Teses / dissertações sobre o tema "Velocity"

To improve the speed measurement of construction equipment, different sensor technologies have been investigated. Many of these sensor technologies are very interesting but to keep the extent of the thesis only two was chosen for testing, magnetic absolute angle sensors using Hall and GMR technology, to investigate if those are a valid replacement for the current measurement system that is using a passive sensor. Tests show that these sensors are capable of speed measurement, but because of noisy angle estimates they need filtering for good speed computation. This filtering introduces a large time delay that is of significance for the quality of the estimate. A Kalman filter has been implemented in an attempt to lower the time delays but since only a very simple model has been used it does not give any improvements over ordinary low pass filtering. For these sensors the mounting tolerance is of great interest. For best performance the offset between the sensor and magnet centres need to be kept small for both sensors. This is due to a non-linearity effect this causes. The distance between the sensors and the magnet is not critical for linearity issues, but only for the quality of the signal, where it might drop out when the distance is too large. This is where the sensor using GMR technology stands out. Compared to the Hall technology sensor, the GMR sensor can handle distances that are more than 10 times larger. The conclusion is that these sensors can be a valid replacement of the current measurement system. They will introduce more functionality with the capability of detecting rotational direction and zero velocity. In an application with more than one sensor they can also be used for more purposes, like detecting slip in clutches etc. Depending on the application, the time delays may not be critical, else more work need to be done to improve the estimate, e.g. with a more advanced model for the Kalman filter.

Magnetic fields at all scales are prevalent in our universe. However, current cosmological models predict that initially the universe was bereft of large-scale fields. Standard magnetohydrodynamics (MHD) does not permit magnetogenesis; in the MHD Faraday’s law, the change in magnetic field B depends on B itself. Thus if B is initially zero, it will remain zero for all time. A more accurate physical model is needed to explain the origins of the galactic-scale magnetic fields observed today. In this thesis, I explore two velocity-driven mechanisms for magnetogenesis in 2-fluid plasma. The first is a novel kinematic ‘battery’ arising from convection of vorticity. A coupling between thermal and plasma oscillations, this non-relativistic mechanism can operate in flows that are incompressible, quasi-neutral and barotropic. The second mechanism results from inclusion of thermal effects in relativistic shear flow instabilities. In such flows, parallel perturbations are ubiquitously unstable at small scales, with growth rates of order with the plasma frequency over a defined range of parameter-space. Of these two processes, instabilities seem far more likely to account for galactic magnetic fields. Stable kinematic effects will, at best, be comparable to an ideal Biermann battery, which is suspected to be orders of magnitude too weak to produce the observed galactic fields. On the other hand, instabilities grow until saturation is reached, a topic that has yet to be explored in detail on cosmological scales. In addition to investigating these magnetogenesis sources, I derive a general dispersion relation for three dimensional, warm, two species plasma with discontinuous shear flow. The mathematics of relativistic plasma, sheared-flow instability and the Biermann battery are also discussed.

The present thesis describes research on quasi-static and low velocity perforation of rolled aluminium plates, where the main objective has been to gain a better knowledge of the physical processes taking place during this type of structural problem. The objective has been met by a combination of laboratory tests, material modelling and non-linear finite element simulations.

The thesis is organized in a synopsis, giving a brief introduction to the problem and summarising the main findings and conclusions, in addition to four independent papers.

Paper I presents an experimental technique for measuring the deformations the plate undergoes during impact and perforation. This information can be used to validate numerical models and to increase the understanding of how energy is absorbed by the plate.

Paper II presents an experimental and numerical investigation on the quasi-static perforation of AA5083-H116 aluminium plates. In the tests, square plates were mounted in a circular frame and penetrated by a cylindrical punch. A full factorial design was used to investigate the effects of varying plate thickness, boundary conditions, punch diameter and nose shape. Based on the obtained results, both the main and interaction effects on the maximum force, displacement at fracture and energy absorption until perforation were determined. The perforation process was then computer analysed using the nonlinear finite element code LS-DYNA. Simulations with axisymmetric elements, brick elements and shell elements were conducted. Slightly modified versions of the Johnson-Cook constitutive relation and fracture criterion were used to model the material behaviour. It was shown that the FEM models were able to predict the trends observed in experiments.

Paper III evaluates methods for determination of the anisotropic properties of polycrystalline metallic materials. Four calibration methods were evaluated for the linear transformation-based anisotropic yield function YLD2004-18p (Barlat et al., 2005) and the aluminium alloy AA5083-H116. The different parameter identifications are based on least squares fits to combinations of uniaxial tensile tests in seven directions with respect to the rolling direction, compression (upsetting) tests in the normal direction and stress states found using the full-constraint (FC) Taylor model for 690 evenly distributed strain paths. An elastic-plastic constitutive model based on YLD2004-18p has been implemented in a non-linear finite element code and used in finite element simulations of plane-strain tension tests, shear tests and upsetting tests. The experimental results as well as the Taylor model predictions can be satisfactorily reproduced by the considered yield function. However, the lacking ability of the Taylor model to quantitatively reproduce the experiments calls for more advanced texture models.

