IROS 2002 2002 IEEE/RSJ International Conference on Intelligent Robots and Systems SEPTEMBER 30 - OCTOBER 4, 2002 EPFL - LAUSANNE. SWITZERLAND
TUTORIAL T1 TOWARDS INTELLIGENT ROBOTIC MANIPULATION
DESIGN ASPECTS FOR ADVANCED ROBOT HANDS L. Biagiotti†, F. Lotti*, C. Melchiorri†, G. Vassura* †
L.A.R. - DEIS *DIEM University of Bologna, Italy
Summary This presentation first recalls some definitions about robotic hands, their dexterity and their possible levels of anthropomorphism. Mechanical design of anthropomorphic hands is then addressed, to the purpose of outlining the main approaches, the open problems, the research tendencies. Integration of sensory equipment into the hand design is finally considered, with a brief review of available sensing concepts and of the most significant design goals and problems.
DESIGN ASPECTS FOR ADVANCED ROBOT HANDS: MECHANICAL DESIGN F. Lotti*, G. Vassura* *DIEM - Department of Mechanical Engineering - University of Bologna, Italy
[email protected],
[email protected]
1.
2.
Introduction
Hand-arm integration
In the design of a robotic hand a basic choice must be done since the beginning: it involves the level of integration between the hand to be designed and the rest of the robotic system. Integration concerns both the physical parts of the system (structural integration) and the way they interact or cooperate in order to accomplish manipulation tasks (functional integration). Structural integration directly determines mechanical design guidelines, while functional integration is mainly a conditioning goal as far as control strategies and task planning procedures are concerned.
With the growth of interest towards humanoid robots, anthropomorphism of robotic hands becomes a necessary design goal, that has been purposely addressed by most recent research projects, e.g. the Robonaut hand by NASA [1, 2], the DLR Hands [3, 4], the University of Tokyo hand [5], the Karlsruhe University ultra-light hand [6], the GIFU hand [7], and others. Consistent levels of anthropomorphism were present, however, in many previous design proposals, e.g. the Utah Hand [8], the Stanford / JPL hand [9], the UB Hand [10, 11] and many others. The first part of this paper presents an attempt to classify robotic hands, proposed so far by research or industry, focusing on their anthropomorphism and dexterity, with the aim of identifying classes with significant differences as far as the main design aspects and the actual manipulation capacity are concerned. Many times we can find hands fully mimicking the human hand as to their aesthetics, but with very reduced functionality, or, on the opposite side, hands with a fair manipulation dexterity but a very reduced resemblance with a human hand. The proposed classification of anthropomorphic hands roughly divides them into classes and levels according to the following points: - type of hand-arm integration; - degree of anthropomorphism (to this purpose an index is defined); - level of dexterity (four main levels are defined). Some hand projects, presented by research in the last twenty years, are then examined according to this classification. After that, the paper focuses on some aspects of mechanical design of robotic hands: it is authors’ opinion that the effort towards design of mechanical solutions inspired to anthropomorphic issues has been so far inadequate and new impulse must be given to research in this direction.
Structural integration Two different concepts about the structural integration of robotic hands are described in the literature. In one case, the hand is considered like an independent and modular device to be applied at the end of an arm: the same hand can be applied to any kind of arm because it has been designed independently of it (examples of this approach are the DLR Hands [3, 4] and the Barret Hand [12 ]). In the other case, reproducing the biological model, the hand is considered a non-separable part of the arm, deeply integrated with it: the hand and the arm are jointly designed and cannot be conceived as separate subsystems (examples of this approach are the Robonaut hand [1, 2], the Utah Hand [8], and the UB Hand [10, 11]). The main difference between these two approaches is that a modular hand must contain all its functional components (actuators, sensors, electronics, etc:), while an integrated system (hand + arm) can distribute these components in the whole structure, placing them where room is available.
1
placement of actuators into the forearm. The size and bulk of stronger actuators is no longer a problem, but many other problems arise due to the need of transmitting the motion through the wrist joints. There are pros and cons on both sides, but it seems that an integrated hand can reach more easily a high level of anthropomorphism, at least with the technical resources available so far. 3.
Considerations on robotic end-effectors
anthropomorphism
of
With the term “anthropomorphism” we intend the capability of a robotic end-effector to mimic the human hand, partly or totally, as far as shape, size, consistency, and general aspect (including colour, temperature and so on) are considered. As the word itself suggests, anthropomorphism is related to external perceivable properties, and is not, itself, a measure of what the hand can do, while, on the contrary, dexterity is related to actual functionality and not to shape or aesthetic factors. We can find in the literature anthropomorphic endeffectors with very poor dexterity levels (even if they are called hands), as the tasks they perform are limited to very rough grasping procedures; at the same time, we can find smart end-effectors, capable of sophisticated manipulation procedures, without any level of anthropomorphism (e.g DxGrip-II-Pisa [13]). Anthropomorphism itself is neither necessary nor sufficient to achieve dexterity, even if it is quite evident that the human hand achieves a very high level of dexterity and can be considered a valid model for dexterous robotic hands. Anthropomorphism is a desirable goal in the design of robotic end-effectors mainly for these reasons: - the end-effector can operate on a man-oriented environment, where tasks may be executed by the robot or by man as well, acting on items, objects or tools that have been sized and shaped according to human manipulation requirements; - the end-effector can be tele-operated by man, with the aid of special purpose interface devices (e.g. a data-glove), directly reproducing the operator’s hand behaviour; - it is specifically required that the robot has a human-like aspect and behaviour (humanoid robots for purposes of entertainment, assistance, and so on). From the observation of the many robotic end-effectors inspired to the human hand, we can conclude that the level of achieved resemblance with a human hand is greatly variable from case to case, although all of them are defined as anthropomorphic hands. An interesting problem arises: what are the components of anthropomorphism and how the achieved level of anthropomorphism can be quantified? Is it more anthropomorphic a hand with five fingers but sharp rigid edges or one with three fingers well shaped and covered with a compliant layer?
