The present invention relates generally to microfabricated electro-pneumatic valves and Microsystems, and more particularly, to an improved microvalve design using footprint efficient layouts that are suitable for bulk microfabrication and for lower cost production.
Interest in Microsystems has increased in recent years because of their potential to reduce system fabrication cost through batch processing, physical size reduction, improved end-product quality, and enhanced performance, for example. Silicon based Microsystems allow mass replication of systems and manufacturing into tiny packages at relatively low costs using conventional IC fabrication techniques. These microfabrication techniques enable a large number of devices to be made on a single silicon wafer thereby significantly driving down production costs when compared to techniques used in the past. Furthermore, advances in plastic microreplication techniques have enabled further cost reductions to be realized in polymer microsystems.
Microsystems comprise microfluidic devices such as microfabricated microvalves for fluid control, which are used in a wide variety of applications. Microactuators, such as microvalves, micropumps, and microsensors, utilizing e.g. mechanical and optical sensing principles, can be used for industrial applications as well as medical applications. Active microvalve devices are devices that typically include flow ducts between a fluidic inlet and a fluidic outlet such that fluid flow is controlled from inlet to outlet by way of transducing a control signal into a change in the pressure-flow characteristics of the flow duct.
One area of industry that holds potential for the introduction of Microsystems is S that of pressure regulation and control. Pressure controllers (also called E/P-converters or I/P-converters, where E stands for electrical, I for current and P for pressure), are basic elements in a vast number of industrial applications. Their basic function is to convert an electrical control signal into a work pressure Pwork. As such, they form the interface between electronic control signals and pneumatic control elements in larger industrial systems.
Current conventional standard pneumatic components are relatively bulky and too expensive for many applications. Microsystem pressure controller devices could benefit from the cost advantages of microfabrication if operating performance can be maintained in terms of pressure and flow characteristics. Efforts have been made in the past to develop microvalves for pneumatic systems, however, most do not meet the demands of industrial use, both technically and economically. To enable the integration in sub-systems like pneumatic cylinders, the outer dimensions of the pneumatic components have to be as small as possible. Ideally, the form factor should be compatible with standard pneumatics, where the width of the valves is standardized and 10 mm is the smallest standard at present. General requirements include a high air flow, a low leak rate, a short response time, a wide temperature range, the ability to adapt unclean environments, and having to operate in standard pressure ranges of e.g. 0 to 8 Bar and requiring as low as possible power consumption. A reason for the low flow/pressure performance-per-cost of most current miniaturisation trials has been a primarily technical issue. By way of example, a popular microvalve type is the so-called seat valve.
Micromachined actuators have been included in many microsystem designs, including microvalves. However, in the past either the actuator's stroke length or the force delivered by the actuator is typically limited. These effects place a limitation on the performance of the majority of microvalve designs i.e. where the actuator directly controls the movement of a boss. A small stroke length constitutes a high flow restriction between the boss and the valve seat, limiting the flow the valve can control. A large stroke length, on the other hand, limits the actuation force, and thus the pneumatic pressure that the valve can control. Furthermore, an increase in the actuator size to improve performance is space consuming and results in higher manufacturing costs, which is undesirable.
A problem that conventional seat type valves must inherently contend with is flow resistance. Flow resistance can be seen as an obstruction in a flow channel or at a flow nozzle. Thus, one of the main problems in microvalve design is to provide a flow obstruction that can sufficiently counteract the pneumatic forces of the flow it controls. Hence, conventional seat type valves require relatively high actuation forces to operate.
Another issue that arises with operating at the micro level scale is that miniaturization of components has specific consequences. The scaling down with a factor N, results in a downscaling of masses and volumes with N3 and of areas with N2. This means that surface tension effects and tribological effects dominate in Microsystems. For this reason it is virtually impossible to use sliding contacting structures at the micro level scale. Therefore, moving structures need to be “free-hanging” to avoid any type of friction.
U.S. Pat. No. 6,592,098 describes a microvalve using a valve seat and diaphragm that is actuated to turn on and “pinch” off the flow. To suitably operate the valve, the diaphragm requires biasing in order to maintain sufficient pressure to operate the valve. Moreover, the diaphragm area lies in the plane of the substrate thus imposing an inherent limit on how much the footprint area of the device can be reduced, thereby preventing significant increases in the number of devices that can be microfabricated on a silicon wafer that would reduce costs.
