International Journal on Mechanical Engineering and Robotics (IJMER)
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Design of Three Fingered Robot-Gripper Mechanism
1Krishnaraju A, 2Ramkumar R, 3Lenin V R
1,2,3Mechanical Engineering, Mahendra Engineering College, Namakkal, India – 637503, Email: 1[email protected], 2[email protected], 3[email protected]
Abstract— The aim of this paper is to study the challenges and to design a three fingered robot mechanism which has the potential to fulfill various demand in industry and factories. So far there are so many mechanisms available for robot gripper in three fingered robot gripper mechanism is a type of mechanism which is used in industrial robots for moving object, which has higher gripper ratio. The kinematic system has been designed for one degree of freedom and the kinematic design of robot structure is developed using SAM mechanism soft ware. The gripper modeling has been designed using Pro-E Wildfire5.0 software and a three finger gripper is fabricated by aluminium material for 5 kg payload. The gripper mechanism has three fingers which are used to hold the object in a balanced way to meet the challenges faced on the industrial life. The fingers are also provided with senses to identify the type of object.
Index Terms—Challenges, Gripper, Kinematic design, Robot mechanism.
I. INTRODUCTION
Robotics is a hodgepodge of geometric transforms, control theory, stepper / DC motors, digital signal processing and real time operating system. Robot is a reprogrammable and multifunction manipulator designed to move materials / work pieces from place to another place through various programmed motions for the performance of a variety of tasks and the robot may also be classified according to its method of applications like control pathway, operating parameters, environmental conditions, structural design, materials of the structure, level of technology. The two major types of control are servo and non-servo. Robot path way may be either continuous or point-to-point. The workspace volume of a robot may be rectangular, cylindrical, spherical, or jointed spherical.
Human labors have always been associated with the acquisition of specific skills, talents & methods and experience. Increasing competition from industrial robots for tasks normally carried out by human hands has led to the need for more effective handling equipment, especially pretension tools (more commonly called Grippers).
Finally, a robot may be classified as low or medium or high-tech based on its number of axes and its level of sophistication in respect of end effectors and grippers.
The gripper is similar to the human hand just as the hand grasps the tool to perform the work the gripper grasps and secures the robot’s work piece while the operation is being performed.
This is one of the reasons why grippers deserve special attention. However, industrial robots are not simply substitute for people often in applications beyond the normal ability (physical or temporal) of conventional manpower.
II. LITERATURE SURVEY
Majid Tolouei-Rad et al [1] described that industrial robots become useless without end-effectors for many instances are in the form of friction grippers. Usually friction grippers concern frictional forces to different objects on the basis of programmers’ experience. This puts a limitation on the effectiveness of gripping force that may result in unclamping of damaging an object. It described the various stages of design and development of a low cost sensor-based robotic gripper that would facilitate the task of applying right gripping forces to dissimilar objects. The gripper is also set with range sensors in order to avoid collisions of the gripper with the objects handled. Gripper is entirely well designed automatic pick and place gripper which can be used in many industrial applications like all automobile industries. This may be induced for further altered or developed in order to suit a better number of industrial activities.
Redwan Alqasemi et al [2] designed and constructed a new robotic gripper for Activities of Daily Living (ADL) which is used with a new wheelchair mounted with a robotic arm and some of the sensible sensors developed at University of South Florida. This kind of new gripper made it as unique by two aspects. The first is the design of the paddles, and the next is the design of the actuation mechanism that produces parallel motion for effective gripping. The designed paddle is to grasp a wide variety objects with different shapes and sizes that are used in
everyday life. The driving mechanism was designed with light-weight, effective, safe, self content and self-regulating of the robotic arm attached to it.
Marinus Maris [3] proposed a method for visual attention selection in mobile robots based on amplification of the selected stimulus. Attention processing is performed on the vision sensors which is integrated on a silicon chip and consists of a contrast sensitive retina with the ability to change the local inhibitory strength between adjacent pixel elements. The sensitivity to visual contrast at a particular region as a result the retina can be adjusted. As the local inhibitory strength can be regulated from outside of the chip, a reconfigurable sensor is realized. This
―attention-retina‖ was tested on an autonomous robot which was given the task of selecting a line to follow while there were two alternatives.
Y. Gene Liao [4] developed a straight line pick and place motions are used for transferring work pieces into & out of an assembly or machine and are most advantageous in such application because of the accuracy of positioning the work pieces at the beginning and end of travel. The linear path or straight-line motion also exhibits great potential for reducing manufacturing cycle time. He also describes the design and analysis of a robotic end-effectors that is capable of grasping objects of varying sizes and the center point of the end-effectors remains as close as possible to the same position, i.e. a straight line, over the range of pick and place movement.
