Re: [PSUBS-MAILIST] Navigation

In a message dated 2/12/2011 5:59:26 P.M. Central Standard Time, cast55@telus.net writes:

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