W.F. Weldon

Nonlinear control of a rigid rotor magnetic bearing system: modeling and simulation with full state feedback

Control strategies for magnetic bearings often focus on linear optimal control techniques. While these methods afford many advantages, the literature is conspicuously sparse with regard to attempts at nonlinear control. Here, we treat the equations of motion of a rigid, horizontal rotor magnetic bearing system. Through feedback linearization and sliding mode control techniques, we formulate a nonlinear control law which maintains the shaft centered in the bearings. This system is then simulated to demonstrate disturbance rejection properties and its robustness to parameter uncertainty and unmodelled dynamics. >

Limits to the velocity of solid armatures in railguns

Velocity limits of a solid armature in a railgun are the result of temperature and internal forces which surpass the yield strength of the material. A two-dimensional finite-element model for the magnetic and temperature fields in a railgun is presented for several rail and armatures designs. Copper rails and a molybdenum armature are identified as candidates for future solid armature testing. All simulations are performed for a 1/2-inch-square-bore railgun. >

Advanced compulsator design

The compulsator or compensated pulsed alternator is the name for a large family of high-energy pulsed-power rotation machinery. It covers an extensive range of currents, voltages, pulse shapes, and also frequency in the case of multipulse generators. The compulsator embodies the single-element philosophy, combining in one element the energy storage, electromechanical energy conversion, and power conditioning. However, inside the machine such functions are done in a staged manner. Two examples are given to illustrate this design philosophy: the flat pulse air-core compulsator, and the high-voltage two stage uncompensated machine (uncompulsator), which is capable of reaching voltages above 100 kV. >

Electromagnetic induction launchers

The electromagnetic launcher consists of a system of stator coils producing a traveling field which accelerates an armature carrying currents induced by the traveling field (induction accelerator [1,2]) or persistent currents supplied from otner sources (synchronous accelerator [2,10]). The fact that their armature has no electrical contact with the stator, essentially riding on the crest of a traveling magnetic wave, makes induction accelerators very attractive for a large number of applications. This paper is devoted exclusively to the accelerator of the induction type. Efficiency considerations require that the traveling wave should accelerate at approximately the same rate as the projectile. This can be achieved either using variable (increasing) winding pitch or a continuously increasing power supply frequency or a combination of both. A new dimension was added to the induction coaxial accelerator technology with the definition at the Center for Electromechanics at The University of Texas at Austin (CEM-UT) of a new electrical machine, the Rising Frequency Generator (RFG) representing a more attractive integrated power source for induction accelerators which had previously been forced to conform to constant frequency power supplies. This paper outlines the principles of design and shows two applications of induction coaxial launchers; a half-scale aircraft launcher in which the system also acts as an electromagnetic brake, stopping the shuttle and driving it in the opposite direction, and a high performance, 18-m long launcher capable of accelerating a 1-kg aluminum projectile to a velocity of 10 km/s at an average acceleration of 250,000 G.

Advanced compulsators for railguns

In order to maximize the penetration of a projectile into a target, the acceleration on the projectile during the launch must be minimized. Low accelerations permit the design of long and slender projectiles which have better penetration capability. From this standpoint, power supplies for electromagnetic launchers must be able to provide rectangular current pulses with a high average to peak acceleration ratio. The authors discuss efforts to obtain the desired pulse shape from a compensated pulsed alternator (compulsator) when it is used as a power supply for railguns. A general theory of the pulse shaping technique is presented first. This is followed by a discussion on the tradeoffs between various equivalent generator configurations. Finally, the electromagnetic design of the compensated pulsed alternator being developed for task C of the Electromagnetic Gun Weapons System Program is presented. >

Application of electromagnetic guns to future naval platforms

Designs for future naval vessels are strongly considering electric drive systems. Already employed in commercial cruise ships, electric drive offers the advantages of increased ship design flexibility, improved efficiency, reduced maintenance and allows ship prime power to be easily diverted to other electrical loads as needed. The ability to use ship prime power generation, which ranges between 40 and 150 MW depending on vessel class, for other electric loads provides the opportunity to electrify many existing functions as well as add new performance enhancing systems. The recent and ongoing emergence of electric gun and guided projectile technologies now allows very long range naval fire support functions to be evaluated for viability. In this paper, conceptual system designs for surface fire support of forces in littoral campaigns are considered. Key advantages of an EM fire support weapon over conventional technologies include reduced logistics burden and cost per round, greater lethality, shorter time of flight, improved survivability and the ability to stow more rounds. Notional mission requirements, projectile, power supply issues and ship integration issues are discussed. Also, other shipboard uses for the pulse power system required for these notional electric gun systems are also reviewed.

Design and testing of large-bore, ultra-stiff railguns

As part of an EM (electromagnetic) gun technology demonstration program, a systematic design approach targeted at identifying critical gun design constraints affecting hypervelocity projectile performance has been pursued. Results of the study led to a laboratory-based EM gun design that provides bore straightness and tolerances characteristic of light-gas guns, dynamic bore deformations comparable to those of conventional guns, the ability to change and test rail/sidewall insulator materials quickly, and superior in-bore performance diagnostics. A high-stiffness, easily maintained, precision-bore 9-MJ EM launcher has been designed and fabrication is virtually complete. A half-scale prototype of this hydraulically prestressed EM gun design has been fabricated and successfully tested. The authors discuss the railgun design approach and performance parameters, the analytical and empirical railgun structured simulation techniques used to validate the full scale gun design, and the fabrication status and initial performance test results. >

Plasma-armature railgun launcher simulations

The authors review three popular loss models currently used at CEM-UT (Center for Electromechanics at the University of Texas at Austin) in modeling EM (electromagnetic) launchers: friction, ablation, and armature drag. In experiments at currents below 500 kA using existing railgun design, the friction model alone was acceptable in predicting performance. In an experiment incorporating a railgun structure modified for higher stiffness and a measured peak railgun current of 700 kA, the effects of each of the loss models were compared to the measured results, and the greatest success at predicting the final projectile velocity and exit time occurred using the velocity-dependent friction model. It is believed that reducing frictional losses and plasma leakage will be instrumental in achieving velocities greater than 6 km/s. >

A compulsator driven rapid-fire EM gun

A compulsator-driven railgun is an attractive alternative to the homopolar generator-inductor-switch configuration, especially for repetitive duty. A conceptual design of a rapid-fire EM-gun system is presented. The generator is sized to accelerate a 0.08-kg projectile to 2 to 3 km/s at a 60 pulse-per-second repetition rate. Initial design parameters are discussed, and example current and velocity waveforms are given. The generator is discharged at the proper phase angle to provide a current zero just as the projectile exits the muzzle of the railgun.

Determination of iron core losses under influence of third-harmonic flux component

The core loss for a magnetic material under excitation can be estimated from the flux density versus the field-strength loops or B-H loops. A method is presented for predicting the position, size, and number of minor loops formed in the B-H loop, while harmonics of known magnitude and phase angle occur along with the fundamental under distorted excitation. The data required for this method are a set of B-H loops at the fundamental excitation frequency and loops formed under third-harmonic excitation frequencies over a range of flux densities.