ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
234   Figure  11: Actuation principle of  the DEXHAND fingers 

The cardanic metacarpophalangeal (MP) base joint is driven by two motors. Due to the coupling of the  tendons in the MP joint the two motor torques can be used together for one DOF of the base joint. This aspect opens the possibility of using the same motors for the base joint and the proximal inter-phalangeal (PIP) joint. Indeed, due to the difference of lever length (pulley radius), the required torques are scaled dependent to each joint. The PIP joint has a fixed coupling with the distal inter-phalangeal joint (DIP) with a ratio of 1:1. In Figure 11 the section lengths, joint positions and definitions are shown. The coupling matrix P, which relates motor velocity θ with joint velocity q is: 

ሶሶ                                                                    (1) 

=ܲ ଵ ௥೛ଶ0 ଶ0 ଷ൱                                            (2) 

Where r p is the motor pulley radii, r 1, r2 and r 3, are the joint pulley radii, and r 13 and r 23 are the pulley  radius of the PIP tendons in the base. Given that the coupling matrix is not configuration dependent, the relationship can be integrated in: 

ݍܲ=ߠ 3)    The motor unit for DEXHAND has been developed based on the DLR / RoboDrive (see section1) [2] ILM 25 motor including the gear of a harmonic drive HFUC8 with a transmission ratio of 100:1. The whole unit fits into a cylinder of 27-mm diameter and a length of 17.5 mm with a weight of 46 g (see section 1 Figure 4). The unit provides a continuous torque of 2.4 N-m with peaks up to 9 N-m which is the maximum peak torque of the gearing. In the DEXHAND, the motor has been electronically limited to 2 Nm for power reasons. Each actuated joint has a reference mark in the middle of its motion range. These reference marks are composed of a small magnet and a Hall-effect sensor located in the actuated joint. The joint torque measurement is implemented using a sensing body and full bridge strain gauges sensors (Figure 12). The torque sensors are all physically located in the proximal finger link. Therefore, the sensor for the PIP/DIP-joints measures the reaction torque of the coupling tendons.
235   Figure 12: a.) Finger without housing, b.) Strain gauge sensor body 

All sensors, except one of the reference sensors of the MP joint, are located and mounted in one mechanical part. This part also contains the pull relief of the wires, as well as the shield connection of the cable from the fingers to the electronics in the palm. This simplifies the assembly and the maintenance of the finger. Special care was taken in the design of the sensor body in order to prevent temperature drift. The force measurements obtained in a thermal chamber from −50° to+70° confirmed the measurement  stability. The palm structure consists of 11 main segments. These segments are massive aluminum parts to improve heat transfer and increase the thermal inertia. The modules of the DEXHAND are four different ensembles representing the ring-, middle-, index- and thumb finger actuation units. A unit includes the tendon guidance from the motor pulleys to the MP and DIP joint. The palm surface mainly consists of the outer shell parts. Furthermore, all parts are designed without sharp edges. They are optimized for ideal thermal allocation and minimum resistivity (Figure 13 shows the hand without the palm grasp pads).  

Figure 13: a.) Section view of DEXHAND with its components, b.) PCB placement in the wrist 

The wrist houses all electronic parts. This includes the analog, the digital and the power circuit boards. The electronic boards are coupled to the palm assembly with connectors. The housing of the DEXHAND is composed of 2-mm aluminum shells. It is fully closed to provide good EM compatibility. During electro static discharge (ESD) tests with 4-kV impulse at the fingertip and palm structure no failure has been detected. The final DEXHAND design is presented in Figure 13a. Furthermore the compact and also fully integrated housing of the electronic inside the wrist is shown in Figure 13b. 

