Rules governing airport noise levels are becoming more restrictive and will soon affect the operation of commercial air traffic. Sound produced by jet engine exhaust, particularly during takeoff, is a major contributor to the community noise problem. The noise spectrum is broadband in character and is produced by turbulent mixing of primary, secondary, and ambient streams of the jet engine exhaust. As a potential approach to controlling the noise levels, piezoelectric bimorph actuators have been tailored to enhance the mixing of a single jet with its quiescent environment. The actuators are located at the edge of the nozzle and protrude into the exhaust stream. Several actuator configurations were considered to target two excitation frequencies, 250 Hz and 900 Hz, closely coupled to the naturally unstable frequencies of the mixing process. The piezoelectric actuators were constructed of 10 mil thick d31 poled wafer PZT-5A material bonded to either 10 or 20 mil thick spring steel substrates. Linear analytical beam models and NASTRAN finite element models were used to predict and assess the dynamic performance of the actuators. Experimental mechanical and electrical performance measurements were used to validate the models. A 3 inch diameter nozzle was fitted with actuators and tested in the Boeing Quiet Air Facility with the jet velocity varied from 50 to 1000 ft/s. Performance was evaluated using near-field and far-field acoustic data, flow visualization, and actuator health data. The overall sound pressure level produced from the 3 inch diameter jet illustrates the effect of both static and active actuators.
Samples of fine grain piezoelectric ceramics (less than or equal to 1 micrometers ) exhibit increased mechanical strength and improved machinability over conventional materials, which should result in actuators which have increased reliability with fewer rejected parts. The focus of the work presented here is to compare the properties of several fine grain and conventional actuators provided by TRS Ceramics. Specimens are constructed of TRS200 (a PZT-5A or DOD Type II equivalent material) and TRS600 (a PZT-5H or DOD Type VI equivalent material). All of the actuators consist of ceramic wafers bonded together with electrodes between them to form a stack. Several actuator overall dimensions and two wafer thicknesses (250 micrometers and 500 micrometers ) are investigated as well as material which has been subjected to hot isopress. The two main figures of merit in the stack actuator comparisons are free strain and blocked stress. Strain and stress loops are measured under a variety of field levels, including negative fields up to the coercive limit (full butterfly loops were not performed). Also compared are values of energy density and hysteresis in the strain, stress and electric displacement vs. field loops. Stack longevity is addressed through duration tests in which stacks are used to drive representative mechanical impedance for an extended period. Results show that fine grain stacks completed 109 continuous actuation cycles with no sign of performance degradation.
A method for estimating the actuation efficiency of a structurally integrated active material is presented. A background literature search revealed many different expressions for efficiency depending upon the application and discipline of interest. Following the review of the literature, an efficiency expression was developed for a piezoelectric actuator in the frequency domain. The actuation efficiency of the piezoceramic actuator was define as the ratio of total mechanical energy imparted to the structure to total electrical energy drawn by the piezoceramic from an electrical power source. The efficiency expression is a function of the piezo electromechanical coupling coefficient, the mechanical impedance ratio of the structural to the piezoceramic material, and the frequency of operation. The developed expression was then used to analytically predict the efficiency of a single degree-of- freedom system actuated by a piezoceramic actuator. Static and quasi-static efficiency results agree with analytical results found in the literature. Dynamic analysis of the efficiency expression, however, produced unexpected and interesting results. For the given definition of efficiency, there exists a combination of material parameters and drive frequency that yield efficiencies greater than one.Further analysis provided evidence that frequency domain formulation provides that while there are instances when relatively large quantities of mechanical energy in the system exist relative to the quantity of electrical energy being drawn by the actuator, total energy is always conserved. The important result of this work was the knowledge gained in the fundamental understanding of power, energy, and efficiency as it relates to dynamic actuation of an electromechanical system. The results shown here support the concept of actuation at or near a system resonance to increase efficiency. This work is on-going; the ultimate goal of which is to develop a tool for aiding active material actuation feasibility studies by using actuation efficiency as a performance metric.
