Abstract:
A linear positioning apparatus includes an intermediate portion having an axis, and first and second end portions mounted with flexure legs thereto. The flexure legs accommodate motion of the intermediate portion relative to the end portions along the axis, but inhibit motion of the intermediate portion relative to the end portions in directions not parallel to the axis. The apparatus can accommodate forces having off-axis components, and produce motion that comprises substantially no off-axis component. The apparatus is useful in, for example, optical systems where precise linear motion is required.

Description:
TECHNICAL FIELD  
         [0001]    The present invention pertains generally to linear positioning, and more specifically to devices that translate applied forces into motion substantially along an axis. The present invention also relates to precision positioning devices useful in optical systems.  
         BACKGROUND OF THE INVENTION  
         [0002]    Linear actuators are utilized for tasks where a linear movement or application of a force is desired. Generally, a linear actuator translates a first element linearly with respect to a second element. Often the second element is fixed with respect to a frame of reference. Linear actuators are utilized in a wide-variety of applications, such as assembly-line processes in which precise linear displacements and reciprocating motions must be generated and maintained. Linear actuators are also utilized in numerous optical systems, such as auto-focus cameras with positioning lenses and laboratory analysis devices, such as interferometers.  
           [0003]    An interferometer is an instrument that provides a means of spectral discrimination by way of a precision splitting and recombination of a beam of light. The interferometer achieves this discrimination by varying the pathlength in one half of the beam with respect to the other, and using the resulting interference of the two beams to derive the intensity distribution of wavelengths within the beam.  
           [0004]    The mechanism used to vary the pathlength of the variable path portion of the beam must provide repeatable and linear motion in order to preserve the phase and spatial relationship between the two beams as a function of time. The better this mechanism performs, (i.e. the straighter and smoother the motion of the moving mirror), the better the resulting information that can be obtained from the instrument. Thus, an improvement to the efficiency and expense of an interferometer&#39;s linear positioning system would be a welcome advance.  
           [0005]    Improved linear actuators are advantageous other systems as well, including generally the adjustment, calibration, pointing, focusing and the like, of various technical or scientific instruments including spectrometers and telescopes.  
           [0006]    The majority of motion-positioning mechanisms utilized in conventional linear actuators are configured from one of three technologies: ball bearings, roller bearings and dovetails. Such technologies provide advantages such as high load capability, and long travel. They all, however, provide varying degrees of friction and stiction, which are undesirable in systems and devices where precise movement over very short distances is required. The use of ball bearings, roller bearings and/or dovetails, for example, can cause wobble, hysteresis, backlash, and an uncertainty in reproducibility, which can all limit their practical usefulness.  
           [0007]    Flexures have also been utilized to implement linear actuators. For examples, flexures have been utilized with auto-focus cameras for the positioning of associated lenses. In general, a flexure is a frictionless, stictionless component that relies upon the elastic deformation (i.e., flexing) of a solid material. Sliding and rolling can thus be eliminated from the design of flexure-based linear actuators. A flexure component or mechanism is generally limited to applications where the required travel is typically no more than 10-15% of the major dimension of the device or system in question. In addition to a lack of internal friction, flexure devices also provide a high stiffness, a high load capacity, and a high resistance to shock. Flexures also exhibit a low sensitivity to vibration. Therefore, because of the frictionless, stictionless nature of a flexure-based positioner, a high degree of vibration can be tolerated. Also, because of the stiffness of a flexure design maintaining a specific position can be greatly enhanced.  
           [0008]    The present inventors thus recognize, based on the foregoing, that a need exists for an improved linear actuator for use in devices requiring the precise movement of components and objects. The present inventors have concluded that improvements over conventional and traditional linear actuator devices and methodologies can be achieved through the implementation of an improved flexure-based apparatus and methodologies thereof, as will be further disclosed herein.  
         BRIEF SUMMARY OF THE INVENTION  
         [0009]    The following summary of the invention is provided to facilitate an understanding of some of the innovative features unique to the present invention, and is not intended to be a full description. A full appreciation of the various aspects of the invention can be gained by taking the entire specification, claims, and drawings as a whole.  
