In order to meet industry demands for improved productivity and part quality, machine tools must be equipped with faster and more accurate feed drives. Over the past two decades, research has focused on the development of new control strategies and smooth trajectory generation techniques. These developments, along with advances in actuator and sensor technology, have greatly improved the accuracy of motion delivery in high speed machine tools. However, further advancement is limited by the vibration of the machine’s structure. The purpose of the research in this thesis is to develop new control techniques that use active vibration damping to achieve bandwidths near the structural frequencies of machine tools, in order to provide better dynamic positioning of the tool and workpiece. Two machine tool drives have been considered in this study. The first is a precision ball screw drive, for which a pole-placement technique is developed to achieve active vibration damping, as well as high bandwidth disturbance rejection and positioning. The pole-placement approach is simple and effective, with an intuitive physical interpretation, which makes the tuning process straightforward in comparison to existing controllers which actively compensate for structural vibrations. The tracking performance of the drive is improved through feedforward control using inverted plant dynamics and a novel trajectory pre-filter. The pre-filter is designed to remove tracking error artifacts correlated to the velocity, acceleration, jerk and snap (4th derivative) of the commanded trajectory. By applying the least-squares method to the results of a single tracking experiment, the pre-filter can be tuned quickly and reliably. The proposed controller has been compared to a controller used commonly in industry (P-PI position-velocity cascade control), and has achieved a 40-55 percent reduction in peak errors during tracking and machining tests. The controller design, stability analysis, and experimental results are discussed. The second drive considered is a linear motor driven X-Y stage arranged as a T-type gantry and worktable. The worktable motion is controlled independently of the gantry using a loop shaping filter. The gantry is actuated by dual direct drive linear motors and is strongly coupled to the worktable position, which determines its inertial characteristics. A 94 Hz yaw mode is handled in the gantry control law using sensor and actuator averaging, and active vibration damping. The stability and robustness of the design are considered using multivariable frequency domain techniques. For the worktable motion along the gantry, a bandwidth of 130 Hz is achieved. The gantry crossover frequency is 52 Hz, which is 3 times higher than the bandwidth that can be achieved using independent PID controllers (16 Hz). The performance of the proposed control scheme has been verified in step disturbance (i.e., rope snap) tests, as well as tracking and contouring experiments.

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Active magnetic regenerator (AMR) refrigeration is an alternative technology to conventional vapor-compression refrigerators that has the potential to operate at higher efficiencies. Based on the magnetocaloric effect, this technology uses the magnetization and demagnetization of environmentally neutral solid refrigerants to produce a cooling effect. To become competitive however, a large amount of research into the optimal device configurations, operating parameters and refrigerants is still needed. To aid in this research, a simplified model for predicting the general trends of AMR devices at a low computational cost is developed. The derivation and implementation of the model for an arbitrary AMR is presented. Simulations from the model are compared to experimental results from two different devices and show good agreement across a wide range of operating parameters. The simplified model is also used to study the impacts of Curie temperature spacing, material weighting and devices on the performance of multilayered regenerators. Future applications of the simplified AMR model include costing and optimization programs where the low computational demand of the model can be fully exploited. / Graduate

We combine optical, X-ray (Einstein and ROSAT) and infrared (IRAS) data to investigate the evolution and spatial distribution of AGN with particular emphasis on the implications for the origin of the diffuse X-ray background. First, we derive the IRAS Seyfert luminosity function to test the continuity of properties between the Seyfert and the QSO population. The QSO luminosity function evolved back to z ~ 0, agrees well with the Seyfert luminosity function. In particular, the similarity of the faint parts of the two luminosity functions, suggests that the optical luminosity function is not severely affected by incompleteness due to reddening. We analyze the clustering properties of the IRAS Seyfert sample in order to probe the AGN clustering evolution. We detect clear clustering (5σ) at scales < 20 h(^-1). Comparing the Seyfert with the QSO clustering results at higher redshifts we find that a comoving model for AGN clustering evolution, where AGN clusters are expanding with the Hubble flow, is probably favoured by the data. Using new faint CCD observations we recalibrate the photometry of the Durham UVX catalogue of Boyle et al. (1990). We show that the luminosity function 'knee' feature claimed by Boyle et al. is not an artefact of photometric errors at faint magnitudes. We evaluate the contribution of Active Galactic Nuclei (AGN) to the X-ray background using this optical luminosity function and evolve it according to Pure Luminosity Evolution models. We estimate that AGN produce around half of the X-ray background at 2 keV. This contribution is consistent with the small scale (arcmin) fluctuations of the X-ray background for both the stable and comoving model of clustering evolution. If a large number of low luminosity AGN with high intrinsic absorption is missed by the optical surveys, AGN could produce all the 2 keV intensity. Although the uncertainty in the estimate of the AGN contribution is high, this work demonstrates, at least, that Pure Luminosity Evolution models are consistent with both the X-ray background intensity and anisotropy constraints. A recent deep ROSAT observation yields a high surface density of X-ray sources (> 100 deg(^-2). Spectroscopic follow up observations show that most of these sources are QSOs. The identified QSOs contribute ~ 30% at 1 keV and therefore this is the lower limit of the AGN contribution to the X-ray background. No other class of sources contributing substantially to the X-ray background has been yet identified.

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