SI Nyquist Plot VI

System Identification VIs

SI Nyquist Plot VI

Installed With: System Identification Toolkit

Produces the Nyquist plot of a system model on an XY graph.

This VI computes the transfer function and frequency response function (FRF) for one input-output pair of a system model. This VI then presents the FRF as a real part versus imaginary part, or Nyquist plot. Use the SI Nyquist Plot Indicator to display the Nyquist plot on the front panel window.

Examples

Place on the block diagram

Find on the Functions palette

freq info specifies the frequency information for the data plot.

start frequency specifies the start frequency of the data plot. The value of start frequency must be in a normalized frequency range of 0–0.5 times the real sampling rate.

end frequency specifies the end frequency of the data plot. The value of end frequency must be in a normalized frequency range of 0–0.5 times the real sampling rate and greater than the value of start frequency.

num of freq bins specifies the number of frequency bins between start frequency and end frequency for the data plot.

interval type determines how the frequency points of the FRF increase from start frequency to end frequency. For a logarithmic interval type, the frequency points are the following:

f₀, f₀*df, f₀*(df)², f₀*(df)³, ..., f_Final

where	f₀ = start frequency
	f_Final = end frequency
	df = (log₁₀(end frequency) – log₁₀(start frequency)) / (num of freq bins – 1)

For a linear interval type, the frequency points are the following:

f₀, f₀+df, f₀+2df, f₀+3df, ..., f_Final

where	f₀ = start frequency
	f_Final = end frequency
	df = (end frequency – start frequency) / (num of freq bins – 1)

0	logarithmic (default) logarithmic (default)
1	linear

system model contains information about the model structure, nominal or estimated parameters, identification result, and so on. Use the Model Management VIs to retrieve the information system model contains.

Note You can use a customized system model probe to view model information that flows through system model wires when you debug a block diagram created with the System Identification VIs. Right-click a system model wire and select Custom Probe»SI System Model from the shortcut menu to use the system model probe.

channel selects the signal pair with which to compute the transfer function.

type specifies whether to compute the transfer function between a stimulus signal and a response signal or the transfer function between a noise signal and a response signal.

0	stimulus (default)—Computes the transfer function between a stimulus signal and a response signal. Use input index and output index to select the stimulus and response signals to use.
1	noise—Computes the transfer function between a noise signal and a response signal. Use input index and output index to select the noise and response signals to use.

input index specifies the index of the system input channel with which to compute the transfer function. input index applies only for MISO or MIMO system models.

output index specifies the index of the system output channel with which to compute the transfer function. output index applies only for MIMO system models.

confidence level (%) specifies the percentage confidence interval for the FRF.

error in describes error conditions that occur before this VI or function runs. The default is no error. If an error occurred before this VI or function runs, the VI or function passes the error in value to error out. This VI or function runs normally only if no error occurred before this VI or function runs. If an error occurs while this VI or function runs, it runs normally and sets its own error status in error out. Use the Simple Error Handler or General Error Handler VIs to display the description of the error code. Use error in and error out to check errors and to specify execution order by wiring error out from one node to error in of the next node.

	status is TRUE (X) if an error occurred before this VI or function ran or FALSE (checkmark) to indicate a warning or that no error occurred before this VI or function ran. The default is FALSE.
	code is the error or warning code. The default is 0. If status is TRUE, code is a nonzero error code. If status is FALSE, code is 0 or a warning code.
	source specifies the origin of the error or warning and is, in most cases, the name of the VI or function that produced the error or warning. The default is an empty string.

Nyquist plot returns the Nyquist plot of the input signal. Nyquist plot returns the imaginary part of the complex frequency response of the input system plotted against its real part. Use the SI Nyquist Plot Indicator to display the Nyquist plot.

	real returns the real part of the frequency response of the system.
	imaginary returns the imaginary part of the frequency response of the system.

error out contains error information. If error in indicates that an error occurred before this VI or function ran, error out contains the same error information. Otherwise, it describes the error status that this VI or function produces. Right-click the error out front panel indicator and select Explain Error from the shortcut menu for more information about the error.

	status is TRUE (X) if an error occurred or FALSE (checkmark) to indicate a warning or that no error occurred.
	code is the error or warning code. If status is TRUE, code is a nonzero error code. If status is FALSE, code is 0 or a warning code.
	source describes the origin of the error or warning and is, in most cases, the name of the VI or function that produced the error or warning.

Examples

Refer to the following VIs for examples of using the SI Nyquist Plot VI:

Model Presentation VI: labview\examples\System Identification\Getting Started\General.llb
Open example Browse related examples
Flexible Arm VI: labview\examples\System Identification\Industry Applications\Mechanical Systems.llb
Open example Browse related examples

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