IIIGareth J.Monkman, Stefan Hesse,Ralf Steinmann, Henrik Schunk

Robot Grippers

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G. J. Monkman, S. Hesse,R. Steinmann, H. SchunkRobot Grippers

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IIIGareth J.Monkman, Stefan Hesse,Ralf Steinmann, Henrik Schunk

Robot Grippers

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The Authors

Prof. Gareth MonkmanFH Regensburggareth.monkman@e-technik.fh-regensburg.de

Dr. Stefan HesseBüro Handhabetechnik, PlauenHhbuerohesse@aol.com

Ralf SteinmannSchunk GmbH & Co. KG, Lauffen/Neckarsteinmann@schunk.de

Henrik SchunkSchunk GmbH & Co. KG, Lauffen/Neckarschunk@schunk.de

Original title:S. Hosse, G. Monkman, R. SteinmannH. SchunkRobotgreifer: Funktion, Gestaltung undAnwendung industrieller GreiftechnikHanser, 2004

Translation:Valentin Petrov

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� 2007 WILEY-VCH Verlag GmbH & Co. KGaA,Weinheim

All rights reserved (including those of translationinto other languages). No part of this book may bereproduced in any form – by photoprinting, micro-film, or any other means – nor transmitted or trans-lated into a machine language without written per-mission from the publishers. Registered names,trademarks, etc. used in this book, even when notspecifically marked as such, are not to be consideredunprotected by law.

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Printed in the Federal Republic of GermanyPrinted on acid-free paper

ISBN: 978-3-527-40619-7

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Preface

This is the English version of the book of the same name previously published in German.However, it is not simply a translation but largely a new work as a comparison with theoriginal will reveal. Much of the former content remains as it were but the organisationhas been altered in line with Anglo-Saxon literary tradition.

Exactly at whom this book is aimed is a little difficult to summarise in a single state-ment. The work acknowledges the maturity of the robotics field and consequently its con-tent is aimed largely at industrial users. Nevertheless, it will certainly also be of value toboth undergraduate students in Mechatronics, mechanical and electrical engineering andpost graduate researchers working in the field of robotic prehension.

References are given per chapter and, where particularly relevant, are sometimes re-peated. Many refer to the original works in English, French, German and Russian, thoughwhere possible additional texts from the same author(s) are given in a second language(where available).

The Authors

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VII

Contents

Preface V

1 Introduction to Prehension Technology 11.1 Grippers for Mechanization and Automation 11.2 Definitions and Conceptual Basics 21.3 Grasping in Natural Systems 101.4 Historical Overview of Technical Hands 14

2 Automatic Prehension 192.1 Active Pair MatingPair Mating 192.2 Strategy and Procedures 272.2.1 Prehension Strategy 27

Example of a prehension strategy 352.2.2 Gripping Procedure, Conditions and Force 362.2.3 Gripper Flexibility 592.3 Gripper Classification 612.4 Requirements and Gripper Characteristics 632.5 Planning and Selection of Grippers 67

3 Impactive Mechanical Grippers 753.1 Gripper DrivesDrives 753.1.1 Electromechanical Drives 783.1.2 Pneumatic Drives 843.1.3 Electrostrictive and Piezoelectric Actuation 923.2 Design of Impactive Grippers 943.2.1 Systematics and Kinematics 943.2.1.1 Parallel Impactive Grippers 1013.2.2 Angular Impactive Grippers 1223.2.3 Radial Impactive Grippers (Centring Grippers) 1313.2.4 Internal Grippers 1323.2.5 Gripper with Self-blocking Capability 1353.2.6 Rotatable Jaw Grippers 1373.2.7 Gripper Finger and Jaw Design 138

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3.2.8 Self Securing Grippers 1423.2.8.1 Securing Through Spring Forces 1423.2.8.2 Securing Through Object Mass 1463.2.9 Three-finger Grippers 1533.2.10 Four-finger Grippers and Four-point Prehension 157

4 Ingressive Grippers 1614.1 Flexible Materials 1614.1.1 Pinch Mechanisms 1624.1.2 Intrusive Mechanisms 1634.1.3 Non-Intrusive Mechanisms 166

5 Astrictive Prehension 1695.1 Vacuum Suction 1695.1.1 Vacuum Production 1705.1.2 Vacuum Suckers 1765.1.3 Passive Suction Caps 1995.1.4 Air Jet Grippers 2025.2 Magnetoadhesion 2045.2.1 Permanent Magnet Grippers 2045.2.2 Electromagnetic Grippers 2075.2.3 Hybrid Electromagnetic Grippers 2155.4 Electroadhesion 2165.4.1 Electroadhesive Prehension of Electrical Conductors 2165.4.2 Electroadhesive Prehension of Electrical Insulators 220

