WO2020190164A1

WO2020190164A1 - Gripper device for a robotic arm capable of operating in two grasp modes

Info

Publication number: WO2020190164A1
Application number: PCT/RU2019/000167
Authority: WO
Inventors: Ivan Igorevich BORISOV; Sergey Alekseevich KOLYUBIN; Oleg Igorevich BORISOV; Sergey Mikhailovich VLASOV; Vladislav Sergeevich GROMOV
Original assignee: Obshchestvo S Ogranichennoy Otvetstvennostyu "Tra Robotics"
Priority date: 2019-03-19
Filing date: 2019-03-19
Publication date: 2020-09-24

Abstract

A gripper device for a robotic arm is provided, the device comprising two or more movable fingers, each movable finger being a mechanism which is a closed kinematic chain. The kinematic chain comprises a ternary link connected to a frame, said ternary link constituting a proximal phalanx; and a binary link constituting a distal phalanx. All connections are in the form of revolute joints. The clutch is configured to open and close, thus breaking or connecting the kinematic chain of the movable finger, correspondingly. The opening and closing of the clutch switches the kinematic chain of the movable finger between an uncoupled mode and a coupled mode. In the uncoupled mode, both the proximal phalanx and the distal phalanx act as output links of the finger. In the coupled mode, only the distal phalanx acts as an output link of the movable finger. Technical result consists in providing a versatile gripper mechanism for a robot which can switch between two grasp modes without employing an excessively complex structure of finger(s) and/or links or joints to actuate the same.

Description

GRIPPER DEVICE FOR A ROBOTIC ARM CAPABLE OF OPERATING IN TWO

GRASP MODES

Field of invention

The invention relates to industrial robotics, and more specifically to a gripper device for a robotic arm to be used, in particular, in an industrial versatile robot to grasp objects with different size, shape, mass, kind of material and firmness.

Prior art

In an automated production line, robots operate with objects, modules, assembly blocks, and perform a variety of actions as determined by general scenario of the manufacturing process. For this purpose, industrial robots use gripper devices which allow grasping various kinds of objects to be manipulated.

A gripper device is a device which enables holding, tightening, handling and releasing a manipulated object. Gripper configuration may be changed in real time to perform the required manipulation. In general, there are two kinds of grasps, which conventional grippers perform: (i) a pinch grasp, also referred to as a parallel grasp or a precision grasp, and (ii) an encompassing grasp, also referred to as a power grasp. Precision grasp, or parallel grasp, enables grasping a wide range of objects, while the encompassing, or power grasp encompasses the object, which allows to handle heavier objects.

A robotic arm is a type of mechanical arm, usually programmable, with similar functions to a human arm. For industrial applications which involve manipulation of various kinds of objects, robotic arms are normally provided with two or more movable and/or flexible fingers. Research and development efforts have been undertaken to create a robotic arm which would be able to closely mimic human hand movements. There are several types of mechanisms which implement finger designs in robotic arms. An underactuated mechanism is one of the most common and suitable variants. The principle of underactuation is a commonly employed strategy for robotic finger design. A mechanism is said to be underactuated when it has fewer actuators than degrees of freedom (DOFs) . A mechanical finger based on an underactuated mechanism makes the gripper self-adaptive. To obtain a determined system, elastic elements and mechanical limits must be provided in underactuated mechanisms. Such fingers envelope the objects to be grasped and automatically adapt to the shape of the objects with only one motor. However, this type of the mechanism has its own advantages and disadvantages .

The second common variant to implement the finger design is a coupled linkage mechanism. Robotic fingers in which motion is mechanically coupled are not underactuated. They are designed to mimic the motion of human fingers, but relative motion of phalanges is determined at the design stage and therefore no shape adaptation is possible.

Some of underactuated mechanisms are based on linkages, while others are based on tendon-actuated mechanisms. The main drawback of tendon-driven underactuated mechanism is that payload to be grasped is limited by tendon-driven transmission system. Tendon systems are generally limited to rather small grasping forces and they lead to friction and elasticity (see e.g. Rea, Pierluigi. "On the design of underactuated finger mechanisms for robotic hands" Advances in mechatronics . InTech, 2011) . Hence, for applications in which large grasping forces are expected, linkage mechanisms are usually preferred (see e.g. Laliberte, Thierry & Gosselin, Clement (2002) . Underactuation in space robotic hands . Canadian Space Agency) .