Fig. 1: Example of modular hand (DLR Hand [2])
The different design approaches have the most evident implications in the placement of the actuators, necessary to move the hand joints. In the first case all the needed actuators have to be placed in the hand, while in the second case the hand design is developed considering the possibility to put the actuator in the forearm. Each of these two modalities has advantages and drawbacks. At present, bulk and performance of available actuators make very difficult to host the required number of actuators inside the hand. As a matter of fact, the size of the proposed modular hands is larger with respect to the human hand, the grasping forces are weaker and the overall design seems complex and not enough reliable.
Fig. 2: Example of hand-arm structural integration (Robonaut Hand [1])
On the other side, the choice of integrating the hand and the arm with simultaneous design allows the 2
Tab. I Example of ant-ind evaluation
With the only aim of trying a comparison between different design proposals presented in the literature, the authors have defined an anthropomorphism index (in the following ant-ind) that is determined considering the following aspects: - the presence or not of the main morphological elements (principal upper fingers, secondary upper fingers, opposable thumb, palm): each of them gives a different contribution to the resultant evaluation score v1, sum of these partial contributions; the lack of articulations inside each finger with respect to the human case causes a reduction on the achieved scores according to correction factors c.f.; the resultant score v1 (0 ≤ v1 ≤ 1) contributes to ant-ind with a weight w1; - extension and smoothness of available contact surfaces, that means the capability to locate contacts with objects all over the surface of the available links; a score v2 is defined, (0 ≤ v2 ≤1), depending on the presence of well-shaped surfaces, absence of sharp edges, availability of external compliant pads; each of this aspects is rated from 0 to 1 and v2 is calculated as the mean of these values; the resultant score v2 contributes to ant-ind with a weight w2; - actual size of the hand compared with the medium size of a human hand and correct size ratio between all the links; a resultant score v3 (0 ≤ v3 ≤ 1), is defined as the mean of these two aspects, rated from 0 to 1, and contributes to ant-ind with a weight w3 (of course Σwi = 1) . The index ant-ind is calculated as the ratio between the weighted sum of the scores achieved by the considered design and the correspondent score of the fully anthropomorphic solution. Therefore the index associated to a given design (e.g. ant-ind = 0.75) gives an immediate idea of how far from the human shape and aesthetics it places. For example, in Tab.I the ant-ind relative to the UBHand, shown in figure 3, is presented [10, 11].
Evaluated aspect
Evaluation elements Main upper fingers
Kinematics Relative weight w1=0.6
Contact surfaces Relative weight w2=0.2
0.3
c.f.
p.s.
1
0.3
Opposable thumb
0.3
0,8
0.24
Palm
0.2
0,8
0.16
Fourth finger
0.1
0
0
Fifth finger
0.1
0
0
v1
0.70
Smoothness of contact surfaces
0,9
Extension of contact surfaces
0,9
Presence of contact pads
0,3 v2
Size Relative weight w3=0.2
Overall size max.1 Size Ratio between links max.1
0.9 v3
ant − ind =
∑v w = v w ∑ ∑v w i
i
i max
4.
