Damping Augmentation Research Articles

Improved damping in sandwich beams through the inclusion of dispersed graphite particles within the viscoelastic core

In this work, the passive damping in a sandwich beam is investigated for the inclusion of dispersed graphite particles within the viscoelastic core. The effective material properties of the viscoelastic particulate composite (VEPC) are estimated using a differential scheme and the elastic–viscoelastic correspondence principle. The corresponding results reveal increased effective storage moduli and decreased effective loss factor of a viscoelastic medium for the inclusion of graphite particles. As it implies both the possibilities of augmentation and deterioration of passive damping in the sandwich beam, the subsequent study is carried out on the variation of damping characteristics of the sandwich beam especially for different volume fractions of inclusion. It is found that the passive damping in the sandwich beam increases significantly for the inclusion of graphite particles within the viscoelastic core, and also an optimal volume fraction of inclusion corresponding to the maximum damping in the sandwich beam appears. Further study on the optimal utilization of VEPC patches within the core of the sandwich beam reveals a minimal change of weight of the overall beam for the replacement of patches of conventional viscoelastic material by VEPC patches while this replacement yields a remarkable improvement of passive damping in the sandwich beam.

Distributed Propulsion Aircraft with Aeroelastic Wing Shaping Control for Improved Aerodynamic Efficiency

This study presents an aeroelastic wing shaping control concept for distributed propulsion aircraft. By leveraging wing flexibility, wing-mounted distributed propulsion can be used to re-twist wing shapes in-flight to improve aerodynamic efficiency. A multidisciplinary approach is used to develop an aero-propulsive-elastic model of a highly flexible wing distributed propulsion transport aircraft. The conceptual model is used to evaluate the aerodynamic benefit of the distributed propulsion aircraft. The initial conceptual analysis shows that an improvement in the aerodynamic efficiency quantity of lift-to-drag ratio is possible with the proposed aeroelastic wing shaping control for distributed propulsion aircraft. Two concepts are studied: single-generator configuration and dual-generator configuration with four propulsors per wing. The baseline aircraft model is NASA Generic Transport Model. A fan performance analysis is developed for propulsion sizing. Cruise performance analysis is conducted to evaluate the potential improvement in the cruise range for the configurations under study. A flutter analysis is performed to address the potential flutter issue as the propulsors are placed toward the wing tip, which would cause a reduction in the wing natural frequencies. Flight control considerations are addressed in the context of the engine-out requirement, yaw and roll controls, and yaw damping augmentation using differential thrust.

Critically stable damping in flexible structure control

The concept of critically stable damping and its application to flexible structure control are presented. Critically stable damping was originally noted from the viewpoint of control-structure integration. It was recognized that a slight augmentation of structural damping would result in substantially more robust control without decreased performance and could guarantee the stability margin for any structural frequency increase. This paper illustrates the wide occurrence of critically stable damping phenomenon, establishes its basic relationship for the insight of the general design procedure, and applies the concept into the practical design of a satellite with a large deployable antenna. It is noted that the critically stable damping concept could provide a feasible approach to overcome the difficulty in ultra-large space structure design and control, and enhance the performance of a variety of excellent control design methodologies for flexible structure control. This critically stable damping provides new insight into the integrated design of control and structure.

Transferring an Earth-based adaptive optics technology to space telescopes

The present work is aimed at studying the application of a contactless voice-coil actuation technology, currently developed for some new-generation earth telescopes, to an hypothetical secondary mirror of a space telescope subjected to nanometric precision requirements. The technology involves a large number of electromagnetic actuators which are not in contact with the optical mirror. Each actuator is made by a fixed coil wound on a cold finger and a moving magnet, glued to the rear surface of the thin adaptive mirror. A stiff backplate provides the position reference. The gap between the mirror and the backplate is measured through co-located capacitive sensors. Since future space telescopes will operate at cryogenics temperatures, where the material damping is extremely small, the design of high-performance and stable active shape controllers without detrimental spillover effects is challenging. Due to the impossibility to exploit aerodynamic damping arising from the squeezed air film between the mirror and the backplate, as done on earth-based mirrors, there is the need of finding an alternative to damp out vibrations. For this purpose, the current-driven technology implemented in earth-based applications is replaced by a voltage-driven solution, which provides damping augmentation by means of eddy currents. This is crucial in achieving a sufficiently stable dynamic response. The related limitation in the control bandwidth is fully compatible with the promptness requirements of the control system for the application under study.

