Multiple direction vibration fixture

An apparatus for simulating a rocket launch environment on a test item undergoing centrifuge testing by subjecting the item simultaneously or separately to vibration along an axis of centripetal force and along an axis perpendicular to the centripetal force axis. The apparatus includes a shaker motor supported by centrifuge arms and a right angle fixture pivotally connected to one of the shaker motor mounts. When the shaker motor vibrates along the centripetal force axis, the vibrations are imparted to a first side of the right angle fixture. The vibrations are transmitted 90 degrees around the pivot and are directed to a second side of the right angle fixture which imparts vibrations perpendicular to the centripetal force axis. The test item is in contact with a third side of the right angle fixture and receives both centripetal-force-axis vibrations and perpendicular axis vibrations simultaneously. A test item can be attached to the third side near the flexible coupling or near the air bag to obtain vibrations along the centripetal force axis or transverse to the centripetal force axis.

BACKGROUND OF THE INVENTION 
The present invention relates to the field of vibration testing, and more 
particularly to the field of combined vibration testing and centrifuge 
testing of electromechanical subsystems to verify capabilities in rocket 
launch environments. 
Vibration testing is a technology that has many applications in industry. 
Many manufactured products are subjected to high frequency vibrations (up 
to 3 kHz) to determine the reliability of the products in a vibration 
environment. Moreover, in simulating a rocket launch environment, 
vibrations are imposed on the tested item in the presence of acceleration 
forces. More specifically, rocket powered missile launch presents a 
variety of acceleration force and vibration force environments which stem 
from missile launch, powered flight, and reentry. Many manufactured 
products form the onboard control systems and the payloads of rockets and 
rocket powered missiles, and such products are tested as to their 
capabilities in the rocket launch environment. 
Presently, ground testing evaluation techniques are known which simulate a 
rocket launch environment and which employ centrifuge test devices that 
permit the addition of vibrational forces. One such known technique 
employs an electrodynamic shaker which is positioned in an in-line or 
cross-arm configuration on a centrifuge to provide selectable vibration 
directions. A problem, however, associated with this technique is the 
presence of an internal bearing or other forms of side load restraints 
which inherently impose undesirable limitations on the shaker performance. 
More specifically, the shaker is prevented from providing a precisely 
controllable, broadband vibration environment in the presence of the 
centripetal force created by the centrifuge. It would be desirable, 
therefore, to provide a precisely controllable, broadband vibration 
environment in the presence of centripetal force to permit a more accurate 
simulation testing environment for testing parts in a variety of 
acceleration and vibration conditions experienced in rocket launch, 
powered flight, and reentry. 
Another known ground testing evaluation technique which simulates a rocket 
launch environment employs two electrodynamic shakers and a slip table to 
provide vibration to test items on a centrifuge. This technique is 
hampered by distortion of acceleration waveforms and decoupling of the 
vibratory motion. The two shakers are rotated on the centrifuge arm to 
produce the various vibration lines of action. 
Generally speaking, the known testing techniques employing electrodynamic 
shakers on centrifuges provide undesirable distortion of vibration 
frequencies. It would be desirable, therefore, to provide an apparatus 
that employs an electrodynamic shaker that provides high fidelity and 
undistorted vibrations to the item under test on a centrifuge. 
Moreover, the known testing techniques employing vibrations imposed on 
centripetal forces do not provide precisely controllable vibrations in a 
high frequency range (up to 3 kHz). It would be desirable, therefore, to 
provide a testing apparatus that provides precisely controllable high 
frequency vibrations to a test item in an environment experiencing 
centrifugal forces. 
SUMMARY OF THE INVENTION 
Accordingly, it is a primary object of the present invention to provide a 
precisely controllable, broadband vibration environment in the presence of 
centripetal force to permit a more accurate simulation testing environment 
for testing parts in a variety of acceleration and vibration conditions 
experienced in rocket launch, powered flight, and reentry. 
Another object of the invention is to provide a precisely controllable, 
broadband vibration environment to permit a more accurate simulation 
testing environment for testing parts in a variety of vibration 
conditions. 
