SE526099C2 - Device for wireless signal and energy transfer for medical implants - Google Patents

Abstract

The invention relates to an inductive link for power and signal transmission to a medical implant, preferably power and signal transmission through the skin or any other physical barrier (3) of a human being or living animal having a medical device implanted in the body. The inductive link comprises at least two coils which are orientated in space in such a way they are located on each side of said physical barrier (3), so that one of the coils, the so-called primary coil (15), is located outside the physical barrier and then forming a part of the primary circuit that generates an electromagnetic field. The other coil, the so-called secondary coil (19), is located on the other side of the physical barrier and then forming a part of the secondary circuit which can absorb and utilize the entire or a part of the magnetic field generated by the primary coil. According to the invention a sensor circuit (11) in the form of a structure of electrical conductors is arranged in a physical close relationship to the primary coil (15) in space so that a part of the magnetic field from the secondary coil (19) is enclosed by the sensor circuit thereby providing an induction in the sensor circuit which can be detected and thereby indicate the condition in the secondary circuit.

Description

O IIOO I o 0 ICO 0000 0 o co Jolo 009 0 O 0 10 _15 20 25 30 52 6 09 9, .._, .._,, .. .. secondary coil, possible voltage drops, etc. How much of the primary field is passes through the secondary coil depends on the size of the players, the distance between the players, the geometric design of the players and any variations in the permeability between the players. The coupling factor fi is denoted as the ratio between the field generated by the primary coil and the tent enclosed by the secondary coil.

The coupling factor can vary between zero and one. The larger the switching factor, the better the efficiency because the primary currents then decrease for a certain secondary voltage, which results in lower resistive losses on the primary side. In general, the larger the coils and the shorter the distance between the players, the higher the coupling factor. But for a certain distance and a certain maximum coil diameter, there are also ways to increase the coupling factor. Perhaps the most used method is to place e.g. ferrite material in the center of the players, thereby shaping the field so that more of the primary field passes through the secondary coil. In a ferrite material, however, eddy currents are induced which give losses, so it is not always so certain that the increased coupling factor which the ferrite material gives actually means a higher efficiency. US 5,889,183 describes a method of winding coils in helical fins and thereby increasing the coupling factor. The fact that the coupling factor is increased here is due to the fact that the spiral winding causes the tent to concentrate more towards the middle of the spool.

Voltage drop between the power source on the primary side and the load on the secondary side consists i.a. of resistance in drive stages, resistive components in the players and the players' leakage inductances. The voltage drop of the drive stage can be reduced by selecting drive transistors with a low voltage drop. However, this entails a larger drive current and slower switching times, so the choice of drive transistor / s must be a balance against this fact. The leakage inductances of the coils can be extinguished by connecting a capacitor in series or parallel to the respective coil and thus making it resonant at the transmission frequency. The frequency you choose on the carrier varies from case to case. It can be said that in order not to get too large coils, the transmission frequency should be a few hundred kHz and up. If you go too high in frequency, the games eventually become self-resonant and then they become more capacitive than inductive. This occurs at frequencies in the range 10 ~ 20 MHz depending on how the players are wound. 0 o oo g.: ... .00-: tat _. n: e .nu 000 Il ho once g U 0 0 I 0 0 0 ono OI 0 0090 000 10 15 20 25 30 526 099 In general, an inductive power and signal transmission link of the kind discussed above should consist of an external power, drive stage , primary and secondary coil. The external power source is usually a battery. However, batteries have a limited operating time and durability and must be replaced at regular intervals. Having a high efficiency between the battery and the load is naturally desirable as it provides a longer operating time and a higher maximum power over the load. The electronics on the inside of the skin require, among other things, a fairly even supply voltage. As mentioned earlier, however, the amount of the induced voltage on the secondary side varies when changing the switching factor, which in turn is caused by a mutual change of position between the primary and secondary coils. A load ring on the inside also affects the voltage due to voltage drops across the link as well as various other series resistances between battery and load.

In order to obtain a reasonably even voltage on the secondary side during load and position variations, some form of voltage control is needed. Such a voltage regulation can take place in different ways. For example, a feedback signal from the secondary side can somehow be sent back to the primary side where the primary field is adjusted in such a way that the secondary voltage is kept within the desired range. Such a type of control is described in patents US 5,876,425 and US 6,442,434.

US 5,735,887 describes another type of control where a load pulse is generated when the voltage on the secondary side falls outside a given interval. This load pulse is sensed on the primary side where regulation takes place.

