Optimizing Servo Motor Holding Torque for Underwater Thrusters

[madacho]

I'm trying to figure out how much a servo motor would have to hold in terms of holding torque, but how would I calculate this?

It will be for a horizontally placed thruster, and the servo motor will be able to turn it 360 degrees... and this will all be underwater.

The thruster itself is about 325 grams, give or take, or about 11.5 oz.

thanks!

 

[stewartcs]

I'm trying to figure out how much a servo motor would have to hold in terms of holding torque, but how would I calculate this?

It will be for a horizontally placed thruster, and the servo motor will be able to turn it 360 degrees... and this will all be underwater.

The thruster itself is about 325 grams, give or take, or about 11.5 oz.

thanks!

The motor will have to hold whatever the torque is that results from the environmental forces acting on the thruster. A diagram would be nice to be sure I have the picture correct in my head!

CS

 

[madacho]

ok here is a really simple diagram i drew up in paint...

and i also realized an issue with the wires... if it keeps spinning, the wires will wrap around and get tangled... i guess i will have to make sure it can only go from 0 to 360 and not complete more than one full rotation

thanks!

 

[stewartcs]

ok here is a really simple diagram i drew up in paint...

and i also realized an issue with the wires... if it keeps spinning, the wires will wrap around and get tangled... i guess i will have to make sure it can only go from 0 to 360 and not complete more than one full rotation

thanks!

I can't see your diagram becasue the site you have it hosted on is blocked by my company. Can you insert it or attach it as a file instead?

As far as the wires go, you can use a slip ring instead so it can rotate all it wants.

CS

 

[madacho]

ah dang, i was using photobucket.

ok hopefully thisll work

 

[madacho]

hmmm a slip ring should be just what i need...

but i still need a calculation for the torque... i don't want to buy a servo motor and see that it can't take the weight of the motor

but that being said, its not going to be supporting the entire weight of it, right? since there will be that seal with the o-rings, the servo motor isn't taking the brunt of the thrusters weight...

 

[stewartcs]

hmmm a slip ring should be just what i need...

but i still need a calculation for the torque... i don't want to buy a servo motor and see that it can't take the weight of the motor

but that being said, its not going to be supporting the entire weight of it, right? since there will be that seal with the o-rings, the servo motor isn't taking the brunt of the thrusters weight...

Are you trying to rotate the propeller as well as making the thruster azimuth?

CS

 

[madacho]

sorry that my knowledge of engineering terms is low, but I am not sure what you mean by azimuth, haha

basically, the thruster itself is one assembly. then, it is attached to a waterproofed servo motor, which will rotate the entire thruster in one axis only. so itll be able to provide thrust backwards, fowards, up, down, etc...

I looked again at the idea of a slip ring and i realized it wouldn't really help... the wires would be going through the hollow shaft but it would still need to exit somehow, and i don't see how a slip ring would help

instead i was thinking that i would just make sure (through programming) that the the servo motor wouldn't rotate past a full 360. as long as it doesn't keep turning in continuous circles, the wires getting tangled shouldn't be a problembut yea, we only have about 4 weeks until this is supposed to be due, and i need to find out what kind of servo motor i need, which will let me build the waterproof casing, then the seal, etc...

thanks!

 

[stewartcs]

sorry that my knowledge of engineering terms is low, but I am not sure what you mean by azimuth, haha

Azimuthing thrusters rotate 360 degrees. Here is a picture of one:

http://www.thrustmastertexas.com/images/photos/azimuthingThrusterCutLarge.jpg

basically, the thruster itself is one assembly. then, it is attached to a waterproofed servo motor, which will rotate the entire thruster in one axis only. so itll be able to provide thrust backwards, fowards, up, down, etc...

Your servo motor (it appears) is allowing the thruster to do the 360 but what is being used to rotate the propeller?

And why do you think the wire will get twisted? Is the motor not mechanically coupled to the thruster?

