The Variable Field Stop (VFS) is a rotary mechanism that reliably positions any of four aperture plates in the optical path of a spaceflight experiment, limiting the amount of light reaching the detectors. This paper discusses the design, operation, and testing of the VFS

Dillman, Robert A.

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DESIGN AND TESTING OF THE
LITE VARIABLE FIELD STOP MECHANISM
Robert A. Dillman*
ABSTRACT
The Variable Field Stop (VFS) is a rotary mechanism that reliably
positions any of four aperture plates in the optical path of a space-
flight experiment, limiting the amount of light reaching the detectors.
This paper discusses the design, operation, and testing of the VFS.
INTRODUCTION
The Lidar In-Space Technology Experiment (LITE) is the primary
payload of a Shuttle mission scheduled to be flown in March of 1994.
LITE will generate a three-wavelength, pulsed laser beam and direct it
down toward the Earth, then catch the return signal from atmospheric
scattering in a 1-meter Cassegrain telescope. The return signal
intensities will be measured by detectors in the Aft Optics subsystem,
mounted behind the primary telescope mirror; later analysis will yield
data on the composition of the upper atmosphere as a function of
altitude.
The VFS is one of the first elements in the Aft Optics encountered by
the beam of light coming in from the telescope. It is designed to limit
the amount of light reaching the sensitive photomultiplier tubes that
are the main sensors in the Aft Optics. On command from ground
controllers, the VFS positions one of three apertures in the beam; a
fourth option is a solid mirrored plate intended for use during any
Shuttle maneuvers that involve pointing the telescope near the Sun.
One of the challenges that this design meets is to allow fine
adjustment of the aperture positions while using a motor with a fixed
step size. The VFS achieves this through use of a flex pivot between the
drive motor and the cam and ratchet that determine aperture
positions. Other requirements on the design include stability and
repeatability of the aperture positions, reliability, fitting in limited
* NASA Langley Research Center
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space in the Aft Optics, survival of shuttle launch and landing
conditions, and operation on-orbit.
VFS HARDWARE
The VFS mechanism is shown in Figure 1, along with an arrow that
represents the light coming from the telescope. The VFS is 22
centimeters (8.5 inches) long, 14 centimeters (5.6 inches) high, and 16
centimeters (6.2 inches) wide, with a total mass of 1.4 kilograms (3.1
pounds). Most visible surfaces are coated with a non-reflective black
paint (see photo in Figure 2.)
The three apertures of the VFS are machined into interchangeable
0.25 centimeter (0.10 inch) thick aluminum plates. Two of the
apertures are circular; the larger is 1.7 centimeters (0.67 inches) in
diameter, and the smaller is 0.53 centimeters (0.21 inches) in
diameter. The third aperture shares both these dimensions, having a
large circular opening with a small obscuration disk supported in the
center. The solid mirrored plate in the fourth position is thicker than
the other three; its thickness was increased to 0.64 centimeters (0.25
inches) to provide more margin for the high thermal load that occurs
while blocking sunlight from the rest of the Aft Optics. The apertures
and the mirrored plate are shown in Figure 3.
These four plates attach to the Aperture Base, which holds them at
45 degrees to the VFS axis of rotation. The active plate is
perpendicular to the incoming light, with the aperture centered on the
beam. The spaces between the plates are blocked by a light baffle, to
prevent stray light from entering the system during aperture changes.
To change apertures, the VFS rotates 90 degrees about its main
shaft, which attaches to the back of the Aperture Base (see Figure 4).
This shaft turns on two bearing rings, which fit inside the shaft
housing. Stop positions are set by a four-position cam, which attaches
to the shaft on the far side of the housing. A ratchet arm rides against
the outside of the cam, held in place by a torsion spring; the rubbing
surfaces of the ratchet and cam are coated with a type III anodize to
reduce wear. The four stop points on the cam each have a protruding
set screw, which is what actually contacts the tip of the ratchet. These
set screws allow adjustment of the cam stop points.
84
Power to rotate the VFS comes from a DC stepper motor, which
moves in 1.5 degree increments. It is connected to the main shaft
through a bellows coupling to prevent axial misalignment from causing
the motor to bind. The motor rotates the cam past each desired stop
point, then backs up to place the ratchet tip in contact with the set
screw from the cam. Since the screw positions are adjustable and the
motor steps are fixed, a flex pivot was also included between the motor
shaft and the main shaft which turns the cam. The stepper motor
backs the cam into contact with the ratchet, and then continues
backing several more steps; the flex pivot takes up the rotation, and its
spring force holds the cam against the tip of the ratchet. This allows
the aperture positions to be controlled by the set screws in the cam,
despite the relatively large step size of the motor. Once the new
aperture is in place, the motor power is turned off; the motor brake
locks it in position until power is applied again.
However, the flex pivot is fairly delicate, and winding it up more
than ten degrees can cause damage. To prevent excessive winding, a
hard stop was added. The hard stop consists of a pair of protrusions
from the coupling that connect the flex pivot to the main VFS shaft.
