Patent Number: 
Section: description

The drive assembly of the present invention provides several advantages over prior art drive mechanisms. By combining the driving mechanism with the slide or shaft on which a driven member travels, one can eliminate relative movement between the driver and the driven member. This arrangement reduces the number of independently mounted parts which must be independently secured and provides a more smooth movement of the driven member. Such a drive assembly is particularly useful in precision drive mechanisms, including the drive mechanisms used in imaging equipment in which beam collimation is carried out by movement of a slit plate relative to an x-ray beam. Movement of the slit plate must be accurate, repeatable and precise throughout all operating speeds and temperatures of the imaging equipment. FIG. 1 illustrates a collimator assembly 10 of an x-ray imaging apparatus which includes a drive assembly 12 according to the invention. The assembly includes a base 14, which is typically made of cast and machined aluminum or brass. The base includes an aperture 16 for admission of a radiation beam from a focal spot of an x-ray source (not shown). A first shaft. 18 is secured to blocks or pads 20 at one side of the aperture on the base with screws or like fasteners 22. A driver 24, shown in FIG. 1 as a stepper motor 24a with associated controller circuitry 24b, is also mounted to the base. The driver is rotatably coupled to a second shaft 26 which is substantially parallel to the first shaft 18. A carrier 28 extends between the shafts and is disposed over the aperture 16 in the base. The carrier is adapted to slide along the shafts 18, 26 in the direction of z axis 30 and is preferably made of cast aluminum or brass. According to the invention, the second shaft 26 is coupled to the driver 24 via a backlash-resistant nut assembly 32. As shown in FIG. 2, the second shaft 26 includes a nonthreaded portion 34 and a threaded portion 36. The non-threaded portion 34 of the shaft 26 is adapted for sliding engagement with the carrier, whereas the threaded portion 36 engages with the nut assembly 32, which is fixed to the carrier 28. The nut assembly 32 is shown in FIG. 3 and includes a first nut portion 38, a second nut portion 40, and a wave washer.42 disposed between them. The first-nut portion 38 has a threaded bore 44 which engages with the threaded portion 36 on the second shaft 26. The second nut portion 40 also has a threaded bore 41 which engages with the threaded portion 36 of the second shaft. The assembly also includes fasteners 46, such as dowel pins, which fit into holes 48 in the first nut portion and slots 50 in the second nut portion. The fasteners join the nut portions together around the wave washer 42, which biases the first and second nut portions away from each other. Means for biasing the nut portions apart other than a wave washer can be used, such as a coiled compression spring. The combined tension and compression of the nut assembly which is created by the counteracting forces of the wave washer 42 and the joined first and second nut portions 38, 40 eliminates substantially all play between the threads of the nut assembly and the threaded portion 36 of the second shaft and prevents any backlash in the movement of the nut over the threaded portion of the shaft 26. The first nut portion 38 includes counterbored holes 52 for receiving fasteners or screws for joining the nut assembly to the carrier, as shown in FIG. 1. The first and second nut portions of the assembly are preferably made of an easily machinable metal, such as brass or phosphor bronze. To reduce friction and facilitate smooth movement of the carrier over the shafts, a ball bearing sleeve 54 may be disposed over each of the shafts. A typical ball bearing sleeve is illustrated in FIG. 4 and in phantom on the shaft of FIG. 2. The ball bearing sleeve 54 includes several apertures extending through the wall of the sleeve. A ball bearing is disposed in each aperture and is movable therein without becoming disengaged from the aperture, so that the ball bearing seems to float within the apertures. The ball bearings provide rolling contact between the shaft inside the sleeve and the carrier outside of the sleeve and substantially reduce the friction between these components. The collimator assembly 10 includes a slit plate 56 which is disposed in the carrier 28 so as to be positioned over the beam aperture in the base. The slit plate 56 includes multiple