Robot End Effector

ROBOT END EFFECTORSBY

R. C. SAINI

www.rcsaini.blogspot.com

TYPES OF END EFFECTORS

1. Grippers– Used to grasp and hold objects.– e.g. loading and unloading, etc

2. Tools– End effectors designed to perform work on the

part rather that to merely grasp it.– e.g. spot welding, spray painting, etc.

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MECHANICA GRIPPERS

• A mechanical gripper is an end effector that uses mechanical fingers actuate by a mechanism to grasp an object.

• The function of the gripper mechanism is to translate some form of power input into the grasping action of the finger against the part.


MECHANICA GRIPPERS• Two ways of

constraining the part in the gripper– Physical Constriction

of the part within the fingers.

– Friction b/w the fingers and the work-part.


TYPES OF GRIPPER MECHANISM

• As per actuating mechanisms :-1. Pivoting Movement.2. Linear or Translational Movement.

• As per types of kinematic device:-1. Linkage actuation2. Gear and rack actuation3. Cam actuation4. Screw actuation5. Rope and pulley actuation6. Miscellaneous


Some Linkages for Robot Grippers


Gear & Rack Method of Actuating the Gripper.


Cam-Actuated Gripper


Screw-Type Gripper Actuation


GRIPPER FORCE ANALYSIS

E.g.1) Suppose the gripper is a simple pivot-type device used for holding the cardboard carton, as fig. The gripper force Fg= 60lb. The gripper is to be actuated by a piston device to apply an actuating force Fa. The corresponding lever arms for the two forces are shown in fig.

Ans) Moments about the pivot arms be summed and made equal to zero.

FgLg - FaLa = 0(60lb)(12in) – (Fa)(3in) = 0

Fa = 720/3 = 240 lb10www.rcsaini.blogspot.com

GRIPPER FORCE ANALYSISE.g.2) The fig. shows the linkage mechanism and

dimensions of a gripper used to handle a workpart for a machining operation. Suppose it has been determine that the gripper force is to be 25 lb. What is required is to compute the actuating force to deliver this force of 25 lb.


GRIPPER FORCE ANALYSIS

Ans2) Due to symmetry of the gripper consider only one half of the mechanism.

Moments :-25(4cos 15°) = F sin 45°(1.5cos15°)

+ Fcos45°(1.5sin15°)

96.6 = F(1.0246+0.2745) =1.2991F

F = 74.4 lb

The actuation force applied to the plunger to deliver this force of 74.4 lb to each finger.

So, Fa = 2 X 74.4 X cos45° Fa = 105.2 lb


OTHER TYPES OF GRIPPERS

1. Vacuum Cups2. Magnetic Grippers3. Adhesive Grippers4. Hooks, Scoops and other miscellaneous

devices


OTHER TYPES OF GRIPPERSVacuum cups, also called suction cups•Requirements on the objects to be handled are that they be flat, smooth and clean, to form satisfactory vacuum.•Gripper made of some elastic material i.e. rubber, plastic , etc.

Venturi Device used to Operate a Suction Cup


OTHER TYPES OF GRIPPERSMAGNETIC GRIPPERS

Stripper Device operated by sir cylinder used with a permanent Magnet Gripper


OTHER TYPES OF GRIPPERS• Adhesive Grippers

– Adhesive substance performs the grasping action– e.g. Handle fabric and other lightweight materials.

• Hooks, Scoops and other miscellaneous devices

Expansion Bladder used to grasp inside of a Cup-Shaped Container


TOOLS AS END EFFECTORS• In most of the robot application in which a tool is manipulated, the

tool is attached directly to the robot wrist. In these cases the tool is the end effector.

• E.g. :-– Spot-welding tools– Arc-welding tools– Spray painting tools– Rotating spindles for operation such as:

• Drilling• Routing• Wire brushing• Grinding

– Liquid cement application for assembly– Heating torches– Water jet cutting tool


ROBOT/END EFFECTOR INTERFACE

• Important aspect– Physical support during the work cycle– Power to actuate must be supplied through the

interface– Control signal to actuate– Feedback signal– High reliability of the interface– Protection against the environment– Overload protection in case of disturbance during the

work cycle



Physical support during the work cycle• Achieved by the mechanical connection between the

end effector and robot wrist.• Ideally, three characteristics :-

1. Strength2. Compliance3. Overload Protection



Physical support during the work cycle1. Strength

– Its ability to withstand the forces associated with the operation of the end effector.

2. Compliance– Its refers to the wrist socket’s ability to yield elastically when

subjected to a force.– In effect, it is opposite of rigidity.– Remote Center Compliance (RCC) devices have been designed to

provide high lateral compliance for centering the object relative to the hole.



Physical support during the work cycle3. Overload Protection

• When some unexpected event happens to the end effector such as a part becoming stuck in a die or a tool caught in a moving conveyor. Whatever the cause, the consequences involve possible damage to the end effector or maybe even the robot itself.

• To eliminate this potential damage– Use Breakaway Feature in the wrist socket.– Sensor to indicate, so somehow take preventive

action to reduce further overloading.



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

Pneumatic Power Using Shop Air Pressure


GRIPPER SELECTION AND DESIGN• Factor that should be consider in assessing gripping

requirements :-1. The part surface to be grasped must be reachable.2. The size variation of the part must be accounted.3. The gripper design must accommodate the change in size the

occurs between part loading and unloading.4. Consideration must be given to the potential problem of

scratching and distorting the part during gripping, if the part is fragile or delicate surface.

5. If there is a choice b/w two different dimensions on a part, the larger dimension should be selected for gripping.

6. Gripper fingers can be deigned to conform to the part shape by using resilient pads or self-aligning fingers.


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THANK YOU

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Robot End Effector