The Microstrain unit allowed you to set the magnetometer gains (as
well as accelerometer and gyro gains, along with a few other things)
in the unit EEPROM, and in fact, I did exactly this when I was using
the unit professionally in a mining context. The unit was installed
on a large mining machine in order to obtain gyro-compensated pitch
and yaw angles. Because of the significant influence of the massive
steel machine, the mags were not particularly useful, so we just set
their gains to zero. We were still able to use the yaw angle
information on an operating cycle basis (i.e. using the vertical
rate gyro) - the gyro bias was permitted to drift over time with no
correction, so we did get accurate swing rate information (most
accurate at the maximum rate during the cycle), we just couldn't
accurately reference this to the world magnetic field to get
absolute heading information. When removed from such magnetic
influences, the unit can provide accurate magnetic heading
information as per a fluxgate compass.
Not sure about your accuracy comment - I presume that in the
operating area of your dive, you are operating visually, or at least
by sonar? The value of knowing your position, IMO, is not that you
can safely navigate around underwater obstructions (if the
visibility is that bad, should you be diving?) while "blind", but
rather that you can mark and return to points of interest within the
navigational error, and also let the surface support know where you
are if you transit from the point of submergence. Thus, the
acceptable positional error circle is very mission dependent. If
you are flying through narrow underwater canyons, then sure, you
need pinpoint accuracy. Then again, perhaps it is enough to fly to
a position known to be within 50' of your target, and use a polar
scan imaging sonar to find it. If you want absolute accuracy near
the bottom, go with a doppler sonar - measuring something is always
better than estimating it.
Accuracy of an IMU depends on the resolution of the instrument, and
the magnitude of the integration error with respect to the magnitude
of the measured accelerations. I suspect that a unit undergoing
very small changes would actually exhibit greater cumulative error
than one undergoing large accelerations. The IMU employs what is
known as a Kalman filter, or rather, an algorithm which fuses the
measurements from the accelerometers, magnetometers and rate gyros
to arrive at the most probable solution for the output that you are
interested in. Unfortunately, this is not absolutely robust. For
example, in an aircraft undergoing a long sustained turn at constant
rate of turn, the accelerometer feedback throws off the filter
algorithm. You can correct problems like this by changing
instrument gains dynamically through some sort of software
algorithm, but this would require some experimentation and tuning to
arrive at the ideal parameters for any given application.
-Sean
On 12/02/2011 2:18 PM, JimToddPsub@aol.com wrote:
Sean,
Would it be better to have the option to
turn off the input from the magnetometers?
One problem I see is that we need a greater
accuracy under water than we do in the surface. Within 50'
is great on the surface, not so satisfactory under water
especially in lower visibility conditions.
I'm speculating that 80% of my underwater
excursions will be within a radius of less than one mile
from the point of submergence. Also the inertial
accelerations (incoming data) for a sub will be much smaller
for a sub than a surface vessel. Do you have any guesses as
to how that will affect the accuracy of the system?
Thanks,
Jim T
In a message dated 2/12/2011 2:59:40 P.M. Central
Standard Time, cast55@telus.net writes: A few
years ago I experimented with an inertial navigation
system I created using a 3DM-GX1 sensor from Microstrain.
I think the newest model is the 3DM-GX3 now - would have
to experiment to see what accuracy gain there is in the
new model. In any case, this is a three axis
accelerometer which incorporates 3 magnetometers and 3
rate gyros. The magnetometers are supposed to be used to
measure the ambient magnetic field and use this as a
quasi-constant input to correct the gyro bias.
Unfortunately, the magnetometers are affected by external
magnetic influences, such as large steel shipwrecks, so
the instrument is least accurate when you tend to need
it. The instrument measures acceleration directly.
Integrating this signal over time gives you your present
velocity, and integrating over time once again gives you
your displacement (position) in xyz space. In my
experiment (conducted on a surface vessel), I started with
a known position obtained from GPS, then shut off the GPS
signal and simply added the position variations as
calculated from the 3DM-GX1 to determine my current
position. This is, in fact, exactly how military
submarines do it, only their inertial navigation units
comprise extremely accurate (and consequently, extremely
expensive) hardware to minimize the integration error.
This is the crux of the inertial navigation problem - you
are essentially determining your position through
dead-reckoning, using the last known good position, and
applying corrections from your IMU instrument. The
problem is that error creeps into the integration, and
since you have to integrate twice, the error starts to get
significant. In my experiment, as soon as the GPS was
shut down, the error started to accumulate, so that the
uncertainty in the calculated position grew with time.
Eventually, you reach a point at which the error in your
calculated position renders the position useless for the
purpose of navigation. The solution? Either spend big
bucks on a more accurate IMU, or periodically correct the
calculated deviations with another input. There are
several possibilities for this:
1) Doppler sonar - limited to low speeds at which the
sonar reading is accurate, but this is more accurate than
inertial navigation when it is implemented. Doing this
would limit the inertial navigation error to that
accumulated during the descent from the surface to a range
from the bottom at which the doppler sonar becomes
effective.
2) Depth transducer - It occurred to me that since you do
know with reasonable accuracy your depth in the water
column (and thus your velocity in the Z direction), you
could use this as a correction input (i.e. do not allow
integrated velocity values in Z direction to exceed this
measured velocity, and rein in the X and Y velocities
accordingly). I have no idea what effect this might have
on accuracy without trying it.
3) Acoustic methods - widely used in industry, but require
surface or seafloor based transmitters. (reference LBL
& SBL navigation). If you have no need to operate
independently of surface support, then acoustic navigation
alone may meet your requirements, but this does require
some hardware and so may end up being more expensive than
an IMU for small submersible navigation at your required
accuracy - depends on what you need.
I would be inclined to try and find a low-cost doppler
sonar for bottom navigation, supplemented by an inertial
unit that isn't hugely expensive. One advantage of the
IMU is that it will output pitch, roll and yaw angles, so
you could use the output to control, for example, a trim
tank system to keep a level keel.
As I recall, the 3DM-GX1 cost somewhere in the
neighborhood of $1400.00. I would be happy to look into
developing a turnkey PSub inertial navigation solution,
but I'd need a 3DM-GX3 (or similar) IMU to play with, as
well as data acquisition hardware that runs at an
appropriate frequency to capture the IMU output with
minimal error. At present, I can't afford to buy these
things for a hobby R&D project, although I might
reexamine this at a later date when my situation changes.
-Sean
On 12/02/2011 10:43 AM, Recon1st@aol.com
wrote:
Jim I think navigation would be a good topic for
this
group to discuss. I for one am at a loss for a
solution.
I so like my gps system on my surface support
boat I want
something decent under water.
How does the military do it? Lots of money I am
sure
but seems something better than a compass could
be
done.
I am leaning towards a tracking beacon on the sub
and
get nav directions from support boat.
Dean
In a message dated 2/12/2011 11:42:17 A.M.
Central Standard Time, kocpnt@tds.net
writes:
Hi All,
I am beginning rebuilding Bionic Guppies
electronics/electrical systems. I am planning
on again using an aircraft directional gyro
for navigation.
Before I go down this road does anyone have
a better/different idea?
Also, Dan Lance, I believe that you were
quite pleased with OTS communication system.
Can you confirm this, and if so provide
contact info for a good supplier.
Best Regards,
Jim K
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