Chapter 8- End Effectors
Landstown High School Governor’s STEM & Technology Academy
Advanced Robotics
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Robot Geometry
• In order to understand how end effectors
work on a robot, you must understand Robot
Geometry.
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What is Robot Geometry?
• The design of a robot arm needs to take into
consideration whether it is to be able to cover
a large area, perform intricate movements, lift
heavy loads or move with great speed or a
combination of these things.
• Robots can be programmed to do any job,
however it is best if a robot is designed with
some particular job in mind.
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• This diagram has the axes
of motion marked with
arrows – there are 3
• A robot may in theory have
any number of axes.
• The more axes there are
the more manoeuvrable
the robot is, however the
more complicated it will be
to program
• Axes of motions do not
need to be rotations – they
can be motions along a
straight line
• Axes that allow rotations
are known as revolute
joints
• Axes that allow movement
in a straight line are known
as prismatic joints
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Axes of Motion
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• The number of
independent directions
in which the end
effector (tool or gripper)
of the robot can move
• Any solid object has a
maximum of six degrees
of freedom
• X, Y and Z represent
movement along a line
• Rx, Ry and Rz represent
rotations
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Degrees of Freedom
x
Rx
Ry
Rz
z
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Types of Robotic Arms
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The Anthropomorphic System
• The articulated system
• Like a human arm (sections
are joined together)
• “Shoulder” and “Elbow” are
used to refer to the two joints
• More sections can be added
if needed.
• Most manoeuvrable
• Used for paint spraying
• Cannot cover a large area
• Difficult to move the end of
the arm in a straight line
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The Cartesian System
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The X, Y Z system – three
independent directions
Best used when a large area
needs to be covered
Not good when intricate
movements are required
X axis allows the arm to move
along the work piece
Y axis allows the arm to move
towards and away from the work
piece
Z axis allows movement upwards
and downwards
The three movements are at 90
degrees to the other two
This robotic arm can move over
the surface of an imaginary
rectangle
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The Cylindrical System
• Similar to the cartesian
except no X axis
• Arm can rotate on a central
support
• Angle of rotation is referred
to by the symbol ø
• Three axis of motion (X, Y,
and Z )
• This robotic arm can move
over the surface of an
imaginary circle
• Can move much faster than
the cartesian robot arm
• Can be used for loading or
unloading
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The Polar System
• Spherical system
• Same Y and Z axes as
cylindrical
• Arm is pivoted so that it
can rotate in the vertical
plane instead of moving up
and down along the z axis
• This robotic arm can cover
the surface of a sphere
• Can move faster in the
vertical direction than a
cylindrical arm
• Range of movement is
much more restricted
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The SCARA System
• Selective Compliant Assembly Robot Arm
• All revolute joints in the arm rotate about the vertical
axes
• Three degrees of freedom
• Used for assembly operations
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The Work Envelope
• The work envelope is the area that a robot can
cover.
• The exact size and shape of the work envelope
will be one of the main factors in deciding
whether the robot is suitable for a particular
job.
• The size and shape vary enormously
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Actuators
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Actuator is the term used for the mechanism that drives the
robotic arm.
There are 3 main types of Actuators
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Electric motors
Hydraulic
Pneumatic cylinder
Hydraulic and pneumatic actuators are generally suited to
driving prismatic joints since they produce linear motion
directly
Hydraulic and pneumatic actuators are also known as linear
actuators.
Electric motors are more suited to driving revolute joints as
they produce rotation
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Hydraulic Actuators
• A car makes use of a hydraulic system. If we look at the
braking system of the car we see that only moderate force
applied to the brake pedal is sufficient to produce force large
enough to stop the car.
• The underlying principle of all hydraulic systems was first
discovered by the French scientist Blaise Pascal in 1653. He
stated that “if external pressure is applied to a confined fluid,
then the pressure is transferred without loss to all surfaces in
contact with the fluid”
• The word fluid can mean both a gas or a liquid
• Where large forces are required we can expect to find
hydraulic devices (mechanical diggers on building sites, pit
props in coal mines and jacks for lifting cars all use the
principle of hydraulics.
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Hydraulic Actuators
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Each hydraulic actuator contains the following parts:
Pistons
Spring return piston
Double acting cylinder
Hydraulic transfer value
And in some cases a hydraulic accumulator
Advantages of the hydraulic mechanism
A hydraulic device can produce an enormous range of forces without the
need for gears, simply by controlling the flow of fluid
2. Movement of the piston can be smooth and fast
3. Position of the piston can be controlled precisely by a low-current
electrically operated value
4. There are no sparks to worry about as there are with electrical motor, so
the system is safe to use in explosive atmospheres such as in paint
spraying or near inflammable materials
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Pneumatic Actuators
• A pneumatic actuator uses air instead of fluid
• The relationship between force and area is the same in a
pneumatic system compared to a hydraulic system
• We know that air is compressible, so in order to build up the
pressure required to operate the piston, extra work has to be
done by the pump to compress the air. This means that
pneumatic devices are less efficient
• If you have ever used a bicycle pump you may have noticed
that it becomes hot as it is used. The heat produced by the
mechanical work done in compressing the air. Heat represents
wasted energy.
