ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Courtesy  Lockheed Martin  Corp.  Figure 4. Springs Showing Various Degrees of Yielding  To find out if performing multiple thermal cycles has resulted in the springs achieving a state of stress  stability, a special test was performed where the torque output was measured after every three thermal  cycles. It was found that the torque became essentially constant between 9 and 15 cycles at a level that  still allowed sufficient torque margin. Additional thermal cycles are planned to confirm this.  Stress and Strain Analysis  The practice of operating constant torque (Neg’ato?) springs at high stress, sometimes beyond yield,  derives from general usage where the design stress level is based on low cycle fatigue requirements and  the number of operational cycles. The invention of this type of spring is credited to Frank A. Votta Jr. In  his technical paper’, he based the number of allowable cycles on a formula for (what he called) stress  factor, fs, shown here in Equation (1).  fs = Stress Factor  t = Spring Strip Thickness  D1 = Fabricated Diameter (Fig.2)  D2 = Drive Spool Diameter (Fig. 2)  This is identical in form to the usual formula from strength of materials for calculating bending strain in the  outer fibers.  &b = Bending Strain  The first term, t/D1, is the bending strain from the change of curvature during the transition from the  original coiled diameter D, to the straight section between the coils. The second term, t/D2, is the strain  from further changing the curvature by reverse flexing the spring from the straight transition to the  diameter of the drive spool DP.  Stress vs. Strain Characteristics of Spring Steel Strip  If one were to calculate the quasi-linear bending stress on the assumption that the modulus of elasticity in  simple tension were constant and equal to the compression modulus, we would come to the impossible  finding that the calculated bending stress would exceed the ultimate stress. The spring represented by  Figure 3 has a calculated bending strain of 1.1 percent. Based on a modulus of elasticity of  E = 193,000 MPa (28,000,000 psi) and a Poisson’s ratio of 0.3, the linear formula for bending stress  165
(Equation 3) would give a value of 2330 MPa (338,000 psi), which is well above the characteristic ultimate  tensile strength of 1860 MPa (270,000 psi) for extra hard Type 301.  &E a, = (1 - v2) (3)  ob = Bending Stress (in Outer Fibers)  E = Modulus of Elasticity in Tension  v = Poisson's Ratio  This formula represents the quasi-linear change in bending stress from the state of residual stress in the  fabricated spring coil. (This method of calculating the bending stresses in a constant torque spring is  summarily discussed in the Associated Spring Design Handbook2, which also cites Votta's paper). The  spring manufacturer can intentionally pre-stress the spring in a process called strain hardening, to induce  residual bending stresses of opposite sign, and thus reduce the stresses when the spring is installed on  the hinge or spring motor. (It is tentatively believed that this was not done for the particular springs  involved in this case of stress relaxation).  Quasi-Linear Stress-Strain Curve for  E = 193,000,000 kPa (28,000,000 psi), nominal  (1 psi = 6.894757 kPa)  (Courtesy of the publishers of the ASMH,  Reference 3. The original data is from  the Allegheny Steel Corp, 1958).  0 0004 0008 0 0004 0.008  Strain. adin.  FIGURE 3.0212. STRESS-STRAIN CURVES FOR SHEET  AND STRIP COLD ROLLED TO FULL-  HARD AN0 EXTRA-HARD TEMPERS (11)  Figure 5. StresdStrain Curves for Type 301, Extra Hard Condition  Tensile and compression test curves for extra hard Type 301, from the Aerospace Structural Metals  Handbook3 (ASMH), are shown in Figure 5. A hypothetical (quasi) linear curve is added to the graph.  From comparison of calculated operating strains for constant torque springs with typical published tensile  or compression test data (as shown in Figure 5), it can be concluded that the behavior of steel strip at  high strain levels is different in bending compared to pure tension and compression. Type 301 extra hard  shows higher values of elastic modulus in tension than in compression. The tensile and compression  curves start to droop near peak values, such that the ultimate failure stress is less than would be  predicted by the nominal assumption that the stress strainkurve is linear all the way to ultimate.  Stressktrain curves published in the ASMH for extra hard Type 301 typically terminate at 1.0 percent  strain, (although some of the numerical data in the ASMH state elongation to failure greater than 1.0  percent).  Another example that infers greater elongation to failure in bending compared to simple tension is the  required bending test for a slightly less cold worked material, Type 301 full hard. (Stressktrain curves for  166
