Patent Publication Number: US-2012034109-A1

Title: System and method for measuring pressure applied by a piezo-electric pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/371,954, filed Aug. 9, 2010, and is hereby incorporated by reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The illustrative embodiments of the invention relate generally to a pump for fluid and, more specifically, to a pump in which the pumping cavity is substantially elliptical in shape having end walls and a side wall 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/or 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 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 elliptical cavity with an acoustic driver at one end, which drives an acoustic standing wave. In such a elliptical cavity, the acoustic pressure wave has limited amplitude. Varying cross-section cavities, such as cone, horn-cone, 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 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 pump has a substantially elliptical cavity comprising a side wall closed at each end by end walls. The 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 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 pump efficiency. The efficiency of a mode-matched pump is dependent upon the interface between the driven end wall and the side wall. It is desirable to maintain the efficiency of such 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 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 elliptical 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 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 pump that decreases dampening of the displacement oscillations to mitigate any reduction of the pressure oscillations within the cavity, that portion being referred to hereinafter as an “skirt” or a “skirt” as described more specifically in U.S. patent application Ser. No. 12/477,594 which is incorporated by reference herein. The illustrative embodiments of the skirt are operatively associated with the peripheral portion of the driven end wall to reduce dampening of the displacement oscillations. 
     Such pumps also require one or more valves for controlling the flow of fluid through the 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 known in the art operate between 150 and 350 Hz. However, many portable electronic devices including medical devices require pumps for delivering a positive or negative pressure that are relatively small in size and quiet during operation so as to provide discrete therapy. To achieve these objectives, such 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 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 the first or second aperture, or both apertures, for controlling the flow of fluid through the 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. 
     BRIEF SUMMARY OF THE INVENTION 
     In addressing measurement and control issues of tissue treatment systems, which may include a disc pump or micro-pump, the principles of the present invention may be utilized to measure the pressure being generated by the disc pump to more effectively and economically control the operation of the disc pump. The disc pump includes an actuator that vibrates within a cavity to generate a radial pressure wave to provide a reduced pressure for application to a load or tissue site as described above. Displacement of the actuator may be measured using one or more sensors. Pressure being generated by the disc pump for the tissue site may be determined in response to the measured displacement of the actuator. A drive signal for the actuator may be adjusted to control operation and, consequently, displacement of the actuator to reach a desired pressure at the tissue site. 
     One embodiment of a disc pump includes a disc pump housing, skirt, actuator, sensor, and electronic circuit. The skirt is fixed to the disc pump housing to support the actuator, and may be any material that is sufficiently flexible to allow the actuator to vibrate. The actuator and the skirt face an opposing base plate to form a cavity within the disc pump wherein radial pressure waves are generated. The actuator may have a first surface and a second surface and be directly or indirectly coupled to the skirt. The sensor may be positioned outside the cavity to sense a position of the actuator with respect to the disc pump housing that corresponds to the pressure being provided. An electronic circuit may be in communication with the sensor and be configured to calculate pressure provided by the disc pump as a function of the position of the actuator with respect to the disc pump housing while the actuator is activated. 
     In another embodiment, a pump body comprises a substantially elliptical shaped side wall closed at one end by a base wall and the other end by a pair of interior plates to form a cavity within said pump body for containing a fluid wherein a first one of the interior plates adjacent the cavity includes a center portion and a peripheral portion. The pump further comprises an actuator formed by the end plates wherein the second one of the interior plates is operatively associated with the central portion of the first interior plate to cause an oscillatory displacement motion thereby generating radial pressure oscillations of the fluid within the cavity in response to a drive signal being applied to said actuator when in use The pump also comprises a skirt flexibly connected between the side wall and the peripheral portion of the first interior plate to facilitate the oscillatory displacement motion. The pump also comprises a first aperture extending through said actuator to enable fluid to flow through the cavity and a second aperture extending through the base wall to enable fluid to flow through the cavity. A valve is disposed in at least one of said first aperture and second apertures and is adapted to permit the fluid to flow through the cavity in substantially one direction to pressurize or depressurize a load as fluid begins flowing through the cavity, thereby causing said actuator to move toward the base wall from a rest position to a biased position with increasing pressure and flexing of the skirt. The pump further comprises a sensor mounted outside the cavity in a fixed position with respect to said pump body for measuring the displacement of said actuator at any position between the rest position and the biased position as fluid begins flowing through the cavity to pressurize or depressurize the load. 
