Abstract:
Large fruits such as watermelons are easily damaged during automated grading, packaging and transfer operations because the fruit container is usually lifted and tipped as a whole, causing the fruit group to roll, collapse and collide with the receiving surface. To reduce this impact while maintaining continuous feeding, a controlled air-resistance flexible tipping-bucket feeding method based on collision sensing with an electronic watermelon model was proposed. The electronic watermelon model was used as a collision-sensing carrier with a size, mass and bulk density close to those of commercial watermelon. It collected acceleration and attitude information during the whole feeding process, and the resultant linear acceleration after gravity removal was used to quantify the impact response of the fruit group. On this basis, a collaborative design framework of material motion, collision sensing and airflow regulation was established. A linear-arc composite guide-rail tipping mechanism was designed to realize a continuous transition of the fruit container from vertical lifting to tipping discharge. The kinematic relationship between bucket angle, roller position and fruit motion stage was analyzed, and an adjustable airflow field was arranged in the unloading region. A motion model of watermelon under the combined action of gravity and air resistance was established to explain how the upward and lateral aerodynamic forces reduce the falling velocity and disperse the concentrated collapse of fruit. According to the evolution of bucket angle and material state, the feeding process was divided into lifting, transition, rolling-sliding, collapse and outflow stages. A staged air-resistance regulation strategy was then proposed: the fan was kept off or at a low output during the stable lifting and transition stages, a higher duty cycle was used during the rolling-sliding stage to suppress rapid acceleration, a medium duty cycle was used during the collapse stage to reduce concentrated impact, and the airflow was reduced during the outflow stage to avoid fruit retention. Experiments were carried out on a tipping-bucket feeding platform. The fan speed, wind speed and thrust were measured under different duty cycles, and the collision response of the electronic watermelon model was compared under different spatial positions, fan duty cycles and staged control conditions. The results showed that fan speed increased approximately linearly with PWM duty cycle, while thrust increased nonlinearly with speed. A stable and controllable airflow buffering field was formed when the fan speed was adjusted within approximately 4,000–36,000 r/min. Without sufficient airflow, the fruit group was prone to concentrated collapse, and high impact peaks occurred mainly near the rear side of the container. With air-resistance regulation, the material motion changed from rapid rolling and concentrated collapse to slower sliding and dispersed release. The peak impact acceleration decreased from 33.34 m/s
2to 19.32 m/s
2 when the duty cycle increased from 20% to 80%, corresponding to a reduction rate of 42.1%. In repeated tests with real watermelons, the groups without and with air resistance each completed 75 cumulative fruit passes. Four fruits cracked in the group without air resistance and one in the group with air resistance. The corresponding cracking rates per fruit pass were 5.33% and 1.33%, respectively, an absolute decrease of 4.00 percentage points. The staged control strategy further improved the coordination between tipping motion and airflow output, reducing severe impact while keeping the discharge process continuous. The study indicates that controlled airflow can be used as an active flexible buffer for large-fruit feeding equipment. The proposed method can reduce the risk of mechanical damage to large fruits while maintaining feeding efficiency and provides a reference for the design of automated equipment for handling agricultural materials with low damage.