           [0010]    A linear positioning apparatus according to the present invention can include an intermediate portion and two end portions. The end portions mount with the intermediate portions with flexure legs, where a flexure leg is configured so that motion of the intermediate portion relative to the end portions along an axis is facilitated while motion not parallel to the axis is inhibited. For example, a flexure leg can mount with an end portion at a first attachment point and with the intermediate portion at a second attachment point. The first and second attachment points can be located such that a line drawn between them is not parallel to the axis. Motion along the axis can thereby be accommodated by flexure of the flexure leg. The resistance of the flexure leg, or of a combination of flexure legs, to flexure along directions not parallel to the axis can be greater than the resistance to flexure parallel to the axis, inhibiting off-axis relative motion.  
           [0011]    Each end portion can mount with the intermediate portion with a plurality of flexure legs, providing greater resistance to off-axis motion. Three flexure legs can provide a balance of complexity and performance for some applications. Also, each flexure leg can have a stiffness parallel to the axis that is less than its stiffness in direction non-parallel to the axis, for example, by having a thicker cross-section normal to the axis than parallel to the axis. Specific regions of a flexure leg, for example regions near the attachment points, can have an even further reduced stiffness (e.g., thickness parallel to the axis) to further ease on-axis relative motion.  
           [0012]    In some embodiments, the end portions and intermediate portion can be coaxial. In some embodiments, the end portion and intermediate portions have circular cross-sections normal to the axis. Many different combinations of materials and dimensions can be used in accordance with the present invention. Travel distance required, limitations on input force available/desirable, off-axis stiffness required, overall size/space requirements, the weight of the moving load and ease of fabrication can influence the design choices. The context of a given application can determine the specific design choices, and even then there will be trade-offs between variables. Aluminum can be suitable for applications with small required travel due to its machinability and relatively low cost, but might not be ideal if larger deformations are required due to its poorer long-term fatigue resistance under larger stresses. Steel and more exotic metal alloys can be used to enhance fatigue resistance and achieve longer travels but might require higher motive forces due to their higher modulus of elasticity and might be more expensive to machine. A molded plastic part can also be used, and might be substantially less expensive, though it would might suffer from poorer off-axis stiffness and fatigue characteristics. The dimensions can be chosen to accommodate the intended application, such as requirements of optical components, required travel and frequency of reciprocation. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    The accompanying figures, in which like reference numerals refer to identical or functionally-similar elements throughout the separate views and which are incorporated in and form part of the specification, further illustrate the present invention and, together with the detailed description of the invention, serve to explain the principles of the present invention.  
         [0014]    [0014]FIG. 1 is a perspective view of a linear positioning apparatus, which can be implemented in accordance with the present invention;  
         [0015]    [0015]FIG. 2 is two perspective views of a flexure web, which can be implemented in accordance with the present invention;  
         [0016]    [0016]FIG. 3 depicts two perspective views of another flexure web, which can be implemented in accordance with the present invention; and  
         [0017]    [0017]FIG. 4 is a high-level flow chart of operations depicting logical operational steps, which can be followed to configure a linear positioning apparatus, in accordance with the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    The particular values and configurations discussed in these non-limiting examples can be varied and are cited merely to illustrate embodiments of the present invention and are not intended to limit the scope of the invention.  
         [0019]    [0019]FIG. 1 is a perspective view of a linear positioning apparatus  100  in accordance with the present invention. Linear positioning apparatus  100  can be configured with a positioning element having material and dimensional properties such that when a force is applied at an axis  112  of linear positioning apparatus  100 , it deforms in a manner that mechanically translates an object attached thereto in a straight path for a distance responsive to the applied force.  
         [0020]    Linear positioning apparatus  100  is shown generally cylindrical in shape and includes a plurality of flexure legs mounted with end portions  102  and  104 . An intermediate portion  110  mounts with the flexure legs and thereby with end sections  102  and  104 . For example, end section  104  mounts with one or more flexure legs  108  (three are shown in the figure) spaced substantially symmetrically about the axis. Likewise, end section  102  mounts with one or more flexure legs  118  (three are shown in the figure) spaced substantially symmetrically about the axis. Flexure legs  108 ,  118  can have a greater thickness measured perpendicular to the axis than their thickness measured parallel to the axis, providing reduced stiffness parallel to the axis and consequently allowing low resistance to motion parallel to the axis. Also, flexure legs  108 ,  118  can have reduced thickness parallel to the axis in selected regions  106 ,  116 , allowing even lower resistance to motion parallel to the axis.  
