Abstract:
An improved implantable medical device system having dual coils in one of the devices in the system is disclosed. The dual coils are used preferably in an external device such as an external controller or an external charger. The dual coils are wrapped around axes that are preferably orthogonal, although other non-zero angles could be used as well. When used to transmit, the two coils are driven (for example, with FSK-modulated data when the transmitting data) out of phase, preferably at 90 degrees out of phase. This produces a magnetic field which rotates, and which reduces nulls in the coupling between the external device and the receiving coil within the implanted device. Moreover, implementation of the dual coils to transmit requires no change in the receiver circuitry of the implanted device. Should the device with dual coils also receive transmissions from the other device (e.g., the implanted device), the two coils are used in conjunction with optional receiver circuitry which likewise phase shifts the received modulated data signals from each coil and presents their sum to typical demodulation circuitry.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a data telemetry and/or power transfer technique having particular applicability to implantable medical device systems. 
       BACKGROUND 
       [0002]    Implantable stimulation devices are devices that generate and deliver electrical stimuli to body nerves and tissues for the therapy of various biological disorders, such as pacemakers to treat cardiac arrhythmia, defibrillators to treat cardiac fibrillation, cochlear stimulators to treat deafness, retinal stimulators to treat blindness, muscle stimulators to produce coordinated limb movement, spinal cord stimulators to treat chronic pain, cortical and deep brain stimulators to treat motor and psychological disorders, and other neural stimulators to treat urinary incontinence, sleep apnea, shoulder sublaxation, etc. The present invention may find applicability in all such applications, although the description that follows will generally focus on the use of the invention within a Spinal Cord Stimulation (SCS) system, such as that disclosed in U.S. Pat. No. 6,516,227, which is incorporated herein by reference in its entirety. 
         [0003]    Spinal cord stimulation is a well-accepted clinical method for reducing pain in certain populations of patients. As shown in  FIGS. 1A and 1B , a SCS system typically includes an Implantable Pulse Generator (IPG)  100 , which includes a biocompatible case  30  formed of titanium for example. The case  30  typically holds the circuitry and power source or battery necessary for the IPG to function, although IPGs can also be powered via external RF energy and without a battery. The IPG  100  is coupled to electrodes  106  via one or more electrode leads (two such leads  102  and  104  are shown), such that the electrodes  106  form an electrode array  110 . The electrodes  106  are carried on a flexible body  108 , which also houses the individual signal wires  112  and  114  coupled to each electrode. In the illustrated embodiment, there are eight electrodes on lead  102 , labeled E 1 -E 8 , and eight electrodes on lead  104 , labeled E 9 -E 16 , although the number of leads and electrodes is application specific and therefore can vary. 
         [0004]    As shown in  FIG. 2 , the IPG  100  typically includes an electronic substrate assembly  14  including a printed circuit board (PCB)  16 , along with various electronic components  20 , such as microprocessors, integrated circuits, and capacitors mounted to the PCB  16 . Two coils are generally present in the IPG  100 : a telemetry coil  13  used to transmit/receive data to/from an external controller  12 ; and a charging coil  18  for charging or recharging the IPG&#39;s power source or battery  26  using an external charger  50 . The telemetry coil  13  can be mounted within the header connector  36  as shown. 
         [0005]    As just noted, an external controller  12 , such as a hand-held programmer or a clinician&#39;s programmer, is used to wirelessly send data to and receive data from the IPG  100 . For example, the external controller  12  can send programming data to the IPG  100  to dictate the therapy the IPG  100  will provide to the patient. Also, the external controller  12  can act as a receiver of data from the IPG  100 , such as various data reporting on the IPG&#39;s status. The external controller  12 , like the IPG  100 , also contains a PCB  70  on which electronic components  72  are placed to control operation of the external controller  12 . A user interface  74  similar to that used for a computer, cell phone, or other hand held electronic device, and including touchable buttons and a display for example, allows a patient or clinician to operate the external controller  12 . The communication of data to and from the external controller  12  is enabled by a coil  17 , which is discussed further below. 
