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HRG by SAFRAN
The game-changing technology
Fabrice Delhaye
Safran Electronics & Defense, Boulogne-Billancourt, France
fabrice.delhaye@safrangroup.com
Abstract—Whereas the world inertial navigation community
was wondering, for decades, if FOG would ultimately replace
RLG, Safran is demonstrating with its HRG that technology
prospective is not such an easy game. With its HRG, Safran is
proving that the HRG innovative approach is a real game
changer in high end navigation. This paper sums up the overall
principles of HRG, how it works and its intrinsic properties.
Current applications of HRG are described to illustrate how
HRG benefits are capitalized in valued-added products. More
prospective aspects of the HRG are also addressed with the latest
tests results of performance limits exploration.
Keywords—Hemispherical Resonator Gyro, Inertial System,
Navigation
I. HRG
P
RINCIPLES
The Coriolis Vibratory Gyroscopes (CVG) [1] are based
upon the following principle: a resonating structure tends to
continue to resonate in the same plane even if its support
rotates.
The HRG is a CVG that follows theses few key principles:
• The resonator is perfectly axisymmetric in order to
exhibit excellent characteristics in terms of balancing,
natural frequency and damping isotropies. In a
practical way, the resonator must be finely tuned in
order to be perfectly balanced.
• The resonator is interfaced to its support through a
vibration node in order to ensure an optimal
decoupling between the resonator and the outer world.
• The flexural waves are controlled through electrostatic
forces thanks to electrodes located at immediate
proximity of the resonator.
• The flexural waves are controlled in Whole Angle
mode in order to minimize the requested energy. This
mode minimizes the errors induced by the electronics,
the detectors and actuators defects. Furthermore, this
mode leads to a very good scale factor (based on the
Bryan coefficient) and allows high measurement range.
The HRG resonator is optimized for performances:
• Its topology is a hemispherical shell for optimal
contribution of each gram of resonator material to the
flexural energy storage. A stem, anchored at the top of
the hemisphere, is used to hold this shell.
• It is made of amorphous fused quartz for optimal
isotropy (no crystallographic direction) and minimal
energy dissipation (no internal friction).
• Its metallic coating, requested to create the electrode
and ensure electrical continuity, is as thin as possible in
order to minimize the energy dissipation inside the
metal.
As a result of these optimizations, the HRG exhibits
outstanding performances. But the most interesting fact is that
these performances are not linked to the size of the resonator.
They are linked to the quality of the resonator rated through the
Q-factor and the quality of the way the flexural waves are
driven, therefore from the electronics.
The major drawback of the HRG is its manufacturing cost
mainly linked to the precise manufacturing and assembly of the
electrodes support which must perfectly match the resonator
shape in order to create the isotropic hemispherical gap
between both parts. Safran solved this drawback with an
innovative design [2], the electrodes are deposited on a plan
and electrostatic forces are generated on the equatorial plan of
the resonator. The assembly of the hemispherical shell and the
electrodes plate is a basic 2D issue.
Fig. 1. Hemispherical electrodes / Flat electrodes
With its disruptive design, Safran HRG can be
manufactured in a cost effective way and makes the intrinsic
performances quite affordable for the mass market.
Fig. 2. Safran HRG
II. S
AFRAN
HRG
APPLICATIONS
The need for extremely reliable space rate gyro dedicated to
AOCS (Attitude & Orbital Control System) is fulfilled by
Regys 20 which takes benefit of the natural space radiation
hardening of the fused quartz of the HRG resonator. Low
angular random walk and ultra-high reliability are the key
characteristics of the Safran HRG which are valued. To date,
more than 100 gyros have been delivered and no failure or
performance degradation is recorded.
Fig. 3. Regys 20 ultra-high reliability HRG space rate gyro unit
The need for ultra-lightweight True North Finder for man
portable observation and targeting systems was fulfilled by
Sterna. Thanks to the excellent bias stability and outstanding
random-walk of Safran HRG, Sterna is able to deliver azimuth
measurement in less than 100 seconds and has an accuracy up
to 0.7 mils. Thanks to the outstanding SWaP (Size Weight and
Power consumption) characteristics of Safran HRG, Sterna
weighs only 1.8 kg (tripod excluded) and is able to operate for
3 days only powered by 4 CR123 cells.
