Parameter estimation of hybrid magnetic bearings system for axial flow blood pump applications with possible self-sensing capability

The active magnetic bearing (AMB) system has been widely employed in industrial applications, among which one of the most important applications is the magnetically levitated rotary blood pumps. It is because AMB can replace traditional ball bearings and realize the non-contact support of rotating impellers/rotors of the pumps. Current state-of-the art ventricular assist devices (VADs) are magnetically suspended rotary pumps, and it is due to the advantages of the magnetic bearings: no mechanical contact between the bearing and rotor, no lubrication needed, less heat generation and lower hemolytic risk. In this research, a completely magnetically levitated axial flow blood pump incorporating a hybrid magnetic bearings (HMBs) system has been designed and developed. The impeller is enclosed in the rotor, which is controlled in five degrees-of-freedom (DOF), among which four radial directions are actively controlled and one axial direction is passively controlled. The HMBs system is composed of two hybrid magnetic bearings for suspending the rotor of the pump with PID controllers, and one brushless and sensorless three-phase permanent magnet (PM) motor for driving the rotor. The design target of the axial flow blood pump is to realize 100 mmHg differential pressure and 5 L/min flow rate. The HMBs system of the axial flow blood pump has been tested to be able to achieve good system performance in air: the pump rotor can rotate in stable suspension at speeds of up to 14,000 rpm. The method of parameter estimation has been proposed to extract the stiffness and damping properties of the HMBs system employed by the axial flow blood pump. A special test rig has been designed such that the rotor of the pump can be perturbed in suspension both in the radial and axial directions, whether it is stationary or rotating. Schroeder Phased Harmonics Sequence (SPHS) is used as the multi-frequency test input signal of the parameter estimation. The dynamic model of the HMBs is validated by the high goodness of fit using statistical analysis. The actuator gain of the HMBs is determined to be linear within 20% of the static eccentricity ratio. Self-sensing parameter estimator for AMB system has been developed and tested. The switching frequency component of the coil current contains the air gap length between the stator and the suspended target. Current demodulation technique is used to extract the air gap length from the coil current with demodulation filters, and the result is the function of the air gap length and the duty cycle of the PWM amplifiers. Self-sensing parameter estimation is composed of two identical demodulation filters, one coil inductance simulator and one PI convergence controller. The output of one demodulation filter is compared with the output of the other demodulation filter, which extracts the simulated coil currents with the input of actual switching voltages and the estimated air gap length. The error between the two demodulation filters is processed by the PI convergence controller to correct the estimated air gap. Benefiting from the close loop characteristics of the self-sensing parameter estimation, not only is the influence of the duty cycle removed, but the dynamic response of the self-sensing system is also greatly increased. ii pump. All of the axial blood pump blades are machined using 5-axis machining with the aid of SolidCAM®. Figure 3.18 shows the user interface of SolidCAM®. 3.3 Development of the HMBs, the rotor and the Lorentz-typed mo r 3.3. 1 Design of the HMBs In order to avoid the use of thrust bearings and make the axial flow blood pump more compact, while at the same time providing strong enough control force in the axial direction, the homopolar HMBs are utilized: an axially polarized permanent magnet is sandwiched in the rotor between the two set of lamination rings to provide the bias flux for the magnetic bearings; meanwhile the radial directions of the HMBs will be actively controlled and the axial direction will be passively controlled. The scheme of flux flow in HMB is shown in Figure 3.19. The cross section view of the stator is like "H" shape, so that the coils can be wound around the L2 as shown in Figure 3.19. The PM provides the bias magnetic flux which starts fro its north pole, passes the left lamination ring on the rotor, then passes the air gap, then flows along the lamination rings: Ll, L2 and L3, after that it returns back from the air gap to the right lamination ring on the rotor, and at last it returns to the south pole to close the flux loop. Besides the primary magnetic field in the working air gap provided by the PM, the electromagnetic coils will also generate magnetic field that add to or subtract from the PM field. The net result of this approach to designing magnetic bearings is lower power consumption for the same load capacity or the same stiffness. 51 ATTENTION: The Singapore Copyright Act applies to the use of this document. Nanyang Technological University Library