Impeller position compensation using field oriented control
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.
1. A cardiac assist device for implanting in a patient, comprising:
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 defining a pumping chamber which receives the impeller, wherein the pump housing includes an inlet for receiving blood from a heart of the patient and an outlet for delivering blood to a circulatory vessel of the patient;
a levitation magnetic structure 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 for providing a plurality of phase voltages to the stator;
a sensing circuit determining respective phase currents flowing in response to the phase voltages; and
a controller configured to calculate successive commanded values for the phase voltages in response to a desired flow output through the outlet, the determined phase currents, and a variable commutation angle, wherein 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.
2. The device of claim 1 wherein the levitation magnetic structure is comprised of permanent magnet material and the levitating magnetic field is substantially constant.
3. The device of claim 1 wherein the angle for calculating the commanded values is determined in response to a phase current characteristic and a rotational speed of the impeller.
4. The device of claim 3 wherein the phase current characteristic is comprised of a peak current flowing in the stator during a predetermined sampling interval.
5. The device of claim 3 wherein the controller includes a lookup table storing predetermined values for the commutation angle corresponding to respective ranges of the phase current characteristic and the rotational speed.
6. A method of operating a centrifugal pump having an impeller rotating suspended within a pumping chamber of a pump housing, comprising the steps of:
providing a first magnetic structure disposed at a first surface of the impeller and a second magnetic structure disposed at a second surface of the impeller;
providing a levitating magnetic field from a first end of the pump housing for axially attracting the first magnetic structure;
providing 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;
supplying a plurality of phase voltages to the stator from an electrical commutator;
determining respective phase currents flowing in response to the phase voltages;
calculating, with a controller, successive commanded values for the phase voltages in response to a desired flow output through the outlet, the determined phase currents, and a variable commutation angle, wherein 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.
7. The method of claim 6 wherein the levitating magnetic field is substantially constant.
8. The method of claim 6 wherein the angle for calculating the commanded values is determined in response to a phase current characteristic and a rotational speed of the impeller.
9. The method of claim 8 wherein the phase current characteristic is comprised of a peak current flowing in the stator during a predetermined sampling interval.
10. The method of claim 8 wherein the controller includes a lookup table storing predetermined values for the commutation angle corresponding to respective ranges of the phase current characteristic and the rotational speed.
11. The method of claim 6 wherein the centrifugal pump is implanted in a patient to supply an assistive blood flow, and wherein the desired flow output is comprised of a desired blood flow rate determined in response to physiological monitoring of the patient.