Panoramic dental X-ray machine employing selectable mode film drive mechanism with shiftable cam followers
CROSS-REFERENCE TO OTHER RELATED PATENT APPLICATIONS 
     Reference is hereby made to Ser. No. 002,148, filed Jan. 9, 1979, for &#34;Panoramic Dental X-Ray Machine X-Motion Drive&#34; of R. Cushman et al., Ser. No. 025,127, filed Mar. 29, 1979, for &#34;Link-Clutch Film Drive Mechanism for Panoramic Dental X-Ray Machine&#34; of R. Cushman et al., both assigned to the assignee hereof. 
     STATEMENT OF THE INVENTION 
     The present invention relates to X-ray apparatus and more particularly concerns film drive mechanism which coordinates the speed of X-ray film travel with patient chair movement to provide undistorted continuous and discontinuous radiographs of dental arch areas. 
     BACKGROUND OF THE INVENTION 
     Prior art panoramic dental X-ray apparatus are well known in the art. Some provide a continuous image of the dental arch area and commonly employ an X-ray source and X-ray film supported on a rotatable carrying arm which orbits a patient situated in the beam path. The patient may remain stationary in the chair, or the chair may be transported in accordance with various X-Y type drive mechanism in order to simulate the generally elliptical shape of the human dental arch, as disclosed in U.S. Pat. No. 4,125,774, assigned to the present assignee. The continuous image radiograph provides the dentist with a panoramic view of the teeth and associated structures and is a useful diagnostic aid in many phases of dental practice. 
     Various other prior art apparatus provide a discontinuous, or split image panoramic radiograph which possesses certain advantages. Here, the dentist is provided with additional interpretive information since two distinctly different views of the incisors, or centrals area are provided. For example, in U.S. Pat. No. 3,045,118, apparatus is disclosed which automatically shifts the patient in order that the line of sight between the X-ray source and film bypasses the patient&#39;s spinal column and permits X-raying of the other half of the dental arch. Apparatus is also disclosed therein for continuously moving an X-ray source and extra-oral film holder about the patient. 
     In U.S. Pat. No. 2,798,958, means are disclosed for reorienting the patient after completion of one-half of the excursion cycle in order to relocate the center of the axis of rotation with respect to the patient&#39;s head prior to X-raying the other one-half of the dental arch in order to provide the discontinuous, or split radiograph images. 
     Regardless of the type radiographic image to be obtained, i.e., continuous or discontinuous, compensation is usually made for the fact that the curvature of the desired area of focus is generally not a true circle or ellipse. Thus, the rate of film travel must be varied in accordance with the chair shift or transport, and the rate of travel of the X-ray source about the patient&#39;s head in order that the radiological projections occupy a distance on the film equal to the linear distance of a curved structure being X-rayed, such as a typical dental arch. 
     The present invention discloses film drive mechanism which is readily adaptable for use with structure disclosed in cross-referenced patent application Ser. No. 002,148, for &#34;Panoramic Dental X-Ray Machine X-Motion Drive&#34;, or with other suitable panoramic dental X-ray machines which provide either or both continuous and discontinuous radiographic images. With slight modification, the present invention may be substituted for the film drive mechanism disclosed in cross-referenced patent application Ser. No. 025,127, for &#34;Link-Clutch Film Drive Mechanism for Panoramic Dental X-Ray Machine&#34;. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of an assembled panoramic dental X-ray machine employing the film drive mechanism of the present invention. 
     FIG. 2 is a plan view, partially sectioned, part in phantom, of the present film drive mechanism. 
     FIG. 3 is a sectional view of FIG. 2 taken along line 3--3 thereof. 
     FIG. 4 is a sectional view taken along line 4--4 of FIG. 3. 
     FIGS. 5, 6 and 7 are sectional views of FIG. 4 taken along lines 5--5, 6--6, 7--7 thereof respectively, looking in the direction of the arrows. 
