Ideas for a good sensor - "plummet" / "waterl

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Asmyldof
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#1
Posted by Asmyldof: Fri. Jul 22, 2011 - 05:56 PM
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Hiya!

Need your collective experience:
I want to keep a platelet exactly level, independent of the unpredictable circumstances it is in.
I have servos and Atmels to swim in, the programming will not be a problem, but I'm having trouble finding a decent electronic solution. I COULD attach a weight to a potentiometer, but that's too much work when I decide to reproduce the solution.
I am, however, fine with a plastic cubelet that contains a low-friction potentiometer and a weight prefabricated.

I need a good amount of accuracy if at all possible.

Searching "waterlevel" and "plummet" end in irrelevant results, for obvious reasons.

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Learning and Information, General Electronics
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ka7ehk
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Posted by ka7ehk: Fri. Jul 22, 2011 - 06:03 PM
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Please define "platelet" a little more. Totally unclear what water has to do with this.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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larryvc
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Posted by larryvc: Fri. Jul 22, 2011 - 06:06 PM
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Did you translate that with Google Translate?

WTF are you talking about. :wink:

Quote:
I want to keep a platelet exactly level

http://en.wikipedia.org/wiki/Pla...

Quote:
I have servos and Atmels to swim in

?

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Asmyldof
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Posted by Asmyldof: Fri. Jul 22, 2011 - 06:19 PM
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I am known for modifying words.
Piglet, small disneyesque pig, thus platelet small plate.

I did not consider the implication of actual platelets.
Sorry.

Waterlevel is the only alternative word I know for a plummet. Very sorry if my lack of being born in the US confuses you, but as someone who makes it a habit of learning new languages (spoken or written) just to help other people out I feel I have to at least resent your comment about Goolge Translate (the great American invention...) a little.

This word ("Waterpas" in Dutch) just isn't in my normal range of things to talk about, so I have to make do with what I know.
It is a device mainly used by constructors to make sure things are level. They contain coloured water with a small bubble. Hence.. Waterlevel.

And I happen to think the other comment you have trouble with is fairly clear. Sure, it's a direct translation of a saying, while "roll in" would be your own version, but being able to swim in something implies at least an equal or greater amount of superfluous objects as being able to roll in them.

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ka7ehk
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Posted by ka7ehk: Fri. Jul 22, 2011 - 06:28 PM
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You can determine "level" with a 3-axis accelerometer (MEMS type). Maybe also 2 axis.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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mckeemj
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Posted by mckeemj: Fri. Jul 22, 2011 - 06:43 PM
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You do not specify update speed or accuracy, but I agree with ka7ehk that a semiconductor accelerometer could be a good fit.

I would not take the question about Google Translate as an insult. It is a usable and powerful service; but it has its issues, mostly due to the fact that it sometimes chooses words that seem to have nothing to do with the context ( e.g. platelet ) even while seemingly translating the rest of the text reasonably intelligibly.

Martin Jay McKee

P.S. From your description, Waterlevel = Bubble Level or Spirit Level

As with most things in engineering, the answer is an unabashed, "It depends."

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larryvc
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Posted by larryvc: Fri. Jul 22, 2011 - 06:45 PM
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Now I understand what you were asking. :oops: Sorry, I didn't before, I do try hard. :)

The ? about "swim in" was me referring to the size of the swimming pool. :wink:

:shock: Google is in Kirkland. I live in Redmond. Try Bing Translator. 8)

Jim's idea is a perfect solution for you.

This is the waterlevel you were talking about

http://en.wikipedia.org/wiki/Spi...

and the plummet

http://en.wikipedia.org/wiki/Plu...

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ka7ehk
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Posted by ka7ehk: Fri. Jul 22, 2011 - 07:14 PM
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Geee, I was thinking of "plummet" as to fall off a high place.

