ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
223 DLR’s Dynamic Actuator Modules fo r Robotic Space Applications 

A. Wedler, M. Chalon, K. Landzettel, M. Görner, E. Krämer, R. Gruber, A. Beyer, H-J. Sedlmayr,   B. Willberg, W. Bertleff, J. Reill, M. Grebenstein, M. Schedl, A. Albu-Schäffer, G. Hirzinger * 

Abstract 

Based on our long-term goal to develop “robonauts” for space and the experiences our research  department accumulated in space robotics, this article describes recent design and development results at DLR’s Robotics and Mechatronics Center. Herein, we focus on lightweight robot arms, articulated hands and highly integrated actuation modules for space applications. We show how the development process started with fully sensorized, highly dynamic joint modules with state feedback control and led to seven degree of freedom (DOF) torque-controlled robot arms that enable innovative Cartesian impedance controllers. Further, we present two space robotics examples that are based on such modular actuation units. The first one is the ROKVISS experiment which has been launched towards the ISS in 2004 and returned to earth in 2011. The second is the Dextrous Robot Hand, DEXHAND, which we developed on contract with ESA. These two projects underline the strong interest to transfer our knowledge and experience from terrestrial robotic developments to innovative space technology. 

Introduction 

Currently, many operations in space, such as maintenance or new module assembly, have to be carried  out by astronauts and expose them to a hazardous environment. These missions are associated with high risks and costs. While robotic systems are not yet ready to replace humans in space, they may provide an excellent support for astronauts. Supported by telemanipulation concepts, robots could be used for many of the Extra Vehicular Activities (EVA). Herein, robotic perception and partial autonomy could strongly simplify tasks for the astronauts. Further, methods like force feedback allow operators haptic interaction at distant locations. Besides near earth orbital activities, we believe that autonomous and semi-autonomous robotic systems will take an important role in planetary exploration scenarios.  In order to bring teleoperated systems to the International Space Station (ISS), the European Space Agency (ESA) is currently investigating different scenarios. We think that humanoid robots, like our terrestrial technology demonstrator “Justin”, (Figure 1) could fulfill ESA’s development goals within this context. Since the ISS is designed for humans, sending up robots with hands and arms is an obvious option that also NASA follows with its Robonaut [1]. In the case of a telerobotic scenario, humanoid robots simplify the mapping from the operator to the robotic system.                                                    * Robotics and Mechatronics Center (RMC), German Aerospace Center (DLR), Wessling, Germany  

Proceedings of the 41st Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012
224   Figure 1: Teleoperater using a head-mounted display to get 3D visualization and lightweight  Robots (LWRs) as haptic input de vice with force feedback and "Jus tin" in telerobotic operation 

This results in an intuitive operation that minimizes learning time as well as user fatigue and improves  execution speed. Upon those arguments, ESA decided to develop an EVA capable hand/arm system. Based on the experiences of our lab with space robotics projects like ROTEX and ROKVISS, ESA assigned the development of the space-qualifiable hand to the DLR Robotics and Mechatronics Center (RMC).  Since space robotics applications require highly integrated, lightweight systems with low power consumption and high performance, DLR's robotic actuation technology and our longtime experience with a space robotic system may provide interesting solutions for future missions. Throughout this article we present our robotic drive train technology and its development process as well as two space related application examples. The next section) of this paper discusses the design of the DLR drive train unit including the in-house developed permanent magnet synchronous motors.   

