Patent Publication Number: US-2017370365-A1

Title: Method of operating a rotor of a blood pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of application Ser. No. 14/510,449, filed Oct. 9, 2014 (pending) which is a divisional of application Ser. No. 13/827,645, filed Mar. 14, 2013 (now U.S. Pat. No. 8,882,477), the disclosures of which are incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to blood pumps and, more specifically, to blood pumps having magnetically levitated and driven rotors. 
     BACKGROUND 
     The human heart is the muscle that is responsible for pumping blood throughout the vascular network. Veins are vessels that carry blood toward the heart while arteries carry blood away from the heart. The human heart consists of two atrial chambers and two ventricular chambers. Atrial chambers receive blood from the body and the ventricular chambers, which include larger muscular walls, pump blood from the heart. A septum separates the left and the right sides of the heart. Movement of the blood is as follows: blood enters the right atrium from either the superior or inferior vena cava and moves into the right ventricle. From the right ventricle, blood is pumped to the lungs via pulmonary arteries to become oxygenated. Once the blood has been oxygenated, the blood returns to the heart by entering the left atrium, via the pulmonary veins, and into the left ventricle. Finally, the blood is pumped from the left ventricle into the aorta and the vascular network. 
     For the vast majority of the population, the events associated with the movement of blood happen without circumstance. However, for many people the heart fails to provide adequate pumping capabilities. These heart failures may include congestive heart failure (commonly referred to as heart disease), which is a condition that results in any structural or functional cardiac disorder that impairs the ability of the heart to fill with or pump blood throughout the body. Presently, there is no known cure for heart disease and long-term treatment is limited to a heart transplant. With only a little over 2,000 patients receiving a heart transplant each year, and over 16,600 more on the waiting list for a heart, there is a persisting need for a cure or at the minimum a means of improving the quality of life of those patients on the waiting list. 
     One such means of bridging the time gap while awaiting a transplant is a circulatory assist system. Circulatory assist systems may also be utilized as a destination therapy for individuals not eligible for a heart transplant. These systems, originally envisioned over thirty years ago, provide assistance to the heart by way of a mechanical pump. In this way, blood is circulated throughout the vascular network despite the diseased heart tissue. Traditionally, these circulatory assist systems include an implantable or extracorporeal pump, a controller (internal or external), and inflow and outflow tubes connecting the pump to the heart and the vascular network. Food and Drug Administration (FDA) approved circulatory assist systems can partially relieve symptoms of breathlessness and fatigue associated with severe heart failure and drastically improve quality of life. 
     The wait time for receiving a heart transplant may be substantial. Therefore, circulatory assist systems, and particular the pumps driving them, must be designed for longevity. Furthermore, it is desirable to provide an ideal and advantageous flow of blood therethrough without damaging the blood. There is therefore a need in the art for a pump and a circulatory assist system which experiences low amounts of inter-component friction during operation and causes less damage to blood than other pumps known in the art. 
     SUMMARY 
     In one embodiment, a device for pumping blood is provided and comprises a housing having a distal end adapted to be coupled to a catheter, a proximal end having an outlet, and a tubular body extending between said first distal and proximal ends along an axis. The device further comprises a rotor rotatably disposed within the housing, a first magnetic bearing operative to levitate the rotor along the axis within the housing, and a second magnetic bearing controlling a radial position of the rotor, and a third magnetic bearing controlling a radial position of the rotor. 
     In another embodiment, a device for pumping blood is provide and comprises a housing having a distal end adapted to be coupled to a catheter, a proximal end having an outlet, and a tubular body extending between the distal and proximal ends along an axis. The device further comprises a rotor rotatably disposed within the housing and a first magnetic bearing further comprising first and second permanent magnets and operative to levitate the rotor to an axial position within the housing. A second magnetic bearing is included and further comprises a plurality of vertically arranged pairs of electromagnetic coils and a pole structure coupled to the rotor. The second magnetic bearing is configured to change or maintain a rotational frequency of the rotor. A third magnetic bearing is provided and further comprises the plurality of vertically arranged pairs of electromagnetic coils and the first permanent magnet. The third magnetic bearing is configured to change or maintain a radial position of the rotor. The device further comprises a Hall Effect sensor sensing the radial position and rotational frequency of the rotor and a controller operably coupled to the Hall effect sensor and configured to communicate with the coils to change or maintain the radial position and rotational frequency of the rotor. 
     A method of operating a rotor of a blood pump is provided and comprises levitating the rotor within a tubular body of the blood pump using a first magnetic bearing, rotating the rotor about an axis within the tubular body using a second magnetic bearing, and maintaining the radial position of the rotor relative to the axis using the third magnetic bearing. 
