Chapter 8- End Effectors

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Chapter 8- End Effectors

• Landstown High School Governors 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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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

Rx
Ry
Rz
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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

• 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

• 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

• 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

• 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

• 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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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

• 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

• 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

• 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?