State of the art in stable accelerometers?

[Peter]

Hello All,

I am doing some basic research on this. I have been doing hardware and
software since the mid-1970s and know a bit about the micromachined
silicon ones, etc, used in camera anti-shake systems etc.

What is the current state of the art, for long distance (light
aircraft) navigation?

I looked into this ~ 3 years ago and the state of the art was about
2-3 orders of magnitude short on long term stability issues.

If one is going to do double integration (acceleration to distance)
one needs a lot of stability.

Then there is a huge gap to fibre optic gyros which are of course fine
but $ 5 digits plus.

Have there been any recent developments which might allow an accuracy
of say 100m of track after 100km of flight at say 200km/hr?

Thank you for any pointers.

I am a private pilot (instrument rated) and was astounded to hear, at
a recent conference, one of the European regulators stating that "new
silicon chips" will make low cost INS systems for light aircraft
possible. This completely suprised me. I think he was dreaming, but
was he?

 

[linnix]

Hello All,

I am doing some basic research on this. I have been doing hardware and
software since the mid-1970s and know a bit about the micromachined
silicon ones, etc, used in camera anti-shake systems etc.

What is the current state of the art, for long distance (light
aircraft) navigation?

I looked into this ~ 3 years ago and the state of the art was about
2-3 orders of magnitude short on long term stability issues.

If one is going to do double integration (acceleration to distance)
one needs a lot of stability.

Then there is a huge gap to fibre optic gyros which are of course fine
but $ 5 digits plus.

Have there been any recent developments which might allow an accuracy
of say 100m of track after 100km of flight at say 200km/hr?

Thank you for any pointers.

I am a private pilot (instrument rated) and was astounded to hear, at
a recent conference, one of the European regulators stating that "new
silicon chips" will make low cost INS systems for light aircraft
possible. This completely suprised me. I think he was dreaming, but
was he?

You can do short term corrections with accelerometers, but GPS will
give much better absolute readings. You need both for INS, and they
are fairly low cost anyway.

 

[joepierson]

Hello All,

I am doing some basic research on this. I have been doing hardware and
software since the mid-1970s and know a bit about the micromachined
silicon ones, etc, used in camera anti-shake systems etc.

What is the current state of the art, for long distance (light
aircraft) navigation?

I looked into this ~ 3 years ago and the state of the art was about
2-3 orders of magnitude short on long term stability issues.

If one is going to do double integration (acceleration to distance)
one needs a lot of stability.

Then there is a huge gap to fibre optic gyros which are of course fine
but $ 5 digits plus.

Have there been any recent developments which might allow an accuracy
of say 100m of track after 100km of flight at say 200km/hr?

Thank you for any pointers.

I am a private pilot (instrument rated) and was astounded to hear, at
a recent conference, one of the European regulators stating that "new
silicon chips" will make low cost INS systems for light aircraft
possible. This completely suprised me. I think he was dreaming, but
was he?

No breakthroughs in MEMS accelerometers.

 

[Paul Keinanen]

On Fri, 03 Oct 2008 21:56:25 +0100, Peter wrote:

The state of the art is GPS aided inertial nav systems, where the GPS
provides the long term stability that the inertial measurements lack, and
the inertial measurements provide the short-term noise smoothing that the
GPS lacks.

I assume you mean pure inertial navigation, though? Why do you need it,
in a world with GPS, Loran, &c?

I hope that you have some independent means on finding a an airfield
and land safely, in case these external systems are suddenly shut down
during your flight.

Paul

 

[Peter]

I assume you mean pure inertial navigation, though? Why do you need it,
in a world with GPS, Loran, &c?

The trigger for this study is a Eurocontrol (the organisation which,
for the most part, rules IFR airspace in Europe) proposal to mandate a
backup for GPS.

Heavy stuff already has INS but the light stuff is GPS only (if you
are talking PRNAV accuracy, VORs don't really count).

Eurocontrol have repeatedly said they will mandate INS or Loran, and
that "new silicon chips" will enable miniaturised INS systems.

I looked into this 3 years ago and found absolutely nothing, and I
still think they are talking nonsense.

In any case, the USA is happy with GPS for the most precision stuff
(using WAAS for vertical guidance applications) so none of the big
avionics manufacturers will make the stuff anyway.... [looking for an
emoticon of a head banging against a wall] but that has never stopped
the Europeans from making some ruling which is impractical, or in this
case excludes general aviation from certain classes of airspace
(PRNAV) which is THE real concern here.

