Campbell Scientific CR1000KD CR1000 Measurement and Control System - Page 265

Section 7. Installation 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 (frequency 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 measurement 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 265