Results of a project to validate and test implementation of Wiener-Hopf optimal controllers for active suppression of vibrations in structures using digital signal processing methods are reported. The Wiener-Hopf design approach employs frequency response models and computational approach for resolving tradeoffs in multichannel vibration control systems based on steady state performance measures. Explicit frequency domain constructions for modeling distributed parameter elastic vibration response in structures can be exploited. Controller design optimization and implementation is achieved using computational algorithms based on data sampling of either model generated or empirically measured frequency response. The experiments described employ nonrecursive, Finite Impulse Response (FIR) algorithms implemented in state-of-the-art Digital Signal Processors (DSP) for realtime vibration control. The results reported provide analytical and experimental validation of the design approach. The experimental results reported were recently obtained in a series of experiments performed on a 12 meter truss experiment at the Wright Laboratories.

A distributed, hierarchical architecture is presented which is particularly suitable for the control of large flexible structures. We assume that large, flexible structures are a collection of smaller substructures for which nominal, probably linear models exist. We advocate the generation of hardware analogs for these substructures based on the nominal models, as building blocks for both the larger structure and for the feedback controllers. Relying on the availability of the substructure model realizations, we advocate the use of local (decentralized) model-based substructure controllers. In addition to the nominal model based nominal controllers outlined above, we propose Neural Network based Identifiers and Controllers to generate a correction signal for state estimation and control. Thus the use of the Neural Network has been relegated to that of being a predictor and corrector to a nominal estimator and controller for the substructure. At a higher level in this architecture is a third Neural Network which has the task of handling larger excursions from the expected response of the system so as to switch to handle faults and other emergencies.

Active control of flexible multi-body systems and structures through utilization of smart materials is considered in this paper. Specifically, utilization of piezoceramics for sensing and actuation is investigated. Experimental setups have been developed at Control/Robotics Research Laboratory (CRRL) to study modeling issues and control design approaches for flexible structures with embedded (or surface-mounted) piezoceramics. In this paper, experimental results on vibration suppression for a clamped-free beam and a rotating flexible beam with surface mounted piezoceramics are presented. We compare and contrast the analytical modeling and control design with the experimental results. It is shown that piezoceramics substantially improve the performance of the systems under consideration. The advocated approaches for control designs are decentralized frequency shaping and self-tuning adaptive controllers. The self-tuning controller is based on identification of the system dynamics in frequency domain utilizing Fast Fourier Transform (FFT). The real-time computing power to evaluate FFTs is provided by digital signal processing boards (TMS320C30 based). Addition of a self-tuning regulator enhances the performance and the robustness of the controlled system to parameter variations.

An active vibration control experiment for precision mirror pointing using smart structure is described. The setup consists of a flexible plate clamped to the shaft of an electric motor. Part of the plate is polished to reflect a laser beam whose direction accuracy is the performance criterion. Electroceramic actuators and sensors are incorporated into the plate to control vibration. The analytical model is generated using the ANSYS program. Six flexible modes are kept to investigate the interaction between the rigid and the flexible modes. Three different control strategies were examined. The goal is to suppress the first and the second mode with very little spillover effects from other modes. Simulation results show that the performance objectives can be met. These analytical studies are verified in actual experiments in the near future.

This paper describes a neural network-based digital control system for a wide range of dynamic applications. Two diverse applications of the digital control of smart structural systems are discussed to illustrate the general nature of this neurocontroller. The first application is reduction of vibration in optical mirrors for large high energy laser installations. Under neurocontrol, multiple actuators and sensors reduce the `jitter' caused by cooling water flow and other vibration sources affecting the mirrors within the laser system. Accelerometers provide control parameters. The control and reduction of high intensity acoustic noise is a similar application of neurocontrollers. In this case, multiple acoustic sources are controlled to reduce the measured noise at multiple microphone locations. Both examples used the same neurocontroller with a filtered-x architecture. Minor adjustments to accommodate either the `macro'-scale dynamics of high-intensity acoustics, or the `micro'-scale dynamics of optical mirror vibrations were made as needed. The paper includes results from actual tests.

This paper presents some initial results based on H^(infinity ) controller designs for a flexible slewing piezoelectric laminate beam. A two-input-one-output model of a piezoelectric laminate is used; slewing torque and piezoactuator voltage are the two inputs and the tip-position is the output. Two control design examples are used to demonstrate the utility of active control in damping the vibrations in a slewing motion.

This paper discusses the development and use of an effective finite element analysis procedure to examine integrated anisotropic piezoelectric actuators for aeroelastic applications where it is essential that torsion and bending be controlled independently of one another. The finite element model consists of three-dimensional isoparametric solid elements (bricks) that allow modeling of tailored piezoelectrics with skewed actuator/sensor axes. This scheme also allows the representation of an anisotropic host structure and can account for material and stacking geometry through the element thickness. Using this finite element representation, it is shown that anisotropic piezoelectric actuators can create sufficient twisting and bending to control aerodynamic loads on a wing, although aerodynamic loads are not included in this discussion.

The use of smart materials to actively control flutter in anisotropic composite panels was examined. An analytical model of an anisotropic panel that can accommodate the effects of the induced strain actuators and supersonic aerodynamics and gust effects was developed. Control design was accomplished with a state-space model that incorporates linear quadratic optimal control. The piezoelectric material lead zirconate titinate was used for the actuators, while various composites and aluminum materials made up the different test panels. The distribution of actuators on the faces on the panel were studied. Actuators segmented in the direction of the airstream were found to be most effective in controlling flutter for square panels; however, actuators should be distributed in both the flow and cross-flow directions for panels with different aspect ratios. A relatively small number of actuators can provide excellent flutter suppression; increasing the number of actuators beyond this amount provides little improvement in flutter speed. The anisotropic characteristics of the composite panels cause some variations in the behavior of the panel as compared to isotropic panels. A design comparison revealed that actively controlled panels provide superior flutter performance to passively controlled panels.

The objective of this research was to experimentally study the capability of adaptive material plate actuators to alleviate buffet loads. This investigation involved a single degree-of-freedom wind-tunnel model consisting of a rigid surface and a flexible mount system which permitted only plunge translation. Actuators made of piezoelectric material and affixed to the mount system provided the actuation mechanism for this system. Command signals, applied to the piezoelectric actuators, exerted control over the damping of the system. The closed-loop response, on average, was 32 percent below the open-loop strain response.

The design of a new aeroservoelastic wing configuration is detailed. The design uses a torque plate mounted within an aerodynamic shell. As the plate is actively twisted, the shell, which is connected to the plate at the tip, undergoes a pitch deflection. An active torque plate for a low aspect ratio subsonic wing section is analyzed and designed with laminated plate theory. Several actuator elements were considered for the plate, including: conventionally attached lead zirconate titanate (PZT), polyvinylidene fluoride (PVDF), piezoelectric fiber composites and directionally attached piezoelectric (DAP) elements. The analytical studies demonstrate that the highest twist deflections are obtained by DAP elements bonded to a beryllium substrate which produces twist deflections and restrained moments that are 67% greater than the next closest actuator material. An aeroservoelastic wing using the DAP torque plate was constructed to demonstrate the concept. The wing used an active plate made from DAP elements bonded to an AISI 1010 steel substrate. The torque plate was mounted within a graphite-epoxy wing with a modified NACA 0012 profile, measuring 2.6' in chord with a 3.66' span. The wing was tested to 160 ft/s and exhibited a stable increase in pitch deflection (in addition to the +/- 3.5 degree(s) static deflection) with theory and experiment in close agreement.

