## 2 Materials and Methods
### Development of Control System  
Based on the planned transplanting requirements, the control system should manipulate the gripper to continuously finish a work cycle of approaching, grasping, extracting, transferring, and planting a seedling. It was also necessary to coordinate the material conveying of the source trays and the destination pots. The seedling gripper was taken as a pneumatic claw structure in use. Some five ports and two positions of solenoid control valves were used to drive these double-acting cylinders. With the help of an overflow-relieve valve, the inlet path of each cylinder was constantly adjusted to meet various working pressures. An exhaust silencer throttle valve was installed on the outlet  
Figure 7: Structure drawing of the chain parallel conveyor: 1, 7, 9, 11. sprocket wheel; 2. connecting shaft; 3. emergency stop switch; 4. frame; 5. servo motor; 6. reducer; 7. synchronous belt; 8, 10. chain; 12. tensioning mechanism; 13: plug seedling; 14: pushing rod; 15. side baffle.  
for different working speeds. The grippers were designed with multichannel switch status detection and on-off controls, which could respond to the operating action in time. In the simplified greenhouse robot transplanting workcell, the mature servo motor control was adopted for the reciprocated displacement of the seedling gripper and the conveying of plug trays/growth pots. The flexible S-shape acceleration and deceleration control algorithm was used to eliminate the motion error [8]. The corresponding starting and limiting locations were collocated with several sensors for positioning.  
Overall, the feeding system was synchronized with the drive system of the seedling pick-up the device so that the gripper extracted the seedlings one by one. For the convenience of manipulation, a well-established PLC system was used as the host controller. A touch screen was allocated for man-machine interaction, corresponding power supply module and air supply power. The final hardware configuration structure is shown in Figure 8.  
The control flow chart is illustrated in Figure 9a. After the power-on and start-up control, a self-test detection was performed to check whether the working states of the actuators and positioning sensors were normal. Before transplanting, the operating modes were set, which mainly included the working trajectories and working velocity of the grippers (Figure 9b). The working trajectories included the displacement of the column, the length and width of displacement, and the rising and falling distances in the upright direction. The horizontal motion of the seedling gripper may be programmed to be linear (straight-line) or curvilinear (joint-interpolated). When a straight-line motion is used, each linear speed may be specified. This flexible automation, through introducing a procedure to accommodate many changing parameters, might become an important alternative to traditional mechanization procedures. Finally, an order transplanting mode was generated and downloaded to the PLC. So, such a greenhouse transplanting mode was constructed on-site. The application scenario was that the entire rows of seedlings were transplanted one by one in order, and the conveyors continuously moved the plug trays/growth pots to the gripper's working space. The transplanting procedure was repeated until all seedlings in the plug trays were transplanted as needed.  
Figure 8: Hardware structure drawings of the control system for the automatic greenhouse robotic transplanter.