Abstract:
An implantable device, such as an infuser device for bidirectional hydraulically controlling a medical artificial sphincter, enhances power transfer characteristics to a secondary coil thereby allowing implantation to greater physical depths and/or enclosing the secondary coil within a housing of the infuser device. The enhanced power transfer is achieved with multiple coils that are longitudinally aligned and physical and electrical parallel to form the secondary loop of a transcutaneous energy transfer system (TET) instead of a single coil. It better optimizes the power transfer from a parallel tuned tank circuit primary coil to an implanted secondary series tuned tank circuit coil.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   The present application is related to four co-pending and commonly-owned applications filed on even date herewith, the disclosure of each being hereby incorporated by reference in their entirety, entitled respectively: 
   “TRANSCUTANEOUS ENERGY TRANSFER PRIMARY COIL WITH A HIGH ASPECT FERRITE CORE” to J. Giordano, Daniel F. Dlugos, Jr. &amp; William L. Hassler, Jr., Ser. No. 10/876,313; 
   “MEDICAL IMPLANT HAVING CLOSED LOOP TRANSCUTANEOUS ENERGY TRANSFER (TET) POWER TRANSFER REGULATION CIRCUITRY” to William L. Hassler, Jr., Gordon E. Bloom, Ser. No. 10/876,038; 
   “LOW FREQUENCY TRANSCUTANEOUS TELEMETRY TO IMPLANTED MEDICAL DEVICE” to William L. Hassler, Jr., Ser. No. 10/867,058; and “LOW FREQUENCY TRANSCUTANEOUS ENERGY TRANSFER TO IMPLANTED MEDICAL DEVICE” to William L. Hassler, Jr., Daniel F. Dlugos, Jr. Ser. No. 10/876,307. 
   FIELD OF THE INVENTION 
   The present invention pertains to a transcutaneous energy transfer (TET) system, in particular, to a TET system used between an external control module and a deeply implanted medical implant. 
   BACKGROUND OF THE INVENTION 
   It is known to surgically implant a medical device in a patient&#39;s body to achieve a number of beneficial results. In order to operate properly within the patient, a reliable, consistent power link between the medical implant and an external control module is often necessary to monitor the implant&#39;s performance or certain patient parameters and/or to command certain operations by the implant. This power link has traditionally been achieved with TET systems that communicate across a small amount of tissue, such as relatively thin dermal tissue across the front of the patient&#39;s shoulder, such as for cardiac pacemakers. 
   In some instances, multiple coils have been suggested in regard to a TET or telemetry system in order to provide additional flexibility in aligning primary and secondary coils. For instance, U.S. Pat. No. 6,058,330 Borza discloses a transcutaneous system in which multiple coils were used in the secondary circuitry and perhaps also in the primary circuitry. However, in this instance secondary coils are spaced about the patient&#39;s body for the purpose of mitigating tissue damage due to long term exposure to strong electromagnetic fields in a continuous application wherein the medical implant is continuously TET powered or continuously engaged in telemetry. Thus, the &#39;330 Borza patent teaches combining the power received from multiple secondary coils that are widely spaced, either a selected one of the secondary coils is receiving a strong electromagnetic signal or multiple secondary coils are simultaneously receiving weaker electromagnetic signals so that the dermal tissue overlying any one secondary coil is not continuously exposed to a strong electromagnetic signal. 
   Another problem with continuously coupling electromagnetic power to a secondary coil is the inconvenience to the patient of having the primary coil externally fixed in place, hampering movement and causing discomfort. U.S. Pat. No. 6,366,817 to Kung discloses using multiple coils in the primary spaced about the patient with circuitry that detects which primary coil is best oriented to efficiently couple electromagnetic energy to the implanted secondary coil and thus switching current to the selected primary coil. 
   U.S. Pat. No. 6,463,329 to Goedeke discloses multiple primary telemetry coils whose major surface, defined by their exterior, are parallel to one another and spaced apart. These coils are used to initiate telemetric communication between the programmer or monitor and the implanted device. At the frequencies disclosed, these coils are employed as loop antennas rather than inductively coupled coils. Since the antenna pattern of a loop antenna includes a “null” when very close to the loop, this approach is used to switch between primary coils when necessary to communicate with the secondary coil, thus primarily addressing problems with medical implants placed under a thin layer of dermal tissue that could coincide with the null of a primary coil placed in contact with the patient. 