6 Contigutive Prehension 2276.1 Chemoadhesion 2276.2 Thermoadhesion 232

7 Miniature Grippers and Microgrippers 2377.1 Impactive Microgrippers 2387.1.1 Electromechanically Driven Impactive Microgrippers 2387.1.2 Thermally Driven Impactive Microgrippers 2407.1.3 Electrostatically Driven Impactive Microgrippers 2457.2 Astrictive Microgrippers 2487.2.1 Vacuum Microgrippers 2487.2.2 Electroadhesive Microgrippers 2497.3 Contigutive Microgrippers 250

8 Special Designs 2538.1 Clasping (Embracing) Grippers 2538.2 Anthropomorphic Grippers 2578.2.1 Jointed finger Grippers 2588.2.2 Jointless Finger Grippers 2648.3 Dextrous Hands 268

Contents

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9 Hand Axes and Kinematics 2799.1 Kinematic Necessities and Design 2809.2 Rotary and Pivot Units 285

10 Separation 29110.1 Separation of Randomly Mixed Materials 29110.2 Separation of Rigid Three Dimensional Objects 29210.3 Separation of Rigid Sheet Materials 29210.3.1 Gripping of Thin Blanks from a Magazine 29210.3.2 Air Flow Grippers 29510.4 Separation of Non-Rigid Sheet Materials 29810.4.1 Roller Grippers 301

11 Instrumentation and Control 30911.1 Gripper Sensor Technology 30911.2 Perception Types 30911.2.1 Tactile Sensors 31011.2.2 Proximity Sensors 31311.2.3 Measurement sensors 31711.2.4 Finger Position Measurement 32311.2.5 Measuring Procedures in the Gripper 32411.3 Sensory Integration 32611.3.1 Discrete and Continuous Sensing 32711.3.2 Software and Hardware Interrupts 32811.3.3 Sensor FusionSensor Fusion 32811.4 Gripper Control 32811.4.1 Control of Pneumatically Driven Grippers 32911.4.2 Control of Electrically Driven Grippers 331

12 Tool Exchange and Reconfigurability 33312.1 Multiple Grippers 33312.1.1 Double and Multiple Grippers 33312.1.2 Multiple Gripper Transfer Rails 33612.1.3 Turrets 33812.2 Specialized Grippers 34212.2.1 Composite Grippers 34212.2.2 Reconfigurable Grippers 34412.2.3 Modular Gripper Systems 34512.3 Gripper Exchange Systems 34812.3.1 Tool Exchange 34812.3.2 Task, Functions and Coupling Elements 35012.3.3 Joining Techniques and Process Media Connection 35312.3.4 Manual Exchange Systems 35412.3.5 Automatic Exchange Systems 35812.3.6 Finger Exchange Systems 36212.4 Integrated Processing 363

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13 Compliance 36713.1 Remote Centre Compliance (RCC) 36813.2 Instrumented Remote Centre Compliance (IRCC) 37213.3 Near Collet Compliance (NCC) 37413.4 Parts Feeding 37513.5 Mechanical Compliance 37713.6 Pneumatic Compliance 38313.6.1 Internal Prehension Through Membrane Expansion 38413.6.2 External Prehension Through Membrane Expansion 38713.7 Shape Adaptive Grippers 39113.7.1 Partially Ccompliant Shape Adaptive Grippers 39113.7.2 Totally Compliant Shape Adaptive Grippers 39313.8 Collision Protection and Safety 39613.8.1 Safety Requirements 39613.8.2 Collision Protection Systems 39613.8.3 Failure Safety 397

14 Selected Case Studies 40114.1 Simple Telemanipulation 40114.2 Grippers for Sheet and Plate Components 40514.2.1 Impactive Grippers for Sheet Metal Handling 40614.2.2 Astrictive Grippers for Sheet Metal 40914.2.3 Astrictive Grippers for Glass Sheet 41214.2.4 Astrictive Grippers for Composite Material Handling 41214.3 Prehension of Cuboid Objects 41314.4 Prehension of Cylindrical Objects 41714.4.1 Serial Prehension of Tubes 41814.4.2 Prehension of Wound Coils 41914.4.3 Prehension of Slit Coils 42014.5 Prehension of Objects with Irregular Topology 42014.5.1 Handling of Castings 42014.5.2 Mounting of Dashboards for Automobiles 42114.5.3 Prehension of Water Pumps 42214.5.4 Astrictive Prehension of Irregular Surfaces 42214.6 Multiple Object Prehension 42314.6.1 Packaging of Candies 42414.6.2 Bottle Palletization 42514.6.3 Multiple Irregular Shaped Objects 42514.7 Prehension of Flexible Objects 42614.7.1 Bag and Sack Grippers 42614.7.2 Gripping and Mounting of Outside O-rings 42814.8 Medical Applications 430

