Patent Publication Number: US-10763701-B2

Title: Inductive power transfer control

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application is a national stage filing under 35 U.S.C. 371 of International Patent Application Serial No. PCT/EP2017/055669, filed Mar. 10, 2017, which claims priority to European application number 16160595.1, filed Mar. 16, 2016. The entire contents of these applications are incorporated herein by reference in their entirety. 
     FIELD OF INVENTION 
     The present invention relates to inductive power transfer control, and in particular to a method of detecting operating conditions at a secondary side of an inductive power transfer system, to a control method using the detection method according to the present invention and to a controller using the control method according to the present invention. 
     BACKGROUND ART 
     Generally, inductive power transfer systems have a number of advantages over conductive power transfer systems. E.g., for electric vehicles it means that a plugging of a cable is no longer required. 
       FIG. 1  shows a schematic diagram of an inductive power transfer system  100  as known in the art. 
     As shown in  FIG. 1 , an inductive power transfer system  100  comprises at a transmission side a DC/AC converter  102 , a transmission side controller  104 , a transmission side compensation capacitor  106 , and a transmitter coil  108  connected in series to the transmission side compensation capacitor  106 . The series connection of the transmission side compensation capacitor  106  and the transmitter coil  108  is connected to the output side of the DC/AC converter  104 . 
     As shown in  FIG. 1 , the inductive power transfer system  100  comprises at a receiving side a receiver coil  110  connected in series to a receiving side compensation capacitor  112 . The series connection of the receiver coil  110  and the receiving side compensation capacitor  112  is connected to an input side of an AC/DC converter  114  which is operated under control of a receiving side controller  116 . Parallel to the series connection of the receiver coil  110  and the receiving side compensation capacitor  112  there is connected a short circuit protection switch  118 . At the output of the AC/DC converter  114  there is connected a load  120 . For the connection of the load  120  there may be provided a DC/DC converter for control of the power level delivered to the load  120  (not shown in  FIG. 1 ). 
     As shown in  FIG. 1 , a wireless communication link  122  may be established from the receiving side to the transmission side for exchange of control data and/or measurement data between from the receiving side to the transmission side. 
     Operatively, the DC/AC converter  102  is adapted to receive a DC input signal and adapted to convert it into a transmission side AC signal. The transmission side AC signal is output to the series connection of the transmission side compensation capacitor  106  and the transmitter coil  108  for generation of an oscillating magnetic field. The transmission side controller  104  is adapted to measure the characteristics of the transmission side AC signal and optionally the DC input signal for control of the DC/AC converter  102 . In more detail, the transmission side controller  104  is adapted to control the DC/AC converter  102  such that the generated magnetic field oscillates at resonant frequency of the series connection of the transmission side compensation capacitor  106  and the transmitter coil  108 . 
     Operatively, the receiver coil  110 , when placed in the magnetic field produced by the transmitter coil  108 , receives energy transmitted by the transmitter coil  108  through inductive coupling. The inductive coupling leads to the generation of a receiving side AC signal. Under control of the transmission side controller  116  the AC/DC converter  114  is adapted to convert the receiving side AC signal into a load side DC signal which is then forwarded to the load  120 . 
     Operatively, the receiving side controller  116  is adapted to measure the receiving side AC signal and optionally the load side DC signal for control of a power delivered to the load  120 . Further, the receiving side controller  116  is adapted to detect an error state at the receiving side for actuation of the short circuit protection switch  118 . Operatively, measurement data and control data may be sent over the wireless communication link  122  to improve the control and to inform the transmitting side on fault conditions at the receiving side. 
     Generally, in inductive power transfer systems there is no direct hardware connection between the transmission side and the receiving side. However, in cases of errors on the receiving side it is essential that a response to the error state is achieved as soon as possible to reduce or stop power transmission from the transmission side. 
     Further, if an open circuit error occurs at the receiving side, the receiving side AC signal will increase to levels that may be destructive to components in the inductive power transfer system or that may be even dangerous. Currently this problem is solved by detecting the over-voltage at the receiving side and by shorting the secondary resonant circuit constituted by the receiver coil  110  and the receiving side compensation capacitor  112  using the short circuit protection switch  118 . Optionally, the error has to be communicated via the wireless communication link  122  to the transmission side to stop the power transmission. This short circuit protection switch  118  can be implemented using dedicated switches or, when available, using two high-side or two low-side active switches in the AC/DC converter  114 . 
     However, communicating the error from the receiving side to the transmission side may be too slow. There are multiple delays added due to the analogue to digital conversion, the processing of the signal and the transmission delay caused by the wireless communication link  122 . 
     Further, while the active short-circuiting by the short circuit protection switch  118  at the receiving side adds some level of safety it is nevertheless possible that the short circuit current increases above the rating of hardware components. Also, using the short circuit protection switch  118  leads to increased costs and complexity. 
     SUMMARY OF INVENTION 
     In view of the above, the object of the present invention is to provide a more efficient solution for detection of operating conditions in an inductive power transfer system for improved control of the inductive power transfer system. 
     According to a first aspect of the present invention this object is achieved by a method of detecting an operating condition at a secondary side of an inductive power transfer system. 
     The method according to the first aspect of the present invention comprises the step of measuring at least one electric variable at a primary side of the inductive power transfer system which is dynamically coupled to the operating condition at the secondary side of the inductive power transfer system. 
     The method according to the first aspect of the present invention further comprises the step of executing a first analysis of the at least one electric variable over time at the primary side of the inductive power transfer system for detection of a change of the operating condition at the secondary side of the inductive power transfer system. 
     According to the present invention the first analysis is executable for detection of the operating condition at the secondary side without use of measurement data measurable at the secondary side of the inductive power transfer system. 
     According to a second aspect of the present invention the object outlined above is achieved by a method of controlling an inductive power transfer system. 
     The method of controlling the inductive power transfer system comprises a step of detecting operating conditions at a secondary side of an inductive power transfer system using the method of detecting an operating condition at a secondary side of an inductive power transfer system according to the first aspect of the present invention. 
     The method of controlling the operation of the inductive power transfer system further comprises a step of controlling an operation of the inductive power transfer system in response to detected operating conditions at the secondary side of the inductive power transfer system. 
     According to a third aspect of the present invention the object outlined above is achieved by a controller using the method of controlling the inductive power transfer system according to the second aspect of the present invention. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       In the following different aspects and examples of the present invention will be explained with reference to the drawing in which: 
         FIG. 1  shows a schematic circuit diagram of an inductive power transfer system as known in the prior art; 
         FIG. 2  shows a schematic circuit diagram of an inductive power transfer system according to a first exemplary embodiment of the present invention; 
         FIG. 3  shows an equivalent circuit diagram of a transformer of the inductive power transfer system shown in  FIG. 2 ; 
         FIG. 4  shows a schematic circuit diagram of an inductive power transfer system according to a second exemplary embodiment of the present invention; 
         FIG. 5  shows a schematic diagram of an detection apparatus for detecting operating conditions at a secondary side of inductive power transfer systems according to a third exemplary embodiment of the present invention; 
         FIG. 6  shows a flowchart of operation of the detection apparatus shown in  FIG. 5  according to a fourth exemplary embodiment of the present invention; 
         FIG. 7  shows a further detailed flowchart of operation of the detection apparatus shown in  FIG. 5  according to a fifth exemplary embodiment of the present invention; 
         FIG. 7 a    shows different operating conditions in relation to a variation of an electrical variable and its maximum allowable amplitude reflected by a threshold; 
         FIG. 7 b    shows that a rate of change of a detected electric variable is independent from the amplitude of the detected variable and thus may be detected independently from the amplitude; 
         FIG. 8  shows further aspects of sampling and measuring at least one electric variable at the primary side of the inductive power transfer system according to the present invention; 
         FIG. 9  shows a variation of self-inductances and mutual inductances as a function of vertical separation between the transmitter coil and the receiver coil; 
         FIG. 10  shows a variation of an induced primary current as a function of vertical separation between the transmitter coil and the receiver coil at a constant secondary side voltage; 
         FIG. 11  shows a schematic diagram according to a first operating condition of the inductive power transfer system according to the present invention and a related equivalent circuit diagram; 
