GRADUATE SCHOOL OF SCIENCES & ENGINEERING
ELECTRICAL AND ELECTRONICS ENGINEERING
PhD THESIS DEFENSE BY HARIS SHEH ZAD
Title: Design and Implementation of Magnetic Bearings in Rotary Blood Pumps
Speaker: Haris Sheh Zad
Time: August 16, 2017, 11:00
Place: MARC Lab, ENG B203
Rumeli Feneri Yolu
Thesis Committee Members:
Prof. Dr. Alper Erdogan (Advisor, Koc University)
Prof. Dr. Ismail Lazoglu (Co-Advisor, Koc University)
Asst. Prof. Dr. Sedat Nizamoglu (Koc University)
Assoc. Prof. Dr. Kaan Güven (Koc University)
Assoc. Prof. Dr. Çetin Yilmaz (Boğaziçi University)
Prof. Dr. Ali Fuat Ercenç (Istanbul Technical University)
Magnetic bearings have applications in the fields of moment wheels, molecular pumps, high speed motors, flywheel energy storage systems and ventricular assist devices. They have the advantages of non-contact operation, lower friction, less heat generation and less power consumption as compared to the conventional mechanical bearings. There are two types of the magnetic bearings, active magnetic bearings and passive magnetic bearings. Passive magnetic bearings make use of permanent magnets to generate the bearing forces. Their advantage is that no electrical power is needed for their operation and also easier control of the bearing stiffness and damping. Passive magnetic bearings can stabilize a rotor in only four out of five degrees of freedom. Therefore, active control is needed in the fifth degree of freedom to completely stabilize the rotor. Electromagnets are used for the operation of active magnetic bearings. The combination of these two types is known as the hybrid magnetic bearings. Hybrid magnetic bearings are used in the blood pumps, vacuum pumps, and machine tools etc.
For designing the hybrid magnetic bearings, the distribution of the magnetic fields across the electromagnets and the permanent magnets is a major factor because the damping and the stiffness of the hybrid magnetic bearings depend upon the strength of the magnetic field. However, the measurement of the magnetic flux density is not easier because of the smaller airgaps between the rotor and the stator. Also, the damping and the stiffness are nonlinearly related to the control currents, magnetic flux and the airgaps. For designing the magnetic bearings, magnetic circuit method is used widely. However, in this method, the leakage flux, eddy currents, effects of fringing, the hysteresis effects, reluctances and the nonlinearities are ignored. As there are lot of assumptions and simplifications involved in the magnetic circuit method, the models obtained through this method do not provide the exact distribution of the magnetic fields. Also, the damping and the stiffness obtained through this method have low precision. Therefore, a field method more precise than the magnetic circuit method should be used for designing the hybrid magnetic bearings. Finite element Method (FEM) analysis is based on numerical tools. As compared to the magnetic circuit method, the models obtained using FEM analysis are more identical to the corresponding physical prototype system.
In this research, a hybrid magnetic bearing prototype setup is designed using the FEM analysis. The damping, stiffness, magnetic flux distribution, forces in the radial and axial direction, the number of turns and the current for the electromagnets are optimized using FEM analysis. Based upon the optimization performed, a prototype setup is manufactured and the different properties of the hybrid magnetic bearing prototype are experimentally verified.
The control of the magnetic bearing system is a critical problem in the presence of the disturbances, unmodeled dynamics and nonlinearities. The classical control techniques such as PID control cannot give robustness against theses disturbances and parameter variations. Therefore, an advanced adaptive sliding mode controller is designed for the hybrid magnetic bearing system and experimentally applied to the prototype setup to see the effectiveness of the designed controller.
The third-generation axial flow blood pumps utilizing the magnetic bearings suffer from the problem of longer rotor size in the axial direction. The rotor of the axial flow pump resembles a column, therefore only one magnetic bearing cannot stabilize the rotor in the tilt direction and in the radial direction. At least two radial magnetic bearings are needed for the axial flow pump to fully stabilize the impeller of the pump with additional motor unit as well to rotate the impeller. Also, at least two bearingless motor units are required for the complete operation of the third-generation axial flow pumps.
The aim of this research is to design a third-generation axial flow blood pump using only one bearingless motor unit for the rotation as well the levitation of the impeller. The computational fluid dynamics (CFD) analysis is performed in order to optimize the design of the impeller and overall pump system. An enclosed impeller based rotor system is proposed based upon the optimization performed, in which the rotor axial length is much smaller as compared to the diameter of the rotor. Therefore, only one bearingless motor system is required for the complete operation of the pump.
Generally, eddy current sensors are used for measuring the rotor radial position in the bearingless brushless DC motors and bearingless permanent magnet synchronous motor systems, while Hall Effect sensors are used for measuring the rotor angular position. The eddy current sensor’s application in the proposed optimized design of the third-generation blood pump is not possible due to the much smaller diameter of the pump. Therefore, a sensor system must be developed in order to measure the radial position of the rotor in the proposed design.
In this research, a novel design for the rotor assembly of the novel blood pump is proposed, in which the rotor radial position is measured with the linear Hall Effect sensors in addition to the rotor angular position. The working principle of the sensor assembly is explained in detail and the rotor control system is explained based upon the Hall Effect radial position sensors. Finite element analysis of the novel design of bearingless motor for the miniature axial flow blood pump is also performed. The passive and active stiffness, damping, magnetic flux distribution, forces and currents are obtained through the finite element analysis and compared with the experimental data. The results obtained through finite element analysis show a good agreement with the experimental data. An experimental setup of the designed bearingless motor system is manufactured. The performance of the proposed bearingless permanent magnet motor is evaluated under various test conditions.
The designed adaptive controller is applied to the novel design of the bearingless motor in which the radial direction displacement of the rotor is measured using Hall Effect sensors. There are two permanent magnet rings in the rotor assembly, one ring is used for measuring the rotor radial direction displacement and one ring is used for generating the torque for the bearingless motor and the radial bearing forces. Therefore, strong magnetic coupling exists in the rotor system and accurate radial position is required for the advanced controller implementation. Therefore, the radial position of the rotor is first adaptively estimated and then a sliding mode controller is designed for the position control of the proposed bearingless motor system. The simulation and experimental results show better position tracking of the proposed bearingless motor using the designed adaptive controller as compared to the classical control techniques.