Mechanical Design of a Rover for Mobile Manipulation in Uneven Terrain inthe Context of the SpaceBot Cup

Marc Manz*, Roland U. Sonsalla*, Jens Hilljegerdes**, Christian Oekermann*, JakobSchwendner*, Sebastian Bartsch*,Stephan Ptacek*

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

**University of Bremen, FB3, Robotics Lab, Bremen, Germanye-mail: firstname.lastname@dfki.de

Abstract

This paper describes the development and test of amobile manipulation platform intended for a terrestrialrobotic competition. While current space missions areplanned to minimize complex manipulation tasks, plansfor future space missions go beyond these restrictions.Infrastructure deployment, human-robot cooperative mis-sions and complex sample collection require increasinglycomplex manipulation capabilities. To meet this need theSpacebot Cup consists of several complex manipulationtasks in unstructured terrain. These requirements werethe main design driver for the presented system. The pre-sented rover consists of a 3-Boogie-Chasis designed to in-crease the maximum stepping size, flexible rubber wheelsto increase the maximal climbing inclination on loose sur-faces and a small six degree of freedom manipulator tohandle objects within the competition. The iterative sim-ulation and experiment process used to develop the flexi-ble rubber wheels is presented. Furthermore experimentsare presented which allow a performance comparison be-tween flexible and rigid wheels on loose surfaces.

1 Introduction

The SpaceBot Cup announced in December 2012 bythe DLR 1 had the goal of transferring knowledge fromother disciplines to the space sciences and vice versa. TheSpaceBot Cup is a robotic competition mimicking an ex-traterrestrial exploration scenario regarding communica-tions delay, operation, and ground surfaces. The DFKIteam ARTEMIS (Autonomous Rover Team for Explo-ration and Manipulation Intended for SpaceBot) took partin the SpaceBot Cup while the system described in thispaper served as autonomous exploration rover.

1German Aerospace Center

1.1 Mission ScenarioWithin the competition the system had to au-

tonomously explore a 29 m x 22 m terrain, starting at apreviously defined lander position. GPS was unavailablefor navigation and localisation. Furthermore, the detec-tion of the system state was possible only via onboardsensors. The robot had to find and identify three differentobjects of known size and color, then grasp two of theseobjects and transport them to a base station (third object)where they were to be assembled. The main parts of thecompetition were:

• Mapping of the area

• Localisation of the rover in the map

• Localisation of mobile and fixed objects

• Moving on unstructured surfaces covered with fine-grained soil and gravel

• Collection of the mobile objects

• Assembly of the mobile and fixed objects

• Drive back to the landing site

These tasks had to be done in one hour. Including threecheckpoints within the ”groundcrew” could interactionwith the system for five minutes. All communication waseffected with a latency of two seconds.

1.2 RequirementsThe overall system mass was limited to 100 kg includ-

ing additional communication devices at the lander site.The number of robots used to solve each task was notsubject to regulations and the type of locomotion was leftto each groups discretion. The system had to be able tomanipulate two different mobile objects: a cup-like ob-ject with a diameter of eighty millimeters and mass of600 g and a ”battery” pack with dimensions of 200 mmx 40 mm x 100 mm and mass of 800 g. The inclinations

on the competition site were between 15° and 30°. Whilethe 15° slope had to be overcome to complete all missiontasks, the 30° slope was optional, constituting the shortestroute.

2 Mechatronic Design

The mechatronic design was directly driven by themission requirements described in the introduction. Tocreate such a system in under eight months the decisionwas made to use existing subsystems and components de-signed in other projects. Apart from the reused adaptedsubsystems, elastic wheels and a robotic gripper were de-veloped from scratch.