In view of the foregoing, it is desirable to provide a microvalve design for use in microsystems that mitigates the aforementioned disadvantages. The design of which can provide high operating efficiency by using low energy actuation that is cost effective by using space efficient layouts that are especially conducive to high volume microfabrication.
Briefly described and in accordance with embodiments and related features of the invention, there is provided a microvalve for providing flow regulation within a microsystem application that uses highly efficient actuation while providing a space efficient layout in a manner that is suitable for cost effective bulk microfabrication In an embodiment of the invention, the microvalve comprises a first substrate layer, a second layer disposed over the first substrate layer cooperating with the first substrate layer to form a flow duct through which the flow traverses and defines a direction of the flow. An obstruction element or knife gate is micromachined into the second layer such that it is attached to the second layer and actuated by a bimorph actuator to displace the gate along a plane that is substantially perpendicular to the direction of the flow and out of plane with respect to the first substrate layer in order to regulate the flow. Moreover, the microvalve of the invention can be actuated by means that include thermal, pneumatic, piezoelectric, electrostatic, and magnetic means. The cross-sectional area of the flow duct is perpendicular to the plane of the substrate that allows the footprint area (FPA) of the device to be reduced substantially since it is independent from the cross-sectional area of the flow duct.
In another embodiment of the invention, a microsystem comprising the microvalve concept of the invention is microfabricated into an IP-converter that can be used in pneumatic high flow/pressure control applications. The microsystem comprises at least three pneumatic ports that includes a supply port, a work port and a vent port whereby the three ports are coupled respectively to a supply pressure (Psupply), a work pressure (Pwork), and a vent pressure (Pvent). The microsystem comprises first knife gate microvalve, which is pneumatically coupled to the supply port and the work port for regulating the flow between supply pressure (Psupply) and the work pressure (Pwork). Moreover, a second knife gate microvalve is pneumatically coupled to the work port and the vent port for regulating the flow between the work pressure (Pwork) and the vent pressure (Pvent). The pneumatic flow within the microsystem is regulated using control signal means that are electrically coupled to the microvalves that selectively actuate the microvalves.
In a method aspect of the invention, a method of operating a microvalve to provide flow regulation of a fluid is described. The microvalve comprises a first substrate layer, a second layer disposed over the first substrate layer and cooperating with the first substrate layer to form a channel through which a main flow traverses and defining a direction of flow. An obstruction element or gate formed from the second layer is connected to a member that is attached to the second layer. An actuator is operative on the obstruction element for displacing the obstruction element along a plane that is substantially perpendicular to the direction of the main flow and out of plane with respect to the first substrate layer.
The invention, together with further objectives and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which:
a illustrates a design depicting a flow direction that is out-of-plane with respect to the substrate and an obstruction element moving in an in-plane direction;
b illustrates a design with an in-plane direction for the flow direction and the obstruction element movement;
c illustrates a design with an in-plane flow direction and out-of-plane obstruction element movement in accordance with the invention;
a illustrates a microvalve design showing a pressure recovery area that reduces generated forces that counteract the operation of the gate assembly;
b shows a top view of an exemplary side gate microvalve having a reduced footprint area in accordance with an embodiment of the invention;
c-6e show side, top, and end view illustrations of microvalves operating in accordance with the invention;
a-9b are diagrammatic illustrations of the microvalves used in the microsystem and the associated packaging in accordance with an embodiment of the invention.
a illustrates a first design that has been used to show a cross-sectional view of a microvalve assembly where the gas flow Qz 450 is out-of-plane with respect to the wafer and the obstruction movement Dx in-plane with respect to the wafer surface. In this design, one or more orifices 400 can be closed with a sliding obstruction plate 420. Here several nozzles are used in parallel in order to reduce the obstruction stroke length Dx. Preferably, the actuation means comprise thermal actuation and electrostatic comb drive means. An exemplary fabrication technique that can be used with this design is Deep Reactive Ion Etching (DRIE) and wafer-through inlet etching using a 2-wafer stack.
b illustrates top view of a second design by which the gas flow Qx 450 is in-plane with respect to the wafer and the obstruction 420 movement Dy is also in-plane with the wafer but is also perpendicular to the gas flow 450. The actuation means preferably can use thermal actuation and electrostatic comb drive actuation. A technique that works well for fabricating this design is DRIE using a 2-wafer stack.
c illustrates a perspective view of a third design where the gas flow Qy is in-plane with the wafer and the obstruction 200 movement Dz out-of-plane. Here, the actuation methods that work well include thermal, magnetic, electrostatic, pneumatic or piezoelectric actuation, where the fabrication can be performed with the DRIE method using a 2-wafer stack. This configuration is often referred to as a knife gate microvalve, which exhibit the principles outlined in the present invention.
