DE4419933A1 - Piezoelectric polymer film acoustic generator for active cancellation - Google Patents

Abstract

A circular plastic diaphragm (1) has PVDF films (10) bonded adhesively (9) to both sides and is stretched over a tyre-shaped circular body (3) with a tensioning ring (6), hooks (2) and nuts (4) whose adjustment determines the tension in the diaphragm. Each film has electrical contacts made on both sides by vapour-deposited Al (7). A Cu contact layer (12) is applied to two clamps (11) between which the piezoelectric or electrostrictive composite film (8) is gripped and wired to a signal generator or pick-up socket (13).

Description

title

Device and method for sound generation, sound detection and active noise reduction.

State of the art

Sound, d. H. Density fluctuations in a medium cause mechanical Vibrations in solid bodies that share a common interface own this medium. Conversely, a mechanically vibrating one solid body emits sound into the surrounding medium. The mechanical Vibrations of the solid body can be excited and electrostrictive on the one hand on the other hand, can be detected piezoelectrically, e.g. B. using Actuator or sensor elements consisting of inorganic, crystalline or polycrystalline solids with electrostrictive or piezoelectric Properties such as B. sintered and polarized lead zirconium titanate polycrystals (piezoceramic actuators or sensors).

According to this principle, both a generation (Bhattacharya & Crocker: Forced vibration of a panel and radiation of sound into a room, Acustica 22, pp. 275-294, 1969/1970) as well as proof of sound (Fricke, Moser, Scheurer & Schubert: Schall und Schallschutz, Physik-Verlag, Weinheim, 1983, 312 p.). Using an appropriate Control system can have the effect of an on a vibrating body attached actuator can be controlled so that the vibration deflection this body despite the sound of the surrounding medium is dampened by the signal of the attached to the same body Deflection sensor is used as a measured variable for the control. To this In this way, devices for active sound attenuation can be implemented  (Fuller, Hanser & Snyder: Active control of structurally radiated noise using Piezoceramic actuators, proc. Inter-noise 89, Newport Beach, Ca., Dec. 4-6, 1989, pp. 483-488).

Instead of piezoceramic actuators and sensors, too piezoelectric polymer films, e.g. B. PVDF (polyvinylidene difluoride) films used. Due to their shape, they can cover a large area mechanical rigid support structure, e.g. B. a plate, applied and with this be frictionally connected. The support structure can then be yours characteristic natural vibrations are excited or these Natural vibrations can be actively damped (Bailey & Hubbard: Distributed Piezoelectric-Polymer Active Vibration Control of a Cantilever Beam, Journal Guidance and Control, Vol. 8, No. 5, pp. 605-611, 1985).

problem

According to the current state of the art for sound generation, Sound detection and active sound attenuation of a mechanical structure used actuators and sensors made of inorganic crystalline or polycrystalline solids (piezoceramic actuators and sensors) limited in their extent and can therefore only in one accordingly restricted area transfer forces to the vibrating body or record its deflections there. So that are not in any number of places of the vibratable body directly apply forces or the deflections of this body can be grasped at the same time. This local positioning of the Actuators or sensors have restrictions in the frequency spectrum of the excitable or demonstrable vibrations and provides a Restriction on the design and optimization of efficiency a sound absorption system. In addition, these actuators and Sensors only to a limited extent due to their elastic properties non-positive with vibrating bodies made of other materials, e.g. B. Plastic membranes to be coupled. You also need a fixed mechanically rigid support structure that can absorb the reaction forces. With Piezoelectric polymer films can overcome these limitations because they are applied to a vibrating structure over a large area and so that forces are distributed in a locally distributed manner into the surface of the Support structure can be initiated. With a mechanically stiff  Support structure such as B. a plate can only Natural vibrations of this support structure are excited because the piezoelectric polymer films themselves are not mechanically rigid bodies. So that the frequency spectrum of these vibrations by the Natural vibrations of the support structure defined and also limited.

