Me 445 Groover Ch8 Industrial Robotics

5/28/2018 Me 445 Groover Ch8 Industrial Robotics

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2008 Pearson Education, Inc., Upper Saddle River, NJ. All rights reserved. This material is protected under all copyright laws as they currently exist.

No portion of this material may be reproduced, in any form or by any means, without permission in writing from the publisher. For the exclusive use of adopters of the bookAutomation, Production Systems, and Computer-Integrated Manufacturing, Third Edition, by Mikell P. Groover.

Ch 8 Industrial Robotics

Sections:

1. Robot Anatomy and

Related Attributes

1. Robot Control Systems

2. End Effectors

3. Sensors in Robotics

4. Industrial Robot Applications

5. Robot Programming

6. Robot Accuracy and Repeatability



Industrial Robot Defined

A general-purpose, programmable

machine capable of possessing certain

anthropomorphic characteristics.

The most obvious anthropomorphic

characteristic of an industrial robot isits mechanical arm, which is used to

perform various industrial tasks.

Other human-like characteristics are:

robots capabilities to respond to sensory inputs,

communicate with other machines, and

make decisions.


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These capabilities permit robots to perform a variety of

useful task.

The development of robot technology followed the

development of numerical control, and the two

technologies are quite similar.

They both involve coordinated control of multiple axes

(the axes are calledjo in ts in robotics), and they both

use dedicated digital computers as controllers.

Typical production applications of industrial robots

include spot welding, material transfer, machine

loading, spray painting, and assembly.




Robots can be substituted for humans in hazardous or

uncomfortable work environments.

A robot performs its work cycle with a consistency and

repeatability that cannot be attained by humans.

Robots can be reprogrammed. When the productionrun of the current task is completed, a robot can be

reprogrammed and equipped with the necessary

tooling to perform an another different task.

Robots are controlled by computers and can therefore

be connected to other computer systems to achieve

computer integrated manufacturing.


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Robot Anatomy

Robot manipulator - a series of joint-link combinations



Robot Anatomy

Manipulator consists of joints and links

Joints provide relative motion

Links are rigid members between joints

Various joint types: linear and rotary Each joint provides a degree-of-freedom

Most robots possess five or six degrees-of-freedom

Robot manipulator consists of two sections:

Body-and-arm for positioning of objects in the

robot's work volume

Wrist assembly for orientation of objects


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Types of Manipulator Joints

Translational motion

Linear joint (type L)

Orthogonal joint (type O)

Rotary motion

Rotational joint (type R)

Twisting joint (type T)

Revolving joint (type V)



Translational Motion Joints

Linear joint

(type L)

Orthogonal joint

(type O)


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Rotary Motion Joints

Rotational joint

(type R)

Twisting joint

(type T)

Revolving joint

(type V)



Joint Notation Scheme

Uses the joint symbols (L, O, R, T, V) to designate joint

types used to construct robot manipulator

Separates body-and-arm assembly from wrist assembly

using a colon (:)

Example: TLR : TR


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Robot Body-and-Arm Configurations

Five common body-and-arm configurations for industrial

robots:

1. Polar coordinate body-and-arm assembly

2. Cylindrical body-and-arm assembly

3. Cartesian coordinate body-and-arm assembly

4. Jointed-arm body-and-arm assembly

5. Selective Compliance Assembly Robot Arm (SCARA)

Function of body-and-arm assembly is to position an end

effector (e.g., gripper, tool) in space.



Polar Coordinate

Body-and-Arm Assembly

Notation TRL:

Consists of a sliding arm (L joint) actuated relative to thebody, which can rotate about both a vertical axis (T joint)and horizontal axis (R joint)


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Cylindrical Body-and-Arm Assembly

Notation TLO:

Consists of a vertical column,

relative to which an arm

assembly is moved up or down

The arm can be moved in or out

relative to the column



Cartesian Coordinate

Body-and-Arm Assembly

Notation LOO:

Consists of three sliding joints,

two of which are orthogonal

Other names include rectilinear

robot and x-y-z robot


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Jointed-Arm Robot

Notation TRR:

General configuration

of a human arm



SCARA Robot

Notation VRO

SCARA stands for Selectively

Compliant Assembly Robot Arm.

Similar to jointed-arm robot except

that vertical axes are used forshoulder and elbow joints.

The arm is very rigid in the vertical direction, but

compliant in the horizontal direction.

