WO2022003727A1

WO2022003727A1 - A compliant mechanism based variable stiffness robotic grasper

Info

Publication number: WO2022003727A1
Application number: PCT/IN2021/050643
Authority: WO
Inventors: Adarsh SOMAYAJI; Asokan Thondiyath; Karthik Chandrasekaran
Original assignee: INDIAN INSTITUTE OF TECHNOLOGY MADRAS (IIT Madras)
Priority date: 2020-07-01
Filing date: 2021-07-01
Publication date: 2022-01-06

Abstract

The present disclosure relates to an apparatus for grasping an object based on structural modulation of a compliant structure. The apparatus (100) includes a pair of grasping jaws (101), a parallelogram based four-bar linkage (110) coupled to the jaws (101). The jaws (101) includes at least two opposing elongated parallelogram shaped envelopes (102), and a set of second parallelogram shaped envelopes (104) disposed within the elongated envelopes (102). A plurality of rotors (106) housed within the envelopes (104) include ribs (108) disposed within the envelopes (104), wherein orientation of the ribs varies stiffness of the grasping jaws (101). The jaws with variable stiffness may be used to perform grasping and manipulation of different types of objects, including rotation and translation along a jaw. A parallelogram-shaped envelope (400) including rotors for modulating stiffness that may be attached to any mechanized or robotic device is also disclosed.

Description

A COMPLIANT MECHANISM BASED VARIABLE STIFFNESS ROBOTIC

GRASPER

CROSS-REFERENCES TO RELATED APPLICATION

[0001] This application takes priority to Indian Provisional Patent Application No. 202041027961 entitled “A Compliant Mechanism based Variable Stiffness Robotic Grasper” fried on July 1, 2020.

FIELD OF INVENTION

[0002] The present disclosure relates to a robotic grasper and in particular to a variable stiffness robotic grasper.

DESCRIPTION OF THE RELATED ART

[0003] Grasping of objects is an essential function of robotic systems and the end effector design is one of the most important aspects of a robotic manipulator, as the capability of the manipulator to execute tasks depends on the features offered by the end effector. Historically, robotic graspers were composed of rigid links and dedicated to performing a particular task. Therefore, traditional industrial robotic graspers have low error tolerance and require precise positioning for grasping objects. However, robots operating in unstructured environments require end effectors equipped with an ability to sense the object characteristics such as deformability, and then accordingly control the force exerted on the object to ensure an optimal grasp.

[0004] The advent of soft robotics has led to the development of novel materials and technologies aimed at building robots with a high degree of compliance. Varying stiffness of soft robotic graspers can lead to new modes of grasping that are more robust and allow the user to vary the amount of force exerted on objects. Current variable stiffness graspers utilize techniques such as structural transformations, granular/layer jamming, shape-memory materials, tendon based stiffening, and usage of different fluids such as electro-rheological and magneto-rheological fluids to achieve variable stiffness. These methods have certain drawbacks such as the requirement of an external power pack for operation for fluid-based systems, slow response for change of stiffness in the case of shape memory materials and low melting point alloys, low range of stiffness variation and continuous power requirements to maintain stiffness for rheological fluids [0005] With advancements in 3D printing, structural morphing of compliant mechanisms to achieve variable stiffness can be a potential solution to resolve some of the drawbacks of existing technologies. A variable stiffness compliant robotic grasper that is simple in design and operation would improve the end effectors used in assistive robotics and prostheses wherein the ability to vary stiffness would benefit in handling a wide variety of objects. This invention uses a compliant mechanism based jaws that is capable of conforming to the surface of an object being grasped while having the ability to vary its stiffness quickly through structural modulation.

SUMMARY OF THE INVENTION

[0006] The present subject matter relates to an apparatus for grasping an object and is capable of conforming to the surface of the object being grasped.