Software and Control  As presented in the previous section, the hand comprises a DSP and a FPGA. The FPGA is used for low level, high speed functional blocks. In the DSP, functional blocks such as calibration tables, impedance control loop, communication, and safety systems are implemented. The controller itself is an impedance controller running at 1 kHz.  The communication to the system is done via a CAN bus, on which no hard real-time data is allowed. The commands to the system are always asynchronous. In order to establish telerobotic scenario, a VxWorks
237 [5] C. Preusche, D. Reintsema, K. Landzettel, M. Fischer, and G. Hirzinger, “DLR on the way towards  telepresent on-orbit servicing,” in Proc. Mechatronics & Robotics 2004, 2004.  [6] C. Preusche, D. Reintsema, K. Landzettel, and G. Hirzinger, “ROKVISS - towards telepresence control in advanced space missions,” in Proc. 3rd. International Conference on Humanoid Robots (Humanoids 2003), Munich and Karlsruhe, Oct. 2003.  [7] B. Schäfer, K. Landzettel, A. Albu-Sch¨affer, and G. Hirzinger, “ROKVISS:  Orbital testbed for tele-presence experiments, novel robotic components and dynamics models verification,” in Proc. 8th ESA Workshop on Advanced Space Technologies for Robotics and Automation (ASTRA), Noordwijk, The Netherlands, Nov. 2-4 2004. [8] K. Landzettel, B. Brunner, R. Lampariello, C. Preusche, D. Reintsema, and G. Hirzinger, “System prerequisites and operational modes for on orbit servicing,” in Proc. ISTS International Symposium on Space Technology and Science, Miyazaki, Japan, May 30-June 6 2004.  [9] G. Hirzinger, K. Landzettel, and et al., “ROKVISS robotics component verification on ISS,” in Proc. of ’the 8th Int. Symposium on Artificial Intelligence, Robotics and Automation in Space - iSAIRAS, Munich, Germany, 2005.  [10] A. Albu-Schäffer, W. Bertleff, B. Rebele, B. Schäfer, K. Landzettel, and G. Hirzinger, “Rokviss - robotics component verification on ISS current experimental results on parameter identification.” in ICRA. IEEE, 2006, pp. 3879–3885.  [11] J. Butterfaß, G. Hirzinger, S. Knoch, and H. Liu, “DLR’s Multisensory Hand Part I: Hard- and software architecture,” Proceedings of the IEEE Int. Conf. on Robotics and Automation, 1998.  [12] C. Borst, M. Fischer, S. Haidacher, H. Liu, and G. Hirzinger, “DLR hand II: experiments and experiences with an anthropomorphic hand,” in ICRA, 2003, pp. 702–707.  [13] Z. Chen, N. Y.Lii, T. Wimboeck, S. Fan, M. Jin, C. H. Borst, and H. Liu, “Experimental study on impedance control for the five-fingered dexterous robot hand DLR-HIT II,” Proceedings - IEEE IROS, 2010.  [14] M. Grebenstein and P. van der Smagt, “Antagonism for a highly anthropomorphic hand arm system,” Advanced Robotics, no. 22, pp. 39–55, 2008.  [15] A. Wedler, M. Chalon, and et al., “DLR’s space qualifiable multi-fingered dexhand,” in Proc.:11th Symposium on Advanced Space Technologies in Robotics and Automation (ASTRA), vol. 11, ESA. ESA/ESTEC, Noordwijk, the Netherlands: ESA, 12 14 April 2011, p. Session 3a.  [16] M. Chalon, A. Wedler, and et al., “Dexhand: A space qualified multi-fingered robotic hand,” in Proc. IEEE Int Robotics and Automation (ICRA) Conf, 2011, pp. 2204–2210.  [17] M. Chalon, T. Wimböck, M. Grebenstein, and G. Hirzinger, “The thumb:  Guidelines for a robotic design,” in IROS, 2010.
239 Mars Science Laboratory Rover Integrated Pump Assembly  Bellows Jamming Failure 

Michael R. Johnson1*, Joel Johnson*, Gajanana Birur*, Pradeep Bhandari* and Paul Karlmann* 

