They might "want" to sync up, but I'm not sure they would. If the platform has rotational acceleration there would be a difference in the acceleration on each device depending on it's distance from the center. That would keep them out of sync.
Rick C.
snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:
If the 'platform' it was mounted to was 'known' (by the software), the positions and differences from the center reference of any arc like motion of the platform would be part of it.
Does anyone remember the guy whom all on his own developed little quad rotor 'birds' that would all act together in a swarm fashion? He showed it off in an ice rink or such and they were extremely controllable and all knew exactly where they were in reference to each other.
Pretty sure DARPA snatched him up. They wanted his software and the algos he used.
It is not just the bulk motion of the body which you have to take into acco unt, but also all the structural vibrations and their harmonics if you want to do averaging of accelerometer values over a distributed space.
For example, if you have three arrays of accelerometers on the nose, wing r oot and tail, all three will experience and report different accelerations if the drone is buffeted by wind and its wings experience a momentary boost in lift. Thing is you don't even know what the wind pressure-space-time di stribution is exactly so you can't account for it.
It would take a supercomputing cluster to do a real-time dynamic analysis o f all the accelerations from bulk + vibrational motion in your platform, ev en IF you knew to a high degree of accuracy what all the forces acting on y our platform are.
The very act of trying to correct for systematic errors in a distributed ac celerometer array introduces new errors in terms of the accuracy of your ma thematical models.
But your idea of a spatially distributed sensor network for navigation soun ds interesting. Migratory birds for example are fascinating because of thei r ability to fly intercontinentally and land in their previous destination last year, with great accuracy and no trial-and-error. I wonder if one bird would get lost whereas as the flock size increases, their likelihood of ge tting lost decreases?
ample size), does having many accelerometers and then averaging their outpu t therefore reduce their overall error?
ers rather than one expensive one like a laser ring gyro?
35 deg/sqrt-hour) and that of a MEMS accelerometer (2.25 deg/sqrt-hour) [1] , you need about 400k MEMS accelerometers to approach the accuracy of a las er ring gyro.chip fab, it should be possible.
ictions by using such an array? Or will ITAR or the like be slapped on such a device once its accuracy is published in a brochure?
a Low-Cost GPS Receiver. Park, M & Gao, Y. [2008] Sensors Vol 8
rm has rotational acceleration there would be a difference in the accelerat ion on each device depending on it's distance from the center. That would keep them out of sync.
If the body is rigid then s = r*theta, and you can take the time derivati ve twice for velocity and then acceleration.
Problem is that real life is not so easy. A long slender missile body for e xample would have very different transient accelerations at the tip and the tail. After you double integrate the accelerations to get displacements, o ver a distance of say 10 - 1000km range, your CEP becomes wide as the side of a barn.
The best thing really for such applications is to either locate all your ch eap accelerometers into a small, rigid box, or use a more niche but higher accuracy accelerometer like a RLG.
That is not my experience. Resistors are tuned in the process, which means that one lot will typically have more or less the same distribution, but it is offset from the nominal. If they measure its's within the specs (1%), t hen they do not alter the process to pull it in. They just press the big "G O" button
Thus more resistors in parallel gets you nowhere
Cheers
Klaus
Which is why he is known as AlwaysWrong.
al vibrations whose amplitude and frequency are position-dependent. While t hese vibrations do not result in a net spatial or angular translation (you would hope so!) for the math to work the accelerometers have to be at the s ame location, the closer the better, sampling the same thing.
Not true. DecadentLinuxUserNumeroUno is called AlwaysWrong by krw because h e's one of the many people who disagree with krw.
DLUNU can be irritating, and the name AlwaysWrong gets thrown at him from t ime to time by other people when he's gotten more irritating than usual - I 've done it - but DLUNU is right some of the time, and more frequently righ t than krw.
He's done some interesting stuff, but he's almost as likely to fly off the handle as Phil Allison.
-- Bill Sloman, sydney
Would it? If they're on the same platform, the coupling is the same no matter what additional force is on them (superposition).
Krw happens to be wrong here. The force imposed by rotational acceleration does vary in proportion to the distance from the centre of rotation, as Rick pointed out, and krw seems to have failed to comprehend.
-- Bill Sloman, Sydney
snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:
My units used a GE IRU (Inertial Reference Unit) They are about a 7 inch cube and run about 50k each in quantity. Singles usually cost you about $100k each. We used them to keep a satellite dish aimed while the base it was mounted on was in motion (mobile sat comms).
