Rotary Axis Orientation Measurement Using Spherical Calibration
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Solution Overview
Problem
Existing machine tool systems face challenges in accurately determining the orientation of rotary axes relative to linear axes due to errors introduced by calibration ring skewness and imperfect mounting surfaces, leading to inaccuracies in probe deflection offset measurements.
Innovation Solution
A method using a spherical calibration device to calibrate the probe, allowing it to measure the center of a sphere at various positions, which eliminates the need for a calibration ring and improves accuracy by using identical geometry for both calibration and measurement, thereby reducing errors in determining rotary axis orientation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a calibration ring is used to calibrate the probe, then the probe can be calibrated to measure the rotary axis orientation, but errors are introduced due to calibration ring skewness and imperfect mounting surfaces
Solution Approach 1:
Instead of using a calibration ring that requires mounting on the rotary axis (which introduces errors from skewness and mounting surface imperfections), the invention inverts the approach by using a sphere mounted on the linear axis to calibrate the probe. The sphere's center is measured at multiple rotary axis positions, and the rotary axis orientation is determined from these measurements. This inversion eliminates the mounting errors associated with calibration rings while achieving the same calibration objective.
Solution Approach 2:
The invention changes the geometric parameter from a circular calibration ring (2D) to a spherical calibration device (3D). The sphere provides a center point that can be measured at multiple angular positions, and its spherical geometry ensures that the center location remains constant regardless of the rotary axis orientation. This parameter change from ring to sphere eliminates the sensitivity to mounting errors while maintaining calibration capability.
2Ease of manufacture
If a calibration ring is used, then probe deflection offset measurements can be performed, but inaccuracies arise from incorrect deflection offsets due to mounting errors
Solution Approach 1:
The invention inverts the calibration sequence: instead of calibrating the probe to the rotary axis directly (which propagates mounting errors to deflection offset measurements), it measures the sphere center at multiple rotary positions first, determines the rotary axis orientation from these measurements, and then uses this determined orientation for accurate deflection offset calibration. This inversion breaks the error propagation chain while maintaining calibration simplicity.
3Reliability
If a spherical calibration device is used instead of a calibration ring, then the need for a calibration ring is eliminated and accuracy is improved, but the measurement process requires measuring the sphere center at multiple positions
Solution Approach 1:
The spherical calibration device serves multiple functions: it calibrates the probe, determines the rotary axis orientation, and provides reference points for measuring linear axis perpendicularity to the rotary axis. By using a single spherical device for multiple measurement objectives, the invention eliminates the need for separate calibration ring procedures, reducing overall measurement time while improving accuracy through the sphere's geometric properties.
Data Source
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AI summary
The present disclosure includes a method for use on a machine tool system having a controller, three linear axes of motion and at least one rotary axis, for determining the orientation of the rotary axis relative to the linear axes, including mounting a sphere to a component of the system that rotates about the rotary axis, rotating the component to move the sphere to at least three positions about the rotary axis, measuring a center of the sphere at each of the positions by using the controller to move a a probe mounted to a spindle of the system into contact with the sphere, computing, using the controller, a plane that fits the center measurements, and computing, using the controller, a vector normal to the plane that passes through a center of rotation of an arc that lies in the plane and fits the center measurements, the vector corresponding to the orientation of the rotary axis.