BLDC for outer space aplication 3

[josehdx]

thread237-347174

Hi there

I was looking for some information regarding how to choose a BLDC motor. I assume there are may post, and yes, i took a look before. But maybe i should explain my self first

This is the first time i do this. So, what i need to do, and what i have done is the next maths

I need to rotate a mass of 0.097kg from 0 to 10rev/s in 60sec. The moment of inertia acording to solidworks is 2.661e^-5KG*M^2 on the rotational axes. The radius is 0.0475mts It’s an outer space application, temperatures can change from -100 to + 200 C (extreme assumptions) and vacuum environment.

output power i found is 1.75e^-3 watts (very low) with a momentum of 2.7866e^-5 N.M. how can I choose a motor for this application? I have seen some models but they look "too powerful" for my requirements.

I have seen this supplier

http://www.micromo.com/brushless-flat-dc-gearmotors

But i dont understand too much this data (torques at differents RPMs)

http://www.micromo.com/media/pdfs/1202_BH_DFF.pdf

I can contact directly to the seller, but first i would like to know your opinion an experience

I made a spreadsheet regarding the data i have. i am attaching a print screen.

I have done what i can but i really need some advice

Thanks so much for your cooperation

Jose

 

[imcjoek]

They are giving you stall torque (torque at zero rpm) and a slope of the torque curve as the torque curve of most brushed/brushless DC motors will roughly be a straight line down to zero torque at no-load rpm. Alternately you can use Kt and Ke to arrive at these values. This part is readily googleable. (DC Motor Torque Curve).

That said, now I'm going to be a bit of a jerk and say this: If you're sending stuff to space and you are stumped with some basic motor torque curve stuff, then you just might be slightly in over your head. Is this a cube-sat or something?

 

[josehdx]

Hi IMCJOEK

Yes. this is a small sat.

It is my first project here and i have never chosen any electric motor before. in fact i just make thermal simulations, but now i need to make this device.

I have done the next tasks: I selected the 1608 BH motor, i have plotted the next graphics

Torque vs speed

Power vs speed

i can or i should plot more like efficiency and current. So far i do not have other values like voltages, but they will be known soon

So, according to my literature review. i should check and compare graphics for various models

Then i should select the best motor when comparing the curves, for example efficiency at "X" number of RPM

is this method right?

Thanks

 

[Brad1979]

You are worried about performance when what you should be worried about is the environment. Make a list of all your requirements and send them to motor manufacturers. They will find the right motor for you or they will tell you how much it will cost to make the right motor for you or they will tell you they don't want your business.

 

[imcjoek]

is this method right?

Depends what the goal is.
Accelerate a small mass to 600RPM in a minute. But what else?
With the highest efficiency? (best conversion of electrical->mechanical power)?
With the lowest mass motor & drive electronics?
How close does it need to be to 10 rev/s? Does it need to maintain this speed?

10 rev/s top speed is pretty slow for all of these motors. They will do it, but they will be way off their peak efficiency and control will not be great if you require fine speed adjustment.
Some kind of gearing or small belt would be beneficial. If you decide to add speed reduction, you will need to check that the efficiency loss and mass gain inherent in the gear/belt does not trump the efficiency gained by operating the motor at a happier speed.

 

[MikeHalloran]

You also have to make sure that the bearings will live in a vacuum and that any needed lubrication scheme will work in zero gravity.

I.e., you really need help from someone who has done this stuff before.


Mike Halloran
Pembroke Pines, FL, USA

 

[josehdx]

Hey thanks so much for your answers

I know some few stuff about this

For example, i don't want speed reduction with gears, because it increases the chances of failure.

About vacuum conditions and lubrication, well it should be dry contact, we should modify the bearings from metal to ceramic. I am in charge also of the friction study

About efficiency, this can be important but not as important as the accuracy on the RPS and to keep this constant speed. And just for the record, if i choose this motor for example, the lifespan would be reduced significantly because it would work out of the optimum performance

I have contacted with some suppliers with my requirements, and for example the operational temperatures of many motors available are not matching with the requirements on space, too cold and too hot) but i will deal with this because i am the thermal engineer ha ha.

On the other hand maybe a stepper motor is not a bad idea. It can have micro stepping and the movement can be smooth enough

Thanks so much for your cooperation!

 

[mikekilroy]

ABSOLUTELY NOT.

Wow.

Right way to pick motor to go to various motor mfgrs with your written requirements (RFQ) and ask for a quote; those who can will contact you back. Then open discussions with them on details. You need something like 50 man years or more of experience to successfully get the motor you need - you cannot do it yourself. For example, you think only ceramic bearings will work in space. You want a motor rated 180C MAX temp to run in 200C and produce output - that is not how it is done for space.

http://www.KilroyWasHere<dot>com

 

[btrueblood]

What Mike said (both of them). Additionally: for some s/c the outgassing of lubricants and coil insulation etc. etc. are no-nos (that vapor will plate out somewhere cooler, often on sensitive optics, or heat shielding reflective surfaces, or....

 

[mikekilroy]

If you are willing to write a spec to send around for quotes, then you should have companies to send to. I can begin a list for you of some I know who are heavily involved in space rated motors:

Moog.com
Kollmorgen.com

Others on here no doubt also know potential sources for you.

http://www.KilroyWasHere<dot>com

 

[josehdx]

ok mike

Thanks so much for the comments

I will writhe the specs