MECH 567 – Systems Architecture
“Modeling Systems Architecture Structures in SysML”
Stephen Milton and Sandra Biedron
Stephen.Milton@colostate.edu , Sandra.Biedron@colostate.edu
(970) 491-8742
Introduction
Modeling is defined as a simplification of reality that has utility.
Models can come in a variety of forms of simplification including mathematical
models (Equations), physical models (mockups), visual models (design drawings/plans),
and text models (specifications, descriptions). Models can also have a variety of utilities
including organization, visualization, documentation, hypothesis testing, etc.
For all of these reasons, we want to be able to model systems architectures. The
most common modeling language used in software systems engineering has been UML
(unified modeling language). SysML (system modeling language) provides a set of
extensions to UML that augments the capabilities of UML in dealing with larger-scale and
more generic systems.
For the purposes of this course, SysML will serve as an exemplar of a systems
architecture modeling language. It will be the primary way that we document and
communicate systems architectures in the course.
Semester-long Laboratory Learning Objectives
1. Exemplify SysML modeling concepts
2. Gain experience with all of the diagrams required to interpret and design systems
architectures using SysML
3. Model physical systems in SysML
4. Model processes/enterprises in SysML
5. Model requirements in SysML
The authoritative reference for using SysML is entitled, “OMG Systems Modeling
Language
(OMG
SysML™).”
It
is
available
for
no
cost
at
http://www.omg.org/spec/SysML/1.3/. You are encouraged to use the language definition
document as the ultimate authority on SysML for this course, although these laboratories,
and the reference text should provide you some value in distilling and exemplifying the use
of SysML. In the laboratory section, we will try to learn how to use SysML by meeting the
semester-long, learning objectives above.
SysML has three kinds of modeling abstractions: structural modeling, requirements
modeling and behavioral modeling. These are show diagrammatically as a SysML Block
Definition Diagram in Figure 1.
1
Figure 1. SysML v1.3 Diagram Taxonomy
SysML has 4 types of structural models. Each will be useful for different parts of the
architecture definition process.
1.
2.
3.
4.
Block Definition Diagram
Package Diagram
Internal Block Definition Diagram
Parametric Diagram
Task 1 – Structural Modeling (in General)
Structural modeling of complex systems is perhaps the most fundamental of all
organizational systems. It is found in many aspect of human thought from biological
classification, to philosophy, to (of course) descriptions of complex technical systems.
Structural modeling attempts to represent a system through its entities and relationships.
There are various means that people have devised to document structural models,
but the essence of each one is this concept of entities and relationships. A rudimentary
example of this form of modeling is a structure chart.
Figure 2. IEEE Structure Chart
In general, the weaknesses of these types of structure charts are that they can only
express a single type of relationship: a hierarchical relationship. Hierarchies are a classical
means to express the relationship among physical entities. Linnaean taxonomy, “modern”
corporate structure, and feudalism are all derived from (and describable by) hierarchies.
2
For more complex systems, we will require a more complex model of the structure of a
system.
Task 2 – Structural Modeling in SysML
We will learn to perform SysML structural modeling by starting out today with the
Block Definition Diagram (BDD).
There are two key elements in the BDD: Blocks (which represent things) and
Associations (which relate blocks). In general SysML blocks should represent nouns,
whereas the associations between them can be verbs.
First, we can seek to understand SysML blocks by modeling a system component
using a SysML block. Let us start this project the same way that you should start any of
your systems architecting projects: with “pencil and paper.” Let’s think about modeling the
structure of the system that allows us to drive to work in a car. To restrict ourselves to
thinking at an architectural level, please aim for 5-7 blocks.
Now of course, we need to connect these blocks to define the relationships between
them. There are a few key relationships that have symbols allocated to them in SysML.
Association is the simplest relationship and is represented by a simple line.
Associations are usually described using verbs and the relationship then has a natural
sentence like syntax.
3
If a system is made up of an aggregation of subsystems, then they are connected
using the symbol “◊“ and can be though of “is made up of”. For instance, the traffic signal is
part of the traffic management system, then we can connect these two blocks with this
aggregation relationship.
