Abstract:
A microvalve sensor for sensing fluid flow therethrough and generating an electrical signal indicative thereof comprises: a housing connectable inline with a fluid passageway; a microvalve disposed in the housing to permit fluid to flow unidirectionally through the housing, the microvalve including: a substrate; an insulating layer disposed over the substrate, the substrate and insulating layer including an orifice to accommodate fluid flow through the housing; and a diaphragm element disposed over the insulating layer, the diaphragm element including: a solid center portion having an area sufficient to cover the orifice, and an outer portion surrounding the center portion having a plurality of apertures for passing fluid from the orifice through the housing, the outer portion being affixed to the insulating layer around a periphery thereof, the diaphragm element and substrate forming opposite plates of a capacitor having a capacitance which changes with fluid flow through the housing; and a circuit coupled across the opposite plates of the capacitor and powered by an electrical source for measuring the capacitance of the capacitor and generating an electrical signal indicative thereof.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 10/994,512, filed Nov. 22, 2004, now U.S. Pat. No. 7,013,726, issued Mar. 21, 2006, the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention is related to fluidic demand apparatus, in general, and more particularly, to fluidic demand apparatus employing a microvalve or micro electro-mechanical system (MEMS) flow sensor, and the microvalve or MEMS flow sensor itself. 
     An example of a fluidic demand apparatus includes an Oxygen conserver which is shown by way of example in the fluidic schematic diagram of  FIG. 1 . An Oxygen conserver controls the flow of Oxygen gas from a source to a patient on demand, i.e. when a patient inhales. Referring to  FIG. 1 , in fluidic demand apparatus, the fluid, like Oxygen gas, for example, is generally provided from a high pressure source, such as a storage tank  10 . From the tank  10 , the fluid is usually regulated by a regulator  12 . A pressure gauge  14  may be provided at the tank  10  as an indication of the fluid remaining in the tank  10 . In the present example, the fluid in the tank  10  is at a pressure of 2,000 pounds per square inch (psi) and the regulator  12  reduces the pressure to approximately 40 psi. 
     The fluid may exit from the regulator  12  at a pressure of approximately 40 psi through two tubes or passageways  16  and  18 . The tube  16  may be coupled to a delivery tank  40  which is coupled through a tube  22  to an input of a shuttle valve  24 . A variable flow restrictor  25  may be disposed at the tube  16 . An output of the shuttle valve  24  is coupled through a tube  26  to a passageway  28  leading to the patient. Within the valve  24  is a piston  30  which is movable from a bottom or closed position to a top or open position (see dashed lines). The tube  18  may be coupled to a tee connection  32  which may be coupled to the top of the valve  24  through a tube  34  and to a bottom of a diaphragm container  38  through a tube  36 . Fixed fluid flow restrictors  40  and  42  may be disposed at the tubes  18  and  36 , respectively. Another tube  44  may couple the bottom of container  38  to the atmosphere through a variable restrictor  46 . Yet another tube  48  couples a top of container  38  to the patient&#39;s tube  28  through a check valve  50 . A diaphragm  52  within container  38  may be in a spring loaded position (solid line) to close off a passage between tubes  36  and  44 . 
     In operation, when the patient starts to inhale fluid through tube  28 , fluid is conducted through the check valve  50  in tube  48  which creates a pressure differential across the diaphragm  52  in container  38 . When the differential pressure overcomes the spring bias force, the diaphragm  52  is forced upwards (see dotted line position) which permits fluid to flow from the regulator  12  through tubes  18  and  36 , through an open passageway in container  38  and through tube  44  exiting to the atmosphere. Thus, the fluidic pressure holding piston  30  in valve  24  in the closed position is relieved allowing piston  30  to rise to the open position (dotted line). In this position, fluid flows from the delivery tank  20  through tubes  22 ,  26  and  28  to the patient. The apparatus will remain in this state while the patient is inhaling. 
     When the patient stops inhaling, the spring bias force on diaphragm  52  forces it downward to block the fluid passageway between tubes  36  and  44 . In this state, fluidic pressure builds up in tube  34  to force the piston  30  to the closed position (solid line), thereby closing off the fluid flow between tubes  22  and  26  and to the patient via tube  28 . The foregoing described operation will repeat itself upon demand. In the present example, this demand results from commencement of inhalation of the patient. Note that the demand should be sufficient enough to overcome the spring bias of the diaphragm  52  in container  38 . Otherwise, no fluid will flow to the demanding entity. The fluid flow in the present example is limited by the various restrictors in the tubes. In some apparatus, the valve  24 , diaphragm container  38  and restrictors  40 ,  42  and  46  may be integrated in a common mechanical unit. 
