Torque-controlled light weight armsand articulated hands -do we reach technological limits now?

G. Hirzinger, N. Sporer, M. Schedl, J. Butterfaß, M. Grebenstein

DLR German Aerospace CenterInstitute of Robotics and Mechatronics, D-82230 Wessling, GermanyGerd.Hirzinger@dlr.de, http://www.robotic.de

Abstract. Based on the long-term goal “robonauts for space” and the experiences DLR hasgained so far in space robotics, the talk describes recent design and development results inDLR’s robotics lab towards a new generation of “mechatronic” ultra-light weight robotswith articulated hands. The design of fully sensorized joints with complete state feedbackwhich has led to 7dof torque-controlled, “soft” robot arms with definable impedance in car-tesian space, is outlined. The just recently completed third light-weight arm generation isbased on a fully modular carbon fibre shell structure, newly designed motors with excitingcharacteristics, and it gives the feeling that it comes close to the limits of what is technicallyfeasible today; in the same way the second generation of DLR’s most highly integrated 4finger-hand with its 13 actuators was completed now and adapted to the new arms. Thus itis hoped that major steps towards a new generation of space as well as service and personalrobots have been achieved.

1 Introduction – The space robotics background

When comparing human skills with those of present-day robots of course humanbeings in general are by far superior, but when comparing the skill of an astronautin a clumsy space-suit with that of the best available robot technology, then thedifferences are already going to disappear, the more if there is a remote control andmonitoring capability on ground with arbitrarily high computational and humanbrain power. Although it is not clear today when a multi-fingered robot hand mightbe as skilled as the human hand without a thick glove and when (if ever) a robotmight show up real intelligence and autonomy, it nevertheless is obvious that evenwith today`s technology and the available telerobotic concepts based on close co-operation between man (e.g. the ground operator) and machine there are manytasks in space, where robots can replace or at least augment human activities withreduced cost at least from a long-term perspective.

Thus we are convinced that automation and robotics (A&R) will become one of themost attractive areas in space technology, it will allow for experiment-handling, in-spection, maintenance, assembly and servicing with a very limited amount ofhighly expensive manned missions (especially reducing dangerous extravehicularactivities). And the expectation of an extensive technology transfer from space toearth seems to be more justified than in many other areas of space technology, themore as space is a driver for a new generation of ultralight weight robots withminimal power consumption.

2 DLR’s early space robot projects

Our first big experience with space robotics had been ROTEX ( Fig. 1 ) the first re-motely controlled robot in space. It flew with Spacelab-Mission D2 inside shuttleCOLUMBIA in April ’93 and performed several prototype tasks (e. g. assembly andcatching a free-floating object) in different operational modes, e. g. remotely pro-grammed, but also on-line teleoperated by man and machine intelligence. Its suc-cess was essentially based on

• multisensory gripper technologies• local autonomy using the above sensory feedback capabilities• predictive graphics simulation compensating for 5 – 7 seconds

Fig. 1 ROTEX – the first remotely controlled space robot flew with shuttle COLUMBIA in 1993

We gained our second big space robot experience with NASDA’s ETS VII project,the first free-flying space robot, who was operable for around two years. In April’99 we got the permission by our Japanese friends to remotely program and controltheir robot from Tsukuba / Japan. The project called GETEX (German TechnologyExperiment) was again very successful (as was the whole ETS VII mission); ourgoals in particular had been:

• To verify the performance of an advanced telerobotic concept (MARCO, seee.g. [10]), in particular concerning the implicit task level programming capabili-ties as well as the sensor-based autonomy and world model update features. Ahighlight was indeed the tele-programming of a peg-in-hole task, where in thevirtual world we intentionally displaced the standby position of the peg fromwhere the robot had to fetch it. Vision processing on ground using NASDA´stracking markers on the task board and the Jacobian matrix learning before-hand based on real images caused the ETS VII robot to automatically and per-fectly adapt to the unexpected situation. The peg-in-hole insertion as such(taking into account the fairly high tolerances) was less critical and of coursemade use of NASDA’S compliant motion commands.

• To verify 6 dof dynamic models for the interaction between a robot and itsfree-flying carrier satellite. A major part of the GETEX experiment time was al-located to these experiments, which consisted of a series of manoeuvres car-ried out by the manipulator while the attitude control system of ETS-VII wasswitched off.

