pdf download - Software and Computer Technology - TU Delft

Diagnosing the Beam Propeller Movement
of the Frontal Stand 5.4 MBD Implementation 1
5.4.2 The Modeled Structure
The structure of the model is shown in Figure 5.3. In short, this block diagram describes the following.
The input of the target system is the speed the user requests (V_user). The controller of
the movement controls the the output of the target system: the angular speed of the frontal stand
(speed). It does so, by using three control loops. Note that the fault detection mechanism is part
of the architecture. It constitutes of the POSITION_ERROR, SPEED_ERROR, CURRENT_ERROR and
POSVAL_ERROR signals. These signals are also chosen as being outputs of the system. During nominal
behavior of the system these four error signals are false (Appendix B.3). The LYDIA model that
defines the nominal behavior is listed in Appendix C.3.5.
This model, that is part of the first MBD implementation, specifies the target system with a
certain granularity. The chosen granularity is motivated by the following two considerations:
• the choice of PMS what components can be replaced with new FRUs, and which cannot. This
is the reason that the potentiometer and encoder are combined to one entity (PEU). The motor
and brake are also combined (MBU), because it is impossible to replace one without the other.
• the possibility to use system data to distinguish between faults. For example, a fault of the
Stand could, on a lower level of abstraction, be caused by numerous anomalities: oil, a
power from the environment, etc. In some cases, these ’lower’ faults can be recovered by the
service engineer, but the system data cannot reveal them. Thus, these low-level faults are not
represented by health variables in the model.
The model listed in C.3.5 is not compositional. Thus, the structure is not explicitly modeled.
This is a practical form of the model during development, but in order to allow long-term maintenance,
a compositional model is much more appropriate. Appendix C.3.2 shows a compositional
version of the model belonging to the MBD-1 implementation. In this model, each health variable
belongs to separate component. Each component is modeled as a separate entity. The structure is
defined by the top level entity of the model; FS_ Beam_ Propeller_ Movement. Each component
defines part of the behavior. The behavior is discussed in the following subsection.
5.4.3 Behavior of the Model
A block diagram as shown in Figure 5.3 only shows structure, and only developers of the system
are able to understand how these components emit the intended behavior. The components of the
compositional model LYDIA model do specify how the components implement the beam propeller
movement of the frontal stand (see Appendix B.3 for a description of nominal behavior in natural
language). It is a weak fault model. None is said about what happens if, for example, the MBU
malfunctions. To give insight in how the models specifies the nominal behavior of the system, the
writing below describes two important building blocks of the model: a control loop and an error
signal.
Modeling a Control Loop
A very interesting issue of the beam propeller movement subsystem is that it consists of control
loops. Because of their recursive nature the model should embed state and time. However, these
issues are still topic of ongoing research. Therefore, the text below suggests an abstraction of
a control loop that does not depend on time and state. Figure 5.4 shows successively the block
diagram of one, two and three control loops.
59