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Robust compliant adaptive grasper and method of manufacturing same — President And Fellows Of Harvard College (US8231158B2)

President And Fellows Of Harvard College · Google Patents

Google Patents · Patents · License: Open Access

patent, google patents, intellectual property, US8231158B2, President And Fellows Of Harvard College, Aaron Dollar, en, 2012

ABSTRACT

Abstract

A multi-fingered underactuated mechanical grasping system driven by a single actuator, yet can grasp objects spanning a wide range of size, shape, and mass. A member for moving a link relative to a base acts in parallel to a direction of compliance of a joint between the link and the base. The joint has a plurality of degrees of freedom. The number of members for moving links in the grasping system is less than the number of degrees of freedom in the grasping system.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and the benefit of the filing date of U.S. Provisional Patent Application Ser. No. 60/864,252 filed on Nov. 3, 2006, which is hereby incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under N00014-98-1-0669 awarded by the Office of Naval Research and DAMD17-01-1-0677 awarded by the U.S. Army. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to robotic graspers or hands and, more particularly, to a robust compliant underactuated mechanical grasper and method of manufacturing the same.

2. Brief Description of the Related Art

After years of experimenting with complex, fully-articulated anthropomorphic hands, researchers have begun to embrace the idea that much of the functionality of a hand can be retained by careful selection of joint coupling schemes, reducing the number of actuators and the overall complexity of the grasping mechanism. Many of these grippers are âunderactuatedâ, having fewer actuators than degrees-of-freedom. These types of graspers, grippers or hands have also been referred to as âadaptiveâ or âselfadaptableâ. Other simplified hands have fixed-motion coupling between joints, reducing the overall degrees-of-freedom of the mechanism. These two classes of simplified grippers can be easier to control, much lighter, and less expensive than their fully-actuated counterparts.

The very nature of unstructured environments precludes full utilization of a complex, fully-actuated hand. In order to appropriately use the added degrees of actuation, an accurate model of the task environment is necessary. A gripper with a reduced number of actuators is not only simpler to use, it is more appropriate based on the quality of information available for the unstructured grasping task.

The joint coupling necessary to allow for underactuation is often accomplished through compliance in the manipulator structure. Compliance is perhaps the simplest way to allow for coupling between joints without enforcing the fixed-motion coupling relationship inherent with gear or linkage couplings. Compliant couplings are a simple way to allow a joint to passively deflect without causing a fixed-motion proportional change in the joints to which it is coupled.

Compliant underactuated grippers show particular promise for use in unstructured environments, where object properties are not known a priori and sensing is prone to error. Finger compliance allows the gripper to passively conform to a wide range of objects while minimizing contact forces. Passive compliance offers additional benefits, particularly in impacts, where control loop delays may lead to poor control of contact forces. See D. E. Whitney, âQuasi-static assembly of compliantly supported rigid parts,â Journal Dyn. Syst. Measurement Control 104, pp. 65-77, 1982 and J. M. Schimmels and S. Huang, âA passive mechanism that improves robotic positioning through compliance and constraint,â Robotics Comput.-Integr. Manuf. 12 (1), 65-71, 1996. Compliance can also lower implementation costs by reducing the sensing and actuation required for the gripper.

