Inertia based underwater positioning system

[Miyaru]

Why not ultrasound?

I looked at the sensors that are used by fish finders and depth sonar, but they're expensive, require a lot of power and the technology is very old (no I2C versions).

 

[high_order1]

The fun is in DIY.

Exactly. You can buy just about anything if the check is long enough. Making something come to life, especially something said not possible... (chef's kiss)

I looked at the sensors that are used by fish finders and depth sonar, but they're expensive, require a lot of power and the technology is very old (no I2C versions).

IP68 Underwater Ultrasonic Obstacle Avoidance Sensor (3m, UART)

Garmin LIDAR-Lite Optical Distance Sensor - V3

I found a couple of I2C versions, but you would have to probably enclose them and use a mica (or similar) window for them.

 

[lowwall]

Would have thought that the speed of a diver relative to the speed of the water would make the measurements very difficult.

For example, jump in at the end of slack water and the tide reverses. If the diver's heading in a constant direction (whilst wagging around, pulsing on each fin stroke, etc.), and the tide slowly changes over 10 minutes or more in the opposite direction, then the inertial system has to be exceedingly sensitive and accurate, not to mention very high resolution to integrate all the propulsion "pulses" and subtle movements of the diver during the dive. The accuracy would very quickly degrade as that 'noise' happens.

The signal to noise ratio will eventually make your results unusable without external correction. For a device without correction, it's just a question of accelerometer capability versus time.

 

[TooCold]

Lots of interesting discussion since I last looked! Almost makes me want to move this sort of project higher in my DIY todo list, but probably never happen unless I move to warm clear waters where I could test (and use...) it regularly. Anyone got a place in the Carribbean they'd let me use for 5 months?

So my first thought on it would be to run 4 cheap IMU sensors in a purely data logging mode to start (a la @TooCold ), and take them for a walk

My very early system (and any system I would attempt) was/would be primararily a machine vision system - the IMU was supplementary to hopefully "fill in" estimates for occassions when zero environmental features were in view. I don't think there's anything to be gained trying an IMU-only system as a first step, since it doesn't really generalize to the diving use-case.

Would have thought that the speed of a diver relative to the speed of the water would make the measurements very difficult.

Yes. Which is why any IMU-based system is doomed to failure without external data inputs (as @tursiops points out).

Why not ultrasound?

When I've periodically noodled around the internet with this project in mind I have never been able to find small enough and/or cheap enough transducers with the necessary range. And capable ones want *lots" of power.

What I don't understand about all these inertial navigation posts:
Accelerometers measure acceleration, which is relative to gravity (i.e., the bottom), not to the motion of the water in which one is swimming.

There are some very good underwater navigation systems that use acoustics;

Your absolutely right. But AFAIK the size, cost and power budget of the systems and even the components, especially the transducers, put them out of the DIY realm - if you know of good (i.e. smallish, cheapish, battery-friendly) candidates, please let me (us) know!

Assuming a "perfect" IMU, that would be enough to know your position for the duration of the dive.

If only. The killer is (as pointed out by at least @tursiops and @James79) that even a "perfect" IMU is blind to your movement due to current.

Suex developed such a unit

Do you know the price, weight, dimensions? When I go to the website I see nothing but a login prompt...

I looked at the sensors that are used by fish finders and depth sonar, but they're expensive, require a lot of power

Yup.

IP68 Underwater Ultrasonic Obstacle Avoidance Sensor (3m, UART)

Garmin LIDAR-Lite Optical Distance Sensor - V3

These sensors (even if you hacked the housing for use at greater depth) typically have a range less than 3m, which I think makes them impractical for diving unless you're hugging the bottom or a wall.

So, a passive camera-based system for track generation (not realtime navigation), with pressure (depth) data and surface GPS endpoints, is the way I'd go if I did this again. Good enough cameras are now cheap and common (think RPi). As long as you're willing to live with the 3 requirements/limitations I listed in my first post I think it could maybe be done today for a BOM cost of around USD 250. For navigation, it would have to add considerable additional computational resources (more $$).

 

[high_order1]

But,
it would limit your system to clearwater/bluesky use without a powerful illuminator, right?

I still think echolocation is going to be the ticket. I just went to the robot sites and hit a couple of things as proof of idea, not even concept really.

Batteries are amazing now. I would not imagine this in a wrist-mount form factor, unless it was like military radios where the brick is somewhere secure, and a small screen goes where you can access it.

I was thinking a handheld tool similar to the pics I posted, or perhaps as another presented, a brick/cannister on the tank or wing. You can shoehorn plenty of new chemistry battery into that form factor.

 

[high_order1]

Oh, consider my underwater wireless comms. They use an AM radio signal propagated by a vibrating transducer. Those run all day on a brace of AA batteries, and have some distance to them.