Fig. 2 ETS VII ground control via the task-level programming system MARCO

In such a mode of operation, a space robot consisting of a manipulator and a satel-lite is generally considered to be free of external forces. The robot therefore is as-sumed to have constant angular momentum, due to the law of the conservation ofangular momentum, which means that if the arm moves and thus introduces angu-lar momentum into the system, the satellite reacts with a compensating motion. Theamount of satellite rotation produced depends on the mass and inertia of the bod-ies which constitute the system. The description of a TCP trajectory in orbit-fixedco-ordinates, as it is necessary e.g. for the capturing of a defect satellite, has to ac-

count for the satellite reaction. The experiments conducted during the GETEX mis-sion aimed at a verification of the existing models of free-floating space robots andat the identification of the dynamic model parameters such as the satellite inertiatensor. A further goal was to obtain some insight into the nature and importance ofdisturbances acting on a robotic satellite in low Earth orbit and to gather data forthe future design of controllers which will combine the manipulator motion controlwith the satellite attitude control. Therefore, a variety of different manoeuvres wereexecuted (an example of which is shown in Fig. 3), which include simple point-to-point operations and closed-loop re-orientation manoeuvres, sequences duringwhich only one joint was active at a time as well as sequences during which alljoints were moving simu ltaneously.

Fig. 3 Example of a Dynamic Motion manoeuvre carried out during the GETEX mission.The shaded robot indicates the reference position.

The satellite reaction to the arm motion is scaled by a factor of 10 in this picture.

The major constraint, due to mission security aspects, was the maximum satelliteattitude error allowed by NASDA, which was limited to ±1.0° around each axis, andthe fact that the maximum tool center point velocity was limited, too. Furthermore,the reaction wheels were turning at a very low but non-zero constant velocity dur-ing the experiments, which introduced undesired torques into thesystem.

In total, over 110 minutes of dynamic motion experiments have been carried out, ofwhich 52 minutes have been spent in free motion mode. The remaining time wasused to repeat the experiments in reaction wheel attitude control mode for verifica-tion purposes. Evaluations of the measurement data confirmed the need to account

for external disturbance forces acting on the satellite, such as the gravity gradienttorque and magnetic.

3 Future steps

Germany has made decisions as to where the next steps in space robotics willmove. There are two major directions:

3.1 Systems at the international space station ISS

The most remarkable system at ISS is Canada’s Mobile Servicing Center, a threearm system with a long (≈ 17m) arm and two smaller arms (≈ 3,5) on top of it. Can-ada has put a major part of its space budget into this remarkable technology; nev-ertheless in the past we have repeatedly criticized, that the arms were only con-trollable by astronauts – typically with 1mm/see, at least in our opinion a waste oftime. As an example, in July 2001 (the base arm was just installed) two astronautsneeded 6 hours to grasp the so called airload out for the shuttle bay and mount itat the ISS. We are very happy that a close cooperation exists meanwhile betweenCSA and DLR aiming at an efficient ground control of the arms implying the ad-vanced telerobotic system MARCO.

Fig. 4 The Canadian multi-arm ISS robotsystem (courtesy CSA)

Simulated Airlock assembly with ourtelerobotic system MARCO

In addition, a German Space station robotic project ROKVISS has just started (Fig.5). It aims at the qualification of DLR’s newest light weight joints (sect. 6 and 7) by

hand of a 2 dof arm on the ISS by the end of 2003 and it also plans a demonstrationof telepresence technologies with real-time stereo-video transmission and tactilefeedback via the ground station.

Fig. 5 The two-joint qualification and telepresence experiment ROKVISS on ISS aims atthe verification of basic technologies for future robonaut applications

Indeed what we are still missing in space are fast signal transmissions via just onerelay satellite yielding coverage times of around 40 minutes and round trip delaysof ≈ 0,5 sec. thus allowing even haptic feedback. Realistic telepresence will beneeded e. g. too, when astronauts have to be remotely investigated using roboti-cally guided 3D-ultrasound probes.

Fig. 6 Telepresence is gaining more importance in space

3.2 Free-flying systems

We have performed an extensive technology study on an experimental servicingsatellite (ESS), which applies robotics to solve the problem of servicing a non-co-operative target in or near to a geostationary orbit, a region of space still out ofreach to manned spaceflight. A three-month demonstration flight of such an ESShas been planned and all phases of its mission have been defined. These includethe acquisition, inspection and servicing of an orbiting satellite through to parkingit in a graveyard – orbit.

