Abstract:
A micropump fabricated in a planar substrate is provided with a valving chamber which is communicated to a pumping chamber. The valving chamber has an inlet and outlet port. Both the valving chamber and pumping chamber have a liquid, electrically conductive piston disposed therein, which liquid piston is nonmiscible with the pumped working fluid and nonreactive with the substrate in which the chambers are formed. The valving piston is magnetohydrodynamically driven to selectively close either the inlet port or the outlet port. The pumping piston is magnetohydrodynamically driven to pull or push the working fluid through one of the inlet or outlet ports, through the valving chamber, into the pumping chamber, back out of the pumping chamber and through the other one of the inlet or outlet ports after activation of the valving piston. Both direct current and inductive magnetohydrodynamic drives are contemplated. The valving and/or pumping chambers may be shaped or narrowed in their dimensions to impose a mechanical bias on the respective valving and/or pumping pistons to assume a preferred position in their respective chambers when the magnetohydrodynamic drive is turned off.

Description:
RELATED APPLICATION 
     The present application relates to U.S. Provisional Patent Application, serial no. 60/114,203, filed on Dec. 29, 1998, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a micropump for delivering fluid at a low and controllable flow rate. 
     2. Description of the Prior Art 
     Science and engineering have been devoted to building machines that mimic human&#39;s functionality to expand our reach. The Industrial Age came about due to the invention of steam engine which freed human from laborious muscle works. With the advent of electronics, computers are revolutionizing the Information Age. Advances in microelectronics processing have opened up a far reaching capabilities in microengineering. 
     In recent years, there has been an explosion of interest in the field of integrated MicroElectroMechanical Systems (MEMS). The field is still so new that there is no commonly accepted definition of the field among researchers. Instead of fashioning devices that simply shunt electrons, moving devices are fabricated. While integrated circuit technology is essentially a two-dimensional or planar process. MEMS works in a three dimensional process. Because much of the key process is not radically different from fabricating microelectronics elements, many essential techniques can be simply copied. 
     Rooted back in the early research effort on materials and processing for the fast emerging field of integrated circuits, the late 1960&#39;s and early 1970&#39;s saw the effort in developing integrated sensors. After early attempts to make temperature and pressure sensors, visible image arrays were produced in large volume. After years of steady improvement, today visible image arrays rival the resolution of photographic films and promise to revolutionize the field of photography. Though they represent some of the largest chips made. Only a few processes and packaging techniques go beyond standard integrated circuit manufacturing. 
     1970&#39;s saw considerable advances in bulk micromachining. The emergence of preferential etch, and impurity based etch-stops took silicon based sensors out of laboratories into mass production. Pressure sensors led the way. Much attention was concentrated on preferential etch and sealing technique to make pressure sensors a reality on silicon. Late 1980&#39;s surface micromachining led to the development of a series of AC resonant sensors. Gradually, accelerometer and flowmeters joined pressure sensors as high-volume production devices. 
     Today, bulk and surface micromachining, in combination with wafer-to-wafer bonding and electroforming technologies offer a designer a rich array of processes for the creation of micromechanical structures in batch and with high precision. It has been established that micromachined sensors can be produced with high yield. They can be merged with integrated electronics, both in monolithic and multi-chip hybrid assemblies. These devices are widely used in high performance instrumentation and control system. To date, VLSI interface circuitry with digital signal processing has pushed some sensors to reach 16-bit accuracy and feature self-testing and digital compensation possible for commercial mass production. 
     Since micromachined sensors are passive devices, a complete mechanical system is not readily implemented. In order to complete the system, actuators, namely machines that cause other devices to move, are badly needed. In 1988, combining surface micromachining, the emergence of electrostatic actuators were widely researched. Later, other actuation methods such as thermal and resonant actuation also demonstrated their possibilities. 
     With the addition of microactuators to microsensors and microelectronics interface circuitry, most of all the elements for a complete MEMS were in place. However, due to the complexity of microactuators, integration has proven to be difficult. Microactuators which were being produced were never fully satisfactory for practical applications. To date, electrostatic microactuators remain as the accepted means of actuation in microscale. Only recently has the possibility of magnetostatic microactuators been realized with reasonable success. 
     The requirements for an ideal microactuator can be overwhelming. A microactuator has to be able to transfer its driving energy to other devices. A low loss energy transmission must be incorporated into the system. The driving voltage for the microactuator must be compatible with integrated circuits, which can mean well below 15 volts, in order to be controlled by on chip electronics. Reliability of the microactuator should be as unquestionable as the driving electronics themselves. And last, the fabrication process should be compatible with electronics fabrication processes. 
