Richard Perraud(1), Alexandre Piche(2), Franck Flourens(3), Daniel Soulard(4), Sébastien Thibaud(3),. Dominique Lemaire(3) and Jean-Charles Garric(3). (1).
Simulation test bench for LIE and HIRF on a communication bus Richard Perraud(1), Alexandre Piche(2), Franck Flourens(3), Daniel Soulard(4), Sébastien Thibaud(3), Dominique Lemaire(3) and Jean-Charles Garric(3) (1)
EADS IW, 12, rue Pasteur ,BP76, 92152 Suresnes Cedex, France EADS IW, 18 rue Marius Terce, BP13050, 31025 Toulouse Cedex 03, France (3) AIRBUS France, 316, route de Bayonne 31300 Toulouse (4) ASTRIUM 31 rue des Cosmonautes, Z.I. du Palays, 31402 Toulouse Cedex 4
(2)
Abstract - This paper deals with the building of a theoretical simulation test bench for LIE and HIRF on a communication bus. For that purpose, we propose a description of the methodology used to predict the induced levels on the BUS for typical scenarii derived from aircraft topology. Models needed to perform such a simulation are coupler model, transformer model, cable model, injection model. To validate the simulation chain before the exploitation, we perform two kind of validations i.e. simulations / measurement comparisons: one at component level and the other one on a setup representative of aircraft. The predicting tool used in this study is Aseris NET. It solves network equations in the frequency domain with multiconductor transmission lines method. Keywords : EMC, Communication bus, Lighting Indirect Effects (LIE), HIRF (High intensity
radiated fields).
I. INTRODUCTION CFRP (Carbon Fibre Reinforced Plastic) panels are used in the aerospace industry on many systems (helicopter, launcher…). However, for aircraft industry, the proportion of metallic panels tends to decrease thanks to the maturity of carbon fibre composite structure engineering. From an electromagnetic point of view, the presence of composite on a whole aircraft implicates new investigations for direct and indirect effects of lightning but also for system installation requirements. The use of balanced signals electrically insulated from the structure is the best solution that need to be clearly specified. We present here a methodology based on simulations and measurements which aims are:
978-1-4244-2737-6/08/$25.00 ©2008 IEEE
x x x
validate individual models with comparisons with measurements understand the coupling mechanisms on the system predict the levels at aircraft level
The system examined in this paper is a 1553 communication bus. Assessments of induced levels on bus interfaces (couplers, transformers) are of great importance to validate the protection of this bus against indirect effects of lightning. We remind that on composite aircraft the waveforms that are encountered are WF5, those which are the most energetic. The components to be considered for a communication bus are: bus interfaces (transformers), physical media (cables) and coupler. We will see also that some questions raise about the ground connection of couplers that could be or not referenced to the common ground. All the simulations are performed with network solvers, spice or internal EADS code Aseris NET. Aircraft levels in the different zones are given from 3D simulations but they are not presented here.
II. INDIVIDUAL MODEL VALIDATION STEP We present the three validations steps concerning the cable model, the coupler model and transformer model. A. Cable model To be able to predict the way voltages and currents on the bus topology, we need accurate cable model, for example for all that concerns the common mode impedance i.e. the impedance of the
internal wires with its shield (capacitive coupling). To obtain a model of cables two ways are possible : x
x
From cable standard ASNE0290 (XM24) we can derive a cable geometrical description with dielectric and conductor interfaces and compute with a 2D FEM code the capacitance and inductance matrix.
winding capacitance. These capacitance are around 42pF. The figure below shows the comparison between measurement and simulation for common mode impedance seen on the bus input (two internal core connected together). 10
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The other way is to build a model from measurement ,
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The second way has been chosen as the first one does not give realistic result as the cable never match the minimum performances specified in the standard. The uncertainties we have on the exact positions of conductors in the cross sections (i.e. proximity from the shield) lead to substantial differences between estimated and measured common mode capacitance. The parameters that are influent are the geometrical distance between internal wire and the shield and also the permittivity between internal conductors and shield.
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Model derived from standard
Model derived measurement
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Common mode impedance on bus coupler measurement (black) / simulation (blue)
C. Transformer model The transformer model are of two types and as the coupler, we must also determine the capacitance between primary and secondary of the transformer. This value is not known and we must also make some measurements with specific boards presented below :
from
The numerical model derived from measurement could then be used for external coupling considerations.
B. Bus coupler model The bus coupler is made with 2 main ports for main bus line (cable and load) and 4 other ports for stubs connected to this coupler. The figure below shows a typical bus coupler with A and B, bus line ports, and C, D1, D2, D3, the stubs lines ports.
Bus coupler functional model
This schematic is purely functional but for common mode threats we must also consider the coupling between each part of the coupler which will occur with parasitic capacitance called inter
PCB board developed for inter winding capacitance measurement on transformer
The measurements of the transformer Microspire are shown hereafter and clearly, for frequencies below 10MHz, the transformer behaves as a capacitance for common mode. A basic post processing step is needed to take into account the PCB trace/ground plane capacitance and the real value of transformer capacitance is 10pF.
|Z|(ohms)
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injection on primary and secondary connected to ground
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Measurement on Microspire transformer model
Open circuit voltage
III. VALIDATION OF THE INJECTION MODEL ON
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To validate our model that contains cable, coupler and transformer models but also to understand the basic phenomena of coupling on typical topologies, we have built the setup below which is representative of common mode injection as LIE.
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Short circuit current : measured (green), computed (red) TP STUB TP Load
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Setup to validate assembly of individual models
A pulse generator (WF4) is placed between the cores of the cable (BUS side) and the ground. At first the residual common mode voltage on the stub line is expected as being the result of a capacitive coupling though the inter-winding capacitance of the bus coupler. A. The injector model In the first measurements, to calibrate the generator (Voc and Isc for a WF4 500V/100A), we have observed that the behaviour of Isc is non ideal as the current becomes negative before returning to zero. In our first computations of the setup we did not see this phenomena which is completely due to the generator ; our first generator model was too ideal. As the short time response was correct (ratio of 100: between Voc and Isc on the peak value), we added a capacitance to our model to represent the late time response. This capacitance explains that at one time, current flows in the opposite direction. We can see below the open circuit voltage and short circuit current compared with our model considering a voltage generator made of an emf, series 100: resistance and 60µF capacitance:
Coupling phenomena The simple setup that was built up is representative of all the future setups whatever their complexity is. The coupling phenomena for common mode coupling injections will be rather the same. If we look on the way that a common mode voltage appear on stub lines when bus lines are under constraint we see that this residual voltage can be expressed as :
Vresidual 1
1 V Ceq threat Ciw
where Ceq is
Ceq
Ciw C stub Ctransfo / ground
Ciw : inter winding capacitance Cstub : common mode capacitances on the stub line Ctransfo/ground : inter winding capacitance for transformer at the end of the line We can note here that longer the stub line, lower the residual voltage. This is observed on the figure below where the generator is a WF4 80V/16A:
7 v3 6 5 4 3 2
B. LIE results on representative topologies Some scenarii have been analysed in these study and we present here the scenario below which represent an architecture for the communication bus. The bus line is 50m and some stub lines can reach 30m. Once we have identified all the component of our network, it is then possible to simulate this architecture.
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Residual voltage on transformer input for stub line length=2m . Measurement (blue), Computation (red).
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IV. LIE PREDICTIONS A.
LIE model Our experience (measurement/simulations comparisons) on 3D simulations prediction gives us confidence to assess the indirect effects on cables inside structures. Our computations are made with Aseris FD which is an FDTD code for which some recent developments have been made to accelerate the computation. For low frequency responses, lightning A waveform especially, we can introduce artificially a dielectric permittivity in the volume space to increase the simulation time step. To simulate the LIE on the bus network, we use a coupling technique called shield current coupling which consider the shield current computed on overbraidings typically on wing stubs. To perform this coupling, we first compute the shield current in the 3D code in time domain and inject this distributed current into Aseris NET knowing the transfer impedance of the cable. The levels presented hereafter are those measured and computed for localized WF 4 injections on representative topologies with real cables lengths.
