Campbell Scientific CR6 CR6 Measurement and Control System - Page 67
Vibrating-Wire Measurements - Overview
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Section 5. System Overview • Period Averaging - Overview (p. 66) • Period Averaging - Details (p. 346) The CR6 can measure the period of an analog signal. U terminals are configurable for period average. : • Voltage gain: 1, 2.5, 12.5, 64 • Maximum frequency: 200 kHz • Resolution: 130 ns divided by the programmed number of cycles to be measured for the average. Note Both pulse-count and period-average measurements are used to measure frequency output sensors. Yet pulse-count and period-average measurement methods are different. Pulse-count measurements use dedicated hardware - pulse count accumulators, which are always monitoring the input signal, even when the CR6 is between program scans. In contrast, period-average measurement instructions only monitor the input signal during a program scan. Consequently, pulse-count scans can usually be much less frequent than period-average scans. Pulse counters may be more susceptible to low-frequency noise because they are always "listening", whereas period averaging may filter the noise by reason of being "asleep" most of the time. Pulse-count measurements are not appropriate for sensors that are powered off between scans, whereas period-average measurements work well since they can be placed in the scan to execute only when the sensor is powered and transmitting the signal. Period-average measurements use a high-frequency digital clock to measure time differences between signal transitions, whereas pulse-count measurements simply accumulate the number of counts. As a result, period-average measurements offer much better frequency resolution per measurement interval, as compared to pulsecount measurements. The frequency resolution of pulse-count measurements can be improved by extending the measurement interval by increasing the scan interval and by averaging. For information on frequency resolution, see Frequency Resolution (p. 341). 5.1.5 Vibrating-Wire Measurements - Overview Reading List: • Vibrating-Wire Measurements - Specifications (p. 94) • Vibrating-Wire Measurements - Overview (p. 67) • Vibrating-Wire Measurements - Details (p. 347) Vibrating-wire sensors impart long term stability to many environmental and industrial measurement applications. The CR6 is equipped to measure these sensors either directly or through interface modules. A thermistor included in most sensors can be measured to compensate for temperature errors. Measuring the resonant frequency by means of period averaging is the classical technique, but Campbell Scientific has developed static and dynamic spectrumanalysis techniques that produces superior noise rejection, higher resolution, diagnostic data, and, in the case of dynamic spectral analysis, measurements up to 333.3 Hz. Dynamic measurements require addition of an interface module. 67