integrated inertial positioning systems
Post on 29-Dec-2021
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Zupt, LLC
Integrated Inertial Positioning SystemsSome facts, some editorial and some biased opinions
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Inertial Tools - What instruments are currently in daily use for Survey?
In our businessInertial navigation systems (INS) for land seismic, Vertical Reference Units (VRU’s) for DP, USBL/SBL and Swath sonar attitude/heave corrections –
Surface heading sensors – Spinning mass gyros as well as strap down Attitude Heading Reference Systems (AHRS)
In other applicationsInertial sensors - anti-lock brakes, anti skid, virtual reality headsets –Analog Devices, Crossbow, Systron, Bosch, BAe and many others
Inertial Navigation systems as (Tactical) short term positioning sensors -Northrop Grumman, Honeywell, Kearfott, BAe, Boeing, etc.
High precision (Strategic) Inertial Navigation systems for long term positioning outages - Northrop Grumman, Honeywell, Thales navigation, etc.
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Available How Long?
In discussing this exciting new technology we must also understand that these tools have been around for a while, even in the seismic and survey business:
Western Geophysical’s W-INS (“we invariably needed SHORAN”) late 1970’s
Shelltech/Itech land seismic use of INS for control (helicopter based Zupt’s) Mid/late 1970’s
Exxon/Honeywell’s DP reference systems – Riser and INS – 1979
British Oceanics/Intersub INS for manned submersible construction positioning (used in place of “the unreliable acoustic systems”) – early 1980’s
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Session Agenda
A few definitions
What is an inertial measurement unit (IMU)?
Overview of inertial sensors
Price versus performance
Where is inertial technology going?
Integrated Inertial Positioning Systems
Loosely, tightly and deeply coupled
Aiding Observations – current and future
Applications for Integrated Inertial Positioning Systems
Current and near term products
Commercial benefits of these systems
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A few definitions
An Inertial Sensor is a position, attitude or motion sensor whose reference are completely internal draft revision to IEEE Std 528
A Gyroscope is a sensor designed to illustrate the dynamics of a rotating body
In Strapdown operations the inertial frame of reference is stored in the computer as opposed to being maintained mechanically by gimbals.Coordinate transformations and sensor compensation have to be completed within the strapdown computer.
Bias - no input, but some level of output
Angle random walk - white spectrum rate detection noise leads to an angle random walk (optical and coriolis gyros)
Aiding - using external non inertial observations to minimize bias
Scale Factor- an error in the assumed scale factor in the instrument output
Schuler Oscillation/Period - 84 minutes – just think about a pendulum centered at the earth’s core and the IMU at the earth’s surface
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What does an IMU consist of?
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How does this fit together?
Measureacceleration
Compensate foraccelerometer
bias and SF
Compensatefor
gravity
IntegrateOnce-VelocityTwice-Distance
Measurerotation rates
Compensatefor
Earth’s rotation
Navigation ComputerAccelerometers
Gyros
Heading
Distance
Speed
Compensate for Gyro Bias, ARW, SF and acceleration sensitivities
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Overview of inertial sensors
Inertial sensors come in many forms and are an excuse for infinite acronym generation.
The two types of sensors within an IMU are:
Gyroscopes - rate of rotation
Accelerometers - linear acceleration
Some examples are:
Gyros Dynamically tuned, (DTG), Fiber Optic (FOG), Ring laser (RLG)
Accels Vibrating Beam (VBA), Quartz Resonating (QRA), Pendulous Mass (PMA)
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Gyro Technology
Angular Rate Sensing technology principles:
Spinning Mass - angular momentum
Vibratory/Resonator - Coriolis
Optical - Sagnac
Micro Electro Mechanical Sensor(MEMS)
primarily vibratory, some spinning mass, some optical
Micro Optical Electro Mechanical Sensor (MOEMS) another variant
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Current Gyro technology
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More Gyro details
Spinning Mass Honeywell, Northrop Grumman, RockwellPros Wide performance range 0.0001 to >100°/hr
Very low noise - (specifically gas bearing)Cons Relatively high cost
Long warm upNot well suited to strapdown applicationsSome types very fragile
Vibratory/Resonant Watson, Systron Donner, Murata, BAePros Relatively small
Minimum moving partsCons Small scale factor
Output noisyRate gyro open loopLimited performance range (getting better though)
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Pros and Cons
Optical Honeywell, Northrop Grumman, (Fibresense), Ixsea, Sagem, etc.
