Statistics-based characterizations of acoustic propagation, namely, fading and coherence, are being developed as
functions of urban terrain zones. The fading and coherence curves are characterized for each of several urban terrain
zones of interest, and the resulting curves are parameterized as a function of frequency and distance from the source.
With the parameters for signal fading and coherence as a function of frequency, distance to source, and urban terrain
zone type, the decision support tool SPEBE (Sensor Performance Evaluator for Battle-space Environments) is extended
to urban areas. Combined with a separate effort characterizing background noise levels as functions also of
urban terrain zones, a tool for predicting probability of detection for various sources in urban areas is demonstrated.
Future US Army ground sensors in urban terrain will process acoustic signals to detect, classify, and locate sources of
interest. Optimal processing will require understanding of the effects of the urban infrastructure on sound propagation.
These include multi-path phenomena that must be accounted for in sensor placement and performance algorithms. This
work applies Fourier analysis to urban acoustic wave-field data from three-dimensional high-performance computations
to generate statistical measures of signal fading caused by scattering. The work calculates these measures from ratios of
Fourier transforms of wave-field signals with and without scattering to isolate the structure-induced scattering.
Piezohydraulic actuation is the use of fluid to rectify the motion of a piezoelectric actuator for the purpose of overcoming the small stroke limitations of the material. In this work we study a closed piezohydraulic circuit that utilizes active valves to rectify the motion of a hydraulic end affector. A linear, lumped parameter model of the system is developed and correlated with experiments. Results demonstrate that the model accurately predicts the filtering of the piezoelectric motion caused by hydraulic compliance. Accurate results are also obtained for predicting the unidirectional motion of the cylinder when the active valves are phased with respect to the piezoelectric actuator. A time delay associated with the mechanical response of the valves is incorporated into the model to reflect the finite time required to open or close the valves. This time delay is found to be the primary limiting factor in achieving higher speed and greater power from the piezohydraulic unit. Experiments on the piezohydraulic unit demonstrate that blocked forces on the order of 100 N and unloaded velocities of 180micrometers /sec are achieved.
Design and analysis of a scalable piezohydraulic actuation system is presented. Efficiency analysis of frequency rectification demonstrates that hydraulic actuation transfers the maximum amount of work from the actuator to the load. The ratio of peak electrical power to average power delivered caries from 8 percent to 25 percent depending on the piezoelectric coupling coefficient, highlighting the need for efficient power electronics to minimize heat dissipation in the system and minimize volume. A lumped parameter system model demonstrates that fluid compliance is the limiting facto in the stiffness of a bidirectional actuator that does not require hydraulic accumulators or four-way valves. A benchtop experiment consisting of a piezoelectric shock actuator, pumping chamber, and a linear hydraulic cylinder is developed and tested to determine the effect of friction on the micron- level motion of the actuator. The effects of friction are minimized by applying a pneumatic precharge to the system and driving the actuator at its maximum voltage level. Friction is not deemed a limiting factor to the development of a piezohydraulic system with stroke outputs on the order of 100 micrometers per cycle.
Future launch vehicle payload fairings will be manufactured form advanced lightweight composite materials. The loss of distributed mass causes a significant increase in the internal acoustic environment, causing a severe threat to the payload. Using piezoelectric actuators to control the fairing vibration and the internal acoustic environment has been proposed. To help determine the acoustic control authority of piezoelectric actuators mounted on a rocket fairing, the internal acoustic response created by the actuators needs to be determined. In this work, the internal acoustic response of a closed simply-supported (SS) cylinder actuated by piezoelectric (PZT) actuators is determined using a n impedance model for the actuator and boundary element analysis. The experimentally validated model is used to extrapolate results for a SS cylinder that emulates a Minotaur payload fairing. The internal cylinder acoustic levels are investigated for PZT actuation between 35 and 400 Hz. Significant reductions in the structural response due to increased damping do not equate to similar reductions in the acoustic SPLs for the cylinder. The sound levels at the acoustic resonant frequencies are essentially unaffected by the significant increase in structural damping while the acoustic level sat the structural resonant frequencies are mildly reduced. The interior acoustic response of the cylinder is dominated by the acoustic modes and therefore significant reductions in the overall interior acoustic levels will not be achieved if only the structural resonances are controlled. As the actuation frequency is reduced, the number of actuators required to generate acoustic levels commensurate to that found in the fairing increases to impractical values. Below approximately 100 Hz, the current demands reach levels that are extremely difficult to achieve with a practical system. The results of this work imply that PZT actuators do not have the authority to control the payload fairing internal acoustics below approximately 100 Hz.
This paper presents the recent research on impedance-based structural health monitoring technique at Center for Intelligent Material Systems and Structures. The basic principle behind this technique is to use high frequency structural excitation (typically greater than 30 kHz) through the surface-bonded piezoelectric sensor/actuator to detect changes in structural point impedance due to the presence of damage. Two examples are presented in this paper to explore its effectiveness to the practical field applications. First, the possibility of implementing the impedance-based health monitoring technique to detect damage on massive, dense structures was investigated. The test structure considered is a massive, circular, three-inch thick steel steam header pipe. Practical issues such as effects of external boundary condition changes and the extent of damage that could be detected were the issues to be identified. By the consistent repetition of tests, it has been determined that this impedance-based technique is able to detect a very small size of hole (4 X 20 mm), which can be considered the mass loss of 0.002% of entire structure. The second example includes the implementation of this technique in the high temperature applications. With high temperature piezoceramic materials, which have a Curie temperature higher than 2000 degrees F, experiments were performed to detect damage on the bolted joint structure in the temperature range of 900 - 1100 degrees F. Through the experimental investigations, the applicability of this impedance-based health monitoring technique to monitor such an extreme application was verified, with some practical issues need to be resolved. Data collected from the tests proved beyond a doubt the capability of this technology to detect both existing and imminent damage.