Paper IV presents an experimental and numerical investigation on low velocity perforation of AA5083-H116 aluminium plates. In the tests, square plates were mounted in a circular frame and penetrated by a cylindrical blunt-nosed projectile. The perforation process was then computer analysed using the nonlinear finite element code LS-DYNA, in order to investigate the effects of anisotropy, dynamic strain aging and thermal softening in low velocity impacts on the present aluminium alloy. Dynamic strain aging has been shown to influence both the predicted force level and fracture, while thermal softening only influences the fracture prediction. No effect of plastic anisotropy was observed.

The main purpose of this report, is to evaluate feasibility of using The Global Positioning System as an aiding tool for vehicle state estimation based on nonlinear techniques, and to develop a potential solution to the road bank angle problem. Previous work within the CEmACS project, includes development of a general nonlinear observer for lateral and longitudinal velocity, and an augmentation in the form of road-tire friction adaptation. Because the existing solutions have been shown to lack robustness with regards to certain disturbances, such as road grade and road bank angle, it has been stated that the estimation scheme should be upgraded, so that these disturbances can be accounted for. By including GPS velocity or a or a roll rate gyro measurement in the observer feedback loop, the possibility of detecting previously inobservable quantities is gained. In simple terms, evaluation of feasibility corresponds to demonstrating improvements and limitations of new solutions, using relatively crude methods in the test procedures. Problems related to the above mentioned task, are approached by means of signal processing and control theory. Following an intuitive sequence of operations, the report presents GPS theory and results first, as this lays the foundation for all subsequent results. Methods used comprise simple differentiaton, rotational kinematics and discrete filtering. Secondly, theory and results related to nonlinear observers, with focus on GPS aiding, are examined thoroughly. Lyapunov theory, known from control engineering, is used to evaluate stability, while data from simulations and actual vehicle tests is used to show how a new observer scheme can improve existing solutions. Before the most important results are presented, something should be said about their accuracy and significance. It has already been pointed out that the methods used are not based on optimality requirements, and consequently the results are best viewed as indicators of potential, rather than absolute solutions. This is especially true for the GPS velocity calculations, which are based on differentiation of position measurements; generally not a desired approach. In this report, it is firstly shown that GPS position measurements can be used to compute receiver velocity in the body-fixed coordinate frame. While this is a crude approach, resulting in relatively poor signal to noise ratio, it is easily implemented on low-level equipment. It is also shown that it is possible to use these velocities as measurements in a nonlinear observer structure, slightly modified from previous solutions within the CEmACS project. By doing this, accurate estimates of road grade and bank v angle are achieved, so long as these vary slowly enough. Stability of the observer is not proven in the general sense, but it is shown that it can be made stable through realistic assumptions and gain selection. Stability is further demonstrated through the use of data sets from actual vehicle tests. Secondly, a mathematical model of roll dynamics is combined with a roll rate measurement to create the possibility of detecting road bank angle. This is done by the now familiar nonlinear observer approach. Usefulness is demonstrated by simulations, but no stability proof is presented. The main conclusion is that it is feasible to use GPS aiding to account for robustness problems in a vehicle state estimation scheme, and that the inclusion of a roll rate measurement opens up possibilities for “cheap” bank angle detection. A direct implementation of results presented in the report may not be ideal, but the fact that the system works for a large set of conditions, suggests that it is worthwhile to develop it further. This is especially true when it is assumed that GPS receivers will become an integral part of new vehicles in the near future. Refinements and upgrades can be made in the form of more advanced GPS technology, new parameter estimation techniques and integration with the road-tire friction adaptation scheme.

When estimating tissue velocities, the conventional autocorrelation method (AM) is only biased if the demodulation frequency is correctly estimated. While AM assumes the received centre frequency to be constant, the modified autocorrelation method (MAM) estimates the received centre frequency continuously from pulse to pulse. Although MAM has shown superior performance in simulation environments, it fails to show consistently better performance compared to AM when applied to experimental data. In order to investigate this lack of consistency, a model for simulation of signal from moving tissue was developed, including realistic aspects such as thermal noise, signal from clutter and aliasing. The simulation model was adapted using experimental tissue data and parameters from a true acquisition system. A 1st order FIR filter was applied for clutter rejection prior to velocity estimation. The investigations using simulation data shown faster performance degradation of MAM compared to AM when the amount of signal from clutter or thermal noise were increased independently. For clutter signal mimicking acoustic noise from reverberations, MAM went from significantly better under low-noise conditions to approaching AM performance when the signal-to-clutter ratio became lower than 10 dB. Analogously, MAM approached AM performance when the signal-to-noise ratio was lower than 15 dB. Velocity estimation of experimental data shown MAM's robustness to frequency dependent attenuation by means of frequency compensation, while AM suffered from bias effects due to erroneously estimated demodulation frequency. The frequency compensation did, however, not succeed to approve lower estimation variance in MAM compared to AM. Statistical analysis based on expected values from simulations, demonstrated correlation between the estimation error in AM and MAM.