The volume of certain types of smart materials proposed for the coating of aerodynamic surfaces can be instantaneously affected if subjected to an electric or a magnetic field, thus creating dramatic changes in the entire flow field. For example, a transonic airfoil that was designed to be shock-free at a given angle of attack and a given flight Mach number will start developing shock waves immediately after either the angle of attack or the flight Mach number are perturbed. In order to maintain the same aerodynamic performance of an airfoil (that is, maintain the identical distribution of surface pressures on the airfoil) over a range of flight speeds, the airfoil shape must readjust continuously. Using our highly accurate and proven transonic airfoil design code we have performed a detailed evaluation of the required local thickness alterations necessary to maintain the shock-free flow field at different flight Mach numbers. A recommendation as to the required volumetric change of the smart skin materials to be used for the continuous shape adjustments of the aerodynamic configurations was established.

Control of the static aeroelastic characteristics of a swept uniform wing in roll using an adaptive structure is examined. The wing structure is modeled as a uniform beam with bending and torsional deformation freedom. Aerodynamic loads are obtained from strip theory. The structure model includes coefficients representing torsional and bending actuation provided by embedded piezoelectric material layers. The wing is made adaptive by requiring the electric field applied to the piezoelectric material layers to be proportional to the wing root loads. The proportionality factor, or feedback gain, is used to control static aeroelastic rolling properties. Example wing configurations are used to illustrate the capabilities of the adaptive structure. The results show that rolling power, damping-in-roll and aileron effectiveness can be controlled by adjusting the feedback gain. And that dynamic pressure affects the gain required. Gain scheduling can be used to set and maintain rolling properties over a range of dynamic pressures. An adaptive wing provides a method for active aeroelastic tailoring of structural response to meet changing structural performance requirements during a roll maneuver.

The so called `adaptive' or `smart structures' are a new generation of structures in which the principles of control engineering are used to make a structure to adapt to a set of required geometric or dynamic constraints. Examples of `adaptive' or `smart structures' are abundant in `organic and biological systems' in nature. One of the problems in shape adaptation of flexible structures is active damping in which the objective is to suppress undesirable disturbances in the way of convergence to a desired shape. Principally, this task is achieved through the application of modal forces which are opposing unstable or undesirable motion of the structure. In this paper active damping of a beam as an example of flexible structures is studied. The purpose is to stabilize significant modes of motion with a piezoelectric actuator- sensor. Using a one dimensional partial differential equation model, a variational method is used to find a dissipative energy function. A criteria for the design of the shape of the piezoelectric material or electrode, which can accomplish damping of significant modes, is derived. Unlike previous efforts, which could find electrode shape for dissipativity of one mode, it is proven that the suggested shape guarantees the dissipativity of energy function for all modes of the system.

Multisegment SMART traversing beams with built-in sensing and actuation capabilities have been considered in various critical applications where weight and deflection are of utmost importance, such as in long support bridges. With such capabilities, light weight SMART segments can be added one at a time to extend the traversing beam over long gaps with minimal deflections and stresses. Each segment of the SMART beam has a set of distributed wire sensors, embedded in its bottom fibers, to sense the beam deflection and another set of wire actuators, embedded in the top fibers, to compensate for the encountered deflections. The theory governing the operation of this class of SMART beams is developed and validated experimentally on a two-segment prototype of the traversing beam whose sides are made of a photoelastic material.

This study investigates the feasibility of using distributed force actuators (in addition to hub- torque) to control the tip deflection of a flexible beam-like structure during a rapid reorientation maneuver. The major objectives of the study are to assess system performance and actuator force requirements. Three feedback control schemes are compared to an open- loop control law corresponding to the rigid-body minimum-time solution. These control schemes use (1) a rate feedback hybrid controller, (2) an LQR-type hybrid controller, and (3) an LQR-type integrated controller. Comparisons between these controllers are made on the basis of actuator force required, increase in closed-loop system damping, and time required to execute a 90 degree reorientation maneuver. It was found that the rate-feedback hybrid control scheme results in high damping, but requires large peak-value forces from the actuators. The LQR-type hybrid control scheme requires lower peak-value forces from the actuators, but yields lower damping. The LQR-type integrated control scheme requires even lower actuator forces and a tip deflection that is significant only at the beginning of the maneuver, but results in an increase in the maneuver time.

This paper investigates the concept of active fatigue control of an adhesively bonded structure. Adhesive bonding has recently become very widely used, especially with the gaining popularity of composite materials. The adhesive bonds are usually the weakest link in the entire bonded structure. Additionally, one of the major causes of failure in the bond is fatigue. With increasing knowledge in the field of intelligent material systems, it is now possible to reduce the stress in the adhesive bond by using induced strain actuators, such as PZT. Experimental results show that the fatigue life of a vibrating, cantilever beam with an adhesive bond can be increased by nearly an order of magnitude with active fatigue control. This paper presents an analytical model which can be used to determine the correct control schemes of induced strain actuators on a cantilever beam vibrating over a range of frequencies. With proper control, the peeling stresses in the adhesive bond can be minimized for a broad frequency range.

This paper develops one-dimensional pure bending, coupled bending and extension, and combined bending, extension and torsion models of isotropic beams with induced strain actuation. A finite thickness adhesive layer between the crystal and beam is included to incorporate shear lag effects. Experimental tests evaluate the accuracy and limitations of the models. The bending and coupled bending and extension models show acceptable correlation with static test results whereas the combined extension, bending, torsion model poorly predicts the system behavior and needs refinement.

NITINOL alloy wire actuation system is discussed in this paper. This actuator is an active component used to achieve a camber change in a NACA 0012 airfoil. The camber changes are used to provide a collective control for a two bladed model helicopter. This means the collective control is moved to the individual blades. A two actuator control system has been designed using H_(infinity ) control techniques.

This paper presents an experimental study on the development of a Froude scale helicopter rotor blade with trailing edge flap as a vibration reduction device actuated by piezoelectric crystals. A fixed wing model with NACA 0012 airfoil, 3.0 inch blade chord and 20% trailing edge flap is fabricated and tested in the open-jet tunnel to determine the dynamic flap response at various blade angles of attack and excitation frequencies. The results show that for a given velocity, flap response does not change appreciably with the excitation frequency and the blade angle of attack.

The objective of this research is to develop a dynamically-scaled helicopter rotor blade with embedded piezoceramic elements as sensors and actuators to control blade vibrations. A 6 ft diameter 2-bladed Froude-scale bearingless rotor model is built where each blade is embedded with banks of specially-shaped piezoelectric crystals at +/- 45 degree angles on the top and bottom surfaces. A twist distribution along the blade span is achieved through in-phase excitation of the top and bottom crystals at equal potentials. The non-rotating static torsional response of the piezoceramic blade is experimentally determined and then correlated with the prediction by theory.

This paper presents results from an exploratory research effort involving active control of rotor noise using blade airfoil shape changes. Using a numerical technique for solving the unsteady transonic full potential flow equation, the rotor blade flow problem is considered to study the effect of blade airfoil shape changes on the mach number distribution over the surface of the blade. In particular, the magnitude and location of maximum shape changes required to eliminate shock waves present on the blade (and hence the associated noise) are identified.

This paper presents an analytical study on natural frequencies of rotating open-section composite beams with actuation by embedded shape memory alloy (SMA) fibers. The analysis includes the essential features of open-section composite beam modeling, such as constrained warping and transverse shear deformation. A general plate segment of the beam with and without SMA fiber-composite ply is modeled using laminated plate theory and the forces and displacement relations of this plate segment are then reduced to the force and displacement of the one-dimensional beam. The natural frequencies of rotating graphite-epoxy I-beams with SMA are calculated for different volume ratios of fibers. The feasibility of maintaining constant frequencies at different rotational speeds by SMA actuation is shown for composite I- beams.

The possibility of using interdigitated surface electrode patterns to improve the transverse actuation capability of electroceramic actuators is investigated. This pattern produces nonuniform electrical fields in the plane of the wafer which utilize the longitudinal piezoelectric effect to generate larger, more anisotropic planar actuation than conventional piezoelectric devices. Analytical models are developed for a representative electroceramic volume element of a piezoelectric wafer with interdigitated electrodes. These models incorporate full electromechanical coupling through the constitutive relations and are solved using approximate energy methods. The analytical models are compared to piezoelectric finite element solutions. The analysis predicts a range of electrode thickness and spacings which can increase the achievable transverse actuation. An experimental program was performed to validate the analytical results. The experimental results verified the analytical prediction of highly orthotropic large magnitude in plane strains.

