RIMRES: A Modular Reconfigurable HeterogeneousMulti-Robot Exploration System

Florian Cordes*, Thomas M. Roehr*, and Frank Kirchner*,**

*DFKI Robotics Innovation Center Bremene-mail: firstname.lastname@dfki.de

**University of Bremen

Abstract

This paper describes the current state of the projectRIMRES1 and is meant to give an overview of the stateof integration, the aspects of modularity and reconfigura-bility as well as the challenges still to be tackled. Withinthe project, a complex earth-based demonstrator systemfor a lunar multi-robot exploration scenario is built up,demonstrated, and evaluated. A novel approach is pur-sued in which a wheeled and a legged system can be com-bined into a single system via an electro-mechanical in-terface (EMI). When detached, both systems can act in-dependently and take full advantage of their respective lo-comotion system. Apart from the mobile units, modularimmobile payload units (so-called payload-items) are de-veloped that can be used to (1) assemble various scientificpayloads and (2) to enhance or expand the capabilities ofthe mobile units. The payload items are interconnectedwith each other or with the mobile units via the same EMI.The scenario that is demonstrated within RIMRES envi-sions a lunar crater exploration where the wheeled systemis used to transport the highly mobile six-legged scout sys-tem to the crater rim. The scout is then deployed and startsto climb down into the crater to explore the permanentlyshaded regions of the crater.

1 Introduction

Considering reconfiguration already in the designphase of a system, higher system performance and ro-bustness can be achieved. Even without reconfigurationexplicitly accounted for in the design phase, there are ex-amples of reusing the remaining capabilities of a space-craft demonstrating that reconfiguration is a possibility torecover from errors. In the Japanese Hayabusa sample re-turn mission2, several reconfiguration actions during themission were necessary. For example, after failure of ionthrusters, the components were reconfigured to combinethe remaining capabilities in order to still be able to cor-rect the trajectories of the spacecraft.

1Reconfigurable Integrated Multi-Robot Exploration System2http://hayabusa.jaxa.jp/e/index.html

Figure 1. Parts of the RIMRES-System:Hybrid wheeled-leg rover Sherpa,legged scout CREX and immobilepayload-items for building infrastruc-ture and expanding the capabilities ofthe mobile systems.

A critical issue for planetary modular and physicalreconfigurable systems is a reliable and robust physicalinterface for attaching and detaching the single modules.This was also identified by Yim et al. [1], however, explicitexperiments concerning dust robustness and reliability ofthe mechanisms is up to now a sparsely covered topic inliterature. For RIMRES, an electro-mechanical interface(EMI) was developed to connect the subsystems with eachother. The EMI is tested with extreme dust accumulationsand proved to be reliable under those conditions [2].

Another critical issue for planetary exploration is sur-face mobility. Stationary probes (pure landing units) canonly provide scientific data for a limited range on thesurface, typically in the range of an attached manipula-tor arm. Opposed to that, mobile systems are capable oftraversing relatively long distances and can therefor ex-tend scientific reward of a mission. Critical issues for mo-bility on planetary surfaces are

• energy efficiency,• robustness and reliability, and• the ability to negotiate a wide range of terrains.

These requirements are partly contradictory: Onthe one hand, wheeled or tracked locomotion providesrelatively energy-efficient locomotion when comparedwith legged locomotion, but is limited in the range oftraversable terrains. On the other hand, legged locomo-tion allows a very strong terrain traversal capability, evenfree climbing on vertical slopes is possible [3], but dueto a higher number of actuators and the need for activelymaintaining the body height even when standing still, theenergy efficiency of those systems is worse compared towheeled systems.