A number of underactuated and fixed-motion coupled robotic hands have been proposed. Those prior devices include the devices described in the following publications: [1] M. Higashimori, M. Kaneko, A. Namiki, M. Ishikawa, âDesign of the 100G Capturing Robot Based on Dynamic Preshaping,â The International Journal of Robotics Research , vol. 24 (9), pp. 743-753, 2005; [2] W. T. Townsend, âThe BarrettHand GrasperâProgrammably Flexible Part Handling and Assembly,â Industrial RobotâAn International Journal , vol 10 (3), pp. 181-188, 2000; [3] M. Rakic, âMultifingered Robot Hand with Self-Adaptability,â Robotics and Computer Integrated Manufacturing, vol. 5(2/3), pp. 269-276, 1989; [4] J. Butterfass, G. Hirzinger, S. Knoch, H. Liu, âDLR's Multisensory Articulated Hand Part I: Hard- and Software Architecture,â Proceedings of the 1998 IEEE International Conference on Robotics and Automation , pp. 2081-2086, 1998; [5] J. Butterfass, M. Grebenstein, H. Liu, G. Hirzinger, âDLR-Hand II: Next Generation of a Dextrous Robot Hand,â Proceedings of the 2001 IEEE International Conference on Robotics and Automation , pp. 109-114, 2001; [6] A. Edsinger-Gonzales, âDesign of a Compliant and Force Sensing Hand for a Humanoid Robot,â Proceedings of the 2004 International Conference on Humanoid Manipulation and Grasping (IMG04), 2004; [7] J. Crisman, C. Kanojia, I. Zeid, âGraspar: A Flexible, Easily Controllable Robotic Hand,â IEEE Robotics and Automation Magazine , pp. 32-38, June 1996; [8] S. Hirose and Y. Umetani, âThe Development of Soft Gripper for the Versatile Robot Hand,â Mechanism and Machine Theory , vol. 13, pp. 351-359, 1978; [9]T. Laliberte, L. Birglen, C. Gosselin, âUnderactuation in Robotic Grasping Hands,â Machine Intelligence & Robotic Control , vol. 4 (3) pp. 1-11, 2002; [10] J. Ueda, Y. Ishida, M. Kondo, T. Ogasawara, âDevelopment of the NAIST-Hand with Vision-based Tactile Fingertip Sensor,â Proceedings of the 2005 IEEE International Conference on Robotics and Automation , pp. 2343-2348, 2005; [11] E. Torres-Jara, âObrero: A platform for sensitive manipulation,â Proceedings of the 2005 IEEE - RAS International Conference on Humanoid Robots , pp. 327-332, 2005; [12] C. S. Lovchik, M. A. Diftler, âThe Robonaut Hand: A Dexterous Robot Hand for Space,â Proceedings of the 1999 IEEE International Conference on Robotics and Automation , pp. 907-912, 1999; [13] K. DeLaurentis, C. Mavroidis, âMechanical design of a shape memory allow actuated prosthetic hand,â Technology and Health Care , vol. 10, pp. 91-106, 2002; [14] D. Caldwell, N. Tsagarakis, ââSoftâ grasping using a dextrous hand,â Industrial Robot: An International Journal vol. 27 (3), pp. 194-199, 2000; [15] R. Crowder, V. Dubey, P. Chappell, D. Whatley, âA Multi-Fingered End Effector for Unstructured Environments,â Proceedings of the 1999 IEEE International Conference on Robotics and Automation , pp. 3038-3043, 1999; [16] M. C. Carrozza, C. Suppo, F. Sebastiani, B. Massa, F. Vecchi, R. Lazzarini, M. R. Cutkosky, P. Dario, âThe SPRING Hand: Development of a self-Adaptive Prosthesis for Restoring Natural Grasping,â Autonomous Robots 16, pp. 125-141, 2004; [17] N. Dechev, W. Cleghorn, S. Naumann, âMultiple finger, passive adaptive grasp prosthetic hand,â Mechanism and Machine Theory 36, pp. 1157-1173, 2001; and [18] F. Lotti, P. Tiezzi, G. Vassura, L. Biagiotti, G. Palli, C. Melchiorri, âDevelopment of UB Hand 3: Early Results,â Proceedings of the 2005 IEEE International Conference on Robotics and Automation , pp. 4499-4504, 2005. Table I provides an overview of some of the features of those underactuated and fixed-motion coupled robotic hands.

TABLE 1

UNDERACTUATED AND FIXED-MOTION COUPLED ROBOT HANDS

Pitch

Coupling scheme

joints per

actuators

(*indicates compliant coupling

Source of compliance

Hand

# fingers

finger

per finger

{circumflex over (â)}indicates adaptive mechanism)

Coupling ratio

and/or adaptability

100G [1]

Â½

prox:*:dist

unknown

tendon routing, spring-loaded joints

Barrett [2]

prox:{circumflex over (â)}:dist

(3:4)

âTorqueSwitchâ differential

Belgrade/USC [3]

4 + 1

3 + 0

Â½ + 1

(prox;med;dist) + (prox;dist)

(~9;8;7)

rocker arm coupling of fingers

DLR I and

med;dist

(1;1)

none

II [4, 5]

Domo [6]

prox;med*:dist

(1;1:passive)

unactuated compliant distal joint

Graspar [7]

prox:{circumflex over (â)}:med:{circumflex over (â)}:dist

(~5:4.2:2.9)

tendon differential mechanism

Hirose [8]

10â

Â½

prox:(all):distal

(55:::28:::10:::1)

tendon routing

Laval 10-DOF [9]

prox:{circumflex over (â)}:med:{circumflex over (â)}:dist

unknown

adaptive linkage mechanism

NAIST [10]

3 + 1

3 + 3

2 + 2

(med;dist) + (med;dist)

(1;1.15)

none

Obrero [11]

prox:*:dist

(4:3)

series elastic actuation

Robonaut [12]

2 + 2 + 1

3 + 3 + 2

2 + 1 + 2

(med;dist) + (prox;med;dist) + 0

(1;1) + (1;1;1) + 0

compliant connector, no adaptability

Rutgers [13]

4 + 1

3 + 3

2 + 2

med:dist

unknown

tendon routing

Salford [14]

4 + 1

3 + 3

2 + 3

(med;dist) + 0

unknown

none

SDM [15]

(prox:*:dist)

(4.5:1)

tendon routing, joints made of

springs

Southampton [15]

prox:{circumflex over (â)}:med:{circumflex over (â)}:dist

unknown

differential unit

SPRING [16]

2 + 1

3 + 2

â + â

(prox:*:med:*:dist) + (prox:*:dist)

(2.9:1.6:1)

series elastic actuation

TBM [17]

4 + 1

3 + 2

1 + 1

(prox;med;dist) + (prox

CROSS-REFERENCE TO RELATED APPLICATIONS

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to robotic graspers or hands and, more particularly, to a robust compliant underactuated mechanical grasper and method of manufacturing the same.

