Armature Velocity Research Articles

Mechanism of Porosity Formation in Transfer Films in Electromagnetic Launchers

Melting occurs at the armature-rail interface during electromagnetic launch due to resistive heating. The liquid metal film at the interface is thought to decrease the friction and contact electrical resistance. The liquid metal is quenched at the rail surface, leaving a solid transfer film with surface properties that may affect the efficiency of subsequent launches. Notably, the transfer film contains pores which increase the overall rail electrical resistance. The reduction of atmospheric water vapor by liquid aluminum to produce hydrogen is thought to be the main source of porosity. To further examine the pore formation mechanism, we performed launch experiments using steel armatures and copper rails lubricated with two silver-bismuth alloys. Spherical pores similar to those observed in aluminum films are also present in silver-bismuth transfer films. Neither liquid silver nor bismuth is capable of reducing water vapor to hydrogen. This strongly suggests that the porosity is not due to hydrogen in these films. Air entrainment occurs in forced wetting when the three-phase contact line velocity exceeds a critical value. The critical wetting velocity is a function of surface tension and viscosity and is estimated to be 4-40 m/s for liquid bismuth at 600 K and a comparable value for liquid aluminum. Armature velocities exceed these values nearly instantaneously during launch. These results indicate that air entrainment is likely the source of porosity in transfer films.

Stress analysis of the rails of a new high velocity armature design in an electromagnetic launcher

In an electromagnetic launcher, the magnetic field creates a dynamic force that moves the armature forward. In an electromagnetic launcher, the armature reaches a critical velocity during the launch which causes high amplitude stress and strain. In addition, high stress and strain damage the rails and reduces its life span. The purpose of this paper is to investigate the effect of armature velocity as well as the rails physical and geometrical properties on the dynamic response of the rails in an electromagnetic launcher. In this study the second moment of inertia of the rails cross-section, Young modulus, foundation stiffness and density of the rails are constant in location and time. In our formulation of governing non-linear differential equations, Maxwell equations and deflection equation are applied to the rails under dynamic loading. To solve the non-linear governing differential equations a finite difference method is utilized.

Dynamic Response and Armature Critical Velocity Studies in an Electromagnetic Railgun

A model is developed to investigate the dynamic response of an electromagnetic railgun structure, induced by a moving magnetic pressure during launch of projectiles. When railgun armature velocity achieves critical velocity, resonance in railgun can occur and cause high amplitude stress and strain in armatures passage and causes life decreasing and damaging in rails. Critical velocity can be considered as one of the railgun dynamic design parameters. In this study, governing equations for rails under dynamic loading conditions has been investigated. Armature critical velocity according to design parameters has also been studied. The results show that dynamic response of the EM barrel becomes critical for increased projectile velocity and lightweight barrel designs. Also, the results of numerical simulation of the bore deformation are presented in this paper under applied loads. The algorithm is used to study the dynamics of tensile stresses near the electrode surface. Accordingly, the model that accounts for projectile velocity and gun construction can be used to guide and improve barrel design

Efficiency and scaling of constant inductance gradient DC electromagnetic launchers

We present efficiency and scaling relationships for dc (i.e., noninduction) constant inductance gradient electromagnetic launchers.We derive expressions for electromagnetic force, efficiency, back-voltage, and kinetic power in terms of electrical circuit parameters. We show that launcher efficiency is a simple function of armature velocity and the launcher???s characteristic velocity. The characteristic velocity characterizes the launcher and is the product of two new parameters: the mode constant and launcher constant. Mathematically, the launcher must operate at its characteristic velocity for 50% maximum efficiency. The mode constant reflects the manner in which the launcher is powered and its maximum efficiency. The launcher constant reflects the geometry of the launcher. We consider two modes of operation: constant current and zero exit current operation. We develop the ideal electromagnetic launcher concept and define it as operation at 100% maximum efficiency at all velocities.We also develop the concept of same-scale comparisons, that is, that electromagnetic launcher comparisons should be done with equal bore diameter, launcher length, projectile mass, and velocity. Finally, we present a comparative analysis based on experimental data of same-scale constant gradient electromagnetic launchers for conventional railgun, augmented railgun, and helical gun launchers in terms of the launcher constant, inductance gradient, bore diameter, bore length, system resistance, and armature (i.e., projectile) velocity.

