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Cycle Average and RMS VI

LabVIEW 2013 Help

Edition Date: June 2013

Part Number: 371361K-01

»View Product Info

Owning Palette: Waveform Measurements VIs

Requires: Full Development System

Returns the average and RMS levels of a selected cycle of a periodic waveform or an array of periodic waveforms. Wire data to the signal in input to determine the polymorphic instance to use or manually select the instance.

Note  The terminology and measurement definitions for this VI comply with IEEE Standard 181-2003, IEEE Standard on Transitions, Pulses, and Related Waveforms.

Examples

Use the pull-down menu to select an instance of this VI.

Cycle Average and RMS 1 chan

cycle number specifies the cycle, or period, of the periodic signal on which to perform the measurement.
signal in is the waveform to measure. Each waveform is required to contain at least cycle number complete cycles, where a cycle is defined as the interval between two consecutive rising mid ref level crossings.
reference levels specifies the high, middle, and low reference levels of a waveform. LabVIEW uses the reference levels to define the measurement interval of one complete cycle. The distance between the mid ref level and the high ref level must equal the distance between the low ref level and the mid ref level. If the two distances are not equal, LabVIEW adjusts either the high ref level or the low ref level to match the smaller of the two distances. For example, if you specify a high ref level of 90%, a mid ref level of 50%, and a low ref level of 20%, LabVIEW uses 80% instead of 90% for the high ref level.
high ref level specifies the high reference level of the waveform in percent (default) or absolute units. After the signal crosses the mid ref level in the rising direction, it must cross the high ref level before the next falling mid ref level crossing can be counted.
mid ref level specifies the middle reference level in percent (default) or absolute units. The interval between consecutive rising mid ref level crossings defines one cycle, or period, of the waveform. At least one high/low reference level crossing must separate each mid ref level crossing.
low ref level specifies the low reference level of the waveform in percent (default) or absolute units. After the signal crosses the mid ref level in the falling direction, it must cross the low ref level before the next rising mid ref level crossing can be counted.
ref units specifies whether the high ref level, mid ref level, and low ref level inputs are interpreted as a percentage (default) of the full range of the waveform or as absolute levels.
error in describes error conditions that occur before this node runs. This input provides standard error in functionality.
percent level settings specifies the method LabVIEW uses to determine the high and low state levels of a waveform. If you select percent ref units, percent level settings determines the reference levels. Otherwise, LabVIEW ignores this input.
method specifies how LabVIEW computes the high and low state levels of the waveform.

0Histogram—Returns the levels of the histogram bins with the maximum number of hits in the upper and lower regions of the waveform. The upper and lower regions of the waveform include the upper and lower 40%, respectively, of the peak-to-peak range of the waveform.
1Peak—Searches the entire waveform for its maximum and minimum levels.
2Auto select (default)—Determines whether the histogram bins that correspond to the high and low state levels each have over 5% of the total hits. If so, LabVIEW returns those results. Otherwise, LabVIEW uses the peak method. This ensures a reasonable answer for either a square wave (ignoring the overshoot and undershoot) or a triangle wave (where a histogram fails).
histogram size specifies the number of bins in the histogram LabVIEW uses to determine the high and low state levels of the waveform. If you select the peak method, LabVIEW ignores this input.
histogram method specifies how LabVIEW computes the high and low state levels of the waveform. Currently, mode is the only available histogram method.

0mode
reserved is reserved for future use.
cycle average is the mean level of one complete period of a periodic input waveform. The average is computed by the following equation.

where i indicates the waveform samples that fall in the single period specified by cycle number and numPoints is given by the following equation.

numPoints = int(period/dt + .5)

where dt is the time between two samples and int( ) is a function that returns the integer portion of a floating-point number.

The cycle average of a perfect sine wave is zero, while the average level of the entire waveform can be nonzero due to partial periods at the boundaries of the waveform.
cycle RMS is the root mean square value of one complete period of a periodic input waveform. The RMS value is computed by the following equation.

where i indicates the waveform samples that fall in the single period specified by cycle number and numPoints is given by the following equation.

numPoints = int(period/dt + .5)

where dt is the time between two points and int( ) is a function that returns the integer portion of a floating-point number.
error out contains error information. This output provides standard error out functionality.
measurement info returns the measurement cycle interval endpoints and the absolute reference levels used to define the measurement cycle.
start time specifies the time of the rising mid ref level crossing that defines the start of the measurement interval.
end time specifies the time of the rising mid ref level crossing that defines the end of the measurement interval.
ref levels returns the three user-defined reference levels of the waveform in absolute units. LabVIEW uses the reference levels to define the interval of one cycle measurement.
high ref level returns the high reference level.
mid ref level returns the middle reference level.
low ref level returns the low reference level.
ref units is always absolute in measurement info.

