Resonant Magnetic Field Sensors Based On MEMS Technology

Microelectromechanical systems (MEMS) technology allows the integration of magnetic field sensors with electronic components, which presents important advantages such as small size, light weight, minimum power consumption, low cost, better sensitivity and high resolution. We present a discussion and review of resonant magnetic field sensors based on MEMS technology. In practice, these sensors exploit the Lorentz force in order to detect external magnetic fields through the displacement of resonant structures, which are measured with optical, capacitive, and piezoresistive sensing techniques. From these, the optical sensing presents immunity to electromagnetic interference (EMI) and reduces the read-out electronic complexity. Moreover, piezoresistive sensing requires an easy fabrication process as well as a standard packaging. A description of the operation mechanisms, advantages and drawbacks of each sensor is considered. MEMS magnetic field sensors are a potential alternative for numerous applications, including the automotive industry, military, medical, telecommunications, oceanographic, spatial, and environment science. In addition, future markets will need the development of several sensors on a single chip for measuring different parameters such as the magnetic field, pressure, temperature and acceleration

Condition Monitoring and Fault Detection for Electrical Machines Using Advanced Sensing Techniques Based on Fibre Bragg Gratings

Emerging techniques are being researched to expand the suite of condition monitoring solutions available for electric machines to adapt to a world of net zero carbon emissions. This research investigates the use of fibre bragg gratings (FBG) for condition monitoring and fault detection in three 2.2kW induction motors (IMs) using stray flux in a non-invasive manner. Optical fibre is immune to electromagnetic interference (EMI) which is an advantage but limits its direct use for magnetic field sensing. A magnetostrictive transducer, terfenol-D was bonded to FBG to form a composite sensor - FBG-T. The FBG-T was inserted into an acrylic tube - which is unaffected by magnetic field - and then positioned both axially and transversely relative to the machine’s rotor shaft at the drive end (DE). The transverse position showed better repeatability and sensitivity over different operating frequencies. Temperature and magnetic flux calibrations of the FBG-T sensor gave sensitivities of 20.77 picometre per degree Celsius (pm/°C) and 19.38 picometre per micro-tesla (pm/μT) respectively. Various investigations were carried out at different operating frequencies and under three motor conditions viz: healthy, broken rotor and inter-turn short circuit conditions. Experimental results confirm that the FBG-T sensor reliably distinguished each of the three machine conditions using different orders of magnitudes of braggshifts. The FBG-T sensor accurately detected faults with the short circuit condition reaching braggshifts of hundreds of pm. Healthy and broken rotor conditions reached braggshifts in the low–to-mid-hundred and high-hundred pm range respectively. Fast Fourier Transform (FFT) analysis performed on the measured stray flux showed that not only its amplitude but also the harmonic component of its spectrum, affected the magnetostrictive behaviour of the magnetic dipoles of the terfenol-D transducer. This effect was translated into strain on the FBG. The investigation proved that FBG technology can reliably and accurately monitor the condition of the motors as well as detect faults in a non-intrusive manner

Adaptive optics for extreme ultraviolet lithography : actuator design and validation for deformable mirror concepts