Another way of voltage regulating is described in patents' US 5,562,714 and US 5.1 17,825. In this case, the voltage regulation takes place by means of a sensor coil which is arranged on the primary side and which senses the primary field and regulates this towards a predetermined value. With this method, a regulation is obtained against load variations but not against position variations.

US 4,679,560 describes a method of achieving stabilized bias by skewing the primary and secondary sides on each side of the transmission frequency.

The oblique tuning means that with a reduction in distance, the poles migrate apart due to the mutual inductance. The pulse separation gives a voltage reduction which counteracts the voltage increase which otherwise a to gg ..

O O 0 00 II o 0 q Ü .OI 0 I one nn 0 Q O Q ÛÛÛ! n 0 con ung; U 0 line one 10 15 20 25 30 526 099 4 E 2 'g'. . ° 2 _! .__ 2 distance reduction gives rise to. Additional patents describing this type of skew are US 5,070,535. Common to the devices described in these two patents is that they have a separate oscillator to generate the transmission frequency.

An inductive link requires some form of restraint device that ensures that the outer and inner unit are held together. The most commonly used method of retention is to use magnets. And to get as small external and internal units as possible, the magnets are usually placed in the middle of the coils. In the magnets, however, eddy currents are induced which give resistive losses. U.S. Pat. No. 6,178,353 describes a method of laminating retaining magnets so as to reduce eddy current losses.

An object of the present invention is to provide a device for wireless signal and energy transmission for medical implants which is significantly simpler and more energy efficient than previously known solutions. The device shall provide an efficient transmission of both signal and energy at low supply voltage and low powers, ie within the power range of a few mW. The device must contain a minimum of active electronics and thus be very energy efficient. The device shall, as far as possible, provide a voltage that is independent of load and position variations both longitudinally and laterally. The device comprises in a known manner two coils which are arranged in space so that they are located on each side of a physical barrier, for example the skin of a person. One coil, the so-called the primary coil, is located outside the physical barrier and forms part of a primary circuit that emits an electromagnetic field.

The second coil, the so-called the secondary coil, is located on the other side of the physical banner and forms part of a secondary circuit which can absorb and utilize all or part of the magnetic field emitted from the primary coil. The primary coil and the secondary coil have a mutual coupling factor k which is less than one.

The coupling factor depends on the relative position of the coils, which can vary with time.

The novel and distinctive feature of the invention is that a sensing circuit in the form of a structure of electrical conductors is arranged in physical proximity to the primary coil in such a manner in space that a part of the magnetic field from the secondary coil is enclosed by o o; oo o o .. '. : "': z 00-: .o ooo oooo oo ooo oo oooo ooo. ..:' lo oooo o; 0 ooo 0 ooo UI oooq oo oo g. 10 15 20 25 30 526 099 o oo oo oo oo oo oo o oo oooooo 5 the shielding circuit and thus gives rise to induction in the shielding circuit, which can be detected to thereby sense the state in the secondary circuit.

The device is further characterized in that the sensing circuit is arranged in space in such a way that the part of the magnetic field from the primary coil enclosed by the sensing circuit does not give rise to any induction in the sensing circuit, so that the sensing circuit thereby shields the state only in the secondary circuit. This is achieved when the integral of the magnetic rail enclosed by the sensing circuit from the primary coil along the contour of the sensing circuit approaches zero.

According to an advantageous embodiment of the invention, the sensing circuit is arranged to sense the instantaneous voltage of the secondary side, i.e. both the phase and amount of the voltage.

According to a further advantageous embodiment of the invention, the sensing circuit is mounted on the primary coil without mutual connection to it and consists of one or more coils.

Unlike the previously mentioned signal and transmission lines, the feedback through the said sensing circuit is purely passive. There is no electronics on the secondary side that send any information back to the primary side, but the primary side shields the secondary side voltage with the said sensing circuit. By feedback of the phase of the secondary tank circuit and controlling the primary side with a certain phase position relative to the phase of the secondary side, a self-oscillating signal transmission is obtained. By also feedbacking the secondary side voltage, any voltage variations caused by a load or position change can be regulated away by adjusting the voltage on the primary side.

