A low frequency torsional suspension system for testing a space structure uses a plurality of suspension stations attached to the space structure along the length thereof in order to suspend the space structure from an overhead support. Each suspension station includes a disk pivotally mounted to the overhead support, and two cables which have upper ends connected to the disk and lower ends connected to the space structure. The two cables define a parallelogram with the center of gravity of the space structure being vertically beneath the pivot axis of the disk. The vertical distance between the points of attachment of the cables to the disk and the pivot axis of the disk is adjusted to lower the frequency of the suspension system to a level which does not interfere with frequency levels of the space structure, thereby enabling accurate measurement

Gold, Ronald R.

Reed, Wilmer H., III

English

NASA Technical Reports Server

United States Patent [19] 
Reed, I11 et al. 
[54] TORSIONAL SUSPENSION SYSTEM FOR 
[75] Inventors: Wilmer H. Reed, 111, Hampton; 
TESTING SPACE STRUCTURES 
Ronald R. Gold, Newport News, 
both of Va. 
[73] Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 357,757 
[22] Filed: May 26, 1989 
[51] Int. 0 . 5  ...................... GOlM 7/02; GOlM 19/00 
[52] U.S. CI. .................................. 73/865.6; 73/866.4; 
73/663 
[58]  Field of Search ..................... 73/865.6, 663, 866.4 
1561 References Cited 
U.S. PATENT DOCUMENTS 
2,708,525 5/1955 Woleslagle ...................... 248/610 X 
3,015,231 111962 Ganahl ............................... 73D65.6 
3,281,964 11/1966 Hewes ................................... 434/34 
3,393,889 7/1968 Ogden ............................. 248/332 X 
3,421,363 VI969 Herr ...................................... 73/663 
4,135,688 1/1979 England .......................... 248/610 X 
12 
[11] Patent Number: 4,995,272 
[45] Date of Patent: Feb. 26, 1991 
FOREIGN PATENT DOCUMENTS 
456169 3/1975 U.S.S.R. ................................ 73/663 
Primary Examiner-Tom Noland 
Attorney, Agent, or Firm-George F. Helfrich; John R. 
Manning; Charles E. B. Glenn 
[571 ABSTRACT 
A low frequency torsional suspension system for testing 
a space structure uses a plurality of suspension stations 
attached to the space structure along the length thereof 
in order to suspend the space structure from an over- 
head support. Each suspension station includes a disk 
pivotally mounted to the overhead support, and two 
cables which have upper ends connected to the disk and 
lower ends connected to the space structure. The two 
cables define a parallelogram with the center of gravity 
of the space structure being vertically beneath the pivot 
axis of the disk. The vertical distance between the 
points of attachment of the cables to the disk and the 
pivot axis of the disk is adjusted to lower the frequency 
of the suspension system to a level which does not inter- 
fere with frequency levels of the space structure, 
thereby enabling accurate measurement. 
8 Claims, 2 Drawing Sheets 
0 
https://ntrs.nasa.gov/search.jsp?R=19910011863 2020-03-24T06:59:00+00:00Z
U.S. Patent Feb. 26, 1991 Sheet 1 of 2 4,995,272 
FIG. I 
FIG. 2 
FIG. 3 
2 . O O l  
. 
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1.00- 
0.00 I I I I I 1 
0.OOO 0.002 0.004 0.006 0.008 0.010 
h/L 
FIG. 4 
PIVOT 
POINT 
-L 
US. Patent Feb. 26, 1991 Sheet 2 of 2 
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4,995,272 
FIG.7 
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4.995,272 
1 
TORSIONAL SUSPENSION SYSTEM FOR 
TESTING SPACE STRUCTURES 
ORIGIN O F  THE INVENTION 
The invention described herein was made in the per- 
formance of work under NASA Contract NAS1-18000. 
In accordance with 35 USC 202, the contractor elected 
not to retain title. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to the general field of 
measuring and testing and, more specifically, to a sus- 
pension system used to suspend large space structures 
for the purpose of testing same in simulated weightless 
conditions. 
2. Description of the Related Art 
The proposed use of large space structure has created 
unique and challenging requirements. The extremely 
large size and light weight of the structures coupled 
with the near zero gravity, vacuum environment of 
space causes difficulty with the control and movement 
of such structures. To understand and document these 
difficulties, a significant number of ground-based tests 
on sample or model large space structures is required. 
