INS gyro alternative or enhancement

Hi,
I am building a INS with three gyros and three acclerometer. The problem
is that the gyro drift and the accelerometer is quite noisy. The accel
noise is something I can deal with because I can filter it out, but the
drift of the gyro is basically screwing me up. (the problems it that it is
drifting in both direction)

I am thinking of adding additional sensors and use a kalman filter to
integrate these sensor values together and hopefully it will be better.
Looking around, I cannot find any cheap sensor (< $100), except of another
accelerometer and gyroscope, but I am not sure if it is going to help.

I did see people building a INS with only six accelerometers mounted with
different orientations, but I don't really want to have something oriented
other than on the axis of xyz. Does another additional accelerometer on the
xyz axis help? Using accelerometer to calculate the orientation will be
nice, but the thing can be moving... Not sure if a low pass filter will
help.. (I don't really think so) so .. any ideas?

I tried using a 3d compass for other projects and it is a nightmare.
Calibration is .... horrible, so I will avoid that as much as possible.

Terence






 
		
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blackmangoes wrote:

> Hi,
> I am building a INS with three gyros and three acclerometer. The problem
> is that the gyro drift and the accelerometer is quite noisy. The accel
> noise is something I can deal with because I can filter it out, but the
> drift of the gyro is basically screwing me up. (the problems it that it is
> drifting in both direction)
> 
> I am thinking of adding additional sensors and use a kalman filter to
> integrate these sensor values together and hopefully it will be better.
> Looking around, I cannot find any cheap sensor (< $100), except of another
> accelerometer and gyroscope, but I am not sure if it is going to help.
> 
> I did see people building a INS with only six accelerometers mounted with
> different orientations, but I don't really want to have something oriented
> other than on the axis of xyz. Does another additional accelerometer on the
> xyz axis help? Using accelerometer to calculate the orientation will be
> nice, but the thing can be moving... Not sure if a low pass filter will
> help.. (I don't really think so) so .. any ideas?
> 
> I tried using a 3d compass for other projects and it is a nightmare.
> Calibration is .... horrible, so I will avoid that as much as possible.
> 
Folks are doing amazing things integrating inertial sensors with GPS. 
Basically the GPS helps to calibrate the sensors, and visa-versa, in one 
great big Kalman filter.  I have absolutely no idea of what's bubbled 
down to being available on the web -- this is still a subject in which 
you can safely base a PhD thesis.

-- 
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Be nice to not depend on GPS because the thing will be mostly (not all the
time) indoor


		
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blackmangoes wrote:

> Be nice to not depend on GPS because the thing will be mostly (not all the
> time) indoor
> 
Well, you didn't mention that.

You may consider using a 3D compass to keep the gyros straightened out, 
if the compass drift is better than the gyro drift.  No matter what you 
do you can't get better than the worst sensor.  If you could set out 
beacons of some sort for the thing you may be able to use them to 
enhance accuracy, but then you'll be restricted to only working where 
the beacons are set up, and you'll have to invent the beacon system.

-- 
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

>> 
>Well, you didn't mention that.

Sorry, I forgot :)

Having a indoor "GPS" is nice. hmm....

Can you think of a way to use an extra accelerometer (maybe of different
brand) to get the gravity vector?
Or use gyroscope (maybe of different brand) to enhance accuracy?

I am pretty stuck
terence
		
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blackmangoes wrote:
> >>
> >Well, you didn't mention that.
>
> Sorry, I forgot :)
>
> Having a indoor "GPS" is nice. hmm....
>
> Can you think of a way to use an extra accelerometer (maybe of different
> brand) to get the gravity vector?
> Or use gyroscope (maybe of different brand) to enhance accuracy?
>
> I am pretty stuck
> terence
>

I guess a ring laser gyro is out of the question..

you may want to reserch Honeywell (was Sperry) and Rockwell Collins
aircraft INS systems.   They do a lot of this stuff.