I am attending a Eurocontrol conference later this year and getting
myself clued up on the state of the art, and if there is nothing I
will tell them, because sure as hell nobody else is going to. This
bizzare proposal has been in the making inside Eurocontrol for about a
year.

 

[Peter]

This is what Boeing was using in their airliners a couple years ago:

http://www.systron.com/pro_QRS11.asp

It was in the news because they were charged with violating
export technology restrictions by selling them (in their
airliners) without a license.

This appears suitable for the applications listed, which are basically
attitude control, not navigation. There are many devices on the market
used for that, for artificial horizons etc. The spec says

Short Term Bias Stability(100 sec at const. temp)< or =0.01°/sec,
typical *

How does one translate that into a positional error?

In any case, a spec over 100 seconds is not useful - a 100 second GPS
outage is a very short time.

 

[Peter]

The state of the art is GPS aided inertial nav systems, where the GPS
provides the long term stability that the inertial measurements lack, and
the inertial measurements provide the short-term noise smoothing that the
GPS lacks.

Is this on the market?

 

[Peter]

Oh, sure. Just Google "AHRS" (attitude and heading reference system).
The Wikipedia article lists about a dozen manufacturers.

A GPS-aided Kalman filter can readily learn and compensate for
systematic errors in the inertial sensors, such as bias and scale
factor. The thing that kills you over the long term (during a GPS
outage) is the "random walk" in your system state that is the
result of integrating the sensor noise.

Any system that can place an aircraft to the accuracy that you're
looking for can also place a missile, and therefore will be subject
to tight military and/or export restrictions (ITAR in the USA).

-- Dave Tweed

OK, I know that one, e.g. Garmin G1000, Avidyne, etc.

But none of these have any navigation capability. They are just
artificial horizons, basically.

Agree re export controls on inertial nav technology. This would be a
problem. They obviously get over it in airliners though, otherwise
Boeing could not sell any planes. But those are FOGs anyway.

Presently I cannot see any sub-FOG technology which is any good for
navigation. The regulators here (Europe) claim this can be done now,
but for an INS which can work from DME/DME fixes one would need say
30-60 mins of autonomous navigation, and that is a very tall order.

Does anyone have a formula for working out the likely accuracy, from
the stability/noise specs of accelerometers?

 

[Peter]

I think Crossbow technologies has something like this for light aircraft
-- dig up their website (searching on "crossbow" and "IMU" or "INS"
should get you there) and take a look.

They may even have, or be willing to cough up when asked, a chart of
error vs. time after GPS blinks out.

I found them at xbow.com and this

http://www.xbow.com/Products/Product_pdf_files/Inertial_pdf/NAV420CA_Datasheet.pdf

seems to be their 'navigation' product. The spec (if I read it right)
is 0.1 degree per second drift, which is insufficient for GPS-lost
navigation by a factor of 100 at least.

 

[Glen Walpert]

No breakthroughs in MEMS accelerometers.

Not much change in the last decade except lower power, smaller and 3
axis packages. But there are still 2 basic designs, the common
piezioresistive type available from many manufacturers, which has
relatively high noise levels, and the much harder to fabricate
capacitive accelerometers (harder due to a much deeper etch). AFIK
the capacitive type are only manufactured by Kistler and not cheap,
but they have more than an order of magnitude lower noise levels than
piezioresistive types of the same range.

<http://www.kistler.com/us_en-us/135...K-Beam-Capacitive-Triaxial-Accelerometer.html>

 

[nobody]

You can do short term corrections with accelerometers, but GPS will
give much better absolute readings. You need both for INS, and they
are fairly low cost anyway.

And if it's too cheap you can probably look at what the weapon industry
is doing:

http://www.mbda-systems.com/mbda/site/ref/scripts/EN_Taurus-KEPD-350_176.html

Terrain reference navigation!

 

[[email protected]]

Hello All,

I am doing some basic research on this. I have been doing hardware and
software since the mid-1970s and know a bit about the micromachined
silicon ones, etc, used in camera anti-shake systems etc.

For the extreme case, see this month's issue of Scientific American,
the article on "The Big Bump". It discusses a super accurate platform
used in a space probe designed to detect the presence of "frame
dragging".

 

[Tauno Voipio]

Hello All,

I am doing some basic research on this. I have been doing hardware and
software since the mid-1970s and know a bit about the micromachined
silicon ones, etc, used in camera anti-shake systems etc.

What is the current state of the art, for long distance (light
aircraft) navigation?

I looked into this ~ 3 years ago and the state of the art was about
2-3 orders of magnitude short on long term stability issues.

If one is going to do double integration (acceleration to distance)
one needs a lot of stability.

