RIMRES - Reconfigurable Integration Multi-Robot Exploration System . 20

inte-grated approach to design and operate reconfigurable multi-robot systems. The project RIM-RES uses reconfigurability as key design requirement. In this context, all robotic systems fea-ture a standardisedEMI, which has been developed by Wenzel, Cordes, Dettmann, et al. (2011).

ThisEMIallows two agents not only to establish a secure mechanical link, but it also estab-lishes the respective communication networks (for Ethernet and RS422) and a shared power bus. Furthermore, it has been designed to operate in the dusty lunar environment.

Electromechanical Interface TheEMIas designed for the project RIMRES is gendered and comes in two variants: a female and a male interface. The female interface variant is also re-ferred to as activeEMIwhile the male is referred to as passiveEMI. The female variant contains a motor-driven, gripper-like mechanism to establish a secure connection with a passive pin at the male interface. Figure 2.2illustrates the two variants. Similar to the iSSI developed by Schervan et al. (2017) the EMI has been developed for space application. However, it it de-signed for the use in a planetary exploration mission on the lunar surface and not as satellite building block. Therefore, it features special design elements to account for an exposure to lunar regolith. According to the general classification scheme, e.g., as used by Liu, X. Zhang, and Hao (2016), this interface permits chain-based structures.

(a)Female (active) variant as part of the end ef-fector(Source: DFKI GmbH).

(b) Male (passive) variant (Source: DFKI GmbH).

Figure 2.2:Male and femaleelectromechanical interfaces(EMIs) in the project RIMRES.

TheEMIembeds a power-bus system, which allows the transfer of energy. All robotic agents feature a power management system which allows to switch between power sources and bal-ance the energy level. All systems connected with anEMIalso share an Ethernet connection; a previous setup of the IP configuration is required. Visual markers have been added to the male interface to facilitate the visual servoing process.

TheEMIis the key element to create reconfigurable multi-robot systems which are the basis for the research of this thesis. The application of an EMI enables modular robotic systems,

(a) Male interface of the back CREX (Source:

DFKI GmbH).

(b)Female interface at the bottom of Sherpa to attach CREX(Source: DFKI GmbH).

Figure 2.3:Interfaces to connect the mobile systems.

(a) Variant 1: CREX attached to Sherpa’s manipulator (Source: DFKI GmbH).

(b)Variant 2: CREX attached to the bottom interface of Sherpa(Source: DFKI GmbH).

Figure 2.4:Docking variants of the mobile systems CREX and Sherpa in the project RIMRES.

which are capable of creating larger composite systems, which can still act as if they were a single unit. Especially a shared communication and energy system is essential to compose a new capable robot from multiple reconfigurable robots.

In contrast to swarm-based systems, no fully distributed control approach is applied for the composite agents in RIMRES: each composite agent uses a centralised control approach. Never-theless, the communication between agent relies on a distributed communication architecture.

In contrast to an androgynous interface design, a female interface can only be connected to a male interface and vice versa. In general, interface compatibility will limit the number of feasible reconfiguration options for the robotic hardware; the implications are discussed in Chapter3. Each female interface contains a camera which can be used for the visual servoing process.

Immobile agents A payload-item, in the sense of the implemented multi-robot system, is a general-purpose module or container with a standardised size of 0.15×0.15×0.15 m³. It comes with a male interface on top and a female interface at the bottom - no interfaces are present at the sides. In principle, each payload-item is an individual agent with a processing unit (an ARM-based processing board is part of the design). Only power modules, however, have their own energy source.

Payload-items have been additionally equipped corresponding to their role in a space explo-ration mission: a battery payload item to serve as energy provider, or a camera payload item as sensing device.

Mobile agents The rover Sherpa as illustrated in Figure2.4 is a wheeled-leg mobile explo-ration system, which features a manipulator on top and comes with an adaptive locomotion platform. The system has four bays to host payload items. Each bay offers a connection via a maleEMI. The robot has one femaleEMIat the bottom so that systems with a male interface can be docked, e.g., as depicted in Figure2.4b. This allows for a marsupial-like transport ca-pability. The end effector of the manipulator is equipped with a femaleEMIas well. Therefore the rover Sherpa is the only system in the overall team of robots which can actively manipulate payload-items and perform reconfiguration of other systems.

The so-called Crater Explorer (CREX) is a six-legged mobile systems with a maleEMIon its back (cf. Figure2.3a). The interface on the back of CREX can be used to dock the system to Sherpa via the rover’s bottom interface, and it can also be used to lift the CREX using Sherpa’s manipulator as shown in Figure 2.4a. This leads to number of reconfiguration states which among other scenarios enable: (a) marsupial transport, (b) lifting systems to previously un-reachable areas, and (c) attaching sensors or tools (here: a payload item or the six-legged robot as gripper device).

