Patent Publication Number: US-9422934-B2

Title: Systems and methods for monitoring a disc pump system using RFID

Description:
The present invention claims the benefit, under 35 USC §119(e), of the filing of U.S. Provisional Patent Application Ser. No. 61/597,493, entitled “Systems and Methods for Monitoring a Disc Pump System using RFID,” filed Feb. 10, 2012, by Locke et al., which is incorporated herein by reference for all purposes. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The illustrative embodiments of the invention relate generally to a disc pump for fluid and, more specifically, to a disc pump in which the pumping cavity is substantially cylindrically shaped having end walls and a side wall between the end walls with an actuator disposed between the end walls. The illustrative embodiments of the invention relate more specifically to a disc pump having a valve mounted in the actuator and at least one additional valve mounted in one of the end walls. 
     2. Description of Related Art 
     The generation of high amplitude pressure oscillations in closed cavities has received significant attention in the fields of thermo-acoustics and disc pump type compressors. Recent developments in non-linear acoustics have allowed the generation of pressure waves with higher amplitudes than previously thought possible. 
     It is known to use acoustic resonance to achieve fluid pumping from defined inlets and outlets. This can be achieved using a cylindrical cavity with an acoustic driver at one end, which drives an acoustic standing wave. In such a cylindrical cavity, the acoustic pressure wave has limited amplitude. Varying cross-section cavities, such as cone, horn-cone, and bulb have been used to achieve high amplitude pressure oscillations, thereby significantly increasing the pumping effect. In such high amplitude waves, the non-linear mechanisms with energy dissipation have been suppressed. However, high amplitude acoustic resonance has not been employed within disc-shaped cavities in which radial pressure oscillations are excited until recently. International Patent Application No. PCT/GB2006/001487, published as WO 2006/111775, discloses a disc pump having a substantially disc-shaped cavity with a high aspect ratio, i.e., the ratio of the radius of the cavity to the height of the cavity. 
     Such a disc pump has a substantially cylindrical cavity comprising a side wall closed at each end by end walls. The disc pump also comprises an actuator that drives either one of the end walls to oscillate in a direction substantially perpendicular to the surface of the driven end wall. The spatial profile of the motion of the driven end wall is described as being matched to the spatial profile of the fluid pressure oscillations within the cavity, a state described herein as mode-matching. When the disc pump is mode-matched, work done by the actuator on the fluid in the cavity adds constructively across the driven end wall surface, thereby enhancing the amplitude of the pressure oscillation in the cavity and delivering high disc pump efficiency. The efficiency of a mode-matched disc pump is dependent upon the interface between the driven end wall and the side wall. It is desirable to maintain the efficiency of such a disc pump by structuring the interface so that it does not decrease or dampen the motion of the driven end wall, thereby mitigating any reduction in the amplitude of the fluid pressure oscillations within the cavity. 
     The actuator of the disc pump described above causes an oscillatory motion of the driven end wall (“displacement oscillations”) in a direction substantially perpendicular to the end wall or substantially parallel to the longitudinal axis of the cylindrical cavity, referred to hereinafter as “axial oscillations” of the driven end wall within the cavity. The axial oscillations of the driven end wall generate substantially proportional “pressure oscillations” of fluid within the cavity creating a radial pressure distribution approximating that of a Bessel function of the first kind as described in International Patent Application No. PCT/GB2006/001487, which is incorporated by reference herein. Such oscillations are referred to hereinafter as “radial oscillations” of the fluid pressure within the cavity. A portion of the driven end wall between the actuator and the side wall provides an interface with the side wall of the disc pump that decreases damping of the displacement oscillations to mitigate any reduction of the pressure oscillations within the cavity. The portion of the driven end wall between the actuator and the sidewall is hereinafter referred to as an “isolator” and is described more specifically in U.S. patent application Ser. No. 12/477,594, which is incorporated by reference herein. The illustrative embodiments of the isolator are operatively associated with the peripheral portion of the driven end wall to reduce damping of the displacement oscillations. 
     Such disc pumps also require one or more valves for controlling the flow of fluid through the disc pump and, more specifically, valves being capable of operating at high frequencies. Conventional valves typically operate at lower frequencies below 500 Hz for a variety of applications. For example, many conventional compressors typically operate at 50 or 60 Hz. Linear resonance compressors that are known in the art operate between 150 and 350 Hz. However, many portable electronic devices, including medical devices, require disc pumps for delivering a positive pressure or providing a vacuum that are relatively small in size, and it is advantageous for such disc pumps to be inaudible in operation so as to provide discrete operation. To achieve these objectives, such disc pumps must operate at very high frequencies, requiring valves capable of operating at about 20 kHz and higher. To operate at these high frequencies, the valve must be responsive to a high frequency oscillating pressure that can be rectified to create a net flow of fluid through the disc pump. Such a valve is described more specifically in International Patent Application No PCT/GB2009/050614, which is incorporated by reference herein. 
     Valves may be disposed in either a first or a second aperture, or both apertures, for controlling the flow of fluid through the disc pump. Each valve comprises a first plate having apertures extending generally perpendicular therethrough and a second plate also having apertures extending generally perpendicular therethrough, wherein the apertures of the second plate are substantially offset from the apertures of the first plate. The valve further comprises a sidewall disposed between the first and second plate, wherein the sidewall is closed around the perimeter of the first and second plates to form a cavity between the first and second plates in fluid communication with the apertures of the first and second plates. The valve further comprises a flap disposed and moveable between the first and second plates, wherein the flap has apertures substantially offset from the apertures of the first plate and substantially aligned with the apertures of the second plate. The flap is motivated between the first and second plates in response to a change in direction of the differential pressure of the fluid across the valve. 
     SUMMARY 
     A disc pump system includes a pump body having a substantially cylindrical shape defining a cavity for containing a fluid, the cavity being formed by a side wall closed at both ends by substantially circular end walls. At least one of the end walls is a driven end wall having a central portion and a peripheral portion extending radially outwardly from the central portion of the driven end wall. An actuator is operatively associated with the central portion of the driven end wall to cause an oscillatory motion of the driven end wall, thereby generating displacement oscillations of the driven end wall in a direction substantially perpendicular thereto, with an annular node between the center of the driven end wall and the side wall when in use. The system includes an isolator inserted between the peripheral portion of the driven end wall and the side wall to reduce damping of the displacement oscillations, the isolator comprising a flexible material that stretches and contracts in response to the oscillatory motion of the driven end wall. The system also includes a first aperture disposed at any location in either one of the end walls other than at the annular node and extending through the pump body, and a second aperture disposed at any location in the pump body other than the location of the first aperture and extending through the pump body. A valve is disposed in at least one of the first aperture and second aperture, and displacement oscillations generate corresponding pressure oscillations of the fluid within the cavity of the pump body, causing fluid flow through the first and second apertures when in use. An RFID tag is operatively associated with the flexible material of the isolator to store and transmit identification data associated with the isolator. 
     A method for tracking components of a disc pump includes manufacturing an isolator comprising an RFID tag, which, in turn, comprises identification data The method includes scanning the identification data using an RFID reader at a first time, storing the identification data in a database, assembling one or more additional components to form a disc pump, and associating the one or more additional components with the identification data in the database. The method also includes tracking the disc pump and components by scanning the disc pump using an RFID reader at a second time that is later than the first time. 
     A disc pump system includes a pump body having a substantially cylindrical shape defining a cavity for containing a fluid. The cavity is formed by a side wall closed at both ends by substantially circular end walls, at least one of the end walls being a driven end wall having a central portion and a peripheral portion extending radially outwardly from the central portion of the driven end wall. The disc pump system includes an actuator operatively associated with the central portion of the driven end wall to cause an oscillatory motion of the driven end wall thereby generating displacement oscillations of the driven end wall in a direction substantially perpendicular thereto. The disc pump system also includes an isolator operatively associated with the peripheral portion of the driven end wall to reduce damping of the displacement oscillations. The isolator comprises a flexible printed circuit material that comprises an RFID tag. The disc pump system also includes a first aperture disposed in either one of the end walls and extending through the pump body, as well as a second aperture disposed in the pump body and extending through the pump body. The disc pump system also includes a valve disposed in at least one of the first aperture and second aperture. 
