ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
514 Modes  7 and 8 are rotational modes  of the elevation axis around the mirror Co G. Modes  9, 10 and 12 are  translation modes  of the azimuth fork and upper fra me combined . Benefits of decoupling the elevation axis  from the rest of the structure can already be observed by looking at the participating mass. Indeed, the  participating mass of these modes is close to the total mass minus the mass of the elevation axi s.  

A frequency response analysis (SOL111) is performed with unit acceleration excitations at the mechanism  interface with the spacecraft. The transfer functions between mirror Co G motion and excitation are  calculated and compared to  the ones  obtained wit h the ball bearings model . In all cases, significant ly lower  responses are observed. To illustrate this, the two transfer functions  shown in Figure 9 are reproduced in  Figure 12 together with the ones  obtain ed with the flexible pivot s concept.  

   Figure 12: Flexible pivot s pointing mechanism response (in blue) and ball bearing s mechanism (in red)  on  elevation axis (Y -rotation) for X -translational (left) and Y -rotational (right) unit acceleration excitation  

The transfer functions  show strong attenuation of the micro -vibration . Most of the pea ks are located in the  10 to 50 Hz bandwidth which is out side of typical micro -vibration spectrums . In the bandwidth between 50  and 200 Hz corresponding to intermediate micro -vibration  excitation from cryo- cooler and reaction wheels  of 0.001 m/s2, the maximal mirror rotation around Y  is 6E-6 µrad at 47Hz , 6E-7 µrad at 50 Hz , and   3E-9 µrad at 200 Hz. Note that these values are for Ry excitation and are smaller for X excitation. The  stability of a compliant pointing mechanism is thus between 3  and 8 orders of magnitude better than  classical ball bearing pointing mechanism.  

To further improve pointing stability , dampers are added on the elevation, azimuth and 3rd axis. These  dampers are acting in rotation but also in radial directions. The effect of these damper s decreases the local  amplification at resonance peak. The elevation rotation due to X excitation can be reduce by a factor 5 on  the mode at 29 Hz and by a factor of 10 for the mode at 49 Hz.
515    Figure 13: Flexible pivot pointing mechanism response with dampers (in light blue) and with ball bearing  (in red) on elevation axis (Y -rotation) for a X -translational (left) and a Y -rotati onal (right) unit acceleration  excitation  

Conclusion  

The development of a novel, highly customizable flexible pivot is opening new design possibilities. Thanks  to a fully automatized optimization software, it is possible to design within a few hours  a tailor-made flexible  pivot that matches specific stiffness, range of motion, and lifetime requirements. In addition, it is possible  to cover a wide range of requirement s, from low torsional to high torsional stiffness and from high to low  torsion to radial stiffness. The application of flexible pivot s is thus numerous, from high- precision pointing  mechanisms , to infinite- life oscillation scanner or to energy storage applications.  

By combining large- angle and small -angle flexible pivot s, micro -vibration excitation can be ruled out from  the pointing accuracy budget. It is shown that it is possible to lower the micro -vibration impact on the  pointing stability by 3 to 8 orders of magnitude between 50 to 200 Hz by the introduction of complianc e at  strategic locations . In the frequency range above 200 Hz , which is the noisiest bandwidth of reaction  wheels, such a novel concept is bringing the level of perturbation angle below 5E -10 rad/m/s2.  

The addition of dampers on the three axes can furthe r improve the pointing stability  by a factor of 5 to 10  in comparison to the pointing mechanism using flexible pivot s alone.  It is thus possible to replace the  combination of coarse and fine pointing mechanisms  by a single mechanism achieving ultra- stable pointing  stability. The total mass of the system can be drastically reduced, as well as the total volume. Finally, the  novel ultra- stable pointing mechanism using flexible pivot s is very cost effective in comparison to solutions  available today on the mark et. The highly customizable flexible pivot design family is an enabling technology  for many applications such as  ultra-high precision pointing mechanisms .  

References  

1. L. Blecha, M. Humphries, Y. Puyol, WO 2017077469A1   2. Schlüter, M. &. (2010). The or acle penalty method. In Journal of Global Optimization, 47(2) (pp. 293325).   3. Schlüter, M. E. (2009). Extended ant colony optimization for non- convex mixed integer nonlinear  programming. In Computers & Operations Research, 36(7) (pp. 2217- 2229).   4. Schlüter, M. G. (2012). A numerical study of MIDACO on 100 MINLP benchmarks. In Optimization, 61(7)  (pp. 873- 900).   5. Puyol, Y. (2019). Innovation in Large angle flexible pivot Design & Material Accelerated Fatigue  Screening Tests Results. In ESMATS 2019 .
517 Compliant Mechanisms M ade by Additive Manufacturing 

Lionel Kiener*, Hervé Saudan* , Florent Cosandier *, Gérald Perruchoud *, Vaclav Pejchal *,   Sébastien Lani* and Antoine Verhaeghe*  

Abstract  

Several compliant mechanisms have been completely redesigned for Additive Manufacturing (AM) and  have allowed CSEM to develop an innovative concept. In addition to the new geometric possibilities offered  by AM, the need for machining and assembly after pri nting are drastically reduced. Support structures under  flexure blades are thus minimised and the overall process becomes more streamlined. Moreover, this idea  allows us to easily design and produce monolithic cross blade flexure pivots with interlocked fl exible blades.  Thanks to this concept, CSEM is now developing and testing new architectures of Compliant Mechanisms   based on Additive Manufacturing (COMAM) for the European Space Agency (ESA).  The development  methodology, the AM process and post -process and the testing approach are detailed in this paper.  

    Figure 1. Compliant  Mechanism built by Additive Manufacturing.  

Introduction  

Mechanisms with friction present significant drawbacks with the need of lubrication, debris generation,  backlash and stick slip. In cryogenic and space environments, suitable lubricants are very limited when not  prohibited. Wear generation can pollute optics, obstruct a smooth motion and c an even lead to early  failures.  

To overcome these important limitations , Compliant  Mechanisms (CM) are usually proposed. They can  achieve macroscopic linear and rotary motion without friction, wear, backlash, and with extremely high  fatigue performance thanks to the elastic deformation of flexible structures. They are used in harsh  environment s such as vacuum, cryogenic and space combining high- precision and a long lifetime  capabilities .  

* Centre Suisse d'Electronique et de Microtechnique (CSEM), Neuchâtel, Switzerland ;   lki@csem.ch  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020
518 To date, the extreme complexity of compliant mechanisms has required highly sophisticated and expensive  manufacturing methods, the gold standard being the Wire Electro -Discharge Machining (WEDM) from a  bulk material block with consecutive large material losses and very long and delicate machining procedures.  Moreover, the assembly has actually to be realized with many precautions to ensure a very precise   positioning between all parts.  

Today, this paradigm is questioned by the possibilities offered by AM technologies, notably the metallic  powder bed processes such as the Selective Laser Melting (SLM). While the largest part of the research  presently repor ted is focused on developing and optimizing designs of what could be described as  “structural or massive parts”, little work has been published up to now to determine the limits related to the  manufacturing of thin, flexible structures used in compliant mechanisms  [1]. 

After more than 30 years of successful developments using compliant mechanisms produced by  conventional manufacturing methods, CSEM demonstrated in 2016 the feasibility of high performances  compliant structures made by AM [ 2]. Over the last few years, CSEM has acquired an expertise in the  computerized optimization of such mechanisms for AM and has proceeded further by inventing a totally  new design concept: interlocked lattice flexures. This new type of compliant structure geometry and  arrangement is such that the flexure elements cross, but never  touch each other, even when deformed.  This new architecture –  made only possible by AM technologies –  creates the opportunity to develop  completely new flexure topologies but also to improve existing ones, as demonstrated with the example of  a redesigned C -flex type pivot (patent US 3073584) illustrated in Figure 2.   

Figure 2. Example of the redesign of a C -flex type pivot with interlocked flexure blades.  

