unit 2

UNIT 2: Robot Anatomy

Prepared by: Mr.SUDHEESH THAMPI, NTTF TTCSEM 51

Syllabus:

2.1 Links, Joints and Joints Notation Scheme

2.2 Degrees Of Freedom, Required DOF in a Manipulator

2.3 Arm Configuration, Wrist Configuration, Work Cell, Work Envelope, and Work Volume.

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Syllabus :

2.4 Robot End Effectors – Definition, Classification of End Effectors, Types of Grippers, consideration in gripper selection and designing.

2.5 General structure of Robot and Specifications of Robots.

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2.1 Robot anatomy:

• The manipulator or robotic arm has many

similarities to the human body.

• The mechanical structure of a robot is like the

skeleton in the human body.

• ROBOT ANATOMY is the study of skeleton of

robot, that is, physical construction of the

manipulator structure.

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•The mechanical structure of a manipulator that consistsof rigid bodies (links) connected by means ofarticulations (joints)is segmented into an arm thatensures mobility and reach ability.

• A wrist that confers orientation, and an end-effectorthat performs the required task.

•Most manipulators are mounted on a base fastened tothe floor or on the mobile platform of an autonomousguided vehicle (AGV).

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2.1 Links

• The mechanicalstructure of a roboticmanipulator is amechanism, whosemembers are rigid linksor bars.

• A rigid link that can beconnected, at most,with two other links isreferred to as a binarylink

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Links

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Joints and joints notation scheme

Many types of joints can be made between twolinks. However, only two basic types arecommonly used in industrial robots.

They are:

• Revolute (R)

• Prismatic (P)

The relative motion of the adjoining links of ajoint is either rotary or linear depending onthe type of joint.

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• Revolute joint: It is sketched in below fig. Thetwo links are joined by a pin (pivot) about theaxis of which the links can rotate with respectto each other.

• Prismatic Joint: The two links are so jointedthat these can slide (linearly move) withrespect to each other. Screw and nut (slowlinear motion of the nut), rack and pinion areways to implement prismatic joints.

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Other types of possible joints used are:

• Planar – one surface sliding over another surface

• Cylindrical – one link rotates about the other at 90º angle

• Spherical – one link can move w.r.t the other in 3D

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2.2 Degrees of Freedom (DOF)

• The number of independent movements that an object can perform in a 3-D space is called the number of Degrees of freedom (DOF). A rigid body free in space has six degrees of freedom – three position and three for orientation.

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2.2.1 Required DOF in a manipulator

• It is concluded from DOF that to position and orient a body freely in 3-D space, a manipulator with less than 6-DOF is required. Such a manipulator is called spatial manipulator. It has three joints for positioning and three for orienting the end-effector.

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2.2.1 Required DOF in a manipulator

• A manipulator with less than 6-DOF has constrained motion in 3-D space. There are situations where five or even four joints (DOF) are enough to do the required job. There are many industrial manipulators that have five or fewer DOF. These are useful for specific applications that do not require 6-DOF.

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2.2.1 Required DOF in a manipulator

• A planar manipulator can only sweep a 2-D space or aplane and can have any number of degrees of freedom.For example, a planar manipulator with three joints (3-DOF) – may be two for positioning and one forOrientation – can only sweep a plane.

• Spatial manipulator with more than 6-DOF havesurplus joints and are known as redundant Manipulator(that can be omitted without any loss of significance).The extra DOF may enhance the performance byadding to its dexterity (skill in using one’s hand).

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2.3 Arm configuration

• The mechanics of the arm with 3-DOF depends on the type of three joints employed and their arrangement. The purpose of the arm is to position of the arm is to position the wrist in the 3-D space and the arm has following characteristics requirements.

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The four basic configurations

i. Cartesian (rectangular) configuration – all three Pjoints

ii. Cylindrical configuration – one R and two P joints

iii. Polar (Spherical) configuration – two R and one P joint

iv. Articulated (Revolute / Jointed-arm) configuration –all three R joints

v. Other configurations

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Cartesian (Rectangular) configuration :

• This is the simplest configuration with all three prismatic joints as shown in just above fig.