With the help of the feedback, a closed loop is obtained which makes it possible to maintain a constant bias on the secondary side by regulating the input power on the primary side. As the sensing coil only senses the tent and thus the voltage on the secondary side and this is done with purely passive electronics, no extra current is consumed for the feedback, which provides a very current-efficient way of controlling the voltage. o oo oo o oo o ooo oo 0 0000 00 0000 00 0 00 00 0000 i I OO C OI O OI OI 0 0 0 0 0 0 I00 I 000 0000 0 0 0 0 0 0 0 0 0 0 00 00 00 000 00 0000 000 10 15 20 25 30 526 099, .._, ..,, .. ..

In the following the invention will be described in more detail in connection with the accompanying drawings which show examples of some different forms of discharge of the device, Figure 1 showing in block form the constituent parts of an inductive link for signal and energy transmission, Figure 2 shows in the form of an electrical diagram the constituent components in more detail, Figure 3 schematically shows the position of the coils in space, in Figure 4a shows the relationship between E J-tïï when calculating the field image around a fl C coil, Figure 4b shows the field image characteristics as a function of the radius of coils wound in two different ways, Figure 5 shows a primary coil with a sensing coil placed in such a way that the total field from the primary coil through the sensing coil becomes zero, Figure 6 shows the sensing characteristic longitudinally between two different coils, Figure 7 shows some different examples of the physical structure (wire structure) of the sensing coil, Figure 8 shows a pancake coil wound of Litz wire, Figure 9 shows the appearance of a shield coil with lateral insensitivity, Figure 10a shows shielding characteristics in radial direction for three different coil distances d, Figure 10 b illustrates two coils at the distance d from each other, and 10 15 25 25 526 099 1 .. ooo ooo no I ooo uocooon oo o I oo 0 cooooo oo oo oo oo Figure 1 1 shows an example of the physical design of the transfer device.

Figure 1 describes in block diagram form the constituent parts of an inductive link for signal and energy transmission. The system consists of a primary side 1 and a secondary side 2. The primary side 1 and the secondary side 2 are physically separated by a physical barrier 3, for example the skin of a living being. The primary side consists of a transmitter 4, a drive circuit 5 and a bias supply 6. The transmitter's task is to convert a varying current into a varying magnetic field, while the drive circuit 5 by means of a control signal controls a current through the transmitter 4 in a desired manner. On the secondary side, the magnetic field must be converted back into electrical energy, which takes place in a receiver 7.

In order to get a direct voltage to drive a load 12 with it, a rectification 8 of the voltage takes place. In order to be able to maintain an iron voltage on the secondary side 2, the voltage on the secondary side tank circuit is fed back to the primary side 1 where the fed-in power can be regulated. The feedback takes place by means of the sensing circuit 11 which shields the field from the tank side of the secondary side and thus the instantaneous voltage. It is possible to send information on the carrier by modulating it, which is done in circuit 9. The simplest way to achieve such modulation is to vary the supply voltage on the primary side. If so, it should be demodulated on the secondary side, as indicated by the circuit 10. The rectified voltage from 8 and the demodulated signal from 10 supply a load 12 with signal and energy. In principle, modulation of the carrier can be done in three different ways, namely by amplitude modulation, frequency modulation or phase modulation. Since it is important to keep the effector roar down, amplitude modulation is used in this case.

Figure 2 describes the system in more detail by means of an electrical circuit diagram.

The generation of the magnetic field on the primary side 1 takes place by means of a coil 15, the so-called the primary coil. The drive to the coil 15 consists of two transistors 16, 17 and a comparator 18. On the secondary side 2, the magnetic field is converted into electric energy by means of the secondary coil 19. The coil 19 and a capacitor 20 form a parallel LC circuit and are tuned to the overdrive sequence for to provide better crossover properties. In order to obtain a direct voltage to drive a load with, a rectification of the voltage takes place with a diode 21. A capacitor 22 smooths out the current so that an even voltage is obtained. The diode 23, the capacitor 24 and the resistor 25 form an envelope detector which is the simplest dernodulator for an amplitude modulated system. The equal voltage and the demodulated signal supply the load 26 with energy and signal. As mentioned above, the amplitude modulation is performed by varying the supply voltage to the drive transistors 16 and 17. This is done by means of the transistor 27. In order to be able to maintain an even voltage on the secondary side, the voltage of the tank circuit on the secondary side is fed back to the primary side where the attenuated power is regulated by the operational amplifier 28 and the transistor 27. The feedback is by the coil 29 from the secondary side tank circuit and thus the instantaneous voltage.

