Patent Publication Number: US-2020282123-A1

Title: Transcutaneous power transmission ulitizing non-planar resonators

Description:
This application is a continuation of co-pending U.S. patent application Ser. No. 16/375,573, filed on Apr. 4, 2019, now allowed, which is a continuation of U.S. patent application Ser. No. 14/796,983, filed on Jul. 10, 2015, now issued as U.S. Pat. No. 10,279,096, which is a continuation of U.S. patent application Ser. No. 13/301,717, filed on Nov. 21, 2011, now issued as U.S. Pat. No. 9,079,043. 
    
    
     FIELD 
     Embodiments of the invention relate to wireless energy transfer; and more specifically, to the transfer of energy from a power source outside the body to an implanted medical device inside the body. 
     BACKGROUND 
     In wireless energy transmission systems, resonators with planar geometries have been used in the power transmitter and receiver units to facilitate the transfer of energy. Such planar resonators have geometries that span a substantially flat surface with no dimensional measurement on an axis orthogonal to the flat surface, or where dimensions on an axis along the flat surface are orders of magnitude greater than a dimension on an axis orthogonal to the flat surface. Transmission of power between the transmitter and receiver units relies on inductively or magnetically coupling the respective planar resonators. When the planar transmitter resonator carries an electrical current driven by an external power source, magnetic flux is generally generated in a direction perpendicular to the plane of the transmitter resonator. A planar receiver resonator is then placed within a vicinity of the planar transmitter resonator and is oriented parallel to the planar transmitter resonator such that the planar receiver resonator is able to effectively intercept the magnetic flux generated by the transmitter resonator to produce an electrical current in the receiver unit. 
     The use of planar resonators is an effective approach for closely coupled wireless power transmission systems to transfer energy between two stationary objects that are in close proximity. For example, in charging pad applications, a planar transmitter resonator is embedded in a charging pad that is placed on a desk. When an electronic device, such as a cellular phone, is equipped with a planar receiver resonator, and is placed on the charging pad, the two planar resonators are orientated parallel to each other along the planes of the resonators. In this parallel orientation, energy can be effectively transferred from the planar transmitter resonator embedded in the charging pad to the planar receiver resonator in the electronic device to charge the electronic device. Because the electronic device is not expected to move on its own, the two planar resonators remain oriented parallel to each other to continuously charge the electronic device until the electronic device is removed from the charging pad. 
     SUMMARY 
     The present invention is directed to a system for omni-orientational wireless energy transfer. The system may be particularly useful in applications where energy is to be transferred wirelessly between two objects, and at least one of the objects is free to move around. One such application is where energy is being transferred from a power transmitter unit to a receiver unit that is part of an implanted medical device. In such an application, while the transmitter unit may be stationary, for example, be plugged into a wall, a patient implanted with the medical device and the receiver unit may be free to move around such that the relative orientation of the transmitter and receiver units is not fixed and may change. 
     In one aspect of the present invention, a system for wireless energy transfer includes a transmitter unit and a receiver unit. The transmitter unit has a resonator with a coil configured to be coupled to a power supply to wirelessly transmit power to a receiver unit. The receiver unit has a resonator with a coil coupled to a device load. At least one of the resonators is a non-planar resonator that spans a non-degenerate two-dimensional surface with at least one concave portion. 
     In another aspect of the present invention, a method for omni-orientational wireless energy transfer includes the following steps. A resonator including a coil coupled to a power supply is provided. Another resonator with a coil coupled to an electrical load is placed at a separation distance from the other resonator. At least one of the resonators spans a non-degenerate two-dimensional surface with at least one concave portion to enable wireless energy transfer between the coils regardless of their respective orientation. 
     In a further aspect of the present invention, a transmitter unit for use in an omni-orientational wireless energy transfer system to wirelessly transfer energy to a receiver unit includes a non-planar resonator with a coil that spans a non-degenerate two-dimensional surface with at least one concave portion. The transmitter unit also includes power circuitry coupled to the coil. The power circuitry is configured to be electrically connected to a power supply source to deliver an electrical current to the coil. 
     In a different aspect of the present invention, a receiver unit for use in an omni-orientational wireless energy transfer system to receive wirelessly transferred energy from a transmitter unit includes a non-planar resonator with a coil that spans a non-degenerate two-dimensional surface with at least one concave portion. The receiver unit also includes an electrical load coupled to the coil. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may best be understood by referring to the following description and accompanying drawings that are used to illustrate embodiments of the invention. In the drawings: 
         FIG. 1  illustrates a non-planar resonator according to one embodiment of the invention; 
         FIG. 2  illustrates a non-planar resonator according to another embodiment of the invention: 
         FIG. 3  illustrates a non-planar resonator according to a further embodiment of the invention; 
         FIG. 4  illustrates a non-planar resonator according to a different embodiment of the invention; 
         FIG. 5  illustrates a conceptual diagram of a system for wireless energy transfer according to one embodiment of the invention; 
         FIG. 6  illustrates a planar resonator used in a system for wireless energy transfer according to an embodiment of the invention; 
         FIG. 7A  illustrates a transcutaneous energy transfer system (TETS) according to one embodiment of the invention; 
         FIG. 7B  illustrates a TETS according to another embodiment of the invention; 
         FIG. 8A  illustrates a TETS arranged in a particular orientation according to an embodiment of the invention; 
         FIG. 8B  illustrates a TETS arranged in a different orientation according to an embodiment of the invention; 
         FIG. 9A  illustrates an method for omni-orientational wireless energy according to one embodiment of the invention; and 
         FIG. 9B  illustrates an method for omni-orientational wireless energy according to another embodiment of the invention; 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     In the following description, numerous specific details are set forth. However, it is understood that embodiments of the invention may be practiced without these specific details. In other instances, well-known circuits, structures and techniques have not been shown in detail in order not to obscure the understanding of this description. 
