Active vibration control is an important problem in structures. One of the ways to tackle this problem is to make the structure smart, adaptive and self-controlling. The main objective of active vibration control is to reduce the vibration of a system by automatic modification of the system's structural response. This work features the modeling and design of a robust decentralized controller for a smart flexible system using periodic output feedback control technique when there is a failure of a system component to function (say, a sensor or a actuator). The entire structure is modeled in state space form using FEM by dividing the master structure into 8 finite elements and placing the sensor / actuator pair at finite element positions 2, 4, 6 and 8. Robust decentralized POF controllers are designed for the various models of the plant by retaining the first two vibratory modes. The effect of the failure of one of the piezo patch is observed. In this proposed control law, the control inputs to each actuator of the multimodel flexible system is a function of the output of that corresponding sensor only and the gain matrix has got all off-diagonal terms zero. The designed robust controller provides satisfactory stabilization of the multimodel smart structure system.

In this paper we propose the use of the genetic algorithm (GA) as a tool to solve multi-objective optimization problems in flexure jointed hexapods. Using the concept of heuristic mutation, a modified GA-based multi-objective optimization technique is proposed and the passive parameters' optimization problems in a flexure jointed hexapod system are solved. The passive parameters found include the spring and the damping parameters in each strut of the hexapod. The results produced by this new approach are compared to those produced by other practical selection techniques, proving that this technique is more flexible. Thus, the genetic algorithm can be used as a reliable numerical optimization tool in such problems.

The effect of a hybrid semiactive control technique on steady state and transient dynamics that occur in a vehicle suspension is studied analytically, using a single suspension model. Hybrid semiactive control refers to a technique that mathematically combines the commonly-used skyhook and groundhook control methods for vehicle suspensions. The steady state analysis evaluates the response of hybrid control with different weighting factors to periodic inputs at the sprung body (vehicle body) and unsprung body (axle) natural frequencies. The transient response analysis includes an evaluation of different aspects of the damper, vehicle body, and axle dynamics to a step input. The damper force-velocity trajectories, vehicle body and axle response, and vehicle body jerk are among the metrics that are examined as part of this analysis. The results of the study show that several tradeoffs exist between improving the response of the suspension at its natural frequencies, and between the response of the vehicle body and axle to transient inputs. The trade offs can be adjusted through changing the hybrid control gain, which adjusts between the emphasis on skyhook and groundhook control. The results further indicate that, in balance, hybrid control can be more effective in controlling suspension dynamics than either skyhook or groundhook control.

This paper presents control strategies and simulations of a two-DOF precision platform as an adaptive thruster mount structure with precision positioning and active vibration suppression capabilities for thrust vector control of space satellites. First, the configuration of the two-DOF precision platform is introduced, which is an intelligent tripod with two in-plane rotational degrees of freedom for the top device-plate. Precision positioning of this platform is achieved using active members that extend or contract to tilt the top device-plate where the thruster is mounted. Kinematic analysis of the platform is then presented and followed by two control strategies; namely local control strategy and global control strategy. In the local control strategy, the motion of each active member is controlled locally according to the kinematical feature of the platform and the local sensor information to achieve the desired tilt of the top device-plate. In the global control strategy, the motion of each active member is adjusted according to the system level information from a tilt sensor. Fuzzy logic control is employed and the two control strategies are simulated and compared.

In this paper, the electromechanical displacements of curved piezoelectric actuators with laminated composite material are calculated using high performance computing technology, and the optimal configuration of composite curved actuator is proposed. To predict the pre-stress in the device due to the mismatch in coefficients of thermal expansion, carbon-epoxy and glass-epoxy as well as PZT ceramic are numerically modeled by using hexahedral solid elements. Because the modeling of these thin layers causes the numbers of degree of freedom to increase, large-scale structural analyses are performed through the PEGASUS supercomputer which is composed of 400 Intel Xeon CPUs. In the first stage, the curved shape of the actuator and the internal stress in each layer are obtained by the cured curvature analysis. Subsequently, the displacement due to the piezoelectric force by an applied voltage is also calculated and the performance of composite curved actuator is investigated by comparing the displacements according to the configuration of the actuator. To consider the finite deformation in the first stage and include the pre-stress in each layer in the second analysis stage, nonlinear finite element analyses will be carried out. The thickness and the elastic constants of laminated composite are chosen as design factors.

This work utilizes a Direct Constant Voltage (CV) Scheme as a versatile tool to study piezoelectric actuator performance effectiveness either surface-mounted or embedded. To achieve this goal calculated control Constant Voltage is employed for active vibration suppression. This paper introduces a closed form formula that replaces Design Charts for a faster and easier way to calculate actuator voltage required for active vibration suppression. To perform a comprehensive study, three finite element models (FEMs) are developed. The first FEM considers non-collocated surface-mounted piezoelectric patches as sensor and actuator for beams with varying thicknesses. The second FEM deals with embedded non-collocated piezoelectric patches as sensor and actuator for beams with varying thicknesses and constant number of constraint layers. For this embedded case one extra layer is needed to cover the piezoelectric patch and one extra cut-out layer to fill the area around the piezoelectric patch compared to the surface-mounted. The third FEM focuses on beams having constant thicknesses with variable number of constraint layers and non-collocated embedded piezoelectric patches. A surface-mounted ACX piezoelectric patch acts as a shaker is all three FEMs. Numerical and experimental results were compared and excellent comparisons were obtained. The Direct Constant Voltage Scheme can offer useful information into the actuator performance effectiveness in terms of: 1) the piezoelectric actuator performance effectiveness with respect to the laminate thicknesses and actuator distance from the beam neutral axis either surface-mounted or embedded, and 2) the influence of the constant and variable number of constraint layers on the embedded piezoelectric effectiveness for beams with variable and constant thicknesses.

The scientific community has put significant efforts in the manufacturing of sensors and actuators made of piezoceramic fibers with interdigitated electrodes. These allow for increased conformability and actuation capability at high field regime. The prediction of their coupled field behavior, however, is so far limited to low field applications, where the piezoelectric coupling coefficient is assumed to be constant. An approach, which takes into account the strain driven nonlinearity of a representative work cycle at high field regime is still lacking. This study presents a nonlinear Finite Element Model to simulate the free strain properties of Active Fiber Composites (AFCs) under high electric field conditions. Input data for the fully parametric model are the Representative Volume Element (RVE) geometry and the material properties of its piezoceramic and epoxy resin components. The high field properties of single PZT fibers under free strain conditions were determined using a novel characterization procedure. Free strain properties of the actuators were measured experimentally, and important geometrical parameters (contact angle between the fiber and the electrode, average spacing between the fibers) were measured using micrographical imaging. The results of the simulation show good agreement with the free strain measurements, allowing for prediction of a representative work cycle hysteresis. The influence of important geometrical parameters on the actuator properties such as electrode spacing and electrode-fiber contact angle was investigated both numerically and experimentally.

