Abstract:
A piezoelectric microvalve employs a valve element formed of hermetically sealed and opposed plates flexed together by a cross axis piezoelectric element. Large flow modulation with small piezoelectric actuator displacement is obtained by perimeter augmentation of the valve seat which dramatically increases the change in valve flow area for small deflections.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     BACKGROUND OF THE INVENTION 
       [0001]    Compact, electrically actuated valves may be used in a variety of applications for example ranging from drug delivery to metering cryogenic gases in cooling systems for future space missions. Such valves may need to be highly reliable, resistant to extremes in temperatures, resistant to contamination from the environment, and energy efficient. 
         [0002]    One promising approach for the production of such valves constructs the valve body from micro-machined silicon using integrated circuit techniques. These micro-machined valve elements may be actuated by a piezoelectric actuator having very low power consumption and yet able to apply a very high force to the valve elements, necessary for high-pressure control in some applications. 
         [0003]    One challenge to the use of piezoelectric actuators is the relatively small displacement that they produce resulting in comparably low flow modulation in the valve, flow modulation being the difference in flow between when the valve is opened and closed. This problem of small displacement provided by piezoelectric actuators can be aggravated when valves are used at cryogenic temperatures which reduce the displacement produced by the piezoelectric element. 
       BRIEF SUMMARY OF THE INVENTION 
       [0004]    The present invention provides an electrically actuated microvalve having a cross-plane piezoelectric actuator working to compress a pair of planar, micro-machined valve elements. The small displacement of the piezoelectric actuator is offset by perimeter augmentation of a valve seat between the valve elements. The result is a valve that can accommodate high actuation pressures and provide large flow modulation. A protective housing may be used to support the piezoelectric actuator and may be matched to the actuator with respect to thermal expansion to preserve actuator operating range over a wide range of temperatures. One movable valve element may be a monolithic silicon wafer providing a continuous membrane preventing contamination between the environment and the fluid controlled by the valve, and allowing the incorporation of electrical sensor elements as integrated circuit components directly in the valve element. 
         [0005]    Specifically, the present invention provides a high flow range microvalve having an opposed first and second plate spaced to provide therebetween a flow channel between an inlet and outlet. A piezoelectric actuator is positioned to press on the first plate to flex the first plate toward the second plate to constrict the flow channel over an actuation area and an augmented length valve seat is positioned between the first and second plate in the actuation area separating the inlet and outlet when the flow channel is constricted. The augmented length valve seat may have a contiguous length greater than four to ten times a square root of the actuation area when the actuation area is an area bounded by contact through the valve seat ridge between the first and second plates. Alternatively the augmented length valve seat may have a contiguous length greater than eight to fifty times a square root of the minimum cross-sectional area of the inlet or outlet. 
         [0006]    Thus it is one feature of at least one embodiment of the invention to provide significantly increased flow modulation range for a micro-machined valve using a piezoelectric actuator. 
         [0007]    It is another feature of at least one embodiment of the invention to provide for greater flow modulation in a valve that may be hermetically sealed by the bonding of two wafers and actuated by a low displacement flexing of one wafer. 
         [0008]    The first plate may be a silicon wafer and the second plate may be glass. 
         [0009]    It is another feature of at least one embodiment of the invention to provide a low cost valve incorporating a glass substrate that may match the coefficient of thermal expansion of a silicon wafer, the latter better suited for micromachining, so that the plates may remain bonded without undue stress over a wide range of temperatures. 
         [0010]    The augmented length valve seat may be a serpentine wall separating the inlet and outlet. 
         [0011]    Thus it is a feature of at least one embodiment of the invention to provide a valve seat topology that may be flexibly tailored to a particular valve configuration and requirement. 
         [0012]    The serpentine walls may be comprised of parallel interconnected line segments or alternatively the serpentine walls may be comprised of concentric interconnected arc segments. 
         [0013]    It is thus a feature of at least one embodiment of the invention to provide for patterns of generating augmented valve seat perimeters that provide for high density and simple construction. 
         [0014]    The valve may include a housing supporting at least one of the first and second plates and containing the piezoelectric actuator to position a first end of the piezoelectric actuator to press on the first plate and restrain a second end of the piezoelectric actuator with respect to the first plate so that dimensional changes of the piezoelectric actuator flexes the first plate. 
         [0015]    It is thus a feature of at least one embodiment of the invention to provide for a sealed microvalve where the housing may also support the actuator. 