There exist various approaches to combine the advan-tages of wheeled and legged locomotion, in the major-ity of the cases by designing hybrid wheeled-leg [4] orlegged-wheel [5] robots. In RIMRES, this is only onepart of the approach: Sherpa is a wheeled-leg rover thatcan actively adapt to the environment. The active suspen-sion system can further be reconfigured to act as a sens-ing device, see section 3.1 for more details. The secondpart of our approach of increasing the surface mobilityincorporates reconfiguration and cooperation: A leggedscout can be attached mechanically and electronically tothe wheeled rover, constituting a combined system. In thisway, the scout can be transported in an energy-efficientmanner to scientifically interesting places such as steepcraters. Once arrived at the destination, the scout is de-tached and both systems act independently [6]. In a for-mer approach with systems not specifically designed forcooperative tasks, this already proved to be a feasible ap-proach [7].

2 System and Mission Overview

The aspired mission in RIMRES tries to simulatetypical elements of a situation in an exploration missionand/or infrastructure build up. The mission is operatedfrom an earth-bound (mission) control center, which com-municates with a system control station at the lunar sur-face. This system control station is the focal point forcommunication of all robotic systems that are part ofthe mission: two mobile subsystems and four immobilepayload-items.

The two mobile subsystems are a wheeled rover anda legged scout. Both systems can act completely inde-pendently from each other, but at the same time a closeelectro-mechanical connection between both systems canbe established combining both separated systems into onecombined system. Further reconfiguration abilities areadded by the introduction of modular payload-items thatcan extend the capabilities of the mobile systems or canbe used to create payloads during the mission. These pay-

loads can either be part of a science mission or representbasic infrastructure elements, e.g. for communication.

The overall system in RIMRES is a technologydemonstration and will be used under earth conditions.However, for demonstration and validation, an artificiallunar crater (surface area 105m2) with realistic slopes andlighting conditions has been set up in the DFKI laborato-ries.

The anticipated mission has the following outline ofactions. The rover transports the scout to the rim of a lu-nar polar crater, where it is detached from the rover. Thescout then climbs into the permanently shaded regions ofthe crater to conduct in-situ measurements for finding wa-ter ice. During the transport by the rover, the scout isfully functional, thus its scientific instruments can alsobe used during the transport phase. The rover’s manip-ulator can be used to assemble scientific payloads fromso-called payload-items. For the demonstration scenario,four types of payload-items are implemented: (1) A bat-tery module for extending the range of the mobile unitsand for powering the assembled science packages, (2) acommunication/navigation item (REIPOS, section 3.3.2),(3) a camera module, simulating a data-generating sciencepayload, and (4) the mole subsurface sampling system thatalready flew on the Beagle-2 mission is planned to be im-plemented in the RIMRES framework.

In RIMRES, reconfiguration aspects are part of vari-ous layers. Firstly, the overall system and team of robotscan be reconfigured by either stacking of payload-items(onto each other or onto the mobile systems) or by dock-ing the legged scout and the wheeled rover. Secondly, onsubsystem level the individual systems are capable of dif-ferent operating modes which we also describe as recon-figuration property: (i) the wheeled rover can be reconfig-ured in the sense that the active suspension system can beused in various ways to propel the robot - in addition itsmanipulator can be used for handling the payload-itemsas well as for locomotion support and supervision of thesystem (ii) the legged scout is reconfigurable in the sensethat the legs used for locomotion are equipped with grip-per elements and using a gripper mode are able to pick upgeological samples at a site of interest.

3 RIMRES – A System of Systems

As stated above, RIMRES is constituted from differ-ent robotic systems that can combine and reconfigure inmultiple ways. The central component for reconfigura-tion is a robust and reliable electro-mechanical interface(EMI). The EMI developed for RIMRES is described indetail in [2], [8]. There is an active and a passive face ofthe EMI which each are identical throughout the subsys-tems of RIMRES.

This section mainly focuses on the wheeled rover

Figure 2. Current state of Sherpas integra-tion study. In this picture the active sus-pension is used to step onto an obstacle.The arm is used to actively influence thecenter of gravity for improved stability.

Sherpa as the central device in the reconfigurable systemRIMRES. The reconfiguration possibilities of the systemitself are highlighted. Furthermore, a brief overview ofthe legged scout robot and the singular payload-items isprovided.

