ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Bi-Axial Solar Array Drive Mechanism: Design, Build and Environmental Testing Noémy Scheidegger*, Mark Ferris* and Nigel Phillips* Abstract The development of the Bi-Axial Solar Array Drive Mechanism (BSADM) presented in this paper is a  demonstration of SSTL’s unique space manufacturing approach that enables performing rapid development cycles for cost-effective products that meet ever-challenging mission requirements: The  BSADM is designed to orient a solar array wing towards the sun, using its first rotation axis to track the  sun, and its second rotation axis to compensate for the satellite orbit and attitude changes needed for a successful payload operation. The tight development schedule, with manufacture of 7 Flight Models within 1.5 year after kick-off, is offset by the risk-reduction of using qualified key component-families from  other proven SSTL mechanisms. This allowed focusing the BSADM design activities on the mechanism  features that are unique to the BSADM, and having an Engineering Qualification Model (EQM) built 8 months after kick-off. The EQM is currently undergoing a full environmental qualification test campaign. This paper presents the BSADM design approach that enabled meeting such a challenging schedule, its  design particularities, and the ongoing verification activities. Figure 1.  BSADM Track  Axis AssemblyFigure 2. BSADM Trim  Axis Assembly Introduction Surrey Satellite Technology Ltd (SSTL) is a key supplier of small satellites based near London (United  Kingdom) providing complete in-house design, manufacture, launch and operation of small satellites.SSTL also has an office and Assembly Integration and Test facilities based in Colorado in the U.S.,SSTL-US. Heritage designs, commercial off-the-shelf technology, combined with a common sense and  pragmatic approach to manufacture and low-cost operations enable SSTL to ensure that program economics are kept as low as realistically possible. The SSTL development approach focuses on the experience gained from previous missions. Extensive portions of new projects are evolved from flight-proven design, enabling SSTL to provide custom-designed solutions with high confidence founded on inorbit performance. Satellite capabilities improve in line with technology developments, allowing the SSTL  satellites to fulfill ever-challenging mission objectives. SSTL has an experienced mechanisms skillset;  proven by the mechanisms successfully operating in orbit including reaction wheels (with both dry and  wet lubrication), Antenna Pointing Mechanisms (APM), imager focusing mechanisms, solar array hold  down and release systems (including hinges), and a variety of optical scanning mechanisms. SSTL has  *Surrey Satellite Technology Ltd, Surrey, United Kingdom /g51/g85/g82/g70/g72/g72/g71/g76/g81/g74/g86/g3/g82/g73/g3/g87/g75/g72/g3/g23/g21/g81/g71/g3/g36/g72/g85/g82/g86/g83/g68/g70/g72/g3/g48/g72/g70/g75/g68/g81/g76/g86/g80/g86/g3/g54/g92/g80/g83/g82/g86/g76/g88/g80/g15/g3/g49/g36/g54/g36/g3/g42/g82/g71/g71/g68/g85 /g71/g3/g54/g83/g68/g70/g72/g3/g41/g79/g76/g74/g75/g87/g3/g38/g72/g81/g87/g72/g85/g15/g3/g48/g68/g92/g3/g20/g23/g16/g20/g25/g15/g3/g21/g19/g20/g23/g3 153
now extended its mechanism’s product range and developed a Bi-Axial Solar Array Drive Mechanism  (BSADM) for advanced Low Earth Orbit missions.   Heritage The first solar array drive mechanism engineering model developed by SSTL - the SADM-Twist - is based on the APM’s azimuth axis (illustrated in Figure 3), and mainly consists of a stepper motor with an integrated planetary gear box driving a spur gear transmission assembly to rotate the central shaft, which  is supported by a duplex bearing. Magnetic encoders are used for position feedback. Like the APM, the  SADM-Twist has a flexible printed circuit board (flexi-PCB), which is coiled up inside the large-diameter bearings and allows transmitting power and telemetry across the rotation axis. The APM’s baseline flexi-PCB was scaled up for the SADM-Twist, to include 20 power lines (rated at 1.5 A), 6 signal lines (rated at 0.5 A) and 5 sections. This allowed the SADM-Twist to transfer 300 W from its rotating part to its stationary part. While the flexi-PCB provides a cost-effective solution, it does have limited rotation range and power handling capabilities – the latter influenced by track sizing and associated stiffness/bending effects over life. SSTL qualified the low-power SADM-Twist over a 350° movement range to 88000  cycles, at which point the flexi-PCB tracks started to degrade. While this proven life was far superior to the requirement of 36000 orbit cycles, it did highlight a limitation to the power-transfer capability of the flexi-PCB technology. Figure 3. Left: APM, Right: SADM-Twist The higher power requirement for the new SADM development and the need for continual rotation forced  the replacement of the flexi-PCB with a more conventional slip ring. In addition to that, the SADM had to be equipped with a second rotation axis to cope with regular satellite orbit and attitude changes. These considerations were the main drivers for the enhancement of the SADM-Twist design leading to the BiAxial Solar Array Drive Mechanism (BSADM) development presented in this paper. The modular nature of  SSTL’s mechanisms allowed using qualified components for most of the BSADM design to retain heritage and reduce risk: • The track / trim axis bearings are from the same family as the APMs bearings • The track / trim axis stepper motor and gearbox are from the same family as used within the APMs  and the Imager Focus Mechanisms • The spur gear transmission is based on the design used within the APMs and the Imager Focus  Mechanisms • The BSADM is commanded by a Bi-Axial Solar Array Drive Electronics, which is based on the APM  drive electronics. Requirements The BSADM key requirements are detailed in Table 1. The BSADM has furthermore to provide full internal electrical redundancy, position feedback, and the capability to sustain a solar array deployment  moment of 50 N-m. In addition, the BSADM had to be modular in design such that the tracking axis can  exist as an entity in its own right (without trim axis) for use as a conventional tracking SADM. 154
Table 1. BSADA Requirements Specification Operation Characteristics Track Axis Trim Axis Motion Range Unlimited continuous rotation ±60° Rotation Speed < 2°/s < 2°/s Position Accuracy Absolute: ±3°  Relative: < 0.01°Absolute: ±3°  Relative: < 0.01° In-Orbit Duty 30800 rotations of 360° 675 sweeps of ±60° Qualification Cycles 64000 rotations of 360° 3160 sweeps of ±60° Physical Properties BSADM BSADM Mass < 6 kg Volume Diameter 150 x 150 mm Power/Signal Transfer Number of Circuits 60 @ 1.6 A max Voltage Nom. 32 V Operation Characteristics Mission Life 5.5 years Temperature Range Operational: -30 to  +60°C Non-Operational: -40 to +80°C Motor Power Consumption 1.6 W (Nominal Operation) Design The Bi-Axial Solar Array Drive Mechanism includes two rotation axis assemblies as illustrated in Figure 4: The lower axis assembly consists of a traditional SADM and is responsible for continual tracking of the sun. The upper axis (hinge) is responsible for the array trimming to compensate the satellite orbit and attitude changes needed for a correct payload operation. Both rotation axis assemblies are characterized  by • A stepper motor generating the torque needed for the axis rotation• A planetary gear box and a spur gear that transmit and amplify the motor torque  • Angular contact bearings to support the rotation axis, lubricated with Maplub pf101A  • A redundant potentiometer that generates an analog signal between 0 to 5 V, proportional to the  absolute angular position of the rotation axis.  The drive electronics shown in Figure 4 are from the APM housed in a standard module tray preventing  the need for any non-recurring engineering. In addition to these ‘standard’ mechanism features, there are some particularities in the BSADM design as presented hereafter. 155
Figure 4. BSADM Design Overview Angular Range Lock During launch, the solar array will be folded and the BSADM hinge oriented perpendicular to the satellite  surface panel as shown in Figure 5.a. Once the solar array has been deployed, the hinge will be rotated towards its nominal operation range which is between +60°/-70°. An angular range lock has been  implemented on the hinge rotation axis to prevent the hinge (and the solar array) to exceed this operation range. This is particularly important as the solar array might collide with other satellite instruments if the  track axis was rotated while the hinge is positioned outside this range. a. Disengaged During Launch b. Enga ged During Nominal Operation Figure 5.  Angular Range Lock Deployment Lock Under conventional circumstances, without damping, the solar array wing used for the SSTL satellite  would bounce back after deployment and come to a rest at an unknown position. A back-driving torque of 50 N-m is needed during the deployment to reasonably limit this solar array wing back-bouncing. In order to accommodate this requirement within a compact and lightweight product, an additional locking mechanism has been incorporated into the hinge assembly. The solar array deployment lock operation  method is illustrated in Figure 6 and includes the following operation steps:  a. During the solar array deployment, the hinge rotation is blocked through a pin which is in contact  with an end stop on the hinge static housing. The translational displacement of this pin is  prevented through an add-on feature of the gear, which forces the pin to remain in its position.  The pin-carrier is mounted to the hinge shaft, onto which the solar array bracket is also attached.spacecraft mounting  planesolar array bracket(solar array not shown) trim rotation range hinge shaft  rotated into  operational  position track axis:  conventional SADMtrim axis: hinge slip ring with integrated  potentiometer External Solar Array Drive  Electronics (SADE) with  full electrical redundancy angular range  lock pin not  engagedhinge shaft part of  static  hinge  housingangular  range  lock pin  engaged track axis rotation  sense,range not limited 0° 0° -90° 156
The rotation of the solar array is thus prohibited, and the required high back-driving torque  resistance is provided through this locked pin. b. Once the solar array has been deployed and settled, the hinge motor is actuated and the gear  begins to rotate. Since at this point the hinge shaft and the gear are still disengaged, the gearrotates, while the pin’s position remains static until it reaches the gear opening allowing the pin to  push through.  c. The pin pushes through into a cavity in the gear add-on feature, forming thus a rigid connection  between the gear and the hinge shaft (on which the solar array is attached). The hinge drive is  now engaged; the rotation of the gear is transmitted through the pin to the shaft and the solar  array. Nominal operation can be started.  a. Hinge Locked b. Un-Locking Operation c. Drive Engaged pin-carrier mounted on hinge  shaft pin against stop on static housinggear rotates pin constrained against housingpin engages into the gear,  locking shaft and gear together pin is no longer constrained by  housing, hinge shaft is free to  rotate Figure 6. Solar Array Deployment Lock Operation Method Track Axis Rear Bearing and Membrane The track axis shaft is mainly supported by its front duplex bearing. These bearings will take most of the  axial and radial loads during launch. An additional single-row bearing has been implemented at the rear end to further restrict radial displacements and guarantee that the shaft (especially the slip ring shaft) remains properly aligned with respect to its stationary counterpart. The rear bearing is supported by a flexible membrane which allows translation along the rotation axis.  This membrane compensates thus for shaft elongation/retractions due to temperature gradients between  the shaft and the housing, and hence prevents significant variations of the bearing load.  Slip Ring The slip ring allows the transmission of power and electrical signals from the stationary to rotating  structure of the track axis. Its core consists of 60 current transfer rings made from gold plated brass, each of them having the capability to transfer 1.6 A. The molding of the rings within a space-qualified epoxy provides a very high electrical insulation between the tracks. The counter parts for these rings are gold brushes, wiping over the gold rings and thus providing electrical connection between the rotating and the stationary part of the track axis. Due to the criticality of the gold-on-gold contact between the brushes and  the gold rings, the slip ring was purchased in order to benefit from existing heritage of such a sophisticated element. The slip ring will none the less be completely re-qualified within the BSADM as its performance significantly depend on the method how it’s supported. 157
a. Rear Bearing Support Arrangement b. Max. Flexible Membrane Deformation Figure 7. Trim Axis Rear Bearing Membrane Verification The BSADM qualification test campaign allows proving the mechanism’s performance during the ground  testing, the launch and its whole orbital lifetime. It includes • A bench test to calibrate both rotation axis and to verify the mechanism’s functional performances  prior to its submission to thermal and mechanical loading  • Vibration tests to demonstrate that the mechanism is able to sustain launch loads• A deployment test to show that the deployment torque generated by the solar array wring will not  damage the mechanism (and in particular the deployment lock pin) • A thermal test to verify the mechanism’s robustness to temperature changes and its capability to  provide the required performance over the whole operational temperature range • A life test performed with temperature changes in vacuum, to prove that the targeted mechanism  performances are provided during the whole orbital lifetime  Functional tests are performed regularly throughout the entire qualification test campaign to closely  analyze and monitor the evolution of the mechanism’s performance under the various  circumstances/operation scenarios.  