Noise reduction results of the ACASIAS Active Lining Panel

Advanced concepts for aero-structures with multifunctional capabilities are investigated within the EU-project ACASIAS. In work package 3 of ACASIAS, components of an active noise reduction system are structurally integrated into a curved sandwich panel by means of 3D printed inserts. This so-called smart lining is intended for application in aircraft as a modular and lightweight interior noise treatment in propeller-driven aircraft. The broad application scenario of smart linings ranges from retro-fitting of current regional aircraft such as ATR 42, ATR 72, DHC-8 Q400 to the application in new short-range aircraft with energy efficient counter rotating open rotor (CROR) engines or with distributed electric propellers. A key feature of the smart lining with integrated active components is its modularity, facilitating a flexible application in the aircraft cabin. This requires a fully self-contained sensing mechanism based on structurally integrated accelerometers. Using the normal surface vibration data from the integrated sensors, the smart lining is able to predict the sound field in front of it. The so-called virtual microphone method with remote sensors and observer filter allows to get rid of real microphones and wiring in the aircraft cabin. This makes retro-fitting easier because it reduces wiring effort and costs which is beneficial for future aircraft as well. However, the use of virtual instead of real microphones might deteriorate the performance or even the stability of the active noise reduction system because it relies on accurate plant models. Laboratory experiments in a sound transmission loss facility are conducted to assess the behavior of the smart lining with virtual microphones and compare it to a smart lining with real microphones. The sensitivity of the smart lining to environmental changes and the noise reduction performance and control system stability are investigated in this study

EXPERIMENTAL RESULTS OF AN ACTIVE SIDEWALL PANEL WITH VIRTUAL MICROPHONES FOR AIRCRAFT INTERIOR NOISE REDUCTION

This work focuses on the reduction of aircraft interior noise by means of actively controlled sidewall panels (smart linings). It was shown in prior work that considerable reductions of interior sound pressure level can be achieved using structural actuators on the lining and microphones distributed in the seat area in front of the linings. Simulation results suggest that the physical microphones in front of the linings can be replaced by so-called virtual microphones. The signals of these virtual microphones are obtained from filtering the normal surface vibrations of the lining through an observer filter. Accelerometers are mounted on the lining structure to obtain the vibration signals. Simulation results of a smart lining with virtual microphones show a mean sound pressure level (SPL) reduction of 10 dB and 5.9 dB(A) in front of the lining. These results must be verified by laboratory experiments applying real-time control because the effects of time variances and imperfect path models will deteriorate the performance of the smart lining. Therefore, the present contribution describes an experimental realization of a smart lining with remote sensors and virtual microphones in a realistic laboratory setup. A double panel system consisting of a primary carbon fiber reinforced plastic (CFRP) structure (fuselage) and a coupled smart lining is installed in the opening of a transmission loss facility. The real-time behavior of the smart lining is tested and the influence of using virtual instead of physical microphones on the SPL reduction is quantified. One focus is on evaluating the robustness of the smart lining under changing environmental conditions

On the noise reduction of active sidewall aircraft panels using feedforward control with embedded systems

Keeping aircraft interior noise on an acceptable level is an important aspect for the passenger comfort and health in modern aircraft design. Generally there is a trade-off in the achievable transmission loss of aircraft and the additional weight of the insulation provision, especially at low frequencies below 500 Hz. In this frequency range, passive methods for sound insulation usually have high weight and volume requirements for suitable performance in aircraft. The present work focuses on aircraft with rotor engines having characteristic low frequency narrow banded noise sources due to the rotational speed of the rotors. A major transmission path of the noise is given by the linings of the aircraft cabin, which are coupled to the fuselage. These large sound emmitting surfaces radiate the noise directly into the passenger zone. Active control is a promising method to reduce at least the low-frequency noise radiated by the panels. These so-called smart linings, augmented with suitable actuators and sensors, are also usable for additional tasks, such as e.g. passenger announcements and noise masking. The feasibility of active feedforward control to reduce narrow banded frequencies below 500 Hz has already been tested successfully at DLR on test panels and in a fully equipped test aircraft on ground, which are excited externally by a loudspeaker array. This paper is dedicated to a discussion of the recent activities to simplify the underlying control strategies in a way that they can be implemented on small embedded systems with limited performance. In the future, such microprocessor units enable the integration of the actuators, sensors and control algorithms directly into smart lining modules, having a positive effect on maintainability and weight footprint of the smart Panels