In accordance with an embodiment of the invention, there is provided a flow control knife gate microvalve suitable for replacing large-scale valves. The device of the present invention, also referred to as a knife gate microvalve, features an increased flow-pressure performance per device footprint area and overcomes the drawbacks of the microvalves described in the prior art.
The gate element 500 is pivotally attached to the second layer of silicon 550 via a piezo bimorph actuator arm 540 with glue at points 560. The movement from the pivot enables sufficient vertical displacement h to be achieved in order to block the flow, or allow it to pass unobstructed. It should be noted that other methods than glue to attach the gate can be used such as soldering, for example. The operation of the knife gate microvalve requires that some spacing be left between the gate 500 and the orifice of the flow duct 535 to avoid friction, which means that a small leakage flow Qleak will exist in the closed state. Fortunately, however, small leakage flows Qleak can be tolerated in many flow and pressure controller applications.
The actuator means in the embodiment preferably uses a piezoelectric bimorph actuator 540 for displacing gate 500. A flow duct extension indicated within dashed line 570 extends the flow duct length 535 and is pneumatically coupled to the device package via an opening 580 that lies in the plane of the second layer. It should be noted that various thermal actuation means such as bimorph actuation, shape memory alloy, or thermopneumatic means could be used to actuate the gates.
The knife gate microvalve microstructure of the embodiment is fabricated in silicon and etched using, for example, Deep Reactive Ion Etching (DRIE). The microfabrication process involves silicon fusion bonding and bulk micromachining, which is a subtractive fabrication procedure where the substrate is used to produce the primary mechanical structures. It should be noted that other techniques can be used such as surface micromachining where thin layers of film are deposited on the surface of the substrate such that the layers are then used as mechanical structures. However, the DRIE etching technique performs particularly well for etching of high aspect ratio features such as narrow and deep grooves, for example.
There are a number of specific challenges to consider in order to optimize batch fabrication of the microvalves. First, the design must be footprint-efficient, as footprint area is one of the primary cost driving factors for the device. The second involves providing reliable and reproducible fabrication of the high-aspect ratio spacing gap between the valve gates and their respective orifices. This gap determines the closed-state leakage flow rate of the microvalve. The microfabrication processes, especially when using DRIE, can be tuned for this feature, In one embodiment of the invention, the closed-state leakage flow can be substantially diminished or even eliminated using an appropriate valve design. By way of example, once the obstruction element is in the closed position it can then be moved laterally against the main flow direction, thereby reducing the gap. By way of example, the “free-hanging” gate or obstruction element, when in a closed position, can be moved laterally a small distance, in a direction substantially parallel to the direction of the direction of the flow, against a jam formed from the second layer. This acts to reduce or block off any leakage flow that would previously escape between the gate and the jam. By way of example, the additional lateral movement of the obstruction element could be effected using, for example, cooperative electrostatic actuation means arranged to induce movement of the obstruction element suitable to block the leakage. Thirdly, the actuator needs to be optimized in terms of power dissipation versus actuator stroke length. Preferably, the system can be actuated with the electrical power delivered via a standard electrical communication bus.
To maintain system controllability may require further design optimization, for example, the existence of hysteresis in the signal-gate stroke relation can be expected. This results from the Bernoulli suction and pressure recovery that occurs in the compressible flow in the structure causing a risk of undesired pneumatic forces on the bimorph actuator or even mechanical instability. Also, thermal cooling of the bimorph actuator due to the gas flow might influence the system controllability by preventing the gate from opening and closing properly. Such effects can be successfully addressed by those skilled in the art using appropriate well-known system design techniques.
a shows a design technique used in some embodiments that show the main flow 615 traversing the flow duct 620 in the plane of the substrate 650 such that the main flow flows into a pressure recovery area 640 that is located far enough from the gate apparatus so as not to affect its operation. This means that the flow is redirected by a barrier 660 at a location sufficiently distant from the gate assembly, i.e. the obstruction 600 and actuating member 610, such that the static pressure build-up resulting from the pressure recovery will not be near enough to counteract displacement of the gate 600, which moves substantially perpendicular to the main flow 615 and out-of-plane with respect to substrate 650.