For that external servicing task high interactivity between man and machine is re-quired, because the remote environment will be mainly unknown. The teleroboticsystem MARCO will be used to give the system the local autonomy by intelligentsensor data processing. Because all the satellites, built so far, are not equipped forservicing, the final stages of approach and the subsequent capture of the target arethe most critical phases of the mission.

To simulate the dynamic behavior of the chaser during robot motions, we have ar-ranged two KUKA robots as shown in Fig. 7. Robot B is used to carry out the cap-turing task, Robot A emulates the entire dynamic relation between the chaser andthe target satellite, where the dynamic coupling with the AOCS is included.

Robot A Robot B

Sensor - control

Dyn. ModelTarget Motion

Dyn. Modelof ESS -

Manipulator

Dyn. Modelof ESS AOCS Model

TCPDifference

Tool CenterPoint

Model ofApogee Motor

Capture Tooland Sensors Video, F/T and

Distance Data

Fig. 7 ESS simulation and testbed

The manipulator of ESS, equipped with a multi-sensory capturing tool, must followthe residual movements of a selected object on the target (e.g. the main thruster or“apogee motor”) by means of an image processing system whose data are passedthrough an extended Kalman filtering process. With the robot controller monitoring

laser distance sensor values, force, torque and travel, the capture tool is insertedinto the cone of the thruster (Fig. 8, Fig. 9).

Fig. 8 Tracking of target’s apogee as seen from the wrist-mounted hand camera.The wireframe model of the target is projected into the live video image at the

currently estimated pose

Fig. 9 An artist’s view of ESS, catching the apogee of TV-Sat-1,one of whose solar panels did not open

The “tumbling” target would not necessarily be in a geostationary object. Indeed afew space systems no longer controllable have been identified on lower orbits,which might become dangerous for earth, as they will not completely burn outwhen passing the atmosphere. Thus grasping them with a robot (e.g. with a morearticulated hand if no apogee motor is usable) and drawing them down in a well-

defined way might become an important service in the framework of future garbagecollection systems (Fig. 10).

Fig. 10 Catching a worn-out satellite to render it harmless

Independent of this special application the next European step should be a newfree-flying space robot mission (experimental or operational), for which however nofinal decisions exist at present. Our proposal for a two-arm/hand freeflyer is shownin (Fig. 11).

Fig. 11 DLR´s Robonaut concept for a free-flying robot satellite with two armsand two articulated hands

4 Light weight robot concepts

What we definitely need for space (as a technology driver) but also for the widevariety of future terrestrial service robot applications, are “soft” and sensor-controlled light-weight arms (in contrast to the stiff and heavy industrial solutions)and articulated, multifingered hands, which come closer and closer to the delicatehuman performance. Two of these arms combined with an arrangement of a stereocamera pair tends to provide such a system with humanoid appearance and thusprovokes the “robonaut” terminology. NASA has recently presented remarkableresults in this context.

From the experiences we made with ROTEX, we see the need that at least a smallsize space robot (1-2m size) should not only sustain itself on ground without sus-pendings, but should also be able to work in a “0g gravity” mode, i.e. compensat-ing gravity by imposing appropriate bias torques onto the joints. And a free-flyingspace robot should be able to make itself compliant when getting in contact with afloating object to be grasped or docked at. These are only a few arguments that cryfor a new generation of torque-controlled, 7dof light-weight arms with minimalpower consumption.

By the way, to compare light-weight actuators in a fair way, it is of crucial impor-tance to take the maximum motion speed in account (as proposed in [2]). With ourthird generation of light weight arms presented in this paper we try the approachthe technological limits of what is feasible today.

DLR’s light-weight robots from the beginning have been kinematically redundant(7dof) and joint-torque-controlled [2], [3].

LWR I (Fig. 12, left) with its 18kg weight, a nearly 1:2 load to weight ratio and itscarbon fibre grid structure links was a highly mechatronic arm, but its double-planetary gearings with a 1:600 reduction turned out to be too critical in terms oftolerance-safe manufacturing. In addition its inductive torque sensing was critical,too, in terms of complexity and robustness.