     FIG. 8 is a perspective view of the film drive mechanism of the present invention, parts omitted for clarity. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1 of the drawings, the panoramic X-ray machine comprises a base B having a stationary platform P disposed generally centrally thereof. Platform P supports a patient chair L including means S for supporting the chin and head of a patient. A column C is caused to rotate around chair L, the column carrying a tubehead T, a camera supporting arm G, and a camera assembly D which houses the film drive mechanism of the present invention. The chair shift and transport mechanism is located below chair L, within shroud K, the mechanism being bolted securely to stationary platform P. The excursion mechanism for causing column C to rotate around stationary platform P at a uniform and constant rate is supported and partially housed in base B. 
     Referring to FIGS. 2 and 8, film drive mechanism 10 may include a suitable housing 12 which contains synchronous, reversible, A.C. motor 14 having a 2 rpm output shaft. Motor 14 drives a pulley 16 causing input pulley 18 to rotate therewith by means of timing belt 20 operably connected therebetween. Control of tension on belt 20 is effected through cooperative action of screws 22 and slots 24. 
     Input pulley 18 is secured to hollow input or drive shaft 28 as is secured linkage drive 30 thereabove. Linkage drive 30 is provided with a laterally extending arm 32, such that rotation of shaft 28 causes a similar movement to linkage drive 30 and linkage drive arm 32. Arm 32 supports a reversible induction cam follower motor 34, appropriately geared down, which causes either one of two identical cam followers 36 and 38 to be individually and separably shifted, later described, for engagement with either a stationary plate cam 40, providing a discontinuous or split image of the dental arch area; or another stationary plate cam 42, providing a continuous image. Cam follower motor is ideally of the direct current type which lends itself readily to dynamic braking. &#34;Split&#34; cam 40 is disposed above cam follower 36, and &#34;continuous&#34; cam 42 is disposed below cam follower 38, each of the cams being provided with a working groove 44 and 46 respectively. 
     Each groove, when operably engaged by its respective cam follower, causes linkage structure 50 to rotate an output shaft 52 at non-constant speeds. An upper portion of output shaft 52 is concentrically disposed within hollow input or drive shaft 28. Non-constant speed of rotation of output shaft 52 governs the non-constant speed of rotation of the film drive shaft which similarly governs the speed of travel of the X-ray film. 
     Linkage structure 50 includes an output member or link arm 60 secured to output shaft 52, connecting link 62, and a C-shaped rotating cam follower carrier 64 (clearly shown in FIG. 6). 
     Carrier 64 is pivotable about two separate axes. At one, cam follower carrier 64 pivots about pivot pins 66, one of which extends axially from each of a spacer stud 68, disposed between the laterally extending arms of the C-shaped carrier 64. Pins 66 extend partially into orifices 70 of bushings 72 mounted in each arm of carrier 64 (FIG. 5). 
     At another axis, a pivot pin 74 permits rotating cam follower carrier 64 to pivot with respect to connecting link 62. Another pivot pin 76 provides the necessary pivotal action between connecting link 62 and link arm 60, the latter, as aforementioned, having an end affixed to output shaft 52. 
     Linkage drive arm 32 is fastened to stud 68 by screws 78. Thus, rotary movement of linkage drive arm 32 causes cam follower carrier 64 to move and simultaneously pivot about its two pivot points in accordance with predetermined complex patterns governed by the groove configuration of the particular cam groove being engaged by its respective cam follower, which cam followers are carried by carrier 64. 
     Split cam 40 is supported by a member 80 which, in turn, is supported by a vertical support 84 (FIGS. 3 and 4). For purposes of easy access to the interior of the film drive mechanism, split cam 40 is hinged to support member 80. Screws 85 secure split cam 40 to support 80. Guide pin 86 is press fitted into support 80, which guide pin extends downwardly through a large central opening in split cam 40 for reception by a central orifice in bushing 87. Output shaft 52 is provided with a pin 89 which extends into the other end of the bushing orifice. When guide pin 86 is thus seated within the orifice, proper positioning of split cam 40 is insured. 
     Referring to FIGS. 3, 4, 5 and 6, as well as to FIGS. 2 and 7, structure for effecting selection of continuous or split mode of radiographing the patient&#39;s dental arch is described below. 
