Abstract:
A multi-axis gimbal has each axis defined by a respective spherical shell driven by a flat, compact motor attached to the driven shell and to a next outer shell (or to an external mounting platform, in the case of the outermost shell). The shells rotate about respective axes. In one configuration, the outermost shell is referred to as the “azimuth” shell because in use it rotates about a vertical axis. The next inner shell is an elevation shell that rotates about a first horizontal axis that is orthogonal to the axis of the camera or other sensor payload. An optional third shell can be used to provide “roll” motion, such as rotating a camera about its axis to obtain a particular rotational orientation with respect to a target.

Description:
BACKGROUND 
     Gimbals are used as rotatable supports for cameras and other sensing payloads in a variety of applications, particularly in vehicle-mounted applications such as helicopters, unmanned aerial vehicles (UAVs), land vehicles such as trucks or armored vehicles, and spacecraft. 
     The basic configuration of a conventional prior art gimbal is one which dictates a necessarily costly and heavy frame structure in order to provide adequate stability. Weight, and very often cost, prevent these gimbals from being used on many platforms such as small helicopters and other lightweight air vehicles. The geometries of the azimuth and elevation axes require a very strong and stiff “yoke” structure. This becomes the large and heavy bulk of the gimbal. Metals, usually steel and aluminum, must be incorporated into the design in order for the gimbal to achieve the stability that an application requires. The manufacturing methods that are necessarily used are expensive and time consuming. This is a cost driver and accounts for much of the overall weight. 
     The elevation axis is anchored by this yoke and resides within the azimuth structure. Most payloads are carried directly by the elevation axis structure. Due to this arrangement, the available payload space tends to be very small in relation to the extents of the azimuth structure. In rare instances a third, roll axis is included. This resides within the elevation structure, further reducing the payload volume. The need for this configuration to be stable and stiff also dictates costly and heavy materials. 
     For these reasons, this standard design is expensive and prevents these gimbals from offering low weight with enough payload space for many applications. 
     SUMMARY 
     A proposed multi-axis gimbal overcomes the above obstacles by means of a novel approach to the geometry of the gimbal. Each axis is defined by a respective spherical shell driven by a flat, compact motor attached to the driven shell and to a next outer shell (or to an external mounting platform, in the case of the outermost shell). The shells rotate about respective axes. In one configuration, the outermost shell is referred to as the “azimuth” shell because in use it rotates about a vertical axis. The next inner shell is an elevation shell that rotates about a first horizontal axis that is orthogonal to the axis of the camera or other sensor payload. An optional third shell can be used to provide “roll” motion, such as rotating a camera about its axis to obtain a particular rotational orientation with respect to a target. 
     The inherent stiffness of the geometry of a sphere is utilized to provide the stability that would be required by even a sensitive optical system. The spheres can be relatively thin-walled and made of plastic, such as polycarbonate. This aspect alone reduces the weight by many pounds and makes production cost-effective. 
     The way the spheres interrelate also makes the three-axis configuration a space effective one. These “nested” spheres allow a tremendous increase in payload volume for a given outer diameter. 
     This geometry also allows modularity, giving the end user the option to delete the roll axis if the application does not require it. Size may vary. The exterior shell diameter can be increased or decreased as space allows. The shells can maintain their relative sizes without compromising stiffness and stability. 
     The disclosed gimbal provides multiple-axis targeting and pointing stability to a variety of sensing applications while being lightweight and low cost. It also offers a larger ratio of payload capacity to overall size than a conventional gimbal. The gimbal has the following main elements: a mechanized drive system to control each axis, and a spherical housing to provide structural support for each axis. The number of axes relies upon the needs of the end user. The gimbal is scaleable in both size and functionality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is an elevation view of a prior art yoke-type gimbal; 
         FIG. 2  is a perspective schematic rendering of a multiple axis gimbal in accordance with the present invention; 
         FIG. 3  is a block diagram of a sensor system employing the multiple axis gimbal of  FIG. 2 ; 
         FIG. 4  is a perspective solid-model view of a first multiple axis gimbal; 
         FIG. 5  is a perspective line drawing of the gimbal of  FIG. 4 ; 
         FIG. 6  is a side view of the gimbal of  FIG. 4 ; 
         FIGS. 6A and 6B  are section views along lines A-A and B-B of  FIG. 6 ; 
         FIG. 7  is a rear view of the gimbal of  FIG. 4 ; 