Bench Test    The bench test focuses on the verification of the BSADM key functions, consisting of the measurement of  the operation accuracy (relative & absolute angular position accuracy), the torque margin and the deployment lock release capability. Vibration Test The vibration tests are started with a resonance search (low-level sine sweep) followed by a high-level  sine vibration conducted to confirm the structural integrity of the BSADM. An intermediate-level random  vibration test is then performed at -6 dB to assess the mechanism’s responses before it is finally submitted to the full-level random vibrations that simulate flight-launch representative loading. The  random vibration spectrum is unique for each test axis. Figure 9 shows the z-axis test level and the  mechanism’s response as example. The resonance searches done before and after the high-level sine and the full-level random vibration did not show significant Eigenfrequency changes, especially for the critical mechanism elements. The visual inspec tion and the performance tests done after the vibration  tests did not reveal any damage and reinforce the confidence that the mechanism is able to sustain the  predicted launch loads. bearing membrane allowing for  axial translation of rear bearing 158
Figure 8. BSADM Vibration Test SetUp Figure 9. BSADM Z-Random Vibration Response Deployment Test A static torque of 60 N-m is applied on the solar array bracket to demonstrate that the deployment lock pin will not be damaged through the loads generated by the solar array deployment. A smooth andcontrolled release of the deployment lock after this test confirms that the deployment lock pin is robust enough to sustain the solar array deployment torque.   Thermal & Thermal Vacuum Life Test The BSADM is submitted to 4 cycles between +50°C and -20°C during the thermal tests, and to 12  additional cycles in vacuum between +80°C and -30°C during the thermal-vacuum life test. The first cycleof each test sequence is used to verify the structural integrity of the mechanism under thermal loading. Mechanism start-up and functional tests are then done at hot and cold temperature during the second  cycle. During the remaining thermal cycles, the mechanism track axis is continuously rotated, while the  trim axis performs sweeps of ±60°. The BSADM performs 64000 continuous rotations of 360° with its  track axis and 3160 sweeps of ±60° with its trim axis in overall, and will therefore be qualified as per  ECSS for the targeted in-orbit life.   Conclusion The BSADM design approach – based on heritage, focused on the analysis of critical elements and the  performance enhancement through particular features – lead to a rapid manufacturing of an EQM that  allowed facing the environmental qualification testing with high confidence. The success of the EQM observed throughout the qualification tests done so far reinforce the expectation that the BSADM will  pass the whole qualification campaign without major issue, and that 7 Flight Model mechanisms will be  delivered ready to fit to the spacecraft by August 2014. This is a 17-month program from kick-off to completion – a good example of the successful, unique and highly efficient SSTL approach to design and  development. 159
Figure 1. What this model does not do is account for a duty factor in the flex tape itself, meaning that if  only a portion of the traces in a tape are in use at full capacity the model must be factored without much engineering basis. This approach yields a robust design but not fully understanding some of these intra-tape thermal effects limit how far we can push the envelope in terms of current carrying capacity and unit density. We are currently involved in a program that is focused on a better understanding of these duty factor effects as well as a more complete model verification. For this paper, tapes were tested in a  vacuum chamber using thermal imaging instruments and specialized view ports and chamber  configurations. Figure 1.   