Mixed Radio-Acoustic Link for Control/Communications

[subchaser]

Clearly it would give us much more freedom if the depth penetration problem of radio waves into water (particularly salt water) was overcome. One potential way is to have the control info carried on radio waves be converted to the same info carried on an acoustic wave within the water.

That envisions a radio to acoustic converter - say a radio receiver on a 'buoy' and an acoustic tramsmitter in the water. The signal is stripped off the radio wave by electronics within the 'buoy' and then used to modulate the acoustic carrier. An audio frequency between 50 and 80 kHz - give or take - would seem about best re attenuation and band width..

I am aware of minature ultrasonic gear which is used to track critters like crayfish. There the minature item is a transmitter whereas in our situation the minaturization is in the receiver on-board the sub, although an optional data link back to the surface requires a transmitter also.

For the distances we expect to operate over, there is no need for a high power acoustic link, which tanslates into a modest battery in the 'buoy'/acoustic transmitter.

In fact it is probably not even required to transmit and a hard wire link to the acoustic link could be from shore to just within the water body (pool, river, lake etc)

Any comments or info of existing capabilities/research projects that might be worth investigating?

[He Who Shall Not Be Named]

Clearly it would give us much more freedom if the depth penetration problem of radio waves into water (particularly salt water) was overcome. One potential way is to have the control info carried on radio waves be converted to the same info carried on an acoustic wave within the water.

That envisions a radio to acoustic converter - say a radio receiver on a 'buoy' and an acoustic tramsmitter in the water. The signal is stripped off the radio wave by electronics within the 'buoy' and then used to modulate the acoustic carrier. An audio frequency between 50 and 80 kHz - give or take - would seem about best re attenuation and band width..

I am aware of minature ultrasonic gear which is used to track critters like crayfish. There the minature item is a transmitter whereas in our situation the minaturization is in the receiver on-board the sub, although an optional data link back to the surface requires a transmitter also.

For the distances we expect to operate over, there is no need for a high power acoustic link, which tanslates into a modest battery in the 'buoy'/acoustic transmitter.

In fact it is probably not even required to transmit and a hard wire link to the acoustic link could be from shore to just within the water body (pool, river, lake etc)

Any comments or info of existing capabilities/research projects that might be worth investigating?

Here's the problem:


Using the high band-pass (amount of information able to be transmitted per unit of time) radio frequencies, we're able to drive servos that multiplex among themselves and each still able to be fed a discrete bit of information at a pretty quick 50 Hz sample rate.


Can you cram the same amount of modulated data into a sonic wave, near real time, like what we're getting from radio frequency equipment? Me thinks not.


If you go with the very long wave be prepared for very, very slow servo response.


Transmitting control signals modulated atop such sonic waves will either increase the time it takes to get the same information across, or you will have to substantially dumb down the information conveyed to the model (less channels and/or a much, much slower servo sample rate).


Pick one.


Anyway, that's what I'm led to understand from those well versed in the art of the, Magic Smoke.


Build your sonic data transmitter and prove me wrong. And the industry will beat a path to your door.


Balls in your corner.

David

[subchaser]

Thanks for the challenge. Will consult my guru. Back to you in a few days.

[He Who Shall Not Be Named]

Fair enough. Good luck with it.

David

[roedj]

Balls in your corner.
David

The importance of an apostrophe:

When I first read your response it may very good sense until I came to your last comment," Balls in your corner". Balls - scrotum - cahones - what?

Seconds later I realized you meant (I think) Ball's as in ,"The ball is...".

Normally, I just let these little grammar f'ups go by but I got a good laugh outta this one.

Sorry for the diversion - back to the thread in progress...

Dan

[Kazzer]

The importance of an apostrophe:

When I first read your response it may very good sense until I came to your last comment," Balls in your corner". Balls - scrotum - cahones - what?

Seconds later I realized you meant (I think) Ball's as in ,"The ball is...".

Normally, I just let these little grammar f'ups go by but I got a good laugh outta this one.

Sorry for the diversion - back to the thread in progress...

Dan

Aha! I spotted it too, but let it go. Normally Mr. M is an excellent writer, but like most, suffers from typo's occasionally.

My pet peeves are -
1. 'The exact same', meaning 'exactly the same' Boy, that really irritates me!
2. The words, your, you're, yore. No one gets them right.

You're on your way to old submarine movie of yore.

[He Who Shall Not Be Named]

Oh, for Christ's sake!

Me right good! You no count!

F both of ya!

David,

[Kazzer]

My pet peeves are -
1. 'The exact same', meaning 'exactly the same' Boy, that really irritates me!
2. The words, your, you're, yore. No one gets them right.

You're on your way to old submarine movie of yore.

These last points I made, weren't referring to His Eminence particularly, it's an American 'thing' generally.

[roedj]

Actually, most Americans don't know the difference between:

Their, there, and they're, and

to and too as well.