A highly underactuated gripping mechanism is disclosed in US 6505870 (Laliberte, Thierry & Gosselin, Clement, 2003-01- 14) . The prior art highly underactuated gripping mechanism has ten degrees of freedom, but requires only two actuators, one for actuating the opening and closing of three fingers and the other one for orientation of two rotatable fingers with synchronization. Underactuation between the fingers is provided by a one-input/three -output differential which is associated with an orientation mechanism so that an orientation-fixed finger is deactivated when the two orientatable fingers are rotated to face each other for a pinch grasp. Each finger is enabled to be self -locked in its closing and opening action when the power is off.

2-fingered gripper (see US8973958, ROBOTIQ INC, 2015-03- 10) may be regarded as the closest analog to present invention, the prior art gripper representing an underactuated structure with a transmission system based on a mechanical linkage. It is universal, and it is able to pick up different objects with random shape. Prior art gripper adapts to the object's shape for a solid grip with force, position and speed control for each individual finger. The prior art gripper can execute pinch grasp and adaptive encompassing grasp, but does not enable switching kinematics schemes with a variable length link (does not include a clutch) , and two of its fingers are limited in range of motion. A drawback of using underactuation alone is that it only guarantees conditional stability. A non- stable grasp would lead to ejection of the grasped object. Moreover, it is difficult to control contact pressure. Underactuation in robotic hands generates intriguing properties. Phalanges cannot always apply forces to the grasped object, and geometrical configurations of the fingers and contact situations can lead to a singular equilibrium.

The invention is focused on extending the capabilities of a mechanical gripper device for an industrial robot by adding functionality via mode switching features to enable switching between a parallel grasp and an encompassing grasp without significant physical changes in the configuration of the gripper device .

Summary of invention

Object of the invention is to provide a gripper device for a robotic arm which is capable of switching between two different grasp modes. Another object of the invention is to provide a method of operating a gripper mechanism of a robot which enables selectively switching between two different grasp modes .

Technical result achieved by the present invention consists in providing for a versatile gripper device for a robot which can switch between two grasp modes without employing an excessively complex structure of finger (s) and/or links or joints to actuate the same.

To achieve the above-mentioned object, in one aspect the invention is directed to a gripper device for a robotic arm. The inventive device comprises the following mechanisms: two or more movable fingers, each movable finger being a mechanism which is a closed kinematic chain, the kinematic chain comprising: a ternary link connected to a frame, said ternary link constituting a proximal phalanx; and a binary link constituting a distal phalanx. The distal phalanx is connected to a ternary coupler link by a revolute joint. The ternary coupler link is connected to an input link, the input link being connected to the frame and to a clutch. The clutch is connected to a ternary rocker link, the ternary rocker link being connected to the frame and to the binary coupler. The binary coupler is connected to the proximal phalanx. All connections are in the form of revolute joints. The clutch is connected to the ternary coupler link and the ternary rocker link. The clutch is configured to open and close, thus breaking or connecting the kinematic chain of the finger, correspondingly. The opening and closing of the clutch switches the kinematic chain of the movable finger between an uncoupled mode and a coupled mode. In the uncoupled mode, both the proximal phalanx and the distal phalanx act as output links of the movable finger. In the coupled mode, only the distal phalanx acts as an output link of the movable finger.

The uncoupled mode enables the two or more movable fingers to perform an encompassing grasp. The coupled mode enables the two or more movable fingers to perform a parallel grasp. Encompassing grasp is used to grasp an object which is in the form of a body of rotation or has a substantially circular cross-section. Parallel grasp is used to grasp a small-sized object or an object having a cross-section that comprises two parallel walls.

The clutch may be embodied as an electromagnetic clutch or a mechanical clutch. The device may further comprise at least one actuator to actuate the movable fingers. The opening and closing of the clutch may be controlled depending on the shape of an object to be grasped. The mechanism of the inventive gripper device may comprise two, three or more movable fingers.

In another aspect, the invention is directed to a method of operating a gripper device according to the first aspect, the method comprising: opening and closing the clutch to break or connect the kinematic chains of movable fingers of the gripper device, correspondingly, wherein the opening and closing of the clutch switches the movable fingers between an uncoupled mode and a coupled mode, wherein, in the uncoupled mode, both proximal phalanx and distal phalanx act as output links of the movable fingers, wherein, in the coupled mode, only distal phalanx acts as output link of the movable fingers .

Brief description of the drawings

Figures illustrate various embodiments of the present invention and should be used only as an aid in understanding its fundamental principles and modes of operation but not for defining or restricting the scope of invention in any manner. Similar elements are denoted with similar reference numerals throughout the figures.

