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Author Topic: Tracking landspeed underwater?  (Read 1388 times)

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Offline OllieRobertsTopic starter

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Tracking landspeed underwater?
« on: January 21, 2015, 05:10:15 PM »
Hi People, I am hoping someone might be able to nudge me in the right direction (apologies for the long post but wanted to get all the information I have gained so far down.

Basically I am after a solution to record the path my vessel took under water for later analysis…like a bread crumb trail.

 
Requirements:

Ideally have a range of at least 30meters however if there were no other options I would accept down to 10meters.

Working fresh and salt water.

The vessel is (25cm x 8cm) so size and power consumption are a factors.

It would be traveling roughly parallel to the sea bed at variable distances from the sea bed (range of 0-30 meters)

Does not need to be super accurate, anything less than 5 meters would be fine.

Measurement speed range of 0 – 4 mph.

Measure direction the object was moving in (i.e. forwards, sideways, backwards)…I am planning to use a compass to ascertain N, S, E, W heading.



Options I have discounted:

Accelerometers:
http://www.robotshop.com/en/10-dof-16g-triple-axis-accelerometer-gyro-barometric-sensor.html

This was my initial thinking but in doing some reading it seems they are not suited to my needs (unless you spend loads of money, and then the solution would end up being too heavy anyway).


Optical Flow:
https://pixhawk.org/modules/px4flow

Looks too new (from a consumer perspective) / complicated. I don’t know what its range would be like. Also requires additional sonar sensor.


Current favorites:

Sonar:
http://www.alibaba.com/product-detail/1mhz-waterproof-transducer-underwater-ultrasonic-sensor_1911722468.html

Simplest use is distance from object, however can use doppler effect to analyse speed of a moving object.

40m range, nice!

Presumptions:
 
If I fired this at an angle to the seabed I could deduce the speed the floor was ‘moving’ below which would give me the speed of my vessel?

I am also presuming that I could interpret direction of movement from the data?

I presume that the sensor would need to be aimed at an angle of around 45 degrees down to the seabed?


Laser Rangefinder:
http://www.parallax.com/product/28043

Although it works differently to the Sonar the premise of use looks the same, and thus I have the same queries with this as I do with the Sonar above.


Presume if I mounted the sensor behind high quality glass (to waterproof it) then performance would not be impacted that much.

This is a lot more costly so if it does not give me any advantage over sonar I guess there is no point.


Water Flow Meter:
http://www.robotshop.com/en/adafruit-water-flow-sensor.html

Super low cost and simple compared with the other options, I would potentially use a funnel to increase water pressure if it needs more sensitivity at low speed.

Would then just need to calibrate the pulses from the sensor to a speed reading.

Significant limitations of this is it would be registering a speed of 0 if the vessel was simply drifting with the current….its speed over the seabed that I am interested in.





Current favorite option is sonar (with the option of using water flow meter as second data source)…however are my sonar presumptions correct, have I missed anything?

Any better ideas?

Offline mklrobo

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Re: Tracking landspeed underwater?
« Reply #1 on: January 23, 2015, 07:08:25 AM »
 :D Cool Project!
I had a opportunity to do something similar with a university, but did not get a
chance to finish it.
The problem is location. I would advise this in the pursudo-part of the project;
(Assuming the distance is 30 meters, total)
Create marker bouys to transmit telemetry information to your underwater robot.(UR)
The UR could use information from the bouys to triangulate its position, as well as depth.
any sensors on the UR to detect the bottom, or other enviromental factors, is in addition
to other computation that would be needed. Good Luck!

 


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