Patent Abstract:
A method and apparatus for balancing a statically unbalanced system, particularly useful in balancing X-ray and mammography systems having an X-ray source and detector mounted to a rotational arm, includes a linear actuator coupled to a drive mechanism which synchronizes the activation of the linear actuator with the rotation of the arm of the mammography system. The drive mechanism can include intermeshed gears, wheels or gears coupled together with a chain or belt, or other types of synchronizing drives. The balancing mechanism reduces the amount of force necessary to rotate the arm and decreases the overall weight of the system by eliminating the need for counterweights to maintain balance of the system.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION  
         [0001]    Not applicable.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    Not applicable.  
         BACKGROUND OF THE INVENTION  
         [0003]    In a number of clinical applications, X-ray apparatuses which include an X-ray source and X-ray detector mounted to a rotational arm are common. These systems provide the ability to rotate the X-ray source and detector to varying angles to obtain images without requiring the patient to move. A mammography system, for example, typically comprises an X-ray source, an X-ray detector, a breast support plate, and a breast compression plate. The source and detector are mounted to opposing ends of an arm, such as a C-arm, and the arm is disposed to rotate around the breast support and compression plates. The breast is positioned between the breast support and breast compression plates to hold the breast in place during mammography, and is arranged between the source and the receiver on the opposing ends of the C-arm. During mammography, the C-arm is rotated about the breast plates such that images of the breasts are acquired from varying angles.  
           [0004]    For construction reasons, and due to the varying weights of the components, the center of mass of the rotatable C-arm is typically spaced apart from the axis of rotation, and is therefore “unbalanced” about the axis of lateral rotation. In an unbalanced system, a significant torsional force must be applied to rotate the arm to a desired position. It is desirable, however, to reduce the amount of force required to rotate the arm, to simplify use of the equipment for medical personnel.  
           [0005]    To balance the system, counterweights are typically used to provide a counteractive torsional force. While counterweights significantly reduce the torsional force that must be applied when rotating the arm, they add significantly to both the weight and cost of the system. Furthermore, the counterweights make it very difficult to move the mammography system from place to place when desired. It is desirable, therefore, to provide alternate methods for balancing a mammography or other imaging system comprising an arm in which the torsional force required for rotation is reduced.  
         SUMMARY OF THE INVENTION  
         [0006]    In one aspect, the invention comprises an X-ray apparatus including an arm rotatably coupled to a pivot point on a base support, the arm including an X-ray source and an X-ray detector coupled to opposing ends. A linear actuator is pivotally coupled to the base support at one end and to a drive mechanism at the opposing end. The drive mechanism synchronizes the axis of rotation of the arm with the axis of rotation of the linear actuator such that a force applied by the linear actuator at a contact point on the drive mechanism balances a torque force of the arm.  
           [0007]    In another aspect, the invention comprise an X-ray apparatus including a base support, a rotational member pivotably coupled to the base support, and an arm having an X-ray source coupled to the first end and an X-ray detector coupled to the second end. The arm is mounted to the rotational member for rotation relative to the base support. A first cogwheel is coupled to the rotational member, and a second cogwheel is meshed with the first cogwheel. A constant force linear actuator is coupled between the second cogwheel and to the base support, an active connection provided at the cogwheel and an inactive connection at the base support. As the arm is rotated the first and second cogwheels maintain a one to one correspondence between an angle of rotation of the arm and an angle of rotation of the active connection point of the linear actuator, such that the constant force linear actuator applies a torsional force to counterbalance the torque force of the arm and to provide a substantially statistically balanced system.  
           [0008]    Another aspect of the invention is to provide a statically balanced mammography system. The mammography system comprises a base support, an arm rotatably coupled to the base support, and a linear actuator rotatably coupled to the base support at a first end and to a rotational member at a second end. The linear actuator applies an upward force at a connection point between the linear actuator and the rotational member. A drive mechanism is coupled between the rotational member and the arm, wherein as the arm is rotated, the drive mechanism synchronizes the angle of rotation of the arm with the angle of rotation of the connection point such that the applied force of the linear actuator counteracts the torque force of the arm to balance the torsional force of the arm.  
