Magnetically Coupled Drive

propulsion

#1

At some point, waterproof shaft seals will fail. The problem is that even if a shaft seal mates well with a shaft, the rotating motion of the shaft inevitably causes momentary pathways for fluid to leak through, and with enough pressure (enough depth), water will push its way past the seal and into the motor housing.

Once solution to this issue is to avoid running a shaft through the waterproof interface altogether. The concept is called a Magnetically Coupled Drive and its most often found on large-scale, industrial ROVs that are intended for use in very deep water. Similar technologies can also be found in the food processing industry as well as in medical equipment. Here's how they work:

Instead of spinning a propeller directly through a shaft, the motor spins a set of magnets that are in a completely sealed container along with the motor. Wires needed to power the motor do not move, so they can enter the water proof housing through potting.


Side cross section, and angled diagram of magnets attached to a gear motor which is encased in a sealed cylinder.

A second set of magnets are mounted inside a ring with an inter diameter just slightly larger then the outer diameter of the cylindrical waterproof motor housing.


Front and side cross section of outer ring with magnets surrounding inner magnets attached to motor shaft. Note that magnets are configured in alternating polarity- this provides greater torque transfer both allowing opposite polarized magnets to attract each-other, but prevents skipping by repelling neighboring magnets

The outer ring can be used as a hub for propeller blades or to transfer motion to a propeller elsewhere. In the configuration shown, there is very little normal force between the two surfaces, so bearings are not needed and abrasion is minimal.

Here's what I built with actual hardware. I intend to post instructions on how people can build these soon.


600 RPM 12v DC gearmotor with inner magnet rotor attached


Right: Motor/ magnet assembly placed inside cylinder- top will soon by potted. Left: outer ring with magnets installed and propeller blades attached. Note, mineral oil was originally used to make the motor housing less compressible, but it turned out to be more of a mess then it was worth and eventually caused the motor to cease.

Here's a video of the thruster working in a bowl of water. The motor used only draws about a tenth of an amp, however the thrust level is not particularly high.


First test of Magnetically Coupled Drive in water. Propeller held in place at end of video to demonstrate that magnetic coupling also helps prevent motor burnout if debris or other problems cause stop the propeller from being able to spin.


#2

Excelent work, some tight ducting around the drive unit will help a lot in the thrust #s, this is an awesome set of ideas!


#3

Chris,

Thanks for such positive feedback! I'm already on it with ducting, but unfortunately the thruster still doesn't seem to be terribly powerful compared to the bulge pump motors with ducted fan propellers I was using before (of course the new motors only draw about 0.1A whereas the bulge pump motors draw closer to 1.25A). More experimentation soon!

In the mean time, I made this quick (and slightly out of focus) video to show you how I'm mounting the four thrusters and how the ducting works. My hope is that the location of the vents going into the housing box will help direct thrust so that the ROV will be able to crab as well as move up and down with the vertical thrusters.

Here's the video:

http://www.youtube.com/user/OpenROV#p/a/u/0/qF0pKs-X39E

I have a lot of concerns relating to thrust and flow rate with this design, but hopefully I'll have much more concrete results to consider after I do empirical testing in the next few weeks.

Thanks for the interest!

Eric


#4

I've been wanting to try to build a mag coupling for my thrusters, so it's interesting to see how your experiment turned out. I rewatched the video several times trying to figure out what the tubing was. It almost looks like copper in some shots, which I would think would have a negative effect because of the eddy currents. Were there any further improvements or developments since this?


#5

Its actually a composite material called Garolite I got off of McMaster-Carr. Will send specs when I can use a real computer (rather then my phone)

Eric


#6

Have you guys already explored using magdrive aquarium pumps such as the Vortech series made by EcoTech Marine? http://ecotechmarine.com/products/vortech/vortech-mp10/

Their miniature MP10 model would be ideal for low, controlled, thrust. You could mount it permanently or semi-permanently to the ROV body using an adhesive or locking mechanism. They are expensive retail, but perhaps you could get just the driver and external prop for less directly from EcoTech Marine since the control module is not needed or useful for ROVs. I'm sure they must have a lot of extra parts from past models as they've updated the pumps several times.

Looking forward to more developments on Open ROV...

Cheers,
Wyatt


#7

I propose moving all replies on this thread into: http://openrov.com/forum/topics/brushless-motors


#8

You were on the right track with the mineral oil. I have a coworker that used to build underwater ROV's for his day job and all of the motors will filled with oil but the catch to an oil filled motor is it needs to be a brushless motor. A brushed motor will cease (as you have found out) from the dielectric breakdown of the oil and the motor's brushes.


#9

Right right, and the realization I had was that if we're going brushless, what's really keeping us from just taking measures to prevent short-circuiting and corrosion and run the think out in the water. The biggest challenge really is just maintaining the bearings. Plastic bearings or bushings could work, but silicone spray on stainless bearings seems to work fine so far.


#10

I'm curious on the bill of materials for this build. I'm working in a class environment, as well as building an ROV for the International MATE competition. I would hopefully like to build a similar thruster to test.

Thank you


#11

This is a nice start and I like the concept of not having a shaft/stuffing box-like setup that is subject to leakage and high maintenance. Can you give us some information on where to purchase the cylindrical magnets?

Brian


#12

I got them from United Nuclear!

E


#13

Thanks Eric. I will look into this and post updates.

Brian


#14

Brian,
Ben also took a shot at a next gen magentically coupled drive. He showed his latest iteration off on the last dev call: http://openrov.com/profiles/blogs/dev-call-recap-1


#15

Thanks David.

Brian


#16

Eric,

I looked into the magnets. I think a magnet set for one motor would cost around $50 and I am looking at using several motors. Do you have any alternatives other than purchasing new ones from United Nuclear?

Thanks,

Brian


#17

Not really. There may be some that can be purchased for a lower cost over seas, but it seems unlikely they'd have reliably high strength for the application. The high cost of sufficient magnets was one of the reasons I ditched the idea as a reasonable means for a low cost ROV.

I continue to hold the opinion that brushless motors - but perhaps ultimately more optimized for the environment - are the best architecture for ROV propulsion systems.

Best of luck!

Eric


#18

Thanks Eric. Regarding brushless motors, would these have shaft seals instead of the magnetically coupled motors?

I have another question, have you ever heard of insurance for an ROV? I imagine they exist for commercial applications (millions), but what about a hobby ROV in the range of 1-2K? I have learned from experience with underwater cameras hard wired to the boat that sometime things happen, leaks, etc. due to cable nicks and you lose your equipment. Like any ROV'er I have the concern of it getting snagged on the bottom, etc.

Thanks again,

Brian


#19

Hi, Mr Stackpole, could You tell me how did You make the prop for mag coupling, cause I intend to take a EDF fan prop, dissmantle it and reattach its blades onto a wider mag. driven hub. Sincerely Yours, Niksa Glavic, Dubrovnik, Croatia.