Dynamic modelling and stability of hingeless helicopter blades with a smart spring

AbstractThe aeroelastic stability of a uniform, untwisted hingeless ‘smart’ helicopter rotor blade in hover has been analysed. The concept of a ‘smart’ blade is achieved by implementing a piezoelectric stack at an appropriate location along a host blade such that upon actuation it enters the load path becoming an integral part of the host structure. Thus, the stiffness characteristics of the rotor are altered causing modal damping augmentation of the blade. The perturbation equations of motion for the ‘smart’ blade that describe the unsteady blade motion about the equilibrium operating condition are obtained using Galerkin’s method. These differential equations with periodic time coefficients are analysed for stability utilising the Floquet method. Six different regimes of actuation are investigated, and a parametric study is carried out by considering six different design cases. It is shown that, compared to a ‘host’ blade the stability characteristics of the ‘smart’ blade are not affected adversely. In fact, a judicious design and actuation of the ‘smart’ spring has the potential of improving the stability boundaries of individual blades.

‘Smart spring’ identification for hovering rotor aeroelastic-stability augmentation

AbstractThis work deals with tailoring of adaptive material included at the roots of hingeless helicopter rotor blades to be used in individual blade control (IBC) strategies.Usually, IBC strategies involving the use of adaptive materials either consider adaptive material embedded in the blade structure for inducing strain deformations, or apply adaptive actuators for controlling segments of the blade (e.g. for moving trailing-edge flaps).Here, the adaptive material is used to provide augmentation of modal damping in a passive control approach, that can be conveniently tuned so as to make it the most suitable for the actual rotor configuration under examination. The presentation of a procedure for tailoring this ‘smart spring’ is the aim of the paper.The aeroelastic blade model considered consists of a cantilever slender beam undergoing flap, lead-lag and torsional motion, coupled with a strip theory approach for the prediction of the aerodynamic loads, based on the very low frequency approximation of the pulsating-free-stream Greenberg’s theory.Starting from this model and applying the Galërkin method, generalised mass, damping and stiffness matrices of the basic blade, as well as the incremental generalised mass, damping and stiffness matrices due to the ‘smart spring’ have been determined, the latter depending on the ‘smart spring’ inertial and elastic characteristics.It will be shown that the application of an optimal control criterion, followed by a low frequency-approximation observer, yields the identification of the most suitable ‘smart spring’ characteristics for augmentation of rotor blade aeroelastic stability. The validity of this procedure will be demonstrated by numerical results concerning the stability analysis of two hovering blade configurations.

Human-in-the-loop evaluation of remote manipulator system active damping augmentation

This paper describes the application of controls-structures integration technology to benefit the on-orbit performance of the Space Shuttle remote manipulator system. Called active damping augmentation, the goal is to reduce the vibration decay time of the remote manipulator system following normal payload maneuvers and operations. Simulation of active damping augmentation was conducted in a real-time human-in-the-loop simulation of the remote manipulator system with the objective of obtaining qualitative and quantitative measurements of operational performance improvements from astronaut operators. Sensing of vibratory motions was simulated using a three-axis accelerometer mounted at the end of the lower boom of the remote manipulator system. The sensed motions were used in a feedback control law to generate commands to the remote manipulator system joint servo mechanisms which reduced the unwanted oscillations. Active damping of the remote manipulator system with an attached 3990-lb payload was successfully demonstrated. Six astronaut operators examined the performance of an active damping augmentation control law following payload maneuvers and Shuttle thruster firings. Significant reductions in the dynamic response were observed and investigation of performance benefits with heavier attached payloads was recommended.

Active Vibration Damping of the Space Shuttle Remote Manipulator System

Controls-structures interaction is a technology currently under development for application to large flexible space vehicles. The goal of this technology is the improvement of spacecraft performance through active control of the structural dynamic response of the vehicle. This goal is particularly important for modern spacecraft designs where large size and reduced stiffness make structural response a significant contributor to vehicle dynamics. Analysis and design methods have been developed to analyze and predict flexible spacecraft performance, but the technology remains largely unvalidated by hardware experiments, demonstrations, or applications, particularly in-space flight applications. One potential application considered is to provide active damping augmentation of the Space Shuttle remote manipulator system. The objective of actively damping the manipulator is to demonstrate improved structural dynamic response following payload maneuvers and Shuttle reaction control system thruster firings. This paper describes an initial analysis effort to determine the feasibility of controlling the flexible dynamic response of the arm. The approach to the study is summarized and results from both linear and nonlinear performance analyses of candidate control laws are presented. Results indicate that significant improvement in dynamic response can be achieved through active control if measured arm tip acceleration was made available for feedback.

Active member bridge feedback control for damping augmentation

An active damping augmentation approach using active members in a structural system is described. The problem of maximizing the vibration damping in a lightly damped structural system is considered using the analogy of impedance matching between the load and source impedances in an electrical network. The proposed active damping augmentation approach therefore consists of finding the desired active member impedances that maximize the vibration damping, and designing a feedback control in order to achieve desired active member impedances. This study uses a bridge feedback concept that feeds back a combination of signals from sensors of the axial force and relative velocity across the active member to realize the desired active member impedance. The proposed active damping augmentation approach and bridge feedback concept were demonstrated on a three-longeron softly suspended truss structure.

Dynamic Analysis to Evaluate Viscoelastic Passive Damping Augmentation for the Space Shuttle Remote Manipulator System

This paper presents a NASTRAN finite element analysis for evaluation of the effectiveness of viscoelastic damping treatments as passive controls for large flexible space manipulators. The passive damping could be used alone or as an augmentation to active control. Perhaps the best existing example of a practical flexible manipulator is the space shuttle Remote Manipulator System (RMS). The authors use the RMS as an example for this investigation, subjecting it to a detailed dynamic analysis which can be used to evaluate the critical modes for control and to distinguish the modes which are good candidates for active control from those which are well suited for passive control. Modal potential energy analysis (MPE) is used to examine the modal energy distribution in each structural member of the complex flexible chained system. The results indicate that the most dominant contributors to end-point oscillations fall into two categories. These include very low frequency modes due to joint flexibility and higher frequency modes due to bending in the booms. Significant end-point motions result from each category, but the most significant motions are associated with joint flexibility. Finally, a finite element analysis is performed to evaluate the effectiveness of constrained viscoelastic layer damping treatments for passive vibration control. Passive damping augmentation is introduced through the use of a constrained viscoelastic layer damping treatment applied to the surface of the manipulator’s flexible booms. It is shown that even the joint compliance dominated modes can be damped to some degree through appropriate design of the treatment.

Control Strategy Design of Large Space Structures

A new control strategy called a generalized energy dissipative control is proposed for the control of linear large space structures with proportional damping. The control strategy consists of two feedback control loops : an optimal stiffness control loop and a generalized direct velocity feedback loop for the augmentation of damping. The validity of the proposed control stategy is shown through numerical simulation of flexible mode control of symmetric elastic panels attached to a space-craft.

Review of Various Mechanical Means Suppressing Wind-Induced Vibration of Structures

A variety of means for suppressing wind-induced vibration of structures could be divided into aerodynamic means and mechanical ones.Out of these two means, the latter is dis-cussed in this paper.First, various mechanical means for suppressing wind-induced vibration are classified as follows;A. augumentation of stiffnessB. addition of massC. augmentation of damping, No.1 untuned typeD.augmentation of damping, No.2 tuned typeE. application of control forceEach vibration-suppressing means classified above is discussed one by one making up a list of proposed ideas and actually applied cases.Finally, the design procedure and things to be considered are described in the case of the planning of these mechanical means for suppressing wind-induced vibration.