LEVITATED IMPELLER

CROSS-REFERENCES TO RELATED APPLICATIONS

This PCT application claims priority U.S. Serial No. 13/778,411 filed February 27, 2013 entitled "STARTUP SEQUENCE FOR CENTRIFUGAL PUMP WITH LEVITATED IMPELLER," of which the entire disclosure is incorporated herein by reference for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates in general to centrifugal pumping devices for circulatory assist and other uses, and, more specifically, to an improved startup of a magnetically-levitated impeller that avoids excessive wear of the impeller against the housing before levitation is obtained.

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, Michigan. The DuraHeart® system employs a centrifugal pump with a magnetically levitated impeller to pump blood from the left ventricle to the aorta. The impeller acts 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.

A control system for varying pump speed to achieve a target blood flow based on physiologic conditions is shown in U.S. patent 7, 160,243, issued January 9, 2007, which is incorporated herein by reference in its entirety. The stator of the pump motor can be driven by a pulse-width modulated signal determined using a field- oriented control (FOC) as disclosed in U.S. application serial no. 13/748,780, filed January 24, 2013, entitled "Impeller Position Compensation Using Field Oriented Control," which is incorporated herein by reference in its entirety.

The centrifugal pump employs a sealed pumping chamber. By levitating the impeller within the chamber when it rotates, turbulence in the blood is minimized. The spacing between the impeller and chamber walls minimizes pump-induced hemolysis and thrombus formation. The levitation is obtained by the combination of a magnetic bearing and a hydrodynamic bearing. For the magnetic bearing, the impeller typically employs upper and lower plates having permanent magnetic materials for interacting with a magnetic field applied via the chamber walls. For example, a stationary magnetic field may be applied from the upper side of the pump housing to attract the upper plate while a rotating magnetic field from the lower side of the pump housing (to drive the impeller rotation) attracts the lower plate. The hydrodynamic bearing results from the action of the fluid between the impeller and the chamber walls while pumping occurs. Grooves may be placed in the chamber walls to enhance the hydrodynamic bearing (as shown in U.S. patent 7,470,246, issued December 30, 2008, titled "Centrifugal Blood Pump Apparatus," which is incorporated herein by reference). The magnetic and hydrodynamic forces cooperate so that the impeller rotates at a levitated position within the pumping chamber. Since the hydrodynamic forces change according to the rotation speed of the impeller, the magnetic field may be actively controlled in order to ensure that the impeller maintains a centered position with the pumping chamber.

Prior to starting rotation of the impeller, the axial forces acting on it are not balanced. Magnetic attraction causes the impeller to rest against one of the upper or lower chamber walls. In many pump designs, it is possible for the impeller to be arbitrarily resting against either one of the walls. When rotation begins, the rubbing of the impeller against the chamber wall can cause undesirable mechanical wear of the impeller and/or wall. The amount of wear is proportional to the rotation angle traversed until the impeller lifts off of the pump housing and to the normal force between the impeller and housing.

In a typical startup sequence of the prior art, the stator coils are energized to produce a strong, stationary magnetic field that rotates the impeller into alignment with a known phase angle. When the impeller moves during alignment, it typically overshoots the desired position due to the strong field and then it oscillates around the desired position until the motion dampens out. Much mechanical wear can occur during this step. Once in the aligned position, the field-oriented control can begin closed-loop control to accelerate the impeller until the bearing forces separate it from the chamber wall. However, the normal force can be high before separation occurs, further increasing the wear.

BRIEF SUMMARY OF THE INVENTION

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 is 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 during a running state in response to a desired impeller speed and an actual impeller phase that is detected in response to the determined phase currents. The controller has a startup interval during which the commanded values of the phase voltages are determined in response to a pseudo impeller phase and in response to a ramping gain factor.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a diagram of a circulatory assist system as one example of an implantable pump employing the present invention.

Figure 2 is an exploded, perspective view of a centrifugal pump.

Figure 3 is a cross section showing an impeller levitated to a centered position within a pumping chamber. Figure 4 is a block diagram showing multiphase stator windings and a control system according to the present invention.

Figure 5 represents a gain function for ramping up the phase voltage commands during startup.

Figure 6 shows one embodiment of a ramping gain during a successful startup of the pump.

Figure 7 illustrates a ramping gain for repeated startup attempts. Figure 8 shows an impeller in the pump housing having a random position prior to startup.

Figure 9 shows the magnetic positioning of the impeller onto a predetermined side of the pump housing.

Figure 10 shows a magnetic reversal for pushing the impeller from the

predetermined side of the pump housing toward the other side in order to achieve liftoff at the time that rotation is started.

Figure 11 shows an initial random phase position of the impeller prior to startup.

Figure 12 is a flowchart showing a method for measuring the initial phase of the impeller.

Figure 13 is a flowchart showing one preferred method for axially transporting the impeller off the pump housing and then initiating impeller rotation.

DETAILED DESCRIPTION OF THE INVENTION

Referring to Figure 1, a patient 10 is shown in fragmentary front elevational view. Surgically implanted either into the patient'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's left ventricle into pumping unit 12.

An outflow conduit 13 conveys blood from pumping unit 12 to the patient's aorta. A percutaneous power cable 14 extends from pumping unit 12 outwardly of the patient'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 stator within pumping unit

12.

Figure 2 shows a centrifugal pump unit 20 having an impeller 21 and a pump housing having upper and lower halves 22a and 22b. 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 22a. 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 22b. Housing 22a includes an inlet 28 for receiving blood from a patient'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 33a and 33b. 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. The cross section of Figure 3 shows impeller 21 located at a centered position wherein disc 27 is spaced from housing 22A by a gap 42 and impeller disc 25 is spaced from housing 22B 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. The present invention is equally applicable to other magnetic levitation structures with or without active control.

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. Figure 4 shows an FOC-based controller 50 which supplies a multiphase voltage signal to a three-phase stator assembly 51. Individual phases A, B, and Care 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 _t ,. 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. Block 57 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 as heart beat.

FOC block 56 generates commanded voltage output values v _a , Vb, and v _c which are coupled to PWM block 53 via a startup filter 61 which is controlled by a startup logic block 60. In a normal running state, voltage commands v _a , Vb, and v _c are unmodified by filter 61. During a startup interval, startup logic 60 alters the operation of FOC block 56 by generating a pseudo impeller position (i.e., phase) for use by the FOC algorithm and by directly modifying the generated values for v _a , Vb, and v _c according to a gradually increasing gain factor.

When the impeller is not rotating under field oriented control, current observer block 57 is unable to estimate the actual position (i.e., the phase angle) of the impeller. For that reason, it is common in the prior art to generate a large, stationary magnetic field pulse from the stator in order to force the impeller into a known position. Based on the known position and an initial impeller speed of zero, the prior art controller was able to accelerate the impeller in a controlled (i.e., closed-loop) fashion. The large, stationary magnetic field pulse is not a problem for a radial type of motor structure or for a motor with a mechanical shaft and bearings. However, in the mechanical-bearingless (i.e., levitating) axial motor drive system for a blood pump, the alignment step is not desirable because the large, stationary magnetic field pulse may create oscillation movement of the impeller as a spring/mass system, and it may increase the normal force between the impeller and pump housing as a result of the large magnetic field. The oscillating movement and increased normal force may induce mechanical damage to the pump housing surface. The roughened blood pump surface may promote blood clot which is one of the severe failure modes of this type of device. In order to avoid the alignment step, the present invention adopts a gradual ramping up of voltage commands as the stator magnetic field is rotated using an arbitrary (i.e., pseudo) phase. The gradually increasing magnetic field will eventually catch the impeller to achieve a rotation that eventually becomes sufficiently locked in to enable observer 57 to detect the actual phase and speed of the impeller. Once the impeller is caught by the magnetic field and starts to rotate, the impeller will be suspended by hydrodynamic forces, and no mechanical wear will happen. Due to the gradually increasing magnetic field, the impeller is not very likely to be pulled in a backwards direction (in part because no movement can be generated until the rotating magnetic field overcomes the static friction between the impeller and pump housing). In addition, the speed of rotation of the magnetic field can be performed with a gradual acceleration to improve the likelihood of "catching" the impeller and rotating it in the desired direction. In the event that the ramping sequence fails to catch the impeller, observer 57 will detect that a valid position has not yet been determined. If no valid impeller position has been detected by observer 57 within a predetermined time, the ongoing startup attempt can be terminated and a second attempt can be made to start the impeller.

More specifically, startup logic block 60 may provide a pseudo impeller phase to FOC block 56, wherein the pseudo phase of the impeller has a chosen initial value and then follows a gradually accelerating rotation rate (not to exceed a target rotation speed).

As shown in Figure 5, filter 61 may provide a variable gain g multiplied separately by each voltage command v in order to gradually ramp up the magnitude of the rotating magnetic field over a certain period of time. As impeller rotation begins, this generates a gradually increasing torque applied to the impeller and a gradual increase in the normal force between the impeller and pump housing. A set of multipliers 62-64 each receives the gain factor g at its first input and each receives a respective voltage command v _a - v _c at its second input. In a normal running state of the pump, gain factor g is equal to one. During a startup interval, gain factor g provides a gradually increasing profile such as shown in Figures 6 or 7.

In Figure 6, gain g starts at zero and then increases along a segment 65 with a predetermined slope. Once gain g reaches a predetermined magnitude at 66 which is less than 1, the ramping ceases. The magnetic field continues to rotate for a predetermined time which is chosen to be long enough to allow the current observer to converge on a valid estimate of the impeller phase and speed if the impeller has been properly "caught" by the rotating magnetic field. If a valid estimate is detected, then gain factor g preferably increases to its full value (e.g., 1) at 67 and then the normal running state commences.

