A disturbance compensation system for satellites based on the drag-free concept was mechanized and flown, using a spherical proof mass and a cam-guided caging mechanism. The caging mechanism controls the location of the proof mass for testing and constrains it during launch. Design requirements, design details, and hardware are described

Debra, D. B.

Hacker, R.

Mathiesen, J.

English

NASA Technical Reports Server

N76-28284
CAGING MECHANISM FOR A DRAG--FP_E SATELLITE
POSITION SENSOR
By R. Hacker t J Mathiesen* D B DeBra _$ • , • •
Guidance and Control Laboratory
STANFORD UNIVERS ITY
ABSTRACT
A disturbance compensation system for satellites based on the drag-free
concept has been mechanized and flown, using a spherical proof mass and
a cam-guided caging mechanism. The caging mechanism controls the location
of the proof mass for testing and constrains i_ during launch. Design re-
quirements, design details, and hardware are described.
INTRODUCTION
A navigation satellite launched by the Applied Physics Laboratory of
Johns Hopkins University incorporated a device built at Stanford University .
for the compensation of external forces, e.g., atmospheric drag and solar
radiation pressure, which perturb a satellite's orbit.
If we enclose an object (proof mass) in a housing which isolates it from
the drag and solar radiation pressure, it will follow an orbit influenced
solely by the gravitational field of the earth• If the satellite is con-
strained to follow the proof mass, it will follow a similar gravitational
orbit• Such an orbit can be predicted for much longer times than the orbit
for a satellite calculated on the best estimates of drag and solar disturb-
_ ance [Refs. 1 and 2].
:_.
t Research Engineer.
/
i * Research Engineer at time of design; deceased. .(_
_.
_ _ Director, Guidance & Control Lab; Professor, Aeronautics and Astronautics, _
and Mechanical Engineering. _
; JPL Technical Memorandum 33-777 1)5 _
_. n • - mmmll
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https://ntrs.nasa.gov/search.jsp?R=19760021196 2020-03-20T02:34:17+00:00Z
Tile accuracy of navigation on the earth's surface by ships or aircraft,
and the extension of measurement resolution in the fie]ds of aeronomy aad
geodesy are all areas which can benefit from the long-term predictable orbits
made possible by DISCOS (Disturbance Co__mpensation _ystem).
The DISCOS consists of a spherical proof mass within a housing, a capac-
itive sensing system to detect proof mass position, a propulsion system to
position the satellite relative to the proof mass, a caging mechanism to
control the location of the proof mass for testing and launch, and the elec-
tronics necessary for operating the control system and providing telemetry
information and receiving commands.
The satellite for which the DISCOS was designed was a three-body gravity-
gradient three axis stabilized configuration with an overall length on orbit
of 7.5 m (Fig. 1). The DISCOS was contained in the center body, a 300 mm
diameter by 300 mm long cylinder attached to the other two bodies by folding
booms.
DISCOS DESCRIPTION
The DISCOS is made up of five major components: the proof mass, proof
mass housing, electronics, propulsion subsystem, and the caging mechanism
(Fig. 2). These components are arranged in the cylindrical housing in a
manner which tends to minimize the mass attraction force of the components
on the proof mass; that is, the parts are kept as far away as possible and
as symmetrical as possible. Our familiarity with mass attraction is, in
general, limited to the earth's, i.e., the weight of objects around us. In
the design of a drag-free satellite, however, the disturbance forces to which
the proof mass is subjected must be kept smaller than the order of Io-llg,
which, for comparison, is the attractive force exerted on the proof mass by
a 15 gm mass at a distance of I00 mm. For this reason, the size and loca-
tion of all parts o£ the DISCOS had to be controlled and known to dimensions
as small as 0.01 mm.
The most important and at the same time the simplest component of the
system is the proof mass_a sphere 22 mm in diameter, mass of 0.11 kg, cast
from a 70 -- 30 gold-platinum alloy and lapped to size. The proof mass is
contained in a beryllium oxide housing of 40 mm inside diameter. The caging
n
1_6 5PL Technical Memorandum 33-777
'% i "
1976021184-133
mechanism penetrates the wall of this housing to provide control of the proof
mass. The inside surface of the proof mass housing is coated with vacuum
deposited chromium in a pattern of three orthogonal pairs of capacitor plates.
The change in capacitance caused by motion of the housing relative to the
proof mass is sensed to obtain satellite position data. The other major items
of the DISCO_ are the electronics and propulsion subsystems.
MECHANISM DESCRIPT ION
The caging mechanism for the DISCOS performs two functions. First, it
is used to move the proof mass in a prescribed path inside the housing so that
the signals from the three pairs of capacitive plates can be checked. To a
degree, this is a simulation of the normal orbit condltiou where the proof
mass would be free to sense the motion of the satellite.
The caging mechanism serves the second purpose of securing the proof
mass against motion during launch and ascent to orbit. To meet this caged
position requirement, the mechanism was designed to exert a force of 54N
when securing the proof mass.
The caging mechanism (Fig. 3) is located below the proof mass housing, .,"
and the caging rod, a beryllium oxide cylinder 12.7 mm diameter by 46 mm
