SI Nyquist Plot VI

Owning Palette: Model Analysis VIs

Requires: Advanced Signal Processing Toolkit or Control Design and Simulation Module

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

	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, ..., fFinal where f0 = start frequency fFinal = 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, ..., fFinal where f0 = start frequency fFinal = end frequency df = (end frequency – start frequency) / (num of freq bins – 1) 0 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 node runs. This input provides standard error in functionality.
	show confidence range? specifies whether to show the confidence range of the plot data. The default is TRUE.
	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. This output provides standard error out functionality.

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
• Flexible Arm VI: labview\examples\System Identification\Industry Applications\Mechanical Systems.llb