Thomas G. Engel

Electrical power generation characteristics of piezoelectric generator under quasi-static and dynamic stress conditions

The electrical characteristics of a piezoelectric power generator are investigated under quasi-static (duration >100 ms) and dynamic (stress duration <10 ms) stress applications. The electromechanical model of piezoelectric generator is presented and used to explain the effects of the two stress conditions. A computer simulation of the piezoelectric generator is used to compare the theoretical and experimental results. The simulation predicts that a quasi-static stress will produce a bidirectional generator output voltage, and a dynamic stress will produce a unidirectional output voltage. The simulation also predicts that, when equal stresses are applied to the generator, the dynamic stress will generate a 10/spl times/ higher output voltage than the quasi-static stress, contradicting results reported by other investigators. The output voltage is different for the two cases because of the generators resistive capacitive (RC) time constant. The dynamic stress is applied in a time that is less than the generators RC time constant, and the quasi-static stress is applied in a time greater than the generators RC time constant. The piezoelectric capacitance has enough time to charge in the quasi-static case, resulting in the lower output voltage. The simulation results are experimentally verified for leaded zirconia titanate PZT 5H and PZT 5A materials. Simulated and experimental results are shown to be in good agreement.

Characterization of the Velocity Skin Effect in the Surface Layer of a Railgun Sliding Contact

We present a characterization of contact velocity skin effect (VSEC), which is a major velocity- and efficiency-limiting effect at a railguns sliding contact. Despite enormous contact forces, the armature remains separated from the rail by a thin layer, 4 to 12 A thick. Evidence suggests VSEC is also the primary mechanism responsible for the contact voltage drop. VSEC effects are seen in both electromagnetic launcher (EML) efficiency and breech voltage. We compare theoretical predictions of system efficiency and breech voltage to experimental measurements for both a conventional and an augmented railgun. The characterization of VSEC extends our previous theoretical work in this area and provides new insights into the physics of EML operation, especially with regards to the armature and sliding contact. VSEC is a significant energy loss mechanism and heat source, possibly contributing to contact erosion and transition. We propose a similar VSEC mechanism to explain velocity saturation and efficiency roll-off in plasma and hybrid armature railguns, as well as arc restrike.

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.

Energy conversion and high power pulse production using miniature piezoelectric compressors

The design, construction, and testing of miniature, high-power piezoelectric compression generators are presented and discussed. The piezoelectric compression generators are located inside high speed, 30-mm projectiles that are launched with a high pressure helium gun to velocities of approximately 300 m/s. The large deceleration force created when the projectile imparts the ground is used to power the piezoelectric compression generator. The peak output power is approximately 25 kW into a 10-/spl Omega/ load with the output pulse length on the order of 1 /spl mu/s. Good agreement is found between the experimental results and the theoretical predictions.

Maximum power generation in a piezoelectric pulse generator

This investigation presents and discusses maximization techniques for a high-power piezoelectric pulse generator. Maximizing the piezoelectric generators output power is done by maximizing the product of generated voltage and output current. The maximization methods are derived from the mechanical and electrical models of the generator and provide design guidelines as to the geometric dimensions of the piezoelectric material and circuital conditions that will produce maximum power in the device. The theoretical results show the peak stack voltage to increase with an increasing thickness to area ratio of the piezoelectric material and with increasing applied force. However, in contrast to the peak output voltage, the peak output current increases with the decreasing of thickness to area ratio of the material. In addition to the physical dimension, the peak stack current increases as the value of the antenna inductor decreases. The output power of the piezoelectric generator, which is the product of output voltage and current, linearly increases with the thickness to area ratio. This result is due to the fact that the output voltage is larger comparing to the output current. Experimental results are also given to verify the theoretical results and represent the performance of several types of piezoelectric materials with different thickness to area ratios. The experimental results show good agreement with theoretical predictions. The results also show the peak power output of the experimental generator ranging from 7 to 28 kW with a corresponding power density from 9 to 173 kW/cm/sup 3/.

The Maximum Theoretical Efficiency of Constant Inductance Gradient Electromagnetic Launchers

The maximum theoretical efficiency of constant inductance gradient electromagnetic launchers (EMLs) is analyzed and discussed. The maximum theoretical efficiency is a parameter needed to calculate the EMLs efficiency. Constant inductance gradient EMLs include the conventional railgun, the augmented railgun, and the conventional helical launcher. The maximum theoretical efficiency of an EML is dependent on its geometry and the manner, or mode, in which it is powered. In the lossless case, the conventional railgun, the augmented railgun, and the conventional helical launcher are capable of 50% maximum efficiency when operating in constant current (CC) mode. Conventional and augmented railguns can achieve 100% maximum efficiency when operating in zero exit-current mode. While zero exit-current mode promotes high efficiency, this mode can reduce EML lifetime since it requires current levels much higher than those found in CC mode. The high-efficiency helical launcher, presented and analyzed here for the first time, combines 100% maximum theoretical efficiency with the low-current benefits of constant-current mode.

