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'[EE]: Navigation sensors'
2004\08\23@101152 by Shawn Mulligan

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I'm working on a sensor pack which will measure azimuth, inclination and
roll. Currently I'm involved in literature review and parts specification
with a leaning towards Honeywell's magneto-resistive magnetometers and an
Analog accelerometer. Both Honeywell and Analog provide excellent
application notes and support, but the input of any board members would also
be appreciated, specifically direction to further online research sources.

Thanks, Shawn

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2004\08\23@103227 by M. Adam Davis

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You can also buy expensive mechanical pendulums ($100 for one) that
output a standard gray code (like a rotary encoder) with an index pulse.
http://www.usdigital.com/products/t2/
http://www.usdigital.com/products/a2t/

Then there are capacitave sensors.
http://www.maxim-ic.com/appnotes.cfm/appnote_number/198

For my most recent design I'm using an ADXL213, which should be good to
1/10 of a degree through its range, but with very slow response time
(1Hz bandwidth).  Once you start needing faster updates the resolution
drops quickly.

There are companies that have done all the hard work (temp compensation,
sensor testing, etc) and have drop in modules to do what you want for $90.

-Adam

Shawn Mulligan wrote:

{Quote hidden}

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2004\08\23@105721 by Shawn Mulligan

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Originally posted by Adam:

For my most recent design I'm using an ADXL213, which should be good to
1/10 of a degree through its range, but with very slow response time
(1Hz bandwidth).  Once you start needing faster updates the resolution
drops quickly.

Thanks for the input, Adam. I've sampled the Maxim part and I'm also using
the ADXL213 in a current design. It's an accurate, easy to incorporate
little accelerometer.

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2004\08\23@121201 by Matt Redmond

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Memsic (http://www.memsic.com) makes some single and dual axis accelerometers.  My coworker has their 2-axis demo board ($25) sitting on his desk - pretty cool.

I think one limitation of accelerometers in a 3-D environment is that centrifugal force can play tricks on them.  For example,  in a coordinated turn in an airplane, you're pulling 1G straight down the yaw axis - your roll-axis sensor wouldn't register the roll I don' think.

I want to say that most manufacturers of 3-axis navigation devices combine gyro inclinometers, accelerometers and solid-state compasses with an alogorithm whose name escapes me - to compute very accurate pitch/yaw/roll angles.

Crossbow (http://www.xbow.com) is one.

-matt redmond




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2004\08\23@123652 by Shawn Mulligan

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Matt Redmond writes:

Memsic (http://www.memsic.com) makes some single and dual axis
accelerometers.  My coworker has their 2-axis demo board ($25) sitting on
his desk - pretty cool.

I think one limitation of accelerometers in a 3-D environment is that
centrifugal force can play tricks on them.  For example,  in a coordinated
turn in an airplane, you're pulling 1G straight down the yaw axis - your
roll-axis sensor wouldn't register the roll I don' think.

Thanks. Yes, the Memsic evaluation boards are very well priced and cool.

I'm using these devices in a directional drilling application, and surveys
are only taken when the sensor is stationary, so centrifugal forces won't be
an issue, but I'll follow that train for future reference.

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2004\08\24@174829 by Peter L. Peres

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>I think one limitation of accelerometers in a 3-D environment is that
>centrifugal force can play tricks on them.  For example, in a coordinated
>turn in an airplane, you're pulling 1G straight down the yaw axis - your
>roll-axis sensor wouldn't register the roll I don' think.

These exists no such thing as a coordinated 1g turn. The sum of the
acceleration vectors for any kind of travel excepting straight line is
significantly different from 1g.

Peter

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2004\08\24@202016 by Robert Monsen
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----- Original Message -----
From: "Peter L. Peres" <KILLspamplpKILLspamspamACTCOM.CO.IL>
To: <RemoveMEPICLISTTakeThisOuTspamMITVMA.MIT.EDU>
Sent: Tuesday, August 24, 2004 9:35 PM
Subject: Re: [EE]: Navigation sensors


{Quote hidden}

No, a descending coordinated turn in an aircraft can feel just like 1g
straight and level flight. I realize its possible to tell the difference
over time using precession effects, but instantaneously, the acceleration
vector is simply down.

Thats why pilots use gyros for flying in clouds.