[0007] In one embodiment of the present subject matter, an apparatus for grasping an object that is capable of conforming to the surface of the object being grasped is disclosed. The apparatus includes a pair of grasping jaws, a parallelogram based four- bar linkage coupled to the pair of the grasping jaws and a gear arrangement connected to the pair of grasping jaws adapted to open and close the grasping jaws. The pair of grasping jaws includes at least two opposing elongated parallelogram shaped envelopes forming the jaws, a set of second parallelogram shaped envelopes disposed within the at least two opposing elongated parallelogram shaped envelopes and a plurality of rotors housed within the set of second parallelogram shaped envelopes. Further, the plurality of rotors includes two or more ribs disposed within the plurality of rotors wherein, the relative orientation of the rib with respect to the envelopes varies the stiffness of the grasping jaws. In some embodiments, the apparatus further includes a first motor for operating the pair of grasping jaws. The gear arrangement transmits torque from the first motor to the grasping jaws for opening and closing the grasping jaws.

[0008] In some embodiments, the ribs disposed within the rotors are parallel to the walls of the second parallelogram shaped envelopes to achieve minimal stiffness of the grasping jaws. The ribs disposed within the rotors are positioned along a minor diagonal of the second parallelogram shaped envelopes to achieve maximum stiffness of the grasping jaws. The rotors can be changed dynamically in the course of grasping the object to vary the stiffness of the pair of grasping jaws.

[0009] In various embodiments, the plurality of rotors are connected by a plurality of flexible transmission band such that consecutive rotors are connected by the flexible transmission band on alternate sides. The plurality of rotors are transmission band powered by a second motor. Further, each of the plurality of rotors are individually powered with dedicated motors to vary stiffness along the length of the grasping jaw and induce localized stiffness modulation at any part of the grasping jaw through actuation of the dedicated motors.

[0010] In another embodiment, an apparatus is configured to modulate stiffness of a surface 401 of a robotic device. The apparatus includes an elongated parallelogram shaped envelopes placed on the surface and configured to modulate stiffness perpendicular to the elongated side thereof, a second set of a parallelogram shaped envelopes disposed within the elongated parallelogram shaped envelopes and a plurality of rotors housed within the second set of parallelogram shaped envelopes such that the rotors are configured to be rotatable to vary a stiffness of the robotic device.

[0011] In some embodiments, the rotors are interconnected using trasnmisson bands and multiple rotos configured to be rotated cooperatively. The rotor are configured to be rotatable by individual motors.

[0012] In various embodiments, the apparatus includes one or more proximity sensors adapted to detect a potential contact and wherein the rotors are configured to be rotated to present a surface of reduced stiffness in response to sensor inputs corresponding to postential contact in a collaborative environment.

[0013] This and other aspects are described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The invention has other advantages and features, which will be more readily apparent from the following detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings, in which:

[0015] FIG. 1 illustrates the apparatus, according to an embodiment of the present subject matter.

[0016] FIG. 2Aand 2B illustrate the grasping jaws at their minimum and maximum stiffness position respectively, according to an embodiment of the present subject matter.

[0017] FIG. 3 A denotes reference axes considered for deformation of the rotors

[0018] FIG. 3B to 3D illustrate different configurations of the ribs to vary the stiffness of the grasping jaws.

[0019] FIG. 4 illustrates the actuation mechanism for varying rotor orientation.

[0020] FIG. 5A and 5B illustrate an object held on the outside surface of the jaws and the inside surface of the jaws.

[0021] FIG. 6 illustrates, the apparatus to modulate stiffness of a surface 401 of a robotic device, according to an embodiment of the present subject matter.

[0022] FIG. 7A and 7B illustrate the two configurations of a truss structure as stiff (7A) and collapsible (7B).

