Abstract:
The present invention relates to the design and fabrication of flexures used to guide motion in mechanical systems. A high specific stiffness flexure includes two narrow and thin flexing sections separated by a longer stiffened section. The present invention provides designs and processes for making flexures and flexure systems with monolithic high specific stiffness frame or box structures for the stiffened sections that creates relatively higher self-resonant frequencies.

Description:
BACKGROUND  
       [0001]     The present invention relates to flexures, and more particularly to a high stiffness flexure and a process of making a high stiffness flexure.  
         [0002]     A flexure is a flexible mechanical member connecting two bodies. They may be used as a special bearing or hinge to guide the linear motion of one or both of the bodies, such as a stage. A properly designed flexure is extremely stiff in every direction except the direction of motion. A major benefit of a flexure guided stage is the complete lack of friction, since no part is moving against another. Further, there is no backlash in a flexure stage. Most flexure systems are designed to guide motion linearly, although rotary flexures also exist. A flexure system can be constructed to be stiff or compliant in any number of allowed axes. A simple linear flexure is a strip of metal, or other strong material, that is securely attached at one point to one body and securely attached to a second body at another point. In a well designed flexure, the strip is made rigid for most of its length, but is weakened so that it can bend in short segments next to the attachment points.  
         [0003]     An inherent limitation of a flexure is its limited motion. Since a flexure is actually a bending beam, its motion is limited by its limited flexural strength. Increased range of motion is allowed by increasing the length of the flexure, but this compromises other qualities of the flexure. Increasing the length of the bending segments also makes the flexure less stiff in the other axes of motion. Increasing the overall length of the flexure increases the mass of the rigid section of the flexure between the two short bending segments. The rigid section between the two bending segments forms a spring-mass system which has resonances. Resonances are undesirable, but higher resonant frequencies are preferable to low resonant frequencies.  
         [0004]     The ways that flexures are usually formed can easily lead to a resonant frequency that is low enough to be a real problem or performance limitation in a motion system. Currently, there are two primary methods of creating a flexure. One method, an additive process, shown in  FIG. 1A , is to start with a relatively thin strip of flexible material  1  and add rigid strips  2  and  3  in the central section leaving thin bending segments  4  and  5  near each end  6  and  7 . Another method, a subtractive process, shown in  FIG. 1B , is to start with a relatively thick bar  8  of material and machine notches or slots  9  and  10  near each of the two ends  11  and  12  to form the bending segments with the relatively rigid original bar in between. There are advantages and disadvantages to both approaches. The subtractive process is relatively more expensive, can make a stiffer flexure, and has no joints. The additive process can be less expensive, the bending elements and the central stiffener can be made of different materials, but it has mechanical joints bonding the parts, which can create additional vibration problems. In both cases, the central stiffener is typically solid which makes it stiff but it is also typically massive so that it has a relatively low specific stiffness (i.e., stiffness to mass ratio), which produces a low self-resonant frequency.  FIG. 1C  shows a flexure which has a center stiffened section created by bending sides  52  and  53  up 90 degrees, forming a “U” channel, leaving segments  50  and  51  to flex. This design does have a higher specific stiffness than a solid bar, but if the sides are made tall to maximize stiffness the sides become cantilevered masses with an additional low self-resonant frequency of their own. With typical flexure design and fabrication practices it is difficult to create a flexure that is compliant enough to act as a flexure and has a high self-resonant frequency.  
         [0005]     It generally takes a system of flexure elements to create a functional unit that allows for motion primarily in one axis and is stiff in the other axes. Two such systems are shown in  FIG. 2A  and  FIG. 2B . The system in  FIG. 2A  allows bodies  15 ,  16  to move laterally with respect to each other, shown by the arrows. The flexure elements  13 ,  14  constrain the motion so that the bodies remain parallel. The system in  FIG. 2B  couples a rigid core  20  with a rigid outer ring  21  with three flexure elements  17 ,  18 , and  19 . This system constrains bodies  20 ,  21  to move concentrically with respect to each other (i.e., in and out of the page). In each case, the flexure elements are formed as described above and shown in  FIG. 1A  and  FIG. 1B . In such systems, each flexure element has resonances due to its shape and material properties (the same if the flexures are the same), and the system has complicated resonances.  
         [0006]     What is needed is a flexure with a high stiffness. Furthermore, what is needed is a method for constructing a high stiffness flexure and/or flexure system that is composed of flexure elements with high self-resonant frequencies assembled in a way that minimizes the additional problems of a system. The present invention solves these and other problems by providing high stiffness flexure and method of making a high stiffness flexure as describe below.  
