Actuators: Newsletter Challenge (10/31/06)

Methinks they forgot the dielectric properties of air versus vacuum.

The devil is in the details.

In general, I believe that the problem is with motor.

Specifically, working in vaccume space has different working parameter.

Hope I am correct.

I think they forgot the space temperature, on earth the temperature is common, -40~+50 Celus degree, but in outer space it is very different, the DC motor can't work at all.

If the system passed the "qualifying tests" and then failled in space, the tests were obviously not good enough. Perhaps the motor requires air for cooling or it could not stand the G forces of launch. So many possible causes can be thought off. But then a space agency should be capable of checking these things before launch.

Quote from OP: "To keep the controls simple, you select a traditional DC motor as the prime mover."

This tells us that it is a brushed DC motor. My guess is that the brushes were not of the type that would last in space - the motor apparently did work for about a month in space.

Another possibility is that the worn off brush material would not 'fall' away like they do under gravity and stick to the commutator, causing short circuits between different sections.

I'm going to go with heat. Jorrie, I like your thought but I think the centrifugal force would tend to disperse the brush shedding and I think it would take longer than a month since brushes, if used within their design limits, last quite a long time so I don't think there would be enough wear in one month to cause a problem even if the particles don't disperse. But a "conventional" DC motor operated at its design spec in a vacum with only radiation as a means of cooling would probably not have sufficient radiating area to maintain a safe operating temperature.

By the way, with regard to the space agency and mistakes, if we can miscalculate course because we used the wrong units...

You are absolutely right. Brush dust may collect at the brushes and collector/conmutator ring. Next time use another aproach, maybe variable frecuency induction motor. Regards

Luis

I think Jorrie is right at the edge of the right answer here.

If I recall waaay back at the Academy, there was mention that our experiment for the shuttle could not use a motor with brushes, since in a vacuum the slight sparking (that caused no problem here in atmosphere) would cause the motor to eventually weld itself into place.

That depends on the brush composition. Carbon brushes, for example, don't weld at all.

-e

System was spec'd for vacuum, but not temperature range or radiation hardness.

Problem delayed one month. Eliminate launch stresses.

Insulation test passed. Eliminate temperature.

Can't get to move at all. Eliminate zero gravity.

Due to inadequate shielding in-situ, my DC motor was de-/re-magnetized by induced current over the course of a month as the vehicle passed W-E through Earth's N-S magnetic field.

"Traditional" DC motor probably means it has a field coil and no magnets to de-magnetize. Even so it seems doubtful that <1 gauss would have an effect.

In the words of Carlos Mencia:

"Dee Dee Dee"

I believe you are right, but the demagnetizing came from the effects of the Van Allen belts radiation.

For those who believe temperature is the cultprit, they need to look at the basic definition of temperature, it is the measurement of kinetic energy. In a vacuum there are very few molecules and thus the temperature is extremely low. The testing eliminated temperature.

Many DC motors are brushless, since we weren't told any specifics about the motors, we shouldn't assume it had brushes. Therefore, we can eliminate brushes as a culprit.

Dielectric failure? I think you will find that a vacuum is an excellent dielectric. Actually we're talking about DC so we should be concerned about insulatuion resistance. Again, a vacuum is an excellent insulator.

Chaz

I tend to agree with rcapper on the lack of heat disbursement. The question doesn't specirfy if proper testing was done on motor starter. Could the lubricants fail to remain adhered properly in a gravity free envirement causing the motor to short out.

It wouldn't short but if it were to get on the commutator it might prevent current flow from the brushes. It would seem that there would be adequate pressure to maintain contact though and I'm not sure by what mechanism the grease would migrate. If anything I would think there would be less migration since gravity causes grease to seek the lowest point if it becomes warm enough to flow. In zero gravity I would expect cohesion to tend to keep it distributed more evenly in place.

By the way, I like your tag line!

I say when you have millions of possibilities for the failure, it's most likely the most simple one that occured.

That said, I would guess some cable ruptured or there's a power suply deficiency.

Perhaps the "commands" are not working?

Else, due to zero g enviroment, the motor could be working faster than normal and overheating the "on earth" defined lubricant fluids?

What about the other extreme of temperature: the lubricants froze?

"The system goes through all its tests with flying colors."

Would have had to pass a cold temperature test.

That's what happened in that late 1940's movie, "Destination Moon".

a DC brush requires air to produce a small amount of arc to keep the conection clean without that the brushes will not keep contact and fail.

Electricity will arc in a vacuum. The potential for jumping a gap is slightly different but since we have contact that shouldn't be an issue.

This may be one of those questions with more than one "right" answer because the challenge failed to be specific enough in its requirements.

So far, it looks like the debris issue could be a winner. Without gravity to remove it, a microscopic debris cloud cold be saturating the inner workings of the motor after one month. However, I would think it would be a fairly poor quality motor to wear that fast. Of course it also depends to some extent on how often and how long it was used, its duty cycle.

Another possible winner is the recent posting from Evar: "a DC brush requires air to produce a small amount of arc to keep the conection clean without that the brushes will not keep contact and fail." However, I do not know enough about DC motors to confirm or dispute that one.