         [0021]    By arranging flexure legs  108  and  118  in this manner, the force required to translate center section  110  with respect to end sections  104  and  102  along the axis is low relative to the force required to translate center section  110  in any other direction with respect to end sections  104  and  102 . In this manner, a single mechanical element (i.e., center section  110 ) tends to travel in a straight path parallel to the axis  112  of the linear positioning apparatus  100  even when subjected to forces that are not parallel to the axis.  
         [0022]    Note the symmetry of the flexure-based element (e.g., center section  110  of FIG. 1) about its mid-plane. In the illustrative example described herein with respect to FIG. 1, the mid-plane of the flexure element coincides with the plane of symmetry in the device or unit (e.g., a spectrometer, interferometer, etc.) of which it is a part. This symmetry can enhance the resistance of the overall system to deformation under thermal loads because variations in the dimensions of the element due to thermal expansion tend to balance about the plane of symmetry, thereby minimizing misalignments of other elements in that plane.  
         [0023]    [0023]FIG. 2 illustrates two perspective views of specific embodiments of a flexure leg  200  according to the present invention. FIG. 2 depicts both an angular perspective view  202  and a side perspective view  204  of flexure leg  200 . Flexure  200  includes a mid portion  214  and two end portions  210  and  212 . A flex portion  206  is shown located between mid potion  214  and end portion  210 , while a flex portion  208  is configured between mid portion  214  and end portion  212 . In use, end portions  212 ,  210  of flexure leg  200  would mount with an end portion and the intermediate portion of the apparatus such that the thicker cross-sectional dimension was substantially normal to the axis of the apparatus. The thicker cross-section, and concordant greater stiffness, would consequently oppose a component of motion of the intermediate portion relative to the end portions that is normal to the axis. In contrast, the thinner cross-section, further reduced by the flex portions  206 ,  208 , and concordant reduced stiffness, would accommodate a component of motion parallel to the axis of the apparatus.  
         [0024]    [0024]FIG. 3 illustrates two perspective views of specific embodiments of a flexure leg  300  according to the present invention. FIG. 3 depicts both an angular perspective view  302  and a side perspective view  304  of flexure leg  300 . Flexure  300  includes a mid portion  314  and two end portions  310  and  312 . A flex portion  306  is shown located between mid potion  314  and end portion  310 , while a flex portion  308  is configured between mid portion  314  and end portion  312 . In use, end portions  312 ,  310  of flexure leg  300  would mount with an end portion and the intermediate portion of the apparatus such that the thicker cross-sectional dimension was substantially normal to the axis of the apparatus. The thicker cross-section, and concordant greater stiffness, would consequently oppose a component of motion of the intermediate portion relative to the end portions that is normal to the axis. In contrast, the thinner cross-section, further reduced by the flex portions  306 ,  308 , and concordant reduced stiffness, would accommodate a component of motion parallel to the axis of the apparatus. In contrast to the flexure leg in FIG. 2, where the reduced cross-sections in the flex portions were oriented substantially radially outward from the apparatus axis, in FIG. 3 the reduced cross-sections in the flex portions are oriented substantially parallel to each other approximately centered on the apparatus axis. Such parallel orientation can ease the fabrication of the apparatus, for example by allowing a single machining orientation for both flex portions. It can also provide different flexure leg stiffness characteristics, and accordingly different apparatus performance characteristics.  
         [0025]    An apparatus according to the present invention can be made by fabricating the various portions separately, and then attaching them appropriately. Alternatively, an apparatus can be made by machining a solid piece, removing material to form the flexure legs which separate the intermediate portion from the end portions. Combinations of these two methods can also be used, for example by forming the intermediate portion and flexure legs from a single piece, then attaching separately-formed end portions. The various portions can be made from a wide range of materials and with a wide range of dimensions, depending on, for example, the desired cost, fabrication time, stiffness properties, durability, thermal response, resistance to off-axis motion, resistance to on-axis motion, etc.  
