Abstract:
A method is disclosed of fabricating a mirror for use in limited rotation motor systems, said method comprising the steps of providing a mirror structure including at least one wall section, and exposing the at least one wall section to a fluid etching agent to thereby provide chemical milling of the mirror structure.

Description:
PRIORITY 
       [0001]    The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/513,274 filed Jul. 29, 2011, the disclosure of which is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present invention generally relates to the fabrication of optical components in limited rotation motor systems, and relates in particular, to the fabrication of such components having high stiffness and low inertia. 
         [0003]    Scanning mirror systems generally include a mirror surface that is either mounted on a substrate or is formed as part of a substrate, wherein the substrate is coupled to a shaft of a motor system. Such motors may either run continuously (e.g., for use with a polygonal mirror) or such motors may be limited rotation motors providing movement within a limited angular range. These applications require that the mirror be as stiff as possible consistent with a minimum of inertia as they are accelerated back and forth over the limited angular range. 
         [0004]    In typical limited rotation scanning systems (also called galvanometer systems) a mirror is mounted to the output shaft of a limited rotation motor and the limited rotation motor is controlled by a control loop that seeks to cause the rotor of the motor, and therefore the mirror, to follow a position and velocity command waveform with arbitrarily high fidelity. 
         [0005]    There are limits, however, on the fidelity with which the system may follow the command. For example, the acceleration of the mirror within the system is limited by the rate of rise of current in the motor windings. The positional precision is limited by the signal to noise ratio of the feedback method. The bandwidth of the system (which is its ability to move from position A to position B at a desired high velocity and to then settle at position B precisely in the shortest possible time), is limited primarily by vibrations in the moving parts. The bandwidth of the system will nominally be ½ the first torsional resonance in the moving structure. 
         [0006]    It is customary, therefore, to make the moving parts as stiff as possible within the constraints of the allowable system inertia. Since the torque required of the motor to reach a specified acceleration is directly proportional to the inertia and is proportional to the current (whose rate of rise is limited as noted above), it is often the case that when the system parameters are optimized for a particular inertia, some component, typically the mirror, even when made of a very high stiffness-to-inertia material, is not as stiff as is required to reach system bandwidth goals. In this case, extra material is added to the mirror to increase its stiffness, but, at the cost of additional inertia, requiring a larger, more expensive motor as well as a control loop that is capable of driving the additional inertia. 
         [0007]    There is a need therefore, for a limited rotation motor system that provides improved bandwidth without requiring a larger, more expensive motor and accompanying control system. 
       SUMMARY 
       [0008]    In accordance with an embodiment, the invention provides a method of fabricating a mirror for use in limited rotation motor systems. The includes the steps of providing a mirror structure including at least one wall section, and exposing the at least one wall section to a fluid etching agent to thereby provide chemical milling of the mirror structure. 
         [0009]    In accordance with another embodiment, the invention provides a mirror for use in a limited rotation motor system, and the mirror includes a backing structure opposite a front of the mirror. The backing structure includes at least one wall section having a tapered shape that tapers as the wall extends away from a front of the mirror. 
         [0010]    In accordance with a further embodiment, the invention provides a mirror for use in a limited rotation motor system that includes a backing structure opposite a front of the mirror, and the backing structure includes features that provide rigidity to the mirror and that have thicknesses that become reduced in a direction away from a front of the mirror. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The following description may be further understood with reference to the accompanying drawings in which: 
           [0012]      FIG. 1  shows an illustrated diagrammatic enlarged view of a portion of a mirror substrate to be processed in accordance with the present invention; 
           [0013]      FIG. 2  shows an illustrated diagrammatic enlarged view of a portion of a mirror substrate following processing in accordance an embodiment of the present invention; 
           [0014]      FIG. 3  shows an illustrative diagrammatic front view of a mirror substrate to be processed in accordance with an embodiment of the present invention; 
           [0015]      FIG. 4  shows an illustrative diagrammatic top view of the mirror substrate shown in  FIG. 3  taken along line  4 - 4  thereof; 
           [0016]      FIG. 5  shows an illustrative diagrammatic side sectional view of the mirror substrate shown in  FIG. 3  taken along line  5 - 5  thereof; 
           [0017]      FIG. 6  shows an illustrative diagrammatic back view of the mirror substrate shown in  FIG. 3 ; 
           [0018]      FIG. 7  shows an illustrative diagrammatic enlarged view of a portion of the back of the mirror substrate shown in  FIG. 6 ; 
           [0019]      FIGS. 8A-8C  show illustrative diagrammatic views of a portion of a mirror substrate following processing in accordance an embodiment of the present invention; 
           [0020]      FIG. 9  shows an illustrative diagrammatic back view of a mirror substrate that has been processed in accordance with an embodiment of the present invention; 
           [0021]      FIG. 10  shows an illustrative diagrammatic enlarged view of a portion of the back of the mirror substrate shown in  FIG. 8 ; 
           [0022]      FIG. 11  an illustrative diagrammatic side view of the mirror substrate shown in  FIG. 9  taken along line  11 - 11  thereof; 
           [0023]      FIG. 12  shows an illustrative diagrammatic isometric view of the mirror substrate of  FIG. 8 ; and 
           [0024]      FIG. 13  shows an illustrative diagrammatic view of a limited rotation motor system using an optical element that includes a mirror substrate of the present invention. 