         FIG. 7C  is a section view along line C-C of  FIG. 7 ; 
         FIG. 8  is a plan view of a motor in the gimbal of  FIG. 4 ; 
         FIG. 8A  is a section view along line A-A of  FIG. 8 ; 
         FIGS. 9 and 10  are perspective, semi-exploded views of the gimbal of  FIG. 4  depicting the configuration of the azimuth motor and upper mounting member; 
         FIG. 11  is a perspective, semi-exploded view of the gimbal of  FIG. 4  depicting the configuration of the elevation motor and elevation sphere within the outer (azimuth) shell; 
         FIGS. 12 and 13  are perspective, semi-exploded views of the inner (roll) shell of the gimbal of  FIG. 4  showing the presence and absence of a payload camera on a payload frame; 
         FIG. 14  is a perspective line drawing of a second multiple axis gimbal; 
         FIG. 15  is a side view of the gimbal of  FIG. 14 ; 
         FIGS. 15A and 15B  are section views along lines A-A and B-B of  FIG. 15 ; 
         FIG. 16  is a rear view of the gimbal of  FIG. 14 ; 
         FIG. 16C  is a section view along line C-C of  FIG. 16 ; 
         FIG. 17  is a plan view of a motor in the gimbal of  FIG. 14 ; 
         FIG. 17A  is a section view along line A-A of  FIG. 17 ; 
         FIGS. 18 and 19  are perspective, semi-exploded views of the gimbal of  FIG. 14  depicting the configuration of the azimuth motor and upper mounting member; 
         FIG. 20  is a perspective, semi-exploded view of the gimbal of  FIG. 14  depicting the configuration of the elevation motor and elevation sphere within the outer (azimuth) shell; 
         FIGS. 21 and 22  are perspective, semi-exploded views of the inner (roll) shell of the gimbal of  FIG. 14  showing the presence and absence of a payload camera on a payload frame; 
         FIG. 23  depicts pertinent portions of a gimbal employing an air bearing between spheres along with low-friction support pads (e.g. TEFLON) and linear motors within the spheres; 
         FIG. 24  is a diagram showing the use of momentum reaction wheels to adjust attitude; 
         FIG. 25  depicts pertinent portions of a gimbal employing wet or dry fluid bearing between; 
         FIGS. 26-27  depict gimbal shells having integral wire channels to hold wires or fiber optic cable(s) for motors, encoders, and payload/sensor wires; 
         FIGS. 28-30  show a gimbal including rotary joint twist capsule electrical and or fiber optic as integral part of a gimbal drive assembly; 
         FIGS. 31-32  depicts a gimbal in which signals from the payload (e.g., camera) are passed optically through a transparent portion of the gimbal shells; and 
         FIG. 33  depicts a gimbal in which signals from the payload are sent wirelessly via a transceiver across the shells to an external electronic transceiver. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a gimbal  10  of the “yoke” type as described above. The gimbal  10  has a frame  12  with an inverted “U” shape, which is coupled to a mount  14  for rotation about an azimuth axis  16 . At the ends of the frame  12  are two inward-facing mounts  18  for a payload (not shown) that can be rotated about an elevation axis  20 . As discussed above, the frame  12  of the yoke type of gimbal  10  is generally large and heavy, and therefore may not be a good candidate for use in certain applications. 
       FIG. 2  is a schematic depiction of a multiple-axis gimbal  22  employing a different type of support structure, namely a set of nested spherical shells. As shown, an outer (azimuth) shell  24  may have a motor  26  for rotating the gimbal  22  (and internal payload) about an azimuth axis  28 . A next inner (elevation) shell  30  is coupled to the outer shell  24  by a motor  32  which provides for elevational rotation. Some applications may benefit from a third (roll) shell  34  and motor  36  to achieve rotation about a “roll” axis  38 . Each motor  26 ,  32  and  36  may be a direct-drive or “torque” motor with a position encoder to sense motor position. The position encoder may be any of different types, including magnetic and optical. 
       FIG. 3  illustrates an overall gimbal system including a gimbal  22  and a controller  40 . The controller  40  includes separate sub-controllers  42 ,  44  and  46  for the azimuth, elevation, and roll motors  26 ,  32  and  36 . Each sub-controller  42 ,  44  and  46  provides respective drive signals to the respective motor  26 ,  32  and  36 , and receives respective position signals from the respective position encoder. An inertial measurement unit (IMU)  47  within the gimbal  22  provides rate and acceleration (RATE, ACCEL.) information back to the gimbal controller  40 . The gimbal controller  40  also receives GPS and compass information from external devices not shown in  FIG. 3 . 
       FIG. 4  shows a first gimbal  48  which is an embodiment of the schematically depicted gimbal  22  of  FIG. 2 . The first gimbal  48  employs a transparent hemisphere  50  on its outer shell  52 . The hemisphere  50  provides protection for the gimbal interior, including the payload. In the illustrated embodiment, the hemisphere  50  is translucent to visible light, and may be made of synthetic sapphire for example. In alternative embodiments, other materials that are translucent in other ranges of the electromagnetic spectrum, such as in the infrared region for example, may be employed. 
