International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 3 - November 2015
Reliability Assessment of a spacecraft mechanism using
Axiomatic Design
Jose Thomas#1, Tina Raju2, SujithKumar.N3, Y. S. Shankar Narayan4
1
2
PG Scholar, Associate Professor, Department of Mechanical Engineering,
MA College of Engineering, Kothamangalam, Kerala, India
3,4
Scientist/Engineer, Reliability Assurance Mechanical Division, Systems Reliability Group,
ISRO Satellite Centre, Bangalore, Karnataka, India
Abstract — Reliability of a system is the ability of a
system to perform its required functions under stated
conditions for a specified period of time. This paper
discusses the reliability assessment of payload door
mechanism using axiomatic design. Evaluation of the
payload door mechanism was done by using axiomatic
design and the critical components were identified.
The reliability of these components is assessed by
using stress strength interference method.
Keywords — Design evaluation, Axiomatic design,
Design reliability, Stress-Strength interference model.
I. INTRODUCTION
System reliability is the ability of a system or
component to perform its required functions under
stated conditions for a specified period of time. It is
associated with unexpected failures of products or
services and understanding why these failures occur is
key to improving reliability. Reliability is a critical
design attribute for space systems and an important
metric in spacecraft design and optimization. It is the
aggregate result of the reliability of spacecraft subsystems, and as such, analysing the reliability of these
subsystems and their relative contribution to
spacecraft failures is important for reliability growth
plans and targeted subsystem improvements. This
paper deals with the reliability assessment of pay load
door mechanism.
Figure 1.Pay load door mechanism
Payload is the part of a vehicle’s load, especially an
aircraft’s, from which revenue is derived; cameras,
cargo etc. payloads are placed inside a stationary
frame and the stationary frame is covered by using pay
load door. Payload door is used as a cover for the
detector during ground testing and during launch.
ISSN: 2231-5381
Deployment mechanism has to open door on orbit and
will be retained with spacecraft. Payload will be fixed
with spacecraft. Payload door will be with hinges at
one end and hold down mechanism with the other end.
Mechanism requirement arises since the door, in
closed position, protects the detectors from radiation
damage during transit to moon and then needs to be
opened upon reaching the lunar orbit to enables the
detectors to view the lunar surface.
Before opening the door, it is planned to calibrate the
detectors using calibrating sources (FE-55). These
calibrating sources are mounted on inner surface of
the door. The door opening mechanism is a single shot
operation and the door is deployed and latched
permanently. Pay load door is under development and
reliability should be assessed functionally for meeting
requirements. However measuring reliability does not
make a product reliable, only by designing in
reliability can a product achieve its reliability targets.
The objective of design for reliability is to design a
given product that meets its requirements under the
specified environmental conditions. The process of
ensuring that the final product conforms to user is
called design validation. Design validation is done to
review the effectiveness and efficiency of a particular
design to meet its functional requirements. Validation
of payload door design can be done by ensuring that
each customer requirement is converted to functional
requirements to design parameters. These design
parameters are met by components realized by capable
processes. Validation of payload door design is done
by confirming that the design parameters are traced to
customer requirements and none of the customer
requirements/functional requirements is left out in the
design. Design validation helps to finds out the critical
factors which can be used to ensure reliability of the
system.
Assessment of the reliability of the payload cover
mechanism has to be carried out to ensure functioning
of the payload since it is a single point failure. So
identifying all functional requirements and design
parameters and addressing the failure modes which
prevents meeting the above requirements are essential
in the product assurance. As part of product assurance
of the payload cover mechanism, validation of the
design is carried out using axiomatic design and
design reliability has to be estimated.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 3 - November 2015
II. DESIGN VALIDATION AND RELIABILITY
In this study, two works are carried out. The first
work is the design validation of pay load door
mechanism using axiomatic design. After evaluating
the design, the critical components should be
identified.
The second work is to calculate the reliability of the
critical components. In order to avoid failure the
critical components should have high reliability.
A. Design validation using Axiomatic Design
Axiomatic Design methodology is used to find the
critical components which causes total failure of the
system Axiomatic Design is a design methodology
using matrix methods to systematically analyse the
transformation of customer needs into functional
requirements, design parameters and process variables.
It was created and popularized by Professor Suh of the
Massachusetts Institute of Technology (Suh 1990,
2000). First the objective of the problem is defined
and then how the axiomatic tool can be applied to find
the critical components. The function of the AD tool
starts from customer requirements and mapping it
through FRs and DPs. This process helps in the
creation of a design matrix for visualizing the
interaction and to find out the critical components in
which failures may occur within the design.
The result of axiomatic design is obtained as coupling
and how each FRs and DPs are physically integrated
to sub-assemblies and components. The listed
couplings helps to find out the effect of change in DPs
and FRs. Till this step the first axiom in axiomatic
design is followed. The second axiom is not followed
because there is only one design to validate. The
information axiom is useful when there is more than
one design that satisfies the independence axiom
equally and the best design is the one with the least
information. In order to built quality into the design,
customer requirements acts as a bench mark for
developing any new product or updating an existing
design.
