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Chapter 8- End Effectors

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Title: Chapter 8- End Effectors


1
Chapter 8- End Effectors
  • Landstown High School Governors STEM
    Technology Academy
  • Advanced Robotics

2
Robot Geometry
  • In order to understand how end effectors work on
    a robot, you must understand Robot Geometry.

3
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.

4
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

5
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
6
Types of Robotic Arms
7
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

8
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

9
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

10
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

11
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

12
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

13
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

14
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.

15
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

16
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.

17
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.

18
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.

19
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

20
End Effectors
21
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.

22
Types of Grippers
23
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

24
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.

25
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.

26
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

27
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

28
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.

29
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

30
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.

31
Citations
  • Robotics Therory and Industrial Applications,
  • mrjob.mathsclass.net?
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