In the figures:

- Fig. 1 is a schematic representation of a kinematic chain of one finger of the inventive gripper device where all main elements which constitute the kinematic chain are diagrammatically represented;

Figs. 2A-C schematically represent three various positions of the kinematic chain of one finger of the inventive gripper device when operated in the coupled mode to perform a parallel grasp; Figs. 3A-C schematically represent three various positions of the kinematic chain of one finger of the inventive gripper device when operated in the uncoupled mode to perform an encompassing grasp;

- Fig. 4 represents an exemplary embodiment of one finger of the inventive gripper device with a clutch in a closed position;

- Fig. 5 represents an exemplary embodiment of one finger of the inventive gripper device with a clutch in an open position;

- Fig. 6A represents an exemplary embodiment of a clutch to be used in the kinematic chain of a finger of the inventive gripper device;

Fig. 6B shows a cross-section of the exemplary embodiment of the clutch of Fig. 6A;

- Figs. 7A-C illustrate an embodiment of the inventive gripper device with two fingers in various positions in the course of performing a parallel grasp;

- Figs. 8A-C illustrate an embodiment of the inventive gripper device with two fingers in various positions in the course of performing an encompassing grasp on an object having a circular cross-section;

- Figs. 9A-C illustrate an embodiment of the inventive gripper device having three fingers arranged at various orientations relative to one another;

- Fig. 10A illustrates an embodiment of the inventive gripper device having two movable fingers and one fixed stopper;

- Figs. lOB-C illustrate the embodiment of the inventive gripper of Fig. 10A in the course of performing a parallel grasp on an object having a rectangular cross-section. Embodiments of invention

Now the invention will be explained in more detail with reference to its exemplary embodiments as illustrated on the above-mentioned figures. It should be noted that this detailed description is intended for providing a deeper understanding of the principles of invention and modes of its operation, but not for defining and/or restricting its scope in any way.

The invention relates, in particular, to an industrial versatile gripper mechanism to grasp objects with different size, shape, mass, kind of material and firmness. Taken in general, the gripper mechanism of the present invention is comprised of two or more fingers. Two- finger grippers are well known in the art (among them e.g. the 2- fingered gripper (US8973958 , ROBOTIQ INC) which may be regarded as the closest prior art to the present invention) . However, the number of fingers in the inventive gripper is not limited by two, and gripper configurations with e.g. three fingers are also encompassed by the scope of the present invention.

Fingers of the inventive gripper are generally regarded as movable, although in some embodiments at least one of the fingers may also be embodied as a fixed stopper, as will be further discussed hereinbelow. Movable fingers are comprised of phalanges, wherein there may be two phalanges included in each of the movable fingers. For the sake of simplicity, most of the embodiments of the present invention will be further described taking into account the number of the phalanges that equals two (further referred to herein as a "proximal" phalanx and a "distal" phalanx, respectively, when viewed from the robot arm in the direction of an object to be grasped) . The phalanges are connected by movable joints. There are also links which are said to actuate the phalanges, thus constituting a kinematic chain, which is diagrammatically represented on Fig. 1.

When several links are movably connected together by joints, they are said to form a kinematic chain. Links containing only two pairs of element connections will be hereinafter referred to as binary links; those having three will be hereinafter referred to as ternary links. The phalanges are connected by joints, in particular by rotational joints of any suitable type which is well known in the art .

The inventive gripper device for a robotic arm comprises two or more movable fingers, each movable finger being a mechanism which constitutes a closed kinematic chain.

Referring to Fig. 1, the kinematic chain of one movable finger of the inventive gripper device comprises:

- a ternary link connected to a frame 1, said ternary link constituting a proximal phalanx 2; and

two binary links, one of the two binary links constituting a distal phalanx 3.

The distal phalanx 3 is connected to a ternary coupler link 4 by a revolute joint. The ternary coupler link 4 is connected to an input link 6, the input link 6 being connected to the frame 1 and to a binary clutch link, which will be hereinafter referred to as clutch 5. The clutch 5 is connected to a ternary rocker link 8, the ternary rocker link 8 being connected to the frame 1 and to the binary coupler 7. The binary coupler 7 is connected to the proximal phalanx 2. All above-mentioned connections are made in the form of revolute joints. The clutch 5 is connected to the ternary coupler link 4 and the ternary rocker link 8. The clutch 5 is configured to open and close, thus breaking or connecting the kinematic chain of the finger, correspondingly. The opening and closing of the clutch 5 switches the kinematic chain of the finger between an uncoupled mode and a coupled mode . In the uncoupled mode, both the proximal phalanx 2 and the distal phalanx 3 act as output links of the finger, while in the coupled mode only the distal phalanx 3 acts as an output link of the movable finger. The term "output link" may be defined in the context of the present invention as such a link of the kinematic chain of the movable finger, which directly acts upon (e.g. contacts and applies a certain force) the object to be grasped. There is only one output link in the coupled mode, whereas there are two output links in the uncoupled mode .

