Abstract:
A system and method for operating a cylindrical acoustic projector is provided which allows efficient operation of the projector over a wide bandwidth. The system and method use multiple power amplifiers each tuned to operate over separate and narrow bandwidths, the number of seperate bandwidths corresponding to the number of amplifiers such that the total bandwidth is covered. Each tuning network assembly includes the power amplifier, a transformer and a tuning inductor, with the tuning inductor selected for proper tuning over the frequency bands the amplifier is to operate at. The narrow bandwidths for each power amplifier result in a substantial reduction in the reactive power dissipated in the amplifiers and also the total power consumption of the acoustic projector.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein 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 therefore. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates generally to acoustic projectors, and more particularly to a system and method for operating acoustic projectors over a wide bandwidth while reducing the power supplied and dissipated. 
     (2) Description of the Prior Art 
     Acoustic projectors of the type having multiple ceramic elements are used to provide wide bandwidth operation. Such projectors are normally powered by an amplifier tuned to the center of the frequency band of operation. For example, U.S. Pat. No. 4,652,786 to Mishiro recites a torsional vibration apparatus having a plurality of electrodes formed on the two surfaces of a circular member of electrostrictive material. Adjacent electrodes are simultaneously polarized so as to be mutually reversed in a circumferential direction. The electrodes essentially form multiple elements from the circular member. A high frequency voltage is tuned to the slide resonance frequency and impressed on the apparatus to induce resonant vibration. The electrodes are connected to a power supply through a transformer having the primary coil connected to the power supply, the midpoint of the secondary coil connected to ground and the ends of the secondary coil connected to the segmented electrodes in an alternating manner such that adjacent electrodes have opposite polarity. In a stack configuration, the ends of the secondary coil would be connected at each end of the stack. The power amplifier load at the frequency band edges is highly reactive with a large phase angle. This results in the power amplifier and its power source supplying substantial amounts of reactive power to the projector, with power being dissipated in the amplifier. A need exists to operate acoustic projectors more efficiently over a wide bandwidth. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object of the present invention to provide a system and method to operate an acoustic projector more efficiently over a wide bandwidth. 
     Another object of the present invention is to provide a system and method to operate an acoustic projector which reduces the power dissipated in the amplifiers. 
     Still another object of the present invention is to provide a system and method to operate an acoustic projector which reduces the power supply requirements of the projector. 
     Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings. 
     In accordance with the present invention, a system and method for operating an acoustic projector is provided which allows efficient operation of the projector over a wide bandwidth. The system and method use multiple tuning network assemblies each operating over separate and narrow bandwidths. Each tuning network assembly has a power amplifier, a transformer and a tuning inductor. The tuning inductor for each tuning network assembly is selected for proper tuning over the frequency bands for that assembly. The number of separate bandwidths corresponds to the number of amplifiers such that the total bandwidth is covered. As is well known in the art, the narrower bandwidths for each power amplifier will result in substantial reductions in the reactive power dissipated in the amplifiers and also in the total power consumption of the acoustic projector. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the invention and many of the attendant advantages thereto will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein corresponding reference characters indicate corresponding parts throughout the several views of the drawings and wherein: 
     FIG. 1 is a prior art flextensional acoustic projector; 
     FIG. 2A is a schematic representation of the system of the present invention for operating a flextensional acoustic projector configured for parallel tuning; 
     FIG. 2B is a schematic representation of the system of the present invention for operating a flextensional acoustic projector configured for series tuning; 
     FIG. 3 is a schematic representation of the system of the present invention for operating a cylindrical acoustic projector; 
     FIG. 4A is a schematic representation of the system of the present invention for operating a split-ring acoustic projector; 
     FIG. 4B is a schematic representation of the system of the present invention for operating a split-ring acoustic projector having an electrical isolation element; and 
     FIG. 5 is a block diagram of the method of operating an acoustic projector with multiple tuning network assemblies in accordance with the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, there is shown a schematic representation of a prior art wide bandwidth flextensional acoustic projector  10 . Acoustic projector  10  has a stack  12  of ceramic elements  12   a  enclosed within shell  14 . Typically, acoustic projector  10  is driven through a tuning network assembly  16  which applies a tuned voltage across stack  12 . Tuning network assembly  16  includes power amplifier  18 , which provides an input signal, indicated by arrow  20 , corresponding to the bandwidth. Transformer  22  receives signal  20  and provides a voltage output which is tuned to the center of the frequency band of operation by tuning inductor  24 . 
     Referring now to FIGS. 2A and 2B, there is shown a schematic representation of a multi-tuned flextensional acoustic projector  100  utilizing the system of the present invention. Electrical isolation element  102  is positioned within the stack  104 , thus forming upper stack  104   a  and lower stack  104   b,  each consisting of multiple ceramic elements  106 . The location of isolation element  102  within the stack will depend on the acoustic properties of projector  100  and the desired acoustic signal. Acoustic projector  100  is driven by tuning network assembly  108  having two power amplifiers  110   a  and  110   b.  Each power amplifier provides a signal, indicated by arrows  112   a  and  112   b,  corresponding to a portion of the bandwidth, such that the total bandwidth is represented by signals  112   a  and  112   b.  Transformers  114   a  and  114   b  receive signals  112   a  and  112   b,  respectively and provide a voltage output. The voltage output of transformer  114   a  is tuned by tuning inductor  116   a  to the center of the portion of the bandwidth for signal  112   a.  Similarly, the voltage output of transformer  114   b  is tuned by tuning inductor  116   b  to the center of the portion of the bandwidth for signal  112   b.  In FIG. 2A, tuning inductors  116   a  and  116   b  are shown in a parallel tuning configuration. In FIG. 2B, tuning inductors  116   a  and  116   b  are shown in a series configuration. The tuned voltage from inductor  116   a  is applied across upper stack  104   a  via electrical connections  118   a  and  120   a,  while the tuned voltage from inductor  116   b  is applied over lower stack  104   b  via electrical connections  118   b  and  120   b.  When compared with prior art acoustic projector  10  of FIG. 1, the reactive power supplied by amplifiers  110   a  and  110   b  is considerably less than that supplied by amplifier  18 . As an example, this system or technique could be utilized for a single projector to transmit two widely separated (in frequency) continuous wave tones with almost no reactive power generated. 
