Abstract:
A gimballed mirror assembly having an inner gimbal unit which carries a reflective mirror, an outer gimbal unit and a housing. The inner gimbal unit rotates within the outer gimbal unit, about a vertical axis. The top and bottom of the inner gimbal unit include conical depressions in which are located ball bearings to allow azimuth rotation. The outer gimbal unit rotates within the housing, about a horizontal axis. The sides of the outer gimbal unit include conical depressions in which are located ball bearings to allow elevation rotation. A first adjusting screw assembly contacts conical depressions in the back of the inner gimbal unit on opposite sides of the vertical axis to maintain the inner gimbal unit in a predetermined fixed orientation. A second adjusting screw assembly contacts conical depressions in the back of the outer gimbal unit on opposite sides of the horizontal axis to maintain the outer gimbal unit in a predetermined fixed orientation.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for government purposes without the payment of any royalties therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     Various optical systems require a reflective mirror for changing the direction of a beam of visible or invisible electromagnetic radiation. In order to accomplish this task, the mirror must be rotatable about two mutually perpendicular axes. Such a function may be provided by a gimballed mirror assembly. 
     By way of example, one use for the present invention is the field of laser radar (LADAR) systems wherein, due to space restraints, a generated laser beam is reflected off of mirrors to emerge at an exit port. A return beam is also changed in direction to impinge upon a receiver system. In such an environment the gimballed mirror assembly must be lightweight and must perform in the presence of extreme vibration. 
     Some typical gimballed mirror assemblies utilize bulky and heavy components and require springs to apply pressure on adjusting mechanisms. Although satisfactory for laboratory use, this method of providing positive contact permits vibratory responses to enter the assembly thereby rendering it essentially inoperative for use in aircraft. 
     The present invention obviates the drawbacks of present day gimballed mirror assemblies by providing a gimballed mirror assembly which is not only lightweight, but also which eliminates the requirement for any adjusting springs. 
     SUMMARY OF THE INVENTION 
     A gimballed mirror assembly in accordance with the present invention includes a housing with an outer gimbal unit positioned within the housing. A pair of ball bearings respectively positioned on opposite sides of the outer gimbal unit between the housing and the outer gimbal unit permits rotation of the outer gimbal unit about an axis passing through the ball bearings. An inner gimbal unit is positioned within the outer gimbal unit and another pair of ball bearings respectively positioned on the top and bottom of the inner gimbal unit between the outer gimbal unit and the inner gimbal unit permits rotation of the inner gimbal unit about an axis passing through the other pair of ball bearings. Preferably the ball bearings seat in conical depressions formed in the various components. 
     A first pair of adjusting screw assemblies are mounted to contact and position the inner gimbal unit and to maintain it in a selected position, and a second pair of adjusting screw assemblies are mounted to contact and position the outer gimbal unit and to maintain it in its selected position. Completing the assembly is a mirror having a reflective surface and carried by the inner gimbal unit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be better understood, and further objects, features and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings, in which: 
     FIGS. 1A and 1B are block diagrams of one use for the gimballed mirror assembly of the present invention. 
     FIG. 2 is an exploded view of one embodiment of the present invention. 
     FIG. 2A is a view, from a different perspective, of the inner gimbal unit of FIG.  2 . 
     FIG. 2B is a view, from a different perspective, of the outer gimbal unit of FIG.  2 . 
     FIG. 2C is a view, from a different perspective, of the housing of FIG.  2 . 
     FIG. 3 is an assembled view of the gimballed mirror assembly. 
     FIG. 4 is a plan view of the gimballed mirror assembly. 
     FIG. 4A is a view along the line  4 A— 4 A of FIG.  4 . 
     FIG. 4B is a view along the line  4 B— 4 B of FIG.  4 . 
     FIG. 5 illustrates the relationship of the mirror with its axes of rotation. 
     FIG. 5A is a view along the line  5 A— 5 A of FIG.  5 . 
     FIG. 5B is a view along the line  5 B— 5 B of FIG.  5 . 
     FIG. 6 illustrates an additional feature of the gimballed mirror assembly. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawings, which are not necessarily to scale, like or corresponding parts are denoted by like or corresponding reference numerals. In addition, terms such as top, bottom, front, back, etc. are used herein for ease of explanation and not as structural or orientation limitations. 
     FIG. 1 is a simplified representation of a LADAR system  10 . A laser unit  12  is operable to generate a beam  13  of electromagnetic energy of a certain wavelength. The beam  13  impinges upon a comer turner in the form of a gimballed mirror assembly  14  having a mirror  15  which is of a material to reflect the wavelength of beam  13 . The redirected beam passes through an optics unit  16  and is again redirected by gimballed mirror assembly  18 . After passing through optics unit  20 , the beam is again redirected by gimballed mirror assembly  22  through optics unit  24 , which internally includes a beam splitter. After two reflections from gimballed mirror assemblies  26  and  28 , the beam exits through port  30 . 
