Home        Store        Docs        Blog

Underwater stepper motors

Has anyone found a stepper motor that can be used outside the sub to control sample collection of water,sand, etc? I have had no luck finding one and cant figure out how i would waterproof a normal one given the spindle needs to turn and would inherently be a leak source thru any enclosure.

At the risk of sounding like one of the people i regularly deal with…Are you sure you want a ‘stepper’ motor? Why not use a simple servo? I flood my servos with mineral oil (which is non-conductive) so they work outside the ROV. The water pressure won’t crush the servo since it’s filled with oil, and water also has no reason to leak in along the ‘spindle’.
For most of the bigger metal-gear ones, i’ve drilled a small hole in the housing and plumbed in some 1/8" aquarium hose, which goes to a resivoir, that i can push fresh mineral oil into after each mission, purging any water which may have leaked into any of my servos. After many hours of use, servos with resistive-strip feedback can become polluted by carbon dust, but by purging fresh mineral oil through them this issue is eliminated. The mineral oil Will leak Out around the spindle, under purging pressure.
I have (2) 9-degree-of-freedom manipulator arms which are actuated wholly by servos, not hydraulics. I find servos turning leadscrews to be plenty powerful, quick and precise.

If i needed more speed and power, i would buy a 20v Li-ion brushless DeWalt drill. After potting the stator in epoxy and replacing the bearings with ceramic ones (for saltwater immersion), you have a powerful brushless motor turning a planetary-gear-reduction gearbox which is very fast. Then turning leadscrew to make a linear actuator. The drill can be bought cheaply (without the battery or charger) on Amazon.

Going a bit further, i use an SSC32 servo controller to power all my servos and lights, controlled by the Raspberry Pi over I2C. I use a 32 channel ADC to get pot inputs into my topside R Pi. The only thing i use the Pixhawk for is thruster control.
I don’t have any pics of my setup cause that laptop locked up, but here is my original design for you to examine…


I mounted microcameras on the manipulator(s) wrists so i can more easily maneuver the gripper into position. Just watch how excruciatingly slooooww any of the professional ROV manipulators move…jerk…move…correct…turn…jerk, drop, and you’ll see…they may have megabucks, but they can’t design for drek.
I have an 8-camera security-system DVR in the ROV connected to a 4-port ethernet switch sending all 8 camera feeds up the tether through the FathomX interface.

Long reply but…i’m here to help.

3 Likes

thats a great idea. thanks so much for the detailed response! what servos are you using, i might as well buy those.

Regular stepper motors will work under water, if the connector is properly sealed. There’s nothing to be shorted. However regular ones will likely be corroded quickly, so there are anti-corrosion stepper motors available on the market. I am likely going to get one to see if it can work reliably under submersion. There are also IP rated sealed motors, but I haven’t found any that are rated for long period of submersion, and it is said that when water do inevitably get in, there’s no way for the water to get out.

An alternate to the stepper would be to instead drive one of the M200 motors directly using a sinewave output and enough holding current to prevent loss of comutation. This is how gimbal motors are driven on many UAVs/Drones. Have a look at the attached link:

Hi Oddmar,
Did you manage to unlock your laptop? :wink:
I am very interested in seeing some pictures of your waterproof servos design. Don’t forget to post some of them if you can please!
Best regards

BM

It’s not a matter of unlocking it, it froze and when i tried to reboot, i can’t even get it to boot into Bios. So i figure it’s toast. Eventually i’ll have to pull it’s 2.5" drive and find an adapter to copy it.

As for pics, the ROV is in storage in Arkansas, and i’m in Ill-Annoys, so not anytime soon. Solly.