There are also electrolytic level sensors. They have a bubble like a sprit level and resistance between various electrodes indicates when it is level. These are pretty simple, but are expensive if you buy one. I am guessing that they are hard to make with good level accuracy.

The MEMS accelerometers will be far less expensive and far easier to interface to, compared to almost all other options.

Jim

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Asmyldof
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Posted by Asmyldof: Fri. Jul 22, 2011 - 07:57 PM
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I'll try spirit level in my searches.

The problem with an accelerometer is that on regaining power you need to know the position of the surface.

I have many accelerometers from the days of fiddling with helicopters.
Hey any of you wanna buy a Helicopter by the way? :-P

But this is a trial project where I want to get the surface level on solar power and a small buffer. It's just a "See what I can do" for now, but I have cause to believe that if I can manage that I can obtain a new assignment to make a self-levelling mechanism for a set of show trucks. Problem is, when the trailer is separated from the truck it loses primary power (which is all the power it has) and if the driver of said truck has to then level the trailer before being able to use auto levelling it's a pretty pointless piece of hardware.
(As trailers are much more abundant than the truck bit that pulls it and they switch over multiple times a day on busy schedules)

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Asmyldof
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Posted by Asmyldof: Fri. Jul 22, 2011 - 07:58 PM
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PS
My swimming pool is exactly big enough to hold every one of my Atmels and servos without overflowing or seeming empty...
Obviously!

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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Asmyldof
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Posted by Asmyldof: Fri. Jul 22, 2011 - 08:01 PM
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ka7ehk wrote:
Geee, I was thinking of "plummet" as to fall off a high place.

Exactly why the stupid search engines give me all kinds of crap about company stocks and all such nonsense.
---EDIT:---
(Try it: {"Linear Technology"|"National Semiconductor"|"Analog Devices"} & "Plummet")
-----------

"Schietlood"/"Schotlood" doesn't seem to work as nice in an English search though.

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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Asmyldof
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Posted by Asmyldof: Fri. Jul 22, 2011 - 08:27 PM
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Contacted these:
http://www.frederickscom.com/sen...

People.

Any other suggestions still welcome, as I have a premonition they may not be the cheapest suppliers of tilt sensors.

---edit:
Look ^^ "tilt sensor" is a good description. Though they tend to end up being a filament switch to detect tilting over a certain angle as a simple button press...

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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dak664
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Posted by dak664: Fri. Jul 22, 2011 - 08:30 PM
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You need to specify the power usage and response time constraints. In the absence of those, a 100 meter long water level with float switches at both ends would be the ideal solution.

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ka7ehk
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Posted by ka7ehk: Fri. Jul 22, 2011 - 10:23 PM
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No, accelerometers do NOT need to know initial position. Two axis will do it

If tilted about the Y axis, then the X axis will read an acceleration of G*sin(theta) - theta = rotation angle about Y axis. If tilted around the X axis, the Y axis will read an acceleration of G*Sin(phi) - phi = rotation angle about X axis.

If absolutely horizontal, there is no projection of the "vertical" G vector onto either the X or the Y axis of the sensor, and those acceleration components measure zero. If you read any of the Analog Devices or Freescale app notes about "calibrating" MEMS accelerometers, it tells you about using gravity to do calibration. Its pretty straight forward.

Jim

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ignoramus
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Posted by ignoramus: Fri. Jul 22, 2011 - 10:40 PM
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If you have a stable mechanical reference against which to measure motion of Your platform then a laser diode might be a good way to get a significant mechanical advantage and ensure platform stability

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js
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Posted by js: Fri. Jul 22, 2011 - 11:19 PM
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Water, swimming pools and Holland seem to have a different meaning there compared to anywhere else in the world.

If that little boy gets tied of keeping his finger in the dyke... :wink:

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Asmyldof
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Posted by Asmyldof: Fri. Jul 22, 2011 - 11:23 PM
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Jim,

Aren't those levels in change of G-force miniscule on small angular displacements, in the order of general ADC noise?