     What is needed to address these requirements is a completely different approach to achieve microactuation. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to address shortcomings relating to other microactuators, the present invention provides microactuation in microscale based on magnetohydrodynamics. What is disclosed is a micromechanical device capable of microactuating a conductive fluid inside capillary channel or chamber. 
     In a preferred embodiment, the microactuator is comprised of a source to produce a constant external magnetic field, a channel or chamber where an electrically conductive fluid flows, and electrodes that make electrical contact with the fluid. The direction of magnetic field, the direction of channel flow, and the direction of the electric current are mutually perpendicular to each other. When electric current is applied to the electrodes, the resulting Lorentz force pumps the conductive fluid towards one end of the chamber. The pumped fluid can be used directly as hydraulic fluid to act on another part of a system, or it can be used to pump other fluids. 
     The pump has no moving parts which are used for pumping the fluid other than two liquid masses or pistons. It has a low operating voltage or current operating mode, and also has a simple and effective energy transfer means to other components. In addition, the microactuator allows the use of a planar process for device fabrication with no specific requirement on different types of substrate materials. 
     More specifically the invention is defined as an apparatus for pumping a working fluid comprising a pumping chamber and a valving chamber communicated to the pumping chamber and having an inlet port and an outlet port. These are microcapillary chambers and may be interchangeably described as channels. A liquid, electrically conductive pumping piston is disposed in the pumping chamber. Similarly, a liquid, electrically conductive valving piston disposed in the valving chamber. The pistons are actually a movable mass of material, such as a low melting temperature metal, such as mercury or gallium alloys. An exterior source of heat may be provided to control the liquid-solid state of the pistons at any given point in time. Two magnetohydrodynamic drives are provided. One for the pumping piston and one for the valving piston. A valve magnetohydrodynamic drive is disposed in proximity to the valving piston to controllably move the valving piston within the valving chamber to control direction of flow of the working fluid into and out of the inlet and outlet ports in the valving chamber. The pump magnetohydrodynamic drive is disposed in proximity to the pumping piston to controllably move the pumping piston within the pumping chamber so that the working fluid is pumped into and out of the pumping chamber. 
     The pump and valve magnetohydrodynamic drive may each be a direct current magnetohydrodynamic drive, each be an induction magnetohydrodynamic drive, or one may be a direct current magnetohydrodynamic drive and the other an induction magnetohydrodynamic drive. 
     In the preferred embodiment the liquid, electrically conductive valving piston and the liquid, electrically conductive pumping piston are comprised of a liquid metal, although this is not necessary. Any liquid conductive material with the appropriate surface tension characteristics to provide a seal in the chambers and remain intact as a single mass may be employed. 
     The pumping chamber and the valving chamber are preferably fabricated in at least one planar substrate, usually the same common substrate although separate substrates could be employed in separate fabrication processes and then joined to communicate the two chambers on later assembly. 
     In an alternative embodiment at least a portion of the pumping chamber has a narrowed dimension as compared to another portion of the pumping chamber so that the liquid, electrically conductive pumping piston is biased to move away from the portion with a narrowed dimension toward the other portion of the pumping chamber. The dimension which is narrowed may or may not correspond topologically with each other in the two portions of the chamber. For example, width of the chamber may be narrowed at one end and the width in an orthogonal direction widened in the opposing end. Any shaping of the chamber which would create a bias to position the piston is contemplated as included in the invention. 
     In one embodiment the valving chamber and pumping chamber are communicated with each other through at least two interior ports. The interior ports are alternatively closed by movement of the valving piston. The valving chamber has a centerline and the interior ports are disposed closer to the centerline than are the inlet and outlet ports. 
     Alternatively, the valving chamber and pumping chamber are communicated with each other by a single interior port or a multiplicity of ports which are in one location. The single interior port or ports at one location is open or uncovered by the valving piston, when the valving piston covers either the inlet port or the outlet port. The valving piston is displaced to completely cover either the inlet port or the outlet port, but not both. 
     The invention is also defined as a method for pumping a working fluid in an apparatus as described above. More specifically, the method comprises the steps of controllably, magnetohyrdodynamically moving a liquid, electrically conductive valving piston disposed in a valving chamber to controllably open or close an inlet port or an outlet port. Similarly, a liquid, electrically conductive pumping piston disposed in a pumping chamber is controllably, magnetohyrdodynamically moved to pump the working fluid through an opened one of the inlet or outlet ports. 
