Navigation Sensors and INS Mechanization Tuesday 12 Feb 2013 NMT EE 570 Location and Navigation Theory amp Practice Slide 1 of 19 Navigation Sensors and INS Mechanization Inertial Sensors ID: 199729
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EE 570: Location and Navigation: Theory & Practice
Navigation Sensors and INS Mechanization
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
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Navigation Sensors and INS
Mechanization
Inertial Sensors
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
As the name implies inertial sensors measure motion
wrt
an inertial frame
Advantages: Self-contained & non-reliant on external fields (e.g. EM radiation, Earth’s magnetic field, …)Disadvantages: Typically rate measurements & expensiveAccelerometers measure linear accelerationActually measure specific force, typically, in the body frameGyroscopes measure angular velocityMost gyroscopes measure angular speed, typically, in the body frame
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Navigation Sensors and INS Mechanization
Inertial Sensors
- Accelerometer
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
Inertial Sensors
Accelerometers
Pendulous Mass
Vibratory
Closed
loop
Open
loop
Closed
loop
Open
loop
Gyroscopes
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Navigation Sensors and INS Mechanization
Inertial
Sensors - Gyroscope
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
Inertial Sensors
Gyroscopes
Rotating Mass
Sagnac
Effect
Coriolis Effect
Floated
DTG
Fluid Suspension
Magnetic Suspension
Mech
Suspension
Resonator
Interferometer
Ring Laser Gyro
Fiber Optic Gyro
Hemispherical Resonator Gyro
magnetohydrodynamic
Conductive fluid based
Microelectromechanical
Accelerometers
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Navigation Sensors and INS Mechanization
Inertial Sensors – Accelerometer
: Pendulous Mass
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
The Pendulous Mass Accelerometer
A mass, a suspension system,
and a sensing element
Displacement appliedforce resolved along thesensitive axis
Modeled as a basic 2nd order systemIn steady state
hence
Acceleration due to gravity
Reaction force
Mass
damper
k
spring
b
displacement (x)
Sense axis
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Navigation Sensors and INS Mechanization
Inertial Sensors – Accelerometer: Pendulous Mass
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
Closed-loop version generates a force to null the displacement
Can improve linearity and measurement range
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Navigation Sensors and INS Mechanization
Inertial Sensors – Accelerometer: Pendulous Mass
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
Pendulous Accelerometer
Closed loop configuration
Improved
linearity
Figure:
Clipp (2006)
Sensitive axis
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Navigation Sensors and INS Mechanization
Inertial Sensors
– Accelerometer: Vibratory
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
Vibratory accelerometers
Vibrating Beam Accelerometers (VBA)
Acceleration causes a change in resonance
frequency
Mass
Sensitive axis
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Navigation Sensors and INS Mechanization
Inertial Sensors -
Accelerometer: Vibratory
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
MEMS Accelerometers
www.ett.bme.hu/memsedu
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Navigation Sensors and INS Mechanization
Inertial Sensors -
Accelerometer: Vibratory
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
MEMS Accelerometers
Spring and mass from silicon and add fingers make a variable differential capacitor
Change in displacement => change in
capacitance
CS1 < CS2
APPLIED
ACCELERATION
SENSOR
ACCELERATING
MASS
SPRING
SENSOR AT REST
FIXED
ANCHOR TO
SUBSTRATE
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Navigation Sensors and INS Mechanization
Inertial Sensors – Gyroscopes: Rotating Mass
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
Rotating Mass Gyros
Conservation of Angular
Momentum
The spinning mass will resist
change in its angular momentumAngular momentumH = I (Inertia * Angular velocity)By placing the gyro in a pair of frictionless gimbals it is free to maintain its inertial spin axisBy placing an index on the x-gimbal axes and y-gimbal axis two degrees of orientational motion can be
measured
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Navigation Sensors and INS Mechanization
Inertial Sensors – Gyroscopes: Rotating Mass
Rotating Mass Gyros
PrecessionDisk is spinning about z-axis
Apply a torque about the x-axis
Results in precession about the y-axis
=
H
x
y
z
Precession rate (
)
H(
t+dt
)
H(t)
dt
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
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Navigation Sensors and INS Mechanization
Inertial Sensors
– Gyroscopes:
Sagnac Effect Gyros
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
Fiber
Optical Gyro (FOG)
Basic idea is that light travels at a constant speedIf rotated (orthogonal to the plane) one path length becomes longer and the other shorterThis is known as the Sagnac effect
Measuring path length change (over a dt) allows to be measured
R
Detector
Transmitter
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Navigation Sensors and INS Mechanization
Inertial Sensors -
Gyroscopes: Sagnac
Effect Gyros
Fiber Optical Gyro (FOG)
Measure the time difference
betw
the CW and CCW paths
CW transit time = tCWCCW transit time = tCCWLCW = 2R+Rt
CW = ctCWL
CCW = 2R-RtCCW
=
c
t
CCW
t
CW
= 2
R/(c-R
)
tCCW= 2R/(
c+R )With N turnsPhase
R
Splitter
Transmitter
Detector
R
Splitter
Transmitter
Detector
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
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Navigation Sensors and INS Mechanization
Inertial Sensors -
Gyroscopes: Sagnac
Effect GyrosTuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
Ring Laser Gyro
A helium-neon laser produces two
light beams, one traveling in the
CW direction and the other in theCCW directionWhen rotatingThe wavelength in dir of rotationincreases (decrease in freq
)The wavelength in opposite dirdecreases (increase in freq)Similarly, it can be shown that
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Navigation Sensors and INS Mechanization
Inertial Sensors - Gyroscopes: Coriolis
Effect
Vibratory Coriolis Angular Rate Sensor
Virtually all MEMS gyros are based on this
effect
Linear motion
Linear motion
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
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Navigation Sensors and INS Mechanization
Inertial Sensors - Gyroscopes: Coriolis Effect
Basic Planar Vibratory
Gyro
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
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Navigation Sensors and INS Mechanization
Inertial Sensors - Gyroscopes: Coriolis Effect
In plane sensing (left)
Out of plane sensing (right)
www.ett.bme.hu/memsedu
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
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Navigation Sensors and INS Mechanization
Inertial Sensors -
Summary
Tuesday 12 Feb 2013
NMT EE 570: Location and Navigation: Theory & Practice
Accelerometers
Measure specific force of the body frame
wrt
the inertial frame in the body frame coordinatesNeed to subtract the acceleration due to gravity to obtain the motion induced quantityIn general, all points on a rigid body do NOT experience the same linear velocityGyroscopesMeasure the inertial angular velocity Essentially, the rate of change of orientationAll points on a rigid body experience the same angular
velocity
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