Hydrodynamics: propeller diameter, rotor blade pitch, Rice nozzles, and matching a motor to all of "that"


#1

Okay, a couple things:

Ordered my Beaglebone and ROV cape yesterday, and to this point, that is the summary of my "ROV experience".

Former employer has acquired a laser cutter (this bodes well), but do not yet know it's capabilities, nor do I have any experience with laser cutters.

Picked up an old Amphibico underwater video housing for cheap.

I have access to a bunch of tools and one mechanical engineer (bonus!)

Since I am not yet committed to the body shape of the OpenROV (and haven't confirmed if I can "properly" cut pieces and assemble one), I have the option of design modification, if beneficial. My thought was to have larger diameter thrust rotors/propellers at slower RPM, housed in Rice nozzles (vs. Kort), with higher torque motors if needed.

Rice nozzle

Possible motor (brushed ESC required)

Alternate motor, but too pricey

Why would I change the original design? 4 inch PVC pipe is cheaper than clear acrylic, this got me thinking about the size of the propeller, then re-thinking motor/propeller combo, then a bunch of over-caffeinated tangents from there..

Including 3d printing a rim driven thruster (Voith, Brunvoll, IntegratedThrusters™, patents from the 70's on, etc.), proper pitch for propeller for efficiency vs. thrust, and single blade propellers.

Ideas, suggestions?

It's perfectly fine to tell me to build the original stock design and lay off the coffee!


#2

No replies? Ouch! Problably ill-formed question on my part.


#3

Ha! Just saw this post after your reply- I'm not sure how I missed it!

OHMYGODYES- please try these alternate propulsion approaches. The props we're using right now are not ideal for the way they're being used, we have no nozzle effect, and we may be drastically over powered. Experimentation is certainly needed, and the tech you're looking into is great. As far as designs we'd like to incorporate into the next revision (if someone comes up with something that works better) we'd like it to be open-source able (non-patented) easy to implement by anybody (doesn't require super special tooling capabilities and is low cost) and safe.

Please let us know if you do any experiments with alternate propulsion designs- I'd love to hear what you come up with!

Eric


#4

Thanks Eric!!

As I don’t have a physics background, it is tough for me to verify information from random Internet pages and forums. Would anyone else from the pro ROV/hydrodynamics fields like to chime in?

Just want to make the ROV have the best prop/motor set up for long run times and reasonable speed. From what I’ve read, there seem to be definite signs that a prop is too big or small: "
The engine fails to achieve designed RPM and is overloaded;
The engine passes designed RPM at full throttle, over-revs and is under loaded;
The propeller is overloaded and shows signs of cavitations and surface erosion. " (http://www.propellerpages.com/?c=articles&f=2006-08-08_Correct_Propeller)

Just received my Beaglebone (received my Cape even before that!), all other parts need to be ordered, so testing props and motors won’t be for a while! Anybody who has got there ROV together want to do some underwater tach/amp readings?


#5

Ill try my hand at making a prop testing rig! Eric and I were planning on making a thrust measurement rig, so Ill also see if we can get a optical tach working underwater (or a hall effect sensor of some sort).


#6

Hey Merrick and Colin:

I was pondering the subject of motor testing and found this thread.

I think a motor/prop test rig is really important, since there are so many variables to play with - voltage, ESC throttle, prop, nozzle, you name it.

I'm thinking that we can measure rpm by clipping a frequency counter to one of the output phases of the ESC and comparing it to ground. Just need to check it with an O-scope first to make sure there are no nasty voltage spikes that would kill the frequency counter, and also filter out the PWM coming from the ESC so that the counter only sees the motor phases. Has anybody in this group ever looked at the ESC output on a 'scope?

If I've got this right, the prop speed (in rpm) would be the ESC frequency in Hz, multiplied by sixty, divided by the number of coil poles on the motor (12?). Or is it the magnet poles (14?)?

Colin, if you can get a test rig set up by the next build day, I've got a frequency counter and a high-current power supply we can use for testing.

-Walt


#7

I think the thrust testing rig off homebuiltrovs.com (digital luggage scale/string), coupled with multimeter would be a simple start. Sounds like Walt has got some equipment waiting to be utilized!

Would a optical tachometer work 'fired' through a clear plastic, water containing storage container?

On nozzles:I've got to get back to thingiverse.com user Ttsalo with more on the particular profiles of Rice nozzles, as he already has a 3d file you can download of a Kort nozzle. Also, have to check if Rice nozzle is a current patent, as THAT could be a problem if we could not print it without infringing. http://www.thingiverse.com/thing:10775

Maybe I'll get a chance to play with propeller variables, as I wait for a cargo ship from Hong Kong for pieces..


#8

the most common way of measuring speed on a brushless servomotors are hall sensors, either digital(hall switch) or analog that give out a sinus signal.

for just speed monitoring hall switches are fine. for position control on servomotors, analog are required.

the pwm signals for the motor looks very similar as pwm signals in frequency drives for async motors, but they dont drive the H-bridge in negative, so it kind of a half-wave signal.

http://www.strategictechgroup.com/PSpice-Simulations/PSpice_simulation_hi_side_PWM_Brushless_DC_Motor_Drive.html

http://robotics.stackexchange.com/questions/261/what-do-the-commutation-waveforms-look-like-for-a-brushless-motor


#9

Happy to learn from those with the experience!


#10

And for those who have a bit of extra time:

Build your own brass props!
http://www.rcgroups.com/forums/showthread.php?t=356575

(You might have to run outdated links through archive.org 's “way back machine”)


#11

Thrusters

Here is something that I just found on eBay that maybe could be used or we could even design our own with a 3D printer. The price is right.

http://www.ebay.com/itm/Mini-ROV-Thruster-Kort-Nozzles-/130844768893?pt=US_Radio_Control_Control_Line&hash=item1e76f4ba7d


#12

If we make our own, or at least talk to the guy on Ebay, we could add some way to mount them.

I want to do some testing of open props, props in straight tubes, and hydrodynamically shaped nozzles to see what the bang-for-the-buck ratio is. First I need to build a hydro-dynamometer tank to measure thrust vs electrical parameters.


#13

Russell Kjell Coffield (interspec.org) is a member of this group, maybe I'll ask him..


#14

Hi

Yes the kort nozzles on ebay are my design we used them with 4inch 3 blade props and 750 series motor,

works very well

i can make them up to any size and i have a design with brackets


#15

Here are the nozzles with a bracket and we can make them any diameter you need

http://www.interspec.org/component/virtuemart/mini-rovs/68mm-kort-nozzles-for-mini-rov-detail?Itemid=608


#16

Russell, Wow this is great I wish I knew about these before I ordered up duct fans. Can you provide the 3 blade Prop to go with this? I would like to give these a try vs. the duct fans as yours have some buoyancy. Would be nice if you could design it with the motor mounts so we could screw the motor onto the nozzel then add the prop and away we go. The ROV I am building can very easily mount these on the side of the tube using the side mount.This is these Duct fans that I have http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=4240 but was hoping it would show the Motor mount better. If you need a better picture I can send one to you.

.