       SUMMARY  
       [0007]     The present invention relates to the design and fabrication of flexures used to guide motion in mechanical systems. A high specific stiffness flexure includes two flexing sections separated-by a longer stiffened frame section. The present invention provides designs and processes for making flexures and flexure systems with monolithic high specific stiffness frame or box structures for the stiffened sections that creates relatively higher self-resonant frequencies. The present invention allows for creating short, thin, flexible segments of a high strength material, and creating the longer central frame section of the flexure with a high specific stiffness from a stiff material formed into a light weight but high stiffness geometry, so that the resonant frequencies are all high.  
         [0008]     The following detailed description and accompanying drawings provide a better understanding of the nature and advantages of the present invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     FIGS.  1 A-C show examples of prior art flexures.  
         [0010]     FIGS.  2 A-B shows two typical prior art architectures of flexure systems.  
         [0011]      FIG. 3A  is an isometric view of an example flexure according to one embodiment of the present invention.  
         [0012]      FIG. 3B  is a side view of the flexure in  FIG. 3A .  
         [0013]      FIG. 3C  is a cross section of the flexure in  FIG. 3A .  
         [0014]     FIGS.  4 A-C illustrate an example process of making a high stiffness flexure according to one embodiment of the present invention.  
         [0015]      FIG. 5  is a flexure according to another embodiment of the present invention.  
         [0016]      FIG. 6  is an example of a system using a flexure according to one embodiment of the present invention.  
         [0017]      FIG. 7  is another example of a system using a flexure according to one embodiment of the present invention.  
         [0018]      FIG. 8  illustrates an example of a flexure system and a process of making a high stiffness flexure according to one embodiment of the present invention.  
         [0019]     FIGS.  9 A-B illustrates another method of forming a flexure according to one embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0020]     Described herein are techniques for improving flexures and systems that use flexures. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of different aspects of the present invention. It will be evident, however, to one skilled in the art that the present invention as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include obvious modifications and equivalents of the features and concepts described herein.  
         [0021]      FIG. 3A  is an isometric view of an example flexure according to one embodiment of the present invention. Flexure  300  may be made from a spring material such as stainless steel or Beryllium Copper, for example, which may be plated with Tin. It is to be understood that other materials may also be used depending on the application. Exemplary spring materials typically have a high strength so the material can flex across a large angle without breaking or becoming permanently bent. Flexure  300  includes a central stiffened section  310  comprising a stiffening frame  311 . Flexure  300  further includes flat spring segments  324  and  325  (i.e., flexing segments). Stiffening frame  311  is a structure that maintains the shape and provides support to the central section  310 . Frame  311  may be an open or closed structure adjacent to the flexure base for stiffening the central stiffened section. For example, the cross section of the frame may be a triangle or other shape as described in more detail below. Holes  322  and  323  illustrate example means to mount the flexure between two bodies. However, other mounting configurations may be used. Flat spring segments  324  and  325  are provided between the end holes and the central stiffened section  310 , and provide the primary means of flexing.  
         [0022]      FIG. 3B  is a side view of the flexure in  FIG. 3A . In one specific embodiment, screws  331  and  332  may be used to mount the flexure in a system to rigid bodies  333  and  334 . When the flexure is mounted, rigid bodies  333  and  334  may move up or down relative to one another, causing screw  331  to move up or down relative to screw  332 . Flexing will occur primarily across flexing regions  311  and  312  of the spring segments  324  and  325 , but central stiffened section  310  is designed to be substantially rigid. Flexure  300  may have a thickness t 1 , and flexing regions may flex across regions  311  and  312  having lengths “L 2 ” and “L 3 .” The length of the central stiffened section  310  is “L 1 .” The spring segments of flexure  300  may have lengths “L 4 A” and “LAB.” 
         [0023]      FIG. 3C  is a cross section of the flexure in  FIG. 3A . In this example, central stiffened section  310  includes a flat base  320  and sidewalls  327  and  329 . Base  320  and sidewalls  327  and  329  form a enclosed frame (e.g., a box-type structure) with an interior region  330 . In one embodiment, the frame structure is triangular. However, it is to be understood that other shapes for the frame structure may be used, such as polygons or curves such as semicircles or other arcs. Additionally, a circular flexure may have a concentric circular stiffening frame. Base  320  may have a width W, and the sidewalls  327  and  329  of the triangular cross section may have length L 5  and L 6 .  
         [0024]     FIGS.  4 A-C illustrate an example method of forming a flexure according to one embodiment of the present invention.  FIG. 4A  shows an unfolded view of an individual flexure including flexing segments  424 ,  425  and the mounting holes  422 ,  423 .  FIG. 4C  illustrates a cross section of the integral flexure. As illustrated in  FIG. 4C , sides  427  and  429  may be folded toward each other to meet at the apex  450  of triangular cross section stiffened section  426 . Lap joint portion  428  is included to provide an overlapping joint (i.e., a “lap joint”) with side  427 .  
