KVH Understanding Specs FOGs 0312

© KVH Industries, Inc.

Understanding Specifications of

Fiber Optic Gyros

Steven R. Emge, FOG Principal Engineer

27 March 2012


Fiber Optic Gyro History

• 1913– Principle used by an interferometric fiber optic gyro

(FOG) to sense rotation first demonstrated by French scientist George Sagnac

• 1960– Invention of laser leads to first practical use of

“Sagnac Effect”

– Ring laser gyroscope (RLG)

• 1976– Interferometer fiber optic gyroscope first proposed

by Vali & Shorthill

– “Fiber Ring Interferometer” Appl. Optics, 15

• 1980-2000– FOG performance refined through advances in

theory & design at universities and by industry

• 2001-2012 and beyond– Continued technological advances develop higher

performance, increased reliability, lower-cost FOGs for commercial and military applications

1


FOG Theory of Operation

The Sagnac Effect

• Over a period Dt:

CW beam traverses

2 p R + W R Dt

CCW beam traverses

2 p R - W R Dt

• Path length difference = CW path - CCW path = 2 W R Dt

Dt = L/c => 2 W R L/c = W D L/c

• Sagnac phase shift = DS = (2 p L D W)/(l c)

(where L is the coil length, D is the coil diameter, c is the speed of light,

l is the optical wavelength and W is the input rotation rate)

2


FOG Sensor Configuration

Minimum configuration, open-loop “all-fiber” design• KVH proprietary polarization-maintaining (PM) fiber used throughout

• Light source: solid state laser; photo-detector used receive signal

• Directional couplers used to launch light into/ receive signals from the sensor

• Phase modulator is a fiber-wrapped disc PZT (Piezo-electric transducer)

• Coil fiber length is 100 to 200 meters wound on a 2.6” (1.4”) average diameter

• Fiber components are fusion-spliced together in just 1 to 3 locations

• Sensor is 3.5”L x 2.3”W x 1.3”H (DSP-3000) or 1.70”D x 0.82”H (DSP-1750)

3


FOG Signal Processing

FOG coil stationary

When stationary,

only even harmonics

are present in

detector output

PZT modulationEven harmonics used to maintain

laser and phase modulator

operating points

4


FOG Signal Processing

FOG coil rotating at rate ΩRotation results in

fundamental and odd

harmonics

proportional to sin Ω

FOG digital signal

processor determines

rotation rate Ω

5


FOG DSP Based Electronics

• DSP architecture block diagram for single FOG axis– DSP-based digital pseudo-closed loop electronics allows key sensor internal components to be

accurately controlled/monitored and sensor output to be precisely calibrated over temperature

– DSP-based electronics allows multiple interfaces (i.e. RS-232 async, RS-422 async, 3-wire synchronous, analog, etc.), output data rates (100 Hz to 2000 Hz), and output bandwidths to be offered

RED: KVH-manufactured polarization-maintaining, elliptical core, single mode optical fiber

GREEN: Printed circuit boards

6


FOG Performance Specifications

• The relative performance between DSP-3000/3100 products andthe DSP-1750 sensor

– Both sensors are based on a similar optical circuit design & technology with the DSP-1750 having some miniaturization of optical components & improved electronics

– Note the expanded input rate range & improved ARW & bias stability of DSP-1750

Performance Specification DSP-3000 DSP-1750

Maximum Input Rate ± 375 deg/sec ± 1000 deg/sec

Bias Offset (25 °C) ± 20 deg/hr ± 10, ± 2* deg/hr

Bias Stability (stable temp) ≤ 1 deg/hr, 1s ≤ 0.05 deg/hr, 1s

Bias Stability (full temp) ≤ 6 deg/hr, 1s ≤ 3 deg/hr, 1s

Angle Random Walk (ARW) ≤ 4 deg/hr/√Hz ≤ 0.8 deg/hr/√Hz

Scale Factor (full temp) ≤ 500 ppm, 1s ≤ 300 ppm, 1s

Scale Factor Non-Linearity ≤ 1000 ppm, 1s ≤ 1000 ppm, 1s

* with magnetic shielding

7



• Bias Offset Error– When a FOG is stationary, it can incorrectly register some rotation

– This is known as “bias offset error”

– Its deviation from zero typically given at 25 °C for an ideal

environment (i.e. no temp change, vibration, shock or magnetic field

applied)