References 433

Subject Index 443

Contents

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1Introduction to Prehension Technology

Human labour has always been associated with the acquisition of specific skills, methods,and tools making the work and its environment easier and more effective. Increasing com-petition from industrial robots for tasks normally carried out by human hands has led tothe need for more effective handling equipment, especially prehension tools (more com-monly called “grippers”). However, industrial robots are not simply a substitute for people.Their relevance is more often in applications beyond the normal ability (physical or tem-poral) of conventional manpower. Examples include, dirty, hazardous and repetitive work.Just as human hands are the organs of human manipulation, so are robot grippers usuallythe only parts in direct contact with the workpiece. For this reason they deserve specialattention – to which this book is dedicated.

1.1Grippers for Mechanization and Automation

Grippers are active links between the handling equipment and the workpiece or in a moregeneral sense between the grasping organ (normally the gripper fingers) and the object tobe acquired. Their functions depend on specific applications and include:� Temporary maintenance of a definite position and orientation of the workpiece

relative to the gripper and the handling equipment.� Retaining of static (weight), dynamic (motion, acceleration or deceleration) or process

specific forces and moments.� Determination and change of position and orientation of the object relative to the

handling equipment by means of wrist axes.� Specific technical operations performed with, or in conjunction with, the gripper.

Grippers are not only required for use with industrial robots: they are a universal com-ponent in automation. Grippers operate with:� Industrial robots (handling and manipulation of objects).� Hard automation (assembling, microassembling, machining, and packaging).� NC machines (tool change) and special purpose machines.� Hand-guided manipulators (remote prehension, medical, aerospace, nautical).� Workpiece turret devices in manufacturing technology.

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� Rope and chain lifting tools (load-carrying equipment).� Service robots (prehension tools potentially similar to prosthetic hands).

In robotics technology grippers belong to the functional units having the greatest varietyof designs. This is due to the fact that, although the robot is a flexible machine, the gripperperforms a much more specific task. Nevertheless, these tasks are not limited to prehen-sion alone which is why the more generic term “end-effector” is often used.

The great number of different requirements, diverse workpieces and the desire for welladapted and reliable systems will continue to stimulate further developments in futuregripper design. Many experts consider the capabilities of the gripper as an essential factorfor the economic effectiveness of automatic assembly systems. Experience indicates thatin the future it will only be possible to respond to practical demands if flexible designs forassembly equipment are available. Consequently, grippers must become ever more flex-ible. Assembly relates not only to prehension and manipulation of objects but also topressing, fitting and joining operations. Many grippers are employed for the loading ofmanufacturing lines, in packaging and storage as well as the handling of objects in labora-tory test and inspection systems.

More recently, miniaturized grippers have been developed in order to handle delicatecomponents in microtechnology. This has gone hand in hand with the emergence of manynovel prehension methods. The number of grippers used in nonindustrial areas, e.g. incivil engineering, space research, handicraft, medical and pharaceutical engineering issteadily increasing. Hand-guided (teleoperation) or automatic manipulators are used inthese areas primarily as handling machines. In addition to conventional grippers, forwhich the gripper jaws are shaped according to the workpiece profile, there exist numer-ous application specific grippers. This explains why an overwhelming proportion of corre-sponding patent literature is devoted to prehension concepts of unconventional design. Ingeneral, end-effectors are not normally within the delivery remit of robot manufacturers.Depending on the specific requirements, they are selected as accessories from toolingmanufacturers or specially designed for the given purpose.

1.2Definitions and Conceptual Basics

Grasping organs or tools constitute the end of the kinematic chain in the joint system of anindustrial robot and facilitate interaction with the work environment. Although universalgrippers with wide clamping ranges can be used for diverse object shapes, in many casesthey must be adapted to the specific workpiece shape.

Grippers are subsystems of handling mechanisms which provide temporary contact withthe object to be grasped. They ensure the position and orientation when carrying andmating the object to the handling equipment. Prehension is achieved by force producingand form matching elements. The term “gripper” is also used in cases where no actualgrasping, but rather holding of the object as e.g. in vacuum suction where the retentionforce can act on a point, line or surface.