With regard to the embodiment, thermal actuation can be used. In-plane thermal actuation exploits the fact that a material expands when heated, as described earlier. In general, thermal actuators tend to exhibit the disadvantage of being relatively slow and slightly more energy consuming than some of the other methods of actuation. Other actuation principles that can possibly used for in-plane fabrication are piezoelectric and magnetic actuation, for example.
The microvalve structures contemplated in the present invention are suitable for use in, among other things, pressure control applications. The design is a key element in a truly miniaturized micro-machined high-performance pneumatic control device. The structure is enhanced with bulk microfabrication using DRIE and silicon fusion bonding. In a further embodiment, the structure is actuated with a glued piezoelectric bimorph gate (500, 540, 560). Flow-pressure tests and flow-gate opening performance measurements were conducted that show very good operating performance with this arrangement. Moreover, it has been shown that the valve flow can be controlled gradually through the gate position with relatively good precision. The fabrication of bulk micromachined pressure controllers with integrated thermoelectric bimorph actuators on silicon wafers allow for a significant improvement in space-efficiency and thereby overall cost.
c-6e show side, top, and end view illustrations of several possibilities in the relative positioning of the obstruction 600 and the member/actuator 610 that connect the obstruction 600 with the second layer. In the figures, 601 indicates a direction perpendicular to the plane of the substrate, and 602 and 603 indicate perpendicular in-plane directions. Direction 602 is the direction of the main flow 615. The relative positioning of the obstruction 600 and the member/actuator 610 may be of importance when considering the required mechanical strength of the member 610.
In the preferred embodiments, member/actuator 610 is designed to be flexible in direction 601 in order to diminish the required actuation force. This flexibility can be accomplished by having the member/actuator 610 relatively thin in direction 601. At the same time, member/actuator 610 is preferably designed to be relatively stiff in the directions 602 and 603 in order to prevent the movement of the obstruction 600 in those directions. This is normally accomplished by designing the member/actuator to be relatively wide in directions 602 and 603. However, in order to reduce the footprint area required by the member, it is preferable to limit the width of the member/actuator 610 in either one of directions 602 or 603. In the preferred embodiment, there must exist a good compromise between the mechanical strength of the member/actuator 610 and the footprint area consumed by the member/actuator 610.
c illustrates a preferred embodiment in which member 610 lays substantially parallel to the direction 602 of the main flow 615 and moves substantially in a plane defined by the directions 601 and 602. The member-gate attachment point 670 lays up-stream from the member fixture point 680 such that a pivotal movement of the member around fixture point 680 will move the obstruction 600 upwards (direction 601) and slightly in the direction 602 of the main flow 615. This design is mechanically robust because the member 610 can be relatively thin in the direction 603 since there are no substantial pneumatic forces acting on the member in this direction.
d illustrates a preferred embodiment in which two members 610 lay substantially parallel to the direction 602 of the main flow 615 and move substantially in a plane defined by the directions 601 and 602. The member-gate attachment point 670 lays down-stream from the member fixture points 680 such that a pivotal movement of the members around fixture points 680 will move the obstruction 600 upwards (direction 601) and slightly in the opposite direction of the main flow 615. This design is mechanically robust because the members 610 can be relatively thin in the direction 603 since there are no substantial pneumatic forces acting on the members in this direction.
e illustrates a preferred embodiment in which the members 610 lie substantially in the direction 603, perpendicular to the direction 602 of the main flow 615 and moves substantially in a plane defined by the directions 601 and 603. As mentioned earlier, footprint area efficiency is of key interest. An example of a very footprint area efficient microvalve is a so-called side-gate knife gate microvalve.
b and 6e show views that illustrate the footprint area of an exemplary side gate microvalve in accordance with a further embodiment of the invention. In this configuration, the microvalve comprises an obstruction element or gate 600 that is displaced out of plane with respect to the substrate by actuator means 610. The minimal flow-path cross-sectional area is formed by a flow duct 620, which is the area that determines the amount of flow that can flow through the valve. The flow duct does not put a limit to the footprint miniaturization, since the valves width W can be reduced without limiting the flow duct cross-sectional area. This is because the width W of the structure is perpendicular to the flow duct cross-sectional area. With the reduction in the width W, it can be seen that the footprint area (AFP) is correspondingly reduced, since AFP=L×W.
The knife gate microvalves of the present invention can be used in various microsystem applications while retaining the benefits described herein. An application where using the microvalves of the invention is particularly advantageous is that of an IP-converter.
