# End-Effectors Developed for Citrus and Other Spherical Crops
## 1 Introduction  
Citrus is one of the most popular fruits in China, with an area of 2788.6 thousand hectares and a production of 51.129 million tons by 2021, ranked first in the world [1]. Harvesting is one of the most important aspects of citrus production, which is seasonal and labor-intensive [2, 3], and the cost of harvesting citrus manually accounts for 55% of the total cost of citrus production [4, 5]. The citrus harvesting time is short, manual work is inefficient, and there is a risk of being cut by branches during operation [6]. However, mechanized harvesting can easily damage citrus fruits and thus fail to meet the market requirements [7]. In recent years, most of the young agricultural workers in China have turned to other industries, resulting in a shortage of agricultural labor, which has seriously affected the development of the fruit and vegetable industry [8]. Therefore, the development of highly intelligent picking robots has become a popular research topic in today's society. Picking robots can not only free labor, reduce production costs and improve operational efficiency but also improve the quality of citrus picking, meet the demand of citrus production in real-time and ensure the freshness of the fruit [9].  
In recent years, scholars at home and abroad have conducted research on end-effectors for fruit and vegetable picking, and different end-effectors have been developed for different kinds of crops such as tomato [10, 11], strawberry [12], apple [13] and kiwifruit [14]. For citrus picking, the end-effector needs to adapt to different sizes and ellipticities of citrus picking needs, and it needs to complete the separation between citrus fruits and stems [15] to achieve stable picking, which is one of the key technologies for the research of end-effectors of picking robots.  
To solve the problem of nondestructive citrus picking, most scholars have used soft materials and flexible actuators [16]. However, due to the inherent instability of the pneumatic system, the problem of fruit dropping due to unstable grasping may occur. Citrus grown in the natural environment can have large differences in size and ellipticity [17, 18]. While thetraditional finger-clamp end-effector mechanism is simple and can effectively grasp the fruit by clamping the fingers, the fruit stalk separation is prone to cause peel breakage of the fruit [19,20].  
For example, the 10-degree-of-freedom under-driven three-finger hand [21, 22, 23, 24, 25, 26, 27, 28, 29, 30] developed by [PERSON]'s group at Laval University in Canada, which is the international leader in the under-driven multi-finger hand control, uses only two motors, one motor is responsible for the grasping opening and closing motion of three fingers, and the other motor completes the finger steering. [PERSON] et al., developed a 15-degree-of-freedom single-motor-driven multi-finger hand, which can achieve safe and reliable grasping without any sensors and feedback control [31]. At present, most multi-finger hands with bending and torsional complex degrees of freedom and active soft control are used for humanoid dexterous operation, integrating multiple sensors, motors and actuators, with complex control and high cost [32, 33, 34, 35].  
In order to provide a picking robot with a simple and inexpensive structure, with a certain degree of flexibility, and that is suitable for a variety of sizes of citrus and other spherical picking end-effectors, the actuator uses a finger bending grasping mechanism to achieve the grasping of different sizes of spherical fruits, such as citrus, and to rotate the cutting between the blade and the stalk. We make a prototype for experimental verification. The focus of this study is on the picking mechanism. The machine's vision system is not in the scope of the study.