Abstract:
Various piezoelectric transducers are provided. In one form, a polymer film or thin ceramic piezoelectric transducer is formed of various piezoelectric (active) and dielectric (inactive) layers in which the piezoelectric effect may be attenuated locally at any given point on the surface of the transducer, by use of printed circuit patterns, preferably made by photolithography. This provides a practical realization of a distributed piezoelectric transducer with a bi-dimensional polarization profile that varies smoothly over the surface of the piezoelectric structure. Flexibility of the fabrication procedure provides a way to optimize the design of a distributed piezoelectric transducer for applications such as active vibration control. In another form, a segmented piezoelectric transducer includes a set of active elements such as piezofilms, electronics and flexible printed circuits and connected with external electronic circuitry. The segmented piezoelectric transducer uses a modulation scheme to combine the individual sensor outputs. Modal coordinates may be extracted from a test structure on which the segmented piezoelectric transducer is attached. As well, it is possible to use a distributed piezoelectric transducer to actuate a structure by using a time-varying polarization profile.

Description:
[0001]    This patent application is a divisional of and thus claims the benefit of U.S. patent application Ser. No. 09/876,650 filed Jun. 7, 2001 entitled Piezoelectric Transducer. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to piezoelectric sensors and actuators and, more particularly, to piezoelectric laminates, segmented piezoelectric film sensors, and the application thereof to modal analysis.  
         DESCRIPTION OF THE PRIOR ART  
         [0003]    During the past decade, distributed piezoelectric sensors and actuators (collectively, transducers) have been used increasingly in the field of active vibration sensing and control. The basic idea is to bond a piezoelectric lamina onto a large surface of a thin vibrating structure. By collecting the piezoelectric charge induced by surface strains, one can estimate the vibrational state of the structure. Reversely or conversely, by applying an appropriate control voltage to the transducer, it is possible to excite (actuate) the structure and thus, for example, to generate an artificial damping effect or monitor structural behavior. Among the advantages of this approach are the light weight of the piezoelectric transducer and the possibility to embed the device in thin, flexible structures that are prone to vibrate. Use of this technology can be found from the aerospace industry to manufacturers of high-end consumer goods such as “smart skis” (i.e. skis that incorporate such transducer technology).  
           [0004]    For this technology to be efficient, it is necessary for the polarization of the piezoelectric lamina to vary in a well-defined manner over the surface of the transducer. Since the piezoelectric sensitivity of a laminate is normally uniform, a weighting function must be applied to the transducer with the help of some controlled physical process.  
           [0005]    It has been demonstrated that by measuring or inducing strains in a thin bending structure (such as a beam or plate) by means of a shaped piezoelectric lamina bonded on its surface, it is possible to sense or actuate directly a given modal contribution of a transverse motion, provided that the boundary conditions satisfy certain requirements that result in the orthogonality of these modes.  
           [0006]    In this method, it can be shown that the local piezoelectric sensitivity of the transducer must follow an appropriate spatial sensing distribution (or weighting) function Λ(α 1 , α 2 ) in order to interact with a unique mode. It should be noted that the use of the two parameters α 1  and α 2  do not limit the domain of Λ to a plane. Instead, α 1  and α 2  are bi-dimensional curvilinear coordinates that can define any point on the neutral surface of a three-dimensional shell. Since the piezoelectric sensitivity of a laminate is usually uniform, a weighting function must be implemented with the help of a controlled physical process. Sometimes, the operation of imposing a given polarization profile on a planar piezoelectric transducer is also called “shading”. There are several available procedures for shading.  
           [0007]    In one procedure, the longitudinal piezoelectric constants of the piezoelectric material can be adjusted to a value proportional to a weight function Λ(α 1 , α 2 ), by means of a manufacturing method such as repoling, doping or the dosage of a two-phase composite. It should be noted however that these techniques are costly and difficult to implement in practice.  
           [0008]    In another procedure, a given area of a piezoelectric transducer is made effective by the presence of a pair of electrodes which can either collect the charges generated piezoelectrically (sensor mode) or impose an electric potential between them (actuator mode). Thus, an elementary type of spatial weighting is obtained by limiting the areas provided with electrodes. In this case, the weighting function takes only two values: Λ=1 inside the shape boundaries, or Λ=0. In practice, a photolithographic process can implement this profile.  
           [0009]    For a beam, it is sufficient to implement a one-dimensional weight function. In this case, the appropriate variation of piezoelectric sensitivity can be obtained by varying the width of the electrode down the length of the beam since there is no deflection along this direction. To impose the correct sign of the weight (spatial sensing distribution) function, areas are defined where the polarization of the piezoelectric material should be accordingly positive or negative, either by bonding the laminate “face up” or “face down”, or by inverting its contacting electrodes.  
