ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
95  

Figure 10.  Minimization of Motor Detent Torque 

 Alignment Concept

Based on the existing tolerance chain between Drive Unit and Base Plate, Drive Shafts and Reflector and  between Base Plate and Feed Horn (Mounting Brackets), the implementation of a suitable alignment concept was mandatory in order to fulfil the pointing and performance requirements.   In order to cope with the schedule constraints duri ng the integration and test phase, it would have been  favorable to avoid as far as possible a time intensive alignment concept. For ex ample, concepts using in  situ shimming and adjustment of the individual com ponents during the integration and test process are  time and effort intensive.  Therefore, two alignment alternatives were discussed in this context:  one was to manufacture the relevant mating parts with sufficiently high accuracy in order to avoid shims wherever possible. This alternative was found not to be adequate, since t he involved tolerance chains did not allow to  manufacture the individual parts with sufficiently  high precision without high additional effort.    The second alternative was to provide active adjustm ent and alignment provisions  already by design, so  to allow alignment by means of suitable adjustment sc rews after integration. This alternative was skipped  due to the fact that it added high design complexity, im posed stiffness and load capability limitations and  would have increased mass and moment of inertia. Therefore it was decided to go for a conventional shimming approach and to adjust first the Reflectors ab out two axis in the interface between Drive shaft  and Reflector and then to shim the feed horn support structures with respect to the Reflectors.   The correct Reflector alignment with respect to the feed horn was supported by using Reflector best fit  analysis data as an input for the 3D measuring machine during the reflector alignment process.  A zero referencing possibility in rotational direct ion was additionally implemented in the electronics  command interface in order to allow for compensati on of rotational misalignment of the Reflectors  between the S/C mechanical and electrical zero.        Detent Tor que  Rotor Rotation An gleDouble stators aligned 

NASA/CP-2010-216272
96 Encoder Supply Voltage   During the last environmental test which was t he common TV test of ASM (Mechanism) and MCDE  (Electronics), an error was observed in the main path of the equipment. The observed phenomenon was  a randomly occurring incorrect start command execution of the scanning function after switching from the  redundant to the main path.  The error was observed at first during cold TV condit ion in the main equipment path however it could later  be reproduced also on the redundant path and also a fter cross-strapping between main electronics and  redundant mechanism. Therefore, it was concluded that a systematic random error had to be considered.   After detailed check of the electronics function and al so of the mechanism integrity, the detailed root  cause investigation led to the suspicion that the erro r must be produced by the power supply used during  equipment test. It was proven by test, that the pro bability to reproduce the random error was depending  on the actually used power supply unit.  Therefore the power supply characteristics were in vestigated in detail and it was found that the encoder  supply voltage (5 V) provided to the encoder had a rise time of >20 ms. Detailed discussion with the encoder supplier led to the conclusion that this fact  could cause a starting issue in the encoder ASIC. The  RC part of the ASIC has time constant of <10 ms so that the encoder start-up could not always be executed in a reliable manner.   As a consequence a switch box was integrated into  the output line of the power supply so to allow  switching the encoder voltage by a separate command after switching on the power supply in order to  guarantee a voltage rise time of < 5 ms. By including the additional switch box, the scanner start up issue was solved and the TV test was successfully repeated. 

Scan Performance during Ground Test 

The key functional performance requirements of the Antenna Scan Mechanism are its high velocity  constancy during the earth scan period and the requested minimum time consumption for the motion time between two earth scans (hot/cold calibration and mo vement between these positions) in order to  maximize the useful observation time. This was achieved by optimizing the motor dimension and its output torque and dynamics in combination with a mini mized moment of inertia of the rotating masses.   The achieved measured velocity and acceleration profile of two subsequent scan periods is shown in  Figure 11. The maximum angular velocity ω is about 9 rad/s, the maximum achieved acceleration is about  150 rad/s 2 while the maximum deceleration is up to 180 rad/s2 (deceleration is supported by bearing  friction torque). 

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97  

Figure 11. Measured Performance Data (Velocity and Acceleration) 

Orbit Performance 

The temperature variation of the mechanism over t he orbit was analyzed during the development phase.  The housekeeping data obtained during the mission show good correlation to the analysis. An active  thermal heater concept for the instrument is not needed due to a trimming radiator. The available orbit  data of the mechanism demonstrate that the 183 GHz Fr ont-end remains at a very stable temperature of  12°C over one orbit while the 150 GHz Front-end temperature varies between 20 and 22°C.  In Figure 12, the average current needed to drive the scan mechanism is plotted over time. Since the current needed to drive the system is a good indicator of the mechanism health status and especially of its bearings, such data are extremely helpful to gain information for similar flight designs requesting non-uniform bearing rotation.  As can be seen, the current is at  about 0.37 A during a first orbital run-in phase lasting for about 3  months. After this time, the current decreases to about 0.35 A and finally to an average value of about  0.34 A after a time of about 9 months. The overall number of scans performed within the time frame  shown in Figure 12 amounts to about 16.5 million revs (almost half of the nominal operational life).  MHS measured performance data -15-10-5051015 Time [ms] Time [s]Velocity [rad/s] -200-150-100-50050100150200 Acceleration [rad/s^2]Velocity [rad/s] Acceleration [rad/s^2] 1 2 3 4Earth scan  Cold space calibration hot calibration  NASA/CP-2010-216272
98   Figure 12. Motor Current Curve from June 2008 to Nov 2009. 

