How to Work with Physical Units

Unlike Simulink® signals, which are essentially unitless, physical signals can have units associated with them. You specify the units along with the parameter values in the block dialogs, and Simscape™ unit manager performs the necessary unit conversion operations when solving a physical network. Simscape blocks support standard measurement systems. The default block units are meter-kilogram-second or MKS (SI).

Simscape software comes with a library of standard units, and you can define additional units as needed (see Unit Definitions). You can use these units in your block diagrams:

  • To specify the units of an input physical signal, type a unit name, or a mathematical expression with unit names, in the Input signal unit field of the Simulink-PS Converter block dialog. You can also select a unit from a drop-down list, which is prepopulated with some common input units. Signal units that you specify in a Simulink-PS Converter block must match the input type expected by the Simscape block connected to it. For example, when you provide the input signal for an Ideal Angular Velocity Source block, specify angular velocity units, such as rad/s or rpm, in the Simulink-PS Converter block, or leave it unitless. If you leave the block unitless, with the Input signal unit parameter set to 1, then the physical signal units are inferred from the destination block.

  • Simscape block dialogs have drop-down combo boxes of units next to a parameter value, letting you either select a unit from the drop-down list, or type a unit name (or a mathematical expression with unit names) directly into the box. These drop-down lists are automatically populated by those units that are commensurate with the unit of the parameter, based on the current list of unit definitions. For example, if a parameter is specified, by default, with the units of meters per second, m/s, the drop-down list of units contains commensurate units, such as mm/s, in/s, fps (feet per second), fpm (feet per minute), and so on, including any other linear velocity units currently defined in your unit registry.

  • To specify the units of an output physical signal, type a unit name, or a mathematical expression with unit names, in the Output signal unit field of the PS-Simulink Converter block dialog. You can also select a unit from a drop-down list, which is prepopulated with some common output units. The system compares the units you specified with the actual units of the input physical signal coming into the converter block and applies a gain equal to the conversion factor before outputting the Simulink signal. The default value is 1, which means that the unit is not specified. If you do not specify a unit, or if the unit matches the actual units of the input physical signal, no gain is applied.

For more information, see How to Specify Units in Block Dialogs.

    Note   Currently, the blocks in the Physical Signals library (such as PS Add, PS Gain, and so on) ignore the physical unit of the input signal and just perform calculations on the value. The output signals of the Physical Signals library blocks are unitless.

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