The lightning waveform generator is WF4 with Voc=500V. For information, the parameters for the double exponential are V0=547Volts, D = 11358 and E=641710. This source simulates an emf due to the flow of lightning current in overshields (we concentrate the lightning emf in one point). It is why the injection has the hot spot on a conductive plate connecting cables shields and grounded components and the cold spot on the ground (grounding plate). The table below gives the maximum values in time domain results for shield currents and voltages that appears in the network. We see here that simulation can predict very well the behaviour of the network.
Vsource Source current Shield current Voltage on transformer input
SIMULATION MEASUREMENT 239.34 201.91 86.95
82.08
48.30
45.50
21.04
19.25
A. Susceptibility levels When an error occurred on the protocol (software error), the frequency is registered and we perform at the same frequency the voltage measurement at the interface input. These two measurements give the CM voltage that is present at the input of the remote terminal when an error appears.
The conclusions that can be established are the following. Regarding the common mode voltages under the shields (those which are constraint the 1553 couplers and transformers), they are the consequence of two major coupling phenomena : 1/ transfer impedance (resistive) coupling due to the current flow in the bundles, 2/ capacitive coupling through the bus coupler. We must note that the main coupling effect in terms of amplitude is the transfer impedance coupling. C. LIE predictions at aircraft level We analysed the same configurations as before with distributed WF5 injections and depending on the bus coupler connection to the ground. The bus coupler in electronic bay is considered here always grounded but coupler between fuselage and wing not. The outputs from the simulations (no measurement in this case) are shield current and common mode voltages on equipment interface. The shield current sources (derived from 3D simulations in worst cases) are WF5 waveforms as specified in ED84 and their amplitude are 2000A on wing bundles. The typical common mode voltage on bus and stub line that were predicted depends on the coupler ground connection. The common mode voltage on the stub line is decreased when the coupler is isolated as the lightning current flows in a larger loop on the network. As a consequence the common mode voltage on the bus line is increased. For WF5, the maximum values are around 300V.
I cc
2hE Zc
with Zc : characteristic impedance of the line h m: height above the ground plane E : field amplitude For h=3mm and the XM24 geometry, Zc computed is 240:. So we find the E field amplitude which must be 590 V/m to get Icc=150mA. The table below shows clearly that in this configuration we have a margin at least equal to 20dB. Susceptbility level vs expected level Vmc measured with error
Vmc measured without error
Expected Vmc
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Vmc measured near RT
Vmc (Volt)
Voltage on the transformer input
B. HIRF predicted levels We determined by simulation the expected level on bus interface when a constant current of 150mA is flowing on the network shields. This level is assumed to be the maximum current level seen on aircraft. The assumptions of the simulation are : shields are connected to the ground at both ends and illumination of the wing stub by an incident field whose characteristics are normal incidence and E // line and H A line and // ground. In that case the maximum current level is
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Aseris Net simulation Ishield Wing OBG = 150mA Ishield Wing IBY = 150mA
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V. HIRF PREDICTIONS VI. CONCLUSION For HIRF, the study was decomposed in two steps : x x
determine the susceptibility levels of a functional bus setup for RF injection predict the HIRF levels on aircraft configuration
This paper reports the modelling steps that have been introduced in the design phase for a communication bus installation in new aircraft. After having characterized with measurement and validated individual models of the simulation chain, we used those models in expected network configurations to assess the LIE and HIRF on that system. This assessment is performed with the
simulation tool Aseris NET. Our main output points are shield currents and common mode voltages created by the common mode threat. We can note here that simulations were performed to predict levels at aircraft levels with a high level of confidence due to the step by step validation phase (measurement vs simulations). The added value from simulations is then to understand how the coupling phenomena occurred on complex network and finally validate the performance of the solution that is specified taking into account the worst situation for each specified parameter. Indeed, this assessment is impossible to do experimentally because all components are usually better than their specification
REFERENCES [1] Tesche, Ianoz, Karlsson, “EMC Analysis Methods and computational models”, John Wiley & Sons, Inc., New York, 1997. [2] Clayton R. Paul, “Introduction to Electromagnetic Compatibility”, Second Edition, Wiley Intersciences, Inc., New York, 2006.