     The invention now having been briefly summarized, an illustrated embodiment of the invention can be better visualized in the following drawings turn to the following drawings wherein like elements are referenced by like numbers. It must be expressly understood, that the invention is not limited by the particular features which are used in the illustrations, but encompasses the full range of equivalents and logical embodiments which are included within the scope and meaning of the following claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic side cross-sectional view of the present invention, showing the micropump comprised of a main pump chamber and a valve chamber. 
     FIG. 2 is a side cross-sectional diagram as seen through lines  2 — 2  of FIG. 1 illustrating magnetohydrodynamic actuation by direct current case in which an external magnetic field that is oriented perpendicular to both the direction of flow and electrical current, which in the illustration of the figure is vertical on the page. 
     FIG. 3 is a highly diagrammatic depiction of an inductor array shown in plan elevational view which is used when the electrical current is induced by a traveling magnetic field. 
     FIG. 4 is a vertical cross-sectional view of the main chamber of the pump as seen through section lines  4 — 4  of FIG. 1 shown in an alternative embodiment where the chamber is provided with at least one narrowing end to reposition the piston when electrical current is turned off. 
    
    
     The invention and its various embodiments can be understood as set forth in the following detailed description. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A micropump  10  is comprised of two micro-capillary tubes coupled to the inlet and outlet ports  20   a  and  20   b  and the pump  10 , and two pistons  32  and  34  driven by magnetohydrodynamic (MHD) mechanisms. Piston  34  operates the opening and closing of the valve ports  20   a  and  20   b , while the other piston  32  changes the volume of the pump chamber  12 . 
     Before reviewing a detailed description of the invention, consider first some of its advantageous features. A first feature of the present invention is the fabrication of the micropump  10  using a planar manufacturing process, which allows miniaturization and mass manufacture of the device using conventional silicon micromachining techniques and integration with other micromachined and circuit components on the same substrate. For example, pump  10  may be fabricated so that the embodiment of FIG. 1 is entirely circumscribed in a volume of 1×1×5 mm. As a result of using a liquid metal or a conducting liquid, the micropump  10  has a reliable means for pumping that is sufficiently small in size. In the illustrated embodiment, the mechanism which converts electrical energy to mechanical energy is implemented by a combination using liquid metal pistons  32  and  34 . The liquid metal pistons  32  and  34  not only facilitate the action of pumping, but also ensures the opening and closing of the flow passages to and from the main pumping chamber  12 . Pistons  32  and  34  also provide adequate sealing to prevent leakage of the working fluid past them. By reversing the sequence of opening and closing of the flow passages to and from main pump chamber  12 , the liquid metal pump  10  can easily perform bidirectional pumping. 
     The second feature of the present invention is that the electrical specifications on the power supply voltage needed to drive pump  10  are relaxed as contrasted to other types of MEMS actuators demanding a special high voltage power supply. This will simplify electronic circuit design for feedback control as well as reduce the potential risk of subjecting the fluid to the high voltage environment. For example, a power supply having a voltage of the order of magnitude of 5 volts and current capacity of the order of magnitude of 1 amp will easily drive pump  10 . 
     In the present invention, the micropump, generally denoted by reference numeral  10 , is comprised of a rectangular main pump chamber  12  and a rectangular valve chamber  14  as shown in the diagrammatic side cross-sectional view of FIG.  1 . The valve chamber  14  is connected to the main pump chamber  12  through multiple openings  16 , which can be a single opening, or two or more openings. In this illustration, two openings  16  have been used. Two additional openings  20   a  and  20   b  defined in the wall  18  of the valve chamber  14  form the inlet  20   a  and outlet  20   b  to and from the main pump chamber  12 . Defined in the chamber wall  22  of main pump chamber  12  the opposite from the valve chamber  14 , is an opening  24  to release pressure when the piston  32  in main chamber  12  moves. All of the openings  16 ,  20   a ,  20   b  and  24  are much smaller than the axial diameters of either chambers  12  or  14 . For example, when the liquid metal is mercury, then the range of sizes of openings  16 ,  20   a ,  20   b  and  24  includes 100 microns. The shape of the cross section of openings  16 ,  20   a ,  20   b  and  24  is arbitrary. 
