Robotic disc grinding path planning method based on multi-objective optimization for nuclear reactor coolant pump casing

Abstract

In the nuclear industry, the finishing grinding work of the nuclear reactor coolant pump (RCP) casing is mainly performed manually. Uncontrollable grinding tasks cause the grinding disc to be easily worn during the grinding process, which will greatly affect the grinding accuracy and efficiency. This paper introduces a path planning method that can efficiently and accurately perform a disc grinding task on an RCP casing. First, we provide a wear model for rigid grinding discs and verify its accuracy through finite element simulations and experiments. It can be used to predict the wear conditions of grinding discs during grinding. Then, a series of linear geodesic offset paths with the shortest path length characteristic can be generated and converted to NURBS interpolation paths. The velocity, acceleration, and jerk of the of the NURBS interpolated path generated by the S-shaped acceleration/deceleration (ACC/DEC) feedrate planning method in Cartesian space can be converted into the corresponding angular velocity, acceleration, and jerk of each joint in joint space to ensure that the grinding tasks can be performed under appropriate kinematic constraints; Then, an improved NSGA-II algorithm is proposed and its performance is verified based on benchmark test problem suite in three indicators. The verification results showed that the solution set generated by the proposed algorithm has good distribution uniformity, is closer to the true boundary, and has good convergence compared with other advanced optimization algorithms; Furthermore, by substituting the multi-objective optimization functions and kinematic constraints into the improved NSGA-II algorithm, the compromise minimization problem of grinding time, impact, and disc wear can be solved. The simulation and experimental results demonstrate the superiority and effectiveness of the optimized geodesic grinding paths in terms of grinding precision, accuracy, stability, and efficiency. In contrast, multi-directional paths, e.g., optimized cycloid paths, will produce varying grinding contact forces and varying disc sliding velocities, which will lead to more complex material removal situations, thus affecting the accuracy of the optimization solution.