The coil 29 is a purely passive sensing coil which reconnects the field in the secondary coil 19 to the primary side. With the help of the feedback, a closed loop is obtained which makes it possible to maintain a constant voltage on the secondary side by regulating the input power on the primary side. The feedback phase is also used to control the primary drive, which eliminates the need for an extreme oscillator. To obtain the correct phase position on the primary side drive, a phase shifting filter is located between the feedback voltage and the primary drive. The filter here consists of the resistor 30 and the capacitor 31. In order that the sensing coil 29 should not receive any contribution from the primary coil 15, the sensing coil is placed over the area where the field from the primary coil changes sign. If the sensing coil there encloses as much positive as negative field, the primary field is zeroed out, which is explained in more detail below.

The secondary field sensing coil 29 only senses the field and thus the voltage on the other hand will be indicated in the sensing coil. As the secondary side and this is done with purely passive electronics, no extra current is consumed for the feedback, which provides a very energy-efficient way of controlling the voltage and the primary drive Figure 3 shows schematically how the various coils are oriented in space. The primary and secondary coils 15 resp. 19 are arranged at a certain mutual distance fi- from each other and where the distance can vary with time. The sensed voltage in the secondary coil 19 will naturally vary with the distance between the secondary coil and the sensing coil 29. However, by a suitable geometry of the primary and secondary coils it is possible to obtain a maximum of the sensed voltage on a certain 10 15 20 25 30 09 »v00 J o nl con: 00 00 on 0 on 0 o 0 a 0 oonoi I 0 0 o 0 øno c 0 o 0 526 099 H 9 300 EO: I ... I g.: ...: O distance. And by ensuring that this distance ends up in the middle of the current distance interval, a core ring characteristic between the primary and secondary coils which is relatively insensitive to distance variations can be obtained.

The sensing coil is located in physical proximity to the primary coil, suitably mounted on the primary coil without mutual connection to it, and in the simplest case has a coil radius that is significantly less than the coil radius of the primary coil.

The sensing coil is peripherally located in relation to the primary coil and over the area where the field from the primary coil changes characters,: see further below.

The coils are held together in a known manner, for example by means of magnets.

The retaining magnets are then suitably placed in the periphery of the coils, since the low field concentration there gives minimal eddy current losses. The magnetic placement in the periphery also provides a better "lcra" car "between the outer and inner unit of the link, which makes the outer unit less prone to fall off. The principle of feedback is something that significantly distinguishes the connection from previously known signal and energy transmission links. Previous descriptions of feedback are based solely on some form of active coupling, i.e. active electronics on the secondary side generate some form of status signal that the primary side senses. In this case, there is no electronics at all on the sectarian side to generate any status signal. The problem with a sensing coil on the primary side to sense the field n from the secondary side is instead that the sensing coil does not want to have any influence fi from the primary coil 15 and that it can be thought to be a certain distance dependence. Sensing from the primary coil can be avoided by placing the shielding coil in such a way that it encloses a field from the primary coil which always has two characters and always has the sum zero. To see how this is possible, the field image is studied in a coil: The field strength at the distance R from a conductor with the length dl looks like: Lid xÉ 47: R * when: 0 00 00 0000 00 0 0 I 0 0 0 000 0 000 0000 I 0 0 0 000 00 0000 000 0000 00 I 0 0 0 0 0 0 0 00 00 OOIO 0 0 0 0 0 00 10 15 20 25 30 35 526 099 10 E 'I. -. .. .. .i i ... where the permeability constant in vacuum i = the current through the conductor Ä = the distance vector between the conductor and the current point .- dl = The direction vector of the conductor element dl The integral of dÉ over the entire conductor length gives the field strength at a certain point.

For a circular loop it becomes: q-dâ: Li dïxUÄ -I-Ép) _ül '* ï kfdqn-RcdpRp cosçv 41: Rß 41: 3 ..

* ”= ° (R: + Rf - ZRCRP cos 40); bl '2' Rf _ RPR, cosçv (0 41: Rz + Rf -ZRCRp coscp R: + Rf -2R, Rp coscp r-Or where Rc, Rp and cosø appear from fi gur 4a.

The integral above is most easily solved numerically in e.g. MATLAB.

