Systems engineering in the product lifecycle

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Int. J. Product Development, Vol. 2, Nos. 1/2, 2005

Systems engineering in the product lifecycle
Conrad Bock
US National Institute of Standards and Technology,
Manufacturing Engineering Laboratory,
100 Bureau Drive, Stop 8263,
Gaithersburg, MD 20899 8263, USA
Fax: 301 975 8273
E-mail: [email protected]
Abstract: This paper introduces basic elements of systems engineering that are
useful in managing the product lifecycle, as expressed in an extension to the
Unified Modeling Language. It presents models of product requirements for
capturing stakeholder needs, system structure for defining the static relations of
its elements, behaviour for the transformation of inputs to outputs, parametrics
for constraining properties of structure, and allocation for assigning behaviour
to structure. The relation of behaviour to structure is identified as a central issue
in the integration of systems and software engineering.
Keywords: product lifecycle; systems engineering; UML; SysML; product
Reference to this paper should be made as follows: Bock, C. (2005) ‘Systems
engineering in the product lifecycle’, Int. J. Product Development, Vol. 2,
Nos. 1/2, pp.123–137.
Biographical notes: Conrad Bock is a Computer Scientist at the US National
Institute of Standards and Technology specialising in product modelling and
UML. He leads efforts on UML process modelling at the OMG, and is
contributing to the submission on OMG’s UML for Systems Engineering.
Bock studied at Stanford, receiving a BS in Physics and a MS in Computer
Science. His previous experience includes expert systems in vehicle design and
nuclear power.

Systems engineering (SE) overlaps a significant portion of product lifecycle management
(PLM). It has existed as a discipline for several decades and been applied successfully to
a wide range of complex products (INCOSE, 2000). This paper introduces SE and recent
work on defining a standard SE modelling language. It also addresses a central aspect of
integrating software development into general engineering practice.
1 Introduction
Both PLM and SE are concerned with managing the multiple views and interrelationships
of product information to maintain coherence across time and place, and to apply to more
than one product. The International Council on Systems Engineering (INCOSE)
describes SE as:

Copyright © 2005 Inderscience Enterprises Ltd.

C. Bock

“…defining customer needs and required functionality early in the
development cycle, documenting requirements, then proceeding with design
synthesis and system validation while considering the complete problem:
operations, performance, test, manufacturing, cost and schedule, training and
support and disposal.” (INCOSE, 2004)
The areas addressed by SE are so diverse that it must be concerned with communication
between people working in them:
“Systems engineering integrates all the disciplines and specialty groups into a
team effort forming a structured development process that proceeds from
concept to production to operation.” (INCOSE, 2004)
SE is currently hampered by a lack of a standard language for coordination across the
product lifecycle and across disciplines involved in product development. Organisations
using multiple languages have less effective communication, increased project cost, and
decreased product quality. Many of the specialties that systems engineering interacts with
have adopted standard languages, most recently software engineering.
To address these issues, INCOSE joined with a major software consortium, the
Object Management Group (OMG), to create a Standard Modelling Language for
Systems Engineering. OMG and INCOSE began by forming the Systems Engineering
Domain Special Interest Group (SE-DSIG) (Friedenthal and Kobryn, 2004;

OMG 2004a). The SE-DSIG developed a request for proposal for an SE modelling
language, issued in March 2003 (OMG, 2003a). The requirements were developed from
an OMG request for information (OMG, 2002) and with INCOSE and the international
organisation for standardisation’s ISO 10303, informally known as the standard for the
exchange of product model data (STEP). In particular, ISO’s 10303-233 application
protocol for systems engineering team (AP-233) participated to align the OMG
requirements with the evolving AP-233 neutral data interchange standard for systems
engineering (ISO, 2004a). This is important in bridging to engineering analysis
disciplines represented in other ISO standards.
The OMG request covers a substantial part of SE:
• requirements
• structure
• behaviour
• parametrics (constraints)
• verification (testing)
• deployment.
The request also identifies OMG’s Unified Modeling Language (UML) (OMG, 2004c) as
a basis for SE modelling, because it combines critical elements needed for SE:
• graphical presentation for communication between a wide variety of disciplines
• extensibility mechanisms for adapting to new domains
• wide range of capabilities, from requirements to deployment
• model repository supporting notations for multiple disciplines, and translatable to
multiple specialised formats.