     One method for controlling a disc pump includes driving an actuator within a housing of a disc pump using a drive signal. The actuator is mounted within the disc pump by the skirt which is flexible. As the actuator vibrates in response to the drive signal, the pressure created in a load increases while airflow decreases from a free-flow state to a stall state. The pressure being built up in the load by the disc pump may be measured by a sensor as a function of the displacement of the actuator from a rest position in the free-flow state to a biased position in the stall state when the pressure forces the actuator away from the rest position as the skirt flexes with the actuator from its fixed position toward the biased position. Because the actuator generates radial pressure waves within the cavity of the disc pump, such a sensor is preferably positioned outside the cavity of the disc pump so that it does not interfere with the operation of the disc pump itself. 
     Other objects, features, and advantages of the illustrative embodiments are disclosed herein and will become apparent with reference to the drawings and detailed description that follow. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Illustrative embodiments of the present invention are described in detail below with reference to the attached drawing figures, which are incorporated by reference herein and wherein: 
         FIG. 1A  is a schematic, cross-sectional view of a first disc pump having an actuator shown in a rest position according to a first illustrative embodiment; 
         FIG. 1B  is a schematic, cross-sectional view of the first disc pump showing the actuator in a biased position according to a first illustrative embodiment; 
         FIG. 2A  is a graph of the axial displacement oscillations for a fundamental bending mode of the actuator of the first disc pump; 
         FIG. 2B  is a graph of the pressure oscillations of fluid within the cavity of the first disc pump in response to the bending mode shown in  FIG. 2A ; 
         FIG. 3  is a zoomed-in view of a first sensor for measuring the displacement of the actuator of the first disc pump according to a first illustrative embodiment; 
         FIG. 4  is a schematic view of an illustrative receiver of the first sensor indicating the position of the actuator when in the rest position and the biased position; 
         FIG. 5  is a schematic, cross-sectional view of the disc pump with the actuator shown in the biased position including a zoomed-in view of a second sensor for measuring the displacement of the actuator according to a second illustrative embodiment; 
         FIG. 6  is a third illustrative sensor including a diffraction grating for measuring displacement of an actuator in a disc pump; 
         FIG. 7  is a fourth illustrative sensor including a magnetic element for measuring displacement of an actuator in a disc pump; 
         FIG. 8  is a block diagram of an illustrative circuit of a disc pump for measuring and controlling a reduced pressure generated by the disc pump; and 
         FIG. 9  is a flow chart of an illustrative process for controlling pressure generated by a disc pump. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       FIGS. 1A and 1B  are illustrations of a cross-section view of an illustrative disc pump  100  in accordance with illustrative embodiments. As shown, the disc pump  100  may include a pump housing  102  having a substantially elliptical shape including a elliptical wall  101  closed at one end by a base wall  103  and mounted at the other end by legs  105  attached to a circuit board  108  or other substrate to support the pump housing  102 . The elliptical wall  101 , the legs  105 , and base wall  103  together form the pump housing  102 . The pump  100  further comprises a pair of disc-shaped interior plates  114 ,  115  supported within the pump  100  by a ring-shaped skirt  130  affixed to the elliptical wall  101  of the pump body. The internal surfaces of the elliptical wall  101 , the base wall  103 , the interior plate  114 , and the ring-shaped skirt  130  form a cavity  116  within the pump  100 . The internal surfaces of the cavity  116  comprise a side wall  118  which is a first portion of the inside surface of the elliptical wall  101  that is closed at one end by end wall  120  wherein the end wall  120  is the internal surface of the end plate  103  and the end wall  122  comprises the internal surface of the interior plate  114  and a first side of the skirt  130 . The end wall  122  thus comprises a central portion corresponding to the inside surface of the interior plate  114  and a peripheral portion corresponding to the inside surface of the ring-shaped skirt  130 . 
     Although the pump  100  and its components are substantially elliptical in shape, the specific embodiment disclosed herein is a circular, elliptical shape. In the embodiments shown in  FIGS. 1A and 1B , the end wall  120  is shown as being a frusto-conical surface, but may also be generally planar and parallel with the end wall  122 . The base wall  103  and elliptical wall  101  of the pump body may be formed from any suitable rigid material including, without limitation, metal, ceramic, glass, or plastic including, without limitation, injection-molded plastic. 