Pros Rapid reaction and turn on(<1s)
Ideally suited for strapdown operation
No moving parts - very rugged
Cons Performance increases with size
RLG is a high voltage device
FOG very temperature sensitive
Micro Electro Mech.Sensors (MEMS) Draper/Honeywell, JPL, BAe, AD, Bosch, etc.(only vibratory discussed)
Pros Very small
No moving parts
Very low cost
Cons Higher precision still under development
Limited performance range (only for a while)
Bias stability
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FOG block Diag.
Gyro Bias (°/hr) is usually proportional to length of fiber
The longer the fiber - the better the FOG
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RLG Block Diag.
Cervit block
Path lengthcontrol mirror
AnodeAnode
Opticalbeams
Mirror
Cathode
-Schematic of ring laser gyro. Input axis is perpendicularto the plane of page.
Detector
Dithermechanism
Gyro Bias (°/hr) is usually proportional to path length
The longer the path length - the better the RLG
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Accelerometer Technology
Linear acceleration sensing technologies:
Pendulous/Translational Mass displacement/rebalanceElectrical Restraint
Rotational Restraint
Elastic Restraint
Resonant Element FrequencyVibrating String
Vibrating Beam
Double Ended Tuning Fork
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Current Accelerometer Technology
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More accel. details
Force Rebalance Accels - Honeywell Q-Flex, Northrop Grumman A4, Kearfott Mod VII
Pros Highly Reliable - relatively low cost
Wide bandwidth
Low bias error
Cons Analog output
self heating under changing acceleration
Power consumption
Pendulous Rebalance Accels.Pros Reliable, rugged, small
Well understood error model
Pendulous Integrating Gyro Accel. (PIGA) as good as it gets
used for ICBM and general missile guidance
Cons PIGA - Cost
Resonant Element Accel. Sundstrand, Allied Signal Adkem
Pros Digital output
Low power
Cons Not good in high shock environment
Detailed calibration required
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Sensor Advances - MEMS
Wafer thick gyros - 400µm
Critical assembly process for MEMS
Assembly issues being worked on to
make a low cost, mass produced “instrument”.
Noise is the challenge I-O have low noise,
low G product – VectorSeis®
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MEMS DoD Development Program
A low cost, high G MEMS and guidance
effort is underway for a DoD joint forces
program. This effort has the following goals:
Phase 1 <75°/hr, >10,000G, <8 cubic inches
This phase should have been delivered
3 vendors selected – 2 delivered 4 months ago
Phase 2 <10 °/hr, >20,000G, <4 cubic inches
This phase should be delivered this/next year
2 vendors selected – one ready to deliver
Phase 3 <0.5 °/hr desired (<1 °/hr acceptable), >20,000G launch survivable, <2 cubic inches volume. DoD’s cost expectation for this IMU is <$1,200
Should have been delivered in 2006 – may not be needed due to deeply coupled Phase 2.
Deeply coupled L1 and L2/Lm, WAAS, SAASM GPS receiver should be incorporated as an option to Phase 2/3
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So how good is a “good” INS?
Once the sensors (just discussed) have been combined to make an Inertial Measurement Unit software has to be added to turn the raw rate (incremental rate - ? ?) and the raw acceleration (incremental velocity - ? V)data into something useful as a:
Attitude Heading Reference System (AHRS), or an
Inertial Navigation System (INS)
The performance of an INS is usually rated in terms of its position error growth rate once the INS is navigating in “free inertial” mode (no aiding).
