Abstract:
An inductive rotating power transfer circuit, preferably for transferring electrical power from the stationary part to the rotating part of a CT scanner comprises an inductive power transformer having a stationary primary side and a rotating secondary side. The secondary side is connected via a rectifier to a filtering capacitor, delivering electrical power to a load. One of the output pins of the filtering capacitor is connected to a secondary ground at the rotating part which is further connected to a stationary protective ground via a galvanic slip ring. In the case of a short circuit between a secondary transformer winding and the secondary ground, the secondary winding is partially short-circuited by one of the rectifier diodes. This causes an asymmetric current load at the primary side and a current flowing through the slip ring. Both currents may be used to detect a failure of the secondary winding.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from and benefit of the pending European Application No. 14198958.2 filed on Dec. 18, 2014, the disclosure of which is incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The invention relates to an inductive power coupling device for coupling electrical power between two units that are rotatable against each other, and, specifically for power couplers used in computed tomography scanners. Such power couplers are also known as rotary joints. 
     2. Description of Relevant Art 
     In computed tomography (CT) scanners and other related machines high-power in the range from 10 kW up to more than 100 kW is transferred from a stationary side to a rotating side. There, a high voltage in the range of above hundred kilovolts is generated to produce x-ray radiation. 
     In U.S. Pat. No. 7,054,411 a multiple channel inductive rotary joint is disclosed. It has inductive channels for transferring power from the stationary side to the rotating side. There is an auxiliary power and a main power circuit. Furthermore a capacitive feedback link for power control is provided. There may be some failure states such as a short circuit of a rotating power channel to protective earth, which may cause dangerous high voltages at the rotating part and which may cause the rotating part of the feedback link to be inoperative and, therefore, may interfere with the communication to a primary power controller. 
     SUMMARY 
     The embodiments of the invention are directed to increasing the safety of devices that utilize inductive power couplers between rotating parts. Such devices may be CT scanners. Specifically, a short circuit of a rotating power channel to protective earth should no more cause excessive voltages at the rotating part. Furthermore, means and methods should be provided to detect such a short circuit from the stationary side without requiring communication from the rotating side. 
     Inductive rotary joints usually are built like power transformers, where one side is rotating against another side. For example, in CT scanners, power has to be transferred from the stationary to the rotating side. Therefore, the power coupler is a transformer having a stationary primary winding and a secondary rotating winding. For simplicity, the following explanations and embodiments refer to a CT scanner rotary joint. The same concepts can be applied to any rotary joint in general and furthermore to a rotary joint configured to transfer power from a rotating side to a stationary side. 
     As a transformer can only transfer AC (alternating current), it is either fed by an AC line voltage or by an inverter, generating an AC voltage of a higher frequency which can better be transferred via a rotating transformer. At the output side, in most cases this AC voltage is converted to a DC voltage to provide a DC output. This may be done by a bridge rectifier, followed by a filtering capacitor to generate a smooth DC voltage. Although the secondary winding of the rotating transformer and the DC voltage generated thereof are floating, there is a significant capacitance between the secondary DC circuit and the mechanical base holding the components of the rotating part. This is specifically the case with a CT scanner, with a large number of electronic components mounted to a rotating disk forming the mechanical base of the rotating part. The mechanical base is further also referred as secondary or rotating ground. Furthermore, there may be capacitors for suppressing noise, which are connected between the DC voltage supply and the mechanical base, which may further be connected by a galvanic slip ring to stationary protective earth. This connection to protective earth further prevents high voltage at the rotating part in the case of certain failures against ground, and therefore prevents electrical shock of persons operating the device when touching the device in such failure state. 
     Basically, the secondary winding is isolated against the mechanical parts, and therefore against the protective earth. Under certain circumstances, the isolation may fail. The applicable circumstances may include, for example, a mechanical failure due to mechanical damaging of the isolation, which may occur at ends of the isolation or at locations where the isolated wire of the secondary winding is connected to the external device, such as a rectifier. There may be other failure modes, such as thermal failures that may be caused by overheating, or electrical failures that be caused by longtime degradation of the isolation, or by sparking or arcing, or even a combination of some of these failure modes. 
