[PSUBS-MAILIST] More on inertial guidancce

[John Brownlee <jonnie@cobweb.scarymonsters.net>]

	Some manufacturers of off-the-shelf accelerometer and rate gyro
units are mentioned in the comp.robotics.misc FAQ, available at:

	http://www.frc.ri.cmu.edu/robotics-faq/TOC.html

	Looking into some of these manufacturers shows that inertial
measurement units (IMU's) with 6 degrees of freedom (three axis rotation
plus three axis translation) are available off the shelf, and are getting
cheaper all the time. Laser ring gyros are usually expensive compared with
the silicon micromachined accelerometers. It shouldn't be too hard to rig
up a computer to read the output from these units and integrate to find
your position and orientation. With the exception of the azimuthal axis,
it should be possible to re-caibrate to a local plumb line during
operations. Of course, these are the two degrees of freedom that play the
least importance in underwater navigation.
	IMU's do not appear to be cheap, though they are pretty reliable
and are not affected by most of the nasties in PSUBS (magnetic fields,
electrical noise, or temperature variation). Also, most of them output an
analog signal, so that means a high fidelity ADC board also. Don't know
how expensive these would be. The software to put it all together and
integrate the position is not hard, given a low power-consumption PC to
run it on. I imagine my IMU system would run Linux. :-)
	Concerns when putting together a system like this are backups and
redundancy. I would hate to be in turbid water unable to see a thing and
have the IMU konk out. On the bright side, integrating GPS into a 6-DOF
gyro system would be pretty easy, so you could head towards shallower
water, get a signal from GPS and re-calibrate the absolute position.
Still, I would like to know if this is the system Phil uses in the
DeepWorkers.

							John

John Brownlee
Chief Systems Administrator
Scary Monsters Network
jonnie at cobweb dot scarymonsters dot net