     References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described may include a particular feature, structure, or characteristic, but every embodiment may not necessarily include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     In the following description and claims, the terms “coupled” along with its derivatives, may be used. It should be understood that the term “coupled” is used to indicate that two or more elements, which may or may not be in direct physical or electrical contact with each other, co-operate or interact with each other. The terms “energy transfer,” “power transfer,” and “power transmission,” and their derivatives, are used interchangeably and refers to the transmission of energy between two devices. 
     Although the use of planar resonators is effective for applications that transfer power between two stationary objects, as the transmitter and receiver resonators are moved or rotated into a non-parallel orientation, the rate of power transfer dramatically decreases. Even in systems that are capable of transmitting power over large separation distances, no net power transfer will occur when the planar transmitter resonator is orientated orthogonal to the planar receiver resonator. Thus, in systems where the relative orientation of the transmitter and receiver devices may vary greatly or is unknown, it is possible for the receiver device to receive no power at all for an extended period of time when the planar resonators are positioned orthogonal to each other. While this result may be inconsequential in certain applications, this result may pose a risk to the health of a patient if the receiver device is an implanted medical device that is used to assist critical bodily functions, and a power storage unit, such as a battery, in the implanted medical device is not promptly recharged. 
     An implanted medical device can be powered or charged using a transcutaneous energy transfer system (TETS) that transfers electrical energy from an external power source to the implanted medical device wirelessly through the skin of a patient. In a conventional TETS that uses planar resonators, a patient is restricted to remain substantially in a fixed or known position relative to a power transmitter device in order to sustain effective power transmission over an extended period of time. For example, suppose a patient is implanted with a medical device with a planar receiver resonator and is resting on a bed. A planar transmitter resonator can be positioned on or embedded in the bed such that when the patient is laying flat on the patient&#39;s back, the two resonators are oriented substantially parallel to each other to effect charging of the implanted medical device while the patient rests. However, as the patient moves or rolls around in bed, the amount of power transfer between the planar transmitter resonator and the planar receiver resonator implanted in the patient can drop to zero if the patient turns to rest on the patient&#39;s side such that the resonators are now orthogonal to each other. If the patient does not move from this position for an extended period of time, the implanted medical device may lose power completely to pose a risk to the patient. 
     In accordance to one embodiment of the present invention, a non-planar resonator to enable omni-orientational wireless energy transfer in a wireless energy transmission system will now be described. A non-planar resonator is a resonator that spans a surface area occupying three spatial dimensions instead of two dimensions. For example, in one embodiment, the non-planar resonator is a resonator that spans a non-degenerate two-dimensional surface with at least one concave portion, such as an elliptical paraboloid surface. An elliptical paraboloid surface is surface that is shaped like a bowl, with the interior of the bowl being the concave portion. It should be noted that in an elliptical paraboloid surface, when the concave portion is extended along the elliptical paraboloid surface, the concave portion can extend out to infinite space. In other words, the concavity of the elliptical paraboloid surface does not wrap around and does not enclose on itself. In three-dimensional Euclidian space, an elliptical paraboloid surface is described by the equation: 
         x   2   /a   2   +y   2   /b   2   −z= 0, 
     where a and b are constants.
 
In an exemplary embodiment as shown in  FIG. 1 , the non-planar resonator  100  spanning an elliptical paraboloid surface has a spiral wire coil  102 , such as a Litz wire coil, that starts at the center of the bottom of an imaginary bowl-shaped surface defined by the above equation, and spirals upwards along the sidewalls of the surface up to the rim of the imaginary bowl-shaped surface.
 
     In some embodiments, there can be gaps  104  in between the loops of the spiral wire coil  102 . The gaps  104  can be uniformly distributed, or there can be portions of the wire coil that have wider gaps and other portions that have narrower gaps. Similarly, the density of the wire windings can also vary over the surface of the non-planar resonator  100 . The geometries of the gaps and the density of the wire windings can be used to tailor the non-planar resonator to have a specific inductance or a specific capacitance to achieve a particular resonant frequency. It should be understood that this is just one example of a wire coil configuration that forms a resonator spanning a non-planar surface. In other embodiments, the wire coil  102  can be configured differently to form a resonator that spans such a surface. 
     Alternatively, as shown in  FIG. 2 , the non-planar resonator  200  can be an elliptical paraboloid or bowl-shaped core  204  made of a magnetic material to direct magnetic flux. A wire  202 , such as a Litz wire, for purpose of exemplary illustration, is wrapped around that core  204 . The wire  202  can be wrapped around the core  204  from one side of the bowl-shaped core  204  to an opposing side, be wrapped around the exterior surface of the bowl-shaped core  204 , or be wrapped along intersecting diameters of the bowl-shaped core  204  as shown. In other embodiments, the wire  202  can be wrapped around the magnetic core  204  in other configurations. As with the above embodiment, the density of the wire windings can be tailored to achieve a particular resonant frequency. 
