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If the 10706A Plane Mirror Interferometers are installed with their measurement centerlines
intersecting on the axis of the fixed cutting tool, measuring probe, lens centerline, etc., (Point
A), then X-Y position errors due to any yaw angle errors can be corrected. For example, if the
X-Y stage undergoes an angular error
6,
then the point of interest A i s displaced to A'. Since
this angular error also generates an X- and Y-axis position error .of
a
and
C I
in the X and Y
directions respectively, point A' can be moved back to point A which will be the correct
position.
2.36
Deadpath Errors
When installing any displacement transducer in a positioning system, temperature effects causing
thermal expansion of the system can be a significant source of error. When using the Laser Trans-
ducer System a s a displacement measuring device, proper positioning of the interferometer and
retroreflector can minimize this source of error. The error manifests itself primarily as a zero shift
in the positioning system and is considered as a component of deadpath. The following informa-
tion i s concerned mainly with the installation aspects of this error source. A more detailed dis-
cussion of deadpath error i s contained in Accuracy Considerations.
I n general, when installing the interferometer and retroref lectors in a positioning system,
make every effort to ensure that these components are almost in contact physically when
the machine is at its coordinate system zero point. In Figure 2-46 the interferometer and
retroreflector are installed with the distance AB at a minimum when the saddle is at its closest
point of travel to the machine table. In addition, place the fixed component of the measuring
system (interferometer or retroreflector) as close as possible to the machine reference system.
This helps compensate for thermal expansion.
If, because of interference or inaccessiblility, it is not possible to mount the interferometer
and retroreflector in close proximity to each other, provision is made to compensate for this
problem. The compensation is made in the interface electronics in conjunction with the
controller.
RETROREFLECTOR
I
INTERFEROMETER
I
Figure
2-46.
Installation
of
Optics
for
Minimum
Deadpath
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