Patent Publication Number: US-2011074534-A1

Title: Transformer for a computer tomography gantry

Description:
FIELD OF THE INVENTION 
     The present invention relates to a transformer for a computer tomography gantry for transfering contactlessly electrical energy from a stationary part of the gantry to a rotary part of the gantry and a computer tomography gantry comprising such a transformer. 
     BACKGROUND OF THE INVENTION 
     For high power computer tomography applications the x-ray tube power must be transferred to the rotating gantry. This is currently done by mechanical slip-rings. For high power applications it is necessary to use a rotary power transformer with a stationary primary winding and a secondary winding on the rotating part of the gantry. The power transformer has a circular outline in which the inner diameter is determined by the given inner bore of the computer tomography system. The rotary power transformer comprises a set of E-cores. 
     SUMMARY OF THE INVENTION 
     Due to the fact that a part of the gantry is stationary and another part of the gantry is rotating mechanical bending and mechanical tilting of the rotating part of the gantry is possible. The bending and tilting is caused by electromagnetic forces between the stationary part of the gantry and the rotary part of the gantry. In case of a high rotation speed of the rotary part of the gantry the bending and tilting can result in mechanical resonance and in a worst case scenario the power transformer can be destroyed. 
     It would be desireable to provide an improved device for prevention of a bending and tilting caused by the rotation of the rotary part of the gantry. 
     The invention provides a transformer for a computer tomography gantry for transfering contactlessly electrical energy from a stationary part of the gantry to a rotary part of the gantry, wherein the transformer comprises a set of primary windings, a set of secondary windings, a set of first cores, a set of second cores, wherein the set of primary windings being arranged at the set of first cores on the stationary part of the gantry, such that a winding of the set of primary windings is adapted to induce a magnetic flux into a core of the set of first cores, wherein the set of secondary windings being arranged at the set of second cores on the rotary part of the gantry, such that a winding of the set of secondary windings is adapted to induce a magnetic flux into a core of the set of second cores, wherein the set of first cores and the set of second cores are adapted to reduce mechanical resonant vibrations caused by the rotation of the rotary part of the gantry. 
     In other words the invention deals with the problem of mechanical fluctuations due to the rotation of the rotary part of the gantry. These mechanical fluctuations result in mechanical bending and tilting of the rotary part of the gantry. Therefore, the speed of rotation can be limited to a maximum speed because of these mechanical problems. The worst case scenario is a rotation of the rotary part of the gantry with the eigenfrequency of the mechanical arrangement of the rotary part of the gantry. In this case the bending and tilting arrives at a maximum. Resulting the above-mentioned these mechanical problems could result in damages to the whole computer tomography gantry. The solution to this problem provided by this invention is to break through the symmetry of the arrangement with the help of a special arrangement of the cores of the primary side of the transformer and the secondary side of the transformer. 
     Further, the invention provides a computer tomography gantry comprising a transformer according to one of the claims  1  to  11 . 
     Further embodiments are incorporated in the dependent claims. 
     According to the present invention it is provided a transformer, wherein the number of the cores of the set of first cores is different to the number of cores of the set of second cores. 
     Different numbers of cores on the primary side of the transformer and the secondary side of the transformer leads to a wished unbalance of the arrangement. This unbalance prevents the appearance of eigenfrequencies of the mechanical arrangement. Therefore, an operation of mode of the computer tomography gantry without limits with respect of the speed of the rotation is possible. 
     According to an exemplary embodiment it is provided a transformer, wherein the dimensions of the cores of the set of first cores are different. 
     According to the present invention it is provided a transformer, wherein the dimensions of the cores of the set of second cores are different. 
     It is also possible to use different kinds of cores with respect to the mechanical dimensions of the cores. Usually, it will be used cores with different cross sectional areas. This leads to differences with respect to the magnetic flux, which will be conducted within the cores. These differences of the magnetic flux prevents the appearance of eigenfrequencies of the mechanical arrangement of the rotary part of the gantry. It does not matter which set of cores is changed with respect to the usual arrangement of a set of cores. But it is important to arrive at different arrangements of cores on the primary side of the transformer and the secondary side of the transformer. 
     Especially, it is advantageously to use cores with different widths of the cross-section. The different widths of the cross-section can be seen best with respect to the cross-section of the rotary part of the gantry and the stationary part of the gantry, respectively. 
     According to an exemplary embodiment it is provided a transformer, wherein the smallest distances between the cores of the set of first cores are different. 
     According to an exemplary embodiment it is provided a transformer, wherein the smallest distances between the cores of the set of second cores are different. 
     It is also a possibility to provide an arrangement of cores with different air gaps between the single cores. Such an arrangement leads also to the effect of prevention of mechanical fluctuations. The air gaps can be considered as gaps between the single cores, wherein the air-gaps can be seen best in a cross-section of the rotary part of the gantry or a cross-section of the stationary part of the gantry. 
     According to an exemplary embodiment it is provided a transformer, wherein the dimensions of the cores of the set of first cores and the dimensions of the cores of the set of second cores are different. 
     According to an exemplary embodiment it is provided a transformer, wherein the cores of the set of first cores and the cores of the set of second cores are E-shaped. 
     According to another exemplary embodiment it is provided a transformer, wherein the cores of the set of first cores and the cores of the set of second cores are U-shaped. 