Concept Initial concepts consisted of two heteroge-neous robots which were to solve the tasks cooperatively.However, it soon became clear that such a multi-robot sys-tem would be too expensive to design to a high reliability.Due to the short time frame this concept was inappropri-ate, so a single robot system was developed (Fig.1). Tomeet the contest requirements the system had to providethree major hardware characteristics: a proper sensor con-cept for proprioception and exteroception, a chassis capa-ble of overcoming uneven terrain and the ability to ma-nipulate the objects. The final system, including the base

Figure 1. The Rover with all subsystemsand the objects to be manipulated, bat-tery (yellow), cup (blue) and basestation(red) [CAD drawing]

station has a weight of 87 kg. The rover itself has a size of830 mm x 1300 mm x 500 mm (width x length x height). Itis equipped with a six degree of freedom manipulator andis able to operate over one hour without external energy.

Components To solve all the contest tasks a variety ofsensors and support devices were intended. The most im-portant are depicted in Figure 1. Mounted on top of a sen-sor mast is a Velodyne HDL-32E LiDAR sensor to mapthe whole area. Three ATV Prosilica enable the system toperform colour based object detection. To provide accel-eration, velocity and orientation an XSens MTi-30 AHRSIMU is mounted under the three main cameras. This po-sition was chosen because it is as far as possible awayfrom all strong electromagnetic fields created in inductorssuch as motors and power supplies. To process all sen-sor data and planning tasks an Intel Core i7 embedded PC(KTQM77) is integrated. In order to avoid collisions thesystem is additionally equipped with two laser scanners(Hokuyo UTM-30LX). They are mounted in the front andrear of the chassis on a servo driven tilt unit. While themanipulator handles objects the forces between gripperand manipulator-arm are sensed by an ATI force-torquesensor (Mini45). The communication with the base sta-tion is realized over a 5.8 GHz 802.11ac WLAN connec-tion. See [1] for more information on the software andautonomy aspects of the system.

2.1 ManipulatorThe manipulation device is one of the integral com-

ponents to solving the given contest tasks. The developedmanipulator consist of six actuators with BLDC2 motorsfrom RoboDrive and Harmonic Drive transmissions witha reduction ratio 1:100 [2]. In order to adapt to devel-opment time all six actuators are identical. This increasesthe weight to payload ratio due to the heavy actuators nearthe end effector. To cope with with this drawback a sup-port spring is attached between the second and third actu-ator (Fig.2).

Figure 2. Manipulator with spring support

If the manipulator is in a vertical position the springdoesn’t affect the manipulator. In a horizontal posturethe spring provided 284 N of support, producing 17.6 N mtorque. The additional weight caused by the springs isabout 130 g. The springs reduces the load for the first actu-ator roughly about 40% (Fig.3). The depicted current for

2Brushless direct current

the first actuator results from a transition from the verticalinto horizontal posture. The nominal torque of the actua-tors is 28 N m with a maximum angular speed of 55 rpm.A single actuator weighs 525 g which results in a weight totorque ratio of 0.02 kg/Nm. The whole manipulator withgripper has a weight of 4.25 kg and can lift a 1 kg payload.

(a) With spring support

(b) Without spring support

Figure 3. Current of the first joint for tran-sition from vertical to horizontal posture

To grasp different objects, a generic two finger gripperprincipal is realized. It is actuated with two Dynamixel-MX64 servo motors (Fig.4). Each of the two fingers has tofinger limbs, these are cupped with the finger limbs of theopposed finger. This allows the actuation of both fingerswith two servos. The fingers are coupled over gear wheelsinside the gripper palm.

Figure 4. Gripper sectional view

2.2 Flexible wheelsThe static stability of the platform and the traction

force between ground and wheel surface are important pa-rameters for locomotion on uneven terrain. To increase thetraction the contact area between ground and wheel can beenlarged by increasing the wheel diameter or via flatten-ing the wheels [3] [4]. This approach is common in spaceapplications where it is implemented with flexible metalwheels. However, a major disadvantage of this approachis the poor performance observed on rigid surfaces due tothe low friction coefficient of metal on hard surfaces.

