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This paper describes recent results in applying robust control techniques to achieve vibration suppression of an active precision truss structure.
Embedded piezoelectric devices may be ideally suited for vibration control of space structures, which lack an inertial ground. When subjected to an input voltage, an embedded piezoelectric actuator changes its dimensions, which in turn generates a pair of forces exerted on adjacent structural members. From the direct piezoelectric effect, an embedded piezoelectric transducer generates an electric charge proportional to the structural dynamic response. In this paper, the implementation, testing and modeling of an active truss structure consisting of piezoelectric sensors and actuators are described. Linear quadratic Gaussian (LQG), second-order, and direct rate feedback control schemes are designed to suppress the vibrations of the active structure. Simulation and test results are presented. It is shown that special model reduction considerations are required to achieve good correlation between test and analysis.
Increasingly complex spacecraft will benefit from integrated design and optimization of structural, optical, and control subsystems. Integrated design optimization will allow designers to make tradeoffs in objectives and constraints across these subsystems. The location, number, and types of passive and active devices distributed along the structure can have a dramatic impact on overall system performance. In addition, the manner in which structural mass is distributed can also serve as an effective mechanism for attenuating disturbance transmission between source and sensitive system components. This paper presents recent experience using optimization tools that have been developed for addressing some of these issues on a challenging testbed design problem. This particular testbed is one of a series of testbeds at the Jet Propulsion Laboratory under the sponsorship of the NASA Control Structure Interaction (CSI) Program to demonstrate nanometer level optical pathlength control on a flexible truss structure that emulates a spaceborne interferometer.
Two active-control concepts incorporated into system for suppression of vibrations in truss structure and regulation of length of optical path on structure to nanometer level. Optical-path-length-control subsystem contains two feedback control loops to obtain active damping in wide amplitude-and-frequency range. Concept described in more detail in number of previous articles, including "Stabilizing Optical-Path Length on a Vibrating Structure" (NPO-19040), "Controllable Optical Delay Line for Stellar Interferometry" (NPO-18686), "Test Bed for Control of Optical-Path Lengths" (NPO-18487).
STS-112 Mission Specialist 2 Sandra H. Magnus is seen during a prelaunch interview. She answers questions about her inspiration to become an astronaut and her career path. She gives details on the mission's goals, the most significant of which will be the installation of the S-1 truss structure on the International Space Station (ISS). The installation, one in a series of truss extending missions, will be complicated and will require the use of the robotic arm as well as extravehicular activity (EVA) by astronauts. Magnus also describes her function in the performance of transfer operations. Brief descriptions are given of experiments on board the ISS as well as on board the Shuttle.
The main objective of the structures group is to provide quality aerospace research with the Center for Aerospace Research - A NASA Center for Excellence at North Carolina Agricultural and Technical State University. The group includes dedicated faculty and students who have a proven record in the area of structures, in particular space structures. The participating faculty developed accurate mathematical models and effective computational algorithms to characterize the flexibility parameters of joint dominated beam-truss structures. Both experimental and theoretical modelling has been applied to the dynamic mode shapes and mode frequencies for a large truss system. During the past few months, the above procedures has been applied to the hypersonic transport plane model. The plane structure has been modeled as a lumped mass system by Doctor Abu-Saba while Doctor Shen applied the transfer matrix method with a piecewise continuous Timoshenko tapered beam model. Results from both procedures compare favorably with those obtained using the finite element method. These two methods are more compact and require less computer time than the finite element method. The group intends to perform experiments on structural systems including the hypersonic plane model to verify the results from the theoretical models.
The structural design and configuration feasibility of the long spacer truss assembly that will be used as part of the Space Station Freedom is the focus of this study. The structural analysis discussed herein is derived from the transient loading events presented in the Space Transportation System Interface Control Document (STS ICD). The transient loading events are liftoff, landing, and emergency landing loads. Quasi-static loading events were neglected in this study since the magnitude of the quasi-static acceleration factors is lower than that of the transient acceleration factors. Structural analysis of the proposed configuration of the long spacer truss with four longerons indicated that negative safety margins are possible. As a result, configuration changes were proposed. The primary configuration change suggested was to increase the number of truss longerons to six. The six-longeron truss appears to be a more promising structure than the four-longeron truss because it offers a positive margin of safety and more volume in its second bay (BAY2). This additional volume can be used for resupply of some of the orbital replacement units (such as a battery box). Note that the design effort on the long spacer truss has not fully begun and that calculations and reports of the negative safety margins are, to date, based on concept only.
Fastener assembly for use in erecting truss structure assembled by robot equipped with one arm and one gripper. Easier for robot or human technician to build structure in outer space or on Earth. Assembly features extremely large capture envelope; accommodates initial lateral misalignments of truss members of as much as 0.4 in. and rotational misalignments large as 28 degrees and still produces sound structural connection. Chuck key is only tool required.