This permits the robot to perform insertion tasks (for

assembly) in a vertical direction, where some side-to-

side alignment may be needed to mate the two parts

properly.


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Joint Notations



Wrist Configurations

Wrist assembly is attached to end-of-arm

End effectoris attached to wrist assembly

Function of wrist assembly is to orient end effector

Body-and-arm determines global position of end

effector

Two or three degrees of freedom:

Roll

Pitch

Yaw


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Wrist Configuration

Typical wrist assembly has two or three degrees-of-

freedom (shown is a three degree-of freedom wrist)

Notation :RRT



SCARA Robot - No wrist

The SCARA robot configuration is

unique in that it typically does not

have a separate wrist assembly.

Since it is used for insertion type

assembly operations in which theinsertion is made from above, the

orientation requirements are

minimal, and the wrist is therefore

not needed.

Orientation of the object to be inserted is sometimes

required, and an additional rotary joint can be provided

for this purpose.


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Work Volume

The work volume (the term work envelope is also used) ofthe manipulator is defined as the envelope or three-dimensional space within which the robot can manipulatethe end of its wrist.

Work volume is determined by the number and types ofjoints in the manipulator (body-and-arm and wrist), theranges of the various joints, and the physical sizes of thelinks.

The shape of the work volume depends largely on therobots configuration. A polar configuration robot tends tohave a partial sphere as its work volume, a cylindricalrobot has a cylindrical work envelope, and a Cartesiancoordinate robot has a rectangular work volume.



Work Volume


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Joint Drive Systems

Electric

Uses electric motors to actuate individual joints

Preferred drive system in today's robots

Hydraulic

Uses hydraulic pistons and rotary vane actuators

Noted for their high power and lift capacity

Pneumatic Typically limited to smaller robots and simple material

transfer applications



Speed

The drive system, position sensors (and speed sensors if

used), and feedback control systems for the joints

determine the dynamic response characteristics of the

manipulator.

The speed with which the robot can achieve aprogrammed position and the stability of its motion are

important characteristics of dynamic response in robotics.

Speed refers to the absolute velocity of the manipulator at

its end-of-arm.

The maximum speed of a large robot is around

2 m/s (6 ft/s).


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Speed

Speed can be programmed into the work cycle so that

different portions of the cycle are carried out at different

velocities.

What is sometimes more important than speed is the

robots capability to accelerate and decelerate in a

controlled manner.

In many work cycles, much of the robots movement is

performed in a confined region of the work volume, so the

robot never achieves its top-rated velocity.

In these cases, nearly all of the motion cycle is engaged in

acceleration and deceleration rather than in constant

speed.



Speed

A term that takes all of these factors into consideration is

speed of response, which refers to the time required for

the manipulator to move from one point in space to the

next.

Speed of response is important because it influences therobots cycle time, which in turn affects the production rate

in the application.

Stability refers to the amount of overshoot and oscillation

that occurs in the robot motion a the end-of-arm as it

attempts to move to the next programmed location.


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Load Carrying Capacity

Load carrying capacity depends on the robots physical

size and construction as well as the force and power that

can be transmitted to the end of the wrist

The weight carrying capacity of commercial robots ranges

from less than 1 kg up to approximately 900 kg (2000 Ib).

Medium sized robots designed for typical industrial

applications have capacities in the range of 10 to 45 kg

(25 to 100 Ib).

One factor that should be kept in mind when considering

load carrying capacity is that a robot usually works with a

tool or gripper attached to its wrist.



Robot Control Systems

The actuations of the individual joints must becontrolled in a coordinated fashion for themanipulator to perform a desired motion cycle.

Microprocessor-based controllers are commonly

used today in robotics as the control systemhardware.

The controller is organized in a hierarchicalstructure, so that each joint has its own feedbackcontrol system, and a supervisory controllercoordinates the combined actuations of the jointsaccording to the sequence of the robot program.

Different types of control are required for differentapplications.


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Robot Control System

Hierarchical control structure of a robot microcomputer

controller




1. Limited sequence control pick-and-place

operations using mechanical stops to set positions

2. Playback with point-to-point control records

work cycle as a sequence of points, then plays back

the sequence during program execution3. Playback with continuous path control greater

memory capacity and/or interpolation capability to

execute paths (in addition to points)

4. Intelligent control exhibits behavior that makes it

seem intelligent, e.g., responds to sensor inputs,

makes decisions, communicates with humans


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1. Limited sequence control

The most elementary control type.

