Active Vibro-acoustic Control Research Articles

Overview of Onera acoustic active control activities in helicopter cabin

Within a helicopter cabin, passengers are in close proximity to disturbing sources that contribute to interior noise: main and tail rotors, engines, main gearbox and aerodynamic turbulence. These sources generate bending vibrations of the entire tail boom, induced vibrations in the cabin at blade passing frequencies, transient vibrations of rotor blades and structure borne noise induced by gear meshing within gearboxes. Conventional passive systems, as trim panels or passive anti-resonance isolation systems, are still the main way to control the cabin noise. Nevertheless, (semi) active control techniques have been the subject of numerous studies for decades, especially to improve the performance of passive concepts in low frequencies. Unfortunately, many turned out to be unreliable or completely inapplicable because problems of robustness, time convergence of algorithms, but also a difficult identification of optimal locations for actuators and sensors or a high added mass and electrical power. The objectives of this paper is to describe in details the research projects in active noise control carried out at Onera since the 1990s, i.e. from numerical modeling and laboratory tests to flight tests in helicopter cabin. This overview will feed into the reflection on the renewal of active vibro-acoustic control within different laboratories.

Modal Sensitivity and Selectivity of Small, Rectangle-Shaped Piezoelectric Transducers Used as Sensors and Actuators in Active Vibroacoustic Control Systems

The paper focuses on some issues regarding the utilization of small rectangle-shaped piezoelectric transducers as both sensors and actuators in active vibration and vibroacoustic control systems of beam, plate and panelled structures with arbitrary (non-homogeneous) boundary conditions. A new form of description of a simple proportional active control system with multiple independent feedback loops is proposed. The modal sensitivity functions of sensors and the modal selectivity functions of actuators are introduced to describe their ability for sensing and exciting specific structural modes of the structures. Basing on the assumed form of cost function and the derived equations of control system the influence of the modal characteristics of transducers on the stability of the system and on the performance of the active control is analyzed. The results of analytical solutions and numerical simulations are compared with the results of the experiments carried out on various beam and plate structures made up of aluminium or composite materials including the actual materials used in aviation, proving usefulness of the presented approach.

General-purpose processors for active vibro-acoustic control: Discussion and experiences

This paper gives an appropriate perspective about why, how, and when low-cost general-purpose computers can be used, with performance similar to dedicated processors, for the implementation of advanced multi-input–multi-output feedback/feedforward active noise and vibration controllers. High performance and real-time capabilities requirements of active vibro-acoustic control systems are critically discussed with respect to the architectural features of general-purpose processors and the actual closed-loop bandwidth of interest. Discussion on proprietary and free real-time operating systems running on general-purpose hardware is included and a free open-source solution is described. An aeronautical application is presented to support paper position and demonstrate feasibility of the approach hereafter proposed.

Distributed control of sound and vibration

The control of sound and vibration in large systems with many actuators and sensors has conventionally been achieved using fully-coupled, centralised, control strategies. The complexity of such a fully coupled controller rises rapidly with the number of transducers, however, and its stability is vulnerable to individual failures. Distributed control systems, of which a decentralised controller, having only local loops, is an extreme example, are not so interconnected, and may not be so susceptible to individual failures. It is possible to design decentralised systems for the active control of sound in which the individual control loops do not significantly interact, but then only local, rather than global control is generally achieved. For active vibration control, however, decentralised controllers can be made stable, with the appropriate choices of actuators and sensors, despite strong interaction between individual control loops, and significant global performance can then be achieved. Examples are considered in active vibration isolation and active vibroacoustic control. Finally, the distributed nature of the active mechanism of hearing in the inner ear is briefly described. Although the mechanisms associated with the individual control loops are poorly understood, it is clear that they are tuned very close to instability in order to achieve the greatest sensitivity.

Thin, low frequency, high displacement actuator panels

An advanced type of thin, high displacement, low frequency actuator panel is being developed for in-air active vibro-acoustic control applications. The 10 cmx10 cm actuator panel consists of cymbal-type flextensional sub-elements placed mechanically and electrically in parallel with each other between two stiff cover plates. The total thickness of the actuator panel is 6.35 mm. Two panel designs are currently under investigation. One consists of sixty-four 12.7 mm diameter cymbals in an 8×8 square arrangement. It is intended for use in the frequency band from 1 kHz to 10 kHz. The other panel has sixteen 25.4 mm diameter elements in a 4×4 square configuration and is designed for use below 1 kHz. Laser Doppler velocimetry (LDV) measurements show peak displacements on the order of 1 μm or more are obtained when driving the panels at one Volt at their respective resonance frequencies.