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Pneumatic Actuators
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Advantages of the Pneumatic system:
Generally less expensive than an equivalent hydraulic system. Many factories
have compresses air available and one large compressor pump can serve
several robots
Small amount of air leakage is ok, but in a hydraulic system it will require
prompt attention
The compressibility of air can also be an advantage in some applications. Think
about a set of automatic doors which are operated pneumatically. If a person is
caught in the doors they will not be crushed.
A pressure relief valve can be incorporated to release pressure when a force is
exceeded, for example the gripper of a robot will incorporate a relief value to
ensure it does not damage itself or what it is gripping
Pneumatic devices are faster to respond compared to a hydraulic system as air
is lighter than fluid.
A pneumatic system has its downfalls and the main one is that it can produce
the enormous forces a hydraulic system can. Another is concerned with the
location of the pistons. As air is compressible heavy loads on the robot arm may
cause the pistons to move even when all the valves on the cylinder are closed.
It is for this reason that pneumatic robots are best suited for pick and place
robots.
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Electric Motors
• Not all electric motors are suited for use as actuators
in robots
• There are three basic characteristics of a motor,
when combined will determine the suitability of a
motor for a particular job. The 3 characteristics are
power, torque and speed. Each of these
characteristics are interdependent, that means that
you can not alter one without affecting the others.
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Electric Motors
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Two types of power: electrical and mechanical, both are measured in
watts.
Torque is how strong a motor is or how much turning force it is able to
produce and is measured in newton-metres.
The speed is measured in revolutions per minute and is rotation of the
motor
There are 3 different types of motors
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AC motor which operates by alternating current electricity
DC motor which operates by direct current electricity
Stepper motors which operates by pulses of electricity
Any type of electric motor could be used for a robot as long as it is
possible to electronically control the speed and power so that it behaves
the way we want.
DC motors and Stepper motors are commonly used in robotics
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End Effectors
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What is an end effector?
• An end effector is the device that is at the end of a
robotic arm.
• There are two main types of end effectors: Grippers
and tools.
• We can think of an end effector like a human hand.
Even though a human hand is very versatile, an end
effector has one great advantage that humans do
not have and that is the interchangablility of end
effectors. If the end effector is not suitable than it
can be changed unlike the human hand.
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Types of Grippers
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Grippers
• Grippers are devices which can be used for holding or gripping
an object.
• They include what you might call mechanical hands and also
anything like hooks, magnets and suction devices which can
be used for holding or gripping.
• Grippers take advantage of point-to-point control (exact path
that the robot takes between what it is picking up and where
it is placing it.
• Grippers should be designed so that it requires the minimum
amount of manoeuvring in order to grip the work piece
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Types of Grippers
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There are four main categories which makes use of a gripper
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
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
Precise gripping – A robot holds the workpiece which requires accurate
positioning for example unloading and loading machine tools
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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Mechanical Grippers
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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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Suction Grippers
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There are two types of suction grippers:
Devices operated by a vacuum – the vacuum may be provided by
a vacuum pump or by compressed air
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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Magnetic Grippers
• Magnetic grippers obviously only work on magnetic objects and therefore
are limited in working with certain metals.
• For maximum effect the magnet needs to have complete contact with the
surface of the metal to be gripped. Any air gaps will reduce the strength of
the magnetic force, therefore flat sheets of metal are best suited to
magnetic grippers.
• If the magnet is strong enough, a magnetic gripper can pick up an irregular
shaped object. In some cases the shape of the magnet matches the shape
of the object
• A disadvantage of using magnetic grippers is the temperature. Permanent
magnets tend to become demagnetized when heated and so there is the
danger that prolonged contact with a hot workpiece will weaken them to
the point where they can no longer be used. The effect of heat will
depend on the time the magnet spends in contact with the hot part. Most
magnetic materials are relatively unaffected by temperatures up to
around 100 degrees.
• Electromagnets can be used instead and are operated by a DC electric
current and lose nearly all of their magnetism when the power is turned
off.
• Permanent magnets are also used in situations where there is an explosive
atmosphere and sparks from electrical equipment would cause a hazard
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Hooks and Scoops
• Hooks and scoops are the simplest type of end
effectors that can be classes 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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Tools
• Tools are devices which robots use to perform operations on
an object, for example, drills, paint sprays, grinders, welding
torches and any other tool which get a specific job done.
• Tools take advantage of continuous path control (the path the
end effector takes needs to careful, steady and continuously
controlled at every moment)
• If we think of a spray gun and if it moves to quickly then the
paint will be too thin on the other hand if it moves to slowly
the paint will be too thick or in blobs.
• Any tool required can be fitted to the end of the robotic arm
and can be programmed to select and change tools without
human intervention
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Types of Tools
• A common tool used as an end effector is the
welding tool. Welding is the process of joining two
pieces of metal by melting them at the join and there
are 3 main welding tools: a welding torch, spot
welding gun and a stud welding tool
• Other common tools are paints praying, deburring
tools, pneumatic tools such as a nut runner to
tighten nuts.
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Citations
• Robotics: Therory and Industrial Applications,
• mrjob.mathsclass.net
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