January 29 th, 2013
P.PRIEUR (CNES, Microscope DFACS architect)
MICROSCOPE Drag-free & Attitude Control System
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau2/13
Contents
Introduction : orbit, attitude, Fep DFACS driving requirements DFACS hardware equipment Life span evaluation and enhancement DFACS control loop Real time attitude estimation A posteriori attitude knowledge Calibration modes Conclusions
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau3/13
1) Introduction : Orbit, Attitude, Fep
Orbit : Microscope will be launched into a quasi-circular 700km dusk-dawn
sun-synchronous Earth orbit
Attitudethe instrument main axis is kept into the mean orbital plane
Fep frequency (modulation of ‘g’ w.r.t. s/c frame)
Inertial pointing : Fepi = Forb ~ 1.7 10 -4 Hz (sessions of Ti=120 orbits) Rotating mode : Fepr = Forb + Fspin ~ 1 10 -3 Hz (sessions of Ti=20 orbits)
Calibration mode : based on inertial pointing
- linear stimulations
- attitude oscillations about s/c axes
eq uator
Xsat
N orth gr
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau4/13
2) DFACS DRIVING REQUIREMENTS
Drag-free
The difference of scale factor between the 2 masses (Kd<1.5 10-4) induces the control of the common acceleration @Fep
This 3 axes linear requirement induces the use of a 3 axes propulsion system The air drag is the main perturbation @Fep : 10 to 30µN depending on the solar
activity => need of rejection factor of 30µN/300Kg/1.10-12m/s2=105 (100dB)
γ1 γ2
21 KK ≠
Hzm/s 3.10 and m/s 1.10 2-10@Fep
2-12@Fep /<Γ<Γ
rr
m/scKd. : that so 21610.2 −≤Γ
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau5/13
2) DFACS DRIVING REQUIREMENTS
Attitude control
The differential excentring of the masses (∆<20µm) induces the control of the angular acceleration and of the angular velocity @Fep
1.10-9rd/s @Fepr is equivalent to an attitude stability of the instrument better than
0.16µrad at very low frequency
This order of magnitude is far below the thermo-elastic deformations between the instrument base-plate and measurement axes of any kind of attitude sensor
Then attitude estimation is clearly a challenge (see later)
mode) (rotating rd/s 1.10
modes) rotating & (inertial rd/s 1.109-
@Fep
2-11@Fep
<
<
ω
ωr
r&
γ1 γ2
21 OO ≠ m/s and : that so 21610.2)^(^^ −≤∆ΩΩ∆Ω
rrrr&r
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau6/13
2) DFACS DRIVING REQUIREMENTS
Attitude control (suite) : frequency and magnitude of the disturbance torques
Aerodynamic and solar pressure torques
are negligible due to the shape of the s/c
Gravity gradient produces large torques@2Fepr due to the satellite non-spherical
inertia
Magnetic torques (s/c permanent andinduced moment) produces large
@Fepr+Forb torques
Because of non linear effects in the δextraction method, all the harmonic signals
are taken into account in the @Fep DFACS
budgets via rejection templates
The most DFACS stringent requirementsare Harmonic ones. Stochastic sources
(on sensors, actuators, perturbations) are
quite low
Rotating mode – out of plane axis – FFT of Cpert
Rotating mode – In plane axis – FFT of Cpert
µN.m
µN.m
30
20
15
10
Fepr+Forb
2Fepr
Fepr+Forb
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau7/13
3) DFACS hardware equipment
In operational mode, the DFACS uses : the scientific instrument as main sensor for linear and angular accelerations
2 star-tracker camera heads (co-pointed but twisted) A set of 8 cold gas proportional thrusters (+cold redundance 1/1)
Range: 1 to 300 µN , τ<250ms, <1µN/rHz
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau8/13
4) Life Span evaluation and enhancement
Intensive Monte-Carlo simulations have been performed to estimate the number of
operational orbits before completing the propellant gas Some learnings have been taken into account in the design :
The satellite centring tolerance (i.e. the position of the s/c CoM w.r.t. the SUs) is
tighten to reduce the gravity gradient and the inertial forces in rotating mode The orientation of the thrusters has been optimized (trade-off between the force
efficiency of the configuration in case of strong solar activity and the torque efficiency
in case of large magnetic momentum) The gas consumption depends on key parameters :
Solar activity (atmospheric density)
Thrusters minimum setpoint (for quick re-start)
Satellite Magnetic momentum, Thusters ISP