In LWR II (Fig. 12, right) we went back to harmonic drive gearings, a strain-gaugebased torque measurement system, embedded into a full state measurement andfeedback system (motor position, link position, joint torque). Again all electronics(signal, power, control) was fully integrated into the arm weighing 17kg and carry-ing 8kg.

Fig. 12 LWR I (left) and LWR II (right)

With this arm, we were fully satisfied with its high-fidelity impedance controllabil-ity, but manufacturing and duplicating the arm was not yet simple enough.

Fig. 13 New DLR Light-Weight-Robot LWR III, virtual (left) and real (right)

The development goal of our light weight robot generation III (Fig. 13) was to real-ize a system which builds up on all the experiences made with robot I and II, butexploits the remaining potentials to approach the technical limits.

5 Modular arms

Thus our new robot arm concept (with future marketing and sales of the arm inmind) aimed at a completely modular assembly system with only a few basic com-ponents concerning joint mechanics, electronics (i.e. mechatronics) and links, sothat completely different configurations can be composed in a short time.

And, as in LWR II the Harmonic drive gearings had been redeveloped in close co-operation with the Japanese manufacturer into a drastically weight-reduced ALUversion (60% weight saving), it was clear that now the key issues to be attackedwere a new motor generation, ultralight-weight brakes and bringing the links backto carbon fibre technology (LWR I had already a grid-type carbon structure, but asalready stated unsatisfactory joint drives).

Indeed the new concept is based on a fully modular joint-link-assembly system,with only a few basic components, namely three one-dof robot joint–link types anda two-dof wrist joint. Still there remain two alternatives: a symmetrical version (Fig.14) and an asymmetrical version (Fig. 15) which is particularly interesting if foldingthe arm (e.g. for transporting it into the outer space) is a key issue.

Fig. 14 Symmetrical Robot

Fig. 15 Asymmetrical Robot

These modularity concepts were supported by a powerful kinematic-dynamicanalysis and design software, based on concepts of concurrent engineering, whichallow to assemble any type of robot using the link component library (with con-tinuously adjustable link lengths) and a set of (typically 3) available gearing typesand a set of (typically 3) available motor types. When an arm has thus been virtu-ally composed (a matter of a few minutes) the arm tip (with a definable mass fixed toit) may be immediately moved around with a 6 dof input device like a Space Mouseor a Phantom, both devices allowing also to impose forces and torques on to thearm’s tip; and the kinematic-dynamic simulation in real-time shows the arisingforces/torques in the joints (Fig. 16, Fig. 17), and - after passing FEM calculations –give hints on necessary material strengths e. g. in bearings and carbonfibrestructures.

Light WeightCFK LinkStructures

HighlyIntegratedDrives

Online TorqueMonitoring

7-Joint Robot Kinematics

Online InteractiveRobot Control

Online Dynamics Simulationand Visualization

Fig. 16 A kinematic-dynamic simulation tool for optimizing the design

Fig. 17 Concurrent Engineering for the arm configuration

Another result came out from the kinematic-dynamic simulations: a ball-shaped twoaxis wrist joint (Fig. 18), imitating the human wrist, but showing up much highermobility. Indeed the most important joints of a robot are the wrist joints. Manipula-bility of the robot is significantly dependent on kinematic configuration. If the dis-tance between wrist-pitch axis to tool-center-point is short, changes of the orienta-tion at the tool-center-point do not lead to big movements of the lower robot joints.And every gram which can be saved in the design of the wrist is a gain of payload.

The new extremely light-weight design of the wrist enables two kinematic configu-rations. A common configuration with roll-pitch-roll axis and optional a roll-pitch-pitch configuration which is easy to reconfigure. The short standard flange is sim-ply to be replaced with an extended 90° bended flange. The cardan joint avoidssingularities in a stretched position. This position is often reached while fine-manipulating with articulated hands like DLR’s 4-finger-hand.