     The foregoing described mechanical fluidic demand apparatus is adequate for controlled delivery of fluid to a demanding entity; however, it has a number of drawbacks. For example, such apparatus is comprised of many individual fluidic components which are complex and expensive to assemble. The overall manufacture of such apparatus generally involves special tooling, and set-up and quality assurance procedures. In addition, the mechanical fluidic apparatus is difficult to service in the field leading to reliability and cost issues. Generally, field service of the apparatus involves replacement of parts. Also, from a clinical perspective, the response to patient inhalation is not considered sensitive enough for triggering fluid flow, i.e. the patient has to draw harder. 
     The present invention overcomes these drawbacks of the current fluidic demand apparatus by replacing the mechanically active parts with miniature, low power electrically operative units as will become more evident from the detailed description of the invention found herein below. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a microvalve sensor for sensing fluid flow therethrough and generating an electrical signal indicative thereof comprises: a housing connectable inline with a fluid passageway; a microvalve disposed in the housing to permit fluid to flow unidirectionally through the housing, the microvalve including: a substrate; an insulating layer disposed over the substrate, the substrate and insulating layer including an orifice to accommodate fluid flow through the housing; and a diaphragm element disposed over the insulating layer, the diaphragm element including: a solid center portion having an area sufficient to cover the orifice, and an outer portion surrounding the center portion having a plurality of apertures for passing fluid from the orifice through the housing, the outer portion being affixed to the insulating layer around a periphery thereof, the diaphragm element and substrate forming opposite plates of a capacitor having a capacitance which changes with fluid flow through the housing; and a circuit coupled across the opposite plates of the capacitor and powered by an electrical source for measuring the capacitance of the capacitor and generating an electrical signal indicative thereof. 
     In accordance with another aspect of the present invention, fluidic demand apparatus for conducting fluid from a fluid source under pressure to a demanding entity comprises: an electrically operative fluidic valve connectable between the fluid source and demanding entity; and a fluid flow sensor connectable in a fluid passageway to the demanding entity, the sensor including a microvalve operative electrically to sense fluid flow demand from the demanding entity through the passageway and to generate an electrical signal to drive the fluidic valve in response thereto. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fluidic schematic diagram of exemplary fluidic demand apparatus, like an Oxygen conserver, for example. 
         FIG. 2  is a block diagram schematic of fluidic demand apparatus suitable for embodying one aspect of the present invention. 
         FIG. 3  is a break-away sketch of a microvalve fluid flow sensor suitable for embodying another aspect of the present invention. 
         FIG. 4  is a cross-sectional, cut-away sketch of an exemplary microvalve suitable for use in the fluid flow sensor embodiment of  FIG. 3 . 
         FIGS. 5A and 5B  are cross-sectional sketches of operational states of the fluid flow sensor embodiment of  FIG. 3 . 
         FIG. 6  is a block diagram schematic of an exemplary circuit embodiment suitable for use in the fluid flow sensor embodiment of  FIG. 3 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 2  is a block diagram schematic of fluidic demand apparatus suitable for embodying one aspect of the present invention. Many of the components of the embodiment of  FIG. 2  remain the same as described in connection with the embodiment of  FIG. 1  and thus, will maintain their same functions and reference numbers. In the present embodiment, an electrically operated, fluidic valve  60  has its input and output fluidic ports coupled to tubes  22  and  26 , respectively, and is driven by a voltage across electrical pins  62  to conduct fluid from tube  22  to tube  26 . The fluidic valve  60  may be of the type manufactured by The Lee Company under model no. LHLX0500300B, for example. In addition, a flow rate sensor  64  is disposed at tube  48  between tube  28  and atmosphere, and employs a MEMS microvalve in its operation which will become more evident from the more detailed description found herein below. The sensor  64  is operative to perform the functions of check valve operation, differential pressure operational valve setting adaptable to the demanding entity, and flow rate sensing. The sensor  64  is operative to produce an electrical signal over signal lines  66  which are coupled to electrical pins  62  of the fluidic valve  60 . 
     In operation, the patient or demanding entity will initially draw fluid from the atmosphere through tubes  28  and  48  and sensor  64 . Note that the unchecked flow direction of the sensor  64  is from the atmosphere to the demanding entity or patient. When the sensor  64  senses fluid flow through the MEMS microvalve therein indicative of fluid demand, it produces the electrical signal over lines  66  at a level sufficient to drive the fluidic valve  60  open to deliver fluid from the delivery tank  20  to the demanding entity or patient through tubes  22 ,  26  and  28 . In this state, the sensor  64  checks delivery of fluid to the atmosphere via lines  26  and  48 . The fluidic valve  60  may be latched in the open position until the flow demand ceases. The foregoing described operation will continue for each flow demand cycle. 
