05 measuring systems 2010.pdf

8/10/2019 05 measuring systems 2010.pdf

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16 October 2010

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OE5663 Dynamic PositioningMeasuring systems

ArjenTjallema Hugo Grimmelius

ir A.R. Tjallema

3mE

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Agenda

Measured data in DP systems

Sensor systems:

Position references

Measuring the environment

Actuator feedback

External forces

Sensor readings:

Filtering

Validation

Sensor weighting

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Sensors in DP systems


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Measured data

sensorsfeed-forwardcontrol / operator

sensorsposition controller

/ operator

sensorsactuator feed-back / operator

sensorsexternal forceestimate / operator

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Position reference systems

Requirements:

Accuracy

Signal / noise ratio

Standard deviation

Reliability

Update rate

Time delay Availability (weather conditions)

(Common) failure modes

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How to determine ones position?

Satellite navigation

Hydro-acoustic Position Reference (HPR)

Mechanical

Relative reference

Inertial sensors

Ship orientation:

Heading

Roll & pitch

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Satellite navigation - GPS

6 orbital planes with 4satellites each

31 satellites currently activefor redundancy

Always min. 6 satellites inline of sight, min. 4 required

Owned and maintained byUS Ministry of Defence

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Satellite navigation GPS in 2D

?

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Error sources

Ionospheric effects: 5 m

Ephemeris (sat position) errors: 2.5 m

Satellite clock errors: 2 m

Multipath distortion: 1 m

Tropospheric effects: 0.5 m

Numerical errors: 1m

Signal obstruction: loss of position

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Source: Trimble Navigation, Inc


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Satellite navigation Enhancing GPS

Differential GPS

Accuracy 1m

Inertial Navigation Systems

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DGPS network

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Satellite navigation other systems

GALILEO -> European system, not fully operational yet

GLONASS -> Limited number of satellites, not fullyoperational

BEIDOU (Chinese) -> Limited coverage

IRNSS (India) -> under development, limited coverage


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Hydro-acoustic Position Reference(HPR)

Beacon(s) on theseabed transmit signals

Transponders on vesseldetermine range ofbeacons

Accuracy, depending onfrequency: 0.1 5 m

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Long baseline system

Baseline 50-2000 m

Pros:

Very high accuracy, even in deep water

High redundancy

Can provide vessel position in 6 DoF

Cons:

Complex system for operators

Large arrays of expensive equipment

Comprehensive calibration at deployment

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Short baseline system

Baseline 10-50 m

Pros

Low complexity

Easy to deploy

Redundant system

Cons:

Low accuracy in deep water

Additional sensors required for absolutepositioning

Detailed offshore calibration required


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Ultra-short baseline system

Baseline


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HPR weaknesses

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Mechanical reference

Measure top angle of wire

Error sources:

Current

Weight of cable

Cable dynamics(accelerations)

Roll & pitch

Accuracy > 1% of waterdepth

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Mechanical reference

Alternative: riser angle


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Relative references

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Relative reference

Laser (Fanbeam). Accuracy: 0.2m, 0.1 degrees

Microwave (Artemis). Accuracy: 1m, 0.02 degrees

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Accurate relative position reference


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Accurate relative position reference

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Inertial sensors

Can be used as position reference once startingposition is know

Only accurate for limited time period

Can be used as dead-reckoning system

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Inertial sensors as support

Combination with HPR (or GPS)

HPR:

Relatively high noise

No position drift

Inertial sensor:

Low short-term noise

High position drift

HPR accuracy improves 2-3 times


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Ship orientation

Vertical Reference Sensor (VRS) -> pitch & roll

Heading reference -> gyro compass

Vertical gyro

Pendulum

Inertia based

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Measuring the environment

Wind normally measured -> feed-forward control

Use of simple wind sensors

Combine number of sensors

Current not measured

Waves not measured

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Measuring wind

Simple reliable sensors, but take into account:

Position of sensors

Operations


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Wind forces

Calculated with wind speed only (ship speed very low)

2

2

2

( )2

( )2

( )2

wind X wind lat wind

wind Y wind front wind

wind N wind lat wind

X C A U

Y C A U

N C A L U

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Source of constants

E.g. Brix:

Often Semi emperical

Wind tunnel tests

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DP current

Wind forces are estimated by control system

Compare to thruster forces

DP current = thruster forces wind forces, so:

Current

Waves

External forces

All non-modeled phenomena


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DP current

Risk of using DP current:

Erroneous thruster feedback (25% of max pitch)

Result: current of 2.5 knots displayed

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DP current

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DP current

Unclear for operator, not harmful for control system

But, if current and waves are known:

Performance can be enhanced

Fuel consumption can be decreased

System failures can better be detected


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Current feed forward

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Current measurements

How to measure current?

Downward or upward lookingacoustic doppler radar

Thruster wash

Current-hull interaction

Slowly varying

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Wave measurements

Wave drift forces relatively high on large vessels

Wave feed-forward improves performance due tofaster reaction to large wave groups

How to measure?

And: how to predict (!)


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Wave measurements

Radar measurements

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Wave measurements

Relative wave height on hull

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Alternative current and wavemeasurement

Separate measurement buoy

Accurate measurements

Only from one direction

Vulnerable system


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External forces

Control system needs to know of significant external forces

Forces need to be included in internal model

Mooring system

Hawser

Drill string

Risers

Pipe

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Other sensors indicating shipbehaviour

Water depth

Draught

Speed

Rate of turn

Trim

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Actuator feed-back

Compare thruster output to control settings

Rpm

Pitch

Direction

Detect failing thrusters

Improve controller performance

Use redundant sensor system

Thrust cannot (yet) directly be measured


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Filtering

Filter sensor data

Kalman filter

Thrusters not capable of reacting at wave-frequency

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Sensor validation

Check whether sensor signal is correct

Easy to detect:

Frozen signal

Signal jumps

No signal

Dangerous failure mode:

Drifting signal

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Sensor validation


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Sensor validation

Compare output of multiple sensors:

Two sensors -> reveal an error

Three sensors -> detect which sensor fails

But: common failure modes not covered

Temperature effects

Noise

GPS: ionospheric disturbance

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Class requirements

3 independent reference systems

Deep water?

2x DGPS + 1x HPR

Inertial sensors: 2.5 independent systems

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Sensor weighting

Some sensors more reliable or accurate than others

Changes over time

Dynamic sensor weighting

Failure modes!


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Sensor weighting

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Sensor failure

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Sensor failure


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Remote measurements

Environmental conditions and weather forecast

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Remote measurements

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Measurement systems

Measurements of:

Position

Environment

Actuator feedback

External forces

Filtering and validation of sensor readings

Combination of sensor readings

Operators decisions crucial


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Questions?

05 measuring systems 2010.pdf

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