     The interior plates  114 ,  115  of the pump  100  together form an actuator  140  that is operatively associated with the central portion of the end wall  122  which is one of the internal surfaces of the cavity  116 . One of the interior plates  114 ,  115  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 electro-strictive or magneto-strictive material. In one preferred embodiment, for example, the interior plate  115  is formed of piezoelectric material that that exhibits strain in response to an applied electrical signal, i.e., the active interior plate. The other one of the interior plates  114 ,  115  preferably possess 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  114  possess a bending stiffness similar to the active interior plate  115  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  115  is excited by an electrical current, the active interior plate  115  expands and contracts in a radial direction relative to the longitudinal axis of the cavity  116  causing the interior plates  114 ,  115  to bend, thereby inducing an axial deflection of their respective end wall  122  in a direction substantially perpendicular to the end wall  122  (See  FIG. 2A ). 
     In other embodiments not shown, the skirt  130  may support either one of the interior plates  114 ,  115 , whether the active or inert internal plate, from the top or the bottom surfaces depending on the specific design and orientation of the pump  100 . In another embodiment, the actuator  140  may be replaced by a device in a force-transmitting relation with only one of the interior plates  114 ,  115  such as, for example, a mechanical, magnetic or electrostatic device, wherein 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 pump  100  further comprises at least two apertures extending from the cavity  116  to the outside of the pump  100 , wherein at least one of the apertures contains a valve to control the flow of fluid through the aperture. Although the apertures may be located at any position in the cavity  116  where the actuator  140  generates a pressure differential as described below in more detail, one preferred embodiment of the pump  100  comprises aperture  126  located at approximately the centre of and extending through the base wall  103 . The aperture  126  contains at least one end valve. In one preferred embodiment, the aperture  126  contains a valve  128  which regulates the flow of fluid in one direction as indicated by the arrow. Thus, for this embodiment, the valve  128  functions as an inlet valve for the pump. 
     The pump  100  further comprises at least one aperture from the cavity  116  through the actuator  140 , wherein at least one of the apertures contains a valve to control the flow of fluid through the aperture. Although these apertures may be located at any position on the actuator  140  from the cavity  116  where the actuator  140  generates a pressure differential as described below in more detail, one embodiment of the pump  100  comprises a single aperture  131  located at approximately the centre of and extending through the interior plates  114 ,  115 . The aperture  131  contains an actuator valve  132  which regulates the flow of fluid in one direction from the cavity  116  as indicated by the arrow so that the actuator valve  132  functions as an outlet valve from the cavity  116 . The actuator valve  132  enhances the output of the pump  100  by supplementing the operation of the inlet valve  128  as described in more detail below. 
     The dimensions of the cavity  116  described herein should preferably satisfy certain inequalities with respect to the relationship between the height (h) and radius (r) of the cavity  116  which is the distance from the longitudinal axis of the cavity  116  to the side wall  118 . These equations are as follows: 
         r/h&gt; 1.2; and 
         h   2   /r&gt; 4×10 −10  meters.
 
     In one embodiment of the invention, 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  116  is a gas. In this example, the volume of the cavity  116  may be less than about 10 ml. Additionally, the ratio of h 2 /r is preferably within a range between about 10 −6  and about 10 −7  meters where the working fluid is a gas as opposed to a liquid. 
     Additionally, the cavity  116  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  140  vibrates to generate the axial displacement of the end wall  122 . The inequality equation is as follows: 
     
       
         
           
             
               
                 
                   
                     
                       
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     wherein the speed of sound in the working fluid within the cavity  116  (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  140  is preferably about equal to the lowest resonant frequency of radial pressure oscillations in the cavity  116 , but may be within 20% that value. The lowest resonant frequency of radial pressure oscillations in the cavity  116  is preferably greater than about 500 Hz. 
     Although it is preferable that the cavity  116  disclosed herein should satisfy individually the inequalities identified above, the relative dimensions of the cavity  116  should not be limited to cavities having the same height and radius. For example, the cavity  116  may have a slightly different shape requiring different radii or heights creating different frequency responses so that the cavity  116  resonates in a desired fashion to generate the optimal output from the pump  100 . 
     In operation, the pump  100  may function as a source of positive pressure adjacent the outlet valve  132  to pressurize a load (not shown) or as a source of negative or reduced pressure adjacent the inlet valve  128  to depressurize a load  150  as illustrated by the arrows. The inlet of the pump  100  as shown is in fluid communication with the load  150  such that the pump  100  functions as a source of negative or reduced pressure adjacent the inlet valve  128 . The load  150  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 pump  100  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. 