Abstract    The Mars Science Laboratory rover and spacecraft utilize two mechanically pumped fluid loops for heat transfer to and from the internal electronics assemblies and the Radioisotope Thermo-Electric Generator (RTG). The heat transfer fluid is Freon R-11 (CFC-11) which has a large coefficient of thermal expansion. The Freon within the heat transfer system must have a volume for safe expansion of the fluid as the system temperature rises. The device used for this function is a gas-over-liquid accumulator. The accumulator uses a metal bellows to separate the fluid and gas sections. During expansion and contraction of the fluid in the system, the bellows extends and retracts to provide the needed volume change.  During final testing of a spare unit, the bellows would not extend the full distance required to provide the needed expansion volume. Increasing the fluid pressure did not loosen the jammed bellows either. No amount of stroking the bellows back and forth would get it to pass the jamming point. This type of failure, if it occurred during flight, would result in significant overpressure of the heat transfer system leading to a burst failure at some point in the system piping. A loss of the Freon fluid would soon result in a loss of the mission. The determination of the source of the jamming of the bellows was quite elusive, leading to an extensive series of tests and analyses. The testing and analyses did indicate the root cause of the failure, qualitatively. The results did not provide a set of dimensional limits for the existing hardware design that would guarantee proper operation of the accumulator. In the end, a new design was developed that relied on good engineering judgment combined with the test results to select a reliable enough solution that still met other physical constraints of the hardware, the schedule, and the rover system.   Introduction 

The Mars Science Laboratory Mission - Overview  The mission consists of four discrete sections or phases:  1. Launch Phase 2. Cruise Phase 3. Entry, Descent, and Landing Phase 4. Surface Operations Phase   An exploded view of the assemblies that make up the spacecraft as it heads to Mars is shown in Figure 1. The Cruise Stage is jettisoned just prior to Entr y, Descent, and Landing (EDL). The Backshell and Heat  Shield are necessary components during the Entry portion of EDL. The Descent Stage contains fuel and rocket engines to provide a powered descent to the surface with a deployed Rover for a soft touchdown. After the Rover is on the surface, the Descent Stage is cut loose and flies away from the Rover’s location.                                                    * Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 

Proceedings of the 41st  Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012
240  

Figure 1.  Mars Science Laboratory Spacecraft Major Assemblies 

The Mars Science Laboratory Mission – Thermally  The cruise portion of the mission consists of the rover and cruise stage traveling from Earth to Mars over  a period of 8½ months. During this time, power is provided to the cruise stage and the rover by solar panels located on the cruise stage and an RTG attached to the rover chassis. The RTG is located inside the spacecraft entry body. Heat generated by the RTG, as well as waste heat from the electronics, is transferred to radiators on the cruise stage using a system of pipes containing Freon R-11. This heat management system is called the Heat Rejection System (HRS). There are two separate fluid systems, one that is part of the cruise stage (CHRS), and one that is part of the rover assembly (RHRS). More  detail on these two systems is provided in references 1 and 2. The CHRS is no longer needed after the several month cruise period to Mars so it is vented and then jettisoned from the entry body with the Cruise Stage just prior to the Entry, Descent, and Landing phase. The separate fluid system within the rover must continue to operate throughout landing and the 690 Earth-day primary surface mission, as well as any extended mission time. The main function of the heat transfer system is to distribute heat around the rover assembly for the purpose of maintaining a smaller operating temperature range for the rover electronics than the environment would otherwise require. Heat is transferred from the RTG to the internal Rover Avionics Mounting Plate (RAMP) during cold periods. The fluid is directed to radiators when excess
241 heat must be exhausted. By utilizing the heat from the RTG and the fluid Heat Rejection System (HRS),  the temperature range for the internal electronics assemblies is substantially reduced from -128°C to +55°C to a smaller range of -40°C to +55°C for the length of the mission.  Thermal Hardware Description  The thermal system for the rover and cruise stage are very similar. The hardware consists of an Integrated Pump Assembly (IPA), a gas-over-fluid accumulator, a significant length of fluid tubing, and heat transfer surfaces. The IPA consists of a motor, centrifugal pump, motor drive electronics, and directional valves for maintaining the coolant temperature. The valves direct more or less Freon to the radiators depending on the temperature of the coolant. A representation of the internal piping of the rover is shown in Figure 2 and the piping on the cruise stage is shown in Figure 3.    

