HP 5501A Operating And Service Manual page 72

NOTE
A rule of thumb which is helpful for approximating the error attrib-
utable to angular motion i s that for each arcsecond of angular
motion, the error introduced is approximately 0.1 microns per 20
mm of offset (5 microinches per inch of offset).
When considering a specific machine, make every effort to direct the measurement path as
close as possible to the actual work area where the cutting or measurement process takes
place. In Figure 2-43 a machine slide is shown with the interferometer and retroreflector
placed to minimize Abbe error. The measurement axis is placed at approximately the same
level as the work table and is also measuring down the center of the machine slide.
I
1
MACHINE SLIDE
1
1
-
,
\
1
\
INTERFEROPAETER
TOP VIEW
/
\
LASER BEAM
WORKING
RETROREFLECTOR
SURFACE
MACHINE SLIDE
/
-
/
SIDE VlEW
Figure 2-43. Positioning of Measurement Axis to Minimize Abbe Error
Although compromises must often be made because of mechanical interference or inaccessi-
bility, the closer the measurement path of the Laser Transducer System i s placed to the work-
ing surface, the smaller the measurement error is due to geometric inaccuracies of the
machine.
With machines that exhibit small geometric errors, it may be adequate to mount the interfero-
meter on the side of the machine slide and below the work surface without significantly af-
fecting the measuring or cutting accuracy.
A special case where the Laser Transducer System can make a significant contribution to
minimizing Abbe error is on high precision X-Y stages. Using the 10706A Plane Mirror Inter-
ferometer in conjunction with plane mirrors, mounted at 90° to each other on the top edges
of the X-Y stage, a very accurate positioning system which almost completely eliminates
Abbe error
is achieved. Figure 2-44 shows a typical installation for an X-Y stage. The principal
advantage of this type of positioning system is that the measurement in both X and Y axes
takes place at the work surface. If there are angular errors in the cross slides of the stage, any
displacement of the work surface due to these errors is measured by the Laser Transducer
System.