Campbell CR9000, Инструкция по эксплуатации

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Section 3. CR9000 Measurement Details
3-26
The resolution of the pulse counters is one count. The resolution of the
calculated frequency depends on the scan interval: frequency resolution =
1/scan interval (e.g., a pulse count in a 1 second scan has a frequency
resolution of 1 Hz, a 0.5 second scan gives a resolution of 2 Hz, and a 1 ms
scan gives a resolution of 1000 Hz). The resultant measurement will bounce
around by the resolution. For example, if you are scanning a 2.5 Hz input once
a second, in some intervals there will be 2 counts and in some 3 as shown in
Figure 3.4-1. If the pulse measurement is averaged, the correct value will be
the result.
3 2 3 2
FIGURE 3.4-1. Varying Counts within Pulse Interval
The resolution gets much worse with the shorter intervals used with higher
speed measurements. As an example, assume that engine RPM is being
measured from a signal that outputs 30 pulses per revolution. At 2000 RPM,
the signal has a frequency of 100 Hz (2000 RPMx(1 min/60 s)x30=1000). The
multiplier to convert from frequency to RPM is 2 RPM/Hz (1 RPM/(30
pulses/60s) = 2). At a 1 second scan interval, the resolution is 2 RPM.
However, if the scan interval were 1 ms, the resolution would be 2000 RPM.
At the 1 ms scan, if every thing was perfect, each interval there would be 1
count. However, a slight variation in the frequency might cause 2 counts
within one interval and none in the next, causing the result to vary from 0 to
4000 RPM!
The POption parameter in the PulseCount instruction can be used to set an
interval period for a running average computation of the frequency output from
the sensor. Example: Scan Rate of 10 mSec is for other measurements. The
output from the Pulse sensor will vary from 1000 Hz to 10 Hz. Set the
POption parameter to 1000 (mSec), and the instruction returns get a running
average of the Pulse outputs (getting 100 samples/second) over a 1 second
period. This would smooth the output.
Another method for measuring frequency is to use the TimerIO instruction
using one of the Pulse channels on the CR9071E Pulse Module to measure the
period of the signal (40 nanosecond resolution). The value returned is in
milliseconds. You can take the reciprocal of the returned value and multiply
by 1000 to get the frequency of the signal.
3.4.1 High Frequency Pulse Measurements
All twelve pulse channels of the CR9070 and CR9071E can be configured for
high frequency inputs. The signal is feed through a filter with a time constant
of 200 (
τ
= 200 nanoseconds
)
nanoseconds to remove higher frequency noise.
It is then feed through a Schmitt circuit to convert the signal to a square wave,
and to guard against false triggers when the signal is hovering around the
threshold level. In the High Frequency mode, the input signal to the Schmitt