  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.
