Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M7/00—Vibration-testing of structures; Shock-testing of structures
  • G01M7/02—Vibration-testing by means of a shake table
  • G01M7/06—Multidirectional test stands
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01M—TESTING STATIC OR DYNAMIC BALANCE OF MACHINES OR STRUCTURES; TESTING OF STRUCTURES OR APPARATUS, NOT OTHERWISE PROVIDED FOR
  • G01M7/00—Vibration-testing of structures; Shock-testing of structures
  • G01M7/02—Vibration-testing by means of a shake table

Definitions

• This invention relates to the simulation of a vibration environment for use in testing products which are subjected to vibration during use, and more particularly, to an improved vibration test machine which, in one embodiment, simulates a vibration environment in multiple degrees of f eedom, at very high frequencies.
• the experienced real world vibration usually includes all six degrees of freedom (DOF), that is, the vibration usually has linear acceleration components along the three orthogonal axes, and angular acceleration components about those axes.
• DOF degrees of freedom
• the best simulation of the vibration environment would include vibration in all six degrees of freedom.
• Typical hydraulic servo valves are limited to 50 or 100 Hz frequency response.
• U.S. Patent 5,343,752 assigned to Team Corporation, discloses a servo valve and double-acting piston actuator that responds to 1000 to 2000 Hz depending on the size of the actuator.
• Several multi- degree of freedom systems are disclosed in that patent. Each uses a high frequency valve to produce higher vibration frequencies than previously attainable by a multiple degree of freedom shaker.
• One of those systems referred to as the Cube test system, comprises actuators on the inside of the vibration table. This system improves on the frequency response of a six DOF vibration test system, raising the controllable frequency from about 50 Hz for the prior art, to about 250 Hz. Even greater frequency response is desired, so the Cube type system forms the basis upon which the improvements of the present invention are compared.
• the present invention Compared to the Cube style multi - axis vibration table, the present invention has higher frequency response and better table uniformity (less distortion) . Electro-dynamic (ED) actuators have better frequency response and better freedom from distortion than even the best electro- hydraulic (EH) shakers; and the smaller, stiffer table of the present invention provides a higher first mode frequency than the Cube test system. This translates to much higher G levels for the tests object.
• ED Electro-dynamic
• EH electro- hydraulic
• the actuators of the cube shaker are located inside the vibration table and are mounted to the reaction mass on legs that protrude through the bottom of the vibration table. It has been found that the mounting structure (legs) for the internal actuators has a relatively low natural frequency that limits the frequency response of that design to frequencies below what the actuators are capable of.
• one embodiment of the present invention comprises a vibration test machine having a stationary supporting frame, a shake table in the form of a rigid structure to which vibration energy is transmitted inside the supporting frame, a driven preload piston supported by the frame and coupled to one side of the shake table, a passive preload piston supported by the frame and coupled to the shake table on a side opposite from the driven preload piston, and an external force generator external to the supporting frame and coupled to the driven piston for inducing high frequency vibrational energy to the driven piston which, in turn, transmits that vibration energy to the shake table.
• the passive preload piston applies a force against the shake table which opposes the driven preload piston force.
• a multiple degree of freedom shaker comprises multiple exterior force generators each coupled to a corresponding driven preload piston and having a related passive preload piston on a opposite side the shake table.
• the preload piston couplings to the shake table comprise self-aligning bearing pads to accommodate the multiple degree of freedom vibrational motion induced in the shake table.
• the present invention surrounds the shake table with a reaction mass (the supporting frame) so that the centers of gravity of the table and reaction mass are very close to each other. Two aspects of the test fixture are improved, compared with the Cube-type shaker. First, the reaction forces act much closer to the center of gravity of the reaction mass, reducing induced moments dramatically, and thereby reducing the required size of the reaction mass. Second, the shake table is smaller and has much lower mass, so the magnitudes of the reaction forces and moments are much lower than for the Cube configuration.
