The NASA Langley Research Center and the Massachusetts Institute of Technology have been working together to advance the state of the art in apply piezoelectric actuators to aeroelastic systems. This paper describes an experimental and analytical investigation into using piezoelectric actuators for tailoring the aeroelastic response of a five-foot span wind-tunnel model. Improvements in the flutter boundary were demonstrated as well as significant reductions in model response at dynamic pressures below flutter.
An inertial actuator, also known as a proof mass actuator (PMA), applies structural forces by reacting against an inertial mass. This paper introduces a class of recently developed piezoceramic PMA and its application to reduce vibration and structure-borne noise. The design incorporates displacement amplification to efficiently achieve low resonant frequency. A method is presented for assessing the efficiency of a piezoceramic PMA and comparing the power density of competing PMA technologies (ie, voice-coil vs. piezoceramic vs. magnetostrictive). The performance of the PMA is demonstrated by measuring the force generated against an infinite impedance and measurements on a structure representative of a turbo-prop fuselage. The experimental testing demonstrates the validity of a simple vibration absorber model in understanding PMA performance on complex structures.
The potential of using volume and shape changes of polymers, gels, and composites as a source of actuating power was evaluated. For an ordinary polymer, volume changes may be induced by simple heating and cooling (thermal expansion) or by allowing the polymer to undergo physical transitions such as glass transition, melting or crystallization, and solid state phase transformation. Shape changes in shape memory polymers may be obtained by a combination of thermal and deformation treatments. Polymer gels may undergo large continuous or discrete volume changes in response to stimuli such as temperature, solvent concentration, pH value, electric field, and light. A composite/bimetal laminate, when configured in a proper stacking sequence, is capable of generating out-of-plane forces or deflections. When constrained from changing the volume or shape, each class of these materials can exert a great counteracting stress or pressure on the constraining body. when free to expand or contract, the material may be prescribed to undergo a large change in volume or shape. The magnitudes of the counteracting stresses and the percentages of achievable volume or shape changes were calculated for several commonly used gels, piezopolymers and composites/bimetals. Methods and procedures for making these calculations were established.
New developments in smart structures and materials have made it possible to revisit earlier work in adaptive and flexible wing technology, and remove some of the limitations for technology transition to next-generation aircraft. Research performed by Northrop Grumman, under internal funding, has led to a new program sponsored by ARPA to investigate the application of smart structures and materials technologies to twist and adapt and aircraft wing. Conceptual designs are presented based on state-of-the-art materials, including shape memory alloys, piezoelectrics, and fiber optic sensors for incorporation in a proposed smart wing design. Plans are described to demonstrate proof-of-concept on a prototype 1/10 scale -18 model that will be tested in a wind tunnel for final validation. Highlights of the proposed program are summarized with respect to program objectives, requirements, key concept design features, demonstration testing, and smart wing technology payoffs and risks.
Theory and experiments to control the static shape of flexible structures by employing internal translational actuators are summarized and plants to extend the work to adaptive wings are presented. Significant reductions in the shock-induced drag are achievable during transonic- cruise by small adaptive modifications to the wing cross-sectional profile. Actuators are employed as truss elements of active ribs to deform the wing cross section. An adaptive-rib model was constructed, and experiments validated the shape-control theory. Plans for future development under an ARPA/AFWAL contract include payoff assessments of the method on an actual aircraft, the development of inchworm TERFENOL-D actuators, and the development of a method to optimize the wing cross-sectional shapes by direct-drag measurements.
