RAT 281 BASICS OF ROBOTICS
MODULE 3
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Syllabus
Robot configurations-PPP, RPP, RRP, RRR; features of SCARA, PUMA
Robots; Classification of robots based on motion control methods and drive
technologies; 3R concurrent wrist;
Classification of End effectors - mechanical grippers, special tools,
Magnetic grippers, Vacuum grippers, adhesive grippers, Active and passive
grippers, selection and design considerations of grippers in robot.
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Robot configurations
• The DOF of a manipulator is distributed into two
subassemblies
• Arm – positioning the end-effector.
• Wrist – orienting the end-effector.
• The four basic arm configurations are:
1. Cartesian or Rectangular
2. Cylindrical
3. Polar or Spherical
4. Articulated
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Cartesian or Rectangular configuration
• When the arm of the robot moves in a rectilinear mode i.e., to
the directions of x, y and z coordinates of the rectangular
Cartesian coordinate system.
• Only linear motion along 3 perpendicular axis
• All 3 joints are prismatic - PPP
• Cartesian robot needs large volume to operate.
• Workspace is rectangular
• Maintenance is difficult
• Expensive
• Movements are:
• travel x
• height or elevation y
• reach z
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Cylindrical configuration
• One revolute and two prismatic joints
• RPP configuration
• Workspace is cylindrical
• Movements are:
• base rotation, θ
• height, y
• radius, z
• Used when a task requires reaching into small
openings or working on cylindrical surfaces
• Eg. Welding pipes
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Polar or Spherical configuration
• One prismatic and two revolute joints.
• RRP configuration.
• Workspace is partial spherical.
• Movements are:
• base rotation, θ
• elevation angle, ϕ
• reach, z
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Articulated or Revolute configuration
• All three revolute joints - RRR
• Best simulates a human arm.
• Workspace is spherical.
• Articulated robots are the most common robot type used in
industrial settings.
• Articulated robots are defined as robots containing rotary
joints.
• These joints are commonly referred to as axes in the robotic
world.
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Robot Configurations
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SCARA configuration
• In SCARA the ccharacteristics of cylindrical and articulated
configuration is combined.
• Selective Compliance Assembly Robot Arm
• It is a simple articulated robot which can perform assembly tasks
precisely and fast. Eg: inserting a round pin in a round hole without
binding.
• SCARA is more or less like a human arm, it cannot rotate along an
axis other than vertical.
• SCARAs are generally faster than comparable Cartesian robot
systems.
• Their single pedestal mount requires a small footprint and provides an
easy mounting.
• On the other hand, SCARAs more expensive than comparable
Cartesian systems
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SCARA
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PUMA
• PUMA is an industrial robotic arm developed at pioneering
robot company Unimation.
• Programmable Universal Machine for Assembly Robots
• PUMA is the most commonly used industrial robot in assembly,
welding operations and university laboratories.
• PUMA resembles more closely to the human arm than SCARA.
• PUMA has greater flexibility than SCARA but with the
increased compliance comes the reduced precision.
• PUMA is preferably used in assembly applications which do not
require high precision, such as stocking operations.
• PUMA is an articulated robot with a chain of members
connected with each other through revolute or rotary joints.
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Structure of PUMA
• PUMA has six degree of freedom.
• Each rotary joint is actuated by DC servomotors and accompanying
gear trains.
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Wrist configuration
• It requires only rotary joints.
• It permits rotation about three perpendicular axis ie.,
1. Roll
2. Pitch
3. Yaw
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3R Concurrent Wrist
• A 3R concurrent wrist consists of a three DOF
rotational joint with all three axes of rotation intersect
at one point.
• It is also called a spherical wrist.
• The non-spherical configuration hand have a shifted
wrist axis of rotation.
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Classification of Robots
• Based on Drive Technologies or Actuation System:
• Electric – Most commonly used
• Pneumatic – In light assembly or packing works
• Hydraulic – Heavy payload applications
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Classification of Robots
• Electric robot employs DC servo motors, stepper motors or
brushless ac motors.
• These robots have the advantage that they are clean and
relatively easy to control.
• Hydraulic robot is preferred for tasks that require a large load
carrying capacity.
• Care and maintenance is required to handle leaks and fluid
compressibility problems.
• Pneumatic robot is often preferred for high speed applications.
• These robots are generally clean, but can be hard to control due
to challenges associated with air compressibility.
