39.4 END OF ARM TOOLING (EOAT)
The best known universal gripper - the human hand
Useful classifications are,
End of arm tooling is typically purchased separately, or custom built.
39.4.1 EOAT Design
Typical factors to be considered are,
pre- and post- processing geometry
potential for part damage
coefficient of friction between gripper and part
maximum accelerations during motion
robot accuracy and repeatability
number of cycles required
use of separate wear components
design for maintainability
cost and ease of fabrication
use of mounting plate on robot
gripper flexible enough to accommodate product design change
The typical design criteria are,
- low weight to allow larger payload, increase accelerations, decrease cycle time
- minimum dimensions set by size of workpiece, and work area clearances
- widest range of parts accommodated using inserts, and adjustable motions
- rigidity to maintain robot accuracy and reduce vibrations
- maximum force applied for safety, and to prevent damage to the work
- power source should be readily available from the robot, or nearby
- maintenance should be easy and fast
- safety dictates that the work shouldn't drop when the power fails
Other advanced design points,
- ensure that part centroid is centered close to the robot to reduce inertial effects. Worst case make sure that it is between the points of contact.
- holding pressures/forces/etc are hard to control, try to hold parts with features or shapes
- compliance can help guide work into out-of-alignment conditions.
- sensors in the EOAT can check for parts not in the gripper, etc.
- the gripper should tolerate variance in work position with part alignment features
- gripper changers can be used to make a robot multifunctional
- multiple EOAT heads allow one robot to perform many different tasks without an EOAT change.
- *** Don't try to mimic human behavior.
- design for quick removal or interchange of tooling by requiring a small number of tools (wrenches, screwdrivers, etc).
- provide dowels, slots, and other features to lead to fast alignment when changing grippers.
- use the same fasteners when possible.
- eliminate sharp corners/edges to reduce wear on hoses, wires, etc.
- allow enough slack and flexibility in cables for full range of motion.
- use lightweight materials, and drill out frames when possible.
- use hard coatings, or hardened inserts to protect soft gripper materials.
- examine alternatives when designing EOAT.
- 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
Movements of the fingers
- pivoting (often uses pivotal linkages)
- linear or translational movement (often uses linear bearings and actuators)
Typical mechanisms
- miscellaneous - eg. bladder, diaphragm
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.
e.g.
Advantages,
- requires only one surface of a part to grasp
- a uniform pressure can be distributed over some area, instead of concentrated on a point
- the gripper is light weight
- many different types of materials can be used
Disadvantages,
- the maximum force is limited by the size of the suction cups
- positioning may be somewhat inaccurate
- time may be needed for the vacuum in the cup to build up
39.4.3 Magnetic Grippers
Can be used with ferrous materials
Electromagnets,
- easy to control, requires a power supply, and a controller
- polarity can be reversed on the magnet when it is put down to reverse residual magnetism
Permanent magnets,
- external power is not required
- a mechanism is required to separate parts from the magnet when releasing
- good for environments that are sensitive to sparks
Advantages,
- variation in part size can be tolerated
- ability to handle metal parts with holes
- requires only one surface for gripping
- can pick up the top sheet from a stack
Disadvantages,
- residual magnetism that remains in the workpiece
39.4.3.1 - Adhesive Grippers
Can handle fabrics and other lightweight materials
These grippers are basically a sticky surface on the end of the robot
As the adhesive gripper is repeatedly used, it loses stickiness, but a tape roll can be used to refresh the sticky surface.
39.4.4 Expanding Grippers
Some parts have hollow cavities that can be used to advantage when grasping.
A bladder can be inserted into a part, and then inflated. This forms a friction seal between the two, and allows manipulation. When done the pressure is released, and the part freed.
Expanding grippers can also be used when gripping externally.
39.4.5 Other Types Of Grippers
Most grippers for manipulation are sold with mounts so that fingers may be removed, and replaced.
Gripper fingers can be designed to reduce problems when grasping.
