Metrology Frame Isolation for Hexapod CMM Accuracy
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Solution Overview
Problem
Non-Cartesian coordinate measurement machines (CMMs) face challenges in maintaining measurement accuracy due to distortions caused by load forces and thermal expansion in the thrust frame, which are transmitted to the metrology frame, leading to inaccurate results.
Innovation Solution
The implementation of a separate metrology frame with a low coefficient of thermal expansion, isolated from the thrust frame using kinematic mounts and attachment means that prevent distortion transmission, ensuring the metrology frame remains undisturbed by load forces and thermal changes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the metrology frame is integrated with the thrust frame, then the device complexity is reduced, but measurement precision deteriorates due to distortion transmission
Solution Approach 1:
The apparatus is divided into two independent frames: a thrust frame for load-bearing and positioning functions, and a separate metrology frame for measurement functions. This segmentation prevents distortion transmission from the thrust frame to the metrology frame, thereby maintaining measurement precision while accepting increased structural complexity.
Solution Approach 2:
The metrology frame is extracted from the thrust frame structure, creating an independent measurement system. This extraction isolates the measurement functions from the distorting influences of the thrust frame, allowing high-precision measurements even though the overall device complexity increases.
2Measurement precision
If the metrology frame is isolated from the thrust frame, then measurement precision is improved, but device complexity increases
Solution Approach 1:
A magnetic coupling mechanism acts as an intermediary between the thrust frame and metrology frame. The magnets provide a fixed spatial relationship and positioning function while allowing thermal and mechanical isolation, thus achieving measurement precision without excessive structural complexity.
Solution Approach 2:
Traditional mechanical connections between the thrust frame and metrology frame are replaced with magnetic coupling. This substitution eliminates mechanical distortion transmission while maintaining the necessary spatial relationship, improving measurement precision without requiring complex mechanical isolation structures.
3Strength
If the thrust frame is made rigid to withstand heavy loads, then strength is improved, but measurement precision deteriorates due to thermal expansion and distortion
Solution Approach 1:
The segmentation of thrust and metrology functions into separate frames allows the thrust frame to be optimized for strength and load-bearing capacity without compromising measurement precision. The metrology frame independently handles measurement functions, isolating it from thermal expansion and distortion issues in the thrust frame.
Solution Approach 2:
The metrology frame is extracted from the thrust frame structure, removing it from the path of thermal and mechanical distortions. This allows the thrust frame to be made highly rigid for heavy loads while the separate metrology frame maintains measurement accuracy.
4Measurement precision
If materials with high thermal stability are used in the metrology frame, then measurement precision is improved, but manufacturing cost increases
Solution Approach 1:
The metrology frame is extracted as a separate component that can be manufactured from expensive, thermally stable materials like Invar or Zerodur. This extraction allows concentrated investment in material quality only where it matters for measurement precision, rather than requiring all components to be made from expensive materials.
Solution Approach 2:
High-quality, thermally stable materials are used specifically in the metrology frame where measurement precision is critical, while the thrust frame can use more cost-effective materials. This local application of quality optimization achieves measurement precision while controlling overall manufacturing cost.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enhances measurement accuracy by isolating the metrology frame from distortions, maintaining its precision and stability, even under varying thermal conditions and heavy loads, thereby providing higher accuracy compared to prior art.
Implementation Method 1
the base attachment means comprises a kinematic mount. A kinematic mount is preferred, although by no means essential, as it constrains the six degrees of freedom between the metrology base and the load carrying base without providing any redundant constraints.
Implementation Method 2
the base attachment means may conveniently comprise at least one magnet. The one or more magnets may be fixed to the metrology base and/or the load carrying base, as appropriate, such that the bases are urged together by the effect of magnetic attraction.
Implementation Method 3
The metrology frame conveniently has a different coefficient of thermal expansion than the thrust frame. Preferably, the metrology frame has a lower coefficient of thermal expansion than the thrust frame.
Data Source
AI summary
Position measurement apparatus, such as hexapod co-ordinate measuring machine, is described that comprises a thrust frame and a metrology frame. The thrust frame comprises a load carrying base connected to a moveable platform by a plurality of powered extendable legs. The metrology frame comprising a metrology base attached to the load carrying base by a base attachment mechanism such as kinematic mounts. The base attachment mechanism is arranged to prevent any distortions of the load carrying base being transmitted to the metrology base.