Hi Sean,
There's a triple axis accelerometer on the Cytron
Robotics site. Needs additional analogue to digital converter.
Cost RM $109, that's probably less than $40- US.
Have bought stuff off them & it's been good quality.
Will these units cope when your sub is moving in a
current?
Regards Alan
----- Original Message -----
Sent: Sunday, February 13, 2011 9:59
AM
Subject: Re: [PSUBS-MAILIST]
Navigation
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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