Anant Grewal

Active Cabin Noise and Vibration Control for Turboprop Aircraft Using Multiple Piezoelectric Actuators

The Active Structural Acoustic Control (ASAC) technique was applied to reduce propeller-induced noise and vibration in the passenger cabin of the deHavilland Dash-8 turboprop aircraft. Piezoceramic elements were used for structural actuation while velocity feedback of the fuselage was achieved through the use of accelerometers. Three actuators comprised of segmented piezoelectric elements were designed with the objective of reducing the noise and vibration levels at the propeller Blade Passage Frequency (BPF). Twelve accelerometers were grouped to effectively form three sensors. Second order classical compensators were designed for each of the three dominant control loops in order to achieve high gain at the BPF, rendering the closed-loop system insensitive to disturbances at the control frequency. The propeller acoustic field on the port side of the aircraft was simulated in a laboratory using a speaker-ring consisting of four loudspeakers. The control system was tested using this acoustic field, producing noise attenuation as high as 28 dB in the interior, and fuselage vibration reduction as high as 16 dB.

Multibody dynamics and robust control of flexible spacecraft

The paper focuses on an approach to the study of the dynamics and control of large flexible space structures, comprised of subassemblies, a subject of considerable contemporary interest. To begin with, a relatively general Lagrangian formulation of the problem is presented. The governing equations are nonlinear, nonautonomous, coupled, and extremely lengthy even in matrix notation. Next, an efficient computer code is developed and the versatility of the program illustrated through a dynamical study of the first element launch (FEL) configuration of the Space Station Freedom, now superseded by the International Space Station. Finally, robust control of the rigid body motion of the FEL configuration using both the linear-quadratic-Gaussian/loop transfer recovery (LQG/LTR) and H/sub /spl infin// procedures is demonstrated. The controllers designed using the simplified linear models, prove to be effective in regulating librational disturbances. Such a global approach-formulation numerical code, dynamics, and control-is indeed rare. It can serve as a powerful tool to gain comprehensive understanding of dynamical interactions and thus aid in the development of an effective and efficient control system.

High Intensity Noise Generation for Extremely Large Reverberant Room Test Applications

A recent operational need for the development of a large (101,000 ft3) reverberant acoustic chamber at the Space Power Facility of NASA Glenn Research Center’s Plum Brook Station with the requirement of generating sound pressure levels (SPL) as high as 163 dB has resulted in the need to re-examine the generation of noise in reverberant rooms. Early in the design stage, it was realized that the acoustic power level capability (10-30 kW) of conventional electrodynamic air modulators, such as those supplied by the Wyle Corporation, would be required in unprecedented numbers to meet the test spectra requirements. The design team then turned to a lesser known modulator, the hydraulically driven air modulator supplied by the Team Corporation, which has 150-200 kW acoustic power capability. The advantage to the project was a significant reduction in the number of modulators required to meet the requirements. However, since only limited characterization of Team modulator’s performance has been reported, a test program was required in order to mitigate the risk of the design of the RATF. Aiolos Corporation, which is responsible for the acoustic design of the RATF, and the Institute of Aerospace Research (IAR) of the National Research Council of Canada (NRC), entered into a collaborative agreement with the objective of characterizing, optimizing and investigating the controllability of the Team modulators. The test program was performed at the NRC-IAR reverberant chamber, a 19,000 ft3 facility located in Ottawa, Ontario, Canada. The current paper provides details of the principle of operation of the Team modulators, including their servo control loops and provides of a summary of the characterization and controllability test program.