         FIG. 12  shows a schematic diagram according to a second operating condition of the inductive power transfer system according to the present invention and a related equivalent circuit diagram; 
         FIG. 13( a )  shows a primary side high frequency current and a primary side high frequency voltage according to the first operating condition of the inductive power transfer system illustrated in  FIG. 11  or according to the second operating condition of the inductive power transfer system shown in  FIG. 12 ; 
         FIG. 13( b )  shows a secondary side high frequency current and a secondary side high frequency voltage according to the first operating condition of the inductive power transfer system illustrated in  FIG. 11  or according to the second operating condition of the inductive power transfer system shown in  FIG. 12 ; 
         FIG. 14  shows a schematic diagram according to a third operating condition of the inductive power transfer system according to the present invention and a related equivalent circuit diagram; 
         FIG. 15( a )  shows a primary side high frequency current and a primary side high frequency voltage according to the third operating condition of the inductive power transfer system illustrated in  FIG. 14 ; 
         FIG. 15( b )  shows a secondary side high frequency current and a secondary side high frequency voltage according to the third operating condition of the inductive power transfer system illustrated in  FIG. 14 ; 
         FIG. 16  shows a schematic diagram according to a fourth operating condition of the inductive power transfer system according to the present invention and a related equivalent circuit diagram; 
         FIG. 17( a )  shows a primary side high frequency current, a primary side high frequency voltage, and a primary side power according to the fourth operating condition of the inductive power transfer system illustrated in  FIG. 16 ; 
         FIG. 17( b )  shows a secondary side high frequency current and a secondary side high frequency voltage according to the fourth operating condition of the inductive power transfer system illustrated in  FIG. 16 ; 
         FIG. 18  shows a schematic circuit diagram of a controller for an inductive power transfer system according to a sixth exemplary embodiment of the present invention; 
         FIG. 19  shows a flowchart of operation for the controller according to the sixth exemplary embodiment of the present invention as shown in  FIG. 18 ; and 
         FIG. 20  shows a further detailed flowchart of operation for the controller according to the sixth exemplary embodiment of the present invention as shown in  FIG. 18 . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     In the following the present invention will be explained in detail with reference to the drawing. Here, it should be understood that such explanation is related to examples of the present invention only and not binding to the scope of the present invention as defined by the claims. As far as reference is made to specific circuit components this is to be considered as example for the underlying functionality such the circuit components are clearly exchangeable as long as the same functionality is achieved. 
     The present invention described here uses the measurement signals available at the transmission side of the inductive power transfer system also referred to as primary side in the following to detect operating conditions, e.g., high and low impedance states, at the receiving side of the inductive power transfer system also referred to as secondary side in the following. 
     According to the present invention the inductive power transfer system can respond to an operating condition, e.g., an error at the secondary side, without mandatory use of measurement data that is measurable at the secondary side. There is no requirement for a wireless communication link or dedicated short circuit protection switch. Overall operating conditions can be detected intrinsically by using electrical signals available on the primary side in an efficient manner at low costs. 
       FIG. 2  shows a schematic diagram of an inductive power transfer system  10  according to a first exemplary embodiment of the present invention. 
     As shown in  FIG. 2 , the inductive power transfer system  10  has a transmission unit  12  at the primary side. The transmission unit  12  comprises a primary side DC/AC converter  14 , a primary side controller  16 , a primary side compensation capacitor  18 , and a transmitter coil  20  connected in series to the primary side compensation capacitor  18 . The series connection of the primary side compensation capacitor  18  and the transmitter coil  20  is connected to the output side of the primary side DC/AC converter  14 . 
     Operatively, the primary side DC/AC converter  14  is adapted to receive a primary side dc voltage U 1,dc  and a primary side direct current I 1,dc  as input and to convert it into a primary side high frequency voltage U 1,hf  and a primary side high frequency current I 1,hf . The primary side high frequency voltage U 1,hf  and the primary side high frequency current I 1,hf  are then supplied to the series connection of the primary side compensation capacitor  18  and transmitter coil  20  for generation of an oscillating magnetic field. 
     Operatively, the primary side controller  16  is adapted to measure the primary side high frequency current I 1,hf  and optionally the primary side high frequency voltage U 1,hf , the primary side direct current I 1,dc  and/or the primary side dc voltage U 1,dc . The primary side controller  16  is adapted to process a measurement result for control of the primary side DC/AC converter  14 . The primary side DC/AC converter is adapted to supply the primary side high frequency voltage U 1,hf  and the primary side high frequency current I 1,hf  to the series connection of the primary side compensation capacitor  18  and the transmitter coil  20 . The control of the primary side controller  16  is such that the magnetic field generated by the transmitter coil  20  oscillates at resonant frequency of the series connection of the primary side compensation capacitor  18  and the transmitter coil  20 . 
     As shown in  FIG. 2 , the inductive power transfer system  10  also has at least one receiving unit  22  separated from the transmission unit  12 . 
     As shown in  FIG. 2 , the receiving unit  22  comprises a receiver coil  24  connected in series to a secondary side compensation capacitor  26 . The receiving unit  22  further comprises a secondary side AC/DC converter  28 . At an input side of the secondary side AC/DC converter  28  there is connected the series connection of the receiver coil  24  and the secondary side compensation capacitor  26 . At the output side of the secondary side converter  28  there is connected a load  30 . The receiving unit  22  comprises a secondary side controller  32  adapted to control the secondary side AC/DC converter  28 . 
     Operatively, the receiver coil  24 , when placed in the magnetic field produced by the transmitter coil  20 , receives energy transmitted by the transmitter coil  20  through inductive coupling. The inductive coupling leads to generation of a secondary side high frequency voltage U 2,hf  and a secondary side high frequency current I 2,hf . 
     Operatively, the secondary side AC/DC converter  28  is adapted to convert the secondary side high frequency voltage U 2,hf  and the secondary side high frequency current I 2,hf  into a secondary side dc voltage U 2,dc  and a secondary side direct current I 2,dc  under control of the secondary side controller  32 . 
     Operatively, the secondary side controller  32  is adapted to optionally measure the secondary side high frequency current I 2,hf  and to optionally measure the secondary side high frequency voltage U 2,hf , the secondary side direct current I 2,dc  and/or the secondary side dc voltage U 2,dc . The secondary side controller  32  is adapted to process a measurement result for controlling the secondary side AC/DC converter  28 . The secondary side AC/DC converter  28  is adapted to supply the secondary side dc voltage U 2,dc  and the secondary side direct current I 2,dc  to the load  30 , e.g., either directly or via a DC/DC converter (not shown in  FIG. 2 ). 
     As shown in  FIG. 2 , according to the present invention the provision of a wireless communication link between the primary side and the secondary side or the provision of a dedicated short circuit protection switch is not mandatory. 
     Generally, according to the first exemplary embodiment of the present invention the primary side controller  16  is adapted to detect operating conditions at the secondary side through intrinsic analysis of electric signals over time which are available on the primary side which are dynamically coupled to operating conditions at the secondary side. 
     E.g., due to the inductive coupling the occurrence of a change of an operating condition at the secondary side will lead to a transient characteristic of the primary side high frequency voltage U 1,hf  and/or the primary side high frequency current I 2,hf . 
     Further, according to the first exemplary embodiment the primary side controller  16  is adapted to control the inductive power transfer system  10  in response to detected operating conditions. In more detail, the primary side controller is adapted to control, e.g., the primary side de voltage U 1,dc , the primary side direct current I 1,dc , the primary side high frequency voltage U 1,hf , and/or the primary side high frequency current I 1,hf  such that the secondary side of the inductive power transfer system enters a safe state upon occurrence of predetermined operating conditions at the secondary side of the inductive power transfer system. 
     In the most general sense, according to the present invention the operating condition at the secondary side of the inductive power transfer system is related to a change from a regular operating condition to a non-regular operating condition. Further, in the most general sense the second non-regular operating condition is an open circuit condition or a short circuit condition at the secondary side of the inductive power transfer system. Further examples of non-regular operating conditions are e.g., an over current, an over temperature, high impedance state, and/or low impedance state at the secondary side. 
       FIG. 3  shows an equivalent circuit diagram of a transformer constituted by the transmitter coil  20  and the receiver coil  24  in the inductive power transfer system  10  using series-series compensation. 
     As shown in  FIG. 3 , L 1s  and L 2s ′ represent the leakage inductances of the transformer and L m  is the mutual inductance of the transformer. The inductances of the transmitter coil  20  is L 1 =L 1s +L m  and the inductance of the receiver coil  24  is L 2 =L 2s ′+L m . If the primary side compensation capacitor  18  and the secondary side compensation capacitor  26  are dimensioned to compensate the self-inductance of the transmitter coil  20  and the self-inductance of the receiver coil  24 , respectively, the inductive power transfer system  10  is operated at a resonant frequency 
     
