Abstract:
A device for inductive transfer of electric energy from a stationary unit including at least one primary inductance to a vehicle that is adjacent to the unit and has at least one secondary inductance. The stationary unit has a device for detecting the presence of an electrically conductive object in a predetermined area that borders the primary inductance. The area can be between the primary and secondary inductance during the inductive energy transfer. The device has at least one measuring inductance, a measuring unit for measuring the impedance of the measuring inductance and an evaluation unit connected to the measuring unit. Preferably, the device is equipped with a plurality of measuring inductances which form a regular two-dimensional arrangement extending approximately in one plane, the plane lying perpendicular to the main direction of the magnetic field that is generated by the primary inductance during operation.

Description:
FIELD OF THE INVENTION 
     The invention relates to a device for inductive transmission of electrical energy. Devices of this kind are used for inductive charging of a rechargeable battery installed in an electric vehicle. During the transmission of energy, a magnetic field of high field strength and flux density is built up between a stationary primary coil and a secondary coil on the vehicle. This is necessary in order to induce a sufficiently high current for the desired transmission power in the secondary coil. 
     BACKGROUND OF THE INVENTION 
     If objects made of metal materials are introduced into the area of such a field, eddy currents are induced in these which lead to heating dependent on the material, time of exposure and level of field strength. In the presence of such conditions, such an object can reach a temperature which can lead to damage, e.g. to melting of plastic surfaces, or to hazards to persons. The latter occur in particular when the secondary side has been removed and heated metal objects are freely accessible and can be touched by persons. 
     Due to the characteristics of existing applications for inductive energy transmission systems, such a hazard arising from metal foreign bodies was assessed as not being relevant or, for example in the case of industrial trucks (AGV) attempts were made to remove such objects from critical field areas by fitting brushes in front of the secondary consumers. In the case of vehicles with a driver, training can be conducted to draw attention to the need to look out for the presence of such objects while in operation and to ensure that these are removed before the inductive transmission is set in operation, or in the event of any doubt to ensure that the inductive transmission is not set in operation. The existing safety measures appear unsuitable or at least inadequate for largely automatic operation or with higher safety requirements which must be assumed in particular when such systems are used in areas accessible to the public. 
     From US 2007/0145830 A1 a system for the wireless transmission of electrical power to electronic devices is known, which comprises a plurality of primary coils, thereby eliminating the need for precise alignment of a primary and a secondary coil relative to each other. In this document, the problem of the presence of metallic objects is mentioned, but a metal detector is dismissed as being unusable. Rather, by suitable circuit design it is ensured that the system only enters its resonant state when a resonant circuit comprising a secondary coil and a tuning capacitor connected in parallel approaches the primary side, thereby causing a steep increase of the primary current, which is concentrated on one or a few primary coils immediately near the secondary coil. A conductive object represents no problem in this case, because it does not constitute a resonant circuit. 
     SUMMARY OF THE INVENTION 
     Therefore, the underlying object of the invention is to improve the operating safety of an inductive energy transmission system with regard to the presence of electrically conducting foreign bodies. 
     According to the invention, with a device for inductive transmission of electrical energy from a stationary unit with at least one primary inductor to a vehicle standing in the vicinity hereof and having at least one secondary inductor, the stationary unit has a device for detecting the presence of an electrically conducting object within a predetermined space adjoining the primary inductor. Primarily, this means the space lying between the primary inductor and the secondary inductor during the inductive transmission of energy. If the selected range of the detection device is increased, it is also possible to detect the presence of a vehicle in the area of the primary inductor in addition to the presence of conducting foreign bodies in the immediate vicinity of the primary inductor. 
     Preferably, a sensor is provided operating on an inductive basis, i.e. with an impedance measurement at a measuring inductor, since such as sensor is particularly suitable for discriminating between conducting and non-conducting objects. In the simplest case, the existing primary inductor can be used as the measuring inductor. A higher sensitivity and approximate location of the position of a metal object allows the use of a regular two-dimensional arrangement of a plurality of separate measuring inductors. These can be embodied for example as planar coils which can be manufactured inexpensively in large numbers on a common substrate. 
     An evaluation device compares the impedance of the measuring inductor or a distribution of the impedances of the individual measuring inductors or groups of measuring inductors with a reference impedance or reference distribution. In the presence of a deviation of a predetermined amount, a signal is produced which indicates the deviation. This signal can be produced on an indicating device and/or used to deactivate the current supply of the primary inductor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, embodiment examples of the invention are described with reference to the drawings in which: 
         FIG. 1  shows a diagrammatic illustration of a charging station for inductive transmission of energy with an electric vehicle in a charging position, 
         FIG. 2  shows a diagrammatic illustration of a sensor for detecting conducting foreign bodies according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a diagrammatic sectional view (at the top) and a diagrammatic plan view (at the bottom) of an electric vehicle  1  standing over the primary coil  2  of a charging station to charge its battery. On the underside of the vehicle  1  in a housing  3  there is a secondary coil  4  which is connected to an electronic charging unit  5 . This converts the parameters of the electric power transmitted inductively to the secondary coil  4  into suitable values for charging the battery of the vehicle  1 . The primary coil  2  is fed by a current supply unit  6  of the charging station and is located in a housing  8  which is positioned statically in a vehicle parking bay. The current supply unit  6  is controlled by a control unit  7  of the charging station. 
