Ground fault protection for EMAT transistor switched magnet pulsers

A fast reacting ground fault interrupter circuit particularly suited for use with high current, high voltage DC pulses generated by transistor switched magnet pulsers driving pulsed electromagnets used with electromagnetic acoustic transducers (EMATs). Whenever a ground fault is detected, a digital shut off signal is sent to the transistor switched magnet pulser to shut it off and prevent transmission of the DC pulse. Additionally, the shut off signal is also provided to a DC current interrupter which shuts off power supplied to the transistor switched magnet pulser itself.

FIELD AND BACKGROUND OF THE INVENTION 
The present invention relates, in general, to electromagnetic acoustic 
transducers and inspection systems utilizing same and, in particular, to a 
new and useful fast reacting ground fault interrupter for use with high 
current pulsers for driving electromagnets used with electromagnetic 
acoustic transducers (EMATs). 
Ground fault interrupter (GFI) circuits are commonly available (and for 
certain circuits in household wiring are required) protection circuits for 
standard power distribution applications. Generally these devices sense 
the difference between the current flowing in the high side (supply side) 
and low side (return) of a circuit, with the low side of the circuit 
connected to safety ground only at the power source. Normally, the current 
in the high side of the circuit equals the current in the low side of the 
circuit so that the difference is zero. When a short or leakage path 
develops between the circuit and safety ground, some of the circuit 
current flows through the safety ground to return to the low side of the 
power source, causing the current in the high side and low side of the 
circuit to become unequal. The ground fault interrupter (GFI) senses this 
difference in currents and shuts off the circuit. The devices that are 
known to the inventors operate with either AC (alternating current) or DC 
(direct current) power circuits. 
EMATs are the basis of a noncontact ultrasonic inspection method that 
requires no fluid couplant because the sound is produced by an 
electromagnetic interaction within the material. The Babcock & Wilcox 
Company has developed a high current, transistor switched magnet pulser 
circuit for use with EMATs. This pulser circuit, in conjunction with a 
pulsed electromagnet, produces the magnetic field necessary for the 
operation of EMATs in many applications. The pulser circuit is capable of 
producing 200 amp pulses at supply voltages of up to 300 V DC. In 
operation, the pulser circuit connects a DC power source to the terminals 
of the pulsed magnet using transistor switches. Current ramps up in the 
pulsed magnet, limited by the resistance and inductance of the pulsed 
magnet. After a sufficient amount of time (typically several hundred 
microseconds) the current and therefore the magnetic field reach a level 
required for the operation of the EMAT. At this time an ultrasonic signal 
is launched and then received by the EMAT. The transistor switches are 
then turned off, shutting off the current flow into the pulsed magnet. By 
using this pulsed operation large magnetic fields are produced with a much 
smaller electromagnet and much less power dissipation than a DC 
electromagnet. Typical rising current pulse lengths are 0.1 to 5 
milliseconds. 
Transistor switched magnet pulsers frequently produce high-voltage and/or 
high-current pulses. The high voltages and high currents can present 
personnel safety hazards to people in contact with the magnets or 
associated structures in the event of a fault in the magnet coil 
insulation or magnet cable insulation. In addition, high fault currents 
may result in damage to the transistor switched magnet pulser circuit 
itself, or the component being tested if the component is grounded and is 
in contact with the pulsed magnet or supporting structure. 
Accordingly, it has become desirable to provide a ground fault protection 
circuit for operation with EMAT transistor switched magnet pulsers to 
quickly shut off the transistor switched magnet pulser when a ground fault 
in the pulsed magnet or connecting cable is detected. 
SUMMARY OF THE INVENTION 
The present invention is drawn to a fast reacting ground fault interrupter 
circuit for use with high current pulsers for driving pulsed 
electromagnets used with Electromagnetic Acoustic Transducers (EMATs). 
This circuit is capable of operation with the pulsed currents generated by 
the transistor switched magnet pulser and is able to shut off the 
transistor switched magnet pulser in only a few microseconds. 
