Patent Publication Number: US-2021187275-A1

Title: System and method for controlling the position of a levitated rotor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/218,791, filed Dec. 13, 2018, and entitled “SYSTEM AND METHOD FOR CONTROLLING THE POSITION OF A LEVITATED ROTOR,” which is a continuation of U.S. patent application Ser. No. 15/042,431, filed Feb. 12, 2016, now U.S. Pat. No. 10,166,318 issued Jan. 1, 2019, and entitled “SYSTEM AND METHOD FOR CONTROLLING THE POSITION OF A LEVITATED ROTOR,” the entire disclosures of which are hereby incorporated by reference, for all purposes, as if fully set forth herein. 
     U.S. patent application Ser. No. 15/042,431, now U.S. Pat. No. 10,166,318 issued Jan. 1, 2019, claims priority to and the benefit of U.S. Provisional Application No. 62/115,603, filed Feb. 12, 2015, and entitled “SYSTEM AND METHOD FOR CONTROLLING THE POSITION OF A LEVITATED ROTOR,” the entire disclosure of which is hereby incorporated by reference, for all purposes, as if fully set forth herein. 
     U.S. patent application Ser. No. 15/042,431, now U.S. Pat. No. 10,166,318 issued Jan. 1, 2019, claims priority to and the benefit of U.S. Provisional Application No. 62/115,324, filed Feb. 12, 2015, and entitled “SYSTEM AND METHOD FOR CONTROLLING THE POSITION OF A LEVITATED ROTOR,” the entire disclosure of which is hereby incorporated by reference, for all purposes, as if fully set forth herein. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates in general to rotary devices, and more specifically, to improved pumping devices and methods for their control. 
     One exemplary type of rotary machine is a centrifugal pumping device or mechanical circulatory assist device for treating patients with heart failure. Many types of circulatory assist devices are available for either short term or long term support for patients having cardiovascular disease. For example, a heart pump system known as a left ventricular assist device (LVAD) can provide long term patient support with an implantable pump associated with an externally-worn pump control unit and batteries. The LVAD improves circulation throughout the body by assisting the left side of the heart in pumping blood. Examples of LVAD systems are the DuraHeart® LVAS system made by Terumo Heart, Inc. of Ann Arbor, Mich. and the HeartMate II™ and HeartMate III™ systems made by Thoratec Corporation of Pleasanton, Calif. These systems typically employ a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. The impeller is formed as the rotor of the electric motor and rotated by the rotating magnetic field from a multiphase stator such as a brushless DC motor (BLDC). The impeller is rotated to provide sufficient blood flow through the pump to the patient&#39;s systemic circulation. 
     Early LVAD systems utilized mechanical bearings such as ball-and-cup bearings. More recent LVADs employ non-contact bearings which levitate the impeller using hydrodynamic and/or magnetic forces. In one example, the impeller is levitated by the combination of hydrodynamic and passive magnetic forces. 
     There is a trend for making centrifugal blood pumps used as the mechanical circulatory support devices more miniaturized to treat a broader patient population, more reliable, and with improved outcomes. To follow this trend, contactless impeller suspension technology has been developed in several pump designs. The principle of this technology is to levitate the pump impeller using one or a combination of forces from electromagnets, hydrodynamics, and permanent magnets. In the meanwhile, the pump should be hemocompatible to minimize the blood cell damage and blood clot formation. To that end, the bearing gap between the levitated impeller and the pump housing becomes an important factor. A small gap may lead to the high probability of the thrombus formation in the bearing or to elevated hemolysis due to excessive shear stress. Likewise, a large gap can compromise the hydrodynamic bearing performance and the pump efficiency. 
     One pump design utilizing active magnetic bearings achieves the desired bearing gap by levitating the impeller using magnetic fields generated by electromagnetic coils. However, in such a design there is the need for a separate bearing control system that includes the position sensors and electromagnetic coils to control the impeller position. Another pump design levitates the impeller using hydrodynamic thrust bearings combined with passive magnetic bearings. However, such a design usually requires a small bearing gap to provide sufficient hydrodynamic bearing stiffness to maintain impeller levitation and prevent contacts between impeller and the pump housing. Such a small gap may result in an insufficient washout and vulnerability to blood clotting thus compromising hemocompatibility. 
     Therefore, a solution is needed to enhance the bearing gap to achieve adequate washout without increasing the complexity of the pump mechanical design and reducing the pump efficiency. 
     There is a need for a pump that includes an integrated control method for controlling the impeller position to enhance the bearing gap without increasing the complexity of the pump mechanical design and reducing the pump efficiency. 
