Abstract:
An inductive power transfer circuit or inductive rotary joint has an inductive rotating coupler with a primary side and a primary winding rotatably arranged against a secondary side and a secondary winding. The secondary side is connected via a rectifier to a load. The stray inductance of the coupler together with a resonance capacitor a series resonance circuit having a series resonance frequency. An inverter in a full bridge circuit is provided for converting a DC input voltage into an AC voltage. The inverter is operable in a full bridge mode to deliver a high power level and in a half bridge mode to deliver a low power level. This results in a broad dynamic range, soft power on and improved safety, as switching between the modes may be controlled by a simple hardware.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from and benefit of the European Application No. 14198921.0 filed on Dec. 18, 2014. The disclosure of this European Application is incorporated by reference herein. 
       BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to an inductive power coupling device for coupling electrical power between two units that are rotatable against each other, specifically for power couplers used in computer tomography scanners. Such power couplers are also known as rotary joints. 
         [0004]    2. Description of Relevant Art 
         [0005]    In computer 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. 
         [0006]    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. 
         [0007]    A contactless rotary joint with safety function is disclosed in EP 2 530 805 A1. The inverter of an inductively coupled rotary joint has two operating states. In a first operating state, it receives a three phase power line input. In a second operating state, it receives a single line power input. Depending on the input signal, a higher output voltage and a lower output voltage are generated at the secondary side, which may be used to distinguish between different operating states. The disadvantage is that high power contactors are required for switching over the input signal. 
         [0008]    A general problem exists in all mentioned inductively-coupled rotary joints when switching the output power on. At the secondary side of the rotating transformer there are a rectifier and a filter capacitor. When the secondary side is switched off, the filter capacitor is discharged. For switching the secondary side on, the filter capacitor must be charged to the nominal output voltage. Without any current limiting means, there would be a very high current when starting the circuit, until the filter capacitor is charged. This may lead to a significant stress or even overload of associated electronic components. 
       SUMMARY 
       [0009]    Embodiments of the invention provide a contactless inductively coupled rotary joint, which has a hardware safety circuit for delivering at least two different output power levels without requiring a high power contactor at the input side. A further problem to be solved is to provide an inductively coupled contactless rotary joint which is able to gradually increase the output power to avoid a large inrush current when switching the output power on. Another problem to be solved is to provide an inductively coupled contactless rotary joint having a significantly improved dynamic range. 
         [0010]    Solutions of the problem are described in the independent claims. The dependent claims relate to further improvements of the invention. 
         [0011]    The inductively coupled rotary joint has a primary side and a secondary side. It is preferred, if the primary side is the stationary side and the secondary side is the rotating side. If required, rotating and stationary sides may be switched, if power is to be transferred from the rotating side to the stationary side. 
         [0012]    At the primary side, preferably a DC power is provided by a DC power source, having a positive output and a negative output, which may be a battery, a DC line, a rectifier like a bridge rectifier coupled to an AC line, or a power factor correction circuit coupled to an AC line. The DC power source supplies the DC power to an inverter circuit. The inverter circuit is basically a full-bridge circuit, also called H-bridge for generating an AC voltage. There are four semiconductor switches and four diodes, one diode in reverse direction parallel to one switch. The switches preferably are IGBTs or MOSFETs. Preferably, a control circuit is provided for generating control signals for the switches. The outputs of the inverter may be coupled via a resonance capacitor and an optional transformer and/or a common mode choke to the primary winding of the rotating transformer. These components preferably form a serious resonance circuit having a resonance capacitance and a resonance inductance. The resonance capacitance preferably is formed by the resonance capacitor. There may be other capacitors, preferably in a serious circuit, for example between the transformer and the primary winding or at the secondary winding. The resonance inductance preferably is formed by the stray inductance of either the transformer and/or the rotating coupler. The resonance capacitance and the resonance inductance determine at least one series resonance frequency. Energy coupled from the primary winding at the primary side is received by a secondary winding at the secondary side of the rotating transformer and is preferably fed to a rectifier. The rectifier delivers a rectified signal via a secondary filter capacitor to a secondary load. It may be a bridge rectifier or a voltage doubler circuit having diodes or controlled semiconductor switches like IGBTs or MOSFETs. If an AC voltage is required at the secondary side, the rectifier and capacitor may be omitted. The primary winding and/or the secondary winding may comprise a plurality of winding sections. 
         [0013]    In a preferred embodiment, the inverter has at least two different operating modes which are most preferably set by the control circuit. In a first operating mode, the inverter is used as a half-bridge circuit, delivering only a lower power level to the secondary side, whereas in a second operating mode, the inverter is used as a full-bridge circuit delivering full power to the secondary side. For a smooth powering-on of the circuit, it is preferred that the inverter is working in a start sequence by starting in the first operating mode, delivering a lower power, and after some time switching to the second operating mode delivering full power. This avoids a large surge current at powering-on. 
