Abstract:
A diamond-shaped actuator for a flexible panel has an inter-digitated electrode (IDE) and a piezoelectric wafer portion positioned therebetween. The IDE and/or the wafer portion are diamond-shaped. Point sensors are positioned with respect to the actuator and measure vibration. The actuator generates and transmits a cancelling force to the panel in response to an output signal from a controller, which is calculated using a signal describing the vibration. A method for controlling vibration in a flexible panel includes connecting a diamond-shaped actuator to the flexible panel, and then connecting a point sensor to each actuator. Vibration is measured via the point sensor. The controller calculates a proportional output voltage signal from the measured vibration, and transmits the output signal to the actuator to substantially cancel the vibration in proximity to each actuator.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    The invention described herein was made by employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to a system and a method for actively controlling the vibration of a flexible panel. 
       BACKGROUND OF THE INVENTION 
       [0003]    The structural vibration of a flexible panel or another flexible component can generate undesirable noise, particularly within an enclosure constructed using such panels. For instance, aluminum panels of an aircraft fuselage can vibrate when the aircraft is in flight, which in turn can generate substantial noise within the aircraft. Mitigation of such acoustical/structural resonance may include the use of passive or active damping techniques. As an example, a compliant damping mechanism may be used to dissipate vibration energy as heat. Other approaches may include the use of sound absorbing materials. Active damping techniques, by way of contrast, involve the active, targeted use of force actuators to produce an actuation force that at least partially counteracts a resonant vibration within a particular range of frequencies. However, conventional approaches to active damping may be less than optimal when used with flexible panels. 
       SUMMARY OF THE INVENTION 
       [0004]    An active vibration control system is disclosed herein that is suitable for reducing the vibratory response of a flexible panel. Non-limiting example panels which may be prone to resonant vibration include an aircraft fuselage bay, a vehicle body panel, and a motor/engine enclosure. The present active vibration control system uses one or more piezoelectric diamond-shaped actuators. Each diamond-shaped actuator includes an inter-digitated electrode (IDE). The IDE is connected to a piezoelectric wafer portion. The diamond-shaped actuators may be adhered or otherwise surface-mounted to the flexible panel. The control system also includes one or more point sensors, e.g., miniature accelerometers, and a controller. The controller is in electrical communication with the point sensor(s) and the actuator(s), and performs the requisite calculations and signal processing steps required for substantially cancelling the vibration of the panel. 
         [0005]    The various point sensors are positioned with respect to a given actuator, e.g., at each apex thereof or in/toward the center of the actuator depending on the embodiment. Multiple point sensors may be used with each actuator to provide the desired response. Likewise, a designated controller may be used with each of the actuators to provide a desired level of control redundancy. 
         [0006]    The controller, which may be embodied as a small printed circuit board assembly that is surface mounted to the flexible panel, processes a vibration signal from each point sensor. The controller then generates a proportional output voltage signal, which in turn is transmitted to the actuator. The proportional output voltage signal generates an out-of-phase vibration-canceling response via the actuator to the measured vibration at the surface of the flexible panel. 
         [0007]    The piezoelectric wafer portion and/or the IDE are substantially diamond-shaped. The IDE applies a predetermined electrical field, for instance in an in-plane direction. Unlike prior art actuators, the diamond-shaped actuators disclosed herein need not be aligned along a fixed edge or boundary edge of the flexible panel to which the actuator is connected, although in some embodiments an apex or edge of the actuator may aligned with the boundary. When aligned in this manner, fewer point sensors may be used with the actuator. 
         [0008]    The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic illustration of an example diamond-shaped piezoelectric actuator having multiple point sensors, along with various point forces generated by the actuators. 
           [0010]      FIG. 2  is a schematic circuit diagram of an example vibration control system having a diamond-shaped actuator and controller. 
           [0011]      FIG. 3  is a schematic circuit diagram of an example flexible panel with multiple independent vibration control systems. 
           [0012]      FIG. 4  is a schematic perspective view of an example diamond-shaped actuator. 
           [0013]      FIG. 5  is a schematic perspective view illustration of another example diamond-shaped actuator. 