       FIG. 2A  shows one possible displacement profile illustrating the axial oscillation of the driven end wall  122  of the cavity  116 . The solid curved line and arrows represent the displacement of the driven end wall  122  at one point in time, and the dashed curved line represents the displacement of the driven end wall  122  one half-cycle later. The displacement as shown in this figure and the other figures is exaggerated. Because the actuator  140  is not rigidly mounted at its perimeter, but rather suspended by the ring-shaped skirt  130 , the actuator  140  is free to oscillate about its centre of mass in its fundamental mode. In this fundamental mode, the amplitude of the displacement oscillations of the actuator  140  is substantially zero at an annular displacement node  42  located between the centre of the driven end wall  122  and the side wall  118 . The amplitudes of the displacement oscillations at other points on the end wall  122  are greater than zero as represented by the vertical arrows. A central displacement anti-node  43  exists near the centre of the actuator  140  and a peripheral displacement anti-node  43 ′ exists near the perimeter of the actuator  140 . The central displacement anti-node  43  is represented by the dashed curve after one half-cycle. 
       FIG. 2B  shows one possible pressure oscillation profile illustrating the pressure oscillation within the cavity  116  resulting from the axial displacement oscillations shown in  FIG. 2A . 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 positive central pressure anti-node  45  near the centre of the cavity  116  and a peripheral pressure anti-node  45 ′ near the side wall  118  of the cavity  116 . 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 has a negative central pressure anti-node  47  near the centre of the cavity  116  with a peripheral pressure anti-node  47 ′ and the same annular pressure node  44 . For a elliptical cavity, the radial dependence of the amplitude of the pressure oscillations in the cavity  116  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  116  and so will be referred to as the “radial pressure oscillations” of the fluid within the cavity  116  as distinguished from the axial displacement oscillations of the actuator  140 . 
     With further reference to  FIGS. 2A and 2B , it can be seen that the radial dependence of the amplitude of the axial displacement oscillations of the actuator  140  (the “mode-shape” of the actuator  140 ) approximates 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  116  (the “mode-shape” of the pressure oscillation). Other symmetric and asymmetric functions may also be used to generate pressure oscillations within the cavity  116 . In any event, by not rigidly mounting the actuator  140  at its perimeter and allowing it to vibrate more freely about its centre of mass, the mode-shape of the displacement oscillations substantially matches the mode-shape of the pressure oscillations in the cavity  116  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  140  and the corresponding pressure oscillations in the cavity  116  have substantially the same relative phase across the full surface of the actuator  140  wherein the radial position of the annular pressure node  44  of the pressure oscillations in the cavity  116  and the radial position of the annular displacement node  42  of the axial displacement oscillations of actuator  140  are substantially coincident. 
     As the actuator  140  vibrates about its centre of mass, the radial position of the annular displacement node  42  will necessarily lie inside the radius of the actuator  140  when the actuator  140  vibrates in its fundamental bending mode as illustrated in  FIG. 2A . 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  116  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 centre of the end wall  122  to the side wall  118 , i.e., the radius of the cavity  116  (“r”). Therefore, the radius of the actuator  140  (r act ) should preferably satisfy the following inequality: r act ≧0.63r. 
     The ring-shaped skirt  130  may be a flexible membrane which enables the edge of the actuator  140  to move more freely as described above by bending and stretching in response to the vibration of the actuator  140  as shown by the displacement at the peripheral displacement anti-node  43 ′. The flexible membrane overcomes the potential dampening effects of the side wall  118  on the actuator  140  by providing a low mechanical impedance support between the actuator  140  and the elliptical wall  101  of the pump  100  thereby reducing the dampening of the axial oscillations at the peripheral displacement anti-node  43 ′ of the actuator  140 . Essentially, the flexible membrane minimizes the energy being transferred from the actuator  140  to the side wall  118  with the outer peripheral edge of the flexible membrane remaining substantially stationary. Consequently, the annular displacement node  42  will remain substantially aligned with the annular pressure node  44  so as to maintain the mode-matching condition of the pump  100 . Thus, the axial displacement oscillations of the driven end wall  122  continue to efficiently generate oscillations of the pressure within the cavity  116  from the central pressure anti-nodes  45 ,  47  to the peripheral pressure anti-nodes  45 ′,  47 ′ at the side wall  118  as shown in  FIG. 2B . 
