Abstract:
The invention relates to a floor covering ( 100 ) comprising: a plurality of coils ( 110 ), each coil ( 110 ) being operable to supply inductive energy to a power receiver circuit ( 200 ); wherein the plurality of coils comprises a transmitter area occupying the largest area of the floor covering ( 100 ); and a charging current through the coils is operable to generate said inductive energy.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to inductive power systems and a floor covering, and more particularly to a floor covering comprising one or more coils of the inductive power system which is operable to supply inductive energy to a power receiver circuit. 
       BACKGROUND OF THE INVENTION 
       [0002]    A large percentage of present-day electronics operates wirelessly, and this trend is expected to increase in the future. Portable appliances such as cell-phones, PDA, remote controls, notebooks, lamps, etc. represent only the beginning of what is expected to be a growing number of wireless devices in various industrial sectors. 
         [0003]    Portable and wireless appliances typically require power for operation, usually coming in the form of portable power storage by rechargeable or replaceable batteries. Rechargeable batteries are particularly advantageous, as they avoid the necessity of frequent replacement. Rechargeable batteries are often recharged by using induction means, wherein an inductive power pad is used to supply inductive energy to a power receiver circuit located within the portable appliance. The inductive power pad itself is usually supplied with energy via connected wires and plugs. 
         [0004]    The use of inductive power pads is not without drawbacks. In particular, conventional inductive power pads emit strong inductive fields which can interfere and produce harmful interactions with other electric and biological systems in close proximity. These fields can produce eddy currents in unprotected electronics and consequently damage or destroy them, and they can interfere with biological systems and implants. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0005]    It may be desirable to provide an improved inductive power system that supplies inductive energy everywhere within a room (e.g. office room), but locally where it is needed. 
         [0006]    This need can be met by a floor covering and an inductive power system as defined in the independent claims. 
         [0007]    In one embodiment of the invention, a floor covering comprises a plurality of coils. If the floor covering is used only for a small area, a single coil may be sufficient. If the floor covering covers a large area of a room, a plurality of coils is preferred. Each coil is operable to supply inductive energy to a power receiver circuit. The plurality of coils comprises a transmitter area occupying the largest area of the floor covering. The charging current through the coils is operable to generate said inductive energy inside the transmitter area. 
         [0008]    In another embodiment of the invention, an inductive power system is presented. The inductive power system includes a power receiver circuit which is operable to receive inductive power, and a floor covering, as described above and hereinafter. 
         [0009]    In a preferred embodiment of the invention, the plurality of coils is embedded in the floor covering, so that the transmitter area of the plurality of coils occupies the largest area of the floor covering. The inductive energy is therefore supplied throughout the transmitter area. The power receiver circuit is operable to receive inductive energy independently of its position on the floor covering. The floor covering further includes a wiring system selectively supplying a charging current from a power supply to each coil of the plurality of coils. The arrangement of coils is preferably as dense as the transmitter area of these coils occupying the largest part of the whole area of the floor covering. 
         [0010]    Examples of features and refinements of the floor covering according to the invention will now be described. However, these features and refinements also apply to the inductive power system. 
         [0011]    In one embodiment, the floor covering further comprises an upper protection layer. In a further embodiment, the floor covering further comprises a wiring system. The wiring system is operable to supply a charging current from a power supply to the plurality of coils. 
         [0012]    In another embodiment, the plurality of coils and the wiring system are integrated in a flexible substrate. This flexible substrate is attached to the protection layer. This allows integration of the plurality of coils in the substrate of the floor covering already during production of the floor covering itself. In a further refinement of this embodiment, the wiring system and the plurality of coils are insulated by an insulating layer. In a further refinement of this embodiment, the wiring system and the plurality of coils are structured by means of photolithography. 
