I am trying to measure the vibrations of a lathe machine using a piezoelectric accelerometer sensor. I want to know if the sampling rate can be set at a value greater than the flat response frequency range of the sensor ?
For example, the datasheet of the sensor I am considering to use says that based on the mounting method (stud, adhesive and magnetic) the frequency response changes. If I decide to use adhesive, the mounting resonance is lower (45KHz) than when a stud(80KHz) is used and the frequency range over which the gain is flat also reduces from 0.002~25kHz in case of stud mounting to 0.002~10kHz in the case of adhesive. In the adhesive mounting the, frequency range increases by 1 dB at 20 kHz to reach its peak at 45 kHz and then falls. Suppose that I want to measure the sensor output at 25 kHz sampling range but the condition of my machine only allows for adhesive type mounting, do I have no choice but reduce the sampling rate ?
Best Answer
You need to sample at a (theoretical!) minimum of double the highest frequency you want to measure (Nyquist) and if there are components above half the sample rate they will be aliased down to lower frequencies.
So you would be best to sample much higher than that and precede the ADC with an analog anti-aliasing filter. For example you could sample a 200ksps or 1Msps. The closer you get to the theoretical minimum sample rate, the more difficult the analog filter become to realize practically.
Once you have the signal in the digital domain and free of aliasing you can apply digital techniques to decimate the signal if you want to reduce the amount of data generated, to flatten the response etc.