Transcript System Suitability Module 2

System Suitability
Lecture: Reliability & FMECA
Lecturer: Dr. Dave Olwell
Dr. Cliff Whitcomb, CSEP
[email protected]
About the Lecturer
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Dr. David Olwell
Chair, Department of Systems
Engineering, NPS
Research interests are primarily
reliability engineering and analysis
Phone: (831) 656 3583 (W)
Email: [email protected]
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Objectives
• Be able to construct a FMECA using Risk
Priority Numbers
• Interpret a Fault Tree
• Be able to perform a simple reliability
prediction for a series system with
independent exponential failure times
• Understand the principles of sequential
testing
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Reading
• B&F (4ed) 12.4-12.6
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FMECA
• Discussion in text (section 12.4.1) is good
• Figure 12.21 can easily be implemented in
spreadsheet, although for large systems
commercial software is useful
• Bottom line: RPN is used to score failure
modes on a set of subjective scales, and the
score orders them. We start by mitigating
the ones with the highest number, and work
down to some stopping point.
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FMECA/FMEA
• Failure Modes and Effects Analysis (FMEA)
and Failure Modes, Effects and Criticality
Analysis (FMECA) are methodologies
designed to identify potential failure modes
for a product or process before the problems
occur, to assess the risk associated with
those failure modes and to identify and carry
out measures to address the most serious
concerns.
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Standards
• MIL-STD-1629A
• SAE International’s J1739 and ARP5580
documents (for automotive and nonautomotive applications, respectively)
• Automotive Industry Action Group’s (AIAG)
FMEA-3
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Figure
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Common Themes
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Item/Process: Identify the item or process that will be the
subject of the analysis, including some investigation into the
design and reliability characteristics. For FMEA analysis of a
product or system, the analysis could be performed at the
system, subsystem, component or other level of the system
configuration.
Functions: Identify the functions that the item or process is
expected to perform.
Failures: Identify the known and potential failures that could
prevent or degrade the ability of the item/process to perform its
designated functions.
Failure Effects: Identify the known and potential effects that
would result from the occurrence of each failure. It may be
desirable to consider the effects at the item level (Local
Effects), at the next higher level assembly (Next Higher Level
Effects) and/or at the system level (End Effects).
Failure Causes: Identify the known and potential causes for
each failure.
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Common Themes
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Current Controls: Examine the control mechanisms that will be in
place to eliminate or mitigate the likelihood that the potential failures
will occur (e.g. end of line inspections, design reviews, etc.).
Recommended Actions: Identify the corrective actions that need to
be taken in order to eliminate or mitigate the risk and then follow up on
the completion of those recommended actions.
Prioritize Issues: Prioritize issues for corrective action according to a
consistent standard that has been established by the organization.
Risk Priority Number (RPN) ratings and Criticality Analysis are
common methods of prioritization and they are described later.
Other Details: Depending on the particular situation and on the
analysis guidelines adopted by the organization, other details may be
considered during the analysis, such as the operational mode when
the failure occurs or the system’s intended mission.
Report: Generate a report of the analysis in the standard format that
has been established by the organization. This is generally a tabular
format similar to the one shown in Figure 1. In addition, the report may
include block diagrams and/or process flow diagrams to illustrate the
item or process that is the subject of the analysis. If applicable, the
criticality analysis may be included in a separate table and various
plots/graphs can be included to display statistics on the modes and
rankings.
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Figure
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RPN (Risk Priority Number)
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Severity (S)
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Occurrence/frequency (O)
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1-10 scale
A rating of the likelihood of occurrence for each potential failure
cause.
Detection (D)
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1-10 scale
A rating of the severity or seriousness of each potential failure
effect.
A rating of the likelihood of detecting the failure cause.
1-10 scale, hard to detect being higher
RPN = S*O*D
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Comment
• Scales are arbitrary
– Implies they can only be used for ranking failure
modes internal to the system, and may not be
not useful for comparing across systems
• Must be well defined enough so that different
people assign the same number
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Example: Battery
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Severity: 8 - Extreme Effect. Product inoperable but safe. Customer
very dissatisfied.
Occurrence: 5 - Low. Occasional number of failures likely; expect
about 2.7 failures per 1000 due to this cause.
Detection: 1 - Almost Certain. The operator will almost certainly be
able to detect the failure
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Criticality Analysis – A different approach
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Criticality = (Q)(FMFR)(PL)
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Item Unreliability (Q): The probability of failure for the item at
the time of interest for the analysis.
Failure Mode Ratio of Unreliability (FMFR): The ratio of the
item unreliability that can be attributed to the particular failure
mode. For example, if an item has four failure modes, then one
mode may account for 40% of the failures, a second mode may
account for 30% and the two remaining modes may account for
15% each.
Probability of Loss (PL): The probability that the failure mode
will cause a system failure (or will cause a significant loss).
This is an indication of the severity of the failure effect and may
be set according to the following scale:
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Actual Loss = 100%
Probable Loss = 50%
Possible Loss = 10%
No Loss = 10%
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Battery
• Criticality is (0.08)(0.25)*(1.0) = (.02)
• Use for relative rankings, not necessarily an
absolute scale
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Recommendations
• An important use of the FMECA is to
document mitigation strategies
• The estimated reduction in the RPN is
shown if the suggested mitigation is adopted
• Useful to compare “before” and “after” risk
• In the early FMECA, recommendations
suggested reduced risk for the first failure
mode from 294 to 28
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Comment
• This FMECA approach is commonly used for
reliability, but can be applied to other
domains where risk identification and
mitigation are of interest
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Fault Trees
• Fault trees emphasize what must occur for
system failures while reliability block
diagrams emphasize what must occur for a
systems to work
• They are logically equivalent
• Partial FTs can be useful when trying to
isolate fault structure
• Discussion in book is pretty good, if short
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B&F Textbook Example
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Here is an example of
the RBDs from Figure
12.10 reworked as
fault trees
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Simple reliability prediction
• Requires either data or assumptions
– If reliability of components is known, use their
data
– Be careful to verify that the conditions of use are
the same as those that generated the historical
data
– If making assumptions, be realistic and
conservative
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Example
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Here is an
example of
reliability
prediction for
Figure 12.10(a)
using the data
from Table 12.2
and a mission
time of 100 hours
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Sequential testing
• Sequential testing allows early termination of
a reliability test
– If the system exceeds requirements or fails
requirements by a large margin, the test
terminates early
– For ‘close calls’, it takes more testing to
accumulate enough data to decide
– Idea covered again in system assessment course
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Example
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The example in the text (Section 12.5.1 and Figure
12.26) is good
Focus on understanding the interpretation of Fig
12.26
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That example assumes exponential failure times….
Know the four key elements for constructing the figure:
producer risk, consumer risk, threshold MTBF, objective
MTBF
The horizontal axis is total time on test, or TTT, and is the
sum of all the running time of the units that have failed or
are still being tested
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Operational Testing
• Study section 12.5.4
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Conclusion
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Reliability is a very important element of suitability
Several NPS courses are available for further study:
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OA4302 Reliability and Weapon System Effectiveness
Measurement
SE3321 Reliability Management and Data Systems
SE3322 Reliability Centered Maintenance
SE4321 Reliability Growth and Accelerated Testing
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