Design and testing of mechanical gripping tools for On Orbit assembling

On-orbit servicing, assembly and manufacturing (OSAM) opens a new frontier for robotic systems. A gripping tool for such applications must meet several requirements of space mechanics, such as safety, precision and reliability, while functioning in space conditions. This paper presents a development cycle of such tools designed for the assembly of a small satellite antenna on the International Space Station. Two different grippers, driven by a common drive unit, are presented, conforming to a Multi-Purpose-Tool (MPT) for orbital robotic systems and meeting the requirements of a defined OSAM mission. Both design drivers and concepts and specific component selection are described. Proposed mechanical solutions for safe gripping of objects including space tribology aspects are covered. To adapted to operations not foreseen, the grippers and the drive unit can be reconfigured. The tool architecture presented promotes modularity, scalability, reusability and convertibility of designs, thus facilitating rapid integration in similar missions. A test campaign for critical requirements will be in place to ensure reliable performance of the tools for use in the space environment.


Introduction 1.Motivation
Since the launch of the first satellite, the specific challenges of space flight have severely limited every space mission in terms of mass, lifetime and ultimately cost.The results are mostly highly integrated, single-use space systems that are difficult to maintain and not upgradeable when needed.It is foreseeable that these challenges will be difficult to address with the currently adopted mission paradigm, resulting in the need for a change in for a change in space missions and spacecraft design morphology.To address this, one solution is the development of new technologies for on-orbit servicing, assembly and manufacturing (OSAM) [1].
Almost all potential applications for on-orbit servicing, assembly and manufacturing (OSAM) require the ability to interact with robotic systems.Extensive R&D activities have been carried out to develop compact general-purpose manipulator arms for orbital use (e.g.CAESAR [2], VISPA [3], PIAP [4]).While the named manipulators represents a kind of generic platform, tools that can be attached to the end effector and operated to perform the contact operations are usually less generic, and designed for specific operations.That limits their use to individual missions [3].

Related Work and State of the Art
A series of research projects formed a roadmap for flexible tool architectures in space missions at German Research Center for Artificial Intelligence (DFKI) as well as in other institutes and private companies worldwide.Whereby the OSAM-1 Mission [1] completing its CDR and OSAM-2 is already in the starting holes, ESA activities as the PERASPERA 1 project are in an earlier mission phase.As part of the Horizon 2020 Strategic Research Cluster (SRC) and PERASPERA projects, the DFKI has completed research in the field of space robotics like testing of generic interfaces [5] or the development of control methods, for the manufacturing of large structures in space [6].The DFKI can also draw on experience in the development of generic gripping systems, such as those used in a humanoid robot called RH5 [7] as well as on experience in the development of actuators for space applications [8].
Notable generic gripping tools in this context have also been developed by the German Space Agency (DLR) [9].More advanced tools for OSAM activities have been developed for refuelling satellites, e.g. for docking and gripping a filler neck [10].

Objectives
The objective of this paper is to evaluate the design and technology of two end effectors for orbital robotic systems.The mission is based on the technology roadmap defined in the Horizon 2020 SRC PERIOD 2 project.The overall goal was to define and develop an orbital demonstrator concept for a European OSAM mission [11] and to increase the technology readiness of the addressed robotic building blocks (Figure 1a).The integration of the tools within the mission structure and during the satellite antenna assembly process is shown in Figure 1b.

Design Approach 2.1. Reconfigurable robotic tool systems in space applications
Li et al. [13] provided an overview of robotic technologies for the OSAM subtask of assembly: They note that the wide variety of tasks foreseen requires a certain adaptability, which can be achieved by self reconfiguration of the tools required.The paradigm of flexible and reconfigurable tools originates from tool machines [14] and could also be applied to OSAM missions.
Typically, the most complex and expensive parts are the actuators and the tool avionics.Thus, the main motivation for building on a modular tool architecture from our project partner Airbus Space and Defence, was to avoid duplication of its implementation.The architecture is based on a generic actuator, the Multi-Purpose-Tool (MPT) [3] and a Versatile Robotic Interface (VRI) to transmit the drive torque and signals.

