**6. Conclusions**

This paper presents a three-fingered apple-harvesting soft gripper with constantpressure feedback inspired by the Fin-Ray effect. First, the structural parameters of the single-soft-finger model were optimized using finite element analysis, and the influence of different Fin Ray finger structural parameters on the contact stress and fingertip displacement was investigated. The optimal structural parameters of the single soft finger were proposed: the front and rear beam thickness is 3.5 mm; the finger width is 16 mm, and the number of beams is 9. A three-fingered apple harvesting soft gripper was designed based on the above-optimized fingers. The determined gripper structure's mathematical model was then statically analyzed, and the relationship between the gripping force, the pulling force, and the servo torque was obtained. Therefore, the finger output force during picking can be controlled by adjusting the servo output torque.

We also propose a dynamic control method for detecting fruit slip during apple harvesting by integrating a distance sensor in this study. The maximum static friction coefficient between the finger and the apple, the detachment force of the apple, and the damaged condition of the apple were obtained through an experimental analysis of the apple's mechanical properties, which provides a theoretical basis for the gripper design. In indoor experiments, the results show that the servo output torque has an approximately linear relationship with the contact pressure between the fingers and the apple, and it is suitable for all sizes of apple. In the outdoor orchard experiments, turning on the constantpressure feedback system can improve the safe grasping performance of the soft gripper, which can effectively ensure non-destructive fruit gripping. Comparing the tests for the three cases of rigid fingers and soft fingers with or without slip detection, the optimized Fin-Ray soft fingers in this paper are able to reduce the fruit damage better, and opening the slip detection can effectively avoid fruit damage. Furthermore, the stable output torque of the servo can ensure that the fruits do not break free due to the gripper loosening during harvesting.

In this study, we believe that the soft harvesting gripper is not only suitable for harvesting apples but also for harvesting some other fruits (e.g., tangerine and kiwi) and vegetables (e.g., tomato) and can provide an application reference. It has a high degree of adaptability and can effectively avoid fruit damage by adjusting the servo output torque.

However, our current research work still has some limitations. On the one hand, the complex structure of the Fin Ray fingers needs further investigation, and we will conduct more in-depth and detailed research on it in the future. On the other hand, the theoretical analysis and design of the gripper are only for the single-pulling fruit harvesting method in this study, which has significant limitations. This is only the first step in our exploration. In future work, combined with the optimal method and the posture of the fruit harvesting, the harvesting method combining gripper rotation and pulling will be studied.

**Author Contributions:** Conceptualization, K.C. and T.L.; methodology, K.C.; software, K.C.; validation, K.C., T.L., Q.F. and Q.Z.; formal analysis, K.C., F.X. and T.Y.; investigation, K.C.; resources, Q.F.; data curation, K.C.; writing—original draft preparation, K.C.; writing—review and editing, T.L.; visualization, K.C.; supervision, T.L. and Q.F.; project administration, C.Z.; funding acquisition, Q.F. and T.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Beijing Science and Technology Plan Project (Z201100008020009); Construction Project of Beijing Key Laboratory of Agricultural Intelligent Equipment Technology in 2022 (PT2022-37); BAAFS Innovation Capacity Building Project, grant number KJCX20210414; Beijing Key Laboratory of Intelligent Equipment Technology for Agriculture, Beijing 100097, China; the Priority Academic Program Development of Jiangsu Higher Education Institutions (Grant No. PAPD-2018-87); the Jiangsu Postdoctoral Sustentation Fund [Grant No. 2020Z378].

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank the editor and the anonymous reviewers for their valuable suggestions to improve the quality of this paper.

**Conflicts of Interest:** The authors declare no conflict of interest.
