Abstract:
An apparatus for moving an optical element in an opto-mechanical system, includes at least one circularly symmetric flexure surrounding the optical element, and a device for moving the optical element along an axis perpendicular to and extending through the center of the flexure. A number of flexures may be provided in a stack with spacers therebetween, and the optical element may be mounted in a tubular member supported by two spaced apart stacks. Structure is also provided for damping end of movement vibrations.

Description:
FIELD OF THE INVENTION 
     This invention relates to an apparatus for moving an element in an opto-mechanical system, and in particular for example to an apparatus for moving a lens in a lens focussing operation, for example for use in a wire bonding machine. 
     BACKGROUND OF THE INVENTION 
     Wire bonding machines and other apparatus used in the semiconductor manufacturing and assembly industry, often use a computerised vision system, for example for monitoring processes and for precise and accurate location of the position and orientation of objects such as semiconductor devices. Such vision systems include an opto-mechanical system for, in particular, moving optical elements such as a lens for the purpose of focussing operations. 
     PRIOR ART 
     Flexures are routinely used in opto-mechanical systems for manipulation of optical elements such as a mirror or a lens in focussing and tracking (U.S. Pat. Nos. 4,927,235, 5,208,703, 5,594,820, 6,064,505). Flexures are especially suited for these purposes due to the excellent inherent repeatability of their motion trajectory devoid of friction and wear. In many applications such as, for example, disc drives, a pair of flat parallel flexures similar to the ones shown in FIG. 1, are used. They closely mimic the classic four bar linkage. Thus, while the free end of each flexure moves along an arc, the rigid coupling between the flexures ensures that there is virtually no rotation at the free end. For a small displacement Δx, this approximates linear motion with a tolerably small parasitic motion Δz, perpendicular to the direction of the gross motion, but with virtually no lens tilt. 
     However, in some applications, such as in a computerized vision system of a wire bonding machine, both the orientation and the position of the optical axis of the moving lens have to be maintained within tight tolerances, over the entire range of lens motion. In such cases, parallel flexures mentioned above are not suitable. Flexures with circular symmetry do not suffer from parasitic lateral motion but instead, give rise to a small angular rotation about the motion axis. Such flexures have been used in long life compressors and cryogenic coolers as disclosed in U.S. Pat. Nos. 5,351,490, 5,492,313, 5,522,214, 5,647,217, 5,920,133, 6,050,556 and 6,129,527. Some typical designs are shown in FIG.  2 . These are in the form of flat discs, fractions of a millimeter thick. Each disc has a specified number of slots (usually but not always, three), of either spiral or straight or arc shape or a combination thereof. They are machined using either wire electro-discharge machining or photo-lithography or any other suitable method, yielding a number of flexing “arms” which bear the load of the moving member. Very high ratios of radial stiffness to axial stiffness can be realized using such flexures. 
     Actuation of the mechanism using flexures of circular symmetry is normally effected by a brush-less linear motor using permanent magnets. The linear motor could be either of single phase (commonly called voice coil motor) or of multiphase design, in any of the several possible topologies but most usually cylindrical. While making best use of the available space, leading to a very compact design, the cylindrical voice coil motor keeps the actuating force virtually aligned with the axis of the flexure discs. A position sensor provides position feedback enabling the motor to be operated in closed loop servo mode for very precise control over the axial position of the optical component mounted on the moving member. 
     A problem associated with flexure bearings is the occurrence of self-excited vibrations at the end of the stroke. In the absence of sufficient damping in the system, these vibrations continue for an undesirably long period of time before their attenuation reaches a level acceptable to the specified objective of the motion stage, such as in the present case, moving a lens to focus an image of interest. This considerably slows down the entire process. In order to substantially speed up the attenuation of undesirable vibrations some kind of damping needs to be introduced into the system. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided apparatus for moving an optical element in an opto-mechanical system, comprising at least one circularly symmetric flexure surrounding said optical element, and means for moving said optical element along an axis perpendicular to and extending through the centre of said at least one flexure. 
     Preferably there may be a stack of circularly symmetric flexures, and preferably the flexures in each said stack may be separated by spacer members. Such spacer members may include rim spacers for spacing apart rim portions of said flexures, and central spacers for spacing apart central portions of said spacers. 
     In a particularly preferred embodiment of the invention the optical element is mounted between two stacks of flexures and the axis of movement of the optical element extends perpendicular to and through the centre of each stack. 
     The means for moving the optical element may comprise a voice coil motor. 
     Means may be provided for sensing the position of said optical element and for providing a feedback control to said moving means. The sensing means may comprise a linear variable differential transducer, an optical sensor, a capacitive sensor or an inductive sensor. 
     Another preferred feature of the invention is that the optical element may be supported within a tubular member that moves with the optical element, such that an end of the tubular member is received within a bore formed as part of a fixed component, the bore having an internal diameter slightly greater than the external diameter of the tubular member to define a narrow annular passage therebetween, whereby movement of the optical element may be damped by the movement of air into and out of the bore through the annular passage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in whic: 
     FIG. 1 shows an example of prior art relating to a pair of straight parallel flexures used for tilt free, approximately rectilinear motion, 
     FIG. 2 shows three designs of flexures with circular symmetry, 
     FIG. 3 shows a preferred embodiment of the present invention, 
     FIGS. 4 a  and  4   b  show two views of a flexure stack and FIGS. 4 d  and  4   e  show respectively a rim spacer and a central spacer to be used with the flexure shown in FIG. 4 c , and 
     FIGS. 5 a  and  5   b  show two views of another flexure stack and FIGS. 5 d  and  5   e  show respectively a rim spacer and a central spacer to be used with the flexure shown in FIG. 5 c.   