Smart structures incorporating active materials have been designed and analyzed to improve aerospace vehicle performance and its vibration/noise characteristics. Helicopter integral blade actuation is one example of those efforts using embedded anisotropic piezoelectric actuators. To design and analyze such integrally-actuated blades, beam approach based on homogenization methodology has been traditionally used. Using this approach, the global behavior of the structures is predicted in an averaged sense. However, this approach has intrinsic limitations in describing the local behaviors in the level of the constituents. For example, the failure analysis of the individual active fibers requires the knowledge of the local behaviors. Microscopic approach for the analysis of integrally-actuated structures is established in this paper. Piezoelectric fibers and matrices are modeled individually and finite element method using three-dimensional solid elements is adopted. Due to huge size of the resulting finite element meshes, high performance computing technology is required in its solution process. The present methodology is quoted as Direct Numerical Simulation (DNS) of the smart structure. As an initial validation effort, present analytical results are correlated with the experiments from a small-scaled integrally-actuated blade, Active Twist Rotor (ATR). Through DNS, local stress distribution around the interface of fiber and matrix can be analyzed.

We produced a new piezoelectric fiber with metal core by the hydrothermal method. The insertion of metal core was significant in view that the fragility of ceramics can be overcome and electrodes are not required in the use as sensors and actuators. And new smart board was designed by mounting these piezoelectric fibers on the surface of the CFRP composite. It was shown that these complex fibers function as sensor and actuator in the CFRP board. In order to evaluate the sensor and actuator. However, by the hydrothermal method, piezoelectric layer can be uniformly formed with about 20μm thickness on the titanium wire surface, the PZT clad is insufficient in thickness for the actuator use. In this paper, we solve this problem using extrusion method. We developed the metal core further thicker PZT clad fiber by the extrusion method. 31 piezoelectric fibers were embedded under CFRP composite materials, and the sensor and the actuator function were evaluated. We showed that self sensing was possible using this smart board.

There is currently a need for compact actuators capable of producing large deflections, large forces, and broad frequency bandwidth. In all existing transducer materials, large force and broadband responses are obtained at the price of small displacements and methods for transmitting very short transducer element motion to large deformations need to be developed. This paper addresses the development of a hybrid actuator which provides virtually unlimited deflections and large forces through magnetorheological (MR) flow control and rectification of the resonant mechanical vibrations produced by a magnetostrictive Terfenol-D pump. The device is a self-contained unit which produces large work output concurrently with stiffness and damping control, and is compact and self-locking when unpowered. To increase the output force, hydraulic advantage is created by implementing a driven piston diameter that is larger than the drive piston. Since the pump operates at high speeds in the low kHz range, a fast-acting MR fluid valve is required. The paper presents a four-port MR fluid valve in which the fluid controls its own flow while carrying the full transducer load. A prototype two-port valve was designed and constructed. Experimental measurements at various pressure levels are presented which demonstrate the new valve concept.

Magnetorheological (MR) fluids can be used in a variety of smart semi-active systems. MR dampers have especially great potential to mitigate environmentally induced vibration and shocks. MR fluid can be used effectively in valve networks to control the flow from a hydraulic source to enable a fully active actuator. These devices are simple, have few moving parts and can be easily miniaturized to provide a compact, high energy density pressure source. The present study describes a prototype MR-piezo hybrid actuator that combines the piezo-pump and MR valve actuator concepts, resulting in a self-contained hydraulic actuation device without active electro-mechanical valves. Durability and miniaturization of the hybrid device are major advantages due to its low part count and few moving parts. An additional advantage is the ability to use the MR valve network in the actuator to achieve controllable damping. The design, construction and testing of a prototype MR-piezo hybrid actuator is described. The performance and efficiency of the device is derived using ideal, biviscous and Bingham-plastic representations of MR fluid behavior. Unidirectional performance, or constant velocity actuator shaft motion, is assessed analytically and compared to experimental data. A experimental assessment of the magnetorheological birectional flow control capability is also provided.

In this work, two different mounts fabricated from two different smart materials are proposed for the vibration control of a flexible structure subjected to external disturbances. A semi-active magneto-rheological(MR) mount is firstly devised and its damping force controllability by the current intensity is evaluated. Subsequently, an active piezoceramic mount is devised and its field-dependent inertial force is experimentally evaluated. Each mount is then installed on the flexible structure and the corresponding control model is established. The LQG controller is designed for each mount system and empirically realized. Control responses of each control system such as acceleration and displacement are investigated in the frequency domain.

Control of torsional vibrations in an automotive crankshaft is a classical vibration control problem. The most common solution is to mount a crankshaft damper at one end of the crankshaft. Typical crankshaft dampers are composed of parallel stiffness and damping elements connecting a rotational inertia to the crankshaft. Appropriate design of the damper elements may result in substantial crankshaft vibration. Conventional couplings include elastomeric spring-damper elements and purely viscous fluid couplings. While those approaches result in satisfactory reduction of crankshaft vibration, it may be that a semi-active approach can achieve improved performance. To that end, an investigation of a semi-active crankshaft damper using magneto-rheological (MR) fluid has been initiated. A torsional MR fluid brake was obtained and applied to a scale model of a crankshaft for a common eight-cylinder engine. Experiments were performed with the MR brake as a fixed-friction device. In addition, a simple stick-slip control algorithm was developed such that the MR brake became an on-line variable friction device. While a good deal of work remains to be performed in future efforts, the preliminary experimental results have demonstrated that a torsional damper composed of an MR fluid brake has potential application in the field of torsional vibration control.

This paper presents a wheel slip control via sliding mode controller for a new anti-lock brake system (ABS) of a passenger vehicle using electrorheological (ER) valve pressure modulator. The principal design parameters of the ER valves and hydraulic booster are appropriately determined by considering braking pressure variation during ABS operation. An electrically controllable pressure modulator using the ER valves is then constructed and its governing equations are derived. Subsequently, the pressure control performance of the new pressure modulator is experimentally evaluated. The governing equations of motion for a quarter car wheel model are derived and the sliding mode controller is formulated for wheel slip control. Hardware in the loop simulation (HILS) for braking performance evaluation is undertaken in order to demonstrate the effectiveness of the proposed ABS associated with the ER valve pressure modulator.