Regards,
Bob Monsen

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2004\08\24@232049 by Howard Winter

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On Tue, 24 Aug 2004 17:20:32 -0700, Robert Monsen wrote:

> ----- Original Message -----
> From: "Peter L. Peres" <spamBeGoneplpspamBeGonespamACTCOM.CO.IL>
>...<
> > These exists no such thing as a coordinated 1g turn.
The sum of the
> > acceleration vectors for any kind of travel
excepting straight line is
> > significantly different from 1g.
>
> No, a descending coordinated turn in an aircraft can
feel just like 1g
> straight and level flight. I realize its possible to
tell the difference
> over time using precession effects, but
instantaneously, the acceleration
> vector is simply down.
>
> Thats why pilots use gyros for flying in clouds.

Indeed, and why the life-expectancy of an inexperienced
pilot entering IMC unexpectedly is very few minutes!
Whoever coined the phrase "flying by the seat of your
pants" was utterly wrong - if you rely on the "feel" of
things when you can't see the horizon or the ground,
you're in real trouble.

Happy Landings...


Howard Winter
St.Albans, England

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2004\08\24@234334 by Joe Geller

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>> These exists no such thing as a coordinated 1g turn.

What about 1g barrel rolls?

(As commanded by a PIC micro control system in an R/C plane :)

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2004\08\25@132418 by Peter L. Peres

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>> These exists no such thing as a coordinated 1g turn. The sum of the
>> acceleration vectors for any kind of travel excepting straight line is
>> significantly different from 1g.
>>
>> Peter
>>
>
>No, a descending coordinated turn in an aircraft can feel just like 1g
>straight and level flight. I realize its possible to tell the difference
>over time using precession effects, but instantaneously, the acceleration
>vector is simply down.

You are right.

Peter

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2004\08\25@140534 by Matt Redmond

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Yes, that is correct - I brain-farted on that one.  Coordination just keeps the centrifugal force / gravity vectors such that you are being pulled straight down into your seat rather than slipping sideways in it.

-matt


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2004\08\25@144914 by Gordon Williams

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----- Original Message -----
From: "Matt Redmond" <mdredmondEraseMEspam.....CHARTER.NET>
To: <EraseMEPICLISTspamMITVMA.MIT.EDU>
Sent: Wednesday, August 25, 2004 2:04 PM
Subject: Re: [EE]: Navigation sensors


> Yes, that is correct - I brain-farted on that one.  Coordination just
keeps the centrifugal force / gravity vectors such that you are being pulled
straight down into your seat rather than slipping sideways in it.
{Quote hidden}

Both are correct.  What will be seen in typical cases is something slightly
over 1g for a non-radical coordinated turn.

Someone made a comment about a coordinated descending turn as being able to
produce a 1g turn.  This is only true if the downward speed increases
constantly (i.e. you accelerate downward).  Of course you can't continue to
do this for very long until something very bad happens ;()

This is all nit picking though.

Regards,

Gordon Williams

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2004\08\25@193811 by Robert Monsen

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----- Original Message -----
From: "Gordon Williams" <RemoveMEg_willTakeThisOuTspamspamCYBERUS.CA>
To: <EraseMEPICLISTspamspamspamBeGoneMITVMA.MIT.EDU>
Sent: Wednesday, August 25, 2004 11:53 AM
Subject: Re: [EE]: Navigation sensors


> ----- Original Message -----
> From: "Matt Redmond" <RemoveMEmdredmondKILLspamspamCHARTER.NET>
> To: <PICLISTSTOPspamspamspam_OUTMITVMA.MIT.EDU>
> Sent: Wednesday, August 25, 2004 2:04 PM
> Subject: Re: [EE]: Navigation sensors
>
>
> > Yes, that is correct - I brain-farted on that one.  Coordination just
> keeps the centrifugal force / gravity vectors such that you are being
pulled
{Quote hidden}

slightly
> over 1g for a non-radical coordinated turn.
>
> Someone made a comment about a coordinated descending turn as being able
to
> produce a 1g turn.  This is only true if the downward speed increases
> constantly (i.e. you accelerate downward).  Of course you can't continue
to
> do this for very long until something very bad happens ;()
>

Pilots call it a graveyard spiral... VFR only pilots who venture into clouds
can end up in one of these fairly easily. The end is when they notice the
altitude decreasing quickly, and attempt to pull the nose up with the
elevator. Unfortunately, the lift vector has to pull 5.8 Gs in order to keep
level, because of the unnoticed 80 degree bank... this will occasionally
pull off the tail, causing the pilot to have a bad day.

{Quote hidden}

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