Compliant Mechanisms Heritage  

CSEM is active in the design and development of very high performance flexural elements and mechanisms  for more than 30 years. Notable examples for space applications are the HAFHA flexural pivot and the  Corner Cube Mechanism which is currently operated in the IASI instrument on board MetOp satellites, to  date wi th more than 1 billion cycles (linear stroke of  ±15 mm) achieved in 14  years. Two flight models of  Corner Cube Mechanism has also been delivered last year for the infrared sounder onboard two Meteosat  Third Generation satellites. Another example is the CLU PI linear Focus Mechanism for ExoMars rover.  Other mechanisms (e.g. , Slit Mask, tip -tilt and chopper) have been developed and produced for ground  based telescopes  as well as for the airborne SOFIA telescope.
519 In the same philosophy, the elaboration of new mechanisms  made by additive manufacturing has been  investigated at CSEM over several years targeting the general goals of assessing the benefits and  weaknesses of the AM fabrication process for compliant mechanisms and getting a sufficient level of  expertis e on AM produced compliant mechanisms in perspective of future projects.  

Design Methodology for AM -Based Compliant Mechanisms  

The methodology  to develop C ompliant Mechanisms  built by AM is not straigtforward since the software  tools do not always have a sufficient maturity for these kind of systems. The development workflow has  been developped by CSEM to best utilize the strengths of  all design, AM process and post -process  simulation soft ware. It consists of multiple steps  as described herein . First, the preliminary design is  performed with the definition of the global compliant architecture and a preliminary sizing. Then, the design  is refined in two parallel processes: topology optimizat ion of the rigid structure and shape optimization of  the flexures. Finally, complete Finite Element Modelling simulations are performed to verify the compliance  to the requirements.  

The principal steps of the design flow that have been elaborated by CSEM  to successfully achieve the  development of a compliant mechanism based on AM are presented hereafter and illustrated by the  example of the Compliant Rotation Reduction Mechanism (CRRM) shown in Figure 3.  

Figure 3. Compliant Rotation Reduction Mechanism (CRRM).  

The development of this CRRM is made for the European Space Agency (ESA) for  a research project.  

Specifications   The principal specification for the CRRM  is that the mechanism shall be totally frictionless. In terms of  performance, the input angle shall be ±10° while the output angle shall be ±1°, meaning that the reduction  ratio of the mechanism shall be 1:10. The input and output are also inverted, for a 10° clockwise rotation,  the 1° output i s counter clockwise.  

The repeatability of the system implies that the parasitic motion at output shall be smaller than 10 µm in the  lateral and axial directions and that the parasitic tilt shall be smaller than 1/100°. Its dimensions shall be  120 mm x 50  mm and its mass shall be a maximum of 0.4 kg. For environmental performances, the  mechanism shall withstand launch sinusoidal vibrations of 24 g, random vibrations of 18.4 gRMS and  shocks of 1000 g.
520 Preliminary design and trade- off  The preliminary design activity of an AM- based compliant mechanism can be divided into two phases. The  first one consists in conventional pre- design activities. The flexure topologies and the overall physical  architecture forming the basis of the design are defined, involvi ng the analytical pre- sizing of various  alternatives. A pre- design example of the CRRM is given in Figure 4.  

Figure 4. Architecture and pre- design of the CRRM.  

Design for Additive Manufacturing   This pre-design is then considered under the perspective of the manufacturing process, i.e. , Selective Laser  Melting (SLM). The following aspects are choosen during this phase:   • optimum build- up orientation,   • identification of the critical geometries ,  • geometry of interfaces; fixation areas, positionning features, reference surfaces,   • AM process strategy (support material and its future separation from the part) . 

This is performed by taking into account support structure minimization in critical locations – where postAM machining could be difficult if not impossible, post -process strategy (thermal treatment before/after  removal) and separation from the build plate.  

These activities are realized in accordance with the general design rules for AM and the specific rules for  compliant structures  which have been developed at CSEM.  

The manufacturability of the design should then be assessed. This is done thanks to SLM process  simulation software. A post -processing sequence, including thermal & mechanical post -process es and a  verification strategy is defined in accordance with the specific requirements for compliant structures, such  as temporary fixation of mobile stages  and the addition of features for metrology.  