2. Brief Description of the Related Art

TABLE 1

UNDERACTUATED AND FIXED-MOTION COUPLED ROBOT HANDS

Pitch

Coupling scheme

joints per

actuators

(*indicates compliant coupling

Source of compliance

Hand

# fingers

finger

per finger

{circumflex over (â)}indicates adaptive mechanism)

Coupling ratio

and/or adaptability

100G [1]

Â½

prox:*:dist

unknown

tendon routing, spring-loaded joints

Barrett [2]

prox:{circumflex over (â)}:dist

(3:4)

âTorqueSwitchâ differential

Belgrade/USC [3]

4 + 1

3 + 0

Â½ + 1

(prox;med;dist) + (prox;dist)

(~9;8;7)

rocker arm coupling of fingers

DLR I and

med;dist

(1;1)

none

II [4, 5]

Domo [6]

prox;med*:dist

(1;1:passive)

unactuated compliant distal joint

Graspar [7]

prox:{circumflex over (â)}:med:{circumflex over (â)}:dist

(~5:4.2:2.9)

tendon differential mechanism

Hirose [8]

10â

Â½

prox:(all):distal

(55:::28:::10:::1)

tendon routing

Laval 10-DOF [9]

prox:{circumflex over (â)}:med:{circumflex over (â)}:dist

unknown

adaptive linkage mechanism

NAIST [10]

3 + 1

3 + 3

2 + 2

(med;dist) + (med;dist)

(1;1.15)

none

Obrero [11]

prox:*:dist

(4:3)

series elastic actuation

Robonaut [12]

2 + 2 + 1

3 + 3 + 2

2 + 1 + 2

(med;dist) + (prox;med;dist) + 0

(1;1) + (1;1;1) + 0

compliant connector, no adaptability

Rutgers [13]

4 + 1

3 + 3

2 + 2

med:dist

unknown

tendon routing

Salford [14]

4 + 1

3 + 3

2 + 3

(med;dist) + 0

unknown

none

SDM [15]

(prox:*:dist)

(4.5:1)

tendon routing, joints made of

springs

Southampton [15]

prox:{circumflex over (â)}:med:{circumflex over (â)}:dist

unknown

differential unit

SPRING [16]

2 + 1

3 + 2

â + â

(prox:*:med:*:dist) + (prox:*:dist)

(2.9:1.6:1)

series elastic actuation

TBM [17]

4 + 1

3 + 2

1 + 1

(prox;med;dist) + (prox;dist)

(~2;1;1) + (~2;1)

none

UB III [18]

2 + 2 + 1

3 + 3 + 3

3 + 2 + 2

0 + (med:*:dist) + (med:*:dist)

(~6:7)

tendon routing, joints made of

springs

An âunderactuatedâ hand has fewer actuators than degrees-of-freedom, and therefore demonstrates adaptive behavior. In these hands, motion of the distal links can continue after contact on the coupled proximal links occurs, allowing the finger to passively adapt to the object shape. In a âfixed-motion coupledâ hand, each actuator controls a single degree-of-freedom, and the mechanism has no âadaptabilityâ (final column). In these hands, motion of one joint always results in a proportional motion of the joint(s) coupled to it. In the same way, if contact occurs on one joint fixing its position, all coupled joints are thereby fixed.

In Table I, the â# fingersâ column gives the number of fingers of each different type used in the hand, separated by â+â. Cases where two types are given indicate that some number of identical fingers and one thumb are used in the design. Cases where three types are given mean that two different finger designs are used in addition to a thumb. For example, the Robonaut hand incorporates two âgraspingâ fingers, two âdexterousâ fingers, and a thumb. The second column indicates the number of âpitchâ joints per finger, leaving out âyawâ and ârollâ joints, if any exist. Entries correspond to the data in the â# fingersâ column. For the Robonaut hand, the grasping and dexterous fingers and thumb have three pitch joints each. The next column corresponds to the number of actuators per finger that control the pitch joints. Note that the degree of underactuation ranges from a single actuator for twenty joints (Hirose's âSoft Gripperâ) to twelve actuators for fifteen joints (UB III hand). The coupling scheme is indicated in the next column. âProxâ indicates the proximal joint (nearest to the base), âmedâ is the medial joint (for three phalanx fingers), and âdistâ is the distal joint (farthest from the base). A â:*:â between two joints indicates that the coupling between the two joints is compliant, such as those hands with joints made of springs. A â:^:â between two joints indicates that the coupling between the two joints is based on a mechanism that allows for decoupling. The BarrettHand, for example, achieves this effect by means of a âTorqueSwitchâ differential gear mechanism that actively decouples the two joints once contact has been made on the inner link and a preset torque limit has been reached. A â;â between joints indicates that the coupling is fixed-motion, and therefore has no adaptability. The next column indicates the coupling ratio (prox:med:dist) between the joints. For a finger with some method of adaptability, this ratio is the relative angular motion between joints when the finger is freely actuated (i.e. no external contact). For Hirose's âSoft Gripper,â every third value is given. The final column indicates the method by which the hand is passively compliant and/or adaptive, if at all.