     As seen in FIG. 1B, the return beam  13 ′ is directed to detector system  32  by means of gimballed mirror assemblies  26  and  28  and optics unit  24 . A visual display of a target may then be accomplished by processing in the detector system. 
     The present invention is able to perform the function of beam redirection in such a LADAR environment where space is at a premium and extreme vibrations are generally encountered. FIGS. 2 and 3 illustrate, in respective exploded and assembled views, a gimballed mirror assembly  40  in accordance with one embodiment of the present invention. 
     Assembly  40  includes an inner gimbal unit  42 , which is generally circular, and which has a front  43  and back  44 , with an internal cavity  45  extending through the gimbal unit from front to back. A mirror  46  is placed in cavity  45  and is secured in position by means of a retaining ring  47 . A pair of diametrically opposed projections  50  and  51  extend laterally from the body of the gimbal unit, with projection  50  extending to the right and projection  51  extending to the left. 
     A depression  54  is formed in the top of the gimbal unit, and is preferably conically shaped. With additional reference to FIG. 2A, a similar conical depression  55  is positioned in the bottom of the gimbal unit  42 , diametrically opposed to conical depression  54  in the top. Additionally, it is seen that the backs of the projections  50  and  51  are provided with respective conical depressions  56  and  57 . 
     Assembly  40  also includes an outer gimbal unit  60  having a back  61  and a peripheral wall  62  defining an internal cavity  63 , into which the inner gimbal unit  42  is placed. The peripheral wall  62  is generally rectangular with rounded corners, to save weight, and includes a top portion  64 , a bottom portion  65  and right and left side portions  66  and  67 . A threaded aperture  70  extends completely through the top portion  64  and is vertically aligned with a conical depression  71  formed in the inner surface of bottom portion  65 . Each side portion  66  and  67  has a respective conical depression  72  and  73  ( 73  not seen in FIG.  2 ), formed in the outer surfaces of the sides  66  and  67 , with these two conical depressions  72  and  73  being directly opposed. 
     As best seen in FIG. 2B, the back  61  of outer gimbal unit  60  includes conical depressions  74  and  75 , vertically aligned, and two apertures  76  and  77 , horizontally aligned. FIG. 2B also shows the conical depression  73 . 
     A housing  80  of the gimballed mirror assembly  40  includes an apertured back  81  and side portions  82  and  83  defining a cavity  84  for receiving the outer gimbal unit  60 . A threaded aperture  85  extends completely through the side portion  83  and is horizontally aligned with a conical depression  86  ( 86  not seen in FIG. 2) in side portion  82 . The housing  80  additionally includes a mounting section  90  having mounting holes  91  and an open passageway  92  for gaining access to the mounting holes. 
     As best seen in the view of FIG. 2C, the back  81  of housing  80  includes vertically aligned apertures  94  and  95 , as well as horizontally aligned apertures  96  and  97 . FIG. 2C also shows the conical depression  86 . 
     The assembly of the components can best be described with additional reference to FIGS. 4,  4 A and  4 B. Mirror  46  and retaining ring  47  are placed within inner gimbal unit  42 , and as seen in FIG. 4B, the retaining ring  47  is threaded to positively secure the mirror  46 . Further, the threads of retainer ring  47  may be provided with an adhesive so that it does not vibrate loose during operation. 
     A first ball bearing  100  is placed within conical depression  71  in the inner surface of the bottom of outer gimbal unit  60 . The inner gimbal unit  42  is inserted into the outer gimbal unit  60  such that conical depression  55  on the bottom of inner gimbal unit  42  rests upon the ball bearing  100 . A second ball bearing  101  is inserted through threaded aperture  70  in the top  64  of outer gimbal unit  60  and is secured in place by means of a threaded set screw  102 , having a concave conical tip portion  103  which contacts ball bearing  101 . With this arrangement, and with the orientation of FIG. 4A, inner gimbal unit  42  is permitted to experience limited rotation in azimuth about a vertical axis. 
     Next, housing  80  may be temporarily placed on its right side and a third ball bearing  110  placed within conical depression  86  (FIG. 2C) in the inner surface of side  82 . The outer gimbal unit  60  is inserted into the housing  80  such that conical depression  72  on the right side  66  of outer gimbal unit  60  rests upon the ball bearing  110 . A fourth ball bearing  111  is inserted through threaded aperture  85  in the side  83  of housing  80 , and is secured in place by means of a threaded set screw  112 , having a concave conical tip portion  113  which contacts ball bearing  111 . With this arrangement, and with the orientation of FIG. 4A, outer gimbal unit  60  is permitted to experience limited rotation in elevation about a horizontal axis. 