Detailed design  The detailed design comprises two main phases:   • Topology optimization of the rigid structure,   • Shape o ptimization of the compliant structure, i.e. , the flexure blades.  

Rigid structures optimization  A topology optimization of the rigid structure is performed on the initial design in order to i mprove its  mechanical characteristics, especially the overall rigidity, together with a mass reduction.   The workflow is the following:   1. Definition of the design and non- design spaces, where the design space is the part of the item where  the optimization solver will be active. The non- design spaces are mainly the interfaces and other  peculiar locations which need to be conserved as  defined in the preliminary design.  See Figure 5
521 2. The boundary conditions and the load cases are defined.   3. The optimization parameters are defined.   4. The results are interpreted.   5. A shape smoothing and/or rebuild is performed at the end as illustrated in Figure 6  6. A final  finite element  analysis with the new shape is performed to ensure fulfilling the requirements . 

Figure 5. Definition of the design spaces for the CRRM . 

Figure 6. Result of the topological optimization ( left); design example after smoothing ( right). 

Flexure blades optimization   The compliant structure shall be optimized separately to ensure an optimum solution with regard to  performances, but also to ease as much as possible the manufacturing and the post -treatments, mainly the  removal from the build plate.  

The need to include suppo rt structures while producing thin flexure blades by AM is a critical aspect that  must be taken into account while designing CM. The support structure is minimized and the attachment  points of the support structure to the flexure are weakened in order to m ake its removal easier. The  separation is performed when the part is cut off from the build plate. This concept has been successfully  tested with several designs .  Top viewOutput (in pink )  Fixed interface (in grey ) Bottom viewInput  (in light red )
522 As no single solution allows for simultaneously optimizing the rigid and the flexible part of  the  mechanism [3, 4], a dedicated procedure is devoted to this task.  

Lattice flexure blades   While looking for the most appropriate design for flexure blades, CSEM innovated with a lattice structure  (patent pending) having the main advantages of:   • Lowering the bending stiffness while maintaining a sufficient thickness for manufacturing,   • Avoiding internal support structure thanks to the overhang angle,   • Ability to be interlocked to form a pivot.  

We start by defining a unitary lattice cell from which the whole blade pattern will be generated applying  symmetry operations. Then, this unitary cell is geometrically parametrized. Next, a large number of different  cells are generated using a Monte Carlo method. Some rules must be respected regarding the  manufacturing and integrity of the structure. Therefore, only the designs that are compliant to those rules  are considered. For these remaining solutions, an objective function is defined based on differen t  mechanical parameters with dedicated weighting factors. Example of such parameters are transverse  stiffness and stresses. Another criterion to be assessed is the constancy of the section area along the  longitudinal axis of the leaf spring.  

The goal is to select a lattice that has a cross -sectional surface as constant as possible in order to avoid  having a polygonal effect, to maintain a constant curvature of the leaf spring and to mimic at best the  behaviour of a plain leaf spring. Finally, one of the r emaining designs is selected as candidate for the final,  detailed design, as shown in Figure 7.  

Figure 7. Stress distribution for one particular design (left); optimal lattice leaf spring pattern (right).  

Interlocked lattice flexible structures   Thanks to these optimized lattice structures as well as the opportunities given by AM, interlocked lattices  flexures as illustrated in Figure 8  can be proposed. This architecture forms a monolithic rotational pivot with  a high axial stiffness and which can be additively build with very little support structure.
523    

Figure 8. Monolithic r otation pivot composed of two latticework blades  (patent pending) . 

Based on these interlocked lattice flexures, the CRRM has been designed and the first prototypes of the  CRRM have been successfully produced by AM- SLM.  

Final simulation results   The simulation resuts after optimization show that the Statement of Work requirements are globally fulfilled,  as presented in Table 1.  