     Some of the field lines  9  of the alternating magnetic field produced by the primary coil  2  while in operation are indicated by the dashed lines in  FIG. 1 . The main direction of the field is the direction of the coil axis of the primary coil  2  and thus the vertical direction. A high magnetic field strength and flux density prevails while in operation in the space  10  immediately above the housing  8  of the primary coil  2 . 
     A metal object  11  lies on the housing  8  of the primary coil  2 . This could have become detached from another vehicle which stood in the charging station before the vehicle  1  for example. It could also be an object lost by a person or an empty drinks can. Not least, the object  11  could also have been left there deliberately by a person with the intention of sabotage. As already explained at the start, the object would heat up due to the eddy currents induced in it when current is supplied to the primary coil  2  and thus become a source of danger. Moreover, it would reduce the efficiency of the energy transmission to the secondary coil  4 . 
     How the present invention overcomes this problem is explained below with reference to  FIG. 2 . This shows a diagrammatic plan view of a charging station of the kind described with reference to  FIG. 1  with a primary coil  2  in a housing  8  which is connected to a current supply unit  6 . A plurality of measuring coils  12  are arranged in the housing  8  between its upper wall and the primary coil  2  in order to detect the metal foreign body  11 . These measuring coils  12  are in each case much smaller than the primary coil  2 . In the illustrated example they are of planar design and can be produced for example in the form of conductor tracks on a circuit board or a foil which is fastened to the upper wall of the housing  8  from the inside. The measuring coils  12  could also be embodied as conductor tracks running directly on the internal surface of the upper side of the housing  8 . 
     The measuring coils  12  form a regular two-dimensional arrangement in the form of a matrix with equal grid spacing in the rows and columns. However, successive rows are in each case offset in relation to one another by half a grid space, so that a higher packing density can be obtained compared with a classic matrix arrangement. In  FIG. 2  it appears that the leads to many measuring coils  12  run through other measuring coils  12 , but this is not in fact the case. To avoid this in spite of the high packing density, the measuring coils  12  can in particular be distributed on two different sides of a circuit board or foil. In this respect, the illustration in  FIG. 2  should be seen not as true to reality but as diagrammatic. 
     The measuring coils  12  are in each case connected with an impedance measuring device  13 . These impedance measuring devices  13  are connected to a central evaluation device  14 . When no energy transmission is taking place, but the charging station is on standby, the measuring coils  12  are supplied with a measuring current of predetermined strength. This produces a measuring field around each measuring coil  12 , which produces eddy currents in a metal foreign body  11  lying on the housing  8  over the respective measuring coil. The magnetic reaction of these eddy currents on the measuring coil  12  concerned produces a change in the impedance. The impedance of each measuring coil  12  is measured continuously by the impedance measuring device  13  assigned to it in each case. 
     The measurement values supplied by the individual impedance measuring devices  13  are continuously compared with one another and when appropriate with a reference value in an evaluation device  14 . In the situation illustrated in  FIG. 2  in which a metal foreign body  11  at least partly covers four of the measuring coils  12 , the four impedance measuring devices  13  assigned to these covered measuring coils  12  and marked in black in  FIG. 2  supply impedance values to the evaluation device  14  which are different to those from the other impedance measuring devices  13  assigned to measuring coils  12  which are not covered. Here, the extent of the deviation in impedance depends not just on the conductivity and the form of the foreign body  11  but also on the individual extent of covering of a measuring coil  12  by the foreign body  11 . In the situation shown in  FIG. 2 , one of the measuring coils  12  is almost completely covered by the foreign body  11 , another is roughly half covered and two others are only covered to a lesser degree, with the result that the four impedance measuring devices  13  marked in black in  FIG. 2  measure different changes of impedance. 
     As an alternative to assignment of an individual impedance measuring device  13  to each individual measuring coil  12 , as shown in  FIG. 2 , the measuring coils  12  could also be connected together in groups so that each group is assigned an impedance measuring device  13  which would measure the resulting total impedance of the group. In this way, fewer impedance measuring devices  13  would be required, but the local measurement resolution would be inferior. 
     As a further alternative, a plurality of measuring coils  12  or groups of such could be measured with one common impedance measuring device  13  by connecting an analogue multiplexer in series with the impedance measuring device  13 . This would mean that even fewer impedance measuring devices  13  would be required, in principle just one. Naturally, multiplex operation would entail multiplication of the time required for complete detection of the impedances of all the measuring coils  12 . However, this could still be acceptable as changing a vehicle at a charging station is not a very rapid operation. 