Selected shape and size of the grasped object ranges from 50 mm to 300 mm in cylindrical diameter. Preferred clamping force is 625 N per jaw when the gripper is at its maximum open position and a maximum lift mass is 70 Kg. The analyzed force applied on clamping, as the result the accuracy of motion trajectory, and stress of the end-effectors were analized.
Krishnaraju A et al [5] described about the opportunities and challenges of reconfigurable robot; which have the potential to lead to various applications. The Robot challenges include analyzing, planning & control, environment and locomotion. Opportunities of robot include in various applications such as military, monitoring, and disaster relief, space, search, find and rescue, transportation, medical, fire fighting, and various commercial applications. Also reviewed the basic robot structure namely lattice, mobile and chain with their degree of freedom.
Jay Lee [6] focused on the lead to creation or invention of new robot hand/gripper mechanisms according to the separation of structures and functions, a kinematic structure reference table of industrial robot/gripper mechanism should be generated. Over hundreds of different industrial robot hand and/or gripper mechanisms have been investigated. By using kinematic structure (graph) theory, a creative design approach has been presented for designing robot hand and/or gripper
mechanism. A design database system with knowledge may also be created through the generated kinematic structure table of robot hand and/or gripper mechanisms.
The most difficult phase of mechanical design is the conceptual phase. An expert system may be created by use of the generated kinematic structure (Design Database) .This knowledge based system contains general information about robot hand and/or mechanisms, kinematic structures, functions, types of drive mechanisms and applications. The designer enters the design requirements, the master will select the appropriate mechanisms and graphs from design database according to the drive mechanisms and its functions, with suitable mechanisms may be then evaluated in depth until a final design is achieved.
Jyh-Jone Lee [7] described about the finger-type gripping mechanisms have been widely used for the industrial robots. He also explained that the design equations are transcendental and the optimization-based numerical technique is applied to solve the design equations. To discuss the kinematic states of the mechanism, the synthesized solutions are illustrated. The path synthesis of a gripper-object mechanism with two rolling pairs is studied. Numerous researchers have been well investigated about the kinematic and force analyses of such mechanisms.
Ho Choi [8] has analyzed and tested a flexible gripper with use of compliant materials (i.e., rubber) with pneumatic inflation. They investigate the effects of process and design parameters, such as pressure, friction, rubber material, initial jaw displacement and parametric Finite Element analyses were conducted. A simple, single rubber pocketed flexible gripper was designed and built based on the FEA results. To demonstrate and obtain an overall understanding about the capability and limitations of the gripper feasibility experiments were performed. It was found that objects with different shapes like cylindrical, prismatic, which weigh from 50 grams to 20 kilograms, and types (egg, steel hemi spheres, wax cylinders) may be picked and placed without any loss of control of the object.
R. Saravanan et al [9] presented to obtain optimum geometrical dimensions of robot gripper and their intelligent technique has been used. The non-linear, complex, multi-constraint and multi-criterion are problems considered for optimization for three robot gripper mechanism configuration. To find the Pareto optimization front for a problem, five objective functions, nine constraints and seven variables are taken into consideration. These problems are divided into segmental small cases. Intelligent optimization algorithms namely Multi-Objective Genetic Algorithm (MOGA), Elitist Non-dominated Sorting Genetic Algorithm (NSGA-II) and Multi-Objective Differential Evolution (MODE) are proposed to solve the problem. Two multi-objective
performance measures (Solution Spread Measure (SSM) and Ratio of Non Dominated Individuals (RNIs)) are used to evaluate the strength of the Pareto optimal fronts.
Optimizer Overhead (OO) and algorithm effort measures the multi-objective performance which is used to find the computational effort of MOGA, NSGA-II and MODE algorithms. Normalized weighting objective functions method is used to select the best optimal solution from Pareto optimal front.
M. Aslam et al [10] designed a miniature Smart Robotic Foot (SRF), equipped with an integrated vacuum pump, a suction cup, a pressure sensor and a micro-valve which is also fabricated and tested. A mock up robot equipped with the SRF was used to study the load-carrying potential of the system. The SRF weighs 35 g with a 40mm suction cup, support weights in the range of 1.2–3.5 kg under various test conditions and surfaces.
