Underwater Navigation .. any ideas where to start?

[Chris_P]

I want to make some sort of navigation aid for me when I go diving. I use a compass at the moment, but it is pretty easy to get lost and worrying about this takes away from my enjoyment. I have 2 ideas so far and I am hoping some of you may have some suggestions on which would be the easiest to do and where to start with the design. I have some experience with programming AVR microcontrollers.

Idea 1/ Something that tracks the direction and distance I swim and will then calculate a return journey when needed. Problems to overcome with this would be that currents will effect distance and speed travelled. Maybe an accellerometer and a digital compass would be enough to work all this out?

Idea 2/ Some sort of beacon system. Place a beacon where the dive starts and then have a handheld device that will point to the beacon and display distance. I like this idea, but how I would do this? I am not sure.

Any ideas, thoughts would be greatly appreciated.

 

[hjames]

Accelerometers have pretty lousy drift, and you'd have to figure out which way is down in order to use the information from the compass (Although I vaguely remember one style of compass that compensated for non-level measurements). It's probably worth a shot.

As for beacons, googling turned up this page:
https://www.qsl.net/vk5br/UwaterComms.htm
with some radio/water numbers. Not terribly encouraging, but bands in the 130KHz-180KHz would be the best bet.

Acoustic might be an option, but I have no idea what sort of powers are involved. The easiest way of implementing this might be to have 3 transmitting beacons that are wired to a circuit that ping at regular intervals, you can use the difference in times to triangulate your position. Each pair of beacons gives you a surface to intersect-> 3 surfaces intersects to a point... I'd guess that the accuracy will get increasingly lousy depending on the angular seperation (the further apart the beacons are, the better), and the timing/receive jitter.

Oh, and obstacles - all bets are off when dealing with obstacles...

 

[Hero999]

Ultrasonic beacons are probably the best option. Radio doesn't travel far underwater and the low frequency you'll have to use makes direction finding pretty impossible.

Blackboxes on airoplanes use ulrasonic beacons that operate on 32768Hz, you'll probably want to use a different frequency to avoid confussion. I don't know how direction finding would work though, maybe a rotating transducer?

 

[ericgibbs]

hi Chris,

Sound passes thru fibre glass hulls quiet well. A transducer internally mounted in the base of the hull will perform well.

Velocity of sound in sea water is approx 1480 mtr/sec. It varies with water temperature and salinity.

If you decide to go the ultrasonic route, resolution is better around 150/200kHz but the depth range falls off fairly quickly, for deeper depths, with lower resolution, around 30kHz is OK.

Pulsed power levels down to 150mtrs depth, require about 100Watts at around 200kHz.
At around 30kHz a depth of 500mtr will require about 500/750 Watts.

These depths and power levels are based on the requirement to detect a reflected depth echo back at the transducer.

Standard echo sounder transducers are designed with acoustic beamwidths of 2-3deg thru 20deg, depending on frequency.
EricG

 

[Hero999]

These depths and power levels are based on the requirement to detect a reflected depth echo back at the transducer.

If those are the requirements for sonar then I'd expect a far greater range for just searching for a beakon.

It doesn't need to be a continuous wave, you'll probably get better efficiency from pulses anyway.

I've also heard that the human hearing range extends to 200kHz at high power levels under water because the middle ear which is responsible for the rolloff is bypassed, so you might even be able to hear it.

I understand that lower frequencies travel further but why is depth such a problem?

 

[ericgibbs]

hi chris, hero

The attenuation of the transmitted signal and its return is about the fourth power. [ I'm talking at this time about depth echo sounding]

As you say, the detection 'range' will be greater from the transducer to the diver.
I just didn't want to guess what that range maybe and possibly mislead Chris.

With the echosounding work I have done I can't say I have ever heard the actual sonic pulse while underwater.
In air you can hear a sharp click from the transducer as its pulsed.

The 200kHz sensors are pulse to transmit for about 200uSec, the 30khZ sensors about 1 to 2mSec.
The repetition rate depends on the depth limit expected, 10pps for 70mtr and 1pps for 700mtr.
[these are for continental shelf depths]

The main problem is that most transducers have a fixed beam width, the 200kHz freq, are around 2 to 8 deg, so it would be quite directional from the boat. So a rotational sensor system, with some azimuth oscillation, maybe needed to cover the 360deg hemisphere around the dive boat.