• the present invention uses electro-dynamic (ED) actuators, but the concept is not limited to ED shakers.
• EH electro-hydraulic
• One embodiment of the present invention relates to a method of preloading a self-aligning hydrostatically lubricated slider bearing which is capable of carrying compression loads only.
• the bearing in this invention, is used for connecting an external force-generating device to the shake table such that its motion is capable of providing multiple degrees of freedom vibration.
• the number of degrees of freedom of the vibration energy transmitted to the shake table may be from one to as many as six degrees of freedom.
• Multiple degree of freedom shake tables are used to simulate vibration and shock conditions on test articles of many types. Six degrees of freedom simulation is the most complete.
• This invention allows multiple external force-generating devices that each produce single degree of freedom motion (push-pull forces in a straight line), such that an ED shaker or an EH shaker can be connected so that both the push and the pull forces are effectively transmitted to the shake table.
• this invention permits force generating devices such as ED shakers, which generally have access to the moving component (armature) from one end only, to be effectively connected to the multiple degree of freedom shake table.
• the invention also minimizes cross coupling connections between the mechanisms that connect the one-degree of freedom force generators to the shake table that moves with multiple degrees of freedom.
• one embodiment of the invention comprises the combination of the stationary outer frame; the multi-degree of freedom shake table; external force generators, each of which can be an ED shaker; a driven preload piston and a passive preload piston used in pairs, one pair associated with each force generator; and hydrostatic self-aligning slider bearings coupling the preload pistons to the shake table.
• FIG. 1 is a perspective view showing a six-DOF shake table incorporating ED single DOF force generators.
• FIG. 2 is a schematic cross-sectional view showing a pair of force generators, a stationary frame, a multi-DOF shake table and a pair of preload generating pistons (driven piston and passive piston) that connect each force generator to the shake table.
• FIG. 3 is a schematic cross-sectional view of the driven preload piston.
• FIG. 4 is a schematic cross-sectional view of the passive preload piston.
• FIG. 5 is a schematic cross sectional view of an alternative form of the invention comprising a single DOF shake table.
• FIG. 6 is a perspective view illustrating one embodiment of a support system for the vibration test machine.
• FIG. 7 is a top view of the support system shown in FIG. 6.
• FIG. 8 is a side elevation view taken on line 8-8 of FIG. 7.
• FIG. 9 is a side elevation view taken on line 9-9 of FIG. 7.
• FIG. 10 is a perspective view showing an alternative support system for the vibration test machine.
• FIG. 11 is a top view of the support system shown in FIG. 10.
• FIG. 12 is a side elevation view taken on line 12-12 of FIG. 11.
• FIG. 13 is a side elevation view taken on line 13-13 of FIG. 11.
• FIG. 1 is a perspective view illustrating a multiple degree of freedom vibration test machine which includes a generally cube-shaped stationary rigid supporting frame 10.
• An internal void space 12 inside the supporting frame 10 contains a shake table 14 in the form of a rigid structure with high stiffness to which vibration energy is transmitted to provide the moving element of the shaker.
• a test table 16 is positioned in a horizontal plane on the top face of the shaker. The test table is rigidly affixed to the shake table, and a unit under test (not shown) is subjected to vibration testing when affixed to the test table.
• the supporting frame 10 and other components of the shaker assembly shown in FIG. 1 are mounted to a lower support structure which rests on the floor. Embodiments of the lower support structure are described below.
• the illustrated embodiment comprises a six degree of freedom shaker in which there are six force generators 18 positioned adjacent five faces of the cube-shaped supporting frame. There are two force generators arranged side-by-side adjacent a lower face of the cube-shaped frame, while the upper face which holds the test unit does not have an adjacent force generator. The other four force generators are positioned adjacent the remaining four sides of the cube- shaped frame.
• the vibration force generators transmit vibration energy at high frequency to the moving shake table element of the test machine.