the solution of the problem

With piezoelectric PVDF polymer films as actuator and sensor elements, which are uniform over the entire interface with the sound-carrying medium non-positive with thin, large-area membranes made of materials small modulus of elasticity, e.g. B. plastic films by clamping in form a frame an oscillatable mechanical system, coupled can be a novel device for generation, detection and active Attenuation of sound can be made. By the resilience of this Membrane can be adjusted variably, there is a possibility that Frequency spectrum of the vibrations the requirements of each Adapt application. The tensioning device also serves as Support structure for absorbing the reaction forces. The electrical Contacting of the actuator and sensor foils can be done on one protruding tab of these foils and mechanically also in the Clamping device can be integrated.

benefits

The advantages of such a device are the large area Structure, of the same shape on the whole vibrating Area with even and small mass coverage of the area, the good acoustic impedance matching in air and compared to a mechanically stiff support structure broadband, due to the variable clamping force adjustable frequency spectrum with regard to excitation and detection of Sound. This can cause problems with a limitation of the area Expansion of actuator and sensor elements with regard to the exact and positioning of the Elements and the electrical contact supply arise, switched off become. These advantages enable the production of large areas  Devices for active noise reduction with a small, area-related Mass occupancy and correspondingly smaller, related to their surface Heat capacity; this allows a high heat transfer with simultaneous Suppression of sound transmission can be achieved. Therefore, with this Device z. B. a noise-reducing hood for machines that need good heat dissipation. Furthermore, by a signal path from the controller to the tensioning device the mechanical Voltage of the membrane and thus the frequency spectrum of an active one Sound damping device can be varied dynamically.

Design

A thin, large-area membrane clamped in a frame Plastic is applied to the entire surface, or even to a part of it Surface, non-positively by a suitable adhesive connection on one side or coupled on both sides with a thin piezoelectric PVDF polymer film. This is done on both sides with a thin metallic coating provided PVDF polymer films used on the edges of the actuator and Sensor areas, e.g. B. on a protruding tab, electrically contacted can be. If the tensioned membrane is glued on one side, by connecting the PVDF film to an AC voltage source Membrane excited to vibrations with variable, adjustable frequency and so that sound is generated. Conversely, this device for Sound detection is used by applying sound to the membrane the electrical voltage signal arising from the contacts is evaluated. At double-sided gluing of the membrane with PVDF films and switching one PVDF film as an actuator and the other film as a sensor can pass through suitable control of the actuator signal using the sensor signal a device for active sound attenuation can be created.

Embodiment

An embodiment of the invention is shown in the drawing and will described in more detail below.

It shows Fig. 1 a circular Kunstoffmembran (1) which is provided with PVDF film (10) glued on both sides and with a clamping ring (6), 5 clamping hook (2) and the associated clamping nuts (4) on a circular, hoop-shaped base body ( 3 ) is spanned. The mechanical tension of this membrane can be changed using the tension nuts ( 4 ). The PVDF film consists of a polyvinylidene difluoride layer with piezoelectric or electrostrictive properties ( 8 ), which is vapor-coated with aluminum ( 7 ) on both sides in order to produce electrical contact. This structure is in turn non-positively connected to the plastic membrane ( 1 ) by means of an adhesive layer ( 9 ). The electrical contacting ( 5 ) takes place via two clamping bodies ( 11 ) on which a copper contact layer ( 12 ) is applied. The PVDF film is stretched between the clamping bodies so that there is good contact between the copper layer ( 12 ) and the aluminum layer ( 7 ). A connection to a BNC socket ( 13 ) is established via wires ( 14 ) ( 16 ) attached to the copper contact, on which the signals can be tapped or applied. The contacting of the second film is constructed in the same way.

If the membrane z. B. is caused to vibrate by external sound excitation, electrical voltage signals are generated in the PVDF films, which can be tapped at the BNC sockets ( 13 , 15 ). Conversely, by applying an alternating voltage to the BNC sockets ( 13, 15 ) and thus to the PVDF films, the membrane is excited to mechanical vibrations, which are then emitted into the room as sound. If you use the detected signal in a control loop as the actual variable (sensor signal), e.g. B. at socket ( 15 ), and the signal generated as a manipulated variable (actuator signal), for. B. on socket ( 16 ), an active structure for vibration damping can be realized.

With the experimental setup shown in FIG. 2 and a controller, this device achieved a damping of the sensor signal characterizing the vibration deflection of 18 dB.

Claims (1)

Device and method for sound generation, for sound detection and for active sound damping, characterized in that piezoelectric polymer films are non-positively connected to a membrane clamped with an adjustable tension force.