Talk:Accelerometer

message: Accelerometers are often used on fan bearings to measure vibration.  I would think this is one of the most commonplace uses of accelerometers.  Is there a separate page dedicated to this?  —Preceding unsigned comment added by 12.41.1.54 (talk) 20:11, 17 April 2009 (UTC)
replies: text: I doubt it. If you honestly think that there is a need for such a page, be BOLD and create one. But if it gets deleted, don't be surprised. I personally don't think we'd need 2 separate pages; I could be wrong. There would be too much repeated material IMHO. 98.202.38.225 (talk) 19:28, 10 June 2009 (UTC)
thread_title: What about accelerometers on fan bearings?

message: The introduction repeats itself when it says: "An accelerometer is an instrument for measuring acceleration" and It feels like there are two different explanations of accelerometers --76.22.79.194 00:42, 15 April 2007 (UTC)
thread_title: The introduction repeats itself

message: i believe the nike ipod thing is a piezo electric force sensor, not an accelerometer,  and the segway's balancing comes from MEMS gyroscopes, not accelerometers... —The preceding unsigned comment was added by 137.189.101.14 (talk) 10:09, 26 March 2007 (UTC)
replies: text: I would suggest merging the Uses section into the Applications section; if it becomes too long two or three subsections could be added. If no one protests, I will do it in one of the following days. Pietrow (talk) 09:45, 24 August 2008 (UTC)

text: As noted above, only the Applications section remains. I was able to remove some redundant information, although the Electronic devices subsection is still a bit out of shape. I hope my edit is not too contentious. I also removed the trivia section, because I think applications of the object described is not at all trivia; in fact I would argue it is the second most important section. Pietrow (talk) 10:37, 1 September 2008 (UTC)
thread_title:  needs some work. 

message: I think this article may confuse gyroscopes with accelerometers in a couple places. The introduction correctly implies that these are different concepts but I would guess image stabilization and segways both use gyroscopes without accelerometers, since I would guess translational shake would be a 2nd order consideration compared to rotational 'shake' in blurring a picture, and I would think the segway would know what its wheels are doing and would only need orientation information (unless it has an anti-slip feature, but even there you'd probably watch sudden changes in torque or rpm instead? You'd probably have to lose balance anyhow at that point...)
If someone can confirm, or can find another example which can be confirmed, then perhaps they could change this.
::I agree. I think another instance is the tilt sensing of mobile phones in the consumer electronics section. The articles on gyroscopes (on three axes) and tilt sensors (on two axes) clearly indicate that it's those two that allow this, not the accelerometer -- if the name "tilt sensor" isn't enough proof already. ;-) Allyddin Sane (talk) 19:16, 19 October 2011 (UTC)
thread_title:  gyroscope versus accelerometer 

message: In the introduction it states: "If dropped in a vacuum it will read zero.".
I think the fact that the accelerometer resides in a vacuum does not necessarily imply that there is no gravitational force taking effect.  —Preceding unsigned comment added by 213.39.233.138 (talk) 19:37, 18 October 2007 (UTC)
replies: text: What that editor means is when something is dropped, while it is dropping, it is weightless. Gravity is always there, but what's lacking is the reaction. What keeps you from falling through the floor? Answer: the floor. So to keep you from falling, the floor is working against gravity, pushing you up, supporting your weight. That's the force an accelerometer is actually measuring at 1g. So in a vacuum, with a freefall, there's no air resistance to slow you down, there is no floor to push you up, perhaps the floor is also falling, and the accelerometer is falling. What's stopping you from falling that an accelerometer can measure? What's stopping the accelerometer from falling? Nothing. And that's what the accelerometer measures. Remember, gravity is affecting everything without difference, you, the falling floor, the accelerometer; and a difference of force is what an accelerometer measures. See weightlessness. MMetro 18:05, 28 October 2007 (UTC)
thread_title:  gravitation and vacuum 

message: This text implies that all accelerometers are able of measuring DC acceleration. This is far from real. All vibration monitoring accelerometers usually don't measure anything below 1 Hz.  —Preceding unsigned comment added by 201.76.191.59 (talk) 18:17, 20 October 2008 (UTC)
thread_title:  About gravity 