• the force generators in the illustrated embodiment comprise electro-dynamic vibration force generators, although other types of force generators, such as electro-hydraulic force generators, can be used.
• each force generator is coupled to the shake table through a corresponding pair of hydraulic pistons affixed to the stationary frame. These pistons, referred to herein as preload pistons, are described in more detail in FIGS. 2 through 4.
• the electro-dynamic vibration force generators are mechanically coupled to the preload piston rods through corresponding conical-shaped connectors 20 which step down the diameters of the corresponding vibration elements of the electro-dynamic shaker.
• the electro-dynamic shaker generally comprises a cylindrical metal outer housing containing a voice coil that vibrates at high frequency inside an electromagnetic coil which also includes a field winding. The vibrating voice coil is of larger diameter than the driven piston rod to which the electro-dynamic shaker is coupled.
• FIG. 2 illustrates one embodiment of a system for mechanically coupling the external force generators to the internal shake table via corresponding preload devices.
• the force generators on opposite sides of the support frame 10 are referred to by reference numerals 18a and 18b.
• the force generator 18a shown on the left side of the frame is coupled to the shake table through a driven preload device 22a.
• Vibration energy from the moving voice coil of the force generator 18a is coupled to a preload piston 24a in the driven preload device.
• the piston 24a is rigidly affixed to the moving element of the force generator through the external connector 20 described above.
• the driven end of the piston 24a inside the frame is coupled to the shake table through a self-aligning hydrostatic slider bearing 26a.
• a passive preload device 28a is aligned axially with the driven preload device 22a.
• the passive preload device 28a is not coupled to a force generator and its function is to oppose the preload force of the driven preload piston 24a.
• the passive preload device includes a passive preload piston 30a coupled to the shake table by a self-aligning hydrostatic slider bearing 32a.
• the force generator 18b shown on the right side of the support frame 10 is coupled to the shake table through preload devices similar to those described for the force generator 18a. The force generator 18b is thus coupled to the shake table by a driven preload device 22b having a driven preload piston 24b coupled to the shake table by a self-aligning bearing 26b.
• a passive preload device On the opposite side of the shake table a passive preload device includes a passive preload piston 30b aligned axially with the driver preload piston 24b.
• the passive preload piston 30b is coupled to the shake table by a self-aligning hydrostatic slider bearing 32b.
• the output forces generated by the force generators 18a and 18b are spaced apart along parallel axes. This represents the push-pull arrangements of the two pairs of force generators aligned on opposite side faces of the cube shaped frame.
• the two force generators at the bottom of the frame have their vibrational force outputs along parallel axes coupled to opposite bottom sides of the shake table.
• the driven preload piston and its self-aligning hydrostatic slider bearing are shown in more detail in FIG. 3.
• the driven preload device 22 comprises the moving piston component 24 that has a stepped shaft, a housing 34, a hydraulic accumulator 36, and the self-aligning hydrostatic slider bearing 26.
• a smaller diameter end 38 of the shaft extends out through the body of the housing. This shaft extension provides the means for connecting the one DOF force generator to the moving piston such that both push and pull forces are transmitted into the driver piston.
• the chamber volume created by the stepped piston profile is connected to the hydraulic accumulator 36 and filled with a fluid (typically hydraulic fluid) by connecting the chamber to an external source of fluid under pressure.
• a fluid typically hydraulic fluid
• the shoulder area created by the difference in the diameters of the piston extension and larger piston diameter provides a surface 38 that is acted upon by the fluid under pressure. This pressure produces a force on the piston and in turn preloads the self-aligning pad bearing 26 against a flat surface on the shake table.
• the accumulator 36 allows the piston 24 to extend and retract and maintain contact with the test fixture assembly. Minimizing pressure changes during operation allows more of the available force supplied by the external force generator to be transmitted to the shake table.