Figure 4b shows the characteristics of the field image as a function of the radius. The även guren also shows measured characteristics of coils wound in two different ways, namely outer radially wound coil (i.e. the winding turns are concentrated to the periphery of the coil) and spirally wound coil (i.e. the turns are spread along the radius of the coil, also called parmkakssole). The field strength is normalized to the field strength that appears in the middle of each coil and the radius is normalized to the outer radius of the coils. The fact that the theoretical characteristics do not completely correspond to the measured one is partly due to the fact that the wire diameter is assumed to be infinitely small in the calculation and partly because the test coil used to shield the field has a certain diameter, which makes it impossible to feel the field very closely. the thread. It can be seen in Figure 4b that for both coil winding types it applies that approximately at the outer radius the field changes characters.

Figure 5 shows a primary coil 15 with a sensing coil 29 positioned so that the total field through the sensing coil originating from the primary coil becomes zero. In the uren clock, the primary coil 15 is wound as a pancake coil with all the turns in one plane. However, this is not necessary to achieve a "zeroing" effect. The main thing is that the field image always has two characters at the same time. The location of a 0000 000 10 15 20 25 30 526 099 ~ 000 000 0 0 0 00 90 0 0 0 I 00 00 00 0 0 0 00 000 000 00 0 000 0 0 0 0 0 0 0 0 11: '.' °. ".. '..' .. '2.3.; - shielding coil on a primary coil where the turns are located in the outer periphery of the coil, however, requires significantly higher precision. This is because the zero passage on such coils takes place much more abruptly, see How the voltage in the sensing coil 29 depends on the distance between the sensing coil 29 and the secondary coil 19 depends on the geometry of the two coils. The best distance insensitivity is obtained if a maximum of the sensed voltage is obtained somewhere in the middle of the current distance interval between primary and secondary This can be achieved by suitable geometry.

Figure 6 exemplifies two different sensing characteristics of two different coil pairs A and B. As can be seen in Figure, one characteristic has a maximum at 5 mm and the other at 10 mm. These two coils are thus best suited for distances of around 5 and 10 mm, respectively.

Figure 7 shows some different examples of the physical structure (line structure) of the sensing circuit ll. In the examples above, the sensing circuit has been described in the form of a sensing coil 29 asymmetrically placed over the primary coil as 7a shows.

The disadvantage of having only one sensing coil as 7a shows is that the lateral shielding variation becomes very noticeable. By having two coils as 7b shows, a significantly smaller sensing variation in the y-direction is obtained. The sensed voltage here becomes the sum of the partial voltages u; and uz. If you add two more coils as shown in 7c, this improvement is also obtained in the x-direction. The sensed voltage here too becomes the sum of all subvoltages, ie ~ u1_ ug, u; and u4. The more sensing coils you have, the smaller the sensing variation will be laterally. Another advantage of your sensing coils is that the signal level increases, which gives an improved signal-to-noise ratio. If you connect all small coils to a large one, a "horseshoe" -shaped sensing coil is obtained, as 7d shows. Here you get a completely angle-independent sensing characteristic, almost as if on the slit that is formed between the two outer edges of the horseshoe. A significant disadvantage of the horseshoe coil is that it can be quite difficult to manufacture. Finally, Fig. 7e shows probably the simplest and best way to design the sensing circuit. The shielding circuit here consists of two coils with different radii centered around the center of the primary coil where the sensed voltage consists of the difference voltage between u | and u2. o to oc Dice I I I I I to! 1000 0 00 o II o O: noob 00 I I 0 a 00 II I I I OI cl 10 15 20 25 30 526 099 -, 12 Preferably, at least one of the primary and secondary coils is designed as flat, pancake-shaped coils. The decoupling coil can also be designed in this way.

Pancake-shaped slim physical construction of the transfer device but also provides the best coupling factor. Namely, it has coils allow one it has been found that a pancake-wound coil pair, i.e. where the turns are radially wound, a significantly higher coupling factor is obtained than a coil pair with more spread turns (axially wound). Figure 8 shows a pancake-wound spool of Litz wire, ie wire consisting of a plurality of small strands, at least one of the strands of the Litz wire being coated with heat glue. It has been found that pancake coils wound from Litz wire impregnated with heat glue obtain sufficiently high Q-values (goodness number). The method also provides the opportunity to manufacture the coils in a relatively simple way.

By winding the spools in a spiral shape and winding with a thread divided into små your small threads where each thread is coated with heat glue as described above, a number of advantages are achieved compared to winding with a homogeneous thread or winding all the turns in the outer diameter of the spool. The spiral shape causes the field image to concentrate to the center and fall off to the outside periphery. The field concentration gives an increased coupling factor and the field reduction to the outside of the periphery means that it is possible to place such details there as one does not wish to place in strong fields, such as holding magnets as described above.

