Of course, no sensor is perfect and errors will always crepe in to a n inertial solution. Bias and Scale factor errors accumulate over time leading to a drift in the calculated position over time. The longer we try to navigate on inertial only data the further the navigation results deviate from reality.The majority of the errors in an inertial calculate are due to gyroscope errors, in general the higher quality the gyroscopes the better will be the navigation accuracy. Therefore we find that a good INS requires good gyroscopes. IXBLUE manufactures fibre optic gyroscopes, we control the whole process, from the manufacture of the fibre itself and the optical components that are essential for the measurement of the optical parameters.IXBLUE manufacture a range of Fibre optic gyroscopes, the FOG90 used in our ROVINS and OCTANS products, the FOG 120 used in PHINS and Hydrins, the FOG180 used in our military product MARINS. And the FOG210 used in our space products STARINS.A FOG 90 based INS typically has a free inertial drift rate of 6m in two minutes, the PHINS with it’s FOG120 has a free inertial drift rate of roughly half that 3.2m in two minutes, (that's 0.6Nmi per hour). Where as the Maris is an order of magnitude better than that at 1Nmi in 24 hours.
PHINS is fast becoming the standard instrument for high accuracy ROV navigation , here we see a PHINS 6000, the standard unit for ROV use. Inside we can see the three cans holding the fibre coils with the accelerometers mounted in the centre of each coil. Between the coils is the electronics stack including optical sources and components, interface cards and main signal processing card.A Kalman filter is implemented to fuse the inertial measurements with external aiding data.
As described earlier, no inertial sensor is perfect, errors will accumulate over time. The kalman filter implemented within IXBLUE inertial products is able to take external aiding data and fuse that data with the inertial measruements in order to calculate the current position.A large buffer within the Kalman filter allows the use of old data within the position calculation so long as the age of the data is known. Typically in the offshore environment, the INS will be initialised on deck with GPS, then when the ROV is deployed aiding will switch over to USBL or LBL, and as the vehicle gets closer to the seabed, Doppler Velocity Log becomes available.The PHINS contains a very accurate real time clock, if necessary the PHINS can operate entirely on it’s own clock, but normally a GPS time signal (ZDA) is used to tie the PHINS time to GPS time. This allows the age of time stamped data to be properly taken in to account in the Kalman filter.PHINS is a fully self contained system, all calculations are done within the subsea unit with no surface equipment required. PHINS is as well suited to AUV operation as it is to ROV operations and no link to the surface is required.
PHINS for DP applications
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PHINS for DP Applications
Jim Titcomb
North Sea Spring Seminars 2013
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PHINS & ROVINS – Inertial Products
What is an Inertial Navigation System (INS)?
An instrument (electronic + sensors) which is using its initial state (position)
and internal motion sensors (gyroscopes + accelerometers) to measure and
calculate its subsequent positions in space with high accuracy, stability and
update rate
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3
INS Introduction
What makes an INS good.. or not?
Internal sensors (gyroscopes & accelerometers) are never perfect, bias and
scale factors accumulate over time
Navigation is mostly about Gyroscopes
Gross figures:
Accelerometers errors are not heavily involved in the position drift – 4m –
Schuller period
Gyroscope are heavily involved in the drift – 400 m
Conclusions
A good INS requires good gyro’s
IXBLUE manufactures FOG (Fiber Optic Gyroscope) and controls the whole
process
A range of FOG’s (FOG90, FOG120, FOG180…) for a range of INS
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What's in an INS?
3 “FOG” gyrometers
monitor rotation and speed in X, Y & Z axis
3 accelerometers
measure acceleration (>> speed >> motion) in 3 axis
Powerful electronic / firmware package
PHINS “knows” in real time its motion in space .
Firmware (Kalman filter) calculates its position in real
time + heading, pitch, roll, heave, etc…
All integrated
small, lean, powerful!
(PHINS 6000 example)
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The problem with INS
The best sensors are still not perfect, accumulating small errors vs. time
makes the system drift on the long term
External sensors (aiding) are required to bound drift within acceptable limits.
PHINS & ROVINS includes interfaces for most common external sensors
GPS
DVL (Doppler Velocity Log)
Pressure sensor
Acoustic positioning references (USBL, LBL)
…and all IXBLUE products!
IXBLUE INS are fully integrated Inertial Positioning solutions designed for
ease of installation & operation, flexible enough to fit most requirements, with
no specialist engineer to install / operate.
