Abstract:
A linear motor driven positioning stage comprises a wedge positioned between a carrier and a base and a linear motor positioned between the wedge and the base for moving the wedge relative to the base along a second axis whereby the carrier moves relative to the base along a first axis.

Description:
RELATED APPLICATION 
     This application is based upon Provisional Patent Application Ser. No. 60/012,422, filed Feb. 28, 1996. 
     BACKGROUND OF THE INVENTION 
     Many motion control applications require translation of a payload in three dimensions. Applications in the semiconductor and laser manufacturing fields require travel in the Z-direction (usually vertical) of less than 1 inch. Current solutions for the Z-direction motion include a traditional ball screw or lead screw driven stage, a lead screw driven wedge type lift stage and a piezoelectric driven lift stage. The traditional style screw driven stage requires substantial height to achieve a relatively small movement. In many cases, this is unacceptable as the stage will interfere with laser processing optics or semiconductor inspection equipment. Lead screw driven wedge devices are manufactured by a few motion control equipment suppliers. These have the advantage of providing small travels in a minimum space envelope. All drive mechanics are located below the payload. These stages have relatively good resolution (as high as 50 nanometers). However, backlash in the nut and screw/coupling torsional windup limit the effectiveness of the high resolution. The acceleration, speed and system bandwidth for these stages are low, limiting process cycle times. Piezoelectric drives have compact profiles and excellent system response. However, the travel provided is limited to less than 0.1 mm and to small payloads, that is, less than 10 pounds. 
     It is an advantage, according to this invention, to provide a vertical lift stage for limited Z-direction motion which dramatically improves acceleration, maximum speed and system bandwidth performance. It is a further advantage to provide a lift stage with drive and feedback components that are noncontacting, thus eliminating wear and maintenance. 
     SUMMARY OF THE INVENTION 
     Briefly, according to this invention, there is provided a linear motor driven lift or positioning stage which comprises a base and a table having a mounting surface, wherein the table is connected to the base by linear motion guides that permit the table to move in the substantially vertical direction of a first axis. The table has a wedged surface opposite the mounting surface. A wedge is positioned between the wedge surface of the table and the base. The wedge is connected to the table and the base by linear motion guides to permit the wedge to move along a substantially horizontal second axis perpendicular to the first axis. A linear motor is positioned between the wedge and the base to move the wedge relative to the base along the second axis so that the table moves relative to the base along the first axis. A linear position encoder is positioned to encode the position of the wedge relative to the base. Preferably, the linear motor is a permanent magnet motor. It is also preferred that linear motion guides are positioned between the wedge and the wedge surface of the table. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and other objects and advantages of the invention will be apparent from the following detailed description made with reference to the drawings in which: 
     FIG. 1 is a side view of a linear motor driven vertical lift stage according to this invention; 
     FIG. 2 is a plan view of a linear motor driven stage with the tabletop and wedge removed; and 
     FIG. 3 is a section view of the linear motor driven stage taken along lines III--III in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1, 2 and 3, a lift stage, according to this invention, comprises a base plate 10 which is, for example, a steel plate sized to resist deflection under the expected loads. End plates 11 and 12 are secured along opposite edges of the base plate to the upper surface thereof (as shown in the drawings) and substantially perpendicular to the base plate. The end plates may be held to the base plate by bolts or other fasteners. Attached to the top surface of the base plate are the elongate tracks or rolling guides 13T and 14T of the linear motion guides 13 and 14. Preferably, the linear motion guides or linear bearings are preloaded, no play, no backlash guides. Linear motion guides have elongate tracks and races that are guided by the tracks. The tracks and races can usually reverse positions. 
     Wedge 15 has two faces, 15U and 15L, the extensions of which would intersect. The rise-to-run ratio of the two faces as shown in the drawings is 1:10. Mounted to the bottom face 15L of the wedge are the outer races 13R and 14R of the linear motion guides 13, 14. Mounted to the top surface 15U of the wedge are the elongate tracks 17T and 18T of linear motion guides 17 and 18. 
     Tabletop 20 has two faces, 20U and 20L, the extensions of which would intersect at the same angle that the faces of the wedge would intersect if extended. Thus, the upper face of the tabletop may be maintained parallel to the upper surface of the base plate and a lower surface of the wedge while the upper surface of the wedge is maintained parallel to the lower surface of the tabletop. Mounted to the bottom surface of the wedge are the outer races 17R and 18R of the linear motion guides 17 and 18. 
     Mounted to the end plates 11 and 12 are the outer races of the linear motion guides 24, 25, 26 and 27. 
     As the wedge 15 moves right to left (as shown by arrows on the wedge in FIG. 1) guided by linear motion guides 13, 14, 17 and 18, the tabletop 20 moves up and down guided by linear motion guides 24, 25, 26 and 27. Every position of the top surface of the tabletop is parallel to every other position of that surface. 
     Mounted to a recess in the bottom of the wedge is the traveling magnet track for the linear motor. The magnet track comprises magnetic yoke 30 with a plurality of permanent magnets 31 and 32 secured thereto presenting alternating north and south poles. The linear motor forcer 34 comprises coils 35 defining magnetic poles according to how they are energized. The coils are secured to a mounting bracket 36 which in turn is secured to the base plate 10. The magnetic forcer may be secured by an air or water cooled mounting bracket 36. Examples of linear motors adaptable to this application are set forth in the book, Linear Electric Motors: Theory, Design, and Practical Applications, by Nasar and Boldea, Prentice Hall 1987. The forcer is a multiple phase, usually three phase, coil structure. By controlling the amount of current in each phase in ways well understood to those skilled in the art of linear motors, the forcer and therefore the wedge can be accurately positioned. 
     Mounted to the base plate is a linear encoder 40 that measures the right-to-left motion of the wedge. Due to the 1:10 rise-to-run ratio of the inclined surfaces on the wedge and table, the encoder output is also a measure of the up-down motion of the table. This results in a ten-fold mechanical amplification of the encoder signal. 
     The embodiment described with reference to FIGS. 1 to 3 offers high precision vertical motion of up to 5 mm. The stage configuration provides direct compression loading of the structural members. This eliminates cantilevering for most conventional payloads and minimizes position errors due to deflection caused by uneven loading. 
     The wedge travels in the horizontal plane guided by linear motion guide bearings. The translation motion is driven by the linear motor which is directly coupled to the base and the wedge. A noncontacting linear encoder on the wedge provides high resolution position feedback information. 
     The wedge incorporates two sets of linear motion guides. The lower set lies in the horizontal plane. The second set is inclined at a 1:10 slope. The horizontal motion of the wedge is converted into vertical motion of the tabletop by the inclined linear motion guides. The 1:10 incline provides a mechanical advantage which increases payload capacity of the linear motor and increases system resolution by a factor of 10. The vertical motion of the tabletop is guided in all angular directions and laterally by the linear guides attached to the wedge. The axial straightness is guided by additional sets of linear motion guides located in the vertical plane between the tabletop and the end plate assemblies. This arrangement provides a compact, extremely rigid, and highly accurate mechanical system. 
     It is an advantage, according to this invention, that the noncontacting linear feedback device can provide hysteresis free, high resolution (10 nanometers) positioning information. It is an advantage of this embodiment over the piezoelectric designs to provide greatly increased travel and payload capability. Very important, the embodiment described includes a wedge design and linear motion guide bearing arrangement which provides constant support geometry to load bearing components. This minimizes deflection and provides excellent trueness of travel. Pitch/yaw/roll errors are minimized when compared to other designs. 
     Having thus defined our invention with the detail and particularity required by the Patents Laws, what is desired protected by Letters Patent is set forth in the following claims.