Real time trajectory generation and interpolation

• Main Author: Siu, Anthony
• Language: English
• Published: University of British Columbia 2011
• Online Access: http://hdl.handle.net/2429/35898

This thesis presents a continuous tool motion trajectory generation algorithm for high speed free form surface machining. A NURBS toolpath generation algorithm is presented to fit the discrete motion commands generated from free-form CAD-models. By using a NURBS representation of the machine part, the toolpath is interpolated continuously to direct the synchronized motion of the 5-axis CNC machine. The higher continuity of the motion trajectory allowed for tighter machining tolerances and reduced feedrate fluctuations and the undesired acceleration harmonics in the overall feed motion and in each of the motor motions. An optimal and feasible feedrate profile have been used to continuously maneuver the cutting tool with the interpolated reference tool position and tool orientation commands such that the kinematic constraints of the drives are not violated. Commonly used least squares curve fitting of discrete data points forces the curve to weave through the data points and results in a fluctuating toolpath. By making use of the defined basis function distributions of the NURBS control points, a higher smoothness fit has been achieved through a minimization on the chord error and the third derivative of the curve. The feasibility of this toolpath generation algorithm has been extended using the double spline representation to represent both the tool position and the tool orientation with minimal fitting error. The real time interpolation of the fitted NURBS toolpath has also been implemented using the multi-segment Feed Correction Polynomial. This method provides an adaptive mapping between the nonlinear relationship of the NURBS curve parameter and the curve displacement to allow for a consistent feedrate in the cutting motion. Additionally, the kinematic compatibility conditions are considered based on the inverse kinematics of the 5-axis CNC machine. The proposed algorithm ensures that an overall efficient feed constraint is placed such that none of the individual drives are overdriven. The results from experiments and simulations are presented to demonstrate the effectiveness of the developed trajectory generation algorithms.