A mathematical model of the reusable linear magnetic flux compressor

The reusable linear magnetic flux compressor not only can offer fairly high energy density and work repeatedly, but also can be statically inert prior to firing. Therefore, it can be regarded as an ideal pulsed power supply for the electric gun. Based on theoretical studies of this magnetic flux compressor, a mathematical model is developed and described in this paper. This model includes a 2-D hydrodynamic treatment, armature motion, dynamic inductance, dynamic resistance, flux loss, circuit quantities and 1-D diffusion for the magnetic field, current and temperature inside the conductors. In this model, the gas pressure, which depends on the propellant burn law, propellant mass, armature position and armature velocity, is modeled by 2-D homogeneous phase fluid-dynamic equations. The electromagnetic force, which depends on the current and the inductance gradient of the helical winding, is described by electromagnetic parameter equations. The 2-D homogeneous phase fluid-dynamic equations are coupled with electromagnetic parameter equations through the armature motion equation. The simulation results show that the proposed model is helpful to the practical design.

Comparison of sulfur hexafluoride and synthetic air in a simulated flux compression generator environment

Expendable, explosively driven shock tubes are used to propel aluminum flyer plates toward dense stainless steel plugs to simulate gas compression in explosive magnetic flux compression generators (FCGs). Armature velocity in a typical FCG is calculated using the Gurney method and shock tubes are designed to drive the flyer plates at similar velocities. Experiments with sulfur hexafluoride and synthetic air are presented.

The effect of impact velocity and steering wheel orientation on the crashworthiness of a steering wheel armature

Experimental testing which examines the effects of variations in the column angle and wheel angle of the steering wheel and the velocity at which a chestform impacts the steering wheel armature is the focus of this research. The capability of predicting loads and absorbed energy in any impact between the driver of a vehicle and the steering wheel is very significant in steering wheel design and occupant safety. Methods of illustrating the effects of the wheel angle and column angle (for a prescribed impact velocity) on peak impact load, peak displacement, and energy absorption are presented in this paper. It was observed that experimental observations (impact load, displacement, and energy absorption) are very dependent upon the orientation of the steering wheel armature and chestform velocity when impact occurs between the two entities.

Electrical and thermal effects of rail cladding

Achieving long rail life may require the use of a thin surface cladding of hard, refractory material. This paper reports numerical simulations of the effect of a resistive cladding on railgun efficiency. Simulations are performed using a three-dimensional (3-D) finite element code, Electro-Mechanical Analyses Program in 3 Dimensions (EMAP3D), that is capable of tracking moving conductors. Current and temperature distributions are calculated in two regions of interest: (i) behind the armature-where current diffuses into the rail in a two-dimensional manner, and (ii) in the portion of the rail adjacent to and under the armature-where current flow is fully three dimensional. The railgun model used for these simulations has a 40 mm square bore with a 1 mm thick cladding on the rail surface. The increase in electrical loss due to the cladding is a function of axial location and is largest where the armature velocity is low. We find that combined 2-D and 3-D effects cause a total additional thermal loss equal to about 5% of the muzzle energy when the cladding resistivity is 10 or 50 /spl mu//spl Omega/.cm. The decrease in total launcher efficiency is less than 1%.