Conclusions and Lessons Learned 

Electron beam welded connections (electron beam) we re found to be not adequate for the application and  should be carefully investigated in view of thei r load capability and durability under the given vibration  loads. If possible, alternative solutions should be considered.  Alignment concepts using accurately manufactured parts provide schedule advantage over alignment concepts using shimming methods, however dependi ng on tolerance chains they are often hard to be  realized. Actively adjustable shimming provisions, e.g., alignment screws, etc., suffer from mass and stiffness constraints and add design complexity. Ther efore, the conventional in situ shimming method  using shim washers during integration and test  process was found to be the most effective one.   The detent torque of the motor using two independent motor stators could be re duced by turning one of  the redundant motor stators by half a stator slot pitch with respect to  the other in order to smoothen the  overall detent torque amplitude.  The output characteristics (voltage rise time) of power supplies might be important for the correct  operation of sensitive equipment (e .g., optical encoders). The voltage rise time of power supplies and  potential requirements from equipment side should be carefully cross checked before using standard  power supplies on sensitive equipment.  The drive concept using liquid lubr ication and two pairs of thin ring ball bearings in face-to-face  arrangement provides good life perfo rmance at continuous extreme accele ration / deceleration conditions  as in the presented application. Based on the available current housekeeping data, the current is very  stable over time and no bearing degradation which woul d result in a higher torque /current level is  observed after 2 years of  continuous scanning operation (24 million revs).      NASA/CP-2010-216272
101 Thermal Stability: During operation, the CPA may poi nt in the direction of the sun or to deep space  for quite long durations. The operational thermal range of the CPA poses a true  challenge for material choice and component or design selection. Furthermore, thermo-mechanical stability throughout the operation at these temperature  extremes must be guaranteed.  Radiation Hardness: Teleco m satellites operating in GEO orbits have generally a comparatively large  life time, often 15 years or more. Thus, radiation hardness of the equipment must be very high, which reduces the number of choices for electronic components and surface coatings of mechani cal or optical parts, or may even  impose the need for shielding.  High Load Capability: To enhance versatility and ext end the scope of application, the quasi-static  loads have been chosen accordingly. 

CPA Design Overview   General Overview

Figure 1 shows the assembled CPA without the MLI thermal cover.  

Figure 1.  Assembled CPA without MLI protection   As shown in Figure 2, the CPA consists of the following main elements:  • Azimuth Actuator Assembly, containing a bearing set, a BAPS, a brushless DC motor and an  optical encoder. This assembly is explained in more detail further on in the text.  • Azimuth Supporting Structure, an ultra light weighted AlBeMet structure that connects the  Azimuth Actuator to the Elevation Actuator  and supports the Azimuth Mirror Assembly  • Cable Wrap, for transmission of power and signals over one rotation axis to the next.  • Azimuth Mirror Assembly, which is composed of the Azimuth Beryllium Mirror with a highreflectivity coating, its iso-static  support and the Azimuth Mirror Support.  • Elevation Actuator Assembly, same conc ept as the Azimuth Actuator Assembly.  • Elevation Support Structure, same concept as the Azimuth Support Structure.  • Elevation Mirror Assembly, same co ncept as the Azimuth Mirror Assembly.  • Park Position Assembly (PPA) incl. Launch Lock, where the functions of the Launch Lock are to  rotationally lock the CPA and to increase the firs t Eigenfrequency to the minimum required value.   Making use of significant synergies, the two Ac tuator Assemblies have been designed to be identical.  NASA/CP-2010-216272
102 Further elements that are not shown in the figures are the Multi- Layer-Insulation and thin film heaters,  which enable the thermal control of the CPA. 

