Abstract:
An anti-backlash device for a positioning stage arranged for the precise movement of components composed of a pair of plates having rail receiving grooves, a pair of rails joining the plates and attached thereto and a work piece mounting table, the anti-backlash device, having a plurality of circumferentially spaced elongate spring members for biasing an interrupted thread into zero clearance engagement with a threaded rod rotatable to move the table along the rails.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention resides in the field of adjustable work piece mounting devices for optical components and more particularly relates to motor driven precise positioning stages having anti-backlash components. 
     2. Description of the Prior Art 
     A precision positioning stage of the type relating to the invention is disclosed in U.S. Pat. Nos. 4,652,095 and 5,077,620 issued to the applicant. The device described therein employs an adhesive assembly concept for a stage utilizing two end plates having grooves for receiving a pair of rails and a work table slideably disposed between the plates. 
     The U.S. Pat. No. 5,077,620 is an improvement over U.S. Pat. No. 4,652,095 in that it provides an electrical motor, gear system, and encoder mounted on an additional plate also disposed between the rails and wherein the motor is operationally connected to the work table by, for example, a threaded rod working in conjunction with an anti-backlash device having engaging threads such that the table is precisely moved by actuation of the motor. 
     Although the device of U.S. Pat. No. 5,077,620 is capable of quite accurate control of the positioning, stability and movement of the work table, any imperfection of the threaded interconnection of the work table and the table drive mechanism, particularly deviations in thread diameters, clearance, alignment, etc. and undesired clearance between the male and female threads will detract from that accurate control. Modern miniature components and parts often require tolerances which are difficult or impossible to meet with constructions such as that of U.S. Pat. No. 5,077,620. 
     Backlash is an inevitable byproduct of imperfect fabrication techniques and allowable tolerances. It is the amount by which the space between threads exceeds the width of the engaging threads and generally tends to substantially effect the engaging threads and associated components upon reversal of gear rotation. As is appreciated by those of ordinary skill in the art, any reduction in backlash between threads, especially in the art of micro adjustable tools, amounts to a considerable increase in the precision with which associated components of the apparatus may be positioned. 
     Precise measurement to achieve the above described positioning entails paying particular attention to the alignment and precision of the engaging and moving parts, in particular between engaging threads. Elimination of backlash between threads is of considerable importance in the reduction of error in precise positioning. 
     Accordingly, it is an object of the present invention to overcome many or all of the thread related limitations of the prior art. 
     SUMMARY OF THE INVENTION 
     The invention may be summarized as an improvement in a work piece positioning stage of the type described above to provide a degree of precision and miniaturization not heretofore obtainable in that the travel of the work piece table is precisely performed and duplicated via the particularly accurate engagement between the threaded rod and an anti-backlash device attached to the table. 
     According to the invention there is provided an anti-backlash device defining a longitudinal axis and comprising at least three elongate spring members spaced apart about said axis and supported at opposed ends by first and second longitudinally spaced member supports, each spring member being fast with a threaded portion, the threaded portions being spaced apart about said axis to form an interrupted thread biased by the spring members for zero clearance engagement with a mating thread. 
     Also according to the invention there is provided in an optical positioning stage having a workpiece mounting table slideably disposed on rails, a motor assembly drivingly connected to a threaded rod wherein rotation of said motor affects lateral movement of said table along the rails; an anti-backlash device comprising, a tubular member having a first end and a second end separated by a plurality of resilient bridges, extending between said first and second ends, separated by a like plurality of slots also extending between said first and second ends, the first and second ends having an outer diameter to facilitate a press fit engagement within a bore in said table and the resilient bridges having an incurvate compressive bias providing a variable diameter decreasing to a central portion of the tubular member; the bridges together define, on an inner surface thereof, an interrupted female thread, for a biased, resilient engagement with the threaded rod to effectuate a resiliently biased substantially zero clearance engagement of the female thread with the thread of the rod. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages and features of the invention will be more fully understood from the description of the preferred embodiment and drawings, given by way of example, in which: 
     FIG. 1 is a plan view of a positioning stage incorporating a preferred embodiment of a anti-backlash device of the invention; 
     FIG. 2 is a perspective view of the anti-backlash device of the present invention; 
     FIG. 3 is a plan view, including an exploded partial view of the anti-backlash device and the central engaging threaded portion thereof; and 
     FIG. 4 is a sectional elevation taken normal to the axis of the anti-backlash device in the central threaded portion thereof. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1, a stage comprising the preferred embodiment is illustrated in which rails  10  and  12  are spaced apart by, and fixedly attached to end plates  14  and  16  in grooves  15  and  17  respectively. Work piece mounting table  18  is slideably disposed between rails  10  and  12 . A fixed motor assembly mounting plate  20  is attached between rails  10  and  12  on the opposite side of end plate  16  from mounting table  18 . 
     Motor assembly  22  consisting of electric motor  24 , motor revolution sensing encoder  26  and speed reducing gear train  28  is mounted through plate  20  in bore  30  and is secured therein. 