         [0014]    The inverter circuit comprises at least two switching branches having the following switches with diodes in parallel. A first branch includes a first switch, which is connected between the positive output of the DC power source and a first inverter output. It further includes a second switch, which is connected between the first inverter output and the negative output of the DC power source. A second branch includes a third switch, which is connected between the positive output of the DC power source and a second inverter output. It further includes a fourth switch, which is connected between the second inverter output and the negative output of the DC power source. 
         [0015]    Preferably the inverter has a first operating mode, operating in a half bridge mode. In this mode one switch of one branch is closed, connecting an inverter output to either the positive output or the negative output of the DC power source. In the other branch the switches are closed alternatingly. The operation will be explained in an example. In this example, the fourth switch is closed, connecting the second inverter output to the negative output of the DC power source. The first and the second switches are closed alternatingly, connecting the first inverter output to the positive output or to the negative output of the DC power source. When connected to the positive output of the DC power source, energy is fed into the resonance circuit. When connected to the negative output of the DC power source, the resonance circuit is short-circuited. Therefore energy may only be delivered into the resonance circuit during the intervals where the first switch is closed. Generally the term closed as used herein relate to conductive or on states of semiconductor switches. The term open relates to isolating or off states of semiconductor switches. 
         [0016]    For starting up the power supply, it is preferred to start in the half bridge mode. It is further preferred to operate the first branch of switches with a first frequency most preferably higher or lower than the resonance frequency. The fourth switch of the second branch is closed. When starting up the inverter, the resonance capacitor must get charged to a voltage corresponding to half of the voltage of the DC power source. To avoid a high charge current, it is preferred to start with low duty cycle of the first switch and increase this duty cycle with time until a certain power level is reached or until a maximum duty cycle of 50% whichever is lower. This way, there are short intervals during which power is delivered into the resonance circuit, providing a low power flow. When increasing the duty cycle, the intervals of power flow and therefore the transferred power increased. Preferably, the half bridge mode is initiated by independent and asynchronous depowering one of the drivers of one half bridge by a circuit independent from the bridge control circuit. 
         [0017]    For further increasing the transfer of power by applying a higher primary voltage and thereby achieving a higher secondary voltage, preferably a transition to full bridge mode is made by alternatingly switching the first and second branch in a full bridge operation and by using a second frequency above or below the resonance frequency. Furthermore, it is preferred to adjust the duty cycle to obtain the required power transfer. The power transfer may also be controlled by adjusting the frequency which may be close to the resonance frequency. Preferably, the second frequency has a larger offset to the series resonance frequency than the first frequency. Most preferably, the second frequency is above the resonance frequency and the first frequency is slightly below the resonance frequency. 
         [0018]    For reverting to the half bridge mode, the operating frequency may be maintained, but after the fourth switch has been closed permanently, the first and the second switches start operating with low duty cycle which is gradually increased. 
         [0019]    By alternating between the half bridge mode and the full bridge mode, the inductively coupled rotary joint has a significantly improved dynamic range over prior art. 
         [0020]    In a preferred embodiment and to implement a safety feature, a hardware circuit may be provided to switch between the half bridge mode and the full bridge mode. This may be done by a hardware circuit for disabling the full bridge mode operation by forcing one switch of a branch to an open state and the other switch of the same branch to a close state. This may simply be done by a switching transistor or by simple logic gates. This may work independently of the control signals of the switches as may be provided by the control circuit. 
         [0021]    It is further preferred, if the secondary side has at least one means for evaluating the power delivered, and therefore for activating certain components like an X-ray tube, similar as disclosed in EP 2 530 805 A1 of the same applicant, which is herein included by reference. The safety circuit may simply block alternatingly switching of the switches of one branch by looking the first switch to an open position and a second switch to a close position. The other branch may operate normally. This forces the circuit to go into the half bridge mode, delivering only a reduced voltage level to the secondary side. There may be a further DC/DC converter at the secondary side to provide a controlled output voltage for certain electronic devices like control circuits and/or computers. 
         [0022]    In a further embodiment, there is a DC/DC converter between the positive output and negative outputs and the load. This DC/DC converter may be an up-converter a down converter or a combination thereof. It also may be switchable between up—and down-conversion. Alternatively, there may also be a DC/AC converter. 
         [0023]    Further embodiments relate to a method for switching and/or controlling the switches of the inverter as described above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]    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. 
           [0025]      FIG. 1  shows a preferred embodiment. 
           [0026]      FIG. 2  shows a first state of the half-bridge operating mode. 
           [0027]      FIG. 3  shows a second state of the half-bridge operating mode 
           [0028]      FIG. 4  shows the switch timing during a startup of the first mode. 
