referring now to the drawings , reference numeral 10 denotes an x - ray tube suspension system commonly referred to as a tubestand which for purposes of illustration is mounted for translational movement on a ceiling attached assembly 12 and includes an x - ray tube 16 directed to a movable x - ray film holder commonly referred to as a bucky 18 located beneath an x - ray examination table 20 upon which a patient 22 or other object under examination is located . as is well known in a tomographic procedure , the tubestand 10 including the x - ray tube 16 and the bucky 18 which contains the x - ray film are moved or translated in mutually opposite linear directions with the tube rotating in order to maintain a constant point or fulcrum 24 in space within the patient 22 , which acts to accurately define an image location 26 on the film in response to the x - ray beam 28 passing through the patient while blurring the surrounding image region . in order to provide a non - mechanical linear link or coupling between the x - ray tube 16 and the bucky 18 so that the central ray of the x - ray beam 28 always points to the same location on the bucky 18 , a tubestand position sensor unit 30 is optically coupled to an optical reflector member 31 in the form of a corner cube prism which is located at a spot which is fixed relative to the focal spot 32 of the x - ray tube 16 . the tubestand position sensor unit 30 is shown in detail in the above - referenced related application and comprises a helium neon laser , not shown , which produces a monochromatic light beam which is directed to the reflector 31 through a beam expander , and interferometer assembly . the tubestand position and more particularly the linear translation of the tubestand 10 results in an interferometer fringe pattern which is converted to an electrical pulse signal which in turn is coupled to an electronic control system 34 by means of electrical signal coupling means 36 . the control system 34 operates in response to operator selected input parameters from a control panel 38 which parameters , for example , comprise fulcrum level , speed of sweep , and angle of sweep to first generate a control signal for operating an electrical drive motor 42 through a tubestand motor drive unit 44 for linearly translating the tubestand 10 to a start position and then in a predetermined sweep direction . the control system 34 responds to the pulse signals from the sensor unit 30 to control a second electrical motor 46 through a rotation motor drive unit 48 for rotating the x - ray tube 16 during the tomographic sweep and a third but particular type of electrical motor 50 for translating the bucky 18 in response to the movement of the tubestand 10 . as noted above , in a tomographic procedure the tubestand 10 moves in an opposite direction in relation to the bucky 18 . the latter accordingly must follow the movement of the tubestand with great precision . heretofore , a mechanical tie - bar linkage typically connected the bucky to the x - ray tube . in order to eliminate the tie - bar linkage and obtain a positional type of servo operation without elaborate feedback networks , the present invention utilizes a permanent magnet stepper motor for the motor 50 and ball screw drive for translating the bucky 18 . this arrangement is furthermore disclosed in fig2 wherein the stepper motor 50 is directly connected to one end of an elongated ball screw shaft 52 which terminates in a bearing assembly 54 beneath the x - ray examination table 20 . the ball screw shaft 52 runs lengthwise under the x - ray table 20 and couples to the bucky 18 by means of a pair of ball screw nuts 56 and 58 as shown in fig3 which are attached to the bucky frame generally designated by reference numeral 60 . the bucky frame 60 , moreover , is constrained by simple translational guide means 62 so that any rotation of the ball screw shaft 50 effects translational movement of the bucky back and forth lengthwise under the x - ray table 20 . the stepper motor 50 is fed electrical drive pulses of first and second directional polarities from a drive unit 64 coupled to a pulse generator 66 under the control of the electronic control system 34 . the stepper motor 50 precisely rotates one increment of motion with each input drive pulse supplied thereto . with the motor 50 directly connected to the ball screw shaft 52 , the result is precise translation in a predetermined direction in incremental steps according to the polarity of square wave type drive pulse signals generated in response to tubestand translation sensed by the position sensor unit 30 . typically the stepper motor 50 comprises a permanent magnet stepper motor having an input response of , for example , 10 , 000 steps per second with no loss of steps and provides one revolution for each 200 steps . the ball screw shaft 52 has 0 . 5 inch thread diameter and a 0 . 200 inch per revolution pitch . as taught in the aforementioned related application , the laser type tubestand position sensor is adapted to generate a pulse train in the order of 12 × 10 6 pulses per each inch of translation of the tubestand 10 . this pulse rate is divided by a predetermined frequency division in the electrical control system 34 where it is then applied to the pulse generator 66 in one of two polarities for driving the stepper motor 50 forward or backward as required . since the stepper motor 50 has a unique inherent braking capability , no other external braking means are required while yet permitting free wheeling when unenergized so that the bucky 18 can be moved manually at will . the latter movement presents no problem , since at the beginning of a tomographic procedure the electical control system 34 is operative to drive the tubestand 10 to the start position via its translational drive motor 42 and its associated drive unit 44 . this movement back to the start position will still be sensed by the sensor unit 30 , causing the electrical control system 34 in turn to couple control signals to the pulse generator 66 which supply pulses of proper polarity to the stepper motor 50 to cause the bucky to assume its respective start position , whereupon the tube and the bucky 18 will be in line prior to the beginning of a tomographic sweep . the ball screw nuts 56 and 58 are prestressed for zero play , which is adapted to simplify alignment in a manner unobtainable heretofore . also the inherent characteristics of a permanent magnet stepper motor results in improved damping characteristics to reduce noise during stepping , as well as providing high torque at relatively high speeds . this permits translation while the x - ray table 20 is in a vertical position without a counterweight system . while the subject apparatus disclosed is relatively simple , it provides a high performance unobtainable in present motorized and non - motorized designs having relatively complicated bucky brackets , wheels , bearings and guiderails . electronic control , moreover , is a minimum yet extremely accurate positioning of the bucky 18 relative to the position of the x - ray tube 16 is obtained without the requirement of a mechanical linkage therebetween . while there has been shown and described what is at present considered to be the preferred embodiment of the subject invention , modifications will readily occur to those skilled in the art . for example , when desirable , a stepper motor and its attendant drive circuitry may be utilized for translating the tubestand and / or rotating the x - ray tube . it is not desired , therefore , that the invention be limited to the specific arrangements shown and described , but it is to be understood that all equivalents , alterations , and modifications coming within the spirit and scope of the present invention are herein meant to be included . accordingly ,
a bucky translated under an x - ray table in radiographic apparatus of the linear tomography type wherein precise positioning with respect to the x - ray tubestand is achieved by a combination of a ball screw drive attached to the bucky and a permanent magnet stepper motor coupled thereto which incrementally rotates one step with each input drive pulse applied thereto from a pulse generator operated in accordance with a control input applied from an electronic control system in response to the sensed position change of the tubestand .