Page 688 - The Mechatronics Handbook

Third, models can provide us with insights into the behavior of a system which might be obscured when
we attempt to see the system in its full complexity. Fourth, they allow us to grasp similarities with other
systems and use prior experience to help solve current problems. Fifth, models provide us a way of
identifying and testing our ignorance of the actual behavior of the system by identifying unknown
parameters and providing hypotheses worthy of testing. In this section, we shall consider these diverse
purposes of modeling and examine the characteristics that cause a particular modeling methodology to
be better or more poorly suited for each of these purposes.

Documentation and Communication
The first purpose of a model that we shall consider is documentation. We often forget that aside from
the various purposes of modeling in the design process, our models are often our best tools for commu-
nicating with our colleagues, our customers, and our successors. Every document generated in the design
process is a model of some part of the system. To the extent that we can avoid double documentation
of the same concept, we not only reduce workload and decrease time to market, but we also reduce the
likelihood of inconsistencies in our documentation. How do our models serve as documentation? Con-
sider a circuit diagram. It is not the circuit, but documentation of the circuit’s design. It is also a model
of the behavior we are seeking from the circuit. Nonidealities and component variations will cause the
actual circuit to behave differently from this model, but the model still serves its purpose. Earlier in the
design process, requirements documents also document the product, but in terms of requirements rather
than the features that satisfy them.
Design documentation is important for at least four distinct groups. The engineering team itself needs
communication among its members to ensure that effort is not wasted on elements that do not inter-
operate or that do not contribute to the overall product meeting requirements. Communication is
required between the design team and the customer to ensure that requirements are correctly understood
and implemented in the design. Management needs to understand the status of the project in terms of
outstanding risks, ongoing expenses, and tradeoffs that would affect market size. Finally, future design
teams can benefit from documentation of earlier design concepts, analyses, and decisions. This role of
documentation as a communication with future workers in the field has been recognized as a key part
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of all scientific disciplines but is often overlooked by engineers focused on meeting short-term deadlines.
What characteristics of a model lead to good documentation and clear communication? Clearly, the
model must be understood by others. This requirement encourages the use of standardized modeling
techniques whenever possible. It also encourages the use of modeling techniques that are interdisciplinary
whenever feasible. When choosing models for documentation purposes, one must remember that each
model highlights certain features of the design while hiding others. For example, the sequence diagram
clearly documents communication between two objects while hiding details of how the objects generate
outgoing messages or interpret incoming ones. If multiple models of the system are used to document
the various aspects of the system, there should be some way to cross-reference items between models
(e.g., determine that timing requirements documented in a sequence diagram model are met by the
dynamics of a system described in a block diagram). Also, if the documentation is to be useful to future
designers, it should be possible to cross reference design models based on a variety of criteria including
system requirements and included components, so that future designs can benefit from current ones.
Clearly, any modeling technique that serves some other purpose in design will also serve as documen-
tation, but the model must be chosen so as to match the features one wishes to document.

Hierarchical Framework
Real mechatronic systems include a large number of components that interact in many ways. Our models
simplify these complex interfaces and describe products and processes in terms of simply interacting
components with well-defined interfaces and behaviors. This simplification allows us to subdivide com-
plex problems into a set of simpler problems whose solutions can be easily integrated. In complex
mechatronic systems where we must consider components that interact in several energy domains, this

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