         [0016]    The housing may have a coefficient of thermal expansion matched to the piezoelectric actuator, for example, by constructing the housing from a ceramic material. 
         [0017]    It is thus another feature of at least one embodiment of the invention to provide housing material that offsets dimensional changes in the actuator with temperature to preserve the small operating range of the actuator. 
         [0018]    The opposed first and second plate may provide between them a sealed flow channel between the inlet and outlet and the first plate may be a monolithic silicon substrate. 
         [0019]    Thus it is a feature of at least one embodiment of the invention to provide an extremely simple fabrication technique employing as few as two wafers that shield the fluid stream controlled by the valve from contamination and that reduces dead volumes in the valve. 
         [0020]    The silicon substrate may support one or more electronic devices fabricated on the silicon and selected from the group consisting of a temperature sensing element and a strain sensing element. 
         [0021]    It is thus a feature of at least one embodiment of the invention to allow electronic sensing elements to be incorporated directly in one of the valve plates. 
         [0022]    These particular features and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a perspective view of the microvalve of the present invention mounted on a header for attachment to standard tubing; 
           [0024]      FIG. 2  is a cross-sectional view along line  2 - 2  of  FIG. 1  showing a piezoelectric stack held by the housing of the microvalve against a first plate which may flex against a second plate to control fluid flow between an inlet and outlet valve communicating with the header; 
           [0025]      FIG. 3  is an exploded perspective view of the first and second plate of  FIG. 1  showing a serpentine valve seat and a pressure sensor chamber formed in the underside of the first plate; 
           [0026]      FIG. 4  is a bottom plan view of the first plate showing an arcuate serpentine valve seat; 
           [0027]      FIG. 5  is a figure similar to that of  FIG. 4  showing a prior art valve seat and comparing the valve seat perimeter to actuation and inlet and outlet areas; 
           [0028]      FIG. 6  is a figure similar to that of  FIGS. 4 and 5  showing a rectilinear serpentine valve seat and comparing the valve seat perimeter to actuation and orifice areas; and 
           [0029]      FIG. 7  is a fragmentary cross-section similar to that of  FIG. 2  showing detailed construction of the first and second plates. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    Referring now to  FIG. 1 , a microvalve  10  of the present invention may provide for a block-shaped housing  12 , for example, defining a 1 cm cube. The microvalve  10  may fit against the upper surface of adapter plate  14 , the latter providing connection points  20  to a standard-sized inlet tube  21  and outlet tube  21 ′ through which a fluid controlled by the valve can pass. 
         [0031]    Referring also to  FIG. 2 , a lower, open face of the housing  12  may be bonded to the periphery of a continuous upper valve plate  22  thereby hermetically enclosing an actuator volume  24  within the housing  12 . The housing  12  may be bonded to the upper valve plate  22  using a high temperature epoxy such as Stycast 2850FT epoxy. 
         [0032]    Positioned within the actuator volume  24  is a piezoelectric stack  26  comprised of a set of piezoelectric elements  28  assembled together along a vertical axis  29  generally perpendicular to the upper surface of the upper valve plate  22 . The piezoelectric stack  26  stretches from the upper surface of the upper valve plate  22  to a lower inner surface of the upper face of the housing  12 . A set of electrical power leads  16  may pass through an upper face of the housing  12  to connect to electrodes sandwiching each piezoelectric element  28  to cause the expansion of the stack  26  along a vertical axis  29 . 
         [0033]    As positioned, the piezoelectric stack  26  will increase in height along axis  29  under the application of electrical power through leads  16 , pushing down on the upper valve plate  22  to compress and thus deform an actuation area of the upper valve plate  22 , to in turn press against an upper surface of a lower valve plate  32  parallel to and bonded to the underside of the upper valve plate  22 . 
         [0034]    The housing  12  may be constructed of a ceramic material, for example, Macor machinable ceramic commercially available from The Morgan Crucible Company plc of Berkshire, United Kingdom. The coefficient of expansion of the material of the housing  12  is selected to have a coefficient of thermal expansion approximately equal to that of the piezoelectric stack  26  so that thermal expansion or contraction of the height of the stack  26  along axis  29  is offset by a corresponding expansion or contraction in the height  30  of the side walls of the housing  12 . In this way, temperature extremes do not adversely affect the operating range of the piezoelectric stack  26  or actuate or deactuate the microvalve  10 . 
