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Micromembrane actuatorRelated Patent Categories: Pumps, Motor DrivenMicromembrane actuator description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060233649, Micromembrane actuator. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND OF THE INVENTION [0001] 1. Field of the Invention [0002] This invention relates to a compact actuator that utilizes small thin shape memory alloy (SMA) diaphragms to provide large force output at high drive frequency. The actuation is based on a shape memory alloy miniature pump, which rectifies liquid to achieve stroke for the actuator. A bias pressure is applied to bend the SMA membrane upward to form a cavity between the membrane and a surface in the actuator body upon which the unbiased membrane sits. A pulse of charge is used as resistive heating so that membrane accomplishes a plunging stroke towards the surface when heated, thereby forcing liquid out of the cavity. High drive frequency is reached by impinging the liquid on the heated membrane to achieve forced convective cooling. The heated liquid flows out via the outlet port. Adding additional membranes in parallel also increases the flow rate. [0003] 2. Description of Related Art [0004] The publications and other reference materials referred to herein to describe the background of the invention and to provide additional detail regarding its practice are hereby incorporated by reference. For convenience, the references are numerically referenced and grouped in the appended bibliography. [0005] Thin film SMA possesses unique characteristics that are attractive for use in actuators. A foremost of those characteristics is a large strain output, which can typically strain up to 8-10%. No other active materials posses this behavior, and studies have shown that fatigue life can exceed million cycles when strains are below 2% [1]. It also has the highest work densities for smart materials, for instance 25.times.10.sup.6 joule/m.sup.3 for NiTi compared to 0.1.times.10.sup.6 joule/m.sup.3 for piezoelectric materials [1]. A less well-known attribute is the thin film's high frequency response due to increased heat dissipation from large surface-to-volume ratio. While bulk SMA typically has frequency responses of less than 1 Hz, thin film SMA can have frequency response on the order of 100 Hz if power delivered to the thin film is carefully manipulated to account for heat transfer issues [2]. These attributes make thin film SMA an attractive material for micro actuation devices. [0006] Typical micro devices require large deformations while exerting sufficient forces from an actuating material. This work density is an inherent attribute of thin film SMA and has been used in SMA based micropumps. One of the first micropumps, the pumping mechanism was based on two antagonistic 3 .mu.m thick NiTi membranes [3-6]. For this pump, the push-pull pumping motions were generated by alternately heating the membranes. The pressure head generated by this motion was 519 Pa while operating at drive frequency of 1 Hz. At higher drive frequencies, the membranes were not sufficiently cooled thereby reducing the pumping motion and the flow rate. The maximum flow rate was 50 .mu.l/min. Therefore, this pump did not produce large flow rates or large force outputs due to the limitations of the system. [0007] Makino and his colleagues also developed a SMA-based micropump but used a single NiTi diaphragm biased with pressurized nitrogen gas [7-10]. Their pump was able to operate at a drive frequency of 0.2 Hz, which was sufficient for cooling and shape recovery of the diaphragm. Separate studies also revealed that 6 .mu.m thick NiTi diaphragms displayed larger force outputs under 500 kPa bias pressure. The flow rate was 4.8 .mu.l/min, which was achieved under bias pressure of 100 kPa. This pump lacked the qualities of large force and large flow rates. [0008] More recent development in thin film SMA micropump was based on a bimorph design where strips of 5 .mu.m thick NiTi were adhered on top of 15 .mu.m thick silicon membrane [11-14]. With this design, a 100 Hz drive frequency was reported, but they also pointed out that insufficient cooling subsequently reduced diaphragm stroke causing reductions in flow rate so that this drive frequency was not sustainable. The maximum flow rate was 350 .mu.l/min at drive frequency of 60 Hz. SUMMARY OF THE INVENTION [0009] The present invention provides a thin film SMA micro pump actuator that has high work density and high frequency response. The invention uses a miniature SMA pump to rectify liquid to achieve stroke. The invention manipulates the fluid flow to have forced convection cooling on the SMA membrane, eliminating the insufficient cooling of prior art designs. The result is an improved design with operation at flow rates well in excess of prior art designs. [0010] Past micropumps have not exploited the critical properties of thin film SMA which are high work density and high frequency response. The actuators of the present invention are able to achieve high output force at large velocities by exploiting both high frequency response and work density properties of thin film NiTi SMA membranes. The actuator uses a miniature SMA pump to rectify liquid to achieve stroke, which past micropumps did not consider. The insufficient cooling associated with past micropumps is also eliminated. This is achieved by manipulating the fluid flow to have forced convection cooling on the SMA membrane. By doing so, the flow rate for the current actuator increases to three times the order of magnitude higher than the past micropumps. [0011] Additional features of the inventions are provided in the following detailed description of the preferred embodiments with reference to the drawings. [0012] The above discussed and many other features and attendant advantages of the present invention will become better understood by reference to the detailed description when taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS [0013] FIG. 1 is an illustration showing assemblage of a single membrane pump actuator with SMA membrane and a retaining lid. [0014] FIG. 2 is a cross-sectional view taken along the planes 1A-1A and 2A-2A shown in FIG. 1. [0015] FIG. 3 is a cross-sectional view of a deformed SMA membrane under bias pressure. [0016] FIG. 4 is a front view of a pumping chamber. [0017] FIG. 5 is perspective view of the four-membrane pump actuator formed accordance with present invention. [0018] FIG. 6 is a perspective view of a SMA membrane that is suitable for use in pump actuators in accordance with the present invention. DETAILED DESCRIPTION OF THE INVENTION [0019] A pump-based compact actuator is provided in accordance with the present invention that is capable of producing a large force output and a large volume flow rate. This following detailed description sets forth exemplary embodiments that are a few of the many considered possible for this actuation system and as such this description is regarded as an example. [0020] The actuators of the present invention utilize at least one thin shape memory alloy membrane for means of actuation. Multiple membranes may be used to increase volume flow rate. The force output of the pump can be controlled by varying the properties of the membrane, such as the thickness of the membrane. The actuator includes an actuator body that includes surfaces that define one or more actuation chambers that hold SMA membranes over a membrane seat surface that includes a liquid inlet port and multiple outlet ports. These ports are located relative to each other so that cool liquid impinges on the hot membrane for faster heat transfer through forced convection cooling. The heated liquid then flows out through an outlet port in the actuator or pump body. Continue reading about Micromembrane actuator... Full patent description for Micromembrane actuator Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Micromembrane actuator patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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