FUNCTIONS OF ROBOTS
Technical aspects of robotics:
Axes of motion
Degrees of freedom
Robotic arms
Actuators
End effectors
Sensors
Robot Vision
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13 April 2015
Robot Geometry
End Effectors
Robotic Arms
Sensors
Actuators
Robot Vision
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Robot Geometry
1. What is robot geometry?
2. Types of robotic arms
3. Actuators
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Robot Geometry
What is Robot Geometry?
The angles that a robot makes with the ground
The degrees of freedom a robotic arm has and the
axes of motion
How many shapes can be seen in a robot
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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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Axes of Motion
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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Degrees of Freedom
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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x
Rx
Rz
z
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Ry
y
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Types of Robotic Arms
Anthropomorphic System
Cartesian System
Cylindrical System
Polar System
SCARA System
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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
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 (Y, Z
and ø)
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 ø 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
1.
2.
3.
Actuator is the term used for the mechanism that
drives the robotic arm.
There are 3 main types of Actuators
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
1.
2.
3.
4.
5.
1.
2.
3.
4.
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
Movement of the piston can be smooth and fast
Position of the piston can be controlled precisely by a low-current
electrically operated value
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
1.
2.
3.
4.
5.
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
1.
2.
3.
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
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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Hardware to control robots
End Effectors
Sensors
Robot Vision
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End Effectors
What is an end effector?
Types of end effectors
Grippers
Tools
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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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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
1.
2.
3.
4.
There are four main categories which makes use of a griper
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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Types of Grippers
Mechanical
Suction
Magnetic
Hooks and Scoops
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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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Suction Grippers
1.
2.
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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Sensors
What is a sensor?
Types of sensors
Optical
Heat
Acoustic
Proximity
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What is a sensor?
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Types of Sensors
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Optical Sensors
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Heat Sensors
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Acoustic Sensor
We are all familiar with the microphone – a
microphone is a device for converting
sound waves into electrical signals.
An acoustic sensor interprets the sound
that it senses and requires a computer
Basic acoustic sensors are used in robots
to responds to a human shout or the noise
of an explosion.
A more sophisticated sensor would be able
to recognise words and take appropriate
action and makes use of the concept of
speech recognition
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Proximity Sensor
A proximity sensor is a device that can
sense the presence of an object, just by
being close to it.
Sensors of this type have been made by
using the magnetic effect induced by the
presence of metals, the effect on electrical
capacitance induced by the presence of
most other materials or by detecting
reflected light or reflected ultrasonic sound.
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Robot Vision
What is robot vision?
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Robot Vision
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CONGRATULATIONS
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TRY AGAIN
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ACTIVITY 1:
Using five of the robots in your
portfolio (from the artificial intelligence
booklet), identify the job it is designed
to do. For example spray painting a
car. Then classify each robot into a
size category; the spray painting robot
can cover a large area.
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ACTIVITY TWO
Now using the five robots you used in
the previous activity, identify the axes
of motion for each, drawing arrows on
your robot diagram. If in doubt
discuss with the person next to you
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ACTIVITY THREE:
Once again use the five robots you
used in the previous activity, identify
the degrees of freedom for each,
drawing arrows on your robot
diagram. If in doubt discuss with the
person next to you. Remember
degrees of freedom is the number of
movements the end effector can
make.
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ACTIVITY FOUR
Think for a moment about the work
envelop of your own arm.
What movements can your arm make?
What shape does it make?
What size is the envelope?
Investigate the TALON V polar geometry
robot and the ASEA industrial robot and
compare their work envelops.
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The Work Envelop
Standing in one place, you can extend you arm
upwards, downwards and to either side.
Obviously it doesn’t reach that far behind you
because the joints only move through limited
angles and some areas cannot be reached
because the rest of your body gets in the way.
You can visualise an imaginary chamber or
‘envelop’ which surrounds your arm; the end of
your arm can reach anything placed within this
envelop but anything outside the envelope is
inaccessible. The shape of this work envelop is
hard to draw, but it is roughly the shape of a
sphere, almost like an apple with a large bite
taken out of it.
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Questions
Name the types of robotic arms
What is robot geometry?
What is an end effector?
What is an actuator?
What do we mean by degrees of freedom?
What do we mean by the term axes of motion?
Describe the term work envelop
Name four types of grippers, giving a brief description
What is the difference between a hydraulic actuator and a
pneumatic actuator?
Describe the Anthropomorphic System
Describe the Cartesian system
Describe the polar system
Describe the SCARA system
Describe the cylindrical system
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