Simulation of railgun gouging

Gouging in railgun rails involves the formation of teardrop-shaped depressions in the rails by a moving armature. Gouging is deleterious to rail performance and is a critical life-limiting defect. The precise cause of gouging is difficult to ascertain due to the highly dynamic nature of its formation and the complex electromechanical environment in which gouges form. A prevalent hypothesis is that gouging is the result of mechanical interaction of protrusions or local asperities with the armature/rail assemblage. Understanding of the gouging process would be appreciably advanced by accurate predictive mechanical modeling of the process. The majority of approaches to mechanical modeling do not embody the necessary features unique to gouging: high velocity/strain rate; elastic-plastic deformation; multiple materials interfaces; fracture capability; and extension to three dimensions. The goal of this research was to computer model the railgun under various conditions of firing, including the effects of armature/rail material pairs and armature velocity. Results of a series of simulations of materials damage are presented.

Experimental validation of a capacitor discharge induction launcher model

This paper describes the early stages of the experimental validation of a mathematical model of a capacitor discharge co-axial induction launching system. The model forms part of a family of computer programs produced to aid the design of launchers, so that either the maximum possible armature velocity on exit from the launcher or a specific performance requirement such as a controlled armature acceleration is achieved. The programs range in complexity from relatively simple models, used for initial capability studies, to detailed representations used in the design and development of launcher structures and their control systems. The model described is one of the more complex under development, being based on filamentary representations of both the stator and armature conducting regions. The single-stage experimental launcher described in the paper has been constructed purely for model validation purposes. To this end its geometry is constrained to a straightforward co-axial form, with the armature being a simple 80 gm aluminium ring of 100 mm diameter. The launcher operates vertically, accelerating the armature in free flight in order to avoid difficulties caused by barrel friction. Current and position data are obtained using digital sampling techniques, with post-processing allowing a direct comparison to be made with simulated results. >

The effect of serrated bore surfaces on plasma armatures

The use of serrated bore surface has the potential to overcome the velocity performance limitations of railgun plasma armatures. The authors evaluated several serrated bore configurations to determine their effect on plasma armature length, stability and velocity performance. They found that plasma armature length and stability are significantly influenced by bore serrations. Plasma armatures in serrated rail-serrated insulator bores are very compact and stable. Many small serrations in the rail also cause compact armatures. These micro serrations gave extremely stable armatures that were highly repeatable. The authors were unable to demonstrate the effect of serrated bores on plasma armature velocity performance. >

Investigation of dynamics and structure of a plasma armature in the railgun by means of Rogowski induction coils

A range of operating conditions was experimentally pursued for the railgun in which the space occupied by the discharge plasma behaves as a quasi-stationary object, so that the notion "plasma armature" appears to be correct. Taking this into account, the measurements of velocity, acceleration, and space structure of plasma armatures have been made with the help of induction detectors (Rogowski coils) of different configurations. The experiment allows determination of the velocity, acceleration and longitudinal space structure of a plasma armature by the correlation analysis technique with accuracy limited only by the fulfilment of quasi-stationary conditions. The simplest tomography variant being realized in the experiment provided the lateral structure of the plasma armature. Four points for current density have been investigated along the channel diameter scale.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transition conditions for solid armatures in railguns with muzzle current feed

The velocity skin effect is a principle reason that solid armatures in railguns transition from smooth metal-on-metal contact with the rails to arcing contact. Transition occurs when armature velocity reaches some critical value which depends weakly on current and strongly on the latent heat of evaporation and electrical conductivity of the armature material. Conductivity of the rail material is also a factor. The authors analysis shows that when current is fed to the armature from the muzzle of a railgun armature to the rail, current distribution is more uniform over the contact face than is the case with normal breech feed. They show that muzzle feed allows transition velocity to be increased three-fold.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The determination of launch induced static and dynamic rail deflection of a Benet 50 mm railgun barrel

The design of railgun projectiles requires a knowledge of rail deflection caused by electromagnetic forces generated during a launch. Sufficient armature-to-rail contact force must be maintained as the projectile passes through the bore. A hydraulic jack has been custom designed and built to fit inside a 5 m/spl times/50 mm composite overwrap railgun barrel designed and built by Benet Weapons Lab. A displacement transducer installed in the jack was used to obtain data leading to the determination of the spring constant of the rail's elastic foundation. This paper presents the use of the rail jack data in determining the static/dynamic rail deflection due to electromagnetic distributed loading in the Benet railgun barrel. Additionally, the concept of critical armature velocity as applied to railguns is introduced. >