3.1 Four-Wheeled Rover SherpaThe wheeled rover Sherpa is a key team member in

our multi-robot system. It is responsible for transport-ing the legged scout to the crater rim and for transport-ing payload-items. The wheeled rover is also required toassemble payloads on demand using the manipulator armattached to the central tower.

Sherpa makes use of an active suspension system thatallows to select from a set of locomotion modes depend-ing on the current terrain situation. These modes rangefrom various postures to enhance the relation of center ofgravity and center of the support polygon to substantiallydifferent drive modes, for example planar omnidirectionalmovements or inchworming modes, [9]. Figure 2 displaysthe current state of the integration of Sherpa.

Several elements of reconfiguration are present in thedesign of Sherpa. First of all, it is obvious that the activesuspension system can be regarded as a reconfigurationdevice: One possibility is to reconfigure the footprint ofthe rover according to the challenges the current terrainimposes on the rover. This can be seen as a posture re-configuration. Furthermore, the active suspension can beused to actually propel the robot: instead of just using thewheel actuators, the suspension actuators can be incorpo-rated into locomotion, as for example in an inchwormingfashion or for undulating behaviors.

A second major part of reconfiguration is the manip-ulator arm attached to the rover’s main body. Its primary

use is to handle the payload-items that are attached to thefour EMIs located around the central tower. By manipula-tion of the payload-items, various scientific and infrastruc-tural payloads can be assembled. Furthermore, the armcan be used as a fifth limb, thus reconfiguring an arm into aleg. In a third configuration, the manipulator’s palm cam-era that is normally used for grasping the payload-itemsin a visual servoing process can be used to allow a hu-man operator to supervise the rover system, thus the armserves as a hazard cam in this configuration. Additionally,payload-items attached to the arm can increase the abili-ties of the manipulator, i.e., scoops, sophisticated grippingelements, or other tools (these types of payload-items arecurrently not planned to be integrated within the RIMRESproject).

In the final stage of expansion, the wheels will be re-configurable subsystems of the rover. There are two ma-jor parts concerning the reconfiguration of the wheels: (1)The wheels actively can adapt their stiffness in order toreact to (a) changes in the environment, for example soft-ness of the soil, and (b) changes in the rover’s mass due to(un)docking of the scout. A further reconfiguration capa-bility is (2) to use the wheels as sensors for characteriza-tion of soil properties. This can be achieved by using thesensory disposition of the wheels needed for case (1) incombination with the active DoFs of the suspension sys-tem. By this, a bevameter-like sensing device can be cre-ated from the combination of wheel and swing unit.

Finally, the rover can be reconfigured using payload-items. For example, additional sensors can be attachedvia one of the in total six EMIs of Sherpa, i.e., using anadditional battery pack to extend the operational time ofthe system.

3.2 Six-Legged Scout CREXThe six-legged scout is the second mobile system

in RIMRES. It is based on the SpaceClimber robot [10]and adapted to the requirements of the multi-robot systemRIMRES, e.g. for reconfiguration and docking to Sherpa,an EMI has been placed at the back of CREX.

Apart from the reconfiguration of the overall systemby (un)docking CREX and Sherpa, CREX also providesseveral reconfiguration capabilities by itself. Firstly, grip-ping elements are employed in the front legs in order to beable to pick up geological samples. Thus, the legs used topropel the robot can be reconfigured to be used as manip-ulation/sampling devices.

Via the EMI on its back, CREX can be connected tothe wheeled Rover. However, the EMI can also be usedin the same manner as on Sherpa: arbitrary payload-itemscan be stacked onto CREX for extending its capabilities.This ranges from additional batteries to specialized sen-sors for a task at hand. In later stages, it is possible to inte-grate a second EMI on the belly of the Scout system in or-

Figure 3. CREX robot in artificial craterenvironment. CREX is equipped withan electro-mechanical interface for at-taching to Sherpa and for carryingpayload-items.

der to be able to dock specific sampling devices. By usingthe high degree of mobility of the system, these devicescan be positioned precisely over a spot of interest. Fig-ure 3 shows the integrated scout robot CREX in DFKI’sSpace Exploration Hall.

