水田平地机刚柔耦合多体动力学建模及验证

Dynamic modeling and verification of paddy leveler based on rigid flexible coupling multibody systems

针对将水田平地机视为纯刚体多体结构不能反映其实际动力学特性,且机械系统动力学计算机仿真结果难以有效验证手段。该文提出刚柔耦合的平地机多体动力学模型及一种基于高速相机的模型运动学参数验证方法。其特点是结构与力学分析来对物理对体系统进行动力学建模,再通过计算机软件实现仿真,及非接触式的刚体质心与其姿态角的动态确定方法。从水田平地机机械结构、工作原理与实践结果出发,将平地机作业时变形较大杆件平行连杆作为柔性体,建立其多体机械系统的刚柔部件与运动副约束,即确定其动力学模型,以调平系统动力学部分为例借助多体动力学建模软件Maple Sim对模型进行仿真,得到典型动态激励作用下的平地铲质心位置点的三维坐标与平地铲的姿态角;然后在实际激励信号作用下利用高速相机及其图像分析软件TEMA测得平地铲表面不在同一直线上的3个目标点的三维坐标,基于这些点的坐标求解平地铲质心位置与姿态角度作为测量结果,与仿真结果对比实现模型验证。验证结果表明:平地铲仿真结果与实际测量结果运动规律基本一致,平地铲质心位置误差最大误差约为10 cm。验证平地机建模方法可行性,该文提出的结构与理论分析建模-计算机仿真-基于图像分析的运动参数测量实现模型验证的机械系统设计方法对农机作业机械动力学建模与验证具有普遍适用性。

Proper dynamic modeling is essential for the design and control of paddy field levelers which maintains a level plow while working regardless field unevenness. The simplified rigid multibody method of dynamic modeling, of which modeling and simulation can be handled by hand generally, does not work well in that they do not produce satisfactory results, and there is not easily available method for verifying the simulation results. This paper proposes a flexible multibody approach for the paddy field modeling and simulation, and a method for model verification based on high speed camera measurement. For the former method, it is simplified to decide which bodies to be classified to be rigid or flexible and the related constraint types by studying the structure and theory of the multibody system, while leaving the tedious tasks of building differential-algebra equations and equation solving to the computer-based simulation tools; and the latter method features a non-contact, stereo image way to find the 3D （three-dimensional） position of center of mass and attitude angles of a rigid body through positioning multiple surface points. 1） Modeling. By analyzing the structure and summing up the past experiences, considering that the 3 parallel rods hanging the installation block and plow showed significant flexibility in many directions, a flexible multibody system with 2 rigid bodies （plow and installation block） and 1 flexible body （representing the 3 rods with a flexible beam） and 3 revolute joints in between was built. It was assumed that the tractor body was stationary, which helped to simplify the subsequent simulation and verification by reducing the flexible beam to be a cantilever beam. 2） Simulation. The MapleSim was used to perform the simulation. By introducing the main multibody library models, i.e. flexible beam, rigid body and rigid body frame, and following general modeling procedures, the leveler model was expressed into MapleSim environment. The model parameters were determined by measurement （for dimensions） and computer software like CATIA （computer aided three-dimensional interactive application） and ADAMS （automatic dynamic analysis of mechanical systems） （for mass values and inertial momentums）. Among simulation results, curves for the position of the leveler’s center of mass and inclination angle were produced. 3） Verification. Formulae finding the position of mass center and attitude angles of the plow by multiple surface points’ 3D positions （at least 3 points that do not fall on the same straight line） were proposed, and the surface points’ 3D positions were determined by the stereo imaging system composed of 2 high-speed cameras and the professional image analysis software TEMA. Laboratory tests on specially designed fixtures were conducted, which produced the plow’s 3D position of mass center and its inclination angle as measured results to be used against the simulated results for model verification. The verification showed that the 2 kinds of results generally coincided with each other well, indicating that the modeling, simulation and verification method proposed is feasible and practical, though a closer check showed the inclination curves agreed quite well, but the position curves of mass center revealed a maximum deviation of 10 cm at times. Some causes for the difference were proposed. The method proposed in this paper, which includes modeling by structure analysis, simulation using software, and verification by measuring 3D position of center of mass and attitude angles of a rigid body using high-speed cameras, is feasible and applicable to similar mechanical virtual prototyping applications featuring modeling, simulation and verification.