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Transcript
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 573 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 575 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.