Cycle Average and RMS N chan

cycle number specifies the cycle, or period, of the periodic signal on which to perform the measurement.
signal(s) in is an array of waveforms containing the signals to measure. Each waveform is required to contain at least cycle number complete cycles, where a cycle is defined as the interval between two consecutive rising mid ref level crossings.
reference levels specifies the high, middle, and low reference levels of a waveform. LabVIEW uses the reference levels to define the measurement interval of one complete cycle. The distance between the mid ref level and the high ref level must equal the distance between the low ref level and the mid ref level. If the two distances are not equal, LabVIEW adjusts either the high ref level or the low ref level to match the smaller of the two distances. For example, if you specify a high ref level of 90%, a mid ref level of 50%, and a low ref level of 20%, LabVIEW uses 80% instead of 90% for the high ref level.
high ref level specifies the high reference level of the waveform in percent (default) or absolute units. After the signal crosses the mid ref level in the rising direction, it must cross the high ref level before the next falling mid ref level crossing can be counted.
mid ref level specifies the middle reference level in percent (default) or absolute units. The interval between consecutive rising mid ref level crossings defines one cycle, or period, of the waveform. At least one high/low reference level crossing must separate each mid ref level crossing.
low ref level specifies the low reference level of the waveform in percent (default) or absolute units. After the signal crosses the mid ref level in the falling direction, it must cross the low ref level before the next rising mid ref level crossing can be counted.
ref units specifies whether the high ref level, mid ref level, and low ref level inputs are interpreted as a percentage (default) of the full range of the waveform or as absolute levels.
error in describes error conditions that occur before this node runs. This input provides standard error in functionality.
percent level settings specifies the method LabVIEW uses to determine the high and low state levels of a waveform. If you select percent ref units, percent level settings determines the reference levels. Otherwise, LabVIEW ignores this input.
method specifies how LabVIEW computes the high and low state levels of the waveform.

0Histogram—Returns the levels of the histogram bins with the maximum number of hits in the upper and lower regions of the waveform. The upper and lower regions of the waveform include the upper and lower 40%, respectively, of the peak-to-peak range of the waveform.
1Peak—Searches the entire waveform for its maximum and minimum levels.
2Auto select (default)—Determines whether the histogram bins that correspond to the high and low state levels each have over 5% of the total hits. If so, LabVIEW returns those results. Otherwise, LabVIEW uses the peak method. This ensures a reasonable answer for either a square wave (ignoring the overshoot and undershoot) or a triangle wave (where a histogram fails).
histogram size specifies the number of bins in the histogram LabVIEW uses to determine the high and low state levels of the waveform. If you select the peak method, LabVIEW ignores this input.
histogram method specifies how LabVIEW computes the high and low state levels of the waveform. Currently, mode is the only available histogram method.

0mode
reserved is reserved for future use.
cycle average is an array containing the cycle average of each waveform in signal(s) in. cycle average is the mean level of one complete period of a periodic input waveform. The average is computed by the following equation.

where i indicates the waveform samples that fall in the single period specified by cycle number and numPoints is given by the following equation.

numPoints = int(period/dt + .5)

where dt is the time between two samples and int( ) is a function that returns the integer portion of a floating-point number.

The cycle average of a perfect sine wave is zero, while the average level of the entire waveform can be nonzero due to partial periods at the boundaries of the waveform.
cycle RMS is an array containing the cycle RMS value for each waveform in signal(s) in. cycle RMS is the root mean square value of one complete period of a periodic input waveform. The RMS value is computed by the following equation.

where i indicates the waveform samples that fall in the single period specified by cycle number and numPoints is given by the following equation.

numPoints = int(period/dt + .5)

where dt is the time between two points and int( ) is a function that returns the integer portion of a floating-point number.
error out contains error information. This output provides standard error out functionality.
measurement info is an array of clusters containing measurement information for each input waveform.
start time specifies the time of the rising mid ref level crossing that defines the start of the measurement interval.
end time specifies the time of the rising mid ref level crossing that defines the end of the measurement interval.
ref levels returns the three user-defined reference levels of the waveform in absolute units. LabVIEW uses the reference levels to define the interval of one cycle measurement.
high ref level returns the high reference level.
mid ref level returns the middle reference level.
low ref level returns the low reference level.
ref units is always absolute in measurement info.

Examples

Refer to the following VIs for examples of using the Cycle Average and RMS VI:

  • Pulse and Transition Measurements VI: labview\examples\Signal Processing\Waveform Measurements
  • N channel Pulse and Transition Measurements VI: labview\examples\Signal Processing\Waveform Measurements

 

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