Measurement Axis (Cosine Error) - HP 5528A User Manual

Model 5528A
Distance Measurements
6-26
Misalignment Between Machine Travel Axis and Laser Measurement
Axis (Cosine Error)
Description
If the Laser beam's path doesn't duplicate the machine's travel path you won' t be measuring the
actual path, but" path offset by some angle. The resulting error is called "Cosine Error" and
depends o n the true path length and the cosine of the offset angle. (See
Figure 6-13 )
Minimizing Effects
Cosine error is reduced by making the Laser beam parallel to the Optics ' travel path. Parallelism is
Indicated by the absence of motion between the return dots from
I
he Int erferometer Assembly
and the Retroreflector
al
the laser Head's bottom port, as the Optics are moved along the
measurement axis.
The " OVERLAPPING DOTS" alignment method. described in Section
15
of this User's Guide.
describes how to minimize Ihe mol ion between the return dots and, therefore, minimize the
cosine error.
In special shari-distance. high-accuracy, measurement situal ions. lack of motion betwccn the
return dots is not a strong enough assurance of minimal cosine error. In these cases, you should use
the "Max imum Measurement Reading" technique. described in Section lS, to reduce cosine error
to
CI
negligible level.
LASER
BEAM
-1
r-
LASER WAVELENGTH
~'i-"
~~t'
,~'i-
p'
.. d'
II
I "', I
III
, I
I
I",
LASER BEAM DIRECTION p"
Figure
6-13. Cosine
Error