           [0009]    These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is an illustration of a mass rotatably mounted to a rotatable arm;  
         [0011]    [0011]FIG. 2 is an illustration of a counteractive force for balancing the system of FIG. 1;  
         [0012]    [0012]FIG. 3 is an illustration of a linear actuator mechanism working directly on the rotatable arm;  
         [0013]    [0013]FIG. 4 is an illustration of a balancing mechanism including a linear actuator and a gear drive for applying a counteractive force to the rotatable arm;  
         [0014]    [0014]FIG. 5 is an illustration of an alternate embodiment of a balancing mechanism;  
         [0015]    [0015]FIG. 6 is a block diagram of a balancing mechanism for balancing a rotatable arm;  
         [0016]    [0016]FIG. 7 is a perspective view of a typical mammography system; and  
         [0017]    [0017]FIG. 8 is a perspective view of a support base for a mammography system incorporating a balancing mechanism as described herein with a housing removed to illustrate the balancing mechanism. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring now to FIG. 1, an arm  10  having a length AB and comprising a rotating body  12  having mass “m”, and a center of mass “B” is rotatable about an axis of rotation  14  or pivot point “A”. When the arm  10  is released from a static position, the arm rotates around the rotation of axis  14  in a direction determined by the force of gravity “g” the arm reaches a stable position when the direction of the arm  10  correlates with the direction of the gravity vector “g”, and therefore when the angle of rotation is either zero or one hundred and eighty degrees. When released from a stable position, as shown, at which the angle of rotation is φ=0°, the direction of motion of the arm around the axis  14  is downward, and, as required by the principle of minimum potential, stabilizes at the angle φ=180°. As the arm rotates, the torque force of the arm (M A  (φ) at a given rotational angle is defined by the following equation:  
           M   A (φ)= m*g*AB *sin φ.  [equation 1] 
         [0019]    As described above, the minimum potential stable position for the mass “B” is φ=180, with the center of mass directly below the axis of rotation  14 . To balance the system to maintain the unbalanced body in a static position which is not equivalent to 180 degrees, an opposing torque must be applied to counter the torque “M A  (φ)” of the center of mass “B”.  
         [0020]    Referring now to FIG. 2, to provide a counteractive torque, a mechanism which is rotatable about the minimum potential static resting point (i.e. the one hundred and eighty degree resting point) of the center of mass B and which provides an active force on the arm  10  in a circular path can be provided to deliver a torque in the direction opposite to and of substantially the same magnitude as the torque “M A  (φ)”. Here, the counteractive force is F C , and is directed at an active contact point  21 (C) on the arm  10 , located a distance AC along the arm  10 . Referring again to equation 1, to balance the torque “M A  (φ)”, the counteractive torque must be equivalent to the downward torque of the arm. Therefore:  
           m*g*AB=F   C   *AC   [equation 2] 
         [0021]    The torque F C *AC provides static balancing, such that properly chosen values of F C  and AC results in a balance in which the sum of the torques in the system ΣM A (φ) equals zero.  
         [0022]    Referring now to FIG. 3, a balancing mechanism  20  for providing the opposing torque described above is shown. The mechanism  20  comprises a linear actuator  22  coupled between a pivotal or rotational axis  28  and a contact point  21  on the arm  10 , the contact point  21  being provided between the axis of rotation of the arm  14  and the body  12  of mass “m” The statically unbalanced mass m, with center of gravity “B” is rotatable around the “A” axis  14 , due to the force of gravity, as described above, to provide a torque “M A (φ)” which, when not in a static position, provides unbalance in the system. The unbalance is dependent on the angle of rotation φ, and is directed to drive the mass “m” to the rotation angle of φ-180°. As the center of mass “B” rotates on a circular path around the axis  14 , the active contact point  21  of the linear actuator  20  also rotates on a circular path, at the same angle, thus the linear actuator  20  provides a nearly constant force in a direction opposite the torque of the arm and opposing the vector of gravity at the active contact point  21 . The linear actuator  20  is coupled to a rotation axis  24  and is able to be rotated or pivoted around it.  
         [0023]    Referring now to FIG. 4, a balancing mechanism  20  in which the balanced body  12  is separated from the linear actuator  22  by a gear drive  26  is shown. The gear drive  26  comprises a first cogwheel  28 , a second cogwheel  30 , and a linear actuator  22  extending from a pivot point  24  to an active contact point  21  on the cogwheel  30 . The cogwheel  28  is fixed to the unbalanced body  12  of mass m and center of mass B and is rotatable about the same axis  14  as the arm  10 . The active point of the balancing linear actuator  22  is connected to the contact point  21  of the cogwheel  30 , which rotates about the axis  32 . The cogwheels  28  and  30  are equivalent in size and each include a plurality of teeth  34  and  35 , respectively, extending from the outer diameter of each of the cogwheels  28  and  30 , and are assembled such that the angle of rotation φ 2  of the linear actuator  22  is equal in magnitude to the angle of rotation φ 1  of the arm  10 , which results in a balancing machine with properties similar to those described above with reference to FIG. 3. As shown here, and in FIGS. 5, 6, and  8  below, the balancing mechanism is shown in a vertical configuration. The balancing mechanism, however, can be built in any direction, provided that when the linear actuator is at a maximum position, the arm is in upward position.  
