Derivation and Application of Force Limits for Vibration Tests
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force limits vibration tests lightweight aerospace↗
Engineering topics
Scharton, Terry D.
Publications and source records attributed to Scharton, Terry D..
Force Limits for Vibration Tests
The theoretical basis of force limited vibration testing is reviewed, the results of a frequency shift method of deriving force limits are presented, and two recent applications of the technique are described.
vibration↗
Vibration Test Force Limits Derived From Frequency Shift Method
The two-degree-of-freedom system (TDFS) method used to derive force specifications for force limited vibration tests at JPL is reviewed and some of the limitations of the method and of the simple TDFS model are discussed. A new improved frequency shift force prediction method is developed and applied to a more complex model where the load and source have both residual and modal masses.
force↗
Vibration Test With Extremal Dual Control
Report discusses use of extremal dual control to prevent overtesting. In extremal dual control, electronic test-control subsystem automatically reduces excitation supplied to shaker subsystem to keep force applied by shaker below preset force limit and to keep acceleration of shaker below preset acceleration limit.
Scharton, Terry D.↗
An assessment of the microgravity and acoustic environments in Space Station Freedom using VAPEPS
The Vibroacoustic Payload Environment Prediction System (VAPEPS) was used to predict the stationary on-orbit environments in one of the Space Station Freedom modules. The model of the module included the outer structure, equipment and payload racks, avionics, and cabin air and duct systems. Acoustic and vibratory outputs of various source classes were derived and input to the model. Initial results of analyses, performed in one-third octave frequency bands from 10 to 10,000 Hz, show that both the microgravity and acoustic environments will be exceeded in some one-third octave bands with the current SSF design. Further analyses indicate that interior acoustic level requirements will be exceeded even if the microgravity requirements are met.
Bergen, Thomas F.↗
Force limited vibration testing
A new method of conducting lab vibration tests of spacecraft equipment was developed to more closely simulate the vibration environment experienced when the spacecraft is launched on a rocket. The improved tests are tailored to identify equipment design and workmanship problems without inducing artificial failures that would not have occurred at launch. These new, less destructive types of vibration tests are essential to JPL's protoflight test approach in which lab testing is conducted using the flight equipment, often one of a kind, to save time and money. In conventional vibration tests, only the input vibratory motion is specified; the feedback, or reaction force, between the test item and the vibration machine is ignored. Most test failures occur when the test item goes into resonance, and the reaction force becomes very large. It has long been recognized that the large reaction force is a test artifact which does not occur with the lightweight, flexible mounting structures characteristic of spacecraft and space vehicles. In new vibration tests, both the motion and the force provided to the test item by the vibration machine are controlled, so that the vibration ride experienced by the test item is as in flight.
Scharton, Terry D.↗
Dual control vibration tests of flight hardware
A vibration retest of a spacecraft flight instrument, the Mars Observer Camera (MOC), was conducted using extremal dual control to automatically limit the shaker force and notch the shaker acceleration at resonances. This was the first application of extremal dual control with flight hardware at JPL. The retest was successful in that the environment was representative of flight plus some margin, the instrument survived without any structural or performance degradation, and the force limiting worked very well. The test set-up, force limiting procedure, and test results are described herein. It is concluded that dual control should be utilized when there is a concern about overtesting in hard-base-drive tests and the instrumentation for force measurement and control is available. Recommendations for improving the implementation of dual control are provided as a result of this first experience.
Scharton, Terry D.↗
Motion and force controlled vibration testing
A technique for controlling both the input acceleration and force in vibration tests is proposed to alleviate the overtesting risks and the problems associated with response limiting in conventional vibration tests of aerospace hardware. Previous research on impedance and force controlled vibration tests is reviewed and a simple equation governing the dual control of acceleration and force is derived. A practical method for implementing the dual control technique in random vibration tests has been demonstrated in JPL's environmental test facility using a conventional digital controller operating in the extremal mode. The dual control technique provides appropriate real-time notching of the input acceleration and a corresponding reduction of the test item response at resonances. Issues concerning the need for force and acceleration phase information, the adequacy of specifying the blocked force, and the derivation of the total force for multipoint supports are discussed.
Scharton, Terry D.↗
Analysis of dual control vibration testing
Analysis of dual control vibration testing is conducted using a two-degree-of-freedom system to represent the interaction between a vibration mode of a test item and a mode of a mounting structure. The results indicate that exact dual control based on specification of the source free acceleration and blocked force is strictly valid only if phase is taken into account. However, extremal dual control without phase alleviates overtesting without risk of undertesting, when the limit acceleration and force specifications are chosen to envelope the coupled system results. Several approaches to deriving appropriate force specifications are explored. Random vibration parametric results from the literature are used to estimate the limit force between coupled oscillators with different source and load masses.
Scharton, Terry D.↗