I'm facing a challenge that requires me to process a signal from an engine knocking sensor. The acquisition is done synchronously with the engine speed (every 0.1 degree) which gives me a range from 50KSa/s to 390KSa/s (related to engine speed range). The sensor bandwidth is 20KHz. My task is to convert this data to a fixed sampling of 200KSa/s for further processing in MATLAB. I'm working in this issue for some weeks now but I couldn't find any solution to it! Is there a common approach to such task?
[WARNING, RUDE OPINION AHEAD] Engine people often record data in a speed-invariant fashion using angular sampling. I have been working in that domain for a while, and I bear with you. I have been testing the performance of $0.1$ CA sensors, most of them doing some weird and undocumented interpolation. I also have tested $6$ CA sensors, at the other end of the camshaft.
My opinion so far is that they don't match well, for different reasons, including the acyclic behavior of an engine:
- some angular devices are crappy,
- the analog filtering (if any) in the acquisition chain is not speed-dependent,
the speed cannot be assumed constant within a cycle, and even within a stroke.
And the measurements can be quite uneven at both ends of a camshaft, indicating that in between, at each cylinder, depending on the engine organisation, you might have various effects. This is all the more detrimental with noisy knock sensors : their spec sheets are often imprecise, their calibration is often neglected, etc.
I have concluded that in a standard speed range (1000-4500 RPM), a precision below 0.2 CA is (often) meaningless. And at high speeds, 0.5 CA is doubtful.
- if you only have 0.1 CA data, do the simplest interpolation that copes with the processing you need. A simple LS interpolation (linear, splines) could do the work
- if you have access to the angular timing for raw files, get rid of the spikes, and use it directly have a non-uniform sampling (but don't forget you only see it from one end of the shaft)
- if you have data models, plug them into some super-resolution tool. -if none of the above, well...
I'd suggest a track anyway, that I did not push to the end (so be careful): try to bound or to "probabilize" uncertainties: speed variation, quantization, filtering, etc. From the given measure, make a fuzzy measurement, ie a simulated multivariate object, that will go through further processing, which can give you a rough approximation of uncertainties pertaining to subsequent processing.