     Other features and advantages of the illustrative embodiments will become apparent with reference to the drawings and detailed description that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side, cross-section view of a disc pump; 
         FIG. 1A  is a detail view of a section of the disc pump system of  FIG. 1A  taken along the line  1 A- 1 A of  FIG. 1 , which shows a portion of a ring-shaped isolator having an integrated RFID tag; 
         FIG. 1B  is a detail, section view of an alternative embodiment of the disc pump wherein the isolator includes an RFID tag and a sensor; 
         FIG. 1C  is a detail, section view of an alternative embodiment of the disc pump that includes an RFID tag mounted to the actuator of the disc pump and coupled to an antenna that is integral to the isolator; 
         FIG. 2A  is a cross-section view of the disc pump of  FIG. 1A , showing actuator of the disc pump in a rest position; 
         FIG. 2B  is a cross-section view of the disc pump of  FIG. 2A , showing the actuator in a displaced position; 
         FIG. 3A  shows a graph of the axial displacement oscillations for the fundamental bending mode of the actuator of the first disc pump of  FIG. 2A ; 
         FIG. 3B  shows a graph of the pressure oscillations of fluid within the cavity of the first disc pump of  FIG. 2A  in response to the bending mode shown in  FIG. 3A ; 
         FIG. 4A  is a detail view of a portion of a disc pump system that includes an actuator in a rest position; 
         FIG. 4B  is a detail view of a portion of a disc pump system that includes an actuator in a displaced position; 
         FIG. 4C  is a side, cross-section view of the portion, of the disc pump shown in  FIG. 4A , whereby the actuator is in the rest position and mounted to an isolator that includes a strain gauge; 
         FIG. 4D  is a side, cross-section view of the portion of the disc pump shown in  FIG. 4B , whereby the actuator is in the displaced position and mounted to an isolator that includes a strain gauge; 
         FIG. 5A  shows a cross-section view of the disc pump of  FIG. 2A  wherein the three valves are represented by a single valve illustrated in  FIGS. 7A-7D ; 
         FIG. 5B  shows a partial cross-section, view of a center portion of the valve of  FIGS. 7A-7D ; 
         FIG. 6  shows a graph of pressure oscillations of fluid of within the cavities of the first disc pump of  FIG. 5A  as shown in  FIG. 3B  to illustrate the pressure differential applied across the valve of  FIG. 5A  as indicated by the dashed lines; 
         FIG. 7A  shows a side, cross-section view of an illustrative embodiment of a valve in a closed position; 
         FIG. 7B  shows a cross-section view of the valve of  FIG. 7A  taken along line  7 B- 7 B in  FIG. 7D ; 
         FIG. 7C  shows a perspective view of the valve of  FIG. 7B ; 
         FIG. 7D  shows a top view of the valve of  FIG. 7B ; 
         FIG. 8A  shows a partial cross-section view of the valve in  FIG. 7B  in an open position when fluid flows through the valve; 
         FIG. 8B  shows a partial cross-section view of the valve in  FIG. 7B  in transition between the open and closed positions before closing; 
         FIG. 8C  shows a partial cross-section view of the valve of  FIG. 7B  in a closed position when fluid flow is blocked by the valve; 
         FIG. 9A  shows a pressure graph of an oscillating differential pressure applied across the valve of  FIG. 5B  according to an illustrative embodiment; 
         FIG. 9B  shows a fluid-flow graph of an operating cycle of the valve of  FIG. 5B  between an open and closed position; 
         FIGS. 10A and 10B  show cross-section views of the disc pump of  FIG. 3A , including a partial, detail view of the center portion of the valves and a graph of the positive and negative portion of an oscillating pressure wave, respectively, being applied within a cavity; 
         FIG. 11  shows the open and closed states of the valves of the fourth disc pump, and  FIGS. 11A and 11B  shows the resulting flow and pressure characteristics, respectively, when the fourth disc pump is in a free-flow mode; 
         FIG. 12  shows a graph of the maximum differential pressure provided by the fourth disc pump when the disc pump reaches the stall condition; and 
         FIG. 13  is a block diagram of an illustrative circuit of a disc pump system for measuring and controlling a reduced pressure generated by the disc pump system. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In the following detailed description of several illustrative embodiments, reference is made to the accompanying drawings that form a part hereof. By way of illustration, the accompanying drawings show specific preferred embodiments in which the invention may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is understood that other embodiments may be utilized and that logical structural, mechanical, electrical, and chemical changes may be made without departing from the spirit or scope of the invention. To avoid detail not necessary to enable those skilled in the art to practice the embodiments described herein, the description may omit certain information known to those skilled in the art. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the illustrative embodiments are defined only by the appended claims. 
       FIG. 1  is a cross-section view of a disc pump system  100  coupled to a load  38 . The disc pump system  100  includes a disc pump  10 , a substrate  28  on which the disc pump  10  is mounted, and a load  38  that is fluidly coupled to the disc pump  10 . The substrate  28  may be a printed circuit board or any suitable rigid or semi-rigid material. The disc pump  10  is operable to supply a positive or negative pressure to the load  38 , as described in more detail below. The disc pump  10  includes an actuator  40  coupled to a cylindrical wall  11  of the disc pump  10  by an isolator  30 , which comprises a flexible material. In one embodiment, the flexible material is a flexible, printed circuit material. 
     Generally, the flexible printed circuit material comprises a flexible polymer film that provides a foundation layer for the isolator  30 . The polymer may be a polyester (PET), polyimide (PI), polyethylene napthalate (PEN), polyetherimide (PEI), or a material with similar mechanical and electrical properties. The flexible circuit material may include one or more a laminate layers formed of a bonding adhesive. In addition, a metal foil, such as a copper foil, may be used to provide one or more conductive layers to the flexible printed circuit material. Generally, the conductive layer is used to form circuit elements. For example, circuit paths may be etched into the conductive layer. The conductive layer may be applied to the foundation layer by rolling (with or without an adhesive) or by electro-deposition. 
       FIG. 1A  is a partial section view taken along the line  1 A- 1 A of  FIG. 1 .  FIG. 1A  shows a top view of a portion of the disc pump system  100  that includes the actuator  40  and isolator  30 . In one embodiment, the isolator  30  is formed from a flexible printed circuit material that includes a Radio Frequency Identification (RFID) tag  51 . In some embodiments, the RFID tag is formed integrally to the isolator  30 . But in other embodiments, the RFID tag is manufactured as a separate component and installed at the surface of the isolator  30  or on the actuator  40 . 
     The illustrative embodiments herein utilize simple RFID or an enhanced type of RFID technology to energize integrated electronic devices. As used herein, the word “or” does not imply mutual exclusivity. RFID traditionally uses an RFID tag or label that is positioned on a target and an RFID reader that energizes and reads signals from the RFID tag. Most RFID tags include an integrated circuit for storing and processing information, a modulator, and demodulator. RFID tags can be passive tags, active RFID tags, and battery-assisted passive tags. Generally, passive tags use no battery and do not transmit information unless they are energized by an RFID reader. Active tags have an on-board power source and can transmit autonomously (i.e., without being energized by an RFID reader). Battery-assisted passive tags typically have a small battery on-board that is activated in the presence of an RFID reader. 
     In an illustrative embodiment, the RFID tag  51  is formed using a silicon-on-insulator (SOI) manufacturing process and embedded within the isolator  30  as an RFID chip. The use of such an SOI manufacturing process provides for the ability to manufacture a very small RFID chip that is on the order of 0.15 mm×0.15 mm in size or smaller, such as the RFID tag introduced by Hitachi (disclosed at http://www.hitachi.com/New/cnews/060206.html). The RFID chip may be manufactured with an antenna, thereby slightly increasing its footprint, or by coupling the RFID chip to an external antenna. The external antenna may be separately manufactured and embedded in the isolator  30  with the RFID chip or formed integral to the isolator  30  and coupled to the RFID chip when the RFID chip is embedded within the isolator. 
     In one illustrative embodiment, the enhanced RFID technology is a Wireless Identification and Sensing Platform (WISP) device. WISPs involve powering and reading a WISP device, analogous to an RFID tag (or label), with an RFID reader. The WISP device harvests the power from the RFID reader&#39;s emitted radio signals and performs sensing functions (and optionally performs computational functions). The WISP device transmits a radio signal with information to the RFID reader. The WISP device receives power from an RFID reader. The WISP device has a tag or antenna that harvests energy and a microcontroller (or processor) that can perform a variety of tasks, such as sampling sensors. The WISP device reports data to the RFID reader. In one illustrative embodiment, the WISP device includes an integrated circuit with power harvesting circuitry, demodulator, modulator, microcontroller, sensors, and may include one or more capacitors for storing energy. A form of WISP technology has been developed by Intel Research Seattle (www.seattle.intel-research.net/wisp/). RFID devices as used herein also include WISP devices. 