Systems engineering in the product lifecycle

The last feature addresses the essential requirement in PLM and SE for a common
repository capturing consistent information accessible across the lifecycle. It also
provides information beyond geometric and mathematical representations commonly
used in specifications of product data (Sriram, 1997). The variety of product data requires
flexible representations available in languages such as UML, or ontologies such as
Ontology Web Language (OWL), and Process Specification Language (PSL) (Staab and
Studer, 2004). These languages support identification of terms, relations, and processes in
a form that is amenable to automated reasoning. In the setting of OMG’s model-driven
architecture (MDA), they can be translated to optimised formats for other applications as
necessary, such as automated manufacturing (OMG, 2004b; Bock, 2003c).
This paper describes a response being prepared to the OMG request called the
Systems Modeling Language (SysML) (SysML Partners, 2004). The first version will
cover requirements, structure, behaviour, parametrics, and the relation of structure to
behaviour (allocation). Each of these is introduced in the sections below. Finally, the
relation of structure and behaviour is examined as a central issue in the integration of
systems and software engineering.
2 Requirements
Requirements modelling includes the translation of textually expressed needs into a
computable form, capturing the evolution of requirements and their relation to the system
design. There are at least three important aspects to this process:
1 Translation of text to model. Requirements usually appear first as a large text
document, structured in some way by headings. The requirements model is a
‘parsed’ form of the document. It has an element for each requirement, containing
the portion of text corresponding to that requirement, with a reference to its location
in the source document. The model contains a single element per requirement, even
when the source text conjoins them.
2 Derivation. Requirements can be more or less specific about the requested system.
For example, a source requirement for transportation might be about safety
generally. For automobiles, braking distance, traction requirements, and so on, are
derived from safety. These further derive requirements on wheel rotation forces and
speeds. SysML calls this a requirements trace. Derivation can include detailed
models, as in UML structure and behaviour. Each stage of derivation will involve
some assumptions about the design of the system (Cantor, 2003). For example, the
derivation of the safety requirement above assumed the transportation mode was an
automobile of the conventional kind.
3 Link to system design. System designs that fulfil requirements are said to satisfy the
requirement. SysML anticipates that system designs will be modelled in UML, but
the satisfaction relationship does not restrict how the design is represented.
SysML adds a requirements model to UML, because UML does not have one.
Figure 1 shows the transportation safety example, with derivation traces based on
increasingly elaborated designs (‘requirements flowdown’). The guillemot notation («»)
in UML is used to indicate what kind of language element a particular rectangle or arrow
represents. For example, the rectangle at the top right labelled «document»

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TRANSPORTATION SAFETY is an original source requirements document for the system
being designed, and «requirement» TRANSPORTATION SAFETY REQUIREMENT 1 is a
part of the model representing one of the requirements in the text document. Dashed
arrows in UML show dependencies between pieces of the specification, where the
element at the arrowhead end is independent and the other end is dependent on it.
For example, the dashed arrow labelled «trace» indicates that the requirement is ‘parsed’
from the original source document, and depends on it.
Figure 1 Requirements

The rectangles marked «design» in Figure 1 represent classes of physical objects, where
the design is specified in the class, and describes the structure of the physical objects that
are members of the class. Designs are constructed to satisfy requirements, as indicated by
the «satisfy» dependencies. Some of the designs are connected by generalisation arrows,
notated with a hollow arrowhead. The design at the arrowhead end is a general case of
the design at the other end, which inherits characteristics from the general design.
For example, the characteristics of physical objects conforming to the SMALL SCALE
VEHICLE design also apply to objects conforming to DRY LAND VEHICLE.
In particular, small-scale vehicles are designed to satisfy TRANSPORTATION SAFETY
REQUIREMENT 1, as shown by the dashed arrow labelled «satisfy», so dry land vehicles
will inherit the characteristics satisfying that requirement also.
Each specialised design introduces new characteristics that satisfy requirements
derived from the general design requirements, as shown by the «trace» dependency.
For example, the additional characteristics introduced by DRY LAND VEHICLE
satisfy the TRACTION requirement derived from TRANSPORTATION SAFETY
REQUIREMENT 1. This pattern also applies to parts of designs, as shown at the bottom