If I was Chris, I would as a trial, firmly mount a 50W, waterproof speaker to the inside base of the ships hull.[keep it dry of couse].
Drive it with a 800hZ signal, pulse ON for 1 sec OFF for 5 sec.

As the velocity of sound thru sea water is about 4.5 times faster than in air, he 'might' be to able to hear a 3600hZ tone.
If he can hear the 'ping', he could with a few trials determine whether the idea is feasible.

It sounds an interesting project.

Eric

 

[Hero999]

With the echosounding work I have done I can't say I have ever heard the actual sonic pulse while underwater.
In air you can hear a sharp click from the transducer as its pulsed.

It was just something I read on Wikipedia.
https://en.wikipedia.org/wiki/Ultrasonic_hearing
https://en.wikipedia.org/wiki/Ultrasound

 

[hjames]

Would fish-finder transducers be the cheapest way of getting an ultrasonic transducer? I see a couple for around US$100 or so.

Also, are there any sources for the relatively small sensor that the diver would actually carry? I'm wondering if a standard piezo disk/speaker would be usable for the sensor.

 

[Roff]

If I was Chris, I would as a trial, firmly mount a 50W, waterproof speaker to the inside base of the ships hull.[keep it dry of couse].
Drive it with a 800hZ signal, pulse ON for 1 sec OFF for 5 sec.

As the velocity of sound thru sea water is about 4.5 times faster than in air, he 'might' be to able to hear a 3600hZ tone.
If he can hear the 'ping', he could with a few trials determine whether the idea is feasible.

Eric

Eric, change in propagation velocity doesn't translate to change in pitch (I think that's what you are implying).

 

[Hero999]

I'm wondering if a standard piezo disk/speaker would be usable for the sensor.

Disc piezo speakers have a sharp resonant peak at around 30kHz in air but I don't know what it'll be in water.

 

[Chris_P]

Thanks for all the thoughts so far, so ultrasound sounds like the way to go. Just a thought, if the handheld receiver had 3 sensors spaced evenly around it, even though they will be very close together would a microcontroller be fast enough to distinguish which sensor received the ping first, and from that work out a direction?

 

[hjames]

Speed of sound in water seems to be ~1500M/sec, so a 10cm seperation would be a ~70uS difference, if the source is aligned with the sensor axis. In order to get a 30 degree resolution, you'd need 70uS * (1-cos(30deg)) = ~9 usec.

[For a 200KHz signal (at the 10cm spacing), this is a phase difference of 600 degrees (and 5000 degrees in the best case). Tweak the spacing between the receiver elements and the pulse frequency so that the phase difference never exceeds a wave.] - this probably isn't when I don't completely screw up the numbers

If the signal is strong enough, and you use the InputCaptureModule (ICP) with some simple logic - an XOR gate fed with a pair of mux's so you can select which channels you want to monitor, it should work.

A resonant tank circuit on each channel - tuned to the pulse frequency (200KHz in this case) would probably be a good idea as well.


[As RonH points out, for some reason I was thinking in mm. Numbers modified... Thing is that a solution that depends on the phase of the incoming signal would be pretty tolerant of differing signal strengths, but with these numbers, an envelope/AM detector is probably good enough]

 

[Roff]

Speed of sound in water seems to be ~1500M/sec, so a 10cm seperation (just to make the math easier) would be a ~7uS difference, if the source is aligned with the sensor axis. In order to get a 30 degree resolution, you'd need 7uS * (1-cos(30deg)) = ~900 nsec.

For a 200KHz signal (at the 10cm spacing), this is a phase difference of 60 degrees (and 500 degrees in the best case). Tweak the spacing between the receiver elements and the pulse frequency so that the phase difference never exceeds a wave.

If the signal is strong enough, and you use the InputCaptureModule (ICP) with some simple logic - an XOR gate fed with a pair of mux's so you can select which channels you want to monitor, it should work.

