Abstract:
A drive unit for a micro valve comprises a housing, a spring and at least one shape memory alloy element which is attached to the housing in a deflectable manner. The shape memory alloy element is loaded by the spring towards a deflected position and is movable in response to a temperature increase and the shape memory effect activated thereby, into a position which is at least less deflected against an increasing load by the spring. A normally closed micro valve includes a fluid housing, at least one valve seat, a sealing element opposite the valve seat, and a drive unit of the type mentioned above. The shape memory alloy element cooperates with the sealing element and exerts an operating force on the sealing element for closing and opening the valve seat with or against a compression spring force.

Description:
RELATED APPLICATION 
     This application claims priority to German Application No. 20 2010 010 747.4, which was filed Jul. 28, 2011. 
     FIELD OF THE INVENTION 
     The invention relates to a drive unit for a micro valve comprising a shape memory alloy element as well as to a micro valve. 
     BACKGROUND 
     Shape memory alloy elements are distinguished in that they may be used as actuators with high energy density. 
     Valves are known in which one or more actuating elements cooperate with shape memory alloy elements in the form of a wire, as is described in DE 102 33 601, for example. 
     Foils made from shape memory alloys are also known. In this case, only a very small travel for controlling the stroke of a valve is available in valves, and only low pressures can be controlled. 
     Moreover, known valves based on the shape memory alloy principle for the actuator are NO designs (i.e. “normally open”) in many cases. As a rule, industrial applications require NC valves (i.e. “normally closed”) which are closed in the state of rest and will not open until they are acted upon by an electrical current. 
     SUMMARY 
     A drive unit for a micro valve and a micro valve comprising a shape memory alloy element are configured to provide a larger available stroke. 
     The drive unit for a micro valve comprises a housing, a spring, and a shape memory alloy element. The shape memory alloy element is attached to the housing in a deflectable manner and is loaded by the spring towards a deflected position. In the event of a temperature increase, the shape memory effect becomes operative: the shape memory alloy element is moved into a less deflected position along with an increasing spring load. 
     The deflection of the shape memory alloy element by the spring results from the fact that the shape memory alloy element is pushed apart by a spring force, for instance. If the temperature increases, the latter contracts to assume its initial shape and hence assumes a position which is less deflected against an increasing spring force. It is also possible, however, that the shape memory alloy element is first compressed by the spring and then expands with an increase in temperature, hence reaching a position which is less deflected against an increasing spring force. The shape “remembered” by the shape memory alloy element in the event of a temperature increase is imparted to said element in a known manner by it being acted upon by force and temperature. 
     In one embodiment, the shape memory alloy element comprises at least one pair of foils stacked upon each other and which are connected to each other at their outer circumference at least in sections, in particular are welded, and are pushed apart by the spring or compressed. Especially in the case of applying the drive unit in valve technology, using a pair of stacked foils has the advantage that a larger valve stroke is made available. Therefore, more than two foils may be stacked upon each other, of course. 
     The individual foils comprise a central passage for the spring or a transmission element cooperating with the spring. Depending on the desired size and force conditions, it is also possible to select a spring having a larger diameter, and the shape memory alloy element may then be placed within the spring. 
     The spring is preferably a compression spring supported by the housing and engaging the foil which is farthest from the support. The shape memory alloy element is pushed apart by the spring by a travel which is defined, for instance, by a stop element. Since the shape memory alloy element is affixed on the same housing side, the height of the foil stack is fully exploited. When acted upon by temperature, the shape memory alloy element reassumes its original height against the spring force. 
     Basically, the spring may be an element with elastic properties, which is manufactured from spring steel or an elastomeric material. 
     In another embodiment, the compression spring and the shape memory alloy element are supported by opposite housing sides. In this case, the compression spring engages the foil adjacent to it, and the foil stack is compressed along with a reduction of its original height. When acted upon by temperature, the foil stack expands and returns to the initial position with larger height, the compression spring being loaded further. 
     One example embodiment comprises at least two pairs of foils stacked upon each other. Neighboring foils of two pairs are connected to each other on the circumference of the central passage for the spring or the transmission element cooperating with the spring. This gives the foil stack a spring-like geometry - it may be compressed or pulled apart. 
     Depending on whether the shape memory alloy element and the spring are supported by the same housing side or opposite housing sides, these two elements will engage the transmission element on the same side or opposite sides. In case that the shape memory alloy element and the transmission element are firmly connected to each other, e.g. are welded, the transmission element follows the movement corresponding to the actuator motion by the shape memory alloy element. If this drive unit is used in a valve, there is the advantage that the valve opens automatically even in the absence of any fluid pressure. 
     The firm connection between the shape memory alloy element and the transmission element may be omitted if the shape memory alloy element and the spring engage the transmission element on opposite sides. In this case, the transmission element rests on the shape memory alloy element and automatically follows any movement of the shape memory alloy element. 
     There is a large design freedom in terms of the geometric design of the individual foils for the foil stack forming the shape memory alloy element. It may be formed in particular to be elliptical, circular, or triangular for example. Other geometries are also conceivable, however. The foils are structured according to known methods, for instance by laser cutting or a wet chemical etching process. 
     It is also possible to use several shape memory elements in a drive unit. Instead of the shape memory alloy element being affixed across a large area on a side of the drive unit housing, two or more shape memory alloy elements may be inserted in the drive unit housing such that they are rotated by 90°, so that the foils have their outer circumference resting against the drive unit housing. The central passage will then be formed between the two or more centrically arranged shape memory alloy elements. 
     According to an embodiment of the invention, the foils are heated by an electric current flowing in their interior. NiTi, CuZn, or FeNiAl alloys are typical materials for shape memory alloys. The transition temperatures are in the range from 100 to 300° C. However, there are shape memory alloys where the transition temperature ranges from only 40 to 70° C. The exposure to temperature may also be achieved without any direct electrical contacting of the shape memory alloy element, for example by a heating element or transfer by contact with a fluid. A further possibility of temperature application of the shape memory alloy element may be achieved in that the spring is electrically contacted and is in electrically conductive connection to the shape memory alloy element. Here, the shape memory alloy element may be designed such that an external foil has no central passage and the spring rests on this foil. 
     Preferably, active or passive cooling elements are used for cooling the shape memory alloy element. In doing so, the switching times when using the drive unit in micro valves may be considerably reduced. Peltier elements are also known in micro designs. 
     The drive unit is particularly suited for use as an actuator in micro valves, in particular in normally closed micro valves. In one embodiment, a micro valve including a drive unit comprises a fluid housing with at least one valve seat and a sealing element opposite the valve seat. The sealing element cooperates with the shape memory element, exerting an operating force on the sealing element for opening the valve against the compression spring force. 
     The sealing element can be formed as a membrane which is sandwiched between the fluid housing and the drive unit housing. The sealing element, however, may also be arranged on the transmission element on the side adjoining the valve seat. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further features and advantages of the invention will be apparent from the following description with reference to the attached drawings, in which: 
         FIG. 1   a  is a sectional drawing taken through a micro valve in a “closed” position; 
         FIG. 1   b  is a sectional drawing according to  FIG. 1   a  in an “open” position; 
         FIG. 2   a  is a sectional drawing taken through a second embodiment of a micro valve in the “closed” position; 
         FIG. 2   b  is a sectional drawing taken through the micro valve according to  FIG. 2   a  in the “open” position; 
         FIG. 3   a  is a top view of a shape memory alloy element according to the invention; 
         FIG. 3   b  is a top view of a further embodiment of a shape memory alloy element; 
         FIG. 4   a  is a top view of one embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 4   b  is a top view of another embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 4   c  is a top view of another embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 4   d  is a top view of another embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 4   e  is a top view of another embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 4   f  is a top view of another embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 4   g  is a top view of another embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 4   h  is a top view of another embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 4   i  is a top view of another embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 4   j  is a top view of another embodiment of a foil for assembling shape memory alloy elements; 
         FIG. 5  shows an alternative embodiment of the drive unit of the micro valve. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1   a  illustrates a cross-section of a micro valve  10  comprising a drive unit housing  20  and a fluid housing  30  with a valve seat  40 . The drive unit housing  20  may be constructed in two pieces. A membrane  50  is arranged between the drive unit housing  20  and the fluid housing  30 . The drive unit housing  20  comprises a chamber  60  which is open towards the fluid housing  30  and in which a spring  70 , a shape memory alloy element  80  and a transmission element  90  are arranged. The shape memory alloy element  80  has one side affixed to a support  110  of the housing  20  and its opposite side affixed to the transmission element  90 . On the same housing side, the spring  70  has its one end resting on a support  120  and its second end resting on the transmission element  90 . Likewise, the spring  70  and the shape memory alloy element  80  are affixed to the same side of the transmission element  90 . 
     The shape memory alloy element  80  comprises at least one pair of foils  130  stacked upon each other, which are connected to each other at their outer circumference at least in sections. Depending on the shape of the foils  130 , two triangular foils  130  may be connected to each other locally at their three corners, for instance, in particular by welding. Circular foils may be connected to each other along their entire outer circumference. In this way, the external shape of the shape memory alloy element  80  resembles a stack of disc springs. Due to the fact that several foils  130  are stacked upon each other, a travel exists which is larger than if only a single foil were used. This travel can be exploited as a stroke when used in valves. 
     A central passage  140  for receiving the spring  70  is arranged in the shape memory alloy element  80 . Depending on the force and the travel which are to be provided by the shape memory alloy element  80 , the spring  70  and the shape memory alloy element  80  may also be designed such that the spring  70  has a larger inner diameter than the shape memory alloy element  80 ; then, the spring  70  may accommodate the shape memory alloy element  80  within its interior. 
     The spring  70  is compressed and pushes the transmission element  90  against the membrane  50  and the valve seat  40 , thereby closing the latter. At the same time, the spring  70  pushes the shape memory alloy element  80  apart towards a deflected position. Here, the spring  70  is a compression spring and, in the shape memory alloy element  80 , engages that foil  130  which is farthest from the support  120 . In this way, the entire height of the shape memory alloy element  80  is made use of. 
