W. Keith Belvin

Improved Concept and Model of Eddy Current Damper

When a conductive material experiences a time-varying magnetic field, eddy currents are generated in the conductor. These eddy currents circulate such that they generate a magnetic field of their own, however the field generated is of opposite polarity, causing a repulsive force. The time-varying magnetic field needed to produce such currents can be induced either by movement of the conductor in the field or by changing the strength or position of the source of the magnetic field. In the case of a dynamic system the conductor is moving relative to the magnetic source, thus generating eddy currents that will dissipate into heat due to the resistivity of the conductor. This process of the generation and dissipation of eddy current causes the system to function as a viscous damper. In a previous study, the concept and theoretical model was developed for one eddy current damping system that was shown to be effective in the suppression of transverse beam vibrations. The mathematical model developed to predict the amount of damping induced on the structure was shown to be accurate when the magnet was far from the beam but was less accurate for the case that the gap between the magnet and beam was small. In the present study, an improved theoretical model of the previously developed system will be formulated using the image method, thus allowing the eddy current density to be more accurately computed. In addition to the development of an improved model, an improved concept of the eddy current damper configuration is developed, modeled, and tested. The new damper configuration adds significantly more damping to the structure than the previously implemented design and has the capability to critically damp the beam s first bending mode. The eddy current damper is a noncontacting system, thus allowing it to be easily applied and able to add significant damping to the structure without changing dynamic response. Furthermore, the previous model and the improved model will lie applied to the new damper design and the enhanced accuracy of this new theoretical model will he proven.

Development of a New Passive-Active Magnetic Damper for Vibration Suppression

Magnetic fields can be used to apply damping to a vibrating structure. Dampers of this type function through the eddy currents that are generated in a conductive material experiencing a time-changing magnetic field. The density of these currents is directly related to the velocity of the change in magnetic field. However, following the generation of these currents, the internal resistance of the conductor causes them to dissipate into heat. Because a portion of the moving conductor s kinetic energy is used to generate the eddy currents, which are then dissipated, a damping effect occurs. This damping force can be described as a viscous force due to the dependence on the velocity of the conductor. In a previous study, a permanent magnet was fixed in a location such that the poling axis was perpendicular to the beams motion and the radial magnetic flux was used to passively suppress the beam s vibration. Using this passive damping concept and the idea that the damping force is directly related to the velocity of the conductor, a new passive-active damping mechanism will be created. This new damper will function by allowing the position of the magnet to change relative to the beam and thus allow the net velocity between the two to be maximized and thus the damping force significantly increased. Using this concept, a model of both the passive and active portion of the system will be developed, allowing the beams response to be simulated. To verify the accuracy of this model, experiments will be performed that demonstrate both the accuracy of the model and the effectiveness of this passive-active control system for use in suppressing the transverse vibration of a structure.

Modeling and Application of Eddy Current Damper for Suppression of Membrane Vibrations

Inflatable space-based structures have become increasingly popular over the past three decades due to their minimal deployed mass and launch volume. To facilitate packaging of the satellite in the shuttle bay, the optical or antenna surface is in many cases a thin-film membrane. Additionally, because the structure holding the membrane is a lightweight and flexible inflated device, the membrane is subjected to a variety of dynamic loadings. For the satellite to perform optimally, the membrane structure must be free of vibration. However, due to the extreme flexibility of the membrane, the choice of applicable sensing and actuation methods to suppress the vibration is limited. The present study investigates the use of an eddy current damper to passively suppress the vibration of a thin membrane. Eddy currents are induced when a nonmagnetic conductive material is subjected to a timechanging magnetic flux. As the eddy currents circulate inside the conductor they are dissipated, causing energy to be removed from the system and thus allowing the system to function as a type of viscous damper. Using this concept, the ability to generate sufficient damping forces in the extremely thin-film membranes used in space is studied. First, a theoretical model of the interaction between the eddy current damper and the membrane is developed. The model is then validated through experiments carried out at both ambient and vacuum pressures. The results show that the model can accurately predict the damping of the first mode as the distance between the magnet and membrane is varied. Furthermore, the results of the experiments performed on the membrane at vacuum conditions show the functionality of the damping mechanism in space and indicate damping levels as high as 30% of critical at ambient pressure and 25% of critical at vacuum pressure.

In-Space Structural Assembly: Applications and Technology

As NASA exploration moves beyond earths orbit, the need exists for long duration space systems that are resilient to events that compromise safety and performance. Fortunately, technology advances in autonomy, robotic manipulators, and modular plug-and-play architectures over the past two decades have made in-space vehicle assembly and servicing possible at acceptable cost and risk. This study evaluates future space systems needed to support scientific observatories and human/robotic Mars exploration to assess key structural design considerations. The impact of in-space assembly is discussed to identify gaps in structural technology and opportunities for new vehicle designs to support NASAs future long duration missions.

