# Robotic Button Mushroom Harvesting Systems: A Review of Design, Mechanism, and Future Directions
## 3 Mechanical System
### End-Effectors  
An end-effector is a tool attached to the wrist of a manipulator, enabling direct interaction between the manipulator and the mushroom [50]. In the case of button mushrooms, the picking action is typically carried out by either grasping the mushroom cap or suctioning it at a contact point on the cap. As a result, most of the end-effectors are either graspers, grippers, or vacuum suction cups. To enhance their functionality, end-effectors are often equipped with additional mechanisms that allow them to perform actions like bending, twisting, or lifting, which are essential for picking mushrooms. A critical consideration in the end-effector design is ensuring that they avoid causing any injury or damage to the mushroom cap [3]. Table 1 provides a list of various end-effectors used for picking button mushrooms, along with their respective actuation mechanisms. Vacuum suction cups are a commonly used type of end-effector, which creates negative pressure at the contact point with the mushroom cap to grip the mushroom from the top. [PERSON] et al. were pioneers in automated mushroom harvesting and employed vacuum suction cup end-effectors in their work [26; 27]. In the early stages of developing mushroom-picking end-effectors, this type was widely used [31; 37; 49; 51]. In 2021, [PERSON] et al. studied hand-picking mechanisms and incorporated these insights into their design of the positioning end-effector [52]. [PERSON] et al. developed an end-effector that combines a motor and gear box combination with flexible suction cups [38]. These vacuum suction end-effectors are also used in commercial mushroom harvesters, such as those by TechBrew Robotics [43]. [PERSON] et al. designed pressure-stabilized flexible suction cups that significantly improved the picking success rate to 100% compared to other suction cups [31]. These end-effectors, with only a single point of contact, simplify picking by allowing them to grasp the mushroom cap from any exposed part, even on inclined mushrooms. The actuation mechanism for these end-effectors was pneumatic, with the pressure required to pick mushrooms ranging from 9 kPa [31] to 172 kPa [52]. The picking success rates typically ranged from 90% to 100%, with minor damages like bruises and stains appearing a few days after harvesting [52]. Graspers, in contrast to suction cups, can be either open grippers with finger-like structures or closed structures resembling tubes with internal fingers [53]. These are the most common types of end-effectors used for picking soft fruits and vegetables, as they can exert greater force when needed and securely hold objects, unlike suction cups. Many graspers used in mushroom harvesting share similarities with those designed for fruit  
Figure 2: Different manipulators are used in button mushroom harvesting: (**a**) Cartesian (reprinted from ref. [40]); (**b**) champ-ON Cartesian-trolley harvester (reprinted from ref. [30]); (**c**) multi-arm Cartesian (reprinted from ref. [38]); (**d**) SCARA (reprinted from ref. [43]); and (**e**) multi-arm SCARA (reprinted from ref. [45]). picking in terms of their design, actuation, and working principles [54]. Unlike suction cups, which grip from the top, graspers typically grab mushroom caps from the side and can have multiple contact points. One popular design is the three-finger gripper, inspired by the human hand's thumb, index, and middle fingers. These bio-inspired grippers can either be soft and flexible, allowing the fingers to conform to the mushroom cap's shape [30; 55], or a hybrid gripper combining rigid and soft components [3; 40; 46]. Soft grippers offer advantages such as being lightweight, easy to control, adaptable to different conditions, and gentle in handling [56; 57]. [PERSON] et al. developed a soft gripper with a ribbed structure on the tension side and a plain structure on the compression side, which enhanced flexibility and allowed the gripper to adjust to different mushroom shapes and sizes [33]. [PERSON] et al.