DESIGN OF AN ACTIVE NOISE CONTROL SYSTEM FOR A BUSINESS JET WITH TURBOFAN ENGINES

An active noise vibration control (ANVC) system is designed for a Dassault Falcon 2000LX business jet. Measurement data from ground tests and recordings from cruise flight reveal narrowband and tonal cabin noise in the bandwidth from 50 Hz to 500 Hz. In both conditions the sound pressure level (SPL) of the tonal components is up to 10 dB above the cabin noise floor. The designed ANVC system is expected to reduce the tonal components to the noise floor. The system is realized on the ceiling panel in the aft of the cabin. A frequency domain filtered-x least mean squares algorithm (FxLMS) is used for control. The main design task is the definition of actuator (inertial exciter) and error sensor (microphone) locations on the ceiling panel and in the cabin fluid. measurement data from a ground vibration test of DLR test aircraft iSTAR is used for the identification of suitable actuator and sensor locations. The system performance is predicted with numerical simulations using sampled sound pressures and identified frequency response functions (FRF). The sound pressures are recorded at 312 locations in the volume of the aft cabin. FRF from 48 actuator locations to 312 microphone locations are available for optimization. The system will be realized in the iSTAR in July 2023. Ground tests will be performed with engine excitation up to 80% N1 (rotational speed of low the pressure compressor shaft)

Selbsteinstellende, robuste Regelung von Strukturschwingungen an Parallelrobotern

Parallelroboter realisieren Trajektorien die hohe Beschleunigungen und Geschwindigkeiten erfordern. Insbesondere bei so genannten Pick-and-Place Anwendungen, in denen hohe Geschwindigkeiten und Präzision gefragt sind, vermindern Schwingungen der Struktur jedoch die Leistungsfähigkeit des Roboters. Der vorliegende Bericht beschäftigt sich mit der Konzeptionierung und Realisierung einer Regelung zur Schwingungsreduktion an zwei hochdynamischen Parallelrobotern für die Handhabung und Montage. Die Herausforderung bei der Erarbeitung einer geeigneten Lösung sind das positionsabhängige Schwingungsverhalten der Roboterstruktur, die Sensitivität der Schwingungseigenschaften gegenüber Veränderungen der Struktur und die unterschiedlichen Topologien der beiden Parallelroboter. Durch den Einsatz einer umschaltenden robusten Regelung gelingt die Schwingungsreduktion des Roboters im gesamten Arbeitsraum. Es wird ein Stabilitätsbeweis hergeleitet, der die Stabilität der Reglerumschaltung nachweist. Eine Automatisierung der Systemidentifikation und Reglersynthese erlaubt eine praxistaugliche Selbsteinstellung des Regelungssystems. Die in der Arbeit generierten Algorithmen und Verfahren werden auf beiden Parallelrobotern umgesetzt. Experimentelle Messergebnisse belegen die Funktionsfähigkeit der erarbeiteten Lösungen

Smart Acoustic Lining Panel

Counter-rotating open rotor (CROR) propulsion systems are a promising concept to reach a resource efficient transport demanded by the European Commission in the Horizon 2020 framework program. Due to their fuel efficiency they are actually discussed as an alternative to common jet engines. The disadvantage of CROR engines is the radiation of annoying multi-harmonic noise which leads to high sound pressure levels in the cabin. The impact of the CROR engines on the sound pressure level in the cabin can be reduced by the integration of an Active-Structural-Acoustic-Control (ASAC) system in lining panels. By means of sensors, actuators and a controller the ASAC system is able to increase the sound transmission loss of the panels. Within the scope of the Horizon 2020 ACASIAS project research is performed to integrate the associated wiring, sensors and actuators in lining panels directly. Concepts are explored to realize a seamless integration during an industrial manufacturing process. The integration of an ASAC system in lining panels will enable the installation of fuel efficient propulsion systems. This presentation gives an overview of the current status of the related work package 3 of the ACASIAS project

On the noise reduction of active sidewall aircraft panels using feedforward control with embedded systems

Keeping aircraft interior noise on an acceptable level is an important aspect for the passenger comfort and health in modern aircraft design. Generally there is a trade-off in the achievable transmission loss of aircraft and the additional weight of the insulation provision, especially at low frequencies below 500 Hz. In this frequency range, passive methods for sound insulation usually have high weight and volume requirements for suitable performance in aircraft. The present work focuses on aircraft with rotor engines having characteristic low frequency narrow banded noise sources due to the rotational speed of the rotors. A major transmission path of the noise is given by the linings of the aircraft cabin, which are coupled to the fuselage. These large sound emmitting surfaces radiate the noise directly into the passenger zone. Active control is a promising method to reduce at least the low-frequency noise radiated by the panels. These so-called smart linings, augmented with suitable actuators and sensors, are also usable for additional tasks, such as e.g. passenger announcements and noise masking. The feasibility of active feedforward control to reduce narrow banded frequencies below 500 Hz has already been tested successfully at DLR on test panels and in a fully equipped test aircraft on ground, which are excited externally by a loudspeaker array. This paper is dedicated to a discussion of the recent activities to simplify the underlying control strategies in a way that they can be implemented on small embedded systems with limited performance. In the future, such microprocessor units enable the integration of the actuators, sensors and control algorithms directly into smart lining modules, having a positive effect on maintainability and weight footprint of the smart Panels

