Diapositivo 1

1
Design and Characterization of an attitude Sun
Sensor SSETI/ESEO
Instituto Superior Técnico Lisboa,
Portugal Pedro M. M. Rodrigues pedro.mouta_at_clix.p
t Professor Pedro M. Ramos pedro.ramos_at_lx.it.pt
2
Motivation

• SSETI Initiative / ESEO Project A student
  satellite!!
• Sun sensors are to help the control of the
  satellite.

from ESEO STRU team, Oporto, Portugal
3
Motivation - Requirements

• SSETI Initiative
• To create a network of students, educational
  institutions and organisations(on the Internet)
  to perform the distributed design, construction
  and launch of(micro)-satellites and other
  spacecraft (S/C). from ESEO
  Phase A Document
• ESEO Project
• Take pictures of the earth and moon.
• Measure radiation and test a thrust vectoring
  propulsion system.
• AOCS Subsystem
• Stabilize the spacecraft after launch.
• Provide accurate control (1º) during manoeuvres,
  for payload operation andfor sun tracking for
  the solar panels.
• Sun Sensors
• Accuracy 0.1º

4
Principle of Operation

• Attitude is the orientation of the satellite in
  orbit.
• The attitude angles are the angles between the
  Attitude Reference Frameand the Satellite
  Reference Frame.

5
Principle of Operation

• Uses sunlight direction of incidence to
  determine the attitude anglesof the spacecraft.
• Assumption Sunbeams are parallel to each other.

6
Block Diagram

• Transducer Converts sunlight energy into
  electrical current.
• Transimpedance circuit Amplifies and converts
  from current to voltage.
• Programmable Gain Equalizes the voltages.
• A/D Conversion Converts from analog to digital.
• Microcontroller Performs the calculations and
  sends the information tothe AOCS main computer
  via CAN bus.

7
Transducer

• A transducer is an electronic device that
  converts a non-electricalmeasure in an
  electrical measure.
• Two-dimension Position Sensing Detector (PSD).
• Uses silicon photodiodes to generate electrical
  currents.

from Hamamatsu S5991-E datasheet
8
Transducer Principle of operation

• Aluminium pinhole with a 400mm diameter
  aperture.
• Light hits a position of the active area and
  generates 4 currents.
• From these currents it is possible to determine
  this position and furtherthe attitude angle.

9
Transducer - Field of View (FOV)

• The maximum incidence angle that the sensor can
  measure.
• Depends on the height of the pinhole and the
  precision on the transducersurface.
• Transducer FOV of 60º x 60º gt Maximum
  precision of ?10mm.
• Total FOV per sensor 116º x 60º.

10
Signal Conditioning Circuit

• Two modules Transimpedance circuit and
  programmable gain block.
• Transimpedance block Amplifies thecurrents and
  converts them to voltage.
• Programmable gain block Places allvoltages in
  a convenient range.
• Used to prevent any inaccurateestimate of the
  sunlight power.
• Uses digital potentiometers.

11
A/D Conversion and Microcontroller

• A/D conversion converts analog voltages into
  digital voltages.
• 4-channel 12 bit A/D converters.
• Microcontroller performs the calculations,
  controls the digitalpotentiometers and
  communicates with the AOCS main computer.
• Dual-CAN microcontroller.

12
Mechanical Design

• Aluminium box with 2 transducers.

13
Characterization and Calibration

• Transducer
• Position resolution
• Position linearity
• Offset currents
• Pinhole effect eye-shaped images.
• Sensor
• Determine the systematic errors
• Compensate offset voltages and other
  non-idealities.

14
Characterization and Calibration - EGSE

• Electrical Ground Support Equipment Automatic
  measure system tocharacterize and calibrate the
  sensor.
• Main problem Simulate the Sun!
• Use of an halogen lamp to simulate the power.
• Use of lens to achieve parallel lightbeams.
• Extremely difficult to achieve the radiation
  spectrum of the Sun!
• Automatic rotating system stepper motor,
  gearing Achieve stepangle better than 0.1º.

15
Finally

• Black-box sensor light in, angles out.
• Timeline ? 6 - 8 months.

QUESTIONS?