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From: Alfred Bovin on 8 Mar 2010 14:29
Hi all.

I'll take the chance and ask this question here in the forum since perhaps
there are someone familiar with INS here.

I'm trying to do some inertial navigation for (what perhaps will become) an
autonomous all-terrain vehicle with a strapdown inertial sensor (an
ADIS16350 from Analog Devices) with a 3-axis gyro and a 3-axis accel. I have
no previous experience with this stuff so I've been reading a bit in
"Strapdown inertial navigation technology" by Titterton and Weston.

My question is what kind of reference frame I should use. Let's say that at
time 0 the platform is at some location (I'm not interested in the absolute
position on the Earth) and the frame attached to the platform coincides with
the frame I will represent the navigation in. Then at times > 0 I want to be
able to reconstruct the motion taken by the platform based on the inertial
data.

In the book, three different reference systems are presented of what appears
to be of increasing complexity: navigation with respect to an inertial
coordinate system located at the center of the Earth, navigation with
respect to a coordinate system located at the center of the Earth but
rotating with it and navigation with respect to a local geographic
coordinate system.

In the summary of the chapter they then write that any of these approaches
can be used for navigation close to Earth but the local geographic
coordinate system is preferable for navigating over large distances.

What I fail to see is the difference between this approaches (except for
their mathematical apperances). Do you have any opinions on what to use when
my vechicle never leaves the surface of the Earth?

Thanks in advance.


From: Tim Wescott on 8 Mar 2010 15:12
Alfred Bovin wrote:
> Hi all.
>
> I'll take the chance and ask this question here in the forum since perhaps
> there are someone familiar with INS here.
>
> I'm trying to do some inertial navigation for (what perhaps will become) an
> autonomous all-terrain vehicle with a strapdown inertial sensor (an
> ADIS16350 from Analog Devices) with a 3-axis gyro and a 3-axis accel. I have
> no previous experience with this stuff so I've been reading a bit in
> "Strapdown inertial navigation technology" by Titterton and Weston.
>
> My question is what kind of reference frame I should use. Let's say that at
> time 0 the platform is at some location (I'm not interested in the absolute
> position on the Earth) and the frame attached to the platform coincides with
> the frame I will represent the navigation in. Then at times > 0 I want to be
> able to reconstruct the motion taken by the platform based on the inertial
> data.
>
> In the book, three different reference systems are presented of what appears
> to be of increasing complexity: navigation with respect to an inertial
> coordinate system located at the center of the Earth, navigation with
> respect to a coordinate system located at the center of the Earth but
> rotating with it and navigation with respect to a local geographic
> coordinate system.
>
> In the summary of the chapter they then write that any of these approaches
> can be used for navigation close to Earth but the local geographic
> coordinate system is preferable for navigating over large distances.
>
> What I fail to see is the difference between this approaches (except for
> their mathematical apperances). Do you have any opinions on what to use when
> my vechicle never leaves the surface of the Earth?

Navigating with respect to a fixed earth seemed to be easiest to me. No
matter what you do you'll be left with coordinate transforms that will
make your head hurt. You can't avoid that -- just go with what you
think will make your head hurt least.

Navigating with respect to some local geographic coordinate system would
work well if you never have to change the coordinate system, but as soon
as you want to make a product that'll work world-wide you're back to
that headache.

With that IMU I don't think you have to pay attention to the earth's
rotation -- IIRC the gyros on that have errors far in excess of 15
degrees/hour, so any signal you'd get from the earth's rotation will be
swamped by the gyro errors.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Rich Webb on 8 Mar 2010 15:15
On Mon, 8 Mar 2010 20:29:28 +0100, "Alfred Bovin" <alfred(a)bovin.invalid>
wrote:

>Hi all.
>
>I'll take the chance and ask this question here in the forum since perhaps
>there are someone familiar with INS here.

Might try over on sci.engr.control if you don't get a good discussion
started here.

>I'm trying to do some inertial navigation for (what perhaps will become) an
>autonomous all-terrain vehicle with a strapdown inertial sensor (an
>ADIS16350 from Analog Devices) with a 3-axis gyro and a 3-axis accel. I have
>no previous experience with this stuff so I've been reading a bit in
>"Strapdown inertial navigation technology" by Titterton and Weston.