     There is the need for a blood pump designed to maintain a centered position of the impeller to limit hemolysis and thrombosis without needing active control of the stationary levitating magnetic field. 
     There is a need for pumps which overcomes the above and other disadvantages of known designs. 
     BRIEF SUMMARY OF THE INVENTION 
     In summary, various aspects of the present invention are directed to a rotary machine including a rotor within a housing and having a rotor magnetic structure; a stator on a side of the housing for generating a rotating magnetic field for applying a torque to the rotor magnetic structure; a commutator circuit for providing a plurality of phase voltages to the stator; and a controller for rotating the rotor using the commutator circuit and a vector control algorithm. In one embodiment the controller is configured to adjust the phase voltages to modify an attractive force of the stator on the rotor magnetic structure to translate the rotor. 
     In various embodiments, the machine further includes a sensing circuit for determining a position of the rotor. The controller may be configured to calculate successive commanded values for the phase voltages in response to determined phase currents from the sensing circuit and a variable commutation angle. The angle for calculating the commanded values may be determined in response to a phase current characteristic and a rotational speed of the rotor. 
     In various embodiments, the rotor is levitated by a substantially constant passive magnetic field. 
     In various embodiments, the controller is configured to move the rotor from a first balanced position to a second balanced position. 
     Various aspects of the invention are directed to a rotary machine including a rotor within a housing and having a rotor magnetic structure; a bearing mechanism for suspending the rotor in the housing in a balanced, non-contact manner; motor coils on a side of the housing for generating a magnetic field to apply a torque on the rotor magnetic structure; at least a first sensing circuit for determining a rotational and axial position of the rotor; a controller for rotating the rotor using the motor coils; and an impeller position control mechanism for adjusting a position of the impeller in the housing. 
     In various embodiments, the bearing mechanism comprises one of a hydrodynamic bearing, magnetic bearing, or combination of the same. In various embodiments, the modification of the attractive force on the rotor magnetic structure by the controller causes the rotor to move from a first balanced position to a second balanced position. The sensing circuit may include a plurality of position sensors for detecting the axial and rotational position of the rotor. The plurality of position sensors may include Hall-effect sensors. The plurality of position sensors may include optical sensors. In various embodiments, the rotor magnetic structure includes a plurality of magnetic members. 
     In various embodiments, the rotary machine is a pump. In various embodiments, the rotary machine is a blood pump. In various embodiments, the rotor is formed as an impeller. 
     Various aspects of the invention are directed to a method of operating the rotary machine described in any of the paragraphs above. 
     Various aspects of the invention are directed to a method of operating a centrifugal pump including a stator having windings and an impeller rotating in a non-contact manner within a pump housing, the impeller including a magnetic structure, the method includes applying a first levitating force on the impeller during rotation; and using electromagnetic windings, controlling the position of the impeller in the pump housing axially and rotationally. 
     In various embodiments, the first levitating force comprises a passive magnetic attractive force. The at least second levitating force may include an active magnetic force created by electromagnetic coils. The coils may be driven by vector control. 
     Various aspects of the invention are directed to a method of operating a centrifugal pump including a stator having windings and an impeller rotating in a non-contact manner within a pump housing, the impeller including a magnetic structure, the method including applying a first levitating force on the impeller during rotation; and using electromagnetic windings, controlling the position of the impeller in the pump housing axially and rotationally. 
     In various embodiments, the stator windings form the electromagnetic windings for controlling the impeller position. In various embodiments, the rotation of the impeller is controlled by interaction between the magnetic structure in the impeller and AC currents in the motor stator windings. In various embodiments, the axial position of the impeller is controlled by interaction between the magnetic structure in the impeller and DC currents in the motor stator windings. In various embodiments, the method includes using the electromagnetic windings to move the impeller axially from a first predetermined position to a second predetermined within the pump housing. 
     Various aspects of the invention are directed to a system, method, or computer-program product as described herein and/or shown in any of the drawings. 
     The systems and methods of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated in and form a part of this specification, and the following Detailed Description of the Invention, which together serve to explain the principles of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an implantable pump as one example of a rotary machine employing the present invention. 
         FIG. 2  is an exploded, perspective view of the exemplary centrifugal pump of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the exemplary pump of  FIG. 1 , illustrating the impeller levitated at a first balanced position generally centered within the pumping chamber in accordance with aspects of the invention. 
         FIG. 4  is a schematic view of the exemplary pump of  FIG. 1 , illustrating the impeller levitated eccentrically in the pump chamber by the main bearing components. 
         FIG. 5  is a block diagram of a pump control system in accordance with the invention. 