i
i
0.7 1
0.95 0.75
i
Considerations on dexterity of robotic hands
Generally speaking, the dexterity of a robotic system is the capacity to autonomously perform complex and sophisticated tasks. For an end-effector, with the term “dexterity” we intend the capability of the end-effector, operated by a suitable robotic system, to perform tasks with a certain level of complexity. An exhaustive review of scientific work done so far about robotic hands dexterity, with a complete list of references, can be found in [14]. Even if the word dexterity has by itself a highly positive meaning, it is useful to consider different levels of dexterity, associated with growing complexity and criticality of performable tasks. The dexterity domain for robotic hands can be roughly divided in two main areas, that are grasping and internal manipulation. Grasping is intended as constraining objects inside the end-effector with a constraint configuration that is substantially invariant with time (the object is fixed with respect to the hand workspace), while internal manipulation means controlled motion of the grasped object inside the hand workspace, with constraint configuration changing with time. Further subdivisions of these two domains have been widely discussed in the literature (different grasp topologies on one side [15], different internal manipulation modes based on internal mobility and/or contact sliding or rolling on the other side [14] ). We want here to remark an important aspect concerning dexterity of a robotic end-effector: actual dexterity is the result of two main factors, that are the intrinsic features of the hand effector itself and the capacity of the whole robotic system to exploit them by means of smart control procedures based on adequate sensory capabilities. A very simple end-effector like a
Fig. 3: The University of Bologna Hand II
3
concerns the modular or integrated design concept:
rigid stick can be used for very sophisticated objectpushing tasks if used by a robot with visual and force feedback, while a complex articulated hand without adequate control can limit its dexterity to trivial selfadapting encompassing grasps. Evaluating the design of a robotic hand, for example examining its kinematical configuration or its sensory equipment, we can define a potential dexterity intrinsically related to its kinematical and morphological features. It is quite evident that the potential dexterity of an articulated five-finger hand is better than that of a rigid stick, but it is obvious at the same time that much of the potential dexterity of such a complex structure can be wasted if proper actuation or sensory system are not adopted inside it and smart control procedures are not implemented. The evaluation of potential dexterity of an articulated hand depending on its kinematical configurations (e.g. evaluation of manipulation ellipsoid) has been widely discussed in the literature, as reported by Bicchi [14]. In the following, the potential dexterity will be roughly divided in two areas: - hands with capability limited to grasping (simplified kinematical configuration or complex kinematical configuration but reduced number of controlled degrees of freedom) - hands that are capable of some kind of internal manipulation. Each of these two areas can be further on subdivided in two parts, distinguishing if the capability is limited to fingertip operation or is extended to the other active elements of the hand (whole hand manipulation). It is a rough subdivision, but can help to distinguish between projects that may look aesthetically similar but in practice achieve quite different levels of operating capability. 5.
MH IH
Modular hands Integrated design hands.
As to the level of potential dexterity, the following categorization adds synthetic information about the achievable domain of dexterity: FTG WHG FTM WHM
Potential dexterity limited to fingertip grasping Capability of static grasping using all the available surfaces of the hand; Capability of internal manipulation using only the fingertips Capability of other kind of internal manipulation using contacts on other links in addition to fingertips.
In addition, the evaluation of the anthropomorphism index for each hand can be provided. The result is that any project will be univocally defined by joining the three items: e.g. IH-0.75-WHM means an integrated design hand with a degree of anthropomorphism fairly high and oriented to whole hand manipulation (the evaluation is related to the authors’ previous project UB Hand II). In table II-(1–7), the main features of well known design proposals for robotic hands are presented, together with their classification according to the above mentioned criteria. It was not authors’ intention to establish any merit scale, but simply to try to evaluate the significant features of each project. The review is limited to those projects that clearly addressed the achievement, at a significant level, of both dexterity and anthropomorphism. Some considerations can be formulated after this categorization effort: - the choice of the kind of integration between the hand and the arm is a heavily conditioning factor, but few projects clearly addressed this point, in one sense (integration) or in the other (modularity), in many cases the design seems to have been concentrated only on the hand itself, for laboratory use, without worrying about is suitability to be hosted on a robotic arm; - there are extremes in the scale of evolution, from very anthropomorphic but low dexterity design, (it is the case of hands simply oriented to adaptable grasp applications) to fairly dexterous but less anthropomorphic design; these extremes are usually associated with very restricted and limiting design specifications; - high levels of both anthropomorphism and dexterity are the result of explicitely addressed design goals; this means that the development of hands for humanoid robots requires a specific design approach, where all the adopted technical solutions must be harmonized according to the rules of simultaneous engineering and cannot be considered independently.
A review of robotic hands with some level of anthropomorphism