[0023] FIG. 8 A and 8B illustrate the variation of jaw dimensions with number and diameter of rotors and q

[0024] FIG. 9 illustrates the variation of jaw displacements with Q₁ for a fixed force input [0025] FIG. 10 illustrates the force-displacement curves for the jaw at maximum and minimum stiffness

[0026] FIG. 11 illustrates the force-displacement data at maximum and minimum stiffness

[0027] FIG. 12 illustrates the variation of jaw stiffness vs. rotor orientation q₂ [0028] FIG. 13A, 13B, 13C and 13D illustrate the apparatus handling a rigid and a soft object (13A) Minimum stiffness: screwdriver (13B) Maximum stiffness: screwdriver (13C) Minimum stiffness: polyester foam (13D) Maximum stiffness: polyester foam. [0029] Referring to the drawings, like numbers indicate like parts throughout the various views.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0030] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from its scope.

[0031] Throughout the specification and claims, the following terms take the meanings explicitly associated herein unless the context clearly dictates otherwise. The meaning of “a”, “an”, and “the” include plural references. The meaning of “in” includes “in” and “on.” Referring to the drawings, like numbers indicate like parts throughout the views. Additionally, a reference to the singular includes a reference to the plural unless otherwise stated or inconsistent with the disclosure herein.

[0032] The present subject matter describes an apparatus for grasping an object based on a compliant mechanism such that the apparatus is capable of conforming to the surface of the object being grasped.. The apparatus has variable stiffness jaws whose stiffness may be changed through structural modulation for grasping a wide range of objects including sensitive objects like fruits, vegetables, etc. The stiffness of the grasper jaws may be changed from stiff to soft and vice-versa by structural modulation through a mechanical input.

[0033] In one embodiment, an apparatus 100 for grasping an object is disclosed as illustrated in FIG. 1. The apparatus 100 includes a pair of grasping jaws 101 that are of variable stiffness is shown in FIG. 2A and 2B. Each of the grasping jaws 101 is connected to a parallelogram-based four-bar linkage 110. The pair of grasping jaws 101 includes at least two opposing elongated parallelogram shaped envelopes 102 forming the jaws 101, and a set of second parallelogram shaped envelopes 104 disposed within the at least two opposing elongated parallelogram shaped envelopes 102. A plurality of rotors 106 is housed within the set of second parallelogram shaped envelopes 104. The plurality of rotors 106 includes two or more ribs 108 disposed within the plurality of rotors 106 so as to modify compliance of the rotors depending on orientation of the ribs. A gear arrangement 112 is connected to the grasping jaws 101 to open and close the grasping jaws 101. The apparatus 100 further includes a first motor 114 for operating the pair of grasping jaws 101. The gear arrangement 112 is connected to the first motor 114 and facilitates in transmitting the torque generated by the first motor 114 to the pair of grasping jaws 101 for opening and closing the grasping jaws 101.

[0034] Rotating the rotors 106 changes the relative orientation of the ribs 108 with respect to the envelopes 104. The placement of the ribs 108 is configured to vary the stiffness of the grasping jaws 101. The ribs 108 disposed within the rotors 106 are parallel to the set of a parallelogram shaped envelopes 104 disposed within the at least two opposing elongated parallelogram shaped envelopes 102 to achieve minimal stiffness of the grasping jaws 101. Maximum stiff of the grasping jaws 101 is achieved when the ribs 108 disposed within the rotor 106 are positioned along a minor diagonal Further, the rotors 106 can be changed dynamically in the course of grasping an object to vary the stiffness of the pair of grasping jaws 101 by rotating the rotor 106.