   U.S. Pat. No. 5,741,316 to Chen et al. discloses a transmitter coil that has half of the windings on one leg of a horseshoe magnetic conductor in series with another half of the windings on the other leg. The magnetic flux contribution of each winding portion is thereby combined. However, the corresponding requirement for a horseshoe-shaped magnetic core in the implanted device is undesirable due to the increased size. Thus, laterally offset, electrically serial windings would not be a benefit for a secondary coil integral to an implanted device that lacks a horseshoe shaped magnetic conductor. 
   While these approaches to improving the effectiveness of electromagnetic coupling to a medical implant have merit, we have recognized an application that does not benefit from spaced apart multiple primary coils and/or spaced apart secondary coils, yet a need exists for enhanced power transfer efficiency. An implantable medical device that may benefit from use of enhanced TET is an artificial sphincter, in particular an adjustable gastric band that contains a hollow elastomeric balloon with fixed end points encircling a patient&#39;s stomach just inferior to the esophago-gastric junction. These balloons can expand and contract through the introduction of saline solution into the balloon. In generally known adjustable gastric bands, this saline solution must be injected into a subcutaneous port with a syringe needle to reach the port located below the skin surface. The port communicates hydraulically with the band via a catheter. While effective, it is desirable to avoid having to adjust the fluid volume with a syringe needle since an increased risk of infection may result, as well as inconvenience and discomfort to the patient. 
   To that end, we have recently developed implanted infuser devices that regulate the flow of saline without requiring injection into the subcutaneous port. This system transfers AC magnetic flux energy from an external primary coil to a secondary coil that powers the pump in the implant connected to the gastric band within the abdomen. Although batteries may be used to power the device, these long-term devices benefit from use of TET, allowing an implanted device of reduced size and complexity. Moreover, these devices may remain unpowered between adjustments, which provides additional advantages. These implantable, bi-directional infusing devices that would benefit from enhanced TET powering and/or telemetry are disclosed in four co-pending and co-owned patent applications filed on May 28, 2004, the disclosure of which are hereby incorporated by reference in their entirety, entitled (1) “PIEZO ELECTRICALLY DRIVEN BELLOWS INFUSER FOR HYDRAULICALLY CONTROLLING AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Ser. No. 10/857,762; (2) “METAL BELLOWS POSITION FEED BACK FOR HYDRAULIC CONTROL OF AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Rocco Crivelli, Ser. No. 10/856,971; (3) “THERMODYNAMICALLY DRIVEN REVERSIBLE INFUSER PUMP FOR USE AS A REMOTELY CONTROLLED GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. 10/857,315; and (4) “BI-DIRECTIONAL INFUSER PUMP WITH VOLUME BRAKING FOR HYDRAULICALLY CONTROLLING AN ADJUSTABLE GASTRIC BAND” to William L. Hassler, Jr., Daniel F. Dlugos, Jr., Ser. No. 10/857,763. 
   Unlike the previously mentioned medical implants, an infuser device for an artificial sphincter is typically implanted below a thicker dermal layer of skin and adipose tissue. This is particularly true for patients that typically receive an adjustable gastric band as a treatment for morbid obesity. Moreover, being more deeply implanted may allow for greater client comfort. However, the thickness of tissue presents difficulties for effective power coupling from a primary TET coil. 
   It is desirable that the secondary coil be encompassed within an outer case of the infuser device to enhance the integrity of the device. It is especially desirable to not have one or more secondary coils detached from the infuser device and implanted more superficially, as this complicates the implantation and explantation of the infuser device. Consequently, these generally known approaches to having spaced apart secondary coils to additively contribute to received signals are not appropriate. Further, there are physical and electromagnetic constraints to configuration of a secondary coil that is encompassed within a medical implant, especially the diameter, the number of turns of the coil, and the diameter of each turn. 