Accordingly, one aspect of the present invention is drawn to an electrical 
circuit for use with transistor switched magnet pulsers driving pulsed 
electromagnets used with electromagnetic acoustic transducers, having a 
fast reacting, ground fault interrupter that is capable of operation with 
high current, high voltage DC pulses generated by the transistor switched 
magnet pulser and which shuts off the transistor switched magnet pulser in 
only a few microseconds when a ground fault is detected in the pulsed 
electromagnet or electrical cable means connected thereto. Transistor 
switched magnet pulser means are provided to produce a high current, high 
voltage, DC pulse. Pulsed electromagnet means receive the DC pulse via 
electrical cable means connected inbetween the transistor switched magnet 
pulser means and the pulsed electromagnet means. Ground fault interrupter 
circuit means are provided for shutting off the transistor switched magnet 
pulser means within a few microseconds to prevent transmission of the DC 
pulse to the pulsed electromagnet means whenever a ground fault is 
detected. 
Another aspect of the present invention is drawn to an electrical circuit 
having a transistor switched pulsed current source for producing high 
current, high voltage pulses and electrical cable means having a first and 
a second conductor for conveying the pulses inbetween the transistor 
switched pulsed current source and a load, a fast reacting, ground fault 
interrupter which shuts off the pulsed current source in only a few 
microseconds and prevents transmission of the pulses when a ground fault 
is detected in the electrical cable means. The ground fault interrupter 
comprises current transformer probe means operatively associated with said 
electrical cable means. The probe means senses a difference in current 
flowing through the two conductors to produce a ground fault signal 
proportional to said difference at an output of said probe means whenever 
current in one of said conductors is different from that in the other 
conductor. Means are provided for supplying said ground fault signal to 
first comparator means for comparing said ground fault signal against a 
first reference voltage to produce a first output signal indicative of 
said comparison. The ground fault signal is also supplied to unity gain 
inverting circuit means for producing an inverted ground fault signal. 
Second comparator means, operatively connected to said unity gain 
inverting circuit means, are provided for comparing said inverted ground 
fault signal against said first reference voltage to produce a second 
output signal indicative of said comparison. The first and second output 
signals from said comparators are provided to flip-flop means, responsive 
to said output signals, for providing a shut off signal to the transistor 
switched pulsed current source to prevent transmission of the pulse 
whenever the ground fault is detected. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of this disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which a 
preferred embodiment of the invention is illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to the drawings generally, wherein like numerals represent the 
same or functionally similar elements throughout the several drawings, and 
to FIG. 1 in particular, there is shown an electrical circuit, generally 
designated 10, incorporating the fast reacting ground fault interrupter 
circuit of the present invention that has been developed and tested. 
Pulsed DC current from a pulsed current supply device or transistor 
switched magnet pulser 12 is supplied to a pulsed electromagnet or pulsed 
magnet 14 contained within magnet housing 16 via line 18 labeled SUPPLY. 
The pulsed DC current returns from the pulsed magnet 14 via line 20 
labeled RETURN, which is grounded to electrical safety ground 22 at the 
pulsed current supply device 12. The magnet housing 16 is also grounded to 
ground 22 via line 24. In practice, the preferred cable for connecting the 
transistor switched magnet pulser 12 to the pulsed magnet 14 surrounds 
lines 18 and 20, as well as ground line 24, within a single, conductive 
braid shield 25 which is also connected to electrical safety ground 24 to 
provide RFI shielding and additional personnel protection. Of course, an 
outer layer of insulation (not shown) surrounds the conductive braid 
shield 25. Lines 18 and 20 are routed through a current transformer probe 
26. Probe 26 is preferably a torodial current transformer probe which 
senses the current flowing through the center of the toroidal coil of the 
probe 26. A preferred model of same is made by ION PHYSICS, a Model No. 
CIM-100-L GFI current probe. 
In normal operation, the magnitude of the pulsed current flowing in the 
RETURN line 20 equals the magnitude of the current flowing in the SUPPLY 
line 18, but flows in an opposite direction. The net current flowing 
through the center of torodial current transformer 26 is therefore zero, 
and no signal is present at an output 28 of torodial current transformer 
26. If the insulation breaks down on the coil of the pulsed magnet 14 or 
in the lines 18, 20, some of the pulsed current returns to the pulsed 
current supply device 12 via the safety ground 24 instead of via the 
RETURN line 20. This results in the RETURN line 20 current being different 
than the SUPPLY line 18 current. Torodial current transformer 26 senses 
this difference in currents and produces a ground fault signal at its 
output 28 proportional to the difference in the SUPPLY line 18 and RETURN 
line 20 currents. The ground fault signal from the torodial current 
transformer 26 is output over lines 30 and 32, schematically shown as two 
lines but which could take the form of a conventional RG-58 coaxial cable. 