1 Introduction to Prehension Technology

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Fig. 1.1: Possibilities for prehension of aspherical object

1 pure enclosing without clamping2 partial form fit combined with clamping

force3 pure force closure4 holding with vacuum air

(pneumatic force closure)5 retention using magnetic field

(force field)6 retention using adhesive media

Three of the most usual forms (impactive, astrictive and contigutive) of object prehensionare depicted in six different examples in Figure 1.1.

One should differentiate between grasping (prehension) and holding (retention) forces.While the grasping force is applied at the initial point of prehension (during the graspingprocess), the holding force maintains the grip thereafter (until object release). In the manycases the retention force may be weaker than the prehension force. The grasping force isdetermined by the energy required for the mechanical motion leading to a static prehen-sion force. The functional chain drive � kinematics � holding system is given, however,only for mechanical grippers. Astrictive vacuum suction grippers require no such kine-matics [1-1].

There are some characteristic terms that are often used in prehension technology. Grip-pers consist mostly of several modules and components. In the following, the most essen-tial terms used will be explained considering as an example a mechanical gripper such asthe one shown in Figure 1.2.

A short glossary of further important terms used in gripper technology is briefly ex-plained below.

Astrictive gripper: A binding force produced by a field is astrictive. This field may take theform of air movement (vacuum suction), magnetism or electrostatic charge displace-ment.

Basic jaw (universal jaw): The part of an impactive gripper subjected to movement. An inte-gral part of the gripper mechanics, the basic jaw is not usually replaceable. However, thebasic jaws may be fitted with additional fingers in accordance with specific require-ments.

1.2 Definitions and Conceptual Basics

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Basic unit: Basic module containing all gripper components which is equipped for con-necting (flange, hole pattern) the gripper to the manipulator. The connecting capabilityimplies a mechanical, power, and information interface. Figure 1.3 shows a flange designin accordance with DIN ISO 9409. This German industrial standard and its subsequentamendments contain design requirements concerning the different overall size, pitchcircle diameter, centring cylinder dimensions, number of threaded holes and respectivethread pitch as well as some position tolerances. The flange can also be drilled to allowfeeding of power and control cables.

Chemoadhesion: Contigutive prehension force by means of chemical effects. Usually inthe form of an adhesive (permatack or single use).

Fig. 1.2: Subsystems of a mechanical gripper1 remote centre compliance, 2 carrier, 3 gripper finger, 4 basic jaw, 5 extended jaw, 6 flange

Fig. 1.3: Example of flangedesign and mounting inaccordance with Germanstandard DIN ISO 9409

1 mating hole for locating pin2 threaded securing hole3 centring cylinder4 flange body5 flange rotation

dA pitch circle diameterdB centring cylinder diameterdC inner cylinder diameter

1 Introduction to Prehension Technology

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Contigutive gripper: Contigutive means touching. Grippers whose surface must makedirect contact with the objects surface in order to produce prehension are termed contigu-tive. Examples include chemical and thermal adhesion.

Control system: In most of the cases a relatively simple control component for analysing orpre-processing sensor information for regulation and/or automatic adjustment of prehen-sion forces.

Dextrous hand: Anthropoidal artificial hand (rarely for industrial use), which is equippedwith three or more jointed fingers and may be capable of sophisticated, programmed or re-mote controlled operations.

Double grippers: Two grippers mounted on the same substrate, intended for the temporaland functional prehension of two objects independently.

Drive system: A component assembly which transforms the applied (electrical, pneu-matic, hydraulic) energy into rotary or translational motion in a given kinematic system.

Dual grippers: Two grippers mounted on the same substrate, intended for the simul-taneously prehension of two objects.

Electroadhesion: Prehension force by means of an electrostatic field.

End effector (end-of-arm tooling): Generic term for all functional units involved in direct in-teraction of the robot system with the environment or with a given object. These includegrippers, robot tools, inspection equipment and other parts at the end of a kinematicchain.

Extended jaw: An (optional) additional jaw situated at the end of an impactive gripper fin-ger. It may, in preference to the finger itself, be modified to fit the profile of the object and itmay be replaceable.

Gripper: The generic term for all prehension devices whether robotic or otherwise. Looselydefined in four categories: Impactive, Astrictive, Ingressive and Contigutive.

Gripper axis: A frame with its origin in the TCP (Tool Centre Point). This coordinate sys-tem is used to specify the gripper orientation. Figure 1.4 shows a gripper with three trans-lational and three rotational degrees of freedom. The gripper frame is normally definedrelative to the flange frame of the industrial robot.

