Abstract:
A centrifugal pump system having an impeller rotating with first and second magnetic structures on opposite surfaces. A levitation magnetic structure is disposed at a first end of a pump housing having a levitating magnetic field for axially attracting the first magnetic structure. A multiphase magnetic stator at a second end of the pump housing generates a rotating magnetic field for axially and rotationally attracting the second magnetic structure. A commutator circuit provides a plurality of phase voltages to the stator. A sensing circuit determines respective phase currents. A controller calculates successive commanded values for the phase voltages in response to the determined phase currents and a variable commutation angle. The angle is selected to correspond to an axial attractive force of the stator that maintains a levitation of the impeller at a centered position within the pumping chamber.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Not Applicable. 
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
       [0002]    Not Applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    The present invention relates in general to centrifugal pumping devices for circulatory assist and other uses, and, more specifically, to an improved method and apparatus for maintaining a centered position of a magnetically-levitated impeller. 
         [0004]    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. One such system is the DuraHeart® LVAS system made by Terumo Heart, Inc., of Ann Arbor, Mich. The DuraHeart® system employs a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. The impeller can act as a rotor of an electric motor in which a rotating magnetic field from a multiphase stator couples with the impeller and is rotated at a speed appropriate to obtain the desired blood flow through the pump. 
         [0005]    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. 
         [0006]    A control system for varying pump speed to achieve a target blood flow based on physiologic conditions is shown in U.S. Pat. No. 7,160,243, issued Jan. 9, 2007, which is incorporated herein by reference in its entirety. A target blood flow rate may be established based on the patient&#39;s heart rate so that the physiologic demand is met. The control unit may establish a speed setpoint for the pump motor to achieve the target flow. 
         [0007]    A typical centrifugal pump employs a design which optimizes the shapes of the pumping chamber and the impeller rotating within the chamber so that the pump operates with a high efficiency. By employing a magnetic bearing (i.e., levitation), contactless rotation of the impeller is obtained and the pumping chamber can be more completely isolated from the exterior of the pump. The impeller typically 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 by reference. 
         [0008]    The impeller has an optimal centered 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). In order to ensure proper positioning, active monitoring and control of the impeller position has been employed 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. With an implanted system, it is desirable to miniaturize the pump as much as possible. It is also desirable to reduce failure modes by avoiding complexity. Thus, it would be desirable to maintain a centered position of the impeller to limit hemolysis and thrombosis without needing active control of the stationary levitating magnetic field. 
       SUMMARY OF THE INVENTION 
       [0009]    In one aspect of the invention, a centrifugal pump system comprises a disc-shaped impeller rotating about an axis and having a first magnetic structure disposed at a first surface and a second magnetic structure disposed at a second surface. A pump housing defines a pumping chamber which receives the impeller. A levitation magnetic structure is disposed at a first end of the pump housing having a levitating magnetic field for axially attracting the first magnetic structure. A multiphase magnetic stator disposed at a second end of the pump housing for generating a rotating magnetic field for axially and rotationally attracting the second magnetic structure. A commutator circuit provides a plurality of phase voltages to the stator. A sensing circuit determines respective phase currents flowing in response to the phase voltages. A controller calculates successive commanded values for the phase voltages in response to the determined phase currents and a variable commutation angle. The angle is selected to correspond to an axial attractive force of the stator that maintains a levitation of the impeller at a centered position within the pumping chamber. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a diagram of a circulatory assist system as one example of an implantable pump employing the present invention. 
           [0011]      FIG. 2  is an exploded, perspective view of a centrifugal pump. 
           [0012]      FIG. 3  is a cross section showing an impeller levitated to a centered position within a pumping chamber. 
           [0013]      FIG. 4  is a block diagram showing multiphase stator windings and a control system according to the present invention. 
           [0014]      FIG. 5  is a flow chart showing one preferred method for controlling pump operation. 
           [0015]      FIG. 6  is a flow chart showing one preferred method for adjusting a commutation angle. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0016]    Referring to  FIG. 1 , a patient  10  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 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 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 . 
         [0017]      FIG. 2  shows a centrifugal pump unit  20  having 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  preferably is comprised of one or more permanent magnet segments providing a symmetrical, static levitation magnetic field around a 360° 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 . 
         [0018]    The cross section of  FIG. 3  shows impeller  21  located at 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 . During pump operation, the center position is maintained by the interaction of attractive magnetic forces between permanent magnets  40  and  41  in levitation magnet structure  34  with imbedded magnetic material  44  within impeller disc  27 , and between stator assembly  35  and imbedded magnet material  45  in impeller disc  25 , and by 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. In order 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 is still needed. 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. 
         [0019]    A typical method for controlling voltages applied to a stator in order to provide the desired rotation for a permanent magnet rotor (i.e., the impeller) 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 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. 
         [0020]    The present invention varies the commutation angle in a manner to compensate for variations in attractive force that would otherwise occur as a result of changes in speed and torque. Varying the commutation angle from 90° slightly reduces overall efficiency, but has no significant affect on overall pump performance. At any particular combination of the 1) magnitude of the phase current and 2) the speed of the impeller, a modified commutation angle for generating the phase voltages applied to the stator can be determined so that the attractive force generated by the stator properly balances they hydrodynamic forces and the magnetic forces of the levitation magnets in order to keep the impeller at the centered position. 
