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Rate-gyro accuracy

Hello,

in the ADXRS614 datasheet I read:

"CALIBRATED PERFORMANCE Using a 3-point calibration technique, it is possible to calibrate the null and sensitivity drift of the ADXRS614 to an overall accuracy of nearly 200°/hour. An overall accuracy of 40°/hour or better is possible using more points."

What does that mean? I don't understand the 200°/hour and 40°/hour values.

I'm going to integrate the rate to obtain an heading. But I need an accuracy better than 2° per hour. Does the sentence above tell me that if I integrate the rate (let's assume with no noise...) after 1 hour I may have an error of about 200° ?

I can't use a compass because there is a lot of iron in the nearby. I've already tried with a (calibrated) HMR-3000 with no chance.

So I'm going to use a rate-gyro to have a relative heading, but I'm not sure if I can get the required accuracy over the time.

Thanks for any hint. Marco

Reply to
Marco Trapanese
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Then MEMS sensors won't fit.

?

Yes. Actually, it could be worse, as the calibration gets invalidated=20 with aging or because of thermal or mechanical shock.

e=20

=20

Use the GPS.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

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Reply to
Vladimir Vassilevsky

Il 26/09/2010 18:58, Vladimir Vassilevsky ha scritto:

:-/ Other options?

I can't. It's an underwater equipment. Anyway, even if I was on the surface I don't know how to get the heading from the GPS. The platform doesn't move, just rotates.

Marco

Reply to
Marco Trapanese

Magnetometer.

Tim

-- Deep Friar: a very philosophical monk. Website:

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Reply to
Tim Williams

An acoustic beacon firmly fixed to the sea-bed nearby? A pair of microphones (three if the equipment rotates in all three dimensions) should be able to to give your the angle between the sound-field and the equipment if the wavelength is longer than the separation between the microphones.

Or maybe you could solve the compass problem with a really big and powerful fixed magnet in the immediate vicinity.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

Il 26/09/2010 19:28, Bill Sloman ha scritto:

Good ideas, but I can't use anything off-board :(

Marco

Reply to
Marco Trapanese

Il 26/09/2010 19:19, Tim Williams ha scritto:

Hi Tim,

as far as I know, the solid-state compasses (like the HMR3000) contain 3 magnetometers. Because you suggest me to use them I'm pretty sure I'm wrong.

So, why magnetometers won't be affected by iron?

Marco

Reply to
Marco Trapanese

?

The sentence says that the manufacturer will not guarantee a performance better than 200 degrees per hour.

Since all you interested in is direction, what is wrong with an old fashioned circular contact array?

Reply to
Richard Henry

Il 26/09/2010 20:27, Richard Henry ha scritto:

Ok. Very bad :(

The wrong thing is that there is no contact :) It's an underwater vehicle.

Marco

Reply to
Marco Trapanese

On a sunny day (Sun, 26 Sep 2010 20:36:14 +0200) it happened Marco Trapanese wrote in :

If it can see ground, you could process a camera image for rotation. Does a fluxgate compass work under water?

Reply to
Jan Panteltje

Il 26/09/2010 20:49, Jan Panteltje ha scritto:

Zero visibility.

yes, it works. But it also affected by the iron around.

Marco

Reply to
Marco Trapanese

On a sunny day (Sun, 26 Sep 2010 20:58:24 +0200) it happened Marco Trapanese wrote in :

Well that leaves sonar, perhaps it can rotate a sonar, and process the sonar image of the bottom for rotation, same priciple as light. Couls also be just three directional sonar beams, usually the sea floor is not 100 % flat... I hope. :-)

Reply to
Jan Panteltje

Oh, OP said iron. Nevermind.

Not entirely impossible, as gigantic steel ships have compasses. But if you don't know where the iron is (say you're swimming around the metal roots of a pylon, or something), it would be rather difficult to compensate for.

Can you grab some of those ex-mil gyros from Electronic Goldmine? Or...talk to whoever they got them from if you need quantity. Or if you need to go to the manufacturer...well, that's going to be pricey...

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

Il 26/09/2010 21:08, Tim Williams ha scritto:

Yep. I have a *lot* of iron just under the bottle. Then, the platform will be very close to such a pylon.

mmm... I think it will be difficult, but I can try.

Marco

Reply to
Marco Trapanese

Il 26/09/2010 21:06, Jan Panteltje ha scritto:

Good idea! Anyway, it requires a quite complex firmware, I bet.

Marco

Reply to
Marco Trapanese

Try anyhing with a FOG; HG1700 works really well but subject to ITAR. KVH DSP-3000 is another option for your spec. Be prepared to pay $10K+.

--
Muzaffer Kal

DSPIA INC.
ASIC/FPGA Design Services

http://www.dspia.com
Reply to
Muzaffer Kal

Il 26/09/2010 21:38, Muzaffer Kal ha scritto:

Eh, if I could pay $10k I don't ask here :)

Marco

Reply to
Marco Trapanese

If it's for a personal or educational project, you might try fusing several independent yaw-rate sensors.

If it's commercial project then something like this

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is what your competition is using.

--
Rich Webb     Norfolk, VA
Reply to
Rich Webb

Hi Marco, to get that accuracy you need a gyro based on FOG or RLG technology. The market is rich of equipment of that kind (look for IMU, AHRS, INS), but they are quite costly. True heading gyros are capable of giving heading respect to geographic North (they measure the earth rotation rate). That's exactly what most underwater vehicles use.

P.S. it's not sufficient to integrate yaw rate to get heading. You must have all three rate axes.

Ciao,

--
RoV - IW3IPD
http://digilander.libero.it/rvise/
Reply to
RoV

What do real gyrocompasses cost these days?

John

Reply to
John Larkin

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