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Autor: Agustín García Sáez
Director: José Manuel Quero Reboul
1. Introduction
2. State of the art
3. Objectives and motivation
4. Theoretical study of the sensor
5. Prototype manufacturing
6. Evaluation
7. Conclusions
Table of contents
Collaboration between MEMS research
group and Instituto Nacional de Técnica
Aeroespacial (INTA) for development of
payloads
Idea of studying of an Earth sensor for
integration into the Microsat mission
Financial support by group own funds
1. Introduction
Earth’s
magnetic field
North
magnetic pole
South
magnetic pole
Sun
Magnetic
vector
Solar
vectorNadir
vector
Vast progress in the space industry and development of small satellites
Need for more integration of attitude control instruments with high accuracy
New concept of Earth sensor based in Earth thermal emission
1. Introduction
2. State of the art 2.1. Attitude control sensors
2.2. Earth sensors
2.3. Simulation tools
2.4. State of art conclusions
3. Objectives and motivation
4. Theoretical study of the sensor
5. Prototype manufacturing
6. Evaluation
7. Conclusions
Table of contents
2.1. Atittude control sensors
Many technologies related to attitude controlling:
Sun sensors: High accuracy but need of the Sun in fov.
Star trackers: High accuracy but complex and expensive devices.
Magnetometers: Difficulty to sense Earth‟s magnetic field with high accuracy.
Earth sensors: Can reach good accuracy. Simple electronics and processing
software. Main technologies: quantum, pyroelectric and thermopile detectors.
MAG-30 magnetometer
by Satrec Initiative
NanoSSOC-D60 Sun
sensor by Solar MEMS
TDP 6 Star Tracker of
Alphasat mission
Jain, Y.K.; Alex, T.K.; Kalakrishnan, B. “Ultimate IR Horizon Sensor”,
IEEE transactions on Aerospace and Electronics System, 1980, Vol.
AES-16, Issue 2, pp. 233-238.
3σ error: 0.022 º
Weight: 4 kg
W.G. Krigbaum, Shu-Jone Lee, A.Y. Okamoto, G.T Sakoda,
“Microbolometer earth sensor assembly”, U.S. Patent 6026337 A, Feb. 15,
2000.
3σ error: 0.137 º
Weight: no data
C. Hersom, Z. Afik, A. Hollinger, P.J. Thomas, “Satellite attitude sensor
using thermal imaging”, U.S. Patent 6066850 A, May. 23, 2000.
3σ error: 0.05 º
Weight: no data
Based on radiant balance on the device
Output signal very small and overshadowed
by noise generated internally → cooling
problem
2.1. Earth sensors: Quantum detectors
Crystalline material: electric charge moving when temperature gradient
Based on Earth – background space transition detection → generation of a
negative/positive voltage pulse
Scanning → moving parts
2.2. Earth sensors: Pyroelectric detectors
𝑑𝑇
𝑑𝑡> 0
2.2. Earth sensors: Pyroelectric detectors
Sensor Mass Power
consumption
Accuracy
(3σ error)
MiDES Servo
Corporation
of America
1.5 kg 0.8 W ±0.04°
Earth
sensor
by SSBV
0.5 kg
plus optics &
mounting feet
1.5 W ±0.2°
IRES
SELEX
Galileo
2.5 kg 4 W ±0.05°
STD16
EADS
SODERN
3.5 kg 7.5 W ±0.06°
Based on radiant balance on the device.
Seebeck effect : p-n union with different
temperature junctions produces a voltage
Dependence of temperature. Must be
taken into account.
V = S · ( Thot - Tcold )
V : Output voltage
S : Seebeck coefficient
Thot , Tcold : Temperature of the junctions
2.2. Earth sensors: Thermopile detectors
2.2. Earth sensors: Thermopile detectors
Sensor Mass Power
consumption
Accuracy
(3σ error)
MAI-SES Maryland
Aerospace
66 g
plus enclosure 0.264 W
±0.25°
(resolution)
IRES-C SELEX
Galileo
1.5 kg 3.3 W ±1° (LEO)
±0.36° (GEO)
PSSCT-2
pico-satellite /
STS-135
mission
No data No data ±0.5°
Siegfried W. Janson, Brian S. Hardy, Andrew Y. Chin, Daniel L. Rumsey, Daniel A. Ehrlich and
David A. Hinkley, “Attitude Control on the Pico Satellite Solar Cell Testbed-2 ”, The Aerospace
Corporation.