Research has shown that at certain frequencies the returned signal from a corner reflector can be canceled by causing the orthogonal surfaces to diverge by 2 to 3 degrees. This paper reports on how this can be achieved using fiber reinforced composite materials and integrated piezoelectric actuators. A range of different bond configurations are examined for use with raw PZT actuators and resin encapsulated actuators. Based on the information gained from this first part of the investigation a 100x100-mm active plate was constructed using 16 actuator elements bonded to a GRP substrate. A total angle of deflection of 0.5 degrees was created through the application of 100 v DC. Extensive research is currently underway to develop and enhanced vision system (EVS) for use on civil airliners to assist the pilots when making landings in very poor visibility. This is likely to be based on either an active or passive millimetric radar system operating at 35 or 94 GHz. If an active EVS is developed, then corner reflectors such as those described here could be used to identify the operational runway and when not in use be switched off. The results presented in this report provide a basic insight into how this can be achieved; however, further work will be required before an operational system could be produced.
the electrical behavior of conducting carbon whisker reinforced thermoplastic elastomer (TPE) composites was investigated. The carbon whiskers were produced by a catalytic chemical vapor deposition (CCVC) process. The electrical properties of the composite were characterized as a function of temperature and deformation. The electrical resistivity of the composite can be varied by uniaxial deformation and by temperature. The temperature-resistivity studies indicated that the resistivity of these composites were influenced by the glass transition temperature of the TPE. the resistivity v. 1/T curves exhibited two distinctive negative slopes that intersected at the Tg of the elastomer. This as correlated to the Tg of the rigid segments in the TPE as obtained by the dynamic mechanical thermal analysis. Further, uniaxial deformation studies at room temperature demonstrated that the resistivity increased exponentially with the deformation. Mechanical and electrical properties of the composites indicated that CCVD carbon whiskers can be used to improve the strength and electrical conductivity of TPEs. The change is resistivity (up to 5 orders of magnitude) of the composites with respect to deformation and/or temperature can find use in electromechanical and electrothermal applications.
This paper presents some findings from work in progress to investigate the efficacy of using active materials to control mechanical vibrations. The device under study is designed to be useful for a range of precision positioning, force cancellation, and vibration isolation tasks typically associated with spacecraft environments. It incorporates shape-memory-alloy springs to provide an adaptive-passive isolator stage, and multilayer electrostrictive ceramic actuators for precise positioning and canceling transmitted forces across a support interface. Device performance in each of these three modes is explored by simulations incorporating characterized properties of the individual active material components.
Future spacecraft systems will require advanced positioning systems to meet stringent reliability, vibration, lightweighting, and cost requirements. Current devices employing stepping motor and gear reduction assemblies may not be able to meet future design needs. A shape memory alloy (SMA) actuated multiaxis gimbal has been developed that provides solutions to these mechanism issues. SMAs utilize a thermally activated reversible phase transformation to recover their original heat-treated shape or to generate high-recovery stresses. when heated above a critical transformation temperature. NiTiCu alloy wires have been wound into helical spring actuators to control gimbal rotation using mechanical elements to convert the linear motion of antagonistic SMA springs into rotation. Analytical models that incorporate the nonlinear hysteretic behavior of SMAs have been generated to aid in spring design and SMA conditioning. Indirect resistive hearing of SMA springs was accomplished using programmable power supplies. A potentiometer sensor attached to the output axis of the gimbal was used to provide angular feedback to a digital controller. An antagonistic approach was used to independently control heating and cooling of the opposing spring element for improved stability and bandwidth response. Proportional-integral derivative control was implemented on the active SMA spring to obtain the desired level of rotation while overcoming an external load. Mechanical testing was conducted on the gimbal to assess control system stability, dynamic response, and power requirements. Torque in excess of 3 in./lb was generated using less than 20 watts of applied power.
In a recently awarded ARPA program to advance the state of the art of parallel actuated next-generation machine tools, a vertically integrated team led by Martin Marietta is applying recent advanced in electroceramic smart materials and advanced composites to achieve leapfrog advanced in precision, flexibility, and speed of machine tools. Specific approached to achieve these advanced include active vibration cancellation, improved control technology, and design optimization using advanced structural and dynamic models. In this program, the team will integrate large high-force actuators, composites, and active vibration control with the Ingersoll Milling Machine Company's innovative Octahedral Hexapod machine to develop to Advanced Reconfiguration Machine for Flexible Fabrication. The enhanced Octahedral Hexapod machine will provide new levels of machining flexibility while still retaining precision and low cost. This technology will have widespread impact on the flexible fabrication of materials--especially those that are tough to machine traditionally--in several industries, e.g., aerospace, defense, aircraft, and automotive.
