D.A. Weeks

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. >

Electromechanical Active Suspension Demonstration for Off-Road Vehicles

The University of Texas Center for Electromechanics (UTCEM) has been developing active suspension technology for offroad and on-road vehicles since 1993. The UT-CEM approach employs fully controlled electromechanical (EM) actuators to control vehicle dynamics and passive springs to efficiently support vehicle static weight. The program has completed three phases (full scale proof-of-principle demonstration on a quartercar test rig; algorithm development on a four-corner test rig; and advanced EM linear actuator development) and is engaged in a full vehicle demonstration phase. Two full vehicle demonstrations are in progress: an off-road demonstration on a high mobility multiwheeled vehicle (HMMWV) and an on-road demonstration on a transit bus. HMMWV test results are indicating significant reductions in vehicle sprung mass accelerations with simultaneous increases in cross-country speed when compared to conventional passive suspension systems. Additionally, original projections of low power requirements for suspension actuators are being confirmed. The 3,400 kg (3.75 ton) vehicle being tested utilizes a 5 kW alternator to provide suspension power. Power conditioning circuits limit the continuous deliverable power to 4 kW, which corresponds to 1.2 kW/metric

Plasma armature railgun launcher simulations at the University of Texas at Austin

A velocity-dependent friction model that accurately predicts the losses associated with a plasma armature railgun is presented. Test results from a 1-m-long, 1.27-cm square-bore, plasma-armature railgun have been used to determine the validity of the model. The divergence between the calculated and measured performance is typically less than 5% at railgun currents below 500 kA; however, at currents greater than 500 kA, the deviation increases. Experimental evidence suggests that the railguns lack of stiffness and subsequent venting of driving pressure rather than the electromechanical model is primarily responsible for the divergence. To test this theory, a rail was built using external preloading rings (Ringfeder) to increase its stiffness. On the first test of the Ringfeder railgun, 625 kA was discharged into the gun and the projectile was accelerated to 5.9 km/s. Test data indicate that the projectile accelerated through the entire length of the railgun and that a minimum amount of plasma leakage occurred during the test. >

Design, testing, and installation of a high-precision hexapod for the Hobby-Eberly Telescope dark energy experiment (HETDEX)

Engineers from The University of Texas at Austin Center for Electromechanics and McDonald Observatory have designed, built, and laboratory tested a high payload capacity, precision hexapod for use on the Hobby-Eberly telescope as part of the HETDEX Wide Field Upgrade (WFU). The hexapod supports the 4200 kg payload which includes the wide field corrector, support structure, and other optical/electronic components. This paper provides a recap of the hexapod actuator mechanical and electrical design including a discussion on the methods used to help determine the actuator travel to prevent the hexapod payload from hitting any adjacent, stationary hardware. The paper describes in detail the tooling and methods used to assemble the full hexapod, including many of the structures and components which are supported on the upper hexapod frame. Additionally, details are provided on the installation of the hexapod onto the new tracker bridge, including design decisions that were made to accommodate the lift capacity of the Hobby- Eberly Telescope dome crane. Laboratory testing results will be presented verifying that the performance goals for the hexapod, including positioning, actuator travel, and speeds have all been achieved. This paper may be of interest to mechanical and electrical engineers responsible for the design and operations of precision hardware on large, ground based telescopes. In summary, the hexapod development cycle from the initial hexapod actuator performance requirements and design, to the deployment and testing on the newly designed HET tracker system is all discussed, including lessons learned through the process.

Design and development of extremely high velocity electromagnetic launch accelerators-GEDI program

Research is described that has centered around 1, 2, or 8 m long, 12.7 mm square-bore railguns powered by one or more of the following power supplies: a 1 MJ capacitor-bank-driven, pulse-forming network; a 6 MJ homopolar-generator (HPG)-charged coaxial inductor; or one or more of six 10 MJ Balcones-HPG-charged coaxial inductors. Several variations of the 1 and 2 m-long railguns were designed, fabricated, and tested. Railgun design parameters, development, and some test results are described. Parameters of primary importance in designs include sidewall insulator and rail materials; gun length and outer structure materials; stiffness and strength; ease of assembly and disassembly; and availability of methods for supporting, straightening, and honing the guns. Both static and dynamic aspects of railgun structural behavior are discussed. >

Testing of a high performance, precision-bore railgun

This paper discusses the results of high-pressure (up to 350 MPa) railgun experiments. The gun is designed both to be capable of high-pressure operation without structural damage and to be readily disassembled for inspection, maintenance and component testing. Considerable effort has been invested to develop techniques to produce and measure smooth, extremely precise (>5µm of variation) bores. We have not deliberately varied bore precision to determine the effect of precision on performance, but our experience suggests that if two otherwise identical railguns, one with a polished, high-precision bore, were fired under identical conditions, the precisely finished gun would achieve a higher muzzle velocity. Very high accelerations have been achieved (>107m/s2). A 2-g projectile has been accelerated to 5 km/s in a 13 mm square-bore gun only 1 m long. Projectiles with an L/D as small as 0.65 have been successfully accelerated. Projectiles with smaller L/Ds have not yet been tested to determine the minimum L/D which can be successfully accelerated. A number of different insulator materials ranging from common float glass to fused quartz have been tested. The best results have been obtained with fused quartz, which shows promise of being reusable. In the course of testing, the importance of gasketing the rail-to-insulator seams to prevent loss of plasma has become apparent. We have made progress in gasket design, but more work is needed. Rail gouging has been a continuing problem. Gouging may be dependent on bore precision, projectile fit, rail mechanical properties, projectile L/D, structural stiffness, operating pressure, velocity, and shape of the current pulse.

