Mechatronics

Module IV

• Industrial Robotics: Basic concepts - Robotics and automation- Specification of Robots -Resolution,-Repeatability and accuracy of manipulator-Classification of Robots- Industrial application-

• Robot drives- Characteristics of end of arm tooling –

• Sensors: Tactile, proximity and range sensors- contact and non contact sensors- velocity sensors- touch and slip sensors- Force and torque sensors-

• Programming Lead through programming-Textual programming- Programming languages On line and offline programming- Intelligent Robots

Industrial Robot Defined

A general-purpose, programmable machinepossessing certain anthropomorphiccharacteristics

• Hazardous work environments• Repetitive work cycle• Consistency and accuracy• Difficult handling task for humans• Multi shift operations• Reprogrammable, flexible• Interfaced to other computer systems

Basic concepts in Robotics

Basic concepts in Robotics

• Manipulator – mechanical unit which performs the movement function in the robot

• The manipulator of an industrial robot consists of a series of joints and links.

• Robots needs 6 degrees of freedom to reach a point with a specific orientation in space.

• Therefore the robot's complexity can be classified according to the total number of degrees-of-freedom they possess.

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)

Types of Joints

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

• a) Linear joint (type L joint)• The relative movement between the input link and the output link is a

translational sliding motion, with the axes of the two links being parallel. • b) Orthogonal joint (type U joint) • This is also a translational sliding motion, but the input and output links

are perpendicular to each other during the move. • c) Rotational joint (type R joint)• This type provides rotational relative motion, with the axis of rotation

perpendicular to the axes of the input and output links. • d) Twisting joint (type T joint)• This joint also involves rotary motion, but the axis or rotation is parallel to

the axes of the two links. • e) Revolving joint (type V-joint, V from the “v” in revolving) • In this type, axis of input link is parallel to the axis of rotation of the joint.

However the axis of the output link is perpendicular to the axis of rotation.

Robot classification

• Structurally the robots can be classified according to the coordinate system of the main frame.

– Cartesian – 3 linear axis

– Cylindrical – 2 linear , 1 rotary

– Spherical – 1 linear , 2 rotary

– Articulated / joint arm - 3 rotary

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

Cylindrical

• 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

Polar Coordinate

• Notation TRL:

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

Jointed-Arm Robot

• Notation TRR:

SCARA Robot

• Notation VRO

• SCARA stands for Selectively Compliance Assembly Robot Arm

• Similar to jointed-arm robot except that vertical axes are used for shoulder and elbow joints to be compliant in horizontal direction for vertical insertion tasks

Wrist Configurations

• Wrist assembly is attached to end-of-arm

• End effector is 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

• Notation :RRT

Example

• Sketch following manipulator configurations

• (a) TRT:R, (b) TVR:TR, (c) RR:T.

Example

Solution:

T

R

T

V

(a) TRT:R

R

T

RT R

TR

R

(c) RR:T(b) TVR:TR

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

Grippers and Tools

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

Hydraulic

• Generally associated with larger robot.

• Provides the robot with greater speed and strength

• Main disadvantage : oil leak , large area

• IT can actuate rotational joint / linear joint

• Rotary vane to rotary motion , hydraulic lift for linear motion

Electric drive

• Electric drive system do not generally provide as much speed or power as hydraulic system.

• Accuracy and repeatability are usually better.

• Required small floor space.

• Stepping motors are limited in power suitable only for small robot. They also tend to be noisy and there fore, are seldom used in pratice.

• Mainly for rotary motion

• It can also actuate linear joints by means of pulley

Pneumatic

• Generally for fewer degrees of freedom.

• Simple pick and place application

• Pneumatic power can be readily adapted to the actuation of piston device to provide translational movements.

• It can also used to operate rotary actuators for rotational joints.

Robot Control Systems

• Limited sequence control – pick-and-place operations using mechanical stops to set positions

• Playback with point-to-point control – records work cycle as a sequence of points, then plays back the sequence during program execution

• Playback with continuous path control – greater memory capacity and/or interpolation capability to execute paths (in addition to points)

• Intelligent control – exhibits behavior that makes it seem intelligent, e.g., responds to sensor inputs, makes decisions, communicates with humans

Transducers & Sensor

• A transducer is a device that converts one type physical variable into another form.

• Sensor is a transducer that is used to make a measurement of a physical variable of interest.

• Analog Transducers & Digital Transducers

Sensors

Tactile sensors - Touch

• Touch sensors are used to indicate thatcontact has been made between two objectswithout regard to magnitude of force.

• Some of the commonly used simple devices astouch sensors are micro – switches, limitswitches, etc.

• In addition, it can be used as an inspectiondevice, which has a probe to measure the sizeof a component.

Slip Sensor

• This is the measurement and detection of the movement of an object relative to the sensor.

• This can be achieved either by a specially designed slip sensor or by the interpretation of the data from a touch sensor or a tactile array.

Force sensors

• To grasp parts of different size.

• To apply appropriate level of force.

• In assembly applications , force sensing could be used to determine if screws have become cross threaded or if parts are jammed.

– Force sensing wrist

– Joint sensing

– Tactile array sensors

Force sensing wrist

• Special load cell mounted between gripper and wrist.

• Can provide information about force and moment

Joint sensing

• Torque being exerted in joints by measuring motor current.

Tactile array sensors

• Form an array of force sensing elements so that shape and other information about contact surface can be determined

Position Sensors

• Potentiometer

• Resolver

• Encoder

Potentiometer

Velocity Sensors

• Tacho meter

Proximity sensors and range

• Proximity sensors are devices that indicate when one object is close to another object.