• An adjustable flow restriction device 40 (such as an adjustable orifice) can be incorporated in a fluid passageway 42 between the accumulator and the piston so that variable damping values can be achieved. Damping is useful in controlling structural resonance in the shake table.
• An upper portion 44 of the accumulator contains gas under pressure. The gas pressure is controlled to apply the equivalent of a low spring rate to enable the driven preload piston to maintain its initial preload force during use.
• the passive preload device 28 is shown in more detail in FIG. 4. It incorporates all of the features of the driven preload piston with the exception of the shaft extension.
• the passive preload device includes the preload piston 30 coupled to the shake table by the self-aligning bearing 32.
• a hydraulic accumulator 46 connects to the piston chamber through a line 48 having an adjustable flow restrictor 50.
• a gas volume 52 in the accumulator is controlled at a low spring rate to maintain the initial preload force. When pressurized it generates a force against the shake table which opposes the driven preload piston force. This force is generally equal to the preload force in the driven preload piston, but may also be adjusted to be less than or more than the preload force in order to produce different operating characteristics of the multi degree of freedom shake table.
• the preload pistons provide a means of transmitting the forces applied by the force generators to the shake table.
• the preload pistons are passive in the sense that they do not cause the motion of the shake table on their own accord. Their function is to apply an initial preload force that is greater than the force that is produced by the external force generators. This initial preload condition is critical for maintaining contact between the self aligning slider bearings and the shake table and properly transmitting the external forces into the shake table. It is desirable for these preload forces to remain as very nearly constant as is practical for the best performance of the machine. One way of maintaining a nearly constant force is accomplished by the use of the gas/oil accumulators, but this is not the only method possible.
• the gas volume contained in the accumulators is used to minimize the change in pressure and therefore the change in preload force in the preload pistons as the pistons extend and retract.
• the dynamic forces created by the external force generators add and subtract with the preload forces in the preload pistons during the operation of the machine. If the dynamic forces were to exceed the initial preload forces, then the self aligning pad bearings might separate from the shake table surface.
• the preload pistons are one example of a preload device applying a fluid spring force; other preload devices for applying mechanical spring forces also could be used.
• the self aligning bearings used with the preload pistons can be spherical bearings of the type described in U.S. patent 5,343,752, assigned to Team Corporation.
• FIGS. 6 through 9 show an arrangement for supporting the vibration test fixture during use, and FIGS. 7 through 13 show an alternative support system. In both embodiments, a three- axis vibration test system is shown for simplicity, but these support systems also can be used for other force generating arrangements, such as a six degree of freedom system.
• FIGS. 6 through 9 illustrate a support system in which the force generators 18 are each rigidly affixed to the stationary frame structure 10. This combination is mounted to a rigid lower support structure 60 which rests on the floor. The combined force generators and stationary frame are mounted on air springs 62 which isolate vibration caused by the shaker from being transmitted to the lower support structure, so that vibration generated during use is not transmitted to the floor.
• FIGS . 10 through 13 illustrate an alternative support system in which the forced generators can be mechanically isolated from the vibrating element of the test system.
• each force generator is surrounded by corresponding U-shaped frame 64.
• Each force generator contains its own spring mechanism within the U-shaped frame for isolating vibration from the stationary frame 10.
• the U-shaped frames 64 and their corresponding force generators are rigidly affixed to the stationary frame 10. This combination is rigidly affixed to a rigid lower support structure 66.
• the entire vibration test fixture and the lower support structure are mounted to the floor on air springs 68 between the legs of the lower support structure and the floor. The air springs isolate the floor from vibration produced during use.
• a computer controls the multi axis vibration system with special software designed specifically for multi axis motion control. These controllers are available from several manufacturers well known in the art.
• the computer estimates a linear model of the dynamics of the system that correlates the motion of the test article to the six drive signals to the force generators.
• the model accounts for the interactions between shakers as well as the dynamics of the test machine and the test article. It uses that model to calculate the drive signals to the force generators to produce the desired test vibration.
