Optimizing Sensor and Actuator Arrays for ASAC Noise Control

NASA Langley Research CenterABSTRACTThis paper summarizes the development of an approachto optimizing the locations for arrays of sensors and actu-ators in active noise control systems. A type of directedcombinatorial search, called Tabu Search, is used toselect an optimal configuration from a much larger set ofcandidate locations. The benefit of using an optimizedset is demonstrated. The importance of limiting actuatorforces to realistic levels when evaluating the cost functionis discussed. Results of flight testing an optimized sys-tem are presented. Although the technique has beenapplied primarily to Active Structural Acoustic Controlsystems, it can be adapted for use in other active noisecontrol implementations.INTRODUCTIONIf Active Noise Control (ANC) systems are to be used inGeneral Aviation (GA) aircraft, they must be cost effec-tive. A primary driver in the cost of ANC systems is thenumber of channels needed to achieve a noise reductiontarget. Additional controller channels increase installedcost and subsequent lifecycle costs. The ANC systemsthat have been fielded successfully in several commercialturboprop aircraft have dozens of channels of sensors(microphones) and actuators (loudspeakers) that are dis-tributed throughout the cabin [1, 2]. An ANC systemdesigned for GA aircraft will be expected to have farfewer channels and yet achieve similar noise reductionperformance. An ANC system with optimized sensor andactuator locations can have a reduced channel count fora specified noise reduction target and thus be cheaper tobuild and maintain.A variant of the ANC system is the Active StructuralAcoustic Control (ASAC) system [3, 4]. The actuators ofan ASAC system (e.g., shakers) work directly on thestructure to reduce the levels of the structural vibrationsthat cause interior noise. NASA's Langley ResearchCenter has investigated the use of ASAC systems tocontrol aircraft interior noise [5,6,9]. These results indi-cate that actuator and sensor placement strongly influ-ence ASAC system performance. However, it has alsobeen found that pressurization (at altitude) of the aircraftcabin can alter actuator transfer functions and therebyreduce the performance of optimized configurations.The following sections describe the optimization proce-dure and its use in developing an ASAC control systemfor the Raytheon 1900D.