First step of the Axiomatic design methodology is to
define the objectives. The objectives of the problem
were defined and then how the Axiomatic design tool
can be applied to find out the critical components is
explained.
Customer Requirements
The customer requirements hold the information
obtained from the customer. The customer
requirements are
To protect the detectors during launch and
transit to lunar orbit
Uncover the detector to map lunar surface in
the lunar orbit.
Functional Requirements
From the above mentioned customer requirements the
information is transferred in to a minimum set of
ISSN: 2231-5381
Functional requirements. The functional requirements
are
Protect the swept charge devices from
electron radiation (80MeV) during transit
orbit.
Door should be in closed position during
launch and transit to lunar orbit.
Release the cover after reaching the orbit
Locking the cover after deployment.
Status monitoring
Door should house the calibrating sources.
Design Parameter
Design parameters are the parameters which satisfies
the functional requirements. The design parameters in
the physical domain are listed below.
Cover
Hold down mechanism
Hinge mechanism
Latching mechanism
Switches
Mounting interface
After the domains are defined appropriately the next
step in axiomatic design is to perform a functionally
based decomposition of the obtained top level
functional requirements. This type of decomposition is
called Zigzagging decomposition. Decomposition
helps to convert the elements of the design into
hierarchy until a complete detailed design is obtained
or till the design is complete.
Figure 2.Decomposition of first level functional requirements
into sub levels
Figure 3.Decomposition of second level functional
requirements into sub levels
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 3 - November 2015
requirements are integrated into components of the
assembly. Designer should integrate functional
requirements into minimum number of components
keeping as much independence as possible (with
minimum coupling). There are mainly two sub
assemblies in payload door mechanism.
1. Deployment hinge and Latch mechanism
The functional requirements related to deployment
and latch mechanism are latching slot for the cam, to
latch the door, energy for deployment, storage of
energy, angle for deployment, angle of locking and
angle of deployment. These functional requirements
are physically integrated to deployment hinge and
Figure 4.Decomposition of third level functional requirements into
sub levels
latch mechanism.
Coupling and physical integration
2. Hold down and release mechanism
Coupling occurs in AD when a functional requirement The functional requirements, included protect SCDs
cannot be easily controlled by changing its from electron radiation, door should be in closed
corresponding design parameters. The independence position during launch and transit, release the cover
axiom helps in pointing out any coupling in the design. after reaching the orbit, house the calibrating sources,
Unwanted
coupling
results
in
unintended ability to take launch load, provide load path, to
consequences and makes a design difficult to control measure the hold down load, to cut the Kevlar rope,
or adjust. When two things are coupled it means that load carrying capacity of the rope and provide pushing
they cannot be adjusted or changed independently. force against rope are physically integrated to hold
The coupling obtained from the design matrix is listed down and release mechanism. The functional
below.
requirement cover stiffness is controlled by snubber
DP: Fork end bracket
location, number and location of hinges & hold down
FRL2: provide load path
and snubber force.
FRL2: Angle for deployment
So the components related to the functional
The change in parameters of the fork end bracket will requirements of these two sub assemblies are critical.
affect the load path provided and deployment angle. Any type of failure to these components leads to the
So any changes in the parameters of the fork end total failure of the system. The critical components
bracket have to accommodate the effect in functional identified are torsion spring, hinge shaft, shaft pin,
requirements.
compression spring, latch spring strip, splice holding
DP: Bearing
bracket, washer, fork terminal, heater and load bracket.
FRL2: provide load path
B. Reliability Assessment
FRL2: storage of energy
Reliability is the probability that an item will perform
FRL2: Angle for deployment
The change in parameters of the bearing will affect the its specified mission satisfactorily for the stated time
energy stored, load path provided and deployment when used according to the specified conditions.
angle. So any changes in the parameters of the torsion Reliability must be designed into a product or service.
spring have to accommodate the effect in functional Most important aspect of reliability is to identify cause
of failure and eliminate in design if possible otherwise
requirements.
identify ways of accommodation. Reliability of the
DP: Eye end bracket
system is the product of reliabilities of its
FRL2: provide load path
subassemblies. Here Stress–strength interference
FRL2: storage of energy
method is used to find the reliability of the syste2m.
FRL2: Angle for deployment
The change in parameters of the eye end bracket will By using the Stress Strength interference method the
affect the energy stored, load path provided and reliabilities of each critical component can be
deployment angle. So any changes in the parameters calculated. It is found to be useful in situations where
of the torsion spring have to accommodate the effect the reliability of a component or system is defined by
the probability that a random variable ―S‖
in functional requirements.
