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RESET
(0
WINT)
"DEADPATH
"MEASUREMENT
PATH"
RESET
(0
POINT)
r
( 8 )
3
*
P
CORRECT
SETUP FOR MINIMIZATION
OF
DEADPATH
ERROR
Figure
2-29.
Deadpath
Error
Compensation
b.
By
correcting
for
the
residual
distance
L,
in
software
in
the
case
of
the
Computer
l
nterface
Electronics
and
the
Calculator
l
nterface
Electronics.
Provisions
are
made
for
this
correction
in
hardware
in
the
English/Metric
pulse
output
card.
In
this
case
re-
lation
,@
is
expanded
to
the
more
general
relation:
0
distance
=lwavelengths
of
deadpath
+
wavelengths
of
motion)
air
wavelength
vacuum
wavelength
X
vacuum
wavelength]-
(deadpath
in
inches
or
millimetres)
You must input the terms "wavelengths of deadpath" and "deadpath in inches or millimetres".
These terms can be determined a s follows:
Quarter
wavelengths
of
deadpath
=
1.61
x
lo5*
L1
(if
L1
i s
in
inches)
Quarter
wavelengths
of
deadpath
=
6.32
x
IW*
L1
(if
L1
is
in
millimetres)
Deadpath
in
inches
or
millimetres
=
L,
in
inches
or
millimetres
The
deadpath
(L,)
need
not
be
measured
with
high
precision.
The
error
in
measuring
L,
simply
shows
up
a s
an
uncompensated
deadpath
(AL,).
For
example,
if
the
deadpath
of
a
particular
machine
axis
were
10.5
inches
but
was
assumed
to
be
10
inches
then
this
would
result
in
0 . 5
inches
(the
error
in
measuring
L,)
of
uncompensated
deadpath.
The
resulting
0
shift
that
occurs
if
environmental
conditions
changed
during
a
measurement
would
be:
0
shift
=
AL, X (change in air wavelength in parts per million)
If atmospheric pressure changed by
0 . 1
in. Hg, this causes a
1
ppm change in laser wavelength
and
0
shift
=
0.5
in. X
1
ppm
=
0.5
x
10-6
in.
(1
ppm
=
1
X
10-6
in./in.
=
1
X
10-6
mm/mm etc.)
The ability to correct for deadpath error in software does not eliminate the necessity of mini-
mizing deadpath by proper location of the interferometer wherever possible. If the deadpath
(L,) is large compared to the distance traveled (L,), then the predominant error is a
0
shift
due to uncertainty in determining the change in air wavelength and this error cannot be
eliminated in software.
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