Several robotic hands, more or less anthropomorphic, have been developed over the past two decades. The goals of each project were most times rather different, and the results are not easily comparable to the purpose of declaring one project better than another. Anyway, in order to point out the effectiveness of each contribution and to trace the historical evolution of this sector of robotics, a classification of the potential dexterity and level of achieved anthropomorphism of each design can help to outline results, tendencies, open problems and goals for future evolution of research. Any hand design has been evaluated and classified in eight classes, according to its design concept (integrated with the arm or modular) and its potential dexterity, mainly depending on kinematical design (available degrees of freedom, capability to locate contacts only on fingertips or all over the available links). The following classes have been identified for what 4
Tab.II/1 Main features of design proposals of anthropomorphic hands
Project Identification
Project denomination
Utah / Mit Hand
Stanford / JPL
Reference Author (s) Research Institute Year of Presentation Reference(s )
Jacobsen Utah University 1983 [8]
Salisbury Stanford University 1983 [9]
● ● ● ● ● 17 16
● ● ● 10 9
16
9
=
=
Fingertips / Phalanges / Palm
Fingertips / Phalanges / Palm
Poor/fair/good / very good
Poor / fair / good / very good
0,84 ● ● ● ● IH-0.84-WHM Exoskel./endoskel./intermediate Inside the finger / Remote Pneumatic actuators Yes / Not Und / Rpd / Apd / Not Tendons Pulleys / Sheaths / Mixed
0,48 ● ● MH-0,48-FTM Exoskel./endoskel./intermediate Inside the finger / Remote Electrical revolute motors (DC) Yes / Not Und / Rpd / Apd / Not Tendons Pulleys / Sheaths / Mixed
Picture
Hand-arm integration
Kinematical scheme
Morphological features
Structurally integrated Modular Main upper fingers Opposable thumb Fourth finger Fifth finger Palm Number of links Number of joints Number of controlled degrees of freedom Size w. r. to a human hand Surfaces suitable to contact with objects Smoothness and continuity of contact surfaces
Ant-Index Fingertip grasp Whole-hand grasp Potential dexterity Fingertip manipulation Whole-hand manipulation Classification Structural design concept Actuator location Actuation type Mechanical design Act. joints back-drivability details Kind of non-actuated joints Type of transmission Transmission routing Designed for… Additional notes
NF = > >>
Research / Ind.applic. / Space applic Research / Ind.applic. / Space applic 16 dof - 16x2 = 32 actuators
Information Not Found The hand has approx. the size of human hand The hand is slightly larger than human hand The hand is sensibly larger than human hand
5
Und Rpd Apd Not
-
Underactuated joint Rigid passive-driven joint Adaptive passive-driven joint There aren’t non-actuated joints
Tab.II/2 Main features of design proposals of anthropomorphic hands
Project Identification
Project denomination
Okada hand
Barrett hand
Reference Author (s) Research Institute Year of Presentation Reference(s )
T. Okada Electrotechnical Laboratory, Japan 1986 [16]
W.T.Townsend Barrett Technology, Inc. 1988 [12]
● ● ● 12 11
● ● ● ● 9 8
11
4
>
=
Fingertips / Phalanges / Palm
Fingertips / Phalanges / Palm
Poor / fair / good / very good
Poor / fair / good / very good
Picture
Hand-arm integration
Kinematical scheme
Morphological features
Structurally integrated Modular Main upper fingers Opposable thumb Fourth finger Fifth finger Palm Number of links Number of joints Number of controlled degrees of freedom Size w. r. to a human hand Surfaces suitable to contact with objects Smoothness and continuity of contact surfaces
0,53 0,48 Fingertip grasp ● Whole-hand grasp ● ● Potential dexterity Fingertip manipulation ● Whole-hand manipulation ● Classification IH-0.53-WHM MH-0,48-WHG Structural design concept Exoskel./endoskel./intermediate Exoskel./endoskel./intermediate Actuator location Inside the finger / Remote Inside the finger / Remote Actuation type Electrical revolute motor Electrical revolute motors (Brushless) Mechanical design Act. joints back-drivability Yes / Not NF details Kind of non-actuated joints Und / Rpd / Apd / Not Und / Rpd / Apd / Not Type of transmission Tendons Spur and worm gear Transmission routing Pulleys / Sheaths / Mixed Research / Ind.applic. / Space applic. Research / Ind.applic. / Space applic. Designed for… Ant-Index
-
Additional notes
NF = > >>
Information Not Found The hand has approx. the size of human hand The hand is slightly larger than human hand The hand is sensibly larger than human hand
6
Patented underactuated mechanism
Und Rpd Apd Not
Underactuated joint Rigid passive driven joint Adaptive passive driven joint There aren’t not-actuated joints
Tab.II/3 Main features of design proposals of anthropomorphic hands
Project Identification
Project denomination
UB Hand
DLR Hand I
Reference Author (s) Research Institute Year of Presentation Reference(s )
Bonivento – Melchiorri - Vassura Bologna University 1992 [10,11]
Butterfass – Hirzinger – Knoch -Liu DLR - German Aerospace Center 1997 [3]
● ● ● ● 14 13
● ● ● ● ● 17 16
13 (2 wrist + 11 hand)
12
=
>>
Fingertips / Phalanges / Palm
Fingertips / Phalanges / Palm
Poor / fair / good / very good
Poor / fair / good / very good
Picture
Hand-arm integration
Kinematical scheme
Morphological features
Structurally integrated Modular Main upper fingers Opposable thumb Fourth finger Fifth finger Palm Number of links Number of joints Number of controlled degrees of freedom Size w. r. to a human hand Surfaces suitable to contact with objects Smoothness and continuity of contact surfaces
0,75 Fingertip grasp ● Whole-hand grasp ● Potential dexterity Fingertip manipulation ● Whole-hand manipulation ● Classification IH-0.75-WHM Structural design concept Exoskel. / endoskel. / intermediate Actuator location Inside the finger / Remote Actuation type Electrical revolute motor Mechanical design Act. joints back-drivability Yes / Not details Kind of non-actuated joints Und / Rpd / Apd / Not Type of transmission Tendons Transmission routing Pulleys / Sheaths / Mixed Ant-Index
Designed for…
Research / Ind.applic. / Space applic. Research / Ind.applic. / Space applic -
Additional notes
NF = > >>