Grasping and manipulating objects in unstructured environments, where object properties are not known a priori and sensing is prone to error, is one of the central challenges in robotics. The uncertainty in the relationship between the object and gripper makes it difficult to control contact forces and establish a successful grasp. One approach to dealing with this uncertainty is through compliance, so that positioning errors do not result in large forces and the grasper conforms to the object. Compliance has most often been implemented through control of manipulator impedance, based on active use of joint sensors for position, velocity and force.

While designing durable robots is rarely addressed in robotics research, it is essential in industrial, space, and military applications. Examples include iRobot's âPackBotâ, University of Minnesota's âScoutâ family of launchable robots, and MIT manipulator arms for the NASA/JPL. Pathfinder and Surveyor Mars missions. This durability would expand the type of experimental tasks that can be reasonably attempted and speed implementation due to the reduced need for careful validation of programs.

Unintended contact that often occurs in unstructured grasping tasks can result in large contact forces unless the gripper is compliant. This contact can occur due to sensing uncertainty in unstructured environments, but can also happen in laboratory experiments, particularly in the debugging phase. Researchers are often reluctant to risk crashes with expensive multi-degree-of-freedom robot hands, so implementations must be carefully validated and experimental scope must be limited.

Compliance conveys two key advantages for robotic grasping: adaptability and robustness. The present invention takes advantage of the adaptability inherent with compliance and enhances it by incorporating further adaptability in the form of underactuation. An underactuated hand has fewer actuators than degrees of freedom, and therefore demonstrates adaptive behavior. In these hands, the transmission design allows motion of other joints to continue after contact occurs on a coupled link, allowing the hand to passively adapt to the object shape during finger closure.

Additionally, many complicated robotic hands suffer from drawbacks of being unreliable and difficult to use. Many simpler robotic hands suffer from a drawback of being aesthetically unappealing. The present invention provides a reliable robotic hand that is relatively simple to use and may be implemented with a molding process that may produce aesthetically acceptable appearance.

SUMMARY OF THE INVENTION

The present invention is a compliant, robust, adaptive mechanical grasping system. The compliance of the fingers is in parallel with the actuator elements, e.g., tendons running in parallel with the compliance. When the grasper or hand is used for exploring, the actuation elements are slack or uncoupled to the fingers and thereby do not interfere with or reduce the compliance. When the actuation members are actuated or pulled, the compliance is reduced thereby stiffening the finger or fingers. This property is desirable since compliance is helpful during acquisition of target objects, but less desirable during grasp since it reduces grasp stability. Another important aspect of the present invention is that the fingers of grasping system are also compliant in the direction normal to the plane of motion of the finger when actuated, allowing the fingers to passively comply to target objects and environmental constraints in this direction, as well as reduce the likelihood of damage to the invention when unplanned contact occurs. Additionally, the underactuated nature of the grasping system was designed to demonstrate an adaptability to a wide range of target object properties. This adaptability allows for extremely simple actuation and control of the hand, enabling robust grasping with even a single actuator and purely feed-forward control. The system is also scalable and may comprise flexural elements such that is may be scaled down to very small sizes such as by using MEMS fabrication processes.

In a preferred embodiment, the invention is a robust four-fingered grasper built using Shape Deposition Manufacturing (SDM). This process uses polymeric materials to simultaneously create the rigid links and compliant joints of the gripper, with embedded sensing and actuation components. In addition to simplifying the construction process, the result is an extremely robust gripper, fully functional after impacts and other large loads due to unintended contact. Other established manufacturing processes such as multi-shot injection molding, overmolding, or insert molding may also be used to create an embodiment of the invention with similar properties.

In a preferred embodiment, the present invention is a compliant underactuated grasper that comprises a base and a plurality of fingers. At least one of said plurality of fingers comprises a link, a joint complaint in a direction and connecting the link to the base, and a member for moving the link. The member acts in parallel to a direction of compliance of said joint such that actuation of the member substantially changes the compliance of the joint in the direction. The member may comprise a tendon cable. The invention may further comprise an actuator, for example a DC motor, for activating the tendon cable. The joint may have a viscoelastic response to provide damping. The compliant underactuated grasper may further comprise a sensor for sensing a position of said joint. Each finger may have more than one link, such that one link forms the base for another. A plurality of sensors may be mounted on or under the surface of the fingers to sense contact.