     An alternative method of operating a rotor of a blood pump is provided and comprises levitating the rotor within a tubular body of the blood pump using a first magnetic bearing, the first magnetic bearing comprising a first permanent magnet and a second permanent magnet, the second permanent magnet operatively coupled with the rotor. The method further comprises commencing rotation of the rotor within the tubular body using a second magnetic bearing. The second magnetic bearing further comprises a plurality of vertically arranged pairs of coils circumferentially disposed around the housing and a pole structure coupled to the rotor. A current is sent to at least one of the pairs the coils, thereby magnetizing the coil in a first pole direction and urging an oppositely magnetized portion of the pole structure towards the coil. The method further comprises sensing a rotational frequency and a radial position of the rotor. When a sensed rotational frequency is below a threshold level, the method further comprises sending a current to at least a portion of the pairs of coils, thereby further rotating the rotor. When the radial position of the rotor deviates from a threshold position about the axis, sending a current to a pair of coils, thereby urging the rotor towards the axis. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagrammatic view of an exemplary method of accessing a cavity of the heart. 
         FIG. 2  is a side view in partial cross-section of one embodiment of a blood pump as described herein. 
         FIG. 3  is a diagram of magnetic fields associated with the pump of  FIG. 2 . 
         FIG. 4  is a side view of the pump of  FIG. 2  including supplementary electromagnetic coils. 
         FIG. 5  is a side cross-sectional view of the pump of  FIG. 2 . 
         FIGS. 6A through 6C  are top views of a schematic representation of the device of claim  1  showing the functionality of the coils. 
         FIG. 7  is a top view of a schematic representation of an alternative embodiment of a device as described herein. 
         FIG. 8  is a schematic diagram of the controllers, sensors and circuitry of the pump of  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Turning now to the figures, and in particular to  FIG. 1 , an implanted circulatory assist system  10  is shown within a chest cavity of a patient  14  with the heart  12  shown in cross-section. For illustrative purposes, certain anatomy is shown, including a right atrium  16 , a left atrium  18 , a right ventricle  20 , and a left ventricle  22 . Blood from the left and right subclavian veins  24 ,  26  and the left and right jugular veins  28 ,  30  enters the right atrium  16  through the superior vena cava  32  while blood from the lower parts of the body enters the right atrium  16  through the inferior vena cava  34 . The blood is pumped from the right atrium  16 , to the right ventricle  20 , and to the lungs (not shown) to be oxygenated. Blood returning from the lungs enters the left atrium  18  via pulmonary veins and is then pumped into the left ventricle  22 . Blood leaving the left ventricle  22  enters the aorta  38  and flows into the left subclavian artery  40 , the left common carotid  42 , and the brachiocephalic trunk  44  including the right subclavian artery  46  and the right common carotid  48 . 
     With respect to the implanted circulatory assist system  10 , two cannulae extend between the vascular network and a pump  50 , which may be any implantable or extracorporeal pump that may be radially- and/or axially-driven. Those skilled in this art, however, recognize that other types of pumps may be used in other embodiments but may include pumps such as those described in U.S. patent application Ser. No. 11/627,444, published as 2007/0197854, which is incorporated herein by reference in its entirety. 
       FIG. 2  illustrates the pump  50  in cross-section. In particular the pump  50  includes an elongate pump housing  52  having a first end  53  coupled to a transition portion of the catheter (shown in hidden lines) and a tubular body  56  extending from the first end  53  along a longitudinal axis  58  of the pump housing  52 . The first end  53  may be secured to the transition portion by rigid barbs  60 , adhesive, or any other coupling technique. In one embodiment of the present invention, the tubular body  56  is defined by a pump inlet  62  at the first end  53  and a pump outlet  64  at the second end  55 . There is a shroud  81  ( FIG. 5 ) encasing at least a portion of the housing  52  and the magnetic components, as disclosed below. Blood flows in the inlet  62  at the first end  53  in the direction of arrows  57  and out of the outlet  64  at second end  55 . 
     Various components of a blood pump  50  are housed within the pump housing  52  to draw blood from the catheter  54  into the tubular body  56 . For example, the blood pump  50  may comprise an impeller  66  and associated impeller blades  68  positioned within the tubular body  56 . It will be appreciated that the impeller  66  is only schematically illustrated and may take many forms, including a form as generally shown herein. The pump  50  may further include a support pin  70  to maintain the axial position therein of the impeller  66  prior to levitation. Notably, as disclosed herein, “impeller” and “rotor” are used interchangeably and are meant to refer to reference number  66 . 
     The impeller  66  further includes a rotor magnet  74  having dimensions suitable such that the impeller  66  may reside and rotate freely within the tubular body  56 . The rotor magnet  74  is a dipole configured to be levitated within the tubular body  56 . In one exemplary embodiment of the present invention, the rotor magnet  74  may be 6 mm in diameter and 3 mm in height for pumps configured to operate as left ventricular assist pumps; yet, it would be readily appreciated that the size of the rotor magnet  74  may vary and depend, at least in part, on the size of the impeller blades  68  and a desired blood flow rate. The impeller blades  68  are configured to prevent damage to the blood traveling through the tubular body  52 . 