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Classification of Robots
• Based on control methods:
• Motion control strategy :
1. Servo controlled – closed loop
2. Non-servo Control – open loop
• Motion path strategy :
1. Point to point motion control
2. Continuous path motion control
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Classification of Robots
• Motion control strategy :
1. Servo controlled – closed loop
• Commands are sent to the arm and the actual movement is
monitored.
• The difference between command and the action is fed back to
the controller for further commands.
2. Non-servo Control – open loop
• A command signal is sent and it is assumed that the robot
reaches the required position.
• This type of control is adequate for positioning of light loads.
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Classification of Robots
• Motion path strategy :
1. Point-to-point motion control
• The robot moves from one point to desired point without regard
to the path taken between them.
• PTP control is used for assembly, palletizing, machine tool
loading/unloading, Spot welding etc.
2. Continuous path control
• The robot moves along a continuous path with specified
orientation.
• Application: Welding of two metal parts along a straight line or a
specified curve.
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Classification of End-effectors
• End-effectors are the parts attached at the end of a robot
manipulator.
• This is equivalent to the human hand.
• The end-effectors can be grippers or special tools like welding
electrode, gas-cutting torch, painting brush, debarring tool or
grinding wheel attached at the end of the arm to perform specific
tasks.
• Grippers are end-effectors which are used to grasp an object or a
tool and hold it.
• Tasks required by the grippers are to hold work pieces and
load/unload from/to a machine or conveyer.
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Assignment 1
• Write notes on any five special tools for robotic
applications.
• Examples:• Welding electrode
• Painting tool
• Gas-cutting torch
• Debarring tool
• Grinding wheel
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Grippers
• Grippers can be mechanical in nature using a combination of
mechanism driven by electric, hydraulic or pneumatic powers.
• Grippers can be classified based on:
• The principle of grasping mechanism.
• Magnetic grippers
• Vacuum grippers
• Adhesive grippers
• How it holds an object.
• External gripper – grasp object on its exterior surface
• Internal gripper - grasp object on its interior surface
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Mechanical Grippers
• Mechanical grippers have their jaw movements through pivoting
or translational motion using a transmission elements like
linkages or gears, etc.
• The function of a gripper mechanism is to translate some form
of power input (electric, hydraulic or pneumatic) into the
grasping action of the gripper against the part.
• The gripper can be of single or double type.
• Single gripper has only one gripping device at the robot's wrist,
the latter type has two.
• The double grippers can be actuated independently and are
especially useful in machine loading and unloading.
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Single and Double Gripper
• Task - A raw part is to be loaded from a conveyor onto a
machine and the finished part is to be unloaded onto another
conveyor.
• With a single gripper, the robot would have to unload the
finished part before picking up the raw part. This would
consume valuable time in the production cycle.
• With a double gripper, the robot can pick up the part from the
incoming conveyor with one of its gripping devices and have it
ready to exchange for the finished part on the machine.
• When the machine cycle is completed, the robot can reach in for
the finished part device, and insert the raw part into the machine
with the other grasping device.
• The amount of time spent in changing the parts of the time to
keep the machine idle is minimized.
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Single and Double Gripper
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Mechanical Grippers
• A mechanical gripper uses its fingers or jaws to hold an object.
• There are two ways a gripper can hold an object
• By physical constriction - contacting surfaces of the fingers
are made of approximately the same shape of the part
geometry
• By friction - the fingers must apply sufficient force to retain
the part against gravity or accelerations. The friction method
of holding a part is less complex and hence less expensive.
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Mechanical Grippers
• The motion of the fingers in mechanical grippers is
obtained by means of any of the following transmission
elements:
•
•
•
•
•
Linkage
Gear and rack
Cam
Screw
Cable and pulley
• The final gripping action of the fingers is achieved by
• Pivoting Movement- In this arrangement, the fingers rotate
about fixed pivot points on the gripper to open and close. The
motion is usually achieved by some kind of linkage
mechanism.
• Translational movement -In the translational or linear
motion, the fingers open and close by moving parallel to each
other.
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Pivoting and translating mechanical grippers
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Pivoting Movement
Translational Movement
Magnetic Grippers
• The principle of a magnetic gripper is based on the magnetic
property of a gripper. Hence, they can be used only for ferrous
objects.
• Advantages:
• Variations in object sizes can be tolerated.
• Operations are very fast.
• Require only one surface to hold an object.