In another embodiment, the compliant underactuated grasper of the present invention further comprises a second link and a second joint connecting the second link to the first link and the second joint is compliant in the direction. Movement of the member substantially changes the compliance of the second joint in the direction. The second link may comprise a proximal link of the finger.

In still another embodiment, the present invention is a compliant underactuated grasper that comprises a base and a plurality of fingers. One or more fingers comprise a first link, a second link, a first joint connecting the first link to the second link, a second joint connecting said second link to said base, and a tendon cable for moving the first and second links. The first joint being is compliant in a first direction and the second joint being compliant in a second direction. The tendon cable is in parallel to the first direction of compliance such that movement of the tendon cable substantially changes the compliance of the first joint in the first direction. The first direction and second direction may be substantially the same. Further, the grasper may comprise an artificial hand.

In another embodiment, the present invention is a compliant underactuated grasper that comprises a base and a plurality of fingers. At least one of the plurality of fingers comprises a link, a joint connecting the link to the base with the joint being compliant in at least two directions, and a member for moving the link. The member for moving the link acts in parallel to a direction of compliance of the joint such that actuation of the member substantially changes the compliance of the joint in the direction. The grasper may further comprise a plurality of sensors.

In yet another embodiment, the present invention is a compliant underactuated grasper that comprises a base, a plurality of links, a joint between the base and each of the plurality of links wherein the joints are compliant in one and/or two directions, and an adaptive transmission that allows some links to keep moving after others have made contact with the object. The grasper may further comprise a plurality of viscoelastic joints.

In another embodiment, the present invention is a method for making underactuated graspers with polymer structures so that actuation and/or sensing components are embedded within the polymer material.

Still other aspects, features, and advantages of the present invention are readily apparent from the following detailed description, simply by illustrating a preferable embodiments and implementations. The present invention is also capable of other and different embodiments and its several details can be modified in various obvious respects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and descriptions are to be regarded as illustrative in nature, and not as restrictive. Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description and the accompanying drawings, in which:

FIG. 1 is a diagram of a four-fingered, underactuated SDM hand in accordance with a preferred embodiment of the present invention mounted on a Whole-Arm Manipulator (Barret Technology Inc., Cambridge, Mass.).

FIG. 2 is a diagram of a robotic finger in accordance with a preferred embodiment of the present invention.

FIG. 3 is a diagram showing joint deflection and link motion for three positions of travel across a travel range of a distal joint of a finger in accordance with a preferred embodiment of the present invention.

FIG. 4 is a graph showing for a finger in accordance with a preferred embodiment of the present invention joint response to a tip step displacement released at time=0.

FIG. 5 is a graph of a force-deflection curve of a tip of a finger of a preferred embodiment of the present invention with a linear trend line.

FIG. 6 is an actuation schematic diagram of a hand in accordance with a preferred embodiment of the present invention.

FIG. 7 is a series of graphs showing forces on a PVC cylinder in an example of the present invention.

FIG. 8 is a series of graphs showing forces on a wood block in an example of the present invention.

FIGS. 9( a ) and ( b ) are histograms of the standard deviation of force measurements for the PVC cylinder and wood block of the examples of the present invention.

FIG. 10 shows the placement of the target objects in a workspace in connection with an experiment illustrating a preferred embodiment of the present invention.

FIG. 11 shows the range of finger postures that can be expressed as combinations of two characteristic configurations or eigengrasps.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

To provide both adapatability and robustness, a four-fingered hand 100 , shown in FIG. 1 mounted on a Whole-Arm Manipulator 102 , features passively compliant joints 110 . The exemplary hand 100 was fabricated using polymer-based Shape Deposition Manufacturing (SDM). SDM is an emerging layered manufacturing technique with which the rigid links and compliant joints of the gripper are created simultaneously, with embedded sensing and actuation components. Elastomeric flexures create compliant joints, eliminating metal bearings, and tough rigid polymers fully encase the embedded components, eliminating the need for seams and fasteners that are often the source of mechanical failure.

A preferred embodiment of a compliant finger in accordance with a preferred embodiment of the invention is described with reference to FIG. 2 . Each finger 200 has a distal link 210 and a proximal link 220 . The concave side of each link

210 , 220 contains a soft fingerpad

212 , 214 to maximize friction and contact area, thereby increasing grasp stability. See K. B. Shimoga, A. A. Goldenberg, âSoft materials for robotic fingers,â Proceedings of the 1992 IEEE International Conference on Robotics and Automation , pp. 1300-1305, 1992 and M. R. Cutkosky, J. M. Jourdain, P. K. Wright, âSkin materials for robotic fingers,â Proceedings of the 1987 IEEE International Conference on Robotics and Automation , pp. 1649-1654, 1987. Links

210 , 220 are connected via elastomer joint flexures 232 , designed to be compliant in the plane of finger motion and stiffer out of plane. Each joint 230 in the preferred embodiment has a compliant elastomer joint flexure 232 , a magnet 234 , a hall- effect sensor 236 and a connector 238 . The finger of this embodiment further has a tendon cable 240 running through a hollow cable raceway 266 . There is a compliant joint 246 between link 220 and a dovetail connector 248 having a magnet 264 connected thereto. It further has a connector 260 and a Hall- effect sensor 262 . FIG. 3 shows the behavior of the distal finger joint 230 through its range of motion.