[0035] The variation in the stiffness of the grasping jaws 101 is achieved by changing the orientation of the rotors 106 as illustrated in FIG. 2A and 2B. The orientation of rotors 106 is changed by changing the relative orientation of the ribs 108 disposed within the rotors 106 with respect to the second parallelogram-shaped envelopes 104. The parallelogram-shaped envelopes 104 are configured to collapse along its diagonals whenever an external load is applied to it i.e. when an object is being grasped by the jaws 101, therefore the orientation of the ribs 108 with respect to the second parallelogram shaped envelopes 104 is essential to obtaining desired stiffness of the grasping jaws 101. The orientation 0₂ of the rotor 106 with respect to the set of second parallelogram-shaped envelopes 104 indicates position of the rotor when the grasping jaws are in operation. The orientation 0₂ of the rotor 106 may also be dynamically changed even during grasping of an object to vary the stiffness of the grasping jaws 101, while qi indicates the orientation of the set of second parallegram-shaped envelopes 104 with respect to the bottom edge 204 of the opposing elongated parallelogram-shaped envelopes 102 when the grasping jaws 101 are in operation. For obtaining minimum stiffness of grasping jaws 101, the ribs 108 are set parallel to the second parallelogram-shaped envelopes 104 allowing the envelopes 104 to collapse resulting in minimum stiffness of the jaws 101. For obtaining maximum stiffness of grasping jaws 101, the orientation of the ribs 108 is set at an orientation as shown in FIG. 2B with respect to the second parallelogram shaped envelopes 104, this prevents the collapse of the envelopes 104 leading to maximum stiffness of the grasping jaws 101. Changing the orientation of the ribs 108 between the aforementioned extremities facilitates in obtaining variation in the stiffness of grasping jaws 101 to different degrees.

[0036] FIG. 3A denotes reference axes considered for deformation of the rotors and FIG. 3B to 3D illustrate different emodiments where the ribs can be positioned in various configurations to with 2 or 3 ribs. FIG. 3B shows the orientation of diametrically positioned ribs 108 disposed in the rotor 106. FIG. 3C shows an embodiment where the ribs 108-1 and 108-2 are positioned slightly off the diameter. FIG. 3D illustrates and embodiment where 3 ribs are used instead of two. In various embodiments, the ribs may be made of the same material or different materials so as to obtain different profiles of stiffness variation. In some embodiments, the thickness of the ribs may be varied to modulate stiffness.

[0037] In some embodiments, the plurality of rotors 106 are connected by a plurality of flexible transmission bands 306 such that consecutive rotors 106 are connected by the flexible transmission bands on alternate sides as illustrated in FIG. 4. The opening and closing of the grasping jaws 101 is effected by the motor 114 which acts as a primary actuator for the grasping jaws 101. The grasping jaws 101 receives the torque generated by the first motor 114 through a gear arrangement 112 comprising of spur gears for transmitting the torque from the first motor 114 to the grasping jaws 101. The plurality of rotors 106 are transmission bands 306 powered by a second motor 304. Further, in some embodiments each of the plurality of rotors 106 are individually powered with dedicated motors to vary stiffness along the length of the grasping jaw 101 and induce localized stiffness modulation at any part of the grasping jaw 101 through actuation of the dedicated motors. Localized stiffness modulation of rotors 106 along the grasping jaws 101 may in some embodiments facilitate movement of the object grasped along the grasping jaw 101. In some embodiments, the modulation in stiffness of the adjacent rotors 106 may produce rotation of the grasped object. In some embodiments, the object may be simultaneously translated along the jaws as well as rotated. Further, the orientation of the plurality of rotors 106 is varied based on mechanical actuation of the second motor 304 such that the stiffness of the pair of grasping jaws 101 is varied without an external power pack. Further, power requirements for rotating the rotors is minmal.

[0038] In various embodiments of the invention, the apparatus 100 may perform sensor less grasping force estimation by measuring the torque applied by the first motor 114 and the deformation of the object that is being grasped. For estimating the grasping force for the embodiment where the rotors have diametricallly oriented ribs (FIG. 3B), the stiffness of the grasping jaws 101 and the torques exerted by the first motor 114 are obtained. A closed form equation of the stiffness of the grasping jaws 101 as a function of the rotor 106 orientation is given by equation 1 and the torque that is exerted by the first motor 114 when grasping an object may be obtained from the motor current drawn.

