ANGEL: A Novel Gripper for Versatile and Light-touch Fruit Harvesting

TL;DR

Addressing labor-intensive fruit harvesting and fruit damage, the paper proposes ANGEL, a drawstring-inspired, cable-driven soft gripper. The design uses 3D-printed TPU pockets with integrated steel wires and low-component count to enable gentle, adaptive grasping with minimal control requirements. Experimental results show 0% immediate damage and bruising under 9% after 5 days for tomatoes within the effective size range, with power consumption around 0.1 W during actuation. The work demonstrates a low-complexity, cost-effective approach to robotic fruit harvesting and outlines future work toward full automation and integration with perception and manipulation systems.

Abstract

Fruit harvesting remains predominantly a labor-intensive process, motivating the development of research for robotic grippers. Conventional rigid or vacuum-driven grippers require complex mechanical design or high energy consumption. Current enveloping-based fruit harvesting grippers lack adaptability to fruits of different sizes. This paper introduces a drawstring-inspired, cable-driven soft gripper for versatile and gentle fruit harvesting. The design employs 3D-printed Thermoplastic Polyurethane (TPU) pockets with integrated steel wires that constrict around the fruit when actuated, distributing pressure uniformly to minimize bruising and allow versatility to fruits of varying sizes. The lightweight structure, which requires few components, reduces mechanical complexity and cost compared to other grippers. Actuation is achieved through servo-driven cable control, while motor feedback provides autonomous grip adjustment with tunable grip strength. Experimental validation shows that, for tomatoes within the gripper's effective size range, harvesting was achieved with a 0% immediate damage rate and a bruising rate of less than 9% after five days, reinforcing the gripper's suitability for fruit harvesting.

Figures (12)

• Figure 1: Visualization of the ANGEL Gripper on Artificial Strawberries. The main body of the gripper is fabricated with 3D-printed PLA. The contact surfaces are 3D printed with Thermoplastic Polyurethane (TPU). The gripper imitates the mechanism of a drawstring bag, using two motors to pull the cables routed through the pockets to control its opening and closing. This visualization explains how it works on strawberry models.
• Figure 2: Gripper Components and Dimensions. Visualized from left to right: the CAD model of the main body of the gripper, the front view with TPU pockets, the top view when the pocket envelope is open, and the bottom view with the Dynamixel motors installed.
• Figure 3: Harvesting Process Explanation and Visualization on Artificial Strawberries. From left to right, the gripper sequentially performs the steps of approaching, aligning, enclosing, securing, and detaching to complete the harvest.
• Figure 4: Detailed View of the Gripper. Subfigure A shows an exploded view of the gripper. Subfigure B illustrates the hollow-tube design of the pockets, which allows wiring and provides a cushioning layer between the pockets and the fruit. Subfigure C presents the cable support structure, which guides cable motion and ensures that the pockets are pulled in the intended direction. Subfigures D and E detail the winding of the steel cable around the motor spool, enabling both pulling and release. Note that cables are not visualized in the figure.
• Figure 5: Alternative Pocket Designs. From left to right: TPU pockets optimized for cushioning, flexure, and contact area.