In this preferred embodiment of the invention, the polyurethane used for these joints demonstrates significant viscoelastic behavior, which reduces joint oscillations and permits the use of low joint stiffness. FIG. 4 shows the joint response of the finger of this preferred embodiment to a large step displacement of the fingertip, released at time t=0. Note that the oscillations are negligible after less than 1 second. In a conventionally-fabricated grasper with metal springs, oscillations due to large step displacements were found to persist for tens of seconds after release.

In an exemplary embodiment of the invention, the two links

210 , 220 of each finger are 70 mm (measured from the center of the joint flexures), with a total hand aperture of 113 mm. Due to the molding process used to create them, the SDM fingers, with embedded sensors and actuation components, are a single part weighing 39 grams, with no fasteners or adhesives. These measurements of the links

210 , 220 are exemplary only and it will be apparent to those of skill in the art that many variations in link size and shape, and well as the process to create them, are possible with the present invention.

FIG. 5 shows the force generated at the tip of the fingers due to displacement in the out-of-plane direction. The tip was displaced at a rate of approximately 1 cm/sec while mounted on an actuated linear slide mechanism, with force measured by a multi-axis force/torque sensor. This data represents force generated due to motion of the tip across the tested range and back for a total of five cycles, low-pass filtered with a cut-off frequency of 1 Hz, to remove sensor noise. Note the hysteresis in the curves and the force relaxation due to viscoelasticity.

This result shows that the SDM fingers, while exhibiting very low tip stiffness, can also undergo large deflections while remaining completely functional. In the test shown in FIG. 5 , the tip was displaced more than 3.5 cm in the out-of-plane direction (approximately 20 degrees) without any degradation of mechanical properties. The advantages of this property are clear when considering the usual result of unplanned contact during use of traditional research robotic hands.

To give a sense of the robustness of the mechanism to impacts and other potentially harmful loads, a number of more informal tests were performed. An SDM finger was repeatedly dropped from a height of over 15 m onto a stone floor, without significant damage. The fully-assembled hand has been hit repeatedly with a hammer, fingers jammed against objects, and even used underwater, without degradation of performance.

The preshape and stiffness characteristics of the hand were determined based on the results of an optimization study. See A. M. Dollar and R. D. Howe, âTowards grasping in unstructured environments: Grasper compliance and configuration optimization,â Advanced Robotics, vol. 19 (5), pp. 523-544, 2005. In this simulation, the joint rest angles and joint stiffness ratio of the fingers were varied and the performance analyzed to maximize the allowable uncertainty in object location (successful grasp range) and size as well as minimize contact forces.

The grasping model combined the inverse kinematics of the mechanism, torque balances for each joint, work balance, and equations describing the geometry of the grasper and object. MATLAB (The Mathworks, Natick, Mass., USA) was used to numerically solve these systems of equations and allow for the performance of the grasper to be simulated over a wide range of variations in grasper parameters.

In order to reduce the parameter space and allow for detailed analysis of parametric trade-offs, a simplified version of our hand was examined: a planar, two-fingered, four jointed gripper with links that are rigid lines between compliant rotational joints. The object to be grasped was assumed to be circular (a frequent assumption in the grasping literature, and a reasonable approximation for many objects), and sufficiently massive such that the gripper contact forces do not displace or rotate it. We ignored inertial effects and assumed quasi-static conditions.

Based on the results of this study, the preshape configuration Ï 1 =25Â° (angle with the horizontal in FIG. 6 ) and Ï 2 =45Â° (angle with the proximal link) was chosen for the preferred embodiment of the finger design. In addition, the results showed that the proximal joint should be much stiffer than the distal joint, keeping the grasping surface concave and contact forces low. These angles and stiffnesses were shown to enable grasping of the widest range of object sizes with the greatest amount of uncertainty in object position, while also exhibiting low average contact force, reducing the likelihood of displacing or damaging the object. Additionally, these results were confirmed experimentally by testing the performance of a reconfigurable aluminum grasper as joint rest angles and stiffnesses were varied.

For actuation, each finger has a pre-stretched, nylon-coated stainless steel cable 240 anchored into the distal link 210 , and running through low- friction nylon 11 tubing 242 to the base ( FIG. 2 ). The grasper is unactuated until contact is made with the target object and a successful grasp is predicted based on the available sensory information. Before actuation, the tendon cable 240 , which is in parallel with the compliant joints 230 , remains slack and the finger is in its most compliant state. This method permits the use of actuators that are not backdrivable and prevents the inertial load of the actuator from increasing the passive stiffness. After actuation, the stiff tendon takes much of the compliance out of the fingers, resulting in a stiffer grasp with greater stability.

