Reliability Module By: Alex Miller and Mark Robinson Material - - PowerPoint PPT Presentation

Reliability Module

By: Alex Miller and Mark Robinson Material Summarized from Reliability Module

Derived from original material on the NASA Space Systems Engineering Website (http://spacese.spacegrant.org/).

Key Concepts

• Understanding the importance of reliability

as an engineering discipline

• Definitions and uses for important

engineering concepts

○ Constant failure rate, MTBF, 'bathtub' curve, etc.

• The meaning and practice of redundancy

(fault tolerance, functional redundancy, fault avoidance)

• Ways to calculate reliability

○ Mathematically ○ Visually (block diagrams)

Source: Exploration Systems Engineering Version 1.0, Reliability Module

Definition of Reliability 1

• Reliability: "The probability that the

system-of-interest will not fail for a given period of time under specified operating conditions."

○ Reliability is an inherent system design characteristic ○ Reliability plays a key role in determining the system's cost effectiveness

Source: Exploration Systems Engineering Version 1.0, Reliability Module NASA's Systems Engineering Handbook definition (1995 edition)

Definition of Reliability 2

• Reliability Engineering: "Specialty

discipline within the systems engineering process."

○

Design - including design features that ensure the system can perform in the predicted physical environment throughout the mission.

○

Trade studies - reliability as a figure of merit. Often traded with cost.

○

Modeling - reliability prediction models, reflecting environmental considerations and applicable experience from previous projects.

○

Test - making independent predictions of system reliability for test planning/program; sets environmental test requirements and specifications for hardware qualification.

NASA's Systems Engineering Handbook definition (1995 edition)

Constant Failure Rate

Probability Distribution is usually described as an exponential function: R=et/M

R = reliability t = time M = Mean time between failures

Source: Exploration Systems Engineering Version 1.0, Reliability Module

For systems that must operate continuously, it is common to express their reliability in terms of the Mean Time Between Failure (MTBF)

"Bathtub" Failure Rate Curve

Three phases:

• 1. Burn in period - higher failure rate due to infant

mortality

• 2. Useful life period - low failure rate due to random events
• 3. Old age - increase failure rate due to parts wearing out

Source: Exploration Systems Engineering Version 1.0, Reliability Module

"Bathtub" Failure Rate Curve

Redundancy

• Redundancy: "Exceeding what is

necessary...characterized by or containing an excess." (source: Merriam-Webster Dictionary)

• Two key ideas for systems engineering:

○ Fault Tolerance - The ability of a system to continue

• perating after a component failure has occurred

■ Implemented through the use of redundant systems ○ Functional Redundancy - Design is robust enough to respond to component failures through work- arounds and the use of systems in ways that they weren't originally intended for ■ Examples: Apollo 13, MER, NEAR Shoemaker

Source: Exploration Systems Engineering Version 1.0, Reliability Module

Fault Avoidance

• Ways to achieve reliability in space:

○ Generous environmental and design margins ○ Use of high-quality design components ■ Class S (space qualified) ■ Commercial-Off-The-Shelf

• Must be careful that OTS parts work in intended environment

○ Highly controlled manufacturing, assembly, and documentation ○ Acceptance testing or inspections on all parts if possible

Source: Exploration Systems Engineering Version 1.0, Reliability Module

Example Calculations

Source: Exploration Systems Engineering Version 1.0, Reliability Module

Example Calculations

Component 1 Reliability: 0.90 Component 2 Reliability: 0.90 Input Input Component 1 Reliability: 0.90 Component 2 Reliability: 0.90 Series: A x B = Total Reliability (0.9) x (0.9) = 0.81 reliability Parallel: (A + B) - (A x B) = Total Reliability (0.9 + 0.9) - (0.9 x 0.9) = (1.80 - 0.81) = 0.99 reliability Both need to work Only one needs to work

Tie-In to Rover project

• All subsystem groups have selected

components based on reliability (OTS, heritage, etc.)

○ Fault avoidance

• When practical, redundancy has been added

to all key mission components

○ We have created designs, made trade-studies, put together models, and intend to lab test components

• We strive to make the rover 'smart' so that it

can enter safe-mode or question the commands from ground when threatened

○ Increases reliability by decreasing the chance for critical mistakes

Source: Exploration Systems Engineering Version 1.0, Reliability Module

Summary

• Reliability is a key attribute of space systems, influencing systems

engineering activities such as design, trade studies, modeling, and test.

• The reliability function, R(t), is determined from the probability that a

system will be successful for at least some specified time.

• The Bathtub curve expresses the failure rate as it depends on the age
• f the system. Early and late in life of the system (similar to the human

body) significantly higher failure rates occur called “infant mortality” and “old age” regions. Between these regions normally lies an extended period of approximately constant failure rate. The reliability of systems

• perating in this region can be simply characterized by an exponential

function.

• Ways to achieve reliability include fault tolerance, functional

redundancy and fault avoidance.

• Block diagrams and event trees are useful tools in calculating
• reliability. An understanding of probability basics is required.

Source: Exploration Systems Engineering Version 1.0, Reliability Module