Emerson Rosemount 8732EM, Справочное руководство

Страница: 145 / 254

Signal Processing Functionality Figure 10-1:
A
B
C
D
F
G
H
1
2
3
4
5
E
A. Flow rate
B. Time (10 samples = 1 second)
C. Upper value
D. Lower value
E. Tolerance band
F. Maximum percent limit
G. Minimum percent limit
H. Time limit
• X = Input flow signal from sensor
• O = Average flow signals and transmitter output, determined by the number of samples parameter
• Tolerance band, determined by the percent limit parameter
• Upper value = average flow + [(percent limit/100) average flow]
• Lower value = average flow - [(percent limit/100) average flow]
1. This scenario is that of a typical non-noisy flow. The input flow signal is within the
percent limit tolerance band, therefore qualifying itself as a good input. In this case
the new input is added directly into the running average and is passed on as a part of
the average value to the output.
2. This signal is outside the tolerance band and therefore is held in memory until the
next input can be evaluated. The running average is provided as the output.
3. The previous signal currently held in memory is simply rejected as a noise spike since
the next flow input signal is back within the tolerance band. This results in complete
rejection of noise spikes rather than allowing them to be “averaged” with the good
signals as occurs in the typical damping circuits.
4. As in number 2 above, the input is outside the tolerance band. This first signal is held
in memory and compared to the next signal. The next signal is also outside the
tolerance band (in the same direction), so the stored value is added to the running
average as the next input and the running average begins to slowly approach the
new input level.
Digital Signal Processing
Reference manual 139