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 Post subject: Scanning underwater
PostPosted: Sep 21st, 2017, 3:18 am 
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Joined: Sep 25th, 2015, 3:24 am
Posts: 3
I am looking for an underwater scanner for home construction, preferably with an arduino and laptop, in our sailing club, things fall into t'water (such as windvans, boat sword, personal things like keys), and I would like to find this
Can you homemade , build a transducer?
Camera is difficult because you are restricted to the surface that you can view
I have some HC-SR04 but these are not waterproof, would it work with these sensors ?????

Kind regards


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 Post subject: Re: Scanning underwater
PostPosted: Nov 23rd, 2017, 1:24 pm 
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Joined: Nov 18th, 2017, 7:18 pm
Posts: 8
HC-SR04 work well in air and are dirt cheap!
I use these a lot for range-finding up to about 4M with resolution of a few mm. Fitted to a servo they make a very cool sweeping sonar for robotics (reducing the need for several devices). This does NOT give a view of the swept area, the device must stop, send a pulse, wait for a set time to allow a return along a narrow beam for a preset maximum expected distance, move/repeat, but looks constant to us!
They are also used on drones for fine, low-level height control/landing and or obstacle avoidance (I don't do this). Allows the pilot to (say) move around at 2m without regard for the topography below the drone (steps, hills etc.)

Apart from the sensor, the electronics would need "potting" to make it water-proof.
You can find solid (waterproof) 40Khz sensors to replace those on the board at about 5x the cost of a complete board (I'd probably not extend the wiring). I've not tried this BTW. I would suspect that the transducers are used in a resonant circuit, so you would need a closely matched replacement (I don't know this for sure), and you would probably need a matched pair to achieve any kind of range.

However! 40Khz is not best suited to working in water and I suspect that if it works, the range would be very short due to the damping effect of the more viscous medium using such a weak resonator.
Placing them in a container with an air gap to the water will not work as the water surface will reflect the signal (they can be used as non-contact liquid height sensors!)

You will note if you research marine "pingers" and sonar that they use a huge amount of power and are either in direct contact with the water or use an oil bath etc. to give them good mechanical connection to the water.

I suspect that a cheap 'Fish Finder' (link) might be a better option, and require far less R&D!

Reading 'shapes' will require a high frequency ping and/or an array of sensors, not impossible, but we are talking nation state development :D
Consider the size of your object, say a bunch of keys randomly falling, occupying a rectangle (with fuzzy edges) 100x100x25mm high (and partly submerged in mud) to see them AT ALL, you need to resolve a height difference of 25mm, preferably half of this over an area less than the area of the bunch! So a height resolution of 12mm would identify "something", whether it's a fish, rock, bit of wood or keys is another matter! And the area of the ping would be problematic (height dependent).
I would suggest that you try to resolve keys from something else on a carpet to get an idea of your problem.
Blade Runner etc. have a lot to answer for when it comes to peoples expectations of what is practicable! (blade runner photo analysis link)
You may find cameras or LIDAR (IR to cut through murk?) to be better sensors for the task you describe. A magnetometer would also work for identifying metallic objects and would be a simple, cheap Arduino project.

I hope that provides food for thought
Cheers
A


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 Post subject: Re: Scanning underwater
PostPosted: Nov 23rd, 2017, 5:17 pm 
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Joined: Sep 25th, 2015, 3:24 am
Posts: 3
Thanks for the good explanation
For the moment I used an analog camera such as a webcam, put it into a housing and filled everything with tex7 to make it watertight (the lens is not Hi) Then via an interface to the computer

mvg
Erwin


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 Post subject: Re: Scanning underwater
PostPosted: Dec 2nd, 2017, 1:02 pm 
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Joined: Nov 18th, 2017, 7:18 pm
Posts: 8
Sounds like a good plan. K.I.S! :D
I bet a potted IP camera would give great results.
I've stuck a go-pro on the end of a long rod before now, and the results are amazing, even in cloudy water because you can get up close to your subject.


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