**1. Introduction**

Due to the rich nutrients and unique flavor, the demand for cherry tomatoes is increasing [1–3]. However, harvesting is the most time-consuming and laborious part of its production. Therefore, realizing mechanization and automation is a significant development direction of fruit harvesting robots. However, the growing environment of cherry tomatoes is complex, and the skin is fragile. The quality of harvest directly affects the economic benefits. Its delicate and fragile biological characteristics make mechanized undamaged harvesting face great challenges [4–7].

In recent years, scholars had developed different fruit-picking end-effectors [8,9]. Xiong et al. [10] designed a strawberry end-effector with cutting function, and the success

**Citation:** Zhang, F.; Chen, Z.; Wang, Y.; Bao, R.; Chen, X.; Fu, S.; Tian, M.; Zhang, Y. Research on Flexible End-Effectors with Humanoid Grasp Function for Small Spherical Fruit Picking. *Agriculture* **2023**, *13*, 123. https://doi.org/10.3390/ agriculture13010123

Academic Editors: Cheng Shen, Zhong Tang and Maohua Xiao

Received: 29 November 2022 Revised: 19 December 2022 Accepted: 20 December 2022 Published: 2 January 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

rate of picking reached 97.1%. Tested and compared three traditional methods of collecting mushrooms by hand, Huang et al. [11] developed a vacuum end-effector, and the results showed that the picking rate of the target mushroom reached 100% by the bending picking method. To reduce the scrapes of picking, Miao et al. [12] developed an apple end-effector with a compliant mechanism whose success rate is about 95.3%. It ensures the constant output force required for low destructive fruit clamping. Roshanianfard et al. [13] created a five-fingered pumpkin manipulator with a grabbing radius of 76.2-265 mm, which was electrically driven and had an internal shock-grabbing mode. In addition, the endeffectors of litchi [14], kiwi [15], chrysanthemum [16], blueberry [17], green pepper [18], cucumber [19], and other fruits and vegetables were designed and studied.

However, fruits and vegetables have different biological characteristics, sizes, and growth conditions. The design of specific end-effectors has gradually become one of the essential research directions based on their factors. According to the principle of bionics, Wang et al. [20] made the citrus picking end actuator with occlusion mode by imitating the snake head. At 46◦ as the optimum harvest posture, the harvest rate reached 74%. By studying the grasping behavior of human hands during tomato picking, Wang et al. [21] developed a new 16-channel data acquisition electronic glove, which showed the maximum pressure contribution from the distal regions of the thumb, middle finger, and index finger by picking each fruit. Gao et al. [22] developed a pneumatic-controlled end-effector. It could pick cherry tomatoes continuously and steadily. The average time of picking single cherry tomato was 6.4 s, and the highest success rate was 84%. In summary, the design of an end-effector according to the characteristics of fruits and vegetables can improve the success rate of picking and reduce damage [23–26].

In this work, fingers with sinusoidal contours were designed, and the picking strategy of the two-stage "Holding-Rotating" picking strategy of finger-end grasping was determined. The end-effector was designed to separate the fruit from the stalk based on the linear motion of the constraint part and the rotating gripper. The clamping regulating system based on clamping pressure feedback was built to realize undamaged picking. The double threshold picking index was proposed, which included the grasping index of the fixed value and the slip detection of variance to mean ratio. The performance of the end effector was verified by cherry tomatoes picking test.