Figure 7 illustrates a sequence wherein a first open-loop attempt to start the impeller rotation (i.e., using a pseudo impeller phase and an accelerating rotation rate while ramping the phase voltage commands) fails to establish the desired rotation that is necessary for the current observer to converge on a valid estimate of the actual impeller phase. Thus, gain factor g is ramped from zero up to a predetermined gain along slope 70 and maintains the constant value at 71. When the current observer does not indicate that a valid estimate for the actual impeller phase has been obtained, then the starting attempt is halted and the stator magnetic field may be turned off at 72. After a time delay to ensure that the impeller is stationary, gain factor g is again ramped upward from zero along segment 73 and is then kept at a maximum value (e.g., 1) at 74 for a predetermined time. In the second attempt at starting the impeller, the success of the startup represent a higher priority than the mechanical wear. Consequently, both the slope at 73 and the maximum gain at 74 may be greater than they were during the first attempt at 70 and 71. In the unlikely event that the second attempt likewise fails to achieve a startup (as identified by the current observer converging to a valid estimate for the actual impeller phase), a third attempt may be made to start the impeller at segment 75. Once one of the attempts has successfully started the impeller rotation, then gain factor g remains at a value of 1 for as long as the normal running state continues. In combination with, or used separately from, the ramping up magnetic field and accelerating rotation rate, the present invention may employ an axial transport maneuver before beginning rotation of the magnetic field in order to lift off the impeller from the pump housing in order to avoid all normal forces during the initial rotation. As shown in Figure 8, the pumping chamber in a pump housing 76 has a first end 77 and a second end 78 for retaining an impeller. The impeller may initially be retained in a position 81 against end 77 or a position 80 against end 78. Due to the permanent magnets in the impeller, when in a rest state the impeller will always be attracted to one end or the other of the pump housing. The axial transport maneuver involves generating the stator magnetic field to shift the impeller position from one end toward the other so that the rotation can be initiated while the impeller is moving between the ends, but first the impeller must be set to a known position. Thus, an external magnetic field 82 is generated as shown in Figure 9 to place the impeller against a predetermined one of the ends such as end 77. By magnetically attracting the impeller to a known position 80, then it becomes possible to reverse the magnetic field as shown at 83 in Figure 10 to propel the impeller between the ends of the pump housing, making it pass through a central levitated position. By controlling the magnitude and slope of the reversed polarity magnetic field, the resulting axial movement of the impeller is slow enough that the desired impeller rotation can be initiated before the impeller reaches the other end of the housing.

In order to ensure that the impeller is attracted and then repulsed as desired, it is preferable to discover the actual impeller phase angle so that an appropriate energization of the stator can be determined that will provide the desired attraction and repulsion of the impeller. As shown in Figure 1 1, the plurality of stator windings 84 are laid out at respective phases around the circumference of the pump housing, while an actual impeller phase 85 initially has some arbitrary orientation. The presence of impeller influences the inductance of each stator coil by an amount that depends on the actual phase of the impeller. Thus, by characterizing the relative inductance between different stator coils, the phase of the impeller can be inferred without requiring any movement of the impeller. As shown in Figure 9, a small excitation current is applied to each phase of the stator in step 90. The inductance of each stator phase is measured in step 91. In step 92, the impeller position is inferred from the measured phase inductances. For example, a table can be generated in advance based on repeatedly measuring all the phase inductances with the impeller placed at different phase angles and then storing the measurement results in a table for later comparison with the actual measured inductances to determine the initial impeller phase for conducting the startup interval. Besides being used to initiate the axial transport maneuver, the estimated impeller phase can be used as an initial value for the pseudo impeller phase used during rotation of the magnetic field during the startup interval as described above, which will further increase the likelihood of obtaining a valid startup of the pump on the first attempt.

Figure 13 shows a preferred method for the axial transport maneuver wherein the impeller phase angle is estimated in step 93 as described above. Based on the estimated phase angle, the stator coils are energized in step 94 to attract the impeller to a predetermined end of the pump housing, e.g., the stator side of the housing. In step 95, the magnetic poles are reversed in order to push the impeller away from the predetermined end of the pump housing. While the impeller is moving between the ends, the magnetic field rotation is started in step 96 to begin to spin the impeller. When rotation of the magnetic field starts, the field oriented control is operated in an open-loop mode using a pseudo impeller phase which rotates with an increasing angular speed as described above. Preferably, the initial pseudo impeller phase uses a value determined from step 93. Preferably, the magnetic field rotation may include the use of the ramping gain factor, but that may not be required if the impeller phase angle is accurately estimated.

Although the present invention is especially useful in a centrifugal pump with a levitated impeller for pumping blood in cardiac assist applications, it is also applicable to other types of centrifugal pumps and for other applications.