ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
© 2016 Sierra Nevada Corporation   127 chamber could be opened up and the load cell for that bearing removed allowing the suspect bearing’s  housing to spin freely with no resistive load.  

Figure 5.  Bearing Life Test Setup  

   Load Cells   Preload Spring   Resistive Arm   Ball in Cup   Counterbalances   Duplex Bearings (6X)   Redundant  Fixture Bearings   Labyrinth Shields   Base Plate   Drive Shaft   Flexible Coupling   Thermocouples
© 2016 Sierra Nevada Corporation   128 Life Test Results  

SMMA Bearing Life testing met and exceeded the 128.4 million revolution requirement for 2X l ife. The  bearings have achieved approximately 202 million shaft revolutions to -date and have not exceeded 1 25%  of the initial setup running torque, nor exhibited credible errati c performance, as defined in the test  termination criteria . 

Bearing Friction Torque   From the start of the test, it was noted that bearings 3 and 6 had noisier load cell data than the remainder  of the bearings. As seen in the Figure 7, both bearings 3 and 6 had significantly higher fluctuation in load  cell output . To determine if those two bearings had significant torque variation or if the load cells were  producing errati c outpu t, the temperature plots  displayed in  Figure 8 were reviewed. Bearing temperature  will fluctuate with real changes in friction torque  as a component of mechanical power.  Since the  temperature of b earings 3 and 6 did not follow the torque variation, it was determined that thos e two load  cells were providing erratic output. Per discussions with the customer , it was mutually determined to  continue the life test on all six bearings and not subject bearings  3 and 6 to test termination requirements.  The team concluded that sufficient life test data could be obtaine d from the remaining four bearings , and  with the life test already extending beyond the launch date, it was not worth the additional schedule to stop the test and replace the faulty load cells.  Bearings 3 and 6 were still evaluated for gross changes in  performance and will still be subjected to a visual inspection at the end of  the life test. As can be seen in  Figure 6, none of the remaining four bearings experienced any sudden increases in friction torque and all  stayed below 125% of the initial torque measurement. Momentary torque spikes were seen any time the  test was paused and restarted due to both the inherent higher starting torque in a lubricated bearing plus  the fact that the lubricant had cooled off , thereby reducing its viscosity.  Since the on- orbit application  operates continuously  without starting and stopping, starting torque spikes do not impact on- orbit  performance and were not considered against the termination criteria.  

Figure 6.  Bearing Friction Torque @ 74 RPM  (Bearings 1, 2, 4, & 5)
© 2016 Sierra Nevada Corporation   129   Figure 7.  Bearing Friction Torque @ 74 RPM  (Bearings 3 & 6) 

Bearing Temperature  As can be seen in Figure 8, power generated due to viscous losses did heat each of the bearings to a  temperature above the 25°C limit to set to achieve Boundary lubrication during steady state operation,  while staying below the 36°C  upper limit. There were typical daily and seasonal fluct uations in  temperature that corresponded to the ambient temperature in the test room, but no upward trends that  would correspond with significant changes in internal bearing losses or  lubricant degradation. Any time  the test was paused, the cryopump  would cool the setup resulting in a cold temperature spike. Following  a lengthy test pause for maintenance and to negotiate the extended duration, the test setup was  repositioned to a different location in the test lab resulting in a higher ambient temperature and a  corresponding increase in the temperature of the bearings.  The average temperature of the bearings was  28.6°C during the 2X portion of the life test, slightly elevated from the on- orbit typical operating range of  15°C -20°C. This temperature increase results in a conservative test since lubricant life is potentially cut in  half for approximately every 10°C rise in temperature as discussed in NASA/CR -2009- 215637 [5].  

Figure 8.  Bearing Temperature
© 2016 Sierra Nevada Corporation   132 calibration at the conclusion of the test . There is still a risk of test equipment falling outside of calibration  parameters at the end of test with the impact to be assessed at that time.  

