Robotic End-Effectors
P. Nagarajan
AP/MTRS
Consider typical robots…
What could a robot do without “end effectors”?
Robot End-Effectors
An end-effector is a device attached to the wrist of a manipulator for
the purpose of holding materials, parts, tools to perform a specific task
End Effectors
Grippers
End-effectors used to grasp and
hold objects
Tools
End-effectors designed to
perform some specific tasks
Ex: Spot welding electrode,
Spray gun
Common end effectors
1. Tools
Spot welding
Arc welding
Drilling, grinding, de-burring, etc.
Cutting device (laser, water jet,
torch, etc.)
Spray painting
Common end effectors (cont)
2. Grippers
Mechanical
Vacuum
Magnetized
Adhesive
Simple (hooks, scoops)
Grippers and Tools
Classification of Grippers
Single gripper and double gripper
• Single gripper: Only one gripping device is mounted on the wrist
• Double gripper: Two independent gripping devices are attached to the
wrist
Double gripper
Example: Two separate grippers mounted on the wrist for loading and
unloading applications
Classification of Grippers
Internal gripper vs. External gripper
Internal gripper
External gripper
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Unilateral vs Multilateral
Gripper
Unilateral– only one point or surface is touching the object
to be handled. (fig 1)
Example : vacuum pad gripper & Electro magnetic gripper
Multilateral – more than two points or surfaces touching the
components to be handled (fig.a)
Classification of Grippers
Soft gripper vs. Hard gripper
Hard gripper: Point contact between the finger and object
Soft gripper: Area (surface) contact between the finger and object
Classification of Grippers
Active gripper: Gripper equipped with sensor
Passive gripper: Gripper without sensor
A Few Robot Grippers
Mechanical Grippers
 Use mechanical fingers (jaws) actuated by some mechanisms
 Less versatile, less flexible and less costly
13
Mechanical grippers
• Uses mechanical fingers actuated by a mechanism to grasp an object.
• If the fingers are attachable, then it can be detached and replaced.
• Jaws, make contact with the object different set of finger can be used for
same gripper mechanism
• Grippers with three or more fingers are less common
Working
Function:
To translate some form of power input to grasping action of finger against
part by pneumatic or electric or mechanical power
It is done by
• Physical constriction method
• Friction method
Physical constriction method
• Here fingers enclose the part to an extent
thereby constraining the motion of the
part.
• Done by designing the contacting surface of
the gripper, to be approximately in the
shape of geometry.
Friction method
• By the friction between the gripper and the work part
• Fingers much apply sufficient force for friction to retain the part
against gravity, acceleration.
Disadvantages:
• slippage
continued
To overcome this, gripper must be designed to exert a force
that depends on
• Coefficient of friction between part surface and finger
surface(µ)
• Acceleration of the part
• Orientation between direction of motion during
acceleration and direction of fingers
continued
continued
Another issue is that,
In high speed applications, acceleration of the part could exert a force, that is
twice the weight of the part.
• Hence a new factor called, g factor is introduced.
• µ𝑛𝑓𝐹𝑔 = 𝑤𝑔
• g = 3.0, when acceleration is applied in the same direction as
the gravity force.
• g = 2.0, when acceleration is applied in horizontal direction.
• g = 1, when the acceleration is applied in opposite direction.
Friction-Based Grasping
Manipulators
●
Can range from
simple parallel jawgrippers to multi
fingered manipulators
●
Simpler models of
end-effectors require
less grasp analysis,
but may provide
worse grasps
2-Point/Planar Contact
Manipulators
●
Uses 2- point or 2planar soft-finger
contact friction
between points on
rigid body to ensure
grasp.
●
Is very versatile, but
may not provide
enough contact points
for robust or delicate
grasping.
2-Point/Planar Contact
Manipulators
●
These manipulators
rely heavily on sensor
data of object to
develop a geometrical
model.
●
The robot can
construct a grasp
map using this model
to achieve a grasp
Multi-Contact Point Hand
Manipulators
●
With more fingers, we
can apply more point
and planar contacts
to object
●
Assuming friction,
with more contacts
we can increase the
chance of a stable
force-closure grasp.
Multi-Contact Point Hand
Manipulators
●
The Barrett hand is
an example of a
manipulator that
allows a great deal of
flexibility
Multiple Contact Points/ Multiple
Cooperative Manipulators
●
As we increase the
number of fingers, we
can often increase
the manipulation
capabilities of the
robot
●
The DaVinci medical
robot features 4 end
effectors working in
cooperation
Pros and Cons of Friction-Based
Hand End Effectors
●
May have a highcomputational cost
associated with grasp
analysis
●
Heavily dependent on
sensor data
●
Is appropriate for
delicate and highly
manipulable
scenarios
Mechanical grippers - example
Gripper with linkage
actuation
Mechanical grippers - example
Gripper with rotary actuation
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Mechanical grippers - example
Gripper with cam actuation
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Main Types of Mechanical Grippers
Parallel
Angular
2 or more
“fingers”
Main Types of Mechanical Grippers
Parallel
Angular
2 or more
“fingers”
Mechanical Grippers: Limitations
•Watch out for dynamic forces and moments when sizing a gripper.
