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Topic: detecting obstructions on the sea (Read 3 times) previous topic - next topic

arbarnhart

Nov 17, 2010, 02:04 pm Last Edit: Nov 17, 2010, 02:04 pm by arbarnhart Reason: 1
I have been looking at ultrasonic transducers like the ones here:
http://www.futurlec.com/Ultrasonic_Sensors.shtml
but even the waterproof ones typically have something like this in the datasheet:
Quote
... do not use this sensor in the following, or similar conditions.
a) In strong shock or vibration.
b) In high temperature and humidity for a long time.
c) In corrosive gases or sea breeze.


I am not looking for anything too sophisticated; navigation is by command and/or GPS. Obstruction detection is mostly a safety feature (try not to hit anything at full speed). I think a couple of wide cone with overlap in front (so if I detect something roughly the same decreasing distance away in both, it is likely in front of me) would work.



jackrae

#1
Nov 17, 2010, 02:13 pm Last Edit: Nov 17, 2010, 02:13 pm by John_Rae Reason: 1
Ultrasonic detectors such as you suggest will be totally useless in this application.  Firstly their range is very short - in the order of say 10 metres maximim.  On a pitching boat their ability to detect an obstruction in anything other than a dead flat calm will be questionable. And lastly, placing ones life (at full speed - your quote -) on a device that will not work is suicidal.
Radar is what you need !

jack

arbarnhart

#2
Nov 17, 2010, 02:24 pm Last Edit: Nov 17, 2010, 02:25 pm by arbarnhart Reason: 1
I have been programming the bot too much lately. By "me", I meant the unmanned USV. I am concerned mostly with close range. I figured I would have to correlate with pitch and yaw, which I have sensors for. The primary use for my device is almost guaranteed not to have an obstruction that isn't known to the GPS routing code. My main motivation to add this is so that I don't destroy a unit during testing if the route is wrong.

jackrae

Does USV = Submarine

If so, and if it's underwater obstructions you are concerned with,  then ultrasonic detectors are exactly what you need.
Howevre you need to get hold of the type used in echo sounders, which for obvious reasons are waterproof.  I believe they work at somewhat lower frequency than the "air" ones.  Somewhere in the order of 22kHz, I think.

jack

arbarnhart

#4
Nov 17, 2010, 02:41 pm Last Edit: Nov 17, 2010, 02:43 pm by arbarnhart Reason: 1
USV = Unamanned Surface Vehicle
UUV = Unamanned Underwater Vehicle

So "unmanned USV" was redundant and needlessly reiterative...

I didn't make these up, they seem to be pretty standard in robotics. It's a boat what drives itself...  :D

jackrae

#5
Nov 17, 2010, 02:46 pm Last Edit: Nov 17, 2010, 02:47 pm by John_Rae Reason: 1
OK back to my original statement.  Not suicidal as no person at risk but could be financially crippling as system will be unreliable, at best.

You could put a big rubber tyre round it to act as a fender

jack

arbarnhart

Do you have a specific recommendation (module or component to consider)?

Searching for radar and Arduino seems to lead me to topics that are technically really ultrasound. Parallax has a X-Band detector but it seems to be a motion detector.

jackrae

How about something along this line - excuse the pun

http://www.tycoelectronics.com/aboutus/news/prodinnov.aspx?id=976

jack

arbarnhart


jackrae

The first thing you need to do is define the size, shape and material of the obstructions that you are attempting to avoid.

Needless to say a "barn door" standing vertical on a raft will be much easier to detect than a small log partially submerged but still afloat on the water.

Once you have defined the obstruction specification you can then start looking at what you need to "see" it.

The Doppler effect radar detectors work on the principal of detecting the relative frequency shift between the transmitted signal and the reflected signal from the object.  If there is no relative movement, there is no frequency shift and hence no detection.  You therefore should be looking at a device that will sense actual reflected signal rather than frequency shift (Doppler)

jack

arbarnhart

Motion detection could work since my unit is moving. Anything "moving" toward me at the speed I am moving forward is likely a non (or slow) moving object in my path. Anything moving faster is a real problem. But are waves "moving" ?

MLuckham

#11
Nov 25, 2010, 07:51 pm Last Edit: Nov 25, 2010, 08:02 pm by mluckham Reason: 1
This sounds similar to a problem I've been thinking about - aircraft anti-collision methods.  In such a case you want to have as many seconds of reaction time as possible.  In your case fortunately the solution is 2 dimensional and restricted to a few degrees of angle off the bow (unless you are detecting torpedoes coming from any direction!) - not 3D as in aircraft.

Usually this is solved with radar or radio-frequency transponders, but I've been thinking "passive detection" - whether detecting motor ignition noise, audible or ultrasonic "wind noise", sonar-like devices, infrared heat detector, cameras looking for moving light or dark objects, etc.

So far the most promising is the camera - using Optical Flow analysis techniques.

In your case, I would imagine something like a forward-looking Fish Finder (sonar) would be the answer?  I have no idea whether it would work, for example would the "clutter" caused by the water-air interface (waves!) make it totally useless?

By the way, you don't mention the speed which your USV is moving.

http://en.wikipedia.org/wiki/Sonar and the related http://en.wikipedia.org/wiki/SODAR (which sounds useless in my application due to the size of the equipment!)

arbarnhart

#12
Nov 26, 2010, 08:53 am Last Edit: Nov 26, 2010, 08:55 am by arbarnhart Reason: 1
Speed will probably be 10-25 mph. Conditions could be sunny and clear or pitch black with rain and high winds. Bad weather probably increases the odds of device deployment.

zoomkat

Some time back somebody posted a project where they made a small bot boat that traveled about in a bay mapping the depth and using gps for surface location. You might find it using a forum search. The last set of conditions you posted makes your project a non starter unless you have a lot of $$$ or have a Navy contract. You probably need to buy an inexpensive "fish finder" and do some testing out on the water. Also, you can study the current commercial equipment available such as side scanning sonar units.
Consider the daffodil. And while you're doing that, I'll be over here, looking through your stuff.   8)

Korman

Your optimism is refreshing. Here's an article about what you try to do: http://www.theengineer.co.uk/news/epsrc-funds-usv-collision-avoidance-system/1004270.article

And we're not talking here about submerged items (like lost containers, floating fishing-nets, whales and uprooted buoys.

Good luck.

Korman

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