Bart - Argonaut Jr's
Underwater ROV

If Simon Lake were building the Argonaut Jr. today he would certainly included an ROV.

ROV's are like remotely controlled submarines with video cameras and often robotic arms that allow the operator to work underwater for the safety and comfort of a boat above or from within a submarine.

Putting a video camera on an ROV and flying it underwater is actually can be done with quite easily using a joystick, some electronics, bilge pumps, and an underwater video camera.  We have already built a simple ROV named Bob.  You can see more about Bob here:  Building Bob

Argonaut Jr's ROV will be name "Bart" after Simon Lake's cousin that help him build and test the Argonaut Jr.   Bart will be build from easy to find and relatively inexpensive parts such IP and USB cameras, motors and servos from the Radio Controlled (RC) hobby suppliers, a play station game controller and those parts will be driven by RoboRealm software that makes connecting and programming as easy as it gets.

The Plan

The Parts

Hitec HS-755HB Karbonite 1/4 Scale Servo Futaba Jr.  $26

1) RoboRealm -- Software suite that processes the video, and has easy to use interface modules for the game controller and the Pololu Micro Maestro controller that runs the servos and motor speed controllers. The game controller will use the joystick module and the Pololu servo controller has a custom Pololu Maestro module. RoboRealm also has a web server to make the video available to the Internet when a connection is available.
2) Pololu, Micro Maestro 6-channel USB Servo Controller and Documentation and Forum -- A very small card that controls the RC motors Electronic Speed Controllers (ESC) and servos.
3) Lowrance StructureScan, sonar imaging is unique in that it use Ethernet network to transfer the signal between the the supplied network box and the display console making to possible transmit the transducers information up the ROVs umbilical.  By slowly rotating the transducers two side scan transducers 180 degrees with a servo; we will be able to capture an 360 degree view of the area around the ROV out to and beyond 100ft even in muddy water.
4) Putting a compass in the field of view of a camera works nicely, but we'll go a step further and add a digital compass that provides the heading over an RS232 serial interface with the Pololu, Micro Maestro.
5) Microseven IP camera has an SD card on it where it can record D1 video as well as a built-in web server enables video live anywhere over the internet.
 

800kv D2830 Brushless Motor  $18 with shipping from China.   Aeolian 45Amp ESC 2A BEC for 1/10 RC Car Brushless Motor $27 with shipping.

Brushless Motor Thrusters

We are using RC (radio controlled) hobby type motors with props added on for Bart's thrusters.  These are widely available and relatively inexpensive, small, but powerful motors than can be run even in salt water with the motor completely exposed to the water. Read more here: DC Motors and Props.

We tested a 1000 Kv Outrunner Brushless RC Motor and 30 amp and 30 amp BEC ESC (Electronic Speed Controller) $8.50 for both, plus another $8 for shipping from China.  The results were mixed.  One of the three motors caused the ESC to shutdown after a short time of running underwater. The cause is likely due to a bad battery we were using.  Also when running at higher seeds, the prop which is actual a fan blade taken from an PC's power supply would cause the motor to run too slow and fall too far out of sync from that the ESC's motor speed sensor would be expecting.

We also learned that we need and ESC for RC Cars that have the ability to run the motor in reverse. Just because the ESC says "Reversing" does not mean it can run the motor in both forward and reverse.  What the Chinese translator means is that you can wire the motor to run in forward of reverse.

Mounting a prop to this motor was fairly easy.  We just turned a PVC plumbing fitting in the lathe to fit over the motor and into the fans hub. It would take more time, but you could do this with a drill, small drum sander attachment to fit inside the PVC pipe so it spins the pipe, and sand paper to cut down the outside dimension of the PCV.

For our second test we purchased 5 - 45 amp ESCs for cars because they can run the brushless motors in forward and reverse.  Add we went with 5, even more powerful motor that runs at 800Kv or slower which is better for turning bigger props and getting more power. Finally we got a couple of 1/4 scale servos for tilting the camera and rotating the sonar transducer.

Power

Battery On Board

Commercial ROV's are powered from the surface with very high AC voltage much like large power transmission lines that must transfer electricity over long distances.  However there is obviously a lot of inherent danger that goes with high voltage AC around water.  In comparison DC voltage at or below 12 volts or less is safe even when directly exposed to salt water.  The problem is that 12 volts will not travel through a long wire without a significant loss of voltage.  We could use a larger wire, but then the motors on the ROV would be dragging a huge cable along and any amount of current would in turn drag the ROV around. We also want to go really deep eventually, like 3000 feet and do it without a large ship to carry around the cable, so putting the batteries on board the ROV is our only real option. 