Friction Torque Measurement   When measuring small friction torques in bearings, the test setup must be isolated from external vibration  as much as feasible and carefully designed to minimize misalignment loads. Several bearing friction  torque measurement is sues were discove red during the initial life test setup and validation and had to be  resolved prior to starting the life test to ensure recording of accurate data.    Vibration coming from the vacuum c hamber equipment created enough noise to appear on the torque  readi ngs com ing out of the chamber. R ather than continuously collect torque data, the friction torque was  captured every millisecond for 60 seconds once each hour with the cryopump and the cryopump  compressor turned off during that short period to minimize any torque disturbance.    The weight of the load arm created an offset load which in turn created a zero offset in the friction torque measurements . In order to be able to directly measure friction torque in the bearings without a zero offset,  a counterbalance weight , as shown in Figure 9, had to be added to the opposite side of the bearing  housing. Additionally, the load cell is very sensitive to off -axis loading. The initial concept of a flat surface  contacting a spherical post  was not sufficient to provide accurate force measurements . The load arm  could not be held exactly parallel to the load cell resulting in a slightly misaligned axis of contact. This misalignment combined with the vertical height of the contact point from the load cell caused a small  amount of moment loading into the load cell which produced erroneous values . As can also be seen in  Figure 9, the contact was modified to incorporate a ball in a spherical cup to better align the load axis with  the load cell, as well as raising the load cell to minimize the vertical moment arm  distance. These  modifications greatly improved the accuracy of the force measurements from the load cell .  

Figure 9.  Test Configuration Changes  

Life Test Automation   When running automated life testing, special attention needs to be paid to the computer and software  portion of the GSE to minimize test interruptions . Since computers accumulate memory usage over time,  the test must be paused periodically for the computer  to be rebooted. This allows the automated software  to reload and clear the memory cache to prevent unplanned and uncontrolled test interruptions due to  automated software crashes . Additionally, other programs running on the computer should be eliminated  or minimized as much as possible  including virus scans and forced software updates . Several computer  crashes were attributed to automatic software updates and their impacts on processor and memory  usage. One solution was to minimize computer updates to only  those deemed highly critical.   Counterbalance   Spherical Post    Ball in Spherical Cup    Original Concept    Final Design
136  

Figure 1 .  Cryo -dedicated Amplified Piezo Actuators  

To be compatible with a vacuum and low temperature environment, an interferometric sensor FPS3010  has been used [3]. It provides reliable measurement from ambient to 40K.  

Stroke  is one of the main performance characteristics  of piezo actuators. It is well know n that stroke i s  reduced with low temperature.  Stroke is measured using full stroke sinus command. Amplitude is  extracted from  the interferometer sensor  (Figure 2) . Full stroke drops from 95 µm to 37 µm (3 9%) for  supplier  #1 whereas it decreases from 81 to 17 µm (20%) for supplier#2.  

Figure 2 .  Stroke and hysteresis versus temperature 

Stroke measurement is also used in order to obtain hysteresis information. Hysteresis is an intrinsic  characteristic of piezo actuators. Two suppliers’ preloaded stacks are compared at temperatures from  ambient (293K) to 35K. It is seen in Figure 2 that hy steresis amplitude is reduced with deceasing  temperature. Moreover, supplier  #1 actuators are producing higher hysteresis. S upplier  #1 actuators  reach 20% of full amplitude compared to 10% for supplier  #2.   Piezo actuators are assimilated to capacitance fr om an electrical driver view. The actuator capacitance is  determining  the bandwidth of the group {electronic  + actuator}, due to amplifier current limit. Therefore,  this characteristic is important considering driving philosophy  or in the case of charge control of  piezo  actuators . Capacitance versus temperature is plotted in Figure  3. It can be observed that capacitance   tends to stabilize at approximately 20% of its ambient value when  decreasing temperature.    Position creep is a slow change of actuator position that appears after a constant voltage has been applied. In open loop  operation , position is slight ly increasing with time,  unfavorable to stability. This  creep is generally characteri zed by a logarithmic law [4]. It has been observed, as presented in Figure 3,  that creep increases with low tem perature. However, this behavior is removed after cycling. Therefore,  cycling seems to be good practice before functional actuation of piezo in a cryogenic environment.
137  

Figure  3.  Capacitance and creep evolution versus temperature  

Reduction of stroke due to temperature limits piezo actuators even more due to their short stroke.  Moreover, even if static  power consumption is low, position is  not kept when power is cut. Therefore,  when larger stroke and/or holding force with no power consumption are needed, piezoelectric motors  become relevant.  