A. Brunete et al [11] described about small cavities, generally in construction it is sometimes necessary to perform inspection and maintenance tasks of small cavities, such as pipes (gas or water) or floor and upper limit sections. It is not possible sometimes to access (either in person or with a tool) to these places. Few of the commercial robots that exist nowadays are not able to performance due to its weight, larger diameter or its limited locomotion gaits. The concepts used in heterogeneous multi-configurable chained micro-robots for pipe inspection are expanded to small cavities. New locomotion gaits to access these small places are proposed, based on the concepts of modularity, multi configurability, multi-drives and heterogeneity locomotion. These locomotion gaits are specifically designed for heterogeneous modular robots which combine snake-like structure, inchworm and helicoidal driven patterns
Fengfeng Xi et al [12] constructed a re-configurable parallel robot with two base tripods. The first tripod is called as slide tripod which is made of three prismatic joints with the fixed-leg length. The next tripod is called as swing tripod which is made of three revolute joints.
This parallel robot can be re-configured from 6 DOFs to 5 or 4 or 3 DOFs by detaching one or two or three branches of the swing tripod separately, while the slide tripod is fixed to the moving platform; the branches of the swing tripod are designed to be detachable from the moving platform in addition, the detached branches can be used to perform collaborative work with the parallel robot. Based on this model a unified method is developed to solve the constraint equations. Further a unified method is may be used to solve the inverse kinematics considering varying configurations including three, four, five and six DOFs.
Minzhou luo et al [13] proposed few design considerations for improving grasping capabilities of a low-cost easy-operation three-finger robotic gripper. By a proper mechanism design, a special planetary gear
mechanism has been designed to adjust the position and orientation of two fingers during the assembly of hand gripper. This significantly improves the flexibility of a robotic hand in terms of sizes and shapes of objects which can be grasped. Both pinching and enveloping grasp configurations can be achieved similarly to human hands.
The proposed mechanism design both in terms of kinematic and dynamic performance may effective simulated by Adams environment
AbdeI-Malek K et al [14] made design aspects of a six degree-of-freedom high-accuracy manipulator arm (also called the UTI-arm) comprises four revolute and two prismatic joints for obtaining high accuracy and a high stiffness-to-weight ratio of the links. Design of links under tension load is carried out to enhance bending stiffness.
Pre- stressing of mechanical elements is performed to transform strength to stiffness. Past researchers have reported inefficiency of prismatic joints due to their high compliance. To verify the theory presented for calculating torsional stiffness of such joints experiments were conducted. Prismatic joints used in this design provide higher bending stiffness with a relatively low loss in torsional stiffness compared to a uniform thin-walled tube.FEM was also used to validate the model.
K. Støy et al [15] described the role-based control which is a general bottom-up approach used to control of the locomotion in self-reconfigurable robots. Role-based control is used to implement a caterpillar, a sidewinder, and a rolling track gait in the CONRO self-reconfigurable robot consisting of eight modules. Based on their experiments and discussion they conclude that control systems based on role-based control are least, robust to communication errors and reconfiguration
Hamid Reza Najia et al [16] extended the concepts of multi agent technology to reconfigurable systems—systems in which the functionality of both the associated hardware and software can be altered some time after the system has been built. The use of this new paradigm has the potential to greatly increase the efficiency, expandability, flexibility, feasibility and maintainability of reconfigurable systems and to provide an attractive alternative to the current set of disjoint approaches that are currently applied to this problem domain which is focused on the suitability of reconfigurable hardware to support hardware agents. It also shows that how hardware agents may be created using a conventional hardware description language that when synthesized produces synchronous and asynchronous hardware constructs.
III. DESIGN OF THREE FINGERED GRIPPER MECHANISM
Mechanical design involves basic tasks that ultimately lead to other tasks like serial servo controller and
manipulator software. The mechanical design covers the following tasks,
(1) Selection of kinematic structure (2) Calculation of degree of freedom (3) Selection skeleton material
(4) Dimensioning through torque requirements
Three finger grippers offer good centering possibilities for the adjustment of the work piece on the gripper axis which is difficult to realize with astrictive systems. This is easy to achieve for work piece with axial symmetry but somewhat more difficult for prismatic work piece For a four-point contact the prehension forces act in two axial directions. Unfortunately not every work piece can be handled in this manner and an alternative is the three-point design for grippers. They can move along a curved path or along a straight line towards the centre.
A. Arrangement of the Gripper
Gripper is an assembly unit and it mounted over the end of the link 7 of the robot. Gripper has mounting flange, accurately machined block act as nut, standard screw rod, three dependent link which converts the linear movement of the block into angular movement to the fingers thus Gripper operating as a single unit with screw rod’s rotary motion. CAD model of robot and gripper assembly are illustrated in Fig. 1 & Fig. 2.