Also, without (natural ) convection currents that exist in an air/gravity environment, the motor could become overheated faster than on earth, even though it may have passed a temperature test. The time period that passed could either be due to a slow buildup in heat, or a chance, one-time overload due to a high tasking period where the normal duty-cycle may not have been exceeded, but the actual lesser duty cycle in the vacuum environment may have been, causing a burnout. On the other hand, there may be sufficient contact between the motor and its mounting and/or the actuator mechanism to allow sufficient heat to be conducted away.

One post argued that the earth's magnetic field might be interfering with the motor, magnetizing, or de-magnatizing it, but that was disputed by another poster due to the weakness of the earth's magnetic field.

There is another possible answer, which JohnDG might have come closest to when he said, "System was spec'd for vacuum, but not temperature range or radiation hardness." I believe that vacuum and temperature would be normal considerations, but radiation hardness might not, especially if this "spacecraft" is unmanned, or if the actuator, being in vacuum, is outside the shielding of the crew area.

It may be that all of the "normal" tests did not consider the effect that cosmic and/or solar radiation might have on the motor's magnetic field or on the breakdown of insulation. There is even the possibility that it was hit by a micro-meteorite, which tore a hole right through the soft casing and windings (aluminum/copper).

Therefore, I will state that my "right" answer is inadequate shielding from external forces, either micro-meteorites or solar/cosmic radiation.

Rad-Hard is a standard operating procedure for all electronics used in manned and unmanned space vehicles. But then again, this is not reality we are dealing with, it's a challenge question.

There is nothing wrong with the motor. The radio receiver crapped out preventing signals getting to the motor.

Traditional carbon brushes require (non condensing) humidity and an adequate temperature range to establish a film on the commutator. Without an adequate film the copper and brushes wear excessively fast. However, if the motor had been in service the existing film should have been erroded by the normal commutation process but could not be re-formed causing permanent damage.

If the motor ran a month we can eliminate thermal issues.

"If the motor ran a month we can eliminate thermal issues."

Not necessarily. See my post #18 above. Varying and excessive duty cycles and/or long-term heat buildup in the vacuum of space could be the culprit.

Dragline MG set brushes exhibit exactly this behaviour. Brush grade needs to be matched to the conditions especially humidity. Wrong grade for changed weather conditions can destroy brushes in less than a month, the resulting flashover isn't a lot of fun to fix.

Folks: The question doesn't say that the failure happened after a month of working. It states that it happened "A month later". A lot of systems on spacecraft are not 24x7. It also doesn't say that the motor failed, just "the actuator system". Yes the motor is part of that system; but NOT the only part.

Having worked as a systems integrator I can tell you that the "marriage" of different components is usually made somewhere South of Hades.

I have to go with Jorrie.

It's the brushes, with this addition.

Graphite acts as a lubricant in a gaseous enviornment.

In a vacuum it has been found to act as an abrasive.

Surprised me too when I first read it, but on consideration, it makes sense.

"No matter how hard you struggle to cram The Theory into your head,
The Practice will find a way to bite you on the ass." - Pragmatist

Could be lots of things:The interface between the motor and command control module, the coupling between motor and actuator could have shattered due to the extreme cold, a travel limit switch could have failed,, allowing the actuator to travel beyond it's limits and jam, or be unable to complete the return circuit.

The only thing the boss told him was it "Failed to respond" I would presume that the software would supervise the operation and generate an error code.In the absence of an error code, I would say a communication failure, either internal or external is the problem.All limit switches would be supervised, as well as voltage and current, so that a remote diagnosis would be possible.

Yup, another lame-o "challenge" with lots of possible "right" answers.

See my motion to petition Global Spec to name Challenge authors in post #49 on the Pharmaceutical Plant "Challenge".

And yet, although I submitted a "Challenge" for consideration weeks ago, and other than, "thank you for your submission", I have yet to hear anything back from CR4 or Global Spec. And my "Challenge" has ONLY ONE possible "right" answer!

Some may already know it, but it can't be any easier than the simple Algebra problem involving a workers paid and unpaid time that was passed off as a challenge a few weeks ago!

Can I second you on this for the benefit of this thread. These challenge questions need a shake up. The only good thing about the algebra question was all the self doubt expressed because it was so easy!

These questions need to be clear with only one answer that fits the circumstances described. In this current example the desire to make a good story ("failed to respond") has left the field wide open for anything from some exotic vacuum induced failure to the whole thing just crashed into the Pacific ocean.

I want to see it!

Obviously, they used Windows Embedded!

The motor failed because of overheating. iin a vacuum the heat does not get dissipated because there is no medium.

Nothing has been said about space rated components, or whether the actuator system was tested in a very cold, hard vacuum. You also have problems with the temperature extremes in space - typically very low temps. If the motor generates heat - which dissipates quickly - you still have a large temperature differential in the motor - so parts which fit and worked at earth ambient temperatures, won't work in a space environment. Another possible problem is - was the moisture baked out of the system before it was sealed. Icing before it sublimes/evaporates can be a very real problem.