Preferably, the sensing coil is wound according to the principle in Figure 7e. Figure 9 shows such an embodiment where the spools are wound of Litz wire. Figure 9 also shows how the coils are connected to obtain the zeroing effect of the primary coil field.

The geometry is well suited both physically and electromagnetically to be assembled with the pancake coil in Figure 8. Provided that the coils are fairly plane-parallel, the design according to Figure 9 is that the sensed voltage is only due to movements in axial and radial direction and is therefore insensitive to rotational changes.

Figure 10a shows measured radiating characteristics in radial direction for 3 different coil distances d as shown in Figure 10b. It is seen in Figure 10a that at a radial variation of 12.5 mm and an axial variation of i 3 mm, the sensed I II II IIII I II IIIIIII III I III IIII IIII II II IIII III OI IIII II IIIIIIOOIOI II II II I 10 15 20 25 30 526 099 13 voltage variation only 15%. The outer radius of the measured players in this case is about 12mm.

Figure l shows an example of the physical design of the transfer device. The transfer device consists of an external part 1 'and an internal part 2' separated by a physical barrier 3, for example the skin. In the external part there is a coil configuration 30, a magnetic configuration 31, electrical circuits 32, a battery 33 and some kind of input signal 34. In the internal part 2 there is a coil configuration 35, a magnetic configuration 36, electrical circuits 37 and some form of utsígnal 38 (stimuli).

The invention is not limited to the examples shown above, but may be varied within the scope of the appended claims. For example, the players could be made according to some lithographic method which is per se known from the circuit board and semiconductor industry, preferably a so-called multilayer procedure.

Claims (11)

10 15 20 25 30 526 099 / q PATENT REQUIREMENTS Device for wireless signal and energy transmission for medical implants, preferably signal and energy transmission through the skin or other physical barrier (3) of a person or other living being with a medical implant installed in the body, comprising at least two coils which are arranged in space so that they are located on each side of said barrier, the coil being placed outside the physical barrier, the so-called the primary coil (15), forms part of a primary circuit which emits an electromagnetic blanket and the second coil which is located on the other side of the physical barrier, the so-called the secondary coil (19), forms part of a secondary circuit which can absorb and utilize all or part of the magnetic field emitted from the primary coil and the coils having an interconnection factor (k) which is less characterized by an eighteen sensing circuit (11) in the form of a structure of electrical conductors is applied in physical proximity to the primary coil (15) over the area where the field from the primary coil (15) changes sign and there encloses as much positive as negative field so that the primary field is zeroed out and only the magnetic field from the secondary coil (19) is shielded in the sensing circuit. and thus gives rise to induction in it which can be detected to thereby sense the state mainly only in the secondary circuit and wherein the sensing circuit (11) comprises one or fl your sensing coils which are xt xt mounted on the primary coil without mutual magnetic connection thereto. Device according to patent claim characterized by the sensing circuit (11) is arranged to sense the instantaneous voltage of the secondary circuit, ie both the phase and the amount of the voltage. Device according to claim 1, characterized in that the sensing circuit (11) comprises a number of sensing coils with a substantially smaller coil radius than the radius of the primary coil and placed scattered around the periphery of the primary coil (15) so that the total field through all the individual players originating from the primary coil (15) becomes zero. 10 15 20 25 526 099 G9 IB Device according to claim 1, characterized in that the sensing circuit (1 l) comprises a coil wound in the "horseshoe shape". Device according to patent claim characterized by the sensing circuit (11) comprises two coils with different radii centered around the primary coil (15) and where the sensing voltage is constituted by the differential voltage. Device according to patent claim, characterized in that at least the primary and secondary coils (15 and 19, respectively) are designed as flat, 'pancake-shaped' coils. Device according to patent claim, characterized in that the smallest of the primary and secondary coils (15 and 19, respectively) are wound of Litz wire. 8. Device according to patent claim? characterized in that at least one of the strands of the Litz wire is coated with heat glue. Device according to claim 1, characterized in that the players are designed such that a maximum of the shielding tensioning line is obtained at coil distances greater than zero. Device according to claim 1, characterized in that the primary and secondary coils (15 and 19, respectively) are held together by magnetic means, the holding magnets being placed in the periphery of the coils. 11. ll. The device according to the patent law is characterized by a coil which is manufactured by, according to some lithographic method which is known per se, from the circuit board and semiconductor industry, preferably a so-called multilayer procedure. IN