         [0026]    A positioning apparatus according to the present invention can be configured to translate applied force to translation substantially along the apparatus&#39;s axis. FIG. 4 is a schematic of such a system. An intermediate portion  410  mounts with flexure legs to two end portions  402 ,  404 . End portions  402 ,  404  are fixedly mounted to a reference, in the figure depicted as ground G. A force F can be applied to intermediate portion  410 ; the force can comprise on-axis and off-axis components. The flexure legs flex in response to force F, allowing motion of intermediate portion  410  along the axis. The flexure legs&#39; resistance to flexure normal to the axis, however, inhibits motion of intermediate portion  410  in any other direction. The roles of the end portions and intermediate portions can be exchanged: fixing the intermediate portion to a reference, mechanically connecting the end portions, and applying a force to the end portions.  
         [0027]    The present invention can be utilized in association with numerous applications where straight-line travel over short distance is generally required, including those applications where long-term stability and repeatability demands are stringent. Non-optical applications of the present invention include micro-positioning of components during assembly of miniature instrumentation or anywhere that small linear displacements or reciprocating motions must be generated and maintained. The manufacture of discrete components in sophisticated fabrication facilities utilizing robotic arms and their associated movement, for example, can be enhanced by application of the present invention to various actuating elements thereof.  
         [0028]    Optical applications of the present invention can include non-contact surface analysis, where a stylus or sensor must be maintained at a fixed small distance from a sample, microscope sample micro positioning or steering mirrors in auto-alignment systems. For example, the invention disclosed herein can be utilized for positioning the moving mirror of a spectrometer. One or more flexure elements thereof can be machined of, for example, aluminum and then mounted within a housing that fixes the end sections (e.g., end sections  104  and  102  of FIG. 1) and allow deformation (i.e., flexure) of a center section (e.g., center section  110 ) on-axis by means of a voice coil actuator. A cube-corner retroreflector or another reflecting device can be mounted at a center section and scanned over a travel time of, for example ±0.15 mm in a reciprocating motion at a rate of approximately 2 Hz. The straight-line travel requirements of a spectrometer can be quite demanding (i.e., on the order of 1 micron tolerances), thus the invention described herein can provide adequate stability to allow for effective, stable scanning during operation of the entire spectrometer system. The present invention can also be utilized in association with many near-infrared spectroscopy and multi-variate analysis devices and systems thereof.  
         [0029]    Many different combinations of materials and dimensions can be used in accordance with the present invention. Travel distance required, limitations on input force available/desirable, off-axis stiffness required, overall size/space requirements, the weight of the moving load and ease of fabrication can influence the design choices. The context of a given application can determine the specific design choices, and even then there will be trade-offs between variables. Aluminum can be suitable for applications with small required travel due to its machinability and relatively low cost, but might not be ideal if larger deformations are required due to its poorer long-term fatigue resistance under larger stresses. Steel, other metals, and metal alloys can be used to enhance fatigue resistance and achieve longer travels but might require higher motive forces due to their higher modulus of elasticity and might be more expensive to machine. A molded plastic part can also be used, and might be substantially less expensive, though it might suffer from poorer off-axis stiffness and fatigue characteristics. The dimensions can be chosen to accommodate the intended application, such as requirements of optical components, required travel and frequency of reciprocation.  
         [0030]    The embodiments and examples set forth herein are presented to explain the present invention and its practical application and to thereby enable those skilled in the art to make and utilize the invention. Those skilled in the art, however, will recognize that the foregoing description and examples have been presented for the purpose of illustration and example only. Other variations and modifications of the present invention will be apparent to those of skill in the art, and it is the intent of the appended claims that such variations and modifications be covered. The description as set forth is not intended to be exhaustive or to limit the scope of the invention. Modifications and variations are possible in light of the above teaching without departing from the spirit and scope of the following claims. It is contemplated that the use of the present invention can involve components having different characteristics. It is intended that the scope of the present invention be defined by the claims appended hereto, giving full cognizance to equivalents in all respects.