A resonant tank circuit on each channel - tuned to the pulse frequency (200KHz in this case) would probably be a good idea as well.

H, did you you slip a decimal point? 1sec/1500m=667usec. 10cm=0.1m -> 66.7usec. Take it from there. It makes everything 10 times easier (or higher resolution).

 

[Chris_P]

Thanks for all your help. I have a lot of experimenting to do now. At least I know its possible.

 

[ericgibbs]

Hi RonH.
(I think that's what you are implying).

You are correct, all the marine work I have done has been with sea water depth measurement.
Never had a request to develop a product for point to point under under water.

Pity!, I would have enjoyed doing it.

I hope Chris posts his underwater comms results.

Regards
EricG

 

[Chris_P]

This one is a long term project for me. Just at the thinking stage at the moment. I will definitely post my results as I go.

 

[Chris_P]

I am really getting down to basics here, my knowledge and understanding of electronics is very poor. So I am going to start right at the beginning and get some books on electronics and learn as much as I can so I can begin to understand what I need to do. I was wondering if someone can just give a bit of tip that will help point me in the right direction. Apart from the frequency and requiring a transducer instead of an aerial, would the transmitter circuit for ultrasound be any different to a transmitter for radio waves? Will learning about transmitters and receivers be a good place to start?

 

[hjames]

I am really getting down to basics here, my knowledge and understanding of electronics is very poor. So I am going to start right at the beginning and get some books on electronics and learn as much as I can so I can begin to understand what I need to do. I was wondering if someone can just give a bit of tip that will help point me in the right direction. Apart from the frequency and requiring a transducer instead of an aerial, would the transmitter circuit for ultrasound be any different to a transmitter for radio waves? Will learning about transmitters and receivers be a good place to start?

There's a couple random sonar-ish posts on this site, and I think some people here have worked with underwater sonar stuff.
https://www.electro-tech-online.com/threads/depth-finder.21799/?highlight=sonar

The transducer that dknugyen posted a link to has something that somewhat resembles a data sheet attached to it.

Googling came across this:
http://www.psubs.org/convention/2004/presentations/garyBoucher/Sonar_Presentation.ppt

After you go through the background info, there's some schematics in there. Note that the only interesting part is the power transmitter and the receiver circuits. The transmitter is a big pull-pull amplifier. Note that (for the 50W ultrasonic transducer) impedance is on the order of 1KOhm - I'll let you work out what sort of voltage needs to be going through that part in order to push 50W of power.

As for the receiver, you have
1) amplifier/buffer with input clamp/protection circuit
2) variable gain amplifier (check DAC1021 data sheet app note)
3) filter and buffer
4) another variable gain amplifier
5) another filter and buffer
6) peak detect/diode detector/averaging and buffer.

I would guess that the two VGA modules are used to compensate for distance attenuation - signals that are further away are weaker, and the gain gets boosted up as time passes.

 

[Chris_P]

Thanks hjames, that is some excellent information. I haven't had a chance to look fully through the ppt but it looks like it has just about all of the information I need.

Maybe this is not the way to go, but I am thinking of starting by using audible sound so I can use standard speakers and microphones to see if I can get the basic principle working. Then I will look at ultrasonics after that. But I'll certainly have a good read of that ppt before I start anything. Thanks again.

 

[Chris_P]

Googling came across this:
http://www.psubs.org/convention/2004/presentations/garyBoucher/Sonar_Presentation.ppt

After you go through the background info, there's some schematics in there. Note that the only interesting part is the power transmitter and the receiver circuits. The transmitter is a big pull-pull amplifier. Note that (for the 50W ultrasonic transducer) impedance is on the order of 1KOhm - I'll let you work out what sort of voltage needs to be going through that part in order to push 50W of power.

Me again, more stupid questions. I am looking at this diagram at the moment. I am guessing this it the circuit your refered to as the power transmitter? **broken link removed**
What I am wondering is that it is using a 192KHz ping, will this circuit still work with a lower frequency? I am also a bit confused by the output, does the output through C4 go straight to the transducer? In this circuit it is driving 3 different transducers I think, hence the relays etc. but I am not sure what TXP-AMP is all about? And what sort of transformer is T1?