• It is constructed by three perpendicular slides, giving only linear motions along the three principal axes.

• There is an upper and lower limit for movement of each link.

• Consequently, the endpoint of the arm is capable of operating in a cubical space called workspace

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Cartesian (Rectangular) configuration

• The workspace represents the portion ofspace around the base of the manipulator thatcan be accessed by the arm endpoint.

• The shape and size of the workspace dependson the arm configuration, structure, degreesof freedom, size of links, and design of joints

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Cartesian (Rectangular) configuration:

• The physical space that can be swept by amanipulator (with wrist and end-effector) maybe more or less than the arm endpointworkspace.

• The volume of the space swept is called workvolume; the surface of the workspacedescribes the work envelope

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Cartesian (Rectangular) configuration

• The workspace of Cartesian configuration iscuboidal and is shown in fig. Two types ofconstructions are possible for Cartesian arm:

a) Cantilevered Cartesian

b) Gantry or box Cartesian

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Cylindrical configuration

• It uses two perpendicular prismatic joints and arevolute joint.

• The difference from the Cartesian one is that oneof the prismatic joint is replaced with a revolutejoint.

• One typical construction is with the first joint asrevolute.

• Rotary joint may either have the column rotatingor a block revolving around stationary verticalcylindrical column.

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The cylindrical configuration offers:

• Good mechanical stiffness

• Wrist positioning accuracy decreases as thehorizontal stroke increases

• Suitable to access narrow horizontal cavities

• Useful for machine-loading operations

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Polar (Spherical) Configuration:

• It consists of telescopic link (prismatic joint)that can be raised or lowered about ahorizontal revolute joint.

• Two links are mounted on a rotating base. Thisarrangement of joints, known as RRPconfiguration.

• It gives the capability of moving the arm end-point within a partial spherical shell space aswork volume.

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Polar (Spherical) Configuration:

• work envelope is a portion of a sphere. Spherical configuration allows manipulation of objects on the floor because its shoulder joint allows its end-effector to go below the base.

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Polar arms are employed forindustrial applications such as :

• Machining, Spray painting and so on.

• Other joint arrangement – RPR

• PRR will not give a spherical work volume

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Articulated (Revolute or Jointed-arm) configuration

• It consists of two straight links, corresponding tohuman “forearm” and “upper arm” with tworotary joints corresponding to “elbow” and“shoulder” joints.

• RRR configuration is called revolute becausethree revolute joints are employed.

• The work volume is spherical shaped and withproper sizing of links and design of joints, the armendpoint can sweep a full spherical space.

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Other Configurations:

• New arm configurations can be obtained byassembling the links and joints differently

• For instance, if the characteristics of articulatedand cylindrical configurations are combined, theresult will be another type of manipulator withrevolute motions, confined to the horizontalplane.

• Such a configuration is called SCARA, whichstands for Selective Compliance Assembly RobotArm.

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2.3.1 Wrist configuration

• Wrist – second part of a manipulator attachedto the end-effector to perform the taskproperly, for example, the gripper must beoriented at an appropriate angle to pick andgrasp a work piece.

• For arbitrary orientation in 3-D space, thewrist must possess at least 3-DOF to give threerotations about the three principal axes.

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2.3.1 Wrist configuration

• Fewer than 3-DOF may be used in a wrist,depending on requirements.

• The wrist has to be compact and it must notdiminish (make smaller or less) theperformance of the arm.

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2.3.1 Wrist configuration

• roll (motion in a plane perpendicular to the end of thearm).

• pitch(motion in vertical plane passing through thearm).

• Yaw (motion in a horizontal plane that also passesthrough the arm.

• This type of wrist is called roll-pitch-yaw or RPY wrist.

• A wrist with the highest dexterity is one where threerotary joint axes intersect at a point

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Workspace

• The end point of the arm is capable of operatingin a cubical space, called workspace.

• The workspace represents the portion of spacearound the base of the manipulator that can beassessed by the arm endpoint.

• The shape and size of the workspace depends onthe arm configuration, structure, DOF, size of linksand design of joints.

