In machining complex dies, molds, aerospace and automotive parts, or biomedical components, it is crucial to minimize the cycle time, which reduces costs, while preserving the quality and tolerance integrity of the part being produced. To meet the demands for high quality finishes and low production costs in machining parts with complex geometry, computer numerical control (CNC) machine tools must be equipped with spline interpolation, feedrate modulation, and feedrate optimization capabilities. This thesis presents the
development of novel trajectory generation algorithms for Non Uniform Rational B-Spline
(NURBS) toolpaths that can be implemented on new low-cost CNC's, as well as, in
conjunction with existing CNC's. In order to minimize feedrate fluctuations during the
interpolation of NURBS toolpaths, the concept of the feed correction polynomial is applied. Feedrate fluctuations are reduced from around 40 % for natural interpolation to 0.1 % for interpolation with feed correction. Excessive acceleration and jerk in the axes are also avoided. To generate jerk-limited feed motion profiles for long segmented toolpaths, a generalized framework for feedrate modulation, based on the S-curve function, is presented.
Kinematic compatibility conditions are derived to ensure that the position, velocity, and acceleration profiles are continuous and that the jerk is limited in all axes. This framework serves as the foundation for the proposed heuristic feedrate optimization strategy in this
thesis. Using analytically derived kinematic compatibility equations and an efficient
bisection search algorithm, the command feedrate for each segment is maximized. Feasible
solutions must satisfy the optimization constraints on the velocity, control signal (i.e.
actuation torque), and jerk in each axis throughout the trajectory. The maximized feedrates
are used to generate near-optimal feed profiles that have shorter cycle times, approximately
13-26% faster than the feed profiles obtained using the worst-case curvature approach, which
is widely used in industrial CNC interpolators. The effectiveness of the NURBS interpolation,
feedrate modulation and feedrate optimization techniques has been verified in 3-axis
machining experiments of a biomedical implant.
| Identifer | oai:union.ndltd.org:WATERLOO/oai:uwspace.uwaterloo.ca:10012/3770 |
| Date | January 2008 |
| Creators | Heng, Michele Mei-Ting |
| Source Sets | University of Waterloo Electronic Theses Repository |
| Language | English |
| Detected Language | English |
| Type | Thesis or Dissertation |
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