     As the actuator  140  vibrates in response to the drive signal, the pressure created in the load  150  increases while airflow decreases from a free-flow state to a stall state. The pressure being built up in the load  150  by the disc pump  100  may be measured by a sensor as a function of the displacement (δy) of the actuator  140  from a rest position  136  in the free-flow state as shown in  FIG. 1A  to a biased position  138  in the stall state as shown in  FIG. 1B  when the pressure forces the actuator  140  away from the rest position as the skirt  130  flexes with the actuator  140  from its fixed position at the side wall  101  toward the biased position  138 . Because the actuator  140  generates radial pressure waves within the cavity  116  of the disc pump  100 , such a sensor is preferably positioned outside the cavity  116  of the disc pump  100  so that it does not interfere with the operation of the disc pump  100 . 
       FIG. 3  is a zoomed-in view of a sensor  331  mounted on the circuit board  108  to face the actuator  140  and measure the displacement of the actuator  140  of the disc pump  100 . The sensor  331  includes an optical transmitter  332  and optical receiver  334  for use in measuring displacement  130  of the actuator  140 . The optical transmitter  332  communicates an optical signal  335  that may be a light wave in a visible or non-visible spectrum. The optical signal  335  is reflected off the surface of the interior plate  115  of the actuator  140  so that the reflected signal is received by the optical receiver  334  regardless of the displacement (δy) of the actuator  140  as shown in  FIG. 4 . When the actuator  140  is in the rest position  136 , a first reflected signal  340  impinges on the optical receiver  334  at the position shown in both  FIGS. 3 and 4 . As the actuator  140  is displaced from the rest position  136  to the biased position  138 , the first reflected signal  340  is correspondingly displaced by a corresponding reflected displacement (δx) as a second reflected signal  342  depending on the displacement (δy) of the actuator  140 . Essentially, the image of the reflected signals that impinge on the optical receiver  334  follow a path from the rest position  136  to the fully biased position  138  as shown in  FIG. 4 . The reflected displacement (δx) is proportional to the displacement (δy) of the actuator  140  which is a function of the pressure provided by the disc pump  100  as described above. 
     In one embodiment, the optical transmitter  332  may be a laser, a light emitting diode (LED), a vertical cavity surface emitting laser (VCSEL), or light emitting element. The optical transmitter  332  may be positioned on the circuit board  108  and oriented to reflect the optical signal  335  off any point of the interior plate  115  of the actuator  140  as long as that the first reflected signal  340  and the second reflected signal  342  are still received and measured by the optical sensor  334 . However, as the actuator  140  oscillates in a fundamental mode to generate airflow as described and shown in  FIG. 2A , the amplitude of the displacement oscillations of the actuator  140  may be substantially zero at any annular displacement nodes  42  that are generated. Correspondingly, the amplitudes of the displacement oscillations at other points along the actuator  140  are greater than zero as also described. Therefore, the optical transmitter  332  should be positioned and oriented so that the optical signal  335  is reflected from a position close to the annular displacement nodes  42  to minimize the effect of the high frequency oscillations of the actuator  140  and more accurately measure the displacement (δy) of the actuator  140  as it moves more slowly from the rest position  136  to the biased position  138 . 
     In one embodiment, the optical sensor  334  may include multiple pixels forming a sensor array. The optical sensor  334  may be configured to sense the position of one or more reflected beams at one or more wavelengths. As a result, the optical receiver  334  may be configured to sense the reflected displacement (δx)  144  between the first reflected signal  340  and the second reflected signal  342 . The optical receiver  334  may be configured to convert the reflected signals  340  and  342  sensed by the optical receiver  334  into electrical signals by the respective pixels of the optical receiver  334 . The reflected displacement (δx) may be measured or calculated in real-time or utilizing a specified sampling, frequency to determine the position of the actuator  140  relative to the pump housing  102 . In one embodiment, the position of the actuator  140  is computed as an average or mean position over a given time period. Pixels of the optical receiver  334  may be sized to provide additional sensitivity to detect relatively small displacements (δy) of the actuator  140  to better monitor the pressure being provided by the disc pump  100  so that it can be controlled in real-time. 