• Disadvantage:
• The difficulty to pick thin sheets one at a time because the
magnetic force penetrates through more than one sheet. As a
result, more than one sheet is picked up.
• To overcome such disadvantages, one needs to take care during
the design stage itself either by limiting the magnetic force or by
introducing some means (mechanical) not to allow more than
one sheet to be picked up.
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Magnetic Grippers
• Magnetic grippers can have
• Permanent magnets
• do not require any external power source to operate the magnets.
• no electric sparks in handling hazardous materials.
• require an external stripping mechanism during the release of the object
• Electromagnets
• easy to control
• require only a dc power source.
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Vacuum Grippers
• Vacuum grippers are suitable to handle large flat objects.
• The material of an object is of no concern with vacuum gripers, except
that the object's surface should not have any holes.
• An example of vacuum gripper which uses suction cups made of
elastic materials is shown in figure.
• For a vacuum gripper, lifting capacity can be determined from the
negative pressure and the effective area of the cups as
f =pA
where f is the force on lift capacity, p is the negative pressure and A is
the total effective area of the suction cups
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Adhesive Grippers
•
•
•
•
An adhesive substance is used for grasping action.
It grasps objects by literally sticking to them.
Used to handle fabrics and other lightweight materials.
Advantage - It is simple. It continue to function without maintenance
as long as the material keeps its stickiness
• Limitation – Adhesive substance loses its effectiveness with repeated
use. Hence it has to be continuously fed like a mechanical typewriter’s
ribbon which needs to be attached to the robot's wrist.
• It cannot readily be disabled in order to release the grasp on an object
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Hooks and Scoops
• Hooks and Scoops are the simplest type of end effectors that can
be classified as grippers.
• A scoop or ladle is commonly used to scoop up molten metal
and transfer it to the mould.
• A hook may be all that is needed to lift a part especially if
precise positioning in not required and if it is only to be dipped
into a liquid.
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Expandable bladder type gripper
• It is a specialized gripper that can be used to go pick up and move rod
shaped or cylindrical objects.
• The main element of the gripper is an inflatable donut shaped or
cylindrical sleeve that resembles the cuff used in blood pressure
measuring apparatus.
• The sleeve is positioned so that it surrounds the object to be gripped
and then the sleeve is inflated tight enough to complete the desired
task.
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Active and Passive Grippers
• Active Grippers
• require continuous pressurization during gripping and
holding.
• require actuation to maintain a grip on an object.
• Passive Grippers
• only requires energy for the release process.
• requires power to open and close but not to maintain a grip.
• passive grippers minimize power consumption.
• In power-limited environments, such as those where robots
rely on battery or solar power or have unreliable access to
power, passive grippers provide a unique solution.
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Selection and design considerations of grippers
• Decide the drive system
• Electric and pneumatic power sources are easily available
• If fast but not so accurate motion is desired then pneumatic should be
preferred.
• For heavy objects, certainly one must prefer hydraulic.
• Object or the part surface to be gripped must be reachable by the gripper.
• Size variation: During machining operations, there will be a change in the
work part size. As a result, the gripper must be designed to hold a work part
even when the size is varied.
• The gripper should tolerate some dimension change. For example, before and
after machining a work piece, their sizes change.
• Quality of surface area must be kept in mind. For example, a mechanical
gripper may damage the surface area of an object.
• Force should be sufficient to hold an object and should not fly away while
moving with certain accelerations. Accordingly, the materials on the surfaces
of the fingers should be chosen.
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Selection and design considerations of Grippers
• The following significant factors must be considered to
determine the necessary gripping force that must be applied to
pick up a work part. .
• Consideration must be taken to the weight of a work part.
• It must be capable of grasping the work parts constantly at
its center of mass.
• The speed of robot arm movement and the connection
between the direction of movement and gripper position on
the work part should be considered.
• It must determine either friction or physical
constriction helps to grip the work part.
• It must consider the co-efficient of friction between the
gripper and work part.
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Previous University Questions
1. Choose a gripper that can handle large flat sheets of any type
of material with justification for the choice and necessary
diagrams (3)
2. Explain the 3R concurrent wrist with a neat diagram. (3)
3. Select a robot suitable for assembly of printed circuit boards
with justification and necessary diagrams (7)
4. Explain the features of a PUMA robot with a diagram. (7)
5. Explain in detain the four basic robotic arm configurations
with necessary diagrams. (14)
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THANK YOU
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