| Department 5.3 |
|
Coordinate Metrology
|
|
|
|
Calibration of 2D artefacts
for optical probing
In acceptance testing and for the periodic inspection of
conventional coordinate measuring machines, ball plates are
widely used by industry. For coordinate measuring machines applying
an optical probing technique, two-dimensional artefacts of adequate
accuracy have not been available to date because a suitable
and traceable calibration method was lacking. Coordinate Metrology
Section has developed
a new method by which artefacts for optical probing can be calibrated
in two dimensions, with high accuracy, on the basis of purely
interferometric length measurements.
|
The method is called multilateration and is practically
applied at Coordinate Metrology Section in a commercial 3D coordinate measuring
machine equipped with CCD camera. For this purpose,
the measuring machine has been provided with a two-beam
plane mirror interferometer and a rotary table.
|
|
|
|
Photograph of the measuring system
|
|
|
The method is based on the measurement of a large
number of distances on the 2D artefact, between
two reference marks each. The measurements can be
performed with interferometric accuracy. In a second
step, a mathematical method composes the distances
to a two-dimensional net of coordinates.
|
|
Composition of the distance measurements to a
two-dimensional net
|
The multilateration method offers the following advantages:
- The exclusive measurement of lengths ensures direct
traceability to the metre, the SI unit. Uncertainty influences
are essentially limited to the laser interferometer and
to optical probing of the reference marks. Errors in orthogonality
and straightness do not affect the measurement result.
- Drift effects on the measurement result are minimized.
- For the time being, the method can be applied to artefacts
(700 x 700) mm2 in size.
- The artefacts may be provided with different structures
for optical probing, e.g. crosses, circles, rectangles.
- An accuracy unattained to date can be reached for large
artefacts.
- The measurement process is fully automatic.
When the artefacts used are Zerodur plates on which circular
marks are to be probed, an uncertainty of measurement of U =
0,5 µm (k = 2) can at present be attained for the coordinates
upon calibration. This is valid for a maximum length of the
diagonals of up to 1000 mm.
Potential applications
Potential applications are the
testing/calibration of PCB assembly machines used by the semiconductor
industry, and flat screen quality control - besides coordinate
metrology based on optical probing, where the calibrated artefacts
can be used to check the geometrical accuracy of the measuring
instruments.
Contact: Dr. K.Wendt Dipl.Ing.(FH)
J. Hirsch
 | © Physikalisch-Technische Bundesanstalt
Page created: 2004-04-26, last update: 2004-07-14, Silke Hube |
|