Tensegrity structures are special truss structures composed of bars in compression and cables in tension. Most tensegrity structures under investigation, to date, have been of Class 1, where bars do not touch. In this article, however, we demonstrate the hardware implementation of a 2 stage symmetric Class 2 tensegrity structure, where bars do connect to each other at a pivot. The open loop control law for tendon lengths to accomplish the desired geometric reconfiguration are computed analytically. The velocity of the structure's height is chosen and reconfiguration is accomplished in a quasi-static manner, ignoring dynamic effects. The main goal of this research was to design, build, and test the capabilities of the Class 2 structure for deployment concepts and to further explore the possibilities of multiple stage structures using the same design and components.

This paper concerns the optimal mass-to-stiffness ratio design of class-2 tensegrity towers. For different loading scenarios, the procedure seeks the topology and geometry of the structure that yields an optimal design satisfying common constraints. The domain of feasible tensegrity geometries is defined by imposing tensegrity equilibrium conditions on both unloaded and loaded structure. Remaining constraints include strength constraints for all elements of the structure and buckling constraints for bars. The symmetry of the design is imposed by restricting the domain of geometric variables and element parameters. The static response of the structure is computed by using a nonlinear large displacement model. The problem is cast in the form of a nonlinear program. The influence of material parameters on the optimal shape of the structure is investigated.

A concept for a high authority shape morphing plate is described and demonstrated. The design incorporates an active back-plane comprising a Kagome truss, capable of changing the shape of a solid face, connected to the back-plane by means of a tetrahedral truss core. The two shape deformations to be demonstrated consist of hinging and twisting. The design is performed by a combination of analytic estimation and numerical simulation, guided by previous assessments of the Kagome configuration. It is shown that, while the structure is capable of sustaining large passive loads at low weight, the demonstrable authority is actuator-limited. The full potential of the system can only be realized by developing and incorporating superior actuators.

An analytical model of a thin annulus made of a dielectric elastomer was derived in our earlier work. It is the focus of this paper to fabricate and test the electroactive polymer actuators presented in the model. Radial displacement measurements were obtained using a digital image correlation technique that involved cross-correlating images of the actuator before and after a change in the electric field. Results showed that the two-term Ogden material model provided a good agreement with experimental data. Hysteresis of the dielectric actuators was observed. Radial displacements of material points on the actuator varied between two consecutive tests which may indicate that strain has history-dependent characteristics.

The current MAVs used servomotors as actuators for the control surfaces, such as elevators and rudders. In this paper, the application possibility of conducting shape memory polymer to smart actuator has been assessed. Our final goal will be to replace the servomotor with a newly developed conducting shape memory polymer actuator. As the first step, a conducting shape memory polymer with high transition temperature and high conductivity was manufactured and its properties were measured. Second, conceptual designs of special actuating systems for control surface of micro aerial vehicle are presented. The conducting shape memory polymer was composed of shape memory polyurethane block copolymer and carbon nanotube or carbon black. Its basic thermo-mechanical and conducting properties are discussed for application as electro-active shape memory polymer actuator.

This paper will discuss the application of various bio-inspired morphing concepts to unmanned aerial vehicle (UAV) designs. Several analysis tools will be introduced to calculate the aerodynamic benefits, dynamic response, and mission-level benefits of morphing shape changes. Empirical relations are employed to calculate the effects of various geometry changes on the aerodynamics of the vehicle. A six-degree-of-freedom simulation will evaluate the stability and dynamic response of each vehicle configuration as well as “snapshots” of the morphing change. Subsequently, an aircraft performance analysis will be conducted for various shape configurations. Specifically, the performance of a bio-inspired wing is compared to conventional designs. The aircraft dynamic improvements that morphing technologies introduce will be discussed.

In this paper, we present our recent progress in application of LIPCA (LIghtweight Piezo-Composite Actuator) to design and demonstration of a flapping wing device. The flapping device has flexible wings actuated by the LIPCA. The device is designed such that it can create twist motion during up- and down-stroke like bird or insect wings. The motion could be generated by using LIPCA actuator pivoted to the wing. The wing can bend and twist due to bending-twist coupling of the specially designed pivot system. Experimental results show that the properly designed flapping device powered by LIPCA can create anisotropic motion that is often required for producing effective thrust and lift forces in bird or insect flight.

Morphing aircraft design - the design of aircraft capable of macroscale shape change for drastic in-flight performance variation - is an extremely broad and underdefined field. Two primary means of developing new concepts in morphing exist at Cornell University: design of broad test platforms with generalized motions that can provide future insight into targeted ideas, and specifically adapted aircraft and shape change mechanisms attempting to accomplish a particular task, or hybridize two existing aircraft platforms. Working with both schools of thought, Cornell research has developed a number of useful concepts that are currently under independent analysis and experimentation, including three devices capable of drastically modifying wing structure on a testbed aircraft. Additional concerns that have arisen include the desire to implement ornithological concepts such as perching and wingtip control, as well as the necessity for a compliant aerodynamic skin for producing flight-worthy structural mechanisms.

This paper outlines the testing of a piezoelectric ultrasonic motor (PUSM). The authors propose the use of a pseudoelastic NiTi thin film layer to replace the traditional fiber reinforced polymer contact layer. In accordance with this goal, a literature review of reported accomplishments in the area of the tribology of NiTi, both shape memory and pseudoelastic, is provided. The results of the baseline testing of the commercially acquired PUSMs are presented. The motors were baseline tested using a dead-weight force system, then disassembled and a 10 μm thick layer of NiTi was deposited on the rotor using a dc magnetron sputtering process. The motors were then reassembled and tested again. A comparison of the initial results is presented and discussed.

Continuously morphing aircraft wings are currently a focus of considerable research. Efforts are being made to achieve effective and optimal wing shape change under different flight conditions such as take off, cruise, dash, and loiter. The present research aims to achieve wing morphing by using an internal structure consisting of actuated tendons and passive struts. An important aspect of this approach is determining the optimal layout of tendons and struts. In this paper a genetic algorithm is developed to optimize the three-dimensional tendon-strut layout for a prescribed wing geometry and shape change. The method is applied to two morphing wing applications, the NASA HECS wing and NextGen TSCh wing.