           [0010]    In the more general case of a structure whose deflection varies along two directions, a bi-dimensional spatial sensing distribution function can be approximated by a lattice of small electrodes that are turned either “on” or “off”. However the task remains to connect electrically the individual active electrodes and to impose the correct function sign. In practice, this is only feasible if the “on” electrodes happen to be grouped in a few contiguous domains.  
           [0011]    In spite of the elegance of their concept, modal sensors and actuators have an important limitation. A manufacturing process allowing control the weight function by repoling, doping, or dosing a two-phase composite is usually not available at the level of the application engineer. Furthermore, these steps would be very costly to implement. As a consequence modal sensors have not been applied to structural elements, such as plates and shells, unless their weight function could be reduced to a one-dimensional function or a product of such functions by separation of variables.  
           [0012]    This difficulty and other factors have promoted the use of an approximated version of modal sensors, called segmented piezoelectric sensors. In this design the distributed effect of a piezoelectric laminate is replaced by an array of size-limited, discrete piezoelectric sensors, each of which are measured separately, and the outputs of which are being sampled, multiplied by discrete weight factors (calculated by the method of modal filtering), and then added. The main advantage of this method is that it shifts the operation of fixing the weight factors from the manufacturing process to an electronic operation, making it much more flexible. Thus, except for the number of channels, it is not more difficult to build such a system for a variety of shells and plates rather than a beam.  
           [0013]    However, segmented piezoelectric sensors also have their shortcomings. For one, segmented piezoelectric sensors are only able to model a finite number of modes (the more transducers in the array, the higher this number). If unmodelled modal contributions (residual modes) are present, they constitute a source of noise. For another, segmented piezoelectric sensors are much less compact than modal sensors. Each channel requires a full measurement chain including coaxial cable, low current or charge sensitive amplifier and analog to digital converter. Furthermore, a digital signal processing board is required to carry out the computations to estimate the modal coordinates. By contrast, a modal sensor simply needs a unique coaxial cable and a low current or charge sensitive amplifier. For this reason it is more difficult to embed segmented piezoelectric sensors as elements of an intelligent structure. Finally, because of the analog to digital conversions involved with segmented piezoelectric sensors, it is difficult to include segmented transducers in sensing-actuating applications, such as the frequency-stabilizing element in a resonator. On the other hand, the use of a modal sensor in a resonating circuit is straightforward and may open the door to applications where the frequency of such a system could be used to monitor physical parameters, like the temperature of the structure or a variation of pressure exercised on it.  
           [0014]    Because of the limitations imposed by these technologies, it is very difficult to obtain a polarization profile whose shape varies in function of two geometric dimensions. Thus, in order to apply this active control scheme to structures whose deflections vary along two independent coordinates, such as vibrating plates and shells, one of the following simplifying methods is normally used.  
           [0015]    One such method is that the behavior of a bending plate may be approximated with the one of a bending beam, so that the electrode shaping method described above may be used.  
           [0016]    Another such method is that an arbitrary, bi-dimensional polarization profile may be approximated with a discrete pattern. Namely, one may juxtapose a finite set of segmented piezoelectric sensors/actuators whose individual contributions are weighted and added up electronically.  
           [0017]    A further such method is that a polarization profile can be roughly approximated by applying a “binary” weighting function, i.e. one in which contiguous areas are multiplied by values of either 0 or 1. This can be realized by bi-dimensional shaping of the measuring electrodes.  
           [0018]    However, in each of these simplified embodiments, the efficiency of the control scheme is eventually affected. Possible problems may range from perturbations by residual modes to instabilities via spillover. The use of segmented sensors/actuators may also pose practical problems if many channels are required because of extra weight added to the test structure by the connect cables. As a consequence, there is a need for a new generation of distributed piezoelectric transducers in which arbitrary polarization profiles could be implemented in a versatile and practical manner.  
           [0019]    There is also a need for a piezoelectric transducer that can efficiently implement a weighting function/distribution.  
           [0020]    There is further a need for a new generation of segmented piezoelectric transducers.  
         SUMMARY OF THE INVENTION  
         [0021]    In one form (referred to herein as Form A), the present invention is a piezoelectric transducer. Particularly, in one form the present invention is a multi-layered piezoelectrically active and inactive lamina structure having selective metalizations that implement a bi-dimensional weighting function of piezoelectric sensitivity.  
           [0022]    When the lamina structure is attached to a thin structural element, the piezoelectric transducer may be used as a modal sensor and/or actuator.  
