Actuators: Newsletter Challenge (10/31/06)

I take it the motor is exsposed to space from the description. xloo If you examinem the fomuala v=IR, R is going to be very low such that I is going to be infinite. Result the motor blew up.r

Weak answer with no conclusion.

Overheating in the vacuum of space. Not even convection to help it along. But as nothing is said about the motor's mounting configuration or whether the designer was actually dumb enough to pick a fan-cooled motor, this is pure speculation.

Temperature extremes of space. In shadow, the thing is probably several hundred degrees below zero. In direct sunlight, several hundred degrees above zero. The temp extremes could span at least 500 degrees between light and shadow. The motor is eventually screwed by thermal variations resulting in differential expansion and contraction which tends to crack and separate things, etc., including the bearings. If the motor were in direct sunlight, one side would be hot whilst the side in shadow might still be a hundred or more degrees cooler. Personally, I'd hate to be that motor for this reason alone. And if I were forced to choose this or a different fate, I'd choose to work with engineers.

You mentioned lubricants not outgassing. Okay. How about the bearings/bearing lube freezing at two-hundred-fifty below zero? (I won't mention the lubes boiling away as this is a form of "outgassing" which you prohibited as a cause of failure.)

"Traditional DC motor" still doesn't say what kind of DC motor. Permanent magnet motor, perhaps? What's the Curie Point of your typical ceramic or rare-earth PM motor magnet?

Regardless, I'd say that particular motor got the shaft.

-E

Hey, Europium! Your statement about part of the actuator being in sunlight and part in shadow, with a huge temperature difference gave me a thought. What if there were Peltier junctions in the circuit, due to joints made with dissimilar metals, pure copper and tin/lead alloy (solder) coated copper, as is often found in cable shielding? Perhaps the Seebeck effect would then generate an EMF that interfered with or even erased some of the memory in the controller.

Can't be any crazier than the guy who is convinced that the Earth's magnetic field caused similar problems. Might even make more sense!

However, at this point I would have to go along with your conclusion that some kind of thermal problem is probably at fault. Indeed, thermal expansion/contraction effects are often overlooked by people who should know better!

It's a thought. But assuming this were the case we're still talking about voltages in the millivolt range (not to mention very low currents). For example, say the materials in the motor formed an accidental E-type thermocouple. This type of TC produces the highest voltages over temperature than most (if not all) the other TC types: such as the iron-constantan J-type thermocouple, for example. Even so, the E-type TC produces only 0.077mV/degree C, at negligible current, which means you're gonna have to stack a lot of them to get an appreciable voltage. This scenario would be highly unlikely (more like impossible?) in the case of our spaceborne motor. Consequently, I think the controller might be more worried about the effects of solar flares and magnetic storms (the latter case assuming the craft is in Earth orbit) than about anything our motor could produce.

-e

Can we be a bit careful with our temperature units, please? Several hundred (^oC) below zero would be verrrry chilly! (your post #108 mentions 0.077mV/degree C).

Sorry to be pickky.

Now, there you go again, John!

I understood my fellow countryman very well, even if he is in Texas! They still use those F-degrees down there when talking in relative terms, like "several hundred degrees below zero". (hoo-doggie, colder than a witch's itty-bitty..... )

Now, he never used those kind of gross (as in very large, not as in vulgar) statements in the post your reference (#108) where he does indeed refer to "degree C" but in clearly more of a scientific/mathematical reference where the use of Celsius is clearly understood. After all, in #101, he was putting himself in the place of the motor when he said, "Personally, I'd hate to be that motor for this reason alone.", so of course he used F-degrees, which he could relate to personally!

Funny, when I travelled in Europe (England, Scotland, Germany, France, Switzerland, Austria), I never could figure out how hot or cold it was going to be with those silly C-degrees on TV, but I have no problem using them in engineering calculations!

I am sory you are so pickky, too!

Sorry, again, chaps.

I think this moan (of mine) has something to do with my problem of getting to grips with the Cs and Fs.

When I was little, ^oF ruled. If it was 30 or below, I could expect pretty frost patterns on the inside of my bedroom window when I woke up. Since some time in my teens, I've had to switch to ^oC, and it's only just becoming comfortable to use. Central heating and insulation (and maybe global warming?) have also killed the frost flowers .

I agree with your comment about using ^oC in engineering - I've never had a problem with that - I generally assume it - which I guess is what prompted me to make my post.

BTW the BBC weather forecasters still occasionally use the F-word.

How about we all use ^oK ?

"How about we all use ^oK ?"

How? Absolutely!

Okay, okay! I'll be consistent!

Rankine it is!

-e

Appy-polly-loggies, my dear Pickky. By degrees I've come to see your point. Viddy this: Sunlit side (@ 1 AU): roughly 250 degrees F (121 C). Shadow side: roughly 250 degrees F (-157 C). I guess it's all just a matter of degree.