   Consequently, in order to provide for a larger power transfer range between the primary and secondary TET coils a significant need exists for enhancing power coupling with a deeply implanted medical device within the dimensional constraints imposed upon a secondary coil. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention overcomes the above-noted and other deficiencies of the prior art by providing a transcutaneous energy transfer (TET) system for an implantable medical device that increases the number of turns on the secondary coil while maintaining the same impedance and Q factor (i.e., ratio of bandpass center frequency to 3 dB cutoff frequency) as a single coil configuration. Power transfer is increased to the secondary coil while relatively maintaining the same power on the primary coil. 
   In one aspect of the invention, a medical implant benefits from enhanced power transfer between an external primary TET coil and its secondary TET coil by splitting the secondary TET coil into two physically and electrically parallel coils. The effective number of magnetic flux collecting turns was doubled while maintaining the impedance, the inductance, capacitance, total tank circuit Q and natural frequency of the original secondary coil and tank circuit. By virtue thereof, a medical implant may be implanted more deeply for therapeutic reasons and for simplified implantation and explantation purposes, yet perform satisfactorily. 
   In another aspect of the invention, a TET system that includes both the implantable medical device and an external primary coil assembly achieve an enhanced power efficiency by the twin, electrically and physically parallel secondary TET coils. 
   In yet another aspect of the invention, an inductive energy transfer system for powering a device separated by a barrier from an external primary coil is enhanced by including a pair of electrically and physically parallel front and back secondary coils that are part of a resonant tank circuit. 
   These and other objects and advantages of the present invention shall be made apparent from the accompanying drawings and the description thereof. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention, and, together with the general description of the invention given above, and the detailed description of the embodiments given below, serve to explain the principles of the present invention. 
       FIG. 1  is a diagram of a generally known primary coil aligned for transcutaneous power transfer and/or telemetry to a single secondary coil contained in an implanted medical device for powering electrical components therein. 
       FIG. 2  is a diagram of a primary coil aligned for transcutaneous power transfer and/or telemetry to a twin secondary coil consistent with aspects of the present invention contained in an implanted medical device for enhanced powering electrical components therein. 
       FIG. 3A  is a line graph comparing secondary power received at 20 kHz by the generally-known single secondary coil of  FIG. 1  versus the twin secondary coil electrically connected in parallel of  FIG. 2 . 
       FIG. 3B  is a line graph comparing primary power of the 310 turn 32-gauge coil to the two 325 turn 34-gauge coils connected in parallel at 20 kHz. 
       FIG. 4  is an impedance-phase plot of the twin secondary coil of  FIG. 2 . 
       FIG. 5  is an impedance-phase plot of the generally known single secondary coil of  FIG. 1 . 
       FIG. 6  is a diagrammatic view of a pump system in accordance with the present invention; 
       FIG. 7  is a cross-sectional view of an implantable pump of the pump system taken along line A—A of  FIG. 6 ; 
       FIG. 8  is a cross-sectional view of the implantable pump taken along line B—B of  FIG. 6 ; 
       FIG. 9  is a front, exploded isometric view showing internal components of a first embodiment of the implantable pump of the present invention; 
       FIG. 10  is a rear, exploded isometric view showing internal components of the first embodiment of the implantable pump of  FIG. 9 ; 
       FIG. 11  is a perspective environmental view of an adjustable artificial sphincter system being closed-loop remotely controlled based upon volume sensing. 
       FIG. 12  is a top plan view of a bi-directional infuser device of the adjustable artificial sphincter system of  FIG. 11 . 
       FIG. 13  is a sectioned side elevation view of the infuser device of  FIG. 12 , taken along section line  13 — 13 , showing a version of a bellows accumulator position sensor based on variable inductance, and showing a bellows in an extended position. 
       FIG. 14  is a sectioned side elevation view of the infuser device of  FIG. 12 , similar to  FIG. 12 , but showing a bellows in a collapsed position. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In  FIG. 1 , a generally-known transcutaneous energy transfer (TET) system  10  provides power and/or telemetry between a primary coil  12 , which is external to a dermal layer  14 , to a secondary coil  16  within an implanted device  18 , which is under the dermal layer  14 . The primary coil  12  is inductively coupled to the secondary coil  16 , as depicted by magnetic flux lines  20 . 
