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| Department 5.3 |
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Coordinate Metrology
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Tactile-optical probe to measure small bores and 3D microstructures
As a result of the increasing
miniaturisation of mechanical features and components, conventional tactile
coordinate metrology is often confronted with major problems. At the
Physikalisch-Technische Bundesanstalt (PTB) a sensor for coordinate measurement
machines has been developed, which combines some advantages of optical and
tactile measuring techniques. With this sensor very small parts (<0,5 mm)
can be measured.
Application fields of this sensor are in particular
the measurement of small bores (e.g. injection nozzles), micro-mechanical components,
threads, plug-in connections (microelectronics and optical wave-guide systems),
and very flexible work pieces (small wall thickness or elastic materials).
The principle of the tactile-optical sensor is
described in the following: The position of the probing element is determined
with the aid of a CCD camera image-processing system. An optical fibre serves
as the probing stylus. Its tip is shaped spherical and illuminated through the
fibre. The optical fibre is mounted on the optical system and the probing
element is adjusted to the focal plane and close to the optical axis. The light
backscattered by the sphere is thus imaged on the CCD sensor as a bright spot.
When the probing element contacts the work piece surface and is thus shifted in
relation to the camera, the position of the light spot on the sensor is
changed. This relative change in position can be evaluated with sub-pixel
accuracy.
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Schematic
view of the 2D microprobe: (1) work piece, (2) probing element, (3) measuring
camera, (4) CCD chip, (5) glass fibre, (6) light source
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In an extended set-up for 3D-measurement a second
camera arranged perpendicular to the fibre allows the measurement of the
Z-displacement of the a second target that has been provided to the probe
shaft.
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| Sensor
for tactile-optical 3D measurements with components: (1) probing element and
target for the XY-measureme--+nt, (2) target for the Z-measurement, (3) optical
system for the measurement of the XY-displacement, (4) optical system for the
measurement of the Z-displacement, (5) reflecting mirror, (6) camera for
XY-evaluation, (7) camera for Z-evaluation, (8) adjusting device |
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First Camera
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Second Camera
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| CCD-images during the
measurement of an injection nozzle (diameter: 210 µm) |
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The principal advantage of the sensor is that all influences due to an
elastic or plastic deformation of the contact stylus are eliminated. The
contact stylus may, therefore, be extremely thin. Fibre diameters <10 µm and
probing ball diameters of 15 µm have been realised. The probing force is very
small (< 1µN). The 2D probing error according to VDI/VDE
2617 is smaller than 0,5 µm.
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