OpenROV X1

propulsion

#1

Here's a glimpse of what I've been working on:


This is going to be a sealed, pressure compensated, magnetically coupled thruster using the current OpenROV motor and the Delta V-15 prop. Currently, the pod diameter is 32 mm, and the overall length is probably going to be just over 120 mm. When it's finished, the thing will be filled with mineral oil and sealed, at which point it should operate at essentially any depth.

The first couple of sets are going to be machined (with the magnetic couplers made using rapid prototyping), but once I feel comfortable with the design I'm going to look into casting some of the pieces out of resin, which should make additional units much cheaper.

Currently there are a few outstanding tasks:

  1. Add the structures needed to attach the thruster to the ROV's hull (and provide power)
  2. Add the duct.

The first task should be pretty straightforward: pick a nice airfoil section and add some mounting hardware. The other is a bit more tricky:

  • Duct design has a substantial impact on thrust
  • I don't have any good simulation tools for testing
  • Manufacturing foil shapes is difficult, slow, and usually expensive - you need a very good machinist (I'm fair, but not that good), a CNC machine (I wish), or an RP machine (again, I wish)
  • Iteration is going to be expensive

For now, I may just punt - use a modular interface to put something on there that is probably going to work acceptably well. Then if I find myself with more time, tools, and some further experience, I can tweak it to my satisfaction.


#2

Ben,


This is very impressive! Have you seen my page on Magnetically Coupled Drive design?

http://openrov.com/forum/topics/magnetically-coupled-drive

Perhaps we can re-post the work you're doing to that thread.

Keep up the good work and take photos!

Eric


#3

Eric,

I did see it, and that's actually what got me started on this design. I'd love to try making one of the ring thrusters, but I figure I should try walking before I run. ;-)


#4

I made a prototype of a magnetically coupled drive like this before. The problem I had was torque. It just did not transfer very well at the scale I was building at. Pretty much the water had way too much resistance per the torque transferred through the mag-coupling. I had the magnets switch polarization like you did too but to no avail. I know it can be made but I just have not had the time for a V2 of my prototype....


#5

I'm working on a mineral oil pressure compensated structure with magnetic coupled thruster. I'm modeling the fluid dynamics and mechanical structure that I plan to build using various 3D printers. I'm currently experimenting with various bouyancy compensation schemes.


#6

To make it cheaper and simpler, you might be able to cut down the magnetic couplers from 2 to 1. The overall thruster would be shorter and you would use the brushless motor's electromagnets and a custom made outrunner that is slightly bigger than the current one. I have actually been working on a design like that. It's for my quadcopter variant.


#7

Zach,

The best solution (and I think this is what you are describing) would be to have a potted stator, with the propeller shaft running through it and the outrunner integrated into the impeller. For the moment though, I'm trying see what I can do with the motor as it comes off the shelf and parts that can be made on a lathe.

Bill,

It sounds like we are taking parallel approaches. I hope you post the details as your build progresses!

Kyle,

I'm a bit worried about that, and my mitigations tie in to the rest of the design: I've stuffed as many magnets as I can into that space and still leave space for the membrane. The membrane is so thin that any significant pressure would compromise it, which forces my hand on pressure compensation. I built a test unit which seems to be able to resist substantially more torque than necessary, but until I have the final parts in hand it's hard to be certain. I'll post the results as I have them.

Ben


#8

Ben, take a wild guess, how much would one of these thrusters cost?


#9

It depends.

  • If I were to give you the bill of materials and a set of plans, and you took them to a machine shop, it would probably be a couple of hundred dollars per thruster.
  • If you had all the tools you needed (lathe and rapid prototyping machine) the material cost is pretty low, maybe $200 for a set of three thrusters. (Ballparking!) Keep in mind that about $75 of that is tied up in three each of motors, props, and spinner nuts
  • If I were to make molds and cast the parts myself, the variable cost would be even lower.

All of these things are highly dependent on the final design, so don't take anything I say here as gospel. My take on it is that the material cost is going to be pretty low compared to the cost of the tools that I have to acquire in order to make this.

I'll send out periodic updates with my progress, and hopefully I'll be able to give you better numbers in the future.