   In developing a more efficient TET and/or telemetry system, and in particular the secondary coils of the system, it is necessary to optimize the combination of coil turns, DC resistance, tank circuit capacitance, tank circuit impedance and total tank circuit Q. Spatially optimizing a secondary coil design is largely dictated by how closely the secondary coil may be placed (distance “D”) and longitudinally aligned (longitudinal axis “L”) to the primary coil  12 . Given constraints on the available volume and placement for the secondary coil  16  within implanted device  18 , further maximization is generally available in this manner for a medical implant. 
   As an example of optimizing a single coil design, coils were wound with different wire gauges and turn ratios in order to create different impedances. These secondary coils  16  were made in a single coil configuration, the best configuration for a secondary coil  16 . The best single coil configuration for the above mentioned infuser implant was 310 turns of 32-gauge magnet wire and was approximately 30 ohms DC resistance. The setup for the 310 turn 32 gauge secondary coil  16  is shown in  FIG. 1 . The highest power output that this coil provided to a fixed load under a set of test conditions was 3.96 Watts. 
   In  FIG. 2 , a TET system  110  consistent with aspects of the invention provides power and/or telemetry between a primary coil  112 , which is external to the dermal layer  14 , to a double secondary coil  116 , within an implanted device  118 , which is under the dermal layer  14 . Magnetic flux lines  120  denote increased power efficiency at a distance d′ between coils  112 ,  116 . A secondary coil was made-up of two coils in parallel, each having 325 turns of 34-gauge wire. This coil was approximately 30 ohms DC resistance and is shown in  FIG. 2 . The highest power that the two 325 turn 34 gauge coils in parallel provided to a fixed load under the same test conditions as the 310 turn 32-gauge coil was 4.46 Watts. A comparison of the power transfer curves of both coil arrangements is shown in  FIGS. 3A and 3B . The power transfer curve for the two 325 turn 34-gauge coil arrangement has higher secondary power output than the single coil arrangement. However, the two 325 turn 34-gauge coil arrangement drew only a slight increase in primary power and is relatively the same size, impedance, and Q as the single coil arrangement, giving it a better efficiency. 
   The total impedances at resonance of both coils were relatively the same. In the illustrative version, a parallel combination of two impedances of 60 OHMS is equivalent to a single impedance of 30 OHMS. It proves theoretically how the 310 turn 32-gauge coil and the two 325 turn 34 gauge coil have the same total impedance. The impedance-phase plots of both coil arrangements were compared and found to be functionally equivalent as shown in  FIGS. 4–5 . Also, the total tank circuit Q value of the impedance plots of both coils were found to be relatively the same. 
   The two 325 turn 34-gauge coil arrangement gave a more optimum power transfer characteristic. It is not completely understood how this was accomplished, and it will probably require the use of electromagnetic finite element analysis (FEA) in order to fully understand this phenomenon. What is postulated is that by spatially spreading out the turns of the secondary coil as well as effectively doubling their number as far as coupling to the magnetic flux produced by the primary coil, the power transfer to the secondary was increased. This effect was not apparent by simple linear circuit analysis, which would conclude that the two coils were equivalent. 
     FIG. 6  provides a diagrammatic view of an implantable pump system  120  in accordance with one embodiment of the present invention. As will be described in more detail below, pump system  120  may be implanted under a patient&#39;s skin and controlled by an active telemetry system to direct fluid flow to and from a therapeutic implant. Although the invention is described herein with specific reference to the use of the implantable pump with an artificial sphincter  121 , such as an adjustable gastric band, such description is exemplary in nature, and should not be construed in a limiting sense. The implantable pump of the present invention may also be utilized in any number of different apparatuses or systems in which it is desirable to provide bi-directional fluid flow between two interconnected subcutaneous components. 
   As shown in  FIG. 6 , the pump system  120  includes an implantable pump device  122  having a generally cylindrical outer casing  124  extending around the sides and bottom portions of the pump device  122 , and an annular cover  26  extending across a top portion. Annular cover  126  may be of varying thickness, with the thickest portion located at the center  130  (shown in  FIG. 7 ) of the cover  126 . Casing  124  and cover  126  may be formed of titanium or another type of appropriate, non-magnetic material, as are the other parts of pump device  122  that are exposed to body tissue and fluids. The use of titanium or a similar material prevents pump device  122  from reacting to body fluids and tissues in which the pump device  122  may be implanted. 
