# Robotic Button Mushroom Harvesting Systems: A Review of Design, Mechanism, and Future Directions
## 3 Mechanical System
### End-Effectors
designed a hybrid three-finger gripper, demonstrating improved picking performance by incorporating various shapes and thicknesses of compression slots on the fingertips [46]. In commercial harvesting, Mycionics in Canada employs a three-finger gripper with asymmetric finger placement, resembling human fingers, where the index and middle fingers are closer together and the thumb is farther apart [40; 58]. [PERSON] et al. analyzed mushrooms of different shapes and sizes to design a compact hybrid gripper suitable for a wide range of mushroom sizes [3]. The hybrid grippers mentioned in this review featured soft pads on the tips made from materials like PDMS [59], Ecoflex silicone rubber [3], or thermoplastic polyurethane (TPU) [46; 47]. These soft pads cushion the gripping force, reducing damage to the mushroom caps while increasing the picking success rate. [PERSON] et al. designed a hyper-elastic actuating closed grasper resembling a tube with three PDMS fingertip-like structures inside, specifically for picking organic objects like button mushrooms [59]. Nanjing Agricultural University developed a two-finger gripper for commercial harvesting [45]. The two fingers, positioned opposite each other, close linearly to grasp the mushrooms from both sides and can be made from soft materials to prevent injuries. [PERSON] et al. designed a specialized end-effector with a drill mechanism for picking smaller mushrooms during bud thinning [60]. The grippers are actuated using either electric motors [3; 60] or pneumatic systems [55; 59]. Dorna Robotics has incorporated both suction cups (two) and two-finger grippers in the development of commercial harvesters. The gripper is motor-controlled and adjusts according to the size of the mushroom. The suction cups can approach mushrooms from different angles, which makes the harvesting of inclined mushrooms more efficient [41]. Soft fingers require a relatively low gripping force, as little as 1 N, whereas other grippers may need up to 18 N. The success rate for picking fully grown mushrooms with these graspers typically ranges between 90-100%. Soft grippers cause minimal to no injuries, while other grippers may leave small bruises and dents when applying higher gripping forces. Figure 3 illustrates the end-effectors used to pick button mushrooms. \begin{table}
\begin{tabular}{c c c c c c} \hline
**End-Effector Type** & **Actuation** & **Picking Time [s]** & **Success Rate [\%]** & **Application** & **References** \\ \hline Suction cup & Pneumatic & 1.7 & 70–100 & Commercial harvesting & [26; 27; 31; 38; 52] \\ \hline Two-finger gripper & - & - & - & Commercial harvesting & [45] \\ \hline Three-finger gripper & - & - & - & Commercial harvesting & [58] \\ \hline Hybrid three-finger gripper & Electric motor & 1.2 & 64–100 & - & [3; 46] \\ \hline Closed graspers & Hyper-elastic & - & 100 & Organic objects & [59] \\ \hline Soft three-finger gripper & Pneumatic & 3-4 & - & - & [55] \\ \hline Conventional hand picking & Manual & 3 & \textasci{}100 & Commercial harvesting & [52] \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of end-effectors used in mushroom picking. Most end-effectors have been designed and tested primarily for picking individual, full-grown mushrooms. However, button mushrooms often grow in clusters, and there are no end-effectors reported that match the success rate and efficiency seen with single mushroom picking when applied to the clusters [3; 52]. A few end-effectors have attempted cluster picking, but the current progress indicates that regardless of the type, additional mechanisms like bending and twisting are necessary to improve the success rate for cluster harvesting.