Gripper changing system: A module for rapid manual, but in most cases automatic, ex-change of an end-effector using a standard mechanical interface. In doing so, all powerand control cables must be disconnected and reconnected.

1.2 Definitions and Conceptual Basics

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Gripper finger: Rigid, elastic, or multi-link grasping organ to enclose or clasp the object tobe handled. Fingers are often equipped with extended gripper jaws at their ends. The grip-per finger is usually (though not always) the active part making contact between the grip-per and the object.

Gripper hand (hand unit): Grippers with multiple jointed fingers, each of them repre-senting an open kinematic chain and possessing a high degree of freedom with ƒ joints,e.g. ƒ = 9.

Gripper jaw: The part of the gripper to which the fingers are normally attached. The jawdoes not necessarily come into contact with the object to be gripped. Note: in some casesgripper fingers may be fitted with an additional small (extended) jaws at their ends.

Gripping area: Area of the prehension (gripper jaw) across which force is transmitted tothe object surface. The larger the contact surface area of an impactive gripper, the smallerthe pressure on the object surface.

Gripping surface: The passive contact surface between object and gripper, i.e. the surfacewhich is subjected to prehension forces.

Holding system: A term often used for an active prehension system including gripper,jaws and fingers. It may also apply to a passive temporary retaining device.

Impactive gripper: A mechanical gripper whereby prehension is achieved by impactiveforces, i.e. forces which impact against the surface of the object to be acquired.

Ingressive gripper: Ingression refers to the permeation of an objects surface by the prehen-sion means. Ingression can be intrusive (pins) or non intrusive (e.g. hook and loop).

Fig. 1.4: Gripper frame

1 Introduction to Prehension Technology

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Kinematic system: Mechanical unit (gear) converting drive motion of the prime mover intoprehension action (jaw motion) with characteristic transmission rates for velocities andforces. The most often used kinematic components are lever, screw, and toggle lever gears.The gear determines the final velocity of the jaw movement, and the gripping force charac-teristics. Grippers without moving elements require no kinematics. Some examples ofgears are shown in Figure 1.5.

Fig. 1.5: Pneumatically drivengripper with kinematic systemfor transmission of motion(Sommer-automatic)a) angle gripper with toggle

lever mechanicsb) parallel gripper with roller

linkc) parallel gripper with two

pneumatic cylindersd)parallel gripper with cam

disk

1 basic jaw or finger2 pneumatic cylinder3 straight guideway4 cam disk

Magnetoadhesion: Prehension force by means of a magnetic field (permanent or electri-cally generated).

Multiple grippers: Several grippers mounted on the same substrate, intended for thesimultaneously prehension of more than two objects.

1.2 Definitions and Conceptual Basics

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Prehendability: The suitability of an object to be automatically gripped. Dependant on thesurface properties, weight and strength when exposed to prehension forces. This propertycan sometimes be enhanced by applying such surfaces or elements (handling adapters)which are required only for a particular procedure.

Prehension: The act of acquiring an object in or onto the gripper.

Prehension planning: Deals with the problem of how to ensure stable mating betweenrobot gripper and workpiece. A prehension strategy must be chosen in such a way that itcan be accomplished in a stable manner and collision free. Post prehension misalignmentof the object is undesirable. In many circumstances, special constraints must be observedin order to avoid contact with certain parts of the object (forbidden zones).

Prehension systems: Complete systems including grippers supplemented with additionalunits (subsystems), e.g. rotation, pivot and short-travel units, changing systems, joining(adjustment) tools, collision and overload protection mechanisms, measuring devices andother sensors.

Protection system: These are elements attached to the inner or outer part of the gripperwhich are activated in case of overload or collision in order to protect the robot and gripperfrom damage (warning signal, emergency stop activation, passive or active evasive move-ment).

Retention: Pertains to the post prehension status of an object already held in the gripper.Note: prehension and retention forces are not always equal.

Sensor system: Sensors pertinent to the task of prehension. This may include sensorsbuilt into the end-effector, possibly with integrated data pre-processing, for position detec-tion, registration of object approach, determination of gripping force, path and anglemeasurements, slippage detection etc.

Sucker: Normally refers to a passive suction element (disk, cap or cup) which does not re-quire active vacuum suction but relies on the evacuation of air by distortion of the elementagainst the object surface.

Suction head: A form of astrictive gripper which may consist of one or more vacuum suc-tion elements (discs, caps or cups) freom which air is actively evacuated by means of exter-nally generated negative pressure.