0.3158 _* jaw displacement, for q₂ = 60°| a)

0.5 _* jaw displacement, for q₂ = 120° j

In alternative embodiments, the the grasping jaws 101 may be configured to measure the force exerted on the object grasped by means of force sensors or other force measuring instruments /devices. [0039] In various embodiments, the variation of the rotor 106 orientation is achieved by utilizing flexible transmission band 306 for actuating the plurality of rotors 106, as illustrated in FIG. 4. Each of the plurality of rotors 106 is connected with the flexible transmission band 306 such that the band 306 forms an integral part of the rotors 106, thereby facilitating rapid change in stiffness of the jaws 101. The flexible transmission band 306 between consecutive rotors 106 are alternated on both sides of a grasping jaw 101 as each rotor 106 can only carry one pair of flexible bands 306 on either side. Usage of flexible transmission bands 306 ensures minimal impact on the stiffness of the jaws 101 when changing the orientation of the plurality of rotors 106. Further, the flexible transmission band 306 also allows collapsing of the plurality of rotors that would facilitate form closure.

[0040] In various embodiments, the grasping jaws 101 may be utilized for external as well as internal grasping of objects while providing stiffness modulation on both the faces of the grasping jaws 101 as illustrated in Fig. 5A and 5B. The stiffness of the jaws grasping 101 may be modulated in both modes for obtaining a force or form closure. As a closed form equation is utilized for providing the stiffness of the grasping jaws 101 as a function of rotor angle, the stiffness of an object may be estimated using the orientation of the rotors 106 and applying known torques. Further, the grasping jaws 101 may be configured to probe the environment as well as changing the jaw stiffness in the process of grasping an object.Additionally, the grasping jaws 101 retains the stiffness attained when grasping an object without additional actuation until the stiffness is varied by changing the orientation of the rotors 106.

[0041] In various embodiments, the apparatus 100 may be used in a number of different ways, as already illustrated, to grasp and manipulate objects using the variability of stiffness while holding. The jaws may be used to grasp an object while applying high stiffness, so as to compress the object, such as a sponge, to improve grip. The jaws may in some embodiments be used to form-grip a delicate object by reducing the stiffness of the rotors 106 in the center of the jaws while the boundaries may be held relatively stiffer to provide gentle but enveloping grasp. In some embodiments, the ability to vary stiffness of adjacent rotors 106 may be used to move an object along the jaw. In some embodiments, the alternate stiffening and softening of adjacent rotors may be used to cause rotation of the object being manipulated. In some embodiments, the translation and rotation may be simultaneously performed.

[0042] In another embodiment, an apparatus 400 that is configured to modulate stiffnees of an object having a surface 420 of a mechanized or robotic device is disclosed as illustrated in FIG. 6. The object may be a robotic arm interacting with humans in a collaborative environment, for example. The apparatus 400 includes elongated parallelogram shaped envelopes 402 that are placed on a surface 420 and configured to module the stiffness perpendicular to the elongated side. The apparatus 400 further includes a second of parallelogram shaped envelopes 404 that are disposed within the elongated envelopes 402 and a plurality of rotors 406 that are housed within the second envelopes 404. The boundaries of the elongated envelopes 402 are represented by 411 in FIG. 6. The envelopes may have an optional skirt 412 to exclude inward curvature of the envelopes 402.The plurality of the rotors 406 are configured to be rotatable so as to vary the stiffness of the surface 420. Further, the rotors 404 may in some embodiments be interconnected by transmission bands so that multiple motors are configured to be rotated cooperatively. The rotors 406 in some embodiments may be rotated by individual motors. On rotation of the rotors 406, changing the relative orientation of the ribs 408 with respect to the second envelopes 402 alters the stiffness of the apparatus 400. The apparatus 400 may be scaled to cover surface of any size and to cover a required stiffness range.

[0043] In various embodiments, the apparatus 400 includes one or more proximity sensors that are adapted to detect a potential contact. The rotors 406 may be configured such that they may be rotated to present a surface of reduced stiffness based on the

-li sensor inputs corresponding to potential contact in a collaborative environment. Here, the proximity sensors detect contact with a human who maybe in the vicinity of the apparatus 400. The proximity sensors maybe connected to the one or more motors which facilitates in altering the orientation of the rotors 406 to vary the stiffness of the apparatus 400. Alternatively, the apparatus 400 may utilize proximity sensors to detect the potential contact.