It can be utilized only for simple motion cycles, such

as pick-and-place operations (i.e.. picking an object

up at one location and placing it at another location).

It is usually implemented by setting limits or

mechanical stops for each joint and sequencing the

actuation of the joints to accomplish the cycle.




1. Limited sequence control

Feedback loops are sometimes used to indicate that

the particular joint actuation has been accomplished

so that the next step in the sequence can be

initiated.However, there is no servo-control to accomplish

precise positioning of the joint.

Many pneumatically driven robots are limited

sequence robots.


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2. Playback with point-to-point control

Playback robots represent a more sophisticated

form of control than limited sequence robots.

Playback control means that the controller has a

memory to record the sequence of motions in a

given work cycle as well as the locations and other

parameters (such as speed) associated with each

motion and then to subsequently play back the work

cycle during execution of the program.





In point-to-point (PTP) control, individual positions of

the robot arm are recorded into memory.

These positions are not limited to mechanical stops

for each joint as in limited sequence robots; instead,each position in the robot program consists of a set

of values representing locations in the range of each

joint of the manipulator.


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Thus, each point consists of five or six values

corresponding to the positions of each of the five or

six joints of the manipulator.

For each position defined in the program, the joints

are thus directed to actuate to their respective

specified locations.

Feedback control is used during the motion cycle toconfirm that the individual joints achieve the

specified locations in the program.




3. Playback with continuous path control

Continuous path robots have the same playback

capability as the previous type.

The difference between continuous path and

point-to-point is the same in robotics as it is in NC.

A playback robot with continuous path control is

capable of one or both of the following:

1. Greater storage capacity for location

Thus, the points constituting the motion cycle can be

spaced very closely together to permit the robot to

accomplish a smooth continuous motion.


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3. Playback with continuous path control

2. Interpolation

In point-to-point systems, the x,y, and z axes arecontrolled to achieve a specified point location withinthe robots work volume.

In continuous path systems, not only are the x,y, andz axes controlled, but the velocities are controlledsimultaneously to achieve the specified linear orcurvilinear path.

Servo-control is used to continuously regulate theposition and speed of the manipulator.

It should be mentioned that a playback robot withcontinuous path control has the capacity for PTPcontrol.




4. Intelligent control

Industrial robots are becoming

increasingly intelligent.

An intelligent robot is one that exhibits behavior that

makes it seem intelligent.

Some of the characteristics that make a robot

appear intelligent include the capacities to interact

with its environment, make decisions when things go

wrong during the work cycle, communicate with

humans, make computations during the motion

cycle, and respond to advanced sensor inputs such

as machine vision.


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End Effectors

The special tooling for a robot that enables it to

perform a specific task.

Two types:

Grippers to grasp and manipulate objects (e.g.,

parts) during work cycle

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

spray painting



End Effectors - Grippers

Mechanical grippers, consisting of two or more

fingers that can be actuated by the robot controller to

open and close to grasp the workpart.

Vacuum grippers, in which suction cups are used to

hold flat objects. Magnetized devices, for holding ferrous parts.

Adhesive devices, which use an adhesive

substance to hold a flexible material such as a fabri.

Simple mechanical devices, such as hooks and

scoops.


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End Effectors

Mechanical Grippers

Mechanical grippers are the most common grippertype.

Some of the innovations and advances inmechanical gripper technology include:

1. Dual grippers consisting of two gripper devices inone end effector for machine loading and unloading.(With a single gripper, the robot must reach into theproduction machine twice.)

2. Interchangeable fingers that can be used on onegripper mechanism.

To accommodate different parts, different fingers areattached to the gripper.



End Effectors

Mechanical Grippers

3. Sensory feedback in the fingers that provide the

gripper with capabilities such as:

(1) sensing the presence of the workpart or,

(2) applying a specified limited force to the workpart

during gripping (for fragile workparts).

4. Multiple fingered grippers that possess the general

anatomy of a human hand.

5. Standard gripper products that are commercially

available, thus reducing the need to custom-design

a gripper for each separate robot application.


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Robot Mechanical Gripper

A two-finger mechanical gripper for grasping rotational

parts.



Tools

The robot uses tools to perform processing operations

on the workpart.

The robot manipulates the tool relative to a stationary or

slowly moving object (e.g., workpart or subassembly).

Examples of the tools used as end effectors by robots toperform processing applications include spot welding

gun; arc welding tool; spray painting gun; rotating

spindles for drilling, routing, grinding, and similar

operations; assembly tool (e.g., automatic screwdriver);

heating torch; ladle (for metal casting); and water jet

cutting tool.