When the kinematic chain of the finger is intact, the respective mode of operation of the finger mechanism may be referred to as "coupled" mode. However, when the kinematic chain of the finger is broken, the respective mode of the finger mechanism may be referred to as "uncoupled" or "underactuated" mode . The uncoupled mode enables the two or more fingers of the gripper device to perform an encompassing grasp. The coupled mode enables the two or more fingers of the gripper device to perform a parallel grasp.

The above-mentioned mechanism allows the movable finger to be either kinematically fully defined or underactuated. A mechanism is said to be underactuated when it has fewer actuators than degrees of freedom (DOFs) . A mechanical finger based on an underactuated mechanism makes the gripper self- adaptive. Such self-adaptive fingers envelope the objects to be grasped and automatically adapt to the shape of the objects with the use of only one motor. Thus, the inventive mechanism enables to change the numbers of degrees of freedom (DOFs) of the distal phalanx of the movable finger between 1 DOF in the "coupled" mode (as shown by one circular arrow in each of Figs. 2A-C) and 2 DOFs in the "uncoupled" mode (as shown by two circular arrows in each of Figs. 3A-C) , without using any complicated mechanisms for this purpose, which makes the inventive gripper device more flexible, allowing it to perform a parallel grasp or an encompassing grasp depending on the shape of an object to be manipulated by the robotic gripper.

The parallel grasp, which may also be characterized as "pinch" grasp, at which DOF of each of the two or more fingers equals one, allows the robotic gripper to grasp any object with dimensions interior to the distance between distal phalanges in initial position. Figs. 7A-C show an exemplary implementation of the inventive gripper device with two movable fingers performing a parallel grasp. In the case of parallel grasp, the working surfaces of distal phalanges of the at least two fingers retain their orientation (see Figs. 7A-C) . In the parallel grasp mode, the gripper device creates a frictional force to hold the object. If the object is too heavy and frictional force is not sufficient, it is necessary to use an encompassing grasp instead, since encompassing grasp, which may also be characterized as "power" grasp, is capable of increasing the payload capacity of the gripper device through geometrical limitation of the fingers .

Figs. 8A-C show an exemplary embodiment of the inventive gripper device having two movable fingers in the process of performing an encompassing grasp in the uncoupled mode of each of its two movable fingers, in order to grasp on an object having a circular cross-section, e.g. a tube, which has a large diameter. It should be noted that, in the uncoupled mode, the positions of gripper fingertips and/or distal phalanx are not controlled and they are only defined by the position of the proximal phalanx, for which reason each of the movable fingers is said to be kinematically underactuated .

Uncoupled mode of each of the fingers of the inventive gripper device allows to perform encompassing grasp on objects of different shapes, such as e.g. tubes and blocks. Coupled mode is preferable when it is necessary to get a stable grasp on a heavy object such as a block. Meanwhile, uncoupled mode is more suitable to get a grasp on objects, in particular, having a cross-section in the shape of a body of rotation, such as tubes, cylinders, balls etc.

To enable switching between the aforementioned coupled and uncoupled modes of the kinematic chains of the movable fingers included in the inventive gripper device, the kinematic chains of each of the fingers are opened and closed by means of a clutch 5. As illustrated on Figs. 4 and 5, the clutch can fix its length or make it variable.

Referring now to Figs. 6A-B, in an exemplary embodiment of the inventive gripper device, the clutch of each movable finger of the gripper device consists of two parts. In the coupled mode, two parts of the clutch 5 are holding together. For example, this can be implemented via at least one electromagnet or at least one pin which is actuated by a pneumatic actuator, a leadscrew or by any other suitable actuator. Fig. 6A shows a side view of the clutch 5 according to the invention, and Fig. 6B shows a cross-section of the clutch 5. In the embodiment represented on Figs. 6A-B, the clutch 5 has two pins, along which the upper part of the clutch 5 is movable with respect to the lower part of the clutch 5, such that the size of the clutch 5 can be regulated, and the two parts of the clutch 5 can either hold together such that the clutch 5 is closed, or move away from each other such that the clutch 5 is open.

When the clutch is closed, such that it can be viewed as substantially one rigid body, the kinematic chain of the movable finger is intact. In the coupled mode provided in the case of the closed clutch as illustrated on Fig. 4, a determined system is obtained at any time during finger bending process, and the finger is said to have 1 DOF. When the clutch is open, such that it is substantially divided in two parts as shown on Fig. 5, the kinematic chain of the movable finger is broken. When the kinematic chain is broken, uncoupled mode is on, and the finger has 2 DOFs.

In an embodiment, the distal phalanx of each of the fingers of the inventive gripper device may further have a fingertip including at least one fingertip pad to ensure stable but delicate grasping of an object to be grasped.