           [0023]    The present invention provides at least the following advantages over known solutions to the problem of sensing or actuating transverse modes of vibration in thin structures, which may be applied to various applications too numerous to list: (1) compared to shaped piezoelectric modal sensors and actuators, it is straightforward to implement with this design any arbitrary two-dimensional weight function for structure such as plates and shells. Also, the problem of forcing the correct sign of the weight function is solved in the invention by collecting piezoelectrically induced charges on both faces/surfaces of active lamina of the piezoelectric laminate in a first configuration, and on two individual laminas of the piezoelectric laminate in a second configuration; (2) modal sensors whose weight functions have been built-in by modulating the piezoelectric sensitivity of the material by a manufacturing means, such as repoling, are very costly and often the technology is not readily available, if at all, to engineers, while the technologies required by the present invention in order to implement the weight function (e.g. photolithography or screen printing) are widely available and economical; (3) segmented piezoelectric sensors require a full measurement chain (i.e. a transmitting cable, an impedance converter such as a charge amplifier, and a sampling circuit converting the analog signal into digital form) for each channel. This prior approach becomes very expensive, and thus not practical for modal filtering because of the high number of channels. As well, perturbations by residual modes are then more likely to occur. By contrast, a very high density of weighted “channels” can be implemented with the present invention at no special cost, the limitation being set by the resolution of the available photolithographic or screen-printing process. Also, prior segmented piezoelectric sensors and actuators require an analog to digital conversion (digital to analog, respectively) and dedicated digital signal processing boards. For the present invention, all that is needed is a low-current or charge sensitive amplifier in the sensing mode and a sine voltage generator in the actuating mode for the present piezoelectric laminate.  
           [0024]    In another form (herein referred to as Form B), the present invention is a segmented piezoelectric film sensor. The segmented piezoelectric film sensor includes a set of active elements mounted onto a base and connected with external circuitry. The set of active elements includes piezofilms/sensors, individual electronics for each segment, and a flexible printed circuit. The external circuitry provides a modulation scheme to combine individual sensor outputs and to extract modal coordinates from a test structure on which the segmented piezoelectric sensor is mounted.  
           [0025]    The invention has advantages over prior segmented piezoelectric sensors and to modal filtering using segmented piezoelectric sensors. In particular, compared to prior segmented piezoelectric sensors, the present invention simplifies the required hardware by replacing N-2 measurement chains—each comprising a low current or charge sensitive amplifier and a coaxial cable—with 2N digital modulation signals and electric wires, and by eliminating the need for a digital signal processing board, the system output being a signal directly proportional to the modal coordinate of interest. Also, the invention is always able to provide an estimate of the modal coordinate in real-time, even if a very large number of piezosensors have to be taken into account. Finally, the use of a flexible printed circuit in the present invention makes it easier to connect the active components mounted on the structure to the external electronic circuitry. As well, compared to shaped piezoelectric modal sensors, the system has the flexibility of being able to modify its weighting function, either to improve the estimate of the modal coordinate of interest, or to monitor a different modal coordinate. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0026]    The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawing comprised of a plurality of figures, wherein:  
         [0027]    [0027]FIG. 1 is a perspective view of a piezoelectric lamina structure in accordance with the principles of Form A of the present invention;  
         [0028]    [0028]FIG. 2 is a an exploded view of the individual laminas of the piezoelectric lamina structure of FIG. 1;  
         [0029]    [0029]FIG. 3A is a block diagram of an exemplary system utilizing the piezoelectric lamina structure in a sensor mode for Form A of the invention;  
         [0030]    [0030]FIG. 3B is a block diagram of an exemplary system utilizing the piezoelectric lamina structure in an actuator mode for Form A of the invention;  
         [0031]    [0031]FIG. 4 is a more detailed diagram of an exemplary system utilizing the piezoelectric lamina structure of FIG. 3A;  
         [0032]    [0032]FIG. 5A relates to Form A of the invention and is a top plan view of an upper surface or face of a lamina of one embodiment of the piezoelectric lamina structure of FIG. 1 having an exemplary electrode pattern thereon;  
         [0033]    [0033]FIG. 5B relates to Form A of the invention and is a top plan view of a lower surface or face of the lamina of FIG. 5A having an exemplary electrode pattern thereon (Form A);  
         [0034]    [0034]FIG. 6A relates to Form A of the invention and is a top plan view of an upper surface or face of a lamina of another embodiment of the piezoelectric lamina structure of FIG. 1 having an exemplary electrode pattern thereon;  
         [0035]    [0035]FIG. 6B relates to Form A of the invention and is a top plan view of a lower surface or face of the lamina of FIG. 6A having an electrode thereon;  
         [0036]    [0036]FIG. 7A relates to Form A of the invention and is a top plan view of an upper surface or face of another lamina of the second embodiment of the piezoelectric lamina structure of FIG. 1 having an electrode thereon;  
         [0037]    [0037]FIG. 7B relates to Form A of the invention and is a top plan view of a lower surface or face of the lamina of FIG. 7A having an exemplary electrode pattern thereon;  
         [0038]    [0038]FIG. 8 is a block diagram of a system utilizing a segmented piezoelectric sensor (Form B) in accordance with the principles of an aspect of the present invention;  
         [0039]    [0039]FIG. 9 is a perspective view of a segmented piezoelectric sensor (Form B) in an exemplary system of the manner of FIG. 8;  
         [0040]    [0040]FIG. 10A is representation of switch positions in accordance with an aspect of Form B of the present invention;  
         [0041]    [0041]FIG. 10B is representation of switch positions in accordance with an aspect of Form B the present invention;  
         [0042]    [0042]FIG. 11A is a graph of an exemplary piezoelectric signal from the system of FIG. 9;  
         [0043]    [0043]FIG. 11B is a graph of an exemplary pulse modulation signal for the system of FIG. 9;  
         [0044]    [0044]FIG. 11C is a graph of the combination of the signals of FIGS. 11A and 11B;  
         [0045]    [0045]FIG. 12A is a frequency domain representation of a baseband signal of the system of FIG. 9;  
         [0046]    [0046]FIG. 12B is a frequency domain representation of a modulated signal of the system of FIG. 9;  
         [0047]    [0047]FIG. 13 is another switch representation for the system of FIG. 9 having a pulse-width modulation scheme; and  
         [0048]    [0048]FIG. 14 is a further switch representation for the system of FIG. 9 having a differential pulse-width modulation scheme. 
