Specialised Hook Gripper For Robotic Grasping of Vine Tomato Trusses

Abstract

In this master thesis, the development of an innovative gripper specifically designed for grasping the delicate peduncles of vine tomato trusses is presented. This research addresses a critical challenge in agricultural automation: efficiently and safely manipulating high-value crops without causing damage. The thesis begins with a comprehensive review of the current state of agricultural automation, particularly focusing on the post-harvest handling of vine tomatoes.
Building on identified limitations in existing robotic grippers and automated systems, a novel gripper design is proposed. This design uniquely approaches grasping the peduncle of vine tomato trusses. Traditionally, trusses have been grasped by the peduncle with standard pinching grippers for pick and place operations. The proposed gripper in this paper grasps the truss with a hook around the peduncle, often optimally at the centre of mass, increasing the success rate of grasping, stability, and avoiding damage to the truss. Furthermore, it has a higher tolerance for detection errors, allowing for inaccurate positioning of the robotic system. A hook-gripper can successfully grasp a wider range of tomato varieties than a pinch gripper.
The hook, consisting of two fingers, allows for more stable lifting of the truss and can handle peduncles in hard-to-grasp positions, such as in a crate filled with tomatoes. In addition to increased reach capabilities, the hook-gripper also enables manipulations like dragging and pushing.
The study involved an iterative design process, prototyping, and rigorous testing of the gripper. Key features include a hook mechanism for secure grasping, enhanced mobility for reaching into cramped spaces like packed crates, and a delicate touch to prevent bruising or damaging the fruit. The research also integrates the gripper with advanced detection systems for precise and effective operation within automated setups.
Results from extensive testing demonstrate that the newly designed gripper not only improves the success rate of grasping and manipulating vine tomato trusses but also significantly reduces the risk of damage compared to conventional pinch grippers. Individual trusses can be grasped with great success.
Testing for the different positions showed an increased range of grasping position that resulted in a successful grip. In practical experiments, the gripper performed well, lifting trusses with ease. Other test results show that the position of the peduncle is of great importance for the success rate. Test results indicated an 80% success rate in grasping trusses positioned near the edges of the crate. Replaying waypoint with "learning from demonstration" improves the grasping of the trusses compared to existing detection possibilities, and that emptying a crate is a challenging task.
The specialized hook-gripper offers insights into a practical solution for picking and placing vine tomatoes using the peduncle. This thesis contributes to the field of agricultural robotics and facilitates a step towards future innovations in the automation of high-value crop handling. The study delves into hook-gripper design and actuation, crucial for optimal performance in robotic manipulation systems.