Ground vibration tests (GTV’s) are necessary to 
characterize a structure’s behavior under dynamic con- 
ditions such as slewing a large space antenna or a dock- 
ing maneuver with a space station. GVT’s provide data 
indicative of the structure’s natural modes, frequencies, 
damping, and mode shapes. These data are critical in 
determining methods of moving and controlling large, 
light weight space structures.. 
One of the major difficulties with ground vibration 
testing of large space structures is suspending the struc- 
ture to allow freedon of motion similar to that found in 
space. Although it is impossible to match the near zero 
gravity environment of space, it is desirable for suspen- 
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10 
15 
20 
25 
30 
35 
sion systems to support the structure in a mannerthat 40 
does not overly constrain its motion. The structural 
members themselves are not designed to take significant 
loads such as the weight loading which occurs in a 
ground test environment. 
structures themselves, on even scale models of the 
structures, further adds to the problems associated with 
suspension systems. For example, elongated beam-type 
space structures typically have extremely low natural 
frequencies. The fundamental frequencies will often be 50 
below 0.5 Hz. To adequately uncouple the rigid body 
modes of the structure from the fundamental flexible 
modes, the rigid body modes should have frequencies 
substantially lower than the first fundamental mode. In 
the past, attempts have been made to suspend structures 55 
with long cables, in a pendulum fashion from a high 
ceiling. However, at the extremely low frequencies 
involved, the required cable lengths become prohibi- 
tively long. 
one plane of motion at a time. This support technique 
ignores the coupling of structural modes in more than 
one plane and does not address the distortion of torsion 
modes of beam-like structures. 
lum suspension methods, a suspension system should 
also allow undistorted motion of the torsional degree of 
freedom of beam-type structures. Ideally, the pendulum 
The extremely large size and relative frailty of the 45 
Pendulum cable supports limit structural testing to 60 
In additional to planar motions provided by pendu- 65 
L 
cables should be attached at the node line of the beam 
first torsion mode. However, since this node line usually 
lies inside of the beam contour, it is likely to be inacessi- 
ble for cable attachments. 
Another alternative is to suspend the beam by means 
of very soft springs and pendulum cables attached to the 
structure on either side of the torsion node line. How- 
ever, this approach is complicated by requirements 
regarding stress and stiffness of the cable/spring design. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a 
suspension syste for testing large space structures which 
allows the structure rigid-body movement and mini- 
mizes coupling with the flexible or vibrational modes of 
the structure. 
Another object of the present invention is to provide 
a suspension system which allows freedom of motion of 
the test structure while adequately distributing the 
weight loading throughout the test structure. 
In a preferred embodiment, a low frequency torsional 
suspension system for testing a space structure having a 
length and a center of gravity includes a plurality of 
suspension stations attached to the space structure along 
the length thereof for suspending the space structure 
from a rigid support, such as a ceiling. Each suspension 
station includes a disk pivotally mounted to the rigid 
support and having a horizontal pivot axis, and two 
cables each having upper and lower ends and the same 
length, the lower ends of the two cables being connect- 
able to the space structure and the upper ends being 
connectable to the disks so that the points of attachment 
for the upper and lower ends of the two cables define a 
parallelogram with the center of gravity of the space 
structure being vertically beneath the pivot axis of the 
disk. 
These objects, together with other objects and advan- 
tages which will be subsequently apparent reside in the 
details of construction and operation of the suspension 
system as more fully hereinafter described and claimed. 
reference being had to the accompanying drawings 
forming a part hereof, wherein like reference numerals 
refer to like parts throughout. 
BRIEF DESCRIPTION O F  THE DRAWINGS 
FIG. 1 is a perspective, schematic view of. a first, 
preferred embodiment of the suspension system accord- 
ing to the present invention; 
FIG. 2 is an end view of a truss beam supported by 
the suspension system of FIG. 1; 
FIG. 3 is a graph illustrating the correlation between 
“h” and the ratio of oc/o,; 
FIG. 4 is an end, schematic view of a suspension 
system according to the present invention; 
FIG. 5 is a side elevational view of another embodi- 
ment of suspension system according to the present 
invention; 
FIG. 6 is a side elevational view of another embodi- 
ment of suspension system according to the present 
invention; and 
FIG. 7 is an end view, partly in section of the FIG. 6 
embodiment, taken along line 7-7 of FIG 6. 