     
    
       [0049]    Corresponding reference characters indicate corresponding parts throughout the several views.  
       DETAILED DESCRIPTION  
       [0050]    While the invention is susceptible to various modifications and alternative forms, the specific embodiment(s) shown and/or described herein is by way of example. It should thus be appreciated that there is no intent to limit the invention to the particular form disclosed, as the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
         [0051]    Referring now to FIG. 1, there is shown an embodiment of a piezoelectric device, generally designated  20 , in accordance with the principles presented herein. The piezoelectric device  20  is functional as a sensor and/or an actuator and will hereinafter be referred to as a piezoelectric transducer  20 . In general, the piezoelectric transducer  20  is a laminate composed of a plurality of layers or lamina as hereinafter described.  
         [0052]    The piezoelectric transducer  20  includes a middle lamina  22 , a first upper lamina  24 , a first lower lamina  26 , a second upper or top lamina  28 , and a second lower or bottom lamina  30 , all of which is provided on a base or structural component  32 . Referring additionally to FIG. 2, the middle lamina  22  is defined by a plate or sheet  22   c  of a piezoelectric active material. The plate  22   c  has an upper surface or face  22   a  and a lower surface or face  22   b.  In accordance with the principles of the present invention, the upper and lower surfaces,  22   a  and  22   b,  are metalized (i.e. layered with an electrically conductive material such as a metal). As described more fully below, the upper and lower metalized surfaces  22   a  and  22   b  are patterned or etched selectively, preferably by using a photolithographic or screen printing process.  
         [0053]    The first upper lamina  24  is defined by a thin layer  24   c  of a piezoelectrically inactive material of preferably a constant thickness. The layer  24   c  has an upper surface or face  24   a  and a lower surface or face  24   b.  Preferably, the lamina  24  is an adhesive such that the lower surface  24   b  is bonded onto the upper surface  22   a  of the lamina  22 .  
         [0054]    The first lower lamina  26  is defined by a thin layer  26   c  of a piezoelectrically inactive material of preferably constant thickness in like manner to the lamina  24 . The layer  26   c  has an upper surface  26   a  and a lower surface  26   b.  Preferably, the lamina  26  is an adhesive such that the upper surface  26   a  is bonded onto the lower surface  24   b  of the lamina  24 . Preferably, the lamina  26  and the lamina  24  have substantially identical dielectric constants.  
         [0055]    The second upper or top lamina  28  is defined by a plate or sheet  28   c  of a piezoelectrically inactive material. The plate  28   c  has an upper surface or face  28   a  and a lower surface or face  28   b.  The upper and lower surfaces  28   a  and  28   b  of the plate  28   c  are metalized but not etched or patterned. The lower surface  28   a  of the plate  28   c  is bonded to the upper surface  24   a  of the plate  24   c .    
         [0056]    The second lower or bottom layer  30  is defined by a plate or sheet  30   c  of a piezoelectrically inactive material. The plate  30   c  has an upper surface or face  30   a  and a lower surface or face  30   b.  The upper and lower surfaces  30   a  and  30   b  of the plate  30   c  are metalized but not etched or patterned. The upper surface  30   b  of plate  30   c  is bonded to the lower surface  26   b  of the plate  26   c.    
         [0057]    The laminate  20  composed of laminas  22 ,  24 ,  26 ,  28 , and  30  is bonded onto the base  32 . The base  32  is defined by a shell, beam, or other structural component. The plate  32   c  has an upper surface or face  32   a  and a lower surface or face  32   b.  The lower surface  30   b  of the lamina  30  is bonded to the upper surface  32   a  of the base  32 .  
