Campbell-scientific CR3000 Micrologger Manuel d'utilisateur télécharger pdf (Page 272)

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Section 7. Installation

272

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Inputfrequencytorunningaverage(normalizedfrequency)=100/250

=0.4

Sin(0.4π)/(0.4π)=0.757(orreadfromfigureRunning‐Average

FrequencyResponse

(p.273),wheretheXaxisis0.4)

Fora100‐HzinputsignalwithanAmplitudeof10‐Vpeaktopeak,a

runningaverageoutputsa100‐Hzsignalwithanamplitudeof7.57‐V

peaktopeak.

There is also a phase shift, or delay, in the AvgRun() output. The formula for

calculating the delay, in number of samples, is:

Delayinsamples=(N‐1)/2

Note N = Number of points in running average)

To calculate the delay in time, multiply the result from the above equation by the

period at which the running average is executed (usually the scan period):

Delayintime=(scanperiod)(N‐1)/2

For the example above, the delay is:

Delayintime=(1ms)(4‐1)/2=1.5ms

Example:

Actualtestusinganaccelerometermountedonabeamwhoseresonant

frequencyisabout36Hz.Themeasurementperiodwas2ms.The

runningaveragedurationwas20ms(fre

quencyof50Hz),sothe

normalizedresonantfrequencyis,

36/50=0.72,SIN(0.72π)/(0.72π)=0.34.

Therecordedamplitudeforthisexampleshouldbeabout1/3ofthe

input‐signalamplitude.Aprogramwaswrittenwithtwostored

variables:Accel2andAccel2RA.Therawme

asurementwasstoredin

Accel2,whileAccel2RAwastheresultofperformingarunningaverage

ontheAccel2variable.Bothvalueswerestoredatarateof500Hz.

FigureRunning‐AverageSignalAttenuation

(p.273)showthetwovalues

plottedinasinglegraphtoillustratetheattenuation(therunning‐

averagevaluehastheloweramplitude).

Theresultantdelay(delayintime)=(Scanrate)(N‐1)/2=2ms(10‐1)/2=

9ms.Thisisabout1/3oftheinput‐signalperiod.

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