       FIG. 5  shows the first gimbal  48  in a perspective, line drawing format. At the top of the outer shell  52  is an azimuth motor assembly  54  that includes the azimuth motor  26  (with position encoder; see  FIG. 2 ).  FIGS. 4 and 5  both show the azimuth motor assembly  54  as including a cylindrical mount  56  by which the gimbal  48  can be mounted to a vehicle or other platform. 
       FIG. 6  shows a front elevation view of the gimbal  48  with two section lines A-A and B-B.  FIG. 6A  shows the view of section A-A, and  FIG. 6B  shows the view of section B-B. In addition to the outer (azimuth) shell  52 , the gimbal  48  includes an elevation shell  58  and a roll shell  60 . The shells  58  and  60  have openings toward the left in  FIG. 6A  for the passage of light to the sensors (not shown), which are to be mounted in a rectangular space  62  within the roll shell  60 . Within the roll shell  60  is a package of electronic circuitry referred to as an “inertial measurement unit” or IMU  64 . The azimuth motor assembly  54  and a roll motor assembly  66  are also shown. Although in  FIG. 6A  the IMU  64  is shown as a single unit, in alternative embodiments the components that collectively provide the IMU function may be spatially distributed within the gimbal rather than being concentrated in a single physical package. 
       FIG. 7  shows a side elevation view of the gimbal  48  with a section line C-C, and  FIG. 7C  shows a view of the section C-C. This view shows the elevation motor assembly  68  which is disposed between the azimuth shell  52  and the elevation shell  58 . 
       FIG. 8  shows a plan view of the elevation motor assembly  68  with a section line A-A, and  FIG. 8A  shows the view along the section line A-A. The elevation motor assembly includes a rotor housing  70 , stator housing  72 , an encoder circuit board  74  and an encoder magnetic strip  76 , which is disposed around a radially outer surface of rotor housing  70 . During operation, magnetic sensing circuitry on the encoder board  74  detects a spatially periodic magnetic field created by the magnetic strip  76  and generates corresponding electrical encoder signals that indicate the rotational position of the elevation motor. These signals are provided as motor position feedback signals to the elevation motor sub-controller  44  ( FIG. 3 ) which uses the signals to control the rotational position of the rotor  70  with respect to the stator  72 . In the illustrated embodiment, the azimuth motor assembly  54  uses a similar arrangement to that shown in  FIG. 8A . 
       FIGS. 9 and 10  show the mounting arrangement in more detail. The azimuth motor assembly  54  including the encoder circuit board  74  fits within the cylindrical mount  56 , and the stator  72  is secured to the mount  56 . The rotor housing  70  is attached to the azimuth shell  52 . 
       FIG. 11  illustrates another aspect of the construction of the gimbal  48 , namely the use of two half-shells  52 -A and  52 -B to form the azimuth shell  52 . The half-shells  52 -A and  52 -B are attached to each other in a suitable manner. In the illustrated embodiment, the mating edges of the half-shells  52 -A and  52 -B form a lap joint, and through-holes  80  accept mounting hardware such as bolts or rivets to secure the two lapped edges together. In the illustrated embodiment, the elevation shell  58  and roll shell  60  are also formed in this manner, i.e., using two half-shells that are mechanically coupled together. 
       FIGS. 12 and 13  illustrate the configuration of the roll shell  60  and its inner payload. The roll shell  60  has two half-shells  60 -A and  60 -B which are coupled together as described above with respect to the azimuth shell  52 . Within the roll shell  60  is mounted a payload frame  82  to which is attached the IMU  64  at attachment points  84 . The payload frame  82  receives a payload such as a camera  86 . An extended front portion  88  of the camera  86  is disposed through an opening  90  of the payload frame. 
       FIGS. 14-22  illustrate a second gimbal  92  of the same general type as depicted in  FIG. 2 . Many of the elements of the gimbal  92  are similar to corresponding elements of the gimbal  48 , so any such elements of the gimbal  92  are referred to using the same reference number with the addition of a single quote mark. Thus the gimbal  92  includes an azimuth shell  52 ′ having two half shells  52 -A′ and  52 -B′, etc. The main differences between the two gimbals  48  and  92  are described below. 
     Referring to  FIG. 14 , it will be seen that the azimuth shell  52 ′ of the gimbal  92  has an elongated slot  94  in contrast to the transparent hemisphere  50  of the gimbal  48 . The slot  94  extends approximately one-quarter of the way around the azimuth shell  52 ′ to provide a range of elevation angles from horizontal (horizon) to straight down. The slot  94  is intended to be left open rather than covered. For some applications, any shell material might interfere with optimal operation of the camera or sensor, for example because of the particular frequencies of electromagnetic energy being sensed. The open arrangement enables operation at such frequencies. 