•Gripper could drop part with loss in air pressure (can order spring clamps to solve
this problem)
•Angular grippers are less expensive, but the arcing motion of the jaws may require
additional tooling clearance and will grip at varying points as part width varies. A
parallel gripper is simpler to tool to compensate for part size variance.
Mechanical Gripper Specifications
Grip force: ranges tiny forces .1 lbs to over 1600 lbs (don’t forget dynamic forces,
moments)
Part sizes: typically .01 to 36 inches
Number of jaws: typically 2 to 4 jaws or fingers
Repeatability: typically +/- .001 to .005 inches
Cycles to failure: up to 10 million cycles
Supporting technologies: air valves, air compressors, sensors, I/O interfaces
Mechanical Grippers





We can think of a mechanical gripper as a robot hand. A basic robot hand will have only two or three
fingers
A mechanical hand that wraps around an object will rely on friction in order to secure the object it is
holding.
Friction between the gripper and the object will depend on two things, First is the type of surface whether it
be metal on metal, rubber on metal, smooth surfaces or rough surfaces and the second is the force which
is pressing the surfaces together.
Mechanical grippers are often fitted with some type of pad usually made from polyurethane as this
provides greater friction. Pads are less likely to damage the workpiece. Pads are also used so to have a
better grip as the polyurethane will make contact with all parts of the surface when the gripper is closed
Mechanical grippers can be designed and made for specific purposes and adjusted according to the size
of the object. They can also have dual grippers. We are all familiar with the saying ‘two hands are better
than one” and robots benefit from having dual grippers as they can increase productivity, be used with
machines that have two work stations where one robot can load two parts in a single operation, operations
in which the size of objects or part change due to the machining processes and where the cycle time of
the robot is too slow to keep up with the production of other machines.
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Advances in Mechanical Grippers
 Dual grippers
 Interchangeable fingers
 Sensory feedback
 To sense presence of object
 To apply a specified force on the object
 Multiple fingered gripper (similar to human hand)
 Standard gripper products to reduce the amount of
custom design required
Vacuum Gripper
• Also called as “Suction cups”, used for handling certain type of
objects.
• The objects should be flat, smooth, clean and condition necessary to
form a satisfactory vacuum between object and suction cup.
• Suction cup is made of elastic material like rubber or soft plastic.
• When the object to be handled is soft, the cup should be made of
hard substance.
• Two devices can be used: Either Vacuum pump or venturi
Vacuum Gripper
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Types of Vacuum Grippers
“A suitable suction cup is available for virtually any situation
– with very few exceptions. Suction cups are very well suited
for a wide variety of needs and circumstances. They can
handle plate, eggs, paper, wood – practically any material. Be
it smooth, irregular, bent, porous, airtight, lightweight, heavy,
angled, hot or cold, the material can be handled by suction
cups safely, gently and quickly.”
Small to
medium
objects
Large, heavy
objects
Uneven
objects
Types of Vacuum Grippers
Level compensator
A level compensator compensates for differences in level and
absorbs shocks. The mounting also reduces the need for accurate
positioning in height.
Ball joint
A ball joint adjusts itself automatically to different contact angles and
reduces the bending moments on the suction cup. The ball-joint is
recommended in the following typical cases:
•for lifting heavy objects or plates
•for handling workpieces that are in motion
•for handling workpieces that are curved or have irregular
surfaces
Vacuum Grippers Application
PACKAGING:
Canning
Tray Making
Bottling
Box Making
Capping
Labeling
Bagging & Sealing
MATERIAL HANDLING:
Auto Manufacturing
Steel Fabricators
Conveyors
Manufacture & Packaging of Compact Disc
Electronics
Heavy Industry
Vacuum Gripper Specifications
Gripping force: generally up to 300 lbs. Multiple cups can pick up to 6,000 lbs.
Pressures: requires –3 to –15 psi gauge
Sizes: suction cups typically range between .05 and 18 inch diameters
Supporting Technologies: Compressors, valves, air filters, controllers
Suction Grippers
•
There are two types of suction grippers:
1.
Devices operated by a vacuum – the vacuum may be provided by a vacuum
pump or by compressed air
2.
Devices with a flexible suction cup – this cup presses on the workpiece.
Compressed air is blown into the suction cup to release the workpiece. The
advantage of the suction cup is that if there is a power failure it will still work as
the workpiece will not fall down. The disadvantage of the suction cup is that
they only work on clean, smooth surfaces.