A servo-tab mechanism was designed for a full-scale rotor blade section, and tested in an open-jet wind tunnel. The intent was to use aerodynamic forces and moments to amplify the stroke and force provided by a compact piezoelectric bender actuator. The design was constrained by the stroke-force characteristics of the piezoceramic benders previously developed for model scale applications. The flap chord was chosen as 25% of the blade chord, and the tab chord was 5% of the blade chord. The flap response to the aerodynamic moment generated by the servotab was predicted using blade element momentum and thin airfoil theory. Testing in an open-jet wind tunnel explored the response of the flap in both steady and oscillating tab conditions.

A new membrane actuator based on our previous diaphragm actuator was designed and constructed to improve the dynamic performance. The finite element analysis was used to estimate the frequency response of the composite membrane which will be driven close to its resonance to obtain a large stroke. The membrane is made of ferromagnetic shape memory alloy (FSMA) composite including a ferromagnetic soft iron pad and a superelastic grade of NiTi shape memory alloy (SMA). The actuation mechanism for the FSMA composite membrane of the actuator is the hybrid mechanism that we proposed previously. This membrane actuator is designed for a new synthetic jet actuator package that will be used for active flow control technology on airplane wings. Based on the FEM results, the new membrane actuator system was assembled and its static and dynamic performance was experimentally evaluated including the dynamic magnetic response of the hybrid magnet.

Synthetic-jet actuators have been intensely studied recently. The interest in these devices is elicited by their usefulness in fluid-control applications, including boundary-layer control, combustion control etc. A synthetic-jet actuator is a zero-net-mass-flux device, and is comprised of a diaphragm mounted to enclose a volume of fluid in a cavity. The diaphragm bends sinusoidally, and fluid is periodically absorbed into and ejected from the cavity through an orifice. The outflow entrains the fluid around it and establishes a mean jet flow at a distance from the source. Piezoceramic materials have been used to drive the vibrating diaphragm, where the piezoceramic is glued directly to a silicon diaphragm. In combustion systems, improved turbulent mixing of air and fuel proper can significantly improve efficiency and reduce pollution. In boundary-layer separation control applications, synthetic-jets are used to improve aerodynamic performance by delaying separation and stall over the airfoil. The current work describes the modeling and design of a single-crystal piezoceramic-driven synthetic-jet actuator, and demonstrates that the efficiency of the actuator increases when single-crystals piezoceramic materials are used instead of the more conventional polycrystalline piezoceramics.

The vital attention of this project is directed to active flow control over micro aerial vehicles using synthetic jet actuators. Previous research has demonstrated that oscillating micro-jets have the potential to influence the flow characteristics and change the aerodynamic performance of the airfoils. The aspiration of this effort is to build on the previous work and support the idea of using synthetic jet actuators to improve the deteriorated aerodynamic characteristics of the low Reynolds number flow regimes and potentially use the same structure as a maneuvering device. The main idea is to actively control the pressure distribution over an aerodynamic surface which could lead to elimination of flow separation, lift increase and drag losses. To understand the efficiency of the synthetic jet actuators and the potential for such applications, it was necessary to investigate the nature and the variety of their performance with different design configurations. This paper covers the characterization of synthetic jets, starting with the analysis of different types of diaphragms, diverse size and shape cavities and different size and shape of the exit nozzles. The diaphragm with the thickness/diameter ratio of 0.17 made out of 0.01 inch thick brass shim was found to be the most effective of those studied. The volume and the shape of the cavity were related and it was noticed that the performance of the synthetic jet actuator increased when the device was tuned such that the resonant frequency of the diaphragm and that of the cavity were close to matching. A square diaphragm supplied better displacement characteristics than the circular diaphragms studied and also showed better results when integrated with the enclosed cavity forming the final device.

This study presents the status and results from an effort to design, fabricate, and test an adaptive jet engine chevron concept based upon embedding shape memory alloy (SMA) actuators in a composite laminate, termed a SMA hybrid composite (SMAHC). The approach for fabricating the adaptive SMAHC chevrons involves embedding prestrained Nitinol actuators on one side of the mid-plane of the composite laminate such that thermal excitation generates a thermal moment and deflects the structure. A glass-epoxy pre-preg/Nitinol ribbon material system and a vacuum hot press consolidation approach are employed. A versatile test system for control and measurement of the chevron deflection performance is described. Projection moire interferometry (PMI) is used for global deformation measurement and infrared (IR) thermography is used for 2-D temperature measurement and feedback control. A recently commercialized constitutive model for SMA and SMAHC materials is used in the finite element code ABAQUS to perform nonlinear static analysis of the chevron prototypes. Excellent agreement is achieved between the predicted and measured chevron deflection performance, thereby validating the design tool. Although the performance results presented in this paper fall short of the requirement, the concept is proven and an approach for achieving the performance objectives is evident.

Experimental tests were conducted on a torque actuator based on ferromagnetic shape memory alloy composite and performance data obtained. Based on experimental results of the initial design, several design changes were made to the actuator that resulted in improved torque capability and maximum angle of twist compared to the original design. A full 3-D finite element method model was undertaken to optimize magnetic flux lines generated by the hybrid magnet.

A new spring actuator composing of two parts: driving unit and ferromagnetic shape memory alloy (FSMA) spring, is designed based on hybrid actuation mechanism. The driving unit, which is based on hybrid magnet, consists of coil, yoke, and ring shape permanent magnet (PM); the FSMA spring can be either composite material, which is made of shape memory alloy of superelastic grade, and soft ferromagnetic material, or FSMAs such as FePd. In this design, driving units are inserted in between ferromagnetic springs. This will provide the actuator with larger force and stroke. FEM analysis under axis symmetric model has been used to optimize the driving components. According to the numerical results, the iron blocks, both on the top and the bottom, can provide larger magnetic forces facing downwards and upwards respectively due to the gradient of magnetic flux density. Advantages of this design are simple, robust, and compact yet providing relatively large force (20N) and stroke (30mm).

The cyclic behavior of shape memory alloys (SMAs) in their austenitic form is studied to determine the most appropriate method of modeling in terms of both accuracy and ease of implementation. Four different models for SMA behavior are evaluated: (a) a simple nonlinear elastic model, (b) a trigger-line model, (c) a one-dimensional thermomechanical model, and (d) a one-dimensional thermomechanical model which accounts for the behavior of SMAs under cyclic loading. Using a two degree-of-freedom bridge model with SMA restrainers and a single degree-of-freedom building model with SMA cross-braces, the effect of using the different models on the seismic response of the bridge and building is evaluated. Using a suite of nine earthquake ground motions, the displacement response histories with the four different models are compared. The results illustrate that although the models show quite different behaviors for the SMAs, the resulting responses of the bridge and building are insensitive to the type of model used. For most of the ground motion records used, the difference in the maximum displacement for the four models was less than 15%. This study lends support to the use of more simplified models when evaluating the effectiveness of the SMAs for seismic response modification.