Figure 2.  Heat Transfer System of Piping in the Rover and Around the RTG 

(Rover Integrated  Pump Assembly)
242  

Figure 3.  Heat Transfer System of Piping in the Cruise Stage and Relationship to the   Rover System of Piping   Components of the Integrated Pump Assembly (IPA)  There are two separate IPA units, one on the Cruise Stage and the other on the Rover. The Cruise Stage IPA is called the CIPA and the Rover IPA is called the RIPA. A model of the RIPA is shown in Figure 4. The pumps provide the circulation power for the Freon R-11, the pressure transducers provide telemetry back to the system indicating the health of the fluid system, the directional valves divide the flow between the radiators and the RTG, and the accumulator provides expansion volume for the fluid in the closed system. The accumulator expansion volume is absolutely necessary because of the large thermal volumetric coefficient of expansion of the Freon of 20% per 100 degree Celsius change in temperature. Providing a volume for the Freon to expand into prevents the following failures:   Mechanical failure of the system tubing due to over-pressurization   Bubble generation from low pressure     

Figure 4.  Rover Integrated Pump Assembly (RIPA) 

Cruise stage Integrated Pump  Assembly (CIPA) is located behind  this radiator  Motor Drive Electronics Pressure Transducer  Directional  Valves  Accumulator Pump Assembly  with Brushless  DC Motor &  Pump
243 The accumulator is a gas-over-fluid design that uses a metal bellows for the barrier between the fluid and  gas. The gas is dry Nitrogen and the fluid is the system coolant of Freon R-11. A cross-section of the accumulator is shown in Figure 5.  

Figure 5.  Accumulator Assembly showing bellows in fully extended position  (maximum fluid volume, maximum pressure of gas)   The free end of the bellows assembly consists of a metallic sweeper to which a Teflon ® guide disk with a  larger outer diameter is mounted to provide low friction sliding on the bore of the outer accumulator housing. The bellows is prevented from stroking too far by a step in the housing bore that corresponds to the maximum design fluid volume increase along with the highest gas pressure. As fluid is removed from the accumulator, the gas pressure compresses the bellows until other features of the assembly act as a compression hardstop. This is the state of minimum fluid volume and minimum gas pressure within the accumulator assembly.  Failure Event  The initial functional testing of the accumulator assembly involved the following steps:   Closing the accumulator assembly without welding (using a mechanical clamp) for a functional  check   Filling the bellows with fluid and removing any air bubbles (input tube upright)   Pressurizing the gas side of the accumulator until the bellows is fully compressed    Pumping fluid into the accumulator to extend the bellows, until the pressure difference across the  bellows reaches a limiting value (this was the point where the bellows was calculated to be at full stroke with the guide seated against the step in the housing bore)   Removing the pump connection   Allowing the bellows to drain into a graduated cylinder to measure the amount of fluid the  accumulator could contain   Verifying that the fluid capacity of the accumulator is within specified tolerances   If the assembly passes the test, weld the bellow end assembly onto the housing for a final flight  seal   Repeat the functional test above to verify no change occurred during welding
244 During final testing of one of the welded accumulator assemblies, the accumulator’s fluid capacity  appeared to be less than half the required value. Upon repeating the test several times, the capacity of the accumulator did not change. No test setup anomalies were observed and the accumulator assembly did not have any discrepancies that could account for the reduced capacity.  The accumulator assembly internals were then imaged using X-rays with the fluid in the accumulator at the limiting pressure difference. The picture in Figure 6 shows what was observed. 

Figure 6.  X-ray of Accumulator Assembly showing bellows shape with the  maximum allowable pressure difference across the bellows.  The accumulator fluid capacity is significantly reduced from the required value. 

The end of the bellows assembly with the sweeper and Teflon® guide had managed to tip to an angle  where the piston would no longer move down the bore and had jammed in place. The force on the piston in the direction of extending the bellows with the maximum allowable pressure difference is 370 N, indicating the jamming force was quite high. When the fluid was removed, the bellows compressed flat against the compression hardstop (the right end in Figure 6) as designed. Repeating the test and measuring the fluid capacity indicated that the failure point was consistent and very stiff – meaning the location along the bore where the piston jammed did not move detectably when cycled between no fluid and the maximum possible fluid, limited by the allowable pressure difference across the bellows.  The X-ray was used to make measurements of the geometry of the failure location so it could be identified when the assembly was opened for inspection. Figure 7 shows some of the measurements made to determine the location and the possible source of the jamming.  

Figure 7.  Measurements made using the X-ray of the Accumulator Assembly
245 It was clear there was some obstruction to the motion of the bellows guide that would hopefully be  identified when the assembly was opened up and inspected. The next step was to carefully remove the bellows assembly from the housing to find the source of the jamming by cutting the weld off the end of the housing. Figure 8 shows what was found inside the housing assembly. The location of the residue seemed to indicate it was the source of the jamming, as indicated in Figure 9.               