     When such a failure of a short circuit occurs, the ground capacitor (the previously mentioned capacitance between the secondary output and the rotating ground) is connected parallel to at least one of the bridge rectifier diodes. The bridge rectifier now acts as a voltage doubler. As a consequence, the DC output voltage may become twice the normal DC output voltage. With a high probability, this will result in a failure of many of the electrical or electronic components attached to the DC output voltage. 
     In a first embodiment, there is a low impedance galvanic connection between a DC output line, which may either be the positive DC output or the negative DC output, and the mechanical base. 
     It is preferred if a galvanic connection is provided between the stationary and rotating sides which is also connected to said DC voltage output. The galvanic connection preferably is a slip ring having a brush sliding on a sliding track. In another embodiment, the galvanic connection may be made by a bearing, which for example may be a ball bearing between the rotating and the stationary parts. Most preferably, this bearing is further complemented by a parallel galvanic low current slip ring. Under normal operating conditions, there is no current flowing through the galvanic ground connection. Therefore, this galvanic ground connection has an extremely long lifetime, as there is not wear of the brushes and the sliding tracks due to arcing which usually occurs under high currents. There is also no wear or corrosion, if a bearing is used. 
     In a further embodiment, a control unit is provided at the primary side of the rotating transformer, which side preferably is the stationary side. This control unit preferably is measuring the current through the galvanic ground connection. In the failure case of a short circuit of the secondary winding towards the secondary ground, there will be significant ripple current flowing through this line, which can easily be detected by the control unit. This control unit may further issue an emergency switch-off signal to disable the power supply from the device. Such a signal may control a primary inverter supplying an AC voltage to the primary winding of the capacitive rotating transformer. In another embodiment, the control unit may be connected to a voltage and/or current sensor at the primary winding and/or at the primary input, detecting abnormal voltages/currents to detect said short circuit. 
     During standstill a ball bearing holding the rotating part may provide a sufficient grounding or protective earth. Grounding may further be increased by a grounding jumper which may be inserted manually for maintenance and service. 
     In a further embodiment, there may be a switch for generating a short circuit as described above, for example by shorting a diode. This switch may be used to trigger a power off at the primary side from the secondary side. It could be used as an emergency shutoff if there is any fault at the secondary side. 
     These embodiments provide a significant improvement in reliability and safety over the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention will be described by way of example, without limitation of the general inventive concept, on examples of embodiment and with reference to the drawings. 
         FIG. 1  shows a circuit diagram of a preferred embodiment. 
         FIG. 2  shows the positive current path in a first failure mode in a first embodiment. 
         FIG. 3  shows the negative current path in a first failure mode in a first embodiment. 
         FIG. 4  shows a circuit known from the prior art. 
         FIG. 5  shows the positive current path in a first failure mode according to prior art. 
         FIG. 6  shows the negative current path in a first failure mode according to prior art. 
         FIG. 7  shows the positive current flow in normal operation. 
         FIG. 8  shows the negative current flow in normal operation. 
         FIG. 9  shows a CT scanner. 
     
    
    
     Specific embodiments of the invention are shown by way of example in the drawings and will herein be described in detail, and are subject to modifications and alternative forms each of which is within the scope of the invention. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     In  FIG. 1 , a circuit diagram of a preferred embodiment is shown. An apparatus like a CT scanner may comprise a primary side  100  which preferably is stationary and a secondary side  200  which preferably is rotating. There is a rotating transformer having a primary winding  110  and a secondary winding  210  for inductively coupling electrical signals from the primary side  100  to the secondary side  200 . The primary winding  110  is fed by an inverter  120  which converts input voltage received via primary inputs  131 ,  132  into an alternating voltage, preferably a voltage in a frequency range from between 1 kHz and 100 kHz, most preferably about 20 kHz. The voltage output of the secondary winding  210  is provided at secondary winding contacts  254 ,  255 , which are connected to a rectifier. Preferably, the rectifier is a bridge rectifier comprising four diodes  221 - 224 . The output of the rectifier preferably is connected to a filtering capacitor  230 . Furthermore, connected to the filtering capacitor  230  may be a positive output voltage pin  251  and a negative output voltage pin  252 , by which a load  240  may be connected. In a CT scanner, the load may comprise an X-ray tube and/or multiple electrical or electronic circuits, like a computer, a detector and imaging processing means. One of the output pins  251 ,  252  is connected to a secondary ground  253 . Preferably, the negative output  252  is connected thereto. The secondary ground  253  is preferably based on mechanical parts at the rotating side, which may be the rotating part of a gantry of a CT scanner. 