Lead Acid or AGM Battery?

AGM Batteries like the Optima brand batteries are tightly coiled sheet of lead and matting that have successuflly been used at great depths as in 12,600 feet when doing work on the wreck of the Titantic.  However, much less expensive lead acid batteries can make that trip too provided they are compensated with oil that floats on the surface of the liquid electrolyte in place of the air normally found inside these batteries.  Lead acid batteries are also less prone to premature death due to deep discharge.

Inside or Outside the Hull?

Both AGM and especially lead acid batteries produce hydrogen gas and oxygen which is explosive.  So placing them with all of the electronic stuff is best avoided.  It would also be convenient if we could quickly and easily swap the batteries out for a pair of batteries that are fully charged.  So outside the hull means we don't need to open up the hull.

How Big A Battery?

There are lots of good sources that explain batteries in detail but one I found very useful is: http://www.gizmology.net/batteries.htm because it has a simple explanation of Peukert's Number and a calculator that will show you how much power you really have.

Here is an comparison between an Optima AGM and a deep cycle lead acid battery with an ROV constantly using 14 amps at 12 volts.  Lead acid batteries generally have a Peukert number that is higher that AGM's meaning that the don't like being discharged at a high rate, so it takes a 75 ah lead acid battery to match a 55 ah AGM battery.  The lead acid will provide 29 minutes when 20% discharged, but unlike the AGM, the lead acid battery can easily survive being discharged to 60%.  So if you think you'll constantly use 14 amps, then you will need 1 of these batteries for every 30 minutes of run time.  In practice, it is unlikely you will actually need 14 amps constantly.

The depth of discharge (DOD) has a major effect on the life expectancy of a battery. Discharging only 80% of the total capacity of the battery will typically get you 25% more cycles than total discharges, and discharging to only 20% will make the battery last essentially forever. Car batteries, however, have to be treated differently - they're not designed to discharge even 20%, and will be damaged if they're deeply discharged. A "deep cycle" battery, on the other hand, can typically survive 400 full discharges.  And as a rule of thumb, AGM batteries are more susceptible to damage from DOD than lead acid batteries.  I have run lead acid batteries completely dead and was able to recharge them.  I've done the same to a couple of AGM's and had to go buy two very expensive new batteries.
 

Main Power Switch

A really easy way to put add a water proof switch is to put a reed switch inside the hull and a magnet outside the hull.  The just move a magnet over the location of the reed switch.  You'll need to keep the magnet over the reed switch as long as you want it on, and you will need the reed switch to trigger the coil on a relay, because most reed switches can only handle about 1 amp.

If you want to avoid having to rig the magnet so it stays over the reed switch then you can buy an expensive locking relay or you can build Awni Zaidoon's cool little circuit.  Then all you need to do is pass a magnet over once to turn on our ROV and again to turn it off.

Awni Zaidoon

Hydrophones and Locating a Sound

Sound waves in water.

A hydrophone is simply a microphone that works underwater.  See our web page of DIY hydrophones and underwater voice communications.  A simple hydrophone on an ROV will add an incredible amount of sound to the experience.  Not only will you be able to hear aquatic life, boat traffic, and thumps when the ROV bumps into objects, but you will also hear the ROV's motors which will quickly tell you if they are working normally. 

With a single hydrophone you can hear the sound, but with two or more hydrophones and a stereo headset, you will be able to tell which direction the sound is coming from.  We humans can tell where a sound comes from based on two things: the amplitude or loudness of the sound at each ear, and the phase shift in the sound's frequency between the ears. Sound in air travels at 768 mph, but our brains can still tell which ear heard that sound first.  In fact, you can listen to multiple sounds at the same time and know approximately the direction of each sound.  Close your eyes and try it. 

