Proceeding of the 6th International Symposium on Artificial Intelligence and Robotics & Automation in Space: i-SAIRAS 2001, Canadian Space Agency, St-Hubert, Quebec, Canada, June 18-22, 2001.

Control of a Three-dof Robotic Gripper

for Space Applications

L. Biagiotti1, C. Melchiorri1, G. Vassura2

1. DEIS, 2. DIEMUniversity of Bologna - Via Risorgimento 2,

40136 Bologna, Italy{lbiagiotti,cmelchiorri}@deis.unibo.it, gabriele.vassura@mail.ing.unibo.it

Abstract

In this paper we present an experimental activity forthe validation of a robotic gripper for space appli-cations, currently under development at the Univer-sity of Bologna with the support of ASI, the ItalianSpace Agency. The gripper is compatible with the EU-ROPA arm, developed by ASI and Tecnospazio, andalso with PaT, the Payload Tutor, proposed again byASI. The kinematic structure and sensory equipmentof this gripper have several interesting features, in-cluding the capability of firmly grasping objects withirregular shapes and with a rather wide range of di-mensions, and the possibility of ‘exploring’ unknownobjects or grasping free-flying objects by simultane-ously applying the contact constraints.Keywords: Space Robotics, Robotic Gripper, Prox-imity Sensors, Force sensors, Vision.

1 Introduction

The use of robotic devices to execute automatic oper-ations in space is foreseen to grow and cover a relevantpart of the activities. However, it is clear that end-effectors with a simple kinematic structure and poorsensoriality can execute only trivial tasks and in anycase are unsuitable to perform unforeseen operations.On the other hand, it is also clear that most of opera-tions of manipulation performed in space are relativelysimple, and can be executed by means of end-effectorswith limited dexterity.Although several dexterous robotic hands have beendeveloped, see e.g. [1]-[3], only a little effort has beendevoted to seek and evaluate alternative solutions,maybe simpler from the mechanical point of view, butwith sufficient dexterity to deal with a wide range of

(possibly unknown) objects.Moreover, referring specifically to the case of spaceapplications, we have to consider a scenario in whichoperations have to be performed in no-gravity con-ditions, where objects could be not constrained andtherefore free to float in space.

Figure 1: The gripper installed on a SMART-3S ma-nipulator.

In order to face some of the above problems, a projecthas started at the University of Bologna in the frame-work of a research programme supported by ASI, theItalian Space Agency, to design and experimentallytest a robotic gripper for space applications, [4]-[8].The gripper is currently installed on a six dof robot, aComau Smart 3S, see Fig. 1, in order to develop suit-able coordinating strategies to “explore” or grasp anobject taking into account the kinematic capabilitiesof the arm and of the gripper, [9]. This paper presentsthe current state of this research activity.

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2 The robotic system

The Gripper. The gripper has a simple and modularstructure, with three one-dof fingers disposed radially,in a symmetric configuration, whose distal phalangecan move along a linear trajectory, see Fig. 2.Despite its simplicity, this kinematic configuration hasseveral interesting features, as described with more de-tails in [4]-[8], including the capability of firmly grasp-ing objects with irregular shapes and with a ratherwide range of dimensions.

Figure 2: The gripper dealing with 2 different objects.

The gripper has been designed considering its instal-lation on the EUROPA arm by ASI, [10], [11], but aredesign of the gripper itself is currently in progressin order to reduce the total size. This new devicecould be compatible within PaT, the Payload Tutor,proposed by ASI (Italian Space Agency) [12]. Thissystem aims to substitute the astronauts in periodicaloperations with a semi-autonomous robotic device.The solution proposed by ASI integrates a small robotarm within a fixed structure, where a set of drawerscan host and protect as many different experiments:the mobility of the robot arm is increased by placingit on slide joints, in order to cover all the front surfaceof the facility even with reduced size of the robotlimbs. The experiments to be performed inside eachdrawer may include manipulation of complex-shape“non-technical” objects, freely floating within theirallowed space. The end-effector for the PaT manip-ulator needs therefore compactness, simplicity andreduced weight as well as capability of operation evenon irregular floating objects.

The sensory equipment of each finger consists of a

position sensor, a proximity sensor and a intrinsicforce/torque sensor, [13], [14]. In this manner, itis possible to control the motion of each finger, itsdistance from the object and the forces applied on itduring the grasp.

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Figure 3: Sensory equipment (a) and kinematic struc-ture of the gripper (b).

