Page 668 - The Mechatronics Handbook

finite element packages to examine the dynamic properties of mechanical constructions. It is only after
reduction to low-order models (modal analysis) that these models can be used for controller design. On
the other hand, typical control-engineering software does not directly support the mechatronic design
process either; in the modeling process the commonly used transfer functions and state space descriptions
often have lost the relation with the physical parameters of the mechanical construction. Tools are required
that allow modeling of mechanical systems in a way that the dominant physical parameters (like mass
and dominant stiffness) are preserved in the model and simultaneously provide an interface to the
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controller design and simulation tools control engineers are used to (Coelingh; Coelingh, De Vries, and
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Van Amerongen ).
Simulation is an important tool to evaluate the design of mechatronic systems. Most simulation pro-
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grams like Simulink use block diagram representations and do not support physical modeling in a way
that direct tuning of the physical parameters of the mechanical construction and those of the controller
is possible as required in the design of mechatronic systems. Recently, programs that allow physical
modeling in various physical domains became available. They use an object-oriented approach that allows
hierarchical modeling and reuse of models. The order of computation is only fixed after combining the
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subsystems. Examples of these programs are 20-sim, described by Broenink as CAMAS and Dymola. 6
In this section the modeling and simulation program 20-sim (pronounced Twente Sim) will be used
to illustrate the simultaneous design of construction and controller in a mechatronic system. 20-sim sup-
ports object-oriented modeling. Power and signal ports to and from the outside world determine each
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object (Weustink, De Vries, and Breedveld ). Inside the object there can be other objects or, on the lowest
level, equations. Various realizations of an object can contain different or more detailed descriptions as
long as the interface (number and type of ports) is identical. Modeling can start by a simple intercon-
nection of (empty) submodels. Later they can be filled with realistic descriptions with various degrees
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of complexity. De Vries refers to this as polymorphic modeling. Submodels can be constructed from
other submodels in hierarchical structures. Proper physical modeling is achieved by coupling the sub-
models by means of the flow of energy, rather than by signals such as voltage, current, force, and speed.
This way of modeling is well suited for mechatronics system design. It will be illustrated with an example.
We want to consider the design of a simple servo system, considering the use of a voltage source, a DC
motor, and a mechanical load driven through a transmission (Fig. 21.5).
The transmission is disregarded for the time being. The belt is considered as infinitely stiff and the
transformation ratio is taken care of by changing the motor constant. If a power amplifier driven by a
signal generator describes the voltage source, we can draw the iconic diagram of Fig. 21.6. At this stage
the different components in this model are still empty. But all components have electrical and/or mechan-
ical “ports.” With the proper interfaces (ports) defined, the components can be connected to each other.

FIGURE 21.5 Simple DC-servo system.

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