     In one embodiment, the levitation of the impeller  66  is accomplished due to a first magnetic bearing. More specifically, the first magnetic bearing includes the ring magnet  76  and the rotor magnet  74  which, in one embodiment, are both permanent magnets. The ring magnet  76  and the rotor magnet  74  are configured such that the oppositely magnetized sides are facing one another. For example, the north pole side  78   a  of the rotor magnet  74  faces the south pole side  80   a  of the ring magnet  76 . Moreover, the configuration of the magnets  74 ,  76  may be chosen such that the interaction between the magnets  74 ,  76  creates an asymmetrical potential energy well, as shown by the magnetic field diagram in  FIG. 3 . The radial potential is shown in the  FIG. 3  as a generally hemispherical shape  73 . The asymmetric potential well is created due to the presence of ring magnet  76  opposing only one side the rotor magnet  74 . The potential well creates stability in the axial direction. Because the depth of the well correlates with the amount of instability in the radial direction, the well may be designed in order to provide the least radial instability while also providing a proper amount of axial stability. In one embodiment, the rotor magnet  74  includes a 6 mm diameter and a 3 mm height. The ring magnet  76  includes an inner diameter of 8 mm, an outer diameter of 14 mm and a height of 3 mm. The potential well creates interactions between the respective magnetic fields of each magnet  74 ,  76 , which results in levitation of the rotor magnet  74  relative to the ring magnet  76 . Moreover, the asymmetric potential well may be configured to provide a levitation force approximately equal and opposite to the fluid forces resulting from the blood flow and pumping action (i.e., rotation of the impeller  66 ). This provides several benefits, including simplification of design. In order to vary the levitation force and the size of the asymmetric potential well, the relative sizes of the rotor and ring magnets  74 ,  76  may be altered. 
     Therefore, while the impeller  66  may be levitated by the potential well ( FIG. 3 ), the rotor magnet  74  and thus the impeller  66  may inherently be unstable in the radial direction unless the rotor magnet  74  is rotating. Even then, the rotor magnet  74  may be unstable such that the rotor magnet  74 , and thus the impeller  66 , may tilt or move radially away from axis  58 . The rotation of the rotor magnet  74  is operative to provide stability in the radial direction of the rotor magnet  74  and thus the impeller  66 . The rotation of rotor magnet  74  is effectuated at least in part by a second magnetic bearing, which is discussed in more detail below. The embodiment shown in  FIGS. 2, 4 and 5  shows the ring magnet  76  being situated distally relative to the rotor magnet  74 . However, in other embodiments, it may be appreciated that the ring magnet  74  may be situated proximally relative to the rotor magnet  74  and still provide the potential well that is operative to levitate the rotor magnet  74 , and thus the impeller  66 , within the tubular body  56 . 
     Alternatively, the levitation of the impeller  66  may be accomplished by use of alternative materials, such as diamagnets. As understood by a person skilled in the art, diamagnets are non-ferrous materials that when placed in a magnetic field, exhibit a repulsion force towards the magnetic source. Therefore, in a preferred embodiment, at least one of the rotor magnet  74  or the ring magnet  76  may comprise a diamagnet. Preferably, in that embodiment, the rotor magnet  74  is a diamagnet while the ring magnet  76  is a permanent magnet as disclosed herein. 
     The magnetic portion or pole structure  72  may further include two or more poles on both top and bottom edges  82 ,  84  of the rotor magnet  74 . In one embodiment, the top and bottom edges  82 ,  84  each include a four pole structure  72 , which may be constructed by magnetic coding of the edges of the rotor magnet  74  by methods such as those taught in U.S. Pat. No. 7,800,471, issued on Sep. 21, 2010, and entitled FIELD EMISSION SYSTEM AND METHOD, such magnetic coding services commercially-available from Correlated Magnetics Research, LLC (New Hope, Ala.). Alternatively, as shown in  FIGS. 5 and 6A -C, the structure may include physically embedded miniature sub-magnets, or pill magnets  88   a ,  88   b  within the impeller  66 . In any event, the pole structure is positioned such that the resultant magnetic field is oriented to oppose the magnetic field of the rotor magnet  74  or radially outwardly from the rotor magnet  74 . The pole structure  72  may be provided to function as one part of a magnetic bearing that interact with the coils  92  as described herein. The pole structure  72  may alternatively include a magnet including several poles, such as a quadropole magnet, which may then be attached or coupled to the rotor magnet such that it is also embedded in the impeller  66 . 
