Abstract:

Objective Greenhouse tomato pruning is still done largely by hand, and the dense canopy makes robotic operation difficult. Conventional shearing-type end effectors must be positioned accurately at the lateral-shoot base, and their jaws need considerable lateral room to open; both requirements restrict their use among leaves, inflorescences and fruits. Tomato lateral shoots also have high moisture content and low flexural rigidity, so they tend to bend, slip or tear during cutting, which degrades cut quality and increases infection risk. A dual-screw propulsion and sliding-cut end effector that combines shoot guidance, axial propulsion and sliding-cut removal in one compact structure is proposed to ease the positioning-accuracy requirement, reduce the required operating space, and stabilize the feeding and cutting of flexible shoots. Methods Tomato plants grown under a wide-furrow and narrow-bed system were characterized for morphological parameters (main-stem and lateral-shoot diameters, branching angle and shoot height) and stem mechanical properties, including Poisson's ratio measured in compression, elastic modulus measured by three-point bending, and the friction coefficient between the stem and a silicone finger, to guide the structural design. Force analysis showed that a double-edged blade provided a lower theoretical cutting force and a more balanced lateral force than a single-edged blade, with less eccentric loading and lateral deflection of the flexible shoot; therefore, a double-edged blade was selected. A sliding-cut mode was adopted to reduce the effective edge angle and instantaneous cutting resistance. Using fresh lateral shoots of 11 mm in diameter, a three-factor, three-level Box-Behnken experiment (blade thickness, cutting speed and sliding-cut angle) was conducted to build quadratic response-surface models for maximum cutting force and cutting time. Screw-propulsion and cutting mechanics models were then established to determine the screw geometry and motor torque, accounting for the effective propulsion coefficient and possible slippage. A prototype was built with 3D-printed AlSi10Mg screws and Cr12MoV blades. Single-arm pruning trials (n=50) and comparative geometric capture-tolerance tests (dual-screw guided, dual-screw unguided and conventional scissor-type, at lateral offsets from -10 to 10 mm) were carried out in a commercial greenhouse. Results and Discussions Both response-surface models were significant (P<0.000 1), with non-significant lack-of-fit and adjusted R² values above 0.96. Among the three factors, blade thickness had the strongest linear effect on maximum cutting force, followed by sliding-cut angle and cutting speed, whereas cutting speed dominated cutting time; the interaction between cutting speed and sliding-cut angle was significant for maximum cutting force. The response surface indicated that the lowest cutting-force region lay near a sliding-cut angle of 42°. Optimization gave a blade thickness of 0.30 mm, a cutting speed of 6.7 mm/s and a sliding-cut angle of 42°; validation showed relative errors below 10% for both indicators. In the 50 pruning trials, shoots of 10-12 mm in diameter were cut with 100.0% success rate and 100.0% smooth cuts rate (mean effective pruning time of 3.1 s), whereas the 12-14 mm group reached 95.5% and 86.4%, with a mean effective pruning time of 3.6 s. Overall, the success rate was 98.0%, the smooth-cut rate 94.0% and the mean effective pruning time 3.32 s. In the capture-tolerance tests on 11-mm-diameter specimens, the guided mechanism reached 90% capture success at an offset of ±10 mm, compared with 85% for the unguided version and 65% for the scissor type, indicating that the guide structure compensates for positioning error in a dense canopy. Conclusions The end effector integrates guidance, propulsion and cutting of tomato lateral shoots within a compact working envelope. It handled shoots of 10-14 mm diameter and tolerated geometric positioning error better than conventional shearing mechanisms, providing a reference for end-effector design in greenhouse tomato pruning robots. Future work should examine robustness, screw-spacing adjustability, and adaptive control to improve cut quality on larger shoots.

Key words: end effector, dual-screw propulsion, sliding cutting, tomato lateral shoot, pruning robot