(roll-)pitch-roll (roll-)pitch-pitch

Fig. 18 Ball like wrist joint

In general the modularity concept gives us a number of advantages, e. g:• rotation symmetric (Fig. 19) components• few single parts, short force transmission from bearing to off-drive

connection, pitch and roll joints identical,• Big hollow shaft in all joints with up to 30 mm diameter, thus optimal cable and

plug links inside the arm.

modular drive between two links 3D-CAD-design of the modular drive

Fig. 19 Modular drives in the new arm

6 The breakthrough in motor technology

As a matter of fact, in the past robot manufacturers have taken the best availablemotors off the shelf for their robots, without being optimized for robotic applica-tions (comparably slow rotational speed though high dynamics, permanently re-versing operation around zero speed) and aiming at minimal weight and powerlosses. Thus we have gone through a two years concurrent engineering and opti-mization process who took in account al the electromagnetic and other physical ef-fects, short copper paths, optimal coil winding and coil filling aspects between themagnetic iron poles. It came out of this optimization process that the stator poleshad to be subdivided and winded separately (Fig. 20).

Fig. 20 Concurrent Engineering for the actuator design

The result is DLR’s high energy ROBOdrive with only half of the weight and halfof the power losses of the (to our knowledge) best available motors (see Fig. 22).Simulative and experimental results differed by only 1-2%.

Fig. 21 Comparing DLR’s ROBODRIVE with the best commercially available motors

Fig. 22 3D CAD-design ofDLR’s ROBODRIVE

Fig. 23 DLR’s new ROBODRIVE withone of the carbon fibre links

Three different sizes are now integrated into LWR III.Other important features and components as developed for LWR II [4] were trans-ferred into the new arm III:• the full state measurement and control in all joints, as are

- straingauge-based torque-sensing and torque feedback in a 3 kHz rate

- motor position sensing using hall sensors

- off-drive position sensing (formerly using optical principles, now movingalternatively to low-cost plastic potentiometer or high-end capacitive sen-sors, respectively)

• the light-weight ALU-Harmonic drive with 60% weight reduction.

The arm has just been assembled and is going to become operable now. Only threecables inside the arm are needed to connect all joints with power supply and theexternal PC controller.

What presently remains to be done is e.g.

- a decision on which types of safetybrake should be finally used. While in [4] aweight reduction of a commercial safety brake from 281 to 155 g was reported,we have in the meantime developed piezoelectric brakes weighing only ≈ 70g(Fig. 25)

Fig. 24 Torque sensor

Fig. 25 A piezo-brake with only 70 g overall weight

However as some moistness problems are inherent with piezobrakes, we havein addition gone through another concurrent engineering and optimizationprocess leading to an extremely weight-reduced electromagnetic brakeversion , too (≈ 30g for the ball-shaped wrist joints)

- miniaturizing the arm electronics further, going far beyond well-known SMD-technologies (with tiny chips anyway) to chipsize-Packaging and flipchip(thus microsystems) technologies by support of leading German Fraunhofer-Institutes specialized on microintegration. All electronics (except the centralPC controller board), is integrated into the arm.

7 Impedance controlled arms

The control structures for this arm have been mainly developed and verified byhand of LWR II (Fig. 26). In particular a flexible joint model is assumed. Fast and re-liable methods for the identification of the joint model parameters (joint stiffness,damping, and friction) were developed, while the rigid body parameters are directlygenerated from the mechanical CAD programs [6]. This leads to an accurate simu-lation of the robot dynamics, so that the controller structures can be developedand tested directly in the simulation.

Fig. 26 controller architecture

The first stage in the controller development was a joint state feedback controllerwith compensation of gravity and friction. The state vector contains the motor po-sition, the joint torques, as well as their derivatives. The conditions for the passiv-ity of this structure were derived in [6]. Under these conditions, the global asymp-totic stability of the controller can be proven [7].

By the appropriate parameterization of the feedback gains, the controller structurecan be used to implement position, torque or impedance control. In the last case,the gains of the controller are computed in every Cartesian cycle, based on the de-sired joint stiffness and damping, as well as depending of the actual value of theinertia matrix. Hence, this controller structure, fulfils the following functionalities:

• It provides active vibration damping of the flexible joint structure.• It maximizes the bandwidth of the joint control, for the given instantaneous

values of the inertia matrix.• It implements variable joint stiffness and damping.

