates the relationship between the gripper's end-opening displacement and the input voltage of the piezoelectric actuator, exhibiting distinct hysteresis characteristics. Consequently, a closed-loop displacement feedback strategy was implemented in subsequent control processes to compensate for this nonlinear hysteresis effect. We have supplemented the manuscript with detailed explanations of this compensation approach, with key additions highlighted in red text.
Comments 12: [Page 10, Figure 8: Page 11, lines 288-292: The claim about force regulation is not sufficiently supported by quantitative data on the accuracy of the regulation.]
Response 12: Thank you for pointing this out. I/We agree with this comment. In the experiment, we employed elastic rubber tubes as the gripping target. However, due to their relatively high stiffness, the applied gripping force was insufficient to induce deformation, thereby preventing the demonstration of force-displacement hybrid control during testing. We intend to conduct further in-depth investigations on this aspect in subsequent studies.
Comments 13: [ Page 13, conclusion: There is no discussion of the implementation's possible applications and economic aspects]
Response 13: Thank you for pointing this out. I/We agree with this comment. Therefore, we have added a paragraph in the Conclusion (in red text ) discussing potential applications and economic considerations of our implementation.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsDear Authors,
Congratulations on your work! I am eager to see the next paper!
Best regards.