Ben


#10

Have you looked at the magnetic impeller motors fond in off the shelf aquarium filters and current makers?


#11

Donald,

I had seen a couple of these, but nothing looked like a good enough fit for what I wanted to spend much time on it. I need something that is brushless, has a small outside diameter, and is pretty cheap. I didn't feel like I could rely on aquarium equipment to meet those criteria.

The other problem is that I'm designing something that needs to work at substantial depths (50 - 100 m). Most "waterproof" equipment is actually only designed to be submerged down to a couple of feet and simply cannot handle the depths I am aiming for.

That said, if you know of equipment that might fit the bill, you should definitely me know.

-Ben


#12

That is a great suggestion to look at how they design that kind of equipment to see if there are any design problem considerations that the aquarium equipment designers added. Donald, you have any examples of that kind of equipment? Can I find that kind of stuff at Petco/Petmart?


#13

Some of it but where you need to look is at the equipment used in large saltwater aquariums. For that, you need to find a shop dedicated to those kind of aquariums. I currently have two saltwater tanks but they are relatively small - 55 and 75 gallons running off a single sump/protein skimmer with several powerheads (http://www.drsfostersmith.com/product/prod_display.cfm?c=3578+4588+23760&pcatid=23760) used to move the water. These are combination impeller / propeller magnetic motors. The largest shown here, the 1200 has a max circulation of 1300 gallons per hour which is a very powerful current. Not sure about depth of operation but at $20 a unit definitely cheap enough to try out in experimentation.


#14

Just and FYI, I know there was some chat about proppler design and thrusters so here is a few links to some Thrusters that I saw online:

shows the propeller used as well.

http://www.crustcrawler.com/products/urov2/index.php

http://www.crustcrawler.com/products/urov2/docs/HiFlow_Thruster_400HFS-L_User_Guide_and_Warranty.pdf


#15

Thanks for the links David. The props continue to be an outstanding issue. We thought that using the Delta V15 rotors would be a good start because they were readily available in hobby shops. However, with our Kickstarter project, we pretty much cleaned out the inventory of the entire country (and are waiting for more to come in).

Ben's magnetically coupled drive is awesome (and I can't wait to hear about further developments on it), but we can do much better than the delta v15.

I think this part of the ROV will be easy to upgrade, and we can look at injection molding our own design if we get enough interest from the community. Just a matter of finding the time to focus on the issue...



#16

I've not actually dropped off the face of the earth yet, and I've managed to get a portion of these built for three units. My initial plan had been to have a friend with a reprap build some of the core parts (rotors), but this didn't work well for a variety of reasons. Plan B was for me to make parts on the mini-lathe in the basement, which required a lot of TLC to get into working condition, then some redesign to account for the fact that they were going to be built differently.

Anyways, that's all been done, and I've made some progress. I've got some build pictures that I've been meaning to post - I'll try to get them up this afternoon.

An interesting note: This design can be easily adjusted for different impellers - in the worst case scenario, the motor enclosure is unchanged and only a couple of parts near the actual prop need to be adjusted.

-Ben


#17

Here's one of the motor (outer) rotors in the process of getting set up. The 22 mm diameter piece and the orange spacers lets me line up all the magnets at positions. Epoxy is then applied and used to attache the magnets to the actual rotor piece. In this picture, I've set the ring of magnets around the rear membrane and placed one of the propeller (inner) rotors inside. The strength of the magnetic linkage seems to be really good - but I'm going to wait until I've tested the thing in water before I sign off on the design.

#18

Hey Ben, did you ever get this thruster in the water?


#19

Hi guys:
About the propellers. For the size we are delaing with, Kaplan propellers are the ones that better suit the requirements.
For reasonable Reynolds numbers (no SciFi speeds), a 4 bladed Kaplan, with AeAo=0.5, inside a 19m series profile, with a chord lenght equal to the propeller diameter, is the best starting point.
Dimensioning the pitch to diameter ratio would not be difficult once drag and desired cruise speed are known.

Regards and … Nice design.
A question, How is the counter rotation of the hull going to be avoided ?

Regards and congratulations again.