message: What is the twisted ribbon FM accelerometer mentioned in this article? __meco (talk) 17:10, 12 December 2008 (UTC)
thread_title:  Twisted ribbon FM accelerometer 

message: Accelerometers are, in general, are based on "effective force" sensing. At this stage, the output will be proportional to (a+g). Then some accelerometers bother removing "g", either through software, or through a voltage offset. Others don't. Stop saying that accelerometers don't feel the 1g. They all feel the 1g. It's just that some choose "1g downwards" to be the reference, and will give you the difference instead, exactly like how you can set a scale to read "−0.454 g" when nothing is on it because the box your packing stuff into has a mass of 0.454 g. Headbomb {ταλκκοντριβς – WP Physics} 19:46, 22 January 2009 (UTC)
replies: text: Nope, accelerometers measure a-g. You can calibrate the g away (if you're stationary or something), but if it changes it changes the opposite way than a- if g goes down, the output goes one way. If a goes down, the output goes the other.- (User) Wolfkeeper (Talk) 20:33, 22 January 2009 (UTC)
replies: text: Sign conventions are trivials.Headbomb {ταλκκοντριβς – WP Physics} 12:39, 23 January 2009 (UTC)
replies: text: No, not in this case, there's some very fundamental physics involved. Basically in GR gravity is a pseudo-force, and accelerometers can't read pseudo-forces at all of any kind. They *can* read a force that is equal and opposite to a pseudo-force, but there isn't always one. There's an equivalent argument in newtonian mechanics that relates to the fact that gravity is the same for all parts of the accelerometer (apart from tidal forces).- (User) Wolfkeeper (Talk) 13:15, 23 January 2009 (UTC)

text: Accelerometers may not always be used by people that understand general relativity, but nevertheless that is the theory that governs their behavior.  They're also not exclusively used on earth, so references to "g" in the definition don't make sense (what's "g" for an accelerometer on a spinning satellite in earth orbit?). Waleswatcher (talk) 13:31, 11 November 2018 (UTC)
thread_title:  A comment on this revert war. 