Piezo Motors  

An APA- based piezo motor is presented with its working principle. First , low temperat ure results are  presented, giving a first indication  about the technology potential. After that, two innovative linear motors,  based on a similar principle are presented.  

Stepping Piezo principle and low temperature compatibility   Stepping Piezoelectric Actuators (SPA) are long- stroke linear inertial piezoelectric motors for micro/nano  positioning applications benefiting from  the advantages and the heritage of the APA®. SPA is  a way to  expand the limited stroke of the APA® to centim eter-length strokes. This feature is achieved using the  IDM principle  [5].   SPAs are formed of only four parts:  the APA ®, a front mass, a clamp,  and a rod. SPA operates through  the accumulation of small steps, using inertial mode, impact forces and stick -slip effects as introduced in  [6]. Typically, a slow APA® actuation generates a slow motion of the mass while the rod sits in the clamp.  A fast APA® actuation induces a fast motion of the rod slipping in the clamp. This allows getting steps,  which gives a long stroke, called the stepping mode (M1). Between each step the actuator is locked in position [ 7].   The load may be fixed on different positions leading to two different motor capabilities thanks to different modes. In a first configuration offering nano positioning (Figure  4), the load can replace the mass or can  be fixed to the mass. So when the long stroke (M1) is performed, the motor can be also operated in a  deformation mode (M2) for a fine adjustment. In this case, the stroke is proportional to the applied voltage, which leads to a nanometer resolution and a high bandwidth (limited by motor blocked force). In a second configuration, the loa d is fixed on the moving rod. In this case, the advantage is a high stiffness,  but fine mode is not  available.    The long stroke stepping mode (M1) is produced by step accumulation with an appropriate 0- 150 V  voltage pattern. The short stroke deformation mode (M2) is produced by deformation of the APA ®, which  is simply proportional to the excitation voltage between - 20V to +150V. Only one amplifier channel per  SPA is required.
138  

Figure 4.  SPA components and principle: stepping mode (M1) and deformation mode (M2)  

The large deformation stroke of the APA® is also an advantage. It provides a useful deformation stroke  mode (M2). It also contributes in getting good speed in long stroke mode (M1);  larger steps per cycle  comp ensate for a lower step frequency. Benefits from amplification in the Impact Drive Mechanism have  been demonstrated in [ 8]. 

Low temperature tests were performed on a  SPA40SM (based on middle range APA®) from 0°C to   -180°C, the lowest temperature allowed by the test rig. Speed is affected with a linear speed reduction  with temperature. This is fully correlated with actuator stroke reduction.  The force results are shown in  Figure 5. It can be seen that a drive force reduction is observed at lower temperatures, but not lower than  75% of nominal force achieve d at ambient temperatures. However, after every temperature cycle , the  performance is recovered upon returning to room temperature, showing no continued reduction in performance after periods of exposure to low temperatures under vacuum.  

Figure  5.  SPA40SM Force versus temperature [ 9] 

Module Stepping Piezo Actuator   In typical SPA motors, the clamp is moving by friction at a variable distance from the actuator. This  creates  a limitation in stroke, hyperstatism with potential bearing, reduction of force with the distance, and  constraint s in new mechanisms design. In comparison, Module SPA (MSPA)  frees the rubbing contact so  the distance between the friction force and the APA® electromechanical force remains at a constant  distance. It is able to drive any type of body by friction in linear or rotary motion depending on the guiding.    First implementation is based on a linear pi ezo stage. This type of stage is relevant for optics motion, or  sample positioning, but is currently limited in stroke. The MSPA configuration is used in order to move a  30x30x10 mm 3 device and is presented on Figure 6. A representative configuration using a dummy lens  to simulate an optical zoom or focus system has shown its capability to perform long stroke (200 mm) with  a speed of 15 mm/s.
139  

Figure 6.  MSPA combined with 200 -mm stroke guiding  in linear configuration  + with dummy lens  

Another possible  requirement is constant speed all along the stroke. MSPA is used to produce a linear  displacement and uses inertia principle to disrupt the friction contact and restart a new ramp of  displacement. This creates a linear constant speed displacement for the moving module, or moving mass.   Figure 7 presents an example of displacement  for a few test masses. Linearity of displacement can be  observed. For more stringent  requirements, a closed- loop solution has been implemented.  

