Development of Parallel Jaw Robotic Gripper

8/8/2019 Development of Parallel Jaw Robotic Gripper

1/15

Author Name: Nitin middha

Coauthor Name: sreejan

College : Laxmi devi institute of engineering andtechnology, ALWAR(Raj.)

Paper Title: Development of Parallel Jaw RoboticGripper (Research paper)

Paper category: This paper is nominated forcategory mechanical, electrical and electronics

(mechnotronics)


2/15

Development of parallel jaw roboticgripper

Abstract

A robot can perform an assembly task only if it is fitted with a device often

called end- effector. The end effector can be either a tool or grasping

device commonly called a gripper. In line with industrial practice the termgripper is used to mean a grasping device, tool or any other end effector. A

gripper usually has no independent degree of freedom as it is anticipated

that all the degree of freedom are provided by other robot elements.

This paper deals the development of a two fingered parallel jaw

robotic gripper and its applications. It also include a brief explanation on the

scope of improvement. The design of gripper is carried out in two stages:

synthesis and analysis.The synthesis state with the need of definitionof problem. In defining the problem we list the various performance

specifications. in this way we obtained our mechanism having eight links and

ten kinematics pairs making two kinematics chains. for carrying out

dimensional synthesis graphical method is used. Finally analysis alsoincorporated for the synthesized mechanism.

1.Introduction:

The increasing demand for robotic application in dynamic unstructured

environments is motivating the need for dexterous end-effectors,

which can cope with a wide variety of tasks and objects encountered in

these environments. Dexterous robotic devices namely grippers and

hands find many applications in this advancing world. Grasping an

object is a problem that has attracted considerable scientific attentionand consequently, the development of grippers and hands, to replace

human in the industrial environment, has been the focus of the robotic

research community the world over. it is desired to impart the

necessary flexibility to the gripper to hold not only the centred load but

also the off-centred ones which would be possible by incorporating the

capability to vary the orientation of the fingers about the base of


3/15

gripper towards the left or right side, achievable by means of link

through which the single input is given to the fingers of the gripper. We

desire to achieve not only the picking, holding and grasping

capabilities of gripper but also to apply sufficient force to achieve

squeezing action of the fingers on a certain grasped object keeping in

view the force limitation of the robotic gripper which is due to limited

load bearing capacity that depends on the clip holding forces which

remains as the characteristic of the object to be grasped.

The focus of this paper has been

achieve two finger grasp of irregular shaped objects that are

light in weight.

2.Gripper Design:

The requirement of the design was to achieve grasp functions of

picking, holding and squeezing of irregular objects with single

actuation control scheme and simple link arrangement with minimum

complexity, easy manufacturability, control and flexibility .

The idea of design was mooted by simple principal of

second class levers in which the force to be applied on the objectto be grasped is between fulcrum and effort. As per the requirement of

the design a two second class levers were used that were coupledtogether. Here there were two fulcrum F1 & F2. The effort E1 was

applied to lever 1 and the derived effort E2 was obtained at lever 2,

which would wrap the linkages about F1. Hence at least two contacting

surfaces could be obtained if the centre of the gravity of the object to

be grasped was at or below F2. With the use of another set of linkages

or the finger, from closure of objects to be grasped was possible. Since

it has single actuation control scheme, therefore total degrees of

freedom of mechanism was greater than the number of the actuators

and hence the gripper was under actuated, with only one actuators for

closing motion of each finger.


4/15

Fig. 1: Principal of second class lever applied togripper

This mechanism was capable of both pinch and power grasp for the objects

of different sizes. The shapes of the linkages were modified without affecting

the designed scheme adopted so far and the modification is shown below.

Fig. 2: Gripper actuation mechanism


5/15

When performing a grasp, the fingers would progressively envelope the

object to be grasped and eventually reach a static equilibrium. A simple

linkage mechanism was used for converting the linear input motion of the

actuator into wrapping motion of the fingers. If a finger grasped an object,

the actuator would continue its motion, with the other finger continuing to

close around the same with the help of actuation being provided.

The link lengths were determined by dimensional

synthesis. The link 7 was assumed to be 40 mm long. The link 6 was

assumed to be fixed and the link length for the four bar mechanism were

determined using motion generation. The length of the upper portion of the

jaw was determined on the basis of the ratio of the adjacent phalanges of

human finger, which is 1.618 using the length of the link 6 as one of the

phalanges. Then the links of forming slider crank mechanism were

determined using the motion generation with the link 1 fixed.

The mechanical stoppers could be used to prevent excessive

extension of the fingers by blocking the joint, thus preventing improper

behavior of fingers or mechanical failure of the same. Also springs could be

used to keep fingers in extension in case no object was in contact but this

world require actuator to overcome the spring force for grasping objects,

Although it has limitation with regard to the grasp forces that can be applied

and limited weight capacity, it still provide the advantages with regard to

simplicity associated with reduced number of links and joints and its capacity

to achieve picking, grabbing, holding and retaining and squeezing tasksbesides performing common grasps such as cylindrical, spherical and pinch

grasps.

3. Description of various links of the gripper:

Link 1: Base (Frame)

The base is the fixed link in the mechanism to which the binary links for

transmitting single actuating motion to the jaws of gripper are pivoted. It

is rectangular in shape and is of appropriate width to provide anadditional surface making contact with the object to be grasped in form

closure of the same.

Link 2: Input link


6/15

The input link provides the single actuating force to actuate the

mechanism. It has sliding motion on the base, which is transmitted

through a set of links to the jaws of gripper for gripping objects.

Link 3: Connecting rod of the slider crank

mechanism.