Don @ NGC

Ordinary D.C. motors use carbon brushes; these become very abrasive at high altitudes or the vacuum of space; consequently chew up the commutator. Both oxygen and water vapor are required for proper commutation of carbon brushes.

Why haven't you considered metal fiber brushes. They eliminate most of the problems of carbon brushes

could be anything, space junk jaming the works, woodpeckers attacking the foam insulation, heat shelding falling off during takeoff, leaky fuel tank seals, flat battery, alein abduction, need a bit more information

After reading all the "second guesses", the best and only answer is-you picked the wrong motor.

The motor requires constant air flow, not only for cooling, but for the removal of copper filings and carbon debris caused by the wearing on the armature; this can not only cause shorting inside of the windings, but the ceasing of the brushes and springs. The other variable not given could also be temperature.

"ceasing of the brushes and springs"

Do you mean "siezing" ? "Ceasing" is a good word, just not the right word to use in this context. In battle, when the officer yells, "Cease fire!" the soldiers merely stop firing, not jam their weapons!

Siezing is a bad word and not the right word to use in this context. However, seizing is a good word and spelled correctly.

You are absolutely right about the spelling! I guess my elementary school teacher who impressed me with "I before E, except after C" can take the blame for this one!

But the word is correctly used, as compared to "ceasing", which was used by Guest #34.

Guest #85, are you the same person as #34? I wish you guys would register! Without a handle to refer to you by, you are more like Ghosts than Guests!

The performance specification DID NOT include all the environmental requirements.

Could have missed a high temp cycle that erased the magnets like a recording tape.

Could be a missed low temp cycle that caused a bearing failure.

Could be the motor was not tested in a vacuum and overheated because of lack of thermal conduction. I've seen a PHD do this one before.

The what went wrong is the specification of requirements was missing all the environmental requirements. No requirement No test of the system in is failure mode.

The requirement was missed because of a lack of design review outside the organization.

The cause as we see it would be because of sub zero temperatures causing the commutator brushes to hold the shaft tight Jamming the rotor ( differential thermal contraction).

Vivek and prashant.

Graphite (used in the brushes of the DC motor) loses its lubricity in vacuum. The slipperiness of graphite depends on the presence of water, which boils off in vacuum. So the commutator is abraded very quickly by the brushes. (There other brush materials, but graphite is the most common.)

The windings on the rotor of DC motors are usually cooled by the flow of air past the rotor. In the vacuum of space there is no convection as there is no air. So the windings melted - they can't burn as there is no air. Insulation may also have decomposed with the high temperature.

"The motor is not responding to commands" Not another metric system/standard system programming snafu?

The brushes arced on start up, caused a radio interference signal, crashed the spacecraft's computer.

You see, Occam? They all got it wrong! This type of second guessing is exactly what your "boss" wants you to do, as he desperately needs to scapegoat you to HIS bosses for at least one of the other 2,000 issues this new spacecraft will face...

"We're sitting on four million pounds of rocket fuel, one nuclear weapon the size of kansas, and a thing that has 270,000 moving parts, all built by the lowest possible bidder..."

"We spend two hundred and fifty billion dollars a year on defense, and here we are, the fate of the planet is in the hands of a bunch of retards I wouldn't trust with a [simple actuator]!"

And my favorite with regard to this thread: "American components, Russian components... all made in Taiwan!"

The real answer to this problem, then, (similar to that of the last question concerning the fate of two engineers working for a drug company...) is that you weren't born your boss. If you were, you could be scapegoating some other poor shmoe who did everything right.

What went wrong? Let me try again.

I did my homework…using references for motor actuated systems operating on Earth's surface. My system tested out fine—on Earth's surface. My system was space rated and passed all qualifying tests, on Earth. So if my system would not have failed at ground level, and it passed all tests at ground level, what operational demand in orbit could not have been tested at ground level? My boss says that the actuator system, not the motor, has failed (has become inoperative and won't move at all). The motor controller has also reported to ground that the motor does not respond to (its) commands so, since commands can be processed by the motor controller, and since the motor controller can report back non-response by the motor, then it is unlikely (even if possible) that the failure exists in the motor controller—such as a fault in a controller or controller/timer card. Most likely, the failure is in the motor itself. But, what would the failure mode be? Since the boss did not mention any BIT responses from the motor (which might give a clue as to the failure mechanism or mode) it is reasonable to start with the assumption that the motor has failed dead. If, for example, the actuator mechanism was jammed (say, by a timing fault), the motor's attempt to move it would be sensed as an out-of-parameter fault and reported to ground. But there are no responses at all. Now, since the system passed all tests on the ground, I am drawn to postulate that the motor would not have failed but for its attempted operation in orbit or space. I am now led to tentatively conclude that I should not question those tests that were performed but, rather, look for a test that was not performed, prior to launch. The only such test which was not (able to be) performed, the only criterion that my design references (my homework) did not address, was operation in a zero-gravity environment. With that realization it occurs to me that my motor was rated for (and tested under) manipulator inertial conditions existing at ground level. It further occurs to me that the motor selected "expected" to encounter loads far in excess of that which it actually encounter in the zero-g environment of space. So, the question I must now resolve is: how could the DC motor be damaged (operate for a time and then fail catastrophically) by moving masses offering no, or far less, resistance to being moved than that for which the motor was designed. And, if the "under loading" of the motor (or over speeding of the manipulator) led to a motor or wiring overheat condition, is it possible that the insulation intended to assure motor operability and availability instead contributed to the total failure of the motor by exacerbating an overheat condition? And, does my boss's anger also reflect anger at himself: either for not knowing and detecting a potential problem when he signed off on my design; or for knowing but failing to exercise appropriate supervisory oversight. (To be continued.)

