Update an Existing Model to Use Units
This example shows how to add units to an existing model. You see how to:
Use an incremental workflow to add units to components in your model
Integrate components that use different unit systems
Specify units for individual elements of a bus object
Troubleshoot unit mismatch problems
The model in the example is a fuel control system. The controller (Fuel Rate Controller) and plant (Engine Gas Dynamics) components of the model are nonvirtual subsystems. Nonvirtual subsystems have the Treat as atomic unit parameter selected. You introduce units to the plant before introducing units to the controller and connecting signals. You also specify units for the individual elements of a bus object in the model.
For the top model, the Allowed unit systems configuration parameter determines the unit systems the model can use. For each of the plant and controller subsystems, a Unit System Configuration block determines the allowed unit systems.
|Component||Allowed Unit Systems|
|Fuel Rate Controller subsystem (controller)|
|Engine Gas Dynamics subsystem (plant)|
In the plant subsystem, on the Signal Attributes tab of each Inport block dialog box, set the Unit parameter to a value appropriate for the connected physical signal.
|Block||Physical Signal||Unit Parameter Setting|
To display units on ports and signals in the model, on the Debug tab, select Information Overlays > Port Units.
In the plant subsystem, you see units on the Inport blocks and connected signals.
Navigate back to the top model. To compile the model, press Ctrl+D, which also performs unit consistency checking.
The model displays a warning to indicate that there is a disallowed
unit for the
throttle angle signal. Clicking the
warning icon displays a link to a Model Advisor report that gives
you more detail.
The model also displays the warning at the bottom of the model editing window.
In the plant subsystem, you specified a unit of
throttle angle signal. However, the warning
message indicates that degrees are not in the SI unit system. As determined
by the Allowed unit systems configuration
parameter, SI is the only unit system that the top
model currently allows. To resolve this warning, you have two options:
In the plant subsystem, specify a unit for the
throttle anglesignal that the SI unit system supports. For more information about supported unit systems and the units they contain, see Allowed Units.
In the top model, change the Allowed unit systems configuration parameter to expand the set of allowed unit systems.
In this case, a unit of
deg for the
angle signal is appropriate. Instead, to resolve the warning,
expand the set of allowed unit systems for the top model. Set the Allowed
unit systems configuration parameter of the top model
all. To recompile the model, press Ctrl+D.
The top model no longer displays warnings.
Now that you have introduced units to the plant and successfully resolved unit
inconsistency problems, you can add units to the controller. In the Fuel
Rate Controller subsystem, set the Unit
parameter of the fuel_rate
Outport block to
kg/s (kilograms per
Navigate back to the top model. To recompile it, press Ctrl+D.
The top model now shows a warning for mismatched units between the controller and plant. To resolve this error, you can:
Explicitly insert a Unit Conversion block between the two components.
Select the Allow automatic unit conversions configuration parameter.
Both options convert units in the same way. A situation in which you might disallow automatic conversions and insert conversion blocks instead is when you are integrating many components in a large system model. In that case, manually inserting conversion blocks can give you an added degree of control of unit conversions in the model. Also, with a conversion block, you can control the data type of the converted signal. This is useful, for instance, when you are modeling for fixed-point precision.
In this case, to enable Simulink® to resolve the unit mismatch automatically, select Allow automatic unit conversions. To recompile the model, press Ctrl+D.
Simulink automatically converts units between the controller and the plant. An automatic conversion icon replaces the warning.
The top model includes a
Simulink.Bus object named
EngSensors that passes various sensor signals as a composite signal
to the controller. To use the Bus Editor to add units to individual elements of the bus
object, on the Modeling tab, under Design, click
Bus Editor. Then, specify the units in the table or in the
Property Inspector pane.
EngSensors bus object, set
the Unit parameter of each element.
|Signal||Unit Parameter Setting|
To recompile the model, press Ctrl+D.
The model shows units on the individual elements of the bus object.
You can also see the units in the interface view of your model. On the Modeling tab, under Design, click Model Interface.
The airflow_calc block of the controller subsystem displays units on the individual elements of the bus object, both at the component interface and within the component.
After you introduce units incrementally and resolve inconsistency and mismatch issues, you can simulate the model.
fuel signal that is connected to
the scope, the plot window displays the associated units of
a y-axis label.