Regenerative chatter is a form of unstable, self-excited vibration that occurs in machining operations such as milling and turning. In high speed milling of many aircraft components, regenerative chatter is the fundamental factor that limits metal removal rate and consequently productivity. Regenerative chatter is essentially a feedback process: the cutting chip thickness produces a force between the tool and workpiece, and the dynamics of these components results in a change in the cutting chip thickness. An exciting method of avoiding chatter is to actively control the workpiece or tool during cutting. On thin walled workpieces, such as those for aerospace structural components, this can be achieved using piezoelectric devices. A wide range of control regimes could be applicable, such as feed-forward actuation, active constrained layer damping, or active damping, using either non-collocated or collocated sensors and actuators. This study focuses on active damping with collocated sensors and actuators. In this article, an experimental study is described whereby workpiece chatter during milling is reduced by using active vibration control with piezoelectric sensors and actuators.

In this paper, the experimental validation of a semi-active vibration reduction strategy is presented where energy is dissipated by dry friction contact surfaces. The aim of this project is to develop a semi-active friction compact device which can be bonded to any light flexible structure. The prototype presented in this paper incorporates two piezoelectric stack actuators used to apply a normal force between a mobile inertial component and two friction pads. This force is controlled so that the distance between the two surfaces is neither too small (to avoid shock and stiction that can cancel the slip between the two surfaces and then the friction effect) or too large (loss of contact surfaces). In order to avoid spillover effects found with the nonlinear control law leading to a bang-bang control, a lead-phase strategy is proposed for the controller. The friction model that was used to develop the controller is first introduced. The parameters of these models are determined experimentally and the friction force obtained from the model is compared with experimental measurements of this force. The piezoelectric stack actuator is then characterized with respect to the friction force generated. The friction model is then used to predict the controller performance for energy dissipation. The implementation of the controller is presented, together with the strategy adopted for the adjustment of the parameters of the controller. Control experiments are conducted on the prototype device and experimental results are presented to assess the performance of the controller.

Most active vibration suppression approaches have attempted to suppress structural vibration by incorporating active material actuators, such as piezoceramic, within the structure to act directly against vibratory loads. These approaches require the piezoceramic actuators to generate significant force and deflection simultaneously to effectively suppress vibration. Unfortunately, successful implementation of these approaches has been hindered by the limited displacement capabilities of piezoceramic actuators. The Smart Spring concept is an unique approach to actively control combinations of dynamic impedance characteristics of a structure, such as the stiffness, damping, and effective mass to suppress vibration in an indirect manner. The piezoceramic actuators employed in the Smart Spring concept are not used to directly counteract excitation loads but rather adaptively vary the effective impedance properties of the structure. Therefore, the piezoceramic actuators in the Smart Spring are not required to produce large forces and deflections simultaneously. This paper demonstrates the ability of the Smart Spring concept to control dynamic impedance characteristics of a structure through numerical simulations and experimental investigations. Mechanical shaker tests using the proof-of-concept hardware verified the controllability of the impedance properties using the Smart Spring device and its ability to suppress vibration. More importantly, the tests conducted in a wind tunnel demonstrated the performance of the Smart Spring under highly varying unsteady excitation conditions. These tests confirmed that the Smart Spring system is able to actively suppress vibration through adaptive control of structural impedance properties.

The use of piezoelectric materials for power harvesting has gained significant interest over the past few years. The majority of research on this subject has sought to quantify the amount of energy generated in power harvesting applications, or to develop methods of improving the amount of energy generated. Usually, a monolithic PZT material with a traditional electrode pattern and poled through its thickness is used in power harvesting research projects. In recent years, however, several companies and research institutions have begun to develop and market a broad range of piezoelectric composite sensor/actuator packages, each conceived for specific operational advantages and characteristics. Commonly, these devices are employed in control and vibration suppression applications, and their potential for use in power harvesting systems remains largely unknown. Two frequently implemented design techniques for improving the performance of such actuators are the use of interdigitated electrodes and piezofibers. This paper seeks to experimentally quantify the differences in power harvesting application performance between several of these new actuators and to identify the reasons for their relative performance characteristics. A special focus on the structural and compositional differences between each actuator is incorporated in the discussion of the effectiveness of each actuator as power harvesting devices.

A new approach of energy reclamation from mechanical vibrations is presented in this paper. The conversion from mechanical energy into electrical energy is achieved using piezoelectric materials. The originality of the proposed approach is based on a nonlinear treatment of the voltage delivered by a piezoelectric insert embedded in a vibrating structure. This nonlinear processing induces a strong increase of the power conversion capability of the piezoelectric insert. The theoretical principle of the nonlinear treatment is exposed, and the analytical model of an electrical generator is developed. The results given by the model are compared to those of an experimental set-up. Experimental results show that the extracted electrical energy may be increased beyond 400%.

This paper discusses a novel concept idea of utilizing smart structure in biomedical, minimum invasive surgery (MIS), MEMS manufacturing assembly line and also as a miniature robotic gripper system. The proposed prototype of a miniature piezoelectric forceps actuator (PFA) is composed of two symmetric slightly curved composite beams which each bonded with piezoelectric ceramic layer. The PFA is an innovative forceps actuator that comes with a data glove. The data glove is simply a custom-made glove with two embedded resistance-bending sensors located on thumb and index fingers. Any users can control opening and closing of the PFA by just wearing the data glove. A thin curved beam theory bonded with piezoelectric ceramic will be derived based on Hamilton's principle and its deflection behavior will be simulated based on distributed transfer function method (DTFM). A feasibility study of simulation open loop data glove-based fuzzy logic controller allows the user to open and close the PFA remotely. The bending movement of the thumb and index finger will be formulated in a table of rules based to produce the necessary output controller gain to control the PFA.

The paper provides a review of the literature dedicated to existing modeling, analysis, synthesis and optimization tools, together with the associated design procedures for MEMS applications, by looking at the approximate analytical algorithms and finite element procedures in the static, dynamical and coupled-field domains as applied to a large compartment of compliant members and their corresponding devices. The paper gives a classification of MEMS according to their structure and another classification as a function of their function. Main architectures of compliant microdevices are also reviewed, including torsion mirrors, bending mirrors, bimorph/multimorph transducers, accelerometers, gyroscopes, scratch drives, out-of-the-plane microcantilevers, sensing devices, resonators, switches or filters. Attention is dedicated to operational means of actuation, such as thermal, electrostatic, magnetic, electromagnetic, piezoelectric or shape-memory-generated, and to their integration into the overall microsystem design. Numerical modeling techniques, such as finite or boundary element model algorithms/codes that are utilized in the analysis and design of MEMS are also presented.