In an exemplary embodiment illustrated on Figs. 10A-C, the inventive gripper device may have two movable fingers and one finger, in particular opposing these two movable fingers, embodied as a fixed stopper. Fixed stopper is useful to provide a precise workpiece location. If one of the fingers is a embodied as a fixed stopper, then basing of the grasped part/detail becomes possible, at the same time still enabling an encompassing grasp for objects of various shapes by virtue of ability of the other fingers to change their mutual orientation, as well as their orientation with respect to the fixed stopper, in the flange plane. The fixed stopper may also have a fingertip provided with at least one fingertip pad to ensure stable but delicate grasping of an object to be grasped.

Figs. 9A-C illustrate one of the possible embodiments of the inventive gripper device having three movable fingers. In this case, the three movable fingers may be arranged at various orientations relative to one another in a flange plane, substantially by rotating each of the fingers about an imaginary vertical axis drawn through a point where each of the movable fingers is coupled to its respective frame 1. In particular, examples of mutual orientations of the three fingers suitable for performing different tasks may be:

- opposing orientation where one movable finger is on one side and two other movable fingers are at the other, opposing side as shown on Fig. 9A, which may be useful for grasping e.g. elongated objects;

- each of the three movable fingers oriented at equal angles in the flange plane, as shown on Fig. 9B, which may be useful for grasping e.g. ball -shaped objects;

- two of the movable fingers opposing each other and one movable finger located at 90 degrees to each of said two movable fingers in the flange plane.

Rotation of the movable fingers in the flange plane, i.e. about a vertical axis drawn through a point where the movable fingers are coupled to their respective frames, may be implemented using various mechanisms which are well known in the art. In an exemplary embodiment illustrated on Figs. 9A- C, a gear mechanism is used for this purpose. However, it should be noted that such mechanism, as well as the mutual orientations of the movable fingers described above and depicted on Figs. 9A-C, are merely exemplary and should not be construed as restricting the scope of the present invention in any manner. Ones skilled in the art shall also appreciate that any other number of movable fingers may be provided in the inventive gripper device, for example there may be four movable fingers.

Having thus described the structure of the inventive gripper and its movable fingers in various embodiments of the claimed invention, the mode of operation of the inventive gripper device will now be further discussed.

As aforementioned, the inventive gripper device comprises at least one movable finger, mechanisms of which constitute a kinematic chain, which enables the movable finger to be either kinematically fully defined or underactuated. In particular, each movable finger of the inventive gripper device is capable of changing the numbers of degrees of freedom (DOFs) of the distal phalanx of the movable finger between 1 DOF in the "coupled" mode (as shown by one circular arrow in each of Figs. 2A-C) and 2 DOFs in the "uncoupled" mode (as shown by two circular arrows in each of Figs. 3A-C) . This is realized by using a binary clutch link, also referred to herein as clutch 5, which can fix its length or make it variable .

Referring again to Fig. 1, the kinematic chain of each movable finger of the inventive gripper device is comprised of the following mechanisms. A ternary link is connected to the frame 1, which is fixedly attached to the flange of the robotic gripper device, by means of a revolute joint, said ternary link constituting the proximal phalanx 2. As illustrated by Figs. 2A-C and Figs. 3A-C, this revolute joint provides one DOF for the phalanges of the movable finger. This ternary link has two more joints, also implemented in the form of revolute joints, connecting one end of the ternary link to the binary coupler 7 and connecting another end of the ternary link to a binary link which constitutes the distal phalanx 3. The revolute joint that connects the proximal phalanx 2 to the distal phalanx 3 provides one more DOF for the phalanges of the movable finger when the latter is operated in the uncoupled mode .

The distal phalanx 3 is also connected to a ternary coupler link 4 by means of another revolute joint. The ternary coupler link 4, in its turn, is connected to an input link 6, the latter being connected to the frame 1 and to the clutch 5 by means of revolute joints. The clutch 5 is connected to a ternary rocker link 8, the ternary rocker link 8 being also connected to the frame 1 and to the binary coupler 7. The binary coupler 7 is connected to the proximal phalanx. As aforementioned, all connections are in the form of revolute joints.

Input link 6 is actuated by means of an actuator (not shown) of any suitable type as is well known in the art. Input link 6 may be referred to as a driving link which drives the whole finger mechanism. The joint which connects the input link 6 to the frame 1 (encircled by a circular arrow on Figs. 2A-C) represents an encoder rotation joint.