Systems engineering in the product lifecycle

right of the figure, using UML’s black diamond line notation. These lines represent the
assembly breakdown of physical objects in the classes being associated. For example,
differentials are parts of cars, and satisfy the corresponding derived requirement
WHEEL ROTATION ON CURVES, even though they are not special cases of cars.
The designs in this example are structural, but some methodologies use functional
design (see Sections 4 and 6). SysML also supports capturing the reason for any
particular derivation or design choice, called the rational. These can include trade-off
analyses. Requirements can specify testing procedures, to ensure the design actually
satisfies the requirement. The trace and satisfaction relationships can be grouped, for
example to identify alternative design choices. Requirements can also be expressed in
tabular formats to improve scaling, which is necessary for typical systems. This technique
is based on the repository for UML and its extensions, which records models in a way
that is independent whether they are presented diagrammatically or textually

(Bock, 2003c).
3 Structure
SysML reuses the UML 2 composite structure model, which SysML calls assemblies.
The UML 2 model adds significant power to the earlier versions of UML, in particular, it
supports the reuse of system elements in multiple assemblies, or in multiple ways in the
same assembly (Bock, 2004b). Figure 2 shows an example composite structure for a
simplified automatic braking system (ABS). The outer rectangle represents the class of
cars, members of which are individual physical cars. It carries the design that each car
conforms to. The labelled rectangles inside the car are its parts, for ABS, wheel assembly,
and so on. Some of the parts are subassemblies, such as the one for wheels. These have
their own components and interconnections inside. The small rectangles on the borders
are ports, which provide access points for connections outside the part. For example, the
ABS has ports for mechanical and hydraulic connections. The lines segments between
ports are connectors, which show how the parts are connected together electronically,
hydraulically, and so on.
Each component of the car is a usage of a generic, reusable subcomponent. For
example, the wheel subassembly might be used four times, though the figure shows only
one for simplicity. Each usage of a wheel subassembly will be connected in different
ways, for example to different ends of the same axle, or to different axles entirely.
To accommodate multiple usages of the same kind of subassembly, the usage of each
component is given a name, which appears to the left of the colon at the top of each part.
The type of part being reused appears to the right of the colon. For example, the generic
WHEELASSEMBLY is used under the name WA. When other wheels are added to the
design they will have different usage names, even though they are all usages of
The UML 2 composition model conforms to the common intuitions of assembly.
For example, the ABS in one car is not connected to the wheels in another, and the ABS
connects only the wheels indicated in the design, which might not be all the wheels
in the car, and some cars might have wheels without ABS systems. The UML class
diagram does not support these intuitions, because class diagrams define artefacts
generically, independently of how they are used. For example, a class diagram showing
an association between ABS systems and wheels would either need to define the linkage

C. Bock

as optional, which would be incorrect for cars that have ABS, or make the
linkage required, which would be incorrect for cars that do not have ABS. The class
diagram must be used in conjunction with the composite structure diagram to show how
generic, reusable components are applied in particular designs (Bock, 2004b;

Baysal et al., 2004).1
Figure 2 Composite

4 Behaviour
One of the purposes of behaviour models is to coordinate or place constraints on other
behaviours.2 For example, a procedure for shaping a piece of metal might have a series of
steps that must happen in a certain order under certain conditions.