     One embodiment of the pole structure  72  is shown in  FIGS. 5 and 6A -C. In the embodiment shown in  FIGS. 5 and 6A -C, there are four sub-magnets, or pill magnets  88   a ,  88   b  on the top and the bottom  82 ,  84  of the rotor magnet  74 , respectively. In alternative embodiments, however, there may be more pill magnets  88   a ,  88   b  on each of the top and the bottom  82 ,  84  such as six or eight. Alternatively, there may be less, such as two. Preferably, as shown in  FIG. 5 , the pill magnets  88   a ,  88   b  are embedded in the impeller  66 . The pill magnets  88   a ,  88   b  are preferably cylindrically shaped and axially magnetized ( FIGS. 6A-C ). In another embodiment, the pill magnets  88   a ,  88   b  may be diametrically magnetized. In yet another embodiment, the pill magnets may be a shape other than a cylinder, such as a triangular or rectangular prism, or another shape. The pill magnets  88   a ,  88   b  may be situated such that the direction of magnetization  87  of the pill magnets  88   a ,  88   b  is tangent to the direction  91  of rotation of the impeller  66 . However, depending on the shape and configuration, the direction of magnetization  87  of the pill magnets  88   a ,  88   b  may be different. 
     The pump  50  further includes a plurality of electromagnetic coils  92   a ,  92   b  disposed on or adjacent the housing  52 . In a preferred embodiment, as shown in  FIGS. 2, 4 and 5 , the pump  50  includes four vertically arranged, equally circumferentially spaced pairs of electromagnetic coils  92   a ,  92   b . Each pair of coils  92   a ,  92   b  includes upper and lower coils  92   a ,  92   b , respectively. Each vertically arranged pair of coils  92   a ,  92   b  is in series and counterwound such that, for example, upper coil  92   a  of a pair is wound in a counterclockwise direction and the lower coil  92   b  is wound in a clockwise direction. However, in an alternative embodiment, the coils may be wound in alternative configurations. For example, coils  92   a ,  92   b  could also be wound the same direction and wired so that the current flows in one direction through the top and the opposite direction in the bottom, thereby producing opposite magnetic fields. The coils  92   a ,  92   b  are preferably encased with housings  93  comprising a non-magnetic material as not to interfere with the functionality of the pump  50 . In an alternative embodiment, however, there may be less than four vertically arranged pairs of coils  92   a ,  92   b . For example, there may be more than four pairs, or less than four pairs. There may be supplemental coils  94 , as shown in  FIG. 4 . 
     In one embodiment, the coils  92   a ,  92   b  may comprise an iron core (not shown) to strengthen the magnetic field emitted by the coils  92   a ,  92   b . The coils  92   a ,  92   b  and the pole structure  72 , such as the pill magnets  88   a ,  88   b , may be the second magnetic bearing that effectuates the rotation of the rotor magnet  74 , and thus the impeller  66 . The rotation of the rotor magnet  74  and impeller  66  provides for axial stability of the levitated rotor magnet  74 , and thus the impeller  66 . The rotation-of the rotor magnet  74  and thus the impeller  66  are described in more detail below. The device further includes a third magnetic bearing which is configured to control a radial position of the rotor  66 . As described in further detail below, the third magnetic bearing includes the rotor magnet  74  and the coils  92 ,  92   b.    
     Preferably, the coils  92   a ,  92   b  are wound from a material, such as copper, capable of conducting electricity such that a current will travel through the coils  92   a ,  92   b  and energize the coils  92   a ,  92   b , thereby magnetizing the coils  92   a ,  92   b . Coils  92   a ,  92   b  receiving current and thereby being magnetized may be referred to herein as “energized” or “magnetized.” The direction of the current flow through the coils  92   a ,  92   b  determines the direction of magnetization, i.e., whether the coils will be magnetized as a south pole or a north pole. For example, in a preferred embodiment, the upper coils  92   a  of a pair are wound in the clockwise direction such that when the upper coils  92   a  are energized, the upper coils  92   a  are magnetized in the north pole direction, as indicated by “N” on  FIG. 6A-C . Similarly, in a preferred embodiment, the lower coils  92   b  are wound in the counterclockwise direction such that when the lower coil  92   b  is energized, the lower coils  92   b  are magnetized in the south pole direction (not shown). 
     Because each vertically arranged pair of coils  92   a ,  92   b  in a preferred embodiment are in series, when the coils  92   a ,  92   b  are energized when receiving a current, the upper coils  92   a  are magnetized in the north pole direction and the lower coils  92   b  are magnetized in the south pole direction. However, as will be recognized by persons skilled in the art, the current may be sent in different directions to the upper and lower coils  92   a ,  92   b , resulting in different magnetization directions of each pair of coils  92   a ,  92   b . Ultimately, changing the direction of the current directed into the coils  92   a ,  92   b  changes whether a coil  92   a ,  92   b  is magnetized in the north or south pole direction. 
