Campbell-scientific CR1000 Measurement and Control System Instrukcja Użytkownika Pobierz pdf (Strona 319)

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101

Section 8. Operation

319

Table 70. Example. E for a 10 Hz input signal

Scan Rising Edge / Scan E

5.0 50 50

0.5 5 5

0.05 0.5 1

TimerIO() instruction measures frequencies of ≤ 1 kHz with higher frequency

resolution over short (sub-second) intervals. In contrast, sub-second frequency

measurement with PulseCount() produce measurements of lower resolution.

Consider a 1-kHz input. Table Frequency Resolution Comparison

(p. 319) lists

frequency resolution to be expected for a 1-kHz signal measured by TimerIO()

and PulseCount() at 0.5-s and 5.0-s scan intervals.

Increasing a measurement interval from 1 second to 10 seconds, either by

increasing the scan interval (when using PulseCount()) or by averaging (when

using PulseCount() or TimerIO()), improves the resulting frequency resolution

from 1 Hz to 0.1 Hz. Averaging can be accomplished by the Average(),

AvgRun(), and AvgSpa() instructions. Also, PulseCount() has the option of

entering a number greater than 1 in the POption parameter. Doing so enters an

averaging interval in milliseconds for a direct running average computation.

However, use caution when averaging, Averaging of any measurement reduces

the certainty that the result truly represents a real aspect of the phenomenon being

measured.

Table 71. Frequency Resolution Comparison

0.5 s Scan 5.0 s Scan

PulseCount(), POption=1 FR = 2 Hz FR = 0.2 Hz

TimerIO(), Function=2 FR = 0.0011 Hz FR = 0.00011 Hz



Q — When more than one pulse is in a scan interval, what does TimerIO() return

when configured to return a frequency? Does it average the measured periods and

compute the frequency from that (f = 1/T)? For example:

Scan(50,mSec,10,0)

TimerIO(WindSpd(),11111111,00022000,60,Sec)

A — In the background, a 32-bit timer counter is saved each time the signal

transitions as programmed (rising or falling). This counter is running at a fixed

high frequency. A count is also incremented for each transition. When the

TimerIO() instruction executes, it uses the difference of time between the edge

prior to the last execution and the edge prior to this execution as the time

difference. The number of transitions that occur between these two times divided

by the time difference gives the calculated frequency. For multiple edges

occurring between execution intervals, this calculation does assume that the

frequency is not varying over the execution interval. The calculation returns the

average regardless of how the signal is changing.

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