In a preferred embodiment, a single actuator drives the four fingers (eight joints 232 ) of the hand. Each finger has links

210 , 220 . This property not only makes the gripper simpler and lighter, but it also allows the gripper to be self-adapting to the target object. FIG. 6 details the actuation scheme, by which motion of the distal links 210 can continue after contact on the coupled proximal links 220 occurs, allowing the finger to passively adapt to the object shape. Additionally, the pulley design in this scheme allows the remaining fingers to continue to enclose the object after the other fingers have been immobilized by contact, ensuring that an equal amount of tension T is exerted on each tendon cable 240 , regardless of finger position or contact state. This adaptive transmission behavior or force balancing behavior can be alternately embodied by a combination of levers pivoted about the attachment of the tendon connected to the motor.

The four fingers in this embodiment are staggered in the out-of-plane direction on the palm to allow them to completely close without interfering with one another. In this simulation, the joint coupling scheme (ratio of torque applied at the distal/proximal joints divided by the stiffness ratio of the joints) was varied in order to maximize the allowable uncertainty in object location (successful grasp range) and size as well as minimize contact forces. The simulation approach followed the kinematics and stiffness study described above.

To keep unbalanced object forces low, the torque ratio (ratio of torques applied at the distal and proximal joints) should be as large as possible. However, as the torque ratio increases, the position range in which an object can be successfully grasped (maximum allowable positioning error) is decreased. This tradeoff in force versus successful grasp range can be weighed by considering the quality of the sensory information available for the grasping task. For a task in which the location of the target object can be accurately sensed, the torque ratio can be large, since the gripper can be reliably centered on the object. However, for tasks in which sensory information is poor, the positioning of the gripper is subject to large errors, requiring that the chosen torque ratio should allow for large positioning errors. Since our hand is intended for grasping in unstructured environments resulting in large expected positioning errors, we chose a lower torque ratio ((Ï 2 /Ï 1 )/(k 2 /k 1 )=0.6). See A. M. Dollar and R. D. Howe, âJoint Coupling Design of Underactuated Grippers,â Proceedings the 30th Annual ASME Mechanisms and Robotics Conference, 2006 International Design Engineering Technical Conferences (IDETC), Philadelphia, Pa., Sep. 10-13, 2006.

While the embodiment described above has the ability to grasp objects in unstructured environments, the range of graspable objects and grasp configurations is limited: only âpower graspsâ of objects larger than a few cm are possible. This embodiment has only a single âwide openâ hand posture and is not capable of precision grasps because the finger tips do not meet. To add functionality, the hand design may be augmented with one or more additional actuators to actively change the rest positions of the fingers to smaller apertures that more closely approximates the size of small objects. One particularly effective means for implementing this ability is to move the finger rest positions to create postures that duplicate the dominant modes of finger motion in human grasp preshaping. See M. Santello, M. Flanders, and J. F. Soechting. Postural hand synergies for tool use. Journal of Neuroscience, 18(23):10105-10115, 1998. For example, through the use of two motors, the fingers can be moved as a combination of two principal components that capture much of the variability of human hand preshaping in anticipation of grasping a wide variety of objects, such as is shown in FIG. 11 .

One reason this approach improves performance is because it allows the fingers to converge on the object before the joint actuation begins. The tendons in the fingers are in parallel with the joint springs, so once the tendons are actuated to close around a small object the fingers become stiffer and large contact forces can result. (This transition to stiff fingers is beneficial in achieving a stable grasp with good disturbance rejection, but should occur only after fingers are correctly positioned around the object. To allow the hand to effectively grasp a wider range of objects we need to actively change the rest positions of the fingers to smaller apertures that more closely approximate the object size. The actuation system to move the finger rest positions can be distinct from the motor that controls the finger closing via cable tendons: its goal is to change the orientation of the fingers with respect to each other and the palm while keeping the fingers compliant, before the finger closing actuation of the tendons makes them stiff. In other embodiments, it is possible to use a single motor that moves the finger rest positions in the first range of actuator movement, then close the finger joints in the succeeding stages.

Joint angle sensing in the robot fingers described above may, for example, be accomplished by embedding a low output impedance linear hall-effect sensor 236 (A3517SUA, Allegro MicroSystems, Inc., Worcester, Mass., USA) on one side of the joint, and a rare-earth magnet 234 (6.35 mm diamÃ3.18 mm, NdFeB, 10,800 Gauss strength, K&D Magnetics, Inc., Boca Raton, Fla., USA) on the other side. Joint motion changes the distance between the two, varying the sensor output. The sensors 236 are wired to exposed connectors 238 (2.5 mm PC board header) for connection to external cables. These sensors give sufficient sensitivity across the entire range of motion of the joints to allow for use in the control of the grasper. The RMS sensor noise was found to be approximately 40 mV. Note that the sensor gives better resolution as the finger opens (Î¸ decreases) in order to optimize sensitivity during passive contact under. This enhances performance of the grasper when used as a âfeelerâ.