     Alternative methods of computing the displacement of the actuator  140  may be utilized in accordance with the principles of the present invention. It should be understood that determining the displacement of the actuator  140  may be accomplished relative to any other fixed-position element in the pump housing  102 . Although generally substantially proportional, the reflected displacement (δx) may equal the displacement (δy) of the actuator  140  multiplied by a scale factor where the scale factor may be predetermined value based in the configuration of the pump housing  102  of the disc pump  100  or other alignment factors. As a result, the reduced pressure within the cavity  116  of the disc pump  100  may be determined by sensing the displacement (δy) of the actuator  140  without the need for pressure sensors that directly measure the pressure provided to a load, but are too bulky and expensive for measuring the pressure provided by the disc pump  100  in a reduced pressure system for example. The illustrative embodiments optimize the utilization of space within the pump housing  102  without interfering with the pressure oscillations being created within the cavity  116  of the disc pump  100 . 
       FIG. 5  is another schematic, cross-sectional view of the disc pump  100  showing the actuator  140  in the biased position  138  including a assumed-in view of another sensor for measuring the displacement of the actuator  140  according to another illustrative embodiment. The sensor is an ultrasonic transceiver  546  that transmits ultrasonic waves  548  to determine the position of the actuator  140  based on the ultrasonic waves  548  reflected by the actuator  140  and received by the ultrasonic transceiver  546 . For purposes of simplicity, the ultrasonic waves that echo back to the ultrasonic transceiver  546  are not shown. The ultrasonic transceiver  546  may send raw measurements or processed data regarding the displacement (δy) of the actuator  140  to one or more electronic devices including, for example, a processor to determine the reduced pressure generated by the this pump  100  and other operational characteristics. 
     With regard to  FIG. 6 , a diffraction grating  602  for measuring displacement (δy) of the actuator  140  in the disc pump  100  is shown. The diffraction grating  602  may be attached to or integrated with the actuator  140 . For example, the diffraction grating  602  may be a reflective optical element attached to or the actuator  140  with adhesives or other fastening means during manufacturing of the disc pump. As shown, a transmitter  607  transmits a multi-spectral optical signal  608  onto the diffraction grating  602 . The diffraction grating  602  diffracts the multi-spectral optical signal  608  into several beams with different wavelengths λ 1 , λ 2 , λ 3 , and λ 4 . The wavelengths of beams λ 1 , λ 2 , λ 3 , and λ 4  are detected by a sensor array  610 . In one embodiment, the sensor array  610  may include multiple pixels  612 ,  614 ,  616 , and  618 . The pixels  612 ,  614 ,  616 , and  618  of the sensor array  610  may also be referred to as a pixel array. Alternatively, the sensor array  610  may be a single sensor or pixel element, such as the pixel  614 . The transmitter  607  and the sensor array  610  may be connected to circuit board  108  or any other fixed-position element of the pump housing  102  to ensure stability during operation. 
     In operation, the transmitter  607  may be a light generation circuit or element that transmits the multi-spectral optical signal  608  in the form of multi-spectrum optical signal onto the diffraction grating. The diffraction grating  602  may be an optical component with a regular pattern, which diffracts light of the multi-spectral optical signal  608  into several beams λ 1 , λ 2 , λ 3 , and λ 4  and reflects the beams in different directions, as shown in  FIG. 6 . As is known in the art, the diffraction grating  602  may include grooves or rulings within the grating of the diffraction grating configured to diffuse the λ 1 , λ 2 , λ 3 , and λ 4  over the sensor, array  610  during normal operation and displacement of the actuator  140 . 
     The sensor array  610  determines the displacement of the actuator  140  based on the one or more wavelengths received by one or more of the pixels  612 ,  614 ,  616 , and  618 . For example, as shown in  FIG. 6  the dispersion of wavelengths λ 1 , λ 2 , λ 3 , and λ 4  on the pixels  612 ,  614 ,  616 , and  618  may correspond to a maximum displacement between the actuator  140  and the circuit board  108 . As the actuator  140  moves toward the housing body (i.e., into the cavity), the pixels  612 - 618  may detect one or more of the wavelengths λ 1 , λ 2 , λ 3 , and λ 4 . In one embodiment, the measurements from the sensor array  610  may indicate the displacement of the actuator  140 . For example, if both λ 3  and λ 4  are detected by pixel  618 , the displacement may be 2 mm indicating optimal displacement for producing a desired pressure in the cavity of the reduced pressure delivery system. The wavelengths λ 1 , λ 2 , λ 3 , and λ 4  detected by each of the pixels  612 ,  614 ,  616 , and  618  may indicate the exact displacement or may provide data utilized to calculate the displacement. In an alternative embodiment, a sensor may be a single pixel configured to sense optical wavelengths in the multi-spectral optical signal  608  so that as the actuator  140  moves, the wavelength sensed by the sensor is indicative of the position of the actuator relative to the housing. In yet another embodiment, an optical sensor with a single cell having known dimensions may be positioned at an optimal location of a certain light spectrum (or any light at all) be sensed by the optical sensor, and, if sensed, a determination may be made that the pump is generating a pressure in a certain tolerance range may be made. 
