The scan rate, or the sampling rate in NI-DAQmx, determines how often an analog-to-digital (A/D) conversion takes place. A fast input sampling rate acquires more points in a given time and can form a more accurate representation of the original signal than a slow input sampling rate.
The basic equation for sampling rate is as follows:
Fs ≥ 2.56 * fmax
Fs = sample rate in hertz
fmax = maximum frequency of interest in hertz
In rotating machinery analysis, maximum rotational speed to analyze and maximum order to analyze are the two key parameters for calculating the sampling rate. For sound and vibration signal acquisition, choose the sampling rate according to the following equation:
sampling rate sound and vibration ≥ 2.56 * max order * max speed (RPM)/60
When you use an analog input channel to acquire a tachometer signal, set the sampling rate to a faster rate. When you perform run-up or run-down tests, the measurement results are highly dependent on the accuracy of the tachometer pulse measurement. You typically want to select a tachometer signal sampling rate at least four times faster than the sound and vibration signal sampling rate. Use the following equation to calculate the tachometer signal sampling rate:
sampling rate tacho ≥ 4 * 2.56 * max order * max speed (RPM)/60
|Note When the number of pulses the tachometer generates in each revolution is very large, the required tachometer sampling rate might be faster than the one you calculate from the above equation.|
When measuring high orders that run at a fast speed, you must set the sampling rate to a very high value. The synchronized analog input channels for tachometer signals and sound and vibration signals usually work at the same sampling rate. The fast sampling rate for the tachometer signal leads to unnecessary processing for the sound and vibration signals that need a lower sampling rate. Some DAQ devices might not be able to set the sampling rate to the required value. In this case, you can use a counter device to acquire the tachometer signal and keep the sound and vibration signal sampling rate at an appropriate value. When you use a counter device synchronized with a DAQ device, the counter can acquire the tachometer signal at a much faster rate than the DAQ acquisition rate. Using this combination of devices can generate more accurate measurement results.