Traditionally, miniature scanning mirrors with which raster-scan displays are designed, oscillate sinusoidally while being operated in resonance. The operation in resonance gives rise to large vibration amplitudes under a small driving force/torque and can thus be realized in MEMS scale to oscillate at 15,000 per second and beyond. Unfortunately, sinusoidal scanning creates images with highly non-uniform intensity levels. It was therefore suggested to create a resonating mirror that performs a near triangular-wave periodic motion. Presented in this paper is a closed-form synthesis procedure with which a suitable multi degrees of freedom scanning mirror can be realized. It is shown that a special topology can be used to generate many periodic oscillatory motions under low operating forces. Specifically, a large scale electromagnetically driven system and a MEMS mirror have been designed and built to demonstrate the applicability of this approach. It is shown that the necessary excitation voltage, in the electrostatically driven MEMS mirror, can be reduced from over 1000 Volt into a more realistic 40 Volt range.

The paper proposes an analytical model-based, lumped-parameter algorithm which enables identifying a microcantilever design that will give the optimized performance with regard to stiffness and resonant frequency values in terms of a corresponding shape and its related geometric parameters. The targeted microcantilever applications are mass variation detection and atomic force microscopy (AFM) whereby the monitored system parameters are the changes in deflection and/or the natural frequency. A design set comprises several configurations, each of them being defined by analytical curves, such as straight lines, circular or elliptical arcs, and which are quantified in terms of compliances/stiffnesses and resonant frequencies by algebraic equations. The model is capable of discerning both the intensity of external excitation and the first resonant frequencies for a given microcantilever. Finite element simulation results of the static and resonant response for these microcantilevers confirm the analytical model predictions. The optimization algorithm, which is based on this model, focuses on maximizing the master bending compliance and on spacing out the first resonant frequency from the subsequent ones in order to increase the response sensitivity of the microcantilever. The model-based optimization algorithm is a relatively low-cost and sufficiently-accurate calculation procedure, which is formulated as an alternative to existing finite element simulation.

Construction of a novel liquid material micro dosing and spraying apparatus and its design optimization is presented in the paper. Its functionality is based on the influence of high frequency vibrations generated by cylindrical piezo-actuator to the flow of liquid substances in a micro tube. New models of the micro dosing elements are presented. Such spray systems provide high accuracy of micro dosing and are applicable in a variety of applications from avionics to medicine. Hybrid experimental-numerical analysis techniques are used for investigation of micro spray systems. Experimental analysis of the investigated systems is based on laser holographic interferometry which is used for the identification and optimization of working regimes of the system.

A torsional actuator has been developed at NRL utilizing the high piezoelectric shear coefficient, d₁₅. This torsional actuator uses an even number of alternately poled segments of electroactive PZT. Under an applied electric field, the torsional actuator produces large angular displacement and a high torque. The solid freeform fabrication technique of the laminated object manufacturing (LOM) is used for rapid prototyping of torsional actuator with potential cost and time saving. First step to demonstrate the feasibility of the LOM technique for the torsional actuator device fabrication is to make near net shape segments. We report a prototype PZT torsional actuator using LOM prepared PZT-5A segments. Fabrication processes and test results are described. The torsional actuator PZT-5A tube has dimensions of 13 cm long, 2.54 cm OD and 1.9 cm ID. Although the piezoelectric strain is small, it may be converted into large displacement via accumulation of the small single cycle displacements over many cycles using AC driving voltage such as with a rotary 'inchworm' actuator or an ultrasonic rotary motor. A working prototype of a full-cycle motor driven by the piezoelectric torsional actuator has been achieved. The rotational speed is 1,200 rpm under 200 V/cm field at the resonant frequency of 4.5 kHz.

Many ultrasonic motors are based on resonators that have two modes of vibration with equal resonant frequencies. Excitation of the two modes with a 90°-phase shift is used to generate elliptical motion at the contact points between resonator and slider. An alternative motor using two modes with a resonant frequency ratio of 1:2 has been demonstrated in the literature. The contact point in this stator describes a Lissajous figure ('8') instead of an ellipse. The motor requires only a single power amplifier and piezoelectric ceramic element. The challenge of designing such a resonator is to achieve the predetermined resonant frequency ratio with corresponding modes having usable motion at the contact points. In this paper, topology optimization was applied to design a resonator that has two modes satisfying these conditions. The topology optimization problem was formulated in an unusual manner with direct minimization of the intermediate density and a constraint on the natural frequency ratio. The design domain was discretized using four-noded membrane elements with drilling degrees of freedom due to their superior efficiency. Manual shape optimization, considering 3-D effects, was performed on the design produced by the topology optimization step. A prototype resonator was constructed and tested to verify the numerical modeling.

A new technique is presented for designing actuators for guided hard-launch adaptive munitions by using actuator and substrate strain limits, static analysis methods and matching the local actuator strains along its length by varying the width. This Load-Matched design technique leads to an exponential area distribution as a function of length which is contrasted against the conventional rectangular actuator shapes that have been used in all adaptive hard-launch munitions up till now. To demonstrate the viability of this new Load-Matched actuator design, ten 600mg, 100mm long rectangular and ten identical mass and length, exponentially shaped, Load-Matched actuator specimens were designed and built to withstand the maximum possible accelerations. Predicted design static strain distributions are presented along with limits, showing that rectangular actuators exhibit a strong strain peak at the root while Load-Matched actuators have a much more even distribution and a gentle maximum near the middle. Shock table testing showed that the rectangular specimens were predicted to fail at 3,500g's, but survived acceleration levels 9.5- 12% higher than expected (3,833 to 3,931g's). The exponentially shaped Load-Matched actuators proved that they could withstand shocks from 17 to 21% over the predicted failure acceleration level of 8,000g's (9,377 to 9,670g's).