Sandia National Laboratories performs R&D in structural dynamics and vibration suppression of precision applications in weapon systems, space, underwater, transportation and civil structures. Over the last decade these efforts have expanded into the areas of active vibration control and 'smart' structures and material systems, In addition, major resources have been focused towards technology to support weapon product development and agile manufacturing capability for defense and industrial applications. This paper will briefly describe the structural dynamics modeling and verification process that supports vibration control and some specific applications of these techniques to manufacturing in the areas of lithography, machine tools and flexible robotics.
A jib crane consists of a pendulum-like end line attached to a rotatable jib. Within this general category of cranes there exist devices with multiple degrees of freedom including variable load-line length and variable jib length. These cranes are commonly used for construction and transportation applications. Point-to-point payload maneuvers using jib cranes are performed so as not to excite the spherical pendulum modes of their cable and payload assemblies. Typically, these pendulum modes, although time-varying, exhibit low frequencies. The resulting maneuvers are therefore performed slowly, contributing to high construction and transportation costs. The crane considered here consists of a spherical pendulum attached to a rigid jib. The other end of the jib is attached to a direct drive motor of generating rotational motion. A general approach is presented for determining the open-loop trajectories for the jib rotation for accomplishing fixed-time, point-to-point, residual oscillation free, symmetric maneuvers. These residual oscillation free trajectories purposely excite the pendulum modes in such a way that at the end of the maneuver the oscillatory degrees of freedom are quiescent. Simulation results are presented with experimental verification.
The buckling of compressively-loaded members is one of the most important factors limiting the overall strength and stability of many structures. This paper presents experimental results showing that active control can be used to stabilize compressive members against buckling, allowing them to be loaded well in excess of their critical buckling load. Experiments conducted using a composite steel/piezo-ceramic column achieved a factor of 5.6 increase in load-bearing capability through active stabilization of the first two uniaxial buckling modes. In addition, a small-scale railroad-style truss bridge was constructed to demonstrate the multiple actively stabilized compressive members may be incorporated into a compound structure. This paper presents an overview of the experimental results, suggests design criteria for actively stabilized members, and discusses potential industrial applications.
NRL's Mechanics of Materials Branch has developed a technology that facilitates sensor selection and placement within a composite structure. The Embedded Sensors for Smart Structures Simulator (ES4) is a tool that relates the output of a finite number of sensors to strain induced structural and material damage. This tool is based on the use of the dissipative part of the bulk nonlinear material behavior. The methodology used to identify this behavior will be briefly described in the present paper. This paper describes the role of strain measurements and their relation to sensor type and location, the conceptual framework of dissipated energy density as the metric employed for assessing material/structural performance. Emphasis is given on the utilization of dissipated energy density for estimating the error between the health of the structure as 'seen' by the sensors and the actual health of the structure. Useful applications of this difference are sensor placement optimization in the case of the design phase and confidence level measure for the case of an on board simulating capability.