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iXBlue underwater positioning solutions
the benefits of data fusion
Data Fusion
Use various and different technologies to measure the same parameter
Blend (fuse all this data (Kalman filter) in order to correlate it and obtain a better
result
A simple example GPS + INS (PHINS, GAPS)
0
0,5
1
1,5
2
2,5
3
0 20 40 60 80 100
time t (s)
positionaccuracy(m)
PHINS pure inertial drift (0.0002 x t^2
m)
Averaging of DGPS data (3 / sqrt(t)
m)
PHINS+DGPS
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7PHINS & ROVINS: Inertial Navigation System
Robust to Acoustic navigation solution
Acoustic positioning can be poor, low update rate, or out of range.
INS + USBL combination / data fusion provides continued high quality
positioning:
Actual USBL data (Hipap) @ 2400 meters water depth
0
0,5
1
1,5
2
2,5
3
3,5
4
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8PHINS & ROVINS: Inertial Navigation System
Robust to Acoustic navigation solution
Acoustic positioning can be poor, low update rate, or out of range.
PHINS + USBL combination / data fusion provides continued high
quality positioning:
Using PHINS + USBL in extreme shallow water. RESON / ADUS 2007
Données de positionnement PHINS et GAPS
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5
10
15
20
25
30
5 10 15 20 25 30 35
X (m)
Y(m)
Phins
Gaps
Données de positionnement PHINS et GAPS
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10
15
20
25
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35
0 5 10 15 20 25 30
X (m)
Y(m)
Phins
Gaps
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15
20
25
30
35
40
5 10 15 20 25
X (m)
Y(m)
Données de positionnement PHINS et GAPS
0
5
10
15
20
25
30
5 10 15 20 25 30 35
X (m)
Y(m) Phins
Gaps
Données de positionnement PHINS et GAPS
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0 5 10 15 20 25 30
X (m)
Y(m) Phins
Gaps
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5 10 15 20 25
X (m)
Y(m)
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DP-PHINS What is it?
Time Signals
Time and Position
For Initialisation Only
Time stamped beacon
positions and heading
Processed
Positions
Enhanced Position
Acoustic
Positions
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DP- PHINS, Features
Relative to Global co-ordinate transforms.
Unlimited number of beacons
Flexible
Expandable
Future Sensors
Taught Wire
Fan Beam
Radascan
Etc. Etc.
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DP-PHINS, Why?
INS improves USBL
USBL improve INS
We still have a single point of failure
INS will drift quickly if the acoustics fail.
3m in 2 min,
20m in 5 min.
0.6Nmi in an hour.
INS can make USBL almost as good as GPS (depending on depth).
Adding INS with USBL alone does not increase redundancy.
What else is available?
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What about LBL?
At Least three ranges are
required for LBL positioning.
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Pure LBL
With only two ranges
positioning is not possible
There are two possible
locations which match the
received ranges.
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INS aided by LBL
Station keeping with only
two ranges is possible with
INS and LBL combined.
The system must be
initialised to the correct
position first.
The two ranges will
constrain the INS drift.
Positioning accuracy is
directly related to the range
measurement accuracy.
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INS LBL Accuracy and Geometry
Accuracy of results is
dependant on a
combination of geometry
and range measurement
accuracy
If the beacons are “in
line” with the vessel
results will be poor.
But accuracy will be
good if there is a high
“Angle of cut”
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INS Velocity Aiding
INS measures rotation rate and acceleration.
First integration produces heading and velocity
Second integration produces position
USBL aids INS at the second integration level
DVL aids INS at the First integration level.
INS aiding data is used to estimate errors. The errors are fed back in to the
calculation in order to correct at a low level.
Aiding at the first integration level has more effect than aiding at the second
integration level.
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Effect of Velocity Aiding
INS free inertial specification is based on time. The more time, the greater
the drift.
DVL aided INS specification is based on distance travelled. If you don’t move
the error can’t grow (as much).
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The problem with DVL
Deep water.
DVL is only available in water depths up to around 1,000m.
In deeper water DVL update rate will be slow.
Remember DVL aids INS at a more fundamental level than other sensors.
The INS only needs a velocity aiding input infrequently in order to correctly estimate
the error on the sensors.
DVL Update
Rate
Error,
% distance
travelled.
Drift speed
m/h
1S 0.03% 0.18
2S 0.13% 0.82
3S 0.26% 1.67
4S 0.27% 1.77
6S 0.32% 2.08
8s 0.30% 1.98
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Acoustic Aiding Examples - Recap
ROV on Seabed.
850m water depth
Well calibrated USBL
About as good as it gets with
USBL
Standard
Deviation
X Y Position
USBL 1.03 0.86 1.34
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INS Aided with USBL - Recap
Spikes in USBL rejected
Automatically.