Fig. 4. Sterna ultra lightweight true northfinder
The commercial marine market is expecting high reliability,
maintenance free, IMO (International Maritime Organization)
certified gyrocompass for 24/7 operations. FOG based products
are only marginally able to fulfil such a need. BlueNaute was
developed in order to address this key requirement. Operational
feedback over 5 years and millions of operation hours, exhibits
a product MTBF in excess of 250.000 hr. BlueNaute takes
benefits of the HRG scalability to address a wide range of
performances in order to fulfil more demanding requirements
of the Offshore Oil & Gas industry such as Dynamic
Positioning. The Titanium version is delivering an azimuth
accuracy of 0.1° rms. No wonder that the paramilitary
organizations (such as maritime polices and cost guards) make
now use of BlueNaute instead of their traditional compass.
Fig. 5. BlueNaute Compass and AHRS
The military land applications are requesting rugged and
compact INS, Sigma 20 takes benefit of the mechanical
robustness of Safran HRG to offer a 4.5 kg INS able to meet
the requirements for azimuth accuracy up to 1mils rms.
Fig. 6. Sigma 20 HRG based Tactical Land INS
For OEM applications, Safran takes benefits of SWaP
characteristics of HRG to offer Primus [5] which with its 420
grams (0.9 lbs) is by far the lightest navigation grade IMU of
the market.
Fig. 7. HRG based Primus IMU
Hammer, Safran Air to Ground Weapon, designed for
stand-off operations in GPS denied environment, takes benefit
of the HRG accuracy to ensure precision strikes at range up to
80 km. The high reliability of HRG contributes to the Hammer
99 % mission success ratio. Hammer is the main Air to Ground
weapon of the Rafale Multirole fighter and is currently in mass
production.
Fig. 8. Hammer Weapons, using HRG for guidance
Firing accurately on inaccurate coordinates would be
nonsense. Joint Terminal Attack Controllers (JTAC) need on
locate theirs targets accurately with theirs man portable target
locators and the only dependable way, in GPS-denied
environment, is through inertial heading measurement.
Therefore the challenge is to design an INS compact enough to
be integrated into a handheld observation multifunction google.
The HRG outstanding SWaP characteristics allowed Safran to
design a highly ruggedized PAVAM (Precision Azimuth &
Vertical Angle Module) that weights only 420g, consumes less
than 5 W and that gives to hand held devices the ability to
locate target with the highest accuracy (TLE CAT I ie 6 m
CEP90).
Fig. 9. Light weight long range multi-function infrared binoculars
To some extent, artillery forward observation vehicles are
subject to similar accuracy requirements except they are
operating at longer distance from the target and therefore need
higher azimuth accuracy. An autonomous inertial position is
also a great benefit since, in combat situation, the GPS is most
likely jammed. The HRG SWaP allowed Safran to integrate an
INS inside the PASEO sight in order to sense the orientation of
the line of sight with an accuracy better than 0.5 mils and to
deliver a positon with accuracy better than 10 meters in GPS
denied environment.
Fig. 10. Paseo modular advanced stabilized sight
The commercials airliners are currently using RLG INS,
typically in triplex or quadruplex configuration. ARINC
704/738 standards define the requested navigation accuracy in
pure inertial mode: 2 Nm/h 95 %. This market is always
looking for weight saving, higher reliability and lower
operation cost. SkyNaute is the Safran answer. This HRG
based INS, currently under development, weighs only 3 kg and
takes benefit of Safran HRG outstanding reliability to offer
disruptive MTBF and ownership cost.
Flight tests campaigns exhibited comfortable inertial
performances margins.
Fig. 11. SkyNaute HRG INS for Commercial Aircrafts
Advanced Space launchers are demanding highly accurate
INS especially to place satellite into a supersynchronous
elliptical geostationary transfer orbit. Safran HRG is well
suitable for this mission since it is able to deliver high accuracy
even under the huge vibration level (above 30 g rms) induced
by the lightened structure of modern launchers.
In 2016, after a deep selection process and analysis of all
potential technological solutions, ArianeGroup decided to
select Safran SpaceNaute and awarded a development and
production contract for the new European space launcher
Ariane 6. The first flight is expected in July 2020. SpaceNaute
takes also benefit of HRG resonator natural space radiation
hardening. SWaP characteristic of Safran HRG allows
SpaceNaute to be twice lighter than its closest competitor.
Further HRG based INS are also currently under
development in order to expand the product portfolio. Some of
them will be unveiled later this year.
III.
