Gary P. Gibbs

Radiation modal expansion: Application to active structural acoustic control

This paper demonstrates active structural acoustic control using multiple input/output adaptive sensoriactuators combined with radiation filters and a feedback control paradigm. A new method of reduced order modeling/design of radiation filters termed radiation modal expansion (RME) is presented. For the experiments detailed in this paper, the RME technique reduced the modeling of the radiation matrix from 400 transfer functions to 6 transfer functions (multiplied by a constant transformation matrix). Experimental results demonstrate reductions of radiated sound power on the order of 5 dB over the bandwidth of 0-800 Hz.

Excitation of thin beams using asymmetric piezoelectric actuators

In this paper, an approximate analytical model is developed for the excitation of a thin beam by a single piezoelectric actuator bonded to the surface of the beam. The premise of this work is to investigate the excitation of beams by piezoelectric actuators on a more fundamental level than present work, and then use the asymmetric model to predict a wave response, rather than a modal response, on more complicated structure/actuator systems. It is determined that the single surface mounted piezoelectric actuator simultaneously excites both flexural and extensional motion in beams whose relative amplitudes are functions of beam/actuator geometry and properties. The model is then applied to the excitation of an infinite beam by two colocated arbitrarily driven actuators. It is shown that this configuration can produce any desired combination of flexural and extensional waves in beams by varying the degree of asymmetry between the actuators.

Experimental Feedback Control of Flow Induced Cavity Tones

Discrete-time, linear quadratic methods were used to design feedback controllers for reducing tones generated by flow over a cavity. The dynamics of a synthetic-jet actuator mounted at the leading edge of the cavity as observed by two microphones in the cavity were modeled over a broad frequency range using state space models computed from experimental data. Variations in closed loop performance as a function of model order, control order, control bandwidth, and state estimator design were studied using a cavity in the Probe Calibration Tunnel at NASA Langley Research Center. The controller successfully reduced the levels of multiple cavity tones at the tested flow speeds of Mach 0.275,0.35, and 0.45. In some cases, the closed loop results were limited by excitation of sidebands of the cavity tones, or the creation of new tones at frequencies away from the cavity tones. The models were not able to account for nonlinear dynamics, such as interactions between tones at different frequencies. Nonetheless, the results validate the combination of optimal control and experimentally generated state space models for the cavity tone problem.

Simultaneous active control of flexural and extensional waves in beams

The simultaneous active control of flexural and extensional vibrations in elastic beams is experimentally investigated. The results demonstrate that using pairs of piezoceramic transducers, whose elements are symmetrically located and independently controlled by a multichannel adaptive controller, enables the high attenuation of both flex ural and extensional response. This capability is due to the nature of the piezoceramic ele ment, which when bonded to the surface of the structure and electrically excited, exerts a surface strain on the structure. This strain enables input of both shear forces and moments into the structural system. The results are applicable to many situations where extensional vibrations couple to large flexural vibrations and subsequently radiate signifi cant sound levels.

Experiments on Active Control of Vibrational Power Flow Using Piezoceramic Actuators/Sensors

The active control of flexural power flow in both semi-infinite and finite elastic beams is experimentall y investigated. The experimental results demonstrate that piezoceramic transducers, when used in conjunction with an adaptive least mean squares narrow-band controller, can effectively control flexural power flow at steady-state single frequencies in thin beam systems. The piezoceramic transducers offer distinct size and weight advantages over conventional transducers. The experiments also demonstrate the use of an axial scanning laser vibrometer to determine out-of-plane velocity and power flow.