Mechanical Design of a Multi-Axis Martian Seismometer  Franck Pecal’, Nicolas Paulin*, Doug Mimoun” and Gabriel Pont”  Abstract  A planetary seismometer intended to fly onboard future missions to Mars has been developed by  SODERN as the industrial partner of IPGP and CNES. This seismometer is a Very Broad Band  seismometer (VBB). Its main purpose is to measure the Martian seismic activity. It is included in the  Geophysical Package of the ESA ExoMars mission (to be launched in 201 1).  This paper describes the mechanical architecture of the seismometer and identifies the main  challenges that were overcome. In particular, the paper points out how SODERN managed the  requirements for the extremely high sensitivity of the seismometer and coped with the very low size  and mass budgets together with the very harsh mechanical and thermal environments specific to  Martian missions (especially very low temperatures and severe shock when landing).  The lessons learned from the full test program results are presented.  Introduction  Since 1992, EADS SODERN has been the industrial partner of IPGP (Institut de Physique du Globe -  Paris) and CNES (Center National d’Etudes Spatiales) for the development and the industrialization of  planetary seismometers. Following the launch failure of the Mars 96 mission and the subsequent loss  of the seismometer “OPTIMISM”, in 2002 CNES and IPGP selected SODERN as their partner in the  development of a new multi-axes seismometer.  This seismometer is a Very Broad Band seismometer (-0 to 20 Hz). It is intended to measure the  Martian seismic activity in a network of seismometers. It was initially developed for the joint  CNES/NASA “NetLander” mission. Although this mission was stopped in 2003, development went OR  until the end of the B phase (preliminary design). The VBB is now part of the geophysical package  instrument suite of the ESA Exomars mission, to be launched in 201 1.  The aims of this B phase were to demonstrate:  0  *  0 The seismometer detection performance with a Breadboard model  The mechanical robustness and stability under thermal and mechanical environments with a  Structural and Thermal Model (STM)  The compliance of the definition of a future Flight model with regard to the main requirements for  the Martian missions  The VBB is currently the only planetary seismometer available for a launch towards Mars in 201 1. Its  characteristics make it compatible with Mars, Moon and Mercury environments. With some  modifications, it may be compatible with the “telluric” satellites of the outer planets such as Europa  and Titan.  f. EADS SODERN, Limeil Brevannes, France + CNES, Toulouse, France IPGP, Saint Maur, France  Proceedings of the 3@ Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  169
General Description  Princide  The seismometer is based on the principle of an inverted pendulum with an angle of 35.25" wrl  vertical axis (Figure 1). This kind of configuration gives a high sensitivity for a low mass and reduced  volume. It also allows detection in the horizontal and vertical directions with the same sensor. The  pendulum equilibrium is realized when the leaf spring moment balances the gravity moment.  Figure 1. Seismometer model  The leaf spring transmits the movement to the mobile mass when a quake occurs. The mobile mass  vibrates and oscillates around the axis of the pivot. The position of the center of gravity is governed by  Eq. (1):  d2a da  dt2 dt J -+ p- + k (a-a,) = -mgD, sin@-a,) + m0, ysof (1 1  with:  cx angular position of mobile part  J moment of inertia of the mobile part  ,O : coefficient of viscous friction  k: stiffness of the assembly pivot-spring  m: mass of the mobile part  g: gravity  The transfer function between the displacement of the mobile part X, and ground acceleration ysol  follows Eq. (2):  k - mgD, cos(@ Q=- 0,z.J  P m; =  J  The displacement of the mobile part is measured by two sensors: a short term sensor called DCS  (Differential Capacity Sensor) and a long term sensor called OCS (Oscillating Cavity Sensor).  170