           [0029]      FIG. 5A, 5B, 5C  show the frequencies and duty cycles of different operating modes. 
           [0030]      FIG. 6  shows schematically a CT (Computed Tomography) scanner gantry. 
       
    
    
       [0031]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. 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 
       [0032]    In  FIG. 1 , the circuit diagram of a preferred embodiment is shown. An inductively coupled contactless rotary joint has a primary side  100  and a secondary side  200 . Preferably, the primary side  100  is the stationary side, whereas the secondary side  200  is the rotating side. It is obvious that stationary and rotating sides may be exchanged for coupling power from the rotating to the stationary side. At the primary side, there is a DC power source  180  having a positive output and a negative output for delivering DC power to an inverter  140 , which generates an AC signal which is further coupled via a resonance capacitor  130 , an optional transformer  120  having a primary winding  121  and a secondary winding  122  to a primary winding  110  of the rotating transformer. 
         [0033]    The DC power source  180  may be a battery, a DC line, a rectifier circuit like a bridge or a one-way rectifier for rectifying an AC power line signal or a power factor correction circuit for generating a DC signal from an AC power line. There may be additional filter capacitor (not shown in here) parallel to the DC power source  180 . 
         [0034]    The inverter  140  comprises a full bridge circuit with four switches  141 ,  142 ,  143 ,  144 , whereas a first branch  145  is formed by a first switch  141  connected to the positive output of the DC voltage source and a second switch  142  connected to the negative output of the DC voltage source to provide a first inverter output  148 . A second branch  146  is formed by a third switch  143  connected to the positive output of the DC voltage source and a fourth switch  144  connected to the negative output of the DC voltage source to provide a second inverter output  149 . Furthermore, four diodes are provided. A first diode  151  is connected parallel to the first switch  141  in reverse direction. Second diode  152 , third diode  153  and fourth diode  154  are connected in parallel to second switch  142 , third switch  143 , and fourth switch  144 , all in reverse direction. 
         [0035]    A control circuit  190  may be provided for generating control signals to control the switching state of the switches. It is preferred to have a first control signal  191  for controlling first switch  141 , a second signal  192  for controlling second switch  142 , a third control signal  193  controlling third switch  143 , and a fourth control signal  194  for controlling fourth switch  144 . 
         [0036]    The outputs of the inverter coupled to a primary winding  110  of a rotating transformer, further having a secondary winding  210 . It is further preferred to have a transformer  120  between the inverter output and the primary winding  110 . This transformer may serve for voltage adapting and for isolation purposes. Furthermore, there is a resonance capacitor  130  collected in series with at least one of the inverter outputs. This resonance capacitor may also be located between the transformer and the primary winding or at the secondary winding. Alternatively, there may be a plurality of such capacitors. 
         [0037]    At the secondary side, there is a secondary winding  210  of the rotating transformer delivering power via a bridge rectifier, comprising four diodes  221 - 224  via a secondary filter capacitor  230  to a load  240  being connected to a positive output  251  and a negative output  252 . Instead of the bridge rectifier shown herein, any other kind of rectifier may be used, for example there may be a voltage doubler circuit. Alternatively, any controlled rectifier with active switches, like MOSFETs or IGBTs may be used instead of diodes. 
         [0038]    In  FIG. 2 , a first state of the half-bridge operating mode is shown. Here, the remaining part of the circuit diagram at the right side (secondary side of transformer  120  including the secondary side) is not shown. Instead, a stray inductance  131  which may be part of transformer  120  and/or the rotating transformer  110 ,  210  is shown. This stray inductance  131  forms a series resonance circuit with the resonance capacitor  130 . The resonance capacitor  130  preferably is at the position indicated, but it may also be at least partially between the transformer  120  and the primary winding  110  and/or in series with the secondary winding  210 . Here, the first switch  141  and the fourth switch  144  are closed. The other switches are open. In this state, current is flowing through a first current path  181  from the power source  180  via first switch  141  into the resonance circuit comprising of resonance capacitor  130  and the stray inductance  131  including transformer  120  back through the fourth switch  144  to the DC power source  180 . In this state, for a first half wave, energy is supplied from the DC power source  180  into the series resonance circuit and to the load. 
         [0039]    In  FIG. 3 , a second state of the half-bridge operating mode is shown. Here, the second switch  142  and the fourth switch  144  are closed. The other switches are open. Current is flowing through a second current path  182  from the resonance circuit comprising resonance capacitors  130  and stray inductance  131 , including transformer  120  through the second switch  142  and the fourth switch  144  back to the resonance circuit. In this state, the series resonance circuit is short-circuited for a half wave. 