     With regard to  FIG. 7 , a magnetic sensor  702  for measuring displacement (δy) of the actuator  140  in the disc pump  100  is shown. The magnetic sensor  702 , which may be a Hall Effect or analogous sensor, is mounted to the circuit board  108  or the pump housing  102 . A conductor  706  may be mounted to an actuator  140 . The conductor  706  may be metallic, magnetic, or otherwise that is capable of providing for magnetic sensing by the magnetic sensor  702 . The magnetic sensor  702  measures a magnetic field  710  between the magnetic sensor  702  and the conductor  706 . The magnetic sensor  702  may be calibrated or configured to measure the changing electric field resulting in the magnetic field  710  to determine the displacement between the magnetic sensor  702  and the conductor  706 . 
     Referring to  FIG. 8 , a block diagram of an illustrative disc pump system  800  that includes a disc pump such as the disc pump  100  described above and a sensor for measuring and, controlling a pressure generated by the disc pump  100  such as the optical sensor  331  including the optical transmitter  332  and the optical receiver  334  is shown. It should be understood that other sensors as described above may also be utilized as part of the disc pump system  800 . The disc pump system  800  also comprises a battery  802  utilized to power the disc pump system  800 . The elements of the disc pump system  800  are interconnected and communicate through wires, paths, traces, leads, and other conductive elements. The disc pump system  800  may also include a processor  804  and a driver  808  where the processor  804  is adapted to communicate with the driver  808  including communicating a control signal  806  to the driver  808 . The driver  808  generates a drive signal  810  that energizes an actuator in the disc pump  100  such as the actuator  140  as described above. The actuator  140  may include a piezoelectric component that generates the radial pressure oscillations of the fluid within the cavity of the disc pump  100  when energized causing fluid flow through the cavity to pressurize or depressurize the load as described above. The processor  804  may be configured to provide in illumination signal  812  to the optical transmitter  332  for illuminating the actuator  140  with an optical beam such as optical beam  335  which is reflected by the actuator  140  to the optical receiver  334  as illustrated by the reflected signals  340 ,  342  which are also described above. When the reflected signals  340 ,  342  to impinge on the optical receiver  334 , the optical receiver  334  provides a displacement signal  814  to the processor  804  corresponding to the displacement (δy) of the actuator  140 . The processor  804  is configured to calculate the pressure generated by the pump  100  at the load as a function of the displacement (δy) of the actuator  140  as represented by the displacement signal  814 . In one embodiment, the processor  804  may be configured to average a plurality of reflected signals  340 ,  342  to determine an average displacement of the actuator  130  over time. In yet another embodiment, the processor  804  may utilize the displacement signal  814  as feedback to adjust the control signal  806  and corresponding drive signal  810  for regulating the pressure at the load. 
     The processor  804 , driver  808 , and other control circuitry of the disc pump system  800  may be referred to as an electronic circuit. The processor  804  may be circuitry or logic enabled to control functionality of the disc pump  100 . The processor  804  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  804  may be a single chip or integrated with other computing or communications elements. In one embodiment, the processor  804  may include or communication 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  140  in the cavity of the disc pump  100  as described above. 
     The disc pump system  800  may also include an RF transceiver  820  for communicating information and data relating to the performance of the disc pump system  800  including, for example, the current pressure measurements, the actual displacement (δy) of the actuator  140 , and the current life of the battery  802  via a wireless signals  822  and  824  transmitted from and received by the RF transceiver  820 . The RF transceiver  820  may be a communications interface that utilizes radio, infrared, or other wired or wireless signals to communicate with one or more external devices. The RF transceiver  820  may utilize Bluetooth, WiFi, WiMAX, or other communications standards or proprietary communications systems. Regarding the more specific uses, the RF transceiver  820  may send the signals  822  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  820  may receive the signals  824  for externally regulating the pressure generated by the disc pump  100  at the load based on the motion of the actuator  140 . 