     In the illustrative embodiment of  FIG. 1 , the disc pump system  100  includes an RFID tag  51  integrated into the isolator  30  and minimizes the addition of other circuit elements, such as sensors, within the isolator  30 . In such an embodiment, the RFID tag  51  may be formed within the isolator  30  during the manufacturing process of the isolator  30 , e.g., as a printed circuit element or as an embedded integrated circuit, and used to store identification data. The identification data may initially only identify the isolator  30  to enable tracking of data relating to the isolator  30 . Once the fabrication of the isolator  30  and installation of the RFID tag  51  is complete, the isolator  30  may be combined with an actuator  40  and installed into the disc pump  10 , as shown in  FIG. 1 . During the subsequent manufacturing and assembly processes that result in the completed disc pump  10 , the RFID data may be monitored and associated with other components of the disc pump system  100 . For example, the RFID data that initially identified the isolator  30  may subsequently identify the actuator  40 , the disc pump  10 , and the disc pump system  100 . The RFID data may be associated with the other components of the disc pump  10  and the disc pump system  100  in one or more external databases, so that only a low power, passive RFID tag is needed to track the isolator  30  and the other components of the disc pump system  100 . For example, when the disc pump  10  is assembled, the RFID tag of the isolator may be scanned using an RFID reader and associated with the disc pump  10 . Subsequently, the disc pump  10  may be scanned with an RFID reader to identify and track the disc pump  10  and its components. 
     In the illustrative embodiment of  FIG. 1B , the RFID tag  51  is an enhanced RFID tag that includes a processor and is electrically coupled to a sensor. In the illustrative embodiment of  FIG. 1B , the RFID tag  51  and the sensor allow sensing and optimization of computational functions. The optimization and computational functions may be based on data collected by the sensor, as described in more detail below. In  FIG. 1B , the isolator  30  includes an optional sensor, such as a strain gauge  50  that is operable to measure the deformation of the isolator  30  and in turn, the displacement (δy) of the edge of the actuator  40 . The isolator  30  may also include other electronic devices or circuit elements, such as a radio-frequency identification a processor or memory, as discussed in more detail below. While  FIG. 1B  shows both the RFID tag  51  and the sensor as being integral to the isolator  30 , either the RFID tag  51  or the sensor may be mounted on the actuator  40 , and electrically coupled to circuit elements on the isolator  30  for the purposes of communicating power or data from a source that is not installed on the isolator  30  or actuator  40 . For example, the RFID tag  51  may communicate with a very small application specific integrated circuit element that is embedded within the isolator  30  for the purpose of sensing data at the isolator  30  or within the disc pump cavity  16  and communicating the data to an external monitoring system (not shown). 
     In the illustrative embodiment of  FIG. 1C , the RFID tag  51  is mounted on the actuator  40  and coupled to an antenna  51   a  that is integral to the isolator  30 . The RFID tag  51  may be active or passive, and in such an application, it may be necessary to specify that the RFID tag  51  that is resistive to malfunctioning as a result of mechanical stress. In another embodiment, the RFID tag  51  is separately assembled and mounted to the isolator  30 . In an embodiment in which the RFID tag  51  is not formed integrally to the isolator  30 , the RFID tag  51  may be a Maxell ME-Y2000 series Coil on Chip RFID system. 
       FIG. 2A  is a cross-section view of a disc pump  10  according to an illustrative embodiment. In  FIG. 2A , the disc pump  10  comprises a disc pump body having a substantially elliptical shape including a cylindrical wall  11  closed at each end by end plates  12 ,  13 . The cylindrical wall  11  may be mounted to a substrate  28 , which forms the end plate  13 . The substrate  28  may be a printed circuit board or another suitable material. The disc pump  10  further comprises a pair of disc-shaped interior plates  14 ,  15  supported within the disc pump  10  by an isolator  30  (e.g., a ring-shaped isolator) affixed to the cylindrical wall  11  of the disc pump body. The internal surfaces of the cylindrical wall  11 , the end plate  12 , the interior plate  14 , and the isolator  30  form a cavity  16  within the disc pump  10 . The internal surfaces of the cavity  16  comprise a side wall  18  which is a first portion of the inside surface of the cylindrical wall  11  that is closed at both ends by end walls  20 ,  22  wherein the end wall  20  is the internal surface of the end plate  12  and the end wall  22  comprises the internal surface of the interior plate  14  and a first side of the isolator  30 . The end wall  22  thus comprises a central portion corresponding to the inside surface of the interior plate  14  and a peripheral portion corresponding to the inside surface of the isolator  30 . Although the disc pump  10  and its components are substantially elliptical in shape, the specific embodiment disclosed herein is a circular, elliptical shape. 
     The cylindrical wall  11  and the end plates  12 ,  13  may be a single component comprising the disc pump body or separate components, as shown in  FIG. 2A . In the embodiment of  FIG. 2A , the end plate  13  is formed by a separate substrate that may be a printed circuit board, an assembly board, or printed wire assembly (PWA) on which the disc pump  10  is mounted. Although the cavity  16  is substantially circular in shape, the cavity  16  may also be more generally elliptical in shape. Substantially circular may include objects that are regular circles, as well as variations on circular shapes, for example, ellipses. In the embodiment shown in  FIG. 2A , the end wall  20  defining the cavity  16  is shown as being generally frusto-conical. In another embodiment, the end wall  20  defining the inside surfaces of the cavity  16  may include a generally planar surface that is parallel to the actuator  40 , discussed below. A disc pump comprising frusto-conical surfaces is described in more detail in the WO2006/111775 publication, which is incorporated by reference herein. The end plates  12 ,  13  and cylindrical wall  11  of the disc pump body may be formed from any suitable rigid material including, without limitation, metal, ceramic, glass, or plastic including, without limitation, inject-molded plastic. 
     The interior plates  14 ,  15  of the disc pump  10  together form the actuator  40  that is operatively associated with the central portion of the end wall  22 , which forms the internal surfaces of the cavity  16 . One of the interior plates  14 ,  15  must be formed of a piezoelectric material which may include any electrically active material that exhibits strain in response to an applied electrical signal, such as, for example, an electrostrictive or magnetostrictive material. In one preferred embodiment, for example, the interior plate  15  is formed of piezoelectric material that exhibits strain in response to an applied electrical signal, i.e., the active interior plate. The other one of the interior plates  14 ,  15  preferably possesses a bending stiffness similar to the active interior plate and may be formed of a piezoelectric material or an electrically inactive material, such as a metal or ceramic. In this preferred embodiment, the interior plate  14  possesses a bending stiffness similar to the active interior plate  15  and is formed of an electrically inactive material, such as a metal or ceramic, i.e., the inert interior plate. When the active interior plate  15  is excited by an electrical current, the active interior plate  15  expands and contracts in a radial direction relative to the longitudinal axis of the cavity  16 . The expansion and contraction of the interior plate  15  causes the interior plates  14 ,  15  to bend, thereby inducing an axial deflection of the end walls  22  in a direction substantially perpendicular to the end walls  22  (See  FIG. 3A ). 
     In other embodiments not shown, the isolator  30  may support either one of the interior plates  14 ,  15 , whether the active interior plate  15  or inert interior plate  14 , from the top or the bottom surfaces depending on the specific design and orientation of the disc pump  10 . In another embodiment, the actuator  40  may be replaced by a device in a force-transmitting relation with only one of the interior plates  14 ,  15  such as, for example, a mechanical, magnetic or electrostatic device. In such an embodiment, the interior plate may be formed as an electrically inactive or passive layer of material driven into oscillation by such device (not shown) in the same manner as described above. 
     The disc pump  10  further comprises at least one aperture extending from the cavity  16  to the outside of the disc pump  10 , wherein the at least one aperture contains a valve to control the flow of fluid through the aperture. Although the aperture may be located at any position in the cavity  16  where the actuator  40  generates a pressure differential as described below in more detail, one embodiment of the disc pump  10  shown in  FIGS. 2A-2B  comprises an outlet aperture  27 , located at approximately the center of and extending through the end plate  12 . The aperture  27  contains at least one end valve  29 . In one preferred embodiment, the aperture  27  contains end valve  29  which regulates the flow of fluid in one direction as indicated by the arrows so that end valve  29  functions as an outlet valve for the disc pump  10 . Any reference to the aperture  27  that includes the end valve  29  refers to that portion of the opening outside of the end valve  29 , i.e., outside the cavity  16  of the disc pump  10 . 
     The disc pump  10  further comprises at least one aperture extending through the actuator  40 , wherein the at least one aperture contains a valve to control the flow of fluid through the aperture. The aperture may be located at any position on the actuator  40  where the actuator  40  generates a pressure differential. The illustrative embodiment of the disc pump  10  shown in  FIGS. 2A-2B , however, comprises an actuator aperture  31  located at approximately the center of and extending through the interior plates  14 ,  15 . The actuator aperture  31  contains an actuator valve  32  which regulates the flow of fluid in one direction into the cavity  16 , as indicated by the arrow so that the actuator valve  32  functions as an inlet valve to the cavity  16 . The actuator valve  32  enhances the output of the disc pump  10  by augmenting the flow of fluid into the cavity  16  and supplementing the operation of the outlet valve  29 , as described in more detail below. 
     The dimensions of the cavity  16  described herein should preferably satisfy certain inequalities with respect to the relationship between the height (h) of the cavity  16  at the side wall  18  and its radius (r) which is the distance from the longitudinal axis of the cavity  16  to the side wall  18 . These equations are as follows:
 
 r/h&gt; 1.2; and
 
 h   2   /r&gt; 4×10 −10  meters.
