| Heart failure is the serious stage in development of heart disease and life-threatening for the patients. Since heart donors are always not ready in time for the heart transplant, blood pump is necessary for most patients to assist the blood circulation and survive till the appropriate heart donor is found. Blood pumps also can help diseased hearts healing, which is an important non-medicine treatment for heart failure. Now the second generation blood pumps are used in the clinical, but their mechanical bearings will cause heat, wear, large damage to blood, hemolysis, thrombus and other problems, which limited their development and application. Magnetic bearings (MB) can support a rotor without contact. This will eliminate the mechanical friction and wear, and reduce the damage to blood and the probability of hemolysis and thrombus, and also increase the use life of blood pumps. Therefore, study on the maglev blood pump for human transplant has very important scientific and social significance.Currently, the research on the maglev blood pump has got many achievements, but a few maglev blood pumps can be used for clinical application and many parts need to be improved and optimized. Maglev blood pump is a mechatronic device, which highly integrated several advanced knowledge, including mechanics, rotor dynamics, fluid mechanics, electromagnetics, automatic control technology, sensor technology, power electronics and so on. Its key components are MBs, impeller rotor and drive motor. The impeller rotor and drive motor can be designed and optimized according to those of the second generation blood pumps. Since there is no systemical theory for MB in blood pumps, the technology of magnetic bearing needs to be studied intensively. Therefore, the study on the magnetic bearing system is a major and important research work in maglev blood pumps.Magnetic bearings can be classified as active magnetic bearings (AMB), permanent magnet bearings (PMB), hybrid magnetic bearings (HMB) and so on. The structure and control system of AMB are complicated with high power consumption, but it can levitate a rotor in five degrees of freedom (DOF) actively. With simple structure, PMB is without control system and power consumption, but it cannot levitate the rotor in five DOFs. HMB also can achieve five-DOF-levitation with low power consumption but more complicated control system. In brief, the levitation system with radial PMB and axial AMB is suitable for maglev blood pump. The emphases and difficulties are how to perfectly combine these two types of MBs in the small space of blood pump.In this thesis, the levitation system with radial PMB and axial AMB for maglev blood pump is studied intensively. A prototype of maglev blood pump is designed and fabricated. Its rotor is levitated stably in five DOFs. A complete method of system design, analysis and modification is studied, including mechanical design, system modeling, controller design, rotor displacement detection and system debugging of maglev blood pump. The characteristics of radial PMB and axial AMB and the coupling between them are systemically studied. Based on the coupling, the scheme of maglev blood pump combined with radial PMB and axial AMB is proposed. The whole system is optimized in detail. Based on the magnetic field coupling between magnet rings of radial PMB, axial displacement detection method based on Hall sensors is presented. Analog PID controller and linear power amplifier is designed for axial AMB. The experiments of rotor levitation and rotation of maglev blood pump prototype are carried out. The main contents of this thesis are listed as follows:(1)The principle, structure and mechanical properties of radial PMB are studied. With the finite element model, the characteristics of capacity and stiffness of radial PMB are studied by finite element method. It is analyzed that the relationship between the stiffness of radial PMB and the relative axial displacement between the rotor ring and the stator ring of radial PMB. The relationship between the stiffness and the radial thickness and axial length of magnet rings is also obtained. According to the rated rotation speed of blood pump rotor and human physiological requirements, radial PMB is designed and optimized.(2) The mechanical characteristics of axial AMB and the coupling of magnetic force and stiffness between axial AMB and radial PMB are studied. The system scheme of MB system is proposed based on the coupling relationship. The model of capability of axial AMB is established and its mechanical characteristics are studied. By analyzing the force balance of rotor in maglev blood pump, the coupling of magnetic force and stiffness between axial AMB and radial PMB is analyzed. Based on this coupling, two design schemes of MB system for maglev blood pump are proposed:low power consumption design scheme and small volume design scheme. The design rules of two schemes are also given. The axial AMB system is designed and optimized. The problems in system design and control caused by the stiffness coupling between axial AMB and radial PMB are solved.(3) Based on the analysis of magnetic field of magnet ring in radial PMB, the method of rotor axial displacement detection with Hall sensor is proposed for the closed loop control of axial AMB. Traditional displacement detection methods need additional target, which will increase the weight and volume of blood pump and lead to system complication. In order for small volume and low power consumption, the relationship between the output voltage of Hall sensor and the magnet field intensity of rotor ring in radial PMB is studied. The designed method utilizes Hall sensor to detect the magnetic intensity variation in the sensor position to reflect the rotor axial displacement. This method eliminated the additional target, which decrease the length, volume and weight of blood pump. The coupling between the rotor radial displacement and axial displacement is analyzed. The method to eliminate the influence of rotor radial displacement on axial displacement detection is proposed. Based on the analysis of magnetic field of magnet ring in radial PMB, symmetrical multi Hall sensor arrangement is proposed to achieve high dectecting accuracy. The system to detect rotor axial displacement from rotor radial direction is designed with good accuracy and linearity. Its output signal will be transmitted to the controller of axial AMB.(4) Control system of axial AMB in maglev blood pump is designed. With analysis of the influence of rotor gravity and blood pressureon rotor movement, the dynamic model of maglev blood pump rotor is created and analyzed, considering the rotor gravity as disturbance. Analog PID controller and linear power amplifier for axial AMB in maglev blood pump are designed and simulated. The performance of the designed control system, such as frequency bandwidth, delay of phase and so on, is tested and debugged until they meet the system requirements.(5) The first axial flow maglev blood pump prototype is designed and fabricated at home. The prototype utilizes MB system combined with radial PMB and single axial AMB, rotor axial displacement detection system based on Hall sensors, analog PID controller and linear power amplifier. Startup experiments, rotor static levitation experiments, anti-jamming experiments and rotor rotation experiments were carried out on the prototype. The prototype rotor reached the middle levitation position quickly. Axial levitation accuracy under static levitation is 1.6μm. After a disturbance, rotor returns to the original levitation position rapidly after a few oscillations. Driving by permanent magnetic brushless DC motor, the rotor reached its maximum rotation speed in air,5100r/min and the maximum amplitude of rotor axial vibration is less than 7μm. The experimental results show that the prototype system has good performance in startup, anti-jamming with good levitation accuracy.A set of methods for designing, analyzing and debugging in MB system of maglev blood pump is established in this thesis. The research results show that the theoretical analysis, experimental research methods, dynamic analysis and control system analysis during the theoretical analysis and design are important and valuable research results for the study on maglev blood pump. The MB system combined with radial PMB and axial AMB and rotor axial displacement detection method are feasible. The research in this thesis lays foundation for study of maglev blood pump and its clinical applications, which has very important theoretical and practical significance. |
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