Mark

blackmangoes wrote:

>>Well, you didn't mention that.
> 
> 
> Sorry, I forgot :)
> 
> Having a indoor "GPS" is nice. hmm....
> 
> Can you think of a way to use an extra accelerometer (maybe of different
> brand) to get the gravity vector?
> Or use gyroscope (maybe of different brand) to enhance accuracy?
> 
Getting the gravity vector is pretty much what an accelerometer will do 
for you.  Gyroscope accuracy correllates pretty directly with cost.  You 
were expressing a disinclination to spend an extra $100 earlier, so 
gyros that cost $1000 or $10000 are probably out of your price range. 
If they aren't, here's some places to go:

Northrop Grumman (already mentioned).  Their 6-axis LTN-200 and 2-axis 
G2000 gyro products are quite nice, but will run $10000 or more for a 
6-axis IMU.

KVG.  They have "open loop" fiber optic gyros that are quite nice, with 
6-axis IMUs in the $4000 range.

Systron Donner.  They make quartz rate sensors, and have an IMU that's 
probably also in the $3000 or $4000 range.

Cloud Cap Technologies makes 6-axis IMUs from Analog Devices chips. 
They use about $200 worth of chips, but then the calibrate the heck out 
of them and sell them for $1000.

Crossbow also uses Analog Devices chips, as well as FOGs (Fiber-Optics 
gyros).

You're probably using the Analog Devices gyro chips.

I've been doing this stuff for the last 10 years.  If you find a way to 
rival the accuracies of the units above at a fraction of the price 
please let me know -- I know a few companies that'll buy them.

-- 
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

blackmangoes wrote:
> I tried using a 3d compass for other projects and it is a nightmare.
> Calibration is .... horrible, so I will avoid that as much as possible.

Don't be so quick to discard the magnetometer. Although its absolute
accuracy may be "horrible" (on the order of +/- a degree or two), it
doesn't really have any inherent drift component, and it can be a
valuable reference for estimating the bias of your rate gyros.

-- Dave Tweed

Are you using the Analog Devices gyros?

Anyway, gyros drift for a reason, are you using any temperature
compensation? The AD gyros are pretty much unusable for an INS unless
you temperature compensation them (they have an internal temperature
sensor for this)

Using six accelrometers for an INS is not ideal and will lower the
quality of your INS (assuming you using inertial sensors of a given
accuracy)

$1000! That�s too much for me. I need to sell my house to get this INS
working!

Yes, I am using ADXRS from Analog Device. 

Magnetometer? Hmm. Maybe I am having a horrible time because I bought a
cheap ass, low quality compass. (It was a 3d analog compass from
Honeywell) The magnetomer of the 3d axis does not have the same gain,
linearity, so I have a horrible time calibrating it. Any suggestion?
(<$100) will be nice. +/- a degree or two is VERY nice. What compass do
you use? Can&#4294967295;t find anything that cheap.

That&#4294967295;s right. Compass does not have drift, but it is hard to get the
gravity vector out of it because anything perpendicular to it can be
&#4294967295;gravity&#4294967295;. A 2D compass reading is off when the thing is tilted.It will
surely be non linear, which will be hard to plug to a kalman filter. How
exactly do you do it? 

> Accelerometer give me a gravity vector

How do you remove the body acceleration. You use a low pass filter? May
works on car and stuff (since acceleration for those is pretty much a
spike). For a flying robot, that&#4294967295;s a little hard (Oh yah, I forget to
mention that). Maybe using some sort of special filtering may help. Any
ideas?

> Use temperature to compensate the gyroscope

Good one! I was thinking of adding a temperature sensor. As you said ADXRS
is really unusable without temperature sensor. But... Hmmm.... no matter it
will eventually drift, after integration. Maybe compass is a good idea if I
can find one cheap.

Does using a high pass filter for the gyroscope helps getting rid of the
drift? Oh there should be a better way?

Terence
		
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