This paper presents a coupled electro-mechanical analysis of piezoelectric ceramic (PZT) actuators integrated in mechanical systems to determine the actuator power consumption and energy transfer in the electro-mechanical systems. For a material system with integrated PZT actuators, the power consumed by the PZT actuators consists of two parts: the energy used to drive the system which is dissipated in terms of heat as a result of the structural damping, and energy dissipated by the PZT actuators themselves because of their dielectric loss and internal damping. The coupled analysis presented herein uses a simple model, a PZT actuator-driven one-degree-freedom spring-mass-damper system, to illustrate the methodology used to determine the actuator power consumption energy flow in the coupled electro-mechanical systems. This method can be applied to more complicated mechanical structures or systems, such as a fluid-loaded shell for active structural acoustic control. The determination of the actuator power consumption can be very important in the design and application of intelligent material systems and structures and of particular relevance to designs that must be optimized to reduce mass and energy.

This paper addresses the micro-mechanical issues associated with embedding optical fiber sensors in a thick composite material. An optical fiber embedded in a thick composite creates a perturbation in the micro-structure of the composite, which in turn alters the strain state of composite in the region surrounding the optical fiber. Moire Interferometry combined with Fourier transform based digital fringe processing is used to measure and analyze the displacement field in the composite material.

The effect of embedded optical fibers on the transverse strength of an AS4/3501-6 composite is experimentally evaluated. The test data are compared with an existing analytical model where applicable. Parametric studies reported on include varying the optical fiber's physical size, coating thickness, and adhesion between the optical fiber and the composite. Results demonstrate that for certain cases the transverse strength can be degraded as much as 50%, however for small diameter optical fibers the strength reduction is negligible. Furthermore, polyimide coated optical fibers appear to have less of an effect on transverse strength than do uncoated optical fibers. It is suggested, both analytically and experimentally, that an optimal coating thickness exists to minimize the degradation in transverse strength.

This paper presents the interactions of a simple vibrating beam host with embedded arrays of micro-actuators. The geometry of the micro-actuator is idealized to be ellipsoidal and Eshelby's classical techniques are used to obtain a first order estimate of the interaction energy of the actuator and the surrounding host, as a result of actuation loads. Preliminary analytical results for piezoelectric actuators embedded in an isotropic host provide important clues regarding the influence of the number of embedded transducers and the excitation voltage on the natural frequencies of the structure. The strain concentration in the host due to the embedded actuators is obtained by using Eshelby's classical result. The results of this study provide crucial insights for: (1) integrating the response of the host and actuator for transient analysis of the adaptive structure; (2) obtaining accurate transfer functions for transient analysis and control of the structure; and (3) assessing the damage (and associated loss of reliability) caused to the host and to the actuator by external as well as actuation loads.

The effect of the location and thickness of twin collocated piezoelectric actuators embedded in a composite structure is analytically studied. An expression for the effective moment induced by the actuators is derived using a static analysis assuming perfect bonding (i.e., zero bonding thickness). The optimal actuator position and thickness which maximizes piezoactuator/substructure coupling is investigated for various actuators/substructure combinations. Results of this study show that the optimal actuator location is at the surface of the composite when the Young's modulus of the actuator is less than approximately 3 to 5 times the modulus of the substructure. However, when the modulus of the actuator is much greater than the substructure, the optimal location of the actuators is within the composite rather than at the surface.

An integrated distributed actuator design methodology based upon the converse piezoelectric effect and aimed at actively controlling the in-vacuo eigenvibration characteristics of rotating helicopter blades is presented. The system of piezoelectric actuators bonded or embedded into the structure produces a localized strain field in response to an applied voltage and, as a result, it yields an adaptive change of dynamic response characteristics. The helicopter blade is modeled as a thin/thick walled closed cross-section untwisted cantilevered beam rotating with constant angular velocity. The considered numerical illustrations reveal the power of the adaptive technology to control the dynamic characteristics of rotating blade structures and, as a result, to avoid their structural resonance as well as any of their dynamic instabilities.

The need for active vibration control for airborne laser systems was demonstrated during the late 1970s by the Airborne Laser Laboratory. Other possible applications include sonic fatigue alleviation, reduction of buffet induced fatigue, vibration control for embedded antennae, and active aeroelastic control. The purpose of this paper is to present an overview of active vibration control technology and its application to aircraft. Classification of classic aircraft vibration problems and currently available solutions are used to provide a framework for the study. Current solutions are classified as being either passive or active and by the methodology (modal modification or addition) used to reduce vibration. Possible applications for this technology in aircraft vibration control are presented within this framework to demonstrate the increased versatility active materials technologies provide the designer. An in- depth study of an active pylon to reduce wing/store vibration is presented as an example. Finally, perceived gaps in the existing technology base are identified and both on-going and future research plans in these areas are discussed.

In this paper the use of Terfenol-D magnetostrictive rods for actuation of an intelligent rotor is being addressed. The challenge in the use of these actuators in intelligent rotor applications is housing them in the structure of the rotor. The packaging of the Terfenol rod actuators, considered in this work, inside the structure of the rotor blade is based on lining the actuators in the center of the spar along the axis of the blade. This is possible due to the fact that most blades have a hollow D or box spar. The two ends of each actuator is sandwiched between two plates which, for flapwise actuation of the spar, are attached to the top and bottom of the spar, alternatively. This arrangement translate the actuator force to a point moment on the blade, at the actuator location.

A new class of distributed sensors is presented which can measure both the linear and angular deflections of composite rotating beams. The sensor relies in its operation on a set of wires which are embedded off the neutral axes of the composite beams. The wires are arranged in a special manner which allows continuous monitoring of the deflection curve of the rotating beam without the need for knowing the modal characteristics of the beam or its rotating speed. The output signals of the wires are processed to determine the linear and angular displacements at critical discrete points along the beam axis. The theoretical and experimental performance of the sensor are presented in both the time and frequency domains. The performance is also monitored at different rotational speeds and tilt angles of the rotating beam.

The utility of helicopter aviation is limited by the high vibration levels caused by interaction of each rotor blade with the wake of preceding blades. Existing full blade actuation using a swashplate has various problems such as insufficient bandwidth, limitations in the number of harmonics controlled, high maintenance, and lack of spanwise lift variation. These problems are avoided by the proposed flap operated, individual blade control system, which uses magnetostrictive actuation technology. Terfenol-D actuation has many advantages over competing technologies such as hydraulic systems, electric motors, and piezoelectric elements. These benefits include all-electric operation, simplicity and reliability, low mass, low voltage, and insensitivity to centripetal acceleration.

Smart structures can be developed using a variety of sensors and actuators such as piezoelectric, fiber-optic, acoustic, and pyroelectric materials, etc. A piezoelectric material produces electric charge when it is mechanically deformed and conversely, an electric potential causes mechanical deformation of the material. The main thrust of research in the area of smart structures has been in vibration control and geometric shape manipulation. Finite element analysis techniques have been introduced recently in vibration suppression and modal control. In the present study a finite element formulation is developed to analyze laminated plates with arbitrarily placed piezoceramic patches. The technique is applied to obtain static response and stress fields due to application of electric field to the piezoceramic patches.

An elasticity solution is presented for the static equilibrium equations of an axisymmetric composite cylinder under loadings due to surface mounted or embedded piezoelectric laminae. Both uniform and non-uniform distributions of the piezoelectric effect are studied and results are verified using a finite element analysis based on axisymmetric 2-D elasticity theory equations. A cylindrical truss element actuator is developed which may be used for damping vibrations of truss type structures. Finally, the effects of a piezoelectric patch have been investigated. The axial forces generated at the fixed ends of a cylinder are found to be proportional to the length of the patch.