In the production of integrated circuits (e.g. computer chips), optical lithography is used to transfer a pattern onto a semiconductor substrate (wafer). For lithographic systems using light in the ultraviolet band (EUV) with a 13.5nm nm wavelength, only reflective optics with multi-layers can reflect that light by means of interlayer interference, but these mirrors absorb around 30% of the incident light. Depending on pattern and beam shape, there is a nonuniform light distribution over the surface of the mirrors. This causes temperature gradients and therefore local deformations, due to different thermal expansions. To improve the throughput (wafers per hour), there is a demand to increase the source power, that will increase these deformations even further. Active mirrors are a solution to correct these deformations by reshaping the surface. This thesis addresses the challenges to accurate deform a mirror with high repeatability, meeting the requirements for implementation in a lithographic illumination machine. The main design criteria are vacuum compatibility, actuator stroke and the distance between actuators. Four different experimental mirrors, with increasing complexity, are successfully designed, realized and validated. All mirrors are equipped with thermomechanical actuators to either bend, or axially deform them. These actuators are free from mechanical hysteresis and therefore have a high position resolution with high reproducibility. Extensive finite element analysis is done, to maximize actuator stroke and minimize input power. All mirrors are tested and validated with interferometer surface measurements and thermocouple temperature measurements. The first experimental mirror with one thermo-mechanical bending actuator is successfully built and tested (chapter 2). To obtain a high mirror deflection at a given inserted actuator power, aluminum is chosen as the actuator material. The mirror is made from Zerodur® like the mirrors in the first EUV lithographic demonstration machines. A mirror deformation of 4:7 nm/C is achieved, where the inserted actuator power is 0.044 C/mW, meaning 0:21 nm/mW. The measured characteristic time constant is 10 s, meaning that for a given input, 63% of the steady state stroke is reached within that time scale. All values are close to the predicted ones from the models and also meet the requirements for implementation. To further investigate the concept and to measure the mechanical and thermal actuator coupling, an experimental mirror with four actuators is designed, developed and validated (chapter 3). It is an extension of the mirror with one actuator. In a single actuator step-response, a mirror deflection of 3.4 nm/C is achieved. A design optimization is proposed and successfully tested which reduces the actuator coupling from 30% to 10%, while the mirror deflection at the same input is reduced to 55%. Actuator speed is demonstrated while simultaneously heating all actuators with 3mW, which correspond with a mirror deformation of 33 pm/s. When using an adaptive mirror in an EUV lithography system, actuator strokes of 1 nm/min are required. The demonstrated actuator speed of 33 pm/s = 2 nm/min meets that requirement. The third and fourth mirror have actuators placed perpendicular to the surface (chapter 4). By placing the actuators on a thin back plate, the force loop is localized and therefore a lower actuator coupling is achieved. The results obtained from the third mirror with 7 actuators are close to the predicted values from the static and thermal models. Based on these good results, this actuation principle is implemented in a smaller deformable mirror with 19 actuators inside a 25mm beam diameter. A linear relation between actuator power and temperature of 0.190 C/mW and between power and averaged interactuator stroke of 0.13 nm/mW is achieved. So, the successfully realized mirror deflection is 0:68 nm/ C and no hysteresis is observed. For both mirrors a support frame is developed, that minimizes introduced surface deformations by temperature variations. Thermal step responses are fitted and both heating and cooling characteristic time constants are 2:5 s. The thermal actuator coupling from an energized actuator to its direct neighbor is 6:0, to their neighbors it is 1:3%. The total actuator coupling is approximated around 10%, based on the good agreement between simulated and measured inter-actuator stroke. Finally, chapter 5 summarizes the main findings from the different deformable mirrors and compares them. Also, suggestions for future research are given for implementation into a lithographic machine

Design, manufacture and test of a magnetic encoder

An new eddy current based magnetic position encoder structure is proposed and studied in this thesis. The encoder is composed of one read head and one scale with metal plates placed periodically on a substrate. The read head contains one emitter and two receiver pairs which are all rectangular planar coils. The electromagnetic coupling between the emitter and receivers were affected by the relative position of the scale. A system level analytical model of the proposed encoder structure has been derived, from which three different encoder signals forms were generated. An amplification and synchronous demodulation circuit has been designed and fabricated. The circuit board was used successfully to process the encoder output signals in the measurement. Four PCB encoder prototypes were fabricated. These encoder structures were studied using the ANSYS MaxwellTM software package. The simulated and measured results were compared. The best accuracy performance of the PCB encoder is -15 μm to 15 μm from the simulation results and -35 μm to 25 μm from the corresponding measurement. An alternative manufacturing process of the magnetic encoder based on multilayer Low Temperature Co-fired Ceramic (LTCC) technology has also been presented. The fabrication process of the LTCC encoder and equipment used were described. Two different methods were used to characterise the LTCC encoder with good agreement between all approaches attempted. The best accuracy performance of the LTCC encoder was -30 μm to 25 μm and after lookup table correction the improved accuracy ranged from -10 μm to 10 μm