In the coupled mode, the whole movable finger is actuated through the input link 6. In the position shown on Fig. 2A, the movable finger is in the initial position, there are no external forces. The input link 6 is connected to the finger frame 1 and drives two links to transmit the motion to the proximal phalanx 2, while the proximal phalanx 2 drives the distal phalanx 3 as depicted on Fig. 2B. Fig. 2C shows what can be regarded as final position of the movable finger in the coupled mode, wherein the working surface of the distal phalanx 3 of the movable finger is substantially pressed to the object to be grasped (schematically depicted by a rectangle) . Operation of two movable fingers in the coupled mode is also depicted on Figs. 7A-C. It should be noted that, in the coupled mode, working surfaces of the distal phalanges of both fingers retain their positions substantially parallel to each other at any point in time when the grasp is performed, which is why this type of grasp can be referred to as a "parallel" grasp.

The opening and closing of the clutch 5 switches the kinematic chain of the movable finger between the coupled mode and the uncoupled mode. When kinematic chain of the movable finger is opened via opening the clutch 5 (breaking the clutch link), underactuation mode is switched on. Figs. 3A-C illustrate the operation of the movable finger in the uncoupled mode at various stages of performing the grasp. In this mode, Fig. 3A shows the movable finger in the initial position, when no external forces are present. Both phalanges of the movable finger rotate as one rigid body about the point where the input link 6 is connected to the frame 1 by the revolute joint, until the proximal phalanx 2 is blocked by the object to be grasped or reaches its final position (see Fig. 3B) . When the proximal phalanx 2 contacts the object, only the distal phalanx 3 rotates around the joint which connects the distal phalanx 3 to the proximal phalanx 2 until it is blocked by the object or reaches the final position (see Fig. 3C) . Straight arrows on Figs. 3B and 3C show the forces which act when the proximal phalanx 2 (see Fig. 3B) and both the proximal phalanx 2 and the distal phalanx 3 (see Fig. 3C) contact the object to be grasped at subsequent stages of performing the encompassing grasp. By- virtue of the operation described above, the movable fingers of the inventive gripper device can grasp objects self- adaptively. Operation of two movable fingers in the uncoupled mode is also depicted on Figs. 8A-C.

In uncoupled mode, relative configuration of both phalanges of the movable finger is determined at any time by external constraints associated with the object. Since the kinematic chain of the movable finger is open, the movable finger has 2 DOFs, and a torsion spring has to be embedded in the mechanism to keep the phalanges aligned under the action of this spring when no external forces are applied to the movable finger.

In summary, it should be noted that, in the uncoupled mode, both the proximal phalanx 2 and the distal phalanx 3 act as output links of the movable finger. In contrast, in the coupled mode, only the distal phalanx acts as an output link of the movable finger. For better understanding of the two grasp modes, it should be taken into account that, when performing a parallel grasp, the gripper creates a frictional force to hold the object. If the object is too heavy and the frictional force is not enough to hold it, encompassing grasp has to be used, since in this mode the payload capacity of the gripper device is increased by geometrical limitation of the movable fingers. For this reason, the encompassing grasp may also be referred to as a "power" grasp.

The opening and closing of the clutch 5 may be controlled by various means well known in the art depending on the particular embodiment of the clutch 5. One non-limiting exemplary embodiment of the clutch 5 is illustrated on Figs. 6A and 6B which show a side view of the clutch 5 consisting substantially of two parts, and a cross-section of such clutch 5, respectively. In the coupled mode, the two parts of the clutch 5 hold together substantially as one solid body, while in the uncoupled mode one of the two parts separates from the other one and moves up along the two parallel shafts which are clearly seen on Fig. 6B. One non- limiting example of such clutch 5 may be an electromagnetic clutch where the two parts are held together by electromagnetic forces e.g. when power is supplied to the electromagnetic mechanism, so as to implement the coupled mode, and are allowed to separate from each other, one part thus moving away from the other one along the above-mentioned shafts, when no power is supplied, so as to implement the uncoupled mode. It should be understood by ones skilled in the art that the embodiment of the clutch as described above is only one example, and a variety of other embodiments of clutch 5 may be apparent for persons skilled in the art after careful reading of the present description with reference to respective figures. The opening and closing of the clutch 5 may be realized physically, mechanically, remotely, in an automated or non- automated manner, depending on the specific implementation (s) of the inventive gripper device. Remotely operated opening and closing of the clutch 5 may be, in particular, manually supervised and/or automatic, based on the shape of object (s) to be grasped and/or on a predefined task.

The switching between the coupled mode and the uncoupled mode may be implemented in the inventive gripper device in dependence on the type, weight and/or shape of the specific object (s) to be grasped by the gripper device in each case. For this purpose, different technical means and methods may be used, depending on the particular implementation of the claimed gripper device. By way of an example, but not limitation, various shape sensing techniques may be employed, such as kinesthetic (defining the internal status of the robot) and tactile sensing techniques, computer vision (contactless sensing) techniques, which may make use of one or more cameras, lidars, sonars, time-of -flight (TOF) cameras etc . Switching between the coupled mode and the uncoupled mode may also be controlled on the basis of the weight of object (s) to be grasped as measured by various means which are well known in the art.

