A robot motion position and posture control method for freeform surface laser treatment based on NURBS interpolation

Laser surface treatment, such as laser cladding, laser quenching and laser cleaning, for the freeform surface can dramatically enhance the properties of the working surface. During the laser surface treatment, the posture of the treatment tool to the surface influences the size and shape of the laser spot, which further influences the quality of the laser surface treatment. Thus, accurate posture and high synchronization between position and posture are crucial. Industrial robots have the irreplaceable advantages of high flexibility, large workspace and cost-effectiveness over the 5-axis machine tools, therefore, they have become more and more popular in freeform surface laser treatment. However, the research on industrial robot motion mainly concentrates on position control, while less attention has focused on posture control and synchronization of position and posture control. In this paper, a robot motion position and posture (P&P) control method based on the non-uniform rational B-spline (NURBS) interpolation is proposed to realize the high-order continuity and synchronization of P&P as well as the self-adaptive motion control. Firstly, simultaneously considering the position and posture, a novel path generation method based on 6-dimensional NURBS is proposed to provide smooth, continuous and collision-free P&P coordinates of the designed scanning path. Next, a self-adaptive path segmentation method is introduced to recognize the corners and divide the whole treatment path into safe and dangerous segments. Based on the kinematical characteristics of the industrial robot, the joint motion constraint is especially considered in addition to the chord error limit and the centripetal acceleration constraint, and the self-adaptive motion parameters are obtained to match with safe and dangerous segments to avoid mechanical shock and ensure the accessibility of joint servos. Moreover, a synchronized look-ahead method with a dynamically refreshed window is employed to obtain the optimal node speed between adjacent segments and relieve the heavy computational burden, after which a series of smooth, accurate and synchronized motion instructions for the freeform surface laser treatment is generated in real time. Finally, simulations and experiments of a NURBS scanning path for a turbine blade laser surface cladding are conducted. The results show the proposed method can realize the high-order continuity and synchronization of P&P as well as the self-adaptive motion control for the freeform surface laser treatment.