• Some of these can be used to measure the distance called range sensors.

Contact and Noncontact type sensor

• Contact – micro switch

• Non contact- IR, Hall effect, Camera

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– Textual programming 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

Lead Through

• the lead through methods require the programmer to move the manipulator through the desired motion path and path is stored to memory.

• “Teach by showing”• Powered lead through• Manual lead through• The control systems for both lead through

operates in two modes.– Teach mode / run mode

Leadthrough Programming

1. Powered leadthrough – Common for point-to-

point robots

– Uses teach pendant

2. Manual leadthrough – Convenient for

continuous path control robots

– Human programmer physical moves manipulator

Powered Lead through

• Use a teach pendant to control the various joint motors, power drive robot arm

• Each point is recorded into memory for play back.

• Teach pendant – small hand held device with combination of toggle switches , dial, button to regulate robots physical movements.

• Use for point to point motion rather than continuous

• Eg – part transfer task, machine loading and unloading , spot welding

• “Walk through method”

• Programmer physically grasp the robot arm and manually moves it through the desired motion cycle.

• Eg spray painting, continuous arc welding

• If the robot is large to physical move , a special programming apparatus is often substitutes for actual robot.

Leadthrough Programming Advantages

• Advantages:

– Easily learned by shop personnel

– Logical way to teach a robot

– No computer programming

• Disadvantages:

– Downtime during programming

– Limited programming logic capability

– Not compatible with supervisory control

Robot Programming

• Textual programming languages

• High level lang

• Enhanced sensor capabilities

• Improved output capabilities to control external equipment

• Program logic

• Computations and data processing

• Communications with supervisory computers

Coordinate Systems

World coordinate system Tool coordinate system

Motion Commands

MOVE P1

HERE P1 - used during lead through of manipulator

MOVES P1

DMOVE(4, 125)

APPROACH P1, 40 MM

DEPART 40 MM

DEFINE PATH123 = PATH(P1, P2, P3)

MOVE PATH123

SPEED 75

Interlock and Sensor Commands

Interlock CommandsWAIT 20, ONSIGNAL 10, ONSIGNAL 10, 6.0REACT 25, SAFESTOP

Gripper CommandsOPEN CLOSECLOSE 25 MMCLOSE 2.0 N

Simulation and Off-Line Programming

Simulation

• Robot simulation is the process of creating the robot workcell in the “virtual world” to check and validate a number of application process parameters prior to implementation– robot placement and reachability

– tooling design validation

– interferences with other workcell equipment

– layout design

– cycle time validation

Off-Line Programming (OLP)

• Off-Line Programming (OLP) is the process of converting the “sequence of operations”, which is the result of the robot simulation phase, and generating the robot program in the native language of the robot manufacturer (ABB, FANUC, Motoman, Kuka, etc).

• The OLPs which have been so generated can be downloaded directly to the robot controller and tested.

Off Line Program Benefits

• Reducing the onsite programming time (thereby freeing up the robot to be used for production, rather than programming)

• Reducing the downtime of equipment when programming new workpieces/variants

• Programming complex paths (for example, deburring, welding in tight spaces, grinding, polishing, etc, which are highly time consuming )

Programming languages

• In 1973, WAVE language was develop• It is used to interface the machine vision system with the robot. • Then AL language was introduced in 1974 for controlling multiple

robot arms during arm coordination. • VAL was invented in 1979, and it is the common textual robot

language. • Later, this language was updated in 1984, and called as VAL II. The

IBM Corporation has established their two own languages such as AML and AUTOPASS, which is used for the assembly operations.

• Other important textual robot languages are Manufacturing Control Language (MCL), RAIL, and Automatic Programmed Tooling (APT)languages.

Specification of Robot

• Resolution

• Accuracy

• Repeatability

• Degrees of freedom

• Envelope

• Reach

• Pay load

Resolution

• The resolution of a robot is a feature determined by the design of the control unit and is mainly dependent on the position feedback sensor.

• programming resolution & control resolution. • The programming resolution is the smallest allowable

position increment in robot programs and is referred to as the basic resolution unit (BRU). For IRB2000 ABB robot it is approximately 0.125 mm on linear axis.

• The control resolution is the smallest change in position that the feedback device can sense. For example, assume that an optical encoder which emits 1000 pulses per revolution of the shaft is directly attached to a rotary axis. T

Resolution

• This encoder will emit one pulse for each of 0.36°of angular displacement of the shaft. The unit 0.36° is the control resolution of this axis of motion. Angular increments smaller than 0,36°cannot be detected.

• Best performance is obtained when programming resolution is equal to control resolution.

• In this case both resolutions can be replaced with one term: the system resolution.

Accuracy

• Accuracy refers to a robot's ability to position itswrist end at a desired target point within thework volume, and it is defined in terms of spatialresolution.

• Accuracy is the measure of how close themanipulator can come to a given point within itsworkspace.

• Accuracy depends on many factors likecomputational errors, machining accuracy in theconstruction of the manipulator, flexibility effectsof the links, gear backlash, controller resolution

Repeatability

• Repeatability is the measure of how close the manipulator can return to a previously taught point.

• Repeatability only depends on the controller resolution.

• It is the ability of a robotic system or mechanism to repeat the same motion or achieve the same position.

Envelope

• A three-dimensional shape that defines the boundaries that the robot manipulator can reach; also known as reach envelope.

Reach

• The maximum horizontal distance from the center of the robot base to the end of its wrist.

Pay load

• The maximum payload is the amount of weight carried by the robot manipulator at reduced speed while maintaining rated precision.