Smart materials' ability to deliver large block forces in a small package while operating at high frequencies makes them extremely attractive for converting electrical to mechanical power. This has led to the development of hybrid actuators consisting of co-located smart material actuated pumps and hydraulic cylinders that are connected by a set of fast-acting valves. The overall success of the hybrid concept hinges on the effectiveness of the coupling between the smart material and the fluid. This, in turn, is strongly dependent on the resistance to fluid flow in the device. This paper presents the results of two and three dimensional simulations of fluid flow in a prototpype hybrid actuator being developed for aerospace applications. The steady simulations show that losses in the device result primarily from three dimensional effects and that two dimensional approaches can underestimate losses by approximately a factor of 40. The effects of varying design parameters like the pumping chamber height, discharge port location, and discharge port chamfer are also explored and are found to have significant impacts on performance. Three dimensional, unsteady simulations demonstrate how resistance to fluid flow in the pump reduces the amplitude of the piezo displacement and thus limits the flow rate of the device.

This paper presents the design and testing of a magnetostrictive-hydraulic hybrid actuator driven by Terfenol-D. The actuator is based on the frequency rectification of small displacements from a Terfenol-D rod by using one-directional check valves. The continuous fluid flow produced from this actuation is then used to drive a hydraulic output cylinder. A transducer was built in order to actuate the Terfenol-D rod in the pump. In addition, an active valve system was designed and built to achieve bi-directional capabilities in the actuator. Successful testing of the actuator in uni-directional and bi-directional modes was carried out. The actuator was found to have an unloaded velocity of 6 in/sec, a blocked force of 10 lbs, and a bi-directional stroke of 0.1 in/cycle at 10 Hz.

This paper presents the health monitoring technique of composite repair patches using small-diameter fiber Bragg grating (FBG) sensors. The composite patch structures consist of carbon fiber reinforced plastic (CFRP) patches, aluminum substrates and adhesive layers. The debonding progressed between aluminum substrate and the adhesive layers under cyclic loading. In this research, two types of the specimen were prepared for evaluation of the debonding size. One is specimen with the patches glued on the both side of the aluminum plate. The other is pre-notched specimen for investigation of the influence of the crack propagation in the substrate. The FBG sensors were embedded into the adhesive layers easily, because the diameter of the sensor was only 52 μm. Reflection spectra from the sensor were measured at various lengths of the debonding. The form of the spectrum changed sensitively with an increase in the debonding size. For confirmation of the measured results, the spectra were simulated theoretically. As a result, the change in the calculated spectrum was consistent with that in the measured spectrum, and the relation between the spectrum and the debonding was clarified. Moreover, the debonding length was evaluated quantitatively by the monitoring of the form of the reflection spectrum.

This paper presents comprehensive studies on sensor performance of an embedded Extrinsic Fabry Perot Interferometer (EFPI) fiber optic strain sensor in an aerospace grade composite system to support fiber optic smart structures (FOSS) development for Structural Health Monitoring (SHM) System. A major portion of this study is focused on establishing the accuracy of the embedded EFPI sensors in a graphite epoxy composite material system at different stress levels under quasi-static loading conditions. The NASA Dryden calibrated EFPI's were used for accurate measurements. Two collocated surface-mounted strain gages and a calibrated surface-mounted EFPI sensor are used to validate the calibrated embedded EFPI sensor. Experimental results suggest that once calibrated, the embedded and surface-mounted EFPI sensors provide robust, reliable and accurate measurement for values up to ~5,400 με higher than sensor's durability limit ~3,000 με at 10⁶ cycles. This validation provides evidence that the sensing information emanating from FOSS can be used to monitor accurate health information.

An effective integrated structural health monitoring system must include a method of sensing and a process of damage identification that are optimized to work together. The result is a system that provides an automated and quantified assessment of damage present in a structure. Two candidates for such a symbiosis of sensing and damage identification are impedance-based measurement and statistical process control. The impedance-based structural health monitoring method uses a high frequency signal to excite a structure through a bonded piezoelectric patch and measures the impedance response of the excited structure across a frequency spectrum. In structural damage cases such as threads loosening or a crack developing, the structure in question will begin to show a change in impedance. Once measured, a damage sensitive feature from this impedance change can be statistically quantified into different damage cases by statistical process control. This paper addresses impedance measurements from experimental structures and a subsequent statistical method for quantitatively determining when the impedance signature of the structures has changed significantly enough to warrant the classification of “damaged”. Simple features and hypothesis testing algorithms are explored in an effort to create real-time solutions and reduce the complexity of damage identification for future use in low resource integrated structural health monitoring systems.

The impedance-based structural health monitoring technique, which utilizes electromechanical coupling properties of piezoelectric materials, has shown feasibility for use in a variety of structural health monitoring applications. Relying on high frequency local excitations (typically>20 kHz), this technique is very sensitive to minor changes in structural integrity in the near field of piezoelectric sensors. Several damage sensitive features have been identified and used coupled with the impedance methods. Most of these methods are, however, limited to linearity assumptions of a structure. This paper presents the use of experimentally identified nonlinear features, combined with impedance methods, for structural health monitoring. Their applicability to for damage detection in various frequency ranges is demonstrated using actual impedance signals measured from a portal frame structure. The performance of the nonlinear feature is compared with those of conventional impedance methods. This paper reinforces the utility of nonlinear features in structural health monitoring and suggests that their varying sensitivity in different frequency ranges may be leveraged for certain applications.

Vibration-based damage identification using embedded sensitivity functions is discussed. These sensitivity functions are computed directly from experimental frequency response functions and reflect changes in the forced response of structural systems when mass, damping or stiffness parameters are changed. The theory of embedded sensitivity functions is reviewed and applied to characterize damage in a simulated three degree-of-freedom system and a full-scale exhaust system with nonlinear characteristics. Linear damage is shown to be properly detected, located and quantified in theory and practice for structures with one damage mechanism by comparing embedded sensitivity functions with finite difference frequency response functions in undamaged and damaged test data. It is also shown using the exhaust system that false indications of damage due to nonlinear amplitude dependence can be avoided by developing nonlinear baseline models. Experimental results indicate that the technique is most effective when changes to frequency response functions are no larger than 10% to avoid distortions in the estimated perturbations due to variations in the sensitivity functions.

Lamb waves have attracted great attention in non-destructive evaluation (NDE) due to its efficiency in interrogating a reasonably extensive distance along the plate. Such waves can be efficiently excited using piezoceramic transducers with good control on the pulse charactertistics to assess the health of structural components, such as the presence of cracks. Through selective generation of Lamb waves within a frequency range, linear cracks can be detected via time-of-flight analysis of the wave, by using plain piezoceramics transducers with strategic positioning. Alternatively, using a well-designed inter-digital transducer (IDT), a single Lamb mode can be generated. It is shown that using IDT enhances detection accuracy and robustness in view of its controllability on the duration and direction of the generated wave. It is thus able to locate curved crack accurately as well as trace its geometry. The performance of both actuators are compared experimentally using both plain piezoceramics and IDT to detect different cracks, namely, linear crack, curved crack and multiple cracks, on aluminum plates. Plain piezoceramics provide accurate detection for linear and multiple cracks, and are able to estimate the geometry of a curved crack reasonably well. However, IDT is more efficient and provides accurate results for these three cases.