0,69 ● ● ● ● MH-0,69-WHM Exoskel. / endoskel. / intermediate Inside the finger / Remote Electrical revolute motor Yes / Not Und / Rpd / Apd / Not Tendons Pulleys / Sheaths / Mixed
Information Not Found The hand has approx. the size of human hand The hand is slightly larger than human hand The hand is sensibly larger than human hand
7
-
Und Rpd Apd Not
Underactuated joint Rigid passive-driven joint Adaptive passive-driven joint There aren’t non-actuated joints
Tab.II/4 Main features of design proposals of anthropomorphic hands
Project Identification
Project denomination
LMS hand
DIST hand
Reference Author (s) Research Institute Year of Presentation Reference(s )
Gazeau – Zeghloul - Arsicualt Université de Poities 1998 [17]
Cafés – Cannata - Casalino DIST Università di Genova 1998 [18]
● ● ● ● ● 17 16
● ● ● ● ● ● 17 16
16
16
=
>
Fingertips / Phalanges / Palm
Fingertips / Phalanges / Palm
Poor / fair / good / very good
Poor / fair / good / very good
Picture
Hand-arm integration
Kinematical scheme
Morphological features
Structurally integrated Modular Main upper fingers Opposable thumb Fourth finger Fifth finger Palm Number of links Number of joints Number of controlled degrees of freedom Size w. r. to a human hand Surfaces suitable to contact with objects Smoothness and continuity of contact surfaces
0,83 Fingertip grasp ● Whole-hand grasp ● Potential dexterity Fingertip manipulation ● Whole-hand manipulation ● Classification MH-0.83-WHM Structural design concept Exoskel. / endoskel. / intermediate Actuator location Inside the finger / Remote Actuation type Electrical revolute motor Mechanical design Act. joints back-drivability NF details Kind of non-actuated joints Und / Rpd / Apd / Not Type of transmission Tendons Transmission routing Pulleys / Sheaths / Mixed Ant-Index
Designed for…
Research / Ind.applic. / Space applic. Research / Ind.applic. / Space applic -
Additional notes
NF = > >>
0,74 ● ● ● MH-0,74-FTM Exoskel. / endoskel. / intermediate Inside the finger / Remote Electrical revolute motor NF Und / Rpd / Apd / Not Tendons Pulleys / Sheaths / Mixed
Information Not Found The hand has approx. the size of human hand The hand is slightly larger than human hand The hand is sensibly larger than human hand 8
-
Und Rpd Apd Not
Underactuated joint Rigid passive-driven joint Adaptive passive-driven joint There aren’t non-actuated joints
Tab.II/5 Main features of design proposals of anthropomorphic hands
Project Identification
Project denomination
Robonaut Hand
Tokyo Hand
Reference Author (s) Research Institute Year of Presentation Reference(s )
C.S.Lovhik - M.A. Diftler NASA Johnson Space Center 1999 [1,2]
Y.K.Lee - I. Simoyama Univ.of Tokyo,bunkyo-ku, J 1999 [5]
● ● ● ● ● ● 22 22 (2 wrist + 20 hand)
● -
17 16
14 (2 wrist +12 hand)
11(hand )+1 (wrist)=12
=
=
Fingertips / Phalanges / Palm
Fingertips / Phalanges / Palm
Poor / fair / good / very good
Poor / fair / good / very good
Picture
Hand-arm integration
Kinematical scheme
Morphological features
Structurally integrated Modular Main upper fingers Opposable thumb Fourth finger Fifth finger Palm Number of links Number of joints Number of controlled degrees of freedom Size w. r. to a human hand Surfaces suitable to contact with objects Smoothness and continuity of contact surfaces
● ● ● ● ●
0,99 0,98 Fingertip grasp ● ● Whole-hand grasp ● ● Potential dexterity Fingertip manipulation ● ● Whole-hand manipulation ● ● Classification IH-0.99-WHM IH-0.98-WHM Structural design concept Exoskel. / endoskel. / intermediate Exoskel. / endoskel. / intermediate Actuator location Inside the finger / Remote Inside the finger / Remote Actuation type Electrical revolute motor (brushless) Pneumatic Mckibben artificial muscle Mechanical design Act. joints back-drivability Yes / Not Yes / Not details Kind of non-actuated joints Und / Rpd / Apd / Not Und / Rpd / Apd / Not Type of transmission Flex-shaft + lead screw Transmission routing Research / Ind.applic. / Space applic. Research / Ind.applic. / Space applic. Designed for… (EVA operations) (Prosthetic hand and service robots) Ant-Index
-
Additional notes
NF = > >>
Information Not Found The hand has approx. the size of human hand The hand is slightly larger than human hand The hand is sensibly larger than human hand 9
-
Und Rpd Apd Not
Underactuated joint Rigid passive-driven joint Adaptive passive-driven joint There aren’t non-actuated joints
Tab.II/6 Main features of design proposals of anthropomorphic hands
Project denomination Project Identification
Reference Author (s) Research Institute Year of Presentation Reference(s )
DRL hand II
Tuat/Karlsruhe Hand
Butterfass–Grebenstein-Liu-Hirzinger Fukaya – Toyama – Asfour -Dillmann DLR - German Aerospace Center Tokyo and Karlsruhe University 2000 2000 [4] [19]
Picture
Hand-arm integration
Kinematical scheme
Morphological features
Structurally integrated Modular Main upper fingers Opposable thumb Fourth finger Fifth finger Palm Number of links Number of joints Number of controlled degrees of freedom Size w. r. to a human hand Surfaces suitable to contact with objects Smoothness and continuity of contact surfaces
-
● -
●
● ● ● ● ●
● ● ● ● 18 17
22 24
13
1
>>
=
Fingertips / Phalanges / Palm
Fingertips / Phalanges / Palm
Poor / fair / good / very good
Poor / fair / good / very good
0,78 0,79 Fingertip grasp ● ● Whole-hand grasp ● ● Potential dexterity Fingertip manipulation ● Whole-hand manipulation ● Classification MH-0.78-WHM IH-0.79-WHG Structural design concept Exoskel. / endoskel. / intermediate Exoskel. / endoskel. / intermediate Actuator location Inside the finger / Remote Inside the finger / Remote Actuation type Electrical revolute motor Electrical revolute motor Mechanical design Act. joints back-drivability Yes / Not …Lightly NF details Kind of non-actuated joints Und / Rpd / Apd / Not Und / Rpd / Apd / Not Type of transmission Harmonic Drive - Gears Link Mechanism Transmission routing Research / Ind.applic. / Space applic. Research / Ind.applic. / Space applic. Designed for… (IVA operations) Ant-Index
Additional notes NF = > >>
Information Not Found The hand has approx. the size of human hand The hand is slightly larger than human hand The hand is sensibly larger than human hand 10
Und Rpd Apd Not
