Magnetic Bearings for Mechanical Cardiac Assist Devices
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By Steven Day, Shanbao Cheng, and Arnold David Gomez
Magnetic bearings are mechatronic devices that produce contact-free electromagnetic force to support a load, such as a moving train or a spinning rotor. Compared to traditional bearings, magnetic bearings offer several advantages: no friction, low heat generation, no required lubrication, quiet operation, and fast and stable rotation. Because of these reasons, magnetic bearings have been used in rotary ventricular assist devices (VADs) to increase design life, reduce or eliminate material wear and bearing maintenance, as well as to increase biocompatibility by eliminating high fluid stresses and heat generation, both of which are associated with hemolysis, platelet activation and aggregation, and thrombus growth. In this monograph, magnetic bearings and their application in VADs are introduced. First, the operating principles of magnetic bearings are introduced.
Typical structures of passive bearings, which are comprised solely of permanent magnets, and active magnetic bearings (AMB), which make use of electromagnets and position sensors to control the position of the rotor, are described. We include some description of all the components of a typical AMB system, including actuator, position sensor, controller, and amplifier, as well as different structures of electromagnet actuators, coils design, and iron selection.
A range of position sensors, including those typically used in blood pumps, as well as self-sensing bearings are discussed. The basics of the hardware and the software (control laws) that comprise a typical magnetic bearing system are described. A section describes the performance considerations of magnetic bearings and the effect on the overall performance of an assist device that uses magnetic bearings. Lastly, the magnetic suspension of existing VADs with magnetic bearings are described: Berlin Heart INCOR, Heartware HVAD, WorldHeart Levacor and MiFlow, Terumo Duraheart, PediaFlow, MiTiHeart, and LEV-VAD.