Bachelor

11/18/2013
Ref.:Andrea Testore
83230910-DOC-TAS-IT-002
SHM based on Ḃ control-law
in near polar orbit
Relatori:
Lorenzo Casalino
Carlo Novara
Candidato:
Andrea Testore

Il documento non deve essere riprodotto, modificato, adattato, pubblicato, tradotto in qualsiasi forma sostanziale, in tutto o in parte,
né divulgato a terze parti senza il preventivo consenso scritto di Thales Alenia Space - © 2013, Thales Alenia Space
12/02/2013
Ref.: Andrea Testore
Safe Hold Mode (SHM) emergency operation mode in case of
failure
Fundamental for satellite reliability, aim: satellite preservation
2
Introduction
To keep positive power balance
Turn off all non-essential devices
To keep correct Sun pointing of solar panels

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Introduction
Evaluate Ḃ based SHM
performance for optical satellite
3
Scenario characteristics
Satellite with quite symmetric
mass distribution
Orbit: 14h SSO
Task
98°

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Contents
Satellite plant definition
Ḃ control law
Safe Hold Mode logic
Performance analysis
Conclusion
4
x
y
z

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Contents
Ḃ control law
Conclusion
5
input output

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6
Conceptual satellite plant
Output
SV, B, Tecl, ...
Input
• Fe
• Me
• MGT
• RW
PLANT
Control
• P
• V
• ω
• qStatus:

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Contents
7
Ḃ control law
Hardware
Traditional
Forced
Conclusion

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BMGT influences MGM measurement.
8
Hardware
Activation NOT concurrent
MGM
MGT
t
MGT
transient
MGM
measurement
MGT
activation
MGT
transient
20s
MGT current

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9
Traditional ḂTraditional ḂTraditional Ḃ
n° Satellite
Initial BRF angular
rate (deg/s)
Initial Attitude Orbit inclination RAAN Initial Polarization GG J2 B_dot Saturation
25a Compact [-0.3 0 0] [0 0 90] 98.112 247.66499 [0 0 0] ON ON ON OFF
0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
x 10
4
−6
−5
−4
−3
−2
−1
0
1
2
3
x 10
−3
time t (s)
Ws
c(rad/s)
Wxsc
Wysc
Wzsc
MMGT: Torque generated by MGT
K: Constant
B: Geomagnetic field
ωSC: Spacecraft angular rate
ωB: Geomagnetic rate of change
MMGT=-K·B×(ωSC - ωB)×B
Torque
ωSC=ωB

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10
0 1 2 3 4 5 6
x 10
4
−2
−1.5
−1
−0.5
0
0.5
1
1.5
x 10
−3
time t (s)
W
s
c(rad/s)
Wxsc
Wysc
Wzsc
n° Satellite
Initial BRF
angular rate
Initial Attitude Orbit inclination RAAN Initial Polarization GG J2 B_dot Saturation
29a Compact [0 0 0 ] [0 0 0] 98.112 247.66499 [-10 0 0] ON ON -ω₀x OFF
Forced Ḃ
Possibility to impose
desired angular rate (i.e.
orbital rate)
Characteristics
MMGT=-K·[B×(ωSC-ωB)+B×(ωB-ωo)]×B
Torque

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11
Contents
Ḃ control law
Submode 1
Submode 2
Submode 3
Submode 4
Conclusion

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12
Safe Hold Mode
Submode 1: Detumbling
Submode 2: Sun Search
Submode 3: Sun Pointing
Sun Pointing/Eclipse loop (SP/E)
Submode 4: Eclipse

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13
Submode 1: Detumbling
Actions
Traditional Ḃ control law
Reaction wheel polarization
SV: Sun vector
êpol
∟
↻
Slow down spacecraft body ①
Align panels axis ⊥ to SV ②
Aims
②①

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14
Submode 2: Sun Search
Forced Ḃ (2.5*ωo) aorund êpolScan all Sun directions
ω0: Orbital rate
Aims Actions
êpol
∟
∟
↻

