Grab arm - how to attach servos


#1

so I’ve seen a few folks commenting on adding a grab arm to their ROVs. I’m curious how they are going to be implemented. I’ve yet to find a servo (other than the thrusters) that will work underwater at a depth of more than 3 feet. (ip67 rating).

If anyone knows of any servos that can be used underwater, please post a link.

thx


#2

Hi @davidsi,

I do not know of anyone who is yet to 100% successfully create or modify a standard RC servo such that it can handle any sort of practical depth long term. I think most peoples attempts involve filling the servo with oil.

I have seen some good gripper designs use peristaltic pumps. Have a look at the more recent posts in this topic OpenROV Gripper, Grabber, Manipulator

In the last few weeks I have been working on a linear actuator/gripper design using magnets. See the video i just posted showing the first open and close test.

https://youtu.be/wmMihi_591k

I’ve used some neodymium magnets and have been able get a linear force of 1.8 kgf before the magnetic bond breaks. Inside I have used a gear motor from ebay like the one in the photo below. All of the yellow is 3D printed ABS.


#3

Hi,

I’ve used Hitec waterproof servos to some depth before. At least 10 ft for a decent amount of time and they seem to hold up great. I have taken them apart and the internals seem sound and everything.

As for filling them with oil, when the mineral oil gets hot, it expands. This causes the oil to leak out of the seals due to the expansion and it then allows water to get in. Not sure if there is a way around this or not. You could maybe try a reservoir of oil for a servo somehow so if any leaks out, some could fill in the missing space. Just an idea.


#4

Hello,

We’ve pushed the HiTec IP67 servos down to 30 meters without leakage. For deeper work, you could consider packing them with marine grease, which won’t expand with heat but could interfere with the servos function if you pack it too tight.


#5

Hey Andrew,

Thanks for the information. If I ever go that super deep, I’ll have to try marine grease. Good to know you can push those Hitec Servos!

-Carter