     FIGS. 7 and 8  are cross-sectional views showing the internal components of a first embodiment of pump device  122 , with  FIG. 8  being a 90° rotation of the  FIG. 7  view. In addition,  FIGS. 9 and 10  provide exploded isometric views from both the forward and rearward directions of pump device  122 , illustrating the relative positions of the components within the pump device  122 . As shown in  FIGS. 7–10 , thickened center portion  130  of cover  126  is molded or machined to include a duct  132 . A catheter port  134  extends laterally from duct  132  in center portion  130  to connect with an external fluid-conveying device, such as, for example, a catheter  136  as shown in  FIG. 6 . Duct  132  connects catheter port  134  with a fluid reservoir  138  in the interior of pump device  122 . Duct  132 , catheter port  134  and catheter  136  combine to provide bi-directional fluid flow between fluid reservoir  138  and a secondary implant. As shown in  FIGS. 6 and 7 , cover  126  includes a port  140  into which a hypodermic needle (not shown) may be inserted either through the patient&#39;s skin, or prior to implantation of device  122 , in order to increase or decrease the fluid volume in reservoir  138 . A septum  142  is disposed in port  140  to enable infusions by a hypodermic needle while preventing other fluid transmissions through the port  140 . Near the periphery of cover  126 , an annular lip  128  extends downwardly in overlapping contact with casing  124 . Casing  124  and cover  126  are welded together along lip  128  to form a hermetic seal. 
   Fluid reservoir  138  comprises a collapsible bellows  144  securely attached at a top peripheral edge  146  to cover  126 . Bellows  144  are comprised of a suitable material, such as titanium, which is capable of repeated flexure at the folds of the bellows, but which is sufficiently rigid so as to be noncompliant to variations in pressure within reservoir  138 . The lower peripheral edge of bellows  144  is secured to an annular bellows cap  148 , which translates vertically within pump device  122 . The combination of cover  126 , bellows  144  and bellows cap  148  defines the volume of fluid reservoir  138 . The volume in reservoir  138  may be expanded by moving bellows cap  148  in a downward direction opposite cover  126 , thereby stretching the folds of bellows  144  and creating a vacuum to pull fluid into the reservoir. Similarly, the volume in reservoir  138  may be decreased by moving bellows cap  148  in an upward direction towards cover  126 , thereby compressing the folds of bellows  144  and forcing fluid from the reservoir into duct  132  and out through catheter port  134 . 
   As shown in  FIGS. 7 and 8 , bellows cap  148  includes an integrally formed lead screw portion  150  extending downwardly from the center of the cap  148 . Lead screw portion  150  includes a screw thread, as indicated by numeral  151 , that operatively engages a matching thread on a cylindrical nut  152 . The mating threads  151  on lead screw portion  150  and cylindrical nut  152  enable the lead screw portion  150  to translate vertically relative to cylindrical nut  152  when the nut  152  is rotated about a longitudinal axis of the lead screw portion  150 . The outer circumference of nut  152  is securely attached to an axial bore of a rotary drive plate  154 . A cylindrical drive ring  156  is in turn mounted about an outer annular edge of rotary drive plate  154  to extend downwardly from the plate  154  on a side opposite to nut  152 . Nut  152 , drive plate  154  and drive ring  156  are all securely attached together by any suitable means, to form an assembly that rotates as a unit about the longitudinal axis formed by lead screw portion  150 . 
