CAD based measurement

Accurate measurement of free-form engineering parts poses a number of challenges that are not adequately met by the conventional inspection methods. Issues include:

• Important to control measurement of each point
• Point distribution must match subsequent processing
• Elimination of outliers
• Need to accurately measure fine features: small radii of curvature
• Measurement process optimisation
  - collision-free path
  - overall measurement time
  - machine-specific issues

Measurement planning tools

• Import and visualisation of CAD models
• User control of measurement distribution according to several sampling criteria 
  - uniform sampling in u & v directions
  - chord-height criterion
  - parameterisation-based criterion
  
  
• Probe path generation (DMIS, CMES) based on selected measurement pattern, stand-off distance etc.
• Probe path verification/simulation executes the CMM code
• Probe radius compensation based on offset surfaces

The Iterative Closest Point (ICP) method was adopted to perform alignment between the the point data and the CAD model. The implemented algorithms involve a number of significant imporovements including for dealing with CAD models defined using NURBS and large measurement data sets:

• Computational efficiency
• Robustness - global convergence
• Accuracy

The following example shows a complex CAD model defined using NURBS (part of a marine propeller) and a large set of measured points. The initial error can be clearly seen in the colour coded visualisation. The final result may require several hundred iterations of the ICP algorithm, emphasising the need for high computational efficiency.