     Distributing a plurality of measuring coils  12  over the upper side of the housing  8  makes it possible to draw conclusions as to the size and the position of a metal object when the impedances of the measuring coils  12  are measured at least in groups or, as assumed in  FIG. 2 , individually. In addition, this increases the sensitivity as a relatively small foreign body  11 , which for example only covers a single measuring coil  12  or a small group of these, causes a significant change in impedance with this measuring coil  12  or group, whereas with a single large measuring coil the measuring field of which would have to cover the entire upper side of the housing  8 , it would only produce a comparatively small change in impedance which would be much more difficult to detect reliably. 
     Independently of the number of measuring coils  12  and their possible combination in groups, whenever the measured impedance distribution deviates from a stored reference distribution by a predetermined minimum amount, the evaluation device  14  always emits an output signal to an indicating device  15  which emits an optical and/or acoustic warning. Apart from this, the evaluation device  14  is also connected with the control unit  7  of the charging station and emits a signal to the latter which blocks the initiation of energy transmission, i.e. the supply of current to the primary coil  2 . If the detected deviation from the specified state persists over an extended period of time, a message is sent to a competent location, e.g. to the operator of the charging station. The transmission of energy can only be set in operation again once the rectification of the faulty state has been confirmed by maintenance personnel. 
     As the measuring coils  12  are exposed to the strong magnetic field of the primary coil  2  when the inductive energy transmission is in operation, it is imperative to prevent high currents being induced in them as a result. For this, the impedance measuring devices  13  must be switched off before the beginning of the transmission of energy, and the measuring coils  12  must be switched to the open circuit state. To this end, the control unit  7  emits a corresponding signal to the evaluation device  14  which causes the measuring devices  13  to be switched off as indicated, and the measuring coils  12  to be switched to the open circuit state in good time before a primary current is switched on. 
     In a highly simplified variant compared with the form of embodiment of the invention previously described, the primary coil  2  is used as the single measuring coil. For this, the primary coil  2  is continuously supplied with a minimal current which is sufficient to build up a magnetic field of low strength as a measuring field around the primary coil  2 . The impedance of the primary coil  2  is measured continuously by means of a suitable measuring device  16  which is shown by the dashed lines in  FIG. 2 . In this variant, the separate measuring coils  12  and the associated impedance measuring devices  13  are eliminated. 
     It is obvious that when the primary coil  2  is used as the single inductive measuring pickup, only comparatively coarse monitoring of the housing  8  for the presence of a metal foreign body  11  is possible, i.e. that only comparatively larger foreign bodies  11  can be detected reliably with this. Moreover, no conclusion as to the position of the foreign body  11  is possible from a change in the impedance of the primary coil  2 , and only a limited conclusion as to its size. For this, the cost of additional hardware is much lower compared to the embodiment with a matrix of separate measuring coils  12  described previously. 
     In any case, when the strength of the measuring field and its physical extent are determined, it must be noted that it only covers a limited vertical range, for example to approx. 50 mm above the housing  8 . This makes it possible to ensure that parking of a vehicle  1  above the housing  8  does not cause a change in the impedance of the measuring coils  12  or misinterpretation by the evaluation unit  14  as indicating the presence of a metallic foreign body  11 . In addition, when the field strength of the measuring field is determined, it must be remembered that it must not be sufficient to heat a metal object  11  to such an extent that it could cause damage or danger. 
     In the case of the form of embodiment described first, the physical distribution of the impedance changes and/or their extent can also be used to detect the presence of a vehicle  1  over the primary coil  2  and differentiate between this and the presence of a metal foreign body  11 . For the purposes of the present invention, a vehicle  1  including its secondary coil  4  represents no more than one specific type of metal object. In this case, the measuring field must be designed so that a larger vertical area can be monitored, for example up to approx. 300 mm above the housing  8 . In this case, a signal sent by the evaluation device  14  to the control unit  7  indicates whether there is a foreign body  11  or a vehicle  1 . 
     The recognition of a vehicle  1  reported by the evaluation unit  14  can be used by the control unit  7  for example to initiate the energy transmission operation. On the other hand, this can also be used to establish when an attempt is made to park a vehicle  1  not requiring charging in the charging station. In the last-named case, measures can then be taken to prevent the definitive parking of the vehicle  1  and so keep the charging option free for other vehicles which need a charge. 
     If a sufficient and lasting change in the impedance or impedance distribution is detected indicating the presence of a vehicle  1 , contact can be made with the detected vehicle  1  for the purposes of identifying it more closely. This can be carried out for example by requesting a vehicle code by means of a transponder (RFID) or other suitable technical methods. At the same time, the communication with the vehicle is used to check whether and in what form there is a need and entitlement to draw energy at the charging station. When a vehicle is detected which has no need for energy transmission or no entitlement to this, the competent locations have the discretion to react appropriately to eliminate the blockage of the charging station. When a vehicle  1  is detected, but no communication can be established with it, the further action will be as in the case of detection of a foreign body  11 .