long with a mass of 0.017 kg, extends upward into the housing. The lower
end of the caging rod is pinned to two circular springs, which are in turn
pinned to a transfer nut. The springs are fabricated from beryllium copper
and the transfer nut is 303 stainless steel. The transfer nut travels on
a 1/4"-16 Acme screw, also fabricated from beryllium copper. The lower end
of the caging rod follows a two-dimensional cam slot cut into two plates on
- the side of the mechanism, thereby imparting the desired motion to the proof
mass for systems tests. The cam slot is designed to move the proof mass
along a path resembling a stnueoid. The beryllium oxide caging rod is fur-
nace brazed to a clevis end fitting of 17-4 PH stainless steel. Initially,
a titanium end fitting was tried but there were excessive voids in the Joint
due to Insufficient wetting. The pl-ocedure which finally was used was to
sputter approximately 4000A of gold on the rod end, 10 _ of gold on the
end fitting, and vacuum braze st 850°C with s gold-copper alloy (Englehard
378). The brazing temperature was held as low as possible to prevent grain 4
growth in the beryllium oxide rod. In spite of this there was an increase :_
JPL Technical Memorandum 33-777 1_ 7 _
1976021184-134
!r 11"
in the rod diameter requiring an additional lapping operation to resize.
The aeme screw is driven by a size 9 permanent magnet d-c motor geared
to produce 23 mm travel of the transfer nut in 30 sec. The motor bearings
are Barden ball bearings with a self-lubricating composite retainer, and the
gear box is lubricated with Microseal 100-1. The motor was being produced for ..
a program which had a similar launch environment and was purchased off-the- i
shelf. A modified Oldham's coupling is used between the motor and the screw.
The frame on which the compouents of the mechanism are mounted was
machined from a piece of 6061-T6 aluminum 62.5 mm diameter by 100 mm long.
The surfaces contacted by the transfer nut were given a hard anodize to mini-
mize wear. An assembly fixture was fabricated for use in obtaining align-
ment of the two cam plates with respect to each other, for checking the switch
trip points, and for measuring the spring load.
During operation in the caging direction, the first 0.86 mm travel of
the transfer nut relieves the spring load which has been applied by stretching
the springs during the uncaging cycle. Free travel then takes place for
21.25 mm during which the cam slot is traversed. At the end of free travel,
t"
the proof mass contacts the proof mass housing and the two springs are com-
pressed a distance of 1.17 mm. This produces the load of 54N giving a
longitudinal acceleration capability of 43 g without motion of the proof
mass away from its caged position. At the end of spring compression, a tab
on the transfer nut contacts a limit switch and power is removed from the
motor. A pair of switches arranged to operate independently is used at each
end of the stroke. Thus, opening either switch of the pair will remove power
from the motor. Should both switches fail to open, the transfer nut will
contact a steel pin after an additional 0.25 mm travel and stall the
mechanism, This is done to prevent cracking or spalling failures of the
beryllium oxide housing and caging rod.
TESTING
The systems check-out and testing phase for the DISCOS required a number
of caging and uncaging cycles for the determination of electronic signal
strength and scale factors for the position sensing system. Tests were per-
formed at ambient conditions, in vacuum at 45°C and -20*C both before and
128 JPL Technical Memorandum 33-777
s
1976021184-135
after vibration testing. Several hundred caging and uncaging cycles were
performed during the test period.
CONCLUDING REMARKS
One caging mechanism was fabricated to serve for both system qualifi-
cation and the flight vehicle. At the completion of testing, the cycle time #
on the unit was within 1 sec of the desired time. Following launch and
orbit injection, the mechanism remained in the caged position for eleven
days, at which time it was uncaged by ground command.
ACKNOWLEDGMENTS
Credit is due to Robert Mattey of the Applied Physics Laboratory (JHU)
for his assistance in determining the correct brazing technique for the cag-
ing rod end fitting; and to Richard Van Patten of the Stanford Guidance
and Control Laboratory for assistance in motor selection.
REFERENCES
1. Lange, B.O., "The Drag-Free Satellite," AIAA J., Vol. 2, No. 9p
Sept. 1964, pp. 1590-1606.
2. Space Dept. of the Johns Hopkins University Applied Physics Lab., and
the Guidance & Control Lab. of Stanford University, "A Satellite Freed
of all but Gravitational Forces: 'TRIAD I'," J. Spacecraft and Rockets,
Vol. II, No. 9, Sept. 1974, pp. 637-644•
_ JPL Technical Memorandum 33-777 129
_
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1976021184-136
_i ¢ i ,, ---- _ _ i ill i • .u= I _
¢,
r
FIG. I NAVIGATION SATELLITE WITH DISCOS HOUSING IN CENTER.
130 JPL Technical Memorandum 33-777
1976021184-137
i I/ ,f "-.
I I1 Y %'B_ ".. .0.,
PROPULSION
SUBSYSTEM
PROOF MASS
HOUSING
PROOF MASS
CAG ING
MECBANZSM
i
f
FIGURE 2 DISC08 SHOWINGLOCATIONOF CAGINGISCHANISII.
JPL Technical Memorandum 33-777 131
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1976021184-138
!i
CAGING
7j/'- /iOD
FIGLP.E 3 CROSS HCTION OF CAGING _CHANISM.
13Z 3PL Technicsl Memoran4um 33-777
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1976021184-139


Caging Mechanism for a drag-free satellite position sensor

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