     Cam follower motor 34 is carried by carrier 64 by means of a support plate 88 affixed thereto. Plate 88 is supported by a gear box 90 containing reduction gears 92. An H-plate 94 (FIGS. 6 and 8) is secured to gear box 90 by screws 96. Gear box 90 and support plate 88 are connected to cam follower carrier 64 but not to stud 68. H-plate 94 supports shafts of gear members 92 in the usual manner as well as selector cam shaft 100 which rotates selector cam 102. As clearly shown in FIG. 6, selector cam 102 is provided with a high-point 104 which, upon rotation of selector cam shaft 100, engages an upper edge of cut-out portion 106 (FIG. 5) provided in cam follower tube 108 to thus cause the tube to shift upwardly such that cam follower 36, carried by cam follower tube 108, engages groove 44 of the split cam. Once so engaged, motor 14 may then be actuated to start input shaft 28 rotating which effects rotation of linkage drive 30 and linkage drive arm 32, and consequently the entire linkage structure 50. Movement of linkage structure 50, in cooperation with the cam follower-groove arrangement, advances or retards the speed of rotation of output shaft 52 connected to link arm 60, and hence, the speed of rotation of the film drive shaft, later described. 
     The rate of change of the distances of grooves 44 and 46 from output shaft 52 as the respective cam follower rides in its respective groove determines the rate of change of speed of rotation of output shaft 52 and hence the film drive shaft. In the case of split or discontinuous images, for example, distance of groove 46 from output shaft 52 will control output shaft speeds, and hence, film drive speeds. To further clarify, if a cam follower rode in a groove which coincided with a circle having its center at the center of the output shaft, the speed of rotation of the output shaft, and hence, the film drive shaft, would be constant. 
     In panoramic dental X-ray machines of the assignee, film travel speed is slower when the cuspid-incisor area is radiographed as compared to the posterior regions of the mouth. Groove configuration to provide desired film travel speeds at various portions of a typical dental arch structure may be calculated by a skilled mathematician and are not described herein. 
     Alternatively, if a continuous radiographic image is desired, linkage structure 50 will be returned to its original starting position by switches and circuitry well known and not herein shown or described. Parenthetically, if another split or discontinuous image of the dental arch area is desired, the present invention is capable of radiographing in the other, or return direction. 
     When selector cam 102 is rotated 180° in a clockwise direction from its position illustrated in FIG. 6, through gearing 92 and cam follower motor 34, high-point 104 will engage a lower edge of cut-out portion 106 in tube 108 to urge the tube downwardly. Thus, cam follower 38 will be in engagement with groove 46 of cam 42 for providing the continuous radiographic images. 
     Cam followers 36 and 38 are rotatable on separate shafts 110 and 112 respectively within cam follower tube 108, although a single shaft would be equally operable. Bearings 114 permit the cam followers to rotate freely within their respective grooves. 
     Structure for effecting automatic cut-off of electric power to cam follower motor 34 after cam follower 36, for example, is seated within groove 44 is described below. 
     Selector cam shaft 100, in addition to mounting selector cam 102, mounts a switching cam 120 (FIG. 7) having a high point which alternately contacts normally closed limit switches 124 and 126. Thus, the seating or engagement of a cam follower within its respective groove is accompanied by an instant braking of cam follower motor 34 when highpoint 122 contacts the proper limit switch. Circuitry to accomplish the power cut-off is well within the skill of the art and is not shown or described herein. 
     In FIG. 3, film drive shaft 130 is shown affixed to output shaft 52, each shaft being rotatable within bearing members 132 and 134 respectively. Rotation of output shaft 52 rotates film drive shaft 130 which, in turn, rotates drum holder disc 136. Secured to drum holder disc 136 is a drum 138 having X-ray film 140 wrapped therearound. It is apparent therefore that rate and speed of travel of film 140 is governed by rotation of the output shafts 52. 
     In order to align the starting position of drum 138 and X-ray film 140, a magnetic slip clutch 142, suitably in the form of an annulus or separate pieces, is secured to flanged portion 144 by pinning 146 or other convenient means. Plate 148 is removably affixed to film drive shaft 130 by screw 148. Alignment of the drum and film is achieved manually by rotating drum 138 to overcome the friction and attraction of magnetic slip clutch 142 to plate 148.