• the static force applied by the preload pistons is controlled by adjusting the pressure regulators described previously to control the hydraulic pressure acting on them.
• the forces sum to zero and the shake table does not move, while at the same time the slider pad bearings are established in contact with the surface of the shake table under compressive loading. If the initial preload forces are adjusted to be not equal, then an external force must contribute additional steady state force in order for the sum of all forces to be zero.
• the external force generators can provide this external force, or it may be supplied by gravity.
• One application of different forces in the passive piston and the preload piston would be to offset the force of gravity on the shake table and the test article mounted on the shake table. Offsetting the force of gravity reserves all of the available driving force for generating motion during the vibration or shock test.
• the fluid under pressure also creates forces on the housing of the driven preload piston and the passive preload piston. These forces are transmitted to the stationary frame.
• the stationary frame connects the housings of the preload piston assemblies such that the forces on the housings sum together. When the preload forces in the driven preload piston and its opposing passive preload piston are equal, the forces in the stationary frame sum to zero. This allows the body of the external force generator to be mechanically isolated from the stationary frame if desired.
• FIG. 5 is a cross-sectional view of an alternative form of the invention showing a simplified embodiment adopting the principles of this invention.
• one driven preload piston and one passive preload piston are connected to the shake table, the supporting frame, and the external force generator.
• the external force generator can be an electro-dynamic shaker, although a hydraulic shaker also can be used. This combination can be repeated in other orientations to produce a multiple degree of freedom system.
• adjacent pairs of the driver preload pistons can be mechanically connected together so as to move together.
• the external force generator would then be connected so as to apply its force symmetrically to the pair of driven preload pistons.
• the external force generator must contribute an additional steady state force in order for the sum of the steady state forces to sum to zero.
• the external shaker would have to apply a temporary tension load to the driven preload piston.
• the external force generator applies its force as a compressive force on the driven preload piston.
• the shaker force and the bias force then add together to produce a larger force (in one direction only), than could be created by the external force generator alone on the shake table. This configuration can be useful for tests such as shock pulses.
• Biasing options that can be produced by the combination of the preload actuators comprise: (1) no biasing loads, i.e., the force of the driven piston is offset by an equal an opposite passive force in the push-pull arrangements described herein; (2) load bias to offset gravity loads; (3) load bias to create higher dynamic forces in one direction (either toward or away from the external force generator); and (4) preloads that are offset to produce controlled angular accelerations.
• the preload force needs to be greater than the shaker force, and "equal and opposite" for use with electro-dynamic shakers or double acting hydraulic shakers.
• the preload needs to be one-half the active cylinder force.
• the external force generator can comprise a variety of devices to induce the desired high frequency vibration.
• electro-dynamic shakers available from the following manufacturers can be used in the present invention: Ling Electronics of Anaheim, California; Unholtz-Dickie of Wallingford, Connecticut; LDS of Great Britain; IMV of Japan; and RMS of Germany.
• Electro-hydraulic shakers available from Team Corporation, Burlington, Washington and described in U.S. Patent No.
• 5,343,752 can be used as a hydraulic force generator.
• the invention has been described in relation to the alignment and use of shakers in the Cartesian coordinate system. The concepts contained herein, however, can be applied to a variety of kinematically appropriate orientations. The invention is not limited to orthogonal orientation of its elements.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Apparatuses For Generation Of Mechanical Vibrations (AREA)

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• 2002
  • 2002-04-26 US US10/133,962 patent/US6860152B2/en not_active Expired - Lifetime
• 2003
  • 2003-04-24 WO PCT/US2003/012741 patent/WO2003091684A1/en not_active Ceased
  • 2003-04-24 JP JP2004500025A patent/JP4217210B2/ja not_active Expired - Lifetime
  • 2003-04-24 CN CNB03809374XA patent/CN100520333C/zh not_active Expired - Lifetime

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