Active control systems that rely on piezoelectric materials are effective in controlling vibrations of structural components. In the present work, a finite element model is developed to study the dynamic behavior of laminated composite plates with integrated sensors and actuators. The model is valid for both continuous and segmented piezoelectric elements that can be either surface banded or embedded in the laminated plate. The formulation is based on the shear deformation plate theory and is applicable for both thin and moderately thick plates. The charge generated by the sensor and the response of the plate to an actuator voltage can be computed independently. These features are then coupled with a constant-gain control algorithm to actively control the transient response of the plate in a closed loop. Finite element solutions are presented to demonstrate the influence of sensor location, actuator location, stacking sequence and boundary conditions on the dynamic behavior of laminated plates.

A theory of sandwich plates with composite-material facings and piezoelectric actuators- stiffeners bonded to the surface or embedded in the facings is developed. The stiffeners bonded to the surfaces are modeled using either the plane stress assumption or a first-order shear deformable theory. The former approach is appropriate if the stiffeners represent thin strips while the latter method can be used in the case where the stiffeners are relatively deep piezoelectric ribs. The stiffeners embedded in the facings in the form of piezoelectric strips are considered using the plane stress assumption.

A three-dimensional finite element model is developed to analyze the interlaminar stress state surrounding an interlaced, active piezoceramic embedded within a unidirectional graphite/epoxy composite laminate. Interlacing increases the strength of composite structures with embedded actuators by redistributing the load around the piezoceramic and softening the material discontinuity between the piezoceramic and the composite structure. The analysis shows that interlacing results in as much as a 34.6% reduction in the magnitude of the maximum interlaminar tensile normal stress and a 25.2% reduction in the magnitude of the maximum interlaminar shear stress in the laminate. Moreover, the critical location of delamination initiation is removed from the interface between the piezoceramic and the composite material to a location away from the embedded actuator, thus maintaining the authority of the actuator after the onset of delamination.

Two global/local finite element modeling procedures are integrated to permit the analysis of localized three-dimensional affects in laminated composite plates with bonded or embedded actuators. The first technique concerns the use of variable kinematic finite elements which are elements that contain several different types of assumed displacement fields. By choosing appropriate terms from the composite displacement field, an entire array of elements with different levels of kinematic complexity can be formed. The different elements can be conveniently connected together in a single domain for global/local analysis. The second technique concerns the use of finite element mesh superposition in which an independent overlay mesh is superimposed on a global mesh to provide localized refinement for regions of interest regardless of the original global mesh topology. Integration of these two ideas yields a very robust, economical analysis tool for global/local analysis of composite laminates.

A theoretical modeling for composite laminates consisting of piezoelectric materials and behaving with flexible deformation properties is presented. The flexible deformation is described by using the geometrical nonlinear theory and the strain-displacement finite deformation relation. The electromechanically coupled laminate constitutive relation is developed based on composite lamination theory, laminar piezoelectric material properties, and laminate design and structural parameters. The developed theory is generic in nature and expected to be applicable to the general flexible deformed piezoelectric composite laminates.

The ACTEX flight experiment is scheduled for launch and to begin its on orbit operations in early 1994. The objective of the ACTEX experiment is to demonstrate active vibration control in space, using the smart structure technology. This paper discusses primarily the hardware development and program management issues associated with delivering low cost flight experiments.

The Inexpensive Structure and Materials Flight Experiment (INFLEX) validates new technologies for space that can reduce cost and improve the performance of future precision structure space programs. These technologies include advanced sensors, structural modeling, actuators, system identification, active control algorithms, health monitoring, passive damping, and advanced composites. The INFLEX payload consists of a 16-foot deployable structure, avionics, control system actuators, and structural sensors. The entire payload structure is hinged with the spacecraft bus and is controlled by an extendible strut. Sensors and proof-mass actuators are distributed on the structure to conduct dynamics and control experiments. Two video cameras (wide and narrow field of view) monitor deployment, assess structural status, and quantitatively monitor structural motion. The data acquisition Remote Units, located on each rib and the central tower, interface to the actuators and sensors mounted nearby. The payload processor is mounted on the thermally controlled bulkhead of the spacecraft bus, and communicates with the Remote Units and the spacecraft to control all experiment hardware.

The Middeck Active Control Experiment is a space shuttle flight experiment intended to demonstrate high authority active structural control in zero gravity conditions. The prediction of on-orbit closed-loop dynamics is based on analysis and ground testing. The MACE test article is representative of multiple payload platforms, and includes two 2-axis gimballing payloads connected by a flexible bus. The goal of active control is to maintain pointing accuracy of one payload, while the remaining payload is moving independently. Current control results on the ground test article are presented. Multiple input, multiple output controllers are designed based on high order measurement based models. Linear Quadratic Gaussian controllers yield reasonable performance. At high authority, however, these controllers destabilize the actual structure, due to parametric errors in the control design model. A robust control design procedure is required to yield high performance in the presence of these errors.

A flight experiment demonstrating vibration suppression using smart structure technology is being flown on a small British satellite in late 1993. Piezo actuators are used to suppress motion of the tip of a cryocooler coldfinger in three dimensions. Two actuation methods are being demonstrated: low voltage piezo translators and applique ceramics. The applique ceramics stretch the coldfinger to cancel the tip motion and are discussed in detail in a companion paper. Commercially available piezo translators displace the entire cryocooler to cancel the motion of the tip of the coldfinger as measured by three eddy current transducers. Two types of control systems are being demonstrated: a real time analog control system using position feedback, and a digital feed forward controller that updates its waveform every second or so.

This paper presents a preliminary overview of the issues relevant to the use of active structures technology for the control of thermal deformations in space structures. Space structures often have extreme dimensional stability requirements, and are exposed to a severe thermal environment in orbit. This environment causes temperature fluctuations and distributions that can lead to unacceptable structural deformations. The orbital environment, its thermal effects on structures, and the resulting deformations are reviewed with examples. It is shown that use of ultra-low coefficient of thermal expansion composites can reduce deformations to low levels, but the deformations that do occur are very difficult to predict. This motivates the addition of active structures technology to actively sense and counteract these deformations. It is found that this is practical with current technology under some circumstances. Problems that must be overcome in sensing and mechanical coupling are also reviewed, as are linkages between thermal and structural dynamic problems.

The development of simultaneous sensing and actuation for a single piezoelectric element, called a sensoriactuator in this paper, provides the opportunity for truly collocated control in adaptive structures. Issues related to collocation are discussed in terms of their effects on active structural acoustic control (ASAC). A variation on earlier feedback ASAC methods, direct radiation feedback (DRFB), is suggested for the sensoriactuator adaptive structure. The DRFB method relies on a discrete-point formulation of the associated radiated energy norm. The influence of the acoustic dynamics on proven sufficient conditions for globally stable collocated velocity feedback is discussed for the first time. Selection of an appropriate Lyapunov function for the stability analysis of collocated DRFB is discussed and compared to previous results for direct velocity feedback in active vibration control.

Active structural acoustic control using adaptive structures has been demonstrated for harmonic disturbances. This paper presents an extension of this work to the attenuation of acoustic radiation from structures subject to broadband disturbances. An adaptive, multi-input multi-output (MIMO), feedforward broadband acoustic control system has been developed based on the least mean squares (LMS) algorithm. The compensators are adaptive finite impulse response (FIR) filters. The control inputs are implemented with piezoelectric ceramic actuators. Both far-field microphones and polyvinylidene fluoride (PVDF) structural sensors designed to observe the efficient acoustic radiating modes are used as error sensors. The disturbance is band-limited zero mean white noise and is implemented with a point force shaker. In the control of harmonically excited systems, satisfactory attenuation is possible with a single-input single-output (SISO) controller. In contrast, for systems excited with broadband disturbances, a MIMO controller is necessary for significant acoustic attenuation. Experimental results for the control of a simply supported plate are presented.

