Journal of Intelligent Material Systems and Structures

The simple phenomenological relation which relates elastic and dielectric properties in an insulating crystalline dielectric takes the form

A shape memory material actuator is being developed to provide active vibration and shape control for large, adaptive space structures. Shape memory alloys have the ability to generate high recovery stresses (>700 MPa) over large strains (> 6 % ), providing potential payoffs for their use in lightweight actuator designs and adaptive com posite structures. However, the utility of these alloys has been limited in past programs due to lack of adequate material characterization studies to optimize mechanism or composite performance, lack of material property stability during transformational cycling, and the dependence on heat transfer rates for rapid cycling capability. In this study, Cory and McNichols' theory of nonequilibrium thermostatics (NET) was applied to quantify and correlate shape memory material behavior for a binary NiTi alloy. NET was then em ployed as a design tool to develop an actuator, utilizing 24 0.5 mm dia. wires (280 mm long) acting against a biasing spring (with stiffness of 305 N/mm), with nominal opera tional stroke of ± 3 mm and force of ± 1000 N. NET state equation parameters were mea sured after stabilizing the NiTi wire pack properties using isothermal transformational cy cling. Isotonic testing in an oilbath was then used to correlate actuator performance with the NET predictions over the design range of force, length, and temperature. Finally, elec trical resistance heating was employed using closed-loop feedback control to assess actua tor response time and mechanical performance in both ambient and vacuum environments. The design methodology employed in this program should provide a viable approach for optimizing the performance of a shape memory actuator specific to its application.

This paper describes recent advances in structural quieting technology as applied to active truss structures intended for high precision space based optics applica tions. The active structure incorporates piezoelectric active members which exert control forces internal to the structure and thereby improve the structure's dimensional stability. The control architecture involves two layers of feedback control. The first utilizes col located measurements of force and velocity at the active member to achieve active damp ing, the second utilizes noncollocated measurements of acceleration at the location of a simulated optical component to achieve structural stabilization. The local control loops are based on the concept of impedance matching, the global control loops are designed using robust control methods. These two levels of control are intended to operate simulta neously; however, in this paper each approach is applied individually. The combined im plementation is left for future work.

The adaptive truss structure of statically determinate topology has a capa bility to perform the required task and to meet the environment by adapting its geometrical configuration and mechanical properties. This article discusses the configuration and workspace reach of the practical model of the geometry adaptive truss of helical mast. At the preliminary stage, the kinematics of the ideal truss model is described. Then, the prac tical structure is investigated for the adaptive truss permitting the offsets at the truss nodes. The kinematics are formulated in the context of incremental analysis. The procedures to analyze the configuration and workspace reach are described in the context of the in cremental kinematics. Some illustrative case studies are also conducted.

This paper describes modern controllers, which are based on digital real- time filters, powerful adaptation algorithms and high speed signal processor systems. Such controllers are required for active shape and vibration control on large flexible space structures.

This paper describes investigations on adaptive vibration control and shape adjustment of truss structures making use of the active member actuator. The actua tor was manufactured in an attempt to achieve these functions by linear actuating. Funda mental characteristics were examined statically and dynamically. Numerical simulations of planar truss model with the configuration of one pair of collocated actuator and sensor shows that a state observer can estimate truncated modes well and the excited vibration can be controlled rapidly.

In this study, the feasibility of using representative box wing adaptive structures for static aeroelastic control is examined. A deformable typical section is uti lized to derive the optimal and suboptimal relations for induced strain actuated adaptive wings, and the relations developed are used to design representative adaptive lifting sur faces which are assessed in trade studies. The optimal relations developed showed that op timal adaptive airfoil designs are possible for some realistic configurations, and effective sub-optimal designs can be achieved for others. In addition, the important parameters associated with inducing curvature and twist, thereby altering the lifting forces on the air foil, are determined. The most important of which were found to be the airfoil thickness ratio, the actuation strain produced by the induced strain actuators, and the relative stiff ness ratio of the actuator to the wing skin for both camber and twist control. The stiffness coupling parameter and the wing aspect ratio were also found to be important for twist control. The potential benefits of using adaptive airfoils for aeroelastic control, rather than conventional articulated control surfaces, is demonstrated in trade studies. It was found that greater control authority along with a lower weight penalty is achievable using adap tive aeroelastic structures for a variety of wing designs. Thus, strain actuated adaptive wings may be used rather than conventional lifting surfaces to increase performance while reducing weight, decreasing loads in critical areas, improving the radar cross section, and maximizing the lift-to-drag ratio for many flight conditions.

Research efforts at Innovative Dynamics have produced sensors and signal processing software that can be integrated into an advanced Health Monitoring System (HMS) to increase the safety of aging aircraft. HMS is a unique distributed system based on "smart structures" technology. By monitoring the vibration signature of a structure, HMS determines structural abnormalities using a network of sensor arrays and distributed processors. Pattern recognition techniques are utilized to classify the sensor signals and determine the type of defect. The memory of the system is formed through a learning pro cess in which a systematic series of experiments is presented to the system.

Active control of sound radiation from a vibrating simply supported rec tangular plate excited by a steady state harmonic point force disturbance is experimentally studied. A variety of test cases were studied for controlling sound radiation due to a distur bance both on and off structural resonance. Control structural inputs are achieved by three piezoceramic actuators bonded to the surface of the plate. The control approach is based upon a Filtered-X version of the adaptive LMS algorithm. Error information for the con troller was taken from two sensor formats, either microphones placed in the acoustic far- field, or polyvmylidene fluoride (PVDF) piezoelectric distributed sensors bonded to the surface of the structure. The two narrow strip PVDF sensors were positioned on the plate such that the dominant observed response was due to the odd-odd and to a lesser degree, the odd-even modes (i.e., the more efficient acoustic radiators). Results from these experi ments indicate that piezoceramic elements provide an efficient method for modification of structural response to attenuate sound radiation. In many cases, the PVDF sensors were as effective as microphones in providing error information necessary to reduce the far-field sound radiation from the structure. In addition, the adaptive LMS algorithm is shown to be an effective narrowband controller, which in contrast to feedback approaches, requires no exact system modelling.