       FIG. 3  is a break-away sketch of sensor  64  shown coupled to tube  48 . Referring to  FIG. 3 , sensor  64  may include two housings or compartments  70  and  72  for containing a MEMS microvalve  74  which may be of the type marketed by iACTIV Corporation under the model no. GP-03X, for example. In the present embodiment, the microvalve  74  is fabricated using MEMS micromaching techniques in a spooked wheel design comprising a center hub  76 , a plurality of radial spokes  78  which extend from the hub  76  to an outer annular surface area  80 . The overall diameter of the microvalve may be approximately 250 micrometers (μm), for example. The center hub  76  may have a diameter of approximately 150 μm and each of the radial spokes  78  may be about 20 μm in width. The center hub  76  and spokes  78  may be approximately 5 μm thick. The spokes  78  are spaced about the periphery of the hub  76  to permit passage of fluid through the open spaces therebetween as will become more evident from the following description. 
       FIG. 4  is a cross-sectional, cut-away sketch of an exemplary microvalve  74 . Referring to  FIG. 4 , the microvalve  74  includes a disc shaped, rigid substrate  82  which may be fabricated from a silicon wafer. The thickness of the substrate  82  is commensurate with the thickness of the fabricating wafer which may vary between 50-100 μm, for example, from wafer to wafer. Disposed over the substrate  82  is an electrically insulating layer  84  which may be silicon nitride, for example, at a thickness of approximately a few μm, for example. A tapered orifice  86  may be micromachined through the substrate  82  and insulating layer  84  to permit fluid to flow therethrough. The diameter of the orifice  86  may be around 60-80 μm at its smallest opening. 
     The hub  76  and spokes  78  may be micromachined from a polysilicon layer over the insulating layer  84  with the hub  76  centered about the orifice  86  and the spokes  78  attached at one end to the hub  76  and at the other end to layer  84 . Note that only one end of each of the spokes  78  is attached to the layer  84 . The thickness of the polysilicon spokes  78  are such to provide an elastic stretching thereof to permit the hub  76  to extend above the layer  80  (as shown) so that fluid may flow through the orifice  86  and the openings between the spokes  78 . In this manner, the hub  76  and spokes  78  act as a diaphragm with openings for fluid to flow through. 
       FIGS. 5A and 5B  are cross-sectional sketches exemplifying operational states of the sensor  64 . The unchecked flow direction is shown by the arrowed line in the  FIGS. 5A and 5B . Referring to  FIGS. 5A and 5B , the housing  70  includes an opening  90  through which one section of tube  48  may be attached. Around the circumference of opening  90  in housing  70  is an annular indented area  92  on which to seat and attach the substrate  82  of microvalve  74  in a permanent position. Housing  70  further includes a cavity  94  above the microvalve  74  and large enough to permit the hub  76  to extend upwards to an open position (see  FIG. 5B ). The height of the hub  76  in the open position may be approximately 20 μm above the orifice  86 . Housing  72  also includes an opening  96  through which the other section of tube  48  may be attached. Housings  70  and  72  may be attached together and sealed around a seam  98  to encase the microvalve  74  therein. 
     In a no flow demand state, the hub  76  of microvalve  74  is seated on layer  84  over the orifice  86  as shown in  FIG. 5A . In the present embodiment, the hub  76  may be held against the layer  84  with an adjustable electrostatic bias force. Referring back to  FIG. 4 , an integrated circuit  100  may be fabricated on the surface of layer  84 , for example, and powered by an electrical source  102  which may be a miniature Lithium battery, for example. The circuit  100  may include an inverter circuit (not shown) to amplify the voltage potential of the battery source  102  to higher output voltage potentials. Output leads  104  and  106  from the inverter circuit of circuit  100  may be connected across the hub  76  and substrate  82  with opposite positive and negative polarities to impose the output voltage potential thereacross and create an attractive electrostatic force to maintain the hub against the layer  84 , thus sealing off flow through the orifice  86 . 
     In the example as shown in  FIG. 4 , lead  104  may be connected to a bonding pad  108  fabricated into the hub  76  and lead  106  may be connected to a bonding pad  110  fabricated into the substrate  82 . Accordingly, the bias force or bias differential pressure to be overcome by the demand may be set by adjusting the voltage potential applied across the hub  76  and substrate  82 . Note that fluid may not flow through the microvalve  74  in the checked direction, i.e. opposite the arrowed line, because the differential pressure in the checked direction will add to the electrostatic force to maintain the hub  76  against the layer  84  and seal off the orifice  86  as shown in  FIG. 5A . 
