robotics and effector

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UNIT-3
ROBOT END EFFECTORS
BY
RAVI KUMAR MANDAVA

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End effectors
An end effector is usually attached to the robot's wrist, and it allows the robot to accomplish a
specific task. This means that end effectors are generally custom-engineered and fabricated for
each different operation. There are two general categories of end effectors viz. grippers and tools.
Grippers grasp and manipulate the objects during the work cycle. Typically objects that grasped
are the work parts which need to be loaded or unloaded from one station to another. Grippers may
be custom-designed to suit the physical specifications of work parts.
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

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Grippers Tools

4. APPLICATIONS FOR GRIPPERS
• Loading
• Unloading
Fig. Robot in loading applications

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Classification of Grippers
1. 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
Example: Two separate grippers mounted on the wrist for loading and
unloading applications

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2. Internal gripper vs. External gripper
Internal gripper External gripper

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3. Soft gripper vs. Hard gripper
Hard gripper: Point contact between the finger and object
Soft gripper: Area (surface) contact between the finger and object

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4. Active Gripper and Passive Gripper
 Active gripper: Gripper equipped with sensor
 Passive gripper: Gripper without sensor

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Types of Gripper Mechanism
 Pivoting Movement
The fingers rotate about fixed pivot points on the grippers to open and close.
 Linear or translational Movement
The fingers open and close by moving in parallel to each other.

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A Few Robot Grippers
1. Mechanical Grippers
 Use mechanical fingers (jaws) actuated by some mechanisms
 Less versatile, less flexible and less costly

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Examples
i. Gripper with linkage actuation

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ii. Gripper with rotary actuation

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iii. Gripper with cam actuation

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2. Vacuum Gripper (used for thin parts)

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•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

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3. Magnetic Gripper (for magnetic materials only. For example: various steels
but not stainless steel)
Can use either electro-magnets or permanent magnets
Advantages:
 Pick up time is less
 Can grip parts of various sizes
 They have ability to handle metal parts with holes (not possible with vacuum
grippers).
 It requires only one surface for gripping.
Disadvantage:
 Residual magnetism
 Picking up only one sheet from a stack.

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Magnetic Gripper
 Stripping device: for separating the part from the permanent magnet
 For separating the part from electro-magnet, reverse the polarity

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4. Adhesive Gripper
 Grasping action using adhesive substance
 To handle lightweight materials
5. Universal Gripper
Example: Human gripper

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Tools as end effectors: A tool is equipped in the robot for carrying out several operations on the
work parts instead of grasping it. A tool acts as an end effector when it is attached directly to
the robot’s wrist. In some applications, there will be a need for multi-tool task, and changing the
tool all the time from the robot wrist will be highly difficult. As a result, a gripper is used in this
process to grasp and manipulate the tool. It certainly helps the robot to handle several tools in an
operation, and thus makes the multi-tooling function possible. Moreover, the time taken to change
the tool is very low.
For example: In a deburring operation, various sizes of deburring tool will be required to hold
for reaching every surface of the work part. Here, the tool is equipped in the gripper for quick
exchange from one tool to another.

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Examples for tools used as end effectors:
In robot applications, the most commonly used three tools as end effectors are listed below:
 Spot welding tools
 Spray painting nozzle
 Arc welding torch
The rotating spindle for routing, drilling, grinding, and wire brushing operations also comes under
this category. Some other examples of tools used as end effectors are liquid cement applicators
water jet cutting tool, and heating torches.
In all the above examples, the actuation of a tool must be coordinated by an industrial robot. An
example of this case includes the spot welding operation in which the robot must control the
actuation as a part of its process. The control of this process is very similar to the opening and
closing of a mechanical gripper.

22. TOOLS DIRECTLY ATTACHED TO ROBOT WRIST
Fig. Arc welding

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End effector Interface:
 Physical support of the end effector during the work cycle must be provided.
 Power to actuate the end effector must be supplied through the interface.
 Control signals to actuate the end effector must be provided.
 Feedback signals must sometimes be transmitted back through the interface to the robot
controller.

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Physical support of the end effector:
 The physical support of the end effector is achieved by the mechanical connection between the
end effector and the robot wrist.
 This mechanical connection often consists of a face plate at the end of the wrist to which the
end effector is bolted.
 There should be three characteristics taken into consideration in the design of the mechanical
connection.
Strength
Compliance
Overload
Strength: It refers to its ability to withstand the forces associated with the operation of the end
effector. These forces include the weight of the end effector, the weight of the object being held
by the end effector if it is a gripper, acceleration and deceleration forces, and any applied forces
during the work cycle.
The wrist socket must provide sufficient strength and rigidity to support the end effector against
these various forces.