     As shown in  FIG. 1   a , it is also possible to use several pairs of foils  130  in the shape memory alloy element  80  which are stacked upon each other. Neighboring foils of two pairs of foils are respectively connected to each other on the circumference of the central passage  140 . In this way, the shape memory alloy element  80  takes on a spring-like geometry. The foils  130  may be pulled apart or compressed in an accordion-like fashion. 
     In one embodiment, the shape memory alloy element  80  and the transmission element  90  are firmly connected to each other. This is why the transmission element  90  follows the movements of the actuator. This drive unit may also be used in those valves where only a small fluid pressure is available. After activating the shape memory alloy element  80 , the valve opens automatically without assistance from the fluid. In case of higher fluid pressures, however, a connection between the shape memory alloy element  80  and the transmission element  90  may be omitted. 
     The foils  130 , which are stacked upon each other to form the shape memory alloy element  80 , comprise electric contacts. When acted upon by an electric current, the foils  130  are heated within their bodies, whereby the “memory” of the shape memory alloy element  80  is activated: the latter assumes the shape imposed on it. 
       FIG. 1   b  shows a cross-section of the micro valve  10  corresponding to FIG. la but in the opened condition. Here, the shape memory alloy element  80  is illustrated in a position which is deflected to a somewhat lesser extent, a position which will be assumed by it in case of a temperature increase/current flow. The shape memory alloy element  80  contracts, along with the spring  70  being loaded to a further extent. As a result, the transmission element  90  and the membrane  50  open the valve seat  40 . 
     In order to shorten the switching times of the micro valve  10 , an active or passive cooling element, in particular a micro-type Peltier element is employed in a favorable embodiment. 
       FIG. 2   a  shows a cross-section of a further embodiment of the micro valve  10  in the closed condition. This micro valve comprises all the components of the valve illustrated in  FIG. 1   a , but with the parts arranged differently. The shape memory alloy element  80  is supported by the support  110  on a side of the drive unit housing  20 , the spring  70  is supported by the support  120  arranged on the opposite side of the drive unit housing  20 . The spring  70  and the shape memory alloy element  80  have their other end resting on opposite sides of the transmission element  90 . The spring  70  engages the foil  130  which is next to the support  120  and compresses the shape memory alloy element  80  to assume a deflected shape. This arrangement has the advantage that the transmission element  90  rests on the shape memory alloy element  80 ; this is why it is not necessarily required (regardless of the pressure conditions of the fluid) that these two components are firmly connected to each other, since the transmission element  90  follows the motion of the actuator. 
       FIG. 2   b  illustrates a cross-section of the micro valve  10  from  FIG. 2   a  in the open condition. The shape memory alloy element  80  is shown here in its less deflected position, which it assumes when acted upon by an electric current. At an elevated temperature, the shape memory alloy element  80  expands by “memory” to the shape imparted to it, the spring  70  being loaded to a further extent. As a result, the transmission element  90  and the membrane  50  open the valve seat  40 . 
       FIG. 3   a  shows a shape memory alloy element  80  which is assembled from foils  130  stacked upon each other and comprises the central passage  140 . Electrical contacts  150  are attached to the external foils  130  on opposite sides. The position of the electrical contacts  150 , however, can be selected and adapted corresponding to the installation situation in the valve  10 . The foils  130  have an elliptical shape. In each of the pairs of foils, the foils  130  are connected to each other at their outer circumference at least in sections. Neighboring pairs of foils stacked upon each other are connected to each other at the circumference of the central passage  140 ; in particular, they are welded. In the embodiment according to  FIG. 3   a , the central passage  140  is formed in a circular shape. The central passage  140  may also have another geometry, as is shown in  FIG. 3   b , for instance. 
     In  FIG. 3   b , the shape memory alloy element  80  is constructed from circular foils  130 . The central passage  140  is likewise configured so as to be circular, but comprises additional radial recesses  160  extending from the center towards the outside. Here, essentially triangular foil sections  170  remain which each have a corner  180  pointing towards the central passage  140 . Neighboring pairs of foils are connected to each other at these corners  180 . 
       FIGS. 4   a  to  4   j  exemplarily show various embodiments of the foils  130  with the central passage  140 . The wide variety of design possibilities is visible here. 
       FIG. 5  shows an implementation of the drive unit of the micro valve, as it is known in its basic construction from  FIGS. 1   a  and  1   b . For the components which are known from the embodiments of  FIGS. 1   a  and  1   b , the same reference numerals are used and reference is made to the above explanations. 
     Unlike the embodiment of  FIGS. 1   a  and  1   b , the shape memory alloy element  80  in the configuration of  FIG. 5  is realized to be closed on the side facing the transmission element  90 . Thus, the spring  70  is supported by the inner side of the lowermost foil  130  which in turn rests against the transmission element  90 . 
     In one example, the external dimensions of the micro valve are approximately 5 mm×5 mm×5 mm. 
     Although an embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.