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2.6 Degrees of Freedom
Prior
to
covering
the
individual units
a
brief
discussion
of
the
degrees
of
freedom
that
an
object
(machine
table,
X-Y
stage,
etc.)
can
experience
as
it
slides
along
a
pair
of
ways
will
clarify
some
of
the
terminology
(Figure
2-5).
As
the
object
moves
linearly
in
the
X-axis
direction
there
are
six
degrees
of
motion
which
will
affect
the
final
position
of
the
object.
Besides
the
posi-
tioning
error
along
the
X-axis
which
relates
directly
to
the
accuracy
of
the
linear
scale,
the
object
can
also
experience
angular
rotations
about
the
X,
Y,
and
Z
axes
known
a s
roll,
yaw,
and
pitch,
respectively.
Pure
translational
motions
in
the
Y
and
Z
axes
are
identified as
vertical
and
horizontal
out-of-straightness
move-
ments
respectively.
In
total,
there
are
six
degrees
of
freedom
of
motion
which
will
affect the
final
position
of
the
object
as
we
command it
to
move
in
the
X
direction.
If
one
considers
a
typical
3-
axis
positioning
system
then
there
are
18
degrees
of
freedom
(6
degrees
of
freedom
per
axis)
plus
errors
introduced
by
out-of-squareness
between
axes,
21
potential
error
sources
in
all,
which
combine
together
to
define
the
final
position
achieved.
Figure
2-5.
The
Six
Degrees
of Freedom
Y-AXIS
OF
VERTICAL
STRAIGHTNESS
X-AXIS
OF
LINEAR
DISPLACEMENT
J
2.7
General
Considerations
for
Mounting
Optics
When deciding where and how to mount the system's optics, keep the following points in
mind:
a.
Vacuum adhesive with low volatility sealant is used to hold the optical components.
b.
Additional information including the method of calculating path loss to ensure that
each axis has sufficient beam power is included in Accuracy Considerations.
c.
If the laser beam has to go through a window (for example into a vacuum chamber)
the window must meet the following requirements:
1. A minimum window diameter of 2 5 mm (1 inch) with a minimum thickness
of 8 mm (0.3 inch). Larger diameter windows must be proportionally thicker.
2.
A figure of transmission of A/20 over 23 mm (0.9 inch).
3. Parallelism of faces of +2 minutes.
4.
Surface quality 60-40 per MI L-0-13830.
5.
But
most
important,
be
sure
there
is
no
strain
in
the
glass.
2.8
5501A Laser Transducer
This paragraph covers only the laser beam orientation and mounting information for the
laser head. Refer to SECTION Ill for the electronic theory of operation of the laser head.
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