         [0058]    In FIG. 3A there is depicted a block representation of a piezoelectric sensor system generally designated  40 . The piezoelectric system  40  includes the piezoelectric transducer  20  in communication with an amplifier  42  such that the system  40  provides a sensing mode for the piezoelectric transducer  20 . The piezoelectric transducer  20  provides signals that are amplified by the amplifier  42 . The amplifier  42  provides an output signal S OUT  that represents piezoelectric signals from the piezoelectric transducer  20 .  
         [0059]    In FIG. 3B there is depicted a block representation of a piezoelectric actuator system generally designated  44 . The piezoelectric system  44  includes the piezoelectric transducer  20  in communication with a voltage generator  46 , preferably generating a sine wave of a given voltage, such that the system  44  provides an actuating mode for the piezoelectric transducer  20 . The sine voltage generator  46  provides its signal to the piezoelectric transducer  20 .  
         [0060]    Referring now to FIG. 4, there is shown an exemplary embodiment of a more detailed piezoelectric transducer system  40  as set forth in FIG. 3A. The piezoelectric transducer  20  is electrically coupled to the amplifier  42  via a coaxial cable  50 . More particularly, the metalized lower surface  28   b  of the lamina  28  is coupled via an electrical line  48   b  to the coaxial cable  50  while the upper surface  30   a  of the lamina  30  is coupled via an electrical line  48   a  to the coaxial cable  50 . The amplifier  42  is preferably a charge sensitive (or low-current) amplifier that provides a voltage output signal V OUT .  
         [0061]    Referring to FIGS. 5A and 5B, and in accordance with an aspect of form A of the present invention, the upper and lower metalized surfaces  22   a  (FIG. 5A) and  22   b  (FIG. 5B) of the middle lamina  22  are formed such that a pattern of openings  52  are formed in each surface. The upper and lower metalized surfaces  22   a  and  22   b  thus form electrodes. The pattern of openings  52  in each metalized surface  22   a  and  22   b  are preferably formed via a photolithography or screen-printing process (patterning). The surface density of the metalized surface, after undergoing patterning, defines the magnitude of a weight function in a given area. By grounding the electrodes (i.e. the upper and lower metalized surfaces  22   a  and  22   b ) a transverse polarization field generated inside the piezoelectric lamina  22  has no external effect wherever the surface is covered by metalization. Thus, the metalization/electrodes locally act as shields to the polarization field. An opening  52 , however, allows the transverse polarization field to extend out of the piezoelectric lamina  22 , through the dielectric layers (laminas  24  and  26 ) by capacitive coupling and finally reaches the inside metalization of the laminas  28  and  30 .  
         [0062]    The following conditions are imposed on the geometry of the etched patterns: if there is an opening  52  on either face/surface ( 22   a,    22   b ) of the piezoelectric lamina  22 , the opposite area facing it must be shielded (see FIGS. 5A and 5B, and note the alignment of the corners ABCD). Since the electrodes (metalization) of the lower surface  28   b  of the lamina  28  and the upper surface  30   a  of the lamina  30  are continuous, they collect the contributions of the transverse polarization field passing through the openings  52  of the upper shield/metalization/electrode  22   a  and the lower shield/metalization/electrode  22   b,  respectively, where they are converted into a total electric charge. It should be noted that a distinction must be made between the electrodes (metalizations) of the lower surface  28   b  of the lamina  28  and the upper surface  30   a  of the lamina  30 : the sign of a given transverse polarization field appears to be reversed if seen from the measuring electrode ( 28   b ) of the lamina  28  to the grounded shield or if it is measured from the electrode ( 30   a ) of the lamina  30  to the grounded shield. Therefore, a given deformation of the piezoelectric lamina results in an accumulation of charges of different signs on each measuring electrode. The electric currents that are formed subsequently on these electrodes can be collected and measured by the low-current or charge sensitive amplifier  42 .  
         [0063]    If the relative size of each opening  52  is small compared to the variations of the transverse polarization field, i.e. if the field is practically constant over the surface of each opening  52 , then it is possible to weigh the piezoelectric effect over a given area by fixing the size of the openings  52  covering a particular area of the lamina  22 . In short, this effect is practically the same or similar to a sensing distribution function Λ(α 1 , α 2 ) weighting the local sensitivity of the piezoelectric lamina  22  (the sign of which is controlled by locating the shield openings  52  on the upper or lower faces  22   a,    22   b  of the piezoelectric lamina  22 ). This weighting function is discrete since the density of the openings  52  in the shield (metalization) is limited by the resolution of the available technology (i.e. photolithography or screen printing process) as well as by capacitive border effects taking place in the dielectric laminas (laminas  24  and  26 ). However, the density of weighting operations per area is still much higher than what can be achieved with typical segmented piezoelectric sensors. For practical purposes, a modal filter implemented in this manner can reasonably be seen as a new type of modal sensor or actuator.  