Furthermore, it describes how a terrestrial seven DOF arm is built based on these actuation units. A later section presents ROKVISS, our robotics component verification experiment that was operated at the outer surface of the ISS from 2004 to 2010. The interesting fact of this experiment is that we used a mixture of space qualified and off-the-shelf components and achieved very reliable long-term operation in space. Receiving the robot back allows us to collect valuable data about the effects of the space environment on the system. Furthermore, this paper describes the development of the EVA qualifiable Dexterous Robotic Hand, DEXHAND, for the ESA. All presented results are based on the Critical Design Review (CDR) status, giving an overview of the technical decisions with respect to the mechanical design.   DLR Drive Train 

Developing actuators for dynamic and sensitive robotic systems is a challenging task. The broad range of applications and the intended interaction with the environment require characteristics that differ from common actuation scenarios in industry. The same robot, for example, has to deliver high torques at low speed in one application while it has to provide high dynamics in another. Since robotic actuators are usually attached to the moving structure, they also need to be small and lightweight. In order to reduce positioning errors and to avoid disturbances during reversing motions all gears should be free of backlash. Thus, the requirements for robotic drive trains are manifold. The most important ones are summarized in the following list: 

• high torques at low speed or stand still • low weight • small sizes • high dynamics • high efficiency / low losses
225 • backlash free gearing  • sensor feedback (position and force-torque) 

In the end of the 1990th no commercially available actuators could sufficiently fulfill those needs. Thus, at our institute, permanent magnet synchronous motors were developed that match the requirements. Being combined with very low backlash Harmonic Drive (HD) gears, they constitute high performance robotic actuation units. These units provide high dynamics and high link side torques due to low inertia parts and the high reduction ratios of the HD gears. Further, their low number of components reduces the probability of failure.  In the following, we present the development of our lightweight robotic arms with a focus on our modular actuation units. We begin at the motor stage and progress via the highly integrated actuation modules towards the complete robot.  The DLR brushless Motors  The key components of the DLR drive train concept are the permanent magnet synchronous motors developed at our institute. The so-called “Innen-Läufer-Motor” (ILM: German for “inside rotating motor”) is well suited for highly dynamic tasks involving frequently reversing motions. For maximum performance, all of our motors are sized to comply with available HD gear dimensions. 

Figure 2: Concurrent engineering for the actuator design and the RoboDrive variations   During the two year development process we strictly followed a concurrent engineering and optimization process (see Figure 2) to achieve high dynamics combined with low losses and low weight. The complete design is based on extensive multi-physics simulations, considering electromagnetic, mechanical and thermal effects. In the final design short copper paths, very dense and optimized windings, strong neodymium rotor magnets and low inertia rotating parts are the main source of the high dynamics. Furthermore, the ILMs consist of a high number of pole pairs (ILM 25 and ILM 38 have 7 pole pairs all others have 10), which allow very accurate positioning and reduce motor ripples. Using wires with a large cross-sectional area allows the motor to handle large currents with low losses and thus, to produce very large torques at low angular velocities. To give a performance example, the torque/current constant of the ILM 25 (stator diameter of 25 mm) is 0.008  N-m/A. The motor uses approximately 12.5 A (without saturation  effect) to provide a torque of 100 mN-m. 

The corresponding HD gears with 1:100 reduction ratio (HFUC8) for example has a nominal torque of  2.4 N-m, a repeatable torque of 4.8 N-m and a momentary peak torque of 9 N-m. The limiting factors for the motor torques are thermal constraints related to the passive cooling. In case of active cooling the torque output of our motors could be even increased. At the time of development the motors achieved a 50% reduction of weight and losses with respect to commercially available units with comparable torque
226 ratings. Currently, our motors are becoming the state of the art for high performance robotic drives and  are commercialized by the company RoboDrive [2].  The DLR Actuator Units  As already mentioned, our motors are developed to comply with the ratings of Harmonic Drive gears. Together with those they form the core of our highly integrated actuation units. Aside those core parts, the units include power, data acquisition, motor control and communication electronics in a very dense package.  The amount of wires has been drastically reduced to two power (one for electronics and one for the motors) two communication lines (SERCOS is a ring) and one emergency stop. Due to the large hollow shaft of the motors these lines can be easily guided through a robot joint and thus remain inside the robot. Furthermore, the actuation units comprise a large set of sensors enabling sophisticated impedance and admittance controllers with underlying high performance torque or position control loops. These sensors are motor side position sensors for commutation and relative position measurement as well as link side absolute position and joint torque sensors (see Figure 3).  Aside the high performance, low weight and low losses, two properties of our actuation units are very interesting for space applications. First, the high torque capacity of the motors allows applying short and very strong “kicks” that could free the unit in case of cold welding within the bearings or the HD stage. Of course, in this case the power electronics has to be designed to supply large currents for a short time.  

