SADA Solar Array Drive Assembly: Functional Testing and Results
The CubeSat satellite is a smart option for reliable and low cost space mission development. Growing CubeSat performances lead to more extensive nanosatellite application. Currently, Telecommunication and Earth Observation missions are under development both in single and constellation configurations. The main targets for the future nanosatellite are: accurate attitude pointing, high data rate transfer, increased power generation. The on board power/energy availability reduces or limits the CubeSat performances in terms of processing capabilities, power transmission and attitude/orbit maneuvers. Following these constraints, the IMT has developed an innovative unit, named nano-Solar Array Drive Assembly (SADA) for 3U CubeSat, with the aim of increasing the photovoltaic energy generation (up to an average 35W EOL). It is composed by two independent Solar Arrays (Wings Assembly) and Rotatory Mechanisms / Logical Unit (SAC – Solar Array Control). The aim of SADA is to align constantly the two Solar Arrays to the Sun direction, around one axis. The rotatory system is composed by drive gear sets, stepper motors and slip rings. The high value of gearhead reduction ratio and two dedicated photodiodes (as solar sensors) allow a fine pointing accuracy (<5°). Several operation modes are implemented and controlled by the On Board Computer through the I2C and CAN buses: autonomous (sun detection and pointing), slave or cooperative. An advanced and smart control algorithm was developed and implemented in the logic unit. The Solar Array points along the maximum solar flux direction, maximum output speed up to 4°/s (step size 0.004°). A system failure control avoids the thermal and power damaging in case one or both wings are blocked. SADA is fully compliant with all CubeSat form factor (3U or greater) and BUS (CSKB – CubeSat Kit Bus). The Solar Wings, during the launch phase, are stowed beside the CubeSat structure (opposite side faces). The overall thickness is less than 9 mm, compliant to ISIPOD dispenser. The Logical and Drive unit (SAC), small (90 x 90 x 12 mm) and light (185 gr), is allocated inside the satellite. The Wings are electrically connected to the SAC, by means of two 16 channels slip rings (1A per contact) for a continuous rotation, without cable saturation. The Alignment Calibration System assures that the unit runs correctly up to 10 mm of misalignment between the SAC and the geometric satellite center, along Z direction. The generated power is not handled by SAC, but by PDU through Standard Molex Connectors. The two wings, stowed during the launch phase, are deployed in orbit. In order to increase the system reliability, the deployment is based on two redundant thermal cutter systems. In the final configuration, the 3U CubeSat has two wings, each one 300 x 300 mm and 36 AzurSpace 3J solar cells.
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SADA Solar Array Drive Assembly: Functional Testing and Results
- 1. SADA SOLAR ARRAY DRIVE ASSEMBLY Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta
- 2. Company presentation Company activities Company capabilities SADA design Mechanical design Electronics design Control law design SADA functional testing Deployment test Sun tracking test Calibrations Results & Conclusions Solar Array Drive Assembly: agenda Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta 1
- 3. • Nano/Microsatellite • On-board unit • Space commercial • Space scientific • Defense • Characterization • Testing • Materials • Processes Company overview… • Equipment for space & defense • Engineering support • Product assurance • QA, software QA • Reliability, safety • Component testing private SME founded in 1991 System Engineering Parts Engineering Det Norske Veritas (DNV) UNI EN ISO 9001:2008 Quality Managment System Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta 2
- 4. Company overview: facilities… Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta RomaValenzano(BA) 13 Employees 6 Employees Main Office: Via C. B. Piazza, 30 00161 Roma - ITALY Tel.: +39-06-44292634 Fax: +39-06-44292634 Email: imtsrl@imtsrl.it URL: www.imtsrl.it Technological Lab.: c/o Tecnopolis Via per Casamassima 70010 Valenzano (BA) Tel: +39-080-4670360 Rome Valenzano (BA) 3
- 5. Company overview: our projects… Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta 4 Scientific Payload Antenna Deployment SystemBattery Pack On Board Computer Electrical Power System
- 6. Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA general architecture… • Rotational resolution of 0.004° • Sun pointing accuracy < 5° • Operational speed max 4°/s • CubeSat standard compliance • SAC thickness < 120mm • Stowed solar array < 8mm 5
- 7. Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA wings assembly… • 18x 3J solar cells • PCB rigid/flex • Yoke • Slip ring • Centering system ±10mm 6
- 8. Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA electronics design: SAC unit… 1:4096 gearhead Full-wave stepper motor control 7 RSP1 RSP2 RSP3 Right Wing Right Motor LSP1 LSP2 LSP3 Left Wing Left Motor TZA SSC Left TZA SSF Left TZA SSC Right TZA SSF Right ADC 12 bit DMA RAM H-bridge Motor stages H-bridge Motor stages Solar Array Control Unit Temperature CAN BUSI2 C MCU core SSC resolution ≈ 700nA SSF resolution ≈ 50nA