Development of a Digitally-Controlled Servo Amplifier for Robots

Kawasaki Heavy Industries, Ltd. has developed a digitally-controlled servo amplifier for robot servo motors. The motor's armature current, position and velocity are controlled by the software in a digital signal processor. The servo is designed for high reliability, performance, flexibility and compactness. To achieve these objectives, we developed an ASIC which controls PWM signals for power transistors and encoder signals. The servo amplifier is currently used in the new Kawasaki AD series of robot controllers.

Cross-correlation-based method for determining the position and velocity of a railgun plasma armature from B-dot probe signals

A method of determining the position and velocity of a plasma armature from B-dot probe signals is presented. It utilizes the physical processes governing the plasma armature current density to model the leading edge of the armature as a unit step function. This model of the current profile is convolved with the impulse response of the probe and compared with B-dot probe data via a local-area cross-correlation (LACC) coefficient. Two-parameter iteration determines the time of passage and velocity of the leading edge of the armature as the values for which the correlation is maximized. Results obtained using this technique indicate that significant improvement in accuracy for time of passage estimates can be obtained over previous ad hoc methods. Velocity estimates from individual B-dot probe signals can also be obtained.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Design and testing of solid armatures for large-bore railguns

A solid metal railgun armature called the Fishbone has been designed and tested at significant velocity, in both square- and round-bore configurations. The name 'fishbone' describes a structure consisting of multiple, independent, compliant contacts. Primary measures of success are low muzzle voltage and post-shot bore integrity. Experiments at armature velocities in excess of 1.9 km/s have resulted in smooth muzzle voltage traces with peak values of 50 to 100 V and negligible bore damage. Design and performance issues important to the successful launch of a solid armature and the features of the Fishbone that address them are discussed. These issues include contact maintenance and gun stiffness, armature compliance, wear, thermal and structural integrity and materials selection. Shot data included are current, velocity, muzzle voltage, and post-mortem physical analysis. >

Why plasma armature railguns don't work (and what can be done about it)

It is noted that conventional plasma armature railguns are limited to velocities of 6-8 km/s by arc restrike. The author argues that to realize the hypervelocity potential of plasma armature railguns, restrike must be eliminated. He describes two promising approaches which are based on demonstrated technology. The near-term solution is some form of circuit arrangement which eliminates current flow in the restrike arc. Both segmented and distributed configurations merit further development, but major issues of electrical and mechanical complexity must be addressed. For the longer term, the development of advanced ceramics coupled with measures to reduce armature power shows promise. Special attention will be required to inject projectiles at a velocity greater than the damage threshold and to perform the plasma armatures to avoid local bore damage. With restrike under control, the only plasma-related limitation will be viscous drag on the armature plasma. Recent MIDI-2 experiments with a hydrogen plasma armature have demonstrated armature velocities in excess of 30 km/s, so armature drag does not appear to be an issue in the 10-20 km/s velocity regime.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Wear of conductors in railguns: metallurgical aspects

The wear debris generated by an aluminum alloy armature sliding on copper rails was studied experimentally. The experimental parameters were set to maximize armature velocity. Preliminary rail and armature wear observations were documented as part of a database for solid armature development. A variety of debris collection and characterization approaches were used. Both in-bore debris and muzzle-exit debris were collected for size and chemical analysis. Debris constituents were copper-rich, and the compound Cu/sub 9/Al/sub 4/ was observed in the splat-quenched muzzle debris. The approach and method are detailed to indicate the potential of wear studies in defining the activity at the sliding interface. Some implications of rail surface roughening for zero wear measurements are also discussed, and the adoption of wear mechanism maps is proposed. >