Signal Coding Choices for Vibration Immunity
A vehicle deck vibrates. A sensor that filters vibration too aggressively loses early-stage signal. The middle ground is where the work is.
IR sensor cells on a vehicle deck see two kinds of motion: the slow drift of ambient temperature and the fast modulation of vessel vibration. A naive low-pass filter handles the second at the cost of latency on the first. We treated it as a band-isolation problem instead.
What the bands look like
- 0.05–2 Hz: vessel motion-induced thermal modulation (slow roll, pitch).
- 2–25 Hz: engine and propulsion-induced vibration.
- > 25 Hz: sensor noise floor and electrical pickup.
- Sub-0.01 Hz: the thermal signature of interest.
The filter chain
A cascaded notch at the dominant vibration band of the vessel class, followed by an EWMA at the cell level, followed by the coherence stage at the segment master. Each stage targets a specific band and does not have to cope with the others.
Questions, answered
Why not just low-pass filter vibration out of the IR signal?+
Because a naive low-pass filter rejects fast vibration at the cost of latency on the slow thermal signal of interest. The cells see vessel motion (0.05–2 Hz), engine/propulsion vibration (2–25 Hz), and noise above 25 Hz, while the thermal signature sits below 0.01 Hz. It is treated as a band-isolation problem, not a single filter.
How does the filter chain isolate the thermal signal?+
A cascaded notch at the vessel class's dominant vibration band, followed by a cell-level EWMA, followed by the coherence stage at the segment master. Each stage targets a specific band and doesn't have to cope with the others, achieving over 40 dB of vibration-band attenuation.
Does filtering vibration delay the thermal signal?+
Minimally. The chain holds phase delay on the thermal signal of interest under 50 ms while delivering more than 40 dB of vibration-band attenuation, with notch centres tuned per vessel class so the filtering matches each ship's dominant vibration signature.
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