     A block diagram schematic of an exemplary integrated circuit  100  suitable for use in the embodiment of  FIG. 4  is shown in  FIG. 6 . Referring to  FIG. 6 , as noted above, an inverter circuit  120  may be included in integrated circuit  100  to boost the DC voltage of the source  102  which may be around 1.5 volts to a higher electrostatic DC voltage applied across the diaphragm  76 / 78  and substrate  82  via leads  104  and  106 , respectively. In the present embodiment, the electrostatic voltage may be adjustable through the inverter circuit  120  depending on the demand application and may vary from 20 volts DC to 80 volts DC, for example. 
     Once the bias electrostatic force on hub  76  is overcome by the demand, e.g. patient inhalation, the differential pressure across hub  76  will force it away from the orifice  86  as shown in the sketch of  FIG. 5B . In this state, fluid may flow in the unchecked direction through the orifice  86  and openings between the spokes  78 . Thus, in the present example, fluid may be drawn from the atmosphere through tube  48  as an indication of fluid demand or commencement of inhalation from the patient. The sensor  74  also includes circuitry to sense this flow rate and generate a voltage signal over lines  66  to drive the fluidic valve  60  (see  FIG. 2 ). 
     It is recognized that the substrate  82  and diaphragm, comprising hub  76  and spokes  78 , of the microvalve  74  form two plates of a capacitor. The distance between these two plates, i.e. substrate  82  and diaphragm  76 / 78 , is held constant by the insulating (dielectric) layer  84  when the hub  76  is maintained against the orifice  86  (see  FIG. 5A ), but changes as the hub  76  moves away from the orifice  86  during fluid flow (see  FIG. 5B ). During fluid flow, the dielectric of the capacitor also changes to include both the insulating layer  84  and the fluid itself. Thus, the capacitance formed by the substrate  82  and diaphragm  76 / 78  changes between the no flow and flow states and is commensurate with the flow rate. A measure of this capacitance will provide an indication of fluid flow through the microvalve  74  in the present embodiment. 
     Referring back to  FIG. 6 , a capacitance measuring circuit  130  may be included in the integrated circuit  100  for measuring the capacitance between the plates  82  and  76 / 78 . The circuit  130  may employ any of the well-known techniques for measuring capacitance comprising determining a resonance frequency of a tank circuit including the varying capacitance or determining time transient behavior of the varying capacitance in the time domain, for example. These techniques generally involve applying a stimulus signal over signal lines  132  across the capacitor and measuring a response signal from the capacitor over signal lines  134 . Since in the present embodiment, the capacitor plates are at a DC voltage higher than the operating voltage of the circuit  130  which may be powered by the source  102 , for example, the stimulus signal  132  may be A/C coupled to leads  104  and  106  through a coupler circuit  136  and the response signal  134  may be decoupled from the DC voltage of leads  104  and  106  by a decoupling circuit  138 . Thus, the coupling circuits  136  and  138  permit the AC stimulus and response signals to modulate the quiescent electrostatic voltage signal of the leads  104  and  106 . 
     In the present embodiment, the capacitance measuring circuit  130  determines the capacitance from the response signal  134  and produces therefrom a signal over line  140  indicative of the flow rate through the sensor  74 . The flow rate signal  140  may be applied to one input of an amplifier circuit  142  to be compared with a set point signal that may be applied to another input of amplifier  142 . The set point signal is adjustable according to the demand application to be commensurate with the minimum fluid flow rate through the sensor  74  for commencement of demand. Accordingly, when the signal  140  exceeds the set point signal, amplifier  142  generates a signal over lines  66  sufficient to drive the latching valve  60  to the open state whereupon fluid is delivered to the demanding entity (e.g. patient) via lines  22 ,  26  and  28  (see  FIG. 2 ). Note that the microvalve  74  in sensor  64  will check fluid flow from dumping to the atmosphere through tube  48 . When the demand is reduced below the minimum flow to keep valve  60  latched, it will close and cease delivery of fluid. The foregoing described cycle will be repeated for each new demand. 
     While the present invention has been described herein above in connection with one or more embodiments, it is understood that such embodiments were presented by way of example and not intended to limit the invention in any way. Accordingly, the present invention should not be limited to any specific embodiment, but rather construed in breadth and broad scope in accordance with the recitation of the claims appended hereto.