# Performance and Effectiveness of the Passive-Compliant Citrus-Picking Manipulator
## 1 Introduction  
Citrus is a widely cultivated economic crop in China, and its production has been increasing yearly [1]. Currently, citrus harvesting is mainly done manually [2]. However, due to the increasingly serious aging population and labor shortage in China, the cost of manual harvesting has risen significantly [3, 4, 5]. The demand for replacing manual labor with machines for harvesting has become more urgent [6, 7, 8, 9]. The harvesting manipulator, as a key component of the fruit-and-vegetable-harvesting robot, greatly affects the harvesting rate and damage rate of the robot. Therefore, research on harvesting manipulators is of vital importance [10, 11, 12, 13]. In recent years, scholars--both domestically and internationally--have conducted extensive research on end-effectors for fruit-and-vegetable harvesting [14, 15, 16, 17, 18], which can be mainly divided into two categories: the first category is cutting-type end-effectors [19], which detach the fruit from the tree by cutting the fruit stem [20, 21, 22]. [PERSON] [23] from Chongqing University designed a snake-mouth-inspired gripping end-effector based on biomimetic principles, which can cut the citrus stem without knowing the exact orientationof the stem in space. However, when picking a citrus from a cluster, it is difficult to cut the stem. [PERSON] [24] from China Agricultural University designed an end-effector for navel orange harvesting, which consists of an adsorption device, a gripping mechanism, and a cutting mechanism. The suction cup picks the fruit surface through a lower cylinder, the gripping mechanism secures the fruit, and an upper cylinder cuts the stem. However, when the stem is out of the cutting mechanism's range, stem-cutting may fail. The second category is twisting-type end-effectors [25], which detach the fruit by grasping or adsorbing the fruit and applying a twisting force to break the stem [25; 26; 27]. [PERSON] [28] from the Chongqing University of Posts and Telecommunications designed an underactuated citrus harvesting end-effector, using a fusion control of three-finger grasping and deflection to harvest citrus fruits of various sizes and ellipsoidal shapes. [PERSON] [29] from Zhejiang Agricultural and Forestry University designed an underactuated joint-type harvesting end-effector, capable of harvesting spherical fruits of different diameters. However, the above-mentioned end-effectors are complex in structure, with multiple transmission mechanisms, leading to poor stability. [PERSON] et al. [30] designed a two-finger end-effector for spherical fruits, incorporating biomimetic fingerprint textures to enhance friction and prevent fruit from slipping. This design showed good adaptability in flexible grasping tests. However, the stability of these methods under different operating conditions still requires further improvement. In addition, some studies have adopted vacuum adsorption technology to improve harvesting adaptability. [PERSON] et al. [31] proposed an apple harvesting end-effector based on vacuum suction cups, replacing traditional funnel-shaped suction inlets with smaller suction cups, making it easier to pass through the tree canopy. However, this method is highly dependent on the adsorption force, and when the robotic arm tilts, the adsorption force decreases, causing the fruit to slip off. In summary, although existing research has made progress in improving the harvesting success rate and reducing fruit damage, there are still several limitations: (1) the adaptability of cutting-type end-effectors is poor in complex fruit stem environments; (2) the harvesting stability of twisting-type end-effectors [32] needs optimization due to the complexity of the transmission mechanisms; (3) vacuum suction end-effectors experience reduced suction capability when the robotic arm tilts, affecting harvesting success. Therefore, this paper proposes a new passive-compliant four-finger robotic hand for harvesting citrus fruits of different sizes and elliptical shapes.