This is a public disclosure of key ideas developed in our laboratory for magnetically suspending a heart pump impeller. This research was supported by the Charles E. Reed Faculty Initiatives Fund.
Our Precision Motion Control Lab designs, builds, and controls electromagnetic systems. We have developed magnetically levitated stages for photolithography, an Angstrom positioning stage for scanned probe microscopy, and high force density linear motors. Recently, we envisioned a six-degree of freedom magnetic suspension capable of large rotations about one axis. Such a system is similar to one of our photolithography stages which is a six-degree of freedom magnetic suspension capable of large translations in two dimensions. However, the application for our rotating system is to magnetically suspend and rotate the impeller in a centrifugal artificial heart pump. This has recently been done with many magnetic actuators and a separate motor to spin the impeller.
Our idea is to use principles from our prior work---namely that the motor can act as the bearings and suspend the impeller without the need for many additional actuators---to accomplish this task more effectively.
Prior Magnetic Suspension
Our flying puck, magnetically levitated photolithography stage is an example of a six degree of freedom stage that integrates drive force and bearings into the same actuators. Won-jong Kim built this stage which is the first to provide all the focusing and alignment motions required for photolithography with one moving part. This is possible because the four linear motors provide suspension as well as driving forces for the 5.6 kg platen supported against gravity. This stage has a 50 mm travel in the x and y directions, 400 µm in the z direction, and can perform milliradian-scale rotations about each of its three axes. Three laser interferometers and three capacitance gauges are used for feedback. The stage has a position noise of 5 nm rms in the x and y directions and can accelerate at 10 m/s2.
Rotating Suspension Idea
A variant on this stage is one in which the suspended platen would be able to spin completely around its vertical (z) axis, and the other five degrees of freedom would be capable of fine adjustments. This could be accomplished by arranging the linear motors in a circular pattern. The suspension force for the moving part is provided by the linear motors just as in the photolithography stage. This spinning six degree of freedom stage could have many applications such as in vacuum pumps, machine spindles, and robotics. The application we are most excited about is as the impeller in a centrifugal artificial heart pump. Magnetic suspensions have recently been used in this regard due to the stringent requirements on such a device. Our proposed suspension and drive technique is superior to those currently used since it reduces the number of actuators required to magnetically suspend and spin the impeller.
Conventional Heart Pumps
Each year approximately 50,000 people need heart transplants, but only 2,000 hearts are available. Artificial hearts and left-ventricular-assist devices can sustain life until a donor heart can be found. Eventually, it is hoped that artificial hearts might be good enough to be a permanent solution themselves. The engineering problem of creating an artificial heart is complex. Since the 1950's researchers have designed many mechanical heart pumps. Reliability problems with mechanical parts, valves, and clotting of blood have hampered these efforts. Many of these machines were also large and cumbersome whereas we would like to have a totally implantable design. In the last fifteen years the biomedical community has focused mostly on rotating pump designs since they are compact and avoid stagnation and clot formation in the blood. Mechanical ball bearings for the rotating pump have led to clotting and cell death, and studies show that these devices have lifetimes of only months. Lately, magnetic bearings have been used to completely suspend the pump's rotor in the heart pump. This solution allows large clearance passages for the blood flow and eliminates mechanical contact and wear.
Others have designed magnetic suspensions that use a brushless motor to spin the impeller and many electromagnets to regulate the other five degrees of freedom. Although all three axes and three rotations are controlled magnetically, the motoring mechanism rotating the impeller is totally separate from the bearing mechanisms used to control the three translations, and two other rotations.
Integrated Drive & Bearings
Our design unifies the bearings and the motor. Our motor spins the impeller and also can regulate the other five degrees of freedom. This results in a simpler, more compact design. Since each segment of the motor can provide drive and suspension forces, it is easier to design for redundancy and robustness which are essential in this application.
Figure 1 shows a possible layout for our integrated motors. We show six motors, but only three are required to generate all required motions. The impeller has a single circular magnet array on one side which faces a set of stators in the heart pump housing. Each stator can generate vertical suspension force and horizontal drive forces as shown in Figure 2. Differential operation of the motors allows control of all six degrees of freedom.
Figure 1: We show the actuators which suspend and spin the impeller. The impeller has a single circular magnet array on one side which faces a set of stators in the heart pump housing. Only three stators are needed to control all six degrees of freedom including full rotations about an axis out of the paper. We show six stators in this design so three of them are redundant.
Figure 2: A cross sectional view of the impeller is shown. Each stator can generate vertical suspension and horizontal drive forces. Differential operation of the motors allows control of all six degrees of freedom.
Figure 3 shows examples of how the same actuators can drive the impeller and act as bearings. In Figure 3 (A), we see that we can generate torque to spin the impeller for its normal operation. This configuration of forces is equivalent to the rotary motor in conventional designs which separate the motor and bearings into different actuators. In Figure 3 (B), we show a fine translation adjustment using just two of our actuators. These are the same actuators that spin the impeller, but now they are acting as bearings. The other four degrees of freedom can be generated in a similar fashion; three of them require the use of the vertical forces shown in Figure 2.
Figure 3: We show two examples of how the forces generated by the actuators can be combined to produce rotations and translations. We use only three actuators in these examples. In (A) we show the forces producing the normal rotation of the impeller. In (B) we show a fine translation adjustment.
The advantages of our design are:
Improved magnetic actuator and suspension design over existing designs comprising a multitude of actuators. The actuators can provide both suspension and drive forces to simplify the magnetic suspension.
Compact and integrated control, sensing, and power electronics so that the system will be implantable.
Redundant and robust actuators and sensors so that the pump will continue to work at some level in case of partial actuator or sensor failure.
Neutrally buoyant impeller to resist disturbance forces which will be encountered as the heart pump is accelerated when its host person moves around.
Low power consumption so that recharging is required less frequently.
Self-sensing motor operation by using the back EMF to get position and allow for sensorless commutation.