     In another exemplary embodiment, the non-planar resonator spans a non-degenerate two-dimensional surface that has at least two concave portions, such as a hyperbolic paraboloid surface. A hyperbolic paraboloid surface is a surface that is shaped like a saddle, with the top of the saddle being one concave portion and the bottom of the saddle being another concave portion. In one embodiment, the concave curvature of one concave portion can be orthogonal to the concave curvature of the other concave portion. It should be noted that in a hyperbolic paraboloid surface, when the concave portions of the surface are extended along the hyperbolic paraboloid surface, the concave portions can extend out to infinite space. In other words, the concavities of the hyperbolic paraboloid surface do not wrap around and do not enclose on themselves. In three-dimensional Euclidian space, a hyperbolic paraboloid surface is described by the equation: 
         x   2   /a   2   −y   2   /b   2   −z= 0, 
     where a and b are constants.
 
In one embodiment as shown in  FIG. 3 , the non-planar resonator  300  spanning a hyperbolic paraboloid surface has a spiral wire coil  302 , such as a Litz wire coil, that starts at the center of an imaginary saddle-shaped surface and spirals outwards along the saddle-shaped surface. In some embodiments, there can be gaps  304  in between loops of the spiral wire coil  302 . In other embodiments, the wire coil  302  can be configured differently to form a resonator that spans such a surface. The geometries of the gaps  304  and the density of the wire windings can also be used to tailor the non-planar resonator to have a specific inductance value or a specific capacitance value to achieve a particular resonant frequency.
 
     Alternatively, as shown in  FIG. 4 , the non-planar resonator  400  can be a hyperbolic paraboloid or saddle-shaped core  404  made of a magnetic material to direct magnetic flux. A wire  402 , such as a Litz wire, is wrapped around that core  404 . The wire  402  can be wrapped around the core  404  from one side of the saddle-shaped core  404  to an opposing side, or be wrapped across the center of the saddle-shaped core  404  as shown. In other embodiments, the wire  402  can be wrapped around the magnetic core  404  in other configurations. As with the other embodiment, the density of the wire windings can be tailored to achieve a particular resonant frequency. 
     More generally, a non-planar resonator according to embodiments of the present invention is a resonator that span a non-degenerate two-dimensional surface defined by one of the following equations in three-dimensional Euclidean space: 
         Ax   2 +By 2   +Cz   2   =D , or 
         Ax   2 +By 2   +Cz= 0, 
     where A, B, C, and D are non-zero numbers.
 
Surfaces defined by these equations are sometimes referred to as non-degenerate quadric surfaces. In addition to the elliptical paraboloid and hyperbolic paraboloid surfaces described above, another exemplary embodiment of a non-planar resonator that spans a surface defined by one of the above equations includes a non-planar resonator that spans a hyperboloid surface. One feature of such surfaces is that these surfaces have a parabolic cross section. A non-planar resonator according to embodiments of the present invention can be implemented with a wire coil that is formed to outline the shape of a non-degenerate two-dimensional surface. The wire that is used to form the coil can be, for example, a Litz wire. A Litz wire is generally referred to as a type of cable used in electronics to carry electric current and may have many individually braided or woven strands of wire in one or more patterns at one or more levels to increase the amount of current passing through each cable by increasing surface area and decreasing the resistance. Alternatively, the wire can be a strand or foil of conductive metal (e.g., such as copper, gold, or silver) with an insulated covering. In other embodiments, the non-planar resonator can be implemented with a wire, for example, a Litz wire, that is wrapped around a core of magnetic material, where the magnetic core has been molded or formed to have a non-degenerate two-dimensional surface. In these embodiments, the core of magnetic material is used to direct magnetic flux in various directions depending on the shape of the magnetic core. In some embodiments, the resonant frequency of the resonator can be in a range of 100 kHz to 10 MHz.
 
     As indicated by the embodiments described above, the phrase that “a resonator spans a non-degenerate two-dimensional surface,” and similar derivatives, do not necessary require the resonator to have a solid surface. Instead, as used herein, the phrase that a resonator spans a non-degenerate two-dimensional surface can mean that the resonator forms an outline of such a surface (e.g., the spiral coil), or mean that the resonator has such a surface (e.g., the magnetic core with a wire wrapped around the core). Furthermore, it should be noted that the phrase that “a resonator spans a non-degenerate two-dimensional surface,” and similar derivatives, also mean that the shape of the resonator may have minor features or deformalities that may deviate from such a surface, without departing from the spirit of the present invention. For example, with respect to the embodiment with the elliptical paraboloid bowl-shaped surface, the resonator may have a flat bottom instead of a curved bottom, or the sidewall may have one or more minor dimple indentations. It should be understood that such minor features or deformalities do not detract away from the scope and spirit of the exemplary embodiments. 
       FIG. 5  illustrates an omni-orientational wireless energy transfer system  500  according to one embodiment of the present invention. The omni-orientational wireless energy transfer system  500  includes a transmitter unit  501  that has a transmitter resonator  502  with a wire coil, and a receiver unit  511  that has a receiver resonator  512  with a wire coil. In this particular embodiment as shown, the receiver resonator  512  is a non-planar resonator that spans an elliptical paraboloid surface. In other embodiments, the receiver resonator  512  may span any of the non-degenerate two-dimensional surfaces described above. Furthermore, while in the embodiment as shown in  FIG. 5 , it is the receiver unit that has a non-planar resonator, in other embodiments, it can be the transmitter unit or both the receiver and transmitter units that have a non-planar resonator. 
