Unit Specification in Simulink Models

Simulink^® enables you to specify physical units as attributes on signals at the boundaries of model components. Such components can be:

Subsystems
Referenced Simulink models
Simulink-PS Converter (Simscape) and PS-Simulink Converter (Simscape) blocks that interface between Simulink and components developed in Simscape™ and its associated physical modeling products
Stateflow^® charts, state transition tables, or truth tables
MATLAB Function blocks
Constant blocks
Data Store Memory, Data Store Read, and Data Store Write blocks
C MEX S-Function

By specifying, controlling, and visualizing signal units, you can ensure the consistency of calculations across the various components of your model. For example, this added degree of consistency checking is useful if you are integrating many separately developed components into a large, overall system model.

In Simulink models, you specify units from a unit database. The unit database comprises units from the following unit systems:

SI — International System of Units
SI (extended) — International System of Units (extended)
English — English System of Units
CGS — Centimetre-gram-second System of Units

Based on the type of system you are modeling, you can use any combination of units from these supported unit systems. For more information about supported unit systems and the units they contain, see Allowed Units.

You can assign units to signals through these blocks:

You can assign units to signals through these objects:

When you add a supported block to your model, the Unit parameter on the block is set to inherit by default. This setting means that the block inherits the unit from a connecting signal that has an explicitly specified unit.

You can explicitly specify units for signals using the Unit parameter of a supported block. For this parameter, the dialog box provides matching suggestions to help you:

If you do not provide a correctly formed unit expression, you get an error. Correctly formed unit expressions are a combination of unit names or symbols with properly balanced parentheses and *, /, and ^ characters. Special characters such as [, ], {, }, <, >, \, ", &, and so forth are not supported.

By default, a block port has an empty (that is, unspecified) unit and the Unit parameter is set to inherit. When you specify a unit for one port, Simulink checks the unit setting of any port connected to it. If a port has an empty unit, you can connect it to another port that has any supported unit. If a port unit parameter is set to inherit, it inherits the unit from a connected port that has a specified unit.

To display the units on the Simulink lines in the model, on the Debug tab, click Information Overlays > Units.

Guidelines for Unitless, Dimensionless, and No Unit Quantities

When dealing with unitless, dimensionless, or no unit quantities, consider these guidelines:

Pure numbers with no units
Consider using 1, the SI standard dimensionless unit. Simulink checks the unit to make sure other dimensionless quantities are propagated in. You can add, multiply, and otherwise manipulate these types of numbers.
Unspecified units
Consider using an empty unit, where the Unit parameter is blank. When this parameter is blank, Simulink accepts any unit that propagates to the port without returning a warning.
Integer counting number unit
Consider using count to denote an integer counting number unit. Use this unit for quantities that are difficult to describe, but that are considered unitless or dimensionless, such as thermally accessible states or PWM count. For more information, see count in Allowed Units.
Dimensionless physical quantities
Dimensionless physical quantities represent a specific physical quantity. Consider using the physical quantities velocity_factor, refractive_index, relative_permeability, strain, Reynolds_number, and logical. Some of these physical quantities, strain andReynolds_number, have corresponding units. To specify units for the other physical quantities, use a format such as 1@physical quantity, for example, 1@refractive_index. Simulink performs a physical quantity check at propagation. For example, if a model tries to add dimensional physical quantities, Simulink returns a warning.
Pure scaled numbers
Consider using ppm, ppb, ppt.

Specify Physical Quantities

When you model a physical system, it is possible to use the same unit expression for two or more signals that represent different physical quantities. For example, a unit expression of N*m can represent either torque or energy. To prevent mistaken connection of two ports with the same unit but representing different physical quantities, you can add a physical quantity to the unit expression. For example, for the same unit of N*m, you can specify different physical quantities of N*m@torque and N*m@energy. Similar to units, the dialog box provides suggestions as you type the names of physical quantities.

Physical quantities help you to enforce an extra degree of unit consistency checking between connected ports. When you attempt to connect ports with different physical quantities, the model displays a warning.

Specify Units in Objects

By default, Simulink.ValueType, Simulink.Signal, Simulink.BusElement, and Simulink.Parameter objects have empty units. In the case of a:

Simulink.ValueType object, the object does not attach a unit to the corresponding signals.
Simulink.Signal object, the empty unit means that the corresponding signal can inherit a unit from an upstream or downstream port.
Simulink.BusElement object, the empty unit means that the corresponding bus element also has an empty unit. You can connect the element to a port with any unit, but the element does not inherit a unit from the port.
Simulink.Parameter object, the object does not attach a unit to the corresponding parameter value.

If you specify a unit in a Simulink.ValueType, Simulink.Signal, or Simulink.BusElement object, Simulink applies the attribute to the corresponding signal line when:

The Simulink.ValueType object specifies the properties for a signal in the model.
The Simulink.Signal object resolves to a signal in the model.
The Simulink.BusElement object is associated with a Simulink.Bus object and you use the corresponding bus element with a Bus Creator, Bus Selector, or Bus Assignment block that applies the Simulink.Bus object.