 
     In one embodiment, the ratio of the cavity radius to the cavity height (r/h) is between about 10 and about 50 when the fluid within the cavity  16  is a gas. In this example, the volume of the cavity  16  may be less than about 10 ml. Additionally, the ratio of h 2 /r is preferably within a range between about 10 −6  meters and about 10 −7  meters, where the working fluid is a gas as opposed to a liquid. 
     Additionally, the cavity  16  disclosed herein should preferably satisfy the following inequality relating the cavity radius (r) and operating frequency (f), which is the frequency at which the actuator  40  vibrates to generate the axial displacement of the end wall  22 . The inequality is as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
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     wherein the speed of sound in the working fluid within the cavity  16  (c) may range between a slow speed (c s ) of about 115 m/s and a fast speed (c f ) equal to about 1,970 m/s as expressed in the equation above, and k 0  is a constant (k 0 =3.83). The frequency of the oscillatory motion of the actuator  40  is preferably about equal to the lowest resonant frequency of radial pressure oscillations in the cavity  16 , but may be within 20% of that value. The lowest resonant frequency of radial pressure oscillations in the cavity  16  is preferably greater than about 500 Hz. 
     Although it is preferable that the cavity  16  disclosed herein should satisfy individually the inequalities identified above, the relative dimensions of the cavity  16  should not be limited to cavities having the same height and radius. For example, the cavity  16  may have a slightly different shape requiring different radii or heights creating different frequency responses so that the cavity  16  resonates in a desired fashion to generate the optimal output from the disc pump  10 . 
     In operation, the disc pump  10  may function as a source of positive pressure adjacent the outlet valve  29  to pressurize a load  38  or as a source of negative or reduced pressure adjacent the actuator inlet valve  32  to depressurize a load  38 , as illustrated by the arrows. For example, the load may be a tissue treatment system that utilizes negative pressure for treatment. The term “reduced pressure” as used herein generally refers to a pressure less than the ambient pressure where the disc pump  10  is located. Although the term “vacuum” and “negative pressure” may be used to describe the reduced pressure, the actual pressure reduction may be significantly less than the pressure reduction normally associated with a complete vacuum. The pressure is “negative” in the sense that it is a gauge pressure, i.e., the pressure is reduced below ambient atmospheric pressure. Unless otherwise indicated, values of pressure stated herein are gauge pressures. References to increases in reduced pressure typically refer to a decrease in absolute pressure, while decreases in reduced pressure typically refer to an increase in absolute pressure. 
     As indicated above, the disc pump  10  comprises at least one actuator valve  32  and at least one end valve  29 . In another embodiment, the disc pump  10  may comprise a two cavity disc pump having an end valve  29  on each side of the actuator  40 . 
       FIG. 3A  shows one possible displacement profile illustrating the axial oscillation of the driven end wall  22  of the cavity  16 . The solid curved line and arrows represent the displacement of the driven end wall  22  at one point in time, and the dashed curved line represents the displacement of the driven end wall  22  one half-cycle later. The displacement as shown in this figure and the other figures is exaggerated. Because the actuator  40  is not rigidly mounted at its perimeter, and is instead suspended by the isolator  30 , the actuator  40  is free to oscillate about its center of mass in its fundamental mode. In this fundamental mode, the amplitude of the displacement oscillations of the actuator  40  is substantially zero at an annular displacement node  42  located between the center of the driven end wall  22  and the side wall  18 . The amplitudes of the displacement oscillations at other points on the end wall  22  are greater than zero as represented by the vertical arrows. A central displacement anti-node  43  exists near the center of the actuator  40  and a peripheral displacement anti-node  43 ′ exists near the perimeter of the actuator  40 . The central displacement anti-node  43  is represented by the dashed curve after one half-cycle. 
       FIG. 3B  shows one possible pressure oscillation profile illustrating the pressure oscillation within the cavity  16  resulting from the axial displacement oscillations shown in  FIG. 3A . The solid curved line and arrows represent the pressure at one point in time. In this mode and higher-order modes, the amplitude of the pressure oscillations has a peripheral pressure anti-node  45 ′ near the side wall  18  of the cavity  16 . The amplitude of the pressure oscillations is substantially zero at the annular pressure node  44  between the central pressure anti-node  45  and the peripheral pressure anti-node  45 ′. At the same time, the amplitude of the pressure oscillations as represented by the dashed line that has a negative central pressure anti-node  47  near the center of the cavity  16  with a peripheral pressure anti-node  47 ′ and the same annular pressure node  44 . For a cylindrical cavity, the radial dependence of the amplitude of the pressure oscillations in the cavity  16  may be approximated by a Bessel function of the first kind. The pressure oscillations described above result from the radial movement of the fluid in the cavity  16  and so will be referred to as the “radial pressure oscillations” of the fluid within the cavity  16  as distinguished from the axial displacement oscillations of the actuator  40 . 
     With further reference to  FIGS. 3A and 3B , it can be seen that the radial dependence of the amplitude of the axial displacement oscillations of the actuator  40  (the “mode-shape” of the actuator  40 ) should approximate a Bessel function of the first kind so as to match more closely the radial dependence of the amplitude of the desired pressure oscillations in the cavity  16  (the “mode-shape” of the pressure oscillation). By not rigidly mounting the actuator  40  at its perimeter and allowing it to vibrate more freely about its center of mass, the mode-shape of the displacement oscillations substantially matches the mode-shape of the pressure oscillations in the cavity  16  thus achieving mode-shape matching or, more simply, mode-matching. Although the mode-matching may not always be perfect in this respect, the axial displacement oscillations of the actuator  40  and the corresponding pressure oscillations in the cavity  16  have substantially the same relative phase across the full surface of the actuator  40 , wherein the radial position of the annular pressure node  44  of the pressure oscillations in the cavity  16  and the radial position of the annular displacement node  42  of the axial displacement oscillations of actuator  40  are substantially coincident. 
     As the actuator  40  vibrates about its center of mass, the radial position of the annular displacement node  42  will necessarily lie inside the radius of the actuator  40  when the actuator  40  vibrates in its fundamental bending mode as illustrated in  FIG. 3A . Thus, to ensure that the annular displacement node  42  is coincident with the annular pressure node  44 , the radius of the actuator (r act ) should preferably be greater than the radius of the annular pressure node  44  to optimize mode-matching. Assuming again that the pressure oscillation in the cavity  16  approximates a Bessel function of the first kind, the radius of the annular pressure node  44  would be approximately 0.63 of the radius from the center of the end wall  22  to the side wall  18 , i.e., the radius of the cavity  16  (“r”), as shown in  FIG. 2A . Therefore, the radius of the actuator  40  (r act ) should preferably satisfy the following inequality: r act ≧0.63 r. 
     The isolator  30  may be a flexible membrane that enables the edge of the actuator  40  to move more freely as described above by bending and stretching in response to the vibration of the actuator  40  as shown by the displacement at the peripheral displacement anti-node  43 ′ in  FIG. 3A . The isolator  30  overcomes the potential damping effects of the side wall  18  on the actuator  40  by providing a low mechanical impedance support between the actuator  40  and the cylindrical wall  11  of the disc pump  10 , thereby reducing the damping of the axial oscillations at the peripheral displacement anti-node  43 ′ of the actuator  40 . Essentially, the isolator  30  minimizes the energy being transferred from the actuator  40  to the side wall  18  with the outer peripheral edge of the isolator  30  remaining substantially stationary. Consequently, the annular displacement node  42  will remain substantially aligned with the annular pressure node  44  to maintain the mode-matching condition of the disc pump  10 . Thus, the axial displacement oscillations of the driven end wall  22  continue to efficiently generate oscillations of the pressure within the cavity  16  from the central pressure anti-nodes  45 ,  47  to the peripheral pressure anti-nodes  45 ′,  47 ′ at the side wall  18  as shown in  FIG. 3B . 
       FIG. 4A  is a detail view of a portion of the disc pump  10  that includes a strain gauge  50  mounted on the isolator  30  of the disc pump  10 . In the embodiment of  FIG. 4A , the isolator  30  comprises a flexible printed circuit material. The strain gauge  50  is attached to the isolator  30  and may be used to compute the displacement of the edge of the actuator  40 . Functionally, the strain gauge  50  indirectly measures the displacement of the edge of the actuator  40 , thereby alleviating the need to include a sensor on the substrate  28 . In this embodiment, the strain gauge  50  measures the strain, i.e., deformation, of the isolator  30  and the measured deformation of the isolator  30  is used to derive the displacement of the edge of the actuator  40 . As such, the strain gauge  50  may comprise a metallic pattern that is integrated into the flexible printed circuit material that forms the isolator  30 . In one embodiment, the strain gauge  50  is integral to the isolator  30  so that the strain gauge  50  deforms as the isolator  30  deforms. In another embodiment, the strain gauge  50  is affixed to the surface of the isolator  30 . The deformation of the strain gauge  50  results in a change in the electrical resistance of the strain gauge  50 , which can be measured using, for example, a Wheatstone bridge. The change in electrical resistance is related to the deformation of the isolator  30  and, therefore, the displacement of the actuator  40 , by a gauge factor. As described in more detail below, the displacement of the edge of the actuator  40  and the associated pressure differential across the disc pump  10  can be determined by analyzing the changes in the electrical resistance of the strain gauge  50 . 