         [0025]     In one embodiment, the flexure pattern may be cut from a sheet of spring material using a die, laser or EDM. Alternatively, the flexure pattern may be chemically etched from a sheet of spring material. If the flexure pattern is formed from a single sheet of material, there may be no preferential bend lines, unless perforations are added. Thus, the flexure may be formed by a machine.  
         [0026]      FIG. 4B  illustrates a method of forming a flexure according to one embodiment of the present invention. As shown in  FIG. 4B , bend lines are created along folding lines to facilitate folding of the flexure elements. For example, a chemical etching process can provide a partial depth etch along folding lines to produce weakened lines to facilitate folding. If the partial etch it sized properly, a small or moderate size flexure can be readily folded by hand, for example.  
         [0027]     FIGS.  4 A-C illustrate a flexure where the stiffened section is formed integral to the flexure from a single piece of material. This technique is preferable because it creates a monolithic flexure. Alternatively, a high specific stiffness frame structure could be formed separately and bonded to a flat flexure. While similar results would be achieved, superior stiffening elements could be used at the cost of an additional bond between the flat flexure and the stiffener.  
         [0028]     The stiffened section of the flexure can take various forms.  FIGS. 3-4  illustrate flexures with triangular cross sectional stiffened sections. Alternatively, an additional side can be added to the pattern that would form a four-sided box structure. The box structure can take the form of a polygon cross section like a triangle, rectangle or trapezoid, or it could be a smooth curve like an arc. A rectangular cross section body has a higher specific stiffness, and therefore a higher self-resonant frequency, than a similar height triangular cross section body. However, for a given base width and total perimeter length, the triangle has the highest specific stiffness. A triangular cross section provides a closed shape so that unsupported sides won&#39;t resonant. Triangular cross sections are also advantageous because they are self-supporting in that such a structure cannot be folded into a parallelogram then collapse.  
         [0029]     Independent of how the flexure is fabricated and what material the flexure is made from, the stiffening frame structure should be a closed shape so that the stiffened section acts as a single stiff monolithic body. If the frame structure is created by folding, the edges that come together to close the shape should be bonded together. A lap joint may be used as shown in  FIG. 4C , and bonded by welding, brazing or soldering. A overlapping tape could be also be used, such as an adhesive tape with a strong substrate, for example.  
         [0030]     A partial chemical etching process may produce weakened corners where the material was made thin to aid bending. This problem can be solved or reduced a couple of ways. One way is to leave the material its full thickness at the corners of the stiffened section adjacent to the flexing segments where the peeling stresses are concentrated. Another way, not mutually exclusive, is by filling, or partially filling, the stiffened section with a material for increasing the strength of the frame. For example, a low density substance like epoxy or rubber may be used to stabilize the structure without adding much mass. This filler material could also have damping qualities to help damp any resonance that gets excited. Alternatively, for damping, a damping material could be added to outside surfaces of the flexure.  
         [0031]     If Tin plated Beryllium Copper alloy is used for the base spring material, a soldered lapjoint may be used to close the body on the stiffened section as shown in  FIG. 4C . Solder can also be used in the inside corners as buttresses like the addition of epoxy as a filler mentioned previously. If steel is used for the base spring material, spot welding may be used. Beryllium Copper alloy is also non-magnetic unlike steel. Since steel is magnetic, it may interact with magnetic fields in the system, such as in a motor, for example.  
         [0032]      FIG. 5  is a flexure according to another embodiment of the present invention. As illustrated in  FIG. 5 , the specific stiffness of the central stiffened section can be further increased by providing lightening holes in the sides and or the base. This decreases the stiffness but can significantly increase the stiffness to weight ratio, which increases the self-resonance frequency.  FIG. 5  shows an isometric view of an individual flexure with lightening holes  530  in the side.  
         [0033]     Flexures according to embodiments of the present invention may be incorporated into improved flexure systems.  FIG. 6  is an example of a system using a flexure according to one embodiment of the present invention. Flexure system  600  includes bodies  636  and  637  that move laterally with respect to each other, shown by the arrows. Flexure elements  601  and  602  constrain the motion so that the bodies remain parallel. Flexure elements  601  and  602  are comprised of flexing segments  631  and  632  and central stiffened section  633 . Flexure element  601  is attached to bodies  636  and  637  by screws  634  and  635 . Flexure element  602  may be attached to bodies  636  and  637  in the same way.  
         [0034]      FIG. 7  is another example of a system using a flexure according to one embodiment of the present invention. The system in  FIG. 7  includes a rigid core  738  coupled to a rigid outer ring  739  using three flexure elements  742 ,  752  and  762 . Each flexure element comprises flexing segments  740  and  741  and a central stiffened section  742 . This system constrains bodies  738  and  739  to move concentrically with respect to each other (i.e., in and out of the page). The rigid core  738  could also be monolithic with the three flexure elements (i.e., a single piece of material), and the flexure elements may take the form of frames or box structures to achieve high stiffness and low mass as described above. Rigid outer ring  739  may be similarly constructed monolithic with the flexure elements.  