Baseline
The architecture of the developed gripping tools is shown in Figure 2. Since two different objects have to be gripped, we end up with two tools, a so-called cleat gripper and the frame gripper.Similar to a pneumatic piston, the GDU converts the rotary motion of the input shaft of the MPT into translatory motion.It has been been discussed whether it would be possible to replace the ball screw kinematics by a a cam disc or a bevel gear to achieve a shorter design solution.However, all of these paths lead to significantly increased risk or mass.Finally, a design was chosen that mechanically decouples the input torque from the gripping force.This design prevents overloading of the gripped objects and of the tool mechanics in an event of a malfunction by getting to high input torque from the MPT.
To avoid losing parts during assembly, the gripper must maintain its holding function in all conceivable states.They are designed to be closed by default by springs and be actively opened by the MPT.To maintain the orientation of the objects, it was decided to use form-fit contact surfaces which also increases the grip stability.
Components for the mechanisms were selected on the basis of available space tolerant technologies and the requirements of vibration loading, as well as for thermal vacuum chamber (TVAC) testing.This brings the design to a "partially verified level" up to TRL5.

Sensor arrangement
To detect the status of the tools and of the grasped object, a set of Baumer micro switches was used, replaceable by a space qualified version by RUAG [15].The sensor assembly consists of at least one sensor per finger to detect whether the object is in contact with the fingers.It also prevents the MPT from hitting the end stops during normal operation.In the case of unpredictable events, an incorrect operation is detectable by additional sensor within the mechanic.It is planned to feed the sensor data directly through the VRI to the MPT.The following five states to be detected were defined: Fully opened, Gripper is being opened, Fully closed, Object grasped successfully, and the Error state.Due to size and space constraints, the final design used one sensor per finger, a sensor inside the cleat gripper mechanism and two sensors inside of the frame gripper mechanism.See Figures 4 and 5.

Gripper Drive Unit
The GDU (Figure 3) is developed as a modular transformer of a rotary input movement to a secured specified linear motion.Studies on space-qualified screw mechanisms can be found in [16][17][18].A modified ball screw from August Steinmeyer GmbH was chosen for the GDU.It has a pitch of 5 mm and a nominal diameter of 16 mm, and is supported by a back-to-back bearing configuration.The ball screw is ready for vacuum, lubricated with Barrierta L55-2 and pre-stressed to withstand vibration loads in a wide range of temperature.The bearings used are high precision 25°paired bearings, manufactured by IBC.The ball screw is connected to adjustable rotary hard limits and pushes the output piston of the GDU.A modified spline shaft was used for quick coupling to the MPT's output shaft.
Two DP4 bushes from GGB supporting the output piston against the spindle and housing.A retainer spring pushes the non actuated output piston into a zero position while it is decoupled from the end effector.A PTFE-bronze counter torque bushing was designed to force the screw  into a linear movement.The GDU movement is be limited to two required working ranges of 7.5 mm and 19 mm.End stops prevent overloading to a maximum of 45 Nm input torque.Sensor cables are routed outside of the housing and protected by a cover.

Cleat gripper
The cleat gripper is designed to grip small round objects or thinner rods with an extra interface by force and form-fit.To convert the linear motion provided by the GDU, the mechanical structure of the gripper is based on a 4-bar-link mechanism.For safety reasons, the GDU piston pushes an interface to move a carriage along a shaft (Figure 4).The movement is supported by a metal-polymer cylindrical GGB-DP4 bushing.The finger levers are also connected by bushings.A single spring provides the closing force, to close the tool by default.Unlike the frame gripper (Section 3.3), there was no need to add further redundancy due to mission requirements.
To detect whether the gripper is closed or if an object is in position, the jaws are equipped with two sensors.A third sensor provides feedback on the state of the internal slide.The conical form closure of the jaws enforces 5 degrees of freedom (DOFs) constraints on the gripped object to determine its position.The total mass and size of the cleat gripper with GDU is 2487 g and the envelope dimensions (W x H x L) are 105 mm x 86 mm x 338 mm.The mechanism exerts a force of 90N on the gripped object.