       
         
           
             
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     If an open circuit error occurs, the secondary side high frequency voltage U 2,hf  will increase to levels that could be destructive to components and even be dangerous. 
     More generally, depending on the compensation strategy of the transmitter coil  20  and the receiver coil  24  and the frequency they are driven relative to their resonant frequency, the power delivered to the secondary side appears as a voltage source, a current source or a mix of the two. In the case of an equivalent voltage source the safe state on the secondary side is an open circuit. In the case of a current source the safe state is a short circuit. 
     Generally, the primary side controller  16  according to the first exemplary embodiment of the present invention is adapted to control the operation at the primary side of the inductive power transfer system  10  such that secondary side enters the safe state upon occurrence of an error condition at the secondary side without exchange of control data and/or measurement data between the primary side and the secondary side. 
       FIG. 4  shows a schematic circuit diagram of an inductive power transfer system  34  according to a second exemplary embodiment of the present invention. 
     Insofar as the inductive power transfer system  34  according to the second exemplary embodiment of the present invention uses the same circuit components as the inductive power transfer system  10  according to the first exemplary embodiment of the present invention, they are referenced using the same reference numerals and explanation of related structure and functionality is not repeated here. 
     As shown in  FIG. 4 , the inductive power transfer system  34  according to the second exemplary embodiment of the present invention comprises a wireless transmission link  36  for exchange of measurement data and/or control data between the primary side and the secondary side or vice versa. 
     As shown in  FIG. 4 , the inductive power transfer system  34  according to the second exemplary embodiment of the present invention further comprises a short circuit protection switch  38  at the secondary side. The short circuit protection switch  38  is connected to the input of the secondary side AC/DC side converter  28  parallel to the series resonant circuit constituted by the receiver coil  24  and the secondary side compensation capacitor  26 . 
     In the following further aspects of the analysis of the operating conditions at the secondary side of the inductive power transfer system  10 ,  34  according to the first and second exemplary embodiment of the present invention will be explained with reference to  FIG. 6  to  FIG. 17 . 
     Here, it should be noted that explanations given in the following with respect to the analysis of the operating conditions are equally applicable to the inductive power transfer systems  10  and  34  according to the first and second exemplary embodiment of the present invention. 
       FIG. 5  shows a schematic diagram of an detection apparatus  40  for detecting operating conditions at a secondary side of inductive power transfer systems  10 ,  34  according to a third exemplary embodiment of the present invention. 
     As shown in  FIG. 5 , the detection apparatus  40  comprises at least one interface  42 , e.g., a radio interface. The interface  42  is suitable for wireless information exchange, e.g., with the secondary side controller of the inductive power transfer system  10 ,  34 , or with an external control station of the inductive power transfer system  10 ,  34 . In some scenarios, the interface  12  may also be used for exchange of information with external systems, e.g., a supervision system operated in a logistics center. 
     As shown in  FIG. 5 , the detection apparatus  40  comprises at least one processor  44  coupled to the interface  42  and a memory  46  coupled to the at least one processor  44 . The memory  46  may include a read-only memory ROM, e.g., a flash ROM, a random access memory RAM, e.g., a dynamic RAM DRAM or a static RAM SRAM, a mass storage, e.g., a hard disc or solid state disc, or the like. The memory  46  also includes instructions, e.g., suitably configured program code to be executed by the at least one processor  44  in order to implement a later described functionality of the detection apparatus  40 . This functionality will be referred to in the following as modules. It is noted that these modules do not represent individual hardware elements of the detection apparatus  40 , but rather represent functionalities generated when the at least one processor  44  execute the suitably configured program code. 
     As shown in  FIG. 5 , the memory  46  may include suitably configured program code to implement a detection module  48 , an analysis module  50 , and an optional threshold module  52 . Further, the suitably configured program code in the memory  46  may also implement a control module for implementing various control functionalities, e.g., for controlling the detection apparatus  40  so as to establish and/or maintain related functionality and connectivity to the outside, or the like. 
       FIG. 6  shows a flowchart of operation of the detection apparatus shown in  FIG. 5  according to a fourth exemplary embodiment of the present invention, for observing an operating condition at a secondary side of an inductive power transfer system  10 ,  34 . 
     As shown in  FIG. 6 , operatively the detection module  48 , in cooperation with the processor  44 , is adapted to execute a step S 10  for measuring of at least one electric variable at a primary side of the inductive power transfer system  10 ,  34  which is dynamically coupled to the operating condition at the secondary side of the inductive power transfer system  10 ,  34 . 
     As shown in  FIG. 6 , operatively the analysis module  50 , in cooperation with the processor  44 , is adapted to execute a step S 12  for implementation of a first analysis of the at least one electric variable over time at the primary side of the inductive power transfer system  10 ,  34  for detection of the operating condition at the secondary side of the inductive power transfer system  10 ,  34  without use of measurement data measurable at the secondary side of the inductive power transfer system  10 ,  34 . 
       FIG. 7  shows a further detailed flowchart of operation of the detection apparatus  40  shown in  FIG. 5 . 
     As shown in  FIG. 7 , operatively the detection apparatus shown in  FIG. 5  is adapted to execute the steps S 10  and S 12  for detection of the at least one electric variable at the primary side of the inductive power transfer system and for execution of related first analysis. 
     As shown in  FIG. 7 , operatively the secondary side controller  32  is adapted to execute a step S 14  for measurement of at least one electric variable at the secondary side of the inductive power transfer system  34  over time for generation of secondary side measurement data in relation to the operating condition at the secondary side of the inductive power transfer system  34 . 
     As shown in  FIG. 7 , operatively the secondary side controller  32  is adapted to execute a step S 16  for communication of the generated secondary side measurement data from the secondary side of the inductive power transfer system  34  to the primary side of the inductive power transfer system  34  over the wireless transmission link  36 . 
