It is highly desirable to measure the voltage and current characteristics of a railgun firing. In any railgun it is important to know the current drawn by the railgun. Since pulse discharge capacitors are usually not meant to withstand much reverse voltage, knowing whether this is occurring is also valuable. In order to know whether transition (from metal to plasma conduction) has occurred on the projectile barrel interface a chart of the muzzle voltage is useful. I have not yet attempted to make a system to capture the muzzle voltage curve.
My instrumentation board consists of four identical pulse capture circuits. On the front end of each circuit is some circuitry specific to each transducer. Below is a description of how the circuit works. Many things changed at the last minute and are not reflected on the schematics. Each pulse capture circuit captures and measures a value between 0 to ± 2 Volts. The polarity of the peak capture being determined by the orientation of the low leakage diode.
The first Op-Amp stage is a unity gain stable high speed OPA627 from the Burr-Brown division of Texas Instruments. It functions as a unity gain peak capture circuit by charging a 0.001 µF polypropylene capacitor through a low leakage diode. The polypropylene capacitor was chosen for voltage stability and extremely low leakage. The capacitor is required to hold the voltage for an extended time, yet it must charge extremely quickly. Therefore it must be small and of low leakage. The Op-Amps are are also high impedance input, the diode is a 1pA leakage PAD1 diode from Vishay-Siliconix On this circuit the potentiometers and resistors next to it used for input offset nulling were the wrong size, due to my misinterpretation of part of the Op-Amp data-sheet. All three were replaced by a single 100 kOhm potentiometer. The reset button was also placed across the diode instead of across the capacitor. This circuit leaks much faster than expected. I believe that is likely due to leakage across the PCB. In order to remedy this a low impedance guard trace needs to be placed around the diode capacitor junction. this trace would be connected to the output of the buffer amplifier and would easily absorb any stray currents.
The second Op-Amp is a self-nulling chopper stabilized precision TLC2652 from Texas Instruments. It serves as a buffer amplifier between the capacitor and the analog to digital converter. The two 0.1 µF polypropylene capacitors are required for the internal nulling circuitry of the Op-Amp. The pairs of 0.22 µF ceramic and 1.0 µF tantalum capacitors are decoupling capacitors to filter noise coming across the power supply from any noisy components. This assortment is almost certainly overkill, but I wanted to be safe and I was in a rush to finish this circuit. I figured it would work and I could best spend my time researching other things instead of decoupling caps. This circuit functioned perfectly without any last minute adjustments.
The final part of this circuit is the analog to digital converter and an LCD display. The TC7106 is a 3½ digit low speed inexpensive AD converter from a company called MicroChip. This chip has a range of ±2 Volts, the rest of the circuit was built with this fact in mind. The filter connected to the potentiometer is meant to supply a 1 Volt reference voltage. The filter is overkill and I would probably replace it and the whole voltage reference circuit with something more elegant and effective. The 21.0 kOhm resistor was accidentally replaced with 21 Ohm resistor, It worked so I left it that way. Connected to the upper left hand side of the circuit is an RC circuit for the chip oscillator, it operates at about 48 kHz, which results in a sample every three seconds. The chip allows for an oscillator to be used in place of the RC combination, which is what I'd do on the next go around which a circuit like this. Pins 29 through 27 have components necessary to the sampling of the input. The component on the right hand side is a 3½ digit LCD display, the AD converter provides the necessary square wave to drive the LCD (they can't be driven with DC). On the AD converter I forgot to connect the Analog input to ground and that was was fixed at the last minute.
The entire board contains four of the peak capture and display sub-circuits, a power supply and a high speed integrator. The top circuit uses a 0.5 milliOhm shunt as a current transducer to measure current pulses up to 4000 Amperes this circuit was the first sub-circuit built and tested, any changes were made on it first before completing assembly of the other three sub-circuits. The second and third sub-circuits are connected to the integrator which connects to a Rogowski coil for measuring very large currents.The current measuring circuit is meant to capture values of ± 1 MegaAmpere. The final circuit captures the value of any negative voltage pulse up to -10 kV.
The 0-4000 Ampere capture circuit works flawlessly. The remaining three circuits can be fairly unstable and slowly climb towards the saturation point of the Op-Amps. I believe this may be due to instability driving a capacitive load (maybe the caps on these circuits are a bit larger) or maybe from improperly nulled input offsets. More work is needed to perfect the circuit.
The transducer for the current pulse measuring circuit is a Rogowski coil. It is similar to a current transformer except it is uses an air core and is left open circuited. The coil produces a voltage proportional to the rate of change of current. In order to get a voltage signal proportional to current, an integrator (either active or passive) must be used. The coil and integrator were thrown together using some simple Rogowski Coil Theory.
The design for the coil calls for a six inch diameter torus, the torus being made of a tube having a cross sectional diameter of 1/8 inch, on the torus are twelve turns of thin wire. The assumed worst case current was 2 MegaAmperes at 40 kHz, at that current and frequency a single turn develops up to 10.4 Volts. So the maximum voltage developed by the coil will be 124.8 Volts, due to a current rate of change of 500 GigaAmperes per second. Using an active integrator with an integration capacitor of 0.001 uF and a resistor of 124.8 kOhms gives a 4 Volt output at 2 MegaAmperes. Since my peak capture circuitry is set to capture a pulse of up to 2 Volts the meter is then limited to ± 1 MegaAmpere. A 2 MegaAmpere pulse won't hurt anything, it will just max out the Display, and possibly push the Op-Amps into saturation. Running a pSpice simulation of the circuit shows that the Op-Amp operates with a maximum voltage slew of 1 V/µs and maximum current of 1 mA. The OPA627 is rated at 55 V/µs and 50 mA.
The leaky integrator is designed to respond to a pulse as
fast as 250 kHz. On the circuit R31, R32, and R33 were replaced with a single
100 kOhm potentiometer for input offset nulling. The same configuration
of a 0.22 µF ceramic and 1.0 µF tantalum capacitor to filter
the power supply are used here. The integrating capacitor was replaced with
a smaller 0.001 µF polypropylene capacitor in order to provide better
high speed response. Resistor R37 was changed to a 1.82 MOhm resistor. Like
all the resistors on the board, R37 is a metal film resistor in order to
minimize noise. R28 was changed to 121 kOhm and moved off the board. The
R28 resistor is actually moved off the board and attached to the Rogowski
coil. I did this after realizing that the creepage clearance from the rowgowski
coil input was too low, the pulse from the coil was expected to be 124.8
volts momentarily, and I didn't want to risk frying my 20 dollar op-amps.
The last change was to make R29 a 3.83 kOhm resistor. As the railgun design
progressed my esimated worse case pulse speed went from 250 kHz to 40 kHz
so the integrator was over-designed. It has not yet been analyzed and proven
using an oscilloscope.
The circuit board was layed out in Orcad Layout. Notice that the AD converters are behind the LCDs on the back side of the board. The blue traces represent copper on top of the board and the red on the bottom of the board. There is a lot of free space on the board. I could have made it a bit smaller had I wanted, but I decided to keep it simple and aesthetic.
Here is the actual Instrumentation board with the case off. In the Instrumentation art work picture I removed the ground planes, but in the picture of the actual instrumentation board the ground planes are visible. The analog and digital sections of the board have different ground planes in order to reduce any noise from the digital circuits interfering with the analog circuitry.
The board safe inside its aluminum case. I chose an aluminum case in order to try and prevent electromagnetic interference from the railgun firing, or any other source.
visitors since June 10th, 2005.