       
    
    
       [0025]    The drawings are shown for illustrative purposes only. 
       DETAILED DESCRIPTION 
       [0026]    Certain high stiffness and low inertia materials are known to be used for making moving (or dynamic) mirrors that move during use, such as mirrors in limited rotation motor systems. It is desired that such mirrors (ideally) have infinite stiffness and zero inertia, and beryllium for example, provides an excellent choice given its high stiffness and low mass. 
         [0027]    It is also known that mirror structures may be machined to further reduce the mass of the mirror, specifically on the back of the mirror and near the edges furthest from the axis of rotation of the mirror. This machining is designed to further reduce the mass of the mirror while not significantly reducing the stiffness of the mirror. In this regard, it is desired to machine the mirror in such a way that walls remain to provide stiffness (e.g., in a honeycomb pattern) that are as thin as possible. 
         [0028]    While materials used for mirror structures have very low specific inertia (gm-cm 2  per unit area), it is difficult to machine certain low inertia materials such as beryllium to very fine thicknesses without cracking. Beryllium is also expensive to machine and produces a dust that is hazardous. Machine tooling is generally unique to specific materials, requiring various speeds, feeds, lubricants, coolants, tool geometries materials and coatings. It is therefore desirable to increase the manufacturing speed of beryllium mirrors and at the same time overcome the limitations of the known machining art. 
         [0029]      FIG. 1  shows at  10  a beryllium mirror substrate, including wall sections  12  and a floor  14  that remain following material removal. As is know in the art, a computer file representing a solid model may be converted directly into machining instructions on a machine tool that mills the part in plan, drills the longitudinal hole, and performs such secondary operations such as drilling and tapping holes as may be required while still a section of a surface of the parent beryllium block. 
         [0030]    The substrate  10  may be further processed by removing it from the parent material block on which it is milled by means of a sawing-off process such as wire EDM (Electrical Discharge Machining) or electro chemical sawing. The separated substrate is then finished typically in accordance with desired tolerances. It may then conventionally be used as-is, or further processed by plating, vacuum coating, or both. 
         [0031]    The present invention is directed to a process for generating the substrate itself. As will be obvious to those skilled in the machining art, the production of a single substratum on a face of a parent block or the production of a multiplicity of substrata on one or more faces of a parent block or in fact the production of a single substratum from a single near-net-shape block all share the same issues and therefore these variants do not depart from the spirit and scope of the invention. As will also be obvious to those skilled in the art, the exact size and shape of the substratum and the precise configuration of the stiffening structure on the back of the mirror are variants which do not depart from the spirit and scope of the invention. 
         [0032]    One of the difficulties in machining beryllium is that the surface develops cracking as a result of the machining forces and the heat developed. With care, using very sharp tools, flood coolant, and spindle speeds in milling under 10,000 RPM these cracks are restricted to the top 10 microns or so of the surface. Even so, they must be removed after machining and before use because otherwise they tend to grow in length and depth, particularly if the part is stressed as during acceleration, until they meet in the interior of the part and cause rupture. 
         [0033]    Unfortunately, when the section thickness of the beryllium is severely reduced (as it must be in order to produce the low inertia desired in mirror substrates) at some reduced thickness, section bending takes place during machining. This bending causes deeper cracking. As a result, the minimum section thickness practical has been approximately 0.5 mm in structures of the scale of mirror substrates as shown at d 1  in  FIG. 1 . This section thickness then essentially puts a lower limit on the inertia that can be achieved. 
         [0034]    The surface cracking caused during machining such as milling may be removed by immersing the clean part after machining in an etchant solution such as 60% concentration hydrofluoric acid (HF) 1 part and 69% concentration nitric acid (HNO 3 ) 9 parts or other suitable etchant. The material removal rate at 20 C+/−5 C is about 18 microns per minute per surface exposed to the etchant. Although this may seem slow compared with typical milling tooth loading of 5 microns at a spindle speed of 10000 RPM, the milling takes place over a single line contact somewhere on the part, whereas the etching takes place simultaneously over the entire exposed surface of the part, and is therefore much faster. Because of the linear relationship between material removed and immersion time, agitation bath composition and temperature remaining constant, it is appropriate for the desired metal removal depth to be controlled by the time the part is immersed in the bath. 