         [0040]    In  FIG. 4 , the switch timing during a startup of the first mode is shown. The upper time line shows switch control signal  191  which controls the first switch  141 . The lower time line shows switch control signal  192  controlling the second switch  142 . During startup of a first half-bridge operating mode, the first switch  141  and the second switch  142  are alternatingly activated. The fourth switch  144  is always on, and the third switch  143  is always off. The high signal shows times, where the associated switch is activated or on. During the low states, the switch is off. During startup, the first switch  141  is activated with small pulses of signal  191  with increasing duration. Between the individual pulses is a pause preferably corresponding to the remainder of the period time of the operating frequency, which preferably is above the resonance frequency of the resonance circuit. Accordingly, during the pulses at the top line corresponding to on-times of the switch  141 , the circuit is in a state as shown in  FIG. 2 , whereas it is in a state as shown in  FIG. 3  during times where the pulses of the bottom line are high and switch  142  is on. Only during the on-times of the top diagram, when the first switch  141  is activated, energy is supplied into the resonance circuit. These times are increased continuously, until they are the same as the times in the bottom curve for the second switch  142 . When this state is reached, the circuit is operating with a duty cycle of approximately 50:50. It is preferred to operate the circuit at a frequency slightly higher than the resonance frequency to get a lower power flow through the resonance circuit, as the series impedance of the series resonance circuit is higher than at its resonance frequency. 
         [0041]    In  FIG. 5 b   , the duty cycle curve  195  of the high side switch  141  is shown in different operating modes. At the first starting time  196 , the circuit powers on from an off-state. It starts as previously explained with a very low duty cycle in half-bridge mode until a duty cycle of 50% or a lower required duty cycle is reached. 
         [0042]      FIG. 5 a    describes the frequency which starts at a 2nd frequency f 2  well above resonance frequency f r  and is lowered to a first frequency f 1  slightly below resonance frequency when normal operation of half bridge mode is reached. 
         [0043]    In  FIG. 5 c   , the duty cycle curve  195  of the high side switch  141  is shown in different operating modes. During half bridge mode operation it is kept to zero. 
         [0044]    At time  197 , the inverter is switched to full-bridge mode. As this time, the frequency is increased back to f 2  and the duty cycle of the high side switch  143  of the second half bridge is ramped up. During the half bridge operating mode, the average voltage at the resonance capacitor  130  is approximately half of the DC power source voltage. When switching over to a full-bridge mode of the circuit, the average voltage at the capacitor has to be decreased to zero. To prevent an excessive current flow, the operating frequency as shown in  FIG. 5 a    of the circuit is increased to a frequency above the resonance frequency when switching over to a full-bridge mode, in which the first switch  141  is switched basically at the same time with the fourth switch  144 , and second switch  142  is switched basically at the same time as third switch  143 , alternating with the first and fourth switch. 
         [0045]    At time  198 , the power is again reduced and the inverter reverts to half-bridge mode. The frequency is the same as the previous full-bridge mode frequency, but the duty cycle of high side bridge ( 143 ,  FIG. 5 c   ) is ramped down from 50% to zero. Generally, if lower power is required, there may be also a lower duty cycle than 50% in the half-bridge mode or full bridge mode and frequency may be changed accordingly. 
         [0046]      FIG. 6  shows schematically a CT (Computed Tomography) scanner gantry. The stationary part is suspended within a massive frame  810 . The rotating part  809  of the gantry is rotatably mounted with respect to the stationary part and rotates along the rotation direction  808 . The rotating part may be a metal disk which supports an X-ray tube  801 , a detector  803  and further electronic and mechanic components. This disk may define a secondary ground. The X-ray tube is for generating an X-ray beam  802  that radiates through a patient  804  lying on a table  807  and which is intercepted by a detector  803  and converted to electrical signals and imaging data thereof. The data obtained by the detector  803  are transmitted via a contactless 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. Other scanners like baggage scanners work in a similar way. 
         [0047]    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 
       [0000]    
       
           100  primary side 
           110  primary winding 
           120  transformer 
           121  primary winding 
           122  secondary winding 
           130  resonance capacitor 
           131  stray inductance 
           140  inverter 
           141  first switch 
           142  second switch 
           143  third switch 
           144  fourth switch 
           145  first branch 
           146  second branch 
           148  first inverter output 
           149  second inverter output 
           151 - 154  diodes 
           180  DC power source 
           181  first half-bridge current path 
           182  second half-bridge current path 
           190  control circuit 
           191 - 194  switch control signals 
           195  duty cycle 
           196  starting time 
           197  switch from half bridge to full bridge mode 
           198  revert to half bridge mode 
           200  secondary side 
           210  secondary winding 
           221 - 224  rectifiers 
           230  secondary filter capacitor 
           240  load 
           251  positive output 
           252  negative output 
           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