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Tools

In each case, the robot must not only control the relative

position of the tool with respect to the work as a function

of time, it must also control the operation of the tool.

For this purpose. the robot must be able to transmit

control signals to the tool for starting, stopping, and

otherwise regulating its actions.

In some applications, the robot may use multiple tools

during the work cycle.

For example, several sizes of routing or drilling bits must

be applied to the workpart.



Tools

Thus the robot must have a means of rapidly changing

the tools.

The end effector in this case takes the form of a fast-

change tool holder for quickly fastening and unfastening

the various tools used during the work cycle.


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Sensors in Robotics

Two basic categories of sensors used in industrial robots:

1. Internal - used to control position and velocity of the

manipulator joints.

These sensors form a feedback control loop with the

robot controller.

Typical sensors used to control the position of the robot

arm include potentiometers and optical encoders.

Tachometers of various types are used to control thespeed of the robot arm.



Sensors in Robotics

2. External - used to coordinate the operation of the robot

with other equipment in the work cell.

In many cases, these external sensors are relatively

simple devices, such as limit switches that determine

whether a part has been positioned properly in a fixtureor that a part is ready to be picked up al a conveyor.

Other situations require more advanced sensor

technologies.


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Sensors in Robotics - External

Tactile - touch sensors and force sensors

Proximity - when an object is close to the sensor

Optical (e.g. Photocell)

Machine vision

Machine vision is used in robotics for inspection.

parts identification, guidance, and other uses.

Other sensors - temperature, fluid pressure, fluid flow,

electrical voltage, current, and various other physicalproperties.



Robot Application Characteristics

General characteristics of industrial work situations that

promote the use of industrial robots

1. Hazardous work environment for humans,

2. Repetitive work cycle,

3. Difficult handling task for humans (e.g., heavy parts),

4. Multishift operations, where workers should be changed,

5. Infrequent changeovers of the physical workplace,

6. Part position and orientation are established in the work

cell, since most robots in todays industrial applications

are without vision capability.


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Industrial Robot Applications

1. Material handling applications

Material transfer pick-and-place, palletizing

(e.g., Transferring parts from one conveyor to another.)

Machine loading and/or unloading

(e.g., Die casting, plastic molding, machining, forging,pressworking, heat treating, ...)

2. Processing operations

Spot welding and continuous arc welding

Spray coating

Other waterjet cutting, laser cutting, grinding, drilling,polishing, wire brushing, ...

3. Assembly and inspection

4. Changing tools, even the work holding devices in themachines.



Arrangement of Cartons on Pallet

The robot retrieves parts, cartons, or other objects from

one location and deposits them onto a pallet or other

container at multiple positions on the pallet.

Although the pickup point is the same for every cycle, the

deposit location on the pallet is different for each carton. Either the robot must be taught each position on the pallet

using the powered leadthrough method, or it must

compute the location based on the dimensions of the

pallet and the center distances between the cartons (in

both x- and y-directions).


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Arrangement of Cartons on Pallet



Depalletizing

Other applications similar to palletizing include

depalletizing, which consists of removing parts from an

ordered arrangement in a pallet and placing them at

another location (e.g., onto a moving conveyor), stacking

operations, which involve placing flat parts on top of eachother, such that the vertical location of the drop-off position

is continuously changing with each cycle, and inspection

operations, in which the robot inserts parts into the

compartments of a divided carton.


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Robotic Arc-Welding Cell

Robot performs

flux-cored arc

welding (FCAW)

operation at one

workstation while

fitter changes

parts at the other

workstation



Robot Programming

Leadthrough programming - work cycle is taught to

robot by moving the manipulator through the required

motion cycle and simultaneously entering the

program into controller memory for later playback

Robot programming languages - uses textualprogramming language to enter commands into robot

controller

Simulation and off-line programming program is

prepared at a remote computer terminal and

downloaded to robot controller for execution without

need for leadthrough methods


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Leadthrough Programming

Leadthrough programming dates from the early 1960s

before computer control was prevalent.

The same basic methods are used today for many

computer controlled robots.

In leadthrough programming, the task is taught to the

robot by moving the manipulator through the required

motion cycle, simultaneously entering the program

into the controller memory for subsequent playback.