Figure 3: DLR drive train concept  a.)  ROKVISS unit - ILM70 - HFUC25,  b.)  Joint module of  the LWR III   The second advantage is that the low number of mechanical components reduces the probability of failure (see Figure 4). In the following, the mechanical components of one joint unit are listed:  • stator • rotor with magnets • rotor bearing • HD wave generator (WG) • HD flex spline (FS) • HD circular spline (CS) • output bearing.  Within these components the rotor bearing and the WG bearing are the most crucial elements since the associated friction has to be overcome by the motor torque directly.
227  

Figure 4: DLR drive train concept  a.)  DEXHAND unit CAD, b.)  DEXHAND unit photo, c.)  ROKVISS unit photo 

In summary, the design of our actuation units allows highly dynamic and powerful operation as well as precise control in fine manipulation tasks. An example for the usage of this drive train concept within a space application is the joints of our ROKVISS experiment that is described in more detail later.  The DLR modular Arm  Developing, building and controlling lightweight robotic arms for a large variety of applications are a research focus at the DLR Robotics and Mechatronics Center (RMC). Starting with the Lightweight Robot I (LWR I) presented in 1994 via the LWR II of 1999 to the current model LWR III from 2003, we gained a lot of experience within this field. Due to the success within the research community and an initial market demand for such technology, in 2006 the KUKA GmbH started to commercialize the LWR III. A first small batch series was mainly provided to different research institutions.   

Figure 5: a.) LWR III, b.) LWR as a KUKA  product, c. ) modular robot system “Justin” with 54 DOF    Currently, around hundred units are sold and the car manufacturer Daimler sees high potential for a factory-wide deployment of the LWRs. For them, especially attractive is the possibility of switching in-between torque-based and position-based control concepts. This is very promising for the solution of challenging assembly tasks.  Besides the innovative impedance and admittance control algorithms developed for our lightweight robots, its modular joint-link concept is one of the reasons for its success. Hereby, the underlying modular assembly system with only a few basic components concerning joint mechanics, electronics and links allows composing completely different configurations in a short time. Thus with three different single degree of freedom (DOF) robot joints and a two DOF wrist joint, different kinematic configurations can be easily assembled. For example, in addition to the symmetric version an asymmetric version (Figure 6) can be built. This is particularly interesting for space applications where the arm needs to be folded for stowing.
228 Again the design process for the joint and link modules followed a concurrent engineering process  including kinematic and dynamic simulations as well as an FEM analysis. Supported by a link component library with variable parameters, virtual prototypes could be assembled, tested and optimized. One special result of this process is the ball-shaped two axis wrist joint (see Figure 7). This joint imitates the human wrist but achieves much higher mobility. Its short distance between the wrist pitch axis and the tool-center-point (TCP), helps to reduce the motion of the lower robot joints in case of changes of the TCP orientation. Furthermore, the wrist design enables two kinematic configurations, a common configuration with roll-pitch-roll axis and optional a roll-pitch-pitch configuration. To reconfigure the joint the short standard flange simply has to be replaced with an extended 90° bend flange. Another advantage of the wrist is that its cardan joint avoids singularities in an extended position and the reduced joint mass allows carrying larger payloads.  