     In one embodiment, each coil  92   a ,  92   b  comprises a #42 AWG copper wire with approximately 750 turns per coil  92   a ,  92   b , made by Precision Ecowind, Inc. of North Fort Myers, Fla. With this diameter and amount of turns, the resistance per coil  92   a ,  92   b  is approximately 50Ω. However, the coil  92  may comprise a different diameter, material, and amount of turns, depending on the desired characteristics of the coils  92   a ,  92   b , which ultimately depend on the desired characteristics of the blood pump  50  (i.e., desired rotational frequency of the blood pump  50  or required force to radially align the impeller  66 ). The current sent to the coils  92   a ,  92   b  to thereby energize the coils  92   a ,  92   b  may be between approximately 0 mA and 200 mA and depends on the characteristics of the coils  92   a ,  92   b  described herein as well as the desired characteristics of the blood pump  50 . 
       FIGS. 6A  through C show an embodiment of the pump  50  showing the upper coils  92   a  and upper pill magnets  88   a  of the pump  50 . A second magnetic bearing is utilized in order to begin rotation of the impeller  66 . A current may be sent to diametrically opposed pairs of coils  92   a ,  92   b  simultaneously by the controller ( FIG. 8 ), thereby magnetizing the pairs of coils  92   a ,  92   b  in a certain direction. The magnetized coils  92   a ,  92   b  then either attract or repel one or more of the pill magnets  88   a ,  88   b , depending on the rotational location of each of the pill magnets  88   a ,  88   b  relative to the magnetized coils  92   a ,  92   b . In order to clarify the functionality of the coils  92   a ,  92   b  during the operation of the device, the specific upper coils  92   a  are labeled  1 ,  2 ,  3  and  4 , while the specific upper pill magnets  88   a  are labeled A, B, C, and D in  FIG. 6A-C . Preferably, to begin rotation, coils  1  and  2  are energized and are thereby magnetized in the north direction. The south poles of pill magnets B, A thereby become attracted to, and are urged towards, coils  1  and  2 , respectively. Because of the diametrically opposed configuration of the energized coils  1 ,  2 , the attractive forces are essentially balanced. Furthermore, because the pill magnets B, A are embedded in the impeller  66 , and the rotation of pill magnets B, A causes the impeller  66  to begin to rotate in the clockwise direction  91 . The interaction between the lower coils  92   b  and lower pill magnets  88   b  would correspond to the interaction between the upper coils  92   a  and upper pill magnets  92   a . For example, in a preferred embodiment as disclosed above, the lower coils  92   b  are magnetized in an opposite pole direction of the upper coils  92   a . Therefore, in order to attract or repel the lower pill magnets  88   b  as discussed herein, the configuration of the lower pill magnets  88   b  may need to be reversed such that the north poles and south poles are facing opposite directions as shown in  FIGS. 6A-C . 
     The Hall Effect sensors  96  sense the magnetic fields of the pole structure  72  (such as the pill magnets  88   a ,  88   b ) as well as the rotor magnet  74 . With this magnetic field information, the Hall Effect sensors  96  may sense the radial and axial positions of the pill magnets  88   a ,  88   b , as well as the rotational frequency of the pill magnets  88   a ,  88   b , and thus the impeller  66 . The Hall Effect sensors  96  essentially determine whether the rotation frequency is at, below, or above a threshold rotational frequency. Further, the Hall Effect sensors  96  communicate with the controller  98  ( FIG. 8 ) to selectively energize certain coils  92   a ,  92   b  to rotate the impeller  66 . Preferably, the Hall Effect sensors  96  use the rotational frequency and position information to communicate with the controller  98  as to which coils  92   a ,  92   b  to energize and/or de-energize. Using the upper pill magnets and coils  88   a ,  92   a  by way of example, as a pill magnet  88   a  approaches a coil  92   a , the controller  98  may de-energize the coil  92   a  until the pill magnet  88   a  rotates past the coil  92   a . More specifically, in  FIG. 6B , pill magnets B, A have rotated past de-energized coils  3  and  4 , respectively. The north poles of pill magnets B, A are facing the coils  3 ,  4 , respectively. After the Hall Effect sensor  96  has sensed that the pill magnets B, A have rotated past coils  3  and  4 , the sensor  96  communicates with the controller  98  regarding the position of pill magnets B and A. The controller  98  then energizes coils  3 ,  4  in the north direction and pill magnets B, A are repelled away from coils  3 ,  4 . Because of the diametrically opposed configuration of the energized coils  3 ,  4 , the repelling forces are essentially balanced and the impeller  66  continues to rotate about the axis  58 . The upper coils  92   a  as shown in  FIGS. 6A-C  may be magnetized in the north direction. However, in alternative embodiments, the selectively energized coils  3 ,  4  may be magnetized in the south direction. Therefore, in that alternative embodiment, the configuration of the pill magnets B, A may need to be altered in order for the repelling and/or attractive forces to occur as coils  3 ,  4  are selectively energized. The orientation and configuration of the pill magnets  88   a ,  88   b  relative to the coils  92   a, b  may be determined by the desired rotational direction. 