DETAILED DESCRIPTION OF THE 
PREFERRED EMBODIMENTS 
Referring to FIGS. 1 and 2, a suspension system for 
testing space structures is referred to generally by the 
* 
3 
4,995,272 
4 
numeral 10. The suspension system includes a plurality 
of suspension stations 12 attached to a space structure, 
such as a truss beam 14, along the length thereof for 
supporting and suspending the truss beam 14 from a 
rigid support, such as a ceiling (not shown). Although 
only the end stations 12 are illustrated in FIG. 1, it is 
understood that a sufficient number of additional sta- 
tions will be disposed between the end stations at spaced 
intervals. The exact number of stations will depend on 
the length and strength of the space structure to be 
tested. 
The system 10 is designed to alleviate the coupling 
between the suspension system and the structure’s tor- 
sion modes. To accomplish this, each station 12 includes 
two cables 16 and 18 of the same length L and a rotat- 
able support disk 20. The cables and disk coact to lower 
the suspension system’s torsion or rotation mode. It is 
desirable to keep the fundamental natural frequency of 
the system significantly lower than the lowest structural 
frequency, which for one 60 meter test mast beam was 
found to be 0.18 Hz. 
The disk 20 is free to rotate about a horizontal axis 0. 
The cables 16 and 18 are arranged to form a parallelo- 
gram abcd such that, at rest, the center of gravity of the 
beam 14 hangs directly below the rotational axis o of the 
disk 20. 
The system has two uncoupled natural pendulum 
modes. One is the “simple pendulum” mode whose 
frequency is described by: 
The second mode is the torsion or “compound pendu- 
lum” mode involving rotation of the body about its own 
center of gravity. The frequency of the compound pen- 
dulum mode can be determined by noting that the cable 
length L has no affect on this uncoupled mode. Thus, 
letting L=O in FIG. 1, the rotational frequency of the 
suspended body can be found by considering the body 
as being suspended from a rotational axis located at a 
distance h, above the center of gravity. This is illus- 
trated in FIG. 2; in FIG. 1, h=L-B. The resulting 
equation for the frequency of the compound pendulum 
is: 
where K is the radius of gyration of the suspended body 
about its center of gravity. As h/L is reduced, the ratio 
of the compound to simple pendulum frequency turns to 
zero as shown in FIG. 3. Preferably, h is adjusted until 
it is nearly zero. A slight amount of h is acceptable to 
provide stability for the suspended test structure. 
In the embodiment of FIG. 1 a plurality of attach- 
ment holes may be provided in the disk in line with the 
cables 16 and 18 so that the height of attachment points 
a and b can be adjusted relative to the pivot axis 0 to 
thereby vary h. 
In order to analyze the effects of the present inven- 
tion, the system was modeled analytically as a rigid 
frame with freedom to rotate about a pivot as shown in 
FIG. 4. The cables at each beam station are attached to 
a pivoted frame 22 which corresponds to the disk 20 of 
FIG. 1. The example used in the study was a 60 meter 
long truss-beam structure, which is generic space struc- 
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15 
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30 
35 
40 
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55 
60 
65 
ture designed for operation in a zero-G environment. 
The beam was cantilevered at one end and supported by 
twenty-eight cables at fourteen stations along the beam 
length. The calculated first torsion mode frequency for 
the unsupported cantilevered beam was 2.01 Hz. With 
the beam suspended by pendulum cables fixed at the 
top, the first torsion mode frequency was 72% higher 
than the unsuspended-beam frequency, whereas with 
the torsion suspension system of the present invention, 
the first torsion mode frequency was within 1 % of the 
unsupported beam frequency. 
FIGS. 5-7 illustrate specific embodiments of the tor- 
sion suspension system combined with other suspension 
concepts which provide lateral and vertical translation 
of the test structure. In FIGS. 5 and 6, the disk is a 
spoked wheel 24 pivotally mounted on a support 26 
which is hung from a ceiling 28 by a bracket 30. The 
bracket provides a soft vertical suspension as an addi- 
tional aspect of the suspension system. Active feedback 
control is provided by means of a servomotor, an active 
spring extending between the servomotor and the sup- 
port 26, a main spring extending between the ceiling 28 
and the support 26, and a compression member disposed 
on opposite sides of the support 26. The active feedback 
control in the vertical degree of freedom provides 
means for augmenting dynamic characteristics of the 
vertical mode. 
In FIG. 6, the spoked wheel 24 is also pivotally 
mounted on a support 26. The support 26 is translatable 
along a horizontal support rod 32 which passes through 
a linear bearing 34. The device illustrated in FIG. 6 is 
also provided with an active feedback control which 
includes a servomotor 36 which drives the support 26 
along the horizontal rod 32 in response to a control 
signal. The torsion suspension system illustrated in FIG. 