[0044] The inventions disclosed herein have many advantages and applications in different fields, as set forth herein. An important advantage of the apparatus is that the parts may be easily manufactured by 3D printing or injection molding. Further, use of a minimal number of actuators facilitates quick assembly of the grasping jaws. The apparatus can provide quick and easy variation of jaw stiffness through a simple mechanical input that facilitates dexterous manipulation of a wide variety of objects. Since the apparatus utilizes only electricity for its operation, it does not require additional hydraulic or a pneumatic power to vary its stiffness.

[0045] The invention finds application in several fields including food harvesting, material handling, minimally invasive surgery, etc. The apparatus is configured to provide functionality with minimal power consumption, as both the jaws and the rotors may be powered by a single battery. The apparatus disclosed may be scaled up or miniaturized based on the application. Further, the apparatus may be used to perform either force closure or form closure of the jaws as required. Force closure ensures precise handling of objects, however, it would not be useful when handling fragile or small objects that may slip out of the grasp. Form closure ensures a non-slip grasp for small or fragile objects such as glassware, fruits, pen, screwdriver, etc. Further, the apparatus 100 or 400 may be used as end-effector in assistive robotic arms and prostheses having a self- contained unit for compact construction and usage. Additionally, the number of grasping jaws 101 in the apparatus 100 may be increased from two to three or more based the field of application for the apparatus. EXAMPLES

Example 1: Structural modulation of jaws

[0046] The stiffness of the grasping jaws 101 was modeled based on the principle of stability of Truss structures. Equations (2) and (3) show the parameters of the Maxwell Truss Equation for the truss structure shown in FIG. 7A. m + r = 20 (2)

2j = 20 (3)

From Equations (2) and (3), m + r = 2j (4)

Where m represents the number of members, j represents the number of joints, and r represents the number of unknown support reactions. Therefore, the structure in FIG. 7A is stable and determinate. Equations (5) and (6) show the parameters of the Maxwell Truss Equation for the truss structure shown in FIG. 7B. m + r = 27 (5)

2; = 34 (6)

From Equations. (5) and (6), m + r < 2j (7)

Therefore, the structure in Fig. 7B is an unstable structure.

From FIG. 7A and 7B, it can be seen that the change in the orientation of members 2, 4, 6 and 8, results in the Truss structure changing from stiff to collapsible.

Example 2: Analysis of Jaws

[0047] To define the grasping jaw 101, the number of rotors 106, length of grasping surface, and the angle Q₁ of walls of the enclosures of the jaw frame needs to be determined. The length F of the grasping surface of the jaw was taken to be 80 mm as illustrated in FIG. 8A and 8B. This jaw length 80 mm is a standard size used in industry for sorting and fruit picking applications. Geometric analysis was carried out to determine the ideal number of rotors 106 for the jaw. As illustrated in FIG. 8A, varying the number of rotors with corresponding diameters 11, 12 and 13 increased the length of the unsupported frame segments (denoted by thick red lines in Fig. 8 A and 8B), and denoted as ul, u2, u3. Conversely, having more rotors 106 with smaller diameters shortened the unsupported segments. These unsupported segments are sources for localized deformation which cannot be controlled by varying the rotor angle q₂. Increasing the number of rotors 106 reduces the unsupported lengths, but also reduces the diameter of the rotors 106. This makes the rotors 106 stiff as the rotor diameter is inversely proportional to its number and therefore, reduces the effective compliance of the jaw 101. Hence, the number of rotors 106 was chosen to be four as it is an optimal number that results in minimal unsupported length while also providing adequate control of the jaw stiffness. The stiffness of the jaw can be varied by changing Q₁ and/or by changing q₂. Of the two, the parameter Q₁ was fixed and cannot be modified once the jaw frame was manufactured. As illustrated in FIG. 8B, the rotor diameter varies to compensate for different q₁. since the length of the grasping surface of the jaw and the number of rotors is fixed.