         [0006]    The external charger  50 , also typically a hand-held device, is used to wirelessly convey power to the IPG  100 , which power can be used to recharge the IPG&#39;s battery  26 . The transfer of power from the external charger  50  is enabled by a coil  17 ′, which is discussed further below. For the purpose of the basic explanation here, the external charger  50  is depicted as having a similar construction to the external controller  12 , but in reality they will differ in accordance with their functionality as one skilled in the art will appreciate. However, given the basic similarities between the external controller  12  and the external charger  50  as concerns this disclosure, they are depicted as a single external device  60  in  FIG. 3 . 
         [0007]    Wireless data transfer and/or power transfer between the external device  60  and the IPG  100  takes place via inductive coupling, and specifically magnetic inductive coupling. To implement such functionality, and as alluded to above, both the IPG  100  and the external device  60  have coils which act together as a pair. When the external device  60  is an external controller  12 , the relevant pair of coils comprises coil  17  from the controller and coil  13  from the IPG. When the external device  60  is an external charger  50 , the relevant pair of coils comprises coil  17 ′ from the external charger and coil  18  from the IPG. In the generic external device  60  depicted in  FIG. 3 , only one coil pair is depicted for simplicity, namely coil  62  from the external device  60  (which can comprise either coil  17  or  17 ′), and coil  64  from the IPG  100  (which can comprise either coil  13  or  18 ). Either coil  62  or  64  can act as the transmitter or the receiver, thus allowing for two-way communication between the external device  60  and the IPG  100 . 
         [0008]    When data is to be sent from the external device  60  to the IPG  100  for example, coil  62  is energized with an alternating current (AC). Such energizing of the coil  62  to transfer data can occur using a Frequency Shift Keying (FSK) protocol for example, such as disclosed in U.S. patent application Ser. No. 11/780,369, filed Jul. 19, 2007, which is incorporated herein by reference in its entirety. Energizing the coil  62  induces an electromagnetic field  29 , which in turn induces a current in the IPG&#39;s coil  64 , which current can then be demodulated to recover the original data. 
         [0009]    When power is to be transmitted from the external device  60  to the IPG  100 , coil  62  is again energized with an alternating current. Such energizing is generally of a constant frequency, and of a larger magnitude than that used during the transfer of data, but otherwise the physics involved are similar. 
         [0010]    Regardless of whether the external device  60  is transferring data or power, the energy used to energize the coil  62  can come from a battery in the external device  60  (not shown in  FIG. 3 ), which like the IPG&#39;s battery  26  is preferably rechargeable. However, power may also come from plugging the external device  60  into a wall outlet plug (not shown), etc. 
         [0011]    As is well known, inductive transmission of data or power can occur transcutaneously, i.e., through the patient&#39;s tissue  25 , making it particular useful in a medical implantable device system. During the transmission of data, the coils  62  and  64  preferably lie in planes that are parallel, along collinear axes, and with the coils in as close as possible to each other, such as is shown generally in  FIG. 3 . Such an orientation between the coils  62  and  64  will generally improve the coupling between them, but deviation from ideal orientations can still result in suitably reliable data or power transfer. 
         [0012]    However, realization of this ideal orientation condition necessarily relies on successful implementation by the user of the external device  60 . For example, and as shown in  FIG. 4 , if the angle θ between the axis  54  of coil  62  and the axis  56  of coil  60  is non-ideal (i.e., non-zero), data or power transfer will be non-ideal. When the axes  54 ,  56 , are perpendicular, theoretically no energy will be transferred, and realistically only a negligible amount of energy will be transferred. Another non-ideal orientation between coil  62  and coil  60  is shown in  FIG. 5 . In this instance, the axes  54  and  56  of the coils are parallel, as are their planes  51  and  52 , but they are not colinear, with the result that the coils are not overlapping. This too adversely impacts the coupling from coil  62  to coil  64 . 
         [0013]    The non-ideal orientations depicted in  FIGS. 4 and 5  illustrate that a user of an external device  60  must be attentive to proper placement of that device relative to the IPG  100 . Requiring correct placement by the user is of course a drawback of such traditional IPG system hardware, because it is unrealistic to assume that any given user will be so attentive, and as a result data or power transfer may be adversely affected. 