         FIG. 6  is a line chart depicting the method of controlling the impeller position in accordance with the invention. 
         FIG. 7 a    is a schematic view of the pump of  FIG. 1 , illustrating the impeller moved to another eccentric position at the bottom of the pump chamber. 
         FIG. 7 b    is a schematic view of the pump of  FIG. 1 , illustrating the impeller moved to yet another eccentric position at the top of the pump chamber. 
         FIG. 8  is a flowchart showing a method of controlling impeller position in accordance with the invention. 
         FIG. 9 a    is a flowchart showing a method of controlling impeller position in accordance with the invention. 
         FIG. 9 b    is a line chart depicting a method for moving the impeller between two balanced positions in accordance with aspects of the invention. 
         FIG. 10  is a flowchart showing a method of controlling impeller position during start-up of the pump in accordance with the invention. 
         FIG. 11  is a cross-sectional view of an exemplary centrifugal flow pump in accordance with aspects of the invention, illustrating electromagnetic bearings to supplement the stator assembly positioning control. 
         FIG. 12  is a cross-sectional view of an axial flow pump in accordance with aspects of the invention, the axial flow pump including mechanical bearings. 
         FIG. 13  is a perspective view of the impeller of  FIG. 12 , with arrows depicting the direction of translation in accordance with the invention. 
         FIG. 14  is a cross-sectional view of another axial flow pump in accordance with aspects of the invention, the axial flow pump including mechanical bearings. 
         FIG. 15  is a cross-sectional view of another axial flow pump in accordance with aspects of the invention, the axial flow pump including passive magnetic and hydrodynamic bearings. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference will now be made in detail to the preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. While the invention will be described in conjunction with the preferred embodiments, it will be understood that they are not intended to limit the invention to those embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims. 
     For convenience in explanation and accurate definition in the appended claims, the terms “up” or “upper”, “down” or “lower”, “inside” and “outside” are used to describe features of the present invention with reference to the positions of such features as displayed in the figures. 
     In many respects the modifications of the various figures resemble those of preceding modifications and the same reference numerals followed by subscripts “a”, “b”, “c”, and “d” designate corresponding parts. 
     As used herein, “gap” generally refers to the secondary flow gaps around the impeller as would be understood by one of skill in the art. The primary flow is through the impeller blade regions. The secondary flow gaps are the other areas of fluid, generally around the impeller. In some respects, the secondary flow gaps are between the impeller and the housing wall and define the hydrodynamic bearing. 
     The term “machine-readable medium” includes, but is not limited to portable or fixed storage devices, optical storage devices, wireless channels and various other mediums capable of storing, containing or carrying instructions and/or data. A code segment or machine-executable instructions may represent a procedure, a function, a subprogram, a program, a routine, a subroutine, a module, a software package, a class, or any combination of instructions, data structures, or program statements. A code segment may be coupled to another code segment or a hardware circuit by passing and/or receiving information, data, arguments, parameters, or memory contents. Information, arguments, parameters, data, etc. may be passed, forwarded, or transmitted via any suitable means including memory sharing, message passing, token passing, network transmission, etc. 
     Furthermore, embodiments of the invention may be implemented, at least in part, either manually or automatically. Manual or automatic implementations may be executed, or at least assisted, through the use of machines, hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware or microcode, the program code or code segments to perform the necessary tasks may be stored in a machine readable medium. One or more processors may perform the necessary tasks. 
     Although aspects of the invention will be described with reference to a blood pump, one will appreciate that the invention can be applied to various rotary machines and other types of pumps. The mechanisms and methods of the invention will be described in relation to blood pumps and in particular the ability to adjust the impeller operating position to address performance, such as the attendant risks for thrombus and hemolysis when pumping blood. One will appreciate from the description herein that the invention can be applied broadly to other pumps, rotary machines, and induction motors. 
     Aspects of the invention enable to the ability to enhance or control the bearing gap. One might wish to increase the bearing gap to adjust the washout rate, lubricate the bearing surfaces, or remove materials (particulates, thrombus, etc.) from the bearing gap. Another use of the invention may be to increase pump efficiency. As is known in the art, the motor efficiency increases as the impeller magnet moves closer to the motor drive coils. Another use of the invention may be to correct impeller malpositioning due to bulk forces or external forces (e.g. bumps or movements of the patient&#39;s body). These and other advantages can be achieved without the need for complex control systems in accordance with the invention. 
     In one embodiment, an electromagnetic force control method is used to change the impeller position and enhance the effective gap between the impeller and the blood chamber. The technique uses the same pump motor stator coils adjust the impeller position as is used to apply a torque to the impeller. No additional control subsystems and components are necessary. 