I'm not going to try and rotate a trailer along it's axis to get it level... That also kind of defeats the purpose.

Anyway, I'll check it out to be sure, can't hurt.
-----
Ig, Trailers drive around, stand outside, the point of auto-levelling these is taking the environment and human intervention out of the equation.
-----
Dak, power usage isn't a big problem, for the small demo-solution I can use any power source. The trailers will of course require an insane amount of electrical power to move the levelling legs, so there's no constraint there. But "making my own fiddly biddly thingermabob" is out. It needs to be reproducible in-factory when it works. And if possible quickly and easily installable.

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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Asmyldof
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Posted by Asmyldof: Fri. Jul 22, 2011 - 11:30 PM
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js wrote:
Water, swimming pools and Holland seem to have a different meaning there compared to anywhere else in the world.

If that little boy gets tied of keeping his finger in the dyke... :wink:

Well, some time, long ago, before teh interwebses, Holland meant the same thing all over the world, as did Brabant, Vlaanderen, etc...
But since someone decided to call 3/4ths of the Netherlands: The Netherlands and 1/4th of them Belgium things went awry.

Swimming pools are different all over the world aren't they? Don't Aussies swim in pig's blood?
----

As for that Dyke... She's been complaining about Jacks 'finger' ever since it first happened...

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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Asmyldof
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Posted by Asmyldof: Fri. Jul 22, 2011 - 11:55 PM
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I need to go to bed... If any noise at all I meant normal noise. ADC noise can be overcome with amplification of the signal.
Anyway, I now see the equation you gave, that is, I see what it says.

This is of course true and I could use that, but it would be a very good thing to know the actual orientation on startup. While mocing the trailer it shakes a bit and this and that and it might take a lot of high-order programming and calibration to make sure it always detects increase and decrease in slope correctly. A small error on my part in this could easily result in a trailer tilting itself to pieces, so to speak.
A trailer is "so soft" that it can bend over a very large angle and rip it's insides if you level it in opposite directions on either end.
Anyway, I'll have a toy-around with some of my resonant G sensors, see if I can get the signal clear enough. Else I'll just have to convince those sensor people to give me a good deal ;-).

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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Plons
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Posted by Plons: Sat. Jul 23, 2011 - 12:21 AM
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Asmyldof, about the accuracy : accelerometers were used when the Eastern Scheld Stormsurge Barrier was built: each pier (10000 tons) being placed by the Macoma/Ostrea combination, needed to hang perpendicular within 0.1 degree. And yes, we used accelerometers sitting on top of the piers to measure that. For the stone-dumpers, a Penny&Giles accelerometer was used to measure the role-movement if the vessel. And such Penny&Giles accelerometer consisted of a potmeter and a weight. Likely with oil for damping. Can't remember that anymore and I never opened one.

So yes, accelerometers are great for that purpose.
Btw, lately there were several action on Samenkopen.net, both analog and digital accelerometers. Analog here: http://www.samenkopen.net/action...

A GIF is worth a thousend words   They are called Rosa, Sylvia, Tessa and Tina, You can find them https://www.linuxmint.com/

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Posted by larryvc: Sat. Jul 23, 2011 - 12:29 AM
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JS, I thought of cautioning you on your choice of words, but your comment was innocent. :)

On the other hand.... :shock:

"I may make you feel but I can't make you think" - Jethro Tull - Thick As A Brick

"void transmigratus(void) {transmigratus();} // recursio infinitus" - larryvc

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ka7ehk
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Posted by ka7ehk: Sat. Jul 23, 2011 - 12:53 AM
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Given initially measured angles, you should be able to determine how much to lift every corner, open-loop. You should not need active feedback during leveling.

And, don't forget that for small radian angles, sine(angle) is approximately angle. That avoids expensive floating point computations. And, for something like this, you only need to drive the two axis accelerations to zero, without knowing the actual angle.