         [0014]    Further exacerbating the potential problem of improper external device-to-IPG orientation is the recognition that improper orientations are not necessarily always the result of user inadvertence. It has so far been assumed that it is relatively easy for the user to understand or infer the positioning of the coils  62  and  64 . For example, when both the external device  60  and the IPG  100  are basically flat, placing the coils  62 ,  64  close to the ideal orientation depicted in  FIG. 3  is not difficult. But what if the external device  60  or IPG  100  is not flat? What if the coils are mounted inside the housings in a manner in which the coil position cannot be inferred? What if the IPG  100  is implanted deep within a patient, such that the orientation of its coil  62  cannot be inferred through the patient&#39;s tissue? What if the IPG  100  moves or rotates within the patient after it is implanted? Any of these effects can make it difficult or impossible for even an attentive user to properly align the coil  62  in the external device  60  and the coil  64  in the IPG  100 . 
         [0015]    From the foregoing, it should be clear that the art of magnetically-coupled implantable medical device systems would benefit from improved techniques for ensuring good coupling between the external device and the IPG, even during conditions of non-ideal alignment. This disclosure provides embodiments of such a solution. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIGS. 1A and 1B  show an implantable pulse generator (IPG), and the manner in which an electrode array is coupled to the IPG in accordance with the prior art. 
           [0017]      FIG. 2  shows wireless communication of data between an external controller and an IPG, and wireless communication of power from an external charger to the IPG. 
           [0018]      FIG. 3  generalizes the external controller and the external charge to a single external device. 
           [0019]      FIGS. 4 and 5  show types of non-ideal orientations between the external device and the IPG which result in poor coupling, and hence poor data and power transfer. 
           [0020]      FIG. 6  shows an embodiment of the disclosed dual transmitter coil approach, in which orthogonal dual coils are used in the transmitter of the external device-IPG system. 
           [0021]      FIGS. 7 and 8  show the transmitter circuitry used in the transmitter, and shows that the two coils are driven with the broadcast data with an approximately 90 degree phase difference. 
           [0022]      FIG. 9  shows in the internal structure of an external device including the dual transmitter coils. 
           [0023]      FIG. 10  shows receiver circuitry useable in a device using dual transmitter coils. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]    The description that follows relates to use of the invention within a spinal cord stimulation (SCS) system. However, the invention is not so limited. Rather, the invention may be used with any type of implantable medical device system that could benefit from improved coupling between an external device and the implanted device. For example, the present invention may be used as part of a system employing an implantable sensor, an implantable pump, a pacemaker, a defibrillator, a cochlear stimulator, a retinal stimulator, a stimulator configured to produce coordinated limb movement, a cortical and deep brain stimulator, or in any other neural stimulator configured to treat any of a variety of conditions. 
         [0025]    As shown in the simplified illustration of  FIG. 6 , the disclosed improved implantable medical device system  200  uses dual coils  62   a  and  62   b  in the transmitting device. In a preferred implementation, the dual coils  62   a  and  62   b  are included in the external device  60  as the transmitter, although the dual coils could also be included in the IPG to improve its ability to back telemeter status data. When the dual coils  62   a  and  62   b  are included in the external device  60 , the external device is most preferably the external controller  12 , but could also comprise the external charger  50  (see  FIG. 2 ). For simplicity sake, and without intention to limit the technique, the foregoing discussion describes an embodiment employing these preferences in which the dual transmitting coils are employed in an external controller for improved data transfer. 
         [0026]    As shown in  FIG. 6 , the dual coils  62   a  and  62   b  are respectively wrapped around axes  54   a  and  54   b  which are preferably orthogonal, i.e., the angle between axes  54   a  and  54   b  is preferably 90 degrees. However, this is not strictly necessary, and the disclosed technique improves over the prior art if any non-zero angle is used between the axes  54   a  and  54   b . That being said, maximal benefit is achieved when this angle approaches 90 degrees, i.e., approximately 90 as close as mechanical tolerances will allow. 