     In one embodiment, the strain gauge  50  is integral to the isolator  30  and formed within the isolator  30  during the manufacturing process. In such an embodiment, the strain gauge  50  may be formed by circuit elements included in an etched copper layer of a flexible printed circuit board material. In another embodiment, however, the strain gauge  50  may be manufactured separately and attached to the isolator  30  during the assembly of the disc pump  10 . 
       FIG. 4B  is a detail view of a section of a disc pump  10  that shows the strain gauge  50  in a deformed state. As opposed to  FIG. 4A , the strain gauge  50  of  FIG. 4B  has a length (l 2 ) that is longer than the initial length (l 1 ) of the strain gauge  50  in its non-deformed state.  FIG. 4C  is a side, detail view of a cross section of the portion of the disc pump  10  shown in  FIG. 4A , and  FIG. 4D  is a side, detail view of a cross section of the portion of the disc pump  10  shown in  FIG. 4B . The initial length (l 1 ) of the portion of the isolator  30  shown in  FIG. 4D  is known and the deformed length (l 2 ) of the portion of the isolator can be computed by analyzing the change in the electrical resistance of the strain gauge  50 . Once the non-deformed and deformed isolator dimensions are known (l 1  and l 2 ), the displacement (δy) of the edge of the actuator  40  may be computed by considering the three dimensions (l 1 , l 2  and δy) as the three sides of a right triangle. 
     The displacement (δy) of the edge of the actuator  40  is a function of both the bending of the actuator  40  in response to a piezoelectric drive signal and the bulk displacement of the actuator  40  resulting from the difference in pressure on either side of the actuator  40 . The displacement of the edge of the actuator  40  that results from the bending of the actuator  40  changes at a high frequency that corresponds to the resonant frequency of the disc pump  10 . Conversely, the displacement of the edge of actuator  40  that results from a difference in pressure on opposing sides of the actuator  40 , the pressure-related displacement of the actuator  40 , may be viewed as a quasi-static displacement that changes much more gradually as the disc pump  10  supplies pressure to (or removes pressure from) the load  38 . Thus, the pressure-related displacement (δy) of the edge of the actuator  40  bears a direct correlation to the pressure differential across the actuator  40  and the corresponding pressure differential across the disc pump  10 . 
     As the pressure differential develops across the actuator, a net force is exerted on the actuator  40 , displacing the edge of actuator  40 , as shown in  FIGS. 2B and 4D . The net force is a result of the pressure being higher on one side of the actuator  40  than the other. Since the actuator  40  is mounted to the isolator  30 , which is made from a resilient material that has a spring constant (k), the actuator  40  moves in response to the pressure-related force. The pressure-related force (F) required displace the actuator  40  is a function of the spring constant (k) of the material of the isolator  30  and the distance (δy) the actuator  40  is displaced (e.g., F=f(k, δy)). The pressure-related force (F) can also be approximated as a function of the difference in pressure (ΔP) across the disc pump  10  and the surface area (A) of the actuator  40  (F=f(ΔP, A)). Since the spring constant (k) of the isolator  30  and the surface area (A) of the actuator  40  are constant, the pressure differential can be determined as a function of the pressure-related displacement of the edge of the actuator  40  (ΔP=f(δy, k, A)). For example, in one illustrative, non-limiting embodiment, the pressure-related force (F) may be determined as being proportional to the cube of the displacement of the edge of the actuator (δy 3 ). Further, it is noted that while the spring characteristics of the isolator are discussed as being linear, non-linear spring characteristics of an isolator may also be determined in order to equate the pressure-related displacement of the edge of the actuator  40  to the pressure differential across the disc pump system  100 . 
     The displacement (δy) may be measured or calculated in real-time or utilizing a specified sampling frequency of strain gauge data to determine the position of the edge of actuator  40  relative to the substrate  28 . In one embodiment, the position of the edge of the actuator  40  is computed as an average or mean position over a given time period to indicate the displacement (δy) resulting from the bending of the actuator  40 . As a result, the reduced pressure within the cavity  16  of the disc pump  10  may be determined by sensing the displacement (δy) of the edge of the actuator  40  without the need for pressure sensors that directly measure the pressure provided to a load. This may be desirable because pressure sensors that directly measure pressure may be too bulky or too expensive for application in measuring the pressure provided by the disc pump  10  within a reduced pressure system, for example. The illustrative embodiments optimize the utilization of space within the disc pump  10  without interfering with the pressure oscillations being created within the cavity  16  of the disc pump  10 . Referring to  FIG. 5A , the disc pump  10  of  FIG. 1A  is shown with the valves  29 ,  32 , both of which are substantially similar in structure as represented, for example, by a valve  110  shown in  FIGS. 7A-7D  and having a center portion  111  shown in  FIG. 5B . The following description associated with  FIGS. 5-9  are all based on the function of a single valve  110  that may be positioned in any one of the apertures  27 ,  31  of the disc pump  10 .  FIG. 6  shows a graph of the pressure oscillations of fluid within the disc pump  10  as shown in  FIG. 3B . The valve  110  allows fluid to flow in only one direction as described above. The valve  110  may be a check valve or any other valve that allows fluid to flow in only one direction. Some valve types may regulate fluid flow by switching between an open and closed position. For such valves to operate at the high frequencies generated by the actuator  40 , the valves  29 ,  32  must have an extremely fast response time such that they are able to open and close on a timescale significantly shorter than the timescale of the pressure variation. One embodiment of the valves  29 ,  32  achieves this by employing an extremely light flap valve which has low inertia and consequently is able to move rapidly in response to changes in relative pressure across the valve structure. 
     Referring to  FIGS. 7A-D  and  5 B, valve  110  is such a flap valve for the disc pump  10  according to an illustrative embodiment. The valve  110  comprises a substantially cylindrical wall  112  that is ring-shaped and closed at one end by a retention plate  114  and at the other end by a sealing plate  116 . The inside surface of the wall  112 , the retention plate  114 , and the sealing plate  116  form a cavity  115  within the valve  110 . The valve  110  further comprises a substantially circular flap  117  disposed between the retention plate  114  and the sealing plate  116 , but adjacent the sealing plate  116 . The circular flap  117  may be disposed adjacent the retention plate  114  in an alternative embodiment as will be described in more detail below, and in this sense the flap  117  is considered to be “biased” against either one of the sealing plate  116  or the retention plate  114 . The peripheral portion of the flap  117  is sandwiched between the sealing plate  116  and the ring-shaped wall  112  so that the motion of the flap  117  is restrained in the plane substantially perpendicular the surface of the flap  117 . The motion of the flap  117  in such plane may also be restrained by the peripheral portion of the flap  117  being attached directly to either the sealing plate  116  or the wall  112 , or by the flap  117  being a close fit within the ring-shaped wall  112 , in an alternative embodiment. The remainder of the flap  117  is sufficiently flexible and movable in a direction substantially perpendicular to the surface of the flap  117 , so that a force applied to either surface of the flap  117  will motivate the flap  117  between the sealing plate  116  and the retention plate  114 . 
     The retention plate  114  and the sealing plate  116  both have holes  118  and  120 , respectively, which extend through each plate. The flap  117  also has holes  122  that are generally aligned with the holes  118  of the retention plate  114  to provide a passage through which fluid may flow as indicated by the dashed arrows  124  in  FIGS. 5B and 8A . The holes  122  in the flap  117  may also be partially aligned, i.e., having only a partial overlap, with the holes  118  in the retention plate  114 . Although the holes  118 ,  120 ,  122  are shown to be of substantially uniform size and shape, they may be of different diameters or even different shapes without limiting the scope of the invention. In one embodiment of the invention, the holes  118  and  120  form an alternating pattern across the surface of the plates as shown by the solid and dashed circles, respectively, in  FIG. 7D . In other embodiments, the holes  118 ,  120 ,  122  may be arranged in different patterns without affecting the operation of the valve  110  with respect to the functioning of the individual pairings of holes  118 ,  120 ,  122  as illustrated by individual sets of the dashed arrows  124 . The pattern of holes  118 ,  120 ,  122  may be designed to increase or decrease the number of holes to control the total flow of fluid through the valve  110  as required. For example, the number of holes  118 ,  120 ,  122  may be increased to reduce the flow resistance of the valve  110  to increase the total flow rate of the valve  110 . 
