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
Replies
@bigkahuna,
Some great observations!
I am still waiting for my Blue Robotics T100 thrusters with the built-in ESC to begin my ROV Project.
Regards,
TCIII AVD
Just new here, but not new to the world of ROVs.
MATE is a program where a wide range of students design, build and compete at regional and international competitions.
I have been working with this program for over 5 years now in various capacities. It really works to get students interested and focused in Science, Technology Engineering and Mathematics (STEM).
These students never cease to amaze me on what they come up with to solve increasingly complex problems the competition comes up with.
If you are interested check out www.marinetech.org
This years international competition is being held in St. John's Newfoundland at Memorial University and the Marine Institute. It will be streamed live through the marinetech.org web site with streaming facility provided by robotics.nasa.gov
Patrick
Hi Patrick,
Welcome aboard so to speak:-)
I have followed the MATE competition in Robot Magazine and I too am amazed at the ingenuity and creativeness of the teams involved in that competition.
Regards,
Tom C AVD
Hi All,
I have some thoughts concerning the following ROV characteristics:
Power: There are two ways to go with providing power to the ROV: local power and remote power.
Installing onboard batteries simplifies powering the ROV, however they add to the weight of the ROV further complicating the buoyancy stability issue.
Using an onshore power supply has the drawback of requiring additional wires in the tether cable to carry the power to the ROV. Some builders have gotten around the heavy power cable issue by using 48vdc onshore power and converting the 48vdc back to 12vdc in the ROV. The use of the higher voltage allows the use of smaller power cables to transmit the same amount of power that a lower voltage higher current (bigger power cables) can transmit. However there is the issue of the power dissipation of the step down voltage converters at the ROV end that will add to the ROV thermal load.
WTC: Many builders use pressure rated PVC pipe in various diameters with 6 inches appearing to be a popular diameter. The PVC pipe wall is usually 1/4" in thickness and the ends are usually sealed with flanges and O rings or acrylic windows. The 6 inch diameter of the tube allows plenty of room for a slide in tray to hold the navigation controller and other electronic accessories.
Regards,
TCIII AVD
Hi Thomas,
There are thicker PVC tubes (usually called Sched 80) which we have used as a WTC up to 100 metres water depth. Deeper usually requires Aluminum tubing, but there are many other new materials that can be used. For depths greater than 100 metres we usually use double o-ring seals and we install the ability to evacuate the WTC slightly so that the seals are compressed.
In order to determine if there is a leak we use an "Aspirin" switch, which is a microswitch held closed by an aspirin. The slightest presence of water will cause the aspirin to disinitegrate opening the switch. The switch can control a master relay or just turn on an LED in camera view, (or switch the APM into surfacing mode).
For power, I have used on-board power, high voltage DC (600VDC), and regular 12V to 120 VDC supplies from the surface. We had to custom build 600VDC to 12 or 24 VDC DC-DC converters, but that proved to be the best way for high power requirements. We used DC because the induction loss in water is so high compared to DC, combined with the cable resistance loss.
Another approach would be to use surface power at low current rates to charge one (or 2) sets of batteries while another set is running the vehicle.
The GPS/Compass package would be useful if we use an INS for Navigation, (position updated by depth, Doppler etc) When at surface the position of the ROV on the surface is recorded, the INS data etc logged and upon return to the surface the position is taken again, Post processing can be used to distribute errors in position throughout the mission.
The one thing to remember if an autonomous ROV is anticipated is that control of the motion has to use track, not the compass heading.
Ethernet connectors are expensive because the distance from cable to cable through the connector has to be as short as possible. Fibre optic is the better way to go. The downside being the cost of sliprings when using a winch for the cable. There are some tricks however if using a single mode cable to make a low low cost slipring.
Control functions can be transmitted from the RC transmitter to the receiver in an ROV using coaxial cable. One of the first ROV's, called TROV had on-board batteries and had 2 coax cables. One for the control, the other for the video signal to the surface. Then, the OSD was a board about 9 inches by 4 inches compared to the minOSD , <1 inch by <1.5 inches.
Regards,
Ken McMillan (just joined the group)
Hi Ken. Can you expand what you mean by this statement?
The one thing to remember if an autonomous ROV is anticipated is that control of the motion has to use track, not the compass heading.
I'm interested to know.
Thanks, Grant.
@Grant,
Consider an ROV wanting to head due north to a target, in a 2 knot easterly current. If it maintains a compass heading of north, the vehicle would end up east of the target. If the vehicle has a speed of 2 knots, the heading would be 315 not 360. To get to the target the vehicle's track has to be north.
This is the same with surface boats. You make your track agree with the value required to get to a waypoint, as wind and current can both affect the boat's direction.
Ken
Ah that's what you mean. Thanks Ken!
Thanks, Grant.
Good success with very long power delivery using Variable frequency 3 phase AC with active feedback.
A small onboard battery for surge loads that is recharged when the power load is low. (Capacitors can be used as well but a battery has a higher power density. (LiPo) The key here is the active control of the 3phase voltage and control of the power factor. Changing frequency can also help to mitigate Xc loss in water and help to reduce noise that can interfere with other signals. Also using ungrounded three phase is safer if one phase becomes shorted to water...
Think Brushless motor speed controller and a 3phase transformer and rectifier! (all off the shelf parts)!
I have had fun with this one and some good results powering small ROVs over very long, very small tether.
Remember Harmonics can be your best friend or your worst enemy...
pr/.
Hi Patrick,
That is an exceptionally novel use of a brushless motor ESC for a DC to AC converter.
I assume that you set a specific PWM frequency for the most efficiency and lowest power factor for a given setup?
I know that MATE allows up to 48vdc on the ROV umbilical to help reduce the size of the cable that transmits the power to the ROV.
Regards,
TCIII AVD
-
30
-
31
-
32
-
33
-
34
of 34 Next