Download A Simple Computational Electromagnetic Analysis Example

A Simple Computational Electromagnetic Analysis Example
of Electromagnetic Coupling to Pyro Circuits.
Reinaldo Perez
Jet Propulsion Labaoratory
California Institute of Technology
Pasadena,California
Abstract
Analysis of electromagneticcoupling to pyro circuits
with the objective of preventing adverseeffects such
as un-intentional tiring, have been studied in the past
using a variety of analytical approaches.It is
proposedthat a much simpler and faster approach,
based on computational electromagnetic,can provide
similar results. A portion of a pyro MOSFET circuit
is first examined addressingfailure modes within the
design that can un-intentionally fire a pyro device if
adverseelectromagneticfields of sufficient strength
are present. The electrical conditions that will also
contribute for the pyro circuits to fire unintentionally
are also discussed.Finally, the method of moments is
used to addressthe field-to wire coupling scenario
and to find if the necessaryinduced voltages within
the pyro circuits are sufficient for a firing condition.
Introduction.
High current (5-15 amps) pyrotechnic devices (or
pyros) are extensively used in many engineering
applications requiring abrupt separationof functional
stagesin the course of a mission. For example, pyro
devices are used in rockets for activating the
separationof f&s, second,and third propulsion
stageswhen fuel and oxidizer are depleted and/or
during insertion maneuvers.Most pyro devices are
modeled after the NASA StandardInitiator (NSI)
whose design was perfected over twenty years ago.
All NSI are activated using a variety of pyro
MOSFET initiator circuits which provide the needed
current (5-7 amps average) in a period of a few
microseconds.Most redundant pyro circuits perform
the NSI activation simultaneously, others are
synchronizedto fire on a prescribed sequence.The
design of pyro circuits have evolved over many years
and more sophisticated designs are still being
developed.
Statement of Problem
In this work the pyro circuits and accompanyingNSI
are used to release a series of parachutesduring the
initial stagesof a high speeddescendof a spacecraft
over a planet surface.A portion of a pyro circuits and
its associatedNSI device is shown in Figure 1. The
illustrated portion of the pyro circuit consists of a
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series of two HEXFET (MOSFET type) transistors
(inhibit # 2 and inhibit # 3 in Figure 1). which serve
as the “enable” and “tire” switches to the NSI when
commanded.The HEXFETs are pulled down by two
NPN transistors which are themselvescommandedby
the buffered outputs of two field programmable gate
arrays (FPGA). In the design of the FPGA it was
discoveredthat an internal failure mode exists within
the FPGA such that a latch-on-high condition could
develop. The output high (about 7.5~) could easily
switch ON both inhibits # 2 and # 3, which could
then be followed by the tiring of the NSI if the
appropriate Vgs voltage for each HEXFET was
present. Preventing this scenario from happening
involves keeping the Vgs of the HEXFET to a low
value of no more than 4V. In Figure 2 this is
accomplishedby the use of a battery enable plug that
keeps battery power interrupted (Vgs = 0). The
battery enable plug is for used at Launch Complex
17 (LC-17) at Kennedy SpaceCenter (KSC) during
final systemscheck out and testings. During flight,
battery power is disabled through software means
which de-activatesseveral solid state powering
switches (some of which are shown in Figure 1).
Lacking battery power (or the means to activate such
power) there is only one more source that could
provide the needed Vgs=4V ---electromagnetic field
coupling of known fields nearby LC-17. The
coupling is unto pyro cabling, portions of which are
shown in Figure 3. The pyro cabling of interest would
be the one located between the battery enable plug
and the first HEXFET (inhibit # 2) in Figures 1 or 2.
Only that portion of the cabling will be consideredin
this work. The nearby electromagneticfields of
interest for the scenario in Figure 2 are those
permanent fields located in LC-17. Figure 4 shows
the worst caseaverageelectric field (W/m*) at LC-17
from all known permanent sources.Figure 5 shows
again the worst case emitters at LC-17 in terms of
electric fields strengths of volts per meter (V/m).
Finally, Figure 6 shows the “average” field strength
(W/m’) of LC-17 known emitters. The fields in
Figure 6 are comparedwith the field strength ueeded
to fire an NSI attached directly (i.e no pyro circuits of
Figures 1 or 2 are present) to a hypothetical simple
h/4 perfectly conducting dipole anterma(maximum
pick-up). Iu the figure this is known as the “99.9 %
NO-tire line”. Notice that with the effects of cable
0-7803-4140-6/97/$10.00
FPGA 8 Telemetry power enable
.