           [0014]      FIG. 6  is a schematic plan view illustration of a diamond-shaped actuator usable with the vibration control systems of  FIGS. 2 and 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    Referring to the drawings, wherein like reference numbers represent like components throughout the several figures, a diamond-shaped actuator  14  as detailed below is shown schematically in  FIG. 1 . Also shown is a pair of example anisotropic triangular actuators  12 . Various point forces (f) are represented with respect to the four apexes of the diamond-shaped actuator  14 . Moments (±m) about a base edge  11  of the example triangular anisotropic actuators  12  are also represented. 
         [0016]    Previous work using the triangular actuators  12  of  FIG. 1  is disclosed in U.S. Pat. No. 7.893,602 to Schiller et al., which is hereby incorporated by reference in its entirety. It is shown in the above listed reference that, if the boundaries of a flexible panel are clamped, for instance a panel that is perimeter-supported or rib-stiffened, then the point forces (f) and line moments (m) along the base edge  11  of each triangular actuator  12  will not couple to the structural response of the panel that is being damped. Therefore, a single point sensor placed at a vertex opposite the base edge  11  can yield a substantially collocated frequency response. That is, the phase of the response will be bounded between ±90 degrees. 
         [0017]    However, if the flexible panel to be damped is not clamped, i.e., if a rib-stiffened or boundary-stiffened panel is not used, or more precisely, and if the triangular actuators  12  shown in  FIG. 1  are placed away from the boundary or perimeter of the panel, then the moment about the base edge  11  couples to the structural response out-of-phase with the point force (f) at higher frequencies, with the signs + and − representing relative direction. When implemented as part of an active vibration control system, the moments (m) of the base edges  11  of the various triangular actuators  12  may combine to destabilize the system. The present design is intended to mitigate that effect, while also allowing greater flexibility in the positioning of the diamond-shaped actuator  14  with respect to the panel to which the diamond-shaped actuator  14  is attached. 
         [0018]    It is recognized herein that a pair of the triangular actuators  12  can be effectively combined as shown to form the diamond-shaped actuator  14  of the present disclosure. Such an approach eliminates the potentially destabilizing base moments along the base edges  11  of the triangular actuators  12 . A collocated transducer pair can be obtained using commonly available point sensors  20 , for instance miniature accelerometers, along with the diamond-shaped actuator  14 . A point sensor  20  is also shown in phantom in FIG. I to represent an optional position, as explained below with reference to  FIG. 2  and the point sensor  20 A shown therein. 
         [0019]    The point forces (½ f) of the triangular actuators  12  combine to form point forces (f) at vertices of the presently disclosed diamond-shaped actuator  14 . Thus, a practical and compact active vibration control system can be created with an operational bandwidth of approximately 20 Hz through approximately 5 kHz. As is understood in the art, at frequencies above approximately 5 kHz, passive noise control treatments may provide a relatively efficient and potentially more cost effective solution. 
         [0020]    The diamond-shaped actuator  14  shown schematically in  FIG. 1  is constructed at least partially of a suitable piezoelectric material. As will be understood by those of ordinary skill in the art, piezoelectric materials are crystalline structures or ceramics which produce a proportional output voltage when a mechanical force or stress is applied thereto. Quartz, tourmaline. lead zirconate titanate, and barium titanate are a few non-limiting examples. Piezoelectric materials produce a proportional voltage in response to an applied mechanical force or pressure. Such materials can also change their shape and/or dimensions in response to an applied electric field, thereby making piezoelectric materials potentially useful as actuators in a host of different applications. 
         [0021]    Since this effect also applies in the reverse manner, an input voltage applied to a sample piezoelectric material such as the diamond-shaped actuators  14  will produce a proportional mechanical force or stress. This force can be imparted to a panel to which the diamond-shaped actuators  14  are mounted. The activation of a typical piezoelectric material can result in a change in dimension of approximately 0.1% for piezo-ceramics and 1% for piezo-polymers. Suitably designed transducer structures made from these particular materials can therefore be made that bend, expand, or contract as desired when a voltage is applied thereto. 
         [0022]    Referring to  FIG. 2 , an active vibration control system  50  includes at least one diamond-shaped actuator  14 . The diamond-shaped actuator  14  is in electrical communication with a controller  118 . Additional diamond-shaped actuators  14  may be connected to a flexible panel  16  (see  FIG. 3 ) and placed in communication with the same controller  118  in this particular embodiment. The diamond-shaped actuator  14  may be relatively thin, e.g., approximately 0.3 mm to approximately 0.4 mm thick in one embodiment or less than approximately 0.5 mm in another example embodiment. As such, the diamond-shaped actuator  14  may be integrated partially or fully within a composite structure. 
