What should I use for motors in a small custom-built ROV?

propulsion

#1

Hi,
I’m really a beginner when it comes to ROVs and hardware (I prefer to code if I have a choice), so please bare with me.

TL;DR: What’s a relatively easy to use & maintain, low cost, goes in salt water solution for ROV motors?

So, I’ve been researching motors for ROVs, and the ones used in OpenROV. The problem is, from what I’ve read, brushless DC motors (like the ones you use) are really complex and expensive to control (with the ESCs and everything). I know that servos are very easy to control, and so are regular DC motors: but I can’t find any servos or regular DC motors that would be strong enough to power an ROV.
I’ve heard you can also use bilge pumps as motors.
Any ideas on what I should do?

Sorry if this is in the wrong category, feel free to move it c:


#2

If it helps, I’d like to do this with an Arduino Uno R3 hooked up via serial to a BeagleBone Black, but I’m open to suggestions.


#3

A lot depends upon the depth (pressure) at which you intend to operate your ROV. If you use bilge pumps for motors this factor becomes critical. Most bilge pumps are water resistant by means of bushings and seals which keep water from entering through the shaft opening. Pressure at significant depth can cause these seals to fail or to deform and bind the shaft. Much work has been done with magnetically coupled drive units where the motor is absolutely sealed from outside water environment. These can be purchased or even manufactured by the hobbyist with some technical and mechanical skill & the proper tools. Magnetically coupled thrusters could theoretically be powered by regular DC motors. However, I feel that brushless motors will offer better control and variety, as well as more durability. If you look on-line at e-bay you will find that you can usually purchase ESC/Motor combos at a reasonable price.

As to the issue of water/corrosion proofing the motors, I have found that the brushless motors are easier to condition for corrosion abatement. If you separate the motor bell from the armature/winding, then coat the armature with a “thin” coat of enamel clear coat paint (rustoleum crystal clear) this process works very well. In adition to this, it is equally important to thoroughly rinse the motors in fresh water after each run and spray a silicone based water disbursant lubricant in to the motor after each fresh water rinse. It sounds like a lot, but really it is quite easy to do.

Good luck…


#4

Hi Ronald,
Awesome post! Thanks for the help.

How deep would a bilge pump go? I guess it varies, but would it break below 50m? I’ve looked at a guide and they said they were “hoping the bilge pumps would go about 10m down,” so I’m pretty sure they wouldn’t go past 50m.
Are the motors you use for OpenROV 2.7 magnetically coupled? Or are they just regular brushless DC motors?

I agree with you about the durability of brushless motors. I read a forum post here that said they had better durability because of less moving parts. Generally, are brushless motors more powerful than brushed ones? I did a little search on Google and found several links to brushless motors with an ESC (some are even waterproof!) on Amazon.

Also (sorry for tons of questions), would a servo motor like this one be powerful enough?

Thanks for the tips on the waterproofing of the motors.


#5

Here is a link to a Homebuilt ROV hobbyist (http://www.homebuiltrovs.com/howstarted.html) They have built several units using both bilge pumps and also they have a design for a homebuilt magnetically coupled thruster. Right now, the OpenROV uses brushless motors. Some have modified their OpenROV’s to use more powerful commercial thruster units.

As far as “power” goes, the power comes from both the combination of the prop and the motor’s ability to turn the prop. There are a number of websites available on-line that deal with optimum prop design for specific applications, marine engineering, etc… The good folks at OpenROV have done some experimenting and testing of various combinations of motors and thrusters and I believe they have come up with a great setup for Hobbyist built micro Observation Class units.

If you are dealing with small ROV design then many components (motors, props, controllers, etc…) are readily available through Hobby stores that specialize in radio controlled boat models.

ROV thrusters (as with all thruster applications) are typically designed to provide sufficient thrust for operations in a particular environment. For example, are you operating in fresh or salt water, what depth, what degree of control you need and will there be significant flow (current) in the water column.

More often than not small observation class ROV’s are low to moderate powered because they are likely to operate in areas that do not have swift moving water or strong current. However, there are some applications where a small ROV would have need of a more powerful thruster combination. More powerful thrusters often cost much more money so most times people build to suit their intended operational need. Think of it like this, If you had an old VW car and intended to only run it around town and at low to moderate speeds then it is likely that you would equip it with an inexpensive stock four cylinder motor (most economical option). But you almost surely would not spend thousands of dollars more to equip it with a 500 hp race motor, unless you intend to race it.

