Grippers in Motion - the Fascination of Automated Handling Tasks - Wolf Part 8 pdf

This type of gripping is characterized by the effort to pick the workpiece up without letting the handling system move into a set position for the pick operation.. In addi-tion to using

sliding grip a) external sliding grip b) internal sliding grip c) centering grip

TCP = tool center point

a)

b)

c)

TCP TCP

90°

3

For applications requiring very high cycle times the workpiece is preferably picked up on the fly This type of gripping is characterized

by the effort to pick the workpiece up without letting the handling system move into a set position for the pick operation The type of movement is generated by “overlapping” positions and is possible only if the workpiece has degrees of freedom along the movement direction

As illustrated in figure 3.22, proper gripping strategies can be developed for picking workpieces up safely By means of these strategies the workpiece can be well positioned within the gripper without having to resort to expensive sensor technology In addi-tion to using gripper movements to adjust workpieces for the pick operation, specially selected gripper jaws can help centering the workpiece

This type of pick operation requires the workpiece to be positioned

at a stop ring which supports positioning with the relative move-ment The gripper jaws can be used as stop rings as well

Gripping Accuracy Control

As detailed above precise presentation of the workpiece and accu-rate gripping during pick operations are essential for reliable place operations Any errors in a pick operation can only be compensated

by appropriate gripper or handling system sensors at a later stage With smaller tolerances picking errors can be compensated by feed rails Three reasons for faulty positioning of the workpiece are distinguished:

Figure 3.22 Gripping strategies

142

1 faulty positioning of the workpiece before pick operation

2 faulty positioning of the gripper in relation to the workpiece

(handling system error)

3 workpiece slipping within the gripper at gripper jaw closing or

caused by faulty contact surface combination or gripping

forces

Faulty positioning of the workpiece might be due to faulty

syn-chronization of the gripper in relation to workpiece movement on a

conveyor or workpiece support as described above Other reasons

could be faulty clamping devices or hazardous materials between

clamping device and workpiece

In any case it is important to pay attention to the degrees of

free-dom the workpiece has while being gripped, i e if the workpiece is

still in the preparatory position when the gripper jaws close or if it

is able to move within certain degrees of freedom If the workpiece

cannot be adjusted, faulty positioning of workpieces may cause

premature wear or damage of gripper or handling system in the

long run

The same applies to faulty positioning of the gripper in relation

to the workpiece Integrating a mechanical collision and overload

protection unit between gripper and handling system is one way

to avoid strain or damage (see Chapter 4) This protection measure

can be applied in case of workpiece tolerances leading to bracings

143



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3 In addition to placing them in stacks the workpieces can be placed in rows if the gripper is first turned into a horizontal position by a

rotary unit or by the kinematics Although this principle of stack gripping cannot be compared to the performance of a parallel gripper it does perform much better than a regular single gripper The advantages of performance have been identified for parallel kinematics, also called delta kinematics, in tables 3.22 to 3.26

A higher workpiece weight is calculated for a multiple gripper because the number of workpieces stacked within the gripper increase the overall weight (table 3.22)

Compared to a regular single gripper a stack gripper can be expected to improve performance by nearly 20 percent if four workpieces are stored within the gripper (table 3.23)

The same stack gripper with a storing capacity of eight workpieces will increase performance by about 30 percent (table 3.24)

If the entire stack is placed at once performance can even be raised by 75 percent if the stack gripper holds four workpieces (table 3.25)

Pick- and place performance can be more than doubled by about

116 percent if the stack gripper can store eight workpieces and place them at the same time (table 3.26)

Workpieces stored

within a stack gripper

Pick operation of stack

gripper

150

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systematically avoid potential errors

problem analysis risk analysis

3

3.5 Safe Gripping

Losing our grip on a coffee-cup or accidentally letting a jar of mixed pickles slip from our hands in the supermarket does not do great harm Losing the grip on workpieces during handling, however, may lead to major financial damage For example, a workpiece acciden-tally lost in a processing machine may cause serious mechanical defect after re-start Just imagine a workpiece within the gripper

of a robot rotating with an action radius of three feet at full speed turning into a kind of projectