Systems Engineering with SysML/UML

Tim Weilkiens.
Systems Engineering with SysML/UML.
Morgan Kaufmann. 2011.

UML, the Universal Modeling Language, was the first programming language designed to fulfill the requirement for “universality.” However, it is a software-specific language and does not support the needs of engineers designing from the broader systems-based perspective. Therefore, SysML was created. It has been steadily gaining popularity, and many companies, especially in the heavily regulated Defense, Automotive, Aerospace, Medical Device, and Telecom industries, are already using SysML or are planning to switch over to it in the near future. However, little information is currently available on the market regarding SysML. Its use is just on the crest of becoming a widespread phenomenon, and so thousands of software engineers are now beginning to look for training and resources. This book will serve as the one-stop, definitive guide that provides an introduction to SysML and instructions on how to implement it for all these new users.

Foreword by Richard M. Soley (former OMG Chairman and Chief Executive Officer)

According to the Boeing Commercial Aircraft Company, a Boeing 747-400 aircraft has a maximum gross take-off weight (including a typical 416 passengers, 171 cubic meters of freight in the cargo hold, and over 200.000 kg of fuel) of nearly 400.000 kg. Four behemoth engines push the bird at up to 88% of the speed of sound for unbelievable distances, up to 13.500 km, without refueling. The length of the aircraft alone (45 m) is longer than the Wright brothers’ entire first flight.

But these amazing statistics, after 30 years finally to be eclipsed by even larger passenger aircraft, are nothing compared to the complexity of the system of systems which makes up the Boeing 747-400. The aircraft’s electrical systems comprise some 274 km of wiring alone; high-strength aluminum and and titanium parts are designed to work both standing still and heated by rapidly passing air. Backup systems keep navigation and life-sustaining systems running if primary systems fail; and even the in-flight entertainment system is one of the more complex systems on earth. In fact, the Boeing 747-400 comprises some six million parts, about half of which are simply fasteners like screws and bolts. It’s no wonder that a Boeing spokesman once quipped, “We view a 777 as a collection of parts flying in close proximity.” As a frequent flyer myself, I constantly and fervently hope that that proximity goal is respected!

All of these facts reflect a fact that all engineers understand: complexity is difficult to manage, and most interesting systems are complex. Worse, the compounding of systems into systems of systems — for example, back to our airplane, the electrical, hydraulic, propulsion, lift surface, life support, navigation and other systems of aircrafts — tends to introduce new complexities in the form of unexpected overlaps of parts and unexpected behavior of previously well-behaved systems.

The net result of the rising complexity of systems is the crying need for ways to express component systems in ways that those designs can be easily shared between design teams. A shared language is a crucial component of the design methodology of any system or process, be it electrical, software, mechanical or chemical. And if that shared language is based on a pre-existing language, the team which must share design will be up to speed more quickly and more able to complete the design task — and the more-demanding long-term maintenance and integration task — better, faster and cheaper.

This line of thinking was the though process behind the Systems Modeling Language (SysML), an open standard developed and managed by the Object Management Group (OMG) and its hundreds of worldwide member companies. OMG’s SysML has quite a lot of positive aspects:


  • it is based on (an extension of a subset of) the OMG’s own Unified Modeling Language, or UML. Thus software developers comfortable with UML can move to SysML easily and tool developers with UML tools can easily support SysML as well;
  • it is graphical. I have always been struck by the way software developers will share design with other developers using graphics (boxes and lines), but then write (for example) “C++ code” when communicating that design to a computer. Likewise product engineers will work out the details of a family of products using graphs, boxes & lines but then outline those choice points using a textual approach, for example PDES/STEP. Graphical languages are a natural choice for design, and have been used for thousands of years (witness building and ship blueprints, electrical circuit diagrams, etc.).
  • it has many implementations. International standards simply aren’t worth the paper they are printed on (or the hard disk space they occupy) if they are not implemented. Even before the SysML final specification became available in early 2006, several companies announced implementation of the standard.

It is important to understand that SysML is not just another software development modeling language. While software is an important component of nearly all complex systems today, it is almost never the only component — planes, trains and automobiles all have software, and so do building controllers and chemical plants, but they also have plumbing, hydraulics, electrical and other systems that must be harmonized and co-designed. Likewise the human and automated processes to use complex systems need to be mapped out as well, in order to get good use from them (as well as maintain and integrate those systems for future requirements and into future systems). And all of these co-design issues need to take into account the fact that all complex systems are available in multiple configurations, based on facilities requirements, personnel hiring issues and myriad other variables.

This is the focus of the SysML language, a language to model all of these different factors in the course of engineering complex systems and systems of systems. The purpose is not to mask the complexity of those systems, but rather to expose and control it in the face of constantly changing business requirements and constantly changing infrastructure.

SysML is likely not the last language for engineering complex systems; the fast-paced change in most engineering fields (and in fact introduction of completely new engineering fields, like bioengineering) will likely cause change in the specification language itself. This shouldn’t be a surprise to any engineer; other long-lasting specification languages have changed over time. One simple example: building blueprints are several hundred years old, but those made before about 1880 didn’t feature the international symbol for electrical outlets, since that feature didn’t exist yet. Languages change to suit changing requirements in engineering, and fortunately SysML itself is designed to change, by being based on a metamodeling infrastructure also standardized by the OMG, the Meta Object Facility (MOF). This same technology can be used to transform and integrate existing designs in older engineering modeling languages, including IDEF (a language heavily used in many fields, including aviation).

Which brings us full circle, back to that collection of six million parts flying in close formation. There is no hope of developing such a large collection of parts — and people, and facilities, and hardware, and software, and hydraulics, and navigation, and HVAC — without a shared design language which takes into account the constant change of component systems. SysML was specifically designed to address that need, and is already doing so in parts from tiny embedded robotics controllers to huge industrial components.

So enjoy learning the language. While not simple — complex problems often require complex solutions — it’s logical and based on good engineering design practice. And keep those parts flying in close proximity!

Richard Mark Soley, Ph.D.
Chairman and Chief Executive Officer
Object Management Group, Inc.

Lexington, Massachusetts, U.S.A.
30 May 2006