## 2 Materials and Methods
### Innovative Design of Transplanting Mechanism
#### 2.2.1 Seedling Gripper  
The successful integration of a robot with seedling transplanting requires an operational pick-up gripper that can grasp and hold a seedling. Figure 3a shows the mechanical structure drawing of the flexible pneumatic gripper using four elastic pick-up pins pulled by the cylinder fingers. It mainly consists of a cross-fixed bracket, several cylinder & rubber fingers, a tightening spring, a limit gasket, some support parts, and other pneumatic parts (quick joints, solenoid valves, magnetic switches, and so on). In the design, four-cylinder fingers of retractable pick-up pins were symmetrically allocated in the crossing form onto the rubber finger that was opening and closing with inner support. One end of each cylinder finger was hinged to the fixed bracket, and another end of the piston rod was consolidated to the pick-up pins threaded with an extension rod. So, the picking pins could  
Figure 2: Structure drawing of the simplified greenhouse robotic transplanting workcell: (**a**) mechanism principle: 1, 3, 4, 6, 7, 9. belt pulley; 2, 5, 8. slider; 10. synchronous belt; (**b**) mechanical structure: 1. transplanting manipulator; 2. seedling gripper; 3. plug seedling; 4. control system; 5. chain conveyor for plug trays; 6. chain conveyor for growth pots; 7. growth pot; 8: working trajectory. be pushed out to penetrate the seedling's root lump and pulled back under the tractive action of the double-acting cylinder. The tightening spring was looped over the cylinder fingers, which could apply a certain pre-tightening force to the retractable pick-up pins. This way, the picking pins could grasp and hold the root lump from four corners. The limit gasket in different sizes was used to modulate the opening degrees of the cylinder fingers. The gripper that was pneumatically driven under the control of some solenoid valves could effectively grasp, hold, and release plug seedlings. The operation of the pneumatic gripper for transplanting seedlings was described as follows. When preparing to pick up a seedling, the rubber finger is inflated (Figure 3b()). Meanwhile, the cylinder fingers withdraw multiple picking pins (Figure 3b()). So, the gripper is in an open state to approach the seedling. When picking up the seedling, the four-cylinder fingers push out their pins to penetrate the root lump of the seedling at a certain depth along the tray cell's wall (Figure 3b()). Then, the rubber bag is forced to deflate and contract (Figure 3b()), and the four-cylinder fingers close due to the spring tension (Figure 3b()). Under the action of the tightening spring, the pick-up pins of four-cylinder fingers can grasp and hold the root lump (Figure 3b()). Finally, the end-effector moves up (Figure 3b()), which pulls out the seedling from its growth tray cell (Figure 3b()). When the seedling is released, the gripper is opened to loosen the tightly-held root lump. Then, the pick-up pins are retracted. So, the extracted seedling naturally falls for planting. Figure 4 illustrates the schematic diagram of the grasping action of the gripper. To successfully grasp the seedling's root lump, the pick-up pins need to penetrate the root-soil as deeply as possible within the depth of the cell [8; 14]. Here, it was assumed that the four pins of the seedling gripper were symmetrically distributed, and the root lump was an equilateral square frustum. According to the geometric relations of grasping action between the pick-up pins and the tray cell, a set of equations can be easily determined as follows.