CNC Two-Stage Interpolation for Path Accuracy
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Solution Overview
Problem
Traditional CNC path generation algorithms face significant accuracy issues when dealing with high speeds and high accuracy requirements, particularly due to adverse effects from acceleration and override control, leading to path errors and undesirable effects such as corner rounding and arc failures.
Innovation Solution
A two-stage interpolation algorithm is employed, where a coarse interpolation step calculates path segments with a speed override factor, followed by a fine interpolation step that generates individual setpoints for each axis at the servo update rate, allowing for independent acceleration control and flexible override management, decoupling acceleration control from programmed move boundaries.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If traditional CNC path generation algorithms are used with acceleration control and override control, then the machine can operate within physical capabilities and respond to manual adjustments, but the accuracy of the commanded path deviates significantly from the desired path
Solution Approach 1:
The algorithm segments the toolpath into discrete segments and processes each segment independently with its own acceleration control parameters. This allows override control to be applied segment-by-segment without affecting the entire path, maintaining accuracy while enabling manual speed adjustments.
Solution Approach 2:
The algorithm pre-calculates acceleration profiles and override factors for each path segment before execution. By preparing acceleration control data in advance for each segment, the system can rapidly respond to override commands without introducing significant path errors during runtime.
2Productivity
If high feedrates are used to increase productivity, then machining speed improves, but path errors increase due to acceleration control filtering effects
Solution Approach 1:
The algorithm dynamically adjusts acceleration profiles based on the current feedrate and path geometry. At high feedrates, the system optimizes acceleration parameters for each segment to minimize path errors while maintaining high speed, rather than using fixed filtering parameters that degrade accuracy.
Solution Approach 2:
The system changes acceleration control parameters dynamically based on feedrate requirements. Instead of using a fixed low-pass filter time constant, the algorithm adjusts acceleration profiles segment-by-segment according to the programmed feedrate, allowing high-speed operation with maintained accuracy.
3Reliability
If traditional low-pass filtering is applied for acceleration control, then acceleration limits are respected, but corner rounding and arc failures occur due to phase lag
Solution Approach 1:
The algorithm divides the toolpath into segments and applies acceleration control independently to each segment. This eliminates the cumulative phase lag effect of continuous filtering, allowing the system to respect acceleration limits while maintaining sharp corners and accurate arc geometries without the rounding errors introduced by traditional filtering.
Solution Approach 2:
Instead of applying low-pass filtering to smooth acceleration, the algorithm inverts the approach by pre-calculating acceleration profiles that inherently satisfy acceleration limits. This reverse engineering of acceleration control eliminates the need for filtering that degrades path geometry.
4Ease of operation
If override control is implemented with traditional filtering, then manual speed adjustment is possible, but the response to override changes is delayed due to filter time constants
Solution Approach 1:
The system applies override control at the segment level rather than through continuous filtering. When an override command is given, the algorithm immediately recalculates acceleration profiles for subsequent segments with the new speed factor, eliminating the response delay inherent in filter-based systems.
Solution Approach 2:
The algorithm pre-calculates how acceleration profiles will adjust to override commands for each segment. This preliminary preparation allows instant response to override inputs without the time constant delays that characterize traditional filtered override control systems.
Data Source
AI summary
A system and method for generating a controlled motion path with a machine-tool system having a controller operating in a servo rate. The method includes a first interpolation step, in which a path segment corresponding to a number of servo update periods is calculated. After the first interpolation step, a speed override factor is established. After establishing the speed override factor a second interpolation step occurs, which includes calculating a number of path setpoints for each path segment. The number of servo update periods per path segment may vary over the path.