message: A better paper is here: http://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-696.pdf
Many configurations of inertial guidance systems are discussed, but in general they use 3 devices that are gyros of one sort or another, or which correspond to gyroscopes (such as the small MEMS devices, which actually measure angular acceleration using the Coriolis effect with tiny silicon vibrating wieghts). Add to this, 3 accelerometers, one for each orthogonal axis.
The box diagrams for all of them are of interest and should be examined. Uniformly, the gyro ouputs are first used to project the measured accelerations onto 3 axes, THEN gravitational acceleration is subtracted out of the Z component, then everything is double-integrated to get velocity and finally displacement (with, of course, need to add in absolute velocity and position at each point in the integration to get rid of the constants-of-integration and give definite results). 
Interestingly, one of the worst causes of error is gyros which don't get the axes precisely right, causing a small component of the subtracted g-acceleration to be subtracted not from the vertical Z-axis (as is proper), but from the X-Y plane, which is where most of the motion of interest is taking place. Since linear accelerations are integrated TWICE to get distance, small errors in integrating incorrect accelerations in the X-Y plane grow rapidly (quadratically with time) and it doesn't take too long for this bias to screw everything up. Here's the direct quote:
[section] 6.2.3 Propagation of Errors  Errors which arise in the accelerometers propagate through the double integration, as described in Section 4.2. This is the obvious cause of drift in the tracked position. Errors in the angular velocity signals also cause drift in the calculated position, since the rotation matrix C obtained from the attitude algorithm is used to project the acceleration signals into global coordinates. An error in orientation causes an incorrect projection of the acceleration signals onto the global axes. This causes several problems. Firstly, the accelerations of the device are integrated in the wrong direction. Secondly, acceleration due to gravity can no longer be correctly removed.In the strapdown algorithm 1g is subtracted from the (globally) vertical acceleration signal to remove acceleration due to gravity before the signal is integrated. A tilt error θ will cause a component of the acceleration due to gravity with magnitude g ·sin(θ) to be projected onto the horizontal axes. This causes a residual bias due to gravity with magnitude g · sin(θ) to remain in the globally horizontal acceleration signals. There will also be a residual bias of magnitude g · (1 − cos(θ)) in globally vertical axis, however this is much less severe since for small θ we have cos(θ) → 1 and sin(θ) → 0. Hence the error in position caused by a small tilt error will occur mainly in the global xy-plane.The propagation of gyroscope errors through to the calculated position is the critical error path in nearly all INS [Inertial Navigation system} systems. In most applications the magnitude of g is much greater than the mean absolute acceleration of the IMU itself. In such cases the critical problem is that a component of the acceleration due to gravity is projected onto the globally horizontal axes. As a concrete example consider a tilt error of just 0.05 [degrees]. This error will cause a component of the acceleration due to gravity with magnitude 0.0086 m/s2 to be projected onto the horizontal axes. This residual bias causes an error in the horizontal position which grows quadratically to 7.7 m after only 30 seconds.
Now, I hope I'm not the only one to notice that this perfectly backs up the other academic paper which has been cited, to the effect that nothing in an inertial guidance system, including the accelerometers in it, can measure gravitational g. It must be removed from the accelerometer readings "by hand" without benefit of any instrument which measures it directly. If there existed an instrument (including the accelerometers themselves) which could do this, they simply would add them to inertial guidance systems, and this growing-bias with time which results from bad orientation and incorrect g-removal angle, would not be the major problem for inertial guidance systems that it is. But it IS a problem. We've discussed the various theoretical reasons why this is not just a difficulty with technology, but rather arrises directly from general relativistic considerations: for any g field point there's an inertial frame in which you can make g vanish. And others in which it is larger or smaller. No instrument, even in theory, can tell you for a planetary g-field which is the "real" and "correct" g, at the same time you're moving up-and-down in it! The trickiness of relativity is that when navigating over a lump of inhomogenous rock, the stuff beneith you imposes on your space-time a complex pattern of "inertial acceleration" along an axis pointing vaguely to the center of the Earth, which changes from place to place, and can't be assessed "on the fly" but only if you physically stop (distance, velocity, and acceleration-wise, with regard to the center of the Earth) to measure it indirection, as you would using a simple scale. But there's no getting away from having to do that. And of course, the scale (like the accelerometer and your butt) is not measuring the g-field directly, but the stress it's causing to the supports of a resting test mass. 
::  for any g field point there's an inertial frame in which you can make g vanish. Inertial??? Matteosistisette (talk) 10:56, 7 April 2012 (UTC)
replies: text:  inertial frame. That reference frame in which things are weightless and all accelerometers read no acceleration (although they might be physically changing velocity due to gravitational effects, as in free-fall). SBHarris 19:00, 7 April 2012 (UTC)
thread_title:  How inertial guidance systems work (section copied from the [[g-force]] talk page, where accelerometers are mentioned) 