Synchronization: In the majority of 2 and 3 finger grippers it is intended that the fingersclose in a uniform manner towards the centre of the gripper. In order to achieve this themotion of the fingers must be synchronized. Pneumatic cylinders, as can be seen from theexample in Figure 1.6, can be moved synchronously by means of a shaft with both rightand left handed threads.

1 Introduction to Prehension Technology

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Fig. 1.6: Synchronization of the gripper fingers by means of a right and left hand threaded shaft

1 cover plate2 pneumatic cylinders3 housing4 thread shaft5 sealing plate6 basic jaw guides7 basic jaws8 securing bolts9 seals

Fig. 1.7: Synchronization by means of adouble swing-yoke-drive (scotch-yoke drive)

1 pneumatic cylinder2 housing or cylindrical boring3 synchronous lever4 sealing5 basic jaw6 jaw guides

Such movement may also be realized by a gear comprising only links and levers (doubleswing mechanism), as shown in Figure 1.7 (see also the solution depicted later in Figure3.15). The basic jaws are again pneumatically driven by means of cylinders integratedwithin the gripper housing.

TCP (tool centre point): Working point at the end of a kinematic chain. The TCP serves alsoas a programmed reference point for an end effector and as a rule determines the origin ofthe tool frame. A coordinate system whose origin coincides with the TCP is called toolframe. Multiple gripper heads may possess several TCPs (Fig. 1.8) or one main TCP withthe rest being defined relative to the main TCP by tool offsets.

Thermoadhesion: Contigutive prehension force by means of thermal effects. Usually inthe form of freezing or melting.

Workpiece or object: A general term which refers to the component or object to be pre-hended or which is already under prehension by the gripper.

1.2 Definitions and Conceptual Basics

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Fig. 1.8: TCPs of multiple grippers

1.3Grasping in Natural Systems

In the course of its evolution Nature has created many different interesting graspingmechanisms. The elephant’s trunk can be regarded as a biomechanical phenomenon. Ac-cording to Brehm’s Life of Animals, it is

“. . . simultaneously a smelling, feeling, and grasping organ. It is composed of ring and longitudi-nal muscles, according to G. Cuvier (1769 –1832) these are about 40 000 separate bundles,which enable not only any twisting but also stretching and contraction”.

In his work on kinematics during the second half of the 19th century, F. Reuleaux (1829 –1905) analyzed (among others) animal mechanisms of motion [1-2]. These included themouths of fish and bird’s beaks which are also used to perform prehension tasks. The useof astrictive force though suction is also nothing new in nature. Such techniques are usedby fauna as suction feet (Fig. 1.9), e.g. in cephalopods. The male of the diving beetle (Dytis-cus marginalis ) possesses stemmed suction cups on its front legs. Applying them to a sur-face causes spreading of the finely chitinous, semispheric caps at their delicate edges.Drawing them back then results in a reduction in pressure which in turn produces the ad-hesion effect. Lizards possess adhesion lamellae on their toes (dry adhesion) which enablethem to traverse glass plates using their surface roughness [1-3]. There are in fact many

Fig. 1.9: Natural grasping, holding, and mastication mechanismsa) bird’s beak, b) fish mouth, c) suction foot

1 Introduction to Prehension Technology

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grippers whose kinematic principles are strongly related to those of Bird’s beaks or elephant’s trunks, for example in paint spraying or to encompass an object (see the soft grip-pers in Chapters 8 and 13). In order to handle fragile objects, grippers which imitate themusculous hydrostates of squid tentacles, have been utilised. The prehension and mas-tication organs of insects (Chelicerae of spiders, Mandibles of biting and chewing insectslike the antlions) resemble impactive grippers [1-4].

If we consider the osprey (Fig. 1.10), we can see that the problem of “grasping undercomplicated conditions” has been solved in the course of biological evolution in a very in-teresting manner. The osprey is able to grasp objects whose surfaces enjoy extremely lowfriction coefficients (specifically to avoid prehension by predators!) during flight.

The grasping foot exhibits long-drawn and sharp claws which make it possible to catchthe prey (ingressive prehension). The lower part of the foot exhibits soft pads with a highcoefficient of friction (buffered impactive prehension). During grasping these pads pro-duce a suction (astrictive prehension) effect against the smooth surface of the object.Hence, in this case several effective prehension principles are combined. Indeed, therealso exist robot grippers which prehend by impactive clamping and simultaneously usevacuum suction (Fig. 1.10b). However, none of the man made grippers possess the wealthof fine details observed in nature. Why?