N. Scheidegger, R. Krpoun, H. Shea, C. Niclass and E. Charbon, “A new Concept for a Low-Cost
Earth Sensor: Imaging Oxygen Airglow with Arrays of Single Photon Detectors”, 30th annual AAS
guidance and control conference. February 3-7, 2007.
Ghose, K. ; Shea, H. , “A micromachined Earth sensor based on measuring the gravity gradient
torque”, ASME 2009 International Mechanical Engineering Congress & Exposition, Lake Buena
Vista, Florida, USA. November 13-19, 2009.
2.2. Earth sensors: Other technologies
Sensor Mass Power
consumption
Accuracy
(3σ error)
Earth Sensor
based on imaging
oxygen airglow
(CMOS camera)
No data No data LEO: ±7.5°
GEO: ±0.9°
Inertial MEMS
Earth sensor 0.5 kg 5 W ±5°
None simulation model useful to the
proposed sensor
Need to develop our own modeling
2.3. Modeling and simulation tools
Simulation of the
airglow
measurement by
the pixel array of
the CMOS sensor
N. Scheidegger, R. Krpoun, H. Shea, C. Niclass and E.
Charbon, “A new Concept for a Low-Cost Earth Sensor:
Imaging Oxygen Airglow with Arrays of Single Photon
Detectors”, 30th annual AAS guidance and control conference.
February 3-7, 2007.
Jain, Y.K.; Alex, T.K.; Kalakrishnan, B. “Ultimate IR Horizon
Sensor”, IEEE transactions on Aerospace and Electronics
System, 1980, Vol. AES-16, Issue 2, pp. 233-238.
J. Rogers, M. Costello, “Low Cost Orientation Estimator Smart
Projectiles using Magnetometers and Thermopiles”, Journal of
the Institute of Navigation, Vol. 59, pp. 9-24, Issue 1, 2012. Thermopile geometry simulation
and electrical behavior
Many technologies used for Earth sensors, most of them based
in Earth thermal emission
Different kinds of thermal detectors:
Scanning (moving parts): Pyroelectric
Static: Quantum and Thermopile
Simulation model needed for studying and adjusting of the
sensor parameters
None simulation model useful to the new concept of Earth
sensor proposed
2.4. State of art conclusions
1. Introduction
2. State of the art
3. Objectives and motivation
4. Theoretical study of the sensor
5. Prototype manufacturing
6. Evaluation
7. Conclusions
Table of contents
3. Objectives and motivation
Earth sensor for integration in
nano and pico-satellites
Good integration, high accuracy
and low power consumption
Study and modeling of a new
concept of Earth sensor consisting
on thermopile detectors with
integrated optics
Microsat mission requirements:
LEO: 700 km
3σ error <0.2°
Nominal temperature range: 15 - 50°C
Detector model
Validation tests
Optics
model
Electronics
model
Radiant balance
model on sensor
Detector-optics model
Validation tests
Detector-optics behavior
model in flight conditions
Adjusting electronics
Detector-optics-electronics behavior model
Sensor behavior model in flight conditions
Sensor manufacturing
Evaluation and environmental tests
1. Introduction
2. State of the art
3. Objectives and motivation
4. Theoretical study of the sensor 4.1. Sensor general description
4.2. Behavior models and validation
4.3 Thermal analysis
4.4 Pointing error simulations
4.5 Theoretical study conclusions
5. Prototype manufacturing
6. Evaluation
7. Conclusions
Table of contents
4. Theoretical study of the sensor
Earth sensor consisting on thermopile detectors with integrated optics
Four thermal detectors orthogonally distributed
Nadir pointing: each detector around 50% of Earth / background space
Cells work in pairs (differential scheme) to achieve dual-axes pointing
Cell 3
FOV
Earth
Cell 1
FOV
Cell 4
FOV
Cell 2
FOV
β (pitch)
α (roll)
Thermal
targetOptics
Signal
conditioning
Thermopile
Thermopile
output (V)
Thermistor
output (Ω)
Thermistor
Detector
Output
signal (V)
Radiant
balance
(W/m2)
Radiant
balance
(W/m2)
IR sensor ZTP-135SR by GE Sensing (integrated thermistor )
Focusing system composed of germanium lenses
Optics optimized in the 14–16 µm range (no cloud thermal effects )
Electronic design to get 0-5 V output signals
4.1. Sensor general description
Radiation balance thermopile - thermal target (parallel flat surfaces):
Thermopile output voltage:
Theoretical/experimental thermopile behavior
4.2. Behavior models and validation: Thermopile
, , · · , ,max
mínNet Target Target Tp Target Target Tpq T T Tr E T E T d
· ·Tp Active Thermopile NetV A S q