A high frequency NDE technique has been under investigation at the Center for Intelligent Material Systems and Structures. Physical changes in the structure cause changes in the mechanical impedance. Due to the electromechanical coupling in piezoelectric materials, this change in structural mechanical impedance cause a change in the electrical impedance of the piezoelectric sensor. Hence, by monitoring the electrical impedance and comparing this to a baseline impedance measurement, we can determine when structural damage has either occurred or is imminent. However, there are still basic research issues that need thorough investigation before full-scale development and commercialization can take place. Included in these is the effect of temperature on this impedance based NDE technique. Since piezoelectric materials exhibit strong temperature dependency and change in temperature results in marked changes in the structural dynamic responses, any variation that is associated with a change in temperature may be confused as damage. In this paper we analyze temperature effects on the electrical impedance of piezoelectric materials and the structures. We have used an empirical approach due to the complexity of the thermo-electrical- mechanical constitutive models for piezoelectric materials. Through the experimental investigations, it was found that a change in temperature modifies both the magnitude and phase of the electrical impedance of the piezoelectric sensors. A computer algorithm was developed which incorporates temperature compensation into our health monitoring applications. This compensation technique minimizes the effect of temperatures on the electrical impedance of piezoelectric sensors bonded on the structure, in the range from 80 to 160 degrees Fahrenheit. In this paper, we show how it is applied successfully to a bolted pipe structure.
The primary purpose of this paper is to present a conceptual design for an adaptive materials driven actuator, to be used for rotorcraft retreating blade stall control. The displacement of a single induced strain actuator is hydraulically amplified and the resulting output displacement is used to actuate a split-flap. Such an actuator is designed to produce high displacements through high amplification factors. The actuator design procedure is presented for a sample case. The required actuator force, displacement and amplification are computed. The effects of the flap deployment on the performance of the helicopter are not presented.
This paper presents the work we have done on a health monitoring technique called active non-destructive evaluation (ANDE) for detecting delaminations in full-scale C-channel structural elements of glass-fiber reinforced polymer (GFRP) composites using active ferromagnetic tagging. Conventional non-destructive evaluation methods are not very effective in monitoring the material conditions of GFRP composite and adhesive joints. A technology that has been proposed to enhance inspection of such non-conductive and non-magnetic GFRP composites is the particle tagging technique. This technique, previously demonstrated on small scale laboratory samples is being developed for full-scale C-channel composite elements. This technique relies on comparing changes in local-area mechanical properties of the structure to identify delaminations. Unlike conventional passive tagging NDE inspection, our technique uses an electromagnetic exciter to interrogate tagged composites over a broad frequency range. As the vibration excitation approaches the resonant frequency of the area of the disbondment, the response amplitude obtained for a given force input increases. Therefore, at frequencies around the transverse resonance of the layer above a defect, the response for a given force will be greater than the response in flawless regions of the structure. Thus, the ANDE test may be based on response measurements alone. The technique is most sensitive if excitation is applied at the plate resonant frequency. Since this frequency is dependent on defect size and depth, a broad band of frequencies must be covered. A laser Doppler vibrometer is a high-sensitivity, high-speed and non-contacting instrument used for detecting surface vibrations. The frequency response curves are obtained using a fast Fourier transform signal processor. An algorithm based on training a neural network to detect significant differences between healthy and damaged structures is then applied to recognize delaminations in full scale structural elements. This method provides the fundamental technology needed for developing a commercial system to monitor the integrity of composite structures, both during manufacturing and during their lifetime as structural elements.
Piezoelectric actuators are being used in increasingly complex structures. An actuator that can be removed and used again would be beneficial in testing actuator placement before the permanent actuator would be attached. Furthermore, an actuator that has similar response characteristics to the permanent actuator would be beneficial in estimating the response characteristics of the actuator before it is attached. A concept for such a removable, reusable actuator has been developed, constructed, and used. This paper describes the differences in authority among three removable, reusable actuators as compared to a permanent actuator. The permanent actuator is bonded to the host structure with only strain gauge cement. This paper also quantifies the changes in authority of the three removable, reusable actuators as they are removed several times from the host structure. When comparing removable and permanent actuators, the stiffer bonding technique typically had greater actuation authority. When comparing authority reduction of removable actuators over ten applications of the actuator, greater reduction occurred with actuators that incorporated a stiffener.
The design and analysis of finite length multiple layered induced strain actuators to further extend the application of piezoelectric actuators in active structural control is investigated. A model of an arbitrary surface bonded multiple layered actuator is utilized to predict the applied force and moment of the i<SUP>th</SUP> piezoelectric layer on a beam. The equations of motion for the transverse vibration of a simply supported beam are derived using Timoshenko beam theory and cast in state space form. The forced response of the one dimensional actuator/substructure system to the piezoelectric induced loads is obtained using an assumed mode technique. The results of experiments performed on a simply supported beam, using various multiple layer piezoelectric actuator configurations for excitation, are compared to analytical predictions for model verification.