SCAT (Sneak Circuit Analysis Tool)
Can You
Afford Not to Do
Sneak Circuit Analysis?
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Imagine..
- One brake in an automobile
suddenly engages because of an unexpected current flow
caused when a turn signal relay disconnects
- The detonation circuitry in a
cruise missile is triggered prematurely because it
shares a ground plane with a high current servo control
circuit
SCAT the
automated tool for Sneak Circuit Analysis will detect
design flaws that can lead to such scenarios by finding
all:
- Reverse current flows
(H-patterns)
- Power source to power source
current flows (Y-domes)
- Ground-to-ground current flows
- Momentary undesired current paths
during switching
For more
information, please review our
brochure.
What
is Sneak Circuit Analysis (SCA)? |
Sneak Circuit Analysis is
performed on systems that include irreversible functions (e.g.,
rocket engine firings, squib ignitions, or release mechanisms).
The objective of SCA is to ensure that there is no possible
system configuration that could induce accidental actuation of
such devices.
A sneak circuit exists
when under some, usually very rare and unexpected, conditions
an output is furnished when not desired, or an output is
inhibited when desired. The worst consequences of sneak
circuits have been in electro-explosive devices (squibs), and
Sneak Circuit Analysis (SCA) has been mandated in many
military applications where squibs are used. MIL-STD-1543
requires SCA for most spacecraft, launch vehicle, and missile
systems. Although the standard is inactive, the provisions
regarding SCA incorporated in the practices of many space and
missile agencies. SCA has also become increasingly important
for the automotive industry as electrical and electronic
devices with complex logic paths are used to control engine,
brake, and other vital functions in cars.
The cause of sneak
circuits is unexpected and unintended direction of current flow
in a branching electrical network. When a load is controlled by
a switch from a single source and is connected to a single
return (ground) there is no possibility of a sneak circuit.
Current will always flow from the source to the return. But as
soon as even a single branch is added, current may flow in the
direction opposite to that which is intended. The undesired and
unintended events occur only under rare conditions or when
established procedures are not followed. The objective of SCA is
specifically the prevention of these rare but high consequence
events. SCA is therefore an appropriate technique for the
investigation of the infrequent and unexplained occurrences.
In the simple example
shown in the figure, the circuit is designed to drop lower the
landing gear and open the cargo door when a normal landing takes
place and to open the emergency door but keep the landing gear
up in an emergency landing. The red path shows a sneak circuit
whereby the closing of both switches will cause the emergency
door top open and the landing gear to be lowered simultaneously,
an action that could result in a disaster.
A complete SCA will
identify, check and verify every possible current path in a
complex system
- Larger systems can have
millions of potential current flows
- Most analysis is done manually today
using valuable design engineering time or costly consultants
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What is SCAT?
SCAT (Sneak Circuit
Analysis Tool) is a Windows program used for automatically
identifying potential sneak paths (i.e., paths which inhibit
desired functions or cause undesired outputs) by processing
circuit schematics.
SCAT has a modern easy
to use GUI that automates both the preparatory work and the
actual analysis that are part of SCA. When user input is
required (such as identifying ports) the GUI clearly prompts
and offers automated menus that induce a streamlined and
smooth work experience.
The SCAT
Approach to Sneak Circuit Analysis
SCAT differs from
conventional SCA (Sneak Circuit Analysis) techniques in that
the latter are based upon the generation (usually automated)
and analysis (mostly manual) of network trees to identify
sneak paths. In contrast, SCAT does not require or even make
use of the traditional network trees.
The automated
procedure provides the design engineer or reliability
analyst with a simple yet powerful tool for rapidly
identifying and correcting sneak paths. The automated
procedure is based on the fact that sneak paths involve
circuit components which can conduct current in either
direction depending upon the switching state of the circuit.
SCAT exhaustively searches for these bidirectional branches
which is more readily automated than searching for specific
topological circuit patterns, as done by conventional SCA
techniques. Furthermore, the analyst's task is reduced to
evaluating the significance of specific potential sneak
paths rather than applying "clue lists" to circuit patterns
for identifying the sneak paths.
A significant issue
that arises is the assurance that sneak paths associated
with interconnecting assemblies or subsystem interfaces are
not overlooked. This issue is addressed in two ways. First,
the system compels the user to identify each interface port
of a switching circuit in terms of it being a power input.
