Steven F. Griffin

Active Structural-Acoustic Control of a Rocket Fairing Using Proof-Mass Actuators

The feasibility of using proof-mass actuators to control noise transmission actively into a small rocket fairing, given practical constraints on actuator power and mass, is explored. The modal-interaction approach was used to develop a fully coupled structural-acoustic state-space model that relates the out-of-plane structural modal velocities to the spatially varying pressure response in the cavity. The dynamics of the proof-mass actuators were included in the structural-acoustic model. The modal-interaction approach also allowed the decomposition of the acoustic response into radiation modes, which proved essential for determining the optimal locations for sensors and actuators. Numerical simulations using linear quadratic Gaussian controllers with collocated proof-mass actuators and displacementsensorsdemonstrated approximately 4.2 dB of attenuation overthe300-Hz bandwidth forthegivenactuatorconstraints.However,thiswasonly slightly morethantheattenuationprovidedby thepassive effects of the proof-mass actuators, which was approximately 3.5 dB.

Active vibration control of large optical space structures

The paper describes techniques for designing and implementing active damping systems for large optical support structures. Each step in the design process is illustrated with results from the Advanced Composites with Embedded Sensors and Actuators (ACESA) vibration control system. The system is installed on a space based laser structural simulator at the Air Force Research Laboratorys Advanced Space Structure Research Experiments facility. The ACESA system consists of three large tubular active members; embedded bad zirconate-titanate (PZTs) wafers in each strut, which allows control of deformation axially and in two bending planes; 400 V drive electronics for each active member; and a nine-channel, digitally programmable, analogue control electronics unit. Design begins with the determination of the critical modes through a gain factor analysis. Actuators are located through modal strain energy analysis. Using piezostructural analysis methods, sensing and actuation functions are included in the open- and closed-loop dynamic simulations. The simulation includes local strain feedthrough effects through a static correction. Damping is applied to all modes in the frequency range up to 100 Hz, with fundamental modes achieving 20% damping, two orders of magnitude greater than the intrinsic damping level. The actuator PZTs used for active damping were also experimentally shunted with resistive elements in an attempt to introduce passive damping, although the effect was barely measurable. This demonstrates that active damping gives three orders of magnitude better performance than a passive resistive shunt in a controlled comparison.

System for enhancing the sound of an acoustic instrument

A system is disclosed that provides sound control for an acoustic musical instrument. Typical to all acoustic instruments, the instruments have a structure or housing that defines a vented acoustic chamber. An input or sound inducing mechanism (such as strings of a guitar) imparts a vibration to the structure which causes acoustic waves to resonate within the acoustic chamber. The motion of air in and out of the vent causes acoustic waves to emanate from the chamber that combine with the acoustic waves emanating from the structure to form sound/musical notes. In accordance with the invention, a system controls the sound emanating from such an acoustic instrument. In accordance with one embodiment of the invention, at least one integral or smart sensor is disposed adjacent a sensing location of the structure, and the sensor is configured to generate sensed electric signals indicative of the magnitude of structural vibration of the structure at the sensing location. A controller in communication with the sensor, includes a processor for processing the sensed electric signals in accordance with a predetermined method (e.g., computer program). In response, the controller produces output electrical signals. At least one integral or smart actuator is disposed adjacent an actuator location of the structure, and the actuator is in communication with the controller and is configured to receive the output electrical signals and induce structural vibration of the structure at the actuator location. As a result of the foregoing structure and operation the induced vibration of the structure at the actuator location creates acoustics that alter the sound emanating from the acoustic chamber as well as that emanating from the structure. Specifically, signature frequency response characteristics of acoustic instruments like damping and frequency values of structural and acoustic resonances can be altered to alter the sound of the acoustic instruments. The use of integral or smart sensors and actuators put no restrictions on the movement of the acoustic instrument player since they are part of the guitar structure.

Feedback control of structurally radiated sound into enclosed spaces using structural sensing

A technique is developed that addresses sensor and actuator placement and feedback control of structural/acoustic problems that can be described as a flexible structure surrounding an acoustic cavity. Specifically, this work is directed at the space launch vehicle problem, where it is assumed that it is not possible to obtain, in advance of a required control output, a coherent measurement of the disturbance or to directly measure the quantity to be controlled. These assumptions necessitate the use of structural sensing to predict the sound pressure in the cavity and of feedback control to reduce the radiated sound. A method for selecting sensor and actuator positions based on a transformation of the problem into radiation modes is covered as well as an optimal feedback control approach which allows the control of radiated pressure into a defined subvolume of the cavity using only structural actuators and sensors. Finally, an example problem is completed which draws on all of the theoretical development t...