     Turning now to the drawings, wherein like components are designated by like reference numerals throughout the various figures, attention is directed to  FIG. 1  which depicts an exemplary pump implanted in a heart failure patient. 
     A typical cardiac assist system includes a pumping unit, drive electronics, microprocessor control unit, and an energy source such as rechargeable batteries and/or an AC power conditioning circuit. The system is implanted during a surgical procedure in which a centrifugal pump is placed in the patient&#39;s chest. An inflow conduit is pierced into the left ventricle to supply blood to the pump. One end of an outflow conduit is mechanically fitted to the pump outlet and the other end is surgically attached to the patient&#39;s aorta by anastomosis. A percutaneous cable connects to the pump, exits the patient through an incision, and connects to the external control unit. 
     Various aspects of the implantable pump are similar to those shown and described in U.S. Pat. Nos. 4,528,485; 4,857,781; 5,229,693; 5,588,812; 5,708,346; 5,917,297; 6,100,618; 6,222,290; 6,249,067; 6,268,675; 6,355,998; 6,351,048; 6,365,996; 6,522,093; 7,972,122; 8,686,674; 8,770,945; U.S. Pub. No. 2014/0205467; 2012/0095281; and U.S. patent application Ser. No. 15/041,987, the entire contents of which patents and publications are incorporated herein by this reference for all purposes. 
     The exemplary system utilizes an implantable pump with contactless bearings for supporting the impeller. Contactless bearings (i.e., levitation) provide a number of potential benefits. Because they reduce rotational friction, theoretically they improve motor efficiency and reduce the risk of introducing particulates into the fluid. In one example, the impeller employs upper and lower plates having magnetic materials (the terminology of upper and lower being arbitrary since the pump can be operated in any orientation). A stationary magnetic field from the upper side of the pump housing attracts the upper plate and a rotating magnetic field from the lower side of the pump housing attracts the lower plate. The forces cooperate so that the impeller rotates at a levitated position within the pumping chamber. Features (not shown) may also be formed in the walls of the pumping chamber to produce a hydrodynamic bearing wherein forces from the circulating fluid also tend to center the impeller. Hydrodynamic pressure grooves adapted to provide such a hydrodynamic bearing are shown in U.S. Pat. No. 7,470,246, issued Dec. 30, 2008, titled “Centrifugal Blood Pump Apparatus,” which is incorporated herein for all purposes by reference. 
     The exemplary impeller has an optimal location within the pumping chamber with a predetermined spacing from the chamber walls on each side. Maintaining a proper spacing limits the shear stress and the flow stasis of the pump. A high shear stress can cause hemolysis of the blood (i.e., damage to cells). Flow stasis can cause thrombosis (i.e., blood clotting). 
     With continued reference to  FIG. 1 , a patient is shown in fragmentary front elevational view. Surgically implanted either into the patient&#39;s abdominal cavity or pericardium  11  is the pumping unit  12  of a ventricular assist device. An inflow conduit (on the hidden side of unit  12 ) pierces the apex of the heart to convey blood from the patient&#39;s left ventricle into pumping unit  12 . An outflow conduit  13  conveys blood from pumping unit  12  to the patient&#39;s ascending aorta. A percutaneous power cable  14  extends from pumping unit  12  outwardly of the patient&#39;s body via an incision to a compact control unit  15  worn by patient  10 . Control unit  15  is powered by a main battery pack  16  and/or an external AC power supply, and an internal backup battery. Control unit  15  includes a commutator circuit for driving a motor within pumping unit  12 . 
     In various embodiments, the commutator circuit and/or various electronics may be on the implanted side of the system. For example, various electronics may be positioned on-board the pump or in a separate hermetically sealed housing. Among the potential advantages of implanting electronics is the ability to control the pump even when communication is lost with the control unit  15  outside the body. 
       FIG. 2  shows exemplary centrifugal pump unit  20  used in the system of  FIG. 1 . The pump unit  20  includes an impeller  21  and a pump housing having upper and lower halves  22   a  and  22   b . Impeller  21  is disposed within a pumping chamber  23  over a hub  24 . Impeller  21  includes a first plate or disc  25  and a second plate or disc  27  sandwiched over a plurality of vanes  26 . Second disc  27  includes a plurality of embedded magnet segments  44  for interacting with a levitating magnetic field created by levitation magnet structure  34  disposed against housing  22   a . For achieving a small size, magnet structure  34  may comprise one or more permanent magnet segments providing a symmetrical, static levitation magnetic field around a  3600  circumference. First disc  25  also contains embedded magnet segments  45  for magnetically coupling with a magnetic field from a stator assembly  35  disposed against housing  22   b . Housing  22   a  includes an inlet  28  for receiving blood from a patient&#39;s ventricle and distributing it to vanes  26 . Impeller  21  is preferably circular and has an outer circumferential edge  30 . By rotatably driving impeller  21  in a pumping direction  31 , the blood received at an inner edge of impeller  21  is carried to outer circumferential  30  and enters a volute region  32  within pumping chamber  23  at an increased pressure. The pressurized blood flows out from an outlet  33  formed by housing features  33   a  and  33   b . A flow-dividing guide wall  36  may be provided within volute region  32  to help stabilize the overall flow and the forces acting on impeller  21 . 