An inertial load would be the same in zero-g

I must apologize for not being more specific...and assuming meaning would be intuited from context. In particular the term was with reference to the gravitational force, at nom. 6370 km between Earth's and the manipulator's centers of gravity, as opposed to the motor's/manipulator's operation under conditions of orbital free fall. It would seem that if a motor was specified (and test qualified) for moving a manipulator (of unspecified mass but more likely than not large) under 1g conditions, then (in order for the ground testing of the motor to be valid), the manipulator might(?) need to be dampered (in some way) to compensate for force application by a motor operating under "inertial" conditions (in the broad rather than textbook, strictly Newtonian sense of the word) for which it (the motor) was (possibly) not designed and (with virtual certainty) not tested on the ground. If this is a reasonable conjecture, then it follows as also reasonable that a failure to consider the change in motor loading in space might have been overlooked...in part because "homework" might have involved resort to (shall we say) "classic" design literature.

"(To be continued.)"

Must you?

I like your investigative style, like Sherlock Holmes, when you eliminate all the possible wrong answers, the remaining answer, no matter how improbable, must be right!

You did leave out one testing condition that could not be simulated on earth, which I mentioned in my post #18 above. That is the bombardment of cosmic and solar radiation as well as micro-meteorites. Without proper shielding these external "forces" could eventually or catastrophically destroy the motor's effectiveness.

See next

Thank you. I had considered radiation, in particular the Van Allen belts (as suggested in another post), but it seemed very unlikely that the craft would be operated in this "environment" or that shielding would have been inadvertently overlooked...or that "homework" during design phase would not have revealed a need for such shielding. In somewhat similar fashion, as regards collisions with meteors/debris (and other "acts of God"), it seemed to me that there was no "clue" within the challenge that would reasonably lead to consideration of such an explanation of "what went wrong" (presumably on the ground, before launch)...so as to cause "my" boss to place blame on me (as opposed to on God). (But, as you have said, it is not unheard of for challenges to involve answers that are either, not exclusive or not logically sustainable--or are even based on unstated "facts" that are un-infer-able, or on assumptions that could only be described as fanciful.)

No, no, no and no. Unless it is a series wound motor, why would it overspeed without another cause of fault eg., open circuit field winding. When a DC motor overspeeds it doesn't burn the wiring or create excess current, the back EMF generated reduces current, but it sure as hell turns the armature windings into a nice bird's nest. The resulting flashover can then cause consequential damage. Failure analysis in flashed DC motors can be a puzzle, but basic theory eliminates improbabilities.

Brushes are not mostly graphite but modernly electrographite. Natural graphite is very seldom used. Special grades are designed to work in low oxygen environments. This kind of failure was first found when planes started flying higher than the atmosphere, in low oxygen media, and their flapper motors lasted very short. This led to the development of special grades. Patina formation needs air to oxidyze copper, and mositure adsorbed to give good sliding and current conducting capacity. If not the commutator surface is attacked by drag and arcs and fails shortly. The solution is to use special grades or brushless motors.

Of course as somebody said in another answer, I'd like to see it because many things may fail and not be described in the question. This is one I consider very likely to happen, having been for 15 years the chief engineer in a brush manufacturing plant and having assisted users on their brushes problems (none of them in the stratosphere, of course, I read about this in specialized books).

Not enough back round on this problem, 270,000 moving parts was a DC motor used in the past?

We are aware of many of the weak points in this type of system.

Brushes, couplings, temp, vibration, encoders, resolvers etc.

Due to the angry boss, we may never know the real fault (PYA).

Example, the coupling if loose would have the motor spinning with no feed back signal. But we don't know if we have a coupling or encoder or resolver, or cheap switches in the linkage for retract or extend position.

If Nasa is monitoring the motor amps they would know the motor is spinning, but we don't have that knowledge.

What are the spec. for the DC Motor, what is the duty cycle(repeat Question).

Do to above lack of knowledge, my guess would be to blame it on the mechanical part of the system, nasa wouldn't have put a DC motor in space without knowing the pit falls.

The answer to this question is very similar to the editor's answer for the prior question. There is no way to determine the cause of failure using the information given. The motor was thouroughly tested in a simulated environment so as far as we know, the design was sound. However, this was a critical system, and there was no way to repair it if it malfunctioned. The engineer designing the system needed to build in redundancy. This way the backup system would have taken over in the event of failure of the primary system.