Development of a sparse-aperture testbed for optomechanical control of space-deployable structures

This paper presents an overview of the development and capabilities of a space-traceable testbed developed for investigation of research issues related to deployable space telescopes. The Air Force Research Laboratory (AFRL) is developing the Deployable Optical Telescope (DOT), which upon completion will be a fully-deployable, sub-scale, space-traceable ground testbed for development and demonstration of critical technologies for the next-generation of space-optics systems. The paper begins with an overview of the DOT project’s technology goals, including the specific performance objectives of the various technologies that are being incorporated into the DOT testbed. The paper presents an overview of the DOT design, including the central integrating structure, deployable primary mirror petals, deployable secondary tower, deployment mechanisms, lightweight mirror segments, metrology, and control systems. The paper concludes with a report on the current status of DOT activities as well as a view of the future research that is planned for the project.

Evacuated enclosure mounted acoustic actuator and passive attenuator

This invention presents a novel means to passively achieve a compact moving-coil actuator with a very low natural frequency. The diaphragm and voice coil are mounted in a sealed enclosure from which the air is partially or completely evacuated. This reduces the air spring effect. The diaphragm is supported by a non-linear, buckling, or collapsible support apparatus. By taking advantage of the non-linear stiffness properties of such structures, the stiffness of the actuator can be designed to be small at the operating point, which when combined with the moving mass, yields a low natural frequency.

Active Acoustic Control of a Rocket Fairing Using Spatially Weighted Transducer Arrays

Apreliminarystudy,includingexperimentalresultsforanovelactiveacousticcontrolapproachtoreducethelowfrequency modal response in a rocket fairing, is presented. The control method uses spatially weighted transducer arrays with H2 feedback control laws to attenuate globally the targeted acoustic modes. The nature of the fairing acoustic problem is described, the theory of the control approach is discussed, and important feasibility issues regarding the actual implementation of the control method are presented. Several controllers were implemented on a full-scale composite model of a small rocket fairing. The results demonstrate that the controller was able to reduce the response of the low-frequency modes by 6 ‐12 dB with very little spillover. In addition, a spatially averaged reduction of the acoustic response of the fairing interior in excess of 3 dB over the 20 ‐200-Hz bandwidth was demonstrated. Thefeasibility studies indicate that limitations on actuatorpowerand volumetric displacement under actual launch conditions are not necessarily prohibitive, but may be satise ed with continued development of actuator technology and placement optimization.

Passive vibroacoustic isolation for payload containers

This paper presents analysis and experimental results which examine key issues related to passive vibroacoustic isolation for container type structures. The key noise reduction principle examined is the passive application of a characteristic impedance mismatch in conjunction with a vibration isolation suspension system to limit structural transmission. The characteristic impedance mismatch is created by imposing a near vacuum condition between partitions of a container structure. Unlike active boundary control techniques, this approach is insensitive to the grazing angle of the source acoustics, is simple, and avoids technology challenges such as the need to develop efficient large stroke actuators. The analysis for this problem is performed by extending Fahys double-leaf partition model. Using the extended model, numerical simulations are conducted to study the effect of various design parameters on acoustic transmission. Guidelines developed from this study are then used to construct an experiment to show the viability of the concept. The experiment demonstrates that at least a 19 dB reduction in sound pressure level (SPL) may be achieved, with a modest level of vacuum.

Fairing Noise Mitigation Using Passive Vibroacoustic Attenuation Devices

This work investigated the application of passive vibroacoustic attenuation devices (PVADs) for fairing noise mitigation. A PVAD is an integrated structural vibration mitigation device and acoustic damper. The PVAD was developed to provide low-frequency noise reduction and to be used as a supplement to acoustic blankets. This work presents modeling of the PVAD and composite cylinder test bed, simulations, optimization studies, experimental results using prototype devices, tests using two component devices, and tests using integrated PVADs on a 2.75-m composite cylinder. Two optimization schemes are presented: one based on radiation-mode analysis and the other using genetic algorithms. The measured experimental data supported conclusions from the model simulations and predictions. The performance of the integrated PVAD devices was better than that of individual components, and exceeded the performance of discrete masses and distributed mass loading. Narrowband reductions of 10 dB were demonstrated at low-frequency acoustic resonances. More than 6 dB of acoustic reduction was measured over the bandwidth of 50‐125 Hz in the cylinder.

Active Structural Acoustic Control of a Launch Vehicle Fairing Using Monolithic Piezoceramic Actuators

This study investigated the feasibility of using active structural acoustic control with monolithic piezoceramic actuators to reduce the low frequency noise transmission through rocket fairings during launch. Closed-loop simulation results are presented using a fully coupled structural acoustic model of a lightly damped composite fairing structure with integrated piezoceramic actuators. Constraints were placed on controller mass and maximum allowable actuator voltage in order to provide a baseline of reasonable expected performance. Realistic disturbance levels were used in the simulations, and two disturbance cases were considered with significantly different spectral characteristics. Simulations were conducted to compare the effects of actuator thickness, covered surface area, and maximum actuator voltage on controller performance and energy requirements. Linear Quadratic Regulator control laws were computed assuming full-state feedback using three design approaches. The results provide significant insight into the noise transmission problem and to the physical dynamics of the control approach. The best-case reduction in the spatially averaged interior acoustic response was determined to be approximately 2.5 dB over the 0-300 Hz bandwidth.