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Algebraic Variable Function

LabVIEW 2012 Control Design and Simulation Module Help

Edition Date: June 2012

Part Number: 371894G-01

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Owning Palette: Implicit Systems Functions

Requires: Control Design and Simulation Module

Solves differential algebraic equations (DAEs) by varying the value of a constraint function over time until the output value equals the input value. To correctly solve a DAE, you must use this function in a feedback cycle by wiring the output value output to the input value input.

You must select the BDF solver in the Configure Simulation Parameters dialog box to use this function. You also must use this subsystem in a Control & Simulation Loop. If you place this subsystem outside a Control & Simulation Loop, LabVIEW returns an error when you run the VI.

Note  The value of the constraint function is the variable λ. The Algebraic Variable function is a subsystem that contains an Implicit Variable function. Refer to the Implicit Variable Function help topic for more information.

Dialog Box Options
Block Diagram Inputs
Block Diagram Outputs

Dialog Box Options

ParameterDescription
Polymorphic instanceSpecifies whether this function is Scalar or Vector. The default value is Scalar.
ParametersLists all the parameters associated with this function. Select a parameter from this list to configure the parameter. When you select a parameter, the parameter and its associated Parameter source control appear in the Parameter Information section of the configuration dialog box.
PreviewDisplays a graphical preview, if available, of the function output or configuration.
Parameter InformationContains the parameters you can configure for this function. You must select a parameter from the Parameters list to make that parameter and its associated Parameter source control visible in the Parameter Information section of the configuration dialog box.
Parameter sourceSpecifies whether you configure this parameter using the Configuration Dialog Box or a Terminal on the simulation diagram. The default value is Configuration Dialog Box. If you select Terminal, LabVIEW displays an input for that parameter on the simulation diagram, and you can wire values to that input to configure this function programmatically. If you select Configuration Dialog Box, LabVIEW removes that input from the simulation diagram. You then must set the value for this parameter inside the configuration dialog box.
initial valueSpecifies the initial value to use for the output value of the Implicit Variable function that this subsystem contains. The default is 0. This value does not affect the output value of the subsystem itself.
Execution ControlSpecifies the execution behavior of the subsystem. You can select from the following options:
  • Normal—(Default) Executes the subsystem without checking for an enabled state or waiting for a trigger.
  • Enable—Checks the value wired to the Enable block diagram input before executing the subsystem. The Enable block diagram input appears on the function when you select this option.
  • Trigger—Waits for a trigger from the Trigger block diagram input of the Trigger Type you specify before executing the subsystem. The Trigger block diagram input appears when you select this option.
  • Enable And Trigger—Checks the value wired to the Enable block diagram input and waits for a trigger from the Trigger block diagram input of the Trigger Type you specify before executing the subsystem. The Enable and Trigger block diagram inputs appear when you select this option.
Trigger TypeSpecifies when to trigger the execution of the subsystem. Set Execution Control to Trigger or Enable And Trigger to enable this option. You can select from the following options:
  • Rising—Executes the subsystem when the Trigger signal crosses 0 on a positive slope.
  • Falling—Executes the subsystem when the Trigger signal crosses 0 on a negative slope.
  • Either—(Default) Executes the subsystem when the Trigger signal crosses 0 on a positive or negative slope.
  • None—Does not specify a trigger.
Trigger OffsetSpecifies the offset from 0 that the subsystem uses when looking for a zero crossing. The default is 0. Set Execution Control to Trigger or Enable And Trigger to enable this pull-down menu.
Hold Previous OutputIndicates the outputs of the subsystem that hold their last known values when the subsystem does not execute. Outputs appear in this list when you set their Disabled Behavior to Hold. Select an output under Execution Control in the Parameters list to configure its Disabled Behavior.
Reset to Initial OutputIndicates the outputs of the subsystem that reset to their initial values when the subsystem does not execute. Outputs appear in this list when you set their Disabled Behavior to Reset. Select an output under Execution Control in the Parameters list to configure its Disabled Behavior.
Disabled BehaviorSpecifies the behavior of an output of a subsystem when the subsystem does not execute. Select an output under Execution Control in the Parameters list to display this option. You can select from the following options:
  • Hold—(Default) Holds the output value to its last known value.
  • Reset—Resets the output value to its initial value.
Use Configured Default ValueSpecifies to use the output value you specify for the value of the subsystem output value. If you set Disabled Behavior to Reset, the subsystem always outputs this value when disabled. If you set Disabled Behavior to Hold, the subsystem outputs this value only on its first execution.
output valueSpecifies the initial value to use for the output value of the subsystem. Place a checkmark in the Use Configured Default Value checkbox to enable this option.

Block Diagram Inputs

ParameterDescription
TriggerSpecifies the trigger signal the subsystem checks for a zero crossing before executing. In the configuration dialog box, set Execution Control to Trigger or Enable And Trigger to display this input.
EnableSpecifies whether the subsystem is enabled. In the configuration dialog box, set Execution Control to Enable or Enable And Trigger to display this input.
input valueSpecifies the input value of the constraint function.
initial valueSpecifies the initial value to use for the output value of the Implicit Variable function that this subsystem contains. The default is 0. This value does not affect the output value of the subsystem itself.

Block Diagram Outputs

ParameterDescription
output valueReturns the output value of the constraint function.

 

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