No no no... you have it all wrong! THIS is what happened:

When using a DC motor, the commutation produces alternating current through the windings. This produces RF energy based on the RPM of the motor. Also, DC motor brushes tend to arc under use. Lets assume that the motor operated (once at steady state, ignore acceleration) at X Hz. This became a serious problem for your motor, since about 1.5 parsecs away there was an ailen vessel whose entire communications system was based upon a transmission frequency of X Hz. This normally friendly race would have ignored the "blank" transmission, however the arcing of the brushes embedded a message into the transmission from your motor. It said something along the lines of "Florg jab glorg bag berlal glugnuujmin!" which is a very bad thing to say to that particular race. Naturally, they brought several warships over (which took about a month) and destroyed the offending DC motor. Then they went home.

(hee, hee, hee, hee.....) That made my day!

You forgot to mention that these warships flew in undetected by Earth Defense because they were the size of a pin-head (no, not the guy who designed the actuator!). In fact, their attack plan was to hide within an incoming micro-meteorite shower, then, when within a few dozen millimeters of the enemy, let go a salvo of photon torpedos (one photon each!), which when placed in the right spot, was enough to fuse the motor's ball bearing in its race, effectively ending the life of the motor and clearing the airwaves (spacewaves?).

As they went home the victorious nano-fleet sent out this warning message (which could just barely be heard above the noise level) to any other would-be Earth aggressors: "Fleegle glib gzob bzzt. Zepht RUK!" , which, roughly translated means, "Now you know how WE ride. Peace OUT!"

You studied the mechanics, but not the electronics.

If we assume the DC motor was chosen because it's easy to reverse the direction of rotation be reversing the connections, then you probably put the motor inside a bridge circuit. Furthermore to avoid all the complexites of power transistor drive circuitry, you chose nice simple power FETs to create the bridge. The Hexfets you used to switch the motor were damaged by radiation, causing the Vgs(on) of the devices to rise above the driving voltage from the actuator circuit (5V TTL).

The actuator thinks it's turning the motor on via the FETs, but the FETs are not actually turning on; not enough current is actually flowing to run the motor.

Wargames,

"I tried that already! Don't you think I would have tried that?"

See, Occam? Now they're thinking logically.

If the answer comes back, as just surmised, that the poor dumb shmoe gets fired because he didn't plan in redundancy, even though his limited budget and consequentially limited design were both "signed off upon" by his boss, then we must look towards a different maxim:

"The first rule of government spending: Why build one for one-and-one-half-trillion dollars when you can build two for twice the price?"

Or, "You don't really think they spend $40 on a hammer, $60 on a toilet seat, do you??"

----------------

"It's cynicism that brings us all together."

"You don't really think they spend $40 on a hammer, $60 on a toilet seat, do you??"

I don't get it.

A quote from Independence Day. The President of the United States was angered that his Chief of Staff was claiming "plausible deniability" in keeping Area 51 a secret even to the President... The President looked around, marveling at all of the space toys retrieved from Roswell and the like and asked rhetorically, "I've seen the bills from Congress; where do you even get the funding for all of this?" To which Judd Hirsch's character replies, "You don't really think they spend $40 on a hammer, $60 on a toilet seat, do you?"

In reference to last week's challenge: If you have a budget of X to build a working Actuator, it is positioned by the Newsletter Challengers that you either spend 2X to get it done right and lose your job for overspending, or you spend X for adequacy and lose your job due to scape goating.

It seems to me, from my experience, that the only way to solve this equation is to spend 0.8X, use parts and solutions from swetheart deal companies that are probably substandard, get the explicit seal of approval from your manager, as well as kudos that you did it all under budget and mostly in-house. Then, 'leak' an anonymous and confidential memo to your boss'es boss'es boss that you were forced to do it this way and there is no way it will work - but you warned both of your bosses even though they wouldn't listen to you. If you get fired for breaking the illusory chain of command, then so be it - but it could just as quickly go the other way, which is advancement. Either way, firing or advancement, at least it will have been done by your own hand - doing something proactive towards keeping your job and/or advancing - and not waiting to be the scapegoat.

While I promised in the other thread (Pharmaceutical Plant) NOT to post anything more (to that thread), since you brought it up here, I have GOT to respond!

I have been the hero and the scapegoat several times in my career, and never took the course you suggest. I am not sure that I could live with myself if I did such a dishonorable, despicable, and dishonest thing like that. Sure, it might help your career, but I for one, would have trouble looking at myself in the mirror every day, knowing what I had done.

Golden Rule: Do unto others as you would have them do unto you. (Treat others the way you would like them to treat you.)

Brass Rule: Do unto others as they have done unto you.

Iron Rule: Do unto others much more than they have done unto you.

Lead Rule: Do unto others before they get a chance to do unto you!

I, myself, subscribe to the Golden Rule, at least I try to. This world would be a much better place if everyone else did the same.

That is a very funny quote about the hammers and the toilet seats, by the way! Thanks!

