Campbell Scientific CR3000 CR3000 Micrologger - Page 325
Pulse Measurement Problems
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Section 8. Operation Table 74. Frequency Resolution Comparison 0.5 s Scan PulseCount(), POption=1 FR = 2 Hz TimerIO(), Function=2 FR = 0.0011 Hz 5.0 s Scan FR = 0.2 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. 8.1.5.4 Pulse Measurement Problems 8.1.5.4.1 Pay Attention to Specifications The table Example of Differing Specifications for Pulse Input Channels (p. 325) compares specifications for pulse-input channels to emphasize the need for matching the proper device to application. Take time to understand signals to be measured and compatible channels. Table 75. Example of Differing Specifications for Pulse-Input Channels Pulse Channels P1, P2 Digital I/O Channels C1, C2, C3, C4, C5, C6, C7, C8 High Frequency Max 250 kHz 400 kHz Max Input Voltage 20 Vdc 16 Vdc State Transition Thresholds Count upon transition from 2.2 Vdc Count upon transition from 3.8 Vdc 8.1.5.4.2 Input Filters and Signal Attenuation Pulse-input channels are equipped with input filters to reduce spurious noise that can cause false counts. The higher the time constant (τ) of the filter, the tighter the filter. Table Time Constants (p. 326) lists τ values for pulse-input channels. So, while TimerIO() frequency measurement may be superior for clean signals, a pulse channel filter (much higher τ) may be required to get a measurement on a dirty signal. 325