Another object is to provide an apparatus that employs an electrodynamic 
shaker that provides high fidelity and undistorted vibrations to an item 
under test on a centrifuge. 
Still another object of the invention is to provide a testing apparatus 
that provides precisely controllable high frequency vibrations to a test 
item in an environment experiencing centripetal forces. 
Additional objects, advantages, and novel features of the invention will be 
set forth in part in the description that follows and in part will become 
apparent to those skilled in the art upon examination of the following or 
may be learned with the practice of the invention. The objects and 
advantages of the invention may be realized and attained by means of the 
instrumentalities and combinations particularly pointed out in the 
appended claims. 
To achieve the foregoing and other objects, and in accordance with the 
purposes of the present invention as described herein, an improved 
apparatus is provided for imparting vibrations to an item on a device 
exerting an acceleration force on the item along a line. The apparatus of 
the invention may be used for simulating accelerations and vibrations of a 
rocket launch environment on a test item. The vibration imparting 
apparatus of the invention includes a source for providing vibrations 
along the line of acceleration force, a means for mounting the vibration 
source onto the device exerting the acceleration force, and a right angle 
fixture in vibrational communication with the vibration source, for 
simultaneously vibrating the item along the line of acceleration force and 
along a line transverse to the line of acceleration force. Thus, the 
invention provides a multiple direction vibration fixture. 
For simulating a rocket launch environment on a test item, the apparatus of 
the invention includes means for retaining the test item on a centrifuge 
arm, whereby the test item is subjected to centripetal force along a 
centripetal force axis. Means are attached to the centrifuge arm for 
simultaneously or separately vibrating the test item along the centripetal 
force axis and along an axis transverse to the centripetal force axis by 
directing vibrations from along the centripetal force axis to an axis 
transverse to the centripetal force axis. 
Still other objects of the present invention will become readily apparent 
to those skilled in this art from the following description, wherein there 
is shown and described a preferred embodiment of this invention. Simply by 
way of illustration, the invention will be set forth in part in the 
description that follows and in part will become apparent to those skilled 
in the art upon examination of the following or may be learned with the 
practice of the invention. Accordingly, the drawings and descriptions will 
be regarded as illustrative in nature and not as restrictive.

DETAILED DESCRIPTION 
With reference to FIG. 1, there is disclosed a preferred embodiment of the 
multiple direction vibration apparatus 10 of the present invention. The 
apparatus 10 may be used for simulating accelerations and vibrations of a 
rocket launch environment on a test item 12. The apparatus includes a 
shaker motor 21 mounted on centrifuge arms 18 and 20. The shaker motor 
vibrates a right angle fixture 28 and the test item 12 which contacts the 
right angle fixture 28. More specifically, the apparatus 10 shown in FIG. 
1 includes mounting irons 14 and 16 that are fixed, such as by welding, to 
the centrifuge arms 18 and 20. The shaker motor 21 is fixed to the 
mounting irons 14 and 16. The shaker motor 21 vibrates flexible coupling 
23 in the direction depicted by double-headed arrow 25. The orientation of 
arrow 25 coincides with the centripetal force axis shown by dashed arrow 
27. It is well known that centripetal force is an acceleration force that 
pulls a centrifuged object toward the center of the circle of 
centrifugation. The vibrations transmitted through the flexible coupling 
23 are imparted to right angle fixture 28. 
The right angle fixture 28 is a key element in an assembly which serves to 
direct vibrations from along the centripetal force axis 27 to an axis 30 
transverse, in this case perpendicular, to the centripetal force axis 27 
and to the centrifuge arm 20. More specifically, the right angle fixture 
28 includes a first side 32, a second side 34 adjoining the first side 32 
and at a right angle 33 with respect to the first side 32, and a third 
side 36 extending between the ends of the first and second sides 32 and 34 
and located opposite the right angle 33. A pivot 38 connects the right 
angle fixture 28 to the mounting iron 16. An air spring 40 is located 
between the second side 34 of the right angle fixture 28 and the 
centrifuge arm 20. 