3.3 Modular Payload-ItemsThe modular payload-items are cubic modules with

an active EMI in the bottom face and a passive EMI inthe top face [2]. By stacking the payload-items, differ-ent scientific payloads and infrastructure elements can beassembled. All payload-items come with a processingunit (Gumstix) for high-level intelligence and a micro-controller to support low-level intelligence. As part of thelow-level intelligence an internal communication protocolhas been designed which allows to infer the current topol-ogy of a stack of payload-items (a so-called payload) fromthe EMI connections, and control basic operations such asopening and closing the mechanic latch to attach an activeEMI to a passive one. These capabilities are exposed tohigher level of control, to allow for more complex recon-figuration activities.

The following sections briefly describe the four spe-cialized module types that are currently under develop-ment for RIMRES.

3.3.1 Battery ModuleWithin the earth demonstration scenario of RIM-

RES, battery modules are used as replacement for energy-harvesting payload-items. In later stages, additional solarmodules for actually harvesting energy are conceivable.A battery module always constitutes the basis of a pay-load stack, since functional modules and energy modulesare separated within the modular framework of RIMRES.

While seemingly a simple device, the battery module re-quires intelligence regarding the power switching. It ispossible to connect multiple systems and multiple bat-tery modules at the same time. Each payload-item canbe a power sink while the battery modules can be a powersource as well. In order to protect the systems from uncon-trolled charging and connecting two power sources withdifferent power levels at the same time, a power manage-ment system has been set up [8].

3.3.2 REIPOSThe REIPOS3 class of payload-items serves two pur-

poses. (1) It can determine the direction and distance ofother REIPOS modules, thus enabling a relative position-ing system [11]. (2) Communication structures can bebuilt up by making use of the REIPOS capability to routemessages between the single nodes. Thus, also a REIPOSpayload-item itself is reconfigurable regarding its func-tionality. A REIPOS item always needs at least a batterymodule to constitute a functioning payload unit.

3.3.3 Science ModulesIn order to simulate science payload-items, a camera

payload-item is integrated in RIMRES as primary exam-ple for a science module. The camera payload-item is aplaceholder for more sophisticated scientific equipment,but it demonstrates the core feature: attached to a batterypayload-item it can form an active payload. This payloadis seamlessly integrated into the system control and com-municates within the framework. Using a general messagebus which is used by all subsystems it can receive controlcommands from the mission control: here, by orientingthe camera that is mounted on a rotational table and re-trieving images. Thus, the acquisition and distribution ofgathered data can be proved.

A secondary scientific device is the mole subsurfacesampling device that already flew on the Beagle-2 mis-sion. This device is made available by DLR-RY andis planned to be integrated into a standardized RIMRESmodular payload-item in order to demonstrate a ”real” sci-ence payload in the modular robotic system RIMRES.

3.4 Combinations of SubsystemsTo conclude the above statements, table 1 displays the

currently possible physical combinations of subsystems inRIMRES. Note that the combination Rover-Manipulatoris static in the current setup and here illustrates a theoreticmodularization. Otherwise, the table shows the range ofreconfiguration the system is capable of. A principle pos-sibility that is crossed out in the table is a connection oftwo rovers: One rover could connect to another rover withits manipulation interface to one of the four payload-baysof the other rover.

3Relative Interferometric Position Sensor

Table 1. Possible combinations of subsys-tems in RIMRES. In principle it wouldbe also possible to connect the manip-ulator of one rover to a payload-bay ofanother rover.

4 Software Foundation for aReconfigurable System

The challenges involved in order to support a recon-figurable team of robots are manifold. Various fields ofresearch tackle individual problems which arise simulta-neously in an application such as the RIMRES project.The key characteristics of the overall robotic team in RIM-RES are distribution, modularity and reconfigurability. Inaddition, the team of robots is heterogeneous and uses amodular hardware design, thus adding complexity by in-troducing a selection of configurations where only a sub-set of these configurations can be active at the same time.

