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The unified EM protection concept is based on enclosing the electronics in an electromagnetic barrier that prevents externally generated electromagnetic fields from degrading the performance of the electronics enclosed by the barrier, in accordance with the system operational requirements. To accomplish this objective, the barrier should be a closed surface that effectively prevents the externally generated environments from leaking into the system interior and thereby causing electronics upset and damage. Once the impact of the externally generated environments is controlled, protection against the internally generated environments must still be provided.
As illustrated in Figure 1, it is not generally practical to build a closed barrier. Penetrations through the barrier are usually necessary for system electrical and mechanical functions. To maintain the barrier effectiveness, penetration protective devices must be provided for all penetrations.
Therefore, for practical systems, the EM barrier is defined by two components: the electromagnetic shield, and the penetration protective devices. In combination, these barrier components must reduce the externally generated environments to levels consistent with the system requirements. Designing for the acceptable levels, and in turn the barrier performance requirements, involves a process of balancing the externally generated environment stresses, the internally generated environment stresses, the electronics immunity, and the margin selected to control risk.
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Figure 1. The Unified EM Protection
Methodology develops protection |
The set of engineering trade studies necessary to achieve this balance is called the EM allocation process. The purpose of EM allocation is to assign performance requirements to all features of the EM protection. This includes the equipment immunity, shielding effectiveness, and penetration protective device performance requirements. In addition, determination of margin, which can be adjusted to cover risk and uncertainties, is part of the allocation process. The general approach is to derive performance requirements that assure that the electromagnetic induced stresses are attenuated below the corresponding equipment immunities by an amount (margin) chosen by the designer to cover risk and uncertainties. This concept is illustrated in Figure 2.
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Figure 2. The barrier performance requirements
limit the external |
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The equations for the allocation process used to compute the Barrier Performance Requirement (BPR) are:
The computations are performed in the frequency domain over a range that covers all of the environments of interest. For the simplest case involving one environment and one subsystem without any penetrations and only one immunity, computation of the barrier performance requirement is almost trivial. However, the process quickly becomes very complex when multiple environments, subsystems and immunities are to be treated. Then it is necessary to perform a unification process to reduce the results for all possible combinations of immunities and environments to a single comprehensive set of requirements. The unification process is described in detail in [1].
The complexity of the allocation process for normal systems, coupled with the need for detailed information on the various EM quantities (environments, immunities, margins and transfer functions), makes the unified EM protection methodology an ideal candidate for implementation in a computer program.
The Unified EM Design software package provides detailed information on environments and immunities as well as the unified EM protection and allocation concepts. As shown in Figure 3, the package provides access to the information in a simple "Explorer" style format. Selecting an item from the list on the left causes the requested information to be displayed on the right. A key element of the software package is the generation of the information tree which allows the user 1) to access the standard environments and immunities, 2) to develop custom descriptions of environments and immunities, 3) to describe system topologies, and 4) to perform barrier allocation processing.
Figure 3.Example of a screen from the
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The software architecture, shown in Figure 4, supports all of these activities, while minimizing maintenance requirements through the use of data driven processing. The software package runs under all windows operating systems and is comprised of custom software developed in Microsoft Visual Basic 6.0 and three different database structures developed in Microsoft Access database. The three databases have different uses, but their structures are very similar, allowing common code processing of information in all three.
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Figure 4. The software architecture
was designed to provide |
A unique feature of the architecture is the inclusion of software query language (SQL) constructs in the database tables. The User Interface software accesses the SQL and uses it to develop the information tree seen on the left side of the form in Figure 3. This allows the format of the entire software package to be modified by simply editing the database table entries, instead of writing new software.
The user has complete control over the data in the User and System Information databases but not the UEMD Information database. This prevents the information on standard environments, immunities and other unified EM quantities from being compromised. The User Information database provides the user with the ability to develop custom EM quantities, starting from the rich collection of descriptions already available.
The user interface includes a simple chart function for plotting the various waveforms. This chart function is comparable with Microsoft Office and was used to generate the comparison figures in this paper (Figures 5, 8, 10, and 11).
The Analytical Models software accesses data in the various databases and generates curves for environments, immunities, transfer functions and barrier performance. The UEMD Information database contains parameters for over 450 distinct waveforms. A very concise set of analytical and numerical representations was developed for the software package. As presented in Table 1, only eight different waveform descriptions were needed to treat all of the environments, immunities and other unified EM quantities required by the process.
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Table 1. Waveform descriptions utillized
in |
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These eight waveform representations were identified through a review of all of the environment and immunity standards of interest. The review included military standards, such as MIL-STD 461 [2] and MIL-STD 464 [3], and commercial standards, such as the series on immunity testing published by the International Electrotechnical Commission (IEC) [4] and the RTCA DO-160 test specification for aircraft electronics [5]. The software includes detailed information (time and/or frequency) on a variety of environments, including:
High Intensity Radio Frequency (HIRF) High-Altitude Electromagnetic Pulse (HEMP) Ultra Wide Band (UWB); High Power Microwave (HPM); Direct Lightning; Nearby Lightning; Precipitation Static (P-static); Electromagnetic Emissions (EME); and Electrostatic Discharge (ESD).
The majority of these are radiated environment specifications. Allocation processing requires a knowledge of the conducted stress on penetration ports due to these radiated environments. Therefore, a simple coupling tool [1] was included in the analytical model capability. As shown in Figure 5, transfer functions for three length lines are provided. These transfer functions are based on an analysis of the worst case coupling response of a conductor exposed to an EM environment.