         [0013]    In one embodiment, wires of said wiring system and/or said plurality of coils are woven and/or embroidered and/or sewn into the upper protection layer. This can preferably be done already during a production process of the upper protection layer itself or afterwards in a subsequent process step, using sewing machines, etc. In a further refinement of this embodiment, the wiring system and the plurality of coils comprise cables with a surrounding insulation. In a further refinement of this embodiment, the insulation is lacquer. 
         [0014]    In further refinements, the wiring system and the plurality of coils are connected by soldering and/or spot-welding and/or non-insulating gluing and/or a connector assembly. 
         [0015]    In a further embodiment, said coils are positioned adjacent to each other. Consequently, the space between two coils is significantly smaller than the diameter of the coils. In a refinement of this embodiment, the coils are arranged in a matrix configuration. To position the coils adjacent to each other, it is advantageous that the coils overlap partly and that the overlapping coils are arranged in different layers. 
         [0016]    In another embodiment, the floor covering further comprises a plurality of switches. Each switch corresponds to at least one coil of said plurality of coils. Each switch is operable to switch the charging current to the at least one connected coil. In a further refinement of this embodiment, the wiring system further comprises at least one power rail connected to each switch and to the power supply. 
         [0017]    In a further embodiment, each coil comprises wire windings or foils. In a refinement of this embodiment, these wire windings or foils are fixed in a certain position within the substrate. Each coil has a spiral or rectangular shape. The wire windings or foils are planar and positioned in-plane of the floor covering, so that the magnetic flux density within the coils is preferably directed perpendicularly to the main plane of the floor covering. 
         [0018]    In one embodiment, the floor covering further comprises a magnetic material which is capable of improving the magnetic coupling between the coils and the power receiver circuit. Such a magnetic material may be soft-magnetic wires, a ferrite polymer compound or a mumetal foil. 
         [0019]    A further embodiment of the floor covering comprises a visual indicator. This indicator is printed on the rear side of the floor covering. In a first refinement of this embodiment, the indicator indicates areas for cutting the floor covering. The indicator indicates where to cut the material best without cutting wires of the coils or wiring system. In a second refinement of this embodiment, the indicator indicates a predetermined point of fracture. Breaking at this predetermined point of fracture disconnects parts of the coils. In this case, the indicator indicates parts of coils that have to be cut when tailoring the floor covering to the exact room dimensions. This will prevent short-circuiting. 
         [0020]    Another embodiment of the floor covering comprises a respective plurality of detector circuits, each detector circuit corresponding to one of the plurality of coils and each detector circuit being operable to electromagnetically sense a power receiver circuit. For example, the detector circuit is or comprises a sensor winding. In a refinement of this embodiment, the sensor winding is embedded in the floor covering so as to detect any electric or electronic device placed on it. In a further refinement of this embodiment, each detector circuit is operable to electromagnetically sense a power receiver circuit. Upon electromagnetically sensing a power receiver circuit, each detector circuit enables or is operable to control switching of its corresponding coil to a power supply, thereby supplying a charging current to its corresponding coil. The charging current is operable to generate inductive energy for transmission to the power receiver circuit. 
         [0021]    Each detector circuit is operable to couple its coil to the power supply when the detector circuit detects a magnetic field emanating from the power receiver circuit. In a further refinement of the invention, each detector circuit includes a detector inductor having a first inductance L 1  in the absence of the magnetic field emanating from the power receiver circuit, which condition is operable to decouple the corresponding coil for the power supply, and a second inductance L 2  in the presence of the magnetic field emanating from the power receiver circuit, which condition is operable to couple the corresponding coil for the power supply. In a further aspect of this embodiment, a resonant capacitor is coupled in parallel with the detector inductor, wherein the inductance of the detector inductor and the capacitance of the resonant capacitor are operable to collectively provide a resonant operating frequency for the detector circuit. Optionally, each detector circuit is operable to receive a reference voltage, and each detector circuit additionally includes a switch which is operable to couple between the transmitting inductor and the power supply, and a differential amplifier which has a first input coupled to the detector inductor and the resonant capacitor, a second input coupled to receive the reference voltage, and an output for controlling the switching state of the switch. 