These protrusions span the joint of the flex pivot and fit into notches in
the end of the bellows coupling, as shown in Figures 5 and 6. When the
flex pivot has wound up six degrees, the protrusions come into solid
contact with the bellows, and prevent further motion of the flex pivot.
An exploded view of the shaft coupling, flex pivot, bellows coupling,
and the pins that attach them is shown in Figure 7.
The VFS attaches to the Aft Optics Bench at three points, using
fasteners which fit through oversized clearance holes in the VFS Base.
Spacer rings between the Bench and the VFS Base adjust the height of
the apertures above the Bench, to roughly center them on the beam of
light; the system is designed to require the spacers even if all parts are
machined perfectly, so it can be lowered as well as raised in height.
Once the VFS Base is positioned properly,, epoxy is injected through
small holes around the three fasteners to lock them in place. Coarse
adjustment of the aperture positions is made by turning the cam set
screws so that the apertures are perpendicular to the incoming beam
of light, and the set screws are then staked in place. Final centering of
each aperture is made by sliding the aperture plates relative to the
Aperture Base, using the clearance on the mounting holes. Once this is
completed, the aperture plate fasteners are also staked in place.
85
TESTING
After assembly, the VFS was functionally tested. Some initial
problems with the stepper motor mis-stepping were fixed by adjusting
the cam set screws so that the coupling hard stop points coincided with
whole steps of the motor. This caused the apertures to tilt slightly
relative to the incoming light and forced them off-center. The
apertures were recentered by opening up the aperture plate fastener
holes to form slots. The aperture tilt could have been corrected by
shimming the aperture plates at the three points where they attach to
the Aperture Base, but calculations showed the tilt to have negligible
effect.
The mechanism was then subjected to random and sine vibration
tests as described in Figures 8 and 9; optical alignment checks were
included to measure the stability of the apertures. The vibration tests
were passed successfully, and the optical tests showed the aperture
alignment to be stable and repeatable to within a few arc-minutes.
While not equal to the stability of a fixed optical mount, this was
deemed acceptable.
Thermal/vacuum testing followed the vibration tests. The VFS was
mounted inside a bell jar (see photo in Figure 10) and put through four
cycles of the profile shown in Figure 11, while under vacuum. The test
included operation of the mechanism at room temperature, after the
one-hour cold soak, and after the one-hour hot soak. The mechanism
operated properly in all cases, and the optical alignment tests were
again acceptable.
After cycling concluded, a strip heater was attached to the mirrored
shield plate; once vacuum was reestablished in the bell jar, this heater
was energized, simulating use of the shield plate to reflect focused
sunlight. Data from the shield test were used to verify the computer
thermal model.
The VFS mechanism was then attached to the Aft Optics Bench and
aligned with the installed optical elements. It operated successfully for
several months, until problems developed with the ratchet spring.
Examination of the spring showed that it had deformed and was no
longer holding the ratchet in contact with the cam. The spring was
replaced and the mounting hardware was modified slightly, giving the
spring a bit more room to expand. Testing of several spare springs
86
indicated no change in spring tension or behavior out to 10,000 cycles,
many times the expected lifetime.
CONCLUSIONS
The VFS achieves its design goal of accurately and reliably
positioning three apertures and one mirrored plate. The mechanism
also allows adjustment of these positions through set screws and three-
point mounting systems. Thorough functional and environmental
testing has qualified the VFS for space flight; the test results indicate
that it will perform as intended in the expected environment.
87
LIGHT BAFFLE
LARGE APERTURE
PLATE
MIRROR
OBSCURED APERTURE
PLATE
-- CAM
RATCHET
SMALL APERTURE
PLATE
Figure 1. VFS Mechanism
Figure 2. VFS Photograph
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LARGE APERTURE SMALL APERTURE OBSCUREO APERTURE MIRRORED PLATE
PLATE PLATE PLATE
Figure 3. Aperture Plates and Mirrored Plate
/--- APERTURE BASE
_>//__--SHAFT
Figure 4. Aperture Base and Shaft
SOLID COUPLING
-- BELLOWS
CAM 7
- \
_ :)
Figure 5. VFS Mechanism
BASE
89
SIX DEGREE GAP
HARD STOP
PROTRUSION
/
Figure 6. Shaft Coupling
//_SET SCREWS (4)
FLEX PIVOT
J
SET SCREWS (2)
F
j
SET PINS
ROLL PINS
Figure 7. Exploded View
90
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60 SECONDS DURATION/AX]S
COMPOSITE: 4.76 Grm$
t I I
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FREOUENCY (Hz)
Figure 8. Random Vibration
FREQUENCY
20 Hz - 80 Hz
80 Hz - 150 Hz
150 Hz - 800 Hz
800 Hz - 2000 Hz
2000 Hz
Spectrum
LEVEL
0.006
• 6 dB/oct
0.02
-7 dB/oct
0.00235
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SWEEP RATE = 4 OCT/MIN
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AXIS LEVEL - 9's
X 12.5
Y 12.5
Z 9.3
20 30 40 SO
FREQUENCY - Hz
Figure 9. Sine Vibration Spectra
91
Figure 10. Thermal Test Photograph
Iii
AMBIENT/
_--- 24"C
(75"F)
FF
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/ F I HR HOLD
RAMP RATE£:
±33"C (60"F)
PER HOUR
I HR HOLD
0 HOURS
Figure 11.
TIME
ThermalTestProfile
6. B HOURS
92


Design and testing of the LITE Variable Field Stop mechanism

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