Efficiency evaluation of an automated mango harvesting unit.

Shortages and high labour cost are constraints to labour-intensive agricultural operations such as soft tree-fruit harvesting. Mechanical solutions have been developed for heavy produce (e.g., potato) and dry fruit (e.g., macadamia), but solutions for soft fruit are nascent and no solution exists for mango fruit. Three activities were undertaken to improve the performance of a prototype mango harvesting solution involving a horizontal array of picking arms that is moved vertically across the face of the tree. 

A method was developed for the low-cost ($20 per piece) production of non-perishable fruit phantoms using silicone (polydimethylsiloxane), along with a preliminary consideration of a phantom fruit stalk for comparative assessment of harvester hardware. Both phantoms achieved comparable characteristics to their real counterparts, in terms of shape, density and firmness for fruit, and required breaking force for stalk. 

The second activity developed a set of performance metrics for evaluation of end effectors designs and harvest strategies, based on: (i) laboratory trials employing fruit phantoms, using metrics termed ‘picking area’, being the cross-sectional area in a plane normal to the direction of approach of the gripper to the fruit in which a fruit was successfully harvested by the gripper, and ‘picking volume’, being the volume of space in which a fruit was successfully harvested by the gripper; (ii) orchard based trials, using metrics termed ‘detachment success’ and ‘harvest success’, i.e., the % of harvest attempts of fruit on tree which resulted in stalk breakage and return of fruit to a receiving area, respectively, and (iii) post-harvest damage, in terms of a score based on the percentage of fruit and severity of damage. The metrics were used in an empirical evaluation of nine gripper designs in the harvest of mango fruit. A design using six flexible fingers achieved a picking area of ~150 cm2 and a picking volume of 467 cm3 in laboratory trials involving a 636 g phantom fruit, and a detachment and harvest efficiency rate of 74 and 65%, respectively, in orchard trials, with no post-harvest damage associated with harvest of unripe fruit. 

The third activity compared strategies involving the coordination of manipulators and fruit picking prioritisation. Cycle time per harvested tree was reduced by picking while the elevator continued movement (51% decrease), with fruit tracking used to anticipate fruit position. Cycle time per harvested fruit was reduced by splitting arm extension into a fast and slow stage (40% decrease). Improved placement of the gripper with the continuous movement of the platform, the collaborative use of adjacent grippers for fruit positioned near the edge of the picking area of a gripper and pushing fruit by up to 15 cm when fruit centre was lower than that of the gripper increased harvest success by up to 24%.