         [0022]    An embodiment of the inductive power system further comprises a remote control device with a transmitter which is operable to remotely control an electronic device wirelessly. In this embodiment, the remote control device comprises said power receiver circuit. In a refinement of this embodiment, the remote control device comprises a switch and/or a push-button and/or a slider. 
         [0023]    A further embodiment of the inductive power system comprises a transmitting circuit connected to the plurality of coils, which circuit is operable to transmit data to the power receiver circuit. In a refinement of this embodiment, the data is transmitted by modulating the charging current. Alternatively, an extra coil for data transmission can be used. In a further refinement of this embodiment, the power receiver circuit comprises means for receiving the transmitted data. In another refinement of this embodiment, the transmitting circuit is operable to transmit and receive data bidirectionally. In a further refinement of this embodiment, the inductive power signal and the transmitted and/or received data are separated by a plurality of frequency-selective filters. 
         [0024]    In a further refinement of the invention, the wiring is connected to each coil so as to supply the charging current to each coil selectively. The charging current is switched only to a coil of the plurality of coils with one or more power receiver circuits. In a further refinement of the invention, the charging current of two, e.g. adjacent, coils differs in phase or frequency so as to reduce unwanted steady-state superposition. 
         [0025]    These and other aspects of the present invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a schematic view of an inductive power system according to the invention. 
           [0027]      FIG. 2  is a schematic cross-section of a floor covering according to the invention. 
           [0028]      FIG. 3  is a schematic view of the circuitry of the floor covering. 
           [0029]      FIG. 4  is a schematic view of a footswitch as a particular appliance. 
       
    
    
       [0030]    For clarity, previously identified features retain their reference signs in subsequent drawings. 
       DESCRIPTION OF EMBODIMENTS 
       [0031]      FIG. 1  is a schematic view of an inductive power system according to the invention. An electronic device as a power receiver circuit  200  is movable across a floor covering  100  used, for example, in an office room. The inductive power system generally includes the floor covering  100 , a power supply (not shown), which is connected to the floor covering  100  by a connecting part  119  of the floor covering  100 , and the power receiver circuit  200 . The floor covering  100  comprises a plurality of coils  110  which are operable to supply inductive energy and operates as a base from which a portable appliance accommodating the power receiver circuit  200  with a rechargeable battery  281  is charged. 
         [0032]    For example, the floor covering  100  may be a flat, wooden base with the plurality of coils on its rear side onto which the portable appliances, e.g. vacuum cleaners, office tables with additional electronic equipment, lamps, thermostats, foot switches, robots, loudspeakers, furniture with integrated or attached electronic devices, movable machines, thermal shoes, etc. are placed for powering and/or recharging. The floor covering  100  has a size which matches the dimensions of the room in which the appliance is used. Instead of a wooden floor covering  100 , a floor covered with linoleum, vinyl or carpet (hand-woven or broadloom) can be used advantageously. 
         [0033]    The floor covering  100  includes a plurality of coils  110 , i.e. 2 or more, e.g. 5, 10, 50, 100, etc., each coil  110  being operable to receive a charging current from the power supply. Each coil is operable to provide the transmission of inductive energy to (i.e. to induce a voltage on) a receiving inductor  210  in the power receiver circuit  200 . The coils  110  and the receiving inductor  210  may be implemented in various forms, for example, as spiral inductors having a particular number of whole or fractional windings. 