     Referring also to  FIGS. 8A-8C , the center portion  111  of the valve  110  illustrates how the flap  117  is motivated between the sealing plate  116  and the retention plate  114  when a force is applied to either surface of the flap  117 . When no force is applied to either surface of the flap  117  to overcome the bias of the flap  117 , the valve  110  is in a “normally closed” position because the flap  117  is disposed adjacent the sealing plate  116  where the holes  122  of the flap are offset or not aligned with the holes  118  of the sealing plate  116 . In this “normally closed” position, the flow of fluid through the sealing plate  116  is substantially blocked or covered by the non-perforated portions of the flap  117  as shown in  FIGS. 7A and 7B . When pressure is applied against either side of the flap  117  that overcomes the bias of the flap  117  and motivates the flap  117  away from the sealing plate  116  towards the retention plate  114  as shown in  FIGS. 5B and 8A , the valve  110  moves from the normally closed position to an “open” position over a time period, i.e., an opening time delay (T o ), allowing fluid to flow in the direction indicated by the dashed arrows  124 . When the pressure changes direction as shown in  FIG. 8B , the flap  117  will be motivated back towards the sealing plate  116  to the normally closed position. When this happens, fluid will flow for a short time period, i.e., a closing time delay (T c ), in the opposite direction as indicated by the dashed arrows  132  until the flap  117  seals the holes  120  of the sealing plate  116  to substantially block fluid flow through the sealing plate  116  as shown in  FIG. 8C . In other embodiments of the invention, the flap  117  may be biased against the retention plate  114  with the holes  118 ,  122  aligned in a “normally open” position. In this embodiment, applying positive pressure against the flap  117  will be necessary to motivate the flap  117  into a “closed” position. Note that the terms “sealed” and “blocked” as used herein in relation to valve operation are intended to include cases in which substantial (but incomplete) sealing or blockage occurs, such that the flow resistance of the valve is greater in the “closed” position than in the “open” position. 
     The operation of the valve  110  is a function of the change in direction of the differential pressure (ΔP) of the fluid across the valve  110 . In  FIG. 8B , the differential pressure has been assigned a negative value (−ΔP) as indicated by the downward pointing arrow. When the differential pressure has a negative value (−ΔP), the fluid pressure at the outside surface of the retention plate  114  is greater than the fluid pressure at the outside surface of the sealing plate  116 . This negative differential pressure (−ΔP) drives the flap  117  into the fully closed position as described above wherein the flap  117  is pressed against the sealing plate  116  to block the holes  120  in the sealing plate  116 , thereby substantially preventing the flow of fluid through the valve  110 . When the differential pressure across the valve  110  reverses to become a positive differential pressure (+ΔP) as indicated by the upward pointing arrow in  FIG. 8A , the flap  117  is motivated away from the sealing plate  116  and towards the retention plate  114  into the open position. When the differential pressure has a positive value (+ΔP), the fluid pressure at the outside surface of the sealing plate  116  is greater than the fluid pressure at the outside surface of the retention plate  114 . In the open position, the movement of the flap  117  unblocks the holes  120  of the sealing plate  116  so that fluid is able to flow through them and the aligned holes  122  and  118  of the flap  117  and the retention plate  114 , respectively, as indicated by the dashed arrows  124 . 
     When the differential pressure across the valve  110  changes from a positive differential pressure (+ΔP) back to a negative differential pressure (−ΔP) as indicated by the downward pointing arrow in  FIG. 8B , fluid begins flowing in the opposite direction through the valve  110  as indicated by the dashed arrows  132 , which forces the flap  117  back toward the closed position shown in  FIG. 8C . In  FIG. 8B , the fluid pressure between the flap  117  and the sealing plate  116  is lower than the fluid pressure between the flap  117  and the retention plate  114 . Thus, the flap  117  experiences a net force, represented by arrows  138 , which accelerates the flap  117  toward the sealing plate  116  to close the valve  110 . In this manner, the changing differential pressure cycles the valve  110  between closed and open positions based on the direction (i.e., positive or negative) of the differential pressure across the valve  110 . It should be understood that the flap  117  could be biased against the retention plate  114  in an open position when no differential pressure is applied across the valve  110 , i.e., the valve  110  would then be in a “normally open” position. 
     When the differential pressure across the valve  110  reverses to become a positive differential pressure (+ΔP) as shown in  FIGS. 5B and 8A , the biased flap  117  is motivated away from the sealing plate  116  against the retention plate  114  into the open position. In this position, the movement of the flap  117  unblocks the holes  120  of the sealing plate  116  so that fluid is permitted to flow through them and the aligned holes  118  of the retention plate  114  and the holes  122  of the flap  117  as indicated by the dashed arrows  124 . When the differential pressure changes from the positive differential pressure (+ΔP) back to the negative differential pressure (−ΔP), fluid begins to flow in the opposite direction through the valve  110  (see  FIG. 8B ), which forces the flap  117  back toward the closed position (see  FIG. 8C ). Thus, as the pressure oscillations in the cavity  16  cycle the valve  110  between the normally closed position and the open position, the disc pump  10  provides reduced pressure every half cycle when the valve  110  is in the open position. 
     As indicated above, the operation of the valve  110  is a function of the change in direction of the differential pressure (ΔP) of the fluid across the valve  110 . The differential pressure (ΔP) is assumed to be substantially uniform across the entire surface of the retention plate  114  because (1) the diameter of the retention plate  114  is small relative to the wavelength of the pressure oscillations in the cavity  115 , and (2) the valve  110  is located near the center of the cavity  16  where the amplitude of the positive central pressure anti-node  45  is relatively constant as indicated by the positive square-shaped portion  55  of the positive central pressure anti-node  45  and the negative square-shaped portion  65  of the negative central pressure anti-node  47  shown in  FIG. 6 . Therefore, there is virtually no spatial variation in the pressure across the center portion  111  of the valve  110 . 
       FIG. 9  further illustrates the dynamic operation of the valve  110  when it is subject to a differential pressure, which varies in time between a positive value (+ΔP) and a negative value (−ΔP). While in practice the time-dependence of the differential pressure across the valve  110  may be approximately sinusoidal, the time-dependence of the differential pressure across the valve  110  is approximated as varying in the square-wave form shown in  FIG. 9A  to facilitate explanation of the operation of the valve. The positive differential pressure  55  is applied across the valve  110  over the positive pressure time period (t p +) and the negative differential pressure  65  is applied across the valve  110  over the negative pressure time period (t p −) of the square wave.  FIG. 9B  illustrates the motion of the flap  117  in response to this time-varying pressure. As differential pressure (ΔP) switches from negative 65 to positive 55 the valve  110  begins to open and continues to open over an opening time delay (T o ) until the valve flap  117  meets the retention plate  114  as also described above and as shown by the graph in  FIG. 9B . As differential pressure (ΔP) subsequently switches back from positive differential pressure  55  to negative differential pressure  65 , the valve  110  begins to close and continues to close over a closing time delay (T c ) as also described above and as shown in  FIG. 9B . 
     The retention plate  114  and the sealing plate  116  should be strong enough to withstand the fluid pressure oscillations to which they are subjected without significant mechanical deformation. The retention plate  114  and the sealing plate  116  may be formed from any suitable rigid material, such as glass, silicon, ceramic, or metal. The holes  118 ,  120  in the retention plate  114  and the sealing plate  116  may be formed by any suitable process including chemical etching, laser machining, mechanical drilling, powder blasting, and stamping. In one embodiment, the retention plate  114  and the sealing plate  116  are formed from sheet steel between 100 and 200 microns thick, and the holes  118 ,  120  therein are formed by chemical etching. The flap  117  may be formed from any lightweight material, such as a metal or polymer film. In one embodiment, when fluid pressure oscillations of 20 kHz or greater are present on either the retention plate side or the sealing plate side of the valve  110 , the flap  117  may be formed from a thin polymer sheet between 1 micron and 20 microns in thickness. For example, the flap  117  may be formed from polyethylene terephthalate (PET) or a liquid crystal polymer film approximately 3 microns in thickness. 
     Referring now to  FIGS. 10A and 10B , an exploded view of the two-valve disc pump  10  is shown that utilizes valve  110  as valves  29  and  32 . In this embodiment the actuator valve  32  gates airflow  232  between the actuator aperture  31  and cavity  16  of the disc pump  10  ( FIG. 10A ), while end valve  29  gates airflow between the cavity  16  and the outlet aperture  27  of the disc pump  10  ( FIG. 10B ). Each of the figures also shows the pressure generated in the cavity  16  as the actuator  40  oscillates. Both of the valves  29  and  32  are located near the center of the cavity  16  where the amplitudes of the positive and negative central pressure anti-nodes  45  and  47 , respectively, are relatively constant as indicated by the positive and negative square-shaped portions  55  and  65 , respectively. In this embodiment, the valves  29  and  32  are both biased in the closed position as shown by the flap  117  and operate as described above when the flap  117  is motivated to the open position as indicated by flap  117 ′. The figures also show an exploded view of the positive and negative square-shaped portions  55 ,  65  of the central pressure anti-nodes  45 ,  47  and their simultaneous impact on the operation of both valves  29 ,  32  and the corresponding airflow  229  and  232 , respectively, generated through each one. 
