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Your own Remotely Operated Vehicle (ROV) Project

Posted by Thomas J Coyle III on September 2, 2014 at 5:42pm

Hi All,

I am sure that many of you have heard of the OpenROV Project which involves building your own ROV from a kit. I am an experienced model builder with plenty of experience and a reasonably equipped workshop, but I was a little taken aback at the complexity of assembling the OpenROV from their kit of parts. It seemed to me to be a high school to college level project requiring a small team of experienced modelers to successfully put the OpenROV together though I could be wrong.

It appeared to me that there were a number of critical assembly requirements that had to be performed spot on as there was no going back to realign or reseal once the assembly was completed. Therefore I have been looking around to see if there might be an easier way to design and build a ROV similar to the OpenROV without having to perform some of the critical assemblies required to complete the OpenROV.

One area of the OpenROV design that I took exception to was the thrusters. They were not going to last very long when immersed in seawater due to the exposed bearings and stator/armature. To me this was a kind of deal breaker as to the questionable lifespan of these thrusters. I know that commercial thrusters are not cheap (think Seabotics or CrustCrawler) and the OpenROV project was just trying to overcome the high price of commercial thrusters with their homebrew design. Still the unknown lifespan of the OpenROV design did not leave me with a warm feeling to say the least.

Then came along Blue Robotics and their T100 Thruster KickStarter Project Link. Their design approach looks good and they have actually tested and characterized prototypes of the thrusters they will deliver to the pledgers. With the addition of the in-thruster water cooled ESC this design becomes very attractive in that it reduces the number of wire penetrations in the Water Tight Compartment (WTC).

Great! Now we have a source of reasonably priced thrusters so what is next? Well, that is where you, the ArduBoat members, come in.

Let's start thinking about the WTC, navigation controller, communication, power, ballast, buoyancy, etc. and attempt to come up with a reasonably priced ROV that the average ArduBoat member might want to consider building.

Regards,

TCIII AVD

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Tags: openrov, operated, remotely, thruster, vehicle
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Replies

    • Patrick Rowe > Thomas J Coyle III February 26, 2015 at 4:37pm

      Tom,

      Correct MATE Explorer class is allowed 48Vdc with current limit for power through the tether.

      Ranger is limited to 12Vdc.

      I have been working on Matt to allow AC in the tether circuit but limit it to 36v and apply a current limit as well, also require it to be ungrounded 3 phase. This would allow for very small tether. Most MATE explorer teams run the 48 to the ROV then use DC-DC buck converters to knock the voltage down for the various needs. This is not very efficient and requires additional hardware that needs to be in a pressure housing, cooled and can produce electrical noise that can effect the processors and other control and sensor circuits in the pressure can...

      As for the PWM frequency that is what makes it a good solution! Very the frequency will allow to adjust around interference and find a good working frequency. Another nice thing is the ability to program the power curves in the ESC. On the down end (Subsea) use a 3 phase rectifier and a series of 1 chip regulators to get the voltages that are required. Keep in mind that there has to be feedback from the down side end that can control the "A-D" input of the ESC to keep the voltage at the subsea end constant as the load changes. A simple inexpensive PIC processor does that very well.

      I have been working with Wild AC devices for many years as a hobby related to alternative energy production. DC transmission is expensive and needs to be heavy duty. AC is much easier to deal with, transmission is much more efficient.

      I work on the  Large Work Class ROVs, 3P/ 3Kv in the range of 200Hp is very common. Oh did I say the Tether is 4.6cm  Dia. and is up to 4Km long. The same can be done for very small ROVs and the Model parts are there right off the shelf to do it.

      Oh and a bit of Trivia:

      On average 1 Cu Ft of:

      Salt water weighs ~64.1 Lb.

      Fresh water is ~ 62.4 Lb.

      So a very basic, back of the match book buoyance equation is:

      {Displaced volume (Cu Ft) * weight of Cu Ft of Water) - Weight of the object displacing the water = Buoyance.

      pr/.