If a system is composed of subsystems such that deleting the part deletes the whole,
then they are connected using the symbol “ “. For instance, the pole of the traffic signal is
part of the traffic signal, and the traffic signal is without value once the pole is deleted, it “is
composed of”. So in this case, we can connect these two blocks with this composition
relationship.
If we would like to express a specialization/generalization relationship, then the
blocks are connected using the symbol “∆ “, “is a type of” in one direction and “has types” in
the other direction. For instance, traveler-controlled systems might include the car, the
garage door, the parking meter. We could also generalize some of our other blocks into
operator-controlled, and uncontrolled systems.
4
A wide variety of other relationships are available in SysML. You can define any
relevant relationship between blocks by labeling the connector. For instance, describe the
relationship between the car and the road.
Task 3 – Starting up UML and SysML Modeling Programs
To add a bit more formality to our models of systems, we are going to use some software
that will allow us to “draw” more accurate and clear representations of our model. Althouhg there
are a number of choices to use out there for such modeling, here we will provide two choices, Visio
and LucidGraph.
In the past we were able to use Visio 2007 on the ENS VirtualLab server.
Unfortunately, Visio 2007 is out-of-date and not supported. To compound matters Visio
2013, the most recent version, is not supported on the Virtual Lab server. As a CSU student
you can get a free copy of Visio 2013 from MicroSoft DreamSpark
https://www.engr.colostate.edu/ens/info/software/dreamspark.html. You will need to
request from ENS a username and password and then you will be able to download the
programs. Also needed are SysML Visio stencils. Copy this set of files1 to a directory. These
files are SysML stencils for Visio, but we will use both SysML and UML to perform systems
architecture modeling. Install and start Visio 2013, and begin a new file by clicking on
File>New>Personal>SysML 1.0, as shown in Figure 2. If you would like to work using the
UML stencils, then you need just double click on the stencil that you would like to use
(“Class” for today’s exercises).
1
http://www.omgsysml.org/
5
Figure 3. Starting the SysML template in Visio
As an alternative to Visio, we have obtained 40 licenses to use LucidChart
www.lucidchart.com. This is a web-based tool that similar capabilities to Visio. To use it
you will need to be added to the “team”. I can add you, but first you will need to send me an
email from your preferred email account. I will add you to the team and send you back
information on how to access the software. LucidChart is new to us, but it might offer us as
a class some interesting additional possibilities as it allows easy exchange of generated
charts. This might be very useful for the distance people in the class as they might be able
to work together in groups. It supports UML and I have imported into it the same Visio
stencils as used in Visio above. As a class we will give it a “test drive” and see how well it
works for us. In the end it will really boil down to a matter of preference.
Task 4 – Structural Modeling in SysML and Visio or LucidGraph
Now that we have a software to develop SysML and UML models, we can start to add
more complexity to our systems models. For example, let’s add more information to the
model of a cat.
In this case, the cat has some behavior that is represented by the function Bite(). We
can link the SysML block definition diagram to a behavioral diagram with the objective of
describing the behavior of the block in more detail.
Cat
Block Attributes and
data types
Block Title
Awake : bool
Bite(you) : bool
6
Operations (operands) :
data type of result
This syntax provides us with the ability to associate behavior with our structural
entities. This will be useful when we start to perform behavioral system modeling.
Task 4 – Simple Structural Modeling in SysML and Visio or LucidGraph
For an example of a simple structural model in SysML, let us interpret the diagram
below together.
Task 5 – Homework
Construct (by hand) 2 block definition diagrams using the SysML BDD components
that we have discussed today for any engineering systems of your choice. One diagram
must represent a simple system and one must represent a complicated system as measured
by the sum of the number of blocks and relationships. Simple systems have 7±2 blocks and
relationships, medium systems have (7±2 )2 blocks and relationships.
I will evaluate your diagrams on their correct usage of all of the aspects of the BDD
that we defined today. Please feel free to include some narrative description, especially
since I may not be an expert on your system.
You may type directly into this document, please replace my name with yours in the
header. Due Jan 27th, 2014, 11:59pm, MST. Turn in by email to milton@engr.colostate.edu
using the subject line “567 HW1 – YourLastName” in pdf format.
7