Developing Depth-rated Servo

Greetings,

I am developing a depth-rated servo for ROVs and USVs, geared toward serious hobbyists and universities. The torque and speed specs will be similar to those of a high-torque, standard-sized R/C servo (roughly 350 oz-in max torque). The electrical interface will be identical (5-7 volts supply voltage, standard PWM interface). I am shooting for a depth rating of 200 meters.

This will be a high-quality product, with all materials and seals being suitable for long-term (up to 1 year) immersion in seawater.

I am trying to gauge interest in a servo like this. If any of you would find this useful, please respond and let me know what you would use it for, what depth it would be likely to see, and for how long it would be deployed. And, of course, I would like to know how much you would pay for it!

Thank you,
Damon

Hi @damon:

Best of luck on your project! Back around the time we were developing OpenROV 2.8, we also tried developing a low-cost underwater servo. The design we came up with worked fine until there was some significant lateral force on the servo shaft- the design we had couldn’t balance low seal drag with the ability to maintain O-ring seal when the shaft was pushed to the side. I suspect some kind of bearing or bushing outside of the sealed cavity is required to make a good shaft seal.

I for one would really like a waterproof servo for one particular application- steering the lights on an ROV. If, for instance, you have a camera mounted inside a dome, and the camera is on a tilt servo, it would be nice to have lights that track the position of the camera. That would allow one to make much brighter lights while keeping power consumption reasonable.

200 meters would be nice, although most of the ROV stuff I’m building these days is targeted for 300-500 meters.

As to how much I’d pay for it, I really don’t know right now. What would, say, $150 get me?

-W

Hi @Walt_Holm,

Thank you for your response. I am in the process of testing shaft seals and am considering the exact issues you bring up (seal drag and sealing ability with side force on the shaft). I think I have a decent solution, but time will tell.

Thanks for pointing out the steerable ROV light application and your target depth. I’m taking notes…

Regarding cost, it’s going to depend on what people are looking for, so it’s good to get your input. I’m considering two different “tiers” of servos:

Lower cost (maybe $200?):

Motor and gear train: brushed motor, torque and speed similar to a $30 R/C servo (which would be more than enough for your ROV light application), suitable for days of continuous operation

Materials: made from materials that are fine for short-term immersion in seawater (anodized aluminum, stainless steel, plastic).

Seals: fine for short-term immersion up to 200 meters. I think to go deeper than that, or for months instead of days, you’d want the servo to be oil-filled, which would be offered for a minimal additional cost.

Cable: the cable would be potted into the enclosure, and may not have a hermetic seal inside of it (meaning that if you got a cut in the cable jacket, the water could migrate into the servo, but with the proper cable jacket material, this risk would be very low).

Higher cost (maybe $500?):

Motor and gear train: brushless motor, torque and speed similar to a $150 R/C servo, suitable for months of continuous operation.

Materials: made from materials that will allow almost indefinite exposure to seawater (engineering plastics, titanium).

Seals: same as above, would be fine for immersion up to 200 meters without oil filling, but going deeper would require oil filling (for a minimal extra cost).

Cable: the cable would at a minimum contain a hermetic seal to protect against water intrusion if the cable jacket is compromised. A better option (which would probably add another $75-$100) is to have a true depth-rated connector and cable system, similar to what you’d see on industrial actuators and instruments.

All servos would be tested under pressure before being shipped. Obviously, I will never be able to match the cost of a mass-produced, imported servo. Rather, my focus will be on quality, reliability, and customer support.

Any further thoughts you have would be greatly appreciated…

Thanks,
Damon

Hi Walt,

It’s been a while since I began this thread asking if there was interest in a depth-rated servo. In the meantime, I have been working hard on that servo and am excited to announce that it’s ready and available for sale. Please check it out at www.bluetrailengineering.com.

The servos I’m currently offering are not oil-filled, but the servo case has provisions for pressure-compensated oil filling, and I have tested this in prototypes. I expect to be offering these oil-filled versions within a few months, and they will be rated to at least 600 meters.

These are made in the USA (Longmont, CO to be specific) and are stocked for immediate availability. I also offer underwater connectors and cables, as well as a high-efficiency thruster for use in ASVs and AUVs. I think if people peruse the website and look at the specifics of the products, they’ll find that this is the real deal: reliable products for underwater robotics at affordable prices.