Mechanical characteristic analysis of drawing and cutting process for greenhouse vegetable harvester based on energy balance theory

Abstract: A novel vegetable cutting mechanism is designed and fabricated, which is capable of measuring cutting force and blade displacement in real time. Rhizome tissue of lettuce is selected as cutting object of vegetable cutting mechanism. And a series of experiments were carried out with the vegetable cutting mechanism under drawing force. The drawing force has contribution to separating rhizome tissue of lettuce from soil slightly, which will facilitate the process of cutting rhizome tissue of lettuce by vegetable cutting mechanism. Cutting speed, cutting method, blade parameters, and so on can’t affect the process phase of cutting. However, larger cutting speed can reduce sampling size of cutting force, resulting in the distortion of the relation curve of cutting force and cutting depth. So, experimental conditions are the cutting speed of 50 mm/min, the drawing force of 30 N, the cutting position of 10 mm above stem of lettuce, and the cutting method of cutting vertically. Blade parameters are blade width of 0.5 mm, wedge angle of blade of 20°, and material of cast iron. According to the experimental results, the cutting process can be divided into 3 phases: Deformation phase, rupture phase and cutting phase. The process that blade cuts into rhizome tissue of lettuce is defined as deformation phase. In the deformation phase, the work done by blade is converting the elastic energy stored in the rhizome tissue of lettuce. The epidermal fiber structure of rhizome tissue is changed under pressure of blade. Therefore, it’s difficult to establish a physical model and an analytical model for deformation phase. However, the cutting force of deformation phase is represented as a typical nonlinear J shape curve, so an exponential function is used to fit it in this paper. Once the blade breaks into the surface of the rhizome tissue, the cutting force suddenly decreases while crack is created, which is defined as rupture phase. The time of rupture phase is very short, so there is almost no energy conversion between the blade and the vegetable. Only elastic energy stored in the deformation phase is released instantaneously. The cutting force shows a huge decrement in a short time, so the cutting force is simplified as a linear decrement. Consequently, the blade cuts into internal fibrous tissue, and the cutting force firstly increases and then decreases with the increment of cutting depth, which is defined as cutting phase. In the cutting phase, the elastic energy stored in the deformation phase changes very little, and the elastic energy produced by drawing force is converted into the energy consumed by the newly formed surface. The mathematical model of cutting force is established according to the energy balance theory in this phase. The test cutting force value is considered as benchmark. The error in deformation stage is -5.3%-7.8%, and the error in cutting phase is -9.6%-8.2%. The cutting force curves of experiment and the mathematical model are well matched, so the correctness of modeling of cutting process is proved. Research results show that the cutting force reaches the maximum critical value in deformation phase; the drawing force has a certain effect on decreasing the cutting force, but excessive drawing force is easy to cause uneven cutting surface and poor harvest effect. The results can help researchers to understand cutting characteristics and cutting mechanism of soft tissue of vegetable, and it provides a possible solution for improving vegetable cutting devices in the future. In addition, although this paper is aimed at mathematical modeling of the cutting process of the rhizome tissue, the analysis and modeling methods of the cutting process are also suitable for other soft tissue materials.