         [0064]    In this design, the openings  52  in the shields (i.e. the metalization on the surfaces  22   a  and  22   b  of the lamina  22 ) are not supposed to touch each other, since it would result in the isolation of metalized areas which could not be grounded any more. This restriction limits the range of the weight function from: −1&lt;Λ&lt;1, to approximately: −0.5&lt;Λ&lt;0.5. Also, the capacitive coupling operation reduces the strength of the polarization field by a factor: γ=1/(1+κ), where κ=(∈ r   1 δ 2 )/∈ r   2 δ 1 ), ∈ r   1 , ∈ r   2  are the relative dielectric constant of the laminas  22 ,  24 , and  26 , and δ 1 , δ 2  are the thickness of the laminas  22 ,  24 , and  26 , respectively. Therefore, the local signal that could be measured with the piezoelectric lamina is multiplied here by a weighting function having values in the following range: −0.5γ&lt;Λ&lt;0.5γ.  
         [0065]    The outside metalization (surface  28   a  and surface  30   b ) of the laminas  28  and  30 , respectively, should be grounded to provide a global electric shield for the piezoelectric laminate  20 . Likewise, the electric connections  48   a  and  48   b  (see FIG. 4) between the measuring electrodes (surface  28   b  and surface  30   a ) of the laminas  28  and  30  respectively, are preferably realized with short coaxial cables, in order to protect their signal against electromagnetic interferences.  
         [0066]    In one form, it is possible to utilize polymer films for the various laminas. The middle lamina  22  can be made out of a bi-metalized piezoelectric polyvinylidene fluoride (PVDF) film. Thin commercial transfer tape may be used as dielectric laminas/adhesives  24  and  26 , while a thin bi-metalized polyester film may be utilized for the outside laminas  28  and  30 .  
         [0067]    In accordance with another aspect or form of the present invention and referring to FIGS. 1 and 2, the piezoelectric transducer  20  again has five (5) layers or lamina that may be used with the systems of FIGS. 3A, 3B, and  4 . However, in this embodiment, the central or middle lamina  22  is made from a thin, piezoelectrically inactive adhesive. The first upper lamina  24  is from a plate of piezoelectrically active material that is bonded to the middle lamina  22 . The first lower lamina  26  is made from a preferably identical plate of piezoelectrically active material that is bonded to the middle lamina  22 . In essence, the lamina  24  and the lamina  26  are effectively bonded together via the middle lamina  22  of adhesive.  
         [0068]    With reference to FIGS. 6A and 6B, the upper and lower surfaces,  24   a  and  24   b,  of the lamina  24  are covered, such as by metalization, by electrodes. The upper surface  24   a  has a pattern of openings  52  and  54  that are preferably formed via photolithography or screen-printing. The metalization of the lower surface  24   b  of the lamina  24  is coupled to a suitable ground. The upper surface  24   a  is coupled via lead  48   b  to the coaxial cable  50  (reference FIG. 4) such that any signal collected via the electrode or metalization is coupled to the amplifier  42 .  
         [0069]    Referring to FIGS. 7A and 7B, the upper and lower surfaces  26   a  and  26   b  of the lamina  26  are covered, such as by metalization, by electrodes. The upper surface  26   a  is coupled to a suitable ground. The lower surface  26   b  has a pattern of openings  52  and  54  that are preferably formed via photolithography or screen-printing. The lower surface  26   b  is coupled via lead  48   a  to the coaxial cable  50  (reference FIG. 4) such that any signal collected via the electrode or metalization is coupled to the amplifier  42 .  
         [0070]    The lamina  28  is made of a piezoelectrically inactive (dielectric) material that is bonded onto the lamina  24 . The outer surface  28   a  of the lamina  28  is provided with an electrode or metalization that is coupled to a suitable ground. The lamina  30  is also made of a piezoelectrically inactive (dielectric) material that is bonded onto the lamina  26 . The outer surface  30   b  of the lamina  30  is provided with an electrode or metalization that is coupled to a suitable ground. The electrodes of the upper surface  28   a  and the lower surface  30   b  act as shields for the high impedance sensor (piezoelectric laminate  20 ) against external electromagnetic interference.  
         [0071]    With respect to this embodiment, the frequency of the openings  52  and/or  54  are varied to control the amount of charge per area collected by the measuring electrodes  24   a  and  26   b,  and therefore to simulate the effect of a spatial weighting distribution function modulating the overall piezoelectric sensitivity of the piezoelectric laminate  20 . However, unlike prior configurations, the openings  52  and/or  54  or “holes” in the electrodes  24   a  and  26   b  are used to inactivate the areas they cover, since the outer electrodes of the laminas  24  and  26  cannot collect a free piezoelectric charge at these locations. The upper layer  24   a  of the lamina  24  is only active over areas where the weight function is positive, whereas the lower layer  26   b  of the lamina  26  is only active over areas where this function is negative.  