         [0021]    The present invention is shown in greater detail in  FIG. 4  wherein a controller  50  uses field oriented control to supply a multiphase voltage signal to a stator assembly  51  shown as a three-phase stator. Individual phases A, B, and C are driven by an H-bridge inverter  52  functioning as a commutation circuit driven by a pulse width modulator (PWM) circuit  53  in controller  50 . A current sensing circuit  54  associated with inverter  52  measures instantaneous phase current in at least two phases providing current signals designated i a  and i b . A current calculating block  55  receives the two measured currents and calculates a current i c  corresponding to the third phase as known in the art. The measured currents are input to an FOC block  56  and to a current observer block  57  which estimates the position and speed of the impeller as known in the art. The impeller position and speed are input to FOC block  56 . A target speed or rpm for operating the pump is provided by a conventional physiological monitor  58  to FOC block  56 . The target rpm may be set by a medical caregiver or determined according to an algorithm based on various patient parameters such heart beat. 
         [0022]    FOC block  56  generates commanded voltage output values v a  , v b , and v c  which are input to PWM block  53 . The v a  , v b , and v c  commands may also be coupled to observer  57  for use in detecting speed and position (not shown). The system in  FIG. 4  generally uses conventional elements as known in the art except for modifications to FOC block  56  which alter the field oriented control algorithm so that a variable commutation angle is provided instead of the conventional 90° angle. In a preferred embodiment, a predetermined lookup table  60  is used to generate a commutation angle to be used at various operating conditions of the pump. 
         [0023]    In a preferred embodiment, the invention proceeds according to a method as shown in  FIG. 5  which highlights a portion of the field oriented control algorithm where a variable commutation angle is adopted. Thus, in step  65  the phase currents are measured. Based on the measured phase currents, the speed and position of the impeller are estimated in step  66 . The phase currents are transformed into a two-axis coordinate system to generate quadrature current values in a rotating reference frame in step  67 . In step  68 , the quadrature current vector is rotated by a desired commutation angle. This angle is selected to provide a proper centering offset from the typical 90° commutation angle according to the phase current and speed as described below. Based on the difference (i.e., error) between the quadrature current values from steps  67  and  68 , the next quadrature voltages are determined in step  69 . In step  70 , the quadrature voltages are transformed back to the stationary reference frame in order to provide the multiphase voltage commands which are output to the PWM circuit. 
         [0024]    According to one preferred embodiment of the invention, the values for the commutation angle which are offset from 90° by a centering offset to properly balance the levitated position of the impeller are determined in advance for various operating conditions of the pump and are compiled into a lookup table for use during normal pump operation. The attractive force applied to the impeller by the stator assembly varies with the magnitude of the magnetic field and the angle by which the magnetic to field leads the impeller position (i.e., the commutation angle). The magnitude of the magnetic field is directly proportional to the phase current. Phase current may preferably be characterized as the peak value for one of the measured phase currents over a sampling interval. In one preferred embodiment, a sampling interval of 1/20 seconds is used. Since the drive currents are always symmetrical, all the phases are driven with the same phase current value so that any one of the phase currents can be used. The phase current values are determined by the FOC algorithm according to the torque requirements of the motor in order to maintain the desired speed. Therefore, the phase currents cannot be used as the primary variable to adjust the axial attractive force. However, commutation angle can be arbitrarily modified to achieve a desired attractive force without otherwise degrading operation of the pump (although a slight reduction in efficiency is produced). 
         [0025]    Entries in the lookup table to be used to determine an offset commutation angle based on the magnitude of the phase current and the current operating speed, can be obtained experimentally during the design of the centrifugal pump system. During normal pump operation, a value for the commutation angle is obtained from the lookup table during each sampling interval using a method shown in  FIG. 6 . Thus, an update routine is periodically entered in step  75  according to the sampling interval. A phase current and speed characterizing the sampling interval are determined in step  76 . In addition to peak current in a single phase, a phase current characteristic such as an RMS value or an average of the square of the current could be employed. Based on the phase current characteristic and the rotational speed of the impeller, an offset commutation angle is looked up in step  77 . The offset can be stored as an absolute commutation angle or can be stored as a difference from a 90° commutation angle. The commutation angle offset is then used in step  78  for performing the field oriented control method of determining the phase voltages for driving the stator assembly until a next update for the following sampling interval. 
         [0026]    In one preferred embodiment, the lookup table includes 16 rows corresponding to the phase current characteristic and 10 columns corresponding to speed. Each row or column covers a respective range of values and all the columns and rows together cover a full operating regime of the pump. The table values can be determined experimentally using an impeller attached to a torque meter. An attractive force measurement fixture is attached to the stator assembly. For each rpm range corresponding to a table column, the phase current characteristic (i.e., the torque) is set to a corresponding range for a table row, with the pump operating using a standard field oriented control algorithm. The commutation angle is manually adjusted while monitoring the change in attractive force until the desired attractive force is obtained. The commutation angle achieving the desired attractive force is then stored in the table. 
         [0027]    The present invention is also useful in the context of a centrifugal pump with a levitating impeller wherein the impeller position can be sensed. Instead of a lookup table, a control loop varying the commutation angle could be employed in order to maintain the desired impeller position.