Active control of turbulent‐boundary‐layer‐induced sound radiation from aircraft style panels

The results of active control of TBL‐induced sound radiation will be presented. In this test, a panel was constructed similar to the sidewall of an aircraft; however, the panel was flat to allow a simple incorporation into the wind tunnel wall. The static in plane stress normally associated with aircraft cabin pressurization (at 40 000 ft) was provided via a tensioning fixture. Thus the smooth ‘‘exterior’’ side was subjected to TBL flow inside the tunnel, and the other (interior) side radiated sound into the model preparation room. Accelerometers were mounted on the interior side of the panel, and microphones were positioned in the model preparation room to monitor sound radiation. Experiments were conducted at both Mach 0.8 and Mach 2.5 flow conditions, and active control was performed on a single bay, and two bays. The results demonstrate reductions in total radiated sound power on the order of 10–15 dB at resonances, and 5–10 dB integrated over the band width of 150–800 Hz. This work represents the ...

Controller Complexity for Active Control of Turbulent Boundary-Layer Noise from Panels

An experimental study of feedback controller complexity vs noise reduction performance for active structural acoustic control of turbulent-boundary-layer (TBL)-induced sound radiation from a panel is described. The reduction of total radiated sound power as a function of the number of actuators, sensors, and controller cost function on a mock aircraft sidewall subjected to TBL excitation are discussed. The results demonstrate total radiated sound power reductions of 15 dB at resonances and 10 dB integrated over 150-1000 Hz for a three-actuator and 15-sensor case. The controller configuration was then simplified to one actuator and four sensors (summed outputs) and was found to produce 10-15-dB reductions in sound power at resonances and 9 dB integrated over the control bandwidth. This result demonstrates the potential for achieving significant reductions in radiated sound power with a relatively simple actuator/sensor topology.

Structural acoustic control of plates with variable boundary conditions: Design methodology

A method for optimizing a structural acoustic control system subject to variations in plate boundary conditions is provided. The assumed modes method is used to build a plate model with varying levels of rotational boundary stiffness to simulate the dynamics of a plate with uncertain edge conditions. A transducer placement scoring process, involving Hankel singular values, is combined with a genetic optimization routine to find spatial locations robust to boundary condition variation. Predicted frequency response characteristics are examined, and theoretically optimized results are discussed in relation to the range of boundary conditions investigated. Modeled results indicate that it is possible to minimize the impact of uncertain boundary conditions in active structural acoustic control by optimizing the placement of transducers with respect to those uncertainties.

Analysis, testing, and control of a reverberant sound field within the fuselage of a business jet

The objective of this project was to develop a speaker-based active control system capable of dissipating reverberant acoustic energy within the fuselage of a typical business jet aircraft. A ground-based fuselage was utilized as the test-bed for the work conducted in this project. Typical aircraft fuselage trim was installed to more accurately reflect the acoustic properties of a flight-ready aircraft. Speaker locations were selected based upon the acoustic modal characteristics of the fuselage and practical considerations concerning available space in the walls of the trim. Preliminary control experiments involving single-input, single-output, feedback loops resulted in a very localized zone of quiet which yielded no global effects. Multi-input, multi-output feedback control with a distributed array of eight pairs of transducers weighted (spatially) to emphasize energy dissipation of select acoustic modes resulted in as much as 6 dB of global attenuation at acoustic resonances corresponding to the secon...

A novel approach to feedforward higher‐harmonic control

A novel adaptive filter structure is proposed for the control of systems characterized by higher harmonic response. The control approach has been designated the higher harmonic least‐mean squares (HLMS) algorithm to differentiate it from the standard multifrequency filtered‐x version of the least‐mean squares approach. In the HLMS algorithm, a single frequency reference at that of the fundamental is all that is required to implement the controller. The remaining harmonics are generated internally based upon simple trigonometric relationships. The filtered‐x LMS algorithm is implemented in parallel for each frequency to be controlled, minimizing or totally eliminating the contribution of the time‐varying terms during the convergence process and increasing the rate of convergence for the higher harmonic control application. Results from the simulation demonstrate that the HLMS approach is far superior to the standard multifrequency, filtered‐x LMS algorithm in adaptive, feedforward, higher‐harmonic control.