Research On Trajectory Optimization Of Electrostatic Spray Robot With Variable Parameter

Electrostatic spray robot is an important equipment for coating, which has been widely applied in coating production line of automobile at home and abroad. Since it can improves the deposition efficiency of charged paint particles via high electrostatic field force, the research on trajectory optimization algorithm, control strategy and off-line programming system of electrostatic spray robot has attracted tremendous interest in recent years. The research of trajectory optimization of spray robot began very late in China and research fields are very limited. The technical level of spray robot and off-line programming system is low, which makes the electrostatic spray robot unfavorable for real applications. This paper will investigates some practical problems in development of spray painting robot. This work is supported in part by the National Natural Science Foundation Program (Grant No.60875052)—"Research on Trajectory Optimization of Electrostatic Spray Robot with Variable Parameter" and in part by the High-tech Research Program of Jiangsu (Grant No.BG2006023)—"Research on Trajectory Optimization and Off-line Programming Technology of Spray Robot". The main content and contributions of the thesis are listed as follows:1. Due to the complex geometry of free-form surfaces, it is still a challenge to generate optimization trajectories of spray gun that satisfies paint uniformity requirements. A method of acquiring the position and orientation parameters of spray painting robot automatically via point cloud slicing technology is proposed. The trajectory of spray gun is usually composed of a set of equidistant curves, which is similar to the cross-section contours created by uniform slicing. The point cloud data of the work-piece is obtained by 3D laser scanning, then, slicing process is applied to the STL model in order to obtain the intersection points. After the slice direction and number of slices are defined by user, the intersection algorithm is adopted to generate the slice profiles of point cloud model. The spraying position is determined by the average sampling of cross-section contours, and the normal vectors of all the sampling points are estimated by using the weighted average method. Finally, the trajectory of spray gun is generated by offsetting the sampling points along their normal vectors. Experimental results illustrate that the method owns good feasibility and effectiveness. The reason is that the spraying trajectory, direction and distance of spray gun can be controlled accurately.2. Due to the big coating thickness difference on free-form surfaces, caused by the change of surface curvature, a method of spray robot trajectory optimization based on the optimization of speed and distance is provided to improve the uniformity. A quadratic function of the paint deposition rate on a plane is proposed according to the experimental data, and a model of paint deposition rate on a free-form surface is also established. In order to select the relative position of the seed curve on the surface, we employ the Gauss-Bonnet theorem. An algorithm of variable spray speed optimization is established to compensate for the curvature change and improve the uniformity of paint deposition along passes. An algorithm for selecting the spacing between the passes is proposed to improve the deposition uniformity vertical to passes, as well as reduce cycle time and paint waste. Finally, the experiment on automobile demonstrates the feasibility and availability of these optimization algorithms.3. Due to the big coating thickness difference and waste on the boundary of irregular polygon plane when the parameters of spray trajectories are constant, a method of trajectory optimization of electrostatic spray robot with variable parameter based on the electrostatic voltage and distance is provided. We first investigate the effect of spray model and transfer efficiency by the parameters of static electricity voltage, rotate speed of ESRB, velocity of ESRB and space between bell and work-piece etc, an then proposed a variable parameter spray model of high-voltage electrostatic rotating bell according to the relationship between the electrostatic voltage and spray model. In order to obtain the optimum spraying effectiveness and efficiency when spraying of irregular polygon plane, we control the distribution of paint and changing the size of the paint deposition graphics by changing electrostatic voltage, moving speed, the distance between passes, etc. Simulations and experiments on the complex non-regular planar polygon demonstrate the feasibility of this trajectory optimization method.4. A conversion method from cartesian space trajectory to joint trajectory of spray painting robot, based on Dijkstra algorithm, is presented to meet the requirements of offline programming spray painting robot and characteristics of spray cartesian space trajectory. Through analyzing the relation between the part coordinates and the robot basal coordinates, the spray cartesian space trajectory is converted to the robot basal coordinates by three points calibration of auxiliary feature points. An optimized model is then proposed to obtain the shortest joint motion distance in robot inverse kinematics solutions. Moreover, the distance among all of the inverse kinematics solutions is described by a directed weighted graph, and the Dijkstra algorithm is employed to obtain the optimal combination of inverse kinematics solution. The feasibility and efficiency of the method are well verified by spray experiments.Based on the technology of trajectory optimization with variable parameter,the theoretical analysis and experiments in the above four sections are derivedfor electrostatic spray robot, which builds the basic frame of spray robot's off-lineprogramming system. Meanwhile, the research will partly establish the theoretical basis for thedevelopment of the electrostatic spray robot.