Table 1: Comparison of requirements with simulation results   Main requirements  Statement of Work  Simulation results   Input angle  ±10° ±10°  Output angle  ±1° ±1°  Diameter  100 mm  120 mm   Length  50 mm  40 mm   Center shifts  < 10 µm   < 350 µrad  < 2 µm   3 µrad   First eigen mode  > 100 Hz   with blocked IF  740 Hz   (400 Hz before optim.)   Input torque  to be minimized  0.24 Nm   Lifetime  min. 100 ,000 cycles   1 mio (goal)  Infinite lifetime at ±10°   Material  Vacuum compatible,   -60° to +80°C  Stainless steel 17- 4PH
524 Manufacturing  

Manufacturing assessment   The additive manufacturing assessment has been made with the help of the Amphyon software tool, which  simulat es the Selective Laser Melting (SLM) process to give information about the internal stresses  generated during layer manufacturing. These stresses c ould be responsible for macroscopic deformations  of the parts  as shown in Figure 9.  

Figure 9. Manufacturing layer by layer thermo- mechanical simulation.  

Based on these simulations, this tool generates a pre- deformed 3D geometry of the part to overcome these  deformations with the aim of having a geometry that conforms  to the nominal ly designed shape. Based on  preliminary tests with thin and flexible structures, the residual deformations were in the range of 0.1 mm. 

Material, process and post -process t esting  

The preliminary material, process and post -process test results have already been presented during  ESMATS 2017  [3]. During the current COMAM project, these results have been consolidated with new tests  such as residual stresses, dissolved gases, tensile, hardness, roughness, general corrosion, stress  corrosion crac king and fatigue. In parallel, the microstructure was  verified as well. These tests are  performed on representative samples which  have been additively manufactured in a high- strength stainless  steel 17- 4PH. They have seen the same post -processing treatment s as foreseen for the final mechanism  (i.e., Hot Isostatic Pressing (HIP),  solution annealing and age hardening).  

In complement, the entire COMAM mechanism will be tested following the classical space approach with  performance, vibrations, shock and thermal cycl ing. The first results are presented in the next chapter  Mechanisms testing . 

Tensile test results   Ten tensile samples machined out of AM -built cylinders were characterised. At room temperatures,  measured values of Yield strength (R p0.2) and Ultimate tensile strength (UTS) were very similar for all  tested samples and varied from 1280 to 1330 MPa and 1380 to 1450 MPa for yield strength and UTS,  respectively  (Figure 10).  For comparison, typical values for extruded forms are Rp0.2: 1070 MPa and UTS  1170  MPa.   

The yield strength was slightly higher at 1410  MPa and 1440 MPa for samples tested at - 40°C while UTS  remained relatively unchanged. Measured Young’s modulus E is between 190- 210 GPa.
525 Elongation at failure exhibited the highest degr ee of variation from 1.2 to 6 %. Fractography revealed the  presence of lack -of-fusion defects in the specimen with the lowest elongation (1.2%). For the rest of the  samples tested at room temperature, necking occurred outside the measured gauge length, whi ch  contributed to the overall spread in measured elongations.  

At low temperature ( -40°C ), the ductility remains relatively high reaching near ly 7%.  

Figure 10. Left: s tress -strain curves of six tensile tests  performed at room temperature;  right:  hardness  measurement  results.  

Hardness test results   Micro -hardness was measured on both ends of tensile samples after machining from cylinders. HV0.3  results lie within 450 and 500 which is a spread in values typical  for micro -hardness measurements  (ca. 10%). HV0.3 between 450 and 500 corresponds to approximately 48 HRC which is near the upper end  of expected hardness values of 17- 4 PH for this thermal condition.  

Roughness test results   The surface quality  has been measured with a surface roughness tester on the fatigue test samples. No  mechanical process has been performed on the surface. The mean Ra value is 8 µm (±1.5 µm) and is  independent of the direction of printing and of the thermal treatments performed after printing. Compared  to surfaces obtained by machining, this value could be seen as much higher but the roughness is only an  indicative value. The fatigue test results are much more important with regard to the behavior of the  compliant mechanism.  

Fatigue test results   The fatigue behavior of this material has already been defined during a previous activity at CSEM with an  alternate bending fatigue test bench.  Additional fatigue tests have been carried out to consolidate the  results, including the lattice flexure blades. The results indicate that the values of these AM-flexure blades   are comparable to the results previously obtained by CSEM [2].