Development of an End-Effector for Robotic Harvesting of Hydroponic Lettuce

Highlights An end-effector prototype was developed for harvesting greenhouse hydroponic lettuce. The end-effector's cutting mechanism was designed for 1.72 Nm torque, accounting for 25% loss. The harvesting test showed that the end-effector can cut and hold simultaneously with 95.15% and 94.45% success rates. The average remnant leaves per plant was 0.55 after cutting and fallen leaves per plant was 0.075 at maneuvering. ABSTRACT. Lettuce is one of the most demanding annual leafy green vegetables. Greenhouse lettuce, meanwhile, is at a much earlier stage of its evolution, with a cost of production still significantly higher than that of field-grown lettuce. Labor accounts for nearly one-third of the total production cost. Decreasing labor availability and increasing labor costs are the two most critical challenges in greenhouse lettuce production. Thus, automation to reduce labor requirements is required for the industry's sustainability. This study aims to develop a prototype of a robotic end-effector for harvesting greenhouse hydroponic lettuce. Lettuce cutting force was obtained using a thin force sensor, which was used to measure the cutting torque to provide guidelines for the end-effector design. The test result suggested that 1.72 Nm torque was required to cut the lettuce stem. An end-effector prototype with a pair of shear blades and a two-finger gripper mechanism operated using a single linear actuator was designed. Cutting and holding mechanisms work together using the first-class lever technique, where applied and acting forces are connected at the pivot point. The end-effector prototype was integrated into a linear arm attached to a two-directional linear manipulator system operated using an embedded controller. Validation experiments of the end-effector for harvesting lettuce growing in a Nutrient Film Technique (NFT) hydroponic system were conducted. The results indicated a 95.15% success rate in cutting and a 94.45% success rate in securely holding the lettuce during harvest. The developed robotic end-effector serves as a pivotal component in lettuce harvesting automation. Keywords: Controlled environment agriculture, Greenhouse lettuce, Leafy green harvesting, Precision agriculture, Robotic end-effector, Stem cutting force.