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page 669

-the EOAT should be recognized as a potential bottleneck, and given extra design effort.

-use shear pins, and other devices to protect the more expensive components.

-consider dirt, and use sealed bearings where possible.

-move as much weight away from the tip of the gripper towards the robot.

39.4.2 Gripper Mechanisms

A gripper is specifically EOAT that uses a mechanical mechanism and actuator to grasp a part with gripping surfaces (aka fingers)

Quite often gripper mechanisms can be purchases, and customized fingers attached.

Fingers are designed to,

1.Physically mate with the part for a good grip

2.Apply enough force to the part to prevent slipping

nfFg

 

=

coeff. of friction between part and grippe

= wgS

=

number of contacting fingers

 

nf

 

Fg

=

gripper force

 

w =

weight of part

g = gravity

S = factor of safety

(for basic applications 2 to 3 should be the absolute minimum, but high speed applications will require more force to resist inertial forces)

Movements of the fingers

-pivoting (often uses pivotal linkages)

-linear or translational movement (often uses linear bearings and actuators)

Typical mechanisms

-linkage actuation

-gear and rack

-cam

-screw

-rope and pulley

-miscellaneous - eg. bladder, diaphragm

page 670

Two Finger Gripper - as the pneumatic cylinder is actuated, the fingers move together and apart.

Parallel finger actuator - as the cylinder is actuated, the fingers move together and apart in parallel.

Two Fingered Pneumatic Actuated - as the cylinder is actuated, it translates to the fingers opening or closing. The extra links help increase holding force.

Two Finger Internal Gripper - as the cylinder is actuated, the fingers move outward.

page 671

39.4.2.1 - Vacuum grippers

Suction cups can be used to grip large flat surfaces. The cups are,

-typically made of soft rubber or plastic

-typically round, or oval shapes

A piston operated vacuum pump (can give a high vacuum), or a venturi valve (simpler) can be used to generate the vacuum.

The surfaces should be large, smooth, clean.

The force of a suction cup depends on the effective area of the vacuum and the difference in the vacuum, and air pressures.

F = PA

F = maximum gripping force

 

P =

difference between vacuum and air pressure

 

A =

total effective area of the vacuum

• e.g.

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