     Referring also to the relevant portions of  FIGS. 11, 11A and 11B , the open and closed states of the valves  29  and  32  ( FIG. 11 ) and the resulting flow characteristics of each one ( FIG. 11A ) are shown as related to the pressure in the cavity  16  ( FIG. 11B ). When the actuator aperture  31  and the outlet aperture  27  of the disc pump  10  are both at ambient pressure and the actuator  40  begins vibrating to generate pressure oscillations within the cavity  16  as described above, air begins flowing alternately through the valves  29 ,  32 . As a result, air flows from the actuator aperture  31  to the outlet aperture  27  of the disc pump  10 , i.e., the disc pump  10  begins operating in a “free-flow” mode. In one embodiment, the actuator aperture  31  of the disc pump  10  may be supplied with air at ambient pressure while the outlet aperture  27  of the disc pump  10  is pneumatically coupled to a load (not shown) that becomes pressurized through the action of the disc pump  10 . In another embodiment, the actuator aperture  31  of the disc pump  10  may be pneumatically coupled to a load (not shown) that becomes depressurized to generate a negative pressure in the load, such as a wound dressing, through the action of the disc pump  10 . 
     Referring more specifically to  FIG. 10A  and the relevant portions of  FIGS. 11, 11A and 11B , the square-shaped portion  55  of the positive central pressure anti-node  45  is generated within the cavity  16  by the vibration of the actuator  40  during one half of the disc pump cycle as described above. When the actuator aperture  31  and outlet aperture  27  of the disc pump  10  are both at ambient pressure, the square-shaped portion  55  of the positive central anti node  45  creates a positive differential pressure across the end valve  29  and a negative differential pressure across the actuator valve  32 . As a result, the actuator valve  32  begins closing and the end valve  29  begins opening so that the actuator valve  32  blocks the airflow  232   x  through the actuator aperture  31 , while the end valve  29  opens to release air from within the cavity  16  allowing the airflow  229  to exit the cavity  16  through the outlet aperture  27 . As the actuator valve  32  closes and the end valve  29  opens ( FIG. 11 ), the airflow  229  at the outlet aperture  27  of the disc pump  10  increases to a maximum value dependent on the design characteristics of the end valve  29  ( FIG. 11A ). The opened end valve  29  allows airflow  229  to exit the disc pump cavity  16  ( FIG. 11B ) while the actuator valve  32  is closed. When the positive differential pressure across end valve  29  begins to decrease, the airflow  229  begins to drop until the differential pressure across the end valve  29  reaches zero. When the differential pressure across the end valve  29  falls below zero, the end valve  29  begins to close allowing some back-flow  329  of air through the end valve  29  until the end valve  29  is fully closed to block the airflow  229   x  as shown in  FIG. 10B . 
     Referring more specifically to  FIG. 10B  and the relevant portions of  FIGS. 11, 11A, and 11B , the square-shaped portion  65  of the negative central anti-node  47  is generated within the cavity  16  by the vibration of the actuator  40  during the second half of the disc pump cycle as described above. When the actuator aperture  31  and outlet aperture  27  of the disc pump  10  are both at ambient pressure, the square-shaped portion  65  of the negative central anti-node  47  creates a negative differential pressure across the end valve  29  and a positive differential pressure across the actuator valve  32 . As a result, the actuator valve  32  begins opening and the end valve  29  begins closing so that the end valve  29  blocks the airflow  229   x  through the outlet aperture  27 , while the actuator valve  32  opens allowing air to flow into the cavity  16  as shown by the airflow  232  through the actuator aperture  31 . As the actuator valve  32  opens and the end valve  29  closes ( FIG. 11 ), the airflow at the outlet aperture  27  of the disc pump  10  is substantially zero except for the small amount of backflow  329  as described above ( FIG. 11A ). The opened actuator valve  32  allows airflow  232  into the disc pump cavity  16  ( FIG. 11B ) while the end valve  29  is closed. When the positive pressure differential across the actuator valve  32  begins to decrease, the airflow  232  begins to drop until the differential pressure across the actuator valve  32  reaches zero. When the differential pressure across the actuator valve  32  rises above zero, the actuator valve  32  begins to close again allowing some back-flow  332  of air through the actuator valve  32  until the actuator valve  32  is fully closed to block the airflow  232   x  as shown in  FIG. 10A . The cycle then repeats itself as described above with respect to FIG.  10 A. Thus, as the actuator  40  of the disc pump  10  vibrates during the two half cycles described above with respect to  FIGS. 10A and 10B , the differential pressures across valves  29  and  32  cause air to flow from the actuator aperture  31  to the outlet aperture  27  of the disc pump  10  as shown by the airflows  232 ,  229 , respectively. 
     In some cases, the actuator aperture  31  of the disc pump  10  is held at ambient pressure and the outlet aperture  27  of the disc pump  10  is pneumatically coupled to a load that becomes pressurized through the action of the disc pump  10 , the pressure at the outlet aperture  27  of the disc pump  10  begins to increase until the outlet aperture  27  of the disc pump  10  reaches a maximum pressure at which time the airflow from the actuator aperture  31  to the outlet aperture  27  is negligible, i.e., the “stall” condition.  FIG. 12  illustrates the pressures within the cavity  16  and outside the cavity  16  at the actuator aperture  31  and the outlet aperture  27  when the disc pump  10  is in the stall condition. More specifically, the mean pressure in the cavity  16  is approximately 1P above the inlet pressure (i.e. 1P above the ambient pressure) and the pressure at the center of the cavity  16  varies between approximately ambient pressure and approximately ambient pressure plus 2P. In the stall condition, there is no point in time at which the pressure oscillation in the cavity  16  results in a sufficient positive differential pressure across either inlet valve  32  or outlet valve  29  to significantly open either valve to allow any airflow through the disc pump  10 . Because the disc pump  10  utilizes two valves, the synergistic action of the two valves  29 ,  32  described above is capable of increasing the differential pressure between the outlet aperture  27  and the actuator aperture  31  to a maximum differential pressure of 2P, double that of a single valve disc pump. Thus, under the conditions described in the previous paragraph, the outlet pressure of the two-valve disc pump  10  increases from ambient in the free-flow mode to a pressure of approximately ambient plus 2P when the disc pump  10  reaches the stall condition. 
     Referring again to  FIGS. 1 and 1A-1B , a method of computing the displacement of the actuator  40  may be utilized in accordance with the principles described above. In an embodiment in which the isolator  30  is formed from a flexible printed circuit material, electronic elements may be incorporated into the structure of the isolator  30 . In one embodiment, the isolator includes a sensor, such as a strain gauge  50 , to gather data related to the performance of the isolator  30  and movement of the actuator  40 . The sensor is coupled to the RFID tag  51 . In one embodiment, the RFID tag  51  is a WISP device that includes a processor. The WISP device may also include a memory and power source that are also formed integrally with, or embedded within, the isolator  30 . Alternatively, the isolator  30  may include an electrical coupling from the sensor to a remote bus and other electronic devices not formed integrally to or affixed to the isolator  30 . The other electronic devices may include a remote RFID tag, a remote processor, a remote memory, and a remote power source. The remote components may be located adjacent the isolator  30  or at a distance away from the isolator  30 . 
     In one embodiment, the sensor measures performance parameters of the disc pump  10 , which may include the maximum and average displacements of the actuator  40  and deformation experienced by the isolator  30  over time, or other parameters. The measured performance parameters are transmitted to the RFID tag  51  in real time, and in turn transmitted to a remote computing unit. 
     In another embodiment, the sensor measures and communicates performance parameters to the WISP RFID tag  51  that is integral to the isolator  30 . In such an embodiment, the performance parameters are stored in a memory that is located on the isolator  30 , and periodically transmitted to a remote computing unit via the RFID tag  51 . The remote computing unit may be the computing unit that includes the processor  56 , discussed with regard to  FIG. 13 , to facilitate the control and operation of the disc pump system  100 , including the disc pump  10 . 
     In an embodiment, the sensor may be the strain gauge  50  that measures displacement of the edge of the actuator  40 , thereby alleviating the need to include a sensor on the substrate  28 . In this embodiment, the strain gauge  50  is a device used to measure the strain of the isolator  30 , and may comprise a metallic coil pattern that is integrated into the flexible printed circuit material. In this embodiment, the strain gauge  50  is integral to the isolator  30 , so that the strain gauge  50  deforms as the isolator  30  deforms. The deformation of the strain gauge  50  results in a change in the electrical resistance of the strain gauge  50 . The change in electrical resistance is related to the pressure-related deformation of the isolator  30 , and therefore the displacement of the actuator  40 , by a gauge factor. Accordingly, the displacement of the actuator  40  and the associated pressure differential across the disc pump  10  can be determined using the stain gauge  50 . 