         [0035]    Referring now also to  FIGS. 2 and 3 , the lower valve plate  32  provides for an inlet  34  and an outlet  36  passing vertically through the lower valve plate  32  and into the adapter plate  14 . Within the adapter plate  14 , the inlet  34  and outlet  36  are received by adapter channels  38  that are sized to accept inlet tube  21  and outlet tube  21 ′. 
         [0036]    Referring now to  FIGS. 2 ,  3 , and  4 , the lower surface of the upper valve plate  22  includes a serpentine valve seat  40  providing a set of ridges extending toward the upper surface of the lower valve plate  32  and aligned with inlet  34  and outlet  36  (shown superimposed on the view of the upper valve plate  22  in  FIG. 4 ). The serpentine valve seat  40  fully surrounds the inlet  34  to separate the inlet  34  from the outlet  36  when the upper valve plate  22  is compressed against the lower valve plate  32  so that the serpentine valve seat  40  seals against the lower valve plate  32 . 
         [0037]    The serpentine valve seat  40 , in a first embodiment, consists of a set of concentric arcuate elements  44  joined by short radial elements  46  to provide a contiguous path around outlet  36  having an extended length far in excess of that needed to enclose the outlet  36 . This extended perimeter provided by the serpentine valve seat  40  greatly increases the flow area of the microvalve  10  when the microvalve  10  is open, for small displacements of the upper valve plate  22 . 
         [0038]    Referring now to  FIG. 5 , in a conventional microvalve  10 , inlet  34  may be separated from an outlet  36  by one or more discontinuous valve seat ridges  54  that provide multiple barriers between the inlet  34  and outlet  36  to reduce leakage. An activation area  56  may be defined as an operably movable portion of the upper valve plate  22  supporting the valve seat ridges  54 , or preferably the smallest continuous area bounded by contact between the first and second plates through the valve seat ridges  54 . The perimeter length of the valve seat ridge  54  corresponding to this activation area will generally be less than or equal to four times the square root of the activation area  56  corresponding to a circular or square valve seat ridge. 
         [0039]    In contrast, as seen in  FIG. 6 , a serpentine valve seat  40  may bound a relatively smaller activation area  56  when compared to its perimeter length as a result of its convoluted route. In this case the serpentine valve seat  40  is composed of a set of parallel rectilinear elements  58  joined by short perpendicular segments  60 . In the present invention the length of the serpentine valve seat  40  will be greater than four times a square root of the activation area  56  and typically more than ten times the square root of this activation area. The depictions of the serpentine valve seat  40  in  FIGS. 4 and 6  are simplified and will typically provide more than eighty rectilinear segments  58  or arcuate elements  44 . 
         [0040]    Referring to  FIG. 5 , this amount of this perimeter augmentation may also be defined with respect to effective area of the outlet  36  and comparing the perimeter length to this effective area. As shown in  FIG. 5 , the cross-sectional area of outlet  36  may be approximately diameter  61  squared. In the prior art, the closest valve seat ridge  54  will have a perimeter length of approximately four times side dimension  70  which may be placed closely around outlet  36 , for example, separated by no more than half the diameter  61  on all sides. In this case, the perimeter length of the closest valve seat ridge  54  will be about eight times the diameter  61  or less than eight times the square root of the area of the outlet  36 . 
         [0041]    Referring to  FIG. 6 , the serpentine valve seat  40  will have a perimeter length that is more then ten to fifty times the square root of the area of outlet  36 . 
         [0042]    This augmented perimeter length provides a large area through which fluid  72  may flow between inlet  34  and outlet  36  so that minor amounts of displacement between the upper valve plate  22  and lower valve plate  32  provide a multiplicatively greater flow modulation. 
         [0043]    Referring now to  FIGS. 2 ,  4 , and  7 , the upper valve plate  22  may be a conventional SOI silicon wafer having an integrated circuit quality top silicon layer  74  on top of an oxide layer  76 . The oxide layer  76  may in turn be on top of a bottom substrate layer  78  (typically also silicon). The top silicon layer  74  may be etched or ground to an arbitrary thickness suitable to provide the desired strength and flexibility for flexing. The top silicon layer  74  may then treated using conventional integrated circuit techniques to add a temperature sensor  80 , for example a patterned platinum metallization providing a resistive temperature detector (RTD) measuring the temperature of the upper valve plate  22  (and hence the temperature of the fluid controlled by the microvalve  10 , for example). The top silicon layer  74  may also be treated to create a piezoelectric resistor based pressure sensor  82  formed to sense strain and positioned over a pressure sensor chamber  48  as will be described below, sensing pressure of the fluid controlled by the microvalve  10 . 