CS

 

[madacho]

Oh well I guess you can't see it because the diagramis a front view, but there is a bilge pump motor that will be spinning the propeller

And yes, the azimuth thruster is what I'm going for

And the wire will get twisted around the shaft since the thruster will be rotatng but the wire will be connected to the power supply, like a thread winding around a spool, will it not?

 

[stewartcs]

Oh well I guess you can't see it because the diagramis a front view, but there is a bilge pump motor that will be spinning the propeller

And yes, the azimuth thruster is what I'm going for

And the wire will get twisted around the shaft since the thruster will be rotatng but the wire will be connected to the power supply, like a thread winding around a spool, will it not?

Ok then, since the bilge pump motor is on the end of the shaft with the propeller, you can use a slip-ring like I said earlier, except you'll need two (one at each end of the shaft). The wire inside the shaft will rotate with the shaft and not get twisted since there is a slip-ring on each end of the shaft.

If you think programming a limit in is easier/cheaper then go for that. Just keep in mind the response will be slower since it will have to make up to a full revolution to go from 360 deg back to 1 deg instead of continuing on around from 360 back to 1.

CS

 

[madacho]

im still not getting the slip ring deal... the whole shaft will be rotating as well, so how would keeping the wire inside the shaft rotating with it help anything? the other end is going to be directly connected with the servo motor

at this point, since i only have a month now to finish, i think programming would be the best way. i realize the limitation from having to go backwards from 360 to 0 instead of just going that 1 extra degree, but it should only be a few seconds worth of time and its not that big of a deal

 

[stewartcs]

im still not getting the slip ring deal... the whole shaft will be rotating as well, so how would keeping the wire inside the shaft rotating with it help anything? the other end is going to be directly connected with the servo motor

at this point, since i only have a month now to finish, i think programming would be the best way. i realize the limitation from having to go backwards from 360 to 0 instead of just going that 1 extra degree, but it should only be a few seconds worth of time and its not that big of a deal

There are two slip rings, one on each end of the shaft. The slip ring has two parts, one moves, the other one doesn't. On the prop end slip ring, the part that doesn't move will be attached to your prop's motor. The part that does move will rotate with the shaft. Same thing happens on the other the other end. The wires rotate with the shaft and with the moveable parts of the slip ring. Make sense?

CS

 

[madacho]

yea i guess so... but i think I am just gunna go with the quick and easy method for the sake of time

but what I really need to know is how strong of a servo motor i need to get... i don't want to make all the waterproofing only to find out the servo won't be able to move the shaft + thruster... what kind of torque/holding torque would I need?

 

[madacho]

and how would the fact that its underwater and that the servo will have to be able to turn through the o-rings friction factor in? again, i don't want to order a servo that will not be able to budge it, but not get an overly expensive/heavy one either.

and the whole purpose of servos is the feedback, right? i will be able to tell at what position it is and where i want it to go?

 

[stewartcs]

yea i guess so... but i think I am just gunna go with the quick and easy method for the sake of time

but what I really need to know is how strong of a servo motor i need to get... i don't want to make all the waterproofing only to find out the servo won't be able to move the shaft + thruster... what kind of torque/holding torque would I need?

The torque required would be equal to the moment of inertia of the thruster/shaft combo times the angular acceleration (i.e. how fast you plan on rotating the assembly), plus the frictional losses you have for bearings and seals.

I'm assuming this is for a school project so above I'm neglecting any environmental effects.

CS

 

[stewartcs]

and how would the fact that its underwater and that the servo will have to be able to turn through the o-rings friction factor in? again, i don't want to order a servo that will not be able to budge it, but not get an overly expensive/heavy one either.

and the whole purpose of servos is the feedback, right? i will be able to tell at what position it is and where i want it to go?

Yes the purpose of a servo is feedback so you should be able to position it based on the feedback loop.

CS

 

[madacho]

alright, I found out that the actual holding torque required really isn't much, like sub-30 oz.-in. I think the main issue will be the friction caused by the O-rings, but hopefully that'll be lessened by some good lubrication. I just hope that it'll keep the servo waterproof...

Thanks for the help, now I am finally getting somewhere with my project