     In the exemplary omni-orientational wireless energy transfer system  500  shown in  FIG. 5 , the transmitter resonator  502  is a planar resonator. A planar top view  600  of the transmitter resonator  502  is shown in  FIG. 7 . As shown, the wire that forms the transmitter resonator  502  in the transmitter unit  501  spans a planar surface. More specifically, the transmitter resonator  502  in this exemplary embodiment is a square spiral wire coil with each loop of the spiral wire coil arranged in substantially the same plane as each other. In other embodiments, other planar transmitter resonators may be used. 
     The transmitter unit  501  is configured to be coupled to a power supply or a power source  520 , for example, a wall electrical outlet, such that when the transmitter unit  501  is powered, the power source  520  drives an electrical current through the coil of the transmitter resonator  502 . Alternatively, the transmitter unit  501  can be coupled to other power sources such as a battery that can be used to drive an electrical current through the coil of the transmitter resonator  502 . Consequently, magnetic flux perpendicular to the plane of the planar transmitter resonator  502  is generated by the electrical current running through the coil of the transmitter resonator  502 . Although not shown, it should be understood that the transmitter unit  501  may have additional power circuitry, for example, to perform alternating current (AC) to direct current (DC) conversion, or vice versa, and may have additional control circuitry, for example, to modulate the power output of the transmitter unit  501 . When the receiver unit  511  is brought within an operating distance of the system, the magnetic flux generated from the transmitter unit  501  induces a current in the receiver resonator  512  in the receiver unit  511 , which causes an electrical current to flow in the coil of the receiver unit  511 . The coil in the receiver unit is coupled to an electrical or device load  513 . The electrical current flowing through the coil in the receiver unit  511  can then be used to power the device load  513 . In the case that the device load  513  is a battery or other types of rechargeable power storage device as shown, the electrical current flowing through the coil in the receiver unit  511  can be used to recharge the power storage device. In this manner, the omni-orientational wireless energy transfer system  500  is able to wirelessly transfer power from the transmitter unit  501  to the receiver unit  511 . 
     By using resonators in the transmitter and receiver units  501  and  511  that have closely matched resonant frequencies, the power transfer efficiency of the system  500  can be improved, and consequently, the range of distances of the omni-orientational wireless energy transfer system  500  through which power can be transferred can be extended beyond conventional inductive coupling techniques. A resonator is an object that has a natural oscillation frequency. More specifically, with respect to wireless energy transfer, a resonator is an object that stores energy in two forms and exchanges the stored energy between the two forms continuously. In an electromagnetic resonator, stored energy is continuously being exchanged or oscillating between the electric fields of a capacitive element and the magnetic field of an inductive element. The oscillation decays or dampens due to resistive losses in the resonator. Because of this oscillation, energy that is not transferred from a transmitter to a receiver in one cycle of the oscillation is not lost completely. In other words, the stored energy in a transmitter can be transferred to a receiver through many cycles of the oscillation. This is in contrast to a pure inductive coupling system where any energy stored in the magnetic field of an inductive element that is not transferred to a receiver at an instance is lost. At large separation distances, this energy loss in a pure inductive coupling system is significant and results in little or no meaningful amount of energy being transferred. In the wireless energy transfer system  500 , the use of resonators allows most of the stored energy in the transmitter unit  501  to be transferred to the receiver unit  511  over many oscillation cycles as described above to improve the overall power transfer efficiency of the system  500 . As a result, the receiver unit  511  can be placed at a separation distance further away from the transmitter unit  501  and still allows a meaningful amount of energy to be transferred over time. 
     While resonators have been used in wireless energy transfer systems to extend the communication/transfer range of the systems, the systems that use planar resonators are susceptible to changes in the relative orientation of the resonators with respect to each other. A comparison of a system that uses a non-planar resonator to a system that uses a planar resonator with both systems having the same separation distance and rated for the same output power may be as follows. The amount of energy that is received at the receiver in the non-planar resonator system in a particular direction can potentially be less than the amount of energy that is received at the receiver in a planar resonator system when the transmitter and receiver resonators are parallel to each other. In an exemplary embodiment, the amount of energy that is received in the non-planar resonator system can be about 25% of the amount of energy that is received in the planar resonator system when the planar resonator system is arranged in its preferred orientation (when the transmitter and receiver resonators are parallel to each other). However, although a planar resonator may be able to transfer more power in a single preferred orientation, the amount of energy that a planar resonator system can transfer drops off dramatically when the system is arranged in other orientations and can drop to zero when the orientation of the transmitter and receiver resonators are arranged orthogonal to each other. 