A feed-back system, based on a magnetic coil actuator, is used to control the mobile part position and  set it at the equilibrium position. A closed-loop system is necessary to widen the measurement  bandwidth of the system and to give a representative output voltage of the ground acceleration.  Another digital feedback loop is used to compensate for the long term and daily thermal effects.  The principle of the seismometer is illustrated by the diagram block Figure 2:  CoiVMagnet Feed-back  Figure 2. Block Diagram of the VBB seismometer  VBB sensor  The Breadboard model of the Very Broad Band sensor developed during the R&D program is  described in Figure 3.  OCS sensor  (shielding) \  Inclined  fixedpart 1 CoiVMagnet  actuator '/- (shielding)  A Counter /  mechanism balance  Fixed  plate AB  Pivot Leaf  spring F7  c  c  -P Connectors 1 Earth compensation gravity  mass  L Mobile part  Figure 3. Breadboard model of the VBB sensor  In addition to the main parts of the seismometer, two other components are essential to operate the  seismometer. A counter balance mechanism is used to adjust the equilibrium position of the mobile  part. This adjustment is necessary according to the uncertainty of the local Martian gravity or the drift  due to the thermal sensitivity. An additional mass is also used to balance the VBB sensor, designed  for Mars gravity, during Earth testing.  Except the design of the housing structure, this breadboard model is very close to the flight model.  171
SDhere  “Sphere” is the name given to the seismic sensor part of the multi-axes Martian seismometer. The  sphere, in flight model configuration, consists of the following components:  0  0 Getters  0 Flex-rigid cables and feed-through  0  0 Radiative screen  0 Spherical covers  0 Sealing pipe 2 VBB seismometers in opposite sensing directions  Inclinometer, vacuum sensor, thermal sensors  Inner plate with insulating blocks  Structural ring (diameter of about 140 mm)  Vacuum is set in the sphere in order to reduce the viscous damping and the buoyancy (Archimede  thrust) on the mobile part. Figure 4 presents an overview of the flight model of the sphere, without the  upper cover.  VBB2  VBB 1  Insulating  block 7 plate J Inner  screen  ring  Figure 4. Flight model of the Sphere  The breadboard model of the sphere offers the same functions as the flight model. The breadboard  sphere, intended to test on the Earth ground, is composed of the breadboard models of the VBB  sensor. Consequently, the breadboard sphere volume and mechanical interfaces are a little bigger.  There is no radiative screen on this model.  172
Figure 5. Breadboard model of the Sphere  Technical Challenges Overcome  Struaalina for a low mass  The total mass allocated to the Martian seismometer, including sphere, installation device and  electronics, is 2.4 kg. The mass budget allocated to the sphere is only 540 g.  At the end of this Phase 6, we managed to get very close to the mass budget with 570 g, which  almost ensures matching the specification mass in a phase C. It is the result of a trade-off between  seismic performance, mechanical resistance and thermal sensitivity. Huge efforts were made to  miniaturize all functions and match with the mass specification while matching at the same time with  the necessary robustness.  Low mass vs. seismic Performance  Theoretically, the bigger the mobile part mass is, the better the seismological performance is for an  open-loop seismometer (the sensitivity of a seismometer varies with m/k, with m the mass and k the  stiffness). For example, the seismometer STS2 which is the current reference on Earth weighs 11 kg.  The use of a feed-back loop on the mobile part position is a way to reduce its mass. Concerning the  fixed part, some rather heavy but, unfortunately compulsory components - the getter and the  shielding for instance - makes the “fight” for a low mass even more challenging.  Low mass vs. mechanical resilience  As an other big issue with regard to a low mass, the structural part has to be stiff enough to not  interfere with the oscillations of the ground and resistant enough to withstand the landing on Mars.  The mass of the structural part has been reduced by calculations on a finite element model and  especially thanks to the use of stuck glued and welded assemblies (instead of bolts and screws).  Low mass vs. thermal sensitivity  Finally, concerning the materials, the choice was led by the thermal sensitivity and the need of  vacuum. Titanium (TAGV or T40) was selected for all the parts participating in the oscillations  measures (ring, plate, fixed part and mobile part) except for the insulating blocks. Titanium is of  course an additional difficulty with regard to the low mass objective.  For the covers, an alternative to the titanium was explored. With a composite material in carbon  covered with a metal deposit, the mass of the covers can be reduced by 40% (35 9). Unfortunately  the current technologies of deposit on composite materials do not guarantee a sufficient level of  173