Example predicted temperature profile (bott om) and material properties (top) along the  length of a flex tape in vacuum at 10A. In the case presented here the radiation has been added in  order to approximate the test c onditions in the experiments conducte d. For a flight application,  the radiative effect would be removed yieldi ng a much higher temperature and therefore a lower suitable maximum current. Experimental A flex tape from a recent solar array drive flight application was used for testing (shown in Figure 2). Jumpers were soldered between the exposed solder pads at the ends of the tape in a “daisy chain” configuration so that a single power source could be used to energize all of the traces in series, but also allow for connection to individual traces using alligator clips. The flex tape (construction details in Table 1) was mounted to an aluminum alloy support structure using  low outgassing double-sided tape. Type K thermocouples (TCs) were attached using aluminum tape to  each end of the flex tape where it contacted the aluminum supports, additional TCs were attached using aluminum tape to the aluminum supports themselves and to the chamber platen. Additionally a single fine-gauge type K thermocouple was attached using Kapton tape to the flex tape under test, near the  center of the length and width of the tape; this was done to calibrate the thermographic camera. The chamber was pumped down and current applied to all traces of the tape, once stabilization had occurred 360365370375380385 0.00.51.01.52.02.53.03.5 0 5 10 15 20 25 W/m.K/g80/g58/g58-.m  Resistivity W Thermal Conductivity 0 50100150200250 0 5 10 15 20 25Temperature, °C  Position, cm 162
a thermographic image was recorded of the region in which the TC had been attached. The emissivity setting in the camera was adjusted so that it indicated the temperature measured with the TC. Once this baseline was established, the mid-tape TC was removed. Figure 2.  Flex tape used in tests. A 40-mm-diameter CaF 2window was used to view the tapes in far IR (where the FLIR operates). This  material is transparent over most of the spectrum in that range. Calcium fluoride does not transmit all wavelengths in the far IR range with complete transparency, but since the previously described calibration  procedure was performed thru the window we expect reasonable results. Table 1.  Construction details of the flex tape used in the testing performed here. Parameter Value Overall Length 0.61 m (24 in) Active Tape Length 0.24 m (9.5 in) Overall Tape Width 0.061 m (2.4 in) Number of Traces 13 Trace Width/Spacing 4.2x10-3m / 8.9x10-4m (0.16 5in / 0.035 in) Trace Thickness Material7.1x10-5m (2 oz = 0.0028 in) Rolled Annealed Copper (IPC W7) Tape Construction Kapton/Acrylic Adhesive/Cu Single Layer Power was supplied to the flex tape via a bench power supply. Current was monitored using a handheld DMM. Temperatures were monitored using handheld thermocouple readers. The thermographic instrument used was a FLIR i3. This FLIR unit was positioned above the IR window in the top of the chamber (Figure 4) using a ball vise. For each test, the chamber was evacuated using a mechanical roughing pump to a pressure of  approximately 0.13 Pa (1 torr) and power was applied. Tests were done with current levels ranging from  1 A to 11 A. The current used for the tests reported on here was 10 A in every case; this value was  chosen because it gave good signal to noise ratio on the FLIR and allowed us to see more subtle variations in unpowered trace temperatures. The temperature of the tape was allowed to stabilize in order  163
to approximate the steady-state condition and a thermographic image recorded, as well as the  temperature of all thermocouples, chamber pressure, current and the thermal image data file were logged. The chamber was brought back to atmospheric pressure trace connections modified and the  process started again. Figure 3.  General tape configuration in ch amber. The boundary temperature thermocouple can be  seen on the right side of this photo. Figure 4. Thermographic (FLIR) in strument positioned above CaF 2port in top of chamber. Results FLIR images were translated to .csv files and then imported into Microsoft Excel for analysis. The output  from the FLIR takes the form of a 60x60 matrix of temperatures. For the data presented here in Figure 6,a column of data near the center of the image was taken. The column number and method of extraction  are consistent for all the results here. During periods of testing, a K-type thermocouple was attached to  the exposed surface of the flex tape using acrylic adhesive Kapton tape, the temperature read was used  to calibrate the FLIR by adjusting the emissivity input. The emissivity of the flex tape Kapton surface was found to be 0.37. No uncertainty was calculated for this result as the absolute value of the emissivity is not critical for this work. In all cases reported here, “temperature rise” indicates raw temperatures that have been modified by subtracting the aluminum support fixture temperature thereby negating any heating of the fixture and chamber. 164