   A bushing frame  158  is provided in pump device  122  and securely connected along a top edge to annular lip  128 . Bushing frame  158  includes a bottom portion  160  extending beneath bellows cap  148 , and a cylindrically-shaped side wall portion  162  spaced about the periphery of bellows  144 . A cylindrical coil bobbin  164  extends about the inner circumference of frame  158 , between the frame and bellows  144 . One or more coil windings may be wound about the circumference of bobbin  164  for providing transcutaneous signal transfer between an external power and communication source and pump device  122 . In the embodiment shown in  FIGS. 7–10 , a first coil winding  166  on bobbin  164  forms a closed loop antenna (“secondary TET coil”) that is inductively coupled to a primary transcutaneous energy transfer (TET) coil in the external interface. When the primary TET coil in the external interface is energized, an RF power signal is transmitted to the secondary TET coil  166  to provide a power supply for driving pump device  122 . A second coil winding  168  on bobbin  64  provides for control signal transfer between pump device  122  and an external programmable control interface. Coil winding  168  forms an antenna (“secondary telemetry antenna”) that is inductively coupled to a primary telemetry antenna in the external device for transmitting RF control signals between the external interface and pump  122  at a fixed frequency. A bushing  172  is press fit into bushing frame  158  to extend between frame  158  and drive plate  154 . Bushing  172  includes an axial opening for nut  152  and lead screw  150 . Bushing  172  separates bushing frame  158  and drive plate  154  to allow the drive plate and nut  152  to rotate relative to lead screw  150  without interference between the bushing frame  158  and drive plate  154 . In addition, bushing  172  prevents nut  152  from moving radially or axially toward cover  126 . 
   As mentioned above, cylindrical nut  152 , drive plate  154  and drive ring  156  form an assembly that translates lead screw  150  of bellows cap  148  when ring  156  is rotatably driven. In the first embodiment of the present invention, drive ring  156  is rotatably driven by one or more piezoelectric harmonic motors that utilize a series of harmonic vibrations to generate rotation in the ring. In the embodiment shown in  FIGS. 7–10 , a pair of harmonic motors  174 ,  176  are placed in frictional contact with the inner circumference of drive ring  156 , so that the harmonic motion of the motors in contact with the ring produces rotation of the ring  156 . Motors  174 ,  176  may be spaced  1800  apart about the inner circumference of ring  156 , beneath drive plate  154 . Motors  174 ,  176  are mounted to a support board  178 , with a tip portion  180  of each motor in frictional contact with the inner circumferential surface of drive ring  156 . When motors  174 ,  176  are energized, tips  180  vibrate against drive ring  156 , producing a “walking” motion along the inner circumference of the ring  156 , thereby rotating the ring  156 . 
   A spring (not shown) within each motor  174 ,  176  biases motor tip portions  180  into continuous frictional contact with ring  156  to enable precise positioning of drive ring  156 , and a holding torque on the ring  156  between motor actuations to prevent position shift in the ring  156 . Drive ring  156  may be manufactured from a ceramic, or other similar material, in order to provide for the required friction with motor tip portions  80  while also limiting wear on the tip portions  180 . 
   It should be appreciated by those skilled in the art having the benefit of the present disclosure that a piezoelectric harmonic motor, or another type of harmonic motor having no intrinsic magnetic field or external magnetic field sensitivity may be used in the present invention to enable patients with the implant to safely undergo Magnetic Resonance Imaging (MRI) procedures, or other types of diagnostic procedures that rely on the use of a magnetic field. The use of a piezoelectric harmonic motor rather than an electromagnetic servomotor in the present invention enables the device to provide the same high resolution, dynamic performance of a servomotor, yet is MRI safe. An example of a suitable piezoelectric harmonic motor for the present invention is the STM Series Piezoelectric Motor produced by Nanomotion Ltd. of Yokneam, Israel. This motor is described in detail in “The STM Mechanical Assembly and the Nanomotion Product/Selection Guide”, both published by Nanomotion, Ltd. Other types of harmonic motors may also be utilized in the present invention without departing from the scope of the invention. Examples of these other motors include, without limitation, the Elliptec motor by Elliptec AB of Dortmund Germany, which is described in the “Elliptec Resonant Actuator Technical Manual. Version 1.2”; the Miniswys motor by Creaholic of Switzerland; the PDM130 Motor by EDO Electro-Ceramic Products of Salt Lake City, Utah which is described in the technical brochure “High Speed Piezoelectric Micropositioning Motor Model PDA130”; and the Piezo LEGS motor which is manufactured by PiezoMotor Uppsala AB of Uppsala, Sweden and described in the brochure entitled “Linear Piezoelectric Motors by PiezoMotor Uppsala AB”. Additionally, piezoelectric inchworm motors may be utilized to drive a ceramic ring or plate, which motion is then translated into movement of a bellows. Examples of suitable piezoelectric inchworm motors include the IW-800 series INCHWORM motors produced by Burleigh EXPO America of Richardson, Tex. and the TSE-820 motor produced by Burleigh Instruments, Inc of Victor, N.Y. In addition, other types of rotary friction motors, and other types of motors which rely upon piezoelectric effects to drive a member may also be used without departing from the scope of the invention. 