Figure  7.  MSPA displacement example  

Force Stepping Piezo Actuator   The second type of configuration is also based on S tepping Piezo Actuators. The internal structure of  FSPA (Figure  8) allows decoupling high external forces from actuation mechanism making the motor  compatible with hi gh load and high levels of vibration. A prototype has been built  to support a more than  600-N external force without losing its position.  

Figure 8.  FSPA motor prototype  

Another major advantage of this second configuration is the strong resolution that can be reached. In order to show  motor resolution, step- by-step signal is applied and position output is controlled using  capacitive sensor. Figur e 9 presents the results obtained  on the motor . Resolution of steps below 20  nm
140 is shown. This corresponds to a 1.2 µm/min speed. The s peed reached with a 20 Hz and full voltage  amplitude signal is 75  µm/min . 

Figure 9.  Motor displacement and resolution (mean step = 20  nm) 

Cryo applications  and perspectives   The presented configurations and associated results are fully compatible with cryo -dedicated actuators.  The i mpact of temperature on SPA motors are mainly about speed (due to actuator stroke reduction), but  not on force. Therefore, performances  obtained on MSPA or FSPA should be fully comparable and allow  targeting  specific cryo needs. One  example is the  GAIA M2M pointing mechanism, with 70- nm resolution  on 550- µm travel in a  temperature range from ambient to 100K  [10]. Other cryo needs ha ve been  identified through the EChO mission  [3]. Low temperatures (30K) are combined with  high shock levels  (2000g), targeted by FSPA  usage.  Typical 10 -100 N actuation force and 200- N to 2000- N holding position  force are fully in the fie ld of competence.  

Conclusion  

In this paper, results obtained on cryo- dedicated Amplified actuators are shown with r espect to  temperature influence  from ambient to 40K. Compatibility of performances with positioning application is  proven but intrinsic lim its in term s of stroke have to be overcome;  proposed solution is piezo motor. The  SPA concept is reviewed and applications within two new configurations are shown. Interesting  advantages and performances obtain ed at ambient temperature are correlated to a cryo-dedicated  Actuator in order to give the reader the potential  of the combination of technologies.  

Acknowledgment 

Cryogenic piezo actuators have been tested within  the R&T program from CNES. TV  facilities at Toulouse  center  have been used. The authors  want to thank Laurent Cadiergues and Daniel Gervaud for their  support.
141 References  

1.  Le Letty R., Claeyssen F., R ., New amplified piezoelectric actuator for precision positioning and active  damping, Proc. SPIE Vol. 3041 Smart Structures and Materials , (1997 ), pp. 496- 504  2.  Allegranza C, Gaillard L. , Le Letty R., Patti S., Scolamiero L., Toso M.,  ”Actuators for Space  Applications: State of the Art  and New Technologies ”, Proc. Actuator 2014, Bremen   3.  Belly C., Hihoud M., Pages A., Argelaguet H., Moreno J., Mangeot C., “Cryogenic piezo  characterisation and integration in Fine Steering Tip/Tilt Mechanism”,  Proc. Actuator 2014, Bremen  4.  Junga H., Gweon  D., “Creep characteristics of piezoelectric actuators ”, REVIEW OF SCIENTIFIC  INSTRUMENTS VOLUME 7 1, NUMBER 4, (2000), pp. 1896- 1900   5.  Higuchi, T.  “Apparatus and method for effective fine movement by impact force produced by  piezoelectric element ”, (1987) EP0292989   6.  Higuchi T., Yam agata Y., Furutani K. & Kudoh K . “Precise positioning mechanism utilizing rapid  deformations of piezoelectric elements", Proc. of IEEE Workshop on Micro Electro Mechanical  Systems , (1990), pp.47 -51  7.  Claeyssen F., Ducamp A., Barillot F., Le Letty R., Porc hez T., Sosnicki O.& Belly C. , “Stepping  Piezoelectric Actuators based on APAs ”, Proc. Actuator 2008, Bremen  8.  Belly C., Charon W.,  “Benefits of amplification in an inertial stepping motor ”, Mechatronics 22 (2) ,  (2012), pp. 177- 183  9.  Belly C., Buttery M., Claeyssen F., “ THERMAL VACUUM  BEHAVIOUR OF A STEPPING PIEZO  ACTUATOR”, 14th European Space Mechanisms & Tribology Symposium – ESMATS, (2011)   10. Compostizo C., López R., Rivera  L., “GAIA M2M POINTING MECHANISM QUALIFICATION”,  14th  European Space Mechanisms & Tribology Symposium –  ESMATS , (2011)
143 Development of a  Self-latching Hold- down RElease Kinematic (SHREK)  