         [0027]      FIGS. 7 and 8  depict the transmitter circuitry  210  used to drive the two coils  62   a  and  62   b .  FIG. 7  describes such circuitry in a basic block diagram form, while  FIG. 8  shows further details as presently preferred in an actual implementation. In either case, it should be understood that other details of the transmitter circuitry are not set forth for clarity, but are well known. 
         [0028]    As shown in  FIG. 7 , the two coils  62   a  and  62   b  are driven with the same signal but out of phase, and most preferably with a 90 degree phase shift between them. For example, consider an application in which the dual coils  62   a  and  62   b  are used in an external controller to serially telemeter data bits to the IPG  100 . Those signals are centered around f c =125 kHz, with a logic ‘1 bit being represented by an approximately 129 kHz input signal  80  (f 1 ), and a logic ‘0’ bit being represented by an approximately 121 kHz input signal  80  (f 0 ). (Such an example illustrates the use of FSK modulation, which is described in further detail in the above-incorporated &#39;369 application). This modulated input signal  80  is split, and is phase shifted by approximately 90 degrees (i.e., by 1/(4*f c ), or 2 microseconds) in the leg that goes to the driver  82   b  for the coil  62   b . This phase shift in the lower leg to coil  62   b  can comprise either a 90 degree lag or a 90 degree lead when compared to the signal in the top leg to coil  62   a ; however, for ease of discussion, a lagging signal is illustrated herein. It should be realized that the phase shift between the two legs is approximately 90 degrees, with the actual angle between them depending on the particular frequency (f 0  or f 1 ) being processed at any given time. 
         [0029]      FIG. 8  discloses a more detailed schematic for transmitter circuitry  210  in a preferred embodiment. Generation of the driving signals for the two coils  62   a  and  62   b  starts with the external device&#39;s microcontroller  150 , preferably Part No. MSP430 manufactured by Texas Instruments, Inc. The microcontroller  150  outputs a string of digital data bits that are ultimately to be wirelessly broadcast using the transmitter circuitry  210 . The digital data is sent to modulation circuitry (oscillator)  90 , preferably Part No. AD9834 manufactured by Analog Devices, Inc. The oscillator  90  converts the digital bits to AC waveforms whose frequency depends on the logic state of the particular bit being processed (again, as is consistent with use of an FSK protocol). In this embodiment, the center frequency f c ′ as output by the oscillator  90  is 250 kHz, or twice the desired center frequency f c =125 kHz to be ultimately broadcast by the transmitter circuitry  210 . When modulated with the logic states, the result is an AC output of either f 0 ′=242 kHz or f 1 ′=258 kHz. This AC output is then turned into a square wave of the same frequency by a comparator  92  as one skilled in the art will appreciate. 
         [0030]    Thereafter, the modulated square wave data signal is split into two legs that ultimately drive the two coils  62   a  and  62   b . Each leg receives the square wave output at a clocking input (CLK) of DQ flip flops  96   a  and  96   b , although the data received at the lower leg is inverted by an inverter  94 . The inverter essentially works a 180 degree shift in the square wave data signal. The complimentary output Q′ of each flip flop  96   a  and  96   b  is coupled to the corresponding input D. Given this arrangement, and appreciating that the flip flops  96   a  and  96   b  can only change data states upon a rising edge of its clock input, the effect is that the outputs (Q/Q′) of the flip flops  96   a  and  96  comprise a square wave signal at half the frequency (i.e., frequencies of f 0 =121 kHz and f 1 =129 kHz), but in which the signal driving the lower leg lags by 90 degrees. This approximately 90 degree shift in the lower frequency (f c =125 kHz) signal stems from the approximately 180 degree shift imparted by the inverter  94  at the higher frequency (f c ′=250 kHz) signal. 
