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Prototype for an Ultra Large Structure Assembly Robot

Periodic Reporting for period 1 - PULSAR (Prototype for an Ultra Large Structure Assembly Robot)

Reporting period: 2019-02-01 to 2021-03-31

The scope of PULSAR is to demonstrate the ability to autonomously assemble large structures in space. To perform the autonomous assembly of a primary mirror, 3 main concepts should be handled.

dPAMT showed how intelligent mirror tiles (having their own 6 dof mirror motion system) can be assembled by robots achieving the right dimensional precision. The demonstrated used (for gravity constraints) fully-functional reduced-size tiles.
dLSAFFE showed the full-scale mirror assembly using non-functional full-scale mirror tiles in a giant neutral-buoyancy facility. The demonstrator provided the confidence that the full-scale operation ispossible.
dISAS simulated in software the complete assembly of the mirror considering space dynamics conditions. The simulator allowed to tune the concepts of operations as well as the controllers for telescope spacecraft and manipulator.
dPAMT has focused on the precise, automated assembly process of high-fidelity segmented mirrors with individual adjustment capability in lab conditions and reduced scale. The segmented mirror tiles are equipped with a mechanism allowing precise adjustment of the mirror surface with respect to the base. Such mechanism is used to demonstrate the possibility to correct the mirror positioning errors induced by the robotic assembly process. The tripod mechanism coupled to flexure elements is able to perform an extremely accurate adjustment of each individual tile. HOTDOCK is a standard robotic mating interface supporting mechanical, data, power and thermal transfer. The core assembly process handles 3 types of connexions: single, double or triple simultaneous matings. Thanks to navigation skills of the platform and the skill-based planning, the Robotic Assembly System is able to successfully address these three types of connections. The key concept shown by dLSAFFE is its representativeness of the real assembly process with respect to scale. In a word, it demonstrates the ability to autonomously perform the assembly of a large structure with full-scale 1.5m diameter mirror tiles with a limited manipulator workspace, executed in a zero-buoyancy facility to mimic microgravity. A modular underwater manipulator was developed, and integrated with a purpose-built linear rail on the demonstrator mockup structure. This robotic assembly system is controlled by motion planning and control software components. The concept of standard interface enabling the segmented mirror tiles assembly was reused here with a set of controllable magnetic interfaces based on the HOTDOCK geometry. The software architecture of dLSAFFE is based on the concepts developed under the previous PERASPERA operational grants. Perception sensors are based on I3DS, while the perception algorithms are derived from InFuse and the overall system is integrated under the TASTE framework. dISAS was designed and implemented as a tool to support the process of in-space robotic assembly mission specification and rapid-prototyping of new concepts. It allows the user to simulate the software and hardware architectures via a powerful simulation engine. Each hardware or software component, such as structural elements, sensors, actuators, standard interfaces, or robotic manipulator is represented in this environment with a high-fidelity, user-customizable model. Generic software components such as the Assembly Manager, maintaining an internal model of the geometry of the demonstrator, and the Sequencer, encoding the assembly sequence in a behaviour tree formalism, have been developed and reused successfully across multiple demonstrators. dISAS has been used to rapidly iterate on the design of efficient AOCS controllers capable of preserving the required pointing accuracy of the spacecraft during the telescope deployment phase, despite the slow evolution of the inertia and frequency modes of the system as well as the disturbances torques that are induced by the motions of the robotic arm. Over the last 2 years, PULSAR has worked to develop, mature and demonstrate key technologies enabling next-generation concepts where large structures are autonomously assembled in orbit. This opens up new possibilities for higher-performance, more cost-effective and more ambitious future space exploration missions.
PULSAR demonstrators have shown that important components needed for autonomous assembly have reached a sufficient level of maturity to be included in the process of developing new missions able to build very large structures in space. PULSAR demonstrators have shown the capability of high level task planning by generating assembly plans based on skills and manipulator global capabilities and by simply describing complex tasks at a high level through the usage of behaviour trees and state machines. The reconfiguration capabilities of such high level task description have been demonstrated during the implementation phase of dLSAFFE. The development of a novel integrated simulation tool with dISAS and the good results it exhibited demonstrate its capability and usefulness for accelerating the development process of new missions. Indeed, dISAS as a tool can be used in early phases of future missions to prototype, test and perform trade-offs between concepts at a minimal cost. Regarding the control aspects, the dPAMT demonstrator showed the benefits of using compliant control based on torque-controlled joints on the robotic arm to achieve robust assembly of the mirror tiles. This technology is also implemented in the Engineering Model of the CAESAR space robot arm, which will be further developed to a space qualified model in order to be employed in future on orbit servicing missions. As part of the dPAMT demonstrator, the development of a tripod positioning mechanism relying on SLM manufactured flexures component demonstrated the capability of such a novel technology to meet stringent requirement in term of positioning accuracy. Such a manufacturing technology present big advantages related to rapid development and production. The good results of the dPAMT demonstrator and its newly-developed HOTDOCK standard interconnect show that the whole concept of in-space assembly is enabled by such an interface, demonstrating the importance of a standard for modular and reusable future components. In this regard, the HOTDOCK, featuring novel capabilities like the form-fit mating guidance, is a good candidate for TRL increase and integration in future missions. The dLSAFFE demonstrator has also allowed PULSAR partners to develop, test and demonstrate a completely new underwater manipulator, based on a modular design allowing rapid adaptation to new use-cases. It has also shown the interest of a new concept of actively controlled magnetic standard interconnect which could be used, for example, as a tool changer end-effector for underwater robotics operations. Moreover, a completely new underwater linear rail design was developed and tested and has potential applications in underwater robotics. On the software side, we believe that the TASTE framework, used in two of PULSAR’s demonstrators and improved with fixes and tweaks, is also a good candidate for an increase in maturity level so that it can become a de-facto standard for software integration of future missions.