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Figure 5. Simple coupling models provide
estimates |
The narrowband and transient immunity standards included in the UEM Design software package provide coverage for all of the ports (Enclosure, AC Power, DC Power, Signal & Control, and Antenna). One challenge faced by the allocation process was how to accurately utilize the wide range of immunity test methods. Depending on the standard, the test specification might be for an open circuit voltage, a conducted current, a magnetic field or a radiated electric field. As depicted in Figure 6, one of the commercial immunity standards typically provides information on the open circuit voltage from a transient source, while most military standards require calibration of the immunity source using a known load. The results from these two very different test specifications must ultimately be compared to the conducted stress coupled to a penetration.
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Penetration port immunity
standards require |
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This led to the adoption of standardized load impedances for penetration port stress calculations and the use of equivalent sources to estimate the current delivered to a subsystem port under test. The IEC series of immunity test specifications provided only general information on source diagrams, such as the circuit shown in Figure 7, but not all of the component values. The complete set of component values listed in the figure were found by fitting the specified open circuit output voltage with the circuit response.
Figure 7.Simplified circuit and component
values
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The source specification also provides for an adjustable series resistance, RS3 , which can have a big effect on the load current, as shown in Figure 8. Depending on the value of the series resistor, the current response is either under damped or over damped. The resistor also affects the peak current delivered to the port under test. Therefore, the Analytical Models function had to be designed to be able to handle variations in circuit parameters as well as be able to compute load currents from open circuit voltage or calibration load test specifications.
All of the allocation processing is performed in the frequency domain, making it necessary to include accurate Fourier transforms for all of the transient waveform representations. The majority of the transient waveform descriptions consist of direct Fourier transform pairs to describe the waveform in the time and frequency domains. One exception to this is the Linear Exponential Product which is a ratio of linear functions with an exponential multiplier (See Table 1).
The load current for IEC-61000-4-5,
Level 1, as a function of the series
resistor, RS3.
The description is used in IEC 1312 [6] to describe the time domain waveform of direct lightning strikes. A closed form transform of the ratio of linear functions could not be identified. Therefore, the leading term of the time domain curve, y(t) = at/(1+ at) , was fitted with a transformable expression. This portion of the equation was represented by a series of exponentials with exponents related to the time domain equation parameter, a, as follows:
The weighting values, ai, are also provided in the table. This function was selected for the curve fit because it simplified the transform of the product of the curve fit and the exponential term. The curve fit is independent of the waveform parameters, a and ß. The actual curve, y(t) , and the percent error, e, in the time domain curve fit are shown in Figure 9. The curves agree within ± 0.5% over the range of at.
Figure 9.
The Linear Exponential Product waveform
was accurately fitted with an analytic expression.
The lightning waveform was also fitted with two other analytical waveforms, the double exponential and the quotient of exponentials. The frequency curves of the three waveforms are shown in Figure 10. The low frequency curves are in close agreement with each other. Above 100 kHz, the quotient of exponentials fit has a pronounced roll-off from the other curves that is characteristic of this expression. In this same frequency range, the linear exponential product deviates slightly from the difference of exponentials representation.
Figure 10.
Frequency domain comparison of three
different representations of a lightning waveform.
All of the analytical and numerical descriptions must include complete information on the units of the EM quantity. Each description may use a simple conversion function which applies a constant impedance, Z, to the data, y(i), raised to the conversion exponent, v, yielding a new EM quantity, z(i) = y(i)v/Z. The conversion function operates on either time or frequency domain data. Including an exponent allows conversion from field or circuit quantities (V/m, A, etc.) to power density (W/cm2) or power (W). This conversion function is used to calculate the load currents from open circuit voltage specifications as illustrated in Figure 6. Provisions are also made to allow each waveform to have a scaling dependence, such as slant range or source level, which may be either logarithmic or linear.
As shown in Figure 4, each system has its own database which contains user-specified information, such as the system topology, and information computed by the software package, such as barrier performance requirements. Examples of the results from the barrier performance requirement computation are shown in Figure 11. These curves show the unification of the protection requirements for four different external environments. The figure shows the unified protection requirement for the enclosure and the requirement when each environment is considered individually. The individual curves are based on the equipment immunities, system margins and the external environments. The unification process selects the maximum protection requirement at each frequency across all of the environments. This ensures that the subsystem immunities will not be exceeded by any of the environments.
Figure 11.
Barrier protection requirement generated by
the Unified EM Design software package showing
the contributions for each environment.
[1] Lubell, J. I. et al., Results of the Combined Battlefield Environmental Effects Program - A Unified Electromagnetic Environmental Effects (E3) Protection Methodology, DSWA-TR-97-33, February 1999.
[2] MIL-STD-461D, "Requirements for the Control of Electromagnetic Interference and Susceptibility", January 1993.
[3] MIL-STD-464, Interface Standard for Systems Electromagnetic Environmental Effects Requirements, 18 March 1997.
[4] IEC-61000-4, "Electromagnetic Compatibility (EMC) - Part 4: Testing and Measurement Techniques."
[5] RTCA DO-160C, Radio Technical Commission for Aeronautics, Environmental Conditions and Test Procedures for Airborne Equipment, Dec 4, 1989.
[6] IEC 1312-1 (1995-03), "Protection Against Lightning Electromagnetic Impulse - Part 1: General Principles."
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