•
There are many more advantages for using a suction cup rather than a
mechanical grip including: there is no danger of crushing fragile objects, the
exact shape and size does not matter and the suction cup does not have to be
precisely positioned on the object
•
The downfalls of suction cups as an end effector include: the robot system must
include a form of pump for air and the level of noise can cause annoyance in
some circumstances
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Suction-Based End Effectors
●
Suction manipulators
are good for lowweight, small objects
●
Acceleration of end
effector affects max
load that suction
gripper can carry for
many objects
Suction-Based End Effectors
●
Suction gripper end
effectors are used
often in industrial
settings, since they
require a vacuum
system.
●
Unless the object has
a planar surface,
precise manipulation
can be difficult
Advantages/Disadvantages
●
Suction based robots
require less grasp
analysis
●
Are usually not
appropriate for nonplanar large objects
●
Can manipulate rigid
planar objects very
well
Adept Quattro Robot
●
The Adept Quattro
robot is a high-speed
industrial robot used
for packing and
sorting
●
It uses a suction end
effector to lift and
place small objects
Magnetic grippers
• Used for magnetic materials only.
For example: various steels( but not stainless steel)




Can use either electro-magnets or permanent magnets
Pick up time is less
Can grip parts of various sizes
Disadvantage
 residual magnetism
 Problem of picking up only one sheet from the stack
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Magnetic grippers
Solution for Problem of picking up only one sheet from the stack:
• This is caused because the magnetic attraction, tend to penetrate
beyond the top sheet of the stack, hence more than one sheet is
picked.
• This can be addressed by
1. Limit the effective penetration to desired depth
2. Using “Fanner” circuit
Fanner circuit
• This circuit uses the magnetic field to induce a charge, in the ferrous
sheets in the stack.
• Each sheet towards the top of the sheet is given with a magnetic
charge, causing them to posses same polarity and repel each other.
• The sheet most affected is the one at the top of the stack.
• Hence it tries to rise above the remainder of the stack, thus
facilitating pick up of the robot gripper.
Types of magnetic gripper
Magnetic
gripper
Permanent
magnet
Electromagnet
Permanent magnet type
• Do not require any external power source.
• Often used in handling tasks at hazardous environment.
• Loss of control on the part.
Ex:
• When a part is to be released at the end of a handling cycle, some
means of separating part from the magnet must be provided.
• This is done using “Stripper or stripping device”.
• Its function is to “mechanically detach the part from the
magnet”.
Magnetic Gripper with stripper
53
Electromagnet type
• Easy to control
• Requires a source of dc power and a appropriate controller unit.
• While releasing the part, the controller reverses the polarity at
reduced power level, before switching off the electromagnet.
• This cancel outs residual magnetism and ensures positive release of
the part.
Adhesive Gripper
 Grasping action using adhesive substance.
 To handle fabrics and lightweight materials.
Disadvantage:
 It looses its tackiness on repeated usage. Hence, reliability is
diminished with successive operations.
 To overcome this, the adhesive material is loaded in a continuous
ribbon form into a feeding mechanism that is attached to the robot
gripper.
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Hooks, Scoops and miscellaneous devices
Hooks:
• To load and unload parts hanging from overhead conveyors.
Scoops:
• Can handle materials in liquid and powder form.
Disadvantage:
• Difficult to control
• Spillage during handling
Hooks, Scoops and miscellaneous devices
Inflatable bladder:
• Fabricated out of rubber or other elastic material, which makes it
appropriate to grip fragile objects.
• The gripper applies uniform pressure over the gripping object rather
than concentrated force as in mechanical gripper.
Types of Grippers
There are four main categories which makes use of a gripper
1.
No gripping – in this situation the workpiece is held in a jig (a specially
designed purpose built holder) and the robot performs an activity on it.
Jobs which use no gripping can include spot welding, flame cutting and
drilling.
2.
Coarse gripping – in this case the robot holds the workpiece but the
gripping does not have to be precise. Jobs which use coarse gripping
include handling and dipping castings, unloading furnaces, stacking boxes
or sacks.
3.
Precise gripping – A robot holds the workpiece which requires accurate
positioning for example unloading and loading machine tools.
4.
Assembly – the robot is required to assemble parts which requires
accurate positioning and some form of sensory feedback to enable the
robot to monitor and correct its movements.
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Hybrid Suction/Friction “Universal
Gripper” End Effector
●
Uses a balloon full of
coffee grounds to
produce an adaptive
grasp that can allow
manipulation of
irregular objects
●
Excellent versatility
Tools
• In most applications, robot is required to manipulate a tool.
• The end effector is designed to grasp and handle these tools.
• In many applications, tool is directly attached to the wrist.
• Some examples are
1.
2.
3.
4.
5.
6.
7.
Spot welding
Arc welding
Spray painting nozzle
Liquid cement applicator for assembly
Drilling , grinding
Heating torches
Water jet cutting tool