       FIG. 3  shows an exemplary pump  10  similar to the pump shown in  FIG. 2 .  FIG. 3  shows impeller  21  located in a balanced position. The balanced position sometimes refers to the position the impeller naturally stabilizes or finds equilibrium during operation. In the exemplary embodiment, the balanced position is at or near the center of the pump chamber. In the balanced position, the forces acting on the impeller are generally balanced to stabilize the impeller. As one will understand from the description above that the hydrodynamic forces on the impeller will change as the rotational speed of the impeller changes. In turn, the magnetic attractive forces on the impeller will change as the impeller moves closer to or away from the magnet structure  34  and stator assembly  35 . Accordingly, the impeller generally finds a new balanced position as the rotational speed changes. 
     As will be described below, however, aspects of the invention are directed to moving the impeller or changing the balanced position for each given rotational speed. For example, the impeller position control mechanisms to be described facilitate moving the impeller axially (up or down) without changing the rotational speed and all other. This has the effect of enabling movement of the impeller independent of rotor speed. An advantage of this technique is that rotor speed can be determined in normal course (e.g. by a physician based on the patient&#39;s physiological needs) without concern for changing the impeller position. Conversely, the impeller position can be changed without affecting pumping throughput. 
       FIG. 3  shows impeller  21  located at or near a centered position wherein disc  27  is spaced from housing  22 A by a gap  42  and impeller disc  25  is spaced from housing  22 B by a gap  43 . In the exemplary embodiment, the center position is chosen as the balanced or balanced position to ensure substantially uniform flow through gaps  42  and  43 . During pump operation, the balanced position is maintained by the interaction of (a) attractive magnetic forces between permanent magnets  40  and  41  in levitation magnet structure  34  with imbedded magnetic material  44  within impeller disc  27 , (b) attractive magnetic forces between stator assembly  35  and embedded magnet material  45  in impeller disc  25 , and (c) hydrodynamic bearing forces exerted by the circulating fluid which may be increased by forming hydrodynamic pressure grooves in housing  22  (not shown). By using permanent magnets in structure  34  a compact shape is realized and potential failures associated with the complexities of implementing active levitation magnet control are avoided. To properly balance impeller  21  at the centered position, however, and because other forces acting on impeller  21  are not constant, an active positioning control may be desired. In particular, the hydrodynamic forces acting on impeller  21  vary according to the rotational speed of impeller  21 . Furthermore, the attractive force applied to impeller  21  by stator assembly  35  depends on the magnitude of the magnetic field and the angle by which the magnetic field leads the impellers magnetic field position. 
     The structures and methods for controlling the motor will now be described with references to  FIGS. 4 to 6 . 
       FIG. 4  shows the main structure of an exemplary centrifugal pump  50  similar to that shown in  FIG. 3 . It is understood that other pump configurations may be employed, including various combinations of permanent magnets, motor stator windings, and hydrodynamic bearings. In the exemplary embodiment, the rotor is formed as an impeller and driven by a motor. The impeller is also levitated by the combined force F aΣ , which can be expressed as the following equation: 
     
       
      
       F 
       aΣ 
       =F 
       hdb 
       +F 
       pm 
       +F 
       cm  
      
     
     Where, 
     F aΣ  is the combined force to levitate the impeller 
     F hdb  is the combination of hydrodynamic forces from the inlet side bearing, the motor side bearing, or both 
     F pm  is the combination of permanent magnet attraction forces 
     F em  is the magnetic attraction force generated from the motor 
     When the impeller is stabilized, F aΣ  should be equal to zero. Usually F em  can be controlled through the electronic system to adjust the impeller position since all the others are the fixed configurations as the passive mode. Therefore, the basic design concept of this invention is to apply the motor vector control (FOC) to control the force F em  so that the impeller position can be adjusted while rotating only using one set of motor coil and drive system. In such way, there is no additional cost in the pump structure. 
