A biologically inspired compliant gripper for kiwifruit harvesting

Abstract

This gripper was designed taking inspiration from the technique agricultural workers use to pick kiwifruit, and designing the fingers of the gripper similar to that of a humans, with optimised dimensions through SolidWorks Simulations. Validating the gripper included testing for Fruit Detachment Force (FDF) and Fruit Contact Pressure (FCP). The FDF tests involved testing kiwifruit over a variety of input pressures (0-0.7 MPa) to see at what point slippage, i.e. failure occurred. This was conducted over a range of differently sized gold and green kiwifruit, resulting in input pressures between 0.3-0.55 MPa achieving the required FDF of 30.074 N. The FCP tests involved applying the same range of input pressures to force gauges on specialised rigs (one to represent a small kiwifruit and one for large) to find at what input pressure, the output pressure exceeded the critical pressure (0.216 MPa) at which kiwifruit would be damaged. From this we found the maximum FCP (at our operating input pressure of 0.6N – rounded up from our maximum FDF input pressure of 0.55N) to be 0.09 MPa, which is much less than the critical damaging pressure of 0.216 MPa. By then manipulating the faces to account for a variety of differently shaped kiwifruit by extending out the location of the force gauges one at a time, the results were largely affected. The majority of the results remained below the critical pressure, with a few rare situations causing failure, specifically when face 4 was protruded a further 3 mm, and face 2 by just 1 mm for the large kiwifruit situation and 3 mm for the small. To further validate this gripper, it was compared to two existing grippers, one rigid, and one compliant. FDF testing was found to be achieved for all sizes of fruit tested for this gripper and the existing compliant gripper, however the existing rigid gripper failed to pick smaller kiwifruit. In regards to FCP testing, all grippers were comfortably under the critical damaging pressure, with our gripper having a slightly higher pressure then the existing grippers.