Positive meniscus geometry: low focal length
Uncoated surface: avoid transmission changes due to aging by
irradiation
Employment optics provides a more narrow
fov in comparison with a no-lens design
Germanium lens:
Good transmission in 14-16 µm band (40% – 45% )
Greater rejection of unwanted wavelengths
Good properties of toughness and strength
As a semiconductor, it provides some electrical
and irradiation shielding to the detector Optical system representation
4.2. Behavior models and validation: Optics
Goal: calculation of the response of
optics-detector set
Minimum spot size determined by
spherical aberration and diffraction
effects
Third-order aberration theory
concludes that aberration effects
prevail → minimum spot on the
focusing plane
Effect of spherical
aberration
Airy disc due to
diffraction effects
Minimum spot on the focusing plane as
function of wavelength
4.2. Behavior models and validation: Optics
Proportion of the spot within the
active area → Ratio
Optimized for 14 -16 µm band
Spectral sensitivity given by:
Experimental fov of ±2.5°
Geometrical analysis as
function of the spot on the
active area of the thermopile
Proportion of the spot on the
thermopile as function of wavelength
4.2. Behavior models and validation: Optics
· ·Tp GeSens Tr Ratio Tr
Thermal sources involved in the balance:
Sensor: 𝜀~1 (Kirchhoff's law of thermal radiation)
Background space: 𝜀=1 / 𝑇𝑆𝑝𝑎𝑐𝑒= 3 K
Earth: 𝜀=0.612 / 𝑇𝐸𝑎𝑟𝑡ℎ= 288.15 K / Spherical surface
Earth thermal emission: surface discretization + divergence theorem
Detail of the Earth surface discretization Detail of the fov of a cell
4.2. Behavior models and validation: Radiant balance model
2D representation of the fov of a cell in
case of 𝛼 = 𝛽 = 0° obtained with MATLAB
4.2. Behavior models and validation: Radiant balance model
max
min
max
1 min
2
, , , · · , ,
, , · · · · , ,n
Net Sensor Sensor Space Space Sensor
AEarth
Sensor Earth Earth Earth SensorA
Earth
q T F Sens E T E T d
RF A Sens E T E T d dA
R d A
Geometrical study: background space and
Earth view factor as function of 𝛼 and 𝛽.
Thermal study: radiant balance modeling
as function of 𝑇𝑆𝑒𝑛𝑠𝑜𝑟.
Equation as function of 𝛼, 𝛽 and 𝑇𝑆𝑒𝑛𝑠𝑜𝑟:
1st term: sensor – background space balance
2nd term: sensor – Earth balance
Balance at 𝑇𝑆𝑒𝑛𝑠𝑜𝑟=25ºC as function of 𝛼 and 𝛽 Balance as function of 𝛽 and 𝑇𝑆𝑒𝑛𝑠𝑜𝑟
Results of radiant balance equation:
Negligible 𝛼 dependence
Great temperature dependence
Goal: minimize temperature dependence
4.2. Behavior models and validation: Radiant balance model
First gain
stage (G1)
+
-
Sensor package
VTp
+
-+
Second
gain stage
(G2)
Thermal
compensation
network
VCell
RThermistorVComp
Thermopile output voltage
Electronics output voltage
Signal amplification and thermal
compensation
Worst Case Circuit Analysis (WCCA)
Microcontroller COTS (calibration
tables, SPI communication) and
thermistor for temperature monitoring Structure of one cell adaptation
4.2. Behavior models and validation: Signal conditioning
Mathematical tool for 𝛼, 𝛽 calculation
Physical meaning: comparison of the area of the Earth within the FOV in complementary cells
Normalized functions
Monotonically increase → inverse functions F can be calculated
3 4
2
3 4
, , , ,,
, , , , 2·
Cell Cell Cell Cell
Cell Cell Cell Cell off Cell
V T V Tf
V T V T V T
1 2
1
1 2
, , , ,,
, , , , 2·
Cell Cell Cell Cell
Cell Cell Cell Cell off Cell
V T V Tf
V T V T V T
Simulated 𝑓1 and 𝑓2 functions
4.2. Behavior models and validation: f functions
F functions allow obtain the pointing
angles from the cell measurements
Needed a microcontroller to storage the
F functions and to do the calculations
Extraction flow of the pointing angles:
Simulated 𝐹1 and 𝐹2 functions
1
2 1 exp 1Real
Real 3 2 exp 2
4
Cell
Cell erimental calibration Out
Cell erimental calibration Out
Cell
V
V f F
V f F
V
4.2. Behavior models and validation: f functions
Considerations:
Thermal control with an imposed base
temperature of 25°C
Low emissivity surface treatment (𝜀=0.1):
lower heat transfer to the environment
Heat flux and temperature distribution
with/without Sun radiation
Simulation results:
Maximum difference in complementary cells with Sun radiation: 1°C