message: [Outdented and sectioned for easier editing]
Wolfkeeper, you wrote above:
 I don't care how awkward it reads, the wikipedia is supposed to contain
 referenced opinions, not simplified statements that 'read nice' or something.
 The wikipedia is verifiability over truth.
Ouch!  I may be new here, but I would expect both good readability and truth from an encyclopedia – and of course, its information  should be verifiable.
First, regarding readability:  You place heavy emphasis on the distinction between inertial and non-inertial accelerations, as I already pointed out. Your article version states:
 However, accelerometers cannot measure inertial accelerations,
 but only non inertial ones caused by mechanical forces.
You simply cannot leave out a clear and lucid definition or reference to either term.  Doing so hinders understanding and increases wikipedia's bad rep in the sciences field.  I suggest to remove or rephrase this.
Second, a side note to your comment on
 gravity is an acceleration of the reference frame
That's a bit harsh. The principle of equivalence concerns itself with "the equivalence, in terms of local experiments, of gravitational forces and reactions to an accelerated noninertial frame of reference ... and the equivalence between inertial frames of reference and local freely falling frames of reference" (Encyclopedia Britannica (C) 1990, vol. 26, p 534, "Relativity").
:It's not harsh, it's the same thing. That this is an accelerated reference frame means that gravity is non detectable, because according to GR the accelerometer isn't 'really' accelerating under gravity, it's the reference frame accelerating. Incidentally, the same is true for other pseudoforces like coriolis acceleration and centrifugal acceleration and euler acceleration; accelerometers don't register them at all. (Tidal forces are detectable though.)- (User) Wolfkeeper (Talk) 15:04, 31 March 2009 (UTC)
replies: text: Now you're more or less getting it, in both cases, accelerometers fail to measure gravity- that's a reasonably deep reason why you're subtracting it off the actual acceleration in the above equation; although the GR position that it's not actually accelerating due to gravity is even deeper ;-)- (User) Wolfkeeper (Talk) 15:04, 31 March 2009 (UTC)
replies: text: We should also note that when the article referenced above says the surface of the Earth is "almost an inertial frame" it is only talking about behavior in 2-D (like the surface of an air hockey table). In the third dimension, there is no inertial behavior at all-- it takes a force just to keep things still on Earth's surface. That's an accelerated frame, not an inertial frame. In truly inertial frames, also called "free fall frames" things are weightless, and they follow Newton's laws in all axes/dimensions. It's the inside of a falling elevator or cabin of an orbiting space shuttle that is "almost an inertial frame," certainly not the surface of the Earth! So that very statement fouls up the meaning we're getting at, with accelerometers. I wish the author hadn't written it, because he was only thinking about behavior on a surface.  Accelerometers, in the end, measure "weight per unit test mass." Also called the specific force or proper acceleration. That is all. Relativity need not be involved, for the preceding statement is Newtonian also. But notice that this means accelerometers don't (cannot!) measure the gravitational field per se. They don't "see" it, and humans don't "feel" it. All accelerometers "see" is proper accelerations away from the local inertial frame, which they sense by means of mechanical forces (due to weight), or from physical deviation of masses from an inertial path (due to proper acceleration of the accelerometer housing, again due to a mechanical force). The only time accelerometers are aware of gravity per se, is when they are allowed to test the difference between the "inertiality" of frames at different places in space, so that they can test for gradients in space, in what counts as an "inertial" frame for each point. That is a gradiometer-- it consists of a bunch of accelerometers all separated by little distances from each other (see GOCE for an example). However, a gradiometer made from accelerometers only measures tides (the gravitational gradient), not the absolute gravity value anywhere. The absolute gravity at a point in an inertial frame is unmeasurable. Newton would say that's because there's no way to do it. Einstein would say the REASON there's no way to to it, is because in that frame gravity actually doesn't exist! Which is to say, that at a carefully chosen single point in an inertial frame constructed just for that point, space-time is flat. That's what is meant by "locally flat." Right at that point (this doesn't apply to "near the point"), there is no gravitation field and no proper acceleration, so there's nothing for an accelerometer to measure: you can't measure an absolute gravitational field at a point, anymore than its possible to measure an absolute velocity. If you feel nothing and measure nothing, you're quite free to say there IS nothing, and challenge your opponents to prove you wrong (just as with velocity). That's the equivalence principle. SBHarris 21:47, 16 January 2011 (UTC)
thread_title:  Review of Approach and Sources 

message: An accelerometer doesn't measure acceleration in the sense that most people think about it (in respect to earths reference frame). I edited the acceleration to read proper acceleration hopefully this will lead to less confusion. Cheers!  —Preceding unsigned comment added by 194.171.252.108 (talk) 10:28, 26 October 2009 (UTC)
thread_title:  acceleration ?? 

message: In reality, accelerometers would barely behave in the way described in the "Physical properties" section.  A parabolic flight is a transient event and the signal of an actual accelerometer would then oscillate or slowly converge towards 0g, without ever truly getting there.  Also, many real accelerometers behave as high-pass filters and thus are unable to capture the earth acceleration (which is essentially a static event) unless they experience a change in orientation.  In other words, the main purpose of the "accelerometers" presented in this section seems to enable some thought experiments (which may be worthwhile to get a better grasp of the laws of dynamics), but they do little to convey the notion that accelerometers are measuring instruments with specific physical limitations.  I submit that more focus should be placed on the working principles of accelerometers as mechanisms, and less emphasis be given to the meanings of gravitation and acceleration as physical concepts (there should be plenty of room in other Wikipedia entries to address these topics).  After all, it be would strange if an article on watches were to expand on the relativistic theory of time. Decan tulahi (talk) 08:32, 3 June 2010 (UTC)
replies: text: The slow convergence bit is wrong- the convergence is typically an exponential decay towards the correct value, nearly always with a low time constant. Convergence happens very rapidly, just 10 time constants gets you within a few ten millionths of the right answer, and another ten within a couple of billionths.- Wolfkeeper 16:29, 3 June 2010 (UTC)
replies: text: If an accelerometer were to have a time constant of 1 second (due to a large proof mass and weak spring element), it would still miss the mark for the first few seconds of the flight. Now, I do agree that the rate of convergence of a standard accelerometer would be more than sufficient in that case. Rather, the problem I see with the examples is the fact that they mislead the reader into thinking that accelerometers must have a large bandwidth (unlike accelerometers used in seismology) and that they can always detect a static acceleration (unlike piezoelectric accelerometers). Decan tulahi (talk) 22:04, 3 June 2010 (UTC)