UML provides three behaviour models that are reused in SysML. Each kind emphasises a
different aspect of system dynamics, making one or the other more suitable for a
particular application, or stage of application development:
activities emphasise inputs and outputs, conditions, and sequence for invoking
other behaviours
state machines show how events cause changes of object state and invoke other
interactions describe message passing between objects that cause invocation of
other behaviours.
Activities are of particular interest to SE because they focus on what tasks need to be
done, in what order, with what inputs, rather than which entity performs each task
(Bock, 1999, 2003b). More specifically, activities are designed to be used with or without
objects (see Section 6), and highlight the dependency of inputs on outputs. This emphasis

Systems engineering in the product lifecycle

corresponds to SE functional flow, in particular Enhanced Functional Flow Block
Diagrams (EFFBD) (Bock, 2003a). Functional flow is closest to requirements expressed
as transformations of inputs to outputs (also called functions).
Activities follow the style of UML 2 composition models in supporting multiple uses
of the same behaviour. For example, system requirements might dictate a subfunction for
heating water that takes water as input and provides it as output at a higher temperature.
This generic, reusable function might be applied in many ways, and in each particular
application it might provide hot water to different downstream functions. For example,
one usage in a ship might provide hot water to a room heating function, another to a
cooking function, and so on.
Figure 3 is an example activity diagram, with some SysML extensions, showing the
dependencies between some subfunctions of an automobile, adapted from

SysML Partners (2004). Round cornered rectangles in activity diagrams represent the
usages of functions, which are called invocations. For example, the one on the upper left
is an invocation of a generic, reusable function that turns the key to the on position.
Arrows, vertical bars, and dots in activity diagrams determine when the function
invocations occur. For example, the dot on the upper left is an initial node, representing
the starting point of the activity. The arrow coming out of it is a control flow, indicating
the first step in the activity, TURN KEY TO ON.
Figure 3 Activity example 1

Vertical and horizontal bars in Figure 3 are forks showing initiation of concurrent flows
of function invocations. For example, after the key is turned on, driving and braking
functions start concurrently, because the control flow coming out of TURN KEY TO ON
is split into two concurrent control flows, to DRIVING and BRAKING. The SysML
keyword «runToDisable» on a function invocation indicates that once the function starts,
it runs until it is turned off (turning the key to off is not shown).

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The arrow coming out of DRIVING is an object flow, also called a data flow,
or item flow. It represents the flow of information, material, or energy between functions.
Item flow is distinguished from control flow by a rectangle indicating what type of thing
is flowing. In this example, brake pressure information is passing from driving to
braking. The information passed between functions is not necessarily implemented
electronically or in software. For example, sending brake pressure information can be
implemented in an analogue fashion, as with conventional hydraulics. The STREAM
properties in curly braces indicate that this information might be passed between the
functions while they are operating, rather than requiring DRIVING to complete before
generating an output, or BRAKING to wait for break pressure to arrive before starting.
The SysML RATE properties specify how fast the information flows. In this example,
break pressure is emitted continuously from the driving function.
The horizontal bar below BRAKE PRESSURE in Figure 3 indicates that the
information is sent concurrently to two function invocations, one for BRAKING and the
other for a special function that controls MONITORING TRACTION. This function is a
control operator, which means it outputs control information that can enable or disable
other functions (Pandikow and Torne, 2001). When the input break pressure is greater
than zero, it outputs an enabling control value to monitor traction, otherwise it emits a
disabling control value. The effect is that traction is only monitored when pressure is
applied to the brake. The SysML keyword «runToCompletion» indicates the control
operator starts when it receives an input, calculates its output, then stops until the next
input arrives. It is not intended to run indefinitely like driving and braking, though it
happens to run repeatedly in this example, because its inputs come in continuously
(also see example in Figure 4). While MONITORING TRACTION is running, it emits
modulation frequencies to BRAKING as necessary to maintain traction.
Figure 4 Activity example 2

Figure 4 shows the definition for the MONITORING TRACTION function used in
Figure 3. The chevron shapes on the lower left represent receipt of signals from outside
the process, which do not come through explicitly represented inputs. When enabled,
MONITORING TRACTION receives signals from the wheel and accelerometer, which
flow to a subfunction that calculates traction. This function is marked as
«runToCompletion», so it will wait for two inputs to arrive, calculate the traction index
output, stop, and wait for two more inputs. The acceleration input comes in faster than the
angular velocity, as shown by the RATE properties, causing acceleration data to queue up

