Campbell-scientific CR1000 Measurement and Control System Uživatelský manuál stáhnout pdf (Strana 265)

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101

Section 7. Installation

265

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.266)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.

Figure 83: Running-average frequency response

1 2 ... 260 261 262 263 264 265 266 267 268 269 270 ... 587 588

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