New Direction on Robotic Hands Design for Space Applications

ELSEVIER

Copyright © IFAC Automatic Control in Aerospace, Saint-Petersburg, Russia, 2004

IFAC PUBLICATIONS www.clscvicr.comllocatclifac

NEW DIRECTIONS ON ROBOTIC HANDS DESIGN FOR SPACE APPLICATIONS L.Biagiotti· F.Lotti·· C.Melchiorri· G.Vassura··

• DEIS, University of Bologna, Via Risorgimento 2, 40136 Bologna, Italy email : {lbiagiotti.cmelchiorri}~deis.unibo.it •• DIEM, University of Bologna Via Risorgimento 2, 40136 Bologna, Italy email: {fabrizio.lotti.gabriele.vassura}~unibo.it

Abstract: Aim of this work is to discuss some possible solutions for the design of robotic hands/grippers for space manipulation. The paper first reviews possible alternative approaches for the design of general purpose robotic end-effectors. They can differ in kinematic architecture, sensory equipment, etc. and are solutions for different needs and different application scenarios (e.g. autonomous operation, telemanipulation) . Then, three examples (which have been developed at the University of Bologna) are taken into account, namely a medium complexity 3-dof robotic gripper for autonomous tasks in IVA/EVA environments, an anthropomorphic gripper for tele-operated and autonomous operations, and finally a human-like robotic hand based on new design concepts (i .e. compliant mechanisms). Copyright ©2004 IFAC Keywords: Space Robotics, Robotic Gripper, Dexterous Hands

1. INTRODUCTION

of computational resources, power , material selection, etc. (Putz , 1999). For these reasons, intermediate solutions between the poor functional capabilities of a 2-jaw gripper and the complexity of a human-like robot hand are, at the moment, necessary. On the other hand, in a long term view, technological improvements and innovative design approaches will be the key points to build robotic hands, that are dexterous and reliable at the same time. Aim of this work is to explain how the above mentioned issues have been faced at the University of Bologna. In particular, three different end-effectors, with different features and different goals , but sharing a common design philosophy (that is reduced complexity) are shown.

In the last decade, in order to replace the crew in the execution of some repetitive and timeconsuming tasks, robotic arms have been designed and tested in space. These manipulators were equipped with simple grippers with very limited grasp and manipulation capabilities (Hirzinger et aI., 1993). On the contrary, the current research for advanced applications in space addresses the development of dexterous hands with anthropomorphic structure and very high complexity (Butterfass et al. , 2002; Ambrose et al., 2000), although the use of such devices in a space mission seems still far away. The space environment imposes severe constraints concerning safety and reliability as well as strong limitations in terms

327

2. GENERAL ISSUES IN ROBOTIC END-EFFECTORS DESIGN The two main key-words used to describe a robotic end-effector are "anthropomorphism" and "dexterity" , and very often they are used as synonyms. Indeed, these two concepts are "orthogonal" and refer to different aspects of robotic devices, which imply different targets and lead to different design solutions. Dexterity concerns the capability of the manipulator of performing grasp and "internal" manipulation (and it depends on the mechanical configuration, as well as the sensory equipment and the control modalities) while anthropomorphism is related to the aspect of the device and its resemblance with the human hand (as to shape , size, number of dofs, and so on). Therefore, it is possible to achieve a high level of dexterity by means of non-anthropomorphic devices, with a proper design of the mechanical, sensory and control systems. Such devices (usually characterized by a kinematic architecture simpler than that of a human-inspired hand) may be particularly suitable for autonomous operations, in particular considering a high-level sensory apparatus (including position, force, proximity sensors). Conversely, anthropomorphism is a desirable goal for two main reasons:

Fig. 1. The gripper dealing with 2 different objects.