Operation of the gripper device for a robot arm as described above may be controlled via at least one controller, computer, processor etc. as widely known in the art in dependence on specific operational conditions that can be sensed by sensor (s) of one or more types, which are also well known in the art. Said at least one controller, computer or processor can control the operation of the gripper device under the control of a computer program or a computer program element which can be implemented in one or more programming languages or in the form of an executable code as known in the art. The computer program may take various forms, and its certain components, modules, elements, functions may be implemented as software, firmware etc. The one or more controllers may comprise or be comprised of one or more processors, integrated circuits, FPGAs etc., where applicable .

All prior art references cited hereinabove as well as in the Citation List provided below are hereby included in the present disclosure by reference in their entirety where applicable .

While the present invention has been shown and described with reference to various embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the present disclosure, which is only defined by the appended claims and their equivalents.

Citation list

1. Underactuated robotic hands. Laliberte, T., Birglen, L. , and Gosselin, C. (2002). Underactuation in robotic grasping hands. Machine Intelligence & Robotic Control, 4(3), 1-11.

2. L. Birglen and C. M. Gosselin, "On the force capability of underactuated fingers," 2003 IEEE International Conference on Robotics and Automation (Cat. No .03CH37422) , 2003, pp. 1139-1145 vol .1.

3. Birglen L, Gosselin CM. Geometric Design of Three- Phalanx Underactuated Fingers. ASME. J. Mech. Des . 2005; 128 (2) :356-364. doi : 10.1115/1.2159029.

4. Laliberte, Thierry & Gosselin, Clement. (0002). Underactuation in space robotic hands. Canadian Space Agency.

5. D. Yoon and Y. Choi, "Underactuated Finger Mechanism Using Contractible Slider-Cranks and Stackable Four-Bar Linkages," in IEEE/ASME Transactions on Mechatronics , vol. 22, no. 5, pp. 2046-2057, Oct. 2017.

6. Tomovic, R. and Boni, G. (1962). An adaptive artiffcial hand. IRE Transactions on Automatic Control, 7 (3) ,3-10.

7. Wang, X.Q., Liu, Y.W., Jiang, L. , Yang, D.P., Li, N. , Liu, H., and Huang, H. (2010). Design and control of a coupling mechanism-based prosthetic hand. Journal of Shanghai Jiaotong University (Science) , 15(5) , 571-577.

8. Ma, R.R., Spiers, A., and Dollar, A.M. (2016) . M 2 gripper: Extending the dexterity of a simple, underactuated gripper. 795-805.

9. Kragten, G.A. , Baril, M. , Gosselin, C., and Herder, J.L. (2011) . Stable precision grasps by underactuated grippers. IEEE Transactions on Robotics, 27(6) , 1056-1066.

10. R. Rizk, S. Krut and E. Dombre, "Grasp-stability analysis of a two-phalanx isotropic underactuated finger, " 2007 IEEE/RSJ International Conference on Intelligent Robots and Systems, San Diego, CA, 2007, pp . 3289-3294.

11. D. Hirano, K. Nagaoka and K. Yoshida, "Design of underactuated hand for caging-based grasping of free- flying object," Proceedings of the 2013 IEEE/SICE International Symposium on System Integration, Kobe, 2013, pp. 436-442.

12. Rea, Pierluigi. "On the design of underactuated finger mechanisms for robotic hands." Advances in mechatronics . InTech, 2011.

13. W. T. Townsend, "The BarrettHand grasper-programmably flexible part handling and assembly," Int . J. Ind. Robot, vol. 27, no. 3, pp. 181-188, 2000

14. H. Shimojima, K. Yamamoto and K. Kawawita, "A Study of Grippers with Multiple Degrees of Mobility, " JSME International Journal, Vol.30, No. 261, pp . 515-522, 1987.

15. Rizk, Rani, Sebastien Krut, and Etienne Dombre. "Grasp-stability analysis of a two-phalanx isotropic underactuated finger." Intelligent Robots and Systems, 2007. IROS 2007. IEEE/RSJ International Conference on. IEEE, 2007. 16. N. Dechev, W.L. Cleghorn and S. Naumann, "Multiple Finger, Passive Adaptive Grasp Prosthetic Hand," Mechanism and Machine Theory, Vol. 36, No. 10, pp. 1157-1173, 2001.