     With the inner and outer gimbal units  42  and  60  freely moveable within housing  80  around respective vertical and horizontal axes, the gimbal units may now be adjusted and secured in a particular orientation for actual use. This is accomplished by a series of adjusting screw assemblies. 
     More particularly, a first adjusting screw assembly  120  includes an adjusting screw  121  preferably of the type having a ball bearing  122  at the end thereof. An adjusting screw holder  123  has a knob portion  124  and a shank portion  125 , which is threaded both externally and internally. Similarly, a second adjusting screw assembly  130  includes an adjusting screw  131  having a ball bearing  132  at the end thereof. An adjusting screw holder  133  has a knob portion  134  and a shank portion  135 , which is threaded both externally and internally. 
     These two adjusting screw assemblies,  120  and  130 , are threaded into respective apertures  76  and  77  in the back of outer gimbal unit  60  (FIG.  2 B), and the adjusting screws  121  and  131  advanced to contact respective conical depressions  56  and  57  in the backs of projections  50  and  51 . Thereafter, the inner gimbal unit  42 , and therefore mirror  46 , may be set to a desired azimuth angle by a push-push operation wherein one of the adjusting screws applying a force is slightly backed out while the other is advanced to compensate. This arrangement eliminates the need for any type of spring as there is always a force being applied by the pushing adjusting screw. Chatter or vibration is minimized by the application of sufficient force to eliminate any movement in the aligned component. 
     A third adjusting screw assembly  140  includes an adjusting screw  141  having a ball bearing  142  at the end thereof. An adjusting screw holder  143  has a knob portion  144  and a shank portion  145 , which is threaded both externally and internally. A fourth adjusting screw assembly  150  includes an adjusting screw  151  having a ball bearing  152  at the end thereof. An adjusting screw holder  153  has a knob portion  154  and a shank portion  155 , which is threaded both externally and internally. 
     These two adjusting screw assemblies,  140  and  150 , are threaded into respective apertures  94  and  95  in the back of housing  80  (FIG.  2 C), and the adjusting screws  141  and  151  advanced to contact respective conical depressions  74  and  75  in the back of outer gimbal unit  60 . Thereafter, the outer gimbal unit  60  may be set to a desired elevation angle by the aforementioned push-push operation. 
     Many gimballed mirror assemblies are constructed such that the mirror rotates about axes other than on the mirror surface. This type of design can add to the cost and complexity of the assembly. In the present invention the design is such that the azimuth and elevation axes lie on the surface of the mirror. This is demonstrated in FIG. 5 which shows mirror  46  in relation to conical depressions  54 ,  55  and  72 , 73 . A line drawn between the apex of conical depression  54  and the apex of conical depression  55  (on top and bottom of inner gimbal unit  42 ) defines the azimuth, or vertical axis V. It is seen in FIG. 5A that axis V lies on the surface  158  of mirror  46 . Similarly, a line drawn between the apex of conical depression  72  and the apex of conical depression  73  (on the right and left sides of outer gimbal unit  60 ) defines the elevation, or horizontal axis H, also lying on the surface  158  of mirror  46 , as seen in FIG.  5 B. 
     The use of conical depressions with mating ball bearings used throughout the design of the present invention ensures that the conical surface mates with a spherical surface of the ball bearing, with the intersection defining a circular line. This design distributes mating forces evenly about the circular line which reduces internal stresses on the conical depression and reduces any potential deflections that may tend to occur during operation. 
     In dynamic environments where the gimballed mirror assembly may be subject to severe vibration it would be desirable to have an indication of such vibration in order to provide proper compensation in the detection system. To this end, and with reference to the sectioned side view of FIG. 6 (similar to FIG.  4 B), the gimballed mirror assembly  40  may additionally include an accelerometer to provide an output signal indicative of encountered vibration. Accelerometer  160  is positioned just behind mirror  46  close enough to obtain an indication of mirror movement but without actually touching the mirror. An accelerometer mount  162  is threaded into an aperture in the back of the outer gimbal unit  60  and extends out the rear of housing  80  through aperture  164 . 
     The metal components of the gimballed mirror assembly may be machined from 6061-T6 aluminum which provides for a lightweight assembly and has a strength-to-weight ratio that rivals steel. The gimballed mirror assembly may be used in environments ranging from the laboratory to vibration intensive operation such as on a jet aircraft. The gimballed mirror assembly requires no springs and can be designed to accommodate various mirror sizes without any mirror distortion. 
     While the invention has been described with reference to certain preferred embodiments, numerous changes, alterations and modifications to the described embodiments are possible without departing from the spirit and scope of the invention as defined in the appended claims, and equivalents thereof.