• Bias Stability (Constant Temp)– This is the stability of the bias offset at any constant temperature and

ideal environment

– Bias stability is best measured using the Allan Variance measurement

technique

– Allan Variance is intended to estimate stability due to noise processes

and not that of systematic errors or imperfections such as frequency

drift or temperature effects

8



• Bias Stability (full temp, -45 to +75°C)– The bias offset of a FOG will vary slightly (few degrees per hr) with

temperature changes

– Error is repeatable in KVH FOGs, enabling KVH engineers to build an internal table of bias offset values versus temperature for each FOG

– An internal temperature sensor compares temperature with values stored in this internal lookup table, and subtracts them from the result to increase the sensor’s accuracy

– Note both Bias Offset and Stability depend on the polarization maintaining quality for the FOG optical circuit construction

• Angle Random Walk (ARW)– The output of a FOG includes a broadband random noise element

– Angle Random Walk (ARW) is defined in terms of degs/hr/√Hz or degs/√hour

– This describes the average deviation or error that will occur as a result of this noise element

– Major contributors to random noise are the active elements of the FOG, such as the laser diode, photo diode and the transimpedenceamplifier

9



• Scale Factor Non-Linearity Error– Scale Factor is defined as the ratio of FOG sensor output rate to applied input

rate

– If the FOG sensor is rotated at 20 degs/sec CW, does it output a reported rate

of 20 deg/sec CW?

– As the rate of rotation increases from zero to the maximum input rate, the

FOG Scale Factor Error increases

– Scale Factor Non-Linearity Error is the deviation of the output characteristic

from a best-fit straight line

• Scale Factor Error (full temp)– FOG Scale Factor can vary with temperature due to the change in length and

diameter of the fiber coil and the wavelength change of the light source

– For KVH FOGs, the Scale Factor Error versus temperature is repeatable and

can be calibrated using an internal look-up table

10


Typical DSP-3000/3100 Sensor Performance

0.01

0.1

1

10

0.01 0.1 1 10 100 1000

Time (Minutes)

Bia

s (D

eg/H

r)

1 Hz Output -1/2 slope

Bias Instability = 0.15 deg/hr

ARW = 0.055 deg/rt-hr = 3.3 deg/hr/rt-Hz

stdev N( ) 1. 77 deg/hr p_p 14. 52 deg/hr

40 20 0 20 40 60 80100

80

60

40

20

0

20

40

60

80

100

Temp (deg C)

Bia

s (d

eg/h

r)

20

20

Ni ER

Ji 8

stdev SF_err( ) 89. 9 ppm p_p 517.3 ppm

40 20 0 20 40 60 805000

4000

3000

2000

1000

0

1000

2000

3000

4000

5000

Temp (deg C)

SF E

rro

r (p

pm

)

1500

1500

SF_erri

Ji 8

-200

-150

-100

-50

0

50

100

150

200

-500 -400 -300 -200 -100 0 100 200 300 400 500

Input Rate (deg/sec)

SF

Lin

eari

ty E

rro

r (p

pm

)

SF Linearity Error = 30 ppm, 1s

11


Typical DSP-1750 Sensor Performance

12


Thank you


FOG Applications & Specifications

Tom Suita, Applications Engineer

[email protected]


Agenda

I. FOG Applications

II. FOG Output Signal

III. FOG Specifications and Allan Variance Analysis

IV. Example FOG Specification

V. Summary


FOG Applications


Rotation Measurement

• Why use gyros?

– Gyros sense rotation in an inertial reference frame

– Highly accurate gyros will sense the Earth’s rotation

– Unlike GPS, inertial sensor (gyros) are not subject to jamming

• Types of gyros

– Rate gyros

– Rate integrating gyros

• Other non-inertial rotation sensors

(rotary relative position sensors)

– Encoders

– Resolvers/Inductosyns

– Potentiometers

– Rotary variable transformers

– Tilt sensors


Gyro Grades

Gyro grades are defined by bias stability specifications

• Industrial/Automotive: 30 – 100 º/hr

• Tactical: 1 – 10 º/hr

• Inertial Navigation: 0.001-0.01 º/hr

• Strategic: < 0.0001 º/hr


Gyro Grades

Courtesy of

Self-

Aligning

Strategic

Missile

Autonomous

Submarine

Navigation

Cruise Missile

Air/Land/Sea

Navigation

Surveying

AHRS

Torpedo

Interceptor

Tactical

Missile

Midcourse

Guidance

Flight Control,

Smart Munitions,

Robotics


Gyro Technology Types

In-production gyro technologies

• Fiber Optic Gyro (FOG)

– Best value performance for price

• Micro-Electro-Mechanical Systems Gyro (MEMS)

– Low cost, small size, low performance

• Ring Laser Gyro (RLG)