     By using a non-planar resonator  512  that spans a surface area occupying three spatial dimensions, for example, in the receiver unit  511 , the same transmitter unit  501  is able to couple or transfer energy to the non-planar receiver resonator  512  over a wider range of spatial orientations as compared to a receiver unit having a planar resonator. Hence, at a given separation distance D that is within an operating range of the omni-orientational wireless energy transfer system  500 , there is no requirement that the resonators  502  and  512  have to be placed in any particular orientation with respect to each other in order to transfer a meaningful amount of power required to power or recharge the device load  513 . In one exemplary embodiment, the system  500  is able to achieve a power transfer between the transmitter resonator  502  and the non-planar receiver resonator  512  at a given separation distance D that is at least 25% of a maximum power transfer (compared to a planar receiver resonator) at that given separation distance D, regardless of the orientation of the transmitter resonator  502  relative to the non-planar receiver resonator  512 . For example, in an embodiment, power in the range of 5 W to 20 W can be transferred from the transmitter resonator  502  to the receiver resonator  512  over a separation distance D in the range of, for example, 2.5 cm to 35 cm. The system  500  is able to transfer a maximum about of 20 W at a distance D of 10 cm when the resonators  502  and  512  are in their ideal alignment. Even when the resonators  502  and  512  are rotated or displaced from their ideal alignment, the system  500  is still able to transfer at least 5 W of power to the receiver unit  511  at the same distance D of 10 cm. In other embodiments, a greater or lesser amount of power can be delivered over other distances by adjusting the size and geometries of the resonators. The same concept of using non-planar resonators is independent of distance and can equally apply in a larger range of distances so long as there is sufficient power for the signal to travel that range. 
     In some embodiments, the resonant frequency of the transmitter resonator  502  can be 100 kHz, 500 kHz, 1 MHz, or 10 MHz. The receiver resonator  512  is designed to have a resonant frequency that closely matches the transmitter resonator  502 . Hence, if the transmitter resonator  502  has a resonant frequency of 100 kHz, the receiver resonator is designed to also have a resonant frequency close to 100 kHz, for example, within ±5% or ±10% of 100 kHz. In other embodiments, the resonant frequency of the closely matched resonators  502  and  512  can be a frequency that is in the range of 100 kHz to 10 MHz. In further embodiments, other resonant frequencies can be used. 
     In the embodiment as shown in  FIG. 5  and as described above, the transmitter and receiver resonators  502  and  512  are shaped differently. Even if both resonators are made of the same materials with the same length of wire, each resonator may have a slightly different natural resonant frequency due to differences in their geometries and configurations. In addition, other factors that may cause the resonant frequency of one resonator to deviate from the resonant frequency of the other resonator include differences in the surrounding environment of the resonator. For example, one resonator may be subjected to a different temperature than the other resonator, or one resonator may be subjected to other extraneous objects that may affect the resonant frequency, such as a nearby sheet of highly conductive material. 
     To improve the performance of the system, a tunable capacitor can be coupled to the coil of at least one of the resonators. A tunable capacitor is a circuit or component with a variable capacitance value that can be changed in response to a control input. The control input may be voltage, current, frequency, or any other input that can cause the materials or circuit of the tunable capacitor to change its capacitance value. Coupling a tunable capacitor to the coil enables the effective capacitance value of the resonator to be adjusted. By adjusting the effective capacitance value of the resonator using the tunable capacitor, the resonant frequency of that resonator can be tuned to match the resonant frequency of the other resonator of the system and to maximize a voltage gain at the receiver unit  511 . Furthermore, not only can the tunable capacitor be used to match the resonant frequencies of the resonators, the tunable capacitor can also be used to define the system to reduce the amount of energy transferred from the transmitter unit  501  to the receiver unit  511  if the operating conditions of the system require less energy to be transferred. In other embodiments, an array or network of capacitive elements can be coupled to the coil of at least one of the resonators to tune the resonant frequency of the system. The array or network of capacitive elements can be configured to form different series and/or parallel arrangements of capacitive elements to achieve an effective capacitance value for the resonator for the same purposes as described above. Alternatively, a tunable or network of inductive elements can be used to adjust the effective inductance of the resonator to change the resonant frequency. 
     In a further embodiment, at least one of the resonators  502  and  512  is coupled to a tunable resistor or an array or network of resistive elements to tune a quality factor “Q” of the system to maximize a voltage gain at the receiver unit  511 . A tunable resistor is a circuit or component with a variable resistance value that can be changed in response to a control input. The control input may be voltage, current, or any other input that can cause the materials or circuit of the tunable resistor to change its resistance value. Similarly, an array or network of resistive elements can be configured to form different series and/or parallel arrangements of resistive elements to achieve an effective resistance value. For example, the resistive elements can be resistors, capacitors with effective resistance values, or a combination of both. In an embodiment, an array of resistive elements can be an array of capacitors, where the capacitors have the same capacitance values but different effective series resistance values. This allows the effective resistance to be adjusted while keeping the effective capacitance the same. 
     The quality factor “Q” describes the inverse power loss of the resonator. Hence, a larger Q means a lower power loss in the resonator and a higher energy transfer efficiency, resulting in a higher voltage gain at the receiver unit  511 . The quality factor “Q” of a resonant system that has a transmitter resonator  502  and a receiver resonator  512  can be described by the square root of the product of the quality factors of the two resonators  502  and  512 . In order to reduce the power loss in the system  500  to maximize the voltage gain at the receiver unit  511 , the quality factor “Q” of the system can be increased by increasing the quality factors of either or both resonators  502  and  512 . This can be achieved by tuning a tunable resistor or an array of resistive elements that is coupled to the coil of the respective resonator to modulate and to match the impedance seen by the resonator. Alternatively, a tunable or network of capacitive and/or inductive elements can be used to adjust the effective capacitance and/or the effective inductance to change the quality factor “Q.” 