To improve traction an to simplify manufacturing,elastic rubber wheels were developed. Each wheel ismade of three disks of water jet cut EPDM (Ethylene-Propylene-Diene-Monomer) with a thickness of 30 mm.These layers were adhesively bonded with elastic materi-als superglue (LOCTITE™480). The first design (Fig.5a)was developed without the help of simulation by using an”educated guess” approach for the evaluation of differentspring-element shapes.

(a) First design iteration

(b) Second design iteration

Figure 5. Simulation results of non-linearrubber wheel simulation

Unfortunately, tests have shown that the requiredforces for the desired deflection cannot easily be predictedby estimation. For a deflection of 15 mm, a Load of≈1000 N, while the given force is ≈165 N (Fig.6).

(a) First design iteration

(b) Second design iteration

Figure 6. Flexible wheel deflection and forces

For the next design iteration, a contact body FEMsimulation was used to predict mechanical properties. Toreduce the simulation complexity, the model was simpli-fied to a two dimensional setup using Tria 6 Elements.A Mooney-Rivlin-Model based on uniaxial, planar shearand simple shear experimental data was used to simulatethe elastomer material properties. The simulation resultswere evaluated using the prototype test of the first designmodel and show an adequate approximation of the test re-sults.With the simulation model, a variety of shapes and wallthicknesses were analyzed until a combination that resultsin a deflection of 17 mm for the given load was found. Thefinal design (Fig.5b) shows roughly linear behavior withinthe expected operating range. Final tests with the roverhave proven the correctness of the given assumptions.

2.3 Suspension System

The locomotion platform is realized as a 3-Bogie pas-sive suspension to compensate unevenness of the terrain.The 3-Bogie concept was originally proposed for the Ex-oMars rover [5] and it was also used to build the mobileplatform for AILA [6]. The main characteristics of thisconcept are the lower mass and higher or equal static sta-bility compared with other concepts like the CRAB, RCL-E, and the Rocker Bogie [7]. This high static stability is

advantageous for mounting the laser scanner at a properheight to provide a maximum field of view over robotand adjacent environment. Each bogie is equipped withtwo active driven wheel units for steering and to drivethe adaptive wheel. This enables the rover to drive inevery direction with any orientation for the whole sys-tem. This allows for much easier and precise position-ing with regards to objects which are to be manipulated.The wheel is mounted off-axis to the active driven steer-ing axis. For reasons of efficiency this axis is held byan electro-magnetic brake. The whole design enables theadaption of different wheels in diameter and width. Anadditional effect of this design is also that the wheel sup-ports rotations of the steering axis.The rockers are passive while in the setup and can be ad-justed by limiting their rotation angle. The decision of thelimitation depends on the wheel size and ground conditionto lift the chassis before ground contact. These parameterswere determined with stepping-over tests. Each bogie isequipped with an absolute encoder to determine the cur-rent position.

3 Experiments

Several experiments were conducted in order to eva-lute components within the locomotion unit. The main fo-cus of these was the evaluation of the passive suspensionand flexible wheels.

3.1 Step ClimbingBesides static stability, the maximum height of con-

querable steps is a significant indicator for the quality ofmobility in unstructured terrain. This value was obtainedfor both the flexible and the rigid wheels. These revealedthat the system is able to overcome steps 320 mm highwith both wheels (Fig.7). This is one and a half times the

Figure 7. Step test with flexible wheels

wheel diameter. State-of-the-art passive suspension unitsallow the system overcome obstacle with double the wheeldiameter [8]. However the results are not directly compa-rable due to influencing factors such as scalability, differ-ences in sensor and component configurations as well asoverall mass and dimensions of the system.

3.2 Static StabilityTo overcome steps with the system as well as to drive

in steep inclinations a high static stability is advantageousfor the system. Other than designing with consideration inregards to static stability, tests were required to ascertainthe actual static stability of the final system. This test isnecessary on account of differences between CAD modelsand the actual system (Fig.8).