     Moreover, it may be appreciated that in embodiments with an alternative configuration or different amount of coils  92   a ,  92   b  and/or pill magnets  88   a ,  88   b , for example, the rotational frequency may be altered or maintained in similar manner such that the forces on the rotor  66  are balanced, thus causing rotation of the rotor  66 . For example, as discussed above, the embodiment as shown in  FIGS. 6A-C  includes four vertically arranged pairs of coils  92   a ,  92   b . One manner of maintaining or altering the rotation of the rotor  66  is, as discussed, sending a current to diametrically opposed pairs of coils  92   a ,  92   b  such that the magnetic forces from each coil  92   a ,  92   b  may be balanced along the axes  100 ,  102  and thus rotation of the rotor  66  occurs. However, which coils  92   a ,  92   b  to energize in order to provide balanced magnetic forces upon the rotor or impeller  66  depends on the configuration and number of the coils  92   a ,  92   b  as well as the configuration of the pole structure  72 , such as the number of pill magnets  88   a ,  88   b . In an alternative embodiment, rather than including a pole structure  72 , such as pill magnets  88   a ,  88   b , for rotation, the coils  92   a ,  92   b  may interact with the dipole moment of the impeller magnet  74  in order to effect rotation of the rotor magnet  74 , and thus the rotor  66 . 
     The Hall Effect sensors  96  also use the magnetic field information of the pole structure  72  (such as the pill magnets  88   a ,  88   b ) and the rotor magnet  74 , to sense the radial position of the impeller  66 , relative to the axis  58  of the pump  50 . To control radial position of the rotor  66 , a third magnetic bearing is utilized. In a similar manner as with respect to the rotational frequency discussed hereinabove, the Hall Effect sensors  96  communicate with the controller  98  to selectively energize certain coils  92  to alter the radial position of the rotor magnet  74 , and thus the impeller  66 , relative to the axis  58 . As described herein, “off-axis” may be used to characterize the position or movement of the impeller  66  where the impeller  66  is positioned radially away from the axis  58  along axes  100  and  102 , which are transverse to the axis  58  of the blood pump  50 . Moreover, axes  100 ,  102  are transverse to one another. Which coils  92   a ,  92   b  are energized depends on the off-axis position of the impeller  66 . 
     As shown in  FIG. 6B , the radial position of the impeller  66  is characterized by an off-axis position along a single axis  100 , or a generally left direction, due to the radial instability caused by the asymmetric potential well ( FIG. 3 ). The Hall Effect sensors  96  may sense that the rotor  66  is not in a threshold position, the threshold position being characterized by the center axis  67  of the rotor  66  being essentially aligned with axis  58 . Therefore, it may be desirable to alter the position of the impeller  66  in the generally right direction along axis  100 . To accomplish this positional alteration, the Hall Effect sensors  96  communicate with the controller  98  to energize coil  4  in the north direction, as indicated by arrows  97 . Because each pair of coils  92  is wired in series and counterwound, the lower coil  92   b  below coil  4  would be magnetized in the south direction. The rotor magnet  74 , as discussed above, is oriented such that the north pole side  80   a  is essentially adjacent the upper coils  92   a  (i.e. coil  4 ), while the south pole side  80   b  is essentially adjacent the lower coils. The north-direction magnetization of coil  4  thereby repels the north pole side of  80   a , while the south-direction magnetization of the lower coil  92   b  associated with coil  4  repels the south pole side  80   b . Therefore, the forces from coil  4  and the associated lower coil  92   b  are balanced angularly with respect to axis  58 . Moreover, energizing the coils generates forces that provide stability in the radial direction. The rotor magnet  74 , and thus the impeller  66 , are thereby urged towards the axis  88  along axis  100 . It may be appreciated that, instead of energizing coil  4  to repel rotor magnet  74  away from coil  4  and towards axis  58  along axis  100 , coil  3  may be energized in a direction to attract rotor magnet  74  towards coil  3  (and the associated lower coil  92   b ), thereby urging the impeller  66  towards the axis  58 . Moreover, where extra force may be needed to urge the rotor  66  in the direction of axis  58 , coils  3  and  4  may be energized such that coil  3  attracts the impeller towards axis  58  and coil  4  repels the impeller  66  towards axis  58 . It may be appreciated that, due to the generally hemispherical shape of the potential well, the impeller magnet  74  may be urged slightly in the axial direction during radial positioning by the coils  92   a ,  92   b.    
     As shown in  FIG. 6C , the radial position of the impeller  66  is characterized by a movement in the left and down directions along axes  100  and  102 , respectively, and radially away from the axis  58 . Therefore, it may be desired to move the impeller  66  in the up and right directions towards the axis  58 . To accomplish this positional alteration, the Hall Effect sensors  96  communicate with the controller  98  to energize adjacent coils  1  and  4  in the north pole direction, as indicated by arrows  97 , and the associated lower coils (not shown) in the south pole direction. Similar to the description of  FIG. 6B  above, magnetizing coils  1  and  4  and the associated lower coils (not shown) thereof urges the rotor magnet towards axis  58  due to the repelling force between the energized coils and the rotor magnet  74 . 