Maximum difference in complementary cells in darkness: 0.2°C
Thermal simulation carried out with Comsol
4.3. Behavior models and validation: Thermal analysis
Earth thermal parameters homogeneous:
Thermal gradients in the cells + WCCA → nadir: ±0.006° / FOV limits: ±0.07°
Variation of Earth thermal parameters:
Emissivity variations of the Earth (0.612±0.05) → nadir: ±0.049° max
Thermal variations of the Earth (15±5°C) → nadir: ±0.042° max
Pointing error analysis for geostationary orbit (thermal gradients in
the cells and WCCA) → nadir: ±0.01° / FOV limits: ±0.2°
ADC influence (quantization error) → ±0.003° @ 12 bits, 𝑇𝑆𝑒𝑛𝑠𝑜𝑟=25°C
4.4. Behavior models and validation: Pointing error simulations
Differential scheme: canceling out common mode effects
Complete behavioral model developed → Adjusting of optics and
electronics for best results
Flexible model adaptable to different geometrical and thermal
conditions
Optics provided by germanium lens:
Fine FOV → Higher sensitivity/accuracy: nadir: ±0.006° / FOV limits: ±0.07°
Lower probability of interference with Sun/Moon (𝑃=0.004).
Thermal study: Heat flux and temperature gradients in the box
Pointing error characterization under non-ideal conditions
4.5. Theoretical study conclusions
1. Introduction
2. State of the art
3. Objectives and motivation
4. Theoretical study of the sensor
5. Prototype manufacturing 5.1. Electrical interface
5.2. Sensor integration
5.3. Prototype manufacturing conclusions
6. Evaluation
7. Conclusions
Table of contents
5. Prototype manufacturing
Mechanical design carried out with Catia V5
Constraints: PCB size, lenses diameter and focal length
#Pin Reference Description
1 CLK SPI CLK
2 TERM- Thermistor negative pin
3 SENS4 Telemetry – Thermopile 4
4 SENS1 Telemetry – Thermopile 1
5 SENS2 Telemetry – Thermopile 2
6 RX SPI Rx
7 TX SPI Tx
8 TERM+ Thermistor positive pin
9 SENS3 Telemetry – Thermopile 3
10 GND Telemetry GND
11 SEL SPI selection signal
12 GND Digital GND
13 VPP Microcontroller programming
14 GND Power GND
15 VCC 5V Power supply
Female D-SUB HD 15-way
connector
Group of signals:
Power supply
Internal thermistor measurement
Cell telemetries
Microcontroller programming
and SPI communication
5.1. Electrical interface
The optical detector block
includes:
Thermopile detector
Teflon cap
Focuser tube
Germanium lens
Toric gasket
Top of focuser tube
Section view of the optics
5.2. Sensor integration: Optics
Thermopile detector, Teflon cap and
focuser tube bonded with EPO-TEK
353ND epoxy adhesive
Focuser tube coiled and bonded on
the main body with epoxy adhesive
5.2. Sensor integration: Optics
Signal conditioning electronics
COTS Microcontroller (calibration tables, SPI communication)
Integrated thermistor
Female D-SUB HD 15-way connector integrated into the PCB
5.2. Sensor integration: Electronics
Germanium lens, toric gasket
and top of focuser tube
mounted on the focuser tube
PCB wired to the thermopile
detectors
Vent-holes to provide a way to
decompress the sensor cavities
5.2. Sensor integration: Final assembly
Final assembly of the Earth sensor
Manufacturing of the Earth sensor prototype based on the
optimization algorithms and simulation results
Taken into account ECSS normative
Employment of space compatible materials (teflon, silicone,
aluminum 7075 compliant and adhesive for space applications)
Low mass (246 g) and power consumption (125 mW)
5.2. Prototype manufacturing conclusions
1. Introduction
2. State of the art
3. Objectives and motivation
4. Theoretical study of the sensor
5. Prototype manufacturing
6. Evaluation 6.1 Evaluation
6.2. Environmental tests
6.3. In-flight calibration procedure
6.4. Evaluation conclusions
7. Conclusions
Table of contents
Test bench includes:
Stepper motor positioner
Hot and cold thermal sources to emulate the Earth and background space
Thermal gradient: 25°C versus 285°C in-flight conditions
3σ error 0.176° (expected improvement in-flight thermal conditions)
Test bench for sensor characterization
6.1. Evaluation
Calibration
f functions
Calibration
F functions
and random
evaluation
points
6.1. Evaluation
Thermal compensation test:
Discrepancy of 1.03% between
theoretical model and test
Taken into account in the theoretical
error calculation.