17. J. Zhang, G. Guo and W.A. Gruver, "Optimal Design of a Six-Bar Linkage for an Anthropomorphic Three-Jointed Finger

Mechanism," Proceedings of the ASME Mechanisms Conference, Phoenix, vol. DE-45, pp . 299-304, 1992.

18. E.N. Haulin, A. A. Lakis and R. Vinet, "Optimal Synthesis of a Planar Four-Link Mechanism Used in a Hand Prosthesis," Mechanism and Machine Theory, Vol. 36, Nos. 11- 12, pp. 1203-1214, 2001.

19. Townsend, W. (2000) . The barretthand grasper programmably flexible part handling and assembly. Industrial Robot: the international journal of robotics research and application, 27(3), 181-188.

20. N. T. Ulrich, Methods and Apparatus for Mechanically Intelligent Grasping, US Patent No. 4 957 320, 1988.

21. C. Gosselin and T. Lalibert'e, Underactuated mechanical finger with return actuation, US Patent No. 5 762 390, 1996.

22. T. Lalibert 'eand C. Gosselin, Actuation system for highly underactuated gripping mechanism, US Patent No. 6505870B1, 2000-05-30

23. Louis-Alexis Allen Demers, Simon Lefrangois, Jean- Philippe Jobin, Gripper having a two degree of freedom underactuated mechanical finger for encompassing and pinch grasping, US Patent No 20140265401 Al, 2011-11-25

24. Aaron Dollar, Robert Howe, Robust compliant adaptive grasper and method of manufacturing same, US patent 8 231 158 B2 , 2006-11-03 25. Hahn Park, Sung Wook Jung, Jae Chul Hwang, Yong Won Choi, Industrial gripper with multiple degrees of freedom, US Patent 8 424 942 B2 , 2009-12-21

26. Takashi NAMMOTO, Kazuhiro Kosuge, Haruaki CHIBA, Robot hand and robot apparatus, US Patent 8 585 111 B2, 2011-

02-14

Claims

1. Gripper device for a robotic arm, the device comprising the following mechanisms:

- two or more movable fingers, each movable finger being a mechanism which is a closed kinematic chain, the kinematic chain comprising:

- a ternary link connected to a frame, said ternary link constituting a proximal phalanx; and

- a binary link constituting a distal phalanx;

- wherein all connections are in the form of revolute joints;

- wherein the clutch is configured to open and close, thus breaking or connecting the kinematic chain of the movable finger, correspondingly,

- wherein the opening and closing of the clutch switches the kinematic chain of the movable finger between an uncoupled mode and a coupled mode,

wherein, in the uncoupled mode, both the proximal phalanx and the distal phalanx act as output links of the finger,

- wherein, in the coupled mode, only the distal phalanx acts as an output link of the movable finger.

2. The device of claim 1, wherein the uncoupled mode enables the two or more movable fingers to perform an encompassing grasp.

3. The device of claim 1, wherein the coupled mode enables the two or more movable fingers to perform a parallel grasp .

4. The device of claim 2, wherein the encompassing grasp is used to grasp an object which is in the form of a body of rotation or has a substantially circular cross-section.

5. The device of claim 3, wherein the parallel grasp is used to grasp a small-sized object or an object having a cross-section that comprises two parallel walls.

6. The device of claim 1, wherein the clutch is an electromagnetic clutch or a mechanical clutch.

7. The device of claim 1, further comprising at least one actuator to actuate the movable finger.

8. The device of claim 1, wherein the opening and closing of the clutch is controlled depending on the shape of an object to be grasped.

9. The device of claim 1, wherein the mechanism comprises two movable fingers.

10. The device of claim 1, wherein the device comprises three movable fingers.

11. A method of operating a gripper device according any one of the claims 1 to 10, the method comprising:

opening and closing the clutch to break or connect the kinematic chains of movable fingers of the gripper device, correspondingly, wherein the opening and closing of the clutch switches the movable fingers between an uncoupled mode and a coupled mode ,

wherein, in the uncoupled mode, both proximal phalanx and distal phalanx act as output links of the movable fingers, wherein, in the coupled mode, only distal phalanx acts as output link of the movable fingers.

12. The method of claim 11, wherein the uncoupled mode enables two or more of the movable fingers to perform an encompassing grasp.

13. The method of claim 11, wherein the coupled mode enables two or more of the movable fingers to perform a parallel grasp.

14. The method of claim 12, wherein the encompassing grasp is used to grasp an object which is in the form of a body of rotation or has a substantially circular cross- section.

15. The method of claim 13, wherein the parallel grasp is used to grasp a small-sized object or an object having a cross-section that comprises two parallel walls.

16. The method of claim 11, wherein the opening and closing of the clutch is controlled depending on the shape of an object to be grasped.