         [0031]    The lower frequency square wave signals are in turn used to resonant the coils  62   a  and  62   b , again, with the signals arriving at coil  62   b  with a 90 degree lag. Resonance is achieved for each coil  62   a  and  62   b  through a serial connection to a tuning capacitor  98   a ,  98   b , making a resonant LC circuit. As one skilled in the art will appreciate, the N-channel (NCH) and P-channel (PCH) transistors are gated by either the output (Q) or the complementary output (Q′) of the flip flops  96   a  and  96   b  to apply the voltage, Vbat, needed to energize the coils  62   a  and  62   b . Such voltage Vbat comes from the battery (or other power source) with the external device  60 . One skilled in the art will appreciate that the disclosed arrangement reverses the polarity of this battery voltage Vbat across the series-connected LC circuit (+Vbat followed by −Vbat followed by +Vbat, etc.), which in turn causes the coils to resonate and therefore broadcast at the frequencies of interest (f 0 =125 kHz; f 1 =129 kHz). It should be understood that transmitter circuitry  210  as depicted in  FIG. 8  could be made in different ways, and therefore what is disclosed is merely one non-limiting example. 
         [0032]      FIG. 9  shows the structure of an external device  60  and the physical orientation of the coils  62   a  and  62   b  as well as some of the other components. As envisioned, the external device  60  as depicted comprises an external controller, but could also comprises an external charger (see  FIG. 2 ). So that the internal components can be more easily seen, the external device (controller)  60  is depicted without its outer housing, and from front, back, and side perspectives. 
         [0033]    As shown, the external device (controller)  60  comprises a printed circuit board (PCB)  120 , whose front side carries the user interface, including a display  124  and buttons  122 . In the depicted embodiment, the operative circuitry, including the coils  62   a  and  62   b  and the battery  126 , are located on the back side of the PCB  120 , along with other integrated and discrete components necessary to implement the functionality of the external controller. As seen in the back and side views, the two coils  62   a  and  62   b  are respectively wrapped around axes  54   a  and  54   b  which are orthogonal. More specifically, coil  62   a  is wrapped in a racetrack configuration around the back of the PCB  120 , while coil  62   b  is wrapped around a ferrite core  128  and affixed to the PCB  120  by epoxy. 
         [0034]    With the transmitter circuitry  210  and the physical construction of the external device (controller)  60  set forth, the theory of operation of the device is briefly explained. By causing the input signals to the two coils to be 90 degrees out of synchronization, the magnetic field produced by the two coils will effectively rotate around a third axis  54   c  ( FIG. 6 ) orthogonal to both of the coils&#39; axes  54   a  and  54   b . The effect can be analogized to a bar magnet spinning around axis  54   c  with an angular velocity of either f 0  (121 kHz) or f 1  (129 kHz) depending on the data state being transmitted at any given time. Because the produced magnetic field spins, the number and severity of nulls in the magnetic field are reduced at the receiving coil  64  in the IPG  100 . In fact, the only significant null condition exists when the axes of the spinning field  54   c  and the axis of the receiving coil  56  are aligned (not shown in  FIG. 6 ). As a result, the system is not dependent on user attentiveness to provide suitable coupling between the coils  62   a  and  62   b  in the external device  60  and the coil  64  in the IPG  100 , with the result that the reliability of data or power transfer is improved. 
         [0035]    Fortunately, use of the disclosed dual-coil technique does not require any changes in the receiver circuitry used in conjunction with the receiving coil  64  within the IPG  100 . This results from the understanding that current can be induced in the receiving coil  64  either by changing the magnitude of the produced magnetic field (as occurs in traditional signal transmitter coil systems), or by changing the direction of the magnetic field (as occurs with the disclosed dual transmitter coil technique). In either case, one skilled in the art should appreciate that Faraday&#39;s law illustrates that the current induced in the receiving coil will be equivalent whether a single transmitter coil is used, or two orthogonal transmitter coils are used but driven 90 degrees out of phase. This assumes however that each of the coils  62   a  and  62   b  in the dual-coil system are capable of generating a magnetic field of the same strength as that produce by the singular coil in a single coil system. Designing the coils  62   a  and  62   b  (number of turns, etc.) and the transmitter circuitry  210  to achieve equal magnetic strength from the two contributing magnetic fields is therefore desirable, but not absolutely necessary. The benefits of the use of dual transmitter coils are still realized even if the coils do not contribute equally to the produced magnetic field. 