   As discussed above, each motor  74 ,  76  in the first embodiment is mounted to a board  78  using a plurality of screws or other type of secure attachment mechanism. While two motors are depicted in the figures, additional motors may be utilized provided the driving member of each motor is in frictional contact with the drive ring. In addition to supporting motors  74 ,  76 , board  78  may also include control circuitry for powering and operating the motors in accordance with signals transmitted from an external device. Alternatively, a separate circuit board could be included in pump device  22  that would include the circuitry for controlling motors  74 ,  76 . The control circuitry on board  78  is electrically connected to coil windings  66 ,  68  for receiving power to drive motors  74 ,  76 , as well as receiving and transmitting control signals for pump  22 . Board  78  is attached to a wire assembly sheath  81 , which is in turn connected by pins  83  to bushing frame  58 . The connection between board  78  and frame  58  forms a mechanical ground to prevent the board and attached motors  74 ,  76  from torquing within pump device  22  when the motors are energized. As shown in  FIGS. 3–5 , board  78  may also include one or more openings  82  for retaining plate supports  84 . Supports  84  extend between motors  74 ,  76 , from board  78  to drive plate  54 , to support the drive plate  54  and constrain the plate  54  from moving axially away from bellows  44 . 
   In  FIG. 11 , an artificial sphincter system  210  regulates the amount of fluid maintained in an implantable artificial sphincter assembly  212  powered by transcutaneous energy transfer (TET) and under telemetry control of an external assembly  213 . In the illustrative version, the artificial sphincter system  210  is used for weight reduction therapy. A stoma is formed between an upper portion  214  and lower portion  215  of a patient&#39;s stomach  216  to slow the passage of food and to provide a sense of fullness. The implantable artificial sphincter assembly  212  includes an expandable gastric band  218  that encircles the stomach  216  to form the stoma. An infuser device  220  is anchored subcutaneously on a layer of muscular fascia within the patient or in another convenient location. A flexible catheter  222  provides fluid communication between the gastric band  218  and the infuser device  220 . 
   It should be appreciated that the gastric band  218  includes an inwardly directed bladder to expandably receive a fluid, such as saline solution, from the catheter  222  to allow adjustment of the size of the stoma formed therein without having to adjust the attachment of the gastric band  218 . The infuser device  220  advantageously prevents fluid moving in either direction between adjustments so that long-term implantation is realized. 
   An advantageous approach to reducing the necessary size of the infuser device  220  is to utilize TET for powering actuation and control circuitry from the external portion  213 . Telemetry relays the amount of fluid in the infuser device  220  to the external assembly  213  for display, and in some applications for closing the loop on volume adjustment. To that end, the external system  213  may include a primary coil  224  positioned outside of the patient proximally placed to the infuser device  220  that is inside of the patient to inductively couple with a secondary coil (not shown) located within the infuser device  220 . A programmer  226 , which is connected via electrical cabling  228  to the primary coil  224 , activates and monitors the primary coil  224 . 
   With reference to  FIGS. 12–14 , an implantable infuser device  230  incorporates inductive volume sensing. Infuser device  230  includes a fluid discharge head  232  and a cylindrical outer casing  234  sealed hermetically thereto, such as by welding. Discharge head  232  has a discharge conduit  236  sealably attached thereto and in fluid communication with a cylindrical bellows fluid accumulator (“bellows”)  238 . Bellows  238  has an open (fixed) end  240  welded to an inner surface of discharge head  232 . Bellows  238  also has a closed (moving) end  242  fixedly attached to a lead screw  244  centered at the longitudinal axis of bellows  238  and extending away from bellows  238 . Lead screw  244  has fine male threads such as ¼″–32 thereon. 