Hey, please don't get me wrong: I'm cynical, but I'm not anti-social. I'd rather lose my job for doing right than keep my job for doing wrong - but as long as you're not straight out of college with a grin from ear to ear about how life is fair and wonderful, you've probably had first hand or second hand experience with what I've described here!

I TOTALLY believe in the Golden Rule, philosophically - and when it comes to MY integrity - I follow the Golden Rule as a not so altruistic course. As you mention... *I* can't look at myself in the mirror without integrity. And yet integrity, alone, doesn't put braces on the kids or pay the mortgage at the beginning of every month!

There's a management creedo that reads, "Be an iron fist in a velvet glove..." Hey, where's the love? Where's the Gold???

So there must be a happy medium in the real world between doing right *ALL OF THE TIME*, and pleasing our Pointy Haired Bosses in this Dilbertized view of the world.

To begin with, the only information that was provided never mentioned anything about the system working for 30 days as some have suggested in their comments.

The problem simple states that we have a DC motor that has been specified with grease and insulation that is compatible with a vacuum. This would seem to me to eliminate brush lubrication issues. The problem statement also says that "homework on all of the mechanics and system dynamics" has been done. It does not say anything about environmental conditions other then a vacuum.

Furthermore the problem states that that the system goes through all of its tests and "passes with flying colors". From this statement, coupled with the statement just before it, I must assume that tests were done in a vacuum environment, which it passed.

What is not mentioned in the problem is any reference to the temperature conditions the motor will be subjected to in space. I typed into Google "what is the temperature of space" and came up with a website that comments on it. The web site is http://www.faqs.org/faqs/astronomy/faq/part4/section-14.html. Below are the main points of what was stated as to the temperature of something in space:

"Empty space itself cannot have a temperature, unless you mean some
abstruse question about quantum vacuums.

However, if you put a physical object into space, it will reach a
temperature that depends on how efficiently it absorbs and emits
radiation and on what heating sources are nearby. For example, an
object that both absorbs and emits perfectly, put at the Earth's
distance from the Sun, will reach a temperature of about 280 K or 7 C.
If it is shielded from the Sun but exposed to interplanetary and
interstellar radiation, it reaches about 5 K. If it were far from all
stars and galaxies, it would come into equilibrium with the microwave
background at about 2.7 K.

Spacecraft (and spacewalking astronauts) often run a bit hotter than
280 K because they generate internal energy. Arranging for them to
run at the desired temperature is an important aspect of design."

Continuing with the analysis of the problem, the original problem statement says that the motor is insulated. According to the information I found concerning the temperature of space an object insulated from the sun but exposed to interplanetary radiation would reach about 5 K or about -450 F (-268 C). I know of no "traditional DC motor" that can operate in such an extreme temperature environment. The tolerances of the bearings etc. would too tight for the motor to properly operate or for that matter "heat up" as some have suggested.

A good comparison would be to the Germans defeat by the Russians in WWII because of the tight tolerances the Germans used in their weapons causing them to jam and not work.

All the other theories are red herrings. They do not adhere to the specific information in the problem statement, as limited as it is, and should eliminate the discussion of them. For example it is true that sun storms can emit radiation sufficient enough to destroy electrical equipment but the actuator system arrive in space only a month prior to using the actuator for the first time. Sun storms with enough radiation to destroy electrical equipment do not occur that often.

Hundreds of satellites orbit the earth and never get hit by a micrometeorite damaging them enough to not work.

Any discussion as to the brushes should be eliminated because the system was tested with the knowledge that it was going into a vacuum.

Unless the motor was specifically designed for -450 F it will NEVER work!!!!!

Had to get to the end of current entries to find agreement with someone (Orino). I thnk the motor simply "froze to death".

"You're careful to specify insulation and lubricants that won't fail due to outgassing in the vacuum environment of space."

At what temperature? Vacuum outgassing rates typically are measured at some common temperature (usually room temperature). If the motor did have a thermal buildup, the outgassing rates would have increased substantially, creating a cycle of mass loss, more heating, more mass loss, more heating, failure. The question also does not specify if the insulation is thermal or electrical.

Obviously "going through the tests" cannot possibly accomodate every possible perameter in a resonable amount of time, otherwise they would have caught the problem on the ground, nor can it accomodate the statistics of the situation. Running one apparatus for a few tests, even with success, is not statistically significant enough to ensure total success. One must also consider these two possibillities:

1) they tested the actuator rigorously for an extended period of time (months? a year?) and sent that same model into space, where it died after one month, but had a total runtime of much longer. Also, the gound tests probably put more wear and tear on the actuator, because it had to work against gravity. That wear and tear could have created more friction ,either directly or by small particulate debris, that lead to more heating

2) they sent an untested replica of the ground tested actuator or completely rebuilt the original after testing (for cleaning purposes) assuming it would perform exactly the same.

What is a magnet in the vacume of space without the earths poles to influnce it?

Traditional DC motor.......Testing before launch..........No statement the motor actualy worked at all in space only that it failed........ Fixed magnet motor with a electro magnetic armature. Is a Magnet a Magnet in space?