Quantitative data on the stress-strain behavior of walls and roofs of underground mines are unavailable. It is established practice to use roof bolts to support roofs and ribs in mines. The U.S. Bureau of Mines and Strain Monitor Systems, Inc. are jointly developing smart materials and passive or active detection systems capable of monitoring the stress/strain history of mine roofs with the goal of detecting structural damage. The systems can be designed for a passive mode to monitor peak strain or an active mode for a real-time alarm response, the smart materials to be used in this application are TRIP (TRansformation Induced Plasticity) steels. TRIP steels are materials which change state from austenitic, nonmagnetic to a martensitic, ferromagnetic phase as the material undergoes straining. There is a direct correspondence of the peak strain level experienced in the material with the percentage of ferromagnetism, hence monitoring the relative amount of the ferromagnetic content will indicate the level of strain (and therefore stress). The phase transition that accompanies the straining is irreversible so the monitor material indicates the peak strain until that value is subsequently exceeded. Materials research discussed will cover the selection of compositions with a suitable ferromagnetic response and the development of thermomechanical treatments to achieve high tensile strength required for this application. Tensile properties and related phase changes are described with preliminary system designs and proposed installation techniques.
This paper addressed the feasibility of using an active piezoelectric buffet suppression system to reduce buffet vibrations in vertical tail aircraft. During the assessment, functional requirements were defined, models were developed, and full-scale piezoelectric buffet suppression systems were designed and evaluated. A variety of actuator distributions, sensor locations and controller architectures were examined and it was found that significant performance improvements could be achieved (greater than 70 percent) with minimal weight penalties (less than 8 percent). This work enabled the evaluation of issues such as system performance versus added weight and piezoelectric actuator control authority and power requirements. The study showed that the added performance benefit (in terms of vibration reduction and fatigue life) are far greater than the weight penalty, and that piezoelectric actuators have the control authority required to suppress high energy buffet forces within aircraft geometry, weight, and power constraints. Further, the high performance achieved (much greater than that defined in the functional requirements) suggests that systems can be designed with a much lower weight penalty (1/2 to 1/4) than that assumed in this study.
The Synthesis and Processing of Intelligent Cost Effective Structures (SPICES) program is comprised of a consortium of industrial, academic and government labs to develop cost effective material processing and synthesis technologies to enable new products using active vibration suppression and control devices to be brought to market. Each team member possesses a specialty in the area of smart structures which has been focused towards the development of several actively controlled smart material systems. Since smart structures involve the integration of multiple engineering disciplines, it is the objective of the consortium to establish cost effective design processes between this multiorganizational team for future incorporation of this new technology into each members respective product lines. To accomplish this task, the disciplines of materials, manufacturing, analytical modeling, actuation, sensing, signal processing, and control had to be synthesized into a unified approach between all ten consortium members. The process developed for intelligent structural systems can truly be classified as multiorganization/multidiciplined Integrated Product Development. This process is described in detail as it applies to the SPICES development articles and smart material fabrication in general.
The displacement performance of individual actuators is well documented. In a multiple actuator composite plate, the individual addressing of actuators, or small groupings of actuators, will result in a global displacement profile which may or may not be similar to the displacement predicted based on the performance of a single actuator. The purpose of this computational study is to determine the displacement performance of a composite plate which includes multiple ceramic actuators. Stresses and displacements resulting from the application to individual actuators of a electrical field of 0.2 MV/m are determined. Differences in displacement performance based on base plate material, support conditions and location in the array are determined.
Recently it has been observed that it is theoretically possible to place long fiber optic displacement sensors on structures that are generally loaded while still maintaining the ability to exactly discriminate signals of interest. The chief advantage of such finite-length fiber optic sensors is that the sensitivity of the measurement scales not only with the maximum strain along the path but the length of the sensor as well. One consequence of this scaling property is that 'smart' structures incorporating such a measurement approach could be sensitive yet still have low maximum strains. In other words, the sensitivity of the measurement is partially de-coupled from the stiffness of the structure. This theoretical result is true for simple prismatic structures composed of a linear elastic homogeneous material with arbitrary end loading, and with perfectly positioned displacement sensors. Two model tail rotor torque tubes have been constructed and verify the essential elements of the analysis. However, the use of long displacement sensors effectively integrates signals of interest along the measurement paths and are thus susceptible to accumulated manufacturing errors. Deviations from this pristine state to more practical structures are further investigated both experimentally and analytically. Minimizing the sensitivity to certain manufacturing errors by careful design of the structure is also discussed.
