Design and Control of a Novel Six-Degree-of-Freedom Hybrid Robotic Arm

TL;DR

The paper tackles the challenge of achieving a large, agile manipulation workspace for agricultural harvesting in unstructured greenhouse environments. It introduces a novel $6$-DoF hybrid robotic arm that couples a meshed-gear parallel mechanism with a single-rail dual-slider and serial joints, enabling obstacle negotiation and pose preservation on mobile platforms. Key contributions include an analytically solved inverse kinematics framework, Monte Carlo workspace validation showing a large irregular ellipsoidal envelope, and ROS-based, real-time control with segmented PID and S-curve motion. Experimental results confirm high precision (end-effector repeatability ~0.109 mm) and accurate trajectory tracking (RMSE ~0.38 mm), underscoring the arm's potential for efficient, autonomous fruit harvesting in constrained agricultural spaces. The work paves the way for enhanced obstacle avoidance and dynamic picking through sensor fusion and reinforcement-learning-based planning.

Abstract

Robotic arms are key components in fruit-harvesting robots. In agricultural settings, conventional serial or parallel robotic arms often fall short in meeting the demands for a large workspace, rapid movement, enhanced capability of obstacle avoidance and affordability. This study proposes a novel hybrid six-degree-of-freedom (DoF) robotic arm that combines the advantages of parallel and serial mechanisms. Inspired by yoga, we designed two sliders capable of moving independently along a single rail, acting as two feet. These sliders are interconnected with linkages and a meshed-gear set, allowing the parallel mechanism to lower itself and perform a split to pass under obstacles. This unique feature allows the arm to avoid obstacles such as pipes, tables and beams typically found in greenhouses. Integrated with serially mounted joints, the patented hybrid arm is able to maintain the end's pose even when it moves with a mobile platform, facilitating fruit picking with the optimal pose in dynamic conditions. Moreover, the hybrid arm's workspace is substantially larger, being almost three times the volume of UR3 serial arms and fourteen times that of the ABB IRB parallel arms. Experiments show that the repeatability errors are 0.017 mm, 0.03 mm and 0.109 mm for the two sliders and the arm's end, respectively, providing sufficient precision for agricultural robots.

Figures (18)

• Figure 1: The novel six-degree-of-freedom hybrid robotic arm; a demonstration video can be found at https://drive.google.com/file/d/1vZgk7NcdaC_Dejxn6mOWbftSCUKCEhgD/view?usp=sharing.
• Figure 2: The meshed-gear parallel mechanism assembly.
• Figure 3: Newly designed single-rail dual-slider mechanism: a) schematic diagram of the arrangement of the inner and outer circular belts; b) top view of the single-rail dual-slider mechanism.
• Figure 4: Prototype of the patented single-rail dual-slider mechanism
• Figure 5: The meshed-gear parallel mechanism design.