Using accelerometer to detect moving/stationary at sea on a vessel

Facelook

Hi,

Does anyone has experience using accelerometer on a ship/vessel to detect whether the vessel is moving or stationary?

The difficult part is that I took some samples at 6Hz and found that all X/Y/Z axes have accelerations going positive and negative. The pattern is exactly the same when the vessel is moving and stationary. I suppose these are caused by ocean waves.

The reason that I have to use an accelerometer to detect movements on a vessel is that my device (a custom embedded system) goes into sleep/hibernation most of the time to save power and it only wakes up to perform some tasks after it detected movements. GPS consumes too much power and therefore it can't be powered on all the time.

Actually I do not need to measure the speed, direction or position, all I need is to identify whether the ship/vessel is moving or stationary at sea. Accelerometer works fine on land, but not at sea probably due to the random ocean waves.

I have also considered using a gyroscope, however, it is too expensive.

Any suggestions?

Waterfox

The accelerometer will only measure acceleration, it cannot tell the difference between being stationary with respect to the Earth or moving at a constant velocity. You can integrate the acceleration data to obtain velocity information, but it will likely become inaccurate over time.

Facelook

Thanks.

I suppose when the ship/vessel is moving at sea, it must accelerate and decelerate slightly and continually. I am trying to spot the difference between moving and stationary.

Compare to a car moving on land, I can tell it is moving simply looking x, y and z accelerations going positive and negative due to bumpy roads.

Unfortunately, I cannot see any difference between a ship/vessel moving or stationary.

haruspex

I'd say you've no chance. How about listening for the ship's engines?

Facelook

I tried to do that by using Fourier Transform to look at the frequency domain. Unfortunately, no significant signals are found above 1Hz (typical vessel engine is around 100rpm), most signals are found under 1Hz.

It looks to me the accelerometer was unable to pick up the engine vibration.

A.T.

Well, why do you think Galiloeo used a ship to explain Galilean invariance?
http://en.wikipedia.org/wiki/Galileo%27s_ship

But in a less idealized case, maybe the directions and/or frequencies of the wave induced oscillations are different?

haruspex

OK, I wasn't necessarily thinking of an accelerometer as the detector of the sound, but I don't know what the alternatives are. OTOH, I imagine it would be important how the accelerometer is mounted. Need a diaphragm that would resonate at about the right frequency.

Facelook

Thanks.

By the way, given the continuous changing accelerations in the 3 axes at sea, does that mean many accelerometer applications may not work well at sea on a vessel? For example, detecting free fall or counting steps.

haruspex

Applications which depend on a threshold magnitude of acceleration might still work.