Campbell Scientific CR1000KD CR800 and CR850 Measurement and Control Systems - Page 305
Pulse Measurement Problems
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Section 8. Operation 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. 305) 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 67. 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. 305) 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. Input filters, however, attenuate the amplitude (voltage) of the signal. The amount of attenuation is a function of the frequency passing through the filter. Higher-frequency signals are attenuated more. If a signal is attenuated enough, it may not pass the state transition thresholds required by the detection device (listed in table Pulse-Input Channels and Measurements (p. 39) ). To avoid over attenuation, sensor output voltage must be increased at higher frequencies. As an example, table Filter Attenuation of Frequency Signals (p. 306) lists low-level ac frequencies and the voltages required to overcome filter attenuation. For pulse-input channels P1 - P2, an RC input filter with an approximate 1-μs time constant precedes the inverting CMOS input buffer. The resulting amplitude reduction is illustrated in figure Amplitude Reduction of Pulse-Count Waveform (p. 306). For a 0- to 5-Vdc square wave applied to a pulse channel, the maximum frequency that can be counted in high-frequency mode is approximately 250 kHz. Table 68. Time Constants (τ) Measurement τ Pulse channel, high-frequency mode 1.2 Pulse channel, switch-closure mode 3300 305