I try to analyze a system's state with acceleration sensors by measuring the frequencies and amplitudes.
Since a cheap sensor is used, I have a noisy signal in the beginning (up to 40Hz - see below). That is the first thing that I don't get. With another sensor (much higher resolution but also different position) this noise couldn't be observed.
- Do you have any idea where this might come from?
Due to the noise and to reduce uncertainty, I perform multiple measurements which I intend to average afterwards. The second question would, therefore, be:
Where do I best average the measurements in order to obtain the "correct" amplitude/frequency? I have the following ideas:
- average the signals and perform a single fft. The maxima result from the maximum of the transformed signal
- average the signals after the transform and take the maxima of the resulting signal
- average only the maximum/frequency of the single transforms of each measurement
Furthermore, I observe a sort of frequency shift within the measurements (the left vertical line is the excitation frequency, while the right one is the response of the averaged fourier transoforms). Here, I excite the system with 183 Hz and expect the maximum at this very frequency (or in a really small bound around it). Instead, the responses occur to be around 189 Hz and sometimes even more. Due to my lack of experience I tried to conduct some tests to "check" if my algorithm works correctly:
I generated a sine wave and added different offsets and noises. The frequencies would be met with an accuracy of 0.15 Hz (see below). Therefore, I guess it's not the algorithm.
- Do you have any experience with this procedure and how much shift in frequency is normal?
Do you have any suggestions for further (pre-) processing of the signal? I was thinking about a highpass-filter to get rid of the noise in the beginning. Also, I think it might be interesting to apply a bandpassfilter only around the "bound" and analyze the signal in the time-domain - what do you think about that?
I am curious to find out if it's possible to observe the switch of a valve position. Since this is a transient behaviour I am wondering which method to apply. I'd try a PSD and to cut the evaluation of the FFT into pieces and perform it for smaller periods. Do you have any ideas?