Quick thought about using servos under water.

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olegodo
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Quick thought about using servos under water.

Post by olegodo »

Hello everybody!

Recently I have spent a lot of time trying to make the manipulator I want for my ROV. Long story short, my first plan failed so I am back to scratch.

My next plan is to try using servos. My idea is to buy IP67 certified RC servos, and go the oil filled pressure compensated route with them.

Each servo individually filled with oil (the entire servo house) add a small hose barb to the housing and some sort of hose and bladder to pressure compensate the whole thing. using parts meant for RC gas engines seems to be a good fit (barbs, hose and bladder)

Anyone tried this yet? any thoughts before I put in my order?
kenl
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Re: Quick thought about using servos under water.

Post by kenl »

The RC engine fitting sound a great idea, much of my resistance to pressure compensating has been the size of fitting I have considered.

If it is oil filled would you need a bladder? I have always thought the volume difference would be tiny and just some silicone hose would do?

Not that I would know, I do hope you share your final design with us!
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olegodo
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Re: Quick thought about using servos under water.

Post by olegodo »

Your'e probably right on only using some silicone hose since the internal volume of an RC servo is relatively low. Will also make it even cheaper.

I am tempted to test out some of these:
http://www.ebay.com/itm/30-33KG-High-To ... 35be952655

Cheap noname servos, but freaky strong according to the specs. Not sure about the quality though.
rossrov
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Re: Quick thought about using servos under water.

Post by rossrov »

Not sure about how the motor brushes and the feedback pot will work in oil. Maybe do an Internet search on this before buying or deciding what sort of oil to use. Something like WD40 might be better (or worse!) than mineral oil.

Alternatively a stepper motor, being brushless, would stand a better chance I think. In the process of trying that myself, and cannot yet say how oil affects the innards. You would also need a "home" sensor. The code to run a stepper is very simple if you use a stepper driver board.

On the compensator tubing, the bladder or soft compressible bulb somewhere in the system can take care of any small air pockets left inside the servo or other housing. Put simply another way, if the air pocket volume is greater than what your silicone tubing can displace, pressure outside will become greater than inside with increasing depth, and so try to force water in.
Last edited by rossrov on May 10th, 2014, 4:24 pm, edited 2 times in total.
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olegodo
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Re: Quick thought about using servos under water.

Post by olegodo »

rossrov wrote:Not sure about how the motor brushes and the feedback pot will work in oil. Maybe do an Internet search on this before buying or deciding what sort of oil to use. Something like WD40 might be better (or worse!) than mineral oil.

Alternatively a stepper motor, being brushless, would stand a better chance I think. In the process of trying that myself, and cannot yet say how oil affects the innards. You would also need a "home" sensor. The code to run a stepper is very simple if you use a stepper driver board.

On the compensator tubing, the bladder or soft compressible bulb somewhere in the system can take care of any small air pockets left inside the servo or other housing. Put simply another way, if the air pocket volume is greater than what your silicone tubing can displace, pressure outside will become greater than inside with increasing depth, and so try to force water in.

This is all true and valid points rossrov. The reason I hope to make the servo's work is the potential ease of getting the units waterproof and depth tolerant over a stepper motor. At least for the stepper motors that I know of, you would still need to make a watertight enclosure and seal the shaft, which I want to avoid if possible :)

Ofc there is also brushless waterproof servos, those would be even better since there are no brushes. But I can only find relatively small and weak ones. Like the Traxxas Waterproof 2075
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TigerShark
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Re: Quick thought about using servos under water.

Post by TigerShark »

Mineral Oil is a very good insulator and should work fine until it gets saturated with carbon if using brushed motors. I have used it to make high voltage capacitors.
rossrov
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Re: Quick thought about using servos under water.

Post by rossrov »

Food for thought: If you put the stepper motor and rotating parts inside the equivalent of a rubber (non-latex) glove full of oil you don't need a shaft seal. Taking that one step further to reduce the risk and/or volume of oil spills, a box, PVC pipe fitting or plastic food jar with a couple of flexible rubber boots, such as used with car handbrake or bike cabling, electrical cabling, fingers off a rubber glove, silicone tubing etc, would be more practical. You could of course put the non-water-sensitive mechanical linkages/joints outside in the water.

Here is a stepper with a leadscrew and nut, which could simplify gearing. Just a case of converting the linear motion to whatever you need using a couple of levers. Scroll down the page and there is a video.

http://www.pololu.com/product/2268

Aside from that, shaft seals: A lathe is not required, in fact it can make things more complicated than need be. Drill a hole in PVC pipe end cap for the motor shaft to stick out of. Once the motor is secured inside, just glue on the shaft seal to the outside of the cap, making sure that the glue or sealant is compatible with the other materials.

Oil as an insulator: That's good, in a way. My concern is that it may affect the brushes over time, and that it will increase brush to commutator resistance, stopping the motor from working, or worse.
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olegodo
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Re: Quick thought about using servos under water.

Post by olegodo »

Quite some interesting ideas! Looking forward to see how it works out! :)

For update, I have ordered some servos and different other parts to test out my ideas. Will take some time before it arrives though.
Jaman42
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Re: Quick thought about using servos under water.

Post by Jaman42 »

Thinking about using a couple of servos for my ROV, did you get anywhere with your ideas? Anyone else have any input on the subject? I plan on having camera tilt and a gripper arm, better to use motors?
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olegodo
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Re: Quick thought about using servos under water.

Post by olegodo »

Hi,

I didn't really find any good solution for using servos in the water.
What I did was to move the tilt servo for my camera inside the camera tube.
And use brushless motors for my gripper. Same motors as for the thrusters I will use.
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