A new higher harmonic control approach is proposed for structural acoustic systems characterized by linear response to a nonlinear input disturbance. The algorithm is called the HLMS algorithm since it is derived from the filtered-x version of the least mean squares (LMS) algorithm. A single frequency reference signal correlated with that of the fundamental frequency of the input disturbance is all that is required to implement the controller. Simple trigonometric relationships are used to generate the remaining harmonics to be controlled in software as opposed to hardware. The filtered-x LMS algorithm is implemented in parallel for each frequency to be controlled, increasing the stability and rate of convergence for the higher harmonic control application. Results from the simulation confirm the applicability of the control approach for active structural acoustic control.

A number of problems in active noise control involve the attenuation of noise radiated from vibrating structures. Although one could conceivably minimize the radiation by minimizing the vibration of the structure, it has been demonstrated that one can often minimize the radiation without any significant reduction in the vibration levels on the structure. Such an approach is attractive, in that it often reduces the control authority which is required for the control actuators. However, to implement such an approach, it is necessary to sense the structural vibration in a manner which allows one to determine the acoustic radiation. This paper outlines a method to sense the acoustic radiation from a structure, using distributed sensors. The approach is based on estimating the wavenumber spectrum associated with the vibrating structure, from which the acoustic radiation can be determined. Some of the difficulties in implementing the method for typical acoustic problems are addressed, along with possible approaches for overcoming some of these difficulties.

This paper presents the details of the transmission loss test results for composite isogrid panels with different acoustic control systems (ACS) evaluated and the potential of these ACSs to function as mold materials. Two isogrid composite panels approximately two feet square with a skin thickness of 0.06 inch and a rib height of 0.6 inch were wound onto a silicon mold and cured in an autoclave at 350 degree(s)F. One panel had a center skin damping layer to enhance the basic damping of the panel. Comparative transmission loss measurements were made on aluminum flat panels (both solid and laminated with a damping material) and on an aluminum isogrid panel. Parallel to the transmission loss evaluations, the acoustic control materials were evaluated as a possible replacement of the silicon rubber as a mold material. These evaluations consisted of using the acoustic mold system during the fabrication and curing of the isogrid structure.

While many new smart materials have been developed over the last few years, the integration of these materials into useful systems must be exploited if this emerging technology is to become viable. Two applications of piezoelectric and electrostrictive materials are proposed that generate both micro and macro positioning control. Additionally, an overview of these smart materials and their properties is given. Precise micro positioning devices have applications to adaptive optical systems. A NASA project called Space Laser Energy (SELENE) involves such a system. This project proposes to control the surface of a power transmission dish comprised of several thousands of small lenses or lenslets. A major objective of the dish is to increase efficiency by compensating for atmospheric disturbances. A second application of piezoelectric materials is presented for micro and macroscopic one dimensional motion. Specifically, a novel design of an inchworm device is presented. This device has the ability to generate macroscopic motion from microscopic piezoelectric expansions and contractions, the frequency of these expansion/contraction cycles should allow the motor to move for significant distances as well as provide incremental micro positioning.

A general mathematical model of distributed transducer shading, or spatial gain-weighting, is developed for 1-D distributed actuators and sensors, valid for both strain and hydrostatic mode devices. Means for constructing shaded distributed actuators and sensors using shaped electrodes are reviewed. Singularity functions and other complete sets of functions are used to parameterize shadings. Modal and wavenumber interpretations of shading kernels are deduced. Hardware applications are presented from structural control, hydrodynamics, and high- resolution sonar sensing. A conformal, real-time hydrodynamic center of pressure sensor is described. A novel sonar sensor is presented that utilizes two coincident, distributed, shaded piezopolymer sensors to provide high-resolution target bearing estimates. In each of these applications, sensor shading is accomplished by suitably shaping the charge collection electrodes deposited on the sensing layer to provide two coincident distributed devices having a prescribed mathematical relationship spatially, yielding devices that cannot be realized using either discrete or unshaded distributed transducers.

The paper describes experimental research in the area of active vibration isolation. The objective of the research is to quantitatively assess the performance gained by augmenting a passive isolator with an active stage. Vibration isolation experiments were carried out on a flexible structure utilizing a proof-mass shaker as the disturbance source and a newly developed active member as the isolator. Broad band force feedback control demonstrated more than 20 dB reduction in force transmissibility over passive isolation alone. The broad band controller was augmented with notch filters which resulted in reducing force transmissibility by 40 dB over the passive stage in select narrow bands.

The development of a ceramic applique for the vibration suppression of a cryocooler coldfinger is a part of technology demonstration flight experiment. Three sectors of a piezoelectric PZT bonded to the coldfinger are used as actuators to control the motion of the tip. A technique for bonding the PZT material to the coldfinger was developed that minimizes tensile stresses in the ceramic during operation. Extensive development testing has been performed to verify the efficiency of the adhesive and the coldfinger performance at cryogenic temperatures. The problems encountered during the fabrication and assembly of the coldfinger are described. Results of tests are summarized.

TRW has been implementing active damping compensators on smart structures for the past five years. Since that time there have been numerous publications on the use of impedance matching techniques for structural damping augmentation. The idea of impedance matching compensators came about by considering the flow of power in a structure undergoing vibration. The goal of these compensators is to electronically dissipate as much of this flowing power as possible. This paper shows the performance of impedance matching compensators used in smart structures to be comparable to that of active damping compensators. Theoretical comparisons between active damping and impedance matching methods are made using PZT actuators and sensors. The effects of these collocated and non-collocated PZT sensors and actuators on the types of signals they sense and actuate are investigated. A method for automatically synthesizing impedance matching compensators is presented. Problems with implementing broad band active damping and impedance matching compensators on standard Digital Signal Processing (DSP) chips are discussed. Simulations and measurements that compare the performance of active damping and impedance matching techniques for a lightly damped cantilevered beam are shown.

Local controllers are good candidates for active control of flexible structures. Local control generally consists of low order, frequency benign compensators using collocated hardware. Positive real compensators and plant transfer functions ensure that stability margins and performance robustness are high. The typical design consists of an experimentally chosen gain on a fixed form controller such as rate feedback. The resulting compensator performs some combination of damping (dissipating energy) and structural modification (changing the energy flow paths). Recent research into structural impedance matching has shown how to optimize dissipation based on the local behavior of the structure. This paper investigates the possibility of improving performance by influencing global energy flow, using local controllers designed using a global performance metric.

Significant damping may be added to truss structures by replacing a small fraction of the members with passive or active damper struts. Although numerous methods for selecting the locations of dampers or actuators have been proposed, all rely on an accurate structural dynamic model. This paper demonstrates the effect of model error on placement when the objective is minimization of rms displacements. A group of placement algorithms is applied to a finite element model of a truss structure. The effect of adding between one and five viscous passive dampers or piezoelectric active struts in the member locations chosen is then demonstrated on the corresponding laboratory test article. Error in the analytical model leads to discrepancies between prediction and test. Predicted differences in achieved performance between simple and more complex placement algorithms are diminished. A specific pair of uncertain physical model parameters is proposed, and the resulting model error is explicitly included in a modified placement algorithm.

An optomechanical model of a segmented reflector telescope is developed. An image quality metric is used to optimize the placement of damping elements in the telescope backup truss. Comparisons with an alternative placement criteria are made.

Active vibration control of structures using piezoelectric materials is a new approach for damping unwanted vibrations in structures lacking sufficient stiffness or passive damping. The finite elements method is used to model active damping elements which are piezoelectric actuators bonded to a box beam. Efficient implementation of these actuators requires that their optimal locations on the structure be determined and that the structure be designed to best utilize the properties of the piezoelectrics. A formal optimization procedure has been developed to address both of these issues. Multiobjective optimization techniques are used to minimize multiple and conflicting design objectives such as mass and energy dissipated by the piezoelectric actuators.