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Compliance: Compliance refers to the wrist socket’s ability to yield elastically when subjected to
a force.
In effect, it is the opposite of rigidity .
RCC is inappropriate for assembly of pegs in
horizontal direction
Insertion angle must be less than 45 degrees
Cannot be used in chamferless insertion tasks

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Overload:
 The third factor which must be considered relative to the mechanical interface between the
robot wrist and the end effector is overload protection.
 An overload results when some unexpected event happens to the end effector such as a part
becoming struck in a die, or a tool getting caught in a moving conveyor.
 Overload protection is intended to eliminate or reduce the potential damage.
 The protection can be provided either by means of a break feature in the wrist socket or by
using sensors to indicate that unusual event has occurred so as to somehow take preventive
action to reduce further overloading of the end effector.

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Power and Signal Transmission
 Pneumatic
 Electric
 Hydraulic
 Mechanical

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Considerations In Gripper Selection and Design:
 The part surface to be grasped must be reachable.
Example: It must not be enclosed with in a chuck or other holding fixtures.
 The size variation of the part must be accounted for, and how this might influence the
accuracy of locating the part.
Example: There might be a problem in placing a rough casting or forging into a chuck for
machining operations.
 The gripper design must accommodate the change in size that occurs between part loading
and unloading.
Example: The part size is reduced in machining and forging operations.
 Consideration must be given to the potential problem of scratching and distorting the part
during gripping, if the part is fragile or has delicate surfaces.
 If there is a choice between two different dimensions on a part, the larger dimension should
be selected for grasping . Holding the part by its larger surface will provide better control
and stability of the part in positioning.

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 Gripper fingers can be designed to conform to the part shape by using resilient pads or self-
aligning fingers. The reason for using self-aligning fingers is to ensure that each finger makes
contact with the part in more than one place. This provides better part control and physical
stability. Use of replaceable fingers will allow for wear and also for interchangeability for
different part models.
A related issue of the problem of determining the magnitude of the grasping force that can be
applied to the object by the gripper.
 The weight of the object
Consideration of whether the grasp can be grasped consistently about its center of mass. If not, an
analysis of the possible moments from off-center grasping should be considered.
 The speed and acceleration with which the robot arm moves (acceleration and deceleration
forces), and the oriental relationship between the direction of movement and the position of
the fingers on the object (whether the movement is parallel or perpendicular to the finger
surface contacting the part).
 Whether physical constriction or friction is used to hold the part.
Coefficient of friction between the object and the gripper fingers.

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What is work cell?
A workcell is an arrangement of resources in a manufacturing environment to improve the
quality, speed and cost of the process. Workcells are designed to improve these by improving
process flow and eliminating waste.
Advantages of work cells include:
 Reduced direct labor cost.
 Heightened sense of employee participation.
 Reduced raw material and finished goods inventory.
 Reduced investment in machinery and equipment.
Disadvantages of work cells include:
 High volume is required because of the large investment needed to setup the process
 There is a lack of flexibility in handling a variety of products or production rates
 Requires good use of group technology and a high level of training and flexibility of
employees
 Either considerable staff support or imaginative employees are needed for the initial
development of the work cells

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Robot Cell Layouts
A robot cell is a complete system that includes the robot, controller, and other
peripherals such as a part positioner and safety environment. Robot cells are sometimes
referred to as workcells.
Robot workcells can be organized into various arrangements or layouts. These layouts
can be classified into three basic types:
 Robot-centred work cell
 In-line robot work cell
 Mobile work cell

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34. ROBOT-CENTRED WORK CELL
Robot is located at the approximate center of the cell
and the equipment is arranged in a partial circle around
it.
 Center of work cell
 High utilization of robot
 Method of work part delivery (eg: conveyor,
part-feeders, pallets)
 Install for single robot servicing 1@more
production machines
Typical situations in which used:
•Machine load and unload
•Machine load
•Machine unload
•Examples: unload die casting machine; wafers (semi-

35. IN-LINE ROBOT WORK CELL
 1 @ more robots located along
in-line conveyor
 Work is organized so each
robot performs assembly
operation on each part (eg:
welding line)
• Robot is located along a moving conveyor and to perform tasks on the product that is coming
on the conveyor. • Many in-line robot work cell involve more than one robot.
One common example is: Car Body assembly.
• Robots are positioned along the assembly line to spot-weld the car body frames
and panels.

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Mobile work
cell

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47. • Modification to other equipment in the cell
• Part position and orientation
• Part identification problem
• Protect of robot from its environment
• Utilities
• Control of work cell
• Safety

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Thanking You