But enough of this .....

Dan

[He Who Shall Not Be Named]

LOL.

Guilty as charged.

David

[vital.spark]

Down here in Kiwi Land, some schools are allowing students to use texting english in exams!! You guy should know by now David is always right even if he's wrong. Same as my Hong Kong girl friend!!

[Kazzer]

Actually, most Amricans don't know the difference between:

Their, there, and they're, and

to and too as well.

But enough of this .....

Dan

Yes, yes, yes! I like this man - he should become a member of the Sub-driver grammar police - to keep all you bums on the straight and narrow!

[He Who Shall Not Be Named]

(great)........

[subchaser]

In response to Dave Merriman's challenge of operating an acoustic link to a model sub or ROV:

The answer is ----'It depends'

Here is the gist of some feedback I received from a friend with a background in electronics plus some of our shared ideas based on work we have done in building a display for my homebuilt aircraft..

His intent in answereing me was to 'Eyeball some numbers.' Not to be overly precise.

He has essentially broken his response, as I asked him, into 2 possible methods. The first is an acoustic copy of the existing radio procedure, and the second is a digital variation on that analog approach. Both of us would probably choose a third approach as outlined below.

A. Analog equivalent of current radio link

Assuming a conventional r/c servo setup, using the "standard" PCM data format.

Recognizing:

1 each servo requires a pulse between 1ms and 2ms to control it. Repeated every 20ms (50Hz)
2. with overhead for synchronization there's room for 8 channels, though 6 or 7 is typical
3. with a resolution of 100 "steps" per servo, timing resolution is 10us or 100khz

So you could directly modulate a 100kHz signal, with a fairly crude 100 steps per servo. This would have terrible noise immunity, ideally you would want to have a 400Khz carrier at minimum.

Conclusion: Dave is essentially correct. Just structuring a water link to mimic the radio approach based on realistic acoustic frequencies is not workable.

B. Setting up a digital link to achieve basic parameters of an analog system i.e Going Digital

Allow for 8 channels and 255 steps per servo. Use 9 bytes per frame same update rate (50Hz).
Note 9th byte is for synchronizing frames

So we have 9 * 50 = 450 bytes/s or 4050 bps which easily fits in a standard 4800 bps
serial link. Using ASK modulation at 16x the datarate gives us 76.8KHz which is within our technical grasp.

In addition if we reduce the number of channels or reduce the update rate a bit, we can add a CRC to the frame which would detect invalid commands and ignore them. For example at 45Hz instead of 50Hz there's room for 2 more bytes for a CRC without changing the data rate significantly.

Conclusion: A digital system can probably get us close to 50Hz, 8 channels of data, error checking with available acoustic transducers

C. Higher-level command system

If you have ever stood on a ship's bridge you have heard commands like "Half speed ahead', 'Left full rudder' "maintain a course 200 magnetic" To that we might add "maintain depth 10 feet" or "dive at 15 degees of elevator/plane .." - in other words, a group of actionable demands on the (for us submersed) vehicle.

The assumption is that, for example, "half speed" would cause the on-board speed controller to go to its 50% point (assuming it is linear). Most others would translate to some device taking in the command (as a coded chain) and setting say a voltage to position a servo or throw a valve (fully or partiially) etc. In some important cases the controlling computer needs data from sensors within the vehicle such as current direction (magnetic compass) or depth (pressure sensor).

The beauty of this procedure is that the data rate is very low. Let me throw out a sample command line

S50L100C20D10 (or perhaps the last command could call for diving E15)

S=steer L=left rudder C=coarse D = depth E=elevator etc

Probably we are talking about bytes/characters per seconds.

Some instructions could be issued more often than others and we benefit from the asynchronous nature of the control.

Conclusion: A command-based acoustic link is desireable and, in terms of the acoustics relatively straight-forward. However the electronic development and minaturization is not an easy matter.

And before someone raises a question like "How the hell do I know where the sub is to tell it to turn or steer a given coarse when I don't know where it is." I will answer that once we kick around the idea of a new kind of communications.

[He Who Shall Not Be Named]

SubChaser,

You've clearly made a serious effort to make a sonic means of r/c submarine control a viable proposition.

Yes, with the current data transmission stream method (in vogue since the early 60's), servo resolution (a device that attempts to achieve an infinite number of commanded positions within the better part of a circle) is 'good enough' and gives the illusion of having real-time response to operator commands. But, as you observed, it will not work well with the longer wave signal -- that wave now carried by (and not blocked) by the medium between transmitter and receiver.

If I'm reading your right, your solution is to abandon the traditional servos infinite position option in favor of a servo with a few, pre-determined positions?

I'll stop here. Want to be sure I'm on the same page with you.

Good discussion -- Eventually the 74, 27, 40, and other kHz bands will be lost to us and we'll need a viable means of controlling our submarines. Your idea may grow into the solution to a big, just-around-the-corner problem.

David