It is the link that forms the connecting rod of slider crank mechanism and

is responsible for transforming the sliding motion of the input link into

rotary motion of another link and transmit it to the jaws.

Link 4: Crank of the slider crank mechanism.

It is the link, which is connected at one end to the base and at the other

end to another link which forms the connecting rod of the slider crank

mechanism and imparts rotary motions to the jaws.

Link 5: Upper part of the jaw of the gripper.

The jaws are the main part of the gripper that performs the task of

actually gripping the object to be grasped. The jaws of the gripper have

suitably curved contacting surfaces for better grasp stability, providing

them the capability to achieve the basic grasp functions of picking,

holding and squeezing of the irregularly shaped objects.

Link 6: Lower part of the jaw of gripper.

This link is used for connecting the link 5 to the base of gripper andprovides the contacting surface to the object to be grasped.

Link 7: Connecting the link 5 to the crank of

slider crank mechanism.

This link is used for connecting the jaw of the gripper to the crank ofthe slider crank mechanism and used for transmitting the applied effort at

the input link to the jaw.


7/15

Fig. 3: kinematic linkage for one jaw/finger.

TABLE 1

SHOWING THE LENGTH OF THE LINKS OBTAINED

USING DIMENSIONAL SYNTHESIS.


8/15

4.TORQUE CALCULATIONS-MOTOR SELECTION:

The designed gripper can lift can be used for lifting a maximum load of

2.45N. It was assumed that the shape of the object to be cylindrical for

which the positions of the links are as shown in figure.

For link 5:

Taking moments about point B in the X,Y&Z directions.

Mx= load*perpendicular distance in X-direction

= 2.45*0.055

=.135 Nm.

My= load*perpendicular distance in Y-direction.

=2.45*.039

=.095 Nm.

Mz= load*perpendicular distance in Z-direction.

= 2.45*0

= 0 Nm.


9/15

For link 6:

Taking moment about point c in the X,Y&Z direction.

Mx= load*perpendicular distance in X-direction.

= load*perpendicular distance in X direction.

= 2.45 * .0205

= 0.05 Nm.

My = load*perpendicular distance in Y direction.

= 2.45 * 0.078

= 0.1911 Nm.

Mz = load * perpendicular distance in Z direction.

= 2.45 * 0

= 0 Nm.

For link 7:

Taking moments about point D in the X,Y&Z directions.

Mx = load * perpendicular distance in X-direction.

= 2.45 * 0.0585

= 0.1433 Nm.

My = load * perpendicular distance in Y-direction.

= 2.45 * 0.078

= 0.1911 Nm.

Mz = load * perpendicular distance in Z-direction.

= 2.45 * 0

= 0 Nm.


10/15

Motor Specifications

Torque: 3.2 N-cm.

Speed: 1500 rpm

Voltage: 5V


11/15

Fig.4 Entire Kinematic diagram along with

Motor

5.FABRICATION AND TESTING

The link length of desired dimensions obtained geometrically were cut

from the pieces of the acrylic sheet. Thejaws of gripper were

machined to have curved contacting surfaces for better grasp stability.

Thus the linkages system were arranged for transmitting the applied

effort to the point of the contact of the gripper jaws with the object to

be grasped and achieve the wrap around motion of the gripper jaws


12/15

around the same. Motor is interfaced with L293D H-Bridge controller

and it can run the motor in bi-directional.

The motor is coupled with power screw and it moves the slider back

and forth.


13/15

Fig.6 Testing of gripper by holding the cuboid.

6. CONCLUSION:

The developed robotic gripper can grasp objects of different shapes

reliably and therefore, has a variety of applications as a gripping device in

retrieval of objects of various shapes.

The capabilities of gripper listed below:

1. Can grasp a wide variety of objects with possibly complex shapes.

2. Can adjust the grasp to counter slip of the objects.

3. Has a simple control scheme that is grasp and release.

4. Can perform both pinch and power grasp.

5. Can achieve grabbing, holding, retaining and squeezing tasks.

6. Has minimum complexity, easy manufacturability, control and

flexibility.

7. Is inexpensive, mechanically simple, easy to maintain and light in

weight.

8. Is under actuated with degree of freedom more than number of

actuators.


14/15

9. Has more degrees of freedom as compared to some of other types of

gripper.

The developed robotic gripper can be used for the graspingapplications alone and does not have the capability to manipulate the

objects. The gripper jaws can be provided with friction pads to prevent

expulsion of objects having smooth surfaces during pinch grasp. This

extends the application of gripper to a wider variety of objects sizes.

In order to ensure the grasping of the objects of different shapes and

sizes, springs can be used to keep the jaws in extension when no

object is in contact with them.

However, the actuator has to overcome the force of the springs.

The springs however must be compliant enough to avoid the expulsion

of the object during grasp and they must also be stiff enough to

maintain the extension of the finger when no object is in contact with

the jaws/finger.

References:

1.Sendor, George N. and Erdman, Arthur G., Advanced mechanismdesign: Analysis and Synthesis, Vol 2,prentice Hall inc,1984.

2.Norton, Robert L, Design of Machinery: An Introduction to synthesisand analysis of mechanism and machines, McGraw-hill inc.,1992

3.Hanafusa, H., and Asada,H Stable prehension by a robot hand withelastic fingers, Proc. 7th ISIR, 361-368,1977.

4.Hanafusa, H., and Asada, H A robot hand with elastic figures and its

applications to assembly process, Proc. IFAC symp. on informationand control problems in manufacturing technology, 127-138.,1977.


15/15

Development of Parallel Jaw Robotic Gripper

Documents

Transcript of Development of Parallel Jaw Robotic Gripper