B. Prehension strategy
A gripping strategy must include the complete prehension plan, taking into account all possible uncertainties relevant to the process involved. Consequently, the main purpose of the strategy is the programmed or autonomous implementation of prehension. The properties of the
location chosen for gripping are essential importance.
This can be characterized as follows
(1) Fixed gripping point, e.g. prehension from a magazine or stack.
(2) Migratory gripping point, e.g. prehension from running conveyor belt.
(3) Oscillating gripping point, e.g. prehension of a part rolling back forth.
(4) Unknown gripping point which is determined by sensory insight, e.g. attain from table top.
(5) Unknown three dimensional gripping point, e.g. take out from the box.
C. Gripping procedure
Gripping denotes the fundamental motion consisting of object prehension and retention. The gripping procedure is divided into four phases,
(1) Preparation for contact, e.g. by appropriate orientation of objects following a predefined motion pattern. Fig. 3 shows the use of constrains to force an object into a predefined position (px) using the motion of conveyor belt.
(2) Prehension by establishing contact between object and gripping surface area. At this phase the work piece is subjected to static forces and moments.
(3) Retention of the object during its manipulation in space or moving, rotating, or even mounting in few cases.
Dynamic forces and moments occur in the course of motion or task related procedures.
(4) Release of the object at its destination, e.g. by switching-off the vacuum supply and possibly using the assistance of an integrated ejection mechanism .
Fig. 1 CAD Model of robot assembly
Fig. 2 CAD Model of Three fingered gripper assembly
Fig. 3 Mechanical constraint for object positioning
IV. KINEMATICS DESIGN OF ROBOT STRUCTURE
Kinematic design was done using kinematic analysis software SAM and entire 3D model Gripper design was done using Solid Works software as mentioned already.
By SAM software we can ensure the path function of the finger to perform adaptive action and vice versa. Force analysis cannot be done by using the simple kinematic mechanism analysis software.
Robot structure was designed in the basic concept of inversion of four bar mechanism. Combination of four bar links and tertiary links used and formed as a robot. Fig. 4 illustrates the kinematic design of robot structure which is developed using SAM mechanism soft ware. In this design two tertiary links and three four bar mechanism implemented to get the robots anatomy.
Fig. 4 Kinematics design of robot structure The weight of the hand should be kept at a minimum so that the largest possible payload can be lifted for a given maximum total load. The gripper weight is typically a large fraction of the force measured by the wrist force sensors which makes measurement of small forces cumbersome. To make an objective comparison between different grippers with different weights and weight capacity, a simple index is;
Gripper ratio =Maximum pay load (Kg)/Gripper weight (Kg)
= 5 / 1.6 = 3.125
The gripper ratio is also influenced by the type of finger
mechanism, the driving power source, and gripper opening and grip actuator. Larger gripper ratios imply better gripper designs.
The majority of designs are modeled on clamping gripper.
The efficiency of mechanical grippers depends on the applied force, the coefficient of friction between the jaws and the cargo [17]. The griping force is independent of the area of contact. The stability and rigidity of the held component, however may improved by increasing the area the contact by altering the mode of application of the force.
The gripping force can be calculated by;
Gripping force = Applied force x Coefficient of friction The gripper is designed to open about an angle of 28°
when actuated using a servo motor (12v) which turns 90°
in either direction, i.e. total of 180° movement. The gripper is simple and robust in design which may handle broad sizes of objects with less amount of power since power is required only to contraction (grasp) and expansion (release) operations. The fabricated robot-gripper assembly set up is shown in Fig. 5 in which above mentioned gripper ratio is obtained.
Fig. 5 Fabricated robot-gripper assembly set up
V. CONCLUSIONS
Many studies have been illustrated in this study which describes the past designed gripper in real time applications. During designing the kinematic system for
the gripper is having only one degree of freedom. So the output of the gripper perfectly delivers its grasping effect without the use of electronic system to grasp the standard used work piece. The material selected to fabricate the gripper is aluminum, is well suited compare to the other materials which has a unique and unbeatable combination of properties (such as light in density, formability, good reflection on light and comparatively stable in heat with plastics, high coefficient of linear expansion etc.), that make it in to a versatile, highly usable and attractive construction material. These types of grippers are mainly applicable for pick and place a work piece or a component. Load carrying capacity will differs depends upon the nature of the application of the robot. These grippers may also use concentric and non concentric application by changing one finger in the offset manner with respect to the gripper axis. The design process is not much complicate due to light in weight for a 5 kg payload.
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