Middeck 0-G Dynamics Experiment

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The increasingly demanding performance requirements on spacecraft require a detailed model of all dynamic components. If a spacecraft structure is to be an element of the plant in a robust closed loop control system, a high premium is placed on the accuracy of the structural model. The problem is that the accuracy of first generation numerical structural models, and even second generation models, cannot be guaranteed within any stated bounds. Accuracy is degraded as a result of poor modeling due to inexact elements and boundary conditions, mismodeling, and nonmodeling of features such as damping and weak nonlinearities. The approach of using ground experimental results to update a structural model is limited in that ground modal identification of complete spacecraft is not always possible, and when conducted, yields results corrupted by gravity and suspension effects.

The objectives of the Middeck 0-Gravity Dynamics structural Experiment (MODE) were to study suspension and gravity influences on the structural dynamics of a modular truss system by comparing the measured response in ground and orbital tests and to quantify the suspension and gravity induced perturbations using analytical models of the suspension and nonlinear effects. The repeatability of measured modal properties from test to test and from test article to test article was also examined. The final MODE objective was to develop a component testing procedure that will yield the information necessary to update second generation structural models to obtain the accuracy desired for the design of robust and high performance closed loop controllers.

MODE was funded by IN-STEP in 1988 and flew on board STS-48 in September 1991. The research, design and development were a combined effort of MIT SERC, Payload Systems Inc. and Mide Technology Corporation.

MODE Hardware Description

MODE Test Article

An objective of the MODE program was to establish a small middeck dynamics laboratory for the The core of this laboratory is the MODE Experimental Support Module. The ESM's computer, 16-key keyboard, alphanumeric display, and excitation, data storage (200 MB WORM drive), signal conditioning (16 channels) functions can be used to examine various types of dynamic test articles in the shirt-sleeve environment of the middeck.

Built up from erectable and deployable modules the MODE structural test article (STA) can be arranged to produce several structural configurations. The simplest arrangement of the modules is called the baseline configuration, for which, two four-bay deployable modules are connected in the center bay with erectable hardware coinponents to form a straight truss. A slightly more complicated configuration, called the alpha configuration, is formed by replacing the erectable hardware of the center bay of the baseline with a rotary joint modeled after the Alpha Joint of the Space Station Freedom. A more complex configuration includes both deployable modules, erectable hardware, and the rotary joint to form a planar truss called the L configuration.

The two deployable truss modules form the bulk of each configuration. The deployable truss resembles one of the designs proposed for the Space Station Freedom solar array truss structure. Weighing approximately eight pounds, each section is four bays in length with a nominal bay composed of an eight inch cubic section. Each Lexan longeron hinges at its midpoint (through a knee joint) and at its attachment points with the batten frames. The batten frames remain rigid when the truss is collapsed. The hinge arrangement allows the truss segment to fold like an accordion for stowage. Tension is maintained throughout the deployable module by pretensioned cables that run diagonally between the batten stations. A single bay of one of the two deployable modules includes a mechanism that allows for varying the preload level in the wires. The purpose of this feature is to permit the study of preload on the joints and its influence on the truss dynamics.

The erectable components are identical to hardware used by the NASA Langley Research Center for their Dynamic Scaled Model Technology structure

The alpha joint was intended to approximate the dynamics of the Rotary Alpha Joint proposed for Space Station Freedoni. The 2.5 lb module is constructed around two aluminum disks connected at their centers by an axle and steel ball bearings. The bearings between the two plates allow the two plates to rotate. Friction between the two plates can be adjusted by a cammed tensioning lever.

Eleven accelerometers and four strain gauges are used to measure the structural response to the force generated by a single proof-mass actuator.

Supporting Ground Experiments and Analysis

Work at MIT SERC extended force-state mapping to the characterization of realistic multiple degree-of- freedom systems whose constitutive relations are nonlinear, dissipative, coupled and depend on memory of past states. Procedures developed at MIT SERC produce the component information necessary for updating second generation structural dynamic models.

The approach characterizes an entire bay of the MODE truss structure undergoing shear, bending, extension and twisting. Treating a bay as a twelve degree-of-freedom generally nonlinear beam element reduces the number of parameters in the identification process and highlights those parameters that are most important to the overall structural response.

Figure 4 depicts the MIT component tester. Six voice coil actuators are mounted between a one ton granite base and a stiffened top plate in a Stuart platform arrangement. The actuators can produce 34 lb shear loads and 140 in-lb moments at strokes up to 0.08 inches. The component (a bay) is connected to the base and top plate through eight six degree-of-freedom loadcells. These loadcells measure the transmitted and resultant forces while displacement sensors measure the deflection of the top nodes of the bay.

The force-state maps are then assembled into the nonlinear finite beam element that can in turn be used to model complete MODE structural configurations.

Contributions of the MODE Program

The shuttle middeck proved to be an excellent environment in which to perform zero-gravity experiments of scaled models of dynamic systems. The pressurized atmosphere of the middeck allowed experimental determination of gravity influences in an earth-like pressure and temperature environment.

MODE established a comprehensive database on the differences in modal characteristics between ground and space and between nominally identical structures. The MODE results lead to a better understanding and modeling of the effects of gravity and suspension systems.

The MODE program also established a multiple degree-of-freedom non-Iinear force-state mapping technique with which sub-component force-state in formation can be asseiiibled to model a complete structure. This teclinique enables updating of second generation modal models yielding the accuracy required for high performance closed loop controllers.

MODE Reflight

The MODE Structural Experiment is scheduled to fly again onboard STS-62 in February 1994. The objectives of the MODE-Reflight experiment are:

The available STS-48 on-orbit time allowed the modal identification of only a few modes, at a few preload settings and excitation amplitudes. MODE-Reflight will expand the STS-48 test matrix and determine repeatability.
Due to unexpected large frequency shifts, the freqtiency windows of the STS-48 protocols missed the modal resonances of the L-Configuration. MODE-Reflight will use wider windows to capture the modal characteristics of this configuration.
Thie MODE-Reflight experiment will be used to investigate the modal characteristics of an Alpha-Joint with a more representative load transfer path.
Special protocols will be used to obtain data to support the NASA LaRC Modal Identification Experiment (MIE) program. The MIE program investigates the use of thruster firings for in-flight modal identification of the Space Station. The MODE-RefIight MIE excitations will match the energy (scaled) levels.
A stable, very low-gravity environment is necessary to assure that material science, astronomical investigations, and lie science investigations aboard Space Station Freedom will yield the desired results. Ciew disturbances of thie micro-gravity environment must be known to project their effect of crew disturbances aboard the space station. During MODE-Reflight, tionlie MODE ESM and three dynamic load sensors; a handhold, a foot-loop and a push-off pad, will be used to record in space crew force and moment disturbances.