Approximately one-third of fresh fruits are lost in the supply chain, with half the losses occurring during in-field harvesting and in-plant grading. The complexity of supply chain, waste and economic losses could be reduced through integrated non-destructive harvesting and in-field quality grading. Here we developed a self-decoupled tactile sensor for robotic soft gripper, embedding three-axis self-decoupled triboelectric soft tactile sensors across fingers. During single harvesting operation, the strategy can measure the gripping force, fruit firmness, weight by tactile sensors. Hence, integrated non-destructive harvesting and in-field quality grading can be achieved. Tactile sensors are bioinspired by the deformation mechanics of octopus suckers under normal and tangential forces. It features dual-mode TENG, integrating vertical contact-separation and horizontal sliding modes. This structure can independently respond to normal and tangential forces. It will generate a superimposed triboelectric signal with three spatially decoupled components. Consequently, the three-axis force can be easily decoupled with three triboelectric signals independently, achieving high-precision measurement within the 0-20 N range. Finally, we demonstrate the practical efficacy of the sensing strategy through in-field experiments. During grasping, fruit firmness is evaluated from the normal force response over time. Upon fruit detachment, tactile sensors monitor both normal and tangential forces in real-time to prevent damage. Finally, with the gripper reoriented palm-down, the fruit weight is non-destructively calculated via static equilibrium between the tangential force and the fruit's gravity. We develop a robotic sensing system for integrated, non-destructive fruit harvesting and in-field grading, thereby streamlining the supply chain, reducing waste, and enhancing food safety.

Apples are one of the most important fruits in China, due to their high nutritional value and storage resistance. The apple industry has been an important pillar of agricultural industry. Mechanical harvesting is also constantly improving in agricultural machinery. However, there are some complex forces among apples and harvesting equipment in the process of mechanical harvesting, even damage to apples. Some of the forces cannot be directly measured using physical experiments. While traditional experiments are required the preliminary verification, leading to the time-consuming and labor-intensive. Discrete element simulation can be expected to evaluate the apple damage. In this article, a series of calibration experiments were conducted on the contact parameter using via the simulation models using the discrete element method. Red Fuji, Hua Niu, and Golden Delicious apple particles were taken as the research objects. A contour map of apple was drawn using SolidWorks, and then imported into EDEM platform. A discrete element model of the apple was established with a smoothness of 3 and a minimum spherical radius of 3 mm. The physical parameters of the apple were then calibrated using drainage and compression tests. The density of the Red Fuji apple was 923 kg/m³, the Poisson's ratio was 0.33, the elastic modulus was 2.1 MPa, and the shear modulus was 1.4 MPa; The density of Hua Niu apples was 924 kg/m³, Poisson's ratio was 0.34, elastic modulus was 2.23 MPa, and shear modulus was 1.49 MPa; Golden Delicious apple density was 871 kg/m³, Poisson's ratio was 0.37, elastic modulus was 3.16 MPa, and shear modulus was 2.09 MPa. Furthermore, the contact parameters between apples and acrylic plates were calibrated using collision bounce, inclined sliding, and a combination of bench and simulation tests. The collision recovery coefficient between Red Fuji apples and acrylic plates was 0.432, the static friction coefficient was 0.536, and the dynamic friction coefficient was 0.0138; The collision recovery coefficient between Hua Niu Apple and acrylic board was 0.473, the static friction coefficient was 0.435, and the dynamic friction coefficient was 0.0077; The recovery coefficient of collision between Golden Delicious apple and acrylic board was 0.46, the static friction coefficient was 0.519, and the dynamic friction coefficient was 0.0088. The steepest climbing test and the three-factor quadratic regression orthogonal rotation combination test were carried out to establish a quadratic regression equation between the stacking angle and the significance factor. The equation was solved with the actual physical experiment stacking angle as the target value. The optimal parameters of simulation were then obtained: the collision recovery coefficient of Red Fuji apple was 0.487, the static friction coefficient was 0.584, and the dynamic friction coefficient was 0.084; The collision recovery coefficient between Hua Niu Apple and Apple was 0.348, the static friction coefficient was 0.869, and the dynamic friction coefficient was 0.096. Golden Delicious apple collision recovery coefficient was 0.469, static friction coefficient was 0.644, and dynamic friction coefficient was 0.093. The calibrated parameters can serve as a strong reference for the discrete element method. The finding can also provide theoretical basis and model support to the apple physical properties and mechanical harvesting.