3 to 5 mil accuracy.snipped-for-privacy@gmail.com wrote in news: snipped-for-privacy@googlegroups.com:
They do when they are multi-gigohm. So two 10G in paralell makes a closer than rated 5G resistor. Usually.
Random doesn't mean equally distributed. If you found that every set you obtained were equally distributed, you'd be forced to conclude that they were not random.
So the question becomes that of how likely it is that a random selection will be distributed in a way that the combined error exceeds what you want.
If I select 1% resistors, I'll be pretty annoyed if more than a very small number show a 2% error.
By contrast, if I select 10% resistors, and group then in sets of ten in parallel, I'll expect the combined result to exceed a 2% error quite often.
Sylvia.
I'd be very surprised if you actually did get 2% error. I'd expect a reel to be skewed one way or another with perhaps very few individual resistors in tens of reels within 5%, even.
sample size), does having many accelerometers and then averaging their outp ut therefore reduce their overall error?
ters rather than one expensive one like a laser ring gyro?
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fade away. Averaging many measurements will result in a lower range of err or with some probability. No measurement is contained in an error window w ith 100% probability.
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en.
If the errors were normally distributed and the 10% tolerance represented t hree standard deviations away from the mean, 67% of the resistors in the sa mple would lie with +/3% of the mean. Putting ten resistors out of such a s election in parallel would mean that 67% of your samples of ten in parallel would be within +/-1% of the mean.
There's no obligation on the manufacturer to make the resistors in a way t hat generates a normal distribution, and some manufacturers are claimed to measure all the resistors that they did make and sort them into bins.
The +/-1% bin would then get the centre of the distributions, the +/-2% bin would get the two bands around it, the +/-5% bins gets the next two bands out from there, and the +/-10% bin gets all the resistors between +5% and +
10% as well as all the resistors between -5% and -10%.I've no idea precisely what they actually do, but anybody whose distributio n is centred 5% or more away from the target value would end up throwing ou t a lot of resistors.
-- Bill Sloman, Sydney
Eh? If there is rotation each sensor will have a separate acceleration with different distances from the center of rotation. The different forces stimulate the different sensors to different frequencies. Don't the frequencies vary with force?
Rick C.
ample size), does having many accelerometers and then averaging their outpu t therefore reduce their overall error?
ers rather than one expensive one like a laser ring gyro?
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ade away. Averaging many measurements will result in a lower range of erro r with some probability. No measurement is contained in an error window wi th 100% probability.
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What's your point? Did you do the math correctly? Maybe that's why you do n't see the right values.
To get a five fold improvement in accuracy, I believe 10 resistors is not t he correct number. I'd have to do some digging to get the right number and come up with a probability of being within 2%, so I'll let you do your own homework. Bottom line is you can get whatever accuracy you desire to what ever probability you desire by combining resistors or any other components if the values are randomly distributed with a known average.
Rick C.
It's 25. The probability of the set being within 2% of the nominal value depends on the way the resistance values are distributed within the set of parts you are selecting from, which is not guaranteed by anybody, and I've never seen it pulbished.
Sadly, that's not what the manufacturers claim. All they say is that none of the resistors that they sell you with +/-10% tolerance lies outside that tolerance.
The joke example is where they have a process that generates nice stable resistors but with a perfect Gaussian distribution around the nominal value, and they measure everything.
+/-1% are samples taken from the peak of the distribution, and probablity distribution would be pretty much flat. +/-2% are actually -2% to -1% and +1% to +2%, with nothing within the +/1% band.And so on.
In reality, tight tolerance resistors are almost always trimmed after manufacture, but if you do it faster you do it, the trimming process is less precise. It presumably leads to rather messy statistics.
-- Bill Sloman, Sydney
If the resistor is supposed to belong to the E12 series, an inaccuracy more than +/-10 % would be falling in next or previous bin and should be labeled as such.
Do they really measure each and every such low cost component individually ?
Depend when the value is marked on the resistor - before or after measurement.
It seems unlikely. I did label it a joke example.
As I've posted elsewhere in the thread, I've no idea precisely what they actually do.
-- Bill Sloman, Sydney
snipped-for-privacy@ieee.org wrote in news: snipped-for-privacy@googlegroups.com:
He said E12 series.
Resistors all follow a standard progression in values.
Usually a full deviation from center spec will NOT take the part into the next value bin.
If the deviation is that wide, then the table of values it fits into would be wider, IOW not E12.
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