(a) Static stability longitudinal (b) Static stability transversal

Figure 8. Static stability test of the whole system

The system offers a static stability at the longitudinaland transversal side of more than 45°.

3.3 Wheel TestTo evaluate the flexible wheels the system was tested

on loose glass sand with a grain size of 1-3 mm diame-ter. This was done on a sand box with adjustable inclina-tion (Fig.9).

Figure 9. Setup for slip experiments

The flexible wheels were compared with rigid wheelsof the same effective diameter hence the distance from thewheel axis is the same for the flattened flexible wheel andthe rigid one. The results are depicted in Figure 10 for0° to 15° inclinations.

(a) Comparison between rigid and flexible wheels with 0° slop

(b) Comparison between rigid and flexible wheels with 10° slop

(c) Comparison between rigid and flexible wheels with 15° slop

Figure 10. Slip experiments for elastic andrigid wheel

The position of the rover was tracked with a Quali-sis motion tracking system. As a reference the rover wasprogrammed to drive 2 m straight ahead with a velocity of0.1 m/s. A slight variation to the reverence time of 20 sfor each run could be explained by the control error ofthe wheel speed controller. The red lines represent therigid wheels and the blue ones the flexible ones. The er-ror bars represent the variance of the measured values re-

lating to the mean value. The errors within the test on10° inclination are conspicuous, overall however the rigidwheels performed better than expected. Tests had to berepeated with a higher number of experiments. These re-vealed that the rover performed significantly better on allinclinations when fitted with the rubber wheels. The im-provement ranged from 16 % at 0° inclination up to 214%at 15° inclination. About 15% inclination the performanceof rigid wheels was so bad they were no longer compara-ble with the flexible wheels. In order to paint a clearerpicture experiments using the flexible wheels were con-tinued until performance deteriorated to more than 90%slippage (Fig.11). Even at 20° inclination the rover moved

Figure 11. Comparision between all flexi-ble wheel performances

forward. For a complete system with sensor, manipulationabilities and storage facilities for the objects to be handledthe performance was adequate and allowed the robot toovercome any inclination or loose surface within the com-petition.

4 Conclusion and Future Work

The paper presents the development of a mobile ma-nipulation platform including a six DOF manipulator,flexible wheels and a 3-bogie chassis. Furthermore ex-periments are presented evaluating the performance of thechassis and flexible wheels on loose surfaces. The cho-sen design results in a high static stability and climbingcapabilities of one and a half times wheel diameter. Theflexible rubber wheels performed better than rigid wheelsof the same effective diameter at any inclination. Themaximum inclination on loose glass sand showed an in-crease from 15° with rigid wheels up to 20° with the flexi-ble rubber wheels. These flexible wheels are not intendedfor space application and function as more of a transfertechnology for terrestrial robotic applications. While theTweels™ developed by Michelin are designed to replaceconventional air tires with the advantage of higher reliabil-

ity, our flexible wheels are developed to provide a highercontact patch between the contact surface and the wheel inorder to gain higher traction on loose surfaces. These flex-ible wheels could be useful for robots in disaster areas andother unstructured terrain. Future development will con-sist of a investigation of different material combinations,shape variations and comprehensive tests.

Acknowledgments

Parts of the presented work were done by theARTEMIS Team to which belong, apart from the au-thors, in alphabetical order, Sascha Arnold, Roman Be-loschewski, Anna Born, Axel Borold, Raul Domınguez,Markus Eich, Stefan Haase, Hendrik Hanff, Janosch Ma-chowinski, Sankaranarayanan Natarajan, Thorben Opfer-mann, Thomas Rohr, Andreas Scholz.

The project ARTEMIS is funded by the GermanSpace Agency (DLR, Grant number: 50RA1318) withfederal funds of the Federal Ministry of Economics andTechnology (BMWi) in accordance with the parliamen-tary resolution of the German Parliament. ARTEMIS is acooperation between the DFKI Bremen - Robotics Inno-vation Center and the University of Bremen.

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