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Jack H.Integration and automation of manufacturing systems.2001.pdf
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page 340

-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.

11.1.2 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

page 341

-screw

-rope and pulley

-miscellaneous - eg. bladder, diaphragm

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.

page 342

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.

11.1.2.1 - 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

page 343

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.

We have a suction cup gripper with two 5”diameter cups that is to be used to lift 1/4” steel plates cut to 2’ by 3’. How much vacuum pressure must be applied to just hold the plates? Suggest a realistic value.

w =

0.28

 

lb

1

 

 

 

36( in) = 60.48( lb)

 

------

--( in) × 24( in) ×

 

 

 

 

 

in

3

4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A =

2

 

 

 

5 2

 

 

2

) ]

 

2

 

 

 

 

 

 

π

--

 

 

= 2[ 19.63( in

 

= 39.26( in )

 

 

 

 

 

 

2

 

 

 

 

 

 

 

 

Minimum to hold plate

 

w

 

 

 

60.48( lb)

 

1.54( psi)

 

P =

---

=

-------------------------

=

 

 

A

39.26( in2)

 

 

 

 

 

 

 

 

 

 

 

Assume low moving speed for robot, use factor of safety = 2.0

PS = 2.0 × P = 3.08( psi)

Note: this is much less than atmospheric pressure (15 psi), therefore it is realistic.