Based on this joint control structure, three different strategies for implementingCartesian compliant motion have been realized: admittance control, which accessesthe joint position interface through the inverse kinematics; impedance control,which is based on the joint torque interface; and Cartesian stiffness control, whichaccesses the joint impedance controller. To combine the advantages of the lasttwo methods regarding geometric accuracy and high bandwidth, a new controlmethod was implemented, which consists of an impedance controller enhanced bylocal stiffness control [8]. This structure consistently takes into account the factthat the cycle of the joint control loop is typically by one order of magnitude fasterthan that of the Cartesian loop. It uses the high bandwidth of the joint impedancecontroller to improve the performance of the Cartesian impedance control. The con-trol strategy is strongly motivated by the way in which humans achieve their com-pliant manipulation skills. It is well known that the mammalian supervisory controlloop has a significant time delay. The passivity property of the muscles, as well asthe fact that the intrinsic stiffness and damping can be varied according to thestiffness of the environment (in this case by the contraction of antagonistic musclepairs), seems to play an essential role in preserving the stability and performanceof the movement [9 ].

Fig. 27 LBR II balances a pole and reacts with a human

Fig. 27 shows a situation, where arm II automatically balances a pole and a humanoperator touches the arm and deflects the elbow joint using the arm’s redundancyand impedance controlled „soft“ behavior.

The new impedance controller enhanced by local stiffness control shows up a bet-ter performance than classical impedance and stiffness control. Compared to admit-tance control, it has lower geometric accuracy, but higher bandwidth and imped-ance range. This makes it well suitable for applications where the robot is incontact with a stiff, unknown environment.

8 Articulated 4-fingered hands

In 1997 DLR developed one of the first articulated hands with completely inte-grated actuators and electronics.

This well known hand has been in use for several years and has been a very usefultool for research and development of grasping. The main problems remaining weremaintenance and the many cables (400) leading out from the Hand. The experienceswith Hand I accumulated to a level that enabled us to design a new hand accordingto a fully integrated mechatronics concept which yields a reasonably better per-formance in grasping and manipulation and therefore accelerates furtherdevelopments.

Due to maintenance problems with Hand I and in order to reduce weight and pro-duction costs the fingers and base joints of Hand II were realized as an open skele-ton-structure. The open structure is covered by 4 semi shells and one 2-componentfingertip housing realized in stereolitography and vacuum mold.

This enables us to test the influence of different shapes of the outer surfaces ongrasping tasks without redesigning finger parts.

The main target in developing Hand II from the beginning has been the improve-ment of the grasping- performance in case of precision- and power-grasp. There-fore the design of Hand II was based on performance-tests with scalable virtualmodels as seen in Fig. 29.

Fig. 28 DLR's Hand I. Fig. 29 Optimization of kinematics withscalable hand model

On the other hand performing precision-grasps/fine-manipulation requires hugeregions of intersection of the ranges of motion and the opposition of thumb andring-finger (Fig. 30). Therefore Hand II was designed with an additional degree offreedom which enables to use the hand in 2 different configurations. This degree offreedom is a slow motion type to reduce weight and complexity of the system. This“adaptive palm” motion of the first and the fourth finger are both realized with justone brushed dc actuator using a spindle gear. In Fig. 32 real precision and powergrasp are shown with the 13 dof hand II.

Fig. 30 Simulation of Hand II in powergrasp and fine manipulation configuration

Fig. 31 Differential bevel gear of thenew basejoint.

The three independent joints (there is one additional coupled joint) of each fingerare equipped with appropriate actuators. The actuation systems essentially consistof brushless dc-motors, tooth belts, harmonic drive gears and bevel gears in thebase joint. The configuration differs between the different joints. The base joint

with its two degrees of freedom is of differential bevel gear type, the harmonicdrive gears for geometric reasons being directly coupled to the motors. The differ-ential type of joint (Fig. 31) allows to use the full power of the two actuators forflexion or extension.

Fig. 32 Precision and powergrasp with Hand II

Since this is the motion where most of the available torque has to be applied, it al-lows to use the torque of both actuators jointly for most of the time. This meansthat we can utilize smaller motors. The actuation system in the medial joint is de-signed to meet the conditions in the base joint when the finger is in stretched posi-tion and can apply a force of up to 30 N on the finger tip. Here the motor is linkedto the gear by the transmission belt.A dexterous robot hand for teleoperation and autonomous operation needs (as aminimum) a set of force and position sensors. Various other sensors add to thisbasic scheme. Each joint is equipped with strain gauge based joint torque sensorsand specially designed potentiometers based on conductive plastic. Besides thetorque sensors in each joint we designed a tiny (20mm diameter, 16mm height) sixdimensional force torque sensor for each finger tip with full digital output. Theforce and torque measure ranges are 10 N for Fx and Fy, 40 N for Fz, 150 Nmm forMx, My and Mz respectively.