Prediction and verification of electromagnetic forces in helical coil launchers

Calculating the circuital parameters and electromagnetic force production between two coupled coils is critical when modeling helical coil launchers (HCLs). Computer tools that calculate the self inductance, mutual inductance, and inductance gradient of a coupled-coil pair are developed for the PSpice circuit simulator. The inductance values and gradient are used to model a short (16.3-mm length), small-bore (19.4-mm diameter) HCL. The HCL is constructed in the laboratory and its performance is measured. The HCL accelerated nominal 16-g projectiles up to 170 m/s using a 6-kJ capacitive energy store. The highest overall electric-to-kinetic conversion efficiency was 5.4%. Comparisons are made between the theoretical and experimentally measured launcher parameters. In general, there is good agreement between the predicted and measured HCL parameters.

High-efficiency, medium-caliber helical coil electromagnetic launcher

Research progress in the development of a 40 mm/spl times/750 mm helical-coil electromagnetic launcher (HCEL) is presented and discussed. Significant technical problems that have been solved in this research include efficient stator commutation methods and the ability to simultaneously implement high-inductance gradient armatures. The HCEL is able to launch a 525-gram projectile to a velocity of 140 m/s. Power for the HCEL is derived from a 62.5 kJ sequentially fired pulse forming network (PFN) of 900 V (maximum) electrolytic capacitors. The experimentally measured HCEL efficiency of 18.2% is substantially greater than a conventional or augmented railgun of similar scale (i.e., equivalent mass, bore-size, and velocity). The HCELs high launch efficiencies result from its 150 /spl mu/H/m inductance gradient, which is approximately 300 times greater than the inductance gradient of a conventional railgun. HCEL computer model predictions are given and compared to experimentally measured HCEL and PFN parameters including peak current, inductance gradient, acceleration time, parasitic mass ratios, and electrical-to-kinetic conversion efficiency. Scaling relationships for the HCEL are also presented and used to predict launcher operation at higher velocity and with a larger diameter bore size.

Design and operation of a sequentially-fired pulse forming network for non-linear loads

While the construction of a linear pulse forming network (PFN) for a constant load impedance is relatively easy, the process is more difficult for a nonlinear or time-varying load. A passive PFN can certainly be synthesized for nonlinear loads, but is usually large and lacks the flexibility to be truly useful in most practical and research applications. This investigation describes the design and construction of a sequentially-fired pulse forming network (SFPFN) that maintains constant voltage and current for a nonlinear load. Operation of the SFPFN consists of charging multiple capacitor banks (or modules) to various levels and sequentially firing these banks into the load at appropriate times. The load and its characteristics determine the module charge voltage. An added benefit with the SFPFN is real-time computer monitoring and control allowing the PFN modules to be charged from a single prime power source via a group of switching relays. The nonlinear load used in this investigation is a helical coil electromagnetic launcher (HCEL). The SFPFN is also tested with a linear load consisting of a pulsed field coil. The nonlinearity of the HCEL is well-known with a factor of 2 change in winding resistance due to joule heating and a large variation in terminal voltage due to changes in the armature back-voltage. Experimental measurements show the SFPFN can deliver a relatively constant current pulse on the order of 5-15 kA into the HCEL load for a pulse length up to 8 ms. The maximum SFPFN operating voltage is 900 V with a total stored energy of 125 kJ. Scaling the SFPFN to larger or smaller pulse amplitudes or lengths is possible.

Solid-Projectile Helical Electromagnetic Launcher

Helical electromagnetic launchers (HEMLs) can operate at significantly lower currents and higher efficiency in comparison to conventional railgun and induction coilgun launchers. The HEMLs versatility is due, in part, to its large inductance gradient which is typically two to three orders of magnitude greater than a conventional railgun and can be tailored to almost any value in that range. Historically, however, HEMLs were not considered practical since they consisted of a hollow projectile (i.e., armature that is accelerated on the outside of a stator coil). This investigation demonstrates, for the first time, a 40-mm bore times 750-mm length solid-projectile HEML, where the armature is accelerated on the inside of a stator coil. The goal of this paper is to demonstrate the practicality of solid-projectile HEML concept and to measure its performance. Numerous successful tests were conducted. The highest velocity measured for a 170-g projectile was 64 m/s.