         [0034]    In the embodiment shown in  FIG. 1 , the floor covering  100  further includes a plurality of detector circuits  111  (referring to 2 or more detector circuits, e.g. 5, 10, 50, 100, etc.), each detector circuit  111  having a corresponding coil  110  (e.g. detector circuit  111  corresponding to coil  110 ), and each detector circuit  111  being operable to electromagnetically sense the presence of a power receiver circuit  200 . “Electromagnetically sense” herein refers to the detection of an electromagnetic signal (i.e. a signal having an electric, magnetic or combined electromagnetic field) which is communicated between the detector circuit  111  and the power receiver circuit  200 . In one embodiment, the electromagnetic signal is a magnetic field which emanates from a magnet located within/on the power receiver circuit  200 . In another embodiment, the electromagnetic signal is an electromagnetic RF signal, e.g. an RFID signal transmitted from the power receiver circuit  200  to the detector circuit  111 . Other embodiments may also be employed, wherein the detector circuit  111  electromagnetically senses the power receiver circuit  200 . For example, the detector circuit  111  may broadcast a signal and the power receiver circuit  200  operates in a conventional transponder manner and transmits a predefined signal when it receives the signal. More generally, any electric, magnetic or electromagnetic field may be used as the detection means for ascertaining the presence of the power receiver circuit  200  proximate to the detector circuit  111 . Each detector circuit  111  comprises a switch which, upon electromagnetically sensing the presence of the power receiver circuit  200 , is operable to control switching of its corresponding coil  110  to the power supply. A charging current is then permitted to flow to the corresponding coil  110 , thereby generating power for transmission to the inductor  210  in the power receiver circuit  200 . 
         [0035]    In an embodiment further detailed below, the detector circuit  111  is switchably coupled between its corresponding coil  110  and the power supply connected to the floor covering  100  via the connecting part  119  of the floor covering  100 . The detector circuit  111  is operable to couple the corresponding coil  100  to the power supply. In another embodiment, the detector circuit  111  is operable to detect a recognized signal (e.g. a recognized RFID signal) and supply it to a receiver (e.g. an RFID receiver), the receiver being operable to control coupling between the corresponding coil  110  and the power supply. 
         [0036]    In a further embodiment, the floor covering  100  is operable to concurrently supply inductive energy to a multiplicity (e.g. 2, 5, 10, or more) of power receiver circuits  200 . In such an embodiment, a respective multiplicity of detector circuits  111  (or multiple respective groups of detector circuits  111 ) is operable to electromagnetically and concurrently sense the presence of the multiplicity of power receiver circuits  200 , each detector circuit  111  being operable to control switching of its respective coil  110  to the power supply so as to receive a charging current, as described hereinbefore. 
         [0037]    The floor covering  100  further includes a power rail or supply line/bus  113 ′,  114 ′ as a part of a wiring system integrated in the floor covering  100  for supplying power to each coil  110 . The coils  110  are connected to one power rail  113 ′ and the receiving circuit  111  with the switch is connected to the other power rail  114 ′. The power supply may be located close to the connecting part  119  of the floor covering  100  and electrically coupled thereto. Each detector circuit  111  is switchably coupled between its corresponding coil  100  and the power supply via the power rail  114 ′. 
         [0038]    The floor covering  100  further includes a magnetic layer  130  (consisting of e.g. a soft-magnetic plate) which is operable to increase the magnetic flux density in the direction of the power receiver circuit  200 . The magnetic layer  130  is preferably positioned beneath the coils  110 . 
         [0039]    The power receiver circuit  200  as shown in  FIG. 1  is arranged on top of the center of a coil  110 , within a housing  290 . The power receiver circuit  200  includes a receiving inductor  210  (e.g. a spiral inductor), a magnetic layer  230 , and power electronics  280 , including a resonant capacitor, a rectifier and a rechargeable battery  281 . The spiral inductor  210  is operable to receive inductive power transmitted by the coil  110 . The magnetic layer  230  (consisting of e.g. a soft-magnetic plate) operates to provide the detectable magnetic field to be sensed by the detector circuits  111 , and may be arranged as a large/wide area of spiral inductors  210 , or alternatively arranged within the center of the spiral inductors  210  to ensure better sensing capability and positioning accuracy. The magnetic layer  230  is further operable to concentrate the magnetic flux density on the receiving inductor  210 . The magnetic layer  230  may be a ferrite plate or formed from a material which can be easily laminated onto a printed circuit board  220  or other substrate providing the bulk of the power receiver circuit  200 . For example, plastic ferrite compounds or structured highly permeable metal foils (e.g. mumetal, metglas, nanocrystalline iron, etc.) may be used. 
