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change. In fact, the model only contains a placeholder for the block, instead of the
block itself, with a link referring to the applicable source library that contains the
actual block contents. This promotes the re-use of existing blocks, and prevents inadvertent
proliferation of multiple versions of the same block (some being properly
updated, some inadvertently remaining unchanged).
To demonstrate the use of the libraries in the FDC toolbox, we will take a closer
look at the block Gravity, which can be found slightly left of the center of figure 7.10.
Double-clicking this block results in the mask-dialog from figure 7.11. As we have
seen before in figure 7.9, the mask-dialog includes a short description of the block, a
110 Chapter 7. Getting started with the FDC toolbox
Figure 7.9: Mask dialog for the system Beaver
Figure 7.10: Level 2 of the system Beaver (main level of the aircraft model)
7.9. Linking to SIMULINK libraries 111
Figure 7.11: Mask dialog for the block Gravity
Figure 7.12: Gravity and wind forces blocklibrary
Figure 7.13: Looking under the mask of Gravity
Figure 7.14: Error when trying to edit a locked library
112 Chapter 7. Getting started with the FDC toolbox
note that the block has been masked, and a note that the block has been linked from
a library. In this case, there is also a parameter field, although it has been locked
because the parameter definition for this particular block can only be changed in the
library in which it is defined.
It is possible to quickly jump from a block in a model (actually: its placeholder) to
the corresponding library by: (i) right-clicking the block and selecting ‘Go To Library
Link’ from the context-menu, (ii) activating the block and selecting ‘Go To Library
Link’ from the Edit menu, or (iii) activating the block and pressing Ctrl + L . If we
apply this method to the Gravity block, figure 7.12 will be opened, with the Gravity
block being activated as shown in the figure. Of course, if you know the name of the
library, it is also possible to open it directly from the command-line. In this example
typing fdclib4 or double-clicking the ‘Gravity and wind forces’ button in the main
FDC library will open the library shown in figure 7.12.
The internal structure of this block can again be shown by unmasking it first.
This yields figure 7.13, which shows the deepest level of this block, i.e. the level
in which the actual equations are evaluated. The expressions within the individual
Fcn blocks will probably look rather cryptic at first sight, but once one knows the
meaning of the input and output signals (as explained in the next chapters and the
on-line help information) and the part of the theory on which these equations are
based, understanding the block-equations actually turns out to be relatively easy.
Notice that it is not necessary to open the blocklibrary to view the deepest level of a
block: it is possible to look beneath the mask of a linked block within a model too.
The only drawback is that this may cause confusion because linked blocks can not
be edited within the model. Libraries are ‘read-only’ or ‘locked’ by default, which
means that it is not possible to change any elements in library blocks, unless the
library is released first by selecting the ‘Unlock Library’ option in the Edit menu. If
the user does try to edit a locked library, or manipulate a linked block directly from
within a model, the error message from figure 7.14 will be displayed.
It is not possible to edit the contents of a linked block within a model, because the
actual block contents are included in the library only. However, it is possible to break
a library link after copying a block from the library, by selecting the option ‘Break
Library Link’ in the Edit menu. In that case, the placeholder containing the library
link will be overwritten with the block’s actual contents – effectively creating a clone
of the block, and editing the block within the model will become possible. Since this
breaks the library link, it will obviously not affect the contents of the library itself.
Unfortunately, the placeholder containing the library link will look exactly the
same as the original block in the library; the only visual difference is the link indication
in the mask-dialog, and that is only present if a mask dialog exists in the first
place. Some subsystems from the FDC toolbox do not have a mask-dialog, even
though they have been linked from a library. To make the user aware of the library
links in those cases, such linked, yet unmasked subsystems have been highlighted
with a light-blue colour.
　
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