     As shown in  FIG. 7 , operatively the primary side controller  16  is adapted to execute a step S 18  to realize a second analysis of the communicated secondary side measurement data over time at the primary side of the inductive power transfer system  34  in combination with the first analysis for a more accurate and/or an accelerated detection of the operating condition at the secondary side of the inductive power transfer system  34 . 
     It should be noted that according to the present invention the execution of the first analysis and the execution of the second analysis may as well be assigned to the primary side controller  16  and the secondary side controller  32  in parallel to realize a parallelized analysis process. Yet a further variation would be to supplement the control of the inductive power transfer system  34  from the primary side by a decoupled control at the secondary side through the secondary side controller  32 , e.g., actuation of the short circuit protection switch  38  upon occurrence of an error state at the secondary side. 
     Further, the specific order of the steps S 10  to S 18  as shown in  FIG. 7  is to be considered as an example only and it should be understood that any revision of this order is as well covered by the scope of the present invention. 
     In the following more detailed aspects of the detection of operating conditions in the inductive power transfer system for improved control of the power transfer system will be explained. 
       FIG. 7 a    shows different operating conditions in relation to a variation of an electrical variable and its maximum allowable amplitude reflected by a threshold. 
     As shown in  FIG. 7 a   , according to the present invention there may be considered a rate of change of at least one electric variable instead of the actual value of the at least one electric variable. 
       FIG. 7 a    illustrates the implications of this feature through non-binding example scenarios. As shown in the upper part of  FIG. 7 a   , there are three operating scenarios (I) to (III) in relation to the changing amplitude of an electric variable and its maximum allowable amplitude reflected by a threshold value Th 1 :
         (I) Amplitude remains constant and lower than threshold Th 1 ;   (II) Amplitude changes over time but remains lower then threshold Th 1 ; and   (III) Amplitude changes over time and exceeds threshold Th 1 .       
     Here,  FIG. 7 a    shows that for scenario (I) and the threshold Th 1  is not exceeded wherein for scenario (III) eventually the threshold of Th 1  is exceeded after elapse of a time period T 1 . 
     The lower part of  FIG. 7 a    shows the rates of change for the different scenarios. For scenario (I) there is no change of amplitude while for scenario (II) the rate of change stays within an upper bound Th 2  and a lower bound Th 3 . Further, for the scenario (III) the increase in the amplitude is too high and the rate of change for the amplitude exceeds the upper bound Th 2 . 
     As shown in the lower part of  FIG. 7 a    the information that a rate of change exceeds its related threshold is immediately available without delay of a time period T 1  until the actual amplitude of the considered electrical value exceeds the threshold Th 1 . The reason for this is that the rate of change is related to the amplitude differential of the electrical value considered and not to the amplitude of the electrical value. 
     Thus, when considering the rate of change of an electrical value the present invention achieves the technical effect of an accelerated detection of an operative state at the secondary side of the inductive power transfer system. 
     A further implication of the consideration of the rate of change of an electrical value is illustrated by  FIG. 7   b.    
       FIG. 7 b    shows the rate of change of a detected electric variable is independent from the amplitude of the considered electrical variable and thus may be detected independently from the amplitude. As shown in  FIG. 7 b   , while in scenario (III) the amplitude is higher than in scenario (IV), nevertheless the actual rate of change in scenario (III) is lower than the one in scenario (IV). 
     From  FIG. 7 b    it should be clear that according to the present invention a fixed threshold scheme may be used. Thus, irrespective of the actual amplitude it is possible to compare the rate of change for case (III) or (IV) with a same threshold Th 2  which is set once for indication of a specific operation state at the secondary side of the inductive power transfer system and need not be modified during operation. This leads to a facilitation of the process of detection of the operating condition at the secondary side of the power transfer system. 
     In conclusion and as explained with respect to  FIG. 7 a    and  FIG. 7 b   , the present invention may increase level of safety through improved detection of operating conditions at the secondary side of an inductive power transfer system while and the same time it reduces costs of system implementation. 
       FIG. 8  shows further aspects of sampling and observing at least one electric variable at the primary side of the inductive power transfer system. 
     As shown in  FIG. 8 , according to the present invention the detection module  48 , in cooperation with the processor, is adapted to measure the at least one electric variable  54  over time at the primary side of the inductive power transfer system by determining sample values  56  of the at least one electric variable and/or a value being based on the at least one electric variable. 
     Further, as shown in  FIG. 8 , the analysis module  50 , in cooperation with the processor  44 , is adapted to execute the first analysis of the at least one electric variable and/or the value being based on the at least one electric variable using the sample values  56  and a predetermined threshold scheme. 
     As shown in  FIG. 8( a ) , according to the present invention the at least one electric variable is not restricted to any specific electric characteristic and may be selected as single electric variable or as a combination of electric variables from a group comprising:
         a primary side current and/or at least one first value based on the primary side current,   a rate of change of the primary side current and/or at least one second value based on the rate of change of the primary side current,   an electric power supplied to the primary side of the inductive power transfer system and/or at least one third value based on the electric power supplied to the primary side of the inductive power transfer system, e.g., a signal based on the power delivered to the primary side coil (I 1,hf *U 1,hf ) or the primary side DC/AC converter (I 1,dc *U 1,dc ),   a rate of change of an electric power supplied to the primary side of the inductive power transfer system and/or at least one fourth value based on the rate of change of the electric power supplied to the primary side of the inductive power transfer system,   a phase difference between a primary side voltage and the primary side current and/or at least one fifth value based on the phase difference between the primary side voltage and the primary side current,   a rate of change of the phase difference between the primary side current and the primary side voltage and/or at least one sixth value based on the rate of change of the phase difference between the primary side current and the primary side voltage,   a resonant frequency at the primary side of the inductive power transfer system and or at least seventh value based on the resonant frequency at the primary side of the inductive power transfer system.       