Simplified PIIJ circuit showing origin of FPGA and Telemetry power
Figure 1. Pyro Crcuit within the Pyro Initiator Unit and its
NSI connection
MSP Pvro Circuit
Two battery enable Plugs one on power and one on return breaks both 28~ and 2851return paths for NSI
Entire circuit is inside Faradaycage when battery enable plug In place
Figure 2. Battery enable plugs configuration which prevents
firing of NSI
shielding no known emitter at LC-17 at KSC can
even come close to the field strength neededto
produce the 1A current for NSI firing. Therefore, as
suggestedby this work, the most realistic scenario of
inadvertent pyre firing is through the activation of the
HEXFET switches.
Once we have a good knowledge of the
electromagnetic environment in LC-17, the question
arises if any of such field strengthscould induce
enoughcurrent in the shielded cable between the
battery enable plug and the “enable” HEXFET
(inhibit # 2) so as to cause a Vgs = 4V or greater.
From the EMC point of view this is a field-to-cable
coupling scenario.Many of such analysesin the past
have been done analytically for pyre circuits [l-4]. It
is proposedthat such analysescan also be done
numerically using the method of moments (MOM).
As shown in Figure 7, the critical cable layout of
interest, which is inside a capsule, can be modeled
using the method of moments where loads have been
574
FPGA & Telemetry power enable
.
Cimniifiarl
PII
I cirmlit
shov
Figure 1. Pyro Crcuit within the Pyro Initiator Unit and its
NSI connection
MSP Pvro Circuit
Two battery enablePlugs one on power and one on return breaksboth 28~and 28~ return
Entire
circuit
is inside
Faraday
cage when battery
enable
plug
paths for NSI
in place
Figure 2. Battery enable plugs configuration which prevents
firing of NSI
shielding no known emitter at LC-17 at KSC can
even come close to the field strength neededto
produce the 1A current for NSI fining. Therefore, as
suggestedby this work, the most realistic scenarioof
inadvertent pyro tiring is through the activation of the
HEXFET switches.
Once we have a good knowledge of the
electromagneticenvironment in LC-17, the question
arises if any of such field strengths could induce
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enoughcurrent in the shielded cable between the
battery enable plug and the “enable” HEXFET
(inhibit # 2) so as to causea Vgs = 4V or greater.
From the EMC point of view this is a field-to-cable
coupling scenario.Many of such analysesin the past
have been done analytically for pyro circuits [l-4]. It
is proposedthat such analysescan also be done
numerically using the method of moments (MOM).
As shown in Figure 7, the critical cable layout of
interest, which is inside a capsule, can be modeled
using the method of moments where loads have been
Worse
Case Average
7
8
9
10
Field
11
12
in W/M*2
13
14
15 16
@ LC 17
17
16
19
20
21
22
I/ RCS OMNI
2/ RCS HELIX
31 FPS.66
4/ FPS-66
51 GPS GROUND STATION
6/ TVCF
71 MILA TRACKING STATION
81 N A S A STDN
9/ GPN-PO
IOITPN (ASR)
ll/ GPN-20
12l WSR-880
13/ WSR-74C
14/RADAR 1.16
15/RADAR 1.178
IBlRADAR 19.14
171 RADAR 19.17
161 RADAR 0.14
191 RADAR 1.39
201 RADAR 26.14
?I/ RADAR 1.39
?2! RADAR CONDO-6
Emitter Frequency in MHz
Figure 4. Known electromagnetic emitters at LC-17 at KSC and
its worst case field strength in Watts/m’.
Parachute
Mortar W iring
appropriately represented.The shielded cable
(grounded at both ends to protect against high
kequency coupling) which protects the enable line is
modeled as a wire mesh [5]. The details of the
modeling will be presentedin the paper.
Modeling
The NEC code [6] used for thin wire modeling is the
main so&are used for modeling. The center
conductor and shield surrounding the main conductor
shown in Figure xx. It is appropriate to use thin wire
modeling. We are only interested in worst case
coupling of an electric field to the center conductor
and not in the accuratecalculation of current
distribution which could be better obtained using a
surfacepatch modeling for the wire shield. The shield
which is grounded at both ends (due to possible high
frequency coupling) partially protects the
electromagneticwave from coupling directly into the
conduction wire which could activate the HEXFET.