The best effort is done on every key factor
in order to increase the worst cases life span
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau9/13
5) DFACS control loop
Cold-GasPropSyst
Drag free control laws
Attitude control laws
Thrust Repartitor
Hybridization filter
Star Tracker
6-axis accelerometer
attitude
ωγ&
accelerations
Satellite dynamics
DisturbingForces &Torques
++T
C
F
THRUSTC
F
8:1i
iFc
=
comF
ωQ ,r
Attitude measurement
Angular Acceleration Measurement
mω&
Estimates
MCA software : 4Hz sampling
comT
c
c
Q
ω -
+
cγ -
+
Linear acceleration measurement mγ
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau10/13
6) Real time attitude estimation
The star-tracker measurements errors @fep are not compatible with the angular velocity
and acceleration requirements (1.10-9rad/s & 1.10-11rad/s2) [see P5]
Fortunately, the scientific instrument complies with such accuracy A first solution consists in using a classic Kalman filter with a forced very low
hybridization frequency, this solution is possible in Inertial mode
In rotating mode, this technique is inefficient because of coupling transfers between in
plane axes. An original method was used to shape the θest/θstr transfer : a deep and wide attenuation around Fep was obtained at the price of very large digital filters
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau11/13
7) ‘A posteriori’ attitude knowledge
After every session, the attitude, angular velocity
and angular acceleration are calculated at the dates of the instrument measurements
The file is used at the SMC for the extraction of
δ process The restitution methods are still under
investigation : Once a posteriori, the measures from the 6 angular
sources (4 test masses & 2 star-trackers) can be transformed to frequential
A ‘frequency by frequency’ hybridization is then possible, keeping the best of each one of the 6 sources
An ‘expertise file’ giving the DFACS
observations and the evaluation of the DFACS
performances is also produced
Red : real time attitude performanceBlue : a posteriori attitude knowledge
performanceAbove : versus time over 20 orbitsUnder : versus frequency
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau12/13
8) Calibration modes
Some instrument parameters like ∆, Kd, θc (alignment between common SU sensitive axes and star-tracker ones) have been identified to be calibrated in-flight
The DFACS provides stimulations on the platform to increase the observability : Attitude oscillations Linear oscillations
The DFACS uses specific tunings, for example to estimate the attitude from the STR measurement at the observation frequency
K21xx, K22xxKdx, ηdy-θdy, ηdz+θdz, K2dxx
ηcy-θcy
etηcz+θcz
Kcx·∆x, Kcx·∆zméthod 2
∆yCalibrated param
noAbout Xsat, Ysat or Zsat
NonnonoLinear acceleration
stimulations
nonoabout Zsatat Fcalang
about Xsatat Fcalang
about Ysatat Fcalang
Attitude oscillationsA=0.05rad
inertialInertialInertial basedInertial basedInertial basedAttitude
XisXisXisXis, ZisXisMeasurement axis
2*f’TM2*2 10-2 Hz
fcallin1.3 10-3 Hz
fCor =fTM – fcalang=8.5 10-3 Hz
fcalang1.3 10-3 Hz
Fcalang1.3 10-3 Hz
Frequency of observation (fcalib)
EDCBASession Name
MICROSCOPE colloquium – 29&30th January 2013 - Palaiseau13/13
Conclusions
DFACS performance is credible and compatible with the overall mission budget :
While some requirements were initially set to easy figures, others were very tough. After some iterations in the frame of the performance group, the requirements have been adjusted
DFACS performance is clearly limited by sensors :
The performances of the propulsion system (accurate 6 axes control, quick response time) allow huge strain to the control loops, the resulting control errors are negligible
Looking for accuracy at low frequency (10-4Hz to 10-3Hz), we cope with low frequency errors on the star tracker associated with platform thermo-elasticity
Fortunately, the scientific instrument provides accurate angular measurements. Thanks to a complex hybridization, the angular velocity and acceleration requirements are fulfilled
Microscope, a very challenging mission: The large interweaving of DFACS and Scientific Instrument functions implies that progress in design
and performance budgets can only be achieved by a co-operative work between teams, which is very stimulating
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