     Threaded rod  32  driven by motor assembly  22  passes through plate  16  and is rotationally engaged with work piece table  18  by an inner threaded bore of an anti-backlash device  40  non-rotatably mounted in bore  35 , the threaded rod then being rotationally attached to end plate  14  via a split ring clamp  34  or other means as would be obvious to those skilled in the art. 
     As will be understood by those skilled in the art, and described in further detail below, rotation of the motor in this embodiment moves table  18  between end plates  14  and  16  using the same combination of gear train  28 , motor  24 , encoder  26  and cable  36  described above. 
     Cable  36  connects assembly  22  to electrical power and control circuitry wherein the output of the encoder is translated into the distance traveled by table  18 . Actual measurements depend upon the physical parameters of the system constructed in accordance with the invention. Motor assembly units and control components of the type suitable for employment in the invention are available from for example Micro Mo Electronics Inc., St. Petersburg, Fla. 33701. 
     Table  18  includes a bore  35  therethrough sized to firmly engage the anti-backlash device  40  which, in turn, engages the threaded rod  32 . The bore  35  is located substantially centrally in the table  18  and is formed parallel with respect to the rails  10  and  12 . The bore  35  is sized to accept, with an interference fit, the anti-backlash device  40  which is designed to eliminate backlash otherwise associated with the threaded engagement of the rod  32  with the table  18  by way of the device  40 . 
     Turning now to FIG. 2, the anti-backlash device  40  consists of a substantially tubular one piece member  42  defining a longitudinal axis L. The tubular member  42  includes spaced apart first and second end ring (spring member supports)  44 ,  46  each defining an outer diameter sized to facilitate a press fit engagement within and along at least a portion of the bore  35 . 
     A series of evenly spaced parallel longitudinally extending elongate slots  48  are cut along the length of the tubular member  42  and extend from the first to the second end ring. The slots  48  pass radially completely through the tubular member  42  and create an evenly spaced plurality of longitudinal elongate spring members (bridges)  50  spanning substantially the length between the first and second end rings  44 ,  46 . In addition, the outer diameter of the spring member  50  may define a diameter which is less than the press fit outer diameter of the end rings  44 ,  46 . 
     Turning now to FIGS. 3 and 4, each of the spring members  50  includes a substantially centrally located inner threaded portion or region  52  the portions together defining an interrupted thread  54  to matingly engage with the threaded rod  44 . The slots are fabricated by a wirecut EDM process. The backlash device is formed of beryllium copper. 
     The length of the anti-backlash device is about the length of the mounting table  18 , about 0.54 across its two end rings and the slots are cut along the tubular member to a length of about 0.44 inch. There are, in a preferred embodiment, eight elongate slots, although as is known to anyone skilled in the art, any number of slots  48  to create a desired resilience in the spring members could be cut. However, a minimum of three slots  48  is required for proper functioning. The central threaded portion of the spring members  50  is approximately one third of the length of the slots. 
     One of the first or second end rings has an abutment ring  51 , having a larger diameter than the outer diameters of the end rings  44 ,  46  of the tubular member  42 , in order that the abutment ring abuts an end of the mounting table  18  to locate the anti-backlash device relative to the mounting table  18  in bore  35 . 
     The spring members  50  are imparted with a desired incurvate compression about the circumference of the tubular member thus reducing the diameter of the backlash device towards its center. The spring members  50  are heat treated and squeezed radially inwardly, by methods well known to those skilled in the art, approximate their center portions, in order to provide an inward compressive force which is substantially centered on the spring members  50  and therefore on the inner threaded portions  52  and interrupted thread  54 . This inward compressive force is imparted to the inner threaded portions  52  of the anti-backlash device and creates a particularly desirable compressive force and the slight hourglass shape of the entire anti-backlash device. Such forces and shape provide zero clearance, between the thread of the anti-backlash device and the mateably engaging thread of the threaded rod  44 . 
     Such an inward 360 degree compressive feature forces this zero clearance whereby the space between the threads is substantially eliminated thus backlash is effectively eliminated. The resilient spring members also follow any deviation in diameter or misalignment between the interrupted thread of the device and threaded rod such that movement of the table  18  is unaffected by these. Thus linearity of movement and accurate positioning of the workpiece as well as repeatability of any particular position is greatly enhanced. 
     The inner compressive force generated by the spring members  50  is not substantial in that although there is imparted to the interrupted thread enough force to eliminate backlash, there is still only a minimal amount of longitudinal force between the engaged threads needed to cause a slippage between the interrupted thread  54  and the thread of the rod  44 . A longitudinal force of approximately 1 lb in the longitudinal direction between the rod and anti-backlash device will cause the inwardly compressed bridges to flex outwardly causing the engaging threads of the bridges and rod  44  to slip longitudinally past one another with a device here described by way of example. This arrangement serves to minimize friction while providing the desired function of the device.