If you have ever been scuba diving you know that it is nearly impossible to tell from which direction a sound comes from when underwater.  The reason is that our ear drums and the air filled middle ear behind the ear drum are designed for delicate vibrations of sound in air.  Water is over 600 times denser than air, so the relatively delicate vibrations of sound waves in water are now much stronger pressure waves. The water pressure is also playing havoc with our ear drum by compressing the air that is behind the ear drum in the middle ear.  These stronger pressure waves in water will actually cause our skulls to vibrate and that vibration shakes the inner ear which is firmly attached to our skull bone.  So we hear the sounds that created the pressure waves, but because both ears are connected to the same skull the ears pick up the sound at the exact same time. Without the change in the phase, we can't tell the direction.  On an ROV the problem is similar to our skull. When two hydrophones are attached to the ROV the pressure waves cause the ROV to vibrate just like our skull.  So one part of the solution is to mount the hydrophones the ROV's with a vibration absorbent material.

In addition, sound wave travels 4.3 times faster in water, so the phase shift we can detect is 4.3 times smaller than in air and our brain which is accustomed to hearing in air can no longer make an accurate determination of direction. So another part of the solution is to separate two hydrophones on the ROV by 4 to 5 feet.   Why 4 to 5 feet?  The distance between your ears is about 7 inches, if sound waves in water travel 4.3 times faster then 4.3 x 7 is about 30 inches.  The reason is that the extra distance helps the brain out by further increasing the delay of the wave's arrival to the second side.  With two hydrophones 4 or 5 feet apart, we can amplify the signal from each hydrophone and send it to a stereo headset.  Now the sounds we are hearing from the water are more like what our brain normally processes and it can determine the direction.

The the amplitude or loudness of the sound at each ear is also a problem.  Because water is over 600 times denser than air, the molecules being moved by a sound wave in water bump into each other much better that air molecules, so sound in water loses less energy and travels much farther.  Because of this the wave pressure at one ear is almost the same strength when it passes by the second ear.  So the brain does not easily detect the difference in amplitude or loudness as it would in air.  Spacing the hydrophones apart will help here too, but not much.  Remember the spacing is to account for 4.3 times faster travel, not 600 times greater density.

So by spacing hydrophones 4 to 5 feet apart and listening to them in stereo we give our brain the best chance for deciding if the sound is coming from the left or right side. If the hydrophones are on opposite sides of the ROV them steering the ROV toward a sound is quite easy. When the hydrophone's sound is the same in both ears then the source of the sound is either directly ahead or directly behind the ROV.  Only by slowly turning the ROV can we determine the correct path to the source, much the same as we would turn our head from side to side in order to better determine the direction of a sound in air. 

Underwater sound wave paths.

So how do we tell if the sound is coming from below or above the ROV?  Well, we don't.  Because sound travels so much better in water, it tends to bounce, or reflect, off everything, like pool table balls hitting the side bumpers.  In water sound mainly reflects off of the bottom, the surface, and the thermoclines.  A thermocline is where water of one temperature meets water of another temperature.  Generally the deeper you go the colder the water.  These layers act like walls or pool table bumpers to sound wave. The reason is that water of different temperatures also has different densities.  Just like a wall in a room is denser than the air that carries your voice.  When the sound of your voice strikes a wall it reflects off.  If it comes right back at you, then you hear an echo.  In water there are lots of walls, some harder than others but most of them like the bottom and the surface are horizontal and that causes the sound waves in water to bounce up and down.  So if we were to put one hydrophone above another we could detect a sound as having come from above us, but we can't be sure that it didn't bounce off a thermocline and is actually coming from below.

Hyperbaric Test Chamber

Steel Pipe Ball Park PSI calculation

P=2*S*t*F/D
P-pressure
S-SMYS (specified minimum yield strength of your pipe, e.g.
52,000 psi)
t-Pipe wall thickness
F--Design Factor (between 0 and 1, generally between .5 and .72 for DOT regulated work)
D-Pipe outer diameter
52,000 psi is a middle of the road standard pipe commercially available steel pipe strength. You can get pipe in the following grades, Grade B (35,000 psi), X42, X52, X60, X65, X70, and a limited number of mills make X80. (where X52=52,000 psi SMYS).

Example 24 in Pipe with a 3/4" thick wall: (2 * 42 000 * .71 * .75) / 24 = 1,863.75 psi

Resources

www.hobbyking.com  Good selection of RC Motors and ESC.
http://www.ertyu.org/steven_nikkel/ethernetcables.html How to wire Ethernet Cables
Battery Resources:
http://www.gizmology.net/batteries.htm  Includes discharge calculators