HP 5501A Operating And Service Manual page 82

2.44
Visual Alignment Procedure
The visual alignment technique relies on the principle that if the measurement beam to the
retroreflector i s not parallel to the direction of travel, it is offset upon recombination with the
reference beam of the interferometer (see Figure 2-57). When the interferometer and retro-
reflector are at the closest point to each other, the laser beam and optics are adjusted such
that the reference and measurement beams completely overlap. When either the retroreflector
or interferometer are moved along the measurement path, any angular misalignment causes
a
displacement of one laser beam with respect to the other which can be visually observed.
Since the human eye can resolve a displacement of the beam of approximately 300 micrometres
(0.01 inch) this technique can be applied for measurement travel of 0.5 metres (20 inches) or
longer. For travel less than this, the sensitivity of this technique is normally not sufficient and
autoreflection alignment should be used. The cosine error (E) in parts per million can be calcu-
lated from the following formula:
Where D i s the distance traveled in millimetres (inches) and S i s the lateral offset of the re-
turning beam in micrometres (thousandths of an inch). For example, if the distance traveled
is 25 inches and this results in an offset of the return beam of 0.050 inches then:
E
=
(50)2
=
0.5 parts per million or 0.5 microinches error per inch of travel
(8)
x
(2512
The techniques describing the two-axis visual alignment procedure can be followed for almost
any measurement configuration. Figure 2-53 i s a typical measurement configuration which
includes a linear interferometer and a single beam interferometer. In general, when the optical
components are installed on the machine, their optical centerlines will be nominally in the
correct relationship and only minor adjustments should be required.
When starting the adjustment procedure, one-axis at a time is adjusted. The first axis to be
adjusted is the axis where any angular adjustment of the laser beam requires adjustment of
the 5501A Laser Transducer (see X-axis, Figure 2-53). After angular adjustment, the laser
head is locked down and any angular adjustment of the laser beam in the other measurement
axes is accomplished by rotating the optical components. For visual alignment of the measure-
ment system in Figure 2-53, perform the following procedure:
NOTE
Steps 1 through 10 constitute the X-axis visual alignment procedure.
1. Place the interferometer alignment target on the laser side of the X-axis interfero-
meter and place the receiver alignment target on the receiver so that it i s not in the
laser beam (see Figure 2-53c, position 1). Place a piece of opaque material between
the interferometer and the retroreflector.
2. With the retroreflector and interferometer at this closest point, adjust the laser head
until the laser beam passes through the 50% Beam Splitter, enters one hole of the
alignment target on the interferometer and exits the other to impinge on the
receiver alignment target centered on the hole over the photodetector. A slight lateral
adjustment of the interferometer or laser head may be required.
3. Remove the opaque material from between the retroreflector and interferometer and
rotate the receiver alignment target to position 2 (see Figure 2-53c).
4. Adjust the retroreflector to center the return measurement beam on the receiver align-
ment target.