       FIGS. 5 to 6  illustrate a method for controlling voltages applied to a stator in order to provide a desired rotation for a permanent magnet rotor (e.g. the impeller)  52  is a field oriented control (FOC) algorithm, which is also known as vector control. It is known in FOC that the stator magnetic field should generally lead the impeller position by 90° for maximum torque efficiency. The magnitude of the attractive force on the impeller is proportional to the magnitude of the phase currents in the stator. Phase current is adjusted by the FOC algorithm according to torque demands for the pump. Since the commutation angle is typically fixed at 90°, the resulting attractive force varies according to torque output from the pump. The exemplary technique varies the I d  current which creates magnetic flux responsible for attracting the impeller. This provides a convenient and accurate mechanism to create a controlled impeller attractive force. 
     At any particular combination of the (1) magnitude of the phase current and (2) the speed of the impeller, modifying the commutation angle for generating the phase voltages can change the attractive force generated by the stator thereby affecting the impeller balance. In turn, the impeller moves until it settles at a new balanced position where the hydrodynamic forces and magnetic forces are balanced. 
       FIGS. 5, 6, 7   a , and  7   b  illustrate an exemplary system in accordance with aspects of the invention. Based on the principle of motor vector control, the torque current that is usually called quadrature current (I q  current) and stator coil flux current that is called direct current (I d  current) can be decoupled and controlled independently. The quadrature current I q  current is used to control the impeller rotational speed. The direct I d  current controls the magnetic flux of electromagnetic coils. 
     In accordance with the invention, I d  current is utilized to control the impeller position by enhancing or weakening the magnetic flux between impeller (rotor) and motor stator coils to adjust the attraction force F em . This in turn changes the impeller position (shown in  FIG. 6 ). 
     In one embodiment, the impeller position control technique is implemented as an open loop control without impeller position sensors. In one embodiment, impeller position control technique is implemented as a closed loop control with impeller position sensors. 
     In order to ensure proper positioning, active monitoring and control of the impeller position has been employed in the exemplary embodiment by adjusting the stationary magnetic field. However, position sensors and an adjustable magnetic source occupy a significant amount of space and add to the complexity of a system. Accordingly, the use of sensors may depend on the design requirements. Suitable sensors may include, but are not limited to, Hall-effect sensors, variable reluctance sensors, and accelerometers. 
     In one embodiment using the open loop control, the impeller is controlled by periodically alternating the position from one side to another (e.g. from inlet side to motor side) by modulating the I d  current as shown in  FIG. 6 . In this manner, the side gaps (Gap 1 and Gap 2) as shown in  FIGS. 7 a  and 7 b    can be increased or decreased. 
     The position control technique can be implemented into the hardware and/or software of the system. Referring to  FIG. 5 , by example, the controller may employ FOC to supply a multiphase voltage signal to the stator assembly  53 . The exemplary stator assembly is a three-phase stator. Individual phases a, b, and c and currents I a , I b , and I c  may be driven by an H-bridge inverter functioning as a commutation circuit driven by a pulse width modulator (PWM) circuit. An optional current sensing circuit associated with the inverter measure instantaneous phase current in at least two phases providing current signals designated I a  and I b . A current calculating block receives the two measured currents and calculates a current I c  corresponding to the third phase. The measured currents are input to Vector Control (FOC) block  54  and to a current observer block (not shown) which estimates the position and speed of the impeller. The impeller position and speed are input to the FOC block from speed control block  55  and position control block  56 . A target speed or revolutions per minute (rpm) for operating the pump is provided by a conventional physiological monitor to FOC block  54 . The target rpm may be set by a medical caregiver or determined according to an algorithm based on various patient parameters such heart beat, physiological needs, suction detection, and the like. FOC block  54  and drive electronics  57  generate commanded voltage output values Va, Vb, and Vc. The Va, Vb, and Vc commands may also be coupled to the observer block for use in detecting speed and position. 
     The exemplary system differs from conventional configurations inasmuch as the FOC block and electronics are configured to alter the field oriented control algorithm so that a direct current (Id) can be varied independently and generate a desired attractive force. 
     In one embodiment, the invention proceeds according to a method as shown in  FIG. 8  which highlights a portion of the impeller position control with the field oriented control algorithm. Thus, in step  65  the phase currents are measured. Based on the measured phase currents, the current speed and rotor angle of the impeller are estimated in step  66 . Based on the measured rotor angle in step  66 , the phase currents are transformed into a two-axis coordinate system to generate quadrature current (I q  current) and direct current (I d ) values in a rotating reference frame in step  67 . Quadrature current is used to control the torque to rotate the impeller and direct current is used to control the attraction force between rotor and stator to control the impeller position. In step  68 , the next quadrature voltage is determined by the quadrature current error between the quadrature current transformed from step  67  and the required current for impeller rotation. In step  69 , the next direct voltage is determined by the direct current error between the direct current transformed from step  67  and the required current for the attraction force alternation to control the impeller position. In step  70 , the quadrature and direct voltages are transformed back to the stationary reference frame in order to provide the multiphase voltage commands which are output to the PWM circuit. 
