Doctoral Thesis: Bearingless Slice Motor and 6-DoF Position Sensor for Extracorporeal Pump

Krishan Kant

Abstract: Bearingless slice motors perform the function of a conventional motor with a suspended rotor, and therefore require no bearings or other mechanical support. Such combined motor and suspension operation requires a motor design that can generate force as well torque in order to stabilize all 6 degrees of freedom. The slice configuration of such motors uses passive stability in the axial and tilt directions. Hence no closed loop control is required for these degrees of freedom motions. In this thesis, the intended application for the bearingless motor is an extracorporeal blood pump. Extracorporeal blood pumps are used to maintain blood flow as a temporary ventricular assist device and/or blood oxygenator. Bearingless pumps can help to avoid blood damage caused by friction and heat generation due to seals and bearings. These pumps are also single use disposable, and reducing the cost of the pump elements reduces the operational cost.
This thesis presents the design, implementation and testing of a novel bearingless blood pump.
Two bearingless motors are considered here: a split teeth flux reversal motor and an interior permanent magnet motor. A novel bearingless motor design with magnet-less rotor is ideated from the conventional flux reversal motor. The stator of this motor contains magnets which enhance the airgap flux and thereby make the motor more power efficient. The new bearingless motor can also generate force on the rotor independent of the rotor angle. This simplifies the radial suspension control design. This motor has a magnet-less rotor and still has a comparable passive magnetic stiffness, torque and force capability to the motors with permanent magnets in the rotor. 

An interior permanent magnet motor (IPM) is the other motor which is designed and tested in this thesis. This motor uses a redesigned rotor derived from an earlier motor version such that the motor can stably operate at higher speeds. It was found in the earlier motor that cogging torque can generate significant disturbances in the radial positions and drive the suspension unstable at higher speeds. Therefore, the cogging torque became an important design parameter for this new IPM rotor.

Another important part of the thesis is the design and development of a 6 degrees of freedom position sensor. The major challenge was to get all the measurements while only accessing the bottom surface of the rotor. The sensor is composed of two parts: the first part measures the radial positions (X/Y) and the other part measures the Z, theta_X, theta_Y and theta_Z motions. Both work on the principle of eddy currents. The radial position (X/Y) sensor has a linear variable differential transformer(LVDT) type structure where the difference in the induced voltages yields the position measurement. This LVDT sensor uses a conductive aluminium target glued to the bottom surface of the rotor. With rotor motion, the target motion varies the induced voltages in the coils which yields the position estimates. Since this sensor is placed under the rotor, the radial and axial motions are strongly coupled. However, with an appropriate target design the coupling is minimized. The 4 DoF (Z, theta_X, theta_Y and theta_Z)  position sensor uses a printed circuit board (PCB) with 8 coils. These coils are driven by inductance to digital converters measuring the inductances of the coils. The conductive target is designed such that the measured inductance variation in these 8 coils can provide measurements for positions in the 4 DoF. The radial sensor is the most critical for bearingless operation. It achieves 1 kHz bandwidth and better than 1.2  micrometers resolution in the X and Y DoF.

Both the motors are integrated with a pump and tested in a closed loop flow circuit. The flux reversal motor is tested with water and the IPM motor is tested with water and also with blood flowing through an oxygenator.

The motor stator coils are driven with custom designed current-controlled switching amplifiers. The amplifiers are designed to achieve a high current control bandwidth. They are also designed and adapted to minimize interference with the position sensors.

All the components required for a blood pump are designed and developed in this thesis. The design and implementation of individual components poses various challenges. However, another major challenge is to integrate and operate them together as a blood pump such that they do not interfere with each other’s operation. All these various challenges are handled in the thesis and a successful operation of the blood pump system is carried out. 

Details

• Date: Thursday, December 8
• Time: 11:00 am - 12:30 pm
• Category: Thesis Defense
• Location: 35-520 (Given Lounge)

Additional Location Details:

Thesis Supervisor: Prof. David Trumper

Precision Motor Control Group