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A preferred embodiment of the present invention will be described with reference to FIG.  3 . 
     The moving optical component (henceforth referred to as “lens”)  1 , is suitably affixed inside the bore of a precisely machined tubular member  2  (henceforth called “tube”). The tube  2 , is suspended from the main support  4 , using stacks  3 , of flat circular flexures  3   a . Normally, two stacks  3 , of flexures  3   a , separated by a suitable distance are used. An LVDT (Linear Variable Differential Transducer)  7 , consisting of a stationary coil assembly  7   a , and a moving ferromagnetic core  7   b , is used as a position sensor. Any other sensor such as a capacitive, inductive or optical sensor may be suitably used in place of the LVDT. 
     Two views of a flexure stack  3 , are shown in FIGS. 4 a  and  4   b , and its constituent parts are shown in FIGS. 4 c ,  4   d  and  4   e . Two views of another flexure stack  3 , are shown in FIGS. 5 a  and  5   b , and its constituent parts are shown in FIGS. 5 c ,  5   d  and  5   e . Each stack  3 , consists of one or more flexures  3   a , interspersed with spacers  5 ,  6 . Each rim spacer  5 , is shaped to cover that part of the flexure  3   a , meant to be stationary. It has holes  5   a , which are used to mount the stacks  3 , on the main housing,  4 . Each central spacer  6 , has a hole  6   a , which mates with the moving tube  2 . It is shaped to cover that portion of the flexure  3   a , which moves but does not flex. Those portions  3   b , (henceforth called “flex-arms”), of the flexure disc  3   a , that are not covered by any of the spacers  5 ,  6 , can flex to yield the desired axial motion. The mutual coupling of the flex- arms  3   b , within a flexure  3   a , and also within different flexures  3   a , in the stacks  3 , imparts a very high radial stiffness to the entire assembly, while keeping the axial stiffness relatively low. 
     Referring back to FIG. 3, the flexure stacks are spaced apart by the coil mount  8 , of the voice coil  9 , in the moving section and by the main support  4 , in the stationary section. The moving core  7   b , of the LVDT  7 , is then mounted on the moving tube  1 , via core mount  7   c , and the whole moving sub-assembly is clamped tight using nut,  10 . The LVDT core  7   b , is ensured to be nominally co-axial with the LVDT coil assembly  7   a.    
     An axially magnetized permanent magnet  11 , in the shape of a ring is glued into the main housing  4 . The permanent magnet  11 , is made of a high energy density material such as neodymium ferrous boron. A ring shaped pole piece  12 , of magnetically permeable iron alloy, is glued on the magnet. The main housing  4 , which is also made from magnetically permeable iron alloy, acts as the outer pole. Thus the annular air gap  13 , between the inner pole piece  12 , and the main housing  4 , contains a radial magnetic field. When the coil  9 , appropriately positioned in the magnetic air gap  13 , is energized by an electrical current, an axial force is induced on it. When the direction of the current is reversed, the force on the coil is also reversed. The above described voice coil motor is thus used to move and position the tube  2 , and the lens  1 , contained in the tube  2 . Alternative topologies of a voice coil motor or a multiphase linear motor may be used in place of the voice coil motor described above 
     Another lens  14 , meant to be stationary, is affixed inside a precisely machined bore of the lens mount  15 . The bore of the lens mount  15 , is accurately sized such that when assembled properly, the moving tube  2 , enters inside it without touching it. In other words, there exists a very small annular gap  16 , of the order of 10-20 micrometers, between the outer cylindrical surface of the moving tube  2 , and the inner cylindrical surface of the bore in the stationary lens mount  15 . Firstly, this ensures that the stationary lens  14 , is adequately co-axial with the moving lens  1 , thus facilitating proper optical function. Secondly, this simple arrangement also introduces desired damping in the system as follows. When the moving tube  2 , moves towards the stationary lens  14 , the air trapped in between the two lenses  1  and  14 , is compressed. The rise in air pressure above ambient, drives out the air through the narrow annular gap between the moving tube  2 , and the stationary lens mount  15 . The friction generated during the passage of air leads to damping. When the moving tube  2 , moves away from the stationary lens  14 , the air trapped in between the two lenses  1  and  14 , is expanded leading to reduced pressure. This forces air from the ambient into the space between the lenses  1  and  14 , leading to friction damping as explained above. Thus any motion of the moving tube leads to flow of air either into or away from the space enclosed between the lenses  1  and  14 , leading to friction damping in proportion to the velocity of the tube. This helps in speeding up the attenuation of the vibration at the end of each stroke, reducing the settling time and speeding up overall operation. 
     It will thus be seen that, at least in its preferred forms, the present invention provides a mechanism for moving an optical component such as a lens using flexure bearings, for use in a wire bonding machine in particular and also in other machines in general. The mechanism is intended to achieve straight line motion of a lens relative to another co-axial lens thus yielding a means whereby the focus of the optical assembly can be altered without manual intervention. The moving part of the mechanism is actuated by a voice coil motor in such a way, that the effective actuating force is nominally co-axial with the moving lens. 
     Another object of the present invention is to provide a simple method of damping down undesirable vibrations of the mechanism at the end of its stroke thus facilitating faster operation. The principle features of the present invention may be employed in various embodiments not covered herein, without departing from the scope of the invention.