     It is further preferred to have a slip ring  280  comprising at least one sliding track  281  and a at least one brush  282  for electrically connecting said secondary ground  253  to a protective earth  134  (which may be a primary ground), which may further be connected via a protective earth connector  133  to a main power system, or a specific ground pad. 
     It is further preferred to have a control unit  150  for controlling the inverter  120  or any other control means at the primary side. The controller  150  may be connected to a ground current sensor  151  for measuring a current between the secondary ground  253  and the protective earth  134 . It may also measure a current through the primary winding  110 , preferably by use of a second current sensor  153 . Based on the measurement results, a trigger signal  152  may be generated. 
     In an inductive rotating coupler, certain faults may occur. One of these faults may be a short circuit of the secondary winding to the secondary ground  253 . In this embodiment, a short circuit of the second secondary winding contact  255  is marked as a dashed line  270  indicating the short circuit. A similar scenario takes place, if the first secondary winding contact  254  has a short circuit to the secondary ground  253 . There may also be a short circuit of any other part of the secondary winding  210  to secondary ground  253 . By the short circuit, depending on the kind of short circuit, one of the rectifier diodes  221 ,  223  is shorted. The function is explained exemplarily by the kind of short circuit as indicated by dashed line  270 . In this case, the rectifier diode  223  is shorted. As the rotating transformer is operated with an AC signal, it delivers positive and negative half waves at its output. When the secondary winding  210  delivers a positive output, where the voltage at the first secondary winding contact  254  is higher than the voltage at the second secondary winding contact  255 , the circuit works as usual, as the rectifier diode  222  lets the current flow into the filtering capacitor  230  and the load  240 . When a negative half wave is delivered, the voltage at the first secondary winding contact  254  is lower than the voltage at the second secondary winding contact  255 , then the diode  224  provides a short circuit of the secondary winding. This short circuit leads to an asymmetrical current flow through the rotating transformer, which may easily be detected at the primary side, for example by second current sensor  153 , but it would also generate a signal which may be detected by the ground current sensor  151  at the primary side. 
     Due to the asymmetrical short circuit of the secondary winding  210  by one of the rectifier diodes, it is impossible that the circuit works as a voltage doubler, as the prior art, as shown in  FIG. 4 . 
     In  FIG. 2 , the positive current path in a first failure mode with a short circuit  270  is shown as a dashed line with arrows indicating the direction of the current. When the output voltage at the first secondary winding contact  254  is higher than the voltage at second secondary winding contact  255 , then a current flows through the circuit as shown. It flows through a rectifier diode  222  into the capacitor  230  and back via secondary ground  253  and the short circuit  270  to the second secondary winding contact  255 . This kind of current flow results in a normal charge of the capacitor  230 . 
     A negative current flow into the opposite direction, as indicated by  FIG. 2  is shown in  FIG. 3  by a dashed line with arrows indicating the direction of the current. The current flows from the second secondary winding contact  255  via the short circuit  270  and secondary ground  253  through diode  224  back to the first secondary winding contact  254 . This is a short circuit via the diode  224  of the secondary winding  210 . There are further parasitic capacitive currents flowing via the slip ring  280  to the protective earth  134  which may be detected by the control circuit  150 . Furthermore, the asymmetrical load can easily be detected by a second current sensor  153  at the primary side of the inductive rotary joint. 
     In  FIG. 4 , an embodiment as known from the prior art is shown. Here, there is no slip ring  280  and no controller  150  with the associated circuits and components. Furthermore, there is a ground capacitor  260 . This capacitor is required to provide a high frequency connection between the output of the circuit and the secondary ground  253 . In this embodiment, the negative output of the power supply is connected to the secondary ground  253 . If a short circuit between the secondary winding  210  and the secondary ground  253  occurs as indicated by dashed line  270 , the circuit acts as a voltage doubler, causing approximately doubling of the regular output voltage at the capacitor  230 . This would affect the operation of a connected load  240 . There is a high probability that sensitive electronic components within the load may be destroyed or at least damaged. 