Systems engineering in the product lifecycle

at CALCULATE TRACTION while it waits for an angular velocity to pair with. To prevent
a stale acceleration value from being paired with a new angular velocity, the acceleration
input is marked with the SysML IS OVERWRITE property, which causes new values of
acceleration to overwrite old ones in the queue to CALCULATE TRACTION.
The diamond shape in Figure 4 is a decision node that routes the output of
CALCULATE TRACTION according to guard conditions notated in square brackets on the
arrows coming out of the decision. One of the guards tests whether enough traction is lost
to justify outputting a modulation frequency for the brakes. If so, the traction index flows
as input to a function that calculates the frequency, the output of which passed out of the
function. If not, the ELSE guard directs the values to a flow construct that discards it,
notated as a circle with an X in it.
The UML repository can support tabular or matrix formats such as the dependency
structure matrix (DSM) (Sharman and Yassine, 2004). These provide a compact way to
show function dependencies, by omitting some control information, but are not restricted
to hierarchical decomposition. A DSM can be derived from activity models,
stored in a repository, analysed, and results presented in either a matrix notation or an
activity diagram.
Functions can satisfy requirements, as shown in the lower left of Figure 3.3 Under
some methodologies system function is determined separately from structural design, and
requirements are satisfied through function, which is then allocated to structure, rather
than satisfying requirements by structure directly, as in Figure 1 (see Section 6)
(USDoDSMC, 2001).
5 Parametrics
A parametric model describes constraints among properties of a system.

These are typically expressed as mathematical equations, for example ‘F = Ma’. UML
provides a constraint language (OMG, 2003b), but it does not currently support for
reusing equations, a critical requirement for SysML. For example, ‘F = Ma’ can be one of
a library of equations that are reused many times in the analysis of a system, with
variables bound to different properties in each case. SysML introduces a constraint model
supporting reusable equations, which is called parametric relations.
The SysML parametric model has two parts:4
• Parametric relations defined as a reusable combination of other relations.
For example, ‘F = Ma’ is a combination of equality and multiplication, both of which
are primitive parametric relations.
• Application of a parametric relation to particular object properties.
The lower part of Figure 5 shows a SysML parametric diagram relating stopping distance
to other parameters in an automobile, along with a partial structure model that it refers to
at the top. The rectangles with keyword «property» represent the properties being
constrained. The rounded rectangles represent parametric equations. The small rectangles
on the borders are the parameters being constrained, which are connected to each other
and to properties, forming a network of constraints. The equation ‘F

is used twice,5 connected to different relations and properties in each case, but defined
once in the repository that stores the diagram (Bock, 2003c). This means that any change

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to the equation is propagated to all uses in all diagrams (not that one would ever change
Newton’s laws).6
Parametrics are intentionally nondirectional, that is, they do not have inputs and
outputs as activities do. For example, the equation ‘F = Ma’ constrains three variables,
but it does not specify which are being calculated. This applies even when using
irreversible relations, as in ‘y = sin(x)’. The model leaves it up to constraint engines
implementing the equations to determine what to do in these cases. For example, in
Figure 5, the coefficient of friction of the tyre could be a dependent or independent
variable, depending on whether it is given as an input to the constraint engine.
Figure 5 SysML parametrics example

For compactness, parametrics can be notated as equations, and still be stored to the
SysML repository. Tools can use the repository to generate the equations from diagrams
such as Figure 5, or vice versa. The SysML repository does not dictate or suggest

Document Outline

  • Systems engineering in the product lifecycle
  • 1 Introduction
  • 2 Requirements
  • 3 Structure
  • 4 Behaviour
  • 5 Parametrics
  • 6 Allocation
  • 7 Integration of systems and software engineering
  • 8 Conclusion
  • Acknowledgements
  • References
  • Notes