Burgers viscoelastic model-based variable stiffness design of compliant clamping mechanism for leafy greens harvesting

Abstract

To prevent mechanical damage during the feeding and conveying process for leafy greens when roots harvesting, it is necessary to redesign the clamping mechanism in consideration of the rheological properties of leafy greens under the mechanical compression. This study takes spinach, a typical representative of leafy greens, as the research object. Firstly, the rheological constitutive equation for leafy greens under clamping is derived using the Burgers viscoelastic model, and the equation coefficient is identified using a spinach creep test. The damage constraints for the feeding process for different feeding widths are constructed with deformation energy based on the rheological constitutive equation. Secondly, a compliant clamping mechanism with variable stiffness properties is proposed, comprising a clamping floating mechanism and a compact cam pendulum mechanism. This mechanism maintains a preconfigured nonlinear force-displacement curve by utilising off-the-shelf torsional spring and cam profile. Furthermore, the mathematical models of interaction between this mechanism and leafy greens are constructed, with which the low-damage and stable clamping as the optimisation objectives are analysed, and the optimal parameters are achieved with particle swarm optimisation algorithm. Finally, an interactive simulation model of the clamping belt with floating roller supported by the compliant clamping mechanism and the Burgers viscoelastic model is built to simulate the feeding process. The dynamic characteristics of the clamping force are analysed for three feeding widths represented by parallel width of spinach. The experimental results show that relative error of the maximum clamping forces between simulation analysis and theoretical computation is less than 10%, and the relative error between test and theoretical computation is less than 13%, indicating the validity of the simulation model and feasibility of the designed compliant clamping.