Page 15 - Shock and Vibration Overview

Data Acquisition (DAQ) System Selection

2) Sample rate
It’s good practice to sample at a rate 10 times greater than the upper interested
frequency range to accurately capture the vibration profile. For most shock and
vibration measurement applications a DAQ system will need a sample rate of at least a

few thousand hertz; but it all depends on what frequency range that your or customer is
concerned about. Take for an example an excerpt of vibration data recorded on a test
aircraft shown in Figure 5. The data sampled at 2,500 Hz is made up of many different
frequencies ranging from 50 to 600 Hz. Now if this same dataset is sampled only at 500
Hz (shown in the dashed red line), the vibration environment looks much different and
would be inaccurately represented.

Figure 5: An excerpt of vibration data was sampled at 2,500 Hz on an aircraft during a test flight.
When sampled at only 500 Hz the dataset becomes grossly misrepresented.

General guidelines on sample rate are over 10,000 Hz for shock testing, over 5,000 for
general vibration, and around 1,000 Hz for slower vibration or movement.

3) Resolution
Resolution is generally specified as bits which can then be used to calculate the
resolution in acceleration units. For example let’s say that an accelerometer system has
16
16-bit resolution; this means that it has 2 (65,536) acceleration levels or bins it can
measure. Figure 6 illustrates the importance of resolution on a simple 60 Hz sine wave
5
with two lines of different resolutions. 5-bit resolution provides 2 discrete acceleration
3
levels that can be detected while 5-bit resolution only provides 2 or 8 discrete levels.

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