     In another embodiment, the power transfer efficiency of the system can be further improved by using a flux concentrator made of high-permeability, low-loss materials to direct the output magnetic flux generated from the transmitter unit  501  towards the receiving unit  511 . In embodiments that lack a flux concentrator, the magnetic flux generated from the transmitter coil is spread around the transmitter coil, even in stray directions that are away from the receiving unit  511 . The use of a flux concentrator on the transmitter unit  501  can create a magnetic path to channel and redirect the generated magnetic flux from those stray directions towards the direction of the receiving unit  511 . Similarly, a flux concentrator can also be used in the receiving unit  511  to redirect magnetic flux around the surrounding areas of the receiver coil towards the receiver coil. 
     To facilitate the tuning of the system using the techniques and components described above, each of the transmitter unit  501  and the receiver unit  511  may include additional communications circuitry such as encoders, decoders, antennae, amplifiers, modulators, and filters, to establish a communications channel between the receiver unit  511  and the transmitter unit  501  to communicate system information between the units to adjust the components in order to modulate the power delivered to the receiver unit  511 . For example, wireless communications channel can be a radio frequency signal or other wireless signal transmissions including Wi-Fi (IEEE 802.11 family). Bluetooth, infrared, and other well-known wireless communications protocols. The system information communicated over the communications channel can include performance data such as the battery level and usage of the device load  513 . Other performance data may include the resonant frequencies, the impedances of circuits in the system, the voltages and loads of the system, the temperature of the resonators  502  and  512 , and other data that can be used to tune and detune the system. In addition, the system information communicated over the communications channel may also include identification information of the transmitter and receiver units  501  and  511 , as well as commands to change the system&#39;s settings or operating modes. 
     While the above embodiments have been described with the receiver unit  511  having a non-planar resonator, in other embodiments, the non-planar resonator spanning the non-degenerate two-dimensional surface can be in both the transmitter unit  501  and the receiver unit  511 , or only in the transmitter unit  501 , because of the symmetry in the resonant coupling of the resonators. Hence, as long as at least one of the receiver or transmitter resonators is a non-planar resonator that spans a non-degenerate two-dimensional surface, power can be transmitted at a given operating distance regardless of one resonator&#39;s orientation relative to the other resonator. This symmetrical property of the system is particularly useful in applications where space is limited in the receiver unit or where other factors such as extraneous objects restrict the types of geometries that can be used in the receiver resonator. 
     One example of such an application is a transcutaneous energy transfer system (TETS), where the receiver unit is part of an implanted medical device. For an implanted medical device such as a ventricular assist device or a blood pump, the receiver resonator is configured for subcutaneous placement within a human. Depending on the implant location of the receiver resonator in the human body, the geometry of the receiver resonator may be spatially restricted by the proximity of nearby organs or other body structure. Hence, in such applications, it may be desirable to have the non-planar resonator in the transmitter unit instead of the receiver unit. Different embodiments are also possible. For example, a non-planar resonator can be implanted in the thoracic/abdomen area, wrapping around a part of, or the entire circumference of the body of a patient. The non-planar resonator can be formed to span a surface that takes after an undulating outline of the ribs and general shape that contours around the rib cage. 
     In an exemplary embodiment, the dimensions of a non-planar implanted receiver resonator may span a depth in a range of 0.5 inches (in.) to 2 in. and be no bigger than 3 in. by 3 in. along the largest possible area spanned by a cross section of the resonator. In other words, the implanted receiver resonator may span a volume that is less than 3 in. by 3 in. by 2 in. In other embodiments, the implanted receiver resonator can have the largest possible area spanned by a cross section of the resonator to be an area as big as the human anatomy can allow as with the case where a non-planar resonator wraps around the abdomen of the body. 
       FIGS. 7A and 7B  illustrates exemplary embodiments of a TETS  700  with a ventricular assist device (VAD)  750  according to embodiments of the present invention. A VAD is a mechanical circulatory device that is used to partially or completely replace the function of a failing heart. For patients suffering from congestive heart failure, the VAD is implanted into the patient for long term use. VADs are designed to assist either the right (RVAD) or left (LVAD) ventricle, or both at once (BiVAD). VADs can be designed with an axial flow or centrifugal flow configuration. An impeller configured in an axial flow or centrifugal configuration can be suspended by journal bearing such as a ball and cup, or by a combination of active and/or passive magnetic forces, or by a combination of passive magnetic forces and hydrodynamic forces. In other embodiments, the blood pump can be an artificial heart, which is designed to completely take over cardiac function and may require the removal of a patient&#39;s heart. 
     The VAD  750  includes a pump assembly  713 , a blood pump  714 , a rechargeable power storage device  716 , and a power receiver unit  711 . The rechargeable power storage device  716  may include two or more rechargeable batteries  715  to provide the VAD  750  with a backup battery in case the stored energy in the primary battery is depleted or if the primary battery fails otherwise. The rechargeable power storage device  716  can be implanted in a location away from the blood pump assembly  713 , for example, in the lower abdominal as shown in  FIG. 7A . The power receiving unit  711  includes a resonator  712  with a coil that is coupled to the power storage device  716 , which is the electrical load of the power receiver unit  711 . In the embodiment as shown in  FIG. 7A , because the receiver unit  711  is implanted in the lower abdominal area where there may be less spatial constraints on the size and shape of the receiver resonator  712 , the receiver resonator  712  is the non-planar one of the resonators in the TETS  700  and spans a non-degenerate two-dimensional surface including any of the surfaces described above. The resonant frequency of the receiver resonator  712  can be in a range of 100 kHz to 10 MHz. In an exemplary embodiment, the resonant frequency of the receiver resonator  712  can be 100 kHz, 500 kHz, 1 MHz, or 10 MHz. In other embodiments, another resonant frequency that is safe for the human body can be used. 