Then there is a huge gap to fibre optic gyros which are of course fine
but $ 5 digits plus.

Have there been any recent developments which might allow an accuracy
of say 100m of track after 100km of flight at say 200km/hr?

Thank you for any pointers.

I am a private pilot (instrument rated) and was astounded to hear, at
a recent conference, one of the European regulators stating that "new
silicon chips" will make low cost INS systems for light aircraft
possible. This completely suprised me. I think he was dreaming, but
was he?

I'd not take the Eurocontrol speculations at face value ...

For an INS, you'd need three very good accelerometers
and a very good AHRS (attitude and heading reference system).
The accelerometers will accumulate all errors twice, as
the acceleration has to be integrated once for velocity and
another time for position.

Due to mechanical and power constraints, a gyro-stabilized
accelerometer platform is out of question in a small airplane,
and a strap-down system has the attitude and heading angle
errors adding to the acceleration errors. There is a good
reason why the airliners are using laser gyros.

The current state of the art for small aircraft (e.g. Garmin 1000)
is using semiconductor AHRS and augmenting it with the traditional
navaids (VOR, NDB & co).

It is still much better than the thingies in my Turbo Arrow
with gyro-based HSI, traditional navaids and a separate GPS.

 

[Peter]

I'd not take the Eurocontrol speculations at face value ...

For an INS, you'd need three very good accelerometers
and a very good AHRS (attitude and heading reference system).
The accelerometers will accumulate all errors twice, as
the acceleration has to be integrated once for velocity and
another time for position.

Due to mechanical and power constraints, a gyro-stabilized
accelerometer platform is out of question in a small airplane,
and a strap-down system has the attitude and heading angle
errors adding to the acceleration errors. There is a good
reason why the airliners are using laser gyros.

I thought laser gyros were effectively strap-down gyros - something
counts the pulses from the diffraction of the beams or whatever, and
the pulse count is counted to give an integrated velocity value
directly.

The current state of the art for small aircraft (e.g. Garmin 1000)
is using semiconductor AHRS and augmenting it with the traditional
navaids (VOR, NDB & co).

Sure, but we are still just talking about an artificial horizon,
aren't we? The G1000 does no inertial nav whatsoever - other than pure
GPS.

It is still much better than the thingies in my Turbo Arrow
with gyro-based HSI, traditional navaids and a separate GPS.

Funny that - I am an FAA CPL/IR but not CFII Always suprises me how
many electronics / hardware / software people fly too. I will try to
drop you an email.

 

[Wim Lewis]

Not much change in the last decade except lower power, smaller and 3
axis packages. But there are still 2 basic designs, the common
piezioresistive type available from many manufacturers, which has
relatively high noise levels, and the much harder to fabricate
capacitive accelerometers (harder due to a much deeper etch).

There are also thermal accelerometers, which have a little bubble of
fluid (air, probably?) with a heater surrounded by temperature sensors;
the warmer (and therefore less dense) part of the fluid moves in the
direction of acceleration. I don't know if these are any more accurate
than the types you mention; I'm just mentioning them for completeness.
I think their main advantage is shock/impact resistance.

 

[linnix]

There are also thermal accelerometers, which have a little bubble of
fluid (air, probably?) with a heater surrounded by temperature sensors;
the warmer (and therefore less dense) part of the fluid moves in the
direction of acceleration. I don't know if these are any more accurate
than the types you mention; I'm just mentioning them for completeness.
I think their main advantage is shock/impact resistance.

50000g (fluid based) vs. 5000g (capacitive based).
I think production cost is more of an issue.

 

[linnix]

I presume we're talking about an explosive-based mechanism
to get to the 50000 g values.

Yes, you would likely rip everything out of the PCB with 50K G
anyway. Ususally, you don't care much about the device after impact,
as in ICBMs.

 

[Falk Willberg]

I presume we're talking about an explosive-based mechanism
to get to the 50000 g values.

IMHO any X-g spec does not mean anything without a second parameter,
like duration or way.

Imagine two pieces of extremely hard material touching eachother. Two
bottles of beer, for example. Not much m/s, but a very small term 1/s,
thus m/s^2 may reach a few thousands of g.

On the other hand, a 40tons truck smashing a compact car will hardly be
exposed to more than 1g.

Falk

 

[joepierson]

I presume we're talking about an explosive-based mechanism
to get to the 50000 g values.- Hide quoted text -

- Show quoted text -

drop a accelerometer 3 feet on a hard surface will get you 10,000+ g's
and kill most mems accelerometers