         [0024]    Referring now to FIG. 5, a second embodiment of the gear drive  26  is shown. Here, the cogwheels  28  and  30  are driven by a synchronous belt or chain  36 , the belt or chain  36  providing a 1:1 ratio of motion between the wheels  28  and  30  such that an angular rotation of the cogwheel  28  results in an equal rotation of the cogwheel  30 . The angle of rotation of the contact point  21  of the linear actuator  22  φ 2  therefore equals the angle of rotation of the arm  10  (φ 1 ).  
         [0025]    Referring now to FIG. 6, a generalized drive mechanism  40  for synchronizing the motion of the linear actuator  22  and the arm  10  is shown. Here, the linear actuator  22  is coupled to a rotational member which can be, for example a wheel, a non-zero length arm or a cogwheel  30  as shown. The cogwheel  30  is coupled to a drive mechanism  40  which provides a synchronized connection between the rotational axes of the unbalanced mass  12  and the balancing mechanism  20 . The “black box” drive mechanism  40  is selected to provide an angle of rotation of the cogwheel  30  that is equivalent to that of the arm  10  such that the angles of rotation of the arm  10  and of the contact point  21  of the linear actuator  12  are equivalent, and the counteractive force provided by the linear actuator  22  therefore counteracts that of the torque of the arm  10 , balancing the system as described above.  
         [0026]    Referring now to FIG. 7, an X-ray apparatus  42  rotatably mounted to an arm  44  is shown. Here, the X-ray apparatus  42  is a mammography system, comprising the arm  44  rotatably mounted to a base support  46  through a rotatable member  48 . An X-ray source  50  is coupled to a first end  51  of the arm  44 , and an X-ray detector  52  is coupled proximate an opposing end  53 , the X-ray source  50  extending substantially perpendicular to the arm and directed toward the X-ray detector  52 , which also extends from the arm such that the detector  52  receives radiation produced by the source  50 . A breast support plate  54 , and a breast compression plate  56 , are positioned between the X-ray source  50  and the X-ray detector  52 .  
         [0027]    Referring now to FIG. 8, the base support  46  is shown with the front housing removed to provide a view of the balancing mechanism  20  including gear device  26 . Here, the gear drive  26  comprises a first cogwheel  28  coupled to the rotating member  48  and disposed to rotate with the rotatable member  48  in the same axis of rotation  14  with the arm  44 . A linear actuator  22  is coupled between a pivot point  24  provided in a wall of the base support  46 , and an active contact point  21  on the second cogwheel  30  such that the contact point  21  of the linear actuator  22  rotates with the second cogwheel  30 . The linear actuator  22  applies the opposing force to the torque of the arm  10  to balance the system, and is therefore selected to provide a relatively constant force, as differentiations in the applied force can degrade the efficiency of the balancing. Suitable linear actuators include a spiral spring, gas spring, pneumatic power cylinder, hydraulic power cylinder, or similar devices which will be apparent to those of skill in the art.  
         [0028]    The first and second cogwheels  28  and  30  are equivalent in size, and each include a plurality of teeth  34  and  35 , respectively extending from the outer diameter of the wheel, such that the teeth  34  of the first cogwheel  28  mesh with the teeth  35  of the second cogwheel  30 . Therefore, as the arm  44  is rotated, the first cogwheel  28  rotates, causing the second cogwheel  30  to rotate. The number of teeth  35  provided on the second cogwheel  30  is equivalent to the number of teeth  34  on the first cogwheel  28 , wherein as the arm  44  is rotated, a 1:1 angular correspondence is maintained between the first and second cogwheels  28  and  30 , respectively. Although the cogwheels  28  and  30  are shown as of the same size with an equivalent number of teeth, varying ways of gearing the drive system  26  provided by the cogwheels  28  and  30  will be apparent to those of ordinary skill in the art.  
         [0029]    In operation, initial images are acquired with the arm  44  in the zero degree position. After images are acquired in this position, the clinician conducting the test typically rotates the arm  44  to a ninety degree position for additional image acquisition. As the arm is rotated, the first cogwheel  28  rotates, causing the second cogwheel  30  also to rotate. The contact point  21  of the linear actuator  20  is also rotated out of position, and applies a nearly constant upward force which, due to the intermeshed cogwheels  28  and  30 , is maintained at an angle equivalent to the angle of rotation of the arm  44 , such that the linear actuator  20  provides a counteractive force to the torque of the arm  44 . The counteractive force is selected to balance the system such that the amount of force required to rotate the arm  44  is significantly reduced. Furthermore, the balancing mechanism eliminates the need for heavy counterweights, also reducing the overall weight of the X-ray system significantly.  
         [0030]    Although the system has been described with reference to an X-ray source and detector, a balancing mechanism as described above can be used in any unbalanced system, including other types of imaging apparatuses, and particularly those in which a source and detector are provided on opposite ends of a rotatable arm. Other applications will be apparent to those of skill in the art.  
         [0031]    While there has been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention defined by the appended claims.

Technology Classification (CPC): 0