Figure 2.  CPA exploded view   Actuator Units

The CPA mechanism core components are the two Ac tuator Assemblies. The requirements to these  assemblies can be summarized as follows:  • Provide the rotation possibility with a bearing system that has:  • a minimum of resistance torque  • very low torque noise  • a long life in terms of time duration as well as number of cycles  • a high tolerance of quite large temperature gradients across the bearings, which is especially  important for the GEO applications  • Allow the optical beam to pass through the centre of the CPA.  • Include the brushless DC motor  • Include the 24-bit resolution optical encoder  • Withstand the challenging launch loads.   Typically, the first point can be reached through a soft and low preloaded bearing system (see Ref. 1). However, this is in contradiction to the last point, which generally asks for stiff, highly preloaded systems preventing too much gapping in the bearings. Together with ESR Technologies, the solution to this challenge was found by using an evol ution of their original Bearing Active Preload System (BAPS) QM  (see Ref. 2). The BAPS can be considered a bearing housi ng consisting of a monolithic titanium structure  of three coaxial rings which are joined by pairs of thin , blade-like flex-struts as shown in Fig. 3. The upper  and lower rings are interfaced to the bearings to be preloaded whereas the middle ‘synchro-ring’ can be rotated through a small angle (typically ~10-15mrad),  thus deforming the flex-s truts and axially displacing  NASA/CP-2010-216272
103 the upper ring with respect to the lower (typically by 30-100 µm) so changing preload whilst retaining tight  control of the bearing ring planarity.  

Figure 3.  Actuator Unit with BAPS   In the high preload state for launch  the flex-struts are slightly deformed from the nominal as-manufactured  fully straight ‘Top Dead Centre’ (TDC) position as show n in Fig 4. In order to actuate the device a torque  or tangential force is applied to the synchro-ring causing its rotational displacement past the straight strut  position “Top Dead Centre” (TDC) initially to low-pr eload balance point (which is optimally low preload  stiffness since the synchro -ring is un-restrained) at which point th e residual preload in the bearing system  is balanced by the elasticity of the flex-struts in bending.   The BAPS structure is inherently stiff and stable in the high preload stat e for launch when the flex-struts  are relatively lightly stressed even by launch vibration loads. The stru ts also serve to synchronize the  motion so that the parallelism of the axial motion of the upper and lower bearing housing is extremely high. 

Figure 4.  Preload States of BAPS   Thus, the BAPS provides an ideal solution for the applic ation, and eliminates the compromise required  with a passive system. In the high-preload state, it provides high stiffne ss for launch in order to prevent  bearing gapping, a concern for bearings of the highes t surface finish quality, and in order to protect the  close clearance between the rotating and static discs of the high precision encoders from touching. On-orbit, the BAPS is switched to low preload and low stiffness in order to provide very low inherent sensitivity to thermal strains, and low torque noise in absolute terms.   The specified design load parameters to the BAPS are shown in Table 1.     NASA/CP-2010-216272
104 Table 1.  Specified Design Load Parameters of the BAPS 

The actuation of the BAPS from high to low preload is  achieved by a bi-directional Shape Memory Alloy  (SMA) actuator, which engages into the synchro ring  with an SMA contact. The SMA actuator activates  upon introduction of heat into  the heater plate. When pow er is switched off, the SM A contact returns to its  original position and leaves  the synchro-ring free, such that the BAPS is in a minimum energy balance  preload condition. The resetti ng of the BAPS is done manually. 

Figure 5.  BAPS SMA Actuator   In order to allow for a large optical beam in the cent er of the mechanism in combination with low torque  noise, the previous experience (see Ref. 1) has b een recalled and thin section angular contact ball  bearings are utilized in back-to-back configuration. Ceramic load carrying balls are used with undersized steel spacer balls. The bearings are lubricated with Fomblin Z25.  As mentioned above, the motion is enabled by a brushless DC torque motor. The motor has dual windings for redundancy reasons. A frameless design has been selected in order to minimize mass. It has to be noted, that the volume constraints are very stringent. In order to minimize the length and diameter of the system, the motor is placed betw een the central Hollow Shaft and the BAPS structure.    Finally, the actuator units are equipped with 24-bit Op tical Encoders from Codechamp, which allow for a  very smooth control and hence contribute to the achievement of the jitter requirements in motion direction.  Cable Wrap

A major challenge for many multi-axis motion systems, especially with large angular motion ranges, is the transmission of power and signals over one rotation axis  to the next. In order to keep the design simple  and close to previous heritage, a flex print design has been utilized in Omega configuration (see also Ref.  1). However, the number of transferred lines is much larger than used in Ref. 1 and the EMC shielding requirements are more complex. Hence, a flex prin t consisting of three ribbons has been implemented.  One advantage of this design is that the power lines and the sensitive enc oder signals can be routed over  different ribbons, providing naturally better protection from interference. Again, for redundancy reasons, the flex print exists in primary and redundant configuration in the CPA. 