-e

I would have assumed use of a brushless DC motor from the start,or was this option excluded by the terms "traditional DC motor"?

Garth Roodt ,City of Capetown

Yes, you're right. Using the word "traditional" has its drawbacks. Even still, I wouldn't consider brushless DC motors as yet having a traditional aspect; especially in light of the relative maturity of D.C. motor technology overall. In terms of deployment, BDCMs stand a better chance when one considers all of the computer hard drives in the world, for example; not to mention the many other applications which now use BCDMs exclusively. But as the word "traditional" also embodies a concept of age, my guess is the poster is speaking of an older, more mature technology. Is this clear as mud?

-e

How long are you really going to be using the actuator in space, was to open cargo doors or to be used during reentry, I have the feeling that information was left out and so poor instruction lead to the failure. Even so 50 hours of continous use would be a lot of cargo door openings. I beleive that it would survive not that I would wish to try it ! Surely the lubicant you would chose would have the correct properties to allow the oxide to occur as well ! So many flaws in the answer !!!

Whew,

Talk about overanaylis! A properly chosen brush type D.C. motor will work fine in both a high altitude, or space enviorment. Prime example is the hydraulic motor used on the DeltaII launch vehicle, one of the most reliable launchers ever built in the United States. This is a serieswound brush type D.C. electric motor produced by Electro Mech. You will also find it on the Beech King Air Airplane as a hydrualic pump motor. I have overhauled dozens of these units. I am not sure if different brushes are used in the two different applications but I do know a run in time is specified in both uses, to create the carbon interface layer on the commutator. On the Delta II second stage it is used to gimbal the exhaust cone for placement in orbit and again for the re-entry burn. So give up on magnetic fields, vacuums and the like. The actuator in question was either used beyond its' duty cycle or, and most likely, the motor controller or power suppply failed.

Thanks mate.

Many of us obviously have plentiful DC experience and know at least theoretically of the limitations. You on the other hand have worked with the actual device optimised for the application. The word traditional can them be referred to the correct traditional device for the job.

I think that I did say the failure could be related to exceeding the duty cycle back in post #18. Many people forget that there are both short-term and long-term duty cycles. The long-term tends to be more of an average versus short-term being related to a spike in usage. So, either the actuator saw a major overuse, right before failure, after operating well for the previous time, or there was a slight but continuous overuse, so that eventually, during one more cycle, we had the effect of "the straw that broke the camel's back".

Too bad the "official" correct answer says the failure is due to lack of the protective film that normally builds up. Is that the same as the "carbon interface layer" you described? I would have thought that the film could be created during the run in period on earth, also. Why would the author of the challenge think this layer would be destroyed during normal operation in space? Obviously the lifetime would be somewhat shortened if the layer cannot renew itself, but that should be accounted for in the motor selection and projected usage. Perhaps the motor was used way more than expected and the film did wear out prematurely, well, not prematurely, since it would have completed its de-rated lifetime if that was the problem.

But I still like my final answer in #18. Are motors really radiation-hardened for space use, or would they normally depend on shielding of an enclosure to do this? And why couldn't a micro-meteorite do enough damage to an externally mounted acutator to cause failure?

In industry, providing the correct air quality to traditional DC motors to sustain good brush life is of major concern. The max and min humidity is spec'd for good brush life. The current density through the brush directly affects the erosion of the film. I have "played" with large DC machines under extreme operating conditions and the balance between film erosion and creation can be a fine line. Once erosion begins it proceeds very quickly over the next week or two, and unchecked quickly leads to permanent damage.

What is the process that causes this erosion? If it is oxidation, that would not take place in the vacuum of space. If it is strictly mechanical friction (rubbing), then that could happen, but should have been accounted for in calculating the MTTF.

The errosion occurs because there is sparking under the brush. Even "black" commutation has sparking but it is totally hidden by the brush. The commutation process has a reversal of current in the winding that is designed to happen at the brush "neutral" position. The higher the motor RPM and the higher the current the larger the dI/dT that occurs and the worse the sparking is. (part of the dI/dT is actually caused by the moving coil!). So every time it sparks there is a small removal of "film" and the tiny pit acts as an abrasive and slowly exposes bare (soft) copper, hence the progressive rate of failure. What we want is to create a new film, but that takes humidity, oxygen, and appropriate temperature. I am not sure if the temperature is tied to the relative humidity or not. I do know that mills that bring in cold arctic air, then bring it up to temperature (to prevent frosting the motors) and do not provide additional moisture, end up with too dry air.

vacuum welding. The "moving parts" of the actuator aren't moving because they are welded together.

I would probaly go with guest #120