The test item 12 may be retained on the right angle fixture 28 at side 36 
by bolts. Other means for securing the test item 12 to the right angle 
fixture 28 may also be employed. 
In operation, vibrations are imparted along the centripetal force axis 27 
by shaker motor 21 as the centrifuge arms 18 and 20 spin. The vibrations 
pass through flexible coupling 23 and are imparted onto the first side 32 
of the right angle fixture 28. In response to the vibrations along the 
centripetal force axis 27, the right angle fixture 28 vibrates around 
pivot 38 as shown by curved, double-headed arrow 39. As the right angle 
fixture 28 vibrates around pivot 38, the vibrations (shown by a 
double-headed arrow 31) are directed to vibrate along axis 30 which is 
perpendicular to the centripetal force axis 27. The air spring 40 serves 
to align the right angle fixture 28 and equalize the centripetal force 
along the centripetal force axis 27. 
As shown in FIG. 1, the test item 12 is in contact with the third side 36 
of the right angle fixture 28. By being in contact with side 36 of the 
right angle fixture 28, the test item 12 is subjected to both types of 
vibration simultaneously: along the centripetal force axis 27; and along 
the perpendicular axis 30. A test item can also be attached to side 32 
near the flexible coupling 23 or alternatively above the air bag 40 on 
side 34 to obtain vibrations, either along the centripetal force axis 27 
or transverse to the centripetal force axis 27, respectively, thus 
subjecting item 12 to separate vibrations depending on its position. 
As described above, the apparatus of the invention is simple in design and 
versatile in application. The vibration source, such as the shaker motor, 
is rigidly mounted on the centrifuge arms so that the source vibrates 
along the centripetal force axis. As a result, the need for internal side 
load support bearings in the shaker motor is eliminated. 
The entire shaker assembly is air cooled thereby eliminating plumbing 
requirements for water or oil cooling media. 
Furthermore, the entire shaker assembly may be a single unit assembly that 
can be attached to and detached from the centrifuge as a unit. In this 
respect, the shaker assembly can be used as a unit outside the confines of 
a testing system employing a centrifuge. 
With the invention, centripetal force axis vibration and perpendicular axis 
vibration are achieved separately or simultaneously with the right angle 
fixture. However, rotational acceleration produced by the right angle 
fixture will be relatively insignificant for small test items that are 
mounted away from the pivot. Rotational acceleration can be enlarged and 
shaker output can be intensified (either force or displacement) by varying 
the ratio of the length of the drive lever arm (side 32 of the right angle 
fixture) to the length of the output lever arm (side 34 of the right angle 
fixture). 
The right angle fixture itself is made from stiff, lightweight material and 
may have added damping material to limit fixture resonance problems in 
high frequency test bandwidths. 
With the invention, apparatus is provided which is capable of producing a 
precisely controllable, broadband vibration environment in the presence of 
centripetal force to permit a more accurate simulation testing environment 
for testing parts in a variety of acceleration and vibration conditions 
experienced in rocket launch, powered flight, and reentry. 
More generally, the apparatus of the invention is capable of producing a 
precisely controllable, broadband vibration environment to permit a more 
accurate simulation testing environment for testing parts in a variety of 
vibration conditions. 
By employing the apparatus of the invention, high fidelity acceleration 
waveforms are produced by a commercially available electrodynamic shaker 
and are transferred to a test item and in a specific line of action 
relative to a centripetal force. The waveforms can be of high frequency 
(up to 3 kHz). The specified line of action can vary from test to test, 
but it remains fixed during any particular test. The acceleration 
waveforms may be sinusoidal or broadband random in character. 
The foregoing description of the invention has been presented for purposes 
of illustration and description. It is not intended to be exhaustive or to 
limit the invention to the precise form disclosed. Obvious modifications 
or variations are possible in light of the above teachings. The embodiment 
was chosen and described in order to best illustrate the principles of the 
invention and its practical application to thereby enable one of ordinary 
skill in the art to best utilize the invention in various embodiments and 
with various modification as are suited to the particular use 
contemplated. It is intended that the scope of the invention be defined by 
the claims appended hereto.