     The TETS  700  also includes a power transmitter unit  701  that is external to the patient. The transmitter unit  701  includes a transmitter resonator  702  with a coil that is configured to be coupled to a power supply source  720  such as an electrical wall outlet. Alternatively, the transmitter unit  701  can be coupled to other power sources such as a battery that can be used to drive an electrical current through the coil of the transmitter resonator  702 . In this particular embodiment, the transmitter resonator  702  is a planar resonator made of a planar wire loop. When the transmitter unit  701  is plugged into the electrical wall outlet  720 , an electrical current is generated in the coil of the transmitter resonator  702 . The resonant frequency of the transmitter resonator  702  can be in a range of 100 kHz to 10 MHz. In an exemplary embodiment, the resonant frequency of the transmitter resonator  702  can be 100 kHz, 500 kHz, 1 MHz, or 10 MHz. In other embodiments, another resonant frequency that is safe for the human body can be used. The transmitter resonator  702  as part of the transmitter unit  701  may be embedded in a stationary object such as a wall, a chair, a bed, or other fixtures such as a car seat or objects that do not move by themselves without external control or human assistance. The source of power for a stationary and embedded transmitter resonator is generally alternating current from an electric outlet, but can also be direct current from a battery source. In other embodiments, the transmitter resonator  702  may be part of a piece of wearable clothing such as a vest or a jacket, or other wearable accessories. In the case of a transmitter resonator that is embedded into a piece of clothing or object wearable by a person that moves with a person, the source of power would be portable sized rechargeable batteries that also could be worn by the patient. 
     When the receiver unit  711  in the patient comes within a separation distance D of the transmitter unit  701 , the TETS  700  is able to wirelessly transfer energy from the transmitter unit  701  to the receiver unit  711  to recharge the power storage device  716  of the VAD  750 . In one embodiment, at a given separation distance D being in the range of 2.5 cm to 35 cm, the transmitter unit  701  is able to deliver power in the range of 5 W to 20 W to the receiver unit  711  to recharge the batteries  715  in the power storage device  716  of the VAD  750 . By using a non-planar coil in the receiver resonator  712  in the receiver unit  711 , the TETS  700  is able to achieve a power transfer between the transmitter coil in the transmitter resonator  702  and the receiver coil in the receiver resonator  712  at a given separation distance D that is at least 25% of a maximum achievable power transfer at that given separation distance D, regardless of the coils&#39; respective orientation to each other. For example, in one embodiment, the TETS  700  is able to transfer a maximum amount of 20 W at a distance D of 10 cm when the respective coils in the receiver and transmitter resonators  702  and  712  are in their ideal alignment. As the patient moves around and causes the coil in the receiver resonator  712  to be orientated at a different angle relative to the coil in the transmitter resonator  702  away from their ideal alignment, the TETS  700  is still able to transfer at least 5 W of power to the receiver unit  711  at the separation distance D of 10 cm. In other embodiments, a greater or lesser amount of power can be delivered over longer distances, for example, separation distances of 35 cm and beyond, by adjusting the size and geometries of the resonators. 
     The use of a non-planar resonator that spans a surface area occupying three spatial dimensions in the receiver unit  711  according to embodiments of the present invention has the advantage over conventional systems that uses only planar resonators, in that the non-planar receiver resonator  712  is able to couple with more magnetic flux generated from the transmitter unit  701  in a wider range of spatial orientations. Hence, at a given separation distance D within an operating range of the TETS  700 , there is no requirement that the resonators  702  and  712  have to be placed in a particular orientation with respect to each other in order to transfer a meaningful amount of power required to recharge the rechargeable storage device  716 . 
     This advantage of the TETS  700  according to an embodiment of the present invention is illustrated in  FIGS. 8A and 8B . In  FIGS. 8A and 8B , the transmitter resonator  702  as part of the transmitter unit  701  of the TETS  700  is embedded in a bed  800 . Implanted in a patient are a VAD  750  and a receiver unit  711  with a non-planar receiver resonator that is coupled to the rechargeable batteries of the VAD  750 . Whether the patient is laying flat on the patient&#39;s back on the bed as shown in  FIG. 8A  or is lying on the patient&#39;s side as shown in  FIG. 8B , the TETS  700  is still able to transfer a sufficient amount of energy to recharge the rechargeable batteries of the VAD  750 . This feature of the TETS  700  according to embodiments of the present invention may not be possible in conventional systems that use only a single planar resonator for each of a transmitter and a receiver. In a conventional system, the planar resonators in the transmitter and the receiver may be oriented parallel to each other when the patient is laying flat on the patient&#39;s back on the bed as in  FIG. 8A  to effect energy transfer between the planar resonators. However, as the patient turns and lies on the patient&#39;s side as in  FIG. 8B , the planar resonators of such a conventional system may then be oriented orthogonal to each other, which would result in zero or close to zero amount of energy being transferred. If the patient falls asleep and remains in this position for an extended period of time, the batteries of the VAD  750  may deplete completely to cause the VAD  750  of a conventional system to fail. This poses a risk to the patient if the VAD  750  losses power completely. Hence, by using a non-planar resonator in a TEST  700  in accordance with embodiments of the present invention, this risk to a patient can be minimized by enabling energy transfer in the system regardless of the orientation of the transmitter and receiver resonators relative to each other when the resonators are within an operating distance range of the TETS  700 . 