[0048] A distributed force of 2.5 N was applied over a rectangular area of 15 A 10 mm² (where 10 mm is the thickness of the jaw) at the tip of the jaw. Contact interactions are defined between the outer surface of the rotor and the inner surface of the jaw frame with a coefficient of friction m = 0.3. For a given q₁, it is possible to define the two orientations of the rotor (q₂) that represent the theoretical maximum and minimum stiffness of the jaw by Equations. (8) and (9) respectively.

For maximum stiffness, q₂ = p — Q₁ (8)

For minimum stiffness, q₂ = Q₁ (9) [0049] Multiple non-linear static FEA simulation studies were performed for 9₁ values ranging from 30° to 70° in steps of 5°. For every 9₁, the FEA was performed at the minimum and maximum stiffness positions of the rotor. The components of the grasper jaw assembly are meshed using tetrahedral elements. The results of the FEA simulation are illustrated in Fig. 9. It can be observed that the limits of jaw stiffness vary for different values of 9₁. Therefore, a suitable 9₁ can be selected based on the required limits of stiffness for an application.

[0050] For further analysis of the jaw behavior, 9 = 60° was chosen as it provides the minimal unsupported length of the grasper segments while also offering sufficient control on jaw stiffness. The force-displacement relationships of the jaw with q = 60°, obtained under the maximum and minimum stiffness scenarios, are illustrated in FIG. 10. It can be observed that while the jaw shows a linear response to the applied force when it is at the minimum stiffness position, it shows a gradual increase in stiffness in the maximum stiffness position.

[0051] A jaw with 9₁ = 60° was 3D printed on a standard FFF printer and multiple trials were performed with the applied force (F) increased from 0 N to 5 N, while the position of the jaw was captured at 0.2 N intervals. The mean readings of the jaw displacements for the applied forces are illustrated in Fig. 11. The rotor orientation q₂ was varied from 0° to 180° in steps of 15° and the force -displacement characteristics were recorded for forces of 2N, 3N and 4N. As the rotor is symmetric about its diametric member, there is a cyclic variation of jaw stiffness with a frequency of p. The variation of jaw stiffness with the rotor orientation q₂ is shown in FIG. 12. It can be seen that the stiffness of the jaw can be increased by approximately twice its minimum value in this example.

Example 3: Modes of grasping an object

[0052] The grasping jaws can clasp a multitude of objects in compliant and rigid modes as illustrated in FIG. 13A to 13D. FIGs. 13A and 13B illustrate the jaws grasping a rigid object (screwdriver) in compliant and rigid modes respectively. In the compliant mode, the jaw conforms to the contour of the screwdriver handle using form closure while in the rigid mode the screwdriver was grasped with force closure. FIG. 13C and 13D illustrates the jaws grasping a soft object (polyester foam) in compliant and rigid modes respectively. In compliant mode, the jaw’s force output was force-limiting. It was also observed that the foam tends to slip out of the grasping jaw’s hold if excess force is exerted when grasping in compliant grasping mode. This mode is suitable for handling very soft delicate objects such as fruits without damaging them. However, objects such as polyester foam can be grasped in the rigid mode. In this case, the grasping jaws were not force-limited and could apply a larger force and compress the foam as illustrated in FIG. 13D.

[0053] While the invention has been disclosed with reference to certain embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material the teachings of the invention without departing from its scope.