     Here, it should be noted that a value based on an electric variable as exemplified above according to the present invention is not restricted in any form and may be a peak value of the electric variable, an rms value of the electric variable, or any other suitable value derived by a functional operation of the electric variable. 
     As shown in  FIG. 8( b )  to  FIG. 8( d ) , according to the present invention there are provided a plurality of methodologies to execute the first analysis using different threshold schemes to be explained in the following. 
     Generally, according to the present invention a threshold scheme comprises at least one of a threshold which is a fixed threshold, an adaptive threshold, or a combination of a fixed threshold and an adaptive threshold. 
     As shown in  FIG. 8( b ) , a first option to execute the first analysis of the at least one electric variable is the use of fixed threshold values which do not vary over time. 
     As shown in  FIG. 8( b ) , according to the first option of the first analysis there is set a first threshold  58  and a rising edge crossing of a first threshold  58  by a sample value of the sample values of the at least one electric variable or of the related value serves for detection of a first operating condition at the secondary side of the inductive power transfer system  10 ,  34 , e.g., an open circuit condition at the secondary side of the inductive power transfer system  10 ,  34 . 
     As shown in  FIG. 8( b ) , according to a second option of the first analysis there is set a second threshold  60  and a falling edge crossing of the second threshold by the peak value of the sample values of the at least one electric variable or related value serves for detection of a second operating condition at the secondary side of the inductive power transfer system  10 ,  34 , e.g., a short circuit condition at the secondary side of the inductive power transfer system  10 ,  34 . 
     As shown  FIG. 8( c ) , according to a third option of the first analysis the at least one electric variable is the primary side current. The third option of the first analysis comprises a step of sampling the primary side current and a step of analyzing absolute values I 1hf_abs (k) of sample values of the primary side current using a threshold scheme dividing into a first lower threshold I 1low (k) and a second higher threshold I 1high (k) according to
 
 I   1low ( k )&lt; I   1hf_abs ( k )&lt; I   1high ( k )
 
 I   1low ( k )= g   m   U   2,hf ( k )− a ( k )
 
 I   1high ( k )= g   m   U   2,hf ( k )+ b ( k )
 
     wherein 
               g   m     =     1     2   ⁢   π   ⁢           ⁢     f   0     ⁢     L   12               
is the trans-conductance, f 0  is the resonant frequency at the primary side of the inductive power transfer system  10 ,  34 , and L 12  is the mutual inductance or a representative value for the trans-conductance; and
 