Remote Sensing for a Lining Integrated Active Structural Acoustic Control System

In the framework of the EU project ACASIAS an aircraft sidewall panel (lining) with structurally integrated actuators and sensors is developed. Each lining has a digital unit which samples the sensor signals, performs filtering operations and supplies the actuators with control signals. The whole system makes up an active structural acoustic control system aiming at the reduction of low-frequency multi-tonal aircraft interior noise. The novelty of this approach compared to past implementations of active noise control (ANC) systems in aircraft is its modularity. Each so-called smart lining is autonomous in the sense that it processes only structural sensor data from its own integrated sensors. The use of external microphones for error sensing is avoided because this conflicts with the modularity of the smart lining. Hence, one important design task is the replacement of the physical error microphones by the integrated structural sensors and an acoustic filter (observer) running on the digital unit. This method, which is called the remote microphone technique for active control, has never been applied to an aircraft interior structure so far. The detailed design of the smart lining module comprises several steps which are taken within work package 3 of the ACASIAS project. Experimental data of an aircraft typical double panel system is captured in a sound transmission loss facility. The system is excited with a loudspeaker array placed directly in front of the fuselage structure. Different acoustic load cases are used for the definition of the sensors and the actuators. A multi-tonal excitation with high sound pressure level is relevant for the actuator dimensioning and a broadband excitation with multiple independent sound sources is relevant for the sensor definition. 19 accelerometers are mounted on the lining and 20 microphones are placed in front of it. All sensor signals are sampled simultaneously for deterministic and broadband load cases. The lining is equipped with two inertial mass actuators which are used for the active control. Measured frequency response functions of actuators at 39 positions are used for the optimization of the actuator locations. The measurement data is also used for the derivation of an observer and for the simulation of a smart lining with remote microphones. In this contribution, the steps undertaken for the detailed design will be described and simulation results of the noise reduction performance of the smart lining with remote microphones will be presented

Active control of counter-rotating open rotor interior noise in a Dornier 728 experimental aircraft

The fuel consumption of future civil aircraft needs to be reduced because of the CO2 restrictions declared by the European Union. A consequent lightweight design and a new engine concept called counter-rotating open rotor are seen as key technologies in the attempt to reach this ambitious goals. Bearing in mind that counter-rotating open rotor engines emit very high sound pressures at low frequencies and that lightweight structures have a poor transmission loss in the lower frequency range, these key technologies raise new questions in regard to acoustic passenger comfort. One of the promising solutions for the reduction of sound pressure levels inside the aircraft cabin are active sound and vibration systems. So far, active concepts have rarely been investigated for a counter-rotating open rotor pressure excitation on complex airframe structures. Hence, the state of the art is augmented by the preliminary study presented in this paper. The study shows how an active vibration control system can influence the sound transmission of counter-rotating open rotor noise through a complex airframe structure into the cabin. Furthermore, open questions on the way towards the realisation of an active control system are addressed. In this phase, an active feedforward control system is investigated in a fully equipped Dornier 728 experimental prototype aircraft. In particular, the sound transmission through the airframe, the coupling of classical actuators (inertial and piezoelectric patch actuators) into the structure and the performance of the active vibration control system with different error sensors are investigated. It can be shown that the active control system achieves a reduction up to 5 dB at several counter-rotating open rotor frequencies but also that a better performance could be achieved through further optimisations

Entwicklung und experimentelle Erprobung eines mikrocontroller-basierten Regelungssystems

Die Regelung aktiver Strukturen (z. B. CFK mit Piezoaktuatoren) ist rechenintensiv und erfolgt z. Z. auf Rapid-Prototyping-Systemen (z. B. dSpace® ) oder auf PCs. Im Rahmen dieser Arbeit wurde ein Mikrocontroller für diese Aufgabe ausgewählt und ein Regler in Form eines Zustandsraummodels implementiert. Benötigte Hardware, wie Tiefpassfilter und Pegelwandler, wurde angefertigt. Das System ist ausgelegt vier Kanäle mit einer Samplingfrequenz von 1 kHz und einer maximalen Auflösung von 12 dB geregelt werden. Durch die Verwendung des Sparse-Matrix-Types CRS konnte eine Reduktion der Rechenzeit erreicht werden. Die Identifikation einer Strecke und die Hinf-Reglerauslegung ist, durch die Anbindung an Matlab® über USB möglich. Die Strecke kann bei aktiver Regelung vermessen werden. Die Steuerung des Controllers erfolgt über USB durch ein Matlab® -Interface