Campbell CR6 Series, Operator'S Manual

Page: 250 / 690

Section 7. Installation
250
For the example above, the delay is:
Delay in time = (1 ms) • (4 – 1) / 2 = 1.5 ms
Example:
An accelerometer was tested while mounted on a beam. The test had the
following characteristics:
o Accelerometer resonant frequency ≈ 36 Hz
o Measurement period = 2 ms
o Running average duration = 20 ms (frequency of 50 Hz)
Normalized resonant frequency was calculated as follows:
36 Hz / 50 Hz = 0.72
SIN(0.72π) / (0.72π) = 0.34.
So, the recorded amplitude was about 1/3 of the input-signal amplitude. A
CRBasic program was written with variables Accel2 and Accel2RA . The
raw measurement was stored in Accel2 . Accel2RA held the result of
performing a running average on the Accel2 . Both values were stored at a
rate of 500 Hz. Figure Running-Average Signal Attenuation
(p. 251)
shows the
two variables plotted to illustrate the attenuation. The running-average value
has the lower amplitude.
The resultant delay, D
r
, is calculated as follows:
D
r
= (scan rate) • (N–1)/2 = 2 ms (10–1)/2
= 9 ms
D
r
is about 1/3 of the input-signal period.