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Department of Mechanical Engineering
Department Of Mechanical Engineering		 K.U.Leuven
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Dimensional Metrology and Quality Control - Research Topics

In recent years research has focussed on 3D digital coordinate measuring technology, and on reduction of thermal deformation of machine tools and measurement equipment. 3D digital coordinate measuring technology includes both mechanical and optical probing. Reduction of thermal deformation is achieved by developing machine design methodologies resulting in a more stable thermal behaviour. Residual thermal deformations (due to uniform static temperature variations, as well as spatial and temporal temperature gradients) are compensated by estimating the relation between temperature and deformation using physical models (i.e. kinematic and thermal models). Alternatively, neural networks or multi-variable linear regression techniques have been used, but they proved to be less suited to estimate and compensate transient thermal deformations.

 

 

Thermal compensation of machine tool centres and coordinate measuring machines

The ever increasing demands for higher quality products necessitate more accurate machines. The relative importance of temperature dependent errors on machine accuracy grows. PMA has presented significant results with indirect thermal correction techniques developed for machine tools as well as for coordinate measuring machines (CMM). The indirect correction uses a mathematical model to calculate the thermal errors based on an on-line recording of the temperature distribution within the machine structure. For a five-axis milling machine software compensation based on a neural net (NN) has been developed. Estimated deformations are fed back to the controller for corrective purposes. As an alternative, an active correction that directly alters the temperature distribution of the machine was implemented. Both techniques resulted in 80% reduction of the thermal errors.

For CMMs thermal models based on purely mathematical algorithms like NN, showed good potential. However, the absence of dominant internal heat sources allowed an approach with a more direct user input. A library of physical expansion equations was created. Based on this a parametric thermal model for a CMM can be readiily created. Practical tests show a significant error reduction under realistic thermal loads, both with steady-state and transient temperatures. More recently the attention shifted to novel direct thermal correction techniques. Hereby the goal is to measure the thermal errors on-line and use the measured deviation for feedback to the controller. The experience with thermal deformations finds application in different areas of precision engineering because temperature variations affect the majority of machining and measurement processes.

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Geometric calibration and software error correction of 3D-CMM

The increased demand for flexible quality control in the workshop results in more CMMs being used by nonspecialised personnel and in a harsh, hostile environment. In this context a yearly periodic verification of the CMM is no longer sufficient to remain confident in the machine's accuracy. Industry needs a standardised approach for a quick "daily" interim check. For this aim, PMA collaborates with different EU partners on the design of a universal artefact. PMA's contribution focuses on the thermal behaviour of the artefact and time reducing measurement and evaluation procedures. Such universal artefact enables objective intercomparison of different 3D measurement devices, especially interesting now that new mobile systems are entering the market. To facilitate performance verification of CMMs, existing machine tool (MT) evaluation software and procedures are adapted for application. Assessing the volumetric accuracy of a device is often problematic. Conventional (laser interferometer) procedures used in the MT field can not be applied blindly to CMMs. The influence of the geometrical and thermal correction packages must be considered. Standardisation is not yet in pace with the market demands in this field. Based on the research, recommendations will be made to the ISO. Updated procedures should give new measurement systems more opportunities to compete with conventional solutions.
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Productions: | Last update: 08/17/2005
URL: http://www.mech.kuleuven.be/metr/research/index.en.html