– Higher cost, good performance

• Spinning Mass/Dynamically Tuned Gyro (DTG)

– moderate price, high performance, lower reliability

• Hemispherical Resonator Gyro (HRG)

– High cost and high performance



Experimental and emerging gyro technologies:

• Magneto-Hydro-Dynamic Gyro (MHD)

– Laboratory proof of concept

• Nuclear Magnetic Resonant Gyro (NMR)

– Laboratory proof of concept

• Cold Atom Gyro

– Early experimental stages



Gyro fundamental operating principles

• Sagnac Effect: FOG & RLG

– Utilize counter-propagating optical beams and interferometry

– No moving parts

• Mechanical Coriolis Vibratory: MEMS & HRG

– Based on vibrating mass and Coriolis effect resulting from rotation

• Mechanical Spinning Mass: DTG & NMR

– Based on conservation of angular momentum of spinning masses


Gyro Applications

• Land Navigation

– Integrated land navigation

– High performance FOG and

accelerometers with aiding sensors

TACNAV II used in

Bradley Fighting Vehicle


Gyro Applications

• Navigation

– Tightly coupled INS/GPS

– Mobile mapping

– Airborne augmented reality

– 6-DOF IMU (3 FOGs) with GPS

CNS-5000

Integrated INS/GPS


Gyro Applications

• Stabilization

– Gun platform stabilization

– Low FOG ARW and high bandwidth improve stability

DSP-3100 used with

CROWS II

Remote Weapons Station


Gyro Applications

• Stabilization

– Airborne/land camera gimbals

– Low FOG ARW reduces image jitter effects

Dual-axis DSP-1750

for EO/IR

camera turrets


Gyro Applications

• Pointing

– Cameras, lasers, antennas

– FOG scale factor linearity key

performance specification

Single-axis DSP-3000

for diverse pointing

applications


FOG Output Signal


FOG Signal and Noise

FOG output comprised of a number of elements

• Gyro rate output− Degrees per second

• Bias offset− Fixed offset from zero, specified in deg/sec

• Earth rate− Earth rotation in deg/sec sensed by gyro and dependant on

• Latitude

• Input axis orientation (east/west, north/south, up/down)

• Noise− Present in all gyros

− Both high (ARW) and low (bias instability) frequency

components


FOG Signal and Noise

-0.03

-0.02

-0.01

0

0.01

0.02

0.03

0.04

Gyro output with Earth rate, bias offset, and noise components

Signal offset

including:

bias offset &

Earth rate

Noise 1σ


Gyro Signal and Noise

• Earth rate contribution

True North

Earth rotation

rate = 15 °/hr

at Equator

= 0 °/hr

East/West

= 0 °/hr

Earth rotation contributes to the

output of the gyro - this effect is

dependent on:

• Orientation of the gyro input

axis relative to the earth’s axis of

rotation

• Latitude

Earth rate = -15.04 sin(latitude)

At 45° N latitude

= 10.6 °/hr

FOG input axis


FOG Specifications and

Allan Variance Analysis


FOG Specifications

• Basis for FOG specification is IEEE std 952:

IEEE Standard Specification Format Guide and Test Procedure for

Single-Axis Interferometric Fiber Optic Gyros

• Allan Variance (IEEE std 952) analysis is a fundamental

method of deriving performance specifications for all major

gyro types (FOG, RLG, and MEMS)

• FOG packaging specifications

– Shock & Vibration

– Thermal

– Power

– Data



• Allan Variance (AVAR) Extracts contribution of different noise

sources from combined signal and noise gyro output

• AVAR concept assumes each noise source has a Power Spectral

Density ( ) with a unique slope in Allan deviation log-log plot

• AVAR based on cluster analysis where the variance of the

difference of two sequential data clusters is plotted; a data cluster

is rate output data collected over time period



Log-Log Allan Deviation Plot from IEEE-std-952



• Angle Random Walk (ARW)

– Quantum limit noise with significantly higher frequency than data sampling frequency

– Variance: Deviation log slope: -1/2

– Narrow ARW bandwidth limits attitude control systems

– Integrated angle error increases over time (~1/√hour) for navigation systems

• Bias instability – Lower frequency “1/f flicker” noise

– Variance:

Deviation log slope: 0

– Zero slope minimum on Allan deviation curve



• Quantization noise

– Result of discrete/quantized sensor output

– Variance: Deviation log Slope: -1

– Minimal impact on systems due to short correlation time

• Rate random walk

– Long correlation time random process not prevalent in FOGs

• Rate ramp

– Long correlation time deterministic process not prevalent in

FOGs - Sometimes associated with environmental changes


Example FOG Specification


DSP-1750 FOG

Single-axis with Magnetic ShieldDimensions:

Sensor: 1.81” dia. x 0.9”

PCB stack: 2.28” dia. x 0.55”

Dual-axis UnshieldedDimensions:

Sensor: 1.7” dia. x 0.82”

PCB stack: 2.85” dia. x 0.55”

The DSP-1750 is a small high performance tactical grade FOG


DSP-1750 FOG Specifications

• Maximum input rotation rate:

±490 °/sec standard rate ±1000 °/sec high rate

– Maximum rotation rate that can be detected and output by FOG

– ±1000 °/sec high rate device requires export license

– 22 bit binary output data word with LSB scaled to 476.8 μ °/sec

– Standard rate device clips at max binary output of ±490 °/sec

• Bias instability: < 0.05 °/hour

– Includes design margin; testing has demonstrated bias stability at

< 0.02 °/hour

– Lower performance MEMS gyros often specified in deg/sec

• Angle Random Walk: < 0.013 °/√ hour or 0.8 °/hour√ Hz

– Specified in degrees/√hour and degrees/hr/√Hz (PSD);

– Conversion: °/√hr x 60 = °/hr/√ Hz


DSP-1750 Allan Variance Analysis

DSP-1750 8 hour Allan Variance test at 25°C

ARW = 0.009 deg/√ hour Bias Instability = 0.02 deg/hour



• Bias Offset:

± 10 °/hour max unshielded ± 2 °/hour max shielded

– Fixed bias associated with all calibrated gyros (RLGs, FOGs,

MEMS)

– FOGs bias offset due to 0.3 to 0.6 gauss Earth’s magnetic field

(Faraday Effect)

– Magnetic shielded version of DSP-1750 recommended if high

magnetic fields are near the FOG

• Bias Over Temperature: < 3°/hour

– Tested with 1°C/minute thermal ramp rate

– Very flat for FOGs over temperature range -45°C to +75°C

– Lower performance MEMS gyro experience extreme bias

variations over temperature



• Scale Factor Non-Linearity:

< ± 490°/sec: < 100 ppm; ± 490 to ± 1000°/sec: < 500 ppm

– Specified in % or parts per million (ppm)

– Root variance deviation from least squares estimate of gyro scale

• Scale Factor Temperature Variation: < 300 ppm

- Specified in parts per million (ppm) or % over the operating

temperature range of -45°C to +75°C

• Bandwidth: 440 Hz

– Output bandwidth is the 3dB roll-off of the output signal

– Acutronic Lab verified DSP-1750 -3dB bandwidth was 475 Hz

• Misalignment: < 4 milliradians

– Misalignment between gyro coil input axis and gyro mounting case

– Test results: 1.44 milliradians average 3 σ



• Data Output:

– RS-422 1000Hz asynchronous; 22 bit two’s complement rate data

• Initialization Time: < 3 seconds

– Time after power-up at which output data is valid

• Misalignment: < 4 milliradians

– Misalignment between gyro coil input axis and gyro mounting case

– Test results: 1.44 milliradians average 3 σ

• Reliability:

Single axis: > 36,000 hours dual axis: > 22,000 hours

– Specified in hours MTBF

• Electromagnetic Compatibility:

– Comprehensive conducted emissions and susceptibility testing per

MIL-STD-461F performed.



• Power:

– +5VDC and ± 8 to ±15VDC; total power ≤ 3.0 W

• Weight

– Sensor: 45 g unshielded, 65 g shielded

– PCB stack : 45 g

• Environmental

– Operating temperature: -40°C to +75°C

– Shock (functional): 25 g, 11 msec (sawtooth)

– Shock (gunfire): 55 g, 1 msec (1/2 sine)

– Shock (endurance): 40 g, 11 msec (sawtooth)

– Vibration, Random (operational): 8 g rms 20-20,000 Hz

– Vibration, Random (endurance): 12 g rms 20-20,000 Hz

– Altitude (operational): -1,000 to +40,000 ft

– Humidity (operational): 95% at 35°C , 48 hours


Summary


• Fiber optic gyros support numerous applications

– Navigation

– Stabilization

– Pointing

• FOG output signal includes

– Rotation rate data with earth rate component

– Bias offset and some noise

• Key FOG specifications are derived from Allan Variance analysis

– Angle Random Walk (ARW)

– Bias instability

• Comprehensive FOG specifications cover a wide range of

performance features

FOG Specifications Summary

KVH Understanding Specs FOGs 0312

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