Two types:

1. Powered leadthrough

Common for point-to-point robots

Uses teach pendant to move joints to desired positionand record that position into memory

2. Manual leadthrough

Convenient for continuous path control robots

Human programmer physical moves manipulator

through motion cycle and records cycle into memory


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Teach Pendants for Powered




Manual Leadthrough Programming

Manual leadthrough programming is convenient for

programming playback robots with continuous path

control where the continuous path is an irregular motion

pattern such as in spray painting.

This programming method requires the operator to physicallygrasp the end-of- arm or the tool that is attached to the

arm and move it through the motion sequence, recording

the path into memory.


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Because the robot arm itself may have significant mass andwould therefore he difficult to move, a specialprogramming device often replaces the actual robot forthe teach procedure.

The programming device has the same joint configuration asthe robot and is equipped with a trigger handle (or othercontrol switch), which the operator activates whenrecording motions into memory.

The motions are recorded as a series of closely spacedpoints.

During playback, the path is recreated by controlling theactual robot arm through the same sequence of points.




Advantages

Advantages:

Can readily be learned by shop personnel

A logical way to teach a robot

Does not required knowledge of computer

programming Disadvantages:

Downtime - Regular production must be interrupted toprogram the robot

Limited programming logic capability

Not readily compatible with modern computer-basedtechnologies


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World and Tool Coordinate Systems

Coordinating the individual joints with the teach pendant isan awkward and tedious way to enter motion commandsto the robot.

For example, it is difficult to coordinate the individual joints ofa jointed-arm robot (TRR configuration) to drive the end-of-arm in a straight-line motion.

Therefore, many of the robots using powered leadthroughprovide two alternative methods for controlling movement

of the entire manipulator during programming in addition tocontrols for individual joints.

With these methods the programmer can move the robotswrist end in straight line paths.



The names given to these alternatives are

(1) world-coordinate system and

(2) tool-coordinate system.

Both systems make use of a Cartesian coordinate system.

In a world-coordinate system, the origin and axes are definedrelative to the robot base.

In a tool-coordinate system, the alignment of the axis systemis defined relative to the orientation of the wrist faceplate(to which the end effector is attached).

In this way, the programmer can orient the tool in a desiredway and then control the robot to make linear moves indirections parallel or perpendicular to the tool.

World and Tool Coordinate Systems


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World Coordinate System

Origin and axes of robot manipulator are defined relative

to the robot base



Tool Coordinate System

Alignment of the axis system is defined relative to theorientation of the wrist faceplate (to which the end effectoris attached)


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Robot Programming Languages

Textural programming languages provide the opportunity to

perform the following functions that leadthrough

programming cannot readily accomplish:

Enhanced sensor capabilities

Improved output capabilities to control external equipment

Program logic not provided by leadthrough methods

Computations and data processing similar to computer

programming languages

Communications with other computer systems



Motion Programming Commands

MOVE P1

HERE P1 - used during leadthrough of manipulator

MOVES P1

DMOVE(4, 125)APPROACH P1, 40 MM

DEPART 40 MM

DEFINE PATH123 = PATH(P1, P2, P3)

MOVE PATH123

SPEED 75


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Interlock and Sensor Commands

Input interlock:

WAIT 20, ON

Output interlock:

SIGNAL 10, ON

SIGNAL 10, 6.0

Interlock for continuous monitoring:

REACT 25, SAFESTOP



Gripper Commands

Basic commands

OPEN

CLOSE

Sensor and and servo-controlled handsCLOSE 25 MM

CLOSE 2.0 N


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Simulation and Off-Line Programming

In conventional usage, robot programming languages still

require some production time to be lost in order to define

points in the workspace that are referenced in the

program

They therefore involve on-line/off-line programming

Advantage of true off-line programming is that the program

can be prepared beforehand and downloaded to the

controller with no lost production time

Graphical simulation is used to construct a 3-D model

of the robot cell in which locations of the equipment in

the cell have been defined previously



Simulation and Off-Line Programming


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Robot Accuracy and Repeatability

Three terms used to define precision in robotics, similar to

numerical control precision:

1. Control resolution - capability of robot's positioning

system to divide the motion range of each joint into

closely spaced points

2. Accuracy - capability to position the robot's wrist at a

desired location in the work space, given the limits of the

robot's control resolution3. Repeatability - capability to position the wrist at a

previously taught point in the work space



Addressableposition

0 CR/2

Desiredposition

Control Resolution, Accuracy, and

Repeatability (from NC Chapter)

Possible

region for the

actual position


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Robot Accuracy and Repeatability

Documents

Me 445 Groover Ch8 Industrial Robotics