This thesis deals with the problem of measuring current distribution in multi-brush solid armatures employed in electromagnetic rail launchers. The knowledge of current distribution is important because it affects the force distribution acting on the armature, the launch velocity, the launcher efficiency and the rail lifetime. Accurate measurements can help to reach a better understanding of the physical processes occurring at the rail-armature interface, that determine the current distribution. A new measurement method is developed, based on the use of external pick-up loops magnetically coupled to the launcher circuit; by integration of the induced voltages on these loops, the corresponding magnetic fluxes are calculated and the current distribution among the armature brushes is reconstructed from them. The main characteristic of this method is its conceptual simplicity (concerning both its physical basis and the circuit geometry) that allows to formulate a fully analytical model for the calculation of the mutual inductances between the loops and the launcher circuit, necessary for the current reconstruction. The possibility to have an analytical model at disposal offers several advantages: firstly, a parametric analysis is carried out providing general results independent of the particular experimental setup; secondly, the method sensitivity and the model uncertainty are analytically evaluated, as well as the propagation of this uncertainty to the uncertainty of the current distribution reconstructed according to the model. This in turn allows to identify in which conditions the method is more suitable to be applied and, on the other hand, in which conditions it is not able to provide meaningful results because of a lack of sensitivity. In particular, this method is compared, on a theoretical basis, to other methods based on local magnetic field measurements provided by directional sensors, such as Hall or B-dot probes. The use of pick-up loops much bigger than the conductor lengths in the launcher circuit is expected to improve the model accuracy because it decreases the effect of the uncertainty associated with loop positioning; in addition, it also decreases the induced voltages on the measurement circuits that connect the loops to the signal acquisition system, because they are sufficiently far from the launcher. These hypotheses are verified by an experimental analysis on a homemade launcher prototype appositely built, with a two-brush armature, confirming the validity of the analytical model and its uncertainty evaluation. Current distribution between the two brushes is successfully measured in static conditions (without armature motion), validated by independent measurements provided by small Rogowski coils placed around each brush. Preliminary satisfying measurement results obtained during launch are also presented, that encourage further developments of this method. Finally, a coaxial shunt with double-cage structure for the total launcher current measurement is also designed, according to the indications provided by a theoretical analysis of the shunt response. Also in this case, an analytical approach is employed for parametric modeling, that allows to formulate an analytical optimization problem and, as a consequence, to identify the best design solution. Furthermore, the analytical model makes also possible to study measurement circuit configurations and to implement suitable post-processing algorithms to compensate the shunt inductive response and extend its frequency bandwidth. The model validity and the inductance compensation are tested on a homemade shunt prototype.

Questa tesi affronta il problema della misura della distribuzione di corrente in armature solide multi-spazzola impiegate in lanciatori elettromagnetici a rotaia. Viene sviluppato un nuovo metodo di misura, basato sull’uso di spire esterne magneticamente accoppiate con il circuito del lanciatore; attraverso l’integrazione delle tensioni indotte su queste spire vengono calcolati i corrispondenti flussi magnetici e da questi viene ricostruita la distribuzione di corrente tra le spazzole dell’armatura. Per ulteriori dettagli si faccia riferimento all'abstract in lingua inglese.

Development of measurement methods for impulse currents in electromagnetic launchers

FERRERO, ROBERTO

Abstract

This thesis deals with the problem of measuring current distribution in multi-brush solid armatures employed in electromagnetic rail launchers. The knowledge of current distribution is important because it affects the force distribution acting on the armature, the launch velocity, the launcher efficiency and the rail lifetime. Accurate measurements can help to reach a better understanding of the physical processes occurring at the rail-armature interface, that determine the current distribution. A new measurement method is developed, based on the use of external pick-up loops magnetically coupled to the launcher circuit; by integration of the induced voltages on these loops, the corresponding magnetic fluxes are calculated and the current distribution among the armature brushes is reconstructed from them. The main characteristic of this method is its conceptual simplicity (concerning both its physical basis and the circuit geometry) that allows to formulate a fully analytical model for the calculation of the mutual inductances between the loops and the launcher circuit, necessary for the current reconstruction. The possibility to have an analytical model at disposal offers several advantages: firstly, a parametric analysis is carried out providing general results independent of the particular experimental setup; secondly, the method sensitivity and the model uncertainty are analytically evaluated, as well as the propagation of this uncertainty to the uncertainty of the current distribution reconstructed according to the model. This in turn allows to identify in which conditions the method is more suitable to be applied and, on the other hand, in which conditions it is not able to provide meaningful results because of a lack of sensitivity. In particular, this method is compared, on a theoretical basis, to other methods based on local magnetic field measurements provided by directional sensors, such as Hall or B-dot probes. The use of pick-up loops much bigger than the conductor lengths in the launcher circuit is expected to improve the model accuracy because it decreases the effect of the uncertainty associated with loop positioning; in addition, it also decreases the induced voltages on the measurement circuits that connect the loops to the signal acquisition system, because they are sufficiently far from the launcher. These hypotheses are verified by an experimental analysis on a homemade launcher prototype appositely built, with a two-brush armature, confirming the validity of the analytical model and its uncertainty evaluation. Current distribution between the two brushes is successfully measured in static conditions (without armature motion), validated by independent measurements provided by small Rogowski coils placed around each brush. Preliminary satisfying measurement results obtained during launch are also presented, that encourage further developments of this method. Finally, a coaxial shunt with double-cage structure for the total launcher current measurement is also designed, according to the indications provided by a theoretical analysis of the shunt response. Also in this case, an analytical approach is employed for parametric modeling, that allows to formulate an analytical optimization problem and, as a consequence, to identify the best design solution. Furthermore, the analytical model makes also possible to study measurement circuit configurations and to implement suitable post-processing algorithms to compensate the shunt inductive response and extend its frequency bandwidth. The model validity and the inductance compensation are tested on a homemade shunt prototype.
BERIZZI, ALBERTO
TELLINI, BERNARDO
22-mar-2013
Questa tesi affronta il problema della misura della distribuzione di corrente in armature solide multi-spazzola impiegate in lanciatori elettromagnetici a rotaia. Viene sviluppato un nuovo metodo di misura, basato sull’uso di spire esterne magneticamente accoppiate con il circuito del lanciatore; attraverso l’integrazione delle tensioni indotte su queste spire vengono calcolati i corrispondenti flussi magnetici e da questi viene ricostruita la distribuzione di corrente tra le spazzole dell’armatura. Per ulteriori dettagli si faccia riferimento all'abstract in lingua inglese.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10589/74302