       FIG. 9 a    is a flowchart showing another method of operating a rotary machine in accordance with the invention. The method includes operating the pump to rotate the impeller by applying a rotating magnetic field in step S 10 . During operation the impeller is levitated and positioned at a balanced position (P 1 ) by a balancing of forces. As described above, in an exemplary embodiment the impeller is levitated by the combination of hydrodynamic forces F 1  and other bearing forces F 2  (e.g. stator attractive force, passive magnetic forces, and/or bulk forces like gravity) in steps S 11  and S 12 . Next, at least one of the forces, F 2 , is modified to place the impeller out of balance in step S 14 . The impeller moves to a new position, P 2 , where the forces are once again balanced in step S 15 . 
     As described above, the impeller is moved from the first balanced position (P 1 ) to the second balanced position (P 2 ) by applying an attractive force or modifying (increasing or decreasing) an existing attractive force on the impeller. In various embodiments, the attractive force modulation is substantially continuously applied to hold the impeller in the second balanced position. In various embodiments, the attractive force modulation is applied periodically (e.g. as pulses) to hold the impeller in the second balanced position. In various embodiments, the attractive force modulation is applied as a single pulse to move the impeller in the second balanced position. The second balanced position can be configured so the impeller remains in the second balanced position in a stable manner even when the attractive force is removed. 
       FIG. 9 b    illustrates an exemplary method for moving the impeller between two balanced positions in accordance with aspects of the invention. In this example, a permanent magnet arrangement is contemplated whereby the two balanced positions are naturally a product of the permanent magnet arrangement, so that the above-mentioned attractive force modulation may not necessarily be needed or required to hold the impeller in a particular balanced position. Rather, a single (or series) pulse may be applied to “push” the impeller from one stable position to another. For example, as shown in  FIG. 9 b   , the impeller is initially, at a time t1, at a stable balanced position A. At a time t2 a pulse of duration (t3−t2) is applied to push the impeller to a new, stable balanced position B. In practice, the impeller may be pushed between stable balanced position A and stable balanced position B in a manner as needed or desired, such as shown in  FIG. 9 b   . Advantageously, such an implementation may save energy and improve pump efficiency. In one embodiment, one of the two stable positions may be relatively close to the inlet of the above mentioned centrifugal pump unit, and another relatively close to the motor. 
     Turning to  FIG. 10 , in one embodiment, the impeller position control technique is used to facilitate start-up of the pump. In step S 20 , the exemplary pump is configured so the impeller rests against the inlet side (top of the housing) when the impeller is not rotating. In a typical pump with hydrodynamic forces alone, or in combination with magnetic forces, the impeller is levitated away from the wall as it rotates. The blood entrained in the gap between the impeller and the house creates hydrodynamic pressure; however, the impeller must be rotating at a sufficient speed to create the hydrodynamic pressure. Until the minimum speed is met, the impeller rubs against the housing wall. In the exemplary pump, by contrast, the impeller is pulled away from the wall prior to, or just after, rotation begins thereby eliminating the deleterious effects of friction. The impeller is pulled away from the wall by applying a force, F, as described above in step S 21 . For example, the commutation angle may be modified to exert an attractive force. Referring to  FIG. 7 b   , by example, the pump can be configured so the impeller rests at the inlet side. By applying an attractive force to the motor side the impeller moves down from the top wall. In step S 22 , the regular start-up sequence is initiated after the impeller is removed from the wall. 
       FIG. 11  shows a rotary machine in accordance with another embodiment making use of electromagnets. Pump  100  in  FIG. 11  is similar in various respects to pump  10  in  FIG. 3 . In the exemplary embodiment, however, pump  100  includes an active electromagnetic (EM) system  1010 . The EM force generated by electromagnets is used primarily or adjunctively to move the impeller. Exemplary electromagnets  101  comprise iron cores and windings. The EM force is modified in a conventional manner by changing the current applied to the windings. The application of the EM force causes the impeller to move to position P E2 . One will appreciate that the EM force can overpower hydrodynamic and passive magnetic forces present in the system. Accordingly, the EM structure must be dimensioned and configured to apply a relatively balanced force. An advantage of using electromagnets over the existing stator assembly is that there is relatively greater positional control over the impeller. By contrast, as described above, the phase currents typically cannot be used as the primary variable to adjust the axial attractive force on the impeller. A disadvantage of this embodiment is the need to provide an entirely separate EM system. This may not be an issue with large industrial rotary machines, but many types of motors have restrictive form factors. For example, implanted pumps must be relatively small in order to address a wider patient population. 