slits 57 having different widths, for defining beams of different thicknesses in the z direction. The carrier and slit plate move along z axis 30 in response to travel of the nut assembly 32 over the second shaft 26. A mask plate 58 is fixed to the base beneath the carrier 28 and is disposed over the aperture 16 in the base. The mask plate includes a single slit 60. In operation, the carrier 28 is moved in the z axis direction by operation of the motor 24a so that one of the slits 57 in the slit plate 56 is aligned with the slit 60 in the mask plate, thereby allowing a collimated beam of radiation to pass through the aperture in the base to an object to be scanned and to a detector bank (not shown) beyond the object to be scanned. The motor 24a preferably comprises a stepping motor controlled by a controller 24b having a counter for calculating which of the plurality of slits 57 of the collimator 56 is aligned with the slit 60 of the mask plate 58 based upon the stepped rotation of the motor. A suitable controller and counter combination is shown, for example, in U.S. Pat. No. 5,550,886 to Dobbs et al. entitled xe2x80x9cX-ray Focal Spot Movement Compensation Systemxe2x80x9d, which is assigned to the assignee of the present disclosure and which is incorporated herein by reference in its entirety. Referring now to FIGS. 5 through 8, an alternative carrier 70 and drive shaft assembly 72 for use with the collimator assembly of FIG. 1 is shown. Referring to FIG. 5, the carrier 70 is preferably made of cast aluminum or brass and includes a first support member 74 having a bore 75 for slidingly receiving the ball bearing sleeve 54 of the first shaft 18. The carrier 70 also includes second and third spaced-apart support members 76, 78 having bores 77, 79, respectively, for slidingly receiving linear-rotary bearings 80 of the drive shaft assembly 72 of FIG. 7. The second and third support members 76, 78 are each relatively-wide, and provide additional stability for the carrier 70 as the carrier slides on the drive shaft assembly 72. The second support member 76 includes an annular recess 82 and fasteners holes 84.for receipt of the backlash resistant nut assembly 32. As shown, the annular recess 82 faces the third support member 78 such that, when assembled, the nut assembly 32 is positioned between the support members 76, 78 to provide further stability. Referring to FIG. 7, the drive shaft assembly 72 includes a second shaft 86, which is similar to the second shaft 26 of FIGS. 1 and 2, for supporting the carrier 70 on the base 14 of the collimator assembly 10. The second shaft 86 includes a first end 88 that is formed to be coupled to the driver 24 for turning the shaft. The shaft 86 also includes journals 90 adjacent opposite ends of the shaft, a centrally located threaded portion 92, and non-threaded portions 94 between the threaded portion and the journals. As shown in FIG. 7, each of the non-threaded-portions 94 of the shaft 86 slidingly and rotatingly receive the linear-rotary bearing 80, which are in turn for being received in the bores of the support members 76, 78 of the carrier 70. The threaded portion 92 of the shaft 86 engages with the backlash resistant nut assembly 32, which is in turn for-being fixed to the annular recess 82 of the third support member 78 of the carrier 70. Each linear-rotary bearing 80 includes an inner ball bearing sleeve 54 received on the shaft 86, and an outer sleeve 96 received on the inner ball bearing sleeve, as also shown in FIG. 8. The linear-rotary bearings 80 allow the shaft 86 to rotate with respect to the support members 76, 78 of the carrier 70 as the motor 24 turns the shaft 86. The linear-rotary bearings 80 also allow the support members 76, 78 of the carrier 70 to linearly slide with respect to the shaft 86 as the backlash resistant nut assembly 32 moves along the threaded portion 92 of the shaft 86. Preferred linear-rotary bearings are available, for example, from Berg Manufacturing of East Rockaway, NY (http://www.wmberg.com). Referring again to FIG. 7, the drive shaft assembly also includes rotary bearings 100, and a thrust bearing 102 received on the journals 90 of the second shaft 86 for rotatably mounting the drive shaft assembly 72 on the base 14 of the collimator assembly 10. Because certain changes may be made in the above apparatus without departing from the scope of the invention herein disclosed, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted in an illustrative and not a limiting sense.