The shape change of a stress free cylindrical flexible rod with embedded SMA actuators is modeled in this paper for rods with single or multiple SMA actuators placed parallel to the long axis of the rod for the first heating of the SMA actuators. The deformed shape of the rod is found by solving the nonlinear equations of equilibrium of the rod. For multiple actuators the resultant forces and moments are considered. A design analysis is performed for the active rod with a single SMA actuator. Key design variables are: E_ROD, the thermal bond strength of the rod to the SMA actuator, and the rod dimensions. Key design parameters are: maximum rod/actuator interfacial shear stress, rod strain, rod slenderness ratio, and rod deflection. An analysis is then performed which investigates the influence of the design variables on each of the design parameters. Optimal values are obtained which yield desired design parameter values. Experimental prototypes are fabricated, tested, and the deformed shape is compared to the theoretical prediction.

The Mori-Tanaka micromechanics method is used to predict the effective properties of composite materials consisting of a polymer matrix reinforced by a fiber made of a transformation shape memory effect (SME) material. The composite response is plotted for combinations of the following scenarios: (1) isothermal longitudinal and transverse stress input, (2) stress-free thermal loading, (3) constant fiber thermoelastic properties, and (4) thermoelastic fiber properties that vary with the martensite volume fraction. For the case of an isothermal stress input, the composite transformation stress, the maximum transformation strain, and the hysteresis are all reduced vis-a-vis the monolithic SME material. In contrast to a monolithic SME material, stress-free thermal loading of a SME composite can produce a transformation strain. It is shown that closed form solutions for the effective martensite and austenite start temperatures can be derived, that they are sensitive to the stress-free reference temperature of the fiber, and that the stress-free austenite and martensite start temperatures are higher than those of the monolithic SME material.

The bending stiffness of composite drive shafts is actively controlled by activating optimal sets of shape memory alloy (NITINOL) wires which are embedded near the outer surfaces of these shafts and parallel to their longitudinal axes. With such active control capabilities, the drive shafts can be manufactured from light weight sections without compromising their transverse load carrying capabilities. These features will be invaluable in producing drive shafts for critical applications, such as in helicopters, where high resistance to whirling is of utmost importance. Finite element models are developed to describe the individual contributions of the composite matrix, the NITINOL wires and the shape memory effect to the overall performance of the drive shafts. The theoretical predictions of the models are validated experimentally on a prototype of the NITINOL-reinforced drive shaft.

Advanced submarine stern configurations require a variety of control surfaces to actively manage aftbody boundary layer flow, vorticity, propulsor inflow and intrapropulsor flow, as well as vehicle attitude. Two necessary attributes of advanced control surface designs include (1) integrated actuation to provide placement flexibility at remote locations with minimal structural interfacing and control interconnects, and (2) improved lift efficiency and flow using variable or adaptive camber control. To demonstrate these attributes, a shape memory alloy (SMA) actuated compliant control fin (CCF) with a planform area of 620 sq. cm was developed for evaluation as rudder and sternplane appendages on a radio control submarine model at velocities up to 5.1 m/s (Reynolds No. approximately equals 1,000,000) and up to 0.2 Hz full cycle actuation. A completely fixed root design was developed to reduce turbulence at the hull/fine interface, with compliant deformation of the foil to improve flow characteristics over the baseline full-flying and trailing-edge-flap designs.

Smart structures is now a vigorous area of study. One type of smart structure employs shape memory alloys, such as Nitinol, in the form of thin wire actuators. To the authors' knowledge, the only control algorithm that has been implemented heretofore has been a simple on-off control law. In this study, other control algorithms are employed, ranging from classical to a robust controller based on the LQG/LTR algorithm. Two mechanical systems are studied. The first is an aluminum cantilever beam for which the free-vibration damping of the fundamental mode was enhanced. The second system studied is a three-mass system with its three natural frequencies below 3 Hz.

The performance of the Advanced Composites with Embedded Sensors and Actuators (ACESA) vibration control system is described. The system consists of: three tubular active members sixteen feet long and five inches in diameter, with embedded piezoceramics (PZTs) allowing control of deformation axially and in two bending planes; a 9-channel digitally programmable analog local vibration control electronics unit; and 400 Volt drive electronics for each strut. The system is installed on a space based laser structural simulator at the AF Phillips Lab's Advanced Space Structure Research Experiments (ASTREX) facility at Edwards Air Force Base. The system has demonstrated ability to settle vibrations after a thruster induced slew in 0.2 seconds.

The experimental research at the Jet Propulsion Laboratory (JPL) aimed at developing and validating new design methodology arising out of NASA's Control Structure Interaction and Orbital Stellar Interferometer programs is presented. Structural and direct optical pathlength controls are combined to maintain the pathlength variation below 10 nanometer rms. The bandwidth of the controller is 500 Hz with the disturbance rejection of over 70 dB at frequencies below 10 Hz and over 20 dB at frequencies near 100 Hz.

Passive damping of structural dynamics using piezoceramic electromechanical energy conversion and passive electrical networks is a relatively recent concept with little implementation experience base. This paper describes an implementation case study, starting from conceptual design and technique selection, through detailed component design and testing to simulation on the structure to be damped. About 0.5 kg. of piezoelectric material was employed to damp the ASTREX testbed, a 5000 kg structure. Emphasis was placed upon designing the damping to enable high bandwidth robust feedback control. Resistive piezoelectric shunting provided the necessary broadband damping. The piezoelectric element was incorporated into a steel flextensional device in order to concentrate damping into the 30 to 40 Hz frequency modes at the rolloff region of the proposed compensator. The effective stiffness and damping of the flextensional device was experimentally verified.

Active disturbance rejection to minimize optical path length error is illustrated by experimental results from the JPL Phase B Test Bed, which incorporates an interferometric sensor and a controllable trolley mounted on a flexible truss structure. The controller actively isolates the optical instruments from structural vibrations induced by external disturbances consisting of linear combinations of sinusoidal signals.

This paper presents preliminary modelling and vibration suppression experiment results for the USAF Phillips Laboratory's Planar Articulating Controls Experiment (PACE) test bed. PACE is a two link flexible multibody experiment constrained to move over the surface of a large granite table. In this paper, an approximate analytical dynamic model of a single slewing flexible body with surface bonded piezoelectric sensors and actuators is developed using Hamilton's Principle with discretization by the assumed modes method. After conversion to modal coordinates, damping is added to the model by including experimental damping measurements. The model is then converted to state-space form for the purpose of control design. The model is verified by comparison of simulated and experimental open loop frequency response data. Both decentralized and centralized controllers are designed for vibration suppression of a single arm of the PACE test bed. The controllers presented in this paper include: a positive position feedback (PPF) controller for controlling the first mode of vibration, a decentralized controller which uses three independent PPF filters for suppressing the first three modes of vibration, and a multiple-input, multiple-output linear quadratic gaussian design. The experiments include both analog and digital implementations of these controllers.

Adaptive control of spacecraft and large structures using Radial Basis Function (RBF) based Artificial Neural Networks (ANN) is investigated. The centers of approximation of the RBFs are allowed to be dynamic in order to provide persistent excitation and a small window of approximation. Both state and time based RBFs are investigated for their ability to identify unmodeled and persistent effects. Integral feedback of the attitude seems necessary, especially when the initial estimates are poor, to eliminate steady state errors for pointing applications. Examples of motion-to-rest as well as tracking maneuvers and vibration suppression are presented.

Terfenol-D is a magnetostrictive material with properties such as large strain, high force, wide bandwidth, and high energy density. Terfenol-D actuator, with its unique characteristics, is therefore potentially a new class of actuator most suitable for embedded actuator applications. This paper deals with the modelling, design, and control of an embedded compact Terfenol-D actuator to suppress unwanted vibration in flexible structures, especially rotorcraft blades. Based on the two dimensional thermal analysis, the general form of the constitutive equation of magnetostrictive material is modified to include the effects of heating due to the current passing through the coil surrounding the Terfenol-D material. A new Terfenol-D actuator is designed with emphasis on compactness and embeddable properties. An accurate mathematical model based on the modified magnetostrictive constitutive equation is developed. Simulation results indicate good dynamic performance. Methodology of designing is also presented in this paper.