Figure 8.  Tape adhesive residue on the  Figure 9.  Location of tape adhesive residue  housing wall, left behind from previous  corr elates well to the tip-initiation point  rework operation    The tape residue was cleaned from the housing bore and the accumulator reassembled. The test was  repeated with the full expectation that the problem had been solved. The result of the test indicated no  change in the behavior of the assembly, including the measured fluid volumes. The following analysis on  the tipping phenomenon was performed (see Figure 10) to understand the sensitivity of the design to the  variables involved. A description of the mechanical behavior model is presented.   Mechanical Behavior Model Description  The free-body diagram variables include the weight of the sweeper (W s), the weight of the fluid-filled bellows (W b), the  friction at the sweeper-to-housing interface (µ s), and the  normal force generated by a squirming bellows (F b).   Prior to the tipping of the end of the bellows, F b is zero. Once  the end starts to tip, the bellows squirms upward and reacts against the housing wall. This additional normal force increases the frictional drag between the housing wall and the bellows surface, including the sweeper-to-housing interface.  An additional aspect of the tipping end of the bellows is how the contact between the Guide Disk and the housing behaves. The contact starts at a point and, as the tipping angle increases, this contact spot increases to an elliptical  area. As the tipping angle is increased more, the contact patch splits into two small contact patches that travel around the housing surface. As the contact points get higher on the housing surface, the normal force necessary to balance the bellows squirm force, F b ,increases substantially. As the normal force  increases, so does the friction force, increasing the tipping moment on the end of the bellows. This runaway behavior demonstrates that the tipping of the end of the bellows will eventually jam, and added force will not help to release it. See Figure 11 for a pictorial description of this phenomenon.   Figure 10.  Free Body diagram of  bellows assembly in housing
246    

Figure 11a.  Bellows straight, normal   Figure 11b.  Start of tipping, normal force on  force on bottom is from weight only (F b = 0)  bottom is a small contact patch (F b = 0) 

      Fb            F b       Figure 11c.  Tipping angle has increased and  Figure 11d.  As tip angle increases more, the  contact patch has split into two discrete areas contact areas move up the side of the  housing. Radial force must increase to balance the bellows squirm force, F b.   The most significant variable in this tipping and then jamming behavior is the friction force. The housing bores were cleaned very well to minimize the possibility for contamination. The Teflon ® Guide Disk was  relied upon for its low friction properties at the bellows-to-housing interface. No other Accumulator Assembly has exhibited this characteristic and it had not shown up in any of the environmental testing either.  Design History of the Accumulator Assembly  This accumulator design was not new. There was a Heat Rejection System on the Mars Pathfinder Lander in 1997, as well as one on each of the rovers, Spirit and Opportunity in the 2003 missions. The accumulator in the MSL mission was larger than any of the prior units because the volume of heat transfer fluid was much greater. The design was scaled up in size to accommodate the volumetric increase.  The mission life requirement for MSL is eight times longer than the Spirit and Opportunity rovers. The larger size of the accumulator and bellows assemblies coupled with the longer operational life required a cycle life test be performed. The cycle life test resulted in a fatigue failure of the bellows assembly. The bellows was redesigned to accommodate the higher life requirement by reducing the thickness of the convolutions a small amount. The bellows displacement then resulted in a lower stress level in the material. This had the additional effect of reducing the bellows stiffness, both axially and laterally.  The Next Version of the Failure Mechanism Hypothesis  When the elimination of the adhesive residue on the housing wall did not affect the failure results, the additional factor of the reduced stiffness of the bellows was considered. This would make the design even more sensitive to bellows tipping under smaller frictional forces, since the bellows stiffness helps keep the end from tipping by resisting the applied moment from friction with the housing bore. The reduced stiffness of the MSL bellows combined with a surface finish in the housing bore that was at the coarse end of the tolerance and some tape adhesive residue all seemed to have pushed this unit over the edge and initiated the tipping of the end of the bellows. Once it had started, the bellows continued to tip and jam at the same location, even though the adhesive had been removed from the bore. While a stress analysis indicated that the tipped distortion of the bellows should not have resulted in any yielding, and