Another consideration is power supply. This factor will also impact what motors can be used for thruster system. Are you using onboard batteries or surface supplied power? Will you be running a 10 to 12 volt system, 18v, or something significantly higher…

Then there is the issue of motor control. If your ROV is going to be digitally controlled by onboard computer then you will need a motor that is compatable with available ESC.

These are just a few considerations when selecting the best fit thruster for your project.


#6

Hi Ronald,
Thanks so much for your help. I always thought it was just the motors power not the design of the propellor to. I’ve had a look at the homebuilt ROVs site and they do have some pretty good resources there.
Your example of the VW car was helpful, thanks c:

I’ve been searching around and found a ton of brushless motors (some without their own ESC included). Are all motors compatible with all ESCs? Like, if I bought Motor A would I have to hunt down a specific model ESC for Motor A? Or could I just buy a random one off Amazon or something?

If you have any specific hobby shops that you recommend for low-powered hobby motors that’d be nice.
Also, what would you recommend in terms of power supply? I was always going to use on-board batteries, but should I do the power from the surface?

Thanks again. (sorry to bug you with all these specific questions)


#7

The selection and matching of ESC to Brushless motor is dependent on several factors. When I have purchased brushless motors, I have usually gone with combo packs where the manufacturer has bundled the ESC, Motor and sometimes a programming card in one package. That way I can be sure the components all work well together. However, it is not always necessary for you to do it this way. Again, since we are dealing with small scale units we can defer to the scale model hobby. I have found several good articles that address pairing motors to ESC. here is a link to one of them. It also addresses (to some degree) the issue of power source: https://www.hobbywarehouse.com.au/articles/how-to-match-esc-motor-lipo-battery-rc-car.html
If you want to research further you can do a google search with something like “How to match ESC with brushless motor”

The power source is like selecting the appropriate thruster for the ROV. We think of it in terms of what we need and what is the “best fit” for our needs. Some folks have been experimenting with providing power through ethernet or other similar arrangements on their OpenROV’s. Others run a separate power feed along side (or as part of ) the tether.

If you want a small, compact and portable ROV that you can use anywhere, then onboard battery power is perhaps best suited. Keep in mind that all onboard systems must be designed to operate on the available battery power and should be designed for max energy efficiency. Also, you may want to keep an extra set of batteries available with you on site.

The larger ROV’s normally run on surface supplied power (often rather high voltage) or a combination of battery and surface supplied. This has the advantage that you don’t need to change out the batteries when they run low. However, you always need to have a constant power source nearby to tap in to, which limits your operational location somewhat.

I’m currently using the Li-FePO4 batteries. Six of them in two stacks of 3. The advantage is that they provide relatively long (2 hour or more) duration per charge. They do require some special handling and safety considerations, but they work well for this application. Others have tried different batteries with success. A good explanation of battery discharge characteristics can be found here: http://www.mpoweruk.com/performance.htm Selection of batteries requires several considerations including (but not limited to) available space (size), power & performance, requited voltage, cost, safety, etc…

Hobby stores are abundant. It is difficult to recommend a particular one. I usually shop at Amazon or e-bay. I’ve purchased from HobbyKing, Tower Hobbies, RC mart, to name a few. For electronics and onboard computers I have used DigiKey, Cables and Kits, Granger, Edmund Scientifics, RobotShop…

Hope this helps.


#8

Hi Ronald, sorry for the very late reply! I’ve been quite busy. (also sorry for reviving an inactive thread!)

I’ve found some great boat motors on HobbyKing that look perfect, and some even come with their own ESCs!

Thanks so much for your help! I will start constructing soon.


#9

Hi guys,

They aren’t as inexpensive as the bare brushless motors used on the OpenROV, but please check out our thrusters: www.bluerobotics.com. They require a brushless ESC, but their simple to operate with standard servo PWM signals.

Best,

Rusty


#10

Thanks Rustom!

I’ve heard about the Blue Robotics thrusters, a friend showed me the Kickstarter campaign (after I posted this).
They’re very fancy and look awesome… I wish I could get some but I don’t think I can afford it. The T-100 thrusters and the T-200 are a bit expensive, but I could look into the M100 Brushless Motor.

Thanks ayways!