Some of the MEMS sensors output digitally via I2C or SPI. No need to involve the ADC.

Make one axis parallel to the axel and one axis parallel to the fore-aft edge of the trailer. You should be able to mount one almost anywhere. Side wall, front or rear wall, floor, ceiling, externally on the frame, what ever.

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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damien_d
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Posted by damien_d: Sat. Jul 23, 2011 - 02:44 AM
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If you need accurate levelling, then I'm going to recommend 2x SCA103T. Claims of 0.1deg out of the box, though I am aware of one application that can achieve 10's of arc-seconds with very thorough calibration.

You can buy them from DigiKey, though they are a little more expensive that your average mems device.

-- Damien

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Posted by Asmyldof: Sat. Jul 23, 2011 - 02:11 PM
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Edit:
I forgot to look at page 2, so this one is directed at Plons' post:
----
I don't do "Samenkopen" anymore, just talking to importers directly, waving a VAT number and registration you get better prices, less effort and no need to trust through a rickety website.

Anyway, what you are talking about are plummets like I first described in my "I want this right now plx kthxbai" post. The accelerometers talked about output angular velocity, not absolute position (well, not directly intended to, but they do in a way as pointed out by Jim).
Anyway, I'll have a look, though I like the thinfilm spiritlevels the people at Fredricks Company make, might be worthwhile to just buy a box of those.

Or ask my Chinese contact how accurate he can rip those off :-D

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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Asmyldof
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Posted by Asmyldof: Sat. Jul 23, 2011 - 02:21 PM
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ka7ehk wrote:
Given initially measured angles, you should be able to determine how much to lift every corner, open-loop. You should not need active feedback during leveling.

My point was that the initial angle could be to the one or the other side without a difference in signal, it has no directionality. On the small plate it will not matter much, just move a little and see what happens, but on the trailer this will take too much time for all 16 sensors that I would need to make sure there is no point that skews or bends.

I'm going to try the procedure as I have all the sensors anyway and I like the idea of using the sensors for the exact thing that datasheets tell me to calibrate out :-). But I may find it too 'complicated' a procedure for the actual trailers.

Thanks for the suggestion.

ka7ehk wrote:

And, don't forget that for small radian angles, sine(angle) is approximately angle. That avoids expensive floating point computations.

Luckily I'm well versed on the subject of mathematics and all the like ;-)
I just turn generally stupid when I haven't slept for nearly a full day.

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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Asmyldof
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Posted by Asmyldof: Sat. Jul 23, 2011 - 02:29 PM
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damien_d wrote:
You can buy them from DigiKey, though they are a little more expensive that your average mems device.

-- Damien

Thanks, am going to buy a set of those during the whole experiment, if they perform nicely I may just be able to use those in the official assignment to such an extend that contacting VTI directly proves profitable.

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Posted by Chris-Mouse: Sat. Jul 23, 2011 - 06:26 PM
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A couple of possibilities for solving this problem come to mind. The expensive method would be an ordinary target type spirit level and an industrial camera connected to a control system for the self-leveling.
Another approach would be four pressure sensors placed as far apart as possible, and connected by fluid filled tubing. If the platform is tilted, the sensors on the low side of the platform will read a higher pressure than those on the high side.

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jayjay1974
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Posted by jayjay1974: Sat. Jul 23, 2011 - 10:19 PM
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Schietlood = Plumb-bob.

Wikipedia can be very handy to translate these kind of words ;) Just look it up on the dutch one, then choose English at the languages pane at the left.