Long-term structural health monitoring of plate-like structures using distributed guided wave sensors

Aircraft, containers, and storage tanks contain plate-like structures that are safety critical. The structures often undergo non-destructive inspections. The inspection frequency tends to be over-conservatively high, and it may be possible to reduce the intervals between inspections to realize cost savings. This goal can possibly be realized by automated structural health monitoring (SHM) of structures using sparse active guided wave sensor arrays. Guided waves are sensitive to small defects and can propagate long distances across feature dense plates. Thus, a guided wave SHM system that enables reliable detection of critical defects or monitoring of their growth can potentially be used to reduce the frequency of inspections for real structures. Industrial guided wave SHM systems must be reliable throughout prolonged exposure to temperature, humidity, and loading changes encountered in operation. Research at Imperial College shows that temperature compensation and subtraction between monitored guided wave signals and baselines acquired from healthy plates enables detection of 1.5% reflection change over areas ~1 m^2 in the presence of thermal swings and uniform liquid layers. These results and findings from scattering studies indicate it may be possible to detect reflections from hole type defects and notches affecting structures during their operation. An issue is that demonstrations of SHM system capabilities have only been shown in controlled laboratory tests within short periods following baseline acquisition. There is concern whether sustained exposure to service conditions will subject transducer elements to irreversible changes and introduce variability in baseline subtraction results that would mask signals due to slowly growing damage. This thesis studies the reliability of guided wave SHM for monitoring plate-like structures over longer time periods. The theoretical characteristics of the fundamental Lamb waves and their use to monitor and detect damage are reviewed. Strategies for sensing and signal processing are described alongside experimental validation of their performance. The effectiveness of the SHM system is tested in experiments where damage-free plates are exposed to British weather as well as thermal variations in an environmental chamber. The monitoring capabilities of bonded piezoelectric sensors are quantified and compared to the performance achieved using electromagnetic acoustic transducers. Experimental results and findings from simulations of bonded piezoelectric transduction establish that performances achieved with bonded sensors degrade due to variations in the properties of adhesives used to attach sensors to plates. EMATs are relatively stable and capable of enabling detection of 1.5% reflection change at points away from the edges of plates after sustained exposure to thermal cycling loads.Open Acces

Study of Orthogonal Fluxgate Sensor in Terms of Sensitivity and Noise

Ph.DDOCTOR OF PHILOSOPH

Time of flight diffraction and imaging (TOFDI)

Time of flight diffraction and imaging (TOFDI) is based on time of flight diffraction (TOFD), adding cross-sectional imaging of the sample bulk by exploiting the scattering of ultrasonic waves from bulk defects in metals. Multiple wave modes are emitted by a pulsed laser ultrasound ablative source, and received by a sparse array of receiving electromagnetic acoustic transducers (EMATs), for non-contact (linear) scanning, with mode-conversions whenever waves are scattered. Standard signal processing techniques, such as band-pass filters, reduce noise. A B-scan is formed from multiple data captures (A-scans), with time and scan position axes, and colour representing amplitude or magnitude. B-scans may contain horizontal lines from surface waves propagating directly from emitter to receiver, or via a back-wall, and angled lines after reflection off a surface edge. A Hough transform (HT), modified to deal with the constraints of a B-scan, can remove such lines. A parabola matched filter has been developed that identifies the features in the B-scan caused by scattering from point-like defects, reducing them to peaks and minimising noise. Multiple B-scans are combined to reduce noise further. The B-scan is also processed to form a cross-sectional image, enabling detection and positioning of multiple defects. The standard phase correlation technique applied to camera images, has been used to track the relative position between transducer and sample. Movement has been determined to sub-pixel precision, with a median accuracy of 0.01mm of linear movement (0.06 of a pixel), despite uneven illumination and the use of a basic low resolution camera. The prototype application is testing rough steel products formed by continuous casting, but the techniques created to facilitate operation of TOFDI are applicable elsewhere