Figure 6: a.) symmetric LWR modules ,b.) unsymmetric LWR modules   The DLR lightweight robots are very well suited for a large variety of service robotics applications with their modular assembly, kinematic redundancy (7 DOF), the torque controlled joints, the low inertia and a load to weight ratio of nearly 1:1,. Some examples at our institute are the humanoid “Justin”, the three arm co-worker scenario, several single arm manipulation tasks as well as the bi-manual input station for robotic experiments. The concept of highly integrated modular actuation units is further improved for the new DLR hand arm system as well as the DEXHAND. With their low power consumption and the above mentioned features our drive trains and complete robots provide interesting capabilities for future space missions [3].  

Figure 7: LWR cardanic hand module joint a.)  ro ll-pitch-roll configuration, b.)  roll-pitchpitch configuration c.) LWR joint modules
229 ROKVISS 

The German space robotic project ROKVISS (German:”Roboter Komponenten Verifikation auf der ISS”)  has just been finished. The system has been returned to earth early 2011 and the hardware is now subject to detailed analysis and advanced tests.  ROKVISS (see Figure 8) aims at the space qualification of DLR’s LWR modules (modified for a space environment) with a reduced setup of a 2 DOF robot arm on the ISS. With this experiment DLR could prove the concept can use partly components-off-the-shelf (COTS) within a very tight and highly integrated mechatronic device based on the modular joint concept discussed previously.  After more than five years of successful operation and without any failure the innovative mechanical and electronic concept seems very promising for the use in space projects. Those projects are in the wide range of future robonaut applications as well as planetary robotic missions. Indeed what we are still missing in space are fast signal transmissions. One relay satellite could provide signal round trip delays of 0.5 seconds within a coverage times of around 40 minutes. This allows even haptic feedback, which is a major idea of our advanced telerobot control strategy using visual and haptic feedback [4].  ROKVISS was also a telerobotic demonstrator with real-time stereo-video transmission and tactile feedback. The telerobot success was the second major achievement of ROKVISS, besides the component verification. We believe that realistic telerobotics probably combined with partial or full autonomy will be needed in future space robotic applications [5-7].  

Figure 8:  ROKVISS photo of the experiment mounted on the ISS 

The following subsection deals with the mechanical design and the detailed structure of the ROKVISS robot describes the friction analysis and shows preliminary results.  Mechanical System Design  In Figure 8a the overall ROKVISS is shown, while Figure 8b shows the joints as described in the previous section. Figure 8c shows ROKVISS in its position outside the ISS at the Zvezda service module.  The ROKVISS experiment consists of a small robot with two torque-controlled joints, mounted on a Universal Workplate (UWP). Each joint module consists of a power supply, a controller board, a power converter, the drive train and a torque sensor. Inside the robot there are 2 joint modules, a stereo camera, an illumination system, an earth observation camera and a power supply. A mechanical contour device is mounted on the UWP for verifying the robot's functions and performance (see Figures8a and 8c).
230   Figure 9:  ROKVISS FM modules with color changes on the surface after return to earth 

These two robot joints have been extensively tested and the joint parameters have been identified by repetitively performing predefined robot tasks in an automatic mode, or based on direct operator interaction. The automatic mode is necessary due to the fact that communication constraints limit the direct link experiment time to windows of only up to seven minutes when the ISS passes over the tracking station German Space Operations Center ( GSOC). Figure 9 shows the returned two joint modules. After five years of operation on ISS differences on surface with its anodic treatment (LN9368 I 2101) of the two different aluminum alloys (Al 7075 I Al 5083) are obvious [8, 9]. 

