PhD thesis defense to be held on July 9, 2024, at 11:00 (Virtually)

Picture Credit: Neoclis Hadjigeorgiou

Thesis title: Circuit and Systems Architectures and Performance Evaluation of AMR Magnetometers

Abstract: Magnetic fields and their measurement are an important part of human evolution. Navigation in the seas and oceans for many years was based on the magnetic compass. MRIs (Magnetic resonance imaging), an essential tool for soft tissue imaging in medicine, rely on the different properties of each material when exposed to a magnetic field. For the above reasons it was considered appropriate to examine the possibility of a contribution to the field of magnetic measurements and magnetic sensors.

As a contribution to the field of magnetic sensors, AMR (Anisotropic Magneto-Resistor) magnetic sensor technology was chosen for a number of reasons. The two main reasons for the above selection are firstly that AMR sensors have a compact size and secondly their manufacturing method is compatible with integrated circuit (IC) manufacturing technology. In addition, the AMR sensor is commercially available in a discrete form and is widely used. Despite the widespread use of the AMR sensor, it seems that so far, its behavior has not been studied in depth. In order to arrive at the choice of the AMR sensor, an extensive literature search was first carried out as well as a study of other sensor technologies used to measure the magnetic field. A large part of the literature review was devoted to magnetic sensor technologies. The remaining part of it refers to the physical phenomena that govern the behavior of the magnetic materials used in magnetic sensors.

This PhD thesis delves into the behavior of the AMR type sensor. The AMR sensor used is the Honeywell HMC100x (HMC1001 and HMC1002). Various circuits and experimental devices were developed to study the behavior of the above sensor. The behavior of the HMC100x sensor in continuous magnetic fields was investigated and characterized. A mathematical and circuit model was developed for its behavior at high frequencies, due to the problems of closed-loop instability at high frequencies. This instability is due to the flat construction of the AMR sensor under study. In addition, the response time of the sensor to high frequency current pulses was examined. These pulses were used to repolarize the sensor. Then, based on the previous results of the study, a new measurement architecture was developed which aims to improve the response of the sensor in question. Finally, three circuits based on this new architecture were developed. These circuits experimentally confirmed the correct and efficient operation of the new architecture in the response of the sensor.

Supervisor: Professor Paul P. Sotiriadis

PhD Student: Neoclis Hadjigeorgiou