     The driver  808  is an electrical circuit that energizes and controls the actuator  140 . For example, the driver  808  may be a high-power transistor, amplifier, bridge, and/or filters for generating a specific waveform as part of the drive signal  810 . Such a waveform may be configured by the processor  804  and the driver  806  to provide a drive signal  810  that causes the actuator  140  to vibrate in an oscillatory motion at the frequency (f) as described in more detail above. The oscillatory displacement motion of the actuator  140  generates the radial pressure oscillations of the fluid within the cavity of the pump  100  in response to the drive signal  810  to generate pressure at the load. 
     In another embodiment, the disc pump system  800  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  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. 
     A method for measuring pressure generated by a pump to a load is also disclosed. The pump includes an actuator mounted within the pump on a flexible skirt that that forms a cavity within the pump. The flexible skirt allows the actuator to oscillate in order to generate air flow through the cavity of the pump and allows the actuator to be displaced with increasing pressure to the load. The method comprising electrically driving the actuator to cause an oscillatory displacement motion of the actuator within the pump to generate radial pressure oscillations of fluid within the cavity. The method further comprises measuring the displacement of the actuator as fluid begins flowing through the cavity causing the actuator to move from a rest position to a biased position with increasing pressure at the load as accommodated by the flexibility of the skirt. The method also comprises calculating the pressure at the load based on the displacement of the actuator. 
     Referring more specifically to  FIG. 9 , a flow chart of an illustrative process  900  for measuring and controlling pressure generated by a disc pump is shown. The process  900  starts at step  902 , where an actuator within a housing of a disc pump may be driven by a drive signal. The actuator may be driven by a piezo-electric actuator or device. The actuator may be driven to generate a reduced pressure for application at a tissue site For example, the disc pump may directly or indirectly communicate with a tissue site covered by a drape, as is understood in the art. At step  904 , displacement of the actuator may be sensed as the actuator moves from a rest position to a biased position as a result of the pressure increasing within the load. In one embodiment, the rest position occurs when the disc pump when is deactivated or unpowered, and the biased position is reached when pressure within the load is at a maximum value. The displacement of the actuator and the corresponding pressure the load varies between these two positions. The drive signal may be configured, shaped, or otherwise generated by a processor, driver, or control logic of the disc pump for controlling the operation of the actuator and the corresponding pressure being applied to the load. 
     At step  906 , the pressure being generated by the disc pump may be determined as a function of the sensed displacement of the actuator. In one embodiment, the displacement may be determined by reflection or refraction of an optical signal between a housing of the disc pump and the actuator. Similarly, ultrasonic, radio frequency, magnetic, or other optical sensors or transmitter and receiver combinations may be utilized to determine displacement of the actuator. The displacement of the actuator may indicate the pressure being generated by the disc pump for the load. Digital and/or analog electronics may be utilized to determine the pressure applied at the tissue site based on the known differential, factors, losses, and other characteristics of the load such as a tissue treatment system that includes the disc pump as a component. The electronics may utilize any number of static or dynamic algorithms, functions, or sensory measurements to determine the pressure. At step  908 , the drive signal is adjusted to control displacement of the actuator in response to determining the pressure being delivered by the disc pump. The drive signal may be generated in response to measurements of feedback signals received from the one or more sensors measuring the displacement of the actuator. In one embodiment, the amplitude of the drive signal may be increased to increase the reduced pressure generated by the disc pump and correspondingly communicated to the tissue site Similarly, the amplitude or the shape of the drive signal may be modified to drive the actuator of the disc pump for decreasing or maintaining pressure at the load. 
     The illustrative embodiments provide a low cost system for indirectly monitoring the pressure generated by a disc pump by interpreting data provided by a sensor in the disc pump that measures the displacement of an actuator relative to fixed-position components within the disc pump when the actuator moves from a rest position to a biased position. It should be understood that the sensor or any component thereof such as the optical transmitter of an optical sensor may be connected directly to the actuator for measuring the displacement by reflecting the optical signal off of the pump housing or any other fixed position on the disc pump. The illustrative embodiments reduce the equipment, space, and cost to monitor pressure being generated by the disc pump beyond that available utilizing traditional pressure sensors and monitors that directly sense the pressure generated by a pump at the load. 
     The previous detailed description is one of a small number of embodiments for implementing the invention and is not intended to be limiting in scope. One of skill in this art will immediately envisage the methods and variations used to implement this invention in other areas than those described in detail. The following claims set for a number of the embodiments of the invention disclosed with greater particularity.