Vibroacoustic modeling of the control of aircraft cabin noise using piezoelectric actuators

A vibroacoustic modeling methodology for the simulation of noise transmission into an aircraft cabin previously developed by the authors is extended to include the effect of segmented piezoelectric actuation. The structural model of the aircraft fuselage is based on a stiffened shell theory developed by Egle and Sewall. The strain and kinetic energies of the shell, stringers and frames, and the strain energy for the extensional motion of the floor beams are used in a Rayleigh-Ritz analysis to obtain the structural model. The forcing term due to the acoustic pressure disturbance of the propellers is determined by using virtual work considerations. The passive and active effects of the segmented piezoelectric actuators are incorporated in the model by including their energies in the variational approach. The development of the acoustic model of the cabin reduces to a 2D analysis due to the presence of parallel fore and aft bulkheads. The coupled vibroacoustic model, which includes terms that represent the inherent fluid- structure interaction, is developed from the structural and acoustic modal models. The control performance of the piezoelectric actuators is evaluated by considering the minimization of the sum of the squares of the interior sound field or structural response at a number of finite locations as the control objective. The acoustic and structural responses to one actuator and sensor arrangement are evaluated and discussed.

Investigation of aircrew noise exposure levels and hearing protection solutions in helicopter cabin

High noise levels in the helicopter cabin adversely affect aircrew communication and reduce comfort in the short-term and may lead to hearing loss in the long-term if flight helmets cannot provide sufficient protection to the aircrew. A cabin noise exposure survey has been performed on a Royal Canadian Air Force CH-147F Chinook heavy lift helicopter to evaluate the noise environment and noise protection performance of the flight helmet. Investigation results showed that the low-frequency noise attenuation provided by the Royal Canadian Air Force flight helmet was marginal in high-speed flight conditions that generate loud cabin noise. Therefore, in-canal earphone integrated with active noise cancellation capability was investigated to provide enhanced noise protection and improve clarity in voice communication. Simulation and proof-of-concept test results verified that active noise cancellation in-canal earphones can serve as a feasible technical solution to provide enhanced noise attenuation to mitigate the low-frequency N/rev tonal noise generated by the aerodynamic pressure from the helicopter rotor blades.

Acoustic Testing and Response Prediction of the CASSIOPE Spacecraft

A high intensity acoustic test in a reverberant chamber was conducted on the CASSIOPE spacecraft in the final stages of integration and test campaign to ensure that it would survive the acoustic loads during launch. This paper describes the acoustic test methodology, the details of the model used for analytical prediction of the structural response for acoustic excitation and discussion of the predicted response comparison with test results that provided confidence in the spacecraft structural design for acoustic loads. The objective of the spacecraft acoustic test was to demonstrate the ability of the structure and avionics to withstand the broadband random acoustic environment experienced within the launch vehicle payload fairing. The CASSIOPE spacecraft was tested in the reverberant chamber at overall sound pressure level up to 142.1 dB. The automatic spectral control system of the acoustic test facility, which used six control microphones, was able to achieve and the maintain target spectrum levels around the spacecraft within tolerances without manual adjustments to the noise generators’ controls. The dynamic response of the CASSIOPE spacecraft during the test was measured using a large number of accelerometers installed on critical locations of the structure. Low level pre-test and post-test structural response signatures as well as electrical integrity checks performed after the exposure to the proto-flight acoustic environment demonstrated the ability of the spacecraft to survive the launch. The acoustic response of the spacecraft was also predicted based on a finite element model analysis to identify the critical components, evaluate structural margins and assess the risks in proceeding with a proto-flight acoustic test based on the specified launch vehicle spectrum. The analysis method used to predict the responses combines the NX/NASTRAN solver and RAYON, a vibro-acoustic simulation software. The RAYON software functionality is based on a boundary element model that enables the creation of an accurate fluid loading on the structure, with consideration of fluid mass and damping effects. The study used a finite element model of the structure that was correlated through an experimental modal survey test and actual spectrum levels achieved during the acoustic test. Responses of most locations compared favourably with the predictions in critical locations such as the solar arrays. Due to the limited availability of the satellite as well as time and cost constraints in a spacecraft development program, it is important to perform both qualification tests as well as analytical predictions in an efficient and timely manner to validate structural designs of spacecraft.