(representing strength) is greater than another random
DP: Torsion spring
variable ―s‖ (representing stress). While it makes
FRL2: storage of energy
intuitive sense that a component is deemed to have
FRL2: Angle for deployment
The change in parameters of the torsion spring will failed when its strength is lower than the applied stress,
affect the energy stored and deployment angle. So any this model is not entirely restricted to stress and
changes in the parameters of the torsion spring have to strength. It can be applied to any situation or problem
where the random variable ―S‖ represents any
accommodate the effect in functional requirements.
performance related characteristic of the system under
Physical integration is the final step of the design
question and ―s‖ serves as a criterion that determines
process. This is the step where the functional
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 3 - November 2015
failure. Any state where ―S‖ falls below ―s‖ represents
the component to be in a state where it is deemed
unacceptable or to have failed. Once the distribution
and parameters of S and s are determined, the
reliability can be calculated by estimating the
probability S>s, which is computed as shown.
R = 1/
Where,
t=
= mean value of strength
= standard deviation
of strength
= mean value of stress
= standard deviation
of stress
Stress should be different for different sections. So by
applying proper cross section equations stress can be
calculated. Strength of a material is always constant. If
the material used for the manufacturing of component
knows the strength of the material can be easily
calculated from the design data book. Standard
deviation of stress is calculated from the test results
conducted by ISRO and the standard deviation of
strength from design data book. Thus the reliability of
each component can be easily calculated. Total
reliability of the system is the product of reliabilities
of its critical components. This is how the total
reliability of the system is calculated.
Since the system is series, total reliability of the
system is the product of all the reliabilities of its
critical components.
Total reliability = reliability of torsion spring x
reliability of Hinge shaft x reliability of shaft pin x
reliability of Fork terminal x reliability of Load
bracket x reliability of splice holding bracket x
reliability of Latch spring strip x reliability of
compression spring x reliability of Flat washer x
reliability of Heater.
Reliability of the system = 1 x1 x 1 x 1 x 0.999996 x
1 x 0.9999987 x 0.999997 x 1 x 0.999999 x
0.9999963
= 0.999998
Reliability of
torsion
spring
1
Reliability of
hinge shaft
1
Reliability of
compression
spring
0.999997
Reliability of
latch spring
strip
0.9999987
Reliability of
shaft pin
1
Reliability of
splice holding
bracket
1
Reliability of
fork terminal
1
Reliability of
load bracket
0.999996
1
Reliability of
Flat washer
1
Reliability of
torsion
spring
0.999999
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Reliability of
torsion
spring
0.9999963
Reliability
of System
0.999998
III. RESULTS AND DISCUSSION
The reliability of pay load door mechanism was
assessed using axiomatic design. Critical components
of pay load door mechanism were identified and the
reliability of these components is calculated using
stress strength interference theory. According to
axiomatic design, the validation started from the needs
of the customer. The obtained customer requirements
are mapped to functional requirements and then in to
design
parameters.
Parent
level
functional
requirements were decomposed into its lower level
functional requirements. Since the payload door
design was large, decomposition was done with the
help of free mind software which helped to visualize
the decomposition process more clearly. The
evaluation of the design was done with the help of
independence axiom. The final design matrix showed
four couplings. The effect of change in the coupled
design parameters on other functional requirements
has been found out design validation was performed.
The final step of axiomatic design is the physical
integration. Through this physical integration, the
critical components are easily identified. The
reliability of payload door mechanism was found to be
0.999998. Here the calculated reliability is almost
equal to one which reveals that the payload door
mechanism is high reliable.
IV. CONCLUSION AND FUTURE WORK
Axiomatic Design provides a systematic way of
evaluating the design of payload door mechanism. The
customer requirements are traced to physical
integration through functional requirements and
design parameters. The process ensured that each
functional requirement is fulfilled by its corresponding
design parameters and no requirement is left out
unsatisfied. The results obtained from the analysis of
payload door mechanism are used to check for
incorporation of functional requirements into subassemblies
and
components.
Checking
for
incorporations, were able to establish how physical
integration is done and physical integration shows
how components are put together to fulfil the
customer needs. Design evaluation of payload door
mechanism was done by using first axiom
(independence axiom) in axiomatic design. The use of
first axiom helps in listing all the coupling in the
design. The information for listing the number of
coupling present in the design is obtained from the
design matrix. The obtained results will help in
making design decisions in future modifications and
scaling of design.
In the present work, the use of process domain has not
been employed. It is a reformulation of the design
parameters in terms of the processes that can generate
the physical realities in the previous domain. It
describes how the elements of the physical domain
will be created, acquired, or manufactured. Tests &
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 3 - November 2015
evaluation and quality assessment strategies can then
be devised for each and every FR-DP relationship.
Introduction of new design ideas for payload door can
be compared with the old design by using the second
axiom i.e. information axiom, in axiomatic design.
The present work deals with only the first axiom
(independence axiom) for the evaluation of pay load
mechanism. The second axiom will help to identify
the strength and weakness of the design when
decomposed functionally. It can be used in the process
domain for establishing the best process available to
fulfil the design parameters. Defining the process
domain and the application of second axiom into the
process domain can be done as future work.
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