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larryvc
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Posted by larryvc: Sat. Jul 23, 2011 - 10:27 PM
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Sh*tload = "A lot" not Plumb-bob. :wink:

"I may make you feel but I can't make you think" - Jethro Tull - Thick As A Brick

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"When you arise in the morning think of what a privilege it is to be alive: to breathe, to think, to enjoy, to love." -  Marcus Aurelius

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ka7ehk
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Posted by ka7ehk: Sat. Jul 23, 2011 - 11:02 PM
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You mis-understand dual-axis MEMS linear accelerometers (the common sort). They do NOT measure angle RATE. THey measure linear acceleration either in a plane (2 axis) or spatially (3 axis). In the absence of dV/dt acceleration, G is the only acceleration they experience. They indicate the vector projection of the G vector onto the XY (and maybe Z) axes of the accelerometer. If the X and Y axes are exactly horizontal, they experience NO G force. If you tilt around the X axis, the Y sensor experiences a force that is positive or negative, depending on whether the angle is positive or negative. No rate, involved. No intertial considerations are involved. No integration (either single or double). They are absolute indicators.

Here is an app note that should clarify it all for you: You don't have to take my word for it, now.

http://www.analog.com/static/imp...

In an application like yours, they are nearly perfect. You don't care about scale factor, only zero offset. Zero offset may take some "calibration" at the "factory" but that can also be handled according to how it is mounted.

Here is a listing of low-G sensors from Analog Devices which is the category probably best suited for your application.

http://www.analog.com/en/mems-se...

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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damien_d
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Posted by damien_d: Sun. Jul 24, 2011 - 12:38 AM
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Asmyldof wrote:
The accelerometers talked about output angular velocity, not absolute position (well, not directly intended to, but they do in a way as pointed out by Jim)

Angular rate => MEMS Gyros

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damien_d
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Posted by damien_d: Sun. Jul 24, 2011 - 12:45 AM
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Asmyldof wrote:
damien_d wrote:
You can buy them from DigiKey, though they are a little more expensive that your average mems device.

-- Damien

Thanks, am going to buy a set of those during the whole experiment, if they perform nicely I may just be able to use those in the official assignment to such an extend that contacting VTI directly proves profitable.

For best performance, use the differential analogue out of the inclinometer. The AD7799 works nicely with it, though there might be better, more modern parts.

-- Damien

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Asmyldof
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Posted by Asmyldof: Wed. Aug 3, 2011 - 05:32 PM
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Thankyou everyone, I'm back.
Apearantly I have some sort of mean vitamin-stealing parasite in my digestive tract, but wanted to thank everyone here for the great suggestions, did some thinking and reading and contacted my importers/suppliers this morning.

I managed to talk them into giving me two pieces of one of the Analog Devices' 3-axis MEMS and a cheaper freescale model.
For those interested the FreeScale model I found is also available from Farnell:
- MMA8452QT
Farnell #: 1842360

I'm expecting a 1024 count per g should be enough for a trailer.

Thanks again, specially you Jim :-)
------
As far as I'm concerned I learned lots and am happy with the end result, so: Topic Closed.

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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js
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Posted by js: Wed. Aug 3, 2011 - 09:35 PM
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Quote:
I have some sort of mean vitamin-stealing parasite in my digestive tract
Many of us have mean money-stealing parasites AKA as politicians and bankers. :evil:

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Asmyldof
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Posted by Asmyldof: Wed. Aug 3, 2011 - 11:21 PM
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I'm hoping you are not talking about "in your digestive tract" and if you are, that they are no longer operational. Else I shall be forced to call you Mr Dinosaur from now on.

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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Asmyldof
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Posted by Asmyldof: Wed. Aug 3, 2011 - 11:35 PM
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js wrote:
politicians and bankers. :evil:

And the whole of Greece...
>.<
I believe I mandatorilly donated an euro or two to them as well.

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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pnp
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Posted by pnp: Thu. Aug 4, 2011 - 12:37 AM
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Asmyldof wrote:
I believe I mandatorilly donated an euro or two to them as well.
I think your translation let you down again... Instead of "donated", try "loaned". Also, while you are at it, try searching the words and phrases:

loan sharking, IMF, banking system, speculator attacks, corrupted politicians, country assets selling, USA debt, external debt of each european country

From what I have seen so far in this forum, each one keeps the nasty comments for his own country. Many fail to realise it, but we Greeks are very touchy these days, because we are in the spotlight even though he try hard as f*** and while others have equal or bigger problems.