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2.3.2 Work envelope &Work volume

• The surface of the workspace describes thework envelope

• The physical space that can be swept by amanipulator (with wrist and end-effector) maybe more or less than the arm endpointworkspace. The volume of the space swept iscalled work volume.

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2.4 Robot End Effectors – Definition

• An end-effector is a device that attaches tothe wrist of the robot arm and enables thegeneral-purpose robot to perform a specifictask.

• It is sometimes referred to as the robot’s“hand”.

• The end-effector is a part of that special-purpose tooling for a robot.

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2.4.1 Classification of end- effector

GrippersDepends on no. of gripper for best applies mechanically• Single Gripper• Multi Gripper• Double Gripper

Depends on whether the part is grasped on its exterior surface or internal Surface

• External Gripper• Internal Gripper Tools

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Gripper

• Grippers are end effectors used to grasp andhold objects.

• These part-handling applications includemachine loading, picking parts from aconveyor, and arranging parts onto a pallet.

• In addition to the work parts, other objectshandled by robot grippers include cartons,bottles, raw materials, and tools.

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Gripper

• Grippers can be classified as single grippersand double grippers although thisclassification applies best to mechanicalgrippers.

• The single gripper is distinguished by the factthat only one grasping device is mounted onthe robot’s wrist.

• A double gripper has two separate objects

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Tools

• Tools are end effectors designed to performwork on the part rather than to merely graspit.

• One of the most common applications ofindustrial robots is spot welding, in which thewelding electrodes constitute the end effectorof the robot.

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Tools

• Other example of robot applications in whichtools are used as end effectors include spraypainting and arc welding.

• Grippers are sometimes used to hold toolsrather than work parts

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2.4.2 Types of grippers

• Mechanical gripper

• Vacuum cups

• Magnetic grippers

• Adhesive grippers

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Mechanical gripper

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Magnetic gripper

• Magnetic grippers can be a very feasiblemeans of handling ferrous materials.

• A magnet does not attract 18-8 stainless steel,so it is not be an appropriate application for amagnetic gripper.

• Other steels, however, including certain typesof stainless steel would be suitable candidatesfor this means of handling,

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Magnetic grippers can be divided into two categories, those using :

• Electromagnet

• Permanent magnet

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Vacuum gripper

• Vacuum cups, also called suction cups, can beused as gripper devices for handling certaintypes of objects.

• The usual requirements on the objects to behandled are that they be flat, smooth andclean, conditions necessary to form asatisfactory vacuum between the object andthe suction cup.

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Adhesive gripper :

• Gripper designs in which an adhesive substanceperforms the grasping action can be used to handlefabrics and other lightweight materials.

• The requirements on the items to be handled are thatthey must be gripped on one side only and that otherforms of grasping such as a vacuum or magnet are notappropriate.

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• One of the potential limitations of an adhesive gripperis that the adhesive substance loses its tackiness onrepeated usage

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2.5 General structure of robot and specifications of robots

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Functional parameters:

Important parameters are:

• Axes of motion.

• Arm movement.

• Wrist movement.

• End of arm speed.

• Weight carrying capacity.

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Functional parameters:

• Control.

• Programming methods.

• Interfacing.

• Positioning accuracy and repeatability.

• End of arm tooling.

• Max. Ambient operating temperature.

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Physical Characteristics:

Mechanical

• Robot configuration

• Number of axes of movement

• Floor space required for mounting

• Weight

• Physical dimensions

• Physical details

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Physical Characteristics:

Power• Power drive system• Power/services requirements

Control• Programming method• Type of control system• External sensors supported• Program backing storage device• Memory size

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Performance Characteristics

Specific

• Accuracy

• Repeatability

• Resolution

• Velocity range

• Operating cycle time

• Load-carrying capacity

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Performance Characteristics

Non-specific

• Life expectancy.

• Reliability.

• Maintainability.

• Mean time between failures (MTBF).

• Mean time to repair (MTTR).

• Performance specification are usually the means by which robots are judged suitable (or capable) for a particular application.

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