Reconfiguration properties have to be considered notonly in the hardware design, but require support by thecontrolling software layers which eventually have to takeadvantage of the reconfiguration capabilities, e.g. by us-ing (re)planning. The usefulness of reconfiguration capa-bilities can best be shown in an error scenario where ateam of robots uses reconfiguration and redundancy in theoverall system in order to compensate for hardware faultor temporarily unavailable resources.

As already mentioned a system control station at thelunar surface represents the focal point for the communi-cation of the robotic team, regarding the link to the mis-sion control. This architecture introduces a centralizedcontrol approach in the first place. However, to achieverobustness a distributed setup has been selected for therobotic team to decrease the impact of a single-point offailure.

The project RIMRES embeds ESA’s Functional Ref-erence Model [12] [13] as general architecture model withthe three layer of subsystem control (Level A), task con-trol (Level B) and mission control (Level C). In the fol-lowing we describe our approach towards a robust subsys-tem control level (Level A), which is designed to allow formission success in different operation modes: (1) manualoperation: the team of robots is driven by given action se-quences that are forwarded by the mission control centerto achieve certain objective (2) semi-autonomous opera-

tion: the mission control relies on complex task sequencesto achieve a mission objective, or to reconfigure systemsto compensate for errors (3) autonomous operation: mis-sion control or system control fails to operate or cannotcommunicate with the robotic team - the architecture al-lows for self-organization of the robotic team, either tocontinue with the still known objectives or to reestablishcommunication with the mission control.

We target the challenges from two different perspec-tives: the intra-robot and inter-robot perspective.

4.1 Intra-robot architectureThe intra-robot perspective deals with the individual

robot. The software stack applied on a single robot inRIMRES is mainly based on Rock4 which itself is basedon Orocos. Rock uses a model-based approach to cre-ate an infrastructure of software components. Designing asystem with Rock has proven to be useful not only for theRIMRES project, but for multiple others in our institutewhich try to solve complex tasks.

Components are software modules that have dedi-cated input and output ports and perform specific tasks.Components represent the lowest level of granularity inthe software stack and are specialized to fulfill a specifictask. Using a managing component - the so-called super-vision - multiple components can be put together to form anetwork, so-called compositions. These compositions cansupport complex tasks and by creating them in an ’on-demand’ fashion allows to reflect hardware modularity inthe software layer. A single component can be combinedin multiple compositions. This not only fosters reuse ofcomponents but represents also reconfigurability on thesoftware level. Additionally, the supervision evaluates ifthere is a need for a component to run and thus also repre-sents a resource-saving mean. Figure 4 outlines the basicsetup of our component network.

While some generic tools such as the supervision cancontribute to reconfiguration capabilities, individual com-ponent design does as well. The ’TelemetryProvider’serves here as an example for a generic packaging compo-nent to support reconfiguration of the sensor data streamtowards the mission control station. Acquiring sensor dataand also forwarding to the mission control center mightbe costly in terms of resources such as processing powerand energy. In addition the concurrent communicationof images might be even not be feasible regarding com-munication bandwidth. Thus, mission control has to de-fine and activate the sensors such as a camera which itwants images from. Images are provided in a commoninternal data-type. On request, sensors will be activatedand the output data stream is dynamically attached to theTelemetryProvider component which itself internally con-verts images to a target format (as expected by the system

4Robot Construction Kit, http://www.rock-robotics.org/

control station) and outputs a generic telemetry containerpackage. This container package is then forwarded to thesystem control station, where it is unwrapped and splitinto the sensor specific packets. The use of the genericpackaging component allows us to aggregate sensor datain a dynamic fashion and improves reconfiguration capa-bilities on the software side.

Figure 4. Schematic of the component ar-chitecture showing the structure of baseelements such as the ’System Core’which performs communication proto-col validation and mediates between thesystem control center and supervision.