Design and Performance Testing of an Advanced Integrated Power System with Flywheel Energy Storage

The University of Texas Center for Electromechanics (UT-CEM) has completed the successful design, integration and testing of a hybrid electric power and propulsion system incorporating a flywheel energy storage device. During testing, the improved drive train was shown to double acceleration rates while simultaneously reducing prime power usage in excess of 25% when compared to the same vehicle without the flywheel energy storage system. While the system was designed for and demonstrated on a transit bus, the technology described herein is applicable to a wide variety of applications, including additional mobile and marine power and propulsion systems. This paper (1) describes the drive train design with an overview of the critical components and (2) presents results from system-level testing of the transit bus with the integrated drive train.

Plasma armature characterization measurements performed at the University of Texas

The Center for Electromechanics at The University of Texas at Austin (CEM-UT) and Austin Research Associates (ARA), in a cooperative effort, have performed a series of high performance railgun experiments in order to measure plasma armature characteristics. The tests were performed at CEM-UT using a 1 m long, 12.7-mm square bore railgun. Twenty experiments were performed. In each experiment plasma pressure, arc voltage, breech voltage, arc spectrum, gun current, and magnetic probe (B-dot) signals were recorded. Glass filled polycarbonate insulators and molybdenum rails were used in all experiments. Gun currents ranged from 150 to 450 kA. Experiments were performed primarily at atmospheric pressure. The projectile in most experiments was a polycarbonate cube approximately 2.5 g in mass. A few experiments were performed in vacuum with a free running arc. In all cases, the arc was initiated by electrically exploding an aluminum foil fuse. Experimental measurements made with a piezoelectric pressure transducer indicates that pressure profile is time correlated with the armature B-dot profile. The measured parabolic pressure profile is consistent with that expected by theoretical models. Peak pressures recorded were between 30 MegaPascals (MPa) (4,300 psi) and 200 MPa (30,000 psi) and correlated well to the pressure determined by dividing the Lorentz force (calculated from the gun current when the projectile passes the transducer) by the bore cross sectional area. Also of interest is that in-bore intensity measurements made with a PIN diode indicate light emission time is comparable to the pressure profile duration. Analysis of the emitted spectrum shows continuum radiation with strong absorption lines that are associated with neutral molybdenum and neutral aluminum. Plasma resistivity in the transducer region was calculated from the gun current, muzzle volts, bore area, and the armature length as determined by the B-dot probes. The resistivity calculated appears to be consistent with previously published articles indicating that plasma resistivity is inversely proportional to the fourth root of the plasmas pressure. Due to the quick turn around afforded by the modified railgun breech, the same experimental parameters were repeated on several tests producing results that typically correlated better than 5%.

Electromechanical Suspension Performance Testing

Abstract : Under contract DAAEO7-98-C-L020 testing was conducted at the U.S. Army Yuma Proving Grounds by the U.S. Army Tank-automotive and Armaments Command, Research, Development and Engineering Center and the University of Texas Center for Electromechanics during 8, 9, and 10 November 1999 between an active (electromechanical suspension) and passive High Mobility Multi-Purpose Wheeled Vehicle (HMMWV) to determine performance improvements. Two tests, RMS Courses and Lane Change Maneuver, produced the most complete performance results for Ride Quality and Maneuverability determination. For the Lane Change Maneuver, the active HMMWV has much less sprung mass (frame) acceleration, over 5 times reduction at higher speeds, than the passive HMMWV. For the active HMMWV, sprung mass acceleration remains mostly constant at around 0.1 gs to 55 MPH while the passive HMMWV shows noticeable increases, at times in excess of I g. For the RMS Courses, a comparison shows a 5 times reduction in absorbed power over courses 2 to 5 with the active HMMWV. The active HMMWV has much less sprung mass acceleration, over 4 times reduction at higher speeds, than the passive HMMWV. For the active HMMWV it remains mostly constant at around 0.75 gs to higher speeds while the passive HMMWV shows noticeable increases, at times in excess of 2 gs. Total peak power usage was in the range of 3 kW (RMS and Lane Change Maneuver Courses) and total peak regeneration in the range of 6 kW (RMS Courses) for the active suspension.

Electric vehicle modeling utilizing DC motor equations

This paper discusses modeling an electric utility vehicle powered by a separately wound DC motor. Many modeling techniques use steady state efficiency maps and torque-speed curves to describe the performance of electric motors, which can overlook transient response dynamics, current limits, and thermal limits that may affect the end vehicle performance. This paper discusses using bond-graph techniques to develop a causal model of an electric vehicle powered by a separately wound DC motor and development of the appropriate feedforward and feed-back controllers required for route following. The causal model performance is compared to a PSAT model of the same electric vehicle, which uses motor torque-speed curve and efficiency map.