3.1.1. Gripper design The gripper , shown in Fig. 1, has a modular structure, with three one-dof fingers disposed radially, in a symmetric configuration, whose distal phalange can move along a linear trajectory, see Fig. 2.b. Despite its simplicity, this kinematic configuration allows to firmly grasp objects with irregular shapes and with a rather wide range of dimensions. The sensory system of the gripper has been designed taking into account both the motion of the fingers and the approach and interaction phases with the grasped object. In particular, each finger is equipped with a Hall effect position sensor, a proximity sensor and a miniaturized force/torque sensor, as shown in Fig. 2.a . The proximity sensor (based on a simple light emitter-receiver) measures the distance of each finger from the object surface and allows to plan the approach motion in order to get synchronous contacts while the forcetorque sensor (which can detect the interaction forces, by measuring the deformation on the fingertip structure by means of the classical strain gauge technology) can be used for the control of grasping forces once contacts have been applied . Note that, being capable of detecting not only the intensity of contact force components but also the position of the contact centroid on the external surface of the finger, the intrinsic tactile sensor can efficiently recognize actual contact conditions, including incipient sliding (Melchiorri , 2000). 3.1.2. Control modalities In order to cope with the main problems of the space environment (e.g. the lack of gravity) and exploit the structural features of the gripper a logic-based switching control has been designed . As a matter of fact, the different tasks are performed by the gripper according to a sequence of the following main controllers:

• the robotic device can operate in a manoriented environment, where tasks may be executed by the robot or by the astronaut as well, acting on items, objects or tools, sized and shaped according to human manipulation requirements; • the hand can be tele-operated by humans by means of special purpose interface devices (e.g. a data-glove), and anthropomorphism becomes a very powerful way to simplify operation , as the hand should simply mimic the operator's hand behavior. On the other hand, the constraints tied to an anthropomorphic structure imply a noticeable growth of the complexity and, if the abovementioned motivations are not present, this kind of design approach results useless.

3. ROBOTIC END-EFFECTORS FOR SPACE: THREE DIFFERENT PROPOSALS

3.1 Non anthropomorphic gripper with medium dexterity level

• position control; • proximity control; • stiffness control.

In order to reduce the gap between simple 2-jaw grippers and dexterous anthropomorphic hands, we have proposed an intermediate solution, i.e. a 3-fingered gripper (Biagiotti et al., 2001) .

The position control of each finger is based on a classical PI regulator, as depicted in Fig. 3. At

328

Fig. 4. Grasp of a floating object by means of the 3-dof gripper .

(b)

(a)

to obtain some noticeable functionalities . In particular the gripper is able to deal with free-floating objects and to autonomously grasp objects, whose shape is unknown . In Fig. 4 a grasp procedure is shown: the fingers are moved until a given distance from the object surface is reached , then the contacts are applied synchronously. Such a procedure allows to avoid undesired interactions with the object, which could cause its loss . Moreover , the simple structure of the gripper is particularly suitable for an automatic grasp synthesis (Ponce and Faverjon, 1995; Rimon and Burdick, 1994a; Rimon and Burdick, 1994b), which on the base of the object shape and the gripper features selects the best grip points.

Fig. 2. Sensory equipment (a) and kinematic structure of the gripper (b). this level, a difficulty has been the compensation of nonlinearities caused by the actuation system, in particular a relevant (and non constant) dead zone and the nonlinear characteristic of the Hall effect position sensors. The same structure has been exploited to accomplish the proximity control of the finger (by simply switching the feedback signal from the position sensor to the proximity one), and in order to guarantee a smooth behavior of the finger a proper trajectory generation has been implemented. In this modality, it is possible to approach the fingertip to the object surface up to the desired distance and keep that constant, thus avoiding undesired interactions. Instead, when an interaction with the environment (usually the grasped object) is desired , the force exerted by the fingers can be regulated by means of a stiffness control , which in steady state assures an applied force proportional to the displacement from the desired position (Xd):