     Altering or maintaining the radial position of the rotor  66  as described herein with respect to  FIGS. 6B and 6C  also assists in counteracting tilt of the rotor  66 . The natural tendency of the south pole side  78   a  of the rotor magnet  74  is to be attracted to the north pole side  80   a  of the ring magnet  76 . Therefore, as the rotor  66  rotates, the rotor  66  may experience tilt, wherein a center axis  67  of the rotor  66  is angularly displaced relative to the axis  58 . The device and method of counteracting off-axis radial movement as described herein is also adapted to counteract tilt. In an alternative embodiment, it may be advantageous to provide additional coils  92   a ,  92   b  for alteration and/or maintenance of rotational frequency, radial position and tilt. In one embodiment, for example, the device may be configured to energize one or more coils  92  from one of the upper or lower sets  92   a ,  92   b  in order to urge the rotor  66  angularly towards the axis  58 . 
     It is appreciated that the manners, frequency and continuity of energizing the coils  92  and the directions of magnetization resulting therefrom may be altered depending on the number of and configurations of the pill magnets  88   a ,  88   b  and coils  92   a ,  92   b . The descriptions hereinabove of altering or maintaining the rotational frequencies and radial positions of the impeller  66 , as well as counteracting tilt, are simply examples and are not meant to limit the device and method described herein to only those examples. 
     One alternative embodiment is shown in  FIG. 7 . For example, where there are more than four vertically arranged pairs of coils  92   a ,  92   b , the amount of ways that the coils  92   a ,  92   b  may be energized to alter or maintain the rotational frequency, radial position, and tilt is increased. By way of example, in one embodiment, there may be eight pairs of vertically arranged coils  92   a ,  92   b . This embodiment may effectuate the control of rotational frequency, radial position and tilt as described hereinabove. As in previously disclosed embodiments, which coils  92   a ,  92   b  are energized depends on the off-axis position of the impeller or rotor  66 . For example, as shown in  FIG. 7 , the impeller  66  has moved off-axis in the downward direction along axis  102 . Because of the additional coils, there is an increased amount of ways in which the rotor  66  may be urged towards the axis  58 . For example, depending on the positions, and magnetization configuration of the rotor magnet  74 , the coils  2 ,  5  and  8  may be energized and attract, or essentially pull the rotor magnet  74 , thus urging the rotor in the direction of the axis  58  along axis  102 . Coils  1 ,  6  and  7  may be concurrently energized such that it provides a magnetic force such that it repels, or pushes, the impeller magnet  74 . This repulsive force from coils  1 ,  6  and  7  may thereby balance the force from coils  2 ,  5  and  8  directing the rotor along axis  102 , thus maintaining the rotor rotating about axis  58 . It is appreciated, as disclosed previously, that the lower coils (not shown) associated with coils  1 ,  2 ,  5 ,  6 ,  7  and  8  may be oppositely magnetized and simultaneously energized with the associated upper coils  1 ,  2 ,  5 ,  6 ,  7  and  8 , thereby preferably balancing the magnetic forces from the coils. Moreover, as previously discussed, the radial alteration and maintenance of the rotor  66  by the coils  92   a ,  92   b  also assists in counteracting tilt of the rotor  66 .  FIGS. 6A-C  and  7  are provided as exemplary embodiments. These figures and the disclosure regarding these figures are provided as an example of just a few manners in which the rotational frequency, radial position and tilt may be altered or maintained using the device as disclosed herein. It is appreciated that the amount of coils  92   a ,  92   b , and the pole structure  72 , such as the amount pill magnets  88   a ,  88   b , may vary. Furthermore, the size, orientation, position and characteristics of the rotor magnet  74  may be varied. Moreover, the manner in which the coils  92   a ,  92   b  are controlled in order to alter or maintain the rotational frequency, radial position and tilt may also vary. It is further appreciated that when varying the amount of coils  92  and the pole structure  72 , additional controllers  98  ( FIG. 8 ) and Hall Effect sensors  96  may be required. 
     Due to the loading on the impeller, it may also be appreciated by persons skilled in the art that the flow of blood through the housing  52  past the impeller, as well as the rotation of the impeller  66 , may cause the impeller  66  to oscillate in the axial direction along axis  58 . The Hall Effect sensors  96  are configured to detect the oscillation of the impeller  66 . One of the embodiments described herein may be configured to counteract oscillation of the impeller  66 . On the other hand, additional coils  92   a ,  92   b  may be provided in order to counteract the axial oscillation of the impeller  66 . 