text: Another confusing statement on the first line of the "Physical properties" section: "Proper acceleration ... is the acceleration felt by people and objects". Yet, the human body is not ideal either and is basically insensitive to accelerations above a few hundred Hertz (see ISO standard 2631-1). Decan tulahi (talk) 01:23, 4 June 2010 (UTC)
replies: text: I've added some language in the lede that notes that real accelerometers are not perfectly sensitive to changes in proper acceleration on small time scales. And of course, neither is the human body. You'll have to add some amplification of that for real instruments, in the article body. But at least the qualifier is now in. SBHarris 19:42, 17 January 2011 (UTC)
thread_title:  Ideal versus real accelerometers 

message: I think this article should use Newtonian Mechanics and references to specific force.  Why should a discussion on accelerometers be considered in the fundamental physics category.  Newtonian Mechanics is perfectly able to explain the basic operation of the accelerometer to the level that most people need.  I know that in satellite systems that relativistic effects are accounted for in the inertial navigation system.Skimaniac (talk) 02:57, 26 September 2010 (UTC)
replies: text: Specific force is just a fancy name for "weight per unit mass." G-force (also in units of acceleration) is exactly the same concept, but with yet another name. An accelerometer does measure g-force, specific force, and weight-per-test-mass. These should all be mentioned in the lede. This type of acceleration is called proper acceleration in relativity theory, to distinguish it from coordinate acceleration (dv/dt) that we all think about when we see the word "acceleration". Accelerometers cannot, and do not, directly measure dv/dt (although they CAN in the very special case where gravitation is absent). That causes a LOT of confusion. SBHarris 19:38, 17 January 2011 (UTC)
thread_title: Why not only Newtonian mechanics terms?

message: Most of the article is just a very long list of applications. I suggest that this is put into a separate article and just a reasonably short summary included here. Furthermore, I came to page to find out what kind of sensors are used in consumer electronics but I could not find the answer by skimming the text. Quite possibly more emphasis should be put on that. Tronic2 (talk) 15:46, 20 June 2012 (UTC)
replies: text: I don't have any problem listing applications in a separate list article, and moving specific material from here to there. Another possibility is to leave them here, with summaries and collapse boxes. SBHarris 00:10, 21 June 2012 (UTC)
replies: text: Collapse boxes are no good for articles as you expect to read it without clicking.  If there s so much text that it has to be collapsed, then another article is the way to go, with a summary in the parent article. Graeme Bartlett (talk) 11:11, 21 June 2012 (UTC)
replies: text:  Collapse boxes are handy when you have >10-20 items (too many to put into a list without wasting space, but < 100-200, items so not really enough to make for a separate list article. You just need to be judicious about it. And have some respect for the reader. He or she will click if they have to, and are warned. Example: . SBHarris 20:58, 21 June 2012 (UTC)
thread_title:  Move applications to a separate page? 

