Active Damping Using Distributed Anisotropic Actuators

A helicopter structure experiences substantial highfrequency mechanical excitation from powertrain components such as gearboxes and drive shafts. The resulting structureborne vibration excites the windows which then radiate sound into the passenger cabin. In many cases the radiated sound power can be reduced by adding damping. This can be accomplished using passive or active approaches. Passive treatments such as constrained layer damping tend to reduce window transparency. Therefore this paper focuses on an active approach utilizing compact decentralized control units distributed around the perimeter of the window. Each control unit consists of a triangularly shaped piezoelectric actuator, a miniature accelerometer, and analog electronics. Earlier work has shown that this type of system can increase damping up to approximately 1 kHz. However at higher frequencies the mismatch between the distributed actuator and the point sensor caused control spillover. This paper describes new anisotropic actuators that can be used to improve the bandwidth of the control system. The anisotropic actuators are composed of piezoelectric material sandwiched between interdigitated electrodes, which enables the application of the electric field in a preferred in-plane direction. When shaped correctly the anisotropic actuators outperform traditional isotropic actuators by reducing the mismatch between the distributed actuator and point sensor at high frequencies. Testing performed on a Plexiglas panel, representative of a helicopter window, shows that the control units can increase damping at low frequencies. However high frequency performance was still limited due to the flexible boundary conditions present on the test structure. INTRODUCTION A helicopter powertrain generates high-frequency mechanical loads, which propagate throughout the primary structure. These loads cause vibrations in the sidewall and windows, which then radiate sound into the cabin. The resulting noise levels in the cabin can be uncomfortable for passengers. Cabin noise in helicopters is often tonal in nature with strong tones between 500 Hz and 3 kHz. In this frequency range, the vibratory response of the structure is typically controlled by structural damping. As a result, manufactures often add constrained layer damping to the sidewall. Unfortunately this treatment is not effective on the windows since they need to be transparent. Fortunately other options are available for the windows. For instance, researchers have shown that embedding viscoeleastic material between layers of Plexiglas can effectively increase the structural damping without impairing visibility [1]. Another option is to use small control units installed around the perimeter of the window to generate active damping, as described in this paper. Active damping is achieved using direct velocity feedback, and is only effective if the actuator and sensor are substantially matched. This means that the actuator and sensor have to couple to the structure in the same way. For instance, a point force actuator and point sensor constitute a matched transducer pair. Unfortunately real transducer pairs are never perfectly matched, which limits the high frequency performance of the control system. The goal of this work is to improve the bandwidth of a relatively simple active control system originally proposed by Gardonio and Elliott [2]. Gardonio and colleagues have shown that small control units, consisting of triangularly-shaped distributed actuators and point sensors, can be distributed around the perimeter of a panel to increase the structural damping [2-5]. However above https://ntrs.nasa.gov/search.jsp?R=20100039496 2017-09-14T02:01:48+00:00Z