Line 32 is connected to ground 34. Line 31 applies the torodial current 
transformer 26 ground fault signal to a negative input of comparator 36, 
pin 8, via a resistor 38 typically rated at 1/4 watt. A resistor 40 
connected inbetween line 31 and ground 42 provides a low impedance load 
for the input to the comparator 36 (and others described, infra) and in 
conjunction with resistor 38, provides a 50 ohm termination for the 
torodial current transformer probe 26. Capacitor 44, connected inbetween 
line 31 and ground 42, acts as a filter for reducing high frequency noise 
pulses. The ground fault signal from current probe 26 is also conducted 
from line 31 to an input of a unity gain inverting amplifier circuit 46 
comprised of the combination of a low noise op-amp 48, such as a Texas 
Instruments Model No. NE5534, or equivalent and resistors 50, 52, and 54. 
This circuit 46 simply inverts the ground fault signal from the current 
probe 26, and also includes variable potentiometer 56 and resistor 58 
which allow the DC offset of the unity gain inverting amplifier circuit 46 
to be set to zero, using reference voltage VCC. Comparators 36 and 60 are 
provided to compare the ground fault signal from current probe 26 and the 
inverted ground fault signal from inverting amplifier circuit 46 to a 
reference voltage VCC set by adjusting variable potentiometer 62. The 
reference voltage VCC is applied via potentiometer 62 via line 64 to the 
positive input, pin 9, of comparator 36, and to the positive input, pin 7, 
of comparator 60. The inverted ground fault signal from inverting 
amplifier circuit 46 is applied to the negative input, pin 6, of 
comparator 60. Comparators 36 and 60 are advantageously a National 
Semiconductor Model No. LM339, or equivalent. 
If the current probe 26 ground fault signal present at pin 8 of comparator 
36 exceeds the reference voltage provided on pin 9, the output signal at 
pin 14 of comparator 36, which is normally high (at approximately 5 
volts), immediately goes low (to approximately 0.5 volts). Likewise if the 
inverted ground fault signal provided on pin 6 of comparator 60 ever 
exceeds the reference voltage on pin 7 thereof, the output signal at pin 1 
immediately goes low. In this manner, either a positive going or negative 
going ground fault signal from the current probe 26 is immediately 
detected. If either output signal, from pin 14 of comparator 36 or pin 1 
of comparator 60 goes low, flip-flop 66, connected thereto via line 68, is 
preset to cause the output at pin 6 thereof to go low, representing a shut 
off signal. This output at pin 6 remains low until a low going reset pulse 
is applied to the reset input provided via line 70 to pin 1 of flip-flop 
66, provided that the ground fault has been corrected. Flip-flop 66 is 
advantageously a Texas Instruments Model No. SN7474, or equivalent. 
When the output at pin 6 of flip-flop 66 goes low, representing a shut off 
signal, three things immediately occur. The shut off signal, via line 72: 
(1) shuts off driver circuits to the transistor switches in the magnet 
pulser 12 (see FIG. 2, infra) to shut off the current flow to the pulsed 
magnet 14; (2) activates DC power interrupting circuitry 74, such as a 
relay, thereby shutting off the DC power supply to the transistor switched 
magnet pulser 12; and (3) an LED (Light Emitting Diode) indicator 76 on a 
front panel of the transistor switched magnet pulser 12 is illuminated, 
indicating that a ground fault has been detected. The transistor switched 
magnet pulser 12 remains disabled until the ground fault is corrected, and 
then a reset switch 78 on the front panel thereof can be depressed, 
sending a reset pulse to the ground fault interrupter circuit means 46 via 
flip-flop 66. 
Referring now to FIG. 2, there is shown a schematic representation of those 
portions of pulser circuitry 80 within the magnet pulser 12 necessary to 
successfully practice the ground fault protection aspects of the claimed 
invention. It will be appreciated by those skilled in the art that certain 
details of the pulser circuitry, involving various signal conditioning and 
other aspects involved in producing an appropriate DC pulse suited for use 
in EMAT inspection methods and not forming a part of the present 
invention, have been omitted herein for the sake of brevity and will not 
be discussed further unless necessary to elucidate the principles of the 
present invention. 