The USAF define INS in the following manner:
INS Classification Position Error Growth Rate Heading Errors
Low > 2nm/hr >0.2°
Medium 0.5 to 2nm/hr 0.05 ° to 0.2 °
Precision <0.5nm/hr <0.05 °Following a standard ground alignment at 50 ° or lower latitude – USAF SNU84-1
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Cost versus PerformancePRICE $K
160 Department of State Controlled Technology Dept. of Commerce Controlled ?Primarily for internationallysourced IMU's only
IMAR .003º $155K 20cm RLG140
120
100 Thales Totem .001ºThales $100K 30cm RLG
T24 .003º $130K Kearfott 24cm RLGLN250 .005º? $90K 1200m? FOG
80 LN100 .003º $80K Northrop 18cm RLGPHINS .003º $80K Ixsea 1200m FOGCIMU .0035 $80K Honeywell 6" path RLG
Sigma 10 .05º SAGEM $65K 10cm RLG 60 Octans 0.01º $60K Ixsea 700m FOG (AHRS only)
40T16 .01º $100K Kearfott 18cm RLG
T90 1º $39K Tamam
20 LN200 1º $22K Northrop 200m FOGBOEING 3º $20K MEMS
0.002 0.015 0.15 1.5 15 BIAS STAB º/hr
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Where is this technology going?
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Maturity of technology
Draper Laboratory’s view
on the state of current
development.
The suggestion is that most
technologies are now mature
except for MEMS gyros.
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Export Control
The fastest way to go to jail (without passing go) will be to flaunt the export controls associated with this technology.
The International Traffic in Arms Regulations (ITAR) and the Arms Export Control Act (AECA) are the governing law that is overseen through the Directorate of Defense Trade Controls (DDTC www.pmdtc.org) within the U.S. Department of State (remember Colin Powell).
Simply put (to me by the DDTC) “if you screw up, it will be you, not the company, that goes to jail”.
DDTC is responsible for all licensing issues if the commodity is controlled by State.
To get a commodity under the more understanding Dept. of Commerce control a “Commodity Jurisdiction” has to be filed with the D.o.State.
Most US manufactured and “high end” international IMU’s will fall under the control of the DDTC
Do not listen to the vendors when they say don’t worry about this –talk to your own export attorney and get their advice.
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What type of sensor do you need to buy/integrate?
• As can be seen from the previous charts IMU’s are available in many flavors. The one underlying suggestion I would make is to:
Only buy the sensor with the performance you really need
Do not over specify the performance requirements of your sensor or it will cost significantly more than it should.
OR ?
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Free Inertial Drift$100K buys apx. 20 meters for 20 minutes free inertial. This level of inertial
performance is not needed if the IMU errors can be bounded with some form
of external aiding – hence the need for Integrated Inertial Positioning Systems.
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System Integration – Coupling?
A very basic “Loosely Coupled” system –
the INS and aiding system provide
position and velocity into the Kalman filter.Example GPS/INS
GPS would provide Position and Velocity
INS would provide position and
velocity
“Tightly Coupled” – the IMU and aiding
system provide raw observations that
are modeled within the Kalman filter.Example GPS/INS
GPS would provide code and phase observations,
the IMU provides rate and acceleration observations.
INS
KalmanFilter
Aiding NavSystems
IMU
KalmanFilter
Aiding NavSystems
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Deeply Coupled
IMU
KalmanFilter
Aiding NavSystems
“Deeply Coupled” – the IMU and aiding
system provide raw observations that
are modeled within the Kalman. The
solution provides feedback into both
the IMU and the aiding observations.
Example GPS/INS
GPS would provide code and phase observations,
the IMU provides rate and acceleration observations.
The GPS receiver is controlled to “window” onto expected arrivals of SV data. Significantly improves the
signal to noise performance of the GPS system. Currently in use for anti jamming and blocking of GPS
signals in defense applications.
Just imagine what a deeply coupled acoustic line of position/INS solution would do for ROV
positioning? Improving the SNR of the system!
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Why spend effort on better coupling?
Assuming life was good and GPS was very visible –a loosely coupled solution will work well – why would we need such a system (GPS/INS)? High update rate position with precision attitude information (LIDAR, Photogrammetry etc.)
Now we start to require GPS observations in a crowded urban area (road survey)– the GPS solution fails, no position or velocity – my loosely coupled GPS/INS solution starts to fail.
The same is true for land survey under canopy. With some visibility, a loosely coupled system should provide a solution as long as the GPS system provides a position and velocity.
Once the canopy thickens such that only occasionally data is available from some SV’s then a tightly or deeply coupled solution will provide a valid solution much longer.
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In Offshore ApplicationsROV Long Baseline construction survey tasks
To go to work a LBL array has to be deployed and calibrated – takes time and equipment, usually on the critical path.
The ROV then positions itself within this array to complete the subsea construction tasks. The conventional LBL solution could be integrated with an IMU to get a higher update rate and precision attitude info. This may have some value. But if if this integration is taken one step further we will be able to offer significant savings to offshore operations.
What if we could reduce the number of beacons in the array and position the ROV with just a tightly coupled INS/Line of Position (with respect to a seabed mounted transponder) solution? Deeply coupled would be even better as we could extend our acoustic range as we improve our SNR due to driving the acoustic transceiver.
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Aiding Observations – what is available?
Many aiding observations are used to bound the error growth of IMU’s. A few of the normal and not so normal observations are listed below – some from air, land and marine (I am sure I have left many out):Conventional:
GPSZero Velocity Update - ZuptDoppler velocity sensors/logs (airborne radar and underwater acoustic) Altimeters (RF and acoustic)Depth sensorsPedometersDistance Measuring IndicatorsRange/Range systems (RF and acoustics)Half Gauge (Rail tracking indicator)Terrain matching – matching to existing terrain data
Not so conventional:Vision – relative position and velocity from CCD or SIT images (already working)Stripe laser illuminated imagery – very high definition=resolution observations (near to working)Swath Sonar - relative position/velocity from image processing of sonar data (working today)Terrain Mapping – establishing the environment around the system and noting changes as they occur (prototypes working at MIT allowing navigation around halls
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Commercial Benefits
Let’s take a look at just a few examples of integrated solutions to try and understand why these systems will make inroads into our business over the next few months/years:
Marine Construction
Dynamic Positioning
Land Seismic Stake Out under Canopy
A list of “No brainer” uses
As you will see all of these applications of an integrated solution are affordable through real operational savings. The benefits are not just better data, higher update rate, more reliable solutions –
The incentives are real dollar savings through the life of projects
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Marine Construction
What are the issues for the marine construction survey community?
Boat time, Boat time, Boat timeAny operational gain that can be made to reduce the amount of vessel/spread time consumed specifically for the survey aspects of marine construction tasks will go straight into savings for the end customer – do these customers really care?
The marine construction survey community need to be able to reduce vessel/spread time while completing tasks similar to the following:
Metrology – providing repeatable, reliable positioning data while consuming minimal ROV/spread time. A local, relative positioning problem well suited to an aided inertial solution.
Local field development positioning - 300mx300m subsea infrastructure relative installation – significantly reduce the operational time currently consumed to deploy and calibrate large LBL arrays, reduce the LBL beacon count significantly.
Wide area deepwater absolute positioning – 3,000mx 3,000m field wide control with significantly reduced acoustic observation sets. Vessel/ROV spread consumption reduced due to less hardware deployed on the seabed. Such an application would be deepwater permanent suction mooring installations.
Deepwater pipeline “as built” survey – improve the survey deliverable from USBL systems by aiding with an inertial observation set – no need for LBL.