Fig. 1.10: Combined grasping methodsa) grasping foot of an osprey Pandion haliaetus, b) hook gripper combined with suction cups1 anti slip pads, 2 claw, 3 pneumatic cylinder, 4 gripped object, 5 hook, 6 suction cup

Crab pincers are another good example often imitated by man. The crab arms end with arobust scissor mechanism which serves for both grasping and pressing. From the point ofview of kinematics, it is simply a matter of the successive coupling of two four-link spheri-cal gears (Fig. 1.11). To these ends crab arms possess the following design properties:� They have a large pivoting angle for a small number of arm links.� They can exert relatively large forces.� The joints between the arm links are free from mechanical play and are capable of

working under pressure over an extended range of motion.

1.3 Grasping in Natural Systems

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The crab has developed an ingenious solution to the articulation between arm members.It is based on two spherical joints of polar cap form housed concentrically within oneanother. These spherical joints consist in turn of several additional shells whose surfacesserve as slip and contact areas. Such joints are of special interest for miniaturized mecha-nisms since joint solutions of the “fork head – pin” type cannot be arbitrarily down-scaled.

Ball-and-socket (spherical) joints in living organisms are often coated with a jelly-likesubstance as a lubricant so that the connection is free from play and smooth running. Inaddition it may exhibit nonlinearities (stick-slip) effects.

The famous Greek philosopher Aristotele (384-322 BC) described the hands as “the toolof all tools”. The 5-finger human hands represent a particularly flexible and useful grasp-ing organ, particularly in conjunction with control though eye-hand feedback.

The bones of the hand are anatomically divided into three groups: the wrist or carpalbones (16 small bones at the root of the hand); the midhand or palm bones, and the firstlink (metacarpus) and finger (phalanx ) bones (Fig. 1.12).

Fig. 1.11: Model construction: crab scissorsleft: crab; right: kinematic scheme (four-joint chain) after [1-5];1 frame, 2 interlink, 3 link, 4 drive swing, 5 coupler

Fig. 1.12: The human handa) mechanical joint system [1-6], b) simplified mechanical representation.1 radius, 2 ulna, 3 finger joint, 4 hand joint, A rotation axis

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There are 8 carpal bones, 5 midhand bones (one for each finger), and 14 links (two forthe thumb and three for every other finger). This anatomic constellation enables a total of22 degrees of freedom in which as many as 48 muscles are involved.

The hand and forearm muscles are involved in practicing, memorizing, retrieval, andvariation in a tremendous number of separate grips. The human hand possesses ulti-mately 27 degrees of freedom. The exact number depends on how the muscles areclassified in independent groups [1-7]. If the finely coordinated muscles are independentlymoved and one defines for each degree of freedom the two end and one mid positions, thisalone will give 327, i.e. more than 7 billion different potential hand positions. Typical handgrips can be grouped, more or less exhaustively, into six grip classes (Fig. 1.13) [1-8 to1-10].

Fig. 1.13: The different hand grip classes

1 cylindrical hollow grip2 tip grip3 hook grip4 three finger grip5 hand palm grip6 tong grip

If the consideration is restricted to human activities necessary for industrial work, adirect relationship between the hand with the necessary tools and the number of fingersinvolved in the specific work may be observed. In other words, fingers can be replaced bytools. This relationship is illustrated in Figure 1.14.

Fig. 1.14: Fingers can be replaced by tools [1-11]

1 number of tools2 grip possibilities

1.3 Grasping in Natural Systems

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Zero fingers in the graph should be understood as movement of the arm joints only. Ascan be seen, the addition of the fifth finger makes negligible contribution to industrialwork. About 90% of the grips involved in industrial applications can be realized with athree finger hand. Furthermore, all fingers do not possess the same strength. The middlefinger is the strongest one and the little finger the weakest. The strength potential is dis-tributed as follows: index finger 21%, middle finger 34%, ring finger 27%, and little finger18%.

Grasping operations are always an integral part of more complicated handling strategieseven in cases when they are performed automatically. Consequently, grippers should al-ways be considered and evaluated for each individual case. As for the assembling of com-ponents, a brief procedure is shown in Figure 1.15, whereby the simple loading of a clamp-ing device can be considered to be equivalent to the final assembly step.

Fig. 1.15: General flow-chart of an assemblycycle [1-12]

1.4Historical Overview of Technical Hands

The first analogies of the human hand were developed as artificial replacements: The “ironfist” of Götz von Berlichingen (1480 –1562) possessed five separate fingers (Fig. 1.16).

