Significant effort has been made to develop a robotic system to harvest fresh market apples in a modern apple orchard at Washington State University. The current operating harvester is a functional 7-degree of freedom (DOF) articulated robotic arm to pick targeted apples in both laboratory testing and field trials. The novelty of this design is that it works in tandem with a 2-DOF planar catching manipulator, since a key task in robotic harvesting is the collection of the picked fruit in a container. Rather than using a harvesting manipulator to place picked apples in a storage container, the secondary system catches and stores apples at the point of fruit separation in order to minimize path length and improve cycle time.  Preliminary data indicated that depositing the fruit in the collection system near the location of fruit separation, an approach termed pick-and-catch, could lead to increased harvesting throughput. The drop height for the previous design was variable and depended solely on the fruit’s height relative to the picking manipulator. Of all reachable fruit in a single harvesting cycle, higher fruit had a greater drop distance compared to fruit located lower in the canopy. As such, the common request is to convert this 2-DOF articulated robotic arm from 2-degrees of motion to full 3-dimensional motion. With this additional degree of freedom, the drop-height can now be adjusted to minimize the chance of damaging the fruit with bruising.

The newly designed 3-DOF catching robot consists of three revolute joints where the forward linkage is a parallelogram mechanism for keeping the catching end-effector parallel to the picking manipulator’s base. The catching end-effector is a plastic funnel lined with flexible baffles manufactured with a 3D printer. The analysis of the 7-DOF picking arm, yielded a virtual apple field of 505 apples, that was used to determine the capabilities of this new catching arm design. The target catching efficiency was 90% for the provided virtual apple field. The target coordinates for each virtual apple were found by computer modeling and simulation. Geometric parameters were selected such that the catching manipulator could reach every possible drop position in the picking manipulator’s workspace. The design was completed, fabricated, and validated, utilizing an elegant mechanical linkage design.  The workspace analysis showed that an acceptable 93% catching efficiency, and with a larger minimum drop height, which is defined by allowable produce bruising, the catching efficiency approaches 100%.  Definitive inverse-kinematics provided exact joint angles required to catch all catchable apples inside of the workspace. Using these angles, the general equation of motion, using Lagrangian mechmanics, yielded the required torque outputs of each of the three motors on the arm.  Validation of these torques through laboratory experimentation was consider adequate.

More work is needed to find an optimal drop height for minimizing bruising. The actuation mechanism for the second joint of parallelogram motion is needed to redesign to order to increase velocity.  Additional work for the tool-pathing will need to be investigations to prevent collision of the catching arm with the 7-DOF picking arm.  Currently, the end-effector is a simple cushioned basket, a redesign of this basket can reduce harvest time, and increase catching/retaining effectiveness. Finally, combining the optimization of the toolpath and the design of the end-effector for harvesting, can eliminate the need to return to a drop-off location, greatly reducing the cycle time.


Analysis and Design of an Auxiliary Catching Arm for an Apple Picking Robot

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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