With the presenceof a shield only capacitive coupling
Figure 3. Cable layout of pyre initiator circuits
576
.
New W/C Emitters
at 17 A/B
Figure 5. Known electromagnetic emitters at LC-17 at KSC and
its worst case field strength in Volts/meter.
8
?
Frequency in MHz
8
8
t-
Figure h. Average field strength (watts/m’) of known emitters at
LC-17 at KSC.
will allow someof the induced current in the shield
to couple into the conductingwire.
The segmentationof the inner conductoris done per
NIX recommendationsof h/IO. The shield is
modeled as a thin wire mesh following the equal area
rule given by the expressiond=2xa for the minimum
wire radius a=1.Omm. The mesh size is about
577
0.02SI.2and of rectangularshape.The term h is the
wavelengthat eachof the frequenciesof interest. As
in the inner conductor,the averagelength segments,
where possible, are not longer than MO. However,
the shortestand longest segments(in meters) in the
wire mesh are given by
STRUCTURE
/
I Incident
of shield using.-
(enable plug input)
input impedance to “enable” HEXFET
Figure 7. Method of Moments Modeling of Shielded Cable with Loads
V (dBuV)
132 (= 4V for Vgs to turn ON HEXFET )
t
I
12.26
I
2.60
I
2.94
I
3.28
I
3.62
I
3.96
4.3
I
4.64
I
I
4.98
Margin = 3.75 V
I
5.32
I
5.66
I
6.0
I
6.34
I)
1.60
F (GHz)
FIGURE 8. Induced Noise Voltage at input of “enable” HEXFET in pyro circuit for NSI
Activation.
578
L SHORTEST
L LONGEST
12
=
30
=
[2] Sabaroff, Samuel, “Worst Case analysis of squibs
in an RF field,” IEEE Trans. on EMC, December
1967.
Fm,(mzj
Fm,
cmzj
(2)
For the Fm~(MHz) of 1.6G& and F,,&MHz) of 6.0
GHz the wire segmentsize will range between 7.5
mm and 5.0 mm. The limit on the wire thickness is
given by
[3] Hirsh R. Stanley, “RF Current induced in an
ordnancecircuit,” IEEE International Symposium on
EMC, August 1965.
[4] Lee, K.S, H.Marin, and Castillo, J.P. “Limitation
on Wire Grid Modeling of a closed surface,”IEEE
Trans. EMC, No. 3,1977.
[5] Ludwig, C. Arthur, “Wire Grid Modeling of
Surfaces,IEEE Trans. AP, No. 9, September, 1987.
which provides a result of 1.6 mm and this is
consistent with the wire radius previously chosen.
In principle there was only a need to look for a worst
case electromagnetic coupling scenario. If for such
scenario the induced coupled noise was significantly
less than the needed Vgs=4.OV, there would be no
need to pursue this issue any furhter from the safety
point of view. The worst case analysis was envisioned
by using the largest electric field magnitude recorded
at LC-17, which correspondedto that of 222.5 V/M
(5.692 GHz radar), and use this level as the incident
plane wave acrossthe frequency spectrum of interest
(1.6--6.0 GHz). The effects of apertureswere also
ignored for a worst case analysis. The calculated
output voltage resulting from the field-to-wire
coupling scenario on the HEXFET side will be
equivalent to the input Vgs of the “enable” switch.
The results of these noise voltages as a function of
frequency are shown in Figure 8. The figure also
shows the safety margin at the worst critical
frequency.
Results of Analysis
The results show that: a) for this particular situation
the induced noise voltage Vgs at the HEXFET input
is very small to turn ON the enable switch, even if
the FPGA failure (latch-to-high output) were to occur
for both the “enable” and “fire” switches. Notice that
the worst case scenario in Figure 7 is for a frequency
that is not even one of the known emitters at LC-17,
and b) computational electromagnetic can serve as a
valuable tool in modeling pyre-initiation events
triggered by electromagnetic field coupling.
References
[l] Baginski, Thomas A. “Hazard of low frequency
electromagnetic coupling of overheadpower
transmission lines to electro-explosive devices, “
IEEE Trans. on EMC, November 1989.
579
[6] Burke, Geral J. “The Numerical Electromagnetic
Code-Method of Moments,” UCRL-MA-109338, Jan
1992, Lawrence Livermore National Labs.