         [0040]    Those skilled in the art will appreciate that levels of integration may be employed. For example, one or both of the detection circuits  111  and the power receiver circuit  200  may be implemented as an integrated circuit (e.g. Si, SiGe, GaAs, etc.), with the aforementioned components being monolithically formed into an integrated circuit by means of a photolithographic semiconductor process. Another possibility is to form a hybrid circuit from discrete components. 
         [0041]    Passive electric components of the floor covering  100  are preferably realized as printed circuit board-integrated components. Semiconductor ICs may be thinned to reduce vertical height and surface area-reduced so as to minimize risk of breakage. 
         [0042]    As mentioned above, the inductive power system of the present invention can be implemented in a wide variety of portable appliances. A particular application of the system is in the field of wireless control modules used in, for example, office rooms in which diversified electronic devices such as computers, phones, lamps, etc. are remotely controlled and supplied with energy. 
         [0043]    Wireless operation is preferred; however, portable power supply via batteries is not reliable and presents maintenance problems, as batteries must be periodically checked and, if necessary, replaced. Use of conventional rechargeable batteries requires an exposed power transfer point to recharge the batteries, which may leak. An inductive power system with a floor covering  100  comprising coils  110  makes inductive energy available throughout the office. 
         [0044]      FIG. 2  is a schematic cross-section of an embodiment of a floor covering  100  according to the invention. The floor covering  100  is made as a textile floor cloth comprising an upper protection layer with a carpet-like surface  150 . If carpeting is used, the attached floor covering  100  is made of a heavy, thick fabric, usually woven or felted, often wool, but also cotton, hemp, straw, or a synthetic counterpart. Polypropylene is a very common pile yarn. It is typically knotted or glued to a base weave  140 . It is made in breadths of typically 4 or 5 meters to be cut, seamed with a seaming iron and seam tape, but formerly it was sewn together and affixed to a floor using nails, tack strips (known in the UK as carpet rods or stair rods, when used on stairs), (grippers) or adhesives, thus distinguishing it from a rug or mat which are loose-laid floor coverings. Carpeting which covers an entire room area is loosely referred to as ‘wall-to-wall,’ but a carpet can be put on any portion thereof while using appropriate transition moldings where the carpet meets other types of floor coverings. 
         [0045]    Alternatively, the floor covering  100  may be made of ‘carpet tiles’, which are squares of carpet, typically 0.5 m square, which can be used to cover a floor. They are usually only used in commercial settings and are often not affixed to a floor in order to allow access to the sub-floor (for example, in an office environment) or to allow rearrangement in order to spread wear. The wiring system  113 ,  114  of these carpet tiles is realized by using flat connectors between each square. 
         [0046]    A flexible substrate  120  includes the wiring system  114  and the plurality of coils  110  in different, laminated layers. The wires  114  of the wiring system and the coils  110  are integrated in the flexible substrate  120 . This flexible substrate  120  is attached to the protection layer with the carpet-like surface  150  and the weave  140 . In the embodiment shown in  FIG. 2 , the flexible substrate  120  is glued to the weave  140  by means of an adhesive layer  124 . Alternatively, the weave itself may be the flexible substrate comprising the wiring system  114  and the coils  110 . 
         [0047]    The flexible substrate  120  is used in the construction of the coils  110 , e.g. polyimide (“Flexfoil”). Electronic components may be located on top or below the coils  110 , or between them, the construction of the floor covering  100  being suitable for heavy loads on its top while remaining operable, because the copper wires  114 , the foils with spiral windings  110  and the magnetic foils  130  are all flexible. The resulting floor covering  100  can be handled right away as any other floor covering, and can be specifically stored on a roll. 