     U 2,hf (k) is the secondary side high frequency voltage of the inductive power transfer system; 
     a(k) is a lower bound for observing regular operation at the secondary side of the inductive power transfer system in relation to g m U 2,hf (k); and 
     b(k) is a upper bound for observing regular operation at the secondary side of the inductive power transfer system in relation to g m U 2,hf (k). 
     As shown in  FIG. 8( c )  according to third option of the first analysis the values of a(k) and b(k) are constant over time. This facilitates the process of detection of the operating condition at the secondary side of the inductive power transfer system  10 ,  34 . 
     As shown in  FIG. 8( d )  according to a fourth option of the first analysis the values of a(k) and b(k) are adaptive over time. 
     According to the present invention the values of a(k) and b(k) may be adaptive, e.g., according to current state of the signal to be measured, e.g., amplitude, frequency, sign of signal, etc. 
     Generally, the values of a(k) and b(k) should reflect a balance between a small value for fast detection of a change of operating conditions at the secondary side of the inductive power transfer system  10 ,  34  a value large enough to avoid faulty detection by providing enough error tolerance. 
     Further, an adaptive setting of the values of a(k) and b(k) allows to maintain relative tolerance for detection of operating conditions irrespective of amplitude changes of the signal to be measured or related values derived therefrom for detection. 
     As shown in  FIG. 8( c )  and  FIG. 8( d ) , according to the third and fourth option, in the first analysis there is detected the short circuit condition at the secondary side of the inductive power transfer system  10 ,  34  when absolute values I 1hf_abs (k) of the sample values of the primary side current are lower than the first lower threshold I 1low (k) over a first predetermined period of time T 1 . 
     As shown in  FIG. 8( c )  and  FIG. 8( d ) , according to the third and fourth option, in the first analysis there is detected the open circuit condition at the secondary side of the inductive power transfer system  10 ,  34  when the absolute values I 1hf_abs (k) of the sample values of the primary side current are higher than the second higher threshold I 1high (k) over a second predetermined period of time T 2 . 
       FIG. 9  shows a variation of self-inductances and mutual inductances as a function of vertical separation between the transmitter coil  20  and the receiver coil  24 . 
     As shown in  FIG. 9 , the mutual inductance L 12  decreases with increasing vertical separation between the transmitter coil  20  and the receiver coil  24 . Further, the trans-conductance gm is inversely proportional to the mutual inductance L 12 . This is the basis for the adaptive setting of the values of a(k) and b(k). 
     As outlined above, the lower threshold I 1low (k) and thus the value of a(k) triggers detection of a short circuit operating condition at the secondary side of the inductive power transfer system  10 ,  34 . 
     Also, the upper threshold I 1high (k) and thus the value of b(k) triggers the detection of an open circuit operating condition at the secondary side of the inductive power transfer system  10 ,  34 . 
     It is the variation of the mutual induction L 12  or equivalently the variation of the trans-conductance that determines the variation characteristics of the primary side current and the adaptation of the values of a(k) and b(k). 
       FIG. 10  shows a variation of an induced primary current as a function of vertical separation between the transmitter coil  20  and the receiver coil  24  at a constant secondary side voltage. 
     As shown in  FIG. 10 , the induced primary side current increases with increasing vertical separation as the mutual inductance L 12  decreases with increasing vertical separation and as the trans-conductance gm is inversely proportional to the mutual inductance L 12 . 
     As outlined above, the tolerance band for the error detection may be adaptive. Therefore, according to the present invention a decrease of the amplitude of the primary side current implies a decrease of the values of a(k) and b(k) in line with a decrease of vertical separation between the transmitter coil  20  and the receiver coil  24 . Further, an increase in the amplitude of the primary side current implies an increase in the values of a(k) and b(k) in line with an increase of vertical separation between the transmitter coil  20  and the receiver coil  24 . 
       FIG. 11  shows a schematic diagram according to a first operating condition of the inductive power transfer system according to the present invention and a related equivalent circuit diagram. 
     As shown in  FIG. 11( a ) , the first operating condition relates to a disconnection of the secondary side AC/DC converter  28  and the receiver coil  24 . 
     As shown in  FIG. 11( a ) , the first operating condition may be represented by an opening of a first model switch  62  inserted at a location of the possible disconnection of the secondary side AC/DC converter  28  and the receiver coil  24 . 
     As shown in  FIG. 11( a ) , should the first operating condition materialize, then the short circuit protection switch  38  may no longer protect the receiver coil  24  from over voltage. 
     As shown in  FIG. 11( b ) , according to the equivalent circuitry modelling the first operating condition the disconnection of the secondary side AC/DC converter  28  and the receiver coil  24  leads to a mere inductive load as seen from the primary side and thus to a phase shift. Also the parallel branch carries no current such that I 1,hf =0 applies and the impedance as seen from the primary side goes up. 
       FIG. 12  shows a schematic diagram according to a second operating condition of the inductive power transfer system according to the present invention and a related equivalent circuit diagram. 
     As shown in  FIG. 12( a ) , the second operating condition relates to a disconnection of the secondary side AC/DC converter  28  from the short circuit protection switch  38 . 
     As shown in  FIG. 12( a ) , the second operating condition may be represented by an opening of a second model switch  64  inserted at a location of a possible disconnection of the secondary side AC/DC converter  28  from the short circuit protection switch  38 . Should the second operating condition materialize, then the short circuit protection switch  38  may still protect the receiver coil  24  from over voltages. 
     As shown in  FIG. 12( b ) , the equivalent circuitry modelling the second operating condition is similar to the equivalent circuit modelling the first operating condition and thus similar explanations as given above apply. 
     As shown in  FIG. 12 , if an open circuit occurs represented by an opening of the second model switch  64 , this second operating condition could be detected by the secondary side controller  32  and the short circuit protection switch  38  may be closed. The primary side controller  16  may then detect either the second open circuit condition or the following short circuit condition triggered by closing the short circuit protection switch or both. 
       FIG. 13( a )  shows the primary side high frequency current I 1,hf  and a primary side high frequency voltage U 1,hf  according to the first operating condition of the inductive power transfer system illustrated in  FIG. 11  or according to the second operating condition of the inductive power transfer system shown in  FIG. 12 . 
     As shown in  FIG. 13( a ) , upon entry of the first operating condition or the second operating condition at the secondary side at time t=0, the primary side high frequency current I 1,hf  increases and changes phase. The primary side high frequency voltage U 1,hf  is supplied from the primary side DC/AC converter  14  and is therefore unchanged. 
       FIG. 13( b )  shows the secondary side high frequency current I 2,hf  and the secondary side high frequency voltage U 2,hf  according to the first operating condition of the inductive power transfer system illustrated in  FIG. 11  or according to the second operating condition of the inductive power transfer system shown in  FIG. 12 . 
     As shown in  FIG. 13( b ) , the secondary side high frequency voltage U 2,hf  increases according to the variations of the primary side high frequency current I 1,hf . The secondary side high frequency current I 2,hf  drops to zero. 
       FIG. 14  shows a schematic diagram according to a third operating condition of the inductive power transfer system according to the present invention and a related equivalent circuit diagram. 
     As shown in  FIG. 14( a ) , the third operating condition relates to a disconnection of the secondary side AC/DC converter  28  from the load  30 . 
