Search

It is well known that a finite-size scatterer has a set of natural resonances, which are uniquely determined by the physical properties of the scatterer. This is also the case for a breast tumor which can be regarded as a dielectric scatterer. Since the scatterer is naturally "tuned" at the resonances, it is expected that an increased electromagnetic coupling would take place at the resonance frequencies compared to other frequencies. For a breast tumor, this would mean a higher power absorption, indicating a faster temperature increase resulting in more efficient hyperthermia. In this dissertation, an adaptive microwave concept is demonstrated for breast cancer applications. The general approach is to detect and identify the tumor-specific resonance, determine the electrical location of the tumor, and apply the focused microwave hyperthermia using the identified resonance and the electrical location. The natural resonances vary depending on the tumor size, shape, and breast tissue configuration. Therefore, an adaptive tuning of the microwave source to tumor-specific resonance frequencies could improve the overall efficiency of hyperthermia treatment by allowing for a faster and more effective heating to achieve a desired therapeutic temperature level. Applying the singularity expansion method (SEM), both the resonances and the electrical location can be obtained from the poles and residues, respectively. This SEM-based approach is computationally inexpensive and can easily be implemented as a combination processing into emerging UWB microwave systems. Alternatively, a relatively simple microwave system based on this concept can potentially be used in conjunction with existing mammography.

Vehicles and mechanisms which must perform very precise tasks or maneuvers require controllers to compensate for their inherent structural flexibility. Many of these applications involve structures that have time-varying dynamics, or have dynamics that are not considered in the traditional off-line controller design. These types of structures necessitate the use of adaptive control algorithms which can redesign themselves on-line in response to changes in the structural dynamics. This work describes an on-line control algorithm that uses the pole-zero spacings of the collocated control-to-output transfer function to design the optimum Positive Position Feedback (PPF) control law. The PPF control law uses second-order filters to add closed-loop damping to resonant structural modes. An on-line PPF design algorithm was developed based on the theoretical model of the collocated control-to-output transfer function. The optimal PPF filter parameters are shown to be a function of the pole-zero spacing in the collocated transfer function. These parameters were found by solving the pole placement problem using a theoretical model for various pole-zero spacings. The parameters are then stored in a lookup table in the realtime controller, and a frequency sweep algorithm identifies the pole-zero spacing on-line and designs the PPF filters using the parameters in the lookup table. A Phase-Locked Loop (PLL) was also studied as a means for adaptively tuning the PPF filters on-line. The PLL behavior in the presence of random and deterministic signals was characterized. The PLL was used experimentally to tune a PPF filter to a changing modal frequency. Analysis of the theoretical model indicated the amount of closed-loop damping a PPF filter can add monotonically increases with the amount of frequency spacing of the pole/zero pair. Experimental results with the on-line optimal PPF control algorithm proved it to be effective at adding damping to structures and suppressing vibration. The poles and zeros of the control-to-output transfer function were accurately identified by the pole/zero identification routine. However, the closed-loop performance was shown to be very dependent on the correct placement of sensor and actuator pairs. Tests with pointing control problems showed the algorithm to be better suited to vibration suppression rather than vibration isolation. Simulations and experiments with the phase-locked loop showed it to be unable to track a modal frequency of a structure excited by broadband noise. Bandpass prefilters would be necessary to eliminate the frequency content of the other modes, limiting the usefulness of the PLL.

A new method for wideband amplifier design has been developed. The significant feature of this method is the simultaneous determination of the optimized pole locations of the input and output side matching equalizer functions based on discrete frequency data. This produces upgraded performance of wideband amplifiers. With the coefficients of the factors of the denominator of the driving-point immittance functions as the independent variables, the Gewertz procedure is used to generate the driving-point immittance functions from their real parts. A method of calculating the exact gradients of the transducer power gain function and performance measure function with respect to these independent variables is presented and illustrated. To perform this study, (a) an optimization routine employing univariate search has been designed, (b) a technique for assuring positive independent variables in a univariate search has been developed, (c) a modified Richards transformation (K transformation) has been presented, which circumvents the difficulties in the realization of large valued inductors in microwave integrated circuits, and (d) practical design examples are given.

Synthesizing over headphones the free-field to eardrum transfer functions, or head-related transfer functions (HRTFs), of the human auditory system poses several difficulties from both signal processing and psychoacoustic perspectives. Viewed as a system approximation problem, an experiment and two analytic approaches are considered for designing digital filter implementations of HRTFs. Compared to an all-zero model of HRTFs, a pole-zero model would seem to promise fewer parameters and, consequently, a lower computational complexity associated with synthesizing 3-D acoustic fields. The number of parameters might be further reduced by using an appropriate error criterion in the design of these HRTF approximations. In order to identify approximation design criteria, results are presented from a psychophysical experiment that measured the ability of listeners to discriminate between measured HRTFs and their pole-zero model approximations. While a significant reduction in the number of parameters is achieved using a pole-zero, model, a least-squares error criterion may not be the most appropriate in designing the HRTF approximations. Instead, error criteria that are sensitive to log-magnitude spectrum differences between the measured and approximated HRTFs are likely to be more subjectively relevant. A pole-zero model design algorithm is presented, incorporating gradient search techniques to minimize both log-magnitude and phase response errors. Application of the algorithm to approximating HRTFs results in smaller pole-zero model orders than those required when using a least-squares error criterion. Moreover, the gradient search techniques provide a means to interpolate pole-zero models. In comparing two proposed interpolation methods to a standard all-zero model interpolation method, one of the proposed methods shows relatively poor performance, and the other shows performance similar to the all-zero model interpolation method. In a separate analysis, principal component analysis (PCA) is used to investigate the common and directional transfer function representation of HRTFs. Based on alternative assumptions from those made previously, results from PCA applied to measured HRTFs show that a common transfer function does not exist. Consequently, an alternative representation of HRTFs is proposed, and comparisons are made to the directional transfer function model.