Fig. 1.16: The iron first ofGötz von Berlichingen, knightof the Frankish Kingdom

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The fingers could be passively bent, fixed and released at the push of a button. Althoughthe hand weighs about 1.5 kg this was not considered particularly heavy for those times.

In 1564 the French physician Ambroise Paré (1510 –1590) designed a mechanical hand,in which the separate fingers were equipped with individual mechanics. At that time theidea caused a sensation because it seemed to demonstrate that humans and machinesoperate in the same manner and there are possibly spheres where they are exchangeable[1-13].

As a result of World War I the demand for hand replacements increased. The first handreplacement driven by external energy was designed by E. F. Sauerbruch (1875 –1951) andappeared in 1916. He utilized the remaining available force of the residual muscles in theamputation stump. The muscle movement was transmitted to the replacement mechan-ics by inserted ivory pivot pins [1-14].

The first successful use of arm stump bio-currents to control a miniaturized electrome-chanical system in a replacement hand was made in 1947. In the meantime such so calledbio-hands are readily available and their carrying capability and functionality are compara-tively good. The basic principle of operation is shown in Figure 1.17. The electromotoricVaduz-hand of the Swiss E. Wilms (1949) had a similar construction.

In addition to electromechanical systems, pneumatic actuation has also been used forhand replacements. Some 60 years ago an arm prosthesis driven by compressed air wasdeveloped at the orthopaedic centre in Heidelberg (Germany). The hand prosthesisdepicted in Figure 1.18 is a part of it. The fluid actuator is a flexible extensible body which,when inflated, pivots the finger into a firm grip. A return spring serves to release it. Until1965 more that 350 patients benefited from this design. The so-called McKibben arm ex-hibits similar characteristics.

In the 1950s the American J. L. McKibben designed a pneumatic muscle intended forprosthetic actuation (Fig. 1.19). The muscle consisted of a rubber tube with a net of inelas-tic threads in rhomboid pattern over, and along the length, of the surface. When pressur-ized the muscle inflates and simultaneously shortens. Wires transmit these lengthchanges to the joints which in turn produce motion in the finger links. The operation ofthe fluid muscle as a gripper actuator is illustrated in Figure 3.15. Unfortunately, such afluid muscle can produce only contraction forces.

Fig. 1.17: Construction of a myoelectric biohand (prosthesis)

1 arm stump2 skin electrodes3 battery4 gear5 amplifier6 motor

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Fig. 1.18: Hand prosthesis from the orthopaedic center in Heidelberg (1948)

Another trend relates to the so-called android hands developed for special figures in-tended for exposition. The automats designed by Pierre Jaquet Droz (1721 –1790), Henri-Louis Jaquet Droz (1752 –1838) and the mechanic Jean Frederic de Leschot (1747 –1824) arefamous androids which caused sensations in their times [1-15]. The figures were equippedwith program control (turn controller). Figure 1.20 shows the hand mechanics of one suchfigure.

All these, however, did not stimulate the development of robotics. Their designs con-tained few functioning parts and served basically to optically imitate the human hands.This said, the “flutist”, a “saloon robot” for the exhibit of J. de Vaucanson (1709 –1782), ac-tually used leather holstered fingers whilst playing the flute.

The artificial hands needed today for robots and remote-controlled manipulators aresubstantially different. A robot hand with skilful fingers is the realization of the ancient

1 gripper housing2 flexible pneumatic actuator3 wooden finger5 fitting flange4 return spring6 compressed air tubep compressed air input

Fig. 1.19: Arm prosthesis with segmented rubber muscles (after McKibben, USA)

1 rubber muscle2 wire and wire lead3 hand prosthesis

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dream to provide machines with human abilities. The first technical hands for automationand research were developed in the 1960s. The robot hands known from such research areusually named after their institution or its place of origin, e.g., Belgrade/USC Hand,Darmstadt Hand, DLR Hand, Rhode Island Hand (for cylindrical components), HitachiHand, Karlsruhe Hand, Odetics Hand, Rosheim Hand, SRC Hand, Stanford/JPL Hand,Utah/ MIT Hand, and Victory-Enterprises Hand. Most of these hands are driven by elec-tric motors. The wiring and coupling of actuator motion, mostly by means of chords, is aserious problem related to producing adequate force in the available space. A full descrip-tion of dextrous hands is given in Chapter 8.

Fig. 1.20: Hand mechanics of a female “musician”, an android playing harpsichord(designed by Jaquet-Droz [father & son] and J.-F. Leschot, 1774)

1.4 Historical Overview of Technical Hands

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