Correction through characterization
Vacuum test at 10−5 mbar carried
out in climatic chambers of INTA →
No behavior change
6.2. Environmental tests
Sensor compensation test with thermal
target at 25°C and 𝜀=0.9
Irradiation:
High energy protons (10 MeV)
Reference value test of space solar cells
Predominant radiation in LEO
Results:
Lost of thermopile sensitivity
Voltage offset in thermopile
Integrated thermistor out of nominal
values at temperatures below to -10°C
TID [krad] 6 32 65 650
Offset [µV] 0 40 160 190
Sensitivity
factor 0.95 0.9 0.85 0.8
Degradation of the f function by irradiation
Cyclotron (Centro Nacional de Aceleradores)
6.2. Environmental tests
Recommended in commissioning phase to compensate for any
deviation of the environmental parameters
Consist on measure the signal generated by cells with 100%
background space in its FOV
Mitigate deviation effects such as aging by irradiation
Limitation: calculation of angles not possible when saturated areas
6.3. Evaluation: In-flight recalibration procedure
Environmental tests:
Thermal compensation
Vacuum
Aging by irradiation
Apt results for space qualification
Pending tests:
Thermal shock
Vibration
6.3. Evaluation: conclusions
1. Introduction
2. State of the art
3. Objectives and motivation
4. Theoretical study of the sensor
5. Prototype manufacturing
6. Evaluation
7. Conclusions 7.1. Contributions
7.2. Comparison with other sensors
7.3. Future work
Table of contents
Study, design and manufacturing of a new concept of Earth sensor
with thermopile and integrated optics
Optics provides lower FOV:
Higher sensitivity and accuracy
Interference reduction with the Earth and Moon
Developing of a model behaviour of the sensor
Structure that allows the integration in nano and pico-satellites
Technology transfer to the company Solar MEMS
Pending embarking:
Microsat mission (Insituto Nacional de Técnica Aeroespacial - INTA)
Recent negotiations with Instituto de Microgravedad “Ignacio Da Riva”
7.1. Contributions
7.2. Comparison between sensors
Parameters taken into account: accuracy, mass and power consumption
Mobile parts sensors („o‟), Static sensors („*‟)
Sensor Accuracy
[3σ]
Mass
[kg]
Power
[W]
MIDES 0.04 1.5 0.8
SSBV 0.2 1 0.5
STD16 0.06 3.5 7.5
IRES 0.05 2.5 4.5
IRES-C 0.36 1.3 3
MAI-SES 0.25 0.066 0.264
mES 0.07 0.246 0.125
Deeper study of the optics to get a tighter bandwidth centered in the
14 -16 µm band
Mechanical study for the integration in nano and pico-satellites
Improvement of the test bench:
Greater thermal gradient between hot and cold surfaces
Calibration with the four cells
Final validation in Microsat mission (INTA)
Use in industrial applications for tracking and distance measurement
7.3. Future work
A. G. Sáez, J. M. Quero, M. Angulo, “Earth Sensor Based on
Thermopile Detectors for Satellite Attitude Determination”, Sensors
Journal, IEEE, Dec. 2015, Vol. 16, Issue 8, pp. 2260-2271.
A. G. Sáez, J. M. Quero, M. Angulo. “Earth thermal modeling for
infrared optical instruments of horizon sensors”, 9th IAA
Symposium on Small Satellites for Earth Observation, April 8-12
2013. Berlin, Germany.
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