Space structures in general experience free-free vibrational boundary conditions that are not readily replicable on the ground. To conduct vibration tests of such structures on Earth, special devices must be used to support the weight of the structures without introducing any constraining forces that impose boundary conditions that detract from the simulation of the desired free-free boundary conditions in outer space. Previous supports all have certain disadvantages. For example, long cables entail very tall ceilings and generate undesirable pendulum effects, the masses of air pads incorporated into a suspended structure can distort the dynamical characteristics of the structure, pneumatic/electric devices are usually highly complex, and springs are limited by small domains of operation (strokes). To overcome these difficulties, a simple and inexpensive support was designed that includes a noncircular cam, a torsion spring, and a cable.
As shown in the figure, a thin cable is wrapped around the circumference of the noncircular disk. This cable passes through the smooth ring and extends downward to suspend the object under test. To prevent the cable from driving the disk and unwinding, a torsional spring is attached to the disk. The cam has a special profile designed in conjunction with the load it is to suspend and with the stiffness of the torsional spring. The torsional spring loads the cam as the cam rotates so that the torque exerted by the spring about the axis of rotation of the cam is exactly counterbalanced by the weight of the object under test.
In this concept, the profile of the disk is designed such that for any given displacement of the object under test from its original static-equilibrium position, its new position is also one of static equilibrium. This imitates what happens in outer space: any object displaced from one position of static equilibrium to another position remains in static equilibrium in its new position.
This device also simulates the behavior of the object under test when it is subjected to an impulse. When a given velocity is imparted to the object, it continues to travel at that same velocity over a considerable range. The velocity remains constant because the tension in the cable remains constant and equal to the weight of the object, so that no net driving force is exerted on the article throughout its entire range of motion. This condition simulates the motion of an object in outer space.
This suspension system is simple and inexpensive to construct. The setup is very compact, precluding the need for high ceilings or large platforms. The range of motion for the object under test can be large, providing considerably longer times for the experimental collection of data. In addition, the inertia of the system is small and therefore does not appreciably modify the dynamical characteristics of the test article. Besides the obvious applications of this system in the aerospace field, it could be applied to many everyday problems in which objects must be balanced vertically by use of systems of relatively low mass and friction.
This work was done by Meng-Sang Chew and Li-Farn Yang of Old Dominion University, and Jer-Nan Juang of Langley Research Center.
This invention has been patented by NASA (U.S. Patent No. 5,207,110). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to
the Patent Counsel
Langley Research Center; (804) 864-3521
Refer to LAR-14272