- 9. • Photodiode • Phototransistor • Sun Sensor Coarse • Sun Sensor Fine • AzurSpace 3J cells • 2.35V @ 0.5A • Average power 40Wh Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA solar wings… 3 SC1 SC2 SC3 SC4 SC5 SC6 SC7 SC8 SC9 SC10 SC11 SC12 SC13 SC14 SC15 SC16 SC17 SC18 Solar Panel 1 Solar Panel 2 Solar Panel 3 PT PD SSC SSF TEMP SP+ SP- SC1 SC2 SC3 8
- 10. • Redundant configuration • Two resistors for each Dyneema arm • 2.7A@5V for each wing • Current telemetries • High power MOSFET • Timed actuations • Optical feedback Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta Current sense VTC LTC1 LTC2 RTC1 RTC2 Left Wing ITC Current sense VTC ITC Current sense VTC ITC Current sense VTC ITC Right Wing ADC SADA thermal cutter actuators… 9
- 11. • ADC@12 bit • 750nA resolution on SSC • 50nA resolution on SSF • 2-pole Butterworth filter • Digital filtering • Multi-region sensitivity Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA sun sensors… Right SSC Left SSC Right SSF Left SSF TZA TZA TZA TZA ADC 12 bit DMA RAM CPU (control law) 10
- 12. • Each wing composed by 3 main panels • Coupled by hinges • Coupled by springs Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA control law: oscillation damping… • Use sliding window in the acquisition • Filter on comparing four measures • Computing variance value using Bessel’s correction avoid biased estimate • Control based on the Mahalanobis distance Sunsensorcurrent time 11
- 13. • Four operating areas • Slope switching levels Two rotary mechanism Two independent controls • Azimuth independent • Maximum Power PointTracking • Standard P&O algorithm Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA control law: sun sensors feedback… (a) (b) (c) (d) 12
- 14. • SSC operation • SSF sensing near 90° elevation (area a) Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA control law: algorithm… Null current on SSC (ε) SSF current greater than threshold (δL) 𝐼 𝑘 − 𝐼 𝑘−1 = Δ 𝑘 • Δk>0, same direction as previous period • Δk<0, invert direction in the next period (a) (b) SSC: slope remain almost constant (area b) 13
- 15. • Actual value stored as previous value • improve positioning (area c) • SSC current slope very tight Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA control law: algorithm… take measures from SSF same code, same routines code efficiency • Execute until solar panels reach the max • Avoid chattering (area d – smaller box) Power saving (area d – greater box) Check if the motion is convenient (c) (d) 14 !
- 16. • Dummy arrays were used: same inertia of the real solar panels same mass of the real solar panels Different stiffness of the real solar panels Avoid complex off-loading system • Deployment test (ECSS-E-ST-33-01C) • Sensors calibration routine • System calibration • Backlash measurement and compensation • Functional tests of tracking algorithm Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA laboratory functional verifications… 15
- 17. • ECSS-E-ST33-01C • Predicted nominal ground test cycles • Nominal in-orbit operation cycles • 3 expected ground deployment cycles • 1 expected deployment cycle Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA deployment test… 3𝑥4 + 1𝑥10 = 22 𝑐𝑦𝑐𝑙𝑒𝑠 16
- 18. Offsets highlighted from analog sensor (analog filtering) Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA sensor calibrations… Coarse sensor Fine sensor Left wing 2µA 20nA Right wing 1µA 500nA 17
- 19. Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA system calibration: slip ring test… • Verify the repeatability • Verify sensor response @different light irradiance • Lamps used for light illuminance • 3 cycles on each wing • 360° rotation on each cycle Slip ring conduction verify • Maximum offset of each curve ≈0.2% • Below 5° pointing specification 18
- 20. Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA system calibration: light response… Lamps used for different illumination beam Repeatability measures during illuminance variations Measures with illuminance variations Repeatability with illuminance variations 19
- 21. Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA backlash compensation… Two cycles of 360° with motion inversion 20µA before – 1.5µA after Measured backlash of 3.8° firmware compensation 20
- 22. Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA tracking algorithm verification… • A complete turn was performed • Maximum current value has been computed • Tracking algorithm executed Pointing accuracy < 5° 21
- 23. • Power transfer output: 43W max • Output torque: 25mNm • Pointing accuracy: <5° • Step size: 0.004° • Configuration: 2 channels, 2 solar arrays • Release mechanism: 2x 1.7A for each solar array • Range of motion: slip rings allow free rotation • ADC acquisition @12 bit (temperature sensing) • CubeSat standard • 3U structure • CAN BUS • I2C • Power consumption: • Idle 160mA • Holding 430mA • Tracking 460mA • Mass: • SAC 185gr • WING 331gr without solar cells • TOTAL 850gr • 2 sun detection, fine and coarse for each wing • 4 independentTZA stages • 2 poles analog filtering • Digital damping filters • Digital measure filters on 16 samples Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta SADA characteristics… 22
- 24. Thank you!!! Questions Michele Marino – IMT srl Rome,Italy / ESA 4S Symposium 2016, La Valletta - Malta