Interfaces to power and ground are labeled as such
regardless of whether they respectively go to power and
ground directly or through switched or un-switched loads,
and they are included within the sneak path search.
SCAT was developed to
enable design engineers to perform SCA early in the project
life so that problems can be corrected early and at minimal
cost.
Performing a
Sneak Circuit Analysis with SCAT
There are three steps in using
SCAT: preparation, analysis and evaluation. These tasks have
been automated utilizing a concurrent engineering
environment comprising the EDIF 2.0 netlist file format for
schematic capture and the SCAT software.
Preparation
Preparation consists of automated pre-processing
of the schematics, netlist generation, netlist processing,
power port selection and optional switch and capacitor
modeling.
Analysis
SCAT can perform sneak circuit analysis with
a single command. A printable session log (audit trail)
describing all actions taken by the user during the analysis
is stored and can be printed out as analysis documentation.
The user can easily change the input parameters to SCAT and
rerun previous analyses. The first step is performing a
sneak circuit analysis which identifies all the source to
sink paths and among them finds all the bi-directional
paths. The next analysis is the power-to-power analysis
which finds all the source-to-source and all the
sink-to-sink paths which are, by definition, bi-directional
paths.
SCAT TOOL ACTIVATION
Evaluation
Evaluation consists of viewing the
asterisked bi-directional paths on the listing and all the
power-to-power (source-to-source and sink-to-sink) paths.
Where such a path is encountered the analyst must decide
whether it is
- An intended bi-directional current
path
- An unintended bi-directional path
through a high resistor that limits the current to a
fraction of the actuation load - or
-
A true sneak circuit
that must be eliminated.
SCAT SNEAK ANALYSIS
PATHS LIST
SCAT POWER-TO-POWER PATHS LIST
Functional Packages:
PREDICTOR
- 5 Standard Operating
Reliability Prediction Methods
- BOM Builder, Temperature
Curves, Mission Profile, and Import
Wizard
- Full set of
ready-to-print Reliability Reports
- Enhanced Report
Generator for user defined reports
RELIABILITY PRO
- 5 Standard Operating
Reliability Prediction Methods
- BOM Builder,
Temperature Curves, Mission
Profile, and Import Wizard
- User Defined
Components Reliability Data Module
- Full set of
ready-to-print Reliability Reports
- Enhanced Report
Generator for user defined reports
- Components Library
Module (over 400k components)
- RBD - Reliability
Block Diagram
MAINTAINABILITY PRO
- Maintainability
Module (according
to MIL-STD-472 Procedure 5A)
- Built in libraries
for Tasks, Times, STE, Skills,
and Materials
- Maintenance
Engineering Report (MEA
Report)
- Maintenance Task
Analysis Report
- Full set of
ready-to-print Reliability
Reports
- ILS Supprt Fields (user
defined)
- BOM Builder,
Temperature Curves, Mission
Profile & Import Wizard
- Enhanced Report
Generator for user defined
reports
- RCM/MSG-3 Module
- FMECA Module
FAILURE
ANALYSIS & ISOLATION
- BOM Builder, and
Import Wizard
- FMECA (covering
both Hardware & Functional)
Module
- Customizable FMECA
Worksheet
- FTA Module
- FMECA
Import/Export
- Testability Module
- Full set of FMECA
& FTA ready-to-print Reports
- Full set of
ready-to-print Testability
Reports including graphical
trouble shooting, fault
isolation, etc.
- FMD-97 Library
- Enhanced Report
Generator for user defined
reports
SAFETY
ASSESSMENT
- SSA supporting IEC
61508 (Safety
Standard for Safety
Instrumented Systems)
and MIL-STD-882D (SHA,
O&SHA), SAE, ARP 4754A,
and ARP4761
- Customizable FMECA
Worksheet
- FTA Module
- RBD Module
- Markov Chains
Module
- FMECA
Import/Export
- Full set of
ready-to-print Safety Reports
- Enhanced Report
Generator for user defined
reports
AVIATION
SAFETY
- SSA & FHA
according to SAE, ARP 4754A,
ARP4761, and FAR 25 1309b
- FTA including
ready-to print reports
- MSG-3 (System,
Zonal, and Structural)
including ready-to-print
reports
- MMEL including
ready-to-print 5-column report
- FMECA including
ready-to-print FMECA reports,
BOM Builder, and Import Wizard
- Enhanced Report
Generator for user defined
reports
- Report Template
Utility for layout
customization
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