     In  FIG. 5 , the positive current path in a first failure mode according to prior art is shown as a dashed line with arrows indicating the direction of the current. In the case of a positive output voltage of secondary winding  210 , current is flowing through rectifier diode  222  into capacitor  230  and therefrom via capacitor  260 , secondary ground  253 , and the short circuit  270  back to the second secondary winding contact  255 . As will be shown in the next Figure, the capacitor  260  was charged by a current of the preceding negative half wave output of secondary winding  210  to a negative voltage having the inverse polarity to the voltage at capacitor  230 . Therefore, the ground capacitor&#39;s  260  positive side is at the secondary ground  253 , whereas its negative side is at the negative output  252 . As the total voltage over the capacitor  230  and the ground capacitor  260  equals to the output voltage of the secondary winding  210 , the capacitor  230  must have twice the output voltage of the secondary winding  210 . This leads to twice the output voltage at the load  240 . 
     In  FIG. 6 , the current flow in a negative direction according to the prior art is shown as a dashed line with arrows indicating the direction of the current. The current flows from the second secondary winding contact  255  via short circuit  270  and secondary ground  253  through ground capacitor  260 , and diode  224  back to the first secondary winding contact  254 . It can be seen how the ground capacitor  260  is charged with a charge current in the opposite direction to capacitor  230 , as mentioned in the description of the previous Figure. 
     In  FIG. 7 , a positive current flow in normal operation of a preferred embodiment is shown. Here, the current flows from the first secondary winding contact  254  to diode  222 , capacitor  230 , and diode  223  back to the second secondary winding contact  255 . 
     In  FIG. 8 , a negative current flow in normal operation of a preferred embodiment is shown. Here, the current flows from the second secondary winding contact  255  via diode  221 , capacitor  230 , and diode  224  back to the first secondary winding contact  254 . 
       FIG. 9  shows schematically a Computed Tomography (CT) scanner gantry  10 . The stationary part is suspended within a massive frame  810 . The rotating part  809  of the gantry  10  is rotatably mounted with respect to the stationary part and rotates along the rotation direction  808 . The rotating part  809  may be a metal disk which supports an X-ray tube  801 , an X-ray detector  803  and further electronic and mechanic components. This metal disk may define a secondary ground. The X-ray tube  801  is for generating an X-ray beam  802  that radiates throurth a patient  804  lying on a table  807  and which is intercepted by the X-ray detector  803  and converted to electrical signals and imaging data thereof. The imaging data obtained by the X-ray detector  803  are transmitted via a contactiess rotary joint (not shown) to an evaluation unit  806  by means of a data bus or network  805 . Electrical power from a stationary power supply unit  811  may be transmitted by an inductive power coupler  800  to the rotating part  809 . 
     Modifications and alternative embodiments of various aspects of the invention will be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the spirit and scope of the invention as described in the following claims. 
     LIST OF REFERENCE NUMERALS 
     
         
           100  primary side 
           110  primary winding 
           120  inverter 
           131 ,  132  primary input 
           133  protective earth connector 
           134  protective earth 
           150  control unit 
           151  ground current sensor 
           152  trigger signal 
           153  second current sensor 
           200  secondary side 
           210  secondary winding 
           221 - 224  rectifier diode 
           230  capacitor 
           240  load 
           251  positive output 
           252  negative output 
           253  secondary ground 
           254 ,  255  secondary winding contacts 
           260  ground capacitor 
           270  short circuit 
           280  slip ring 
           281  sliding track 
           282  brush 
           800  inductive power coupler 
           801  x-ray tube 
           802  x-ray beam 
           803  x-ray detector 
           804  patient 
           805  network 
           806  evaluation unit 
           807  patient table 
           808  rotation direction 
           809  rotating part 
           810  frame 
           811  power supply unit 
           10  Gantry