         [0035]      FIG. 8  illustrates an example of a flexure system  800  and process of forming a flexure according to one embodiment of the present invention. A flexure system may include a rigid central core  830  coupled to a plurality of flexures  801 ,  802  and  803 . In the present example, the core  830  is an advantageous triangular shape and three flexures  801 ,  802  and  803  are positioned in parallel with each side of the core. The core may be attached to a first body at the inner flexure segments (e.g., using holes  831 ,  832  and  833  or holes  834 ,  835  and  836 ), and the outer segments of each flexure may be attached to a second body (i.e., using holes  851 ,  852  and  853 ) so that the first and second bodies may move laterally (in and out of the page in  FIG. 8 ) with respect to each other. Additional holes  837 - 838  may optionally be positioned around the core for attaching other elements of the first body or other bodies to the core. In one embodiment, the core includes an opening  890 . The opening may be centered in the triangle to achieve a balanced center of gravity, for example. In one embodiment, both the core  830  and the central opening  890  are triangular (e.g., an equilateral triangle), and the sides of the central opening  891 - 893  are parallel with the sides of the core  851 - 853  and the flexure elements  801 - 803 , respectively. Additionally, each apex of the triangular central opening may be flattened (i.e., each apex of the triangular central opening consists of a flat edge such as edges  854 ,  855  and  856 ) to increase the rigidity of the core.  
         [0036]      FIG. 8  also illustrates another aspect of the present invention. In one embodiment, individual flexures, or in this example the whole flexure system  800 , may be produced from a single sheet of spring material and then formed into a final product, wherein the core, flexures and stiffening frames comprise a single piece of material. For example, in one embodiment a single sheet of spring material is chemically etched or cut into the desired pattern (e.g., a triangle or other shape of the desired core and flexure(s)). The pattern may include stiffening sections  821 - 826  and lap joint sections  841 - 843 , which may be patterned and folded to form triangular frames for stiffening a central section of each flexure across a length L 1 . The stiffening sections  821 - 826  may be bonded to lap joint sections  841 - 843  along the entire length L 1  to form tightly coupled frame units on each flexure with relatively few and relatively high self-resonant frequencies.  
         [0037]     In another embodiment, the core  830  may include stiffening frames as well. For example, a single piece of material may include core stiffening sections  861 - 866 , which may be folded to form triangular stiffening frames across the sides  891 - 893  of opening  890  and the parallel sides  851 - 853  of core  830  after the system has been patterned (e.g., by stamp cutting or etching). In this case, lap joint sections  841 - 843  may be divided into three sections as illustrated by lapjoint sections  841 A,  841 B and  841 C, with the end sections (e.g., lap joint sections  841 A and  841 C) forming lap joints with the stiffening sections (e.g., stiffening section  861 ) and the middle lap joint sections (e.g., lap joint section  841 B) forming lap joints with the core stiffening sections  861 - 866 . Similarly, the region between the flexure core and the inner flexure segments may also include stiffening frames illustrated by sections  897 A,  898 A and  899 A. Sections  897 A,  898 A and  899 A may be included at each corner of the triangle, for example, and folded to stiffen the corner from flexing.  
         [0038]     FIGS.  9 A-B illustrates another method of forming a flexure according to one embodiment of the present invention.  FIG. 9A  illustrates a force F exerted when the flexure is bending. In this case, a downward force is exerted that may tend to cause the stiffening frame  910  to separate from the base of the flexure  911  at the corner (i.e., a peeling stress).  FIG. 9B  illustrates a solution to this potential problem. In  FIG. 9B , bend lines  901  and  902  are set back a distance “d” from the corners. Thus, if the bend lines are formed by a partial etch as described above, or by some other method that tends to weaken the material, the corners of the stiffening frame will be unaffected. Consequently, the flexure will be more resistant to tearing at the corners. In one embodiment, a mask may be used to etch the bend lines, wherein the mask window is set back from the corner created at the intersection of the flexing segment  924  and the frame sides  927  and  928 .  
         [0039]     The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. Furthermore, embodiments of the present invention may be used in many different applications. For example, one of the many possible applications for the present invention is as guidance for a voice coil actuator used as either a positioning stage or a reciprocating pump.  
         [0040]     Thus, the above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. For example the flexure pattern could be cut from a relatively thick sheet material and then the short flexing segments formed by etching the material thinner in those spots. Any necessary bonding could be done by welding or gluing. Other means of attaching to the flexure could be used. Dowel pins or sets of two screws, instead of single screws, could be used to attach the flexure to a body to prevent rotation about a single screw, for example. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations and equivalents will be evident to those skilled in the art and may be employed without departing from the spirit and scope of the invention as defined by the claims.