Ruggero Cassanelli*  

Abstract    SHREK (Self-latching Hold- down Release Kinematic) , is an innovative shape memory actuated hold down  and release device,  aimed to latch and release solar panels. It is  specifically conceived to fit  the small  satellite  market requirements, as well as overcome some recurring shortcomings of existing similar  devices.   This paper describes SHREK’s  unique design characteristics  and preliminary test results .    

Figure 1.  SHREK      Introduction  

Small ( mini and m icro) satellites represent  a rapidly expanding market, which extends the access to space  to a number of comparatively small organizations . Its commercial nature brings in requirements that, at no  detriment of technical performance, are focused on o perability and c ost. For this market,  AEREA has  devised a mechanism for latching and releasing deployables  while pursuing the following goals:   • Commercially attractive   • ITAR free   • Automatic manual latching  (self-latching)   • Simple manual release  • Remote release with shape memory alloy  technology   • Full control over the applied preload   • Ground and f light operable  without parts replacement   • On-site ground reset operations without parts replacement   • 50 remote actuations  without degraded performance (operating time and shock)   • Functional temperature range: - 56°C to +70°C ( -56°C to 100°C desirable)   • Disconnection shock : < 300 g’s [negligible shock desirable]   • Preload force: up to  5 kN                                                    * AEREA S.p.A.,  Turate, Italy  

Proceedings of the 43rd Aerospace Mechanisms Symposium, NASA Ames Research Center, May 4-6, 2016
144 • Panel mass : up to 10 kg  • Mechanism mass < 0.5 kg [0.3 kg desirable]   • Nominal power consumption:  15 W   • Actuat ion time: < 60 s   • Envelope: diameter < 100 mm, height < 40 mm  

SHREK Description  

The following is based on an application where the solar panel  is hinged to the satellite on one side and  held by  the SHREK mechanism  on the opposite side.   

Mating principle   The mechanism is composed of  a Panel m odule and a Satellite module. The Panel module is a lightweight  metallic ring permanently connected to the internal surface of the panel. The Satellite module is a metallic  cylinder 35- mm long and 85 mm in diameter, mechanically and electrically connected to the external surface  of the satellite.  The two modules are axially mated  by inter position of an array of spheres (ball lock concept) .  The spheres are housed in the Satellite module, where they can be radially displaced in either one of two  positions : retracted (Unlatched status) or  extracted  (Latched status) . The position of the spheres is  determined by the rotation of a Spool, internal and coaxial to the Satellite module. The Spool has radial  pockets which, when aligned with the spheres, allow the spheres  to retract . 

Figure 2.  Mating concept (Top=Latched – Bottom= Unlatched)   Self-latching  (reference Figure 3)  This is a unique feature of SHREK , operable by manual ly pushing the p anel against the satellite,  and its  goal is  to simplify installation, integration and ground testing of the mechanism . 

The Spool is permanently biased,  by a T orsion spring (10), to rotate toward the latched position. The torsion  applied to the Spool  is reacted by the spheres . The radial component s of such reactions  permanently push  the spheres toward the extracted (Latched) position.  In the Unlatched condition, the spheres are kept into  the Spool (5) pockets by an external, coaxial, spring- loaded Active sleeve (9) which physically blocks  the  spheres ’ passages.    For latching,  the Panel module (1) is pushed against  the Active sleeve (9) , which consequently  slides axially  toward the base of the mechanism , against the force exerted by the Kick-Off spring (7). At the bottom end  of the Active sleeve movement, where the Kick -Off spring reaches its maximum compression, radial motion  of the spheres is possible . In this position,  the torsion spring rotates the Spool. The rotation of the Spool  results in the expulsion of  the spheres from their  pockets  due to the pocket having an angled side that