     In another alternative embodiment, a blood pump  50  includes a supplemental set of coils  94  (shown in phantom in  FIG. 4 ) circumferentially disposed about the housing  52 . Preferably, the supplemental coils  94  comprise four coils  94  equally circumferentially spaced, wherein each secondary coil  94  is offset ninety degrees from adjacent pairs of vertically arranged coils  92 . In the embodiment shown in  FIG. 4 , the vertically arranged pairs of coils  92   a ,  92   b  are utilized for maintaining or altering the radial position of the impeller  66  in the same manner as substantially described herein. The supplemental coils  94  in this embodiment may be utilized for maintaining or altering the rotational frequency of the impeller  66  in the same manner as described substantially herein. Because of the supplemental coils, one or more additional Hall Effect sensors  96  may be required. Alternatively, the vertically arranged pairs of coils  92   a ,  92   b  may be utilized for maintaining or altering the rotational frequency of the impeller  66 , while the secondary coils  94  may be utilized for maintaining or altering the radial position of the impeller  66 . 
     The rotational frequency of the pill magnets  88   a ,  88   b , and thus the impeller  66 , are essentially continuously sensed or monitored by the Hall Effect sensors  96 . The Hall Effect sensors  96  essentially continuously communicate with the controller  98  to energize diametrically opposed sets of coils  92   a ,  92   b , depending on the positions of the pill magnets  88   a ,  88   b , in order to change the rotational frequency of the impeller  66  or to maintain the rotational frequency of the impeller  66 . The required rotational frequency of the impeller  66  depends on certain variables such as the physiological needs of the patient and the dimensions of the impeller  66  and of the blood pump  50 , for example. In one embodiment of a blood pump  50  having an inner housing diameter of 5 mm, a rotational frequency of 17,000 to 32,000 revolutions per minute produces a flow of 0.3 to 2.5 LPM at normal physiological pressures as known to those skilled in the art. The configuration of the blood pump of the aforementioned embodiment allows the blood pump  50  to be smaller than the blood pumps known in the art. The smaller size of blood pump  50  provides a less invasive configuration and can lower costs. 
     More specifically, there is a plurality of Hall Effect sensors  96  circumferentially disposed on the device  50 . Preferably, there are at least two Hall Effect sensors  96  equally circumferentially disposed on the device  50 . As shown in  FIGS. 2, 4 and 5 , the device includes four Hall Effect sensors  96 , where each Hall Effect sensor  96  is essentially aligned with a vertically arranged pair of coils  92   a ,  92   b . Each Hall Effect sensor  96  may be used to sense at least one of the rotational frequency and the radial position of the impeller  66 . However, in one embodiment, one or more of the Hall Effect sensors  96  may be used to sense only the rotational frequency and another portion of the Hall Effect sensors  96  may be used to sense the radial position of the impeller  66 . 
       FIG. 8  shows a control loop  104 . As disclosed herein, and with reference to  FIG. 8 , the Hall Effect sensors  96  communicate with at least one controller  98  in order to selectively energize, or send current through, the coils  92 . In a preferred embodiment, there is a plurality of controllers  98  communicating with the sensors  96 . More preferably, each controller  98  is a proportional-integral-derivative (PID) controller which, based on the information sent it from the Hall Effect sensors  96  based on the magnetic field information, calculates the present, past and future errors. To adjust for the present, past and anticipated future errors (such as off-axis rotation), the PID controllers  96  then selectively energize, or send a current through, one or more coils or pairs of coils  92   a ,  92   b  by way of the H-Bridge  106 . 
     More specifically, the Hall Effect sensors  96  receive the magnetic field information from the rotor magnet  74  and the pole structure  72 . With the magnetic field information from the Hall Effect Sensors, the controller  96  is able to determine the position and rotational frequency of the rotor magnet  74  and the pole structure  72 , and thus the impeller  66 , and compare such with threshold data. The radial position may be sensed in the X and Y positions ( FIG. 8 ), such as along axes  100  and  102  ( FIGS. 6A through 6C ). The threshold data may also be obtained through observance of the current traveling in the coils  92   a ,  92   b . In at least one embodiment, it is observed that as the impeller  66  approaches the threshold position, the current passing through the coils reduces. Observing that phenomenon may allow a person skilled in the art to detect the position and rotational frequency of the impeller  166 . The threshold data may include a predetermined, desired rotational frequency and radial position. When the rotational frequency and radial position as sensed by the Hall Effect sensors deviate from the predetermined, desired values, the controller sends a signal over the H-Bridge, thereby selectively energizing coils  92   a ,  92   b.    
     The coils  92   a ,  92   b  which are energized depends on the desired outcome as described above with respect to at least  FIGS. 6A-C , such as altering or maintaining the radial position or rotational frequency of the impeller  66 . In one embodiment, there are four PID controllers  98 , each being part of a control loop  104  including an H-bridge  106 . However, the number of, type, and arrangement of the controllers  98  as described herein is but one possibility of controlling the device  50  as described herein and the disclosure is not meant to be limited to only the embodiments described herein. 
     While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in some detail, it is not the intention of the Applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will readily appear to those skilled in the art. The various features of the invention may be used alone or in any combination depending on the needs and preferences of the user. This has been a description of the present invention, along with the preferred methods of practicing the present invention as currently known. However, the invention itself should only be defined by the appended claims.