I don't think you intented to insult me, but I felt I had to reply. We can drop this here and continue happily. But if you wish to discuss this further, it's fine with me, although please send me a PM so we don't get more off-topic here.

-Pantelis

Professor of Applied Murphology, University of W.T.F.Justhappened.

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Asmyldof
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Posted by Asmyldof: Thu. Aug 4, 2011 - 02:05 AM
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Since the topic is done this thread has been off topic for a full 100%.

And for the reference I also think JS didn't mean to insult politicians and bankers personally. They can't help being born in a politician's (or banker's) body.

If I made you or anyone else feel bad in any way I can only apologise.

So,
I apologise to you and anyone who may take offence at my comment. (I will not delete it, as I feel that is a dishonest way to 'solve' this)

As I made the comment, as with many others, I did so good naturedly and I see the fun in people moaning about the Dutch whenever certain J names resurface, a drug lord get sentenced, or whatever have you.
Where I can point out that it frustrates me greatly to be asked so often in a serious way what the best place is to score MJ in certain touristic neighbourhoods of bigger cities.

I see a difference in commenting on a country due to recent occurrences and a direct personal comment, however if I were to directly and publicly imply you personally were a cheap bastard living of my bank account I would expect nothing less than you to do every single thing you can think of to get me banned.
This not to imply, once again, that there is anyone or any nation I consider to be living off my personal money.
(except possibly in a small way those that respond to my other hardware post).

I also think you should realise that in the world Greece is far from the only country being talked about when it comes to loans and debts, but that the recent loaning to Greece makes it the go-to country name for the next month or so when comic reference are made from within those countries that decided to make the loan. This too shall pass.
And at the moment my thoughts are also riddled with the problems the USA are facing as I do direct business with people there, but they, unfortunately**, do not (yet) fit this specific turn of phrase.

Just to make sure:
**: I do not wish any problems onto the US, specially not more financial ones. But since the USA is clearly to blame for my 7kg overweight with their stupid McDonalds and BurgerKing and whatnot that I HAVE to eat when I'm stressed, because they told me so, I much prefer making fun of them.

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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If it helps, I can PM you my answer in a number of different languages. Ask if you have trouble reading my high-speed-babble in English.

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pnp
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Posted by pnp: Thu. Aug 4, 2011 - 02:39 AM
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Well, I think I also overreacted a bit, so I also apologize for "exploding"... I understand your comment was not personally against me, but since May 2010 I hear all kinds of things not only online and in general, but also personally when I travel abroad. So that has made me a bit edgy. Now I understand your views better and I actually agree in most parts. Thanks for writing them down. :)

-Pantelis

Professor of Applied Murphology, University of W.T.F.Justhappened.

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Asmyldof
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Posted by Asmyldof: Thu. Aug 4, 2011 - 04:48 AM
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I just wanted to make things clear. I never mean to assume or infer anything about specific people, nor normally of a people in general.

I understand your reaction, there are days I have to actively refrain from sniping at people inferring I'm stoned when I say something completely weird or incoherent, just because of my home town.

Now, I am tired and I shall go to bed knowing once again I witnessed a sunrise. *sigh*

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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js
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Posted by js: Thu. Aug 4, 2011 - 06:21 AM
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Quote:
I also think JS didn't mean to insult politicians and bankers personally.
I think I did. :wink:

John Samperi

Ampertronics Pty. Ltd.

www.ampertronics.com.au

* Electronic Design * Custom Products * Contract Assembly

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Asmyldof
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Posted by Asmyldof: Thu. Aug 4, 2011 - 06:57 AM
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Sorry John, I hope I haven't insulted you.