Underactuated joint Rigid passive-driven joint Adaptive passive-driven joint There aren’t non-actuated joints
Tab.II/7 Main features of design proposals of anthropomorphic hands
Project Identification
Project denomination
Ultralight Hand
GIFU Hand
Reference Author (s) Research Institute Year of Presentation References
Schultz - Pylatiuk - Bretthauer Reaserch center of Karlsruhe, D 2001 [7]
Kawasaki – Shimomura - Shimizu Gifu University, Japan 2001 [6]
● ● ● ● ● ● 17 18
● ● ● ● ● ● 21 20
13 (2x5 fingers + 3 wrist)
16
>>
=
Fingertips / Phalanges / Palm
Fingertips / Phalanges / Palm
Poor / fair / good / very good
Poor / fair / good / very good
Picture
Hand-arm integration
Kinematical scheme
Morphological features
Structural integrated Modular Main upper fingers Opposable thumb Fourth finger Fifth finger Palm Number of links Number of joints Number of controlled degrees of freedom Size w. r. to a human hand Surfaces apt to contact with objects Contact survace smoothness and continuity
Ant-Index Fingertip grasp Whole-hand grasp Potential dexterity Fingertip manipulation Whole-hand manipulation Classification Structural design concept Actuator location Actuation type Mechanical design Act. joints back-drivability details Kind of not-actuated joints Type of transmission Transmission routing Designed for…
0,88 0,94 ● ● ● ● ● ● IH-0.88-WHG MH-0.98-WHM Exoskel. / endoskel. / intermediate Exoskel. / endoskel. / intermediate Inside the finger / Remote Inside the finger / Remote Pneumatic Built-in DC Maxon servomotors Yes / Not Yes / Not Und / Rpd / Apd / Not Und / Rpd / Apd / Not Direct drive Worm gear Research / Ind.applic. / Space applic. Research / Ind.applic. / Space applic. (Prosthetic hand and service robots) -
Additional notes NF = > >>
Information Not Found The hand has approx. the size of human hand The hand is slightly larger than human hand The hand is sensibly larger than human hand 11
Und Rpd Apd Not
Underactuated joint Rigid passive-driven joint Adaptive passive-driven joint There aren’t non-actuated joints
6.
adapt to this need, developing rules of “design for sensory integration” and not, how often we can see, placing sensors where an independently-developed mechanical design allows.
Mechanical design of anthropomorphic hands based on biological models
Provided that nice anthropomorphic hands have been developed with success adopting mechanical solutions very far from biological models, it is authors’ opinion that a greater effort in developing biomorphic mechanics will greatly improve the actual performance of humanoid hands and make them simpler and cheaper, more compatible with the needs of application on humanoid robots. This opinion seems to be spreading in the scientific community and recent work is more clearly oriented to this purpose than in the past. Examining the mechanical design of most of the anthropomorphic hands developed so far, it’s clear that their design has been heavily conditioned by the lack of valid technological solutions for substituting some important functions of the human hand: one of them is the actuation system, and the lack of muscle-like actuators and of transmissions really comparable with performance of human tendons is so far a very conditioning factor. As a consequence of that, designers have developed hand architectures by assembling non-biomorphic mechanical solutions, with great difficulty of integration inside the available space, limited efficiency and high complication. In general, it seems that the technological development of the basic mechanical subsystems did not receive the same effort that was dedicated to the development of other important components, like tactile sensors or vision. In order to stimulate discussion and growth of interest towards the development of anthropomorphic robotic hands based on biomorphic structural solutions, the authors propose a list of design issues, inspired both by previous experience in developing robot hands and by attention to emerging tendencies in the scientific community. - A human-like hand can exploit its potential dexterity if used by a humanoid robot, that is a purposely designed system, therefore the structural and functional integration of the hand and the arm are highly recommended as a powerful way to obtain, on one side, more potential dexterity from the system, and, on the other side, to reduce the system complexity and the miniaturization problems; - in the perspective of a smart use of sensory and visual feedback coming from the hand during operation, the traditional design requirements imposed to robotic mechanical structures (that means very high stiffness, high precision, etc.) can be substituted or at least integrated by different requirements, like local or distributed compliance, reduction of mass, structural and assembly simplification, reduction of cost. In particular, in the case of a robotic hand, an important issue is the availability of compliant contact pads that can greatly enhance grasp adaptability and stability, thus contributing to achieve actual dexterity. - sensory integration is an indispensable tool for dexterity achievement and mechanical design must
The applicability of the above defined issues is greatly dependent from the adopted conceptual approach in the hand structural design. Any articulated structure, no matter if natural or artificial, is made by a frame of rigid links connected through joints, and by other components connected to links that cause joint motion (actuators and transmissions), feel what is happening (sensors of different kind), transmit forces to/from external environment (compliant contact pads), protect from external environment (skins, protective covers, and similar). Two main design concepts can be identified from the examination of biological models: - the exoskeletal design, in which the rigid links are at the same time an external protecting shell, and all the other components are mainly hosted inside the structure; this model applies to the majority of arthropodes like insects, crustaceans, etc.; - the endoskeletal design, where the articulated structure made by bones connected through ligaments is externally surrounded by muscles, tendons, soft tissues, skin; a direct example is the human hand (Figure 4).