A linear actuator system for multi-dimensional structure control using the magnetostrictive material Terfenol-D has been designed, built, and tested by the Intelligent Automation, Inc. The actuator assembly incorporates an instrumented Terfenol-D rod, an excitation coil to provide the magnetic field, a permanent magnet assembly to provide a magnetic bias field, and a mechanical preload mechanism. The prototype of the actuator is 2.0 inches in diameter and 8 inches long, and provides a peak-to-peak stroke of 0.01 inches. A linear model was also established to characterize the behavior of the actuator for small motion. Based on the prototype of the actuator, we have performed a study of a six degree-of-freedom active vibration isolation system using a Stewart Platform in a new configuration. IAI's final system is intended for precision control of a wide range of space-based structures as well as earth- base systems.

We are working to understand Terfenol-D's behavior in dynamic applications. Experimental results demonstrate that the amplitudes of harmonics present in Terfenol-D transducer output displacement spectra increase with increasing drive-current amplitude. An empirical mathematical model which predicts harmonics is presented. The model approximates both the nonlinear strain versus magnetic flux density behavior (c vs B), and the magnetic hysteresis (B vs H) of the material. Trends are shown for displacement versus drive-current amplitude as a function of frequency. Generally, displacements increase with increasing drive-current amplitude, and decrease with increasing drive frequency (when operated away from mechanical resonance). Single frequency drive-current results are compared with the trends predicted by the mathematical model.

Magnetostrictive adaptive materials have many benefits over competing materials such as piezoelectric types. These advantages include very low power and voltage, high toughness and strength, and the ease of composite manufacture. The amorphous metal Metglas was chosen for fabrication of tubular and bimorph samples of magnetostrictive composites. Metglas composites produced magnetostrictive strains greater than 50 ppm in tests. The composite Young's modulus was 7.8 X 10⁶ psi, which is 78 percent of aluminum. Specific stiffness is 48 X 10⁶ in, which is 47 percent of aluminum. This specific stiffness is favorable because the composite replaces heavy actuator components, such as high density piezoelectric materials, as well as supplying primary load carrying capability. Very low power requirements are anticipated because of the 90 percent conversion efficiency from magnetic to mechanical energy.

A new modeling method for two-dimensional distributed transducers with arbitrary spatial distribution is presented. The spatial weighting of a distributed transducer is defined using multi-dimensional distributions with composite functions as arguments. A differentiation theorem is derived for one-dimensional distributions of composite functions and is extended to multi-dimensions through the use of partial distributional derivatives and the product rule. The resulting theory is used to determine the differential operator describing the distributed transducer's spatial dynamics. The methodology, which is valid for both uniaxial and biaxial transducers, is applied to several two-dimensional problems.

The semi-active control approach has been recognized to be effective for vibration suppression of flexible structures. The electrorheological (ER) fluid-based device is a good candidate for such applications. In this research, a new control law is developed to maximize the damping effect of ER dampers for structural vibration suppression under actuator constraints and viscous-frictional-combined damping. Both numerical simulations and experimental work have been carried out to evaluate and validate the theoretical predictions.

A self-sensing magnetostrictive actuator design based on a linear systems model of magnetostrictive transduction for Terfenol-D is developed and analyzed. Self-sensing, or the ability of a transducer to sense its own motion as it is being driven, has been demonstrated for electromechanical transducers such as moving voice coil loudspeakers and, most recently, piezoelectric distributed moment actuators. In these devices, self-sensing was achieved by constructing a bridge circuit to extract a signal proportional to transducer motion even as the transducer was being driven. This approach is analyzed for a magnetostrictive device. Working from coupled electromechanical magnetostrictive transduction equations found in the literature, the concept of the transducer's `blocked' electrical impedance and motional impedance are developed, and a bridge design suggested. However, results presented in this paper show that magnetostrictive transduction is inherently non-linear, and does not, therefore, lend itself well to the traditional bridge circuit approach to self-sensing.

Automated smart systems to monitor the structural health of military aircraft have the potential to reduce life cycle costs, improve turnaround times, and enhance survivability. However, before on-board structural health monitoring systems (SHMS) can be a reality, further advances in several technology areas, as well as careful integration of the component technologies, are required. This paper presents an assessment of the component technologies and discusses the requirements, architecture, technology integration, feasibility, and payoffs of SHMS for military aircraft.

An automated on-board structural health monitoring system (SHMS) for military aircraft promises significant benefits including reduced life cycle costs, improved survivability, and reduced turnaround times. However, current sensors and sensing technologies are not fully matured to develop such a system. This paper presents an assessment of the most promising current sensor and sensing technologies applicable to an aircraft SHMS. The assessment is based upon the work performed under an on-going joint Air Force/Navy contract awarded to Northrop and entitled `Smart Structures Concept Requirements Definition' (SSCORE). The overall objective of the SSCORE program is to develop an ideal SHMS architecture applicable to an aerospace vehicle, establish its feasibility and benefits, assess current technologies and identify technology gaps, develop a first generation system based on current technologies, and demonstrate SHMS concepts. The assessment was made by evaluating the capabilities of the sensors and sensing technologies against the required capabilities of a smart SHMS. The SHMS requirements are driven by the integrity requirements and the geometric complexity of the structure being monitored, the availability of space in the structure for sensor attachment, and the expected type of damage. An overall assessment of SHMS concepts are presented in a companion paper.

This paper reports the results of an ongoing experimental program for damage assessment and mitigation of composite structures using smart materials. One consequence of all damage to a composite structure is a change in its stiffness. This change in stiffness is measured by modal analysis which can be carried out using piezoelectric films as both sensor and actuator on the structure member. It is shown that the presence of damage can be detected by the change in natural frequencies and the location of the damage can be detected from the ratio of changes in frequencies between two successive modes.

One objective of smart structures development is to demonstrate the ability of a mechanical component to monitor its own structural integrity and health. Achievement of this objective requires the integration of different technologies, i.e.: (1) structures, (2) sensors, and (3) artificial intelligence. We coordinated a team of experts from these three fields. These experts used reliable knowledge towards the forefront of their technologies and combined the appropriate features into an integrated hardware/software smart structures wingbox (SSW) test article. A 1/4 in. hole was drilled into the SSW test article. Although the smart structure had never seen damage of this type, it correctly recognized and located the damage. Based on a knowledge-based simulation, quantification and assessment were also carried out. We have demonstrated that the SSW integrated hardware & software test article can perform six related functions: (1) identification of a defect; (2) location of the defect; (3) quantification of the amount of damage; (4) assessment of performance degradation; (5) continued monitoring in spite of damage; and (6) continuous recording of integrity data. We present the successful results of the integrated test article in this paper, along with plans for future development and deployment of the technology.

This paper develops an approach for the health monitoring of a smart structures with multiple embedded sensing and actuation capability. For such a structure the consideration of failure consequences is an important component of any real application. The approach developed here is an integrated control and monitoring procedure whereby the sensors, which are assumed to be distributed spatially across the structure, are processed by a set of spatial modal filters which automatically track the modal coordinates of desired, specified modes, and similarly track changes in modal characteristics such as modal frequency, damping, and mode shape. The adaptive modal filter is formulated and applied to tr:a.ck the time varying behavior of specified modes, thereby indicating in some general sense, the health of the structural system. The adaptive modal filter is insensitive to failures or calibration shifts in individual sensors and will automatically ignore failed sensors. It can also be used to detect disturbances entering the system as well as to identify failed actuator locations. A modal controller based on these estimates is then able to adapt to a changing structure and in addition is insensitive to failures in the sensors and actuators. Both the tl1eory and experimental results from a test structure is discussed.