     Referring back to  FIG. 7A , although not shown, the TETS  700  may include any of the additional components and techniques described above to tune or detune the system to modulate the power delivered to the VAD  750  according to the needs and performance requirements of the patient. In an exemplary embodiment, the transmitter and receiver units  701  and  711  each include additional communications circuitry to establish a wireless communications channel between the receiver unit  701  and the transmitter unit  711  to communicate system information between the units. The system information can be used to adjust the tunable components of the system to modulate the power delivered to the receiver unit  801 . The wireless communications channel can be a radio frequency signal or other wireless signal transmissions that are safe for the human body. 
       FIG. 7B  illustrates a different embodiment of a TETS  760  in accordance with the present invention. In this embodiment, the rechargeable power storage device  716  is implanted near the back shoulder of a patient as shown. Because the receiver unit  711  is implanted near the back shoulder of a patient, the geometry of a planar resonator may be more suitable for implantation at that location than a non-planar resonator. As a result, instead of using a non-planar resonator in the receiver unit  711 , the non-planar resonator is used in the transmitter unit  701  instead. Hence, in this embodiment, it is the transmitter unit  701  that has a non-planar transmitter resonator  762  to enable omni-orientation wireless energy transfer to a planar resonator  772  in the receiver unit  711 . Because of the symmetry in the resonance coupling of the resonators, as long as at least one of the receiver or transmitter resonators is a non-planar resonator that spans a non-degenerate two-dimensional surface, power can be transmitted at a given operating distance regardless of one resonator&#39;s orientation relative to the other resonator. The operation of the TETS  760  is similar to those of the TETS  700  of  FIG. 7A , and hence a description of the operation of the TETS  760  will not be repeated here. 
     The TETS in embodiments of the present invention can be advantageous in other scenarios as well. For example, a transmitter unit can be integrated into a wearable vest that a patient can wear, and the transmitter unit can be powered by a battery embedded in the vest. The patient can move around freely and go about the patient&#39;s daily activities while charging or powering an implanted VAD without requiring wires to connect the vest to the body and without the patient having to worry about the particular alignment of the vest to the body. A transmitter unit can be integrated into an office desk or other office furniture. As a patient moves around within the vicinity of the office desk or other office furniture at work, an implanted VAD can remain fully charged throughout the work day. A transmitter unit can also be integrated into an automobile such that a driver or a passenger with an implanted VAD can freely move around and adjust the person&#39;s sitting position within the automobile while charging an implanted VAD. Transmitter units can also be integrated into an airplane to allow a traveler to freely move around the cabin on long international flights while charging an implanted VAD. 
       FIG. 9A  shows a method for omni-orientational wireless energy transfer according to one embodiment of the present invention. At step  902 , a transmitter resonator including a transmitter coil coupled to a power supply is provided. In this particular embodiment, the transmitter coil that is part of the transmitter resonator is a coil that spans a non-degenerate two-dimensional surface with at least one concave portion. When the power supply drives an electrical current through the transmitter coil, magnetic flux is generated around the transmitter resonator. At step  904 , a receiver resonator is placed at a separation distance from the transmitter resonator. The receiver resonator includes a receiver coil that is coupled to an electrical load. When the separation distance is within an operating range, magnetic flux generated from the transmitter unit induces an electrical current to flow in the receiver coil of the receiver resonator. The electrical current flowing through the receiver coil can then be used to power the electrical load that is coupled to the receiver coil. Hence, power is transferred wirelessly from the transmitter resonator to the receiver resonator. By using a non-planar resonator that spans a surface area occupying three spatial dimensions in the transmitter resonator, magnetic flux is generated over a wider range of directions to enable a receiver resonator to intercept more of the generated magnetic flux in a wider range of spatial orientations relative to the transmitter resonator. 
     In a different embodiment, for example, the method  950  as shown in  FIG. 9B , instead of the transmitter resonator, it is the receiver resonator that spans a non-degenerate two-dimensional surface with at least one concave portion. In both instances, the principles of the operations for omni-orientational wireless energy transfer are the same. When at least one of the resonators spans a non-degenerate two-dimensional surface such as those surfaces disclosed herein, energy can be transferred between the resonators regardless of the orientation of one resonator relative to the other resonator. While the flow diagrams in the figures show a particular order of operations performed by certain embodiments of the invention, it should be understood that such order is exemplary (e.g., alternative embodiments may perform the operations in a different order, combine certain operations, overlap certain operations, etc.). 
     Although some specific embodiments of the invention has been described in relation to a transcutaneous energy transfer system (TETS) to wirelessly transmit power to an implanted medical device through the skin of a patient, the advantages of using a non-planar resonator in a wireless energy transfer system can be applied to other applications as well. For instance, embodiments of the present invention can be used to power or recharge robots that are free to roam around a manufacturing facility, where the orientation of a receiver unit in the robots may vary relative to a stationary transmitter unit installed in the manufacturing facility. In general, embodiments of the present invention can be used in any application to wirelessly transfer energy between two objects, where at least one of the objects is free to move around such that the relative orientation of the two objects may change. 
     While the invention has been described in terms of several embodiments, those skilled in the art will recognize that the invention is not limited to the embodiments described, can be practiced with modification and alteration within the spirit and scope of the appended claims. The description is thus to be regarded as illustrative instead of limiting.