Operations The project RIMRES led to a space mission driven operation and allowed ex-ploitation of the overall system capabilities using manual operation and semi-autonomous se-quences. The project illustrated the feasibility and benefit of using anEMIand analysed the performance of automated docking of the scout to the exploration rover (see (Roehr, Cordes, and F. Kirchner2014)). Hence, the project served as primary evaluation step for a reconfig-urable multi-robot system which is composed of capable individual robotic systems, and a mixture of mobile and immobile systems. Operating the robots in RIMRES also verified the first iteration of the communication architecture which is described in Chapter5.

Limitations Operation of the reconfigurable system in RIMRES is limited to the semi-auto-nomous, time-line based control which is triggered from the ground station. Direct manual control and visual feedback is used by an operator to perform safe operation, e.g., no au-tonomous simultaneous localization and mapping (SLAM)-based navigation approaches are used. As result of this project Roehr, Cordes, and F. Kirchner (2014) suggested the introduc-tion of an organisaintroduc-tion model for reconfigurable multi-robot systems to raise the autonomy level.

2.2.3 TransTerra & FT-Utah - Semi-Autonomous Cooperative Exploration of Plan-etary Surfaces

The project TransTerrA in combination with a field trip in the scope of the project FT-Utah led to a reconfigurable multi-robot system which is capable of semi-autonomously performing exploration missions (cf. Chapter6).

The robotic team in TransTerrA is designed for the operation of a logistics chain to support the long term operation of terrestrial exploration. The team consists of the mobile systems Sher-paTT and Coyote III which are depicted in Figure2.5a. Further team members are a number of immobile systems: battery payload item, camera payload item, gas sensor array payload item, soil sampling payload item, aDifferential Global Positioning System (DGPS)payload item, and an additional manipulator arm calledSymmetrical Interface Manipulator (SIMA)(Brinkmann, Cordes, et al.2018). A base camp serves as logistics hub, and supports the setup of an infras-tructure based on placing payload items on base camps. Figure2.6ashows a base camp. Each base camp has its own computing and communication unit and comes with five male interfaces to couple payload items. Each male interface has pins for guiding a docking process (a) and a central pin for establishing the mechanical connection (b), redundant (spring mounted) pins for the power and communication connection (c), and an optional set of visual markers (d). The rover’s end effector also embeds the female variant as illustrated in Figure2.6b. Furthermore, it comprises LEDS (a) and a camera (d) to support a visual docking process, the central pin of a male interface is received and locked in (c). The redundant set of connection pads (b) estab-lishes the electrical contact with the corresponding pads on the male interface, while guiding holes (e) receive the guiding pins of the male interface. The redundancy of the pads permits 90° stepwise angular rotational difference when connecting a male and a female interface.

Apart of a successively improved electromechanical design, the major difference between Sherpa and SherpaTT is a change of the locomotion system, and the application of a significantly im-proved automation, parts of which will be presented in Chapter6.

Operations Both systems SherpaTT and Coyote III can perform exploration fully autonomously, and both cooperate for a distributedSLAM-based mapping approach. A decentralised commu-nication architecture described in Chapter5is the foundation for the distributed operation of the overall system. SherpaTT can manipulate payload-items, i.e., pick and place onto the Coy-ote III. Chapter6illustrates the performance of a fully integrated approach of an autonomously executed sample mission sequence. Furthermore, it describes the evaluation of the multi-robot system’s abilities and identifies remaining shortcomings.

Limitations The increase of autonomous capabilities of the individual systems in the project TransTerrA allows for significantly more advanced operations in comparison to the projects LUNARES and RIMRES. The overall team has continuously been improved, but remains an evaluation system of high complexity. For this reason, several limitations exist on low-level and on high-level posing a risk of failure. Failures can affect any automated operation so that autonomous robots require local failure handling routines, but might still fall back to direct operator interaction. Chapters6and Chapter7continue the discussion with detail.

(a)The mobile agents of the reconfigurable multi-robot system in TransTerrA while exchanging a payload between SherpaTT (left) and Coyote III (right).

(b) The manipulator SIMA comprises two female inter-faces for docking, here at-tached to a base camp. With connection to a power source, it can serve as agent with lim-ited mobility, e.g., wander by attaching to male interfaces.

Figure 2.5:Mobile and immobile agents in the project TransTerrA.

(a)Base Camp with five male interfaces. (b) End effector with female inter-face.

Figure 2.6:Variants ofelectromechanical interfacesin the project TransTerrA.