F

= Ke(x -

3. 2 Anthropomorphic gripper with medium-high dexterity level The MARVISS 1 project represents an attempt to build an end-effector inspired by the human hand but with a degree of complexity compliant to space conditions . As a matter of fact, anthropomorphism can be obtained in robotic hands at different degrees: in this case a reduced anthropomorphism has been adopted . This means that only some functions and/or only some parts of the human hand are reproduced; in particular this choice leads to a robotic device with a reduced number of fingers and with a reduced internal mobility, so that only a limited set of grasping functions can be performed. On the other hand, it is important to note that the operations that MARVISS (and, more generally, robotic end-effectors in the space context) has to perform are limited to the grasp of regularly shaped objects and to the accomplishment of simple tasks (e .g. push buttons, actuate knobs, slide drawers) . Therefore high-dexterity manipulation is not required and the total number of internal degrees of freedom can be reduced with respect to fully anthropomorphic hands. Moreover, the required specifications

Xd)

In this way each finger behaves like a programmable spring, whose stiffness Ke can be modified according to the desired task. By means of a proper switching logic between the control modalities above introduced , it is possible

···· · - (l;fl·· ->:< } ...... -

. F..................

• . F.

"

f

1

' L'===-r 1:-

mm )

.,

' .'Jr;:l 1" [

' r j 'J" :.. r p, lv~·· · =:- · l )(

i: --

' :'

~

~ ...

,

q.•

:

:- \' 'I.::::. r

./)

....

.... ......... .. 1 Anthropomorphic hand and virtual reality for robotic system on the International Space Station .

Fig. 3. Control scheme of the gripper.

329

,--- '

I

{ ., ',

..

/

i

I :

'.

(a)

(b)

,. ',

'~ /)

\ (c)

(d)

Fig. 6. Different grasp configurations performable by the MARVISS robot hand.

- (5) - Yaw motion of opposable thumb. The yaw motion of the opposable thumb is needed to realize some important kinds of grasps like the lateral grasp shown in Fig. 6.c. Besides, it allows moving the thumb in a lateral configuration to permit the complete closure of the upper fingers against the palm , Fig. 6.c. - (6) - Flexion of proximal opposable thumb phalange. This motion is fundamental to close the fingers together and realize the most common kinds of grasp .

(b)

Fig. 5. Kinematic structure (a) and mechanical design (b) of Marviss .

in terms of overall dimensions , shapes and power consumption are very strict. The envisaged MARVISS hand has six degrees of freedom. It consists of 2 two-dof upper fingers (index and middle) and of a two-dof opposable thumb. Fig. 5.a reports the kinematic model of the device, and in particular shows how the independent degrees of freedom have been arranged in order to obtain the prescribed capabilities:

In order to obtain a modular hand , the motors 2 are placed between the palm and the flange , and the motion is transmitted by means of a solution based on "Worm + spring", see Fig. 5.b . Basically it consists of a worm gear set (worm and wheel) in which the addition of deformable member permits to accumulate the deformation energy. This energy can be used in case of power failure to save the grasp. Moreover the presence of the deformable member allows to use this device like a (very reliable) force sensor , in fact , knowing .t he entity of deformation (e.g. by means of an optIcal sensor) is possible to derive the loads acting on the output shaft . Beside this kind of force sensing, the complete sensory equipment includes position sensors located in the joints (necessary because of the use of under-actuated mechanisms and elastic elements in the kinematic chain) and a 3-axis force sensor in the fingertips. This is the minimum set of sensors necessary to make the robot hand suitable for the interaction with the environment, even if it is worth to underline once again, dexterous ~anipulation capabilities are not required and not achieved .

- (1) and (2) - Flexion of the proximal phalanges of the upper fingers. A symmetric yaw motion of both the upper fingers is linked to the proximal phalanges flexion . The relationship between the value of the flexion and the yaw angles is fixed , When the proximal phalange is completely extended, the yaw angle is maximum (about 15° ), but when the proximal phalange angle passes a certain value (about 40°) the upper fingers become parallel and so remain until the end. This functionality is very important for the MARVISS grasping tasks. In fact, in grasping large size objects, a wide yaw angle allows to realize a stable grasp (power grasp, see Fig. 6.a) ; instead , when working with little objects, the parallel motion of the upper fingers allows using the fingertips (precision grasp, Fig. 6.b). - (3) and (4) - Flexion of the medial phalanges of the upper fingers . The medial phalange flexion is independent in each of the upper fingers . This is important to obtain a good capability of fitting to the shapes of several objects.