         [0048]    Additionally, the floor covering  100  comprises the magnetic material  130 , which is capable of improving the magnetic coupling between the coils  110  and the power receiver circuit  200 . The magnetic material may be a magnetic foil  130  made of a ferrite polymer compound. 
         [0049]      FIG. 3  is a schematic view of the circuitry of the floor covering  100  and other parts of the inductive power system. The floor covering  100  of the embodiment shown in  FIG. 3  comprises sixteen coils  110  arranged in a matrix configuration. The wiring system connecting the coils  110  comprises four row wires  114  and four column wires  115 . Each wire  114 ,  115  of the wiring system is connected to a connecting part  118  for the rows  11 ,  12 ,  13 ,  14  and a connecting part  119  for the columns c 1 , c 2 , c 3 , c 4 , respectively. 
         [0050]    Optical indicators  115  (on the rear side) indicate where to cut the material best without cutting wires unnecessarily. Cut wires may deactivate complete rows or columns of coils  110 . The indicators  115  can also indicate predetermined points of fracture, which are marked X to allow disconnection of parts of coils  110  which have to be cut when tailoring the floor covering  100 . 
         [0051]    The floor covering  100  is connected to a control circuit  300  via a parallel bus  318  having a number M of wires corresponding to the number of rows and via a parallel bus  319  having a number N of wires corresponding to the number of columns. The control circuit  300  comprises at least (M+N) switches  311  to connect each coil  110  to the power supply  310 . The control circuit  300  required to operate the coils  110  may be integrated in the base board. 
         [0052]    The embodiment shown in  FIG. 3  uses a wireless network (not shown) such as ZigBee or WLAN for the specific coils  110  to which the charging current has to be switched by the control circuit  300 . The control circuit  300  switches a current temporarily to a specific coil  110 , with a modulated identification of this coil  110 . The power receiver circuit  200 , which needs to be charged or supplied, receives this code if it is above the corresponding coil  100 . Along with other data, the power receiver circuit  200  sends the identification to the control circuit  300  via the wireless network. The control circuit  300  then just has to switch the charging current to the corresponding coil  110 . Additionally, the control circuit  300  is operable as a transmitting circuit transmitting data to the power receiver circuit  200 . This data transmission may be one-directional or bi-directional. Alternatively, the floor covering  100  may comprise detector circuits  111  similarly as in the embodiment shown in  FIG. 1 . 
         [0053]    The coils may also have different shapes. For example, they may comprise wires from one to the other end of the floor covering, resulting in an elongated coil shape. Several of these elongated coils may be arranged in different, e.g. perpendicular directions so as to form an array. The wires of a plurality of coils can be connected by using a single terminal on at least one side of the floor covering. 
       Examples of Applications 
       [0054]    As mentioned above, the floor covering and the inductive power system of the present invention can be implemented in a wide variety of portable appliances. A particular application of the system is in the field of wireless control modules. For example, the wireless control module may be implemented as a footswitch for controlling movement of a medical instrument or device, such as a patient&#39;s chair in a dental office, or to control aspects of an X-ray diagnostic system, such as a patient&#39;s table movement, gantry movement, release of X-rays, and the like (such instruments being referred to collectively as “medical devices”). Another application is in the industrial field, in which machines may be controlled by a wireless remote control unit. Further examples of applications are (automatic) vacuum cleaners, office tables with additional electronic equipment, lamps, thermostats, foot switches, robots, loudspeakers, furniture with integrated or attached electronic devices, movable machines, thermal shoes, etc. for powering and/or recharging. 
         [0055]    Conventional foot switches, which provide control by wired means, are disadvantageous, because they require a significant effort to clean and disinfect. Wireless operation is preferred; however, portable power supply via batteries is not reliable and presents maintenance problems, because batteries must be periodically checked and, if necessary, replaced. Use of conventional rechargeable battery requires an exposed power transfer point to recharge the batteries, which may leak. An inductive power system in which the control unit is sealed provides the best solution. 
