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Model
5526A
Distance
M
eas urements
Uncompensate
d Measurement Path
Le
ngth (Dead path
Error)
Descriptio
n
Thi
s is
iI
comp lication of the
"Veloci
l
y
"
O
r
~
Light
"
error.
Compensa tion
for
ve
locit
y-or-ligh
t error
is
applied on ly in
proportion
10
the
distance
measured
.
The
error, however, exists ove
r
the
who le
path between the
Optics,
including th
ose
parts not
measured
.
Any ti me
the laser Mea surement
System
is
reset
with some
di
st
an
ce
c)(isting
between
the O
pt
ics, the length of
the
"ope n"
di
stance
will
be
unco mpensa ted,
and
w
il
l resul
t
in
an error.
The
nam
e
for this kind
of
error
is
"
Deadpa
th Error
".
Minimizing Effects
To minimize
the amount or
thi
s
error, the only time
you should
reset the laser Measurement
Sys
tem
is
when
the
Optics are as dose
IOgelher
as
possible.
The on ly way
this
error can
be
elimina ted
is to reset
th
e
Laser Measurement
System w
hen
the
Optics
j
not
thei
r
bases)
are
ac
tuall
y touching.
ror most
appliciil tions, reselli ng when the
Opti
cs
are wiThin
five
centimeters
[2
inchesl
will
be
enoug
h
to
nullify
th
e
error.
Using
The "
Error Budget
Worksheet"
The
objective of this
worksheet is to maximi
ze
th
e
La ser Measurement
Syste
m
accuracy
level
by
"
budgeting
"
am0l18 potential
sources of
error.
Use
this
worksheet
for
each ax is you
measure
.
Follow this
procedure.
usin
g
Figure
6-14
as
an
example.
The
numbers
in
pare ntheses in
th
e procedure
below
are those used
in
Figure 6-'14.
1.
Ident
ify
the axis
('"X'').
2.
Identify
the length of
tra
vel
(SOB
mm,
20
inches).
3.
Determine
the
dccuracy
you
need
.
You
will ge nera
ll
y
need
an
accuracy at
least three
tim
es
betler
than
the
accuracy
specificat
ion
s
of what you are measu ring
iO.00424
mm
,
or
0.000167
inch
).
4.
Identify the accuracy of
yo
ur
compensa tion for
velodty-of-li~ht
(V.O.l.
)
changes.
If
yOll use
the
10751
Air
Sen~or
,
use
the
accuracy value give
n in
the
worksheet fOI
yo
ur
temperature range.
(These
accuracy
Viii
lues
arc
also
given in
th
e
I.
a~er
Measuremenl
System's
Service
Manual.)
If
you
don't
use
the
Air
Sensor. you
should
sti
ll
be
able
to
determ
ine
th
e accuracy va
lu
<,
by
estima
tin
g how accurately you
ca
n
measure
the
ambient
air
's
propert
ies,
and then
relating
thi
s
10
the effect on Distance
mea surement
accuracy.
(See
"V.O.
l.
[R ROR
DESCRIPTIONS"
above.
)
iii.
Fnter
the accuracy
in piilrt s-per
-million
(ppm
)
in
colu
mn
" A".
for our
e)(illllp le, let
's
assume that
you arc using
the
HP
1075 1A Air
Sensor
. and
that
you
arc opera
ting
in the
15-25 Q C
(59-77
Q
FI
range.
The
accuracy
then
is
1.5
ppm
.
6-27
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