Claims

WE CLAIM:

1. An apparatus (100) for grasping an object and is capable of conforming to the surface of the object being grasped, the apparatus (100) comprising: a pair of grasping jaws (101) of variable stiffness, the grasping jaws (101) comprising: at least two opposing elongated parallelogram shaped envelopes (102) forming the jaws (101); a set of second parallelogram shaped envelopes (104) disposed within the elongated parallelogram shaped envelopes (102); and a plurality of rotors (106) housed within the second set of parallelogram shaped envelopes (102); a parallelogram based four-bar linkage (110) coupled to the pair of grasping jaws (101); and a gear arrangement (112) connected to the pair of grasping jaws (101) adapted to open and close the grasping jaws (101).

2. The apparatus (100) as claimed in claim 1, wherein the plurality of rotors (106) comprises two or more ribs (108) disposed within the plurality of rotors (106) wherein, changing the relative orientation of the ribs (108) with respect to the envelopes (104) is configured to vary the stiffness of the grasping jaws (101).

3. The apparatus (100) as claimed in claim 1, wherein the apparatus further comprises a first motor (114) for opening or closing the pair of grasping jaws (101).

4. The apparatus (100) as claimed in claim 1, wherein the gear arrangement (112) transmits torque from the first motor (114) to the grasping jaws (101) for opening and closing the grasping jaws (101).

5. The apparatus (100) as claimed in claim 2, wherein the ribs (108) disposed within the rotors (106) are parallel to walls of the second parallelogram shaped envelopes (104) to achieve minimal stiffness of the grasping jaws (101).

6. The apparatus (100) as claimed in claim 2, wherein the ribs (108) disposed within the rotors (106) are positioned along a minor diagonal of the second parallelogram shaped envelopes (104) to achieve maximum stiffness of the grasping jaws (lOl).

7. The apparatus (100) as claimed in claim 5, wherein the plurality of rotors (106) are connected by a plurality of flexible transmission bands (306) such that consecutive rotors (106) are connected by the flexible transmission bands (306) on alternate sides.

8. The apparatus (100) as claimed in claim 7, wherein the rotors (106) are rotatable dynamically in the course of grasping the object to vary the stiffness of the grasping jaws (lOl).

9. The apparatus (100) as claimed in claim 8, wherein the plurality of rotors (106) are powered by a second single motor (304).

10. The apparatus (100) as claimed in claim 1, wherein each of the plurality of rotors (106) are individually powered by dedicated motors to vary stiffness along the length of the grasping jaws and induce localized stiffness modulation at any part of the grasping jaw (101) through actuation of the dedicated motors.

11. The apparatus (100) as claimed in claim 10, wherein, localized stiffness modulation of rotors along the grasping jaws (101) is configured to faciliate movement of the object grasped along the grasping jaw, or rotation of the object, or a combination thereof.

12. The apparatus (100) as claimed in claim 1, wherein the at least two opposing elongated parallelogram shaped envelopes (102), the second envelopes (104), the plurality of rotors (106) and the plurality of ribs (108) are composed of thermoplastic polyurethane (TPU) or nylon or other flexible material.

13. The apparatus (100) as claimed in claim 1, wherein the grasping jaws (101) are configured to provide sensorless grasping force estimation by measurement of the torque applied by the first motor (114) and the deformation of the object grasped.

14. An apparatus 400 configured to modulate stiffness of a surface 420 of a robotic device, comprising: elongated parallelogram shaped envelopes (402) placed on the surface (420) and configured to modulate stiffness perpendicular to the elongated side thereof; a second set of parallelogram shaped envelopes (404) disposed within the elongated envelopes (402); and a plurality of rotors (406) housed within the second envelopes (402), wherein the rotors (406) are configured to be rotatable to vary a stiffness of the robotic device.

15. The apparatus of claim 14, wherein the rotors are interconnected using transmission bands and multiple rotors are configured to be rotated cooperatively.

16. The apparatus of claim 14, wherein the rotors are configured to be rotatable by individual motors.

17. The apparatus of claim 14, comprising one or more proximity sensors adapted to detect a potential contact and wherein the rotors are configured to be rotated to present a surface of reduced stiffness in response to sensor inputs corresponding to potential contact in a collaborative environment.