Embedded sensor technology in polymer-based structural composites has received much attention in recent years. Embedded sensors are critical to structural health monitoring in high performance applications, e.g. 'smart' skins and structures. To date, optical fiber sensors have been the principal sensing technique for these applications. However, some notable shortcomings of these sensors include difficulties with ingress and egress from part, and interdependence of strain and temperature measurements. This paper discusses an alternate approach for structural health monitoring: remote query of silicon microsensors embedded in composite structures. The technologies involved will first be reviewed, followed by a listing of the potential benefits of applying this technology. Next, a newly initiated program to demonstrate the feasibility of remote query techniques for structural monitoring of composites will be described.
In this paper we focus on topics related to the control system design to the control system design for the SPICES demonstrations: (1) The hierarchical control system, its motivation and design; (2) SRI's SWAPS (neutralized feedforward) design methodology, its necessity in the SPICES high frequency demonstration, and its properties; and (3) simulated performance of preliminary designs of the vibration control loop for the demonstrations.
The development of control technology specifically for smart materials has lagged substantially behind that of the base materials, transducers and embedding techniques. Still, development of materials with ever- greater numbers of embedded elements continues, spurred by potential uses that require large arrays of sensors and actuators. No control technology suitable for such large arrays exists, however, and this presents a barrier to future applications. In this paper we report on work aimed at developing and demonstrating technology capable of controlling hundreds or thousands of sensors and actuators embedded in the base material. We have dubbed this the 'KIKO control problem' (Kilo- Input/Kilo-Output) for smart materials. This paper focuses on a new multiscale/multirate theory of hierarchical design based on the wavelet transform. In the context of this theory, we develop efficient and highly scalable implementations of control systems using multiprocessor architectures. The paper covers: a description of our multiscale control approach, simulation results on an Euler-Bernoulli beam, and open issues.
Smart structures research and development, with the ultimate aim of rapid commercial and military production of these structures, are at the forefront of the Synthesis and Processing of Intelligent Cost-Effective Structures (SPICES) program. As part of this ARPA-sponsored program, MDA-E is using fiber placement processes to manufacture integrated smart structure systems. These systems comprise advanced composite structures with embedded fiber optic sensors, shape memory alloys, piezoelectric actuators, and miniature accelerometers. Cost-effective approaches and solutions to smart material synthesis in the fiber-placement process, based upon integrated product development, are discusses herein.
A manufacturing capability has been established for 1-3 PZT-polymer composite materials and transducers. Uniform arrays of identical PZT rods are formed by a cost-effective ceramic injection molding process. Sintered and poled 1-3 ceramic preforms, containing 361 PZT rods 1.1 mm diameter on a 50 mm square base plate, are arranged to produce 15 or 30 PZT volume percent composite materials with a hard or soft polymeric matrix. More than 2000 identical PZT preforms were produced and more than thirty 250 mm square SonoPanel transducers have been manufactured. The transducers have been found to be well suited for a variety of underwater acoustic applications. Fifteen SonoPanels have been incorporated into a 3 X 5 array as part of a Navy system demonstration.
AVX is the largest US manufacturer of multilayer ceramic capacitors, producing 10's of millions per day. Multilayer ceramic actuators are manufactured using virtually identical fabrication methods. Fabrication from this ceramic tape allows tremendous latitude in device shape, size and material choice. This paper will discuss several different actuator configurations-including stacks, plates and chips- with respect to performance and cost tradeoffs. Virtually all developing smart material applications are 'technology driven,' however the widespread availability of devices at commercial scale relies on 'market pull' to achieve a balance of high annualized volumes and low cost. Given sufficient demand, devices can be produced such that the raw materials themselves dominate the unit cost. Generalized price-volume-performance relationships for the different actuator configurations can both guide system designers and focus long-term component development efforts.