       FIG. 17A  shows that the motor assemblies of the gimbal  92  (e.g. the elevation motor assembly  68 ′ as shown) employ a different type of position encoder. An encoder optical read head  96  is disposed on the stator housing  72 ′, and an annular diffraction grating  98  is secured to the rotor  70 ′. The optical read head  96  detects an optical pattern created by light reflected from the diffraction grating  98  and generates the electrical position signals that are provided to the corresponding sub-controller  42 ,  44  and  46  ( FIG. 3 ). 
     The gimbals  48  and  92  may optionally have or employ any of a variety of other features. As an example, there may be some type of sealing between the azimuth motor  54  and the mount  56 , e.g. using O-rings or brushes. The innermost rotatable support element (i.e., roll or elevation spheres) may have other than a spherical shape, or may be only a partial sphere. For example, the roll support element could be planar, cubic, or a more arbitrary shape attached inside the elevation sphere via the roll motor. 
     In the illustrated embodiment, the hemispheres of each sphere are joined with lap joints. Alternative joining techniques include butt joints, slot-in-groove joints, etc. Additionally, in alternative embodiments the spheres may be formed from more than two elements, or from two complementary partial spheres other than hemispheres that are joined at other than an equator. 
     Also in the illustrated embodiment, each sphere is supported by only a single corresponding motor that couples the sphere to the next outer sphere at one pole of a rotational axis. The gimbal may employ additional support for one or more of the spheres, such as by using a bearing at the other pole of the rotational axis from the motor to reduce cantilever stresses. Ribbing or other structure may also be added to improve stiffness. 
     The disclosed gimbal is ideally suited for manufacture using conventional injection molding techniques. As an alternative to standard injection-molded plastics, composite materials and even metals may be employed instead. 
     The surface of each sphere need not be continuous and featureless such as shown in the Figures. Alternative embodiments may employ a ribbed or pocketed structure to reduce weight and increase stiffness. Such structure may be either in addition to or entirely instead of the continuous, featureless structure such as shown in the Figures. 
     The gimbal may employ other types of drive mechanisms including the following: 
     a. Harmonic drive 
     b. Belt drive 
     c. Gear drive 
     d. Viscous coupling 
     The gimbal may also employ other types of position encoders, including: 
     a. Inductosyn encoder (multiphase electrical) 
     b. Optical encoders using Talbot effect 
     c. Optical encoders using transmission gratings 
     The gimbal may employ other support and drive mechanisms including the following: 
     a. Air bearing  100  between spheres ( FIG. 23 ) with low-friction support pads  102  (e.g. TEFLON) 
     b. Linear motors  104  within the shells ( FIG. 23 ) 
     c. Momentum reaction wheels  106  to adjust attitude ( FIG. 24 ) 
     d. Wet or dry fluid bearing  108  between shells ( FIG. 25 ); high surface tension; may employ attractant and repellant on shell surfaces to adhere fluid to one and avoid shearing with the other. 
     The gimbal shells can have integral wire channels  110  to hold wires or fiber optic cable(s) including flat /ribbon for motors, encoders, and payload/sensor wires ( FIGS. 26-27 ). Channels can be depressions in shell, either left uncovered or covered. 
     The gimbal may include rotary joint twist capsule  112  electrical and or fiber optic as integral part of the drive assembly ( FIGS. 28-30 ). Slip rings can also be used to pass signals across the shells. 
     Signals from the payload (e.g., camera) can be passed optically through the transparent portion  114  of the gimbal shells ( FIGS. 31-32 ). 
     Signals from the payload can be sent wirelessly via a transceiver  116  across the shells to an external electronic transceiver  118  ( FIG. 33 ). 
     In gimbals employing a transparent window such as the first gimbal  48 , it may be desirable to implement the transparent window in a segmented fashion rather than a single continuous sheet. The segments may be relatively small, flat window segments joined together at their edges. 
     The gimbal can employ inertial stabilization using an inertial reference measurement unit (IRMU) on the innermost gimbal. An IRMU enables geolocation of where the sensor is pointing in inertial space. Different types of IRMU may be employed, including the following: 
     i. MEMS based 
     ii. FOG based 
     iii. Mechanical based 
     iv. Integrated optics based 
     v. Laser Ring Gyro based 
     The gimbal may employ an inertial control design that includes one or more of the following: 
     a. Linear multivariable techniques such as:
         i. H-Infinity   ii. H-2   iii. Mu-Synthesis   iv. Loop-Transfer Recovery (“LTR”)   v. Individual channel design (“ICD”)       

     b. Nonlinear techniques such as:
         i. Inverse kinematic control   ii. Feedback linearization   iii. Sliding mode control       

     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.