2 DC gear-motor produced by MAXON , with a diameter of 13 mm.

330

stiffness of the structure is reduced also with respect to transverse bending and torque loads; • structural design must cope with the need of large hinge displacements, with non trivial problems, i.e. fatigue life; • there are deep interactions between all the parts of the system (articulated structure, compliant cover, actuation and control modalities) so that a co-design of all these subsystems must be performed.

!

/

1~1

(a)

In addition to the clear advantages related to structural simplification and reliability enhancement, a strong motivation to apply the compliant mechanism concept to robotic hands design comes from the consistency of this approach with an (b) efficient reproduction of the endoskeleton model offered by the human hand. An endoskeletal strucFig. 7. Sketch of a robotic finger based on compliture obtained with a reduced cross-section inant mechanisms (a) and its actual implementernal frame, instead of the frequently adopted tation. exoskeletal design, offers higher possibility to 3.3 A fully anthropomorphic hand with high dexterity host distributed sensory equipment and compliant pads, both essential for dexterity achievement and level operation reliability improvement. If MARVISS is an attempt to find the optimal trade-off between task specifications and the over3.3.1. Hand prototyping and preliminary experiall complexity (and therefore the safety and the ments Based on the above concept, an innovareliability) of the device, the philosophy underlytive robotic hand, the UB Hand III (University ing this new project is quite different. In this case of Bologna Hand, version Ill), is currently under the goal is again a reduction of the complexity, development .. This device has a structure which which characterizes most of the robotic hands tries to reproduce the human hand , as regards so far introduced in the literature (Ambrose et number of fingers and of dof. Therefore it is comal., 2000; Butterfass et al. , 2002) , but the design posed by four upper fingers and one opposable guidelines adopted to achive this aim are quite thumb , see Fig. 8. In the present implementation, different (Lotti and Vassura, 2002) . In particufor which a patent application has been filed, a lar, in this last case the key idea has been the modular solution based on five identical fingers design of a the finger structure based on the so has been adopted . This allows to further simcalled "compliant mechanisms" , i.e. chains of rigid plify the manufacturing process, which consists in links connected by means of elastic hinges allowmoulding plastic elements (the phalanges) around ing relative motion between them, see Fig. 7.a . the compliant hinges, made of steel. Previous applications in robotics were limited to Each articulated finger is characterized by four small-scale manipulation grippers (Goldfarb and joints: Celanovic, 1999), but this concept seems promis• one for the abduction/adduction movement; ing also for application in robotic hands, with • three for curling motions . these main advantages: The motion transmission is obtained by means of tendons. This encourages the use of a remote actuation, and the integrated design of the hand and of the robotic arm . In particular the motors 3 can be located in the forearms (similarly to the human muscles); moreover, in the forearm can be hosted all the electronics, necessary for the actuation and the sensory system. This is composed by a basic set of position sensors (bending sen-

• design simplification, with reduction of part count, avoidance of part-interface problems (friction, backlash), size and bulk reduction, adoption of alternative materials; • improvement of the reliability level, in particular wit respect to part loosening; • manufacturing enhancement and overall cost reduction. On the other hand, some drawbacks remain and/or emerge:

3 In the present design four "linear" motors are used for each finger, but the degrees of freedom can be easily coupled in order to simplify the actuation and also the control system.

• the compliance of the hinges introduces severe problems in modelling and control the equivalent kinematic chains and the overall

331

complexity due to the adoption of innovative mechanical tools (Le. compliant mechanisms) and a design approach (concerning the development of the sensor apparatus, the adoption of suitable control modalities) consistent with this choice, lead to a device able to perform internal manipulation operations, but characterized by an high level of reliability and safety.