         [0044]    After the formation of the temperature sensor  80  and pressure sensor  82 , the bottom substrate layer  78  which will provide the lower surface of the upper valve plate  22 , may be deep-etched to define a valve boss  84  extending downward toward the lower valve plate  32 . The valve boss so etched, is joined to the remaining portion of the first valve plate  22  only by the oxide layer  76  and the top silicon layer  74 . The oxide layer  76  may form a stop for the deep etching of the bottom substrate layer  78  avoiding risk of etching of the top silicon layer  74  or the need for precise process control. 
         [0045]    The lower surface of the valve boss  84  may provide the etched pattern of the serpentine valve seat  40  as a set of downwardly extending ridges. For example, each ridge may be approximately 50 μm wide and 120 μm deep. In turn, the upper surface of the lower valve plate  32  beneath the valve boss  84  may have an etched recess  86  holding the inlet  34  and outlet  36  beneath the valve boss  84  so the downward flexure of the upper valve plate  22  by the piezoelectric stack  26  causes the serpentine valve seat  40  to contact the upper surface of the etched recess  86  and block passage of fluid between inlet  34  and outlet  36 . The range of travel of the valve boss  84 , for example, may be on the order of 2 μm. 
         [0046]    The upper surface of the lower valve plate  32  is bonded at its periphery to the periphery of the lower surface of upper valve plate  22 . Similar thermal expansion characteristics of silicon and glass materials of the upper valve plate  22  and lower valve plate  32 , respectively, prevent delamination or undue stress over a wide range of temperatures. 
         [0047]    Referring to  FIGS. 2 and 4 , the inlet  34  may communicate through a short path to the pressure sensor chamber  48  also formed by deep etching or similar techniques and providing a thin upper membrane formed of the oxide layer  76  and thin top silicon layer  74  flexing under pressure as sensed by the piezoresistors or conventional resistive devices for example formed as strain gauges in a Wheatstone bridge or the like. As shown in  FIG. 1 , signal leads  18  may pass through the upper face of the housing  12  to communicate with temperature sensor  80  and pressure sensor  82 . 
         [0048]    During the bonding of the upper valve plate  22  and lower valve plate  32 , the upper face of the etched recess  86  facing the boss  84  may be coated with aluminum to prevent unintentional bonding of the boss  84  to the lower valve plate  32 . This aluminum may then be etched or dissolve away. 
         [0049]    Referring to  FIG. 2 , the piezoelectric stack  26  may be attached to the lower face of the upper face of the housing  12  by epoxy placed to bridge a narrow gap  90  between the upper face of the housing  12  and the upper surface of the piezoelectric stack  26 . During a curing of the epoxy, the piezoelectric stack  26  is energized at a maximum voltage to expand the piezoelectric stack  26  along axis  29  thereby closing the microvalve  10 . This activation of the stack  26  removes the effects of tolerance differences in dimensions between the piezoelectric stack  26  and the housing  12  allowing the assembly of these different elements while preserving precise positioning of the piezoelectric stack  26  necessary to ensure closure of the microvalve  10 . 
         [0050]    The piezoelectric stack  26  does not need to be affixed to the upper surface of upper valve plate  22  and thus there is no danger of stresses being generated between these elements caused by differences in thermal expansion rates. 
         [0051]    While the above description has been with respect to a normally-open valve, it will be understood that normally-closed valves or valves that are partially open (exploiting both positive and negative piezoelectric actuation voltages) may be created by simple adjustments in the geometry and/or actuation voltages. The perimeter augmentation of the present invention is applicable to these embodiments as well. Further, while a continuous upper valve plate  22  has been described which substantially reduces valve “dead-volume”, in an alternative embodiment, the upper valve plate  22  can be fabricated to be separate from the remainder of the silicon wafer suspended by means of cantilevered arms etched free from the wafer to reduce the necessary actuation pressure and/or distortion of the valve seat. 
         [0052]    The present invention has been described in terms of the preferred embodiment, and it is recognized that equivalents, alternatives, and modifications, aside from those expressly stated, are possible and within the scope of the appending claims.