tightness for the vacuum desired inside the sphere. Consequently the covers are made of titanium  (T40) by hydroshaping with a 0.3-mm thickness only. This thickness represents the very limit of this  kind of technology. The mass of the DCS and OCS shielding has been optimized by the use of  aluminium with a 0.4-mm thickness.  Hiah thermal insulation from Mars environment  The thermal environment on Mars is very tough. The minimum temperature can be as low as -120°C  in the polar region and the daily amplitude is about 110°C. This environment imposes a very high  thermal insulation to allow the operation of the electronics and to reduce the thermal sensitivity of the  sensors. A thermal regulation inside the sphere is not possible because of the perturbations it would  cause on the seismic signal.  The internal insulation of the sphere has been reached by a drastic reduction of the conductive and  radiative heat transfer. In fact, there are mainly three thermal paths between the inside and the  outside of the sphere.  The first thermal path is the conduction through the structure. Although the titanium TA6V has a rather  low conductivity, it is not sufficient. The high thermal resistance expected has been raised up thanks  to four insulating blocks in TorlonTM (composite material with short fibers). This assembly (Figure 6)  gives a huge thermal resistance of about 2000 WW. Such value also requires that the block be fixed  by an adhesive joint on the structural ring.  Insulating dr  Figure 6. Insulating block assembly  The second thermal path is the conduction through the internal harness. Indeed, many electrical  signals have to be shielded and these shields constitute a real thermal path. The thermal resistance  on this path has been increased thanks the use of flex-rigid cables. Also interesting for the integration,  this solution allows an optimal section of the wires and especially the thermal insulation of the  electronic shielding. The electrical connection of the shielding is made in a very local way with a thin  track.  Finally, the third main thermal path is the radiative exchange with the covers. This exchange was  reduced by the set-up of radiative screens and by the use of a gold coating. The 2 radiative screens  (upper and lower) are made from a sheet of titanium of only 50 pm in thickness. Then, this sheet is  shaped and welded by point to recreate the spherical shape of the covers. This part cannot be  obtained by machining or hydroshaping. Little insulating blocks in TorlonTM are also used for the  screen assembly (Figure 7). They are stuck on both sides on the covers and the screen.  174
radiative  screen block  Figure 7. Upper radiative screen assembly  Withstandina he shock when landing  When the surface module that contains the seismometer, lands on Mars a very violent shock is  generated. This shock, equivalent to a half-sine 200 g - 20 ms, is relatively long and thus very  energetic. The seismometer resistance to this shock is a real challenge for all the subassemblies but  especially for the mobile part because of the absence of a locking mechanism. The power and mass  budgets do not allow using any locking devices.  Good performance was obtained by stiffening the structure and by placing mechanical final stops.  Much work was carried out on the pivot because it is the most sensitive element as far as mechanical  requirements. The pivot Figure 8 is based on a flexible blades assembly. The flexible blades are stuck  on two rigid parts (fixed and mobile part). Consequently, the pivot supports all the loads of the mobile  part and permits only a limited movement.  Final stop L- -  I  -  Lowerbody \ Flexible (mobile part) - _-  blade  Figure 8. Pivot 20 blades  In order to have acceptable stresses in blades, mechanical final stop are designed to limit the  deformation of the blades at f50 vm.  175
Thermal sensitivitv avoidance  Because of the great amplitude of temperature and the small stroke of the mobile part (200 pm), the  VBB sensor has to be insensitive as possible to temperature variations. Our requirement is a mobile  part displacement of 2 pm for a temperature variation of 1°C (measured at the DCS level).  To reach this very critical requirement, material homogeneity is essential in order to avoid any shift of  the center of gravity of the mobile part. All structural parts of the VBB sensor are titanium TA6V.  However, this is not enough and the use of a very specific material for the leaf spring, the  THERMELAST, is necessary. The advantage of this material is the possibility to adjust the  thermoelasticity coefficient j3 (Eq. (3)) with an appropriate heat treatment.  (3) p=-.- 1 AE  E AT  with:  E: Young modulus  T temperature  For the THERMELAST, /3 values can be adjusted in the * 16.1 0-6 K’ range. For example, when the  temperature is cold (lower than the integration temperature), the dilatation makes the distance to the  center of gravity smaller and pulls the mobile part upward. If the leaf spring is less rigid when the  temperature is cold (/bo), a moment balancing can be found.  Obtainina and keepina vacuum  To reduce the viscous damping and the buoyancy (Archimede thrust) on the mobile part of the  seismometer, vacuum is necessary inside the sphere. The vacuum level at the end of the nominal  mission (2 years) must be less than lo-* mbar. This requirement imposes a sealed volume, a very low  outgassing for the materials placed inside the sphere, and a passive pumping after the sphere  sealing. The tightness is obtained by laser welding for the covers and by brazing for the feed-throughs  and sealing pipe assemblies. The sealing pipe (copper tube) is used for the pumping and the closing  of the sphere.  Structural  ring w  Figure 9. Section of the sealing pipe assembly  A low outgassing requires adapted material (plastics have to be avoided or minimized) and a  preparation before sphere sealing. A preliminary outgassing under vacuum is performed for all the  sphere components in order to evacuate the solvents molecules. Then an oven drying of the sphere  at 120°C during the pumping is done to evacuate the water molecules. The oven drying at 120°C is  extremely challenging for the material and adhesive joint choice.  176
In spite of all these preparations, the materials keep on outgassing inside the sphere which is not  acceptable. Getters are necessary to absorb these gasses and thus ensure vacuum (the getter  material works as a sponge with the gas molecules).  Unfortunately, the use of this technology leads to some very tough thermal constraints. When  activated, the getter’s temperature raises up to 900°C (current of 8 A). As a consequence, the getter  has to be strongly thermally insulated. The getter assembly (Figure 10) was designed with insulating  braces made out of ceramic, and surrounded with radiative screens. Screens are opened and placed  so as to mask the getter from the outside.  Getter  connection  Inner  radiative  screen  Outer  radiative  screen  screw  Figure 10. Getter assembly  Hiqh level of intearation  Beyond the studies, the seismometer integration is also a big challenge. For example, the OCS and  DCS assemblies (Figure 11) are made with M1.6 screws. The gaps between the fixed and mobile  electrodes of these sensors (6 electrodes) are adjusted at 250 pm with an accuracy of +lo pm. These  adjustments are reached thanks to measurements on an optical bench.  Fixed DCS  Setting  shims  Figure 11. DCS and OCS assembly  177
The wiring is also an example of the high level of integration. First, the pivot is used to make the  electric interconnection between the fixed and mobile parts. It requires a precise number of blades for  the pivot and also requires that the blades be insulated from the structure. The need of 20 blades for  the VBB sensor makes the pivot assembly very hyperstatic and thus leads to a relatively wide  distribution of the pivot stiff ness.  On the other hand, the wiring is made by using flex-rigid cables. The flex-rigid cables make their way  through the structure towards each component (OCS, DCS, coil, mechanism.. .). These flex-rigid  cables are composed of 4 layers of 12.5 pm in thickness for the electric signals and their shielding.  Figure 12 shows the wiring of the mobile part with the pivot. b- 1 - Flex-rigid  cable  Pivot  wiring  LA  Figure 12. VBB mobile part wiring  Finally the wiring requires specific feed-throughs to take out the electric signals of the sphere under  vacuum. There are 2 feed-throughs for the getter power supply and 2 feed-throughs of 42 pins for the  VBB sensors wiring (Figure 13).  Figure 13. VBB Sensor feed-through  Modelling  VBB sensor modellinq  The mechanical modelling of the VBB sensor is essential to reach the performance required for the  pendulum resonance, the mechanical sensitivity, the thermal sensitivity and the shock withstanding.  The VBB sensor was modelled on ABAQUS because of non-linear calculations (contacts on the final  stop and large displacements for the leaf spring). Great care was taken to achieve the simplified  model of the VBB (Figure 14) in order to give the best predictions. The model is based on shells,  beams and concentrated masses. Each component was updated on tests or on detailed models.  178
Figure 14. Finite element model of the VBB sensor  Concerning the performance, Table 1 presents the modelling results. After the stiffening of the pivot  (upper and lower body), the shock calculations give positive margins on stresses and efforts in the  assemblies. These results validated the final stop setting at 50 vrn.  Table 1. Modelling results of the VBB sensor  Pendulum 0.35 Hz 0.70 Hz  Spring 81 Hz 81 Hz  Pivot 90 Hz 96 Hz  Fixed Dart 137 Hz  sensitivity I Earth config. Mars config.  1.39 1 o-2 s2 2.38 s2  1.99 1 O2 s2 3.38 102 s2 Vertical  sensitivity  Horizontal  sensitivity  0.9 pm/OK  B= +16.1 O6 K-'  Mechanical desicln  The mechanical design of the sphere structure was verified and optimized based on a finite element  model of the sphere (Figure 15). The model was developed on I-DEAS.  179