The arm and vision system. Besides the three dof grip-per, the overall system consists of the six dof industrialrobot Comau Smart-3S, with an “open control” archi-tecture, and a vision system with a camera installedon the wrist of the robot arm. These components areschematically shown in Fig. 4.

Figure 4: The overall system.

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3 Control system

Control of the gripper. The real time control of thegripper is based on standard HW/SW components.The control is performed with a DSP (TMS320C32)board connected to the motor drives and to an in-put board for the sensors. This board has been pur-posely designed because of the relatively high numberof signals (30) to be acquired in real-time. The DSPis hosted on a PC, which provides an high-level userinterface, under Windows or Linux OS, developed inorder to monitor the real time control process runningon the DSP board and allow the communication withthe other component of the whole system (in particu-lar with the control system of the arm).The control of the gripper can be subdivided in anhierarchical structure, in which three main levels canbe considered:Servo control level, in which the basic posi-tion/proximity/force controllers are implemented;Supervision level, which schedules the activationof the proper control in order to perform some basictasks (e.g. approach to an object in the work spaceof the gripper or grasp with a certain force);Task planning level, defining the general proce-dures of the gripper in order to accomplish morecomplex tasks like exploration of an unknown ob-ject or selection of the “best” grasp. In this levelit is obviously performed the coordination betweenthe control of the gripper and that of the carrying arm.

At the moment, the servo control level has beenimplemented considering a simple logic switchingbetween three different controllers:- a position control (based on the position sensor)- a proximity control (based on the proximity sensor)- a force control (based on the force/torque sensor)

The position control of each finger is based on a stan-dard PI controller, as depicted in Fig. 5. In order tocarry out this control the main difficulties have beenthe compensation of nonlinearities caused by the ac-tuation system, in particular a relevant (and non con-stant) dead zone, and the nonlinear characteristic ofthe Hall-effect position sensors. The set points andthe feedback signals (from position and proximity sen-sors) are used according to two main modalities: po-sition control or proximity control. In the first case,the absolute position of the fingertip is controlled byplanning the desired motion with a fourth-order poly-nomial function and assigning the desired motion time

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Figure 5: Position/force/proximity control scheme.

in order to obtain suitable velocities and accelerations.The controlled variable is the position x (the radialdistance from the center of symmetry of the gripper)of the fingertip obtained by means of the forward kine-matics from the joint position measured by the Halleffect sensor, see Fig. 3.In the second case, the controlled variable is the dis-tance of the finger with respect to the approached ob-ject. This modality is activated when the finger issufficiently close to the object (e.g. 5 mm). The con-trolled variable is now the distance from the object, asmeasured by the proximity sensor. This informationcan be used both to start the grasp of the object (ifall the fingers are at the same distance from it), asshown in Fig. 6, or to maintain constant the distancebetween the finger and the object (e.g. if the object ismoving), see Fig. 7.

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Figure 6: Grasp of a floating object: the fingers aremoved until a given distance from the object surface isreached, then the contacts are applied synchronously.

The force control is based on the same PI structure ofthe position and proximity controllers, and at the mo-ment can be classified as a simple compliance controlobtained by specifying the compliance parameter K,see Fig. 5.Obviously, a proper switching logic between the abovethree control modalities and a suitable generation ofset points must be adopted in the different phases of

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Figure 7: Tracking a moving object without graspingit.

the execution of the tasks in order to ensure a smoothbehavior of the gripper.

Control of the arm. The prototype of the gripper hasbeen installed on a 6 dof industrial robot, a COMAUSMART 3S with a open-control architecture, a PCconnected to the standard robot controller C3G9000,and equipped with a force/torque sensor on the wrist,see Fig. 1. The open control architecture allows inparticular to synchronize the tasks of both the gripperand the arm for micro-motion during task execution.The real time OS chosen for this application is RTAI-Linux [17], running in our case on a 450 Mhz PentiumIII PC. This PC may carry out the robot position con-trol, based on the feedback provided by the positionsensors, the wrist force/torque sensor and by the vi-sion system. At the same time, the operating systemallows the communication between the robot controltask (executed as real-time procedure in the RTAI-Linux environment) and the corresponding routineson the DSP board for the gripper control.It is possible to control the robot under RTAI-Linuxin two main modalities. In the first, the servo loopsfor each actuator are performed by the C3G9000, thestandard robot controller. In this modality, the usercontrol task running on the PC can only generate po-sition set-points and send them to the C3G9000. Inthe second case, the PC performs directly the controlof each actuator, with a sampling period of 1 msec.Note that with this control structure, using a PC anda high level OS like Linux, it is also very simple to con-nect the arm/gripper system using Internet to othercomputational resources or robotic devices, for exam-ple to perform teleoperation tasks.