A brief overview of some structural results that came from space station skunk works is presented. Detailed drawings of the pressurized modules, and primary truss structures such as deployable single fold beams, erectable beams and deployable double folds are given. Typical truss attachment devices and deployable backup procedures are also given.
The results of a test program to assess the potential of manned extravehicular activity (EVA) assembly of erectable space trusses are described. Seventeen tests were conducted in which six "space-weight" columns were assembled into a regular tetrahedral cell by a team of two "space"-suited test subjects. This cell represents the fundamental "element" of a tetrahedral truss structure. The tests were conducted under simulated zero-gravity conditions. Both manual and simulated remote manipulator system modes were evaluated. Articulation limits of the pressure suit and zero gravity could be accommodated by work stations with foot restraints. The results of this study have confirmed that astronaut EVA assembly of large, erectable space structures is well within man's capabilities.
The results of closed loop experiments that use piezoelectric active-members to control the flexible motion of a precision truss structure are described. These experiments are directed toward the development of high-performance structural systems as part of the Control/Structure Interaction (CSI) program at JPL. The focus of CSI activity at JPL is to develop the technology necessary to accurately control both the shape and vibration levels in the precision structures from which proposed large space-based observatories will be built. Structural error budgets for these types of structures will likely be in the sub-micron regime; optical tolerances will be even tighter. In order to achieve system level stability and local positioning at this level, it is generally expected that some form of active control will be required.
The 'dual keel' design currently under consideration for NASA's Space Station program is of rectangular configuration to yield greater space along the truss structure for the attachment of payloads, by comparison with a previously considered single-keel truss design. The placing of the pressurized modules much nearer the center of gravity of this station configuration allows accelerations that are closer to 1 micro-g than the earlier alternative. The U.S. Servicing Facility and Telerobotic System, and the Canadian Mobile Servicing Center, will be incorporated in order to meet a large variety of Space Station servicing requirements.
The present conference on adaptive structures discusses piezoelectric and electrostrictive sensors and actuators for adaptive structures and smart materials, real-time control for composite structures with embedded actuators and sensors, a laminated-shell theory incorporating embedded distributed actuators, traveling-wave power flow techniques, uncertainty modeling for the control of an active structure, and active vibration isolation in the presence of unmodeled structural dynamic response. Also discussed are the control of flexible beams via free-free active truss, truss structure control using member actuators with latch mechanism, neural processors for smart-structure control, the effect of imperfections on the static control of adaptive structures, adaptive structures for segmented optical systems, and the shape-control of flexible structures.
This paper presents the results of closed loop experiments that use piezoelectric active-members to control the flexible motion of a precision truss structure. These experiments are directed toward the development of high performance structural systems as part of the Control/Structure Interaction program at JPL. Order of magnitude reductions in dynamic response are achieved with relatively simple control techniques. The practical implementation of high stiffness, high bandwidth active-members in a precision structure highlights specific issues of importance relating to the modelling and implementation of active-member control.
Control system design and closed-loop test results for the Mini-Mast truss structure located at the NASA Langley Research Center are presented. The simplicity and effectiveness of a classical control approach to the active structural control design are demonstrated by ground experiments. The concepts of robust nonminimum phase compensation and periodic disturbance rejection are also experimentally validated. The practicality of a sensor output decoupling approach is demonstrated for the inherent, multivariable control problem of the Mini-Mast.
Practical approaches for establishing large, precise antenna reflectors in space are described. Reflector surfaces consisting of either solid panels or knitted mesh are considered. The approach using a deep articulated truss structure to support a mesh reflector is selected for detailed investigations. A new sequential deployment concept for the tetrahedral truss is explained. Good joint design is discussed, and examples are described both analytically and by means of demonstration models. The influence of curvature on the design and its vibration characteristics are investigated.
The optimal placement of discrete actuators and sensors is posed as a combinatorial optimization problem. Two examples for truss structures were used for illustration; the first dealt with the optimal placement of passive dampers along existing truss members, and the second dealt with the optimal placement of a combination of a set of actuators and a set of sensors. Except for the simplest problems, an exact solution by enumeration involves a very large number of function evaluations, and is therefore computationally intractable. By contrast, the simulated annealing heuristic involves far fewer evaluations and is best suited for the class of problems considered. As an optimization tool, the effectiveness of the algorithm is enhanced by introducing a number of rules that incorporate knowledge about the physical behavior of the problem. Some of the suggested rules are necessarily problem dependent.
An adaptive structures approach for a large segmented optical system with a 20-meter primary reflector made of lightweight hexagonal composite panels is considered. This approach makes it possible to vary the static and dynamic characteristics of a structural system through actuators and sensors integrated within the structure. Topics discussed include quasi-static shape adjustment and active damping augmentation of the primary backup truss structure, deformable panels for long spatial wavelength quasi-static figure correction, and an on-orbit identification system for large segmented optical systems.