Frame Gripper
The frame gripper was designed to grip larger objects such as sheets and frames with a triangular design as the contact surface.The body has been chamfered to fit into small pre-defined gaps.The frame gripper can be hooked into a cut-out, to grip and hold an object with two rotating fingers and one passive finger.The configuration is asymmetrical, so that the objects to be gripped are below the axis of the GDU and MPT.
The design shown in Figure 5 was extended with additional features in redundancy to the cleat gripper.It has a redundant spring and it is almost impossible to retract the two moving fingers due to a toggle linkage.The mechanism also helps to keep the required spring forces low, allowing us to include the springs into the limited assembly space.The frame gripper consists of five micro switches to monitor its operation.The GDU pushes the carriage which slides between two brackets to open the fingers.Similar bushes to those used in the cleat gripper are used to support the translational movement of the carriage, while the fingers pivot on ball bearings.A picture of the tool fully assembled with the GDU can be seen in Figure 7.The total mass and size of the frame gripper with GDU is 2521 g and the envelope dimensions (W x H x L) are 114 mm x 87 mm x 354 mm.The mechanism exerts a force of 30 N on the gripped object, which is significantly lower than that of the cleat gripper.However, the prevention of reverse rotation and the outwardly positioned fingers enable the frame gripper to grip much higher loads than expected, as shown in Figure 8.  Tests with the tools were executed according to the test logic shown in Figure 6.The objective was to gain experience and to pre-qualifying the key elements of the developed mechanisms.Figure 8 shows in an exemplary way how to check whether an object can be held under external loads.For this purpose, a frame was used, which allows to apply forces and torques to the frame gripper.Similar tests were also carried out with the cleat gripper, but the cleats turned out to be the weakest link in the chain.Due to the extensive use of interconnected moving elements like springs, bearings, and bushings, the vibration loads were expected to be the most critical for testing the mechanisms.The vibration levels were based on the Bartolomeo platform loads and additionally on the superseded ECSS-E-10-03A loads.This gives a root mean square (RMS) load of 11.22 G rms for the cleat gripper and 10.72 G rms for the frame gripper.As Figure 7 shows, the GDU window also allowed measurements to be taken directly on the spindle carriage.

EASN-2023
Testing of the two GDU units provided an opportunity to assess differences in manufacturing and assembly.Although the number with three samples was small, it was found that one of the input shaft bearings was not secured as intended.The cause appears to be the assembly procedure defined by the manufacturer (IBC), which prescribes loosening and re-tightening of these bearings with a low torque, leading into a lower bearing preload in the case of small deviations.
The vibration tests were followed by life time testing under atmosphere in which each tool was subjected to 800 cycles.No degradation was detected during this test.It should be noted that the tools were designed to be tested under life-cycle in a thermal vacuum chamber.
The functional tests were carried out by our project partners Airbus Defence and Space in combination with the MPT and a KUKA LBR iiwa manipulator.As the tools are designed to apply forces many times higher than the weight forces of the objects, gravity was not a factor.

Summary and Outlook
In retrospect, the torque provided by the MPT, which was not matched to the gripping tools demand, proved to be a critical point in the modular design approach.The need of handling higher input torques resulted in a more robust and heavier design than was probably necessary.Design solutions for safety reasons, such as the use of a non back drivable toggle lever mechanism, redundant springs or the decoupling of the gripping tools from the MPTs drive train, proved to be essential.They made the control of the tools by the MPT significantly easier and safer.However, for the next iteration, which could already be at TRL6 level, a lighter and shorter design could be realized.The selection of components has been successfully validated during testing, so only the micro switch sensors need to be replaced.
In summary, it has been possible to evaluate critical components and the general structure of interchangeable gripper tools.As a next step, an adopted engineering model (EM) could be built to analyse deeper functional properties during the antenna assembly process and behaviour EASN-2023 Journal of Physics: Conference Series 2716 (2024) 012093 under TVAC conditions.It is the aim of each of the project partners to continue working together and to participate in the PERIOD mission or similar projects.We are optimistic that within the next decade the number and potential of OSAM activities will take spacecraft design to the next level.
(a) Overview of PERIOD's significant achievements.The tools presented are related to the assembly of a cubesat antenna.(b)Visualisation of the manufacturing process during the mission.The interchangeable tools are stored in a magazine box (yellow) and can be used by the manipulators' multi-purpose tool (white) to assemble the antenna.

Figure 1 :
Figure 1: Intended use of the tools within the PERIOD mission [12].

Figure 2 :
Figure 2: Architecture of the elements developed.The tools are divided into a frame gripper for larger objects and a cleat gripper for smaller objects.Both use the same Gripper Drive Unit (GDU), which converts the rotational motion of the MPT into a translational motion.

Figure 3 :
Figure 3: Gripper Drive Unit design and components to transfer rotary input movement to linear output movement.

Figure 5 :
Figure 5: Frame gripper design and components.

Figure 6 :
Figure 6: Tests logic for the tools during the STARLIT project.

Figure 7 :
Figure 7: Frame gripper on shaker during vibration loads tests.The mounting is realized with contacting all fingers enabling pre stress on all mechanical elements and representing a thinkable configuration for launch.

Figure 8 :
Figure 8: Testing the stability of the frame gripper under side loads.