         [0023]    Each diamond-shaped actuator  14  may include one or more point sensors  20 . As noted above, the point sensors  20  may be embodied as miniature accelerometers configured to measure a linear acceleration of a portion of a flexible panel to which the point sensor  20  is attached. When positioned away from the boundary or outer perimeter of a given flexible panel, four point sensors  20  may be used to achieve a collocated transducer pair. However, if the vibration control system  50  of  FIG. 2  is mounted along the rigid boundary of such a panel, a point sensor  20  need not be used at the boundary. Additionally, if the vibration control system  50  is designed to target a narrow frequency band, then a single point sensor  20 A may be used, as shown in phantom, e.g., in or toward the middle of the diamond-shaped actuator  14  instead of at one of the vertexes. 
         [0024]    The weighted sum of the point sensors  20  yields an equivalent sensor matched with the diamond-shaped actuator  14 , regardless of the boundary conditions of the flexible panel being damped. In other words, unlike the example triangular anisotropic actuators  12  shown in  FIG. 1 , which are limited to boundary positioning on a rib-stiffened panel, the present diamond-shaped actuator  14  can be positioned anywhere on the surface of the panel, including away from the rigid boundary or perimeter of the panel. 
         [0025]    The controller  118  of  FIG. 2  provides the necessary power electronics for signal conditioning, filtering, and amplification of the measured vibration signals (arrows  22 ) received from the various point sensors  20 . All of the requisite control structure, including any required processors, diodes, transistors, busses, etc., may be embodied as a printed circuit board assembly (PCBA)  60 , for instance by using a mix of surface mount technologies and through-hole components to sufficiently miniaturize the controller  118 . 
         [0026]    The example diamond-shaped actuator  14  of  FIG. 2  applies a force to a flexible panel, for instance the flexible panel  16  shown in  FIG. 3 , in response to a proportional output voltage signal (arrow  24 ) from the controller  118 . As used herein, the term “proportional voltage” describes a scaled negative voltage producing motion in a flexible panel that effectively cancels or at least partially offsets/dampens the vibration that is measured, detected, or otherwise determined by a given point sensor  20 . 
         [0027]    The controller  118  of  FIG. 2 , as well as the controller  18  shown in  FIG. 3  and described below, may be configured as a closed-loop proportional control device. As such, the controller  118  has the necessary operational amplifiers, transistors, resistors, capacitors, diodes, and/or other necessary electronic circuit components required for manipulating one or more control variables. The controller  118  processes the raw acceleration data transmitted from a corresponding point sensor  20 , and then determines a linear acceleration value of a portion of a flexible panel in close proximity to that point sensor  20 . The controller  118  also calculates a linear velocity value using the linear acceleration value. From this intermediate value, the controller  118  can then generate a scalar negative or proportional voltage signal as the proportional output voltage signal (arrow  24 ) which can be modified via a calibrated applied gain. i.e., a constant of proportionality, as needed to thereby affect the desired vibrational attenuation. 
         [0028]    The controller  118  may be specifically designed for use with point sensors  20  configured as standard Integrated Electronics Piezo Electric (IEPE) accelerometers. Although not shown for illustrative simplicity, the PCBA  60  receives power from a main power bus, for instance a typical 28-volt DC bus used aboard a typical aircraft, and provides reduced power to each of the point sensors  20 . Subsequent stages amplify and combine the response from all of the point sensors  20 . 
         [0029]    Each point sensor  20  generates a measured vibration signal (arrow  22 ) which can be normalized (+1, −1) by the controller  118  in a like manner for oppositely-positioned point sensors  20 . The normalized vibration signals (arrows  122 ) are then fed into a summation node  21  to generate a single normalized vibration signal (arrow  222 ). The normalized vibration signals (arrow  222 ) from multiple diamond-shaped actuators  14  (not shown) may be integrated by an integration module  31  of a PCBA  60  to generate a proportional signal (arrow  224 ) that is proportional to velocity. Similar summation nodes can provide the same function for other diamond-shaped actuators  14  used in conjunction with the same flexible panel. 