     Chamber walls  22  itself can be fabricated from any electrically insulating material provided that the substrate material in which pump  10  is fabricated has no surface reaction to the fluids in chambers  12  or  14 . Any electrically conductive fluids, such as liquid metals, alkalis, or electrolytes, can be serve as the magnetohydrodynamic fluid. A certain degree of conductivity may be necessary when the external magnetic field is weak and internal flow friction is high. However, when electrolytes are used, care must be taken so that electrolysis does not occur at the main chamber electrode pair  28  or valve chamber electrode pair  30 . Main chamber electrode pair  28  or valve chamber electrode pair  30  comprise each a pair of opposing electrodes mounted in main or valve chambers  12  and  14  respectively. Main chamber electrode pair  28  or valve chamber electrode pair  30  are disposed on opposing walls of their respective chambers  12  and  14  and are electrically coupled only when their respective pistons  32  or  34  move between them. As will be described in connection with FIGS. 2 and 3, the current flow through pistons  32  and  34  provided by electrode pairs  28  and  30  in combination with an external applied magnetic field result in a mechanical force which moves pistons  32  and  34  and will hence pump the working fluid. Electrodes  28  and  30  are assumed in the illustrated embodiment to be simple planar, sheet electrodes, but any pattern, form or design for an electrode can be substituted, such as circular, elliptical, interdigitated, banded or the like. 
     Liquid metals show the best promise for use as pistons  32  and  34 , since it has the lowest resistivity. An incompressible hydraulic fluid can be used as the working fluid in pump  10  to deliver mechanical energy to other devices. However, this does not limit the possibility of using a compressible fluid, such as air, to further enhance the efficiency of the energy delivery. 
     In the example of the liquid metal pump shown in FIG. 1, both chambers are partially filled with a low melting temperature metal alloy, such as mercury or gallium alloys. It is to be expressly understood that the invention may use any conducting fluid consistent with the teachings of the invention as the material for pistons  32  and  34 . The pump and valve pistons  32  and  34  respectively are made out of droplets or pools of the low melting point metal alloy. Exceptionally high surface tension exists in liquid metal to prevent the liquid metal from passing through the small openings, such as openings  16 ,  24 ,  20   a  and  20   b , which thus act as a flow stop for the liquid metal, yet other fluids with lower surface tension pass unimpeded. At the same time, high surface tension inside the liquid metal causes pistons  32  and  34  to press tightly against the walls  22  of the chambers  12  and  14  preventing the pumped fluid from leaking pass pistons  32  and  34 . 
     The properties of solid-liquid phase transition in liquid metal can be further taken advantage of for sealing chambers  12  and  14  against any liquid passage. Microheating elements can be fabricated in the proximity of chambers  12  and  14  to raise the temperature of the metal above its melting point to allow the liquid metal to move freely in chambers  12  and  14 . However, as the temperature drops below the liquid metal&#39;s melting point, the metal enters solid phase and pistons  32  and  34  cease to move freely. This can provide full dead-stop valving action. 
     Consider now the operation of pump  10 . As piston  32  is pulled away from the valve openings  16 , there is a volumetric increase in the main pump chamber  12 . If valve piston  34  is moved to the right in the illustration of FIG. 1, Fluid will flow from the inlet  20   a  and opening  16  through valve chamber  14  into main chamber  12 . As the piston  32  is pushed towards openings  16 , and if valve piston  34  is moved to the left in the illustration of FIG. 1, the fluid inside the pump chamber  12  is expelled through opening  16  into valve chamber  14  and out of outlet valve  20   b.    
     Since the inlet valve  20   a  and outlet valve  20   b  are symmetric and identical, the inlet  20   a  can be treated as outlet  20   b  and vice versa depending only on the action of pistons  32  and  34 . 
     It is desirable to have inlet  20   a  and outlet  20   b  to the valve chamber  14  offset further away from the center line of the main pump chamber  12  as shown in FIG.  1 . This allows the valve piston  34  to fully close opening  16  leading to the main pump chamber  12  while still allowing fluid trapped at the end of the valve chamber  14  to leak out of valve chamber  14 . In the simplest case, only one opening  16  leading to the pump chamber  12  is needed. 