Fig. 4: Endoskeletal structure of the human hand
These concepts can be reproduced in mechanical design of articulated robotic structure, and in particular in design of robotic anthropomorphic hands. Most of the examined hand projects are prevalently inspired to the exoskeletrical approach and the shape of links is usually oriented to contain and protect actuation and transmission components, that are hosted inside them. In the usual approach, the material of the link is distributed around the internal hollow with a shape mainly oriented to closed cross sections, with a good ratio between stiffness and weight but with low capability to host external equipment. Most times only a thin rubber layer can be applied on intermediate phalanges, just in order to increase contact friction and the use of thick compliant pads is possible only on the fingertips. Placing external distributed sensing equipment or thick compliant layer for contact adaptation and stabilization becomes a very hard job. Several examples of this design approach can be found 12
relative rotation is the consequence of rolling between two conjugated profiles, both convex or forming a concave-convex couple; In both cases, the kinematical pairs need to be integrated with ligaments, capable of creating a sort of continuity in the structure even if the pairs themselves can resist loads only in one sense. Work in this direction has been proposed along the years, with recent significant contributions where the bone structure of the human hand is directly reproduced [5]. The second class of solutions, that use compliant parts acting as joints, aim to simplify structure design, obtaining articulated structures with a reduced number of parts, easy to be manufactured and assembled, cheap, yet fully efficient and compatible with the required functions. Different kinds of compliant articulations can be defined, based on deformation of properly sized flexural or torsional hinges. This approach, that exhibits however severe design problems, has been adopted so far in different cases for small-displacement manipulators. The major goal was to develop articulations with no friction and no backlash and to use such design solutions in manipulators or end effectors dedicated to highprecision tasks [20,21]. Detailed contributions to elastic hinge design can be found in [22,23,24,25,26]. Recent work is being performed by the authors at the University of Bologna, investigating, both theoretically and experimentally, the potential of integrated design, with finger-frames obtained by links connected through elastic hinges with reduced flexural stiffness and actuation provided by elastic flexures guided inside the fingers [27] (Figure 6).
in the literature. The adoption of mechanical design criteria inspired to the endo-skeletrical architecture seems a promising way to overcome many problems related to design of dexterous hands, limiting their mechanical complexity on one side and increasing their potential dexterity on the other, due to a potentially easier introduction of distributed surface compliance and to better distribution of sensing equipment. Strictly limiting to the consideration of mechanical design aspects, three main topics attract the attention of the designers and show interesting ways for the development of research, concerning both theoretical aspects and technological development and application. a) Alternative design of articulated structures The first topic is related to alternative design of articulated structures with rigid links serially connected through rotary joints. Referring to hands, articulated finger structures can be obtained using joints quite different from the classical revolute pairs based on pin-and-bearing design (fig 5a): alternative solutions can be found according to different concepts, often suggested by nature itself. Two main classes of solutions can be identified: - joints in which the relative motion between adjacent rigid links is obtained by means of kinematical pairs (that means contact surfaces between the two links and a discontinuity of material); - joints in which the relative motion is allowed by the deformation of a compliant part of the structure that permanently connects the two rigid links; in this case the connecting compliant part can be made of a different material (e.g. rubber or steel spring) or simply obtained by introducing structural compliance inside a continuous structure (compliant mechanism) (Figure 5e).