Be advised that an inexpensive navigation-grade INS will run on the
order of $100K. The free-inertial performance of MEMs isn't quite good
enough, yet, although it may be someday given the potential demand.

>My question is what kind of reference frame I should use.

Unless you're using a specialized unit, your GPS fixes will be in WGS-84
latitude and longitude (assuming that's the fix source you'll be using)
so it's probably easiest to stick with that reference frame.

--
Rich Webb Norfolk, VA
From: Tim Wescott on 8 Mar 2010 18:47
Rich Webb wrote:
> On Mon, 8 Mar 2010 20:29:28 +0100, "Alfred Bovin" <alfred(a)bovin.invalid>
> wrote:
>
>> Hi all.
>>
>> I'll take the chance and ask this question here in the forum since perhaps
>> there are someone familiar with INS here.
>
> Might try over on sci.engr.control if you don't get a good discussion
> started here.
>
>> I'm trying to do some inertial navigation for (what perhaps will become) an
>> autonomous all-terrain vehicle with a strapdown inertial sensor (an
>> ADIS16350 from Analog Devices) with a 3-axis gyro and a 3-axis accel. I have
>> no previous experience with this stuff so I've been reading a bit in
>> "Strapdown inertial navigation technology" by Titterton and Weston.
>
> Be advised that an inexpensive navigation-grade INS will run on the
> order of $100K. The free-inertial performance of MEMs isn't quite good
> enough, yet, although it may be someday given the potential demand.
>
I had not thought to point that out in my response.

Note that a MEMS sensor _will_ let you improve on the accuracy of GPS
measurements, but it has to be a good one. I honestly don't know if
that Analog Devices one is good enough. If you're getting enough random
acceleration you'll be able to do a good job of finding down and north,
though.

>> My question is what kind of reference frame I should use.
>
> Unless you're using a specialized unit, your GPS fixes will be in WGS-84
> latitude and longitude (assuming that's the fix source you'll be using)
> so it's probably easiest to stick with that reference frame.
>
Trying to translate from your INS frame of reference into GPS's WGS-84
frame will make your head hurt. Trying to write a system equation to
express the motion of an accelerated, rotating body in the WGS-84 frame
will make your head _really_ hurt. (Say "Coriolis". Say "Coriolis, ow
ow ow").

Translating both to earth-centered, earth-fixed, then back, will make
your head hurt -- each way -- but both of those together may well make
your head hurt less than _just_ trying to write a system equation that
stays in WGS-84 coordinates. Particularly if you want your system to
work close to the poles.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
From: Vladimir Vassilevsky on 8 Mar 2010 20:50


Alfred Bovin wrote:

> Hi all.
>
> I'll take the chance and ask this question here in the forum since perhaps
> there are someone familiar with INS here.
>
> I'm trying to do some inertial navigation for (what perhaps will become) an
> autonomous all-terrain vehicle with a strapdown inertial sensor (an
> ADIS16350 from Analog Devices) with a 3-axis gyro and a 3-axis accel. I have
> no previous experience with this stuff so I've been reading a bit in
> "Strapdown inertial navigation technology" by Titterton and Weston.

That won't work. The accuracy of ADIS is typical for MEMS sensors, i.e.
~1 cm/c^2 or so. This is inadequate for innertial navigation.


> My question is what kind of reference frame I should use. Let's say that at
> time 0 the platform is at some location (I'm not interested in the absolute
> position on the Earth) and the frame attached to the platform coincides with
> the frame I will represent the navigation in. Then at times > 0 I want to be
> able to reconstruct the motion taken by the platform based on the inertial
> data.
>
> In the book, three different reference systems are presented of what appears
> to be of increasing complexity: navigation with respect to an inertial
> coordinate system located at the center of the Earth, navigation with
> respect to a coordinate system located at the center of the Earth but
> rotating with it and navigation with respect to a local geographic
> coordinate system.
>
> In the summary of the chapter they then write that any of these approaches
> can be used for navigation close to Earth but the local geographic
> coordinate system is preferable for navigating over large distances.
>
> What I fail to see is the difference between this approaches (except for
> their mathematical apperances). Do you have any opinions on what to use when
> my vechicle never leaves the surface of the Earth?

You are right: math is the same regardless of coordinate system. I would
do the calculations wrt the center of the Earth; then convert the result
to WGS 84.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

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