:-P

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
-------
If it helps, I can PM you my answer in a number of different languages. Ask if you have trouble reading my high-speed-babble in English.

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Brutte
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Posted by Brutte: Thu. Aug 4, 2011 - 03:43 PM
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Quote:
The problem with an accelerometer is that on regaining power you need to know the position of the surface.

You didn't get the hint. What you want to measure is an influence of gravitational field on a MEMS sensor. It has nothing to do with inertial forces. Thus no (double) integration is needed and no initial condition must be known (as in helicopters you used) as it is not the double integrate of force + x0 you are interested in, but the force itself (no need to integrate).

These MEMS sensors cannot distinguish between gravitational acceleration or inertial acceleration - they add both accelerations algebraically.
So if the output is 10m*s^-2, you can have -5m*s^-2 from gravitational acceleration and 15m*s^-2 from inertial acceleration for example. Or 9.81m*s^-2 from gravitational and 0.19m*s^-2 from inertial acceleration.. No difference for them and the output is going to be exactly the same.

No RSTDISBL, no fun!

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DO1THL
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Posted by DO1THL: Thu. Aug 4, 2011 - 05:18 PM
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Quote:
These MEMS sensors cannot distinguish between gravitational acceleration or inertial acceleration

Nobody can, this principle is the foundation of Einstein's whole theory of general relativity.

Einstein was right: "Two things are unlimited: the universe and the human stupidity. But i'm not quite sure about the former..."

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ka7ehk
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Posted by ka7ehk: Fri. Aug 5, 2011 - 12:15 AM
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If the sensor is stationary (non-moving trailer, for example), the ONLY acceleration the sensor detects is gravity. That Analog Devices App Note tells all about it.

Other than ( possibly ) correcting for offsets in the zero point of the axes, you need to know NOTHING about the initial position at turn on.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Brutte
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Posted by Brutte: Fri. Aug 5, 2011 - 09:06 AM
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However..
Gravitational acceleration varies.
Our small planet is quite mountainous. What is more - it spins around its own axis, around the Star, and there is this big rock flying around it..

All of these have influence on effective gravitational acceleration.

That is why you cannot calibrate precision gravitational scale in the factory - you need to make in on the field, as weigh scales use g to measure the mass (these divide force by g to get the mass).

Because of the same reason it is easier to launch a rocket from the equator than from poles. Preferably with some help of the Moon.

No RSTDISBL, no fun!

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JohanEkdahl
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Posted by JohanEkdahl: Fri. Aug 5, 2011 - 09:14 AM
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Quote:

Because of the same reason it is easier to launch a rocket from the equator [...]

... and eastward(-ish), I suppose?

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Brutte
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Posted by Brutte: Fri. Aug 5, 2011 - 10:08 AM
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Quote:
and eastward(-ish), I suppose?

I think so. But I have no idea if this extra 1700km/h is a significant factor.

There is a:
http://en.wikipedia.org/wiki/Lis...
nearby. They move enormous amounts of mass from one place to the other (about 800m) for a living. Twice a day. Although the horizon looks about the same each day, it varies by some factor because of that.
The question is: does it matter when you are leveling a trailer, or building a skyscraper nearby?

No RSTDISBL, no fun!

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Asmyldof
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Posted by Asmyldof: Fri. Aug 5, 2011 - 04:15 PM
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Lads,
I already sample ordered 2 chips of both brands, the absolute value of G does not matter, as long as it does not show up at all in the X or the Y axis of the chip.
These chips have a 3axis system, so exact orientation isn't even an issue.

Anyway, feel free to continue discussing, but no need to quote me on anything any more, did a lot of reading while in bed. ;-)

But, once again thanks for all the hinst and suggestions.

Embedded design is as much a life choice as any other and I demand the right to legally marry my work.
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If it helps, I can PM you my answer in a number of different languages. Ask if you have trouble reading my high-speed-babble in English.

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