An investigation was performed to develop a technique for using built-in piezoelectrics to detect delaminations and to estimate their size and location in laminated composite structures. Both experimental and analytical work were conducted in the study. Piezoceramics were utilized as sensors for receiving signals and as actuators for dispatching diagnostic waves. A diagnostic technique was developed which combines an electromechanical structural model with an iterative damage identification algorithm to form a closed loop. The structural model was used to predict the frequency response of normal and delaminated structures excited by actuators. The identification algorithm compares the calculation with the data to find a best estimate of delamination size and location. The technique first compares the measured dynamic response to a baseline. If they disagree, the technique searches through the possible locations and sizes of delaminations using the structural model and compares the results with the measurements. The loop terminates when the calculated and the measured frequency responses agree. Tests on composite beams with implanted delaminations were conducted to verify the model and predictions. Overall, the predictions agreed with the data.

A computer program entitled SHARP (structural health assessment and review program) has been developed to demonstrate smart structures concepts for automated aircraft health monitoring. SHARP is an object-oriented design, combining C++ and FORTRAN routines d veloped and implemented on a Motorola 68030 platform. SHARP combines unique analytic flaw generation algorithms with more traditional durability and damage tolerance routines to process data from a variety of sensors (acoustic emission, strain, acceleration etc.) and assess the aircraft's structural integrity. This paper outlines the basic system architecture, program assumptions and constraints. The analytic methods used to monitor aircraft structural integrity are then discussed in the context of current military aircraft structural integrity programs (ASIP). Finally, the potential payoffs of an on-board structural health monitoring system (SHMS) are investigated.

In this paper we describe a new algorithm for the detection, location, and estimation of damage on a space truss using parameter identification techniques. The concept is to start with a model of the truss which is known to accurately model the healthy truss. This healthy model of the truss is periodically updated using dynamic response data. When damage has occurred, a damage model of the truss is constructed from dynamic response data using an identification algorithm. By comparing the parameters in the truth model with the parameters of the damage model, the damage is detected, located, and the extent of the damage accessed. The algorithm proposed here uses an explicit model of the damage to the truss. This description of the damage allows the algorithm search over a smaller set of possible damage models with attendant increase in performance. In addition to locating the damage, an estimate of the damage is also given. The performance of this algorithm for locating and estimating damage is presented in the form of three example problems including a 44 dof planar truss.

By using a modular maltibody dynamics code, dynamical behavior of space structure elements is investigated to get an effective construction scenario for large space systems in orbit. Deployment construction of a triangular key structure for a solar power satellite in low earth orbit is treated, and the dynamical responses through the deployment in orbit plane, orbit normal or parallel plane are simulated. Illustrative examples clearly show the large variety of resulting dynamical behavior during and after deployment construction.

A new feature in the design of smart structures is the capability of the structure to respond autonomously to undesirable phenomena and environment. This capability is often synonymous to the requirement that the structure should assume a set of different geometric shapes or adapt to a set of kinematic constraints to accomplish a maneuver. Systems with these characteristics have been referred to as `shape adaptive' or `variable geometry' structures. The present paper introduces a basis for the kinematics and work space studies of statically deterministic truss structures which are shape adaptive. The difference between these structures and the traditional truss structures, which are merely built to support the weight and may be modelled by finite element methods, is the fact that these variable geometry structures allow for large (and nonlinear) deformations. On the other hand, these structures unlike structures composed of well investigated `four bar mechanisms,' are statically deterministic.

This paper describes design, analysis, and test experience on a large amplitude, tridirectional, distributed suspension system for testing large flexible space structures in a simulated weightless environment. Three cables are attached to the test article at each attachment point, as opposed to one vertical cable in a unidirectional system. Horizontal as well as vertical isolation are thereby achieved without requiring a large overhead clearance. Rotary actuators are employed to utilize overhead space efficiently. Position and torque-controlled motors are used in combination with torsional springs to achieve a large amplitude, high frequency isolation capability.

This experimental investigation is focused on characterizing a class of electro-rheological (ER) fluids and also characterizing a class of smart structures featuring this class of suspensions. These studies involved the imposition of different applied voltages on the ER fluid domain and various concentrations of the suspension particles. Electro-rheological fluids belong to a class of colloidal suspensions whose global characteristics can be controlled by the imposition of an appropriate external electric field upon the fluid domain. Therefore, when these fluids are embedded within a smart beam-like structure, the global properties of the beam, and hence its vibrational response, can also be controlled. In this work, the energy dissipation characteristics of smart cantilever beam specimens were measured. Different ER fluid smart beam specimens with various concentrations were employed in these investigations to provide insight on the relationship between the particle concentration of the suspension with damping ratio of the smart beam and the electric field intensity imposed on the beam. The coupled electrical and mechanical dynamic properties of smart materials featuring hydrous ER fluids were experimentally studied using a Rheometrics RMS 800 mechanical spectrometer to gain insight into their effectiveness in vibration control applications. THe experimental results demonstrate the non-Newtonian rheological behavior of ER fluids, and the ability of this class of smart beams to dissipate energy increases with the increase of particulate concentration and also the applied electric field.

Smart structures are being increasingly applied in active vibration control. Typically, motion ranges produced in such systems are in the micro-range. This work proposes a mechanism to accomplish macro-motion ranges using smart materials. A new concept for automotive active suspension has been developed using smart structures featuring piezoceramics. A candidate design is analytically investigated for performance. The work presented in this paper includes the concept, its illustration, development of a design geometry based on this concept, and its finite element analysis and results. It is shown that by a proper synthesis of smart structure, macro-motion outputs needed for this application are possible.

Maintaining an optimum-wing cross section during transonic cruise can dramatically reduce the shock-induced drag and can result in significant fuel savings and increased range. Our adaptive-wing concept employs actuators as truss elements of active ribs to reshape the wing cross section by deforming the structure. In our previous work, to derive the shape control- system gain matrix, we developed a procedure that requires the inverse of the stiffness matrix of the structure without the actuators. However, this method cannot be applied to designs where the actuators are required structural elements since the stiffness matrices are singular when the actuator are removed. Consequently, a new method was developed, where the order of the problem is reduced and only the inverse of a small nonsingular partition of the stiffness matrix is required to obtain the desired gain matrix. The procedure was experimentally validated by achieving desired shapes of a physical model of an aircraft-wing rib. The theory and test results are presented.

This paper presents work completed in the area of integrally damped composites. The objective of the project was to evaluate the effectiveness of integrated damping layers in composites, to identify the effect of the damping layer on the strength of the composite, and to define fabrication problems associated with integrating the damping layer into the composite. The study addressed two types of composite panels which included flat panels and filament wound rectangular isogrid panels fabricated from IM-7 graphite fiber and ICI Fiberite 977-2 epoxy resin. With each of the selected fiber orientations, one panel was fabricated with a 0.002 inch layer of damping material as the center layer of the composite; while a second panel, a control panel, did not have the damping layer installed. The test specimens were cured in an autoclave at 350 degree(s)F. All of the composite panels were tested dynamically to determine the dynamic stiffness and modal damping as a function of temperature. Tensile, flexure, and compression tests also were run on the panels at room temperature. The paper presents the details of the fabrication and curing procedures used in the construction of the composite panels and the detailed results of the static and dynamic tests on the damped and undamped panels.

The object of this study is to investigate the feasibility of construction of a mechanical actuator to be used for vibrational control of finite-dimensional dynamical systems. The actuator will produce the forces generated by two oppositely rotating eccentric masses of variable eccentricity and equal but opposite angular velocities (omega) .

This study develops a method for the simultaneous structural and control optimization of a strut used in a laser communication cross link structure. This strut combined with others form an isolation system which isolates the pointing element of the laser cross link from the spacecraft disturbances. Included in the optimization of the strut are the weight, thickness, and orientation of the fibers in the composite plies. This study includes the effects of using composite tailoring to promote the coupling of the axial-twisting-bending modes to enhance the damping system. The strut is modelled by a two dimensional composite cylindrical shell theory with higher order approximations for transverse shear. The results of the study are compared to a baseline strut which was designed solely for thermal stability and axial stiffness.