CMM Rotary Table Calibration Using 6 DoF Error Mapping
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Solution Overview
Problem
Conventional Coordinate Measuring Machines (CMMs) face challenges in accurately measuring difficult-to-reach points on objects due to error-inducing factors such as dynamic thermal and mechanical stress, humidity, and inconsistent material response, particularly when additional rotation axes like rotary tables are introduced, leading to limited measuring accuracy and increased costs.
Innovation Solution
A method and system for autonomously calibrating rotary tables in CMMs by generating an error map through frequent measurements of the 6 degrees of freedom (6 dof) pose of a jig, using the CMM itself as a reference, allowing for error correction and improved accuracy without requiring high-precision bearings, and enabling overall calibration of the CMM system.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a rotary table with additional rotation axes is introduced to measure difficult-to-reach points, then the measuring capability and accessibility are improved, but the measurement precision deteriorates due to error-inducing factors such as dynamic thermal stress, mechanical stress, and humidity
Solution Approach 1:
The patent performs preliminary calibration of the rotary table by measuring a calibration object with known geometry at multiple angular positions before actual measurements. This preliminary action characterizes and stores error patterns (such as circularity errors, concentricity errors, and axial runout) in advance, allowing the system to compensate for these errors during subsequent measurements, thereby maintaining high measurement precision despite the presence of additional rotation axes
Solution Approach 2:
The system implements feedback by continuously comparing measured positions with expected positions based on the calibrated error model. The measured deviations are used to update and refine the error compensation parameters, creating a closed-loop system that actively corrects for thermal, mechanical, and humidity-induced errors, thus maintaining measurement precision while utilizing the rotary table's enhanced versatility
2Measurement precision
If conventional factory calibration or long-term stable construction is used to reduce errors, then the measurement precision is improved, but the manufacturing cost and device complexity increase
Solution Approach 1:
The system performs self-calibration by using its own measurement capabilities to characterize and correct its own errors. The CMM measures a calibration object mounted on the rotary table, automatically generates error maps, and applies corrections without requiring external calibration equipment or expert intervention. This self-service approach reduces both device complexity and manufacturing costs while maintaining high measurement precision
Solution Approach 2:
The patent creates a digital copy or model of the calibration object's known geometry and compares it with actual measurements to identify errors. This virtual reference model serves as a template for error characterization and compensation, allowing the system to achieve high precision without requiring physically complex or expensive calibration hardware
3Measurement precision
If factory calibration is performed to generate an error map, then the measurement precision is improved, but the productivity decreases due to time-consuming calibration processes
Solution Approach 1:
The system performs automated self-calibration using its own measurement system without requiring external calibration equipment or expert intervention. The CMM automatically measures calibration features, generates error maps, and applies corrections, significantly reducing calibration time and improving productivity while maintaining high measurement precision
Solution Approach 2:
The patent implements periodic calibration at predetermined intervals or triggers rather than requiring continuous or frequent calibration. This periodic approach maintains measurement precision over time while minimizing interruptions to productivity, allowing the system to operate efficiently between calibration cycles
Data Source
AI summary
A Coordinate Measuring Machine (CMM) system comprising a CMM, a rotary table, and a rotation arrangement, wherein the CMM system is configured to be calibrated by determining the 6 dof pose of a jig, and to a method for calibrating. The jig may be mounted such that a current pose of the jig is associated with a current pose of the rotary table with respect to the CMM. The rotary table may be moved into multiple poses, and the 6 dof pose of the jig is measured for each of the multiple poses of the rotary table. An error map is generated, based on the angles associated with the poses of the rotary table, and is used to generate a coordinate transformation from the CMM coordinate system to the part coordinate system, which is associated with the rotary table, based on the error map.