Somat eDAQlite, Руководство пользователя

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I2773-4.
4
en 113
Somat eDAQ
lite
Pulse Frequency Mode Notes
Consider the following when using a pulse counter in frequency mode.
•
Because the ELDIO uses a 32-bit counter, it has an effective working range in this mode of less than 0.002 Hz.
•
The accuracy of the frequency measurement is inversely proportional to the measured frequency. For example, at a
frequency of 50 kHz, 100 counts are accumulated in the counter resulting in a 1.0% measurement accuracy; at a
frequency of 5 kHz, 1000 counts are accumulated in the counter resulting in a 0.1% measurement accuracy.
•
When the input signal pulse frequency is greater than the sample rate, the eDAQ
lite
latches the first pulse frequency
measured until it reads the value effectively ignoring subsequent pulses that occur in the sample period after the
latched pulse. When the input signal pulse frequency is less than the sample rate, the eDAQ
lite
uses the stored pulse
frequency for the previous output sample.
•
When the input signal pulse train stops for an extended period of time, the following occurs. The ELDIO counters use
32-bit registers, which saturate in about 850 seconds and keep track of the last output pulse period value and the time
elapsed since the last pulse was detected. If sufficient time passes with no new pulse detected, the output is adjusted
downward to account for the fact the pulse period is greater than what was previously latched. This results in the output
pulse frequency value continuously decaying until a new pulse is detected.
High Frequency Inputs in the Pulse Rate Mode
Use the pulse rate mode to cover the high frequency input range that cannot be accurately covered using the frequency or
time modes (i.e., 50 kHz to 1 MHz). The resolution of the average input frequency is equal to the input sample rate. For
example, for an input with a 100 Hz sample rate and 6543 pulses in one sample period, the average input frequency over the
sample period is 654300 Hz (i.e., the resolution is 100 Hz). To make use of the pulse rate mode, the channel must have a
large number of pulses in each sample period.
To cover both low and high frequency inputs, route the input signal into two pulse counter channels—one configured in
frequency mode and the other in rate mode—and use the following expression in a Desk Calculator computed channel (see
“Desk Calculator” on page 130 ).
(X_r > R_max) * X_r + (X_r <= R_max) * X_f
where, X_r is the rate mode, X_f is the frequency mode and R_max is the crossover point where X_r is used instead of X_f.
Set R_max to minimize accuracy loss. For example, if the sample rates are both 100 Hz, then set R_max to 25000 Hz, where
the accuracy of X_f is 0.5% and the accuracy of X_r is 0.4%.
6.3 Analog Input Channels
6.3.1 Bridge
Add a bridge channel to any ELBRG connector. For more information on the ELBRG
layer, see
“ELBRG (Bridge Layer)” on page 94 .
NOTE
Each channel on the ELBRG layer can have an independent sample rate and digital
filter selection.
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