    • Admin
      Thomas J Coyle III > Patrick Rowe February 26, 2015 at 5:50pm

      Hi Patrick,

      You are right about AC being the best method of power transmission, however the Pacific Intertie Link, that terminates north of Los Angeles, Ca, uses 1 million volts DC (+/-500Kv 2 pole) which allows the transmission of 1 Megawatt per Amp of current.

      Regards,

      Tom C AVD

    • Rustom Jehangir > Thomas J Coyle III February 26, 2015 at 4:31pm

      Patrick,

      I agree with Tom, that's a really clever use of a brushless controller. I imagine it would be possible to use one on the vehicle side as an active rectifier?

      Why did you choose 3-phase AC over high voltage DC? So that you can use a transformer?

      -Rusty

    • Patrick Rowe > Rustom Jehangir February 26, 2015 at 4:51pm

      If you go with active feedback then all you need is a 3P rectifier and a filter cap to convert back to DC. As the voltage drops the feedback loop "speeds up" the ESC and the Subsea output goes up. Very simple feedback. Converse when the DC voltage is above a set level the ESC throttle is backed off and the voltage drops. The DC voltage will very within limits of the sample rate and limits set in the feedback loop. But if you have the need for several DC voltages to run things subsea like 3.3, 5, 12, 36. Single chip regulators are available to bust down and Isolate the delivered DC. If you want to get really fancy you could use a 3p Multi drop output transformer and break it down that way. But that means you have to have the extra weight of the transformer subsea.

      3702162824?profile=originalBesides a car alternator rectifier is a easy to get, off the shelf 3P Rectifier.

      pr/.

    • Admin
      Thomas J Coyle III > Ken McMillan December 28, 2014 at 4:54am

      Hi Ken,

      Welcome to the Group!

      You have provided some great observations and pertinent information concerning ROV construction.

      Much appreciated.

      Regards,

      TCIII AVD

  • Rustom Jehangir September 3, 2014 at 3:46pm

    Hi Thomas,

    This is exactly why we designed our thruster in the first place. There are tons of platforms and components for UAVs but there's almost nothing for marine robotics that is affordable to hobbyists.

    We'll be happy to do whatever we can to assist. Here are my initial thoughts:

    WTC: I think this is one of the most challenging parts because if it fails, you could lose hundreds of dollars of electronics. I think plastic tubes, like polycarbonate or PVC, are pretty easy to work with and you can use O-rings to seal them easily. Our BlueROV has a polycarbonate WTC with an aluminum backplate and O-ring to seal it.

    Navigation: The APM or Pixhawk will work great for this. My only complaint is that 3DR combines the compass and GPS unit. GPS won't work underwater so it's not needed. The air pressure sensor could possibly be used to detect leaks in the WTC.

    For an ROV, you generally want live video also. A Raspberry Pi or BeagleBone provide an Ethernet connection for communication and can send live video pretty easily.

    Communication: We're using an Ethernet cable because they are cheap, readily available in long lengths, and provide a high-bandwidth connection. Unfortunately, underwater Ethernet connectors are really expensive ($150+) so the cable is potted directly through the vehicle's hull. A better solution for that would be great.

    Power: We considered sending power over the tether, either directly or with PoE, but a long tether with small wires just isn't great for that. It's a lot easier to put a lipo in the WTC and have 1-2 hours endurance.

    Ballast & Buoyancy: There's lot of options here. Syntactic foam is usually used for added buoyancy.

    User Interface: I think the user interface is really important considering that it's really your only source of information once the ROV is in the water. OpenROV has a pretty cool one. I'm not sure how easy it would be to use with other ROV designs. We'd like need a MAVLink-esque communication standard.

    I'll try to post more about our BlueROV and SolarSurfer in the coming week or two.

    Best,

    - Rusty

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