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High precision pointing and vibration control of future spacecraft and satellites

The Middeck Active Control Experiment (MACE) is a United States Space Shuttle flight experiment manifest for launch on STS-67 in February, 1995. MACE (Figure la) was designed by the Space Engineering Research Center at the Massachusetts Institute of Technology, in collaboration with Payload Systems Incorporated, the NASA Langley Research Center, and Lockheed Missiles and Space Company. The goal is to explore approaches to achieving high precision pointing and vibration control of future spacecraft and satellites. In particular, MACE extends the bandwidth of conventional rigid body instrument pointing and attitude controllers to include the flexible modes of the satellite. Since the success of such flexible control is intimately dependent upon the accuracy of the spacecraft model used for control design, MACE is essentially a spacecraft modeling validation effort where success is determined by the control performance and predictability that is achieved in earth orbit. MACE builds upon the concept of the Middeck 0-Gravity Dynamics Experiment (MODE), which flew on STS-40, STS-48 and STS-62 as a dynamics test facility to characterize fluid, Space Station structure, and crew motion dynamics in zero- gravity. MACE augments the MODE facility with real-time, digital control capabilities.


Suspension and gravity influences on the structural dynamics of a modular truss system

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.