Much higher update rate.
Smoother positioning.
Reduced spread of positions.
5 X improvement with INS
Standard
Deviation
X Y
USBL 1.03 0.86 1.34
INS 0.16 0.2 0.26
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INS Aided with USBL & DVL
DVL update only every 3
seconds
Positioning now better than
high accuracy GPS even in
850m
17 x Better than raw
acoustics.
Standard
Deviation
X Y
USBL 1.03 0.86 1.34
INS 0.16 0.2 0.26
With DVL 0.05 0.06 0.07
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What if we lose acoustics?
DVL update only every 3
seconds
Positioning now better than
high accuracy GPS even in
850m
Standard
Deviation
X Y
USBL 1.03 0.86 1.34
INS 0.16 0.2 0.26
With DVL 0.05 0.06 0.07
INS DVL 0.04 0.02 0.05
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Conclusions
INS is a proven technology on Land, Under water, in Space, why not in DP?
INS has a long track record, Modern FOG based systems bring extreme
robustness and reliability.
INS can produce heading and attitude data as well as positioning.
INS Should not be aided just by GPS for DP applications.
INS can make your acoustics as good as high accuracy GPS.
The biggest benefits can be obtained by combining aiding sensors.
With the addition of DVL, INS can now be considered a stand alone PME for
a significant period of time.
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Nautronix Cooperation
NASDrill RS925 SBL / LBL Overview
Premier DP acoustic positioning reference for
deepwater operations
Uses Nautronix ADS2 signalling for robust and
reliable operation in extreme acoustic noise
High level of system redundancy (hardware and
acoustics)
Independent SBL only, LBL only or combined
SBL/LBL operation
Compatible with all major DP systems
Now with PHINS Inertial Navigation
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Nautronix Cooperation
NASDrill RS925 SBL / LBL Overview
Proven performance since 1998 with operation in
water depths down to 5,830m.
Fast position update (SBL <2 seconds)
High Accuracy:-
SBL only = 0.15% slant range
LBL only = 1m irrespective of water depth
SBL/LBL combined = 2.5m @ 3,500msw
Allows for integration with:
NASNet® DPR
NASeBOP
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Nautronix Cooperation
NASDrill RS925 – Stability
• Standard deviation on the
calibration was 1.7m RMS
• Spread of position data very small,
approx 5m
• Highlights suitability of RS925 as
reliable position reference input for
DP systems which demand
regular, stable & accurate position
data.
• Meets Rig Positioning
requirements in deep water on
acoustics alone
The following results were taken during calibration of the NASDrill RS925
system on board the Deepwater Discovery in 2,870m water.
5m radius
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Nautronix Cooperation
NASDrill RS925 Position Accuracy - INS
Interface between NASDrill RS925 and PHINS created
Single PHINS (no redundancy):
Non-redundant
RS925/PHINS
configuration.
Provided in addition
to standard RS925
output(s) to DP
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Nautronix Cooperation
NASDrill RS925 Position Accuracy - INS
Interface between NASDrill RS925 and PHINS created
Dual PHINS (full redundancy):
Dual redundant
RS925/PHINS
configuration.
Provided in addition to
standard RS925
output(s) to DP
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Nautronix Cooperation
NASDrill RS925 PHINS Testing
Initial integration tests completed
RS925 SBL simulated data fed into PHINS
Simulated SBL data configured to have a noise level of 4.5 m RMS.
Equivalent to a SBL system operating in 3000m depth of water, with a noise
level of 0.15% slant range
SBL position output compared to SBL aided PHINS output...
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Nautronix Cooperation
NASDrill RS925/PHINS Test Results
SBL raw position data with a noise level of 4.5m RMS (3000m depth
equivalent)
PHINS output data with a noise level of 1.6m RMS
Approximately three fold increase in precision (as is typical of various
acoustically aided INS systems)
Above are static tests with simulated data. Dynamic results are expected to
be very similar, with a slight increase in accuracy expected
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Nautronix Cooperation
Summary
NASNet ® DPR & PHINS:
PHINS used to enable sparse array positioning capability from a standard
NASNet ® deployed field
Increase in accuracy of less importance, due to existing capability of
NASNet ® BLBL
• NASDrill RS925 & PHINS:
PHINS used to significantly increase the precision of SBL. Providing a
higher accuracy, higher update rate, input to DP
Sparse array capability of less importance, due to the deployment of
beacons dedicated for the drill rig/vessel – risk of no acoustic return
extremely low
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