         [0056]      FIG. 4  shows a foot switch controller  1000  on a floor covering  100  incorporating an inductive power system according to the invention. The foot switch controller  1000  is operable for wireless communication with a wireless receiver  1050  and includes a power receiver circuit  200  for receiving power from coils  110  of the floor covering  100 . In a particular embodiment, the foot switch controller  1000  is operable to wirelessly control an X-ray apparatus  1050  regarding, for example, the movement of a patient bed, gantry or release of X-ray radiation in a CT system. 
         [0057]    The floor covering  100  may be constructed as a loose mat partly covering the room or fixed to the floor and cover it completely (collectively “transmitter area”) on which the foot switch controller  1000  is placed for operation and/or periodical charging. If the covering is constructed as a loose flexible mat, a flexible substrate is used in the construction of the coils  110 , e.g. polyimide (“Flexfoil”). The electronic components may also be located on top or below the coils  110 , or between them, the construction of the mat—the protection layer and the coils—being suitable for heavy loads on top while remaining operable. The mat may be covered with a thin, anti-slip rubber layer on the back and a sealed protection layer on its top surface. The mat may also be hermetically sealed so as to allow easy cleaning. 
         [0058]    To achieve a uniform height, which allows a good pressure distribution, an additional layer may be added to the flexible mat. This layer is made of a material which is not compressed when stepping on it, and as it must accommodate electric components, this layer has a height which is approximately equal to that of such components. In this manner, the components are buried in and protected by the holes of the layer. The holes may be additionally filled with epoxy to provide further protection. 
         [0059]    The mat may further include an inclined area without coils at the edges so as to avoid a step from the floor to the charging area. The edges may be made of a flexible material (e.g. rubber) to achieve a sealing function with respect to contaminating fluids, such that the bottom surface of the mat stays clean. 
         [0060]    When the floor covering  100  is fixed to the floor, the transmitter area may be equipped with borders so as to facilitate retention of the foot switch controller  1000  within this area. Furthermore, the gap between the plane of the floor and the coils  110  is filled with a material, such as an epoxy plastic, which is fluid during installation and then fills all gaps and holes with minimal air gaps. 
         [0061]    The housing  290  of the foot switch controller  1000  is preferably constructed from non-conducting material in order to avoid induced eddy currents that might cause unintended losses. In order to reduce loss of the induced energy, the receiving coil (e.g. a spiral inductor)  210  is arranged in a hole which has a slightly larger diameter than the spiral coil  110 . In an alternative embodiment, the housing  290  has a recess which contains a matrix of spiral coils, each of which faces the exterior of the housing. The foot switch controller  1000  may be equipped with an indicator lamp indicating that inductive power is being received and that the battery is charged (when so equipped). In one embodiment, the foot switch controller  1000  contains no local energy storage and is only powered by the received inductive energy. Operation without a rechargeable power source simplifies the controller design and reduces cost and maintenance needed for checking and, if necessary, replacing a rechargeable battery. 
         [0062]    Electromagnetic sensing may be realized by means of an RFID tag located within the portable foot switch  1000  (or the power receiver circuit  200  therein), and an RFID receiver  111  within the floor covering  100 . For example, the RFID tag and corresponding RFID receiver  111  may be tuned to a unique signal, thereby preventing unauthorized use of the foot switch controller  1000  in other areas, or interference from another foot switch controller. 
         [0063]    It should be noted that use of the verb “comprise” and its conjugations does not exclude other features, and the indefinite article “a” or “an” does not exclude a plurality, except when indicated. It is to be further noted that elements described in association with different embodiments may be combined. It is also noted that reference signs in the claims shall not be construed as limiting the scope of the claims. 
         [0064]    The foregoing description has been presented for purposes of illustration and elucidation. It is not intended to be exhaustive or limit the invention to the precise form disclosed, and obviously many modifications and variations are possible within the scope of the invention. The described embodiments were chosen in order to explain the principles of the invention and its practical application so as to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined solely by the appended claims.