   NASA/CP-2010-216272
105 Launch Lock   When utilizing a BAPS, a launch lock system in the classical sense of a launch  protection system or a  bearing off-load system is not required. The reasons  for implementing a launch lock into the CPA  nonetheless are the following:  • The COG of the rotating parts, especially arou nd azimuth axis, is out of center. Hence, lateral  accelerations can cause rotational movements, which need to be prevented.  • With the introduction of a pure rotational lock, the required minimum Eigenfrequency of the CPA  could not be met, mainly due to the nodding and bending modes across to the azimuth axis.  •   For the launch lock design key design drivers were minimizing the required volume and the particular  location of the CPA with respect to the terminal’s support structure. The CPA is placed with its azimuth  axis onto the centre of this support  structure and in launch configuratio n the elevation axis points to one  of the supported corners of the plate.  In summary, the requirements to the implemented launch lock are:  • Provide a rotational lock to both axes  • Increase the Eigenfrequency of the CPA above the min. required  • Do not bypass loads of the support structure via the CPA bearings  • Only use one actuator that can be  reset and reused during ground testing  • Minimize the overall used volume   The final implementation is shown in Figure 6. The rotation locking is achieved by a latch that engages with a lock on the moveable part of the ESU struct ure. In addition, the latch positively preloads the  Elevation structure into the flex supported contacts on either side, which thus stop rotational motion  around the Elevation Axis. The preloading is achieved by a lock spring. The release of the system is performed by a HOP actuator and monitored by mi cro-switches. With this design, all of the above  mentioned requirements are met.  

Figure 6.  Launch Lock   Optical Elements

Along with the mechanization units, the performance of the mirrors is of critical importance. Figure 7 depicts the azimuth mirror assembly in its holding jig  ready for integration on the CPA. The core of the  mirrors is of pure Beryllium coated with Nickel. The Nickel is then polished to the required surface quality of around 10 nm RMS and later covered with a pr otective silver coating. The key performance  characteristics of the optical coating are high refl ectivity in the target frequency band (>99.5%) and  polarization efficiency (>0.99). One particularly noteworthy aspect of the mirror units is the iso-static mounting concept. While providing the necessary stab ility, stiffness, and str ength for the high launch  loads, the mounting concept also provides thermal decoupling and by this means high tolerance towards thermo-mechanical influences.  NASA/CP-2010-216272
106 Maximized stray light suppression is achieved by blac kening all elements in the optical path. Thereby, the  establishment of stable processes for black Titanium and black Nickel on AlBeMet proved to be veritable  challenges. Eventually, both key processes were successfully space qualified. 

Figure 7.  The azimuth mirror unit ready for integration on the CPA.     Achieved CPA Major Performance Characteristics   The CPA is currently under testing. Based on a nalyses and tests performed so far the following  performance data can be reported: Pointing Range: ±175° (azimuth), -20/+200° (elevation) Pointing Accuracy: ± 270 μrad, under TV conditions  Jitter performance: ≤ 2 μrad RMS  Mass: < 15.2 kg including MLI and cables 1st Eigenfrequency: > 180Hz Max. power consumption: < 5W The following environmental conditions are covered: Quasi-static Launch loads: 70 g Vertical Random Inputs: 15.2 g RMS Lateral Random Inputs: 9.6 g RMS Operational Temperature Range: -30°C to +45°C at interface  Non-Operational Temperature Range: -30°C to + 55°C at interface Operation Temperature Tolerance: Geo-Geo co ntinuous operation, direct view of the  sun; cold operation in earth shadow, view of deep space.  Max. thermal gradients across bearings: -11°C to +11°C Operational life time: > 15 years  Various sub-unit level tests have been performed so far, among which the most interesting is the torque and torque noise test. Figure 8 shows that the bearing system, running at low preload, has a mean resistive torque of < 0.04 Nm and that  the torque at high preload is ~ 0.2 Nm (i.e., factor of 5). It also can  be seen, that the torque noise is in the order  of 0.01 Nm peak to peak or 0.003 Nm RMS. 

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107  

Figure 8.  Torque and Torque noise measurements 

Figure 9 shows the achieved in plane and out of plane random vibration input levels: 

Figure 9.  Random vibration tests: achieved inputs in plane (left side) and out of plane (right side) 

Lessons Learned   During previous phases, RUAG has achieved considerable success in optimizing and further developing existing designs to meet the above discussed tight requirements. Some examples are discussed in the  subsequent sections.  Flex Prints for Rotary Power / Signal Transfer 

As mentioned above, the cable wrap of the CPA is made of flex prints. This solution was chosen based  on heritage with the previously developed CPA (see Re f. 1). For this CPA, three ribbons are staggered on  either side and connected to the same connector. Th is led to the situation that initially manufactured  prints formed bumps and buckles and their roll-do wn had impacts on the torque noise of the global  system, which would reduce the sm oothness and thus the accuracy of the CPA. Therefore, RUAG Space  improved the design together with the manufacturer. Subsequently the optimal shape of the flex prints was found.                    NASA/CP-2010-216272