     As shown in  FIG. 14( a ) , the third operating condition may be represented by an opening of a third model switch  66  inserted at a location of a possible disconnection of the secondary side AC/DC converter  28  from the load  30 . Should the third operating condition materialize, then the short circuit protection switch  38  may still protect the receiver coil  24  from over voltages. As shown in  FIG. 14( a )  at the output of the secondary side AC/DC converter  28  there remains a capacitive behavior that may be modelled by a capacitor  68 . 
     As shown in  FIG. 14( b ) , the equivalent circuitry modelling the third operating condition is comparable to the equivalent circuit modelling the first operating condition or the second operating condition. However, as the disconnection is at the output side of the secondary side AC/DC converter  28 , there remains an input resistance Zi seen from the input side of the secondary side AC/DC converter  28  which has to be added to the equivalent circuitry as shown in  FIG. 14( b ) . Here, the value of the input resistance Zi is determined by the transfer characteristic of the secondary side AC/DC converter  28  and the capacitance of the capacitor  68 . 
       FIG. 15( a )  shows the primary side high frequency current I 1,hf  and the primary side high frequency voltage U 1,hf  according to the third operating condition of the inductive power transfer system illustrated in  FIG. 14 . 
     As shown in  FIG. 15( a ) , at the primary side the primary side high frequency current I 1,hf  increases and changes phase. Compared to the first operating condition the increase of the primary side high frequency current I 1,hf  is reduced due to the additional parallel branch with the impedance Zi. The primary side high frequency voltage U 1,hf  is supplied from the primary side DC/AC converter  14  and therefore unchanged. 
       FIG. 15( b )  shows the secondary side high frequency current I 2,hf  and the secondary side high frequency voltage U 2,hf  according to the third operating condition of the inductive power transfer system illustrated in  FIG. 14 . 
     As shown in  FIG. 15( b ) , at the secondary side the amplitude of the secondary side high frequency current I 2,hf  does not change significantly while the secondary side high frequency voltage U 2,hf  increases. Here, the increase in the secondary side high frequency voltage U 2,hf  is potentially slower due to charging the dc link of the secondary side. This causes an increase of the primary side high frequency current I 1,hf  at the primary side of the inductive power transfer system. 
       FIG. 16  shows a schematic diagram according to a fourth operating condition of the inductive power transfer system according to the present invention and a related equivalent circuit diagram. 
     As shown in  FIG. 16( a ) , the fourth operating condition may be represented by a closing of a fourth model switch  68  inserted at a location of a possible short circuit at the input of the secondary side AC/DC converter  28 . 
     As shown in  FIG. 16( b ) , the equivalent circuitry modelling the fourth operating condition comprises reactive components only. The value of the secondary side high frequency voltage U 2,hf  is zero. A phase shift between the secondary side high frequency current I 2,hf  and the primary side high frequency voltage U 1,hf  will approach a value of 90°. Further, the effective power supplied to the inductive power transfer system has a value of zero. 
       FIG. 17( a )  shows the primary side high frequency current I 1,hf , the primary side high frequency voltage U 1,hf , and a primary side power according to the fourth operating condition of the inductive power transfer system illustrated in  FIG. 16  at time t=0. 
     As shown in  FIG. 17( a )  the primary side high frequency voltage U 1,hf  is constantly supplied by the primary side DC/AC converter  14 . The primary side high frequency current I 1,hf  changes phase with respect to the primary side high frequency voltage and has reduced amplitude. Thus also the power P 1_fil  supplied at the primary side is reduced. 
       FIG. 17( b )  shows the secondary side high frequency current I 2,hf  and the secondary side high frequency voltage U 2,hf  according to the fourth operating condition of the inductive power transfer system illustrated in  FIG. 16 . 
     As shown in  FIG. 17( b ) , the secondary side high frequency voltage U 2,hf  is zero due to the short circuit operating condition. The secondary side high frequency current I 2,hf  oscillates due to the series resonance characteristics of the secondary branch in the equivalent circuit as shown in  FIG. 16( b ) . 
       FIG. 18  shows a schematic circuit diagram of a controller apparatus  70  for an inductive power transfer system  10 ,  34  according to a sixth exemplary embodiment of the present invention. 
     As shown in  FIG. 18 , the controller apparatus  70  comprises at least one interface  72 , e.g., a radio interface. The interface  72  is suitable for wireless information exchange, e.g., with a remote controller in the inductive power transfer system  10 ,  34 , or with an external control station of the inductive power transfer system  10 ,  34 . In some scenarios, the interface  72  may also be used for exchange of information with external systems, e.g., a maintenance system. 
     As shown in  FIG. 18 , the controller apparatus  70  comprises at least one processor  74  coupled to the interface  72  and a memory  76  coupled to the at least one processor  74 . The memory  76  may include a read-only memory ROM, e.g., a flash ROM, a random access memory RAM, e.g., a dynamic RAM DRAM or a static RAM SRAM, a mass storage, e.g., a hard disc or solid state disc, or the like. The memory  76  also includes instructions, e.g., suitably configured program code to be executed by the at least one processor  74  in order to implement a later described functionality of the controller apparatus  70 . This functionality will be referred to in the following as modules. It is noted that these modules do not represent individual hardware elements of the controller apparatus  70 , but rather represent functionalities generated when the at least one processor  74  execute the suitably configured program code. 
     As shown in  FIG. 18 , the memory  76  may include suitably configured program code to implement a detection module  78  and a control module  80 . 
       FIG. 19  shows a flowchart of operation for the controller apparatus  70  according to the sixth exemplary embodiment of the present invention as shown in  FIG. 18 . 
     As shown in  FIG. 19 , operatively the detection module  78 , in cooperation with the processor  74 , is adapted to execute a step S 20  for detection of operating conditions at the primary side of the inductive power transfer system  10 ,  34  according to the present invention as outlined above. 
     As shown in  FIG. 19 , operatively the control module  80 , in cooperation with the processor  74 , is adapted to execute a step S 22  for control of an operation of the inductive power transfer system in response to detected operating conditions at the primary side of the inductive power transfer system  10 ,  34 . 
     Further, operatively the control module  80 , in cooperation with the processor  74 , may be adapted to execute a control of the inductive power transfer system  10 ,  34  in response to operating conditions detected at a secondary side of an inductive power transfer system  10 ,  34 . 
       FIG. 20  shows a further detailed flowchart of operation for the controller apparatus  70  according to the sixth exemplary embodiment of the present invention as shown in  FIG. 18 . 
     According to the sixth exemplary embodiment of the present invention the inductive power transfer system  10 ,  34  comprises the primary side DC/AC converter  14  for converting a direct current input signal into the high frequency primary side current I hf,1  and the high frequency primary side voltage U hf,1  and the at least one electric variable for detecting the operating state at the secondary side is the high frequency primary side current I hf,1 . 
     As shown in  FIG. 20 , operatively the detection module  78 , in cooperation with the processor  74 , is adapted to execute a step S 24  to control a sampling process of the primary side high frequency current I hf,1  with a phase shift relative to the primary side high frequency voltage U hf,1 . 
     As shown in  FIG. 20 , operatively the detection module  78 , in cooperation with the processor  74 , is adapted to execute a step S 26  to analyze absolute values I 1hf_abs (k) of sample values of the high frequency primary side current I 1,hf  by using a threshold scheme set up by a first lower threshold I 1low (k), a second higher threshold I 1high (k), and a third maximum threshold I 1max  according to
 