On the analytical modeling of the nonlinear vibrations of pretensioned space structures

Abstract Linear, quasi-linear and nonlinear analyses have been used to investigate a relatively simple two-dimensional cable-stiffened structure under sinusoidal excitation. For linear vibrations, both an exact and a simplified analysis in which each cable is modeled as a spring with its mass lumped at the ends have been compared. The exact analysis, which accounts for distributed cable inertia, indicates a mass lumping procedure which is valid for both low and high ratios of cable to joint mass and represents an improvement to using a consistent mass lumping. The quasi-linear analysis extends the simplified linear model to account for cable slackening by removing a cable when its axial load vanishes. The nonlinear analysis accounts for distributed cable mass which permits large cable deformations and the natural collapse of a cable when it slackens. Appropriate dimensionless quantities are derived from the nonlinear differential equations and results are presented in terms of these quantities. The nonlinear analysis predicts softening of the overall structural mode due to cable slackening and hardening of strongly coupled cable-structural modes due to raised tension levels during large cable deformations. Quasi-linear analysis predicts the softening mode provided motions are not very large, but it cannot predict hardening modes. Hardening modes can involve structural motions of the same magnitude as the overall structural mode and can be the fundamental mode of the cable-stiffened structure. Prediction of the hardening mode amplitude requires nonlinear analysis, however, the amplitude of the softening mode can be predicted by linear analysis even through it fails to predict the downward change in frequency.

High Leverage Technologies for In-Space Assembly of Complex Structures

In-space assembly (ISA), the ability to build structures in space, has the potential to enable or support a wide range of advanced mission capabilities. Many different individual assembly technologies would be needed in different combinations to serve many mission concepts. The many-to-many relationship between mission needs and technologies makes it difficult to determine exactly which specific technologies should receive priority for development and demonstration. Furthermore, because enabling technologies are still immature, no realistic, near-term design reference mission has been described that would form the basis for flowing down requirements for such development and demonstration. This broad applicability without a single, well-articulated mission makes it difficult to advance the technology all the way to flight readiness. This paper reports on a study that prioritized individual technologies across a broad field of possible missions to determine priority for future technology investment.

Integration issues for high-strain actuation applications

Strain actuators, used to induce a relative structural displacement, are most effective when integrally embedded into the host structure. The paper summarizes progress and challenges in the field of embedding smart materials into structures. The difficulties of embedding active elements within composite structures are described from a system design perspective. Issues associated with the application of strain actuators in primary aircraft structure are discussed. Development of new embedding technology to combat the difficulties with high strain applications is advocated.

New semi-active damping concept using eddy currents

A damping effect can be induced on a conductive structure that is vibrating in a magnetic field. This damping effect is caused by the eddy currents that are induced in the material due to a time varying magnetic field. The density of these currents is directly related to the velocity of the conductor in the magnetic field. However, once the currents are formed the internal resistance of the conductive material causes them to dissipate into heat, resulting in a removal of energy from the system and a damping effect. In a previous study, a permanent magnetic was fixed in a location such that the poling axis was perpendicular to the beams motion and the radial magnetic flux was used to passively suppress the beam’s vibration. Using this passive damping concept and the idea that the damping force is directly related to the velocity of the conductor, a new semi-active damping mechanism will be created. This new damper will function by allowing the position of the magnet to change relative to the beam and thus allowing the net velocity between the two to be maximized and the damping force significantly increased. Using this concept, a model of both the passive and active portion of the system will be developed, allowing the beams response to be simulated. To verify the accuracy of this model, experiments will be performed that demonstrate both the accuracy of the model and the effectiveness of this semi-active control system for use in suppressing the transverse vibration of a structure.

In-space assembly application and technology for NASA's future science observatory and platform missions

Significant developments are being made for the in-space assembly (iSA) of lightweight structures and spacecraft systems that are needed for NASA’s future science observatory and platform missions. Technology advances in autonomy, robotic manipulators, and modular architectures now make iSA and servicing possible at an acceptable risk and cost. Future in-space system capabilities will be needed for large optical observatory and science platform missions in order to assess key structural design considerations. This paper discusses possible NASA applications of iSA, capability and technology needs for telescope assembly, and emerging technologies for space systems in the future.

Model of Eddy Current Damper for the Suppression of Transverse Membrane Vibrations

In most applications, the additional mass and stiffness that are induced due to the use of a vibration damper, while undesirable are not major design criteria. However, in recent years there has been a growing interest in the development of ultra large lightweight deployable structures. Because these structures are deployable, they are packaged in their launch configuration and deployed once in space. The packaging of the satellite leads to the requirement that the metrology surface be flexible and has typically been a very thin membrane. Because this membrane is to be used as a metrology surface it must hold extremely strict surface tolerances which can easily be exceeded if the membrane is subjected to structural vibrations. This requirement that for the membrane to perform optimally it must be free of vibration, leads to difficult control issues brought on by the membranes extremely flexible nature. This extreme flexibility places severe limitations on the actuation methods compatible with the membrane. These limitations are due to the fact that the bonding of an actuator to the membrane would result in surface aberrations and that if a point actuation method were used only local deformations would result. The eddy current effect can lead to an ideal damping mechanism, however due to the ineffectiveness of the previously developed eddy current damping mechanisms; their potential has not been realized. The ability of a recently developed eddy current damping mechanism to generate sufficient damping forces in the extremely thin film membranes used in space will be studied. A theoretical model will be developed and validated through experiments carried out at both ambient and vacuum pressures. The results of these experiments show that the model can accurately predict the damping of the first mode as the distance between the magnet and membrane is varied. Furthermore, the results of the experiments performed on the membrane at vacuum conditions show the functionality of the damping mechanism in space.