A tunable solid state piezoelectric vibration absorber and an active tuning method were developed and demonstrated. A passive vibration absorber generally acts to minimize structural vibration at a specific frequency associated with either a tonal disturbance or the response of a lightly damped structural vibratos mode. Because this frequency is rarely stationary in real applications, damping is usually added to ensure some level of effectiveness over a range of frequencies. Maximum response reductions, however, are achieved only if the absorber is lightly damped and accurately tuned to the frequency of concern. Thus, an actively-tuned vibration absorber should perform better than a passive one and, furthermore, could be made lighter. In its simplest form, a vibration absorber consists of a spring-mass combination. A key feature of the tunable vibration absorber described herein is the use of piezoelectric ceramic elements as part of the device stiffness. The effective stiffnesses of these elements was adjusted electrically, using a passive capacitive shunt circuit, to tune the resonance frequency of the device. The tuning range of the absorber is thus bounded by its short- circuit and open-circuit resonance frequencies. An alternative tuning approach might employ resistive shunting, but this would introduce undesirable damping. Another feature of the device is the ability to use the piezoelectric elements as sensors. A control scheme was developed to estimate the desired tuning frequency from the sensor signals, to determine the appropriate shunt capacitance, and then to provide it. The shunt circuit itself was implemented in ten discrete steps over the tuning range, using a relay-driven parallel capacitor ladder circuit. Experimental results showed a maximum 20 dB, and a 10 dB average improvement in vibration reduction across the tuning range, as compared to a pure passive absorber tuned to the center frequency, with additional benefit extending beyond the tuning range.
A coupling coefficient is a measure of the effectiveness with which a shape-changing material (or a device employing such a material) converts the energy in an imposed signal to useful mechanical energy. There are different kinds of material and device coupling coefficients, corresponding to different modes of excitation and response. Device coupling coefficients are properties of the device and, although related to the material coupling coefficients, are generally different from them. It is commonly held that a device coupling coefficient cannot be greater than some corresponding coupling coefficient of the active material used in the device. A class of devices was recently identified in which the apparent coupling coefficient can, in principle, approach 1.0, corresponding to perfect electromechanical energy conversion. The key feature of this class of devices is the use of destabilizing mechanical pre- loads to counter inherent stiffness. The approach is illustrated for a piezoelectric bimorph device: theory predicts a smooth increase of the apparent coupling coefficient with pre-load, approaching 1.0 at the buckling load. An experiment verified the trend of increasing coupling with pre-load. This approach provides a way to simultaneously increase both displacement and force, distinguishing it from alternatives such as motion amplification, and may allow transducer designers to achieve substantial performance gains for some actuator and sensor devices.
A shunting method has been developed and experimentally verified for tuning the natural frequency and damping of a piezoceramic inertial actuator (PIA). Without power, a PIA behaves much like a passive vibration absorber (PVA). PVAs typically minimize vibration at a specific frequency often associated with a lightly damped structural mode. Large response reductions, however, may only be achieved if the PVA is accurately tuned to the frequency of concern. Thus, an important feature of a PVA is the ability to be accurately tuned to the possibly varying frequency of a target vibration mode. Tuning an absorber requires a change in either the mass or stiffness of the device. The electromechanical properties of the piezoceramic forcing element within a PIA in conjunction with an external passive electrical shunt circuit can be used to alter the natural frequency and damping of the device. An analytical model of a PIA was created to predict changes in natural frequency and damping due to passive electrical shunting. Capacitive shunting alters the natural frequency of the actuator only, while resistive shunting alters both the natural frequency and damping of the actuator. Experiments using both passive capacitive and passive resistive shunt circuits verified the ability to predictably shift the natural frequencies of the piezoceramic inertial actuator by more than 5%.
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.