         [0072]    Now, provided that the two piezoelectric laminas  24  and  26  share the same poling orientation, the polarization imposed by the respective position of ground and measuring electrodes ensures that the signals generated on each layer have the proper sign. The overall weighted response is finally obtained by adding the respective charges at the node or junction of the electrical leads  48   a  and  48   b,  and the coaxial cable  50 . It should be appreciated that in this configuration, the piezoelectric signals need not be coupled capacitively to transmit the signal to the amplifier  42 , so that the piezoelectric sensor  20  will have a higher signal to noise ratio than the other embodiment.  
         [0073]    Again, commercially available piezoelectric PVDF film and thin transfer tapes may be used to build the present piezoelectric laminate  20 . The piezoelectric laminate  20  along with the amplifier  42  provides a piezoelectric sensor. Conversely, the piezoelectric laminate  20  along with an actuator or sine voltage generator  46  (reference FIG. 3B) provides a piezoelectric actuator.  
         [0074]    Referring to FIG. 8, there is depicted a block diagram of a piezoelectric sensor system for form B of the invention, generally designated  60 , in accordance with an aspect of the present invention. The system  60  includes a segmented piezoelectric sensor having integral circuitry  62  in accordance with the present principles. A signal generator  64  is coupled to the segmented piezoelectric sensor  62  and provides configured signals or sets of modulated signals to the piezoelectric sensor  62 . An integrator  66  is coupled to the piezoelectric sensor  62  that receives signals from the various segmented piezoelectric units of the segmented piezoelectric sensor  62 . A filter  68  is provided for the output of the integrator  66 , the filter  68  providing a piezoelectric output signals represented by block  70 .  
         [0075]    In particular, and referring to FIG. 9, there is depicted an exemplary system, generally designated  80 , implementing the system  40  of FIG. 8. The system  80  includes a segmented piezoelectric sensor made in accordance with the principles presented below, generally designated  82 . The segmented piezoelectric sensor  82  includes a polymeric thin film, substrate, or the like  96  that is bondable to a test structure  94 . The polymeric film  96  may be a polyester film, conductive epoxy, or the like. A plurality of piezoelectric sensors  100  are disposed on the polymeric film  96  and spaced from each other (i.e. segmented). A plurality of microelectronic circuits  102  are also mounted on the polymeric film  96 , the number of which preferably corresponds with the number of the plurality of piezoelectric sensors  100 . Preferably, a microelectronic circuit  102  is adjacent each one of the piezoelectric sensors  100 . A patterned plurality of electrodes and conductive lines  98  are formed on the polymeric film  96  preferably by deposition, but which may take the form of any printed circuit or the like. The plurality of electrodes and conductive lines  98  connect each piezoelectric sensor  100  with an associated microelectronic circuit  102 , communicate with an output, and communicate with an input.  
         [0076]    The system  80  further includes a signal generator  84  that is coupled to the pattern of conductive lines  98  such that the signal generator  84  is in communication with each microelectronic circuit  102 /piezoelectric sensor  100  pair via a conductor  86  at the input of the conductive lines  98 . The conductor  86  is preferably a ribbon cable. Connected at the output of the conductive lines  98  are two conductors that are preferably coaxial cables  92   a  and  92   b  for collecting the currents generated on the surface electrodes of each piezoelectric sensor  100 , such as that shown and described above, and processed by the respective microelectronic circuit  102 . The conductors  92   a  and  92   b  are coupled to a differential charge amplifier  88  that integrates the collected piezoelectric currents. The differential charge amplifier  88  is, in turn, coupled to a low-pass filter  90  to demodulate the output signal U O .  
         [0077]    The signal generator  84  is operative to generate a set of distinct modulating signals for each microelectronic circuit  102  in the manner set forth below. The modulation scheme in conjunction with the segmented piezoelectric sensor  82  combines the piezoelectric signals. While the modulation signals are generated externally via the signal generator  84 , the modulation signals may be generated by circuitry/logic integral with the segmented piezoelectric sensor  82 . It is possible to provide dedicated signal generators via one or several application specific integrated circuits (ASICs) and to mount the components on the structure. These signals may be generated with bi-stable multivibrators, programmable logic devices, a microprocessor board, or the like.  