     The system of providing a multi-tuned acoustic projector can be used with other types of acoustic projectors. FIG. 3 shows a schematic representation of the preferred embodiment for multi-tuned cylindrical acoustic projector  200 . Projector  200  consists of a tangentially polarized ceramic cylinder  202  having multiple ceramic elements  202   a  alternating circumferentially with conductive stripes  202   b , as is well known in the art. Tuning network assembly  108  is used to drive projector  200  with connections  118   a  and  120   a  driving two adjacent ceramic elements  202   a  and connections  118   b  and  120   b  driving alternating pairs of ceramic elements  202   a.  It can be seen that leads  120   a  and  118   b  feed the same alternating conductive stripes  202   b  and thus can be connected into a single lead  204 . Leads  118   a  and  120   b  connect to every fourth conductive stripe  202   b,  such that the pattern ( 118   a,    204 ,  120   b,    204 ) of feeds to conductive stripes  202   b  is repeated four times about the cylinder. FIG. 4A shows a schematic representation of multi-tuned split ring projector  300  having an inner ceramic ring  302  surrounded by adjacent outer ceramic ring  304 , which in turn is surrounded by shell  306 . In this configuration, tuned voltage from inductor  116   a  is applied over inner ceramic ring  302  and tuned voltage from inductor  116   b  is applied over adjacent outer ceramic ring  304 . As in FIG. 3, leads  120   a  and  118   b  are connected to form lead  204 . FIG. 4B shows a schematic representation of multi-tuned split ring projector  300  having electrical isolation ring element  308  between inner ceramic ring  302  and outer ceramic ring  304 . Again, tuned voltage from inductor  116   a  is applied over inner ceramic ring  302  and tuned voltage from inductor  116   b  is applied over adjacent outer ceramic ring  304 . However, leads  120   a  and  118   b  are not connected due to the presence of isolation ring element  308 . 
     In the general case, the method of providing a multi-tuned acoustic projector is illustrated by the steps shown in FIG.  5 . Step  400  provides the wide bandwidth acoustic projector which will be multi-tuned. In step  402 , the number of tuning bands are determined based on the bandwidth and number of ceramic elements in the projector. For example, in a flextensional acoustic projector such as FIG. 1, the upper limit to the number of tuning bands is the number of ceramic elements  106  in the stack  104 . Similarly, for a split ring acoustic projector such as FIG. 4A, the upper limit to the number of tuning bands is the number of ceramic rings. For a cylindrical projector such as FIG. 3, the upper limit to the number of tuning bands is the number of pairs of ceramic elements  202   a.  The number of tuning bands will also depend on the power savings desired. Additional power can be saved utilizing additional tuning bands, however, the driving circuitry becomes increasingly complex. To provide the greatest reduction in reactive power requirements, the number of tuning bands should be a whole number divisor of the number of ceramic elements, rings or pairs of elements. Once the number of tuning bands is determined, the bandwidth is divided into a corresponding number of portions at step  404 . Step  406  divides the acoustic projector into a corresponding number of sub-elements. For example, the flextensional acoustic projector of FIG. 2 was divided into two stacks, or sub-elements, corresponding to the two tuning bands. Step  408  provides a tuned voltage corresponding to each portion of the bandwidth across a corresponding sub-element of the acoustic projector. Step  408  may also be broken into the intermediate steps of: providing at step  408   a,  for each portion of the bandwidth, a corresponding amplified signal; transforming each of the amplified signals to a voltage at step  408   b;  tuning the voltage to the center of the corresponding portion of the bandwidth at step  408   c;  and applying the tuned voltage across the corresponding sub-element at step  408   d.    
     The invention thus described provides a system and method for driving an acoustic projector with reduced power being dissipated in the amplifiers and reduced overall power supply requirements. The acoustic projector is driven by multiple tuning network assemblies each driving a sub-element of the projector over a corresponding portion of the bandwidth. Since power supplies generally increase in size and weight with increasing power requirements, an acoustic projector of the current invention is useful in applications which are space and weight limited, such as broadband noise acoustic countermeasures. 
     Although the present invention has been described relative to specific embodiments thereof, it is not so limited. The multi-tuned acoustic projector system and method can be used to drive most wide bandwidth acoustic projectors consisting of multiple sub-elements which can be independently driven. Also, though the embodiments shown in FIGS. 2-4 utilize an inductor for tuning the voltage, any method of tuning can be employed. As in FIG. 2B, the embodiments of FIGS. 3-4 can be configured for series tuning. 
     Thus, it will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.