     According to an exemplary embodiment it is provided a transformer, wherein the cores of the set of first cores are arranged in a first circle with a centerline, wherein the cores of the set of second cores are arranged in a second circle with the centerline, wherein a core out of the group consisting of the set of first cores and the set of second cores is rotated with an angle around the axis of rotation of the core, wherein the axis of rotation is parallel with the centerline. 
     According to another exemplary embodiment it is provided a transformer, wherein the angle is between −10 and +10 degrees. 
     It may be seen as a gist of the present invention to provide a mechanical arrangement, which avoids bending and tilting of the rotary part of the gantry especially the bending with eigenfrequencies. 
     It should be noted that the above features may also be combined. The combination of the above features may also lead to synergetic effects, even if not explicitly described in detail. 
     These and other aspects of the present invention will become apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in the following with reference to the following drawings. 
         FIG. 1  shows a part of a transformer, 
         FIG. 2  shows schematically a part of a transformer, 
         FIG. 3  shows an arrangement of cores. 
         FIG. 4  shows a computer tomography gantry. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       FIG. 1  describes a part of a transformer  104 . It is depicted a first winding  105  and a second winding  103 , wherein the first winding  105  is supplied by a current Il and the second winding  103  is supplied by a current I 2 . It is also depicted a cross-section of the first winding  106 , wherein different strands  102  can be recognized. The transformer  104  comprises a core  101 , which is E-shaped. 
       FIG. 2  shows a part of a transformer  201 . It is depicted a first winding  203 , which is supplied by a current I 1 . It is also depicted a second winding  204 , which is supplied by a second current I 2 . The arrangement is completed by an arrangement of cores  202 . The arrangement can be interpreted as part of the primary part of the transformer  201  or as a part of the secondary part of the transformer  201 . 
     Typically the total core of a rotary power transformer comprises a plurality of E-cores  202 , which are arranged in a circle. The diameter of this circle is determined by the inner bore of the computer tomography system. There is a circle of cores  202  at the primary side of the power transformer and at the secondary side of the power transformer. 
     If equal numbers of E-cores  202  are arranged at the primary and the secondary side of the rotary transformer, mechanical oszillations can occur during rotation of the rotary part of the gantry. 
     A first embodiment of the invention uses unequal numbers of E-cores  202  at the primary and the secondary side of the transformer. As an example the primary side of the transformer can comprise 12 cores  202  and the secondary side can comprise 13 cores  202 . With the use of unequal numbers of cores on the primary and the secondary side the resulting cogging torque can be reduced for a desired rotational speed or range of speed. Using unequal number of E-cores on the primary and secondary side the noise generated by the rotation will be reduced also. 
     A second embodiment of the invention uses U-shaped cores, which can be used for lower power applications. A different number of cores on the secondary and the primary side of the power transformer results in reduction of bending and tilting and noise generation. 
     A third embodiment of the invention uses E-cores  301  which have a slight angular rotation (e.g. 1 . . . 10°) around their center line  302 . One advantage of this arrangement is the improved magnetic coupling, since there is more overlap between the E-cores  301  during rotation. A further advantage can be seen in the better cooling of the E-cores  301  which is caused by more airflow generated during rotation. 
       FIG. 3  shows an arrangement of cores  301 , wherein the cores are rotated by an angle around the center line  302 . The center line  302  is parallel to the center line  303  of the circle, which is achieved by the arrangement of the cores  301 . 
     The invention solves especially two problems. Especially, the mechanical bending and tilting of the rotary transformer caused by magnetic forces between the rotary part of the gantry and the stationary part of the gantry will be reduced or eliminated. Further, the noise, generated by the rotation of the rotary part of the gantry will be reduced or eliminated. 
     According to the inventive concept the fact, that the air-gaps between the mechanical and magnetic components of the rotary contactless transformer could lead to acoustical noise due to the rotation of the rotary part of the gantry and the air accelerated by the rotation, can be avoided. 
       FIG. 4  shows an exemplary embodiment of a computer tomography gantry  91  arrangement. The gantry  91  comprises a stationary part  92  connected to a high frequency power source  98  and a rotary part  93  adapted to rotate relative to the stationary part  92 . An X-ray source  94  and an X-ray detector  95  are attached to the rotary part  93  at opposing locations such as to be rotatable around a patient positioned on a table  97 . The X-ray detector  95  and the X-ray source  94  are connected to a control and analysing unit  99  adapted to control the X-ray detector  95  and the X-ray source and to evaluate the detection results of the X-ray detector  95 . 
     It should be noted that the term ‘comprising’ does not exclude other elements or steps and the ‘a’ or ‘an’ does not exclude a plurality. Also elements described in association with the different embodiments may be combined. 
     It should be noted that the reference signs in the claims shall not be construed as limiting the scope of the claims. 
     LIST OF REFERENCE SIGNS 
       91  Computer tomography gantry, 
       92  Stationary part of the gantry, 
       93  Rotary part of the gantry, 
       94  X-ray source, 
       95  X-ray detector, 
       97  Table, 
       98  High frequency power source, 
       99  Control and analysing unit, 
       101  Core, 
       102  Strand, 
       103  Winding, 
       104  Part of transformer, 
       105  Winding, 
       106  Cross-section of a winding, 
       201  Part of transformer, 
       202  Core, 
       203  First winding, 
       204  Second winding, 
       301  Core, 
       302  Center line of a single core, 
       303  Center line of an arrangement of cores.