Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1998-03-23
2001-03-13
Tamai, Karl I (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C417S423700, C417S423100, C600S016000, C623S003100
Reexamination Certificate
active
06201329
ABSTRACT:
The present invention relates generally to magnetic bearings for pumps such as, for example, blood pumps implanted in the human body to assist blood flow.
References (in addition to those cited hereinafter) which may be of interest in the development of magnetic bearings include my U.S. Pat. Nos. 5,084,643; 5,133,527 (with others); 5,202,824; and 5,666,014; U.S. Pat. Nos. 5,175,457 and 5,521,448, which above patents are hereby incorporated herein by reference; M. Marinescu et al, “A New Improved Method for Computation of Radial Stiffness of Permanent Magnet Bearings,”
IEEE Transactions on Magnetics
, vol. 20, no. 5, September, 1994, pp 3491-3494; and C. Henrikson et al, “Magnetically Suspended Momentum Wheels for Spacecraft Stabilization,” AIAA paper no. 74-128, AIAA 12
th
Aerospace Sciences Meeting, Washington, D.C., Jan. 30-Feb. 1, 1974.
A major limitation of continuous flow blood pumps utilizing conventional (non-magnetic) bearings is the seal required for separating the blood from the bearing and the lubricant. Seals fail mechanically or by the build-up of amorphous material at the rotating and non-rotating interface.
While various blood lubricated supports such as conventional hydrodynamic bearings and sapphire jewel bearings have been proposed, deposition and homolysis in the interface due to frictional heat and shear stress may nevertheless be excessive.
Relative movement between parts of mechanical bearings or high sheer stresses or friction at interfaces of impellers with glands may cause blood cells to undesirably rupture.
Magnetic suspension of the rotor has been proposed as a suitable bearing means for a blood pump. In such a pump, blood may flow between the stator and rotor since the rotor is suspended relative to the stator. It is considered desirable to have a path through such a pump for free flow of blood so that it does not undesirably stagnate and thus coagulate and which path is large enough to prevent shearing of individual blood cells.
One magnetic suspension concept for blood pump bearings utilizes five actively controlled bearing axes. See P. Allair et al, “Prototype Continuous Flow Ventricular Assist Device Supported on Magnetic Bearings,”
Artificial Organs
, vol. 20, no. 6, 1996, pp 582-590. Such a suspension system is mechanically and electrically complex and consumes much power. Another suspension concept utilizes four active radial axes, and still another utilizes four active axial electromagnets. See R. Hart et al, “A Magnetically Suspended and Hydrostatically Stabilized Centrifugal Blood Pump,”
Artificial Organs
, vol. 20, no. 6, 1996, pp 591-596; and K. Nishimura et al, “Development of a Magnetically Suspended Centrifugal Pump as a cardiac Assist Device for Long-Term Application,”
ASAIO Journal
, 1996, pp 68-71, respectively. These systems are still electrically complicated.
U.S. Pat. Nos. 4,944,748; 5,078,741; and 5,385,581 to Bramm et al disclose a magnetically suspended and rotated rotor for a blood pump which is supported by permanent magnets on the impeller and pump housing at each end of the pump to provide two passive journal or radial bearings, and the axial position stabilized by an electromagnetic on each end of the pump housing which interact with the permanent magnets on the impeller respectively to provide an actively controlled thrust bearing. The permanent magnets and electromagnets on the pump housing are radially spaced across a gap between the pump housing and the impeller from the impeller magnets respectively.
The radial and thrust bearings of the Bramm et al pump appear to be very soft while inefficiently using a lot of permanent magnetic material. In this regard, it has been suggested that a permanent magnet bearing stiffness is proportional to the permanent magnet cross-sectional area squared but inversely proportional to the fourth power of the average distance between the two magnets. See J. Yonnet, “Permanent Magnet Bearings and Coupling,”
IEEE Transactions on Magnetics
, vol. Mag-17, no. 1, January, 1981. Judging from the pump configuration, it is believed that this distance would have to be 0.5 inch or more thereby providing very soft radial bearings. Since the pump rotor of the Bramm et al patents may be super-critical (operating above a critical speed), slight shock load may cause the Bramm et al rotor to undesirably undergo large lateral excursions due to the bearing softness and lack of damping. The Bramm et al thrust bearing does not appear to be stiff or electrically efficient. The make-up of Bramm et al's axial sensor of infrared diodes and photo receivers is an indication of such thrust bearing softness due to its lacking sensing resolution or accuracy. It is also believed that the Bramm et al motor may be of the induction type which would not be electrically efficient due to the large gaps between the rotor and stator.
It is accordingly an object of the present invention to provide blood pump bearings which are suitably stiff.
It is another object of the present invention to provide blood pump bearings which will not damage the blood cells.
It is still another object of the present invention to provide blood pump bearings which are efficient (require little power consumption).
It is a further object of the present invention to provide blood pump bearings which are rugged, dependable, and maintenance-free.
It is yet another object of the present invention to provide blood pump bearings which are non-complicated and inexpensive.
In order to provide such blood pump bearings wherein a suitable stiffness may be obtained, in accordance with the present invention, at least one radially extending gap is provided between a stationary portion of the pump and the rotor, magnetic means for a journal bearing are provided on each of the stationary portion and the rotor and disposed in interacting facing relationship on opposite sides of at least one of the at least one gap, and magnetic means for a thrust bearing are provided on each of the stationary portion and the rotor and disposed in interacting facing relationship on opposite sides of at least one of the at least one gap.
Also in order to provide such bearings, in accordance with the present invention, the pump motor has a bearing span which is at least about 2½ times greater than an average radius of the distributed magnetic force.
Also in order to provide such bearings, in accordance with the present invention, the stator has at least one axial extension, an axially extending gap is provided between the stator and rotor, and interactive magnetic means for a journal bearing are disposed on opposite sides of the gap.
In order to provide such a blood pump thrust bearing, in accordance with the present invention, the thrust bearing comprises means responsive to axial displacement of the rotor for moving at least one pair of permanent magnets on the stator relative to at least one pair of permanent magnets on the rotor which are oriented in an attractive relationship thereto.
The above and other objects, features, and advantages of the present invention will be apparent in the following detailed description of a preferred embodiment thereof when read in conjunction with the accompanying drawings wherein the same reference numeral will denote the same or similar parts throughtout the several views.
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p
Mohawk Innovative Technology, Inc.
Simmons James C.
Tamai Karl I
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