     In another embodiment, the RFID tag  51  may be located on the actuator  40  at a displacement node (described above). In such an embodiment, the strength of the signal from the RFID tag  51  may be measured by a remote sensor placed at a static location for the purposes of determining the displacement of the actuator  40 . Using the signal strength as a measure of the displacement of the actuator  40 , allows the RFID tag  51  to determine the associated pressure differential across the disc pump  10 . 
       FIG. 13  is a block diagram that illustrates the functionality of the disc pump system  100  that includes a sensor and the RFID tag  51 . The sensor may be, for example, the strain gauge  50 , that is operable to measure the displacement of an actuator  40 , as described above. Other sensors may also be utilized as part of the disc pump system  100 . The disc pump system  100  comprises a battery  60  to power the disc pump system  100 . The elements of the disc pump system  100  are interconnected and communicate through wires, paths, traces, leads, and other conductive elements. The disc pump system  100  also includes a controller or processor  56  and a driver  58 . The processor  56  is adapted to communicate with the driver  58 . The driver  58  is functional to receive a control signal  62  from the processor  56 . The driver  58  generates a drive signal  64  that energizes the actuator  40  in the first disc pump  10 . 
     As noted above, the actuator  40  may include a piezoelectric component that generates the radial pressure oscillations of the fluid within the cavities of the disc pump  10  when energized causing fluid flow through the cavity to pressurize or depressurize the load as described above. As an alternative to using a piezoelectric component to generate radial pressure oscillations, the actuator  40  may be driven by an electrostatic or electromagnetic drive mechanism. 
     The isolator  30  of the disc pump  10  is formed from a flexible, printed circuit material and may include the integrated sensor. The optional sensor may be coupled to the RFID tag  51  via a processor or a bus. In such an embodiment, the RFID tag  51  and the processor are in turn coupled to a power supply. When the disc pump  10  is operational, or when a pressure differential is developed across the valve of the disc pump  10 , then the isolator  30  of the disc pump  10  will be deformed in accordance with the displacement of the actuator  40 . For example, if the actuator  40  is displaced from a rest position, the isolator  30  will be under tensile strain. Thus, if the sensor is the strain gauge  50 , the electrical resistance sensed by the sensor will be indicative of the displacement of the actuator  40 . As such, the sensor may be used to measure performance data related to the operation of the disc pump  10 , including data related to the displacement of the actuator  40  and the deformation of the isolator  30 . The measured data may have a dynamic value or a static value, depending on whether the pump is operational, in a free flow state, or in a stall state. In the free flow state, the sensor may return dynamic performance data that can be used to determine the pressure differential created by the pump or the condition of the isolator  30 . For example, the performance data may be used to determine the maximum pressure differentials generated by the disc pump  10  over time, as well as the average pressure differential over time as the disc pump  10  transitions from the free-flow condition to the stall condition. Alternatively, the sensor may be used to determine the pressure differential across the disc pump  10  in a static condition, i.e., when the disc pump  10  is stopped or when the pump has reached the stall condition. 
     The performance data measured by the sensor may be transferred to a remote computing unit via the RFID tag  51 . To facilitate the transfer of data from the RFID tag  51 , the disc pump system  100  includes an RFID reader  49 , which is a receiver or transceiver that receives RFID communications from the RFID tag.  51 . In one embodiment, the RFID reader  49  may also wirelessly supply power to the RFID tag  51 . The transmitted power is stored by the power supply, and used to power the devices located on the isolator  30 , including the RFID tag  51 , the processor, the memory, and the sensor. Data transmitted from the RFID tag  51  to the RFID reader  49  is transmitted to the processor  56  of the disc pump  10 . 
     In one embodiment, the processor  56  may utilize the data as feedback to adjust the control signal  62  and corresponding drive signals  64  for regulating the pressure at the load  38 . In one embodiment, the processor  56  calculates the flow rate provided by the disc pump system  100  as a function of the received data that indicates, for example, the pressures generated at the disc pump  10 , as described above. 
     The processor  56 , driver  58 , and other control circuitry of the disc pump system  100  may be referred to as an electronic circuit. The processor  56  may be circuitry or logic enabled to control functionality of the disc pump  10 . The processor  56  may function as or comprise microprocessors, digital signal processors, application-specific integrated circuits (ASIC), central processing units, digital logic or other devices suitable for controlling an electronic device including one or more hardware and software elements, executing software, instructions, programs, and applications, converting and processing signals and information, and performing other related tasks. The processor  56  may be a single chip or integrated with other computing or communications elements. In one embodiment, the processor  56  may include or communicate with a memory. The memory may be a hardware element, device, or recording media configured to store data for subsequent retrieval or access at a later time The memory may be static or dynamic memory in the form of random access memory, cache, or other miniaturized storage medium suitable for storage of data, instructions, and information. In an alternative embodiment, the electronic circuit may be analog circuitry that is configured to perform the same or analogous functionality for measuring the pressure and controlling the displacement of the actuator  40  in the cavities of the disc pump  10 , as described above. 
     The disc pump system  100  may also include an RF transceiver  70  for communicating information and data relating to the performance of the disc pump system  100  including, for example, the flow rate, the current pressure measurements, the actual displacement (δy) of the actuator  40 , and the current life of the battery  60  via wireless signals  72  and  74  transmitted from and received by the RF transceiver  70 . Generally, the disc pump system  100  may utilize a communications interface that comprises RF transceiver  70 , infrared, or other wired or wireless signals to communicate with one or more external devices. The RF transceiver  70  may utilize Bluetooth, WiFi, WiMAX, or other communications standards or proprietary communications systems. Regarding the more specific uses, the RF transceiver  70  may send the signals  72  to a computing device that stores a database of pressure readings for reference by a medical professional. The computing device may be a computer, mobile device, or medical equipment device that may perform processing locally or further communicate the information to a central or remote computer for processing of the information and data. Similarly, the RF transceiver  70  may receive the signals  72  for externally regulating the pressure generated by the disc pump system  100  at the load  38  based on the motion of the actuator  40 . 
     The driver  58  is an electrical circuit that energizes and controls the actuator  40 . For example, the driver  58  may be a high-power transistor, amplifier, bridge, and/or filters for generating a specific waveform as part of the drive signal  64 . Such a waveform may be configured by the processor  56  and the driver  58  to provide drive signal  64  that causes the actuator  40  to vibrate in an oscillatory motion at the frequency (f), as described in more detail above. The oscillatory displacement motion of the actuator  40  generates the radial pressure oscillations of the fluid within the cavities of the disc pump  10  in response to the drive signal  64  to generate pressure at the load  38 . 
     In another embodiment, the disc pump system  100  may include a user interface for displaying information to a user. The user interface may include a display, audio interface, or tactile interface for providing information, data, or signals to a user. For example, a miniature LED screen may display the pressure being applied by the disc pump system  100 . The user interface may also include buttons, dials, knobs, or other electrical or mechanical interfaces for adjusting the performance of the disc pump, and particularly, the reduced pressure generated. For example, the pressure may be increased or decreased by adjusting a knob or other control element that is part of the user interface. 
     In accordance with the embodiments described above, the implementation of a sensor on the isolator  30  can negate the need for a remote pressure sensor to measure the displacement of an actuator  40  in a disc pump  10 . The measured displacement or other data can be used to determine the pressure differential generated by the disc pump  10 . By mounting the actuator  40  on an isolator  30  that is formed by a flexible circuit material, a sensor and wireless transmission system can be manufactured directly onto the isolator  30  and used to directly measure, for example, the strain on the isolator  30 . The measured strain on the isolator  30  may be used to determine the corresponding displacement of the edge of the actuator  40 , which enables the computation of the differential pressure generated by the disc pump  10 . Where the sensor is a strain gauge  50 , the null electrical resistance of the strain gauge  50  may be measured before the disc pump  10  is coupled to the load  38  to ensure that there is not an externally generated, or pre-existing pressure differential across the actuator  40 . The null resistance may then be compared to the electrical resistance of the strain gauge  50  over time to detect changes in isolator  30 . In one embodiment, this strain gauge data may be gathered to indicate the condition of the isolator  30 . For example, strain gauge data that indicates that the resiliency of the isolator  30  is diminishing may indicate a worn or damaged isolator  30 . Similarly, strain gauge data indicating that there is less strain, or less deformation of the isolator  30  despite the application of a drive signal to the actuator  40  may indicate a pump defect, such as delamination of the isolator  30 . In addition, the rate of change of pressure, which can be measured using data gathered by the strain gauge, may be used to indicate a flow rate of the disc pump. 
     It should be apparent from the foregoing that an invention having significant advantages has been provided. While the invention is shown in only a few of its forms, it is not just limited but is susceptible to various changes and modifications without departing from the spirit thereof.