EXPLORING
HRG
ACCURACY LIMITS
In the mass production configuration, Safran HRG has
widely proven its ability to fulfil performances requirements
usually achieved by optical technologies (RLG and FOG), with
better SWaP characteristics, much higher reliability and always
in a cost effective way.
The Safran HRG design offers quite significant growth
potential. Without change in architecture and size, the
resonator manufacturing can be further improved for better
isotropy and lower damping characteristics. This leads to more
refined fused quartz grinding, metal coating and resonator
balancing. The digital electronics can take benefit of the
semiconductor industry progress (Moore’s law continues to roll
on!) in order to design more efficient and sophisticated digital
control laws. Such R&T activities already lead to interesting
results which are unveiled hereafter.
A preliminary assessment of the grade of a gyro can be
made through the Allan variance analysis techniques. Angle
Random Walk (ARW) and bias instability can be obtained
from the plot below.
Fig. 12. Allan deviation plot of Safran HRG
It can be noted that ARW = 0.0002 °/√h and that the test is
too short (even if the time requested to collect the requested
data was 2000 test hours!) to evidence the bias instability
(minimum value not reached over the test time slot).
As Safran HRG works in Whole Angle mode, its scale
factor error is intrinsically excellent: 1 ppm is commonly
achieved. Recent improvements lead to the following results
(fig.13).
Fig. 13. Scale Factor Stability of Safran HRG
It can be noted that Scale Factor error can be reduced down
to 0.1 ppm (rms).
The bias optimization is currently subject to intensive R&T
activities. Multiples axes are simultaneous pursued including
improvements on control electronics and implementation of
more sophisticated calibration/compensation models. Progress
perspectives in this area are quite significant and the results
obtained on an HRG triad integrated inside a strapdown INS,
during year 2017 (fig. 14), are already extremely promising.
Fig. 14. Biais Stability of Safran HRG
It can be noted that Bias Stability of Safran HRG, measured
over a 2000 h period, can be reduced down to 0.0001 °/h (rms).
It is known that the ultimate performance technology is the
Electrostatically Suspended Gyroscope (ESG). Today, R&T
studies are conducted on Cold Atoms especially with the aim to
replace ESG in the future. From the 80’s, Safran developed
ESG and therefore has a deep knowledge how to use it in
systems. Inspired by the way ESG short-medium-long term
errors are compensated, Safran designed a prototype of
navigation system based on its HRG and assessed
performances, in operational conditions, at sea.
Technical data as well as actual performances are
unfortunately classified but it was proven that HRG was able to
reach performances ESG actually achieves and Cold Atom
gyros are still contemplating.
IV. C
ONCLUSION
If the scientific community always regarded the HRG as a
premier gyro with outstanding accuracy and reliability
characteristics, few anticipated that HRG would be capable to
successfully address the mass market. Safran, thanks to
innovative design [2] and massive industrial investments, was
capable to achieve it. With its 20 mm diameter resonator
(fig. 2), Safran is able to address an unmatched range of
applications. From very cost effective marine compass to ESG
grade strategic submarine navigation, from tripod mounted
north finder to space launcher navigation, Safran HRG is able
to fulfill needs for accuracy even in harsh environmental
conditions and always in a very cost effective way.
Safran has proven that HRG is much more than an
innovative gyro technology; it is a disruptive technological
breakthrough. HRG is a technology not only able to replace
RLG and FOG but also to replace ESG as well as the
promising Cold Atom technology for ultimate performances
level. HRG is therefore able to recompose the inertial gyro
technology landscape which could be soon pictured as per the
fig 15.
Fig. 15. Future gyro technology applications
[1] G. Remillieux, F. Delhaye, Sagem Coriolis Vibrating Gyros: a vision
realized, in Proceedings of Karlsruhe Conference on Inertial Sensors and
Systems, 2014
[2] A. Jeanroy, P. Leger, HRG flat electrodes, Patent US 6474161
[3] A. Jeanroy, A. Bouvet, G. Remillieux, HRG and Marine applications, in
Proceedings of 20th Saint Petersburg International Conference on
Integrated Navigation Systems, 2013
[4] Alain Jeanroy, Gilles Grosset, Jean-Claude Goudon, Fabrice Delhaye
HRG by SAFRAN - From Laboratory to Mass Production, in
Proceedings of Inertial Sensors, 2016
[5] PRIMUS : SWAP-oriented IMUs for multiple applications A. Lenoble,
T. Rouilleault, in Proceedings of Karlsruhe Conference on Inertial
Sensors and Systems, 2016