The motor did not freeze as it passed it's tests which would have included cold. The motor is not responding to control commands. It does not say it started to and then failed. Rather it won't respond to commands. I would consider a computer glitch or it could be another metric system/standard system programming glitch. This can never be resovled with the given information becuase there is no follow up investigation for us to consider. Since we have no more info as to, did the motor work when the systems were tested after launch? Did it first start and then stop? Then we are wasting our time quessing. Since we don't have any other info to go on if I was the engineer with my neck on the line I would tell my boss. "My/Our system is fine the problem lies in the computer controlling the spacecraft which the engineers in charge of it's design parameters failed to account for my/our system and the computer can not 'talk' to my/our actuator." Also as the program coordinator did not specify a backup system for redundency I would lay the the fault on his/her desk.

Wow, is everyone really into laying fault, and blame? Why not just admit that you do not know what the problem is, defend your actions as best you can, point out the possibility that there could be other factors outside of your design, and move on. Tell your boss you will head up the investigation, if he approves, and work with the others involved (where the problem might be) to determine the cause and any possible corrective action.

Laying the fault at the desk of the program co-ordinator who did not specify a backup system" my just come back to bite you in the butt. How do you know the PC didn't ask your boss for a redundant system and he politely refused, expressing confidence in your design? May be HIS neck was stuck out, and you KNOW what rolls downhill, right?

One of the highest compliments I ever got was from a Project Manager who told a colleage of mine about me that, unlike other engineers on his team, if there was ever a problem with one of my designs, I don't just complain that "it can't be my fault, my design was perfect", but that I say I will find the problem and fix it, even if it was not my problem. And 9 times out of 10 the problem will lie with someone else, but what everyone remembers is not who was to blame, but who got things done!

"And 9 times out of 10 the problem will lie with someone else, but what everyone remembers is not who was to blame, but who got things done!"

Damn it if that ain't the truth!

Brush motors require a certain amount of water vapor in the air to keep the brushes conductive. This was first discovered in WW2 aircraft, where the low pressure and cold temperatures at high altitude would reduce the water vapor pressure. There are special brush materials that will work in vacuum, but I imagine a "traditional DC motor" does not have special brushes. -Carl Spearow

Simple defense of one's position is normal. I was defending 'my' design and project. By bring up other factors to consider will get my boss to think of other avenues of system failure that could attribute to 'my' actuator's failure. We would then pursue this with the rest of the engineering\design staff of the spacecraft to try and find out what failed. this will bring out the information we are lacking. Contract specifications, redudant systems. Based on the small amount of info available in the challenge and the fact that a traditional DC motor was chosen leads me to think that the failure might be from the brushes. Arc weld, RFI of the on-board system from arcing or simple dimensional change caused by the temperture extremes causing it to bind. It also could be from the motor going through earth's magnetic field causing it to spin up creating back electromotive force or in effect turning it into a generator and sending current back thru it's system causing a breaker to trip and now it will not respond to commands.

"It also could be from the motor going through earth's magnetic field causing it to spin up creating back electromotive force or in effect turning it into a generator and sending current back thru it's system causing a breaker to trip and now it will not respond to commands."

Really? I thought one of the EE's put that one to rest already because the Earth's magnetic field is too weak to have any significant effect on a motor here on Earth, let alone out in space where the field density of the magnetic flux must be less due to the inverse square law.

However, I do agree that there could be many possible correct "answers" besides what they will eventually publish next week is THE correct "answer".

I did some looking around and found that in 1867 a Cromwell Varley, an English Engineer, was able to get a generator going by using the earth's magnetic field to get enough juice, so to speak, in the windings. Also there have been examples as far back as Ben Franklin of people getting motors to work using the earth's magnetic field. So I'm still leaning towards this theory. The motor spun up by going thru the magnetic field, creating a backflow of electricty popping a breaker. Hence it could not respond to signals to actuate and deploy what ever it was supposed to.

Oh, come on! Sure, the earth's magnetic field can have an affect on sensitive instruments, including primative motors and generators, which are constructed only to show off a scientific principle, not to do any real work, or generate any significant energy! Do you really believe that a standard DC motor "going thru the magnetic field" of the Earth, far out in orbit or further, could create such a significant "backflow of electricity" to pop a breaker?

I still have this bridge in Brooklyn I have been tring to unload. Interested?

Rather than attack someone, Golden Rule, why not conduct a little research on Cromwell Varley, and Ben Franklin's motor experiments.

I apologize if you felt I was attacking you personally. That was not my intention. But by creating a sarcstic nick-name for me, it seems that you are doing just that!

Surely, you had to know by my reference to the Brooklyn Bridge that I was only joking with you?

My apology to you as well. It wasn't a knickname but a reference to yoour post about abiding by the "Golden Rule". I try to as well. But find after 50 years I treat people as they treat me. So if I could I would hold out my hand to shake yours and say peace friend.

