Model
5528A
Distance
Measurements
5.
Calculate the amount
of
cosine
error
as
follows -
a.
Judge the amount of motion
between
the dots.
The
smallest
motion
you are likely
to
perceive
is
about 0.
25
mm
(0.010 inch
).
let
this
distance be
"5
" in the formula
below
.
b.
Determine the length of machine (
Optic
) travel.
let
Ihis be
"D"
in the formula
below.
c. Your cosine
error
("E
n)
will
then
be-
E =
~,
in parts-per-million,
or-
802
E X 0 , in absolute terms,
For
elCample,
assume that -
The movement of the dots
on
the
laser Head's
turret
is
0.5 mrn (
0.020
inch),
and-
The amount
of
machine (
Optic
) travel that
produced
th
is
movement
is
500
mm
(
20
inches
).
The amount
of
cosine
erro
r
wou
ld be -
0.13
ppm,
or
0.00006
mm
(0.000003
inc
h)
6.
Usc the "E
rror
Budget Worksheet"
(See
"W
HAT
AFFECTS
ACCURACY?") to find out
jf
the
cosine
error
in
your setup
is
within
the limits you need for your measurement. If the cosine
error
is
larger than you
can
tolerate, use the "OVERLAPPING DOTS" a
lignm
e
nt
method
(described in Section
15
of
this User's
Guide
) to reduce any
motion
between the dolS.
tn some
cases
where the travel path
length
is
ve
ry short, there may still be cosine error, even
though
there
seems
to
be no
molion
between the dots. For elCample,
if
-
5
=
0,2
mm
(0.008
inch), and -
0
"'"
125
mm
(
4.92
inches), then -
E
= 0.3ppm, and the
absolute error
::::
0.3
ppm
X
125
mm
==
0.
00004
mm (0.
000002
inch
).
In
these
cases,
you should
use
the "
Maximum
Measurement Reading"
methou
to reduce
cosine error.
Measurements Along A Perpendicular
Axis
When a measurement
axis
Is
perpendicular
to
the
Laser
beam,
mount
the
Interferometer
in
line
with th e laser, and
at
the end
of
the
alCis
you
will
be measuring (
see
Figure 6-9
for
illustrations
of
set-ups
for
Distance measurements
perpendicular
to
the
Laser
beam
).
Be sure
to
rotate the
Interferometer
so
the arrows
on
its side
line
up
with
the
Laser
beam and
the
measurement
a)(is.
To align the
La
ser
Measurement System along
an
alCls
perpendicular
to
the
laser beam, attach
the
Interferometer
target to the opening facing
the
Laser
Head (
see
Figure 6-
7)
and
follow
the
instructions presented in the preceding few pages.
6-
15
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