 I   1low ( k )&lt; I   1hf_abs ( k )&lt; I   1high ( k )&lt; I   1max  
 
 I   1low ( k )= g   m   U   2,hf ( k )− c ( k )
 
 I   1high ( k )= g   m   U   2,hf ( k )+ d ( k )
 
     wherein 
               g             m       =     1     2   ⁢           ⁢   π   ⁢           ⁢     f   0     ⁢     L   12               
is the trans-conductance, f 0  is the resonant frequency at the primary side of the inductive power transfer system, and L 12  is the mutual inductance or a representative value for the trans-conductance;
 
     c(k) is a lower bound for observing regular operation at the secondary side of the inductive power transfer system  10 ,  34  in relation to g m U 2,hf (k); and 
     d(k) is a upper bound for observing regular operation at the secondary side of the inductive power transfer system  10 ,  34  in relation to g m U 2,hf (k). 
     As shown in  FIG. 20 , operatively the detection module  78 , in cooperation with the processor  74 , is adapted to execute a step S 28  to detect the short circuit condition at the secondary side of the inductive power transfer system  10 ,  34  when absolute values I 1hf_abs (k) of the sample values of the high frequency primary side current I 1,hf  are lower than the first lower threshold I 1low (k) over a first predetermined period of time. Then, operatively the control module  80 , in cooperation with the processor  74 , is adapted to execute a step S 30  to decrease the electric power supply at the primary side of the inductive power transfer system  10 ,  34  for protection of hardware circuitry in the inductive power transfer system  10 ,  34 . 
     As shown in  FIG. 20 , operatively the detection module  78 , in cooperation with the processor  74 , is adapted to execute a step S 32  to detect the open circuit condition at the secondary side of the inductive power transfer system  10 ,  34  when absolute values I 1hf_abs (k) of the sample values of the high frequency primary side current are higher than the second higher threshold I 1high (k) over a second predetermined period of time. Then, operatively the control module  80 , in cooperation with the processor  74 , is adapted to execute a step S 34  to limit the peak value of the primary side current for protection of hardware circuitry in the inductive power transfer system  10 ,  34 . Optionally, the control module  80 , in cooperation with the processor  74 , may also be adapted to decrease the power supplied to the inductive power transfer system  34 . 
     As shown in  FIG. 20 , operatively the control module  80 , in cooperation with the processor  74 , is adapted to execute a step S 38  to interrupt the operation of the inductive power transfer system  10 ,  34  when the absolute values I 1hf_abs (k) of the sample values of the high frequency primary side current I 1,hf  exceed the third maximum threshold I 1max  over a third predetermined period of time for protection of hardware circuitry in the inductive power transfer system  10 ,  34 . 
     As shown in  FIG. 20 , operatively the control module  80 , in cooperation with the processor  74 , is adapted to execute a step S 40  to control the first lower threshold I 1low (k) and the second higher threshold I 1high (k) in an adaptive manner according to a time varying trans-conductance of the inductive power transfer system  10 ,  34 . 
     As shown in  FIG. 20  the controller apparatus  70  is adapted to iterate the steps S 24  to S 40  for a continuous control of the inductive power transfer system. 
     In conclusion, the present invention adds an extra level of safety to inductive power transfer systems and supports instantaneous detection of operating conditions at the secondary side of the inductive power transfer systems, e.g., an error state. According to the present invention such operating conditions at the secondary side may be detected in a very fast manner. 
     This extra level of safety may be used to reduce costs of system implementation, e.g., costs of the extra short circuit protection switch at the secondary side. Additionally the present invention avoids generation of a short circuit operating condition at the secondary side of inductive power transfer systems which can lead to very high currents and/or voltages at the moment of switching, due to EMI problems during switching and related disturbance of communication and control channels. 
     While above the present invention has been described with reference to the drawings and figures, it should be noted that clearly the present invention may also be implemented using variations and modifications thereof which will be apparent and can be readily made by those skilled in the art without departing from the scope and spirit of the present invention. E.g., functionalities described above may be realized in software, in hardware, or a combination thereof. 
     Accordingly, it is not intended that the scope of claims appended hereto is limited to the description as set forth herein, but rather that the claims should be construed so as to encompass all features of presentable novelty that preside in the present invention, including all features that would be treated as equivalent thereof by those skilled in the art to which the present invention pertains.