         [0078]    The charge sensitive amplifier(s)  88  integrate the currents produced by the piezoelectric sensors  100  at the nodes defined by the cables  92   a  and  92   b,  and convert them into two low impedance voltage signals that are then subtracted from each other. The virtual grounds of these amplifiers also hold the voltage across the piezosensors constant, so that parasitic capacitors in the cable or in transistors (MOSFETs) do not affect the signal. The time integration performed by these components has the effect of canceling out the high frequency noise originating from the modulation signals and coupled through the parasitic capacitors of the MOSFETs (see below).  
         [0079]    Each microelectronic circuit  102  includes, operates, and functions in the manner set forth below. Referring to FIGS. 10A and 10B, there is shown two states, positions, or modes of an exemplary embodiment of a microelectronic circuit  102  as coupled to a piezoelectric sensor  102 . Specifically, pairs of analog switches S 1k  and S 2k  can be implemented so that the output of the kth sensing piezoelement is either directed to the remote charge amplifier  88  (position I, FIG. 10A) or grounded (position II, FIG. 10B). In these cases, the ON/OFF signals m 1k (t) and m 2k (t) actuating the switches S 1k  and S 2k  are in phase opposition so that m 1k (t)=m k (t) and m 2k (t) and −m k (t). By alternating the positions I and II, the output current i k (t) (see FIG. 11A) can be modulated by the signal generator  84  into a measured current i k   A (t) (see FIG. 11C) with a pulse-width T k  and a period T m  (see FIG. 11B) or pulse-width modulation (pwm).  
         [0080]    Each analog switch may be implemented with a solid-state device, such as a metal oxide semiconductor field effect transistor (MOSFET). Since these devices are very small, a pair can be surface-mounted or embedded in the vicinity of each piezofilm without interfering with the dynamics of the test structure  94  (see FIG. 9).  
         [0081]    Referring to FIG. 12A, there is shown a representation in the frequency domain of the output current I k (f). Referring to FIG. 12B, there is shown a representation in the frequency domain of the measured current I k   A (f). The baseband signal (FIG. 12A) can be recovered by utilizing the low-pass filter  90  to low-pass filter the measured current, provided that the sidebands do not overlap with the baseband. This is satisfied under the conditions that: i) the signal measured from the k th  piezoelement in the sensor is bandlimited, i.e. there is a maximum frequency f max  such that I k (f)=0 if f&gt;f max , and ii) the modulating frequency f m =1/T m  is at least twice as high as the maximum frequency: f m ≧2 f max . It should be understood that the amplitude of the demodulated current Demod[i k   A (t)] differs from the baseband signal by a scaling factor related to the pulse width and modulation frequency. Specifically:  
         Demod[ i   k   A ( t )]=( T   k   /T   m ) i   k   A ( t ).  
         [0082]    Each piezoelectric sensor  100  output can be modulated in the same manner, but with different pulse-widths T k , so that after demodulation each signal appeared as multiplied by a specific factor L k =(T k /T m ). It is thus possible to add together the modulated currents at a common node A, demodulate their sum i A (t) with a low-pass filter and then time integrate the resulting current and convert it into a voltage with a charge amplifier  88   a  as shown. In practice, however, it is more convenient to first pass the modulated current i A (t) through a charge amplifier and then proceed with the demodulation, as shown in the FIG. 13 (by linearity, the order of these operations makes no difference).  
         [0083]    In this system, the output yields the voltage:  
             U   out          (   t   )       =       (     1   /     C   f       )                       ∑     k   =   1     N            L   k            q   k          (   t   )               ,                         
 
         [0084]    where C f  is the feedback capacitor of the charge amplifier and q k  is the charge generated by each piezosensor  100 .  
         [0085]    If the factors L k  in the above equation are identical with the i th  line of a gain matrix of a modal filter, then the output voltage would be proportional to the i th  modal coordinate of the system. However, in this scheme the factors L k  display all the same sign, since the ratio T k /T m  is always positive. For modes of higher order than one, though, there will always be a number of negative entries. In order to remedy this problem, a differential configuration  88   b  as shown in FIG. 14 may be used.  
         [0086]    In this circuit the currents are not grounded during the phase II (see FIGS. 10A and 10B), but instead are being summed at a second node B and are then directed to a second charge amplifier. By subtracting u B  from u A  with a differential amplifier, the output voltage yielded by the system is the same as given in the above equation, where now:  
           L   k =( R   2   /R   1   C   f )[2( T   k   /T   m )−1].  
         [0087]    As indicated in the above expression for the range of the coefficients, it is possible via the configuration of FIG. 14 to select a negative value for I k :  
         − R   2   /R   1   C   f   ≦L   k   ≦R   2   /R   1   C   f    
         [0088]    With an appropriate set of modulation signals m k , i.e. a corresponding set of values for the pulse-widths T k &#39;s, the voltage output is made proportional to a modal coordinate of the system. Another modal coordinate can be monitored by selecting another set of values for the parameters T k .  
         [0089]    While this invention has been described as having a preferred design and/or configuration, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the claims.