Triton Systems Inc. has teamed with Virginia Tech to develop smart active materials for mirror shape control and stabilization systems utilized in thin film, space-based, polymeric mirrors. The development of novel lightweight space-qualified optics and support structures is of vital importance to science, industry, and national defense. Primary mirrors are one of the main drivers of the mass of space based optical systems. Therefore, lightweight optics is an essential component to reducing launch costs while increasing payload utility. Electroactive polymers represent a special class of “smart materials” whose electronic and physical properties such as conductivity, charge distribution, and shape can be changed in response to the environment (voltage and stress). The ability of electroactive polymers to change structure within a matrix in response to electrical stimulation has several applications for large ultra-lightweight optics. The Triton/VT team has begun the development of castable electroactive materials that do not depend on aqueous systems. These novel non-aqueous materials allow both an increase in voltage limits, and the ability to be used in environments where water would rapidly evaporate. These materials will allow the adjustment of the shape, to remove aberrations, as well as solve issues such as damping vibrations after re-pointing of large space telescopes.

This paper describes the application of self-sensing control to a cardiac assist device. We propose to improve the pumping performance of diseased or weakened hearts by applying direct cardiac compression using artificial muscle. This particular application imposes strict limitations on size, weight and system complexity, therefore employing self-sensing could offer advantages over separate sensors and actuators. Many electromagnetic actuators produce a back-e.m.f. proportional to velocity. Using a simple system model, it is possible to separate this back-e.m.f. from the supply voltage, thus the actuator velocity can be self-sensed. Furthermore, using a more detailed model, it also is possible to self-sense the force being applied. Experimental results are presented for linear moving-coil actuators and miniature d.c. motors. Estimation of position has been performed by numerical integration of self-sensed velocity, and shown to compare favourably to data from displacement sensors. Force estimation has also been shown to closely agree with data from a load cell. Combined force and position control has been implemented, without using sensors. Unfortunately, since self-sensed position is derived by integrating velocity, the estimated position can suffer from drifting. An automatic re-calibration scheme is proposed for the cardiac assist application.

An inchworm actuator is described which uses complementary configurations for the two clamping sections. In one configuration clamping and release are achieved using high and low voltage respectively while for the other clamping and release are achieved using low and high voltage respectively. The resulting inchworm actuator can be driven by a two-channel controller with the two clamps sharing the first channel and the extender piezoelectric actuator using the second channel. In the coarse positioning mode the direction of motion is determined by whether the extender voltage pulse overlaps the leading or trailing edge of the common clamp pulse. A fine positioning mode can be realized with the common clamp voltage set to 0V and continuous feedback control applied to the extender actuator. The paper also describes a diode-shunted delay circuit that causes unclamping to occur more slowly than clamping. It is shown that by using the delay circuit in series with each clamp, the overall force drive capability of the actuator is increased. The paper presents simulated and experimental results of clamp surface displacement and force vs. time during the switching transient.

Many forced systems are prone to undesirable levels of oscillation if lightly damped modes are present with their frequency range of operation. Rotary systems, for example, can experience these problems during the speed up or speed down stage of operation. Resonant motion can damage or effect the accuracy of operation of such systems and is, therefore, highly undesirable. Many closed-loop controllers avoid this by suppressing the mode itself such that at resonance the modal vibration amplitude is small (i.e. highly damped). The current work presents an alternative novel switching controller, which suppresses the system not by applying a high amount of damping but rather by moving the resonant mode such that it is never excited. From the basis of an accurate plant model, two pole-placement controllers are designed and implemented both in simulation and experiment on a cantilever smart structure. These controllers are shown to successfully change the natural frequency value, while retaining the same damping ratio value. A novel switching system is employed that calculates the optimal switching position by running a simulation of the desired systems in parallel to the controlled open-loop system. Moreover, the system minimises transients occurred by switching back and forth between controllers, thus increasing the efficiency of the system. By comparing the experimental results to a conventional high damping pole-placement controller that applies a similar amount of control effort, a lower overall level of amplitude suppression can be seen.

In this paper we describe a new type of strain sensor which can be embedded in civil structures for structural health monitoring applications. This strain sensor is a passive device that can be embedded in a structure and remotely interrogated using RF signals via an attached antenna. Such a sensor has the advantage of requiring no permanent physical connection between the sensor and the data acquisition system. The sensor is a metal coaxial cavity that can be embedded or bonded to the structure in which strain is to be measured. The design presented here exhibits a dominant mode of electromagnetic resonance for wavelengths two times its cavity length. When the material in which the sensor is embedded is strained, the strain will be reflected in changes in the sensor dimensions, and hence will cause a shift in the resonant frequency of the cavity. The resonant frequency, or shift therein, can be easily obtained by various methods. The acquired resonant frequency is then used to calculate the strain on the structure. The sensor presented in this paper operates at a frequency of approximately 2.4 GHz, and exhibits a shift in resonance of 2.4 kHz per microstrain. The sensor has been embedded in concrete test cylinders and interrogated using external antenna. Experimental results show a strain resolution of better than 1 microstrain with a bandwidth of 30 Hz on this sensor. We will also present results of different interrogation systems including a simple switched detector circuit, a gated detector circuit and also using lock in techniques. This new class of embeddable sensor will have application in monitoring the health of and assessing damage in civil structures.

This paper presents experimental and analytical studies focused on the development of a structural health monitoring system to assess the condition of mechanical fasteners of a thermal protection system. A realistic thermal protection system component, consisting of a carbon-carbon panel bolted through 15 brackets to a backing structure, is utilized. Mechanical states considered include all bolts fastened to a nominal torque value, or one of the 15 bolts loosened. Four transducers on the backing structure provide actuation and sensing signals. Spectral functions are computed from all single and pair-wise signal combinations. Automated analysis of the spectral functions shows frequency intervals exist over which the function values are indicative of the mechanical state of the test article. These frequency intervals are used to provide features for the structural health monitoring classifier. Finite element analyses provide a physics-based understanding of these features. Statistical pattern recognition methods select a subset of the features. The overall localization accuracy of the structural health monitoring system on test data is 99.1% with 99.7% probability of detecting a damaged condition at a 0.2% probability of a false alarm.