I once had a '71 Chevy with a 4-speed stick shift. After the battery died I was able to get a couple of beer buddies to help me "push start the thing while I steered, they pushed, and I popped the clutch while in first. Maybe, a team of Nasa's finest should take the 1971 General Motors' team up in a space shuttle and push start this thing to fix it?

STL, I apologized to you but since I used a different comp it came up as Guest. My dumb.

Soon as I sell my west Arizona ocean front property I'll buy that bridge ;}.

But I'm still stickin with my theory. Alot of the converstations here talk like the motor ran for awhile then quit. But it says they could not get a response. So if you remove 'it ran then quit' from the table you're left with heat/cold failure, except it was specced for the enviroment or radiation(don't know how that would affect it) or vibrational stress or magnetic field. The 'brushes caused the problem' comes under the 'it ran then quit' profile which was not in the challange. The only thing that stands out is "Traditional DC Motor". What is it about a Traditional DC Motor that could cause failure prior to a command to actuate? Since a spacecraft is tight on space an assumption that the motor also was small, fractional hp or less. Then it is quite probable that the magnetic field caused the problem I described. I've even seen people talking about a stepper motor but this was not specified in the challange. I'm also assuming that in it's testing it was tested for heat, cold, vacuum and stress. But I'll bet there is very little magnetic field testing done. Also a point to consider is he specced all the componets for cold and vacuum.

"Traditional DC motors" are only rated for altitudes below 3300ft. For higher altitudes, the temperature rise will generally increase approximately 1% for each 330ft above 3300ft. (-from American Electricians Handbook)

Temperature rise?

Yeah, rise! Above 3300ft the air density decreases, so the air's ability to cool the motor decreases. Higher grades of insulation or motor derating is required, and DC motors require special brushes for operation at high altitudes.

Oh, the running motor's temperature.

Still, I hope this doesn't extrapolate to orbital heights!

Maybe not, I did some researching and found the two major concerns with stepper motors in space are outgassing and mechanical failure. Since outgassing was covered in the design, mechanical failure from shock, vibration and thermal cycling would be the best guess. I found that these factors cause severe metal distortion and warping which lead to mechanical failure and motor lock down.

I think Jorrie and Sandman probably have it right - the dust from the brushes is a problem in a weightless environment. Another possible source of the problem is lubrication. But this probably would not have shown up in a month, it might have been months to a couple of years. It's fairly well known in the aerospace business that lubrication in the space environment is substantially different than on earth and there are experts out there who specialize in this area. I have heard of really oddball failures in space due to poor design of contact surfaces, selection of the materials or lubrication of the those same surfaces.

JB

I believe the hint is here as to what went wrong. "You're careful to specify insulation and lubricants that won't fail due to outgassing in the vacuum environment of space. The system goes through all its tests with flying colors." This says the system goes through all the test, My guess is NASA would have been the one testing the WHOLE unit for all the conditions it would be going through. I'd say some tech preparing the system used the wrong lube. The motor locked up due to improper lubrication.

Did any one suggest solar flares? or a EMP?

yes :-)

Another stab

This guess is based on the assumption that when Challenge states that my boss states that the motor is "not responding to control commands," it is commands in the broadest sense, commands from the ground, that is or might be being referred to; and that it is the entire manipulator subsystem (of the space craft)—including the motor, the motor controller, the motor telemetry and communication node, and the manipulator system's power subsystem—that is being referred to.

Reflecting on the fact that "my baby," comprising the manipulator subsystems, including in particular the prime mover, were designed and tested for maximum reliability and availability (including maximum motor duty cycle and unlimited duty interval). Now it occurs to me that the ground testing…was done under conditions of constant power—and constantly available power cell/cell-battery regeneration. What was overlooked in my zeal for maximized reliability of the prime mover, was the fact that the manipulator's power subsystem would be subject to regularly recurring intervals of discharge and recharge. What might be happening then—I tell my boss—is that the motor was being overused, with the result that after a month of manipulator use the power discharge has overtaken regeneration of the battery, leaving the battery discharged and unable to power the motor, and motor telemetry unable to be communicated to ground. Selecting a DC motor also increased the rate at which the batteries would be discharged relative to their ability to regenerate when the solar collectors were not in earth shadow darkness. A fail safe should have been included in the manipulator's design to limit the motor's duty intervals so as to always have surplus power reserves and prevent full discharge of the battery.

If this proves to have been the problem, I explain to my boss, there is a bright side for him and our little design group. Such an error is one that should have been readily discovered and brought to attention (loudly) by the systems engineering group; so now it will be possible—and mutually advantageous—to spread accountability and dilute responsibility over multiple departments—which will now join together in confronting the problem with the customer. Further, while it is true that the company's performance award might take a hit, cost reimbursement will still be forthcoming…and there are too many people and departments involved for any one manager, boss, or worker to fear having to take "all the heat" alone. And, since the customer was also involved in the design approval process, there is a good likelihood that another contract will be awarded to fix the problem!

(Note: this guess was inspired while watching a NOVA program about the Mars rovers in which JPL became concerned when one rover, during an anomalous failure, was failing to shut down (as designed) to prevent nighttime battery over-discharge.)