Ch-1ppt2 Anatomy

Robotics

Chapter 1 (Part A): Robot Anatomy

ETME -404

Overview

Robot anatomy

kinematic chain

Links

Joints

Degree of Freedom(DOF)

Joint Notation Scheme

Arm Configuration

Wrist Configuration

Work Volume

The End-effector

Robot anatomy Robot anatomy is the study of skeleton of robot that is

the physical construction of the manipulator .

The mechanical structure consists of rigid body(links)connected by means of joints.

Main parts of structure1. Arm ensure mobility and reach ability i.e forpositioning the EE.

2. Wrist for orientation of the EE

3. End effector (EE)for performing tasks eg holding,lifting etc.

Robot anatomy

4. Base ( generally fastened to the floor)

Robot anatomy

Robot Manipulators are composed of links connected by joints to form a kinematic chain

Open kinematic chain

Closed kinematic chain

Robot anatomy

Links (Assumed to be rigid)

Binary link(connected with at most another 2 links)

Tertiary link(connected with at most 3 links)

Robot anatomy

Joints:

Two basic types of joints that are commonly used inindustrial robots are:

1. Linear joint

2. Rotating joint

Linear joints involve a sliding or translational motionof the connecting links.

(achieved in no. of ways for eg rack and pinion, bypiston, screw and nut or mechanism etc.

Robot anatomy

Robot anatomy

Rotating joint

3 types of rotating joint

I. Rotational (R) joint: the axis of the rotation isperpendicular to the axes of the two connectinglinks.

II. Twisting (T) joint : the axis of the rotation isparallel to the axes of the two connecting links.

III. Revolving (V) joint: in which input link is parallelto the axis of rotation and o/p link isperpendicular to the axis of rotation.

Robot anatomy



The number of independent variables required to specify the location and orientation of EE in 3D space.

A rigid link in space has ..dof

A rigid link in plane has .. dof


In open kinematic chain: the DOF is equal to the numbers oflinks or number of joints.(it is assumed that each joint hasonly 1 DOF)


Required DOF in a Manipulator

In order to position and orient a body freely in 3D space, amanipulator should have at least 6DOF such a manipulator iscalled Spatial manipulator.

A manipulator with less than 6 DOF has constrained motion in3D space. There are many industrial manipulators that have 5or less DOF

Spatial manipulator with more than 6DOF have surplus jointsand are known as redundant manipulators.


The extra DOF may enhance the performance by adding to its dexterity/flexibility.

Joint Notation Scheme

Uses the joint symbols (L or P, R, T, V) to designate joint typesused to construct robot manipulator

Separates body-and-arm assembly from wrist assembly usinga colon (:) .Example: TLR : TR

Notation = RR

Arm Configuration

The purpose of the arm is to position the wrist in the 3Dspace.

According to joint movements and arrangement of links, fourwell-distinguished basic structural configurations are possiblefor the arm.

named according to the coordinate system employed or theshape of the space they sweep.

Arm Configuration

The four basic configurations are:

(i) Cartesian (rectangular) configuration

(ii) Cylindrical configuration

(iii) Polar (spherical) configuration

(iv) Articulated (Revolute or Jointed-arm configuration)

Arm Configuration

This is the simplestconfiguration with all threeprismatic joints

The endpoint of the arm iscapable of operating in acuboidal space, calledworkspace.

The workspace representsthe portion of space aroundthe base of the manipulatorthat can be accessed by thearm endpoint

A 3-DoF cantilever type Cartesian arm configuration and its workspace

(i) Cartesian(rectangular) configuration

Arm Configuration

The volume of the space swept is called work volume; the surface of the workspace describes the work envelope

Notation LLL or PPP

Gantry or box Cartesian

Gantry configuration is used when heavy loads must be precisely moved. The Cartesian configuration gives large work volume but has a low dexterity.

Notation LLL and High rigidity

(i) Cartesian(rectangular) configuration

Arm Configuration(ii) Cylindrical Configuration

Uses two perpendicular prismaticjoints, and a twisting joint Notation TLL

The cylindrical configuration offers goodmechanical stiffness and the wristpositioning accuracy decreases as thehorizontal stroke increases.

It is suitable to access narrowhorizontal cavities and, hence, is usefulfor machine-loading operations.

Arm Configuration

It consists of a telescopic link(prismatic joint) that can beraised or lowered about ahorizontal rotary joint.

These two links are mounted ona rotating base.

Notation TRP or TRL

gives the capability of movingthe arm end-point within apartial spherical shell space aswork volume

(iii)Polar (Spherical) Configuration

Arm Configuration

This configuration allows manipulationof objects on the floor because itsshoulder joint allows its end-effector togo below the base.

Its mechanical stiffness is lower thanCartesian and cylindrical configurations

the wrist positioning accuracy decreaseswith the increasing radial stroke.

The construction is more complex. Polararms are mainly employed for industrialapplications such as machining, spraypainting and so on.

(iii)Polar (Spherical) Configuration

Arm Configuration

Its configuration is similar to that of humanarm

It consists of two straight links,corresponding to the human "forearm"and "upper arm" with two rotary jointscorresponding to the "elbow" and"shoulder" joints.

Notation TRR

The work volume of this configuration isspherical shaped, and with proper sizing oflinks and design of joints, the armendpoint can sweep a full spherical space

Ability to extend its arm beyond its base

(iv) Articulated (Revolute or Jointed-arm) Configuration

Arm Configuration

characteristics of articulated andcylindrical configurations are combined

The result is SCARA(SelectiveCompliance Assembly Robot Arm)

Notation VRP

The SCARA configuration has verticalmajor axis rotations such thatgravitational load, Coriolis, andcentrifugal forces do not stress thestructure as much as they would if theaxes were horizontal.

(v) Other Configurations

Arm Configuration

This advantage is very important at high speeds and highprecision.

This configuration provides high stiffness to the arm in thevertical direction, and high compliance in the horizontalplane, thus making SCARA ideal for many assembly tasks.

Wrist Configuration

Wrist assembly is attached to end-of-arm

End effector is attached to wrist assembly

Function of wrist assembly is to orient end effector properlywith respect to the task to be performed.

Two or three degrees of freedom:

Roll

Pitch

Yaw

Wrist Configuration

Notation :RRT

Wrist Configuration

Work Volume

Work Volume

The work volume is determined by the following :

1. The robots physical configuration.

2. The size of the body , arm and wrist components.

3. The limits of joint movement.

The End-effector

The special tooling for a robot that enables it to perform aspecific task

The end-effector is external to the manipulator and its DOF donot combine with the manipulator's DOF.

Different end-effectors can be attached to the end of the wristaccording to the task to be executed.

These can be grouped into two major categories:

1. Grippers 2. Tools

The End-effector

Two types:

Grippers to grasp and manipulate objects (e.g., parts) duringwork cycle

Tools to perform a process, e.g., spot welding, spray painting

The End-effector

A two-finger mechanical gripper for grasping rotational parts

Thank You

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Ch-1ppt2 Anatomy