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15
Submode 3: Sun Pointing
①
②
Keep angle between Sun vector
an solar panel constant
Aims Actions
SVx
y
x
x
y
x
Impose:
SVy = 1 ①
SVx = 0 ②
y
y

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16
Submode 4: Eclipse
Forced Ḃ (-1.5*ωo)Avoid losing Sun direction
Aims Actions

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Contents
17
Hardware
Ḃ control law
Worst case
High initial angular rate
Conclusion
α
SV

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Worst case
18
n° Satellite
Initial BRF angular
rate (deg/s)
Initial Attitude i Ω Initial Polarization GG J2 B_dot Saturation
9 Compact [0 2 0] [-21.81 -106.25 122.74] 98.112 30 [0 0 0] ON ON ON ON
Characteristics
0 0.5 1 1.5 2 2.5
x 10
4
0
20
40
60
80
100
120
140
160
180
X: 0
Y: 179.6
X: 1.604e+04
Y: 7.626
time t (s)
Halfconey
S
C(°)
sun−presence−sensor
half cone Solar Panel −YSC
0 0.5 1 1.5 2 2.5 3
x 10
5
0
20
40
60
80
100
120
140
160
180
time t (s)
Halfconey
S
C(°)
half cone Solar Panel −YSC
Worst SC orientation
Time until stabilization (3 orbits)
SP/E loop efficiency

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19
Worst case
n° Satellite
Initial BRF angular
rate (deg/s)
9 Compact [0 2 0] [-21.81 -106.25 122.74] 98.112 30 [0 0 0] ON ON ON ON
0 0.5 1 1.5 2 2.5 3 3.5 4
x 10
4
−300
−200
−100
0
100
200
300
time t (s)
DipolemomentvectorCMDAm
2
mxCMD
myCMD
mzCMD
0 0.5 1 1.5 2 2.5 3 3.5
x 10
4
1
1.5
2
2.5
3
3.5
4
time (t) s
submodes
Different MGTs effort
Characteristics

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20
0 0.5 1 1.5 2 2.5 3
x 10
5
−0.15
−0.1
−0.05
0
0.05
0.1
0.15
0.2
0.25
time t (s)
WSCvector(rad/s)
Wx
SC
Wy
SC
Wz
SC
n° Satellite
Initial BRF angular
rate (deg/s)
9 Compact [0 4.5 0] [0 0 0] 98.112 30 [0 0 0] ON ON ON ON
Characteristics
0 0.5 1 1.5 2 2.5 3
x 10
5
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
time (t) s
submodes
Ineffective detumbling
New stability condition

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21
n° Satellite
Initial BRF angular
rate (deg/s)
B field value seen
B field value seen
0 0.5 1 1.5 2 2.5 3
x 10
5
−0.15
−0.1
−0.05
0
0.05
0.1
0.15
0.2
0.25
time t (s)
WSCvector(rad/s)
Wx
SC
Wy
SC
Wz
SC
MGT activation in quadrature
conditions
4.5 deg/s
>13.5 deg/s (0.24 rad/s)
Characteristics

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22
0 50 100 150 200 250 300 350 400
−0.08
−0.06
−0.04
−0.02
0
0.02
0.04
0.06
0.08
time (t) s
x component applied by Bdot algorithm
z component applied by Bdot algorithm
x component of B field derivative
z component of B field derivative
n° Satellite
Initial BRF angular
rate (deg/s)
MGT activation in quadrature
conditions
Characteristics
0 200 400 600 800 1000 1200 1400 1600 1800
−0.02
−0.015
−0.01
−0.005
0
0.005
0.01
0.015
0.02
time (t) s
x component applied by Bdot algorithm
z component applied by Bdot algorithm
x component of B field derivative
z component of B field derivative

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Conclusions
Very effective for different initial attitude with moderate angular rate
Ineffective for quadrature phenomenon with high angular rate
23
Possible solution
Reduce measurement/activation period

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24

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