6 includes suspension cables like those described in 
FIG. 1. The cables includes two sets of pins 38, 40 and 
42,44, such that the length of the suspension cables can 
be adjusted by hanging the respective sets of pins in 
notches provided in the periphery of the spoked wheel 
24. The active feedback control system is employed to 
lower the pendulum frequency by controlling horizon- 
tal motion of the pendulum suspension point. A similar 
arrangement is found in FIG. 5. 
FIG. 7 is a cross-sectional view taken along line 7-7 
of FIG. 6 and simply shows the cooperation between 
the linear bearing 34, the support rod 32, and the driv- 
ing connection between the servomotor 36 and its pul- 
ley 37. 
The many features and advantages of the present 
invention are apparent from the detailed specification, 
and thus, it is intended by the appended claims to cover 
all such features and advantages of the suspension sys- 
tem which fall within the true spirit and scope of the 
invention. Further, since numerous modifications and 
changes will readily occur to those skilled in the art 
based upon the disclosure herein, it is not desired to 
limit the invention to the exact construction and opera- 
tion illustrated and described. Accordingly, all suitable 
modifications and equivalents may be resorted to falling 
within the scope and spirit of the invention. 
What is claimed is: 
1. A low frequency torsional suspension system for 
testing a space structure having a length and a center of 
gravity, the system comprising: 
a plurality of suspension stations attached to the space 
structure along the length thereof for suspending 
4,995,272 
5 6 
the space structure from an overhead support, each 
suspension station including a disk pivotally 
mounted to the overhead support and having a 
horizontal pivot axis, two cables each having upper 
and lower end and the same length, the lower ends 
of the two cables being connectable to the space 
structure and the upper ends being connectable to 
the disk so that the points of attachment for the 
upper and lower ends define a parallelogram with 
the center of gravity of the space structure being 
vertically beneath the pivot axis of the disk. 
2. A low frequency torsional suspension system ac- 
cording to claim 1, wherein the attachment points of the 
upper ends of the two cables are vertically adjustable. 
3. A low frequency torsional suspension system, ac- 
cording to claim 1, wherein each suspension station 
further comprises an arm having one end slidably con- 
nected to the overhead support and the other end rotat- 
ably mounting the disk. 
4. A low frequency torsional suspension system ac- 
cording to claim 3, wherein each suspension station 
further comprises a linear bearing providing on the 
overhead support end of the arm and slidably receiving 
a horizontal rod mounted in the overhead support, the 
horizontal rod and linear bearing providing means for 
moving each disk of each suspension station laterally. 
5. A low frequency torsional suspension system ac- 
cording to claim 4, further comprising means coupled to 
the arm for moving the arm positionally relative to the 
horizontal rod. 
6. A low frequency torsional suspension system ac- 
cording to claim 5, wherein the moving means com- 
prises a servomotor, a pair of pulleys mounted on the 
overhead support adjacent opposite ends of the hori- 
zontal rod, and a belt having two opposite ends coupled 
to the arm and extending around the two pulleys. 
7. A method of suspending a space structure having a 
length and a center of gravity, comprising: 
attaching a plurality of suspension stations to the 
space structure- along the length thereof thereby 
suspending the space structure from an overhead 
supportp the attaching step comprising pivotally 
mounting a disk above the space structure, con- 
necting two cables to the space structure and to the 
disk so that the points of attachment for the upper 
and lower ends of the two cables define a parallelo- 
gram with the center of gravity of the space struc- 
ture being vertically beneath the pivot axis of the 
8. A method of testing vibrational frequencies in a 
space structure in a simulated, weightless environment 
comprising: 
attaching a plurality of suspension stations to the 
space structure along the length thereof thereby 
suspending the space structure from an overhead 
support, the attaching step comprising pivotally 
mounting a disk above the space structure, con- 
necting two cables to the space structure and to the 
disk so that the points of attachment for the upper 
and lower ends of the two cables define a parallelo- 
gram with the center of gravity of the space struc- 
ture being vertically beneath the pivot axis of the 
disk; 
adjusting the distance between the pivot axis of the 
disk and the points of attachment of the two cables 
to the disk until the frequency of the suspension 
system is substantially below the frequency of the 
space structure; and 
5 
10 
15 disk. 
20 
25 
30 
35 subjecting the space structure to vibrational tests. 
* I * * *  
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45 
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60 
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Torsional suspension system for testing space structures

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