     Actuation of pistons  32  and  34  is provided by means of magnetohydrodynamics. Magnetohydrodynamic actuation can be direct current or induction. In the direct current case as depicted in FIG. 2, an external magnetic field that is oriented perpendicular to both the direction of flow and electrical current, which in the illustration of the figure is vertical on the page. The magnetic field can be provided by either permanent magnet or by electromagnet. When direct current is passed through liquid metal of pistons  32  or  34  between electrode pairs  28  and  30  respectively, the resulting Lorentz force pushes the liquid metal itself. By reversing the direction of flow of the electrical current between electrode pairs  28  or  30 , or reversing the direction of the external magnetic field, the direction of the Lorentz force on pistons  32  and  34  can also be reversed. The circuitry used to produce the direct current between the electrodes in proper synchronization with pistons  32  and  34  is entirely conventional and will not be further described. 
     In the case where magnetic induction is used to create eddy currents in pistons  32  and  34  as shown in FIG. 3, a linear array  36  of inductors  38  is located in the proximity to and parallel with the flow direction of the liquid metal or pistons  32  and  34 . Array  36  is substituted for electrode pairs  28  and  30 . One array may be provided in place of each electrode or for the electrode pair. Arrays  36  can be provided on the exterior of walls  22  of both valve chamber  14  and main chamber  12 , or at least in a manner which electrically insulates inductors  38  from pistons  32  and  34  while leaving array  36  in close proximity to pistons  32  and  34 . An electrical current is sequentially pulsed in one direction through every spiral inductor  38  in the inductor array  36 . It must be understood that although inductor  38  is depicted diagrammatically as a spirally shaped inductor, that any shape or form for a magnetic inductor now known or later devised may be substituted. Thus, a spatially traveling magnetic field is thus produced along linear inductor array  36 . The traveling magnetic field induces a current flowing inside the liquid metal of pistons  32  and  34 , sometimes referred to an eddy current. As before an appropriately oriented external magnetic field is also provided. Consequently the induced force applied to pistons  32  and  34  moves pistons  32  and  34  in the chambers  12  and  14  to either ends depending on the direction of the pulsed current in array  36 . The circuitry coupled to inductors  38  to provide the sequence of traveling magnetic field and hence the eddy currents in pistons  32  and  34  is conventional and shall not be further described. 
     To further enhance the micropump&#39;s functionality, the chambers or channels holding the liquid metal or pistons  32  and  34  can be tapered gradually at their ends  40  as diagrammatic depicted in FIG.  4 . Again due to surface tension of the liquid metal comprising pistons  32  and  34 , the liquid metal will tend to move to the part  42  of the channel with wider opening. In doing so, the position of piston  32  or  34 , inside the channel will be determined when the electrical current is removed. This can be particularly important when it is necessary to have a normally off or on valve. In addition, it provides an easy resting place for the liquid metal to cool down and enter its solid phase. 
     Piston  32  and main chamber  12  can be used as disclosed above independently from piston  34  and valving chamber  14 . For example, movement of the working fluid into and out of main chamber  12  may be the only action required in a particular application. In addition, piston  32  can be solidified at a controlled position within its movement range within main chamber  12  by means of temperature control of the substrate in which pump  10  is fabricated or located. The control of the position at which piston  32  can be solidified is then a substitute in some applications for the function of valving chamber  14  and piston  34 . 
     Many alterations and modifications may be made by those having ordinary skill in the art without departing from the spirit and scope of the invention. Therefore, it must be understood that the illustrated embodiment has been set forth only for the purposes of example and that it should not be taken as limiting the invention which could be more broadly or narrowly defined by patent claims. 
     The words used in this specification to describe the invention and its various embodiments are to be understood not only in the sense of their commonly defined meanings, but to include by special definition in this specification structure, material or acts beyond the scope of the commonly defined meanings. Thus if an element can be understood in the context of this specification as including more than one meaning, then its use in a claim must be understood as being generic to all possible meanings supported by the specification and by the word itself. 
     The definitions of the words or elements of the following claims are, therefore, defined in this specification to include not only the combination of elements which are literally set forth, but all equivalent structure, material or acts for performing substantially the same function in substantially the same way to obtain substantially the same result. In this sense it is therefore contemplated that an equivalent substitution of two or more elements may be made for any one of the elements in the claims or that a single element may be substituted for two or more elements in the defined claims. 
     Insubstantial changes from the claimed subject matter as viewed by a person with ordinary skill in the art, now known or later devised, are expressly contemplated as being equivalently within the scope of the invention. Therefore, obvious substitutions now or later known to one with ordinary skill in the art are defined to be within the scope of the defined elements. 
     The invention is thus to be understood to include what is specifically illustrated and described above, what is conceptionally equivalent, what can be obviously substituted and also what essentially incorporates the essential idea of the invention.