In order to determine the effectiveness of the hand at grasping objects in unstructured conditions, we experimentally evaluated the ability of the hand to grasp three-dimensional objects in a three-dimensional environment with significant errors in the sensed target object location and a very simple control scheme. Specifically, we examine the amount of positioning error allowable in order to obtain a stable grasp on the object, and record the forces on the object during the grasping task.

EXAMPLE 1

The SDM Hand was mounted on a low-impedance robotic arm (Whole-Arm Manipulator (WAM), Barrett Technology, Cambridge, Mass., USA) for positioning ( FIG. 1 ). Only three of the four joints of the WAM were used for a total of three positioning degrees of freedom: the base roll, shoulder pitch, and elbow pitch. Since there is no wrist, orientation of the hand was not controlled and was determined based on the kinematics of the manipulator at the target position.

The WAM was controlled using a 1000 Hz servo loop running on a DSP co-processor board (DS1103 PPC, dSPACE Inc., Novi, Mich.). The desired position was achieved using a PID controller with gains chosen so that the overall stiffness was dominated by the remote environment stiffness. To increase performance and allow for the use of lower gains, the robot controller uses a feedforward model of the forces on the arm (before contact with the ob

CLAIMS

Claims ( 18 )

1. A compliant underactuated grasper comprising:

a base; and

a plurality of fingers, wherein at least one of said plurality of fingers comprises:

a link;

a joint connecting said link to said base, said joint being compliant in first and second directions, wherein said joint comprises a joint flexure and a compliance of said joint flexure in said second direction is stiffer than a compliance of said joint flexure in said first direction; and

a member for moving said link;

wherein said member acts in parallel to said first direction of compliance of said joint such that actuation of said member substantially changes said compliance of said joint in said first direction, and wherein said grasper has fewer actuators than degrees of freedom.

2. The compliant underactuated grasper according to claim 1 , wherein said member comprises a tendon cable.

3. The compliant underactuated grasper according to claim 2 , further comprising an actuator for actuating a tendon cable, wherein said actuator actuates a plurality of tendon cables and wherein each of said tendon cables changes a compliance in a joint in a different one of said plurality of fingers.

4. The compliant underactuated grasper according to claim 3 , wherein said actuator comprises a DC motor.

5. The compliant underactuated grasper according to claim 1 , wherein said joint has a viscoelastic response to provide damping.

6. The compliant underactuated grasper according to claim 1 , further comprising a sensor for sensing a position of said joint.

7. The compliant underactuated grasper according to claim 1 , wherein said link comprises a distal link of said finger.

8. The compliant underactuated grasper according to claim 1 , wherein a plurality of sensors are mounted on or under the surface of the fingers to sense contact.

9. The compliant underactuated grasper according to claim 1 , wherein said at least one finger further comprises:

a second link; and

a second joint connecting said second link to said link, said second joint being compliant in said first direction;

wherein movement of said member substantially changes said compliance of said second joint in said first direction.

10. The compliant underactuated grasper according to claim 9 , wherein said second link comprises a proximal link of said finger.

11. The compliant underactuated grasper according to claim 1 , wherein said base comprises a second link.

12. A compliant underactuated grasper comprising:

a base; and

a plurality of fingers, wherein as least one of said plurality of fingers comprises:

a first link;

a second link;

a first joint connecting said first link to said second link, said joint being compliant in a first direction;

a second joint connecting said second link to said base, said second joint being compliant in a second direction; and

a tendon cable for moving said first and second links;

wherein said tendon cable is in parallel to said first direction of compliance such that movement of said tendon cable substantially changes said compliance of said first joint in said first direction, and wherein said grasper has fewer actuators than degrees of freedom.

13. The compliant underactuated grasper according to claim 12 , wherein said grasper comprises an artificial hand.

14. A compliant underactuated grasper comprising:

a base; and

a plurality of fingers, wherein at least one of said plurality of fingers comprises:

a link;

a member for moving said link; and

wherein said member acts in parallel to said first directions of compliance of said joint such that actuation of said member substantially changes said compliance of said joint in said first direction, and wherein said grasper has fewer actuators than degrees of freedom.

15. The compliant underactuated grasper according to claim 14 , wherein said two directions of compliance comprise a first direction of compliance in a plane of movement of said one of said plurality of said fingers and a second direction of compliance normal to said plane of movement of said one of said plurality of said fingers.

16. The compliant underactuated grasper according to claim 15 , further comprising a plurality of sensors.

17. The compliant underactuated grasper according to claim 14 , further comprising:

an adaptive transmission that allows some links to keep moving after others have made contact with an object; and wherein said grasper has fewer actuators than degrees of freedom.

18. The compliant underactuated grasper according to claim 17 , further comprising a plurality of viscoelastic joints.

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