Presented are a number of novel designs for large-motion shape-memory alloy (SMA) actuators that may minimize the performance constraints associated with such actuators. Shape-memory alloy (SMA) actuators suffer from two performance constraints, namely small strains (epsilon) , such that (epsilon) <EQ 6%, and heat transfer problems in computer- controlled ohmic heating of contractile SMA fiber bundles to induce contraction and/or expansion type linear actuators. Intelligent material systems and structures have become important in recent years due to some potential engineering applications. Accordingly, based on such materials, structures and their integration with appropriate sensors and actuators, novel applications, useful for a large number of engineering applications have emerged. In the present paper a number of conceptual designs and their respective mathematical models are presented for large motion SMA actuators. The dynamic modeling is preceded by a model of a small motion SMA linear actuator. This model considers the dynamic response of contractile fiber bundles embedded in or around elastic springs that are either linear helical compression springs or hyperelastic springs such as rubber-like materials. The proposed theory presents a description of such processes for resilient shape-memory alloy fiber bundles. We consider the fiber bundle of SMA to be either in a serial configuration with a linear tension spring or a parallel configuration circumscribed inside a helical compression spring with flat heads or in parallel with a number of helical compression spring, end-capped by two parallel circular plates with embedded electrodes to which the ends of the SMA fibers are secured. Thus, the fibers can be electrically heated and subsequently contracted to either expand the tension spring or compress the helical compression spring back and forth. Design details are first described. In essence the dynamic behavior of the actuator depends on the frictional interference effects as well as the interaction between the current supplied to the wires and the heat transfer from the wires. Further, a mathematical model is presented to simulate the electro-thermomechanics of motion of such actuators. The proposed model takes into account all pertinent variables such as the strain (epsilon) , the temperature of the fibers T(t) as a function of time t, the ambient temperature T0, the martensite fraction (xi) , the helical compression spring constant k, the frictional effect and the coefficient of friction (mu) and the overall heat transfer coefficient h. Numerical simulations are then carried out and the results are compared with experimental observations of a number of fabricated systems.
Widespread use of smart structures technology in industrial and commercial applications requires significant reductions in systems cost and complexity. For knowledge and control of the shape and vibration of structures, one would ideally employ a number of discrete strain sensors placed at critical locations. One significant advantage of optical fiber sensors is that they can be multiplexed along a single fiber and be individually addressed, greatly simplifying the ingress-egress- connection problems. Multielement strain sensor arrays have been fabricated during the drawing of an optical fiber from a preform at a sustained rate of approximately equals 2,000 per hour. These arrays have been surface- mounted and embedded in composites, and distributed static and vibrational strain measurements have been made. This paper will report recent developments and improvements in this low-cost technique for fabricating and applying strain sensors for smart structures applications.
The recent progress in the performance and reliability of the fiber optic-based extrinsic Fabry-Perot interferometric (EFPI) strain sensor is reported. The developments include refined fabrication techniques and improved quality of constituent elements for enhanced durability and greater operating temperature range, higher strain sensitivity using high-finesse cavities, modified sensor-head for complete strain-field characterization, absolute, real-time and inexpensive measurements employing white light interferometry, and multipoint, distributed sensing using CDMA and path-matching multiplexing techniques. It is shown that these improvements have assisted in overcoming the limitations of the conventional EFPI sensor and made possible the large- scale commercialization of the state-of-the-art EFPI-based strain sensing system.
A method to detect linear flaws in piezoelectric actuators is demonstrated. Piezoelectrically induced vibrations are excited in the actuator at a large number of closely spaced frequencies. At frequencies of mechanical resonance there are steep impedance peaks and a peak pattern is generated that is characteristic of a particular actuator configuration. It is shown that when the amplitudes of such peaks are abnormally low, i.e. 50% or less, or when there are peaks > 2kHz removed from 'normal' peak locations, significant internal flaws are present.