message: A rambling relativistic discussion of acceleration is not necessary twice in the article, and certainly not in the lead.  Might it not sbe more encyclopediac to refer to whatever artcile we have on [[acceleration] and leave this article to talk about its nominal subject, namely, accelerometers? --Wtshymanski (talk) 22:00, 31 October 2012 (UTC)
replies: text: You cannot discuss accelerometers without noting that they actually do not measure acceleration, but rather weight per mass. Relativity doesn't need to be discussed, and actually isn't in the lede. But it's necessary to discuss the fact that gravity is not felt by either people or accelerometers. If you're falling down an elevator shaft in vacuum, you'll be weightless. You won't feel gravity even though it is acting on you, and your accelerometer will read "zero" even though you're clearly accelerating due to gravity. See the point? This is not a minor detail. It is explained by the equivalence principle, but that can be left until later. The present lede sentence: "An accelerometer is a device that measures proper acceleration, the combined effect of gravity and change of speed," is not informative. Proper acceleration is not a "combined effect of gravity and change of speed" unless you explain that they can "combine" in tricky ways to cancel each other out. And that takes a lot of explaining. It's easier just to say that accelerometers measure the types of acceleration that produce weight. SBHarris 23:14, 31 October 2012 (UTC)
replies: text: So how does an inertial navigation system work? Missiles are in free fall for much of their trajectory...surely the INS needs to find Moscow accurately? The current version of the lead is much better and I don't think the lack of mention of "four-acceleration" hurts it at all; after all, physics grad students aren't going to be looking up accelerometers on the Wikipedia and the rest of us have no idea what a "four-acceleration" is. --Wtshymanski (talk) 13:53, 1 November 2012 (UTC)
replies: text:  Inertial nav systems use gyroscopes to measure angular motion and the accelerometers measure "weight per mass" (which has the unfortunate name of "specific force"-- this actually means "mass-specific force" which means F/m, which has units of acceleration). In order to go from weight/mass to coordinate acceleration (what you really want to integrate, in order to get velocity and position) you need to add or subtract gravitational acceleration from "weight/mass" acceleration, and coordinate acceleration, dv/dt, is what's left. How do you know gravitational acceleration? You have to calculate it from your position and some kind of gravity model, since you can't actually measure it. The gravity model tells you the value of "g" for altitude and latitude and it would be different for every planet. We use Earth. The position is your initial reference position plus the change in distance you calculated from your inertial nav system, so it's a bootstrap thing. But it's a quantity in the system that needs to be compensated for, wherever gravitation is an issue. This can be done in realtime if you're on a slow machine like a submarine. For an ICBM which has no guidance, all the calculations for gravity compensation over the path it's supposed to take, are done on the ground, and fed into the missile as a sort of set of parameters that it should be seeing from its accelerometers and gyros. The ballistic missile then performs a few course corrections to try to minimize deviations from this, in a process called Q-guidance. The process ends up essentially doing the integrations described (although much of the problem has been "pre-computed"). For (quite different) guided missiles and cruise missiles, of course, there are external corrections that can be used to keep the thing on track. Ballistic missiles go by dead-reckoning, but they use a gravity model of SOME sort, as pre-crunched part of input. If you put a giant mountain in their path that wasn't part of the gravity model they use, they wouldn't know about it, and would miss their targets. A new mountain would not show up in pre-computed parameters that come from from prior assumptions about the gravity field of the Earth along the ballistic path.  By the way, some of the arguments above have convinced me not to mention proper acceleration in the lead, or at least qualify it. Specific force (weight per mass) is very close to proper acceleration for most circumstances, but technically is not quite the same if there are influences on the proof mass which affect its motion, but aren't macro-gravitational, and at the same time, don't show up as force between mass and case. Natually these are small, but a list of them is given above, and they aren't quite zero. SBHarris 00:06, 3 November 2012 (UTC)
thread_title:  Lead 

message: In the Consumer Products Section within Applications, the following sentence appears:
.
'.... It is also used in some data loggers to monitor handling operations for shipping containers. ...'
.
Am I missing out on something, do all the other good consumers have their own shipping containers? Where are they hiding these?  What do they do with/in the containers?  Why the effort to conceal this from me (and probably others)?
.
Maybe someone would be kind enough to move this to the Industrial section, which will allow me to pretend like I'm not missing out on some cool consumer application of shipping containers.
.
70.185.109.98 (talk) 23:10, 22 February 2013 (UTC)
thread_title:  Consumer Products Section 

message: In the Consumer Products Section within Applications, the following sentence appears:
.
'.... It is also used in some data loggers to monitor handling operations for shipping containers. ...'
.
Am I missing out on something, do all the other good consumers have their own shipping containers? Where are they hiding these?  What do they do with/in the containers?  Why the effort to conceal this from me (and probably others)?
.
Maybe someone would be kind enough to move this to the Industrial section, which will allow me to pretend like I'm not missing out on some cool consumer application of shipping containers.
.
70.185.109.98 (talk) 23:10, 22 February 2013 (UTC)
thread_title:  Consumer Products Section 

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thread_title:  External links modified