4 Experimental activity

A number of laboratory experiments has been per-formed both on single finger modules and senso-rial/actuation subsystems in order to test the effi-ciency of each finger and of the control system. Thevalidation has also included verification of the proce-dures for the object approach (based on the use of boththe distance and the position sensor information) andgrasp (by means of the force/torque sensors).The first experiments include executions of the follow-ing demonstrative procedures:1. use of proximity sensors for coordinating the ap-proach phase of the fingers;2. control of the approach/contact phase with floatingobjects;3. scanning of the object surface for shape recognitionby means of the proximity sensors;4. choice of optimal grasp configuration according toa criterion of maximum area of friction cones convex[15, 16];5. control of the applied force(s);6. simultaneous application of contacts and test ofgrasp accuracy and stability. Typical results of the

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Figure 8: Identification of objects by exploration withthe proximity sensors and computation of the normaldirections.

exploration of the objects surface are reported in Fig.8. Although still not optimal, the proximity sensorsallow a precise reconstruction of surfaces at a distanceup to 1 cm, and the detection of obstacles up to 5 cm.Concerning the approach and contact phases, it mustbe observed that the possibility of independentlymoving the fingers has noticeably increased thecapability of grasping moving objects. As a matterof fact, the object may be tracked (if moving) witha coordinated movement of both the arm and thefingers. Once the motion is tracked (i.e. the fingersmove synchronously with the object), the grasp maybe firmly applied without loosing contact.

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Examples of this procedure are shown in Fig. 7-Fig.9. In Fig. 7 the positions of the three fingers areshown while tracking a moving objects, maintaininga fixed distance from it. In Fig. 6, the three fingersfirst approach a fixed object until each of them is ata desired distance from it (8 mm), then the contactsare applied. In Fig. 9 the signals from both the po-sition and proximity sensors are reported. The fingeris moved towards a moving object, plot (a), until adesired distance (10 mm) is reached and maintained,plot (b), also with the object in motion.

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Figure 9: Measurements of the position (a) and prox-imity (b) sensor. The task is to approach a movingobject and then to maintain a desired distance (10mm) from it. The output signal of the proximity sen-sor is saturated at 23 mm.

Finally, an experiment involving force control is shownin Fig. 10. Again, an object is approached under po-sition control (phase 1), then the proximity controlis switched on (2) and finally, once contact has beenestablished, the applied force is controlled (3, 4). Inthis case, the reference force is changed during ma-nipulation (from fd = 12 N to fd = 15 N) to showthe effectiveness of the force control. At the end, theobject is released and the force is null (phase 5).

5 Conclusions

Robotic activity in space is expected to grow in thenext future. For this reason, new types of robotic de-vices may be of interest for the automatic execution

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Figure 10: Motion of the finger (a) during an approachand a grasp and force applied on the object (b).

of tasks or to help the astronauts. With this respect,it is important to study new end-effectors that conju-gate a significant manipulation capability with bothreliability and a limited complexity. In this paper, anexperimental activity for the development and valida-tion of a three-finger, three degrees of freedom roboticgripper for space applications has been presented.Although this activity is not concluded yet, the systemconfirmed so far some very interesting properties. As amatter of fact, it is relatively simple in the kinematics,actuation and control, since it has only three actuatorsand three degrees of freedom; it can provide adaptableand synchronous application of contacts to objects ofany shape, thus allowing to grasp objects not centeredwith respect to the gripper axis of symmetry, withoutdisturbing their initial posture; it presents a very largeworkspace with respect to its body size, and is capableto deal with both small and large objects; its sensoryequipment seems to be sufficiently rich and more thanadequate for the expected tasks.Future activity will concern the refinement of thecurrent version of the gripper and the conclusion ofthe verification phase, in particular with respect tothe force control and to the possibility of applyingsimple manipulation procedures on the graspedobjects. A longer term project is the re-design of thegripper, in order to obtain a more compact device andto improve the capability of applying a firm grasps.

Acknowledgments. This research is supported by ASI

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(Italian Space Agency), and by MURST, under the MIS-

TRAL project. The authors thank F. Torelli for the help

given in part of the experimental activity.

References

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