FIG. 2 details how the transistor switched magnet pulser 12 is immediately 
shut off by the signal from pin 6 of flip-flop 66 (FIG. 1). The pulser 
circuitry 80 comprises, inter alia, the following elements. A transistor 
switch drive logic pulse generator 82 generates a logic pulse which allows 
the magnet pulser 12 to transmit a DC pulse to the pulsed magnet 14. The 
logic pulse from logic pulse generator 82 is applied to one input of a 
logic AND gate 84; the other input of gate 84 is provided with the signals 
from pin 6 of flip-flop 66 via line 72. The output of AND gate 84 is 
applied to a pair of transistor driver circuits 86 and 88, which in turn 
control a pair of transistor switches 90 and 92, respectively. To activate 
the transistor switched magnet pulser 12 during normal operation (no 
ground fault) a logic pulse is generated by the transistor switch drive 
logic pulse generator 82, by means of trigger means 94. This logic pulse 
from logic pulse generator 82 determines when the transistor switches 90 
and 92 are to be switched to their conducting state. This logic pulse is 
applied to the transistor drive circuits 86 and 88 via the logic AND gate 
84. When no ground fault has been detected, the logic signal from 
flip-flop 66 is high, allowing the output of AND gate 84 to go high when 
the output from logic pulse generator 82 is high. The high output from AND 
gate 84 is applied to the inputs of transistor driver circuits 86 and 88. 
The transistor driver circuits 86 and 88 then provide the proper voltages 
and currents to the inputs of transistor switches 90 and 92 to cause the 
transistor switches 90 and 92 to go into their conducting states, allowing 
DC current to flow therethrough to lines 18 and 20 and then on to the 
pulsed magnet 14 (FIG. 1). When the output from the logic pulse generator 
82 goes low, the output of AND gate 84 goes low, causing the transistor 
driver circuits 86 and 88 to turn off their respectively controlled 
transistor switches 90 and 92, immediately shutting off the DC current 
flowing therethrough to the pulsed magnet 14 (FIG. 1). If at any time the 
output from pin 6 of flip-flop 66 (FIG. 1) goes low (ground fault 
detected), the output from AND gate 84 goes low causing transistor driver 
circuits 86 and 88 to drive their respectively controlled transistor 
switches 90 and 92 to go into their nonconducting states, shutting off the 
DC current flowing therethrough to the pulsed magnet 14 (FIG. 1). This 
turn off time is typically only a few microseconds. As long as the signal 
from 66 is low, AND gate 84 is prevented from allowing the high going 
logic pulse from 82 from being applied to the transistor driver circuits 
86 and 88, therefore preventing their respectively controlled transistor 
switches 90 and 92 from being switched into their conducting states. 
While ground fault interrupter devices are commonly available for standard 
power distribution applications, to the present inventors' knowledge none 
are available that can deal with situations where the power source 
waveform can consist of arbitrary waveform pulses of extremely short 
duration (on the order of tens of microseconds), the typical operating 
mode in EMAT inspection systems. The ground fault detection circuit of the 
present invention is very fast acting, capable of shutting off the magnet 
pulser output transistor switches in a few microseconds, i.e. shutting off 
the current flow within an individual pulse, providing maximum protection 
to personnel, equipment, and components. To the present inventors' 
knowledge, existing ground fault interrupter devices are much slower 
reacting, requiring several milliseconds to shut off the power source, and 
are better suited or adapted to monitoring a continuous current having a 
defined average range. 
While the ground fault detection circuit of the present invention is 
particularly suited for use in connection with EMAT inspection systems, it 
clearly has potential application in any high current output circuits. The 
ground fault detection circuit of the present invention is capable of 
operating with any waveform, including AC and DC currents, and would thus 
be useful in any critical applications where very fast reaction times (on 
the order of a few microseconds) are needed to disable the current output. 
This would include high power output linear amplifiers as well as other 
types of transistor switched pulsed current sources, and is applicable to 
various electrical circuits wherein such sources provide high current, 
high voltage pulses via electrical cable means having plural conductors 
for conveying the pulses to a load. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles. By way of example and 
not limitation, other current probes and/or transformers could be used for 
current transformer probe 26. Likewise, there are a wide range of op-amps, 
comparators, and flip-flops that could be substituted for comparators 36 
and 60, flip-flop 66, and op-amp 48. Accordingly, all such embodiments 
have been deleted herein for the sake of conciseness and readability but 
properly fall within the scope of the following claims.