         [0036]    From the foregoing, and because of the desire to maintain a consistent magnitude of induced current in the receiving coil, the disclosed dual coil approach may take more power (e.g., twice the power) than approaches using single coils. This additional power requirement is generally not problematic, as the battery power within the external device is not critical and can be easily recharged during periods in which the external device  60  is not used. In any event, it is clearly beneficial that implementation of the dual-coil technique does not require any re-tooling of the IPG or its receiver circuitry. 
         [0037]    While the receiver circuitry in the IPG  100  does not require modification, the receiver circuitry in the external device  60  may be changed to account for the two coils  62   a  and  62   b , assuming that such coils are used as the antennas for so-called “back telemetry” (e.g., status data) received from the IPG  100 . (Obviously, the external device  60  would contain no receiver circuitry in an IPG system lacking back telemetry capability). 
         [0038]    Exemplary receiver circuitry  220  useable with the dual coils  62   a  and  62   b  in the external device  60  and for receiving a wireless modulated data signal from the IPG  100  is shown in  FIG. 10 . As with the transmitter circuitry  210  ( FIGS. 7 and 8 ) the receiver circuitry  220  comprises two legs coupled to each of the two coils. Pre-amplifiers (pre-amps)  130   a  and  130   b  initially amplify the received modulated signals from the two coils  62   a  and  62   b  respectively. Thereafter, the amplified signal from pre-amp  130   b  is shifted 132 by 90 degrees, which shift can be imparted by any number of circuitry approaches as one skilled in the art will appreciate. As with the transmitter circuit  210 , this phase shift  132  can comprise either a lagging or leading of the comparable signal as received from coil  62   a ; a delay is preferred because it is easier to implement. 
         [0039]    Thereafter, the amplified signals, with the phase shift applied between them, are added together at a summer circuit  134 , which again can comprise any well known analog summer circuitry known in the art. The resulting signal is then subject to a band pass filter (BPF)  136 , which removes frequencies component from the signal outside of the frequency band of interest (e.g., outside of the range from 121 to 129 kHz). This signal is then demodulated back into digital bits at a demodulator block  138  operating under the control of a local oscillator  140 . Noise is removed from these digital bits at a low pass filter block  142 , which then allows the received data to be input to the external controller&#39;s microcontroller  150  for interpretation and processing. One skilled in the art will appreciate that summer  134 , the BPF  136 , demodulation block  138 , local oscillator  140 , and LPF  142 , or any combination of these blocks, can collectively comprise demodulation circuitry. 
         [0040]    Receiver circuitry  220  of  FIG. 10  is not the only manner in which data can be received at the two coils  62   a  and  62   b . For example, during data reception periods, each antenna (coil)  62   a  and  62   b  could be sequentially monitored during a preamble portion of the communication protocol to assess the signal quality at each antenna coil. Thereafter, the coil  62   a  or  62   b  with the best signal quality could be used for reception, with the other coil disconnected during the remainder of the data reception period. 
         [0041]    Other embodiments of the invention can be varied from the preferred embodiments disclosed. For example, and as noted earlier, neither the physical angle between the axes  54   a  and  54   b  of the transmitter coils  62   a  and  62   b , nor the phase angle between the signal driving them, need be exactly 90 degrees. 
         [0042]    While disclosed in the context of a medical implantable device system for which the invention was originally contemplated, it should be recognized that the improved dual-coil approach herein is not so limited, and can be used in other contexts employing communications via magnetic inductive coupling, such as in Radio-Frequency Identification (RFID) systems, etc. The disclosed circuitry can further be used in any context in which magnetic inductive coupling could be used as a means of communication, even if not so used before. 
         [0043]    Although particular embodiments of the present invention have been shown and described, it should be understood that the above discussion is not intended to limit the present invention to these embodiments. It will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the present invention. Thus, the present invention is intended to cover alternatives, modifications, and equivalents that may fall within the spirit and scope of the present invention as defined by the claims.