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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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