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Marine Construction Example
Using an example of a project that has multiple frame sets including an array installation and calibration at 4 locations:
Each location: 5 Far field transponders, 6 Near field transponders2.8 boat/ROV spread days per location for deployment/calibration, 4 locations$65K/day complete spread rate - 4(2.8x65) = $728K
If proven aided inertial tools are availableEach location: 2 near filed transponders (absolute array orientation taken care of with good IMU), 1.3 boat/ROV spread days per location, 4 locations$65K/day complete spread rate 4(1.3x65) = $338K
A very conservative estimate of savings = $390K on a single job – this savings would nearly pay for the system development
OR
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The DP – IMU/ERA Case
Riser Operating inNon Bi-Stable Mode
Riser ModelIncluding:Riser Angle TensionMud WeightSlip JointPosition
IMU
Patented in 1979, not used since – Class 2 and 3 DP
Requires “3 independent ref systems from 2 different
Operating principles”.
Today in deep water we only have two systems to
choose from - GPS and Acoustics. The numbers work for
this solution as well.
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The $ Benefit for DP Operators
Graceful disconnect/ shut down of operations $1,000,000/incident
Short term DGPS outage $Operator penalty or disconnect
Slow down acoustic update
Assist the “Multi-user” problem $To work or not, penalty
Extend battery Life $45,000/year
Additional reference sensor $100, 000 or penalty saving
DP model “smoothing”
Less fuel consumption $Does the client care?
Less “wear and tear” $1,000,000/repair incident
(Seal failure * recent example)
Heading, pitch, roll, heave $40,000 + $50,000
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Land Seismic Stake Out Under Canopy
This is one of the sectors where aided inertial systems have been trying to find a home for the past several years. What are the issues for the land seismic community?
When seismic acquisition moves into wooded or forested areas the canopy starts to impact the reception of the direct GPS signals as well as the radio link for RTK.
The primary system will consist of a very good IMU bounding it’s error through the use of Zupt’s. Occasional RTK GPS may be available, but in many cases the days survey is completed with just the IMU and Zupt’s. These systems deliver sub 1m post processed accuracy with good initial calibrations and good closing RTK calibrations.
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Land Seismic Stake Out Under Canopy
Survey time to stake out under canopy requires cutting of the canopy to allow for conventional optical survey tools to be used. Using backpack based inertial instruments significantly reduces the amount of time taken to survey locations.
Conventional cutting and surveying will achieve perhaps a mile a day or less under canopy
Inertial based systems allow between 3 to 4 miles per day (4 times conventional production), and in some instances up to 8 miles per day.
Cost per mile $1,000/mile, line miles on an average (no such thing) 3D land survey will be (a line every .25 miles – 10x10 mile survey) 400 line miles.
Other very significant issues are:
Environmental issues – minimal cutting (low impact seismic) is being specified more often
Safety issues – significant issues associated with HS&E
OR
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Some more examples of “No Brainers”
Some of the current issues facing the survey and positioning community that will benefit from Integrated Inertial Positioning Systems:
EM or 4C/D seabed station installationsLarge numbers of nodes that require precise installation in deep water. Currently primary options are very large LBL arrays as USBL cannot provide the accuracy required. A combined DVL, LOP, Depth and IMU solution will subtracts days from the deployment and calibration of such systems
Metrology/Spool piece measurementSome valiant efforts (CDL – Subsea7) to use systems. The introduction of a proven and fullyaccepted capability would reduce many hours from each measurement set. Just taking a look at some of the west African field development should pay for the development and proving of such a solution.
Acoustic PollutionSlow down update rates, make acoustic bandwidth available through the use of less acoustic channels, DP, construction, ROV tracking and seafloor positioning will all benefit significantly with even a loosely coupled IMU in the loop. No need for massive, commercially confusing, pseudo ranging “Seabed GPS” systems.
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