Referring now to
a shows the actuation of the microvalves is provided by cantilevered thermal bimorphs 910, where one X-Valve provides flow regulation at the supply fluid duct 990 and a second X-Valve provides flow regulation at the vent fluid duct 980. The work area 985 guides the work flow. When the control signal closes the supply port and opens the vent port, the work area 985 is evacuated. When the control signal opens the supply port fully open and closes the vent port the maximum work pressure is generated in the work area 985 and at the work port 940. The two X-Valves can be actuated either together or independently to achieve the work flow required. Electrical connections are included to provide the control signal to the actuators 910 through contacts 950. In the embodiment shown in
b illustrates a package 960 around the micromachined chip containing the microvalves. The supply port 920, vent port 930 and work port 940 are integrated within the package 960 whereby coupling means 970 are formed as part of the package for connecting to external fluid ducts. The supply port 920 is connected with the supply fluid duct 990 in a pneumatically sealed fashion. The vent port 930 is connected with the vent fluid duct 980 in a pneumatically sealed fashion. Furthermore, the work area 985 is connected with the vent port 940 in a pneumatically sealed fashion.
In IP converters, the spacing gap between the valve gates and their respective orifices determines the pressure range that can be controlled as well as the contribution to the total pneumatic energy losses of the system. Theoretical studies have shown that even a relatively large leak flow does not significantly hinder a large work pressure range. However, to avoid overall pneumatic energy loss, leakage should be minimized and effectively controlled to the greatest extent possible.
When using knife gate microvalves of the type described, the flow leakage of the valves influences the IP-controller's static pneumatic energy loss and reduces the dynamic pressure range of the device, which can be seen in the following equation:
ΔPdyn=Pmax−Pmin<Psupply=Psupply−Patm
and,
Psupply>Pmax>Pwork>Pmin>Pvent
For flow rates and for device dimensions of interest, frictional losses in the leakage gap are negligible. This is because the low ratio of the gate-orifice spacing (g) over leakage path length and the smoothness of the micromachined surface of the leakage path. Although at high Mach numbers frictional losses do still occur, however, this causes a decrease in the leakage rate, which is beneficial.
Both the main flow and leakage flow can be modeled as isentropic compressible flow in a sudden expansion, in which the mass-flow is described by:
with Acs being the minimal cross-sectional area of the flow path and γ the gas specific heat ratio [8,9]. The leak rate can then be quantified as:
for w>>hmax, with hmax the maximum gate opening, w the nozzle width, and the indices leak and max referring to the conditions and dimensions at the gate-nozzle spacing and the maximum nozzle opening, respectively.
For a pressure controller comprising two identical control valves with leak rate η, Pmax and Pmax can be calculated using the mass flow continuity equation:
η{dot over (Y)}supply=η{dot over (Y)}work+η{dot over (Y)}leak=η{dot over (Y)}atm (3)
at zero work flow. Pwork=Pmin if the vent port is open and the supply port is closed, in which case Acs
Graphically solving these equations for 1 bar (relative) supply pressure shows that for a leak rate η=20%, Pmax=0.9815 bar and Pmin=0.0376 bar, resulting in a pressure range of
The two knife gate valves are preferably actuated using a thermal bimorph actuator that is a well-known technique in the art. Power is provided to the contact pads that are in electrical contact with a heater in contact with or integrated with the thermal bimorph actuator. When a current is sent through the heater, the bimorph temperature rises. The temperature change causes the bimorph to bend due to the difference in thermal coefficients of expansion between materials such as aluminum and silicon, for example. It should be noted that other actuation methods might be applicable with the invention such as piezoelectric, magnetic, electrostatic actuation or other thermal actuation principles. In the embodiment, the footprint-efficiency of the device is significantly increased due to the displacement of the gate in a plane perpendicular to the main flow and the main flow path orifice being perpendicular to the substrate (out-of-plane with respect to the substrate) thus eliminating the relatively large orifice as being a factor that negatively affects the footprint area.
The foregoing description of the embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, since many modifications or variations thereof are possible in light of the above teaching. Accordingly, it is to be understood that such modifications and variations are believed to fall within the scope of the invention. The embodiments were chosen to explain the principles of the invention and its practical application, thereby enabling those skilled in the art to utilize the invention for the particular use contemplated. It is therefore the intention that the following claims not be given a restrictive interpretation but should be viewed to encompass variations and modifications that are derived from the inventive subject matter disclosed.
Number | Date | Country | Kind |
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0301637-5 | Jun 2003 | SE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB04/50854 | 6/7/2004 | WO | 12/5/2005 |