c o m m u n i c a t i o nc o n t r o l l e r

ADC16 ch

ADC8 ch

sensorcircuitry

sensorcircuitry

sensorcircuitry

ADC8 ch

sensorcircuitry

ADC8 ch

Motor

powerconverter

MotorMotor

powerconverter

powerconverter

sensorcircuitry

power supply

serialto hand base

Fig. 33 The fingertip sensor Fig. 34 Electronics and communicationin a finger

All electronics needed locally is integrated into the hand. However the control ofthe fingers and the hand is done by an external computer. In order to use the handfreely on different manipulators and to reduce cables and the possibility of noise inthe sensor signals, we decided to design a fully integrated serial communicationsystem. Each finger holds one communication controller in its base unit.

This controller is responsible for the collection and distribution of all informationof interest. Furthermore it does some reasonable signal processing.It collects the data of all five ADCs per finger with together 40 channels of 12 bitresolution each and transmits these data to the communication controller in thehand base (see Fig. 35 and Fig. 36). On the other hand it distributes the data fromthe control scheme to the actuators for finger control. The communication control-ler in the hand base links the serial data stream of each finger to the data stream ofthe external control computer. By this hardware architecture we are able to limit thenumber of external cables of DLR's Hand II to a four line power supply and an eightline communication interface since the data is transmitted via differential lines. Thisinterface even provides the possibility of using a quick-lock adaptor for autono-mous tool exchange. Reducing external cabling from 400 (in Hand I) to 12 here, isone of the major steps forward in our new hand.

CommunicationController(FPGA)

Motor

hand dof

fingers 1-4

serial

serial/differentialIEEE 1355-1995

Fig. 35 The communication controller in the hand base links the fingersto external computers

Fig. 36 Signal pre-processing and transferin the hand II

When a robot hand performs any fine manipulation, there is always need that thefingertip should be soft in the direction normal to the contact surface and hardtangential to the contact surface. Thus the impedance should be adaptable to theorientation of the fingertip. Therefore, a cartesian impedance controller has beendeveloped, where the fingertips behave like a programmable spring.

Fig. 37 Block diagram of the hand’s cartesian impedance controller

9 Conclusion

After more than 10 years of torque-controlled light weight robot development weare near to finishing the third generation, where we have tried to use all present-day available simulation and computational technologies to approach the technicallimits. Taking into account its weight of 13-14kg, its typical power consumption oflittle more than 100 Watt, its load capability of around 10kg, its motion speed basedon maximal joint speeds of ≈ 180°/sec, it is probably one of the lightest robots thathave been built so far.Arm III is equipped now with DLR’s articulated hand II. With its 13 active joints in-tegrated into the palm it allows to verify strong power grasps in the same way asdelicate fingertip grasps. A special adapter was designed so that the only 12 wiresleading out of the hand are fully guided inside the hollow shafts of the robot joints.To our knowledge this extremely high degree of mechatronic integration in arm andhand has not been realized before. It is the basis for DLR’s future space robot de-velopments aiming at systems similar to NASA’s robonaut [5]. Torque-controlledarm and hand technologies are also the basis for a new surgical robot system, themore as these new robot systems can be used as "force-reflecting hand control-lers", too. This observation led to the decision to combine the above arm and handtechnologies to create an innovative surgical robotics system comprising five iden-tical, extremely slender, variably compliant, but also very powerful robot arms,where two arms are used as force-reflecting hand controllers, two as instrumentcarriers and one as an endoscope guide system (Fig. 8). This system is supposedto help minimally invasive surgery make a major breakthrough in the future, be-cause it not only enables the surgeon to intuitively transfer his finger and handmovements to the inside of the patient, (just like when a large, traumatic, opening ismade in the body), but also restores his tactile perception.

Fig. 38 A surgical robotics system for the future, based on LWR IIIand Hand II technologies

Acknowledgement

We would like to thank the German space agency and Bavaria’s ministry of tech-nology, who has made mechatronics a key topic of Bavaria’s high tech offensivewith light weight robotics and articulated hands as central demonstrators.