4.2 Inter-robot architectureFor the inter-robot challenges we take advantage of

the experience which exists in the multi-agent community.FIPA5 has brought up a number of standards that havebeen applied mainly in the domain of software agents.We try to widen the field of application and use elementsof the abstract architecture described by FIPA to build upour inter-robot infrastructure. We implemented the bit-efficient message standard of FIPA and use the idea of so-called message transport services (labeled MTA in Fig. 4).Each robotic system, i.e. Sherpa, CREX and payload-itemhas a local message transport service. All local servicesconnect to a network and effectively create a messagebus. Each message transport service announces its exis-tence on the network using the zeroconf solution Avahi.This mechanism enables the message transport servicesto dynamically find each other in a network and can bealso used for other components (or services) to announcetheir presence. This allows us to handle dynamically ap-pearing or disappearing components and fulfills a spe-cific requirement we have regarding modules which canbe build during the mission and should be available andvisible to other modules after powering up. This mecha-nism comes with an additional benefit: appearing and dis-

5Foundation of Intelligent Physical Agents

appearing systems due to communication losses, powerdown or similar can be detected in the network.

In the current context we use this setup to create a dy-namic communication network between all participatingrobots, which are able to communicate via FIPA messages(which take any type of messages as payload).

4.3 Modularization and HeterogenityAs already mentioned the supervision represents a

major contribution towards reconfiguration. Yet, in thefirst place our general development approach (and the oneof Rock) outputs highly modularized software, which al-lows to reuse components to a large extend within theoverall system. Modularization starts by designing low-level drivers that will be embedded into the Orocos mod-ules, e.g. to control the EMI a dedicated (framework-independent) driver has been developed along with a cor-responding (framework-specific) Orocos module. TheOrocos module exposes the EMI functionality and allowsto be used withing the supervision. Modularization con-tinues regarding robot capabilities. While there is com-mon functionality for all systems, e.g. to create our com-munication infrastructure, specific skills per system ex-ist. At supervision level we using a hierarchical struc-ture which allows to bundle functionalities respectivelyand minimize the effect of heterogenity. This structur-ing allows us to run the same basic software stack onSherpa, CREX, and all payload items in RIMRES. Whilestill some configuration and optimization has to be per-formed to account for lower system resources on the pay-load items or for CREX, we achieve a common high-levelsoftware framework for the robotic team in RIMRES.

4.4 Configuration and optimizationWe use the Orocos framework as a basis for our com-

ponents and as such we can use full capabilities of the sys-tem. Still, careful management is required for the setup ofa large component network since the configuration spaceis quite large. First of all, every component can either runwith a fixed period to evaluate data on its port and trig-ger some activities, or it can be event driven i.e. triggeras soon as data is available on a specific port. This setupcannot be mixed at the current stage, but the period settingallows for an optimal usage of resources or in case of anuninformed usage can also create unwanted delays in thecommunication flow.

5 Conclusion and Future Work

This paper presents the state of the RIMRES project.Within this project, a novel approach of tightly cooperat-ing heterogeneous robotic systems is pursued. Reconfig-uration of the subsystems themselves and the overall sys-tem by combining the single subsystems is considered in

the design phase and enables a wide range of actions to betaken in cases of failure, and a high adaptability to terrainsthat should be covered in planetary exploration. Reconfig-urability and modularity can be found on different levelsof the overall system.

The main pieces of hardware and control software arenow available, the next phase of the project includes in-tensive experimentation and validation of the systems.

Acknowledgements

The project RIMRES is funded by the German SpaceAgency (DLR, Grant number: 50RA0904) with federalfunds of the Federal Ministry of Economics and Technol-ogy (BMWi) in accordance with the parliamentary resolu-tion of the German Parliament. RIMRES is a cooperationbetween the DFKI Bremen – Robotics Innovation Centerand ZARM – Center of Applied Space Technology andMicrogravity. Further partners are DLR-RY, EADS As-trium, and OHB System.

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