         [0035]    In accordance with an embodiment of the invention, a beryllium mirror structure (such as shown in  FIG. 1 ) may remain in the etching bath for an extended period of time of, for example, 5 to 6 minutes. Following such a treatment, sections of the beryllium structure become etched to much smaller dimensions as shown at  20  in  FIG. 2 . The etched walls are shown at  22  and the etched floor is shown at  24 . The thickness of the floor section, for example, may be reduced from d 2  (e.g., about 0.5 mm) to d 4  (e.g., about 0.4 mm). Further, the wall sections become tapered, having triangular cross-sectional shapes. The thickness of the wall sections, for example, may be reduced from d 1  (again e.g., about 0.5 mm) to walls having a variable thickness that is for example., about 0.25 mm at the thickest part (d 3 ) down to possibly zero at the top. This has the desirable effect of further reducing the mass of the ribbing and therefore the inertia of the part, particularly since the more mass is removed further from the axis of rotation of the mirror. 
         [0036]    Mirrors formed of such a process were tested and found to be exceptionally low in specific inertia. During further development of the process, it was found that the ribs could be reliably reduced to a line at the top without reducing their height (stiffness) simply by controlling the immersion time. The triangular shaping of the rib cross-section is attributed to partial exhaustion of the etchant inside the closed cells. 
         [0037]    Although the removal of material from the reflective face of the substrate reduces its inertia, it also reduces its stiffness: however, a minimum stiffness is required in order to support the forces produced during polishing of the reflective surface. In keeping with the requirement that the reflective surface be flat to ¼ wavelength or better at the wavelength of use, it follows then that the required minimum section thickness (the inverse of stiffness first order therefore inertia all other things remaining constant) will vary with the intended wavelength of use. Conventionally, it was necessary to machine a mirror substratum to a section thickness in inverse proportion to the intended wavelength of use (¼ of a shorter wavelength is a smaller absolute allowed departure from flatness, and so requires a stiffer substrate). 
         [0038]      FIGS. 3-7  show a beryllium mirror structure  30  having a front side  32  that provides a highly reflective surface and a back side  34 . The opening  33  may include mirror damping material as disclosed, for example, in U.S. Patent Application Publication No. 2010/0271679, the disclosure of which is hereby incorporated by reference in its entirety. As shown at  36  in  FIGS. 6 and 7 , the back side  34  is machined to removed beryllium material in a honeycomb pattern from areas  36 , leaving wall sections  38 . 
         [0039]    With reference to  FIGS. 9-12 , following chemical treatment in the fluid etching agent (as shown in  FIGS. 8A-8C ), the wall sections  46  and the floor  48  of the mirror structure  40  are etched to provide tapered shapes as shown in  FIGS. 2  (at  22  and  24  respectively). The wall sections, in particular, have a shape that tapers as the wall extends away from the axis of rotation of the mirror. The front side  44  of the mirror  40  remains unetched due to masking. 
         [0040]    Because the etching process is fast and un-attended, the present invention is very economical compared with the machining of discrete individual section thicknesses, so that even if in the future a method is found which allows the direct machining of thinner sections than those now possible, processes of the invention will continue to provide a more economical approach to the production of very thin very low inertia mirrors. In practice therefore, a single rather thick substratum is milled or otherwise machined in multiple units, and the individual units are then processed using etching to the final dimensions required for a particular wavelength or wavelength interval of use. 
         [0041]    In accordance with various embodiments of the invention, therefore, a section of beryllium mirror stiffening ribs and/or a face may be reduced to a desired thickness by etching, and/or by masking the exposed mirror face to effectively deepen the ribs without causing cracking in the reflecting face. In accordance with further embodiments, a rate of mechanical agitation may be controlled to control the partial exhaustion of the etchant so that triangular cross-sections thinner at the open end are produced. In accordance with further embodiments, selected areas of the mirror substrate may be masked prior to etching to prevent etching in those areas, and in further embodiments, the etching substrata may be machined to a uniform over-size section thickness to a variable final desired section thickness. 
         [0042]    As further shown in  FIG. 12 , in a limited rotation motor system  50 , the optical element  52  (e.g., the mirror  40 ) is coupled to a limited rotation motor  54  via mirror mounting structure  56  (e.g., via a clamp, threaded mounting structure and/or a tapered mounting structure as disclosed for example in U.S. Pat. No. 7,212,325, the disclosure of which is hereby incorporated by reference in its entirety) for rotation about the motor rotor axis A R . The system  50  also includes a position transducer  58  that is coupled to a feedback control system  60  that provides a command signal  62  to the motor  54  responsive to an input command signal from an input node  64  and a feedback signal  66  from the position transducer  58  to control the speed and/or position of the motor shaft, and therefore the optical element  52 . 
         [0043]    The feedback control system is used to cause the rotor of the motor, and therefore the mirror, to follow a position and velocity command waveform with arbitrarily high fidelity. There are limits however, on the fidelity with which the system may follow the input command signal. The acceleration of the mirror in the system for example, is limited by the rate of rise of current in the motor windings, and the positional precision is limited by the signal to noise ratio of the feedback system. The ability of the system to move the mirror from a position A to a position B at a desired high velocity and to then settle at position B precisely in the shortest time (the bandwidth of the system) is limited primarily by vibrations in the moving parts. Providing a mirror substrate in accordance with the invention advantageously permits the mirror to be very high in stiffness yet low in inertia. 
         [0044]    Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the invention.