For the Simulink.Parameter object, Simulink does not apply any attribute. For all objects, if the Unit parameter has a value that is not formed correctly, you see an error. If the unit is formed correctly but is undefined, you see a warning when you compile the model. If the unit expression contains special characters such as [, ], {, }, <, >, \, ", &, and so forth, Simulink replaces them with underscores (_).

Custom Unit Properties

Notes on the Unit and DocUnits properties starting in R2016a:

The DocUnits property is now Unit for Simulink.Parameter and Simulink.Signal objects. If, in a previous release, you used the DocUnits parameter of a Simulink.Parameter or Simulink.Signal object to contain text that does not now comply with units specifications, simulation returns a warning when the model simulates.
To suppress these warnings, set the configuration parameter Units inconsistency messages to none. This setting suppresses all units inconsistency check warnings.
If you have a class that derives from Simulink.Parameter, Simulink.Signal, or Simulink.BusElement with a previously defined Unit property, Simulink returns an error like the following:
```
Cannot define property 'Unit' in class 'classname' because
 the property has already been defined in the superclass 'superclass'.
```
If you use this property to represent the physical unit of the signal, delete the Unit property from the derived class in the R2016a or later release. Existing scripts continue to work, unless you are assigning incorrectly formed unit expressions to the Unit field. In this case, replace the use of Unit with DocUnits to continue to be able to assign the unit expression.
Note
If you store existing data in a MAT- or .sldd file, in a release prior to R2016a, copy the contents of the Unit property to the DocUnits first. Then, save the file in the earlier release before loading the model in R2016a or later release.

Specify Units for Temperature Signals

When modeling absolute temperature quantities, use units such as K, degC, degF, and degR. When modeling temperature difference quantities, use units such as deltaK, deltadegC, deltadegF, and deltadegR. If you connect a signal that has a temperature difference unit to a block that specifies an absolute temperature unit, Simulink detects the mismatch.

Specify Units in MATLAB Function Blocks

You specify the units for the inputs and outputs of a MATLAB Function block by specifying the units in the Unit (e.g., m, m/s^2, N*m) parameter in the Property Inspector. Double-click the MATLAB Function block to open the MATLAB Function Block Editor. In the Modeling tab, in the Design section, click Symbols Pane. Right-click the variable name and select Inspect. By default, this property is inherit, which means that the unit equals the signal sent to the port. If you select the Data must resolve to signal object property for an output, the signal connected to the output port determines the port unit and the other properties except for the Name and Scope.

MATLAB Function blocks check if the Simulink signal unit matches the unit assigned to the corresponding input or output of the block. If the units do not match, Simulink displays a warning when you update the model. Otherwise, unit settings do not affect MATLAB Function block execution.

Specify Units in Constant Blocks

You can specify units for output data of Constant blocks by using the Unit property in the Simulink.Parameter object.

Specify Units for Logging and Loading Signal Data

You can include units in signal data that you log or load.

You specify units for logging and loading using Simulink.SimulationData.Unit objects. When you log using Dataset or Timeseries format, Simulink stores the unit information using Simulink.SimulationData.Unit objects. If you create MATLAB^® timeseries data to load, you can specify Simulink.SimulationData.Unit object for the Units property of the timeseries object.

For details, see Load Signal Data That Uses Units.

Restricting Unit Systems

By default, you can specify units from any of the supported unit systems. However, in large modeling projects, to enforce consistency, you might want to restrict the unit systems that certain components of your model can use. To specify available unit systems for a model, in the configuration parameter Allowed unit systems, enter all or a comma-separated list containing one or more of SI, SI (extended), CGS, and English. Do not use quotation marks. If your model contains referenced models, you can use the Allowed unit systems to restrict units in each of those referenced models. If your model contains subsystems, you can use the Unit System Configuration block to restrict units in the subsystems. You can also optionally use a Unit System Configuration block in a model. In this case, the settings in the Unit System Configuration block override whatever you specify in Allowed unit systems.

To restrict unit systems in a model:

In the Unit parameter of the Inport, Outport, or Signal Specification block, click the link.
If a Unit System Configuration block exists in your model, this link opens the block dialog box. Otherwise, the link opens the Allowed unit systems configuration parameter.
Specify one or more the desired unit systems, SI, SI (extended), English, or CGS, in a comma-delimited list, or all, without quotation marks.

In a parent-child relationship (for example, a top model with a referenced model or subsystem), you can specify different unit systems for each component. However, if a child propagates a unit into a parent that is not in the unit systems specified for the parent, you get a warning.

To check whether there are unit mismatches caused by restricted unit systems in your model hierarchy:

Press Ctrl+D and visually inspect the model for warning badges.
Use the Model Advisor check Identify disallowed unit systems.