       FIGS. 12 and 13  illustrate another implantable pump in accordance with the invention. Pump  200  is similar in various respects to pumps  10  and  100  described above except pump  200  is an axial flow pump. Blood flows from in through inlet  201  and out through outlet  202  in a generally linear, axial direction. Pump  200  includes an impeller  210  having blades for moving blood through the pump housing and imparting kinetic energy in the fluid. 
     Impeller  210  is fixed within the housing by ball-and-cup bearings  212  and  214 . The ball-and-cup bearings are closely toleranced and generally fix the impeller in a specific position. However, the exemplary bearings are lubricated and washed by the blood flow around the impeller. Accordingly, there is some fluid between the ball and cup surfaces. 
     Torque is applied to the impeller by a stator assembly  205 . The stator assembly  205  includes windings and is driven using a FOC algorithm in a similar manner to the stator assemblies described above. In practice, the impeller position is adjusted proceeding according to the method shown in  FIG. 9 . Using the FOC technique described above the impeller is rotated in the pump housing. At a desired time the I d  current is modulated to adjust the attractive force on the impeller in the axial direction. As long as the attractive force is sufficient to squeeze blood out from a respective bearing gap, the impeller will move axially towards inlet  201  or outlet  202 . The bearing gaps of pump  200  are relatively small compared to Gap 1 and Gap 2 of pump  50  in  FIG. 4 . However, even relatively small impeller movement may be beneficial to enable control of the bearing gaps. 
       FIG. 14  illustrates another pump  300  similar to pump  200 . Pump  300  includes an impeller fixed between two mechanical bearings  212  and  214 . Pump  300  is slightly different than pump  200  because the outlet extends at an angle from the inlet. Pump  300  is configured in a relatively compact design compared to pump  200  including a relatively smaller stator assembly; however, the same general principles can be applied to control the motor and adjust the impeller position. 
       FIG. 15  is a cross-sectional view of another pump  400  similar to pumps  200  and  300 , except pump  400  is an axial flow pump with non-contact bearings. Pump  400  includes a pump housing having an inlet  401  and outlet  402 . An impeller  411  is positioned within a pump chamber for imparting flow to the blood fluid within the housing. The impeller is entirely formed of a magnetic material which is driven by interaction with a stator assembly  405 . 
     Impeller  411  is stabilized in the pump chamber by a combination of hydrodynamic and passive magnetic forces. Impeller  411 , which is a magnetic material, interacts with the magnetic material in stator assembly  405  to provide an axial centering force. A pump ring  452  with a chamfer surface is positioned at the leading end of the impeller to create hydrodynamic stabilization forces in the axial direction (left to right) and radial direction (up and down on page). A permanent magnet ring  450  is provided at the trailing edge of the impeller is oriented with a north pole facing a north pole of the impeller. This arrangement creates an axial bias force to push the impeller against the pump ring  452 . The magnet ring  450  also provides a radially centering force. Finally, the impeller includes deep hydrodynamic grooves to generate a hydrodynamic pressure force against the inner walls of the pump chamber for radial stabilization. 
     In operation, the impeller remains stable in the axial and radial directions. There may be some axial movement as the rotational speed of the impeller changes or as a result of other forces (e.g. the native pulse), but generally the impeller remains centered below the stator assembly. 
     Using the FOC control technique described above, the attractive force of the stator assembly  405  on impeller  411  can be modified. In one embodiment, pump  400  is configured so impeller is eccentric when centered below the stator assembly  405 . In this example, increasing the attractive force amounts to an increase in the axial stiffness to resist axial movement. In one embodiment, the attractive force is modified to actually move impeller  411  axially. For example, the impeller can be moved closer to pump ring  452  to squeeze blood out of the gap between impeller  411  and a surface of ring  452 . The impeller may also be moved away from ring  452  to increase the blood gap therebetween. 
     In this manner, the impeller position control technique adds an element of active position control otherwise not possible with the passive bearing configuration of pump  400 . 
     Although aspects of the invention have been described in connection with blood pumps, one will appreciate from the description herein that the invention can be applied equally to other types of rotary machines such as washing machines, manufacturing machines, computer drives, and more. 
     The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.