References

[1] G.Hirzinger, B.Brunner, J.Dietrich, J.Heindl, „ROTEX - The First Remotely ControlledRobot in Space“, IEEE Int. Conference on Robotics and Automation, San Diego, Cali-fornia, May 8-13, 1994

[2] G.Hirzinger, J.Butterfaß, M.Fischer, M.Grebenstein, M.Hähnle, H.Liu, I.Schaefer,N.Sporer,, “A Mechatronic Approach to the Design of Light-Weight Arms and Mul-tifingered Hands”, IEEE Int. Conference on Robotics and Automation (ICRA 2000), SanFrancisco, April 2000

[3] G.Hirzinger, A.Albu-Schäffer, M.Hähnle, I.Schaefer, N.Sporer: On a New Generation ofTorque Controlled Light-Weight Robots. ICRA International Conference on Robotics &Automation, Seoul, Korea, May 21-26, 2001, IEEE, COEX Seoul Korea, ConferenceProceedings ICRA 2001, S. 3356-3363, (2001)

[4] G.Hirzinger, B.Brunner, K.Landzettel, I.Schaefer, N.Sporer, G.Butterfaß, M.Schedl:Space robotics – DLR’s telerobotic concepts, lightweight arms and articulated hands.10th International Conference on Advanced Robotics (ICAR 2001) Budapest, Hungary,August 2001

[5] M.Bluethmann, R.Ambrose, R.Askew, M.Goza, C.Lovechik, D.Magruder, M.A.Differ,F.Rehnmark: Robonaut: a robotic astronaut’s assistant. 10th International Conference onAdvanced Robotics (ICAR 2001) Budapest, Hungary, August 2001

[6] A.Albu-Schäffer, G.Hirzinger: Parameter identification and passivity based joint controlfor a 7DOF torque controlled light weight robot. ICRA International Conference on Ro-botics & Automation, Seoul, Korea, May 21-26, 2001, IEEE, COEX Seoul Korea, Con-ference Proceedings ICRA 2001, S. 2852-2858, (2001)

[7] A.Albu-Schäffer and G.Hirzinger.: State feedback controller for flexible joint robots: Aglobally stable approach implemented on DLR's light-weight robots. IROS, 2000

[8] A.Albu-Schäffer and G.Hirzinger: Cartesian Impedance Control Techniques for Torque-Controlled Light-Weight Robots” IEEE International Conference on Robotics and Auto-mation (ICRA), May 11-15, 2002 Washington D.C.

[9] R.Koeppe, A.Albu-Schaeffer, C.Preusche, G.Schreiber, G.Hirzinger: A New Generea-tion of Compliance Controlled Manipulators with Human Arm Like Properties. TheTenth International Symposium of Robotics Research, ISRR, Lorne, Victoria, Australia,Nov. 9-12, 2001

[10] Hirzinger, G., Brunner, B., Butterfaß, J., Fischer, M., Grebenstein M., Landzettel, K.,Schedl., M.: Space robotics - towards advanced mechatronic components and powerfultelerobotic systems. i-SAIRAS 2001, 6th International Symposium on Artificial Intelli-

gence and Robotics & Automation in Space, St-Hubert, Quebec, Canada, June 18-22,2001, Canadian Space Agency, CD i-SAIRAS 2001, (2001)

[11] Hirzinger, G., Sporer, N., Albu-Schäffer, A. Hähnle, M., Krenn, R., Pascucci, A.,Schedl, M.: DLR’s torque-controlled light weight robot III – are we reaching the techno-logical limits now? IEEE International Conference on Robotics and Automation (ICRA),May 11-15, 2002 Washington D.C.

[12] D.Vischer, O.Khatib: Design and Development of Toruqe-Controlled Joints. In: Ex-perimental Robotics I. ed. V.Hayward and O.Khatib, Springer-Verlag, pp. 271-286, 1990

[13] P.Dario, E.Guglielmelli, C.Laschi, G.Teti (SSSA): MOVAID: a personal robot in eve-ryday life of disabled and elderly people. Technology and Disability Journal, No. 10,1999, IOS Press, Olanda, pp.77-93

[14] M.E.Rosheim: Design of an Omnidirectional Arm, Proceedings of IEEE Conference ofRobotics and Automation Magazine June 1996, S. 32-38