REFERENCES Ambrose, R.O ., H. Aldridge, R.S. Askew, R .R. Burridge, W . Bluethmann, M. Diftler, C. Lovchik, D. Magruder and F. Rehnmark (2000). Robonaut: Nasa's space humanoid . IEEE Transactions on robotics and automation. Biagiotti, L., C. Melchiorri and G. Vassura (2001) . A novel approach to mechanical design of articulated fingers for robotic hands. In: Proc. IEEE Int. Conf. Robotics and Automation, ICRA01. Seoul, Corea. Butterfass, J., M . Grebenstein, H. Liu and G. Hirzinger (2002) . Dlr-hand ii: next generation of a dextrous robot hand. In: Proc. IEEE Int. Conf. Robotics and Automation, ICRA02. Washington. Goldfarb, J . M . and N. Celanovic (1999). A ftexure-based gripper for small-scale manipulation . Robotica. Hirzinger, G., B . Brunner, J. Dietrich and J . Heindl (1993). Sensor-based space roboticsrotex and its telerobotic features. IEEE Transactions on robotics and automation. Lotti, F . and G. Vassura (2002) . A novel approach to mechanical design of articulated fingers for robotic hands. In: Proc. IEEE/RSJ Int. Conf. on Intelligent Robots and Systems, IROS02. Lausanne, Switzerland . Melchiorri, C . (2000). Slip detection and control using tactile and force sensors. IEEE Trans. on Mechatronics, special Issue on Advanced Sensors for Robotics. Ponce, J. and B. Faverjon (1995). On computing three-finger force-closure grasps of polygonal objects. IEEE Transactions on robotics and automation. Putz, P. (1999). Space robotics. In: Laboratory Astrophysics and Space Resear·ch (P. Ehrenfreund at al., Ed.). Kluwer Academic Publishers. Rimon , E. and J . Burdick (1994a). Mobility of bodies in contact i: a new 2nd order mobility index for multiple-finger grasp. In: Proc. IEEE Int . Conf. Robotics and Automation, ICRA94 · Rimon, E. and J. Burdick (1994b) . Mobility of bodies in contact ii: How forces are generated by curvature effects. In: Proc. IEEE Int. Conf. Robotics and Automation, ICRA94.

Fig. 8. First prototype of the new hand (UB Hand Ill) . sors based on piezo-resistive technology, located in the joints) and of force sensors (miniaturized load cells, which measure the tension of tendons) but further integrations, in particular as concerns force and tactile sensing, are planned. Despite the overall robotic hand is not complete yet, a number of experimental tests has been performed on single fingers. By means of a testbed with three linear motors, the finger has been actuated in order to evaluate both the mechanical behavior of the structure (durability of the hinges and overall stiffness) and its kinematic behavior (e.g. trajectory accuracy and repeatability) . The results have been very satisfactory: the hinges have performed thousands of cycles without any problem due to mechanical fatig1,le, and as to the functional capabilities, the use of external sensors (bending sensors) and a proper choice of the control algorithms (impedance control) has allowed to compensate for difficulties on (kinematic) modelling.

4. CONCLUSIONS In this paper, an overview about the design activity of robotic end-effectors for space at University of Bologna is given. The authors try to highlight different ways to achieve one of the most critical goals in the field of space robotics, that is a general simplification (and accordingly an improvement of reliability and safety) of this kind of devices. Three different cases are reported : a gripper for autonomous tasks, an anthropomorphic not-dexterous robotic hands (that is an hand able to perform grasping but not manipulation operations) and finally a fully anthropomorphic and dexterous hand. In particular, the 3-dof gripper seems, in the short period, a suitable choice to perform manipulation tasks in a remote space environment, considering also that its dexterity may be enhanced by means of a proper functional integration with the robot arm. On the other hand, the "UB Hand Ill" project is particularly promising for the near future: the reduction of

332