         [0030]    A low-pass filter  32  may be used to process the proportional signal (arrow  224 ) into a filtered signal (arrow  124 ). For instance, a cutoff of approximately 11 kHz may be used to limit the impact of any higher frequency mismatches between the proportional signal (arrow  224 ) and the diamond-shaped actuator  14 , e.g., caused by actuator shaping errors, misplacement of point sensors  20 , or high-frequency sensor dynamics. An amplifier  34  may be used to boost the filtered signal (arrow  124 ) to form the proportional output voltage signal (arrow  24 ), which is then transmitted to the diamond-shaped actuator  14  as noted above. Energy is dissipated in the control system  50  in the form of heat within the controller  118 . 
         [0031]    Referring to  FIG. 3 , a vibration control system  150  is shown in another example embodiment. Here, each diamond-shaped actuator  14  has its own dedicated controller  18 , thus providing a measure of control redundancy. Each diamond-shaped actuator  14  may be adhered or bonded to the surface of a flexible panel  16  having an outer perimeter or boundary  17  using adhesive or other suitable means. The flexible panel  16  may be configured as a bay of an aircraft fuselage in a non-limiting example embodiment, and thus constructed of a sufficiently light weight material such as 6061-T6 aluminum. Other embodiments may include a Plexiglas or other flexible window pane, an aircraft, road, or water vehicle body panel, or any other substantially flexible structure which may vibrate at times during operation. Those of ordinary skill in the art will appreciate the noise-reducing potential of the control system  150 , as well as other uses such as stabilizing of optical devices or other sensitive instrumentation. 
         [0032]    In the simplified example of  FIG. 3 , four diamond-shaped actuators  14  may be positioned around the flexible panel  16  as shown, with each diamond-shaped actuator  14  having a point sensor  20  positioned at each of its four apexes. Localized control is provided over each actuator  14  by a corresponding controller  18 . This particular embodiment, although involving a larger number of controllers  18  per flexible panel  16  than the embodiment of  FIG. 2  described above, may provide added control redundancy. That is, if a given controller  18  should happen to fail, the remaining controllers  18  of the same panel  16  can continue to function properly. 
         [0033]    Referring to  FIGS. 4 and 5 , in two possible embodiments the requisite shape of the diamond-shaped actuators  14  described above with reference to  FIGS. 1-3  may be provided by using a diamond-shaped actuator  14 A ( FIG. 4 ) or a diamond-shaped actuator  14 B ( FIG. 5 ). The diamond-shaped actuator  14 A of  FIG. 4  has a diamond-shaped piezoelectric wafer  30  with a thickness (T). The diamond-shaped actuator  14 B of  FIG. 5  has a rectangular piezoelectric wafer  130  with thickness (T). As noted above, the diamond-shaped actuator  14  may be relatively thin, and therefore the dimension (T) may be in the range of approximately 0.3 mm to approximately 0.5 mm in an example embodiment. The diamond-shaped actuator  14  may be integrated partially or fully within a composite structure. 
         [0034]    In  FIG. 4 , an IDE  28  with a plurality of electrode members  29  is mounted to the primary surface  55  of the diamond-shaped piezoelectric wafer  30 . The reverse primary surface  155  may have mounted thereto another IDE  28 . Likewise, in  FIG. 5  a generally diamond-shaped IDE  128  with a plurality of electrode members  129  is mounted to the primary surface  55  of the rectangular piezoelectric wafer  130 . Unlike a conventional monolithic-shaped transducer in which the electrical field couples to both in-plane directions equally. the IDE pattern of  FIGS. 4 and 5  enables the application of an electrical field in a preferred in-plane direction. 
         [0035]    Referring to  FIG. 6 , a diamond-shaped actuator  14  is shown in another possible embodiment. The diamond-shaped actuator  14  is affixed to a membrane  37 . IDEs  228  with electrode members  229  are connected to a power source (not shown) via solder pads  42 . Various dimensions d 1 -d 7  are included to detail possible scale according to a particular embodiment. Other dimensions may be contemplated without departing from the intended inventive scope. 
         [0036]    In an example embodiment, d 1 =approximately 1-2 mm, d 2 =approximately 28-30 mm, d 3 =approximately 16-18 mm, d 4 =approximately 2-3 mm, d 5 =approximately 31 to 33 mm. d 6 =approximately 64 to 65 mm, and d 7 =approximately 69 to 71 mm. One of ordinary skill in the art will appreciate that the actuator  14  can be scaled to the specific application, and thus the above example ranges are not limiting. 
         [0037]    While the best modes for carrying out the invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.