   Connected to and extending from discharge head  232  surrounding the circumference of bellows  238  is a cylindrical member  246  having a rigid bottom surface  248  and a clearance hole  250  centered therein through which lead screw  244  passes. Press-fit inside cylindrical member  246  and outside the perimeter of bellows  238  is a cylindrical bobbin  252  for housing spaced-apart secondary telemetry and transcutaneous energy transfer wire coils (not shown) in annular coil cavities  253 ,  254  formed with the cylindrical member  246 , for receiving an actuation signal and induced power respectively from outside the patients body to operate the infuser device  230 . 
   Cylindrical outer casing  234  has a base  256  substantially parallel to the inner surface  257  of discharge head  232 . Fixedly attached to this base  256  is control circuitry, depicted as a circuit board  258 , which contains a microprocessor and other electronic devices for operating the infuser device  230 . Attached to circuit board  258  are two piezoelectric motors  260  symmetrically spaced about lead screw  244 , having drive mechanisms frictionally contacting an inner rim  262  of a disk  264  centered about lead screw  244 . Disk  264  has an internally threaded boss  266  extending therefrom toward bellows  238 . Threaded boss  266  has matching ¼″–32 threads, which accurately mate with threads of lead screw  244  to form a nut which when rotated with disk  264  by motors  260  about lead screw  244 , drive lead screw  244  and bellows  238  axially to expand or collapse the bellows  238 . Motors  260  and TET/telemetry coils (not shown) are electrically connected to circuit board  258 , all contained within outer casing  234 . 
   It is desirable to sense the extended or collapsed position of bellows  238  to closed-loop control that position in order to accurately transfer a desired volume of fluid to and from the bellows  238 . To that end, a pancake inductance coil  268  is placed in fixed position parallel to and axially aligned with closed end  242  of bellows  238 . Coil  268  is preferably attached to a rigid bottom surface  270  of cylindrical member  246 , for example, to minimize the distance between the coil  268  and the closed end  242  of the bellows  238 . A parallel tuned tank circuit on circuit board  258 , commonly known in the electronic controls art, oscillates at a frequency of resonance depending on the number and diameter of turns in inductance coil  268 , the electrical capacitance in parallel with coil  268 , and the closeness of closed end  242  to coil  268 , forming an inductive position sensor  280 . In the illustrative version, inductance coil  268  is a spiral shaped coil of about 200 turns made of 40 gauge copper wire. A microprocessor on the circuit board  258  measures the frequency of oscillation and compares it to a table of frequencies in order to provide an error signal to indicate how close the actual bellows position is to the command position desired. Piezoelectric motors  260 , combined with driven disk  264  and threaded boss  266 , actuate the bellows  238  via lead screw  244 , forming a bellows actuators  290 . 
   It should be appreciated that a position sensor that is not dependent upon the presence and/or rotation of a lead screw such as the afore-described inductive position sensor may have application in an infuser device that is thermodynamically actuated, such as described in the afore-mentioned cross-referenced applications. 
   It should be appreciated that any patent, publication, or other disclosure material, in whole or in part, that is said to be incorporated by reference herein is incorporated herein only to the extent that the incorporated material does not conflict with existing definitions, statements, or other disclosure material set forth in this disclosure. As such, and to the extent necessary, the disclosure as explicitly set forth herein supersedes any conflicting material incorporated herein by reference. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material set forth herein, will only be incorporated to the extent that no conflict arises between that incorporated material and the existing disclosure material. 
   While the present invention has been illustrated by description of several embodiments and while the illustrative embodiments have been described in considerable detail, it is not the intention of the applicant to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications may readily appear to those skilled in the art. 
   For example, while the TET system  16  described has particular advantages for an implantable medical device system  10 , aspects consistent with the present invention have application to other scientific and engineering scenarios including inanimate physical boundaries. For instance, in a processing apparatus it may be desirable to monitor and/or control an actuator that is contained within a vessel without compromising the integrity of the vessel with wires or conduits passing therethrough. 
   For another example, TET for the purposes of power transfer to operate implanted devices has been illustrated above, although applications consistent with aspects of the invention may be directed to TET for communication purposes (i.e., telemetry). Thus, the power coupling efficiencies enhance the reliability and performance of the resultant communication channel. 
   For a further example, additional power transfer efficiencies may be realized by adding additional coils in physical and electrical parallel to the two described above with circuit optimization to maintain an appropriate Q and impedance, and thus a secondary twin coil is not limited to only two coils.