Friction Analyses  The preliminary results of the on-orbit identification show that the total friction for joint 1 in space increased by about 50% compared to the friction on ground, taken at 20°C, under normal atmospheric pressure (Figure 10). However, only a small further degradation of the parameters has been observed so far during the mission. This friction change will be analyzed in the upcoming tests to determine if the lubricant (Braycote 601) and/or the guidance condition influenced this effect. The temperature dependency of the parameters is close to the range of identification uncertainty (Figure 10) [10].  Conclusion for the Friction Identification  The friction in the two joints increased initially, already at the first experiment in space compared to the values measured on ground, but remained rather constant afterwards. All the controllers were robust with respect to these changes. The friction changes of joint 1 and joint 2 are different. Friction in joint 1 has the same structure as on ground, but values increased by 50%. The friction in joint 2 shows no more significant viscous friction and the total increase is lower than 20%. Since the same lubricant was used for both joints (Braycote 601), further experiments are planned in order to explain the different behavior.  To provide a better fit of measured and simulation data, a nonlinear (third order polynomial) function is required. However, due to the higher number of optimization parameters, their variance over different experiments is greater. A trajectory with a higher number of distinct velocities is needed in order to reduce the variance of the results.  Stiffness Identification  The main sources of elasticity in the joints are the flex splines of the HDs and the torque sensors. The elasticity is identified by contacting a rigid surface with the tip of the robot and by commanding a slowly changing force to the joints. Since the torque is measured after the gear-box, the stiffness can be easily identified with the available torque and position signal. The model torque is computed as a product of position increment and stiffness. The stiffness for joint 2 has a value around 4900  N-m/rad in space.
231   Figure 10: Friction analysis of the FM joints (a. Joint one, b. Joint two) a.) b.)05101520253035404550 02.11.04 24.03.05 12.05.05/1 27.05.05 08.06.05 16.06.05 22.06.05 07.07.05 22.07.05 08.09.05 12.10.05 30.11.05 21.12.05 22.12.05 11.05.06 16.08.06 17.08.06 04.10.06 17.10.06 19.10.06 25.10.06 17.01.07 18.01.07 23.01.07 13.02.07/1 13.02.07/2 14.02.07/1 14.02.07/2 19.02.07 20.02.07 17.04.07 19.04.07 23.05.07 24.10.07 05.12.07 06.12.07 23.01.08 22.04.08 23.04.08 28.04.08 17.06.08/1 17.06.08/2 25.06.08 22.07.08 24.07.08 30.07.08 20.08.08 27.08.08 18.09.08 16.09.09 03.12.09 02.02.10 30.03.10 13.05.10 07.09.10 20.09.11/1 20.09.11/2ROKVISS - friction identifikation joint 1Nm-20°C<T<=-14°C-14°C<T<=  -8°C-8°C<T<=  -2°C-2°C<T<=  +4°C+4°C<T<=+10°C+10°C<T<=+16°C+16°C<T<=+22°C+22°C<T<=+28°C+28°C<T<=+34°C+34°C<T<=+40°Cviscos friction at 1°, 5° [deg/s] coulomb frictionload-dependent friction 05101520253035404550 02.11.04 24.03.05 12.05.05/1 12.05.05/2 27.05.05 08.06.05 16.06.05 07.07.05 22.07.05 08.09.05 12.10.05 14.10.05 09.11.05 21.12.05 22.12.05/1 22.12.05/2 17.08.06 04.10.06 17.10.06 19.10.06 25.10.06 17.01.07 18.01.07 23.01.07 13.02.07/1 13.02.07/2 14.02.07/1 14.02.07/2 19.02.07 20.02.07 17.04.07 19.04.07 23.05.07 24.10.07 05.12.07 06.12.07 23.01.08 22.04.08 23.04.08 28.04.08 17.06.08/1 17.06.08/2 15.06.08 22.07.08 24.07.08 30.07.08 20.08.08 27.08.08 18.09.08 16.09.09 03.12.09 02.02.10 30.03.10 13.05.10 07.09.10 20.09.11/1 20.09.11/2ROKVISS - friction identifikation joint 2Nm-20°C<T<=-14°C-14°C<T<=  -8°C-8°C<T<=  -2°C-2°C<T<=  +4°C+4°C<T<=+10°C+10°C<T<=+16°C+16°C<T<=+22°C+22°C<T<=+28°C+28°C<T<=+34°C+34°C<T<=+40°Cviscos friction at 1°, 5°, 10°, 20°, 30° [deg/s] coulomb frictionload-dependent friction