b)
c)
d)
e)
a)
Fig. 5: Alternative design concepts for finger joints Fig. 6: Prototype implementation of a finger structure based on integrated elastic hinges
In the first class of solutions some biomorphic articulation models can be found, capable of one or more degrees of freedom, with two basic patterns: - sliding-contact pairs (Figure 5b), in which the relative rotation between the two links is mainly due to sliding between two fitting surfaces, convex and concave respectively, with eventual interposition of low-friction layers; - rolling-contact pairs (Figure 5c, 5d) in which the
b) Conceptual and technological development of tendon transmissions Transmission of motion by means of items with stiffness very high in the longitudinal direction and very low in the transverse direction, like ropes, wires, and so on is present in nature and has been used by 13
For what concerns theoretical analysis of tendon operated structures, valid contributions have been provided recently [28, 29], but the full development of adequate control procedures and application experience seems to be still to be achieved . As a final remark about this point, it must be noticed that the use of tendon transmission inside robotic fingers is strictly dependent on the availability of proper linear actuators and is congruent with the adoption of the endo-skeletal design discussed in the previous paragraph.
man since the beginning of its technological development. In the natural model, transmission of motion by tendons is characterized by two main aspects: - each tendon is a part of a complex system of interacting elements, that together contribute to the motion of the articulated structure, being arranged into a net of cooperating elements; the whole system works according to the agonistic-antagonistic scheme (Figure 7) and a typical parameter is the degree of co-contraction of the tendon system, that results in stiffness variation of the whole finger;
c) Mechanical interface with the external world Many authors have dedicated their research effort to evaluate the role of compliant flesh layers and skin in the human hand, in order to determine the properties of layers to be externally applied to robotic hands in order to reproduce, at least in part, the human hand behaviour [30,31,32]. Besides the surface properties (high friction is greatly contributing to grasp robustness), the resultant compliance of the external layer is of paramount importance for grasp adaptability (that means capability to obtain stable non punctual contacts with grasped objects even in case of shape singularities like edges, holes, etc.) and for grasp stability (the viscoelastic properties of pads can greatly help to adsorb shock and vibration energy, stabilizing the contact). The authors share the opinion that a properly designed compliant layer can greatly contribute to the potential dexterity of a hand and see three main goals to address: - further investigation on materials to be used for the compliant pads; in particular, it has been demonstrated that visco-elastic parameters must be contained in a very narrow range and rather soft materials are required for pads (Shore hardness of a human pulp has been estimate as about 6-9 Sh°); at the same time, suitable materials like low-density foams or gels offer poor behaviour as to tear and wear resistance, therefore composite pads with outer skin-like layers should be developed; - integration of distributed tactile sensing into external compliant layers; as it will be discussed in the second part of this presentation, the distribution of sensitive tactile elements (taxels) all over the external surface is a way to obtain a precise map of actual contact distribution; the solution of the technical problem seems to be achievable according to two main approaches, that are on one side to consider the compliant pad just a passive medium that contains distributed active taxels made with different material; on the other side, to introduce load sensing properties in the compliant material itself, exploiting local stress/strain to generate information on local contact conditions; - development of design solutions for practical manufacturing of integrated endoskeletrical structures and compliant covers: it must not be forgot that acceptable solutions must present high simplicity, easy manufacturability, low cost; to this purpose, visco-elastic materials that can be formed at
Fig. 7: A scheme of agonistic-anthagonistic tendons
- tendon routing inside the finger is provided by internally lubricated sheaths connected to the bones (Figure 8); this biological solution allows high adaptability during finger bending, with high efficiency and very reduced cross section.
Fig. 8: A scheme of biological sheath routing of tendons
In robotic hands tendon transmission has been so far widely used, but in most cases in its poorest form, that is not exploiting the concept of net cooperation, limiting co-contraction to single couples of agonisticanthagonistic tendons (in most cases fixed pre-load couples of tendons are used), adopting pulley-based routing schemes (when sheath routing has been used, its technological implementation was in general rather poor). It clearly seems that the development of tendon transmission for robotic hands was not intensively developed and that great potential of improvement is still left. Part of the work to be done concerns technological development of tendons and sheaths together with their manufacturing and assembly problems inside the hand. 14
ambient temperature and low pressure, like polyurethane gels, could offer very interesting perspectives, e.g. to include in a single mould a previously assembled hand skeleton complete with jounts, tendons distributed sensors and wiring. 7.
[5]
[6]
Conclusions
[7]
On authors’ opinion, the design of anthropomorphic robotic hands has not been enough inspired to anthropomorphic and biological models. Even if they fully agree that mimicking biological solutions is not necessary in principle, however they feel that it could be useful, helping to overcome some limitations that actual mechanical solutions show. Scanning the literature and comparing the available knowledge and technology with the hand-designers expectations, the main goals to which address research and technological development in mechanical design are related to alternative concepts of mechanical solutions as far as structure and actuation are concerned. The adoption of an endoskeletrical structure may be a useful way to help the development of dexterous hands. From this point of view, the general impression is that hand designers did not directly face some mechanical problems related to imitation of biological models (e.g. the development of sheath-guided frictionless tendons) but rather avoided them adopting non biomorphic solutions (e.g. routing tendons by pulleys and not by sheaths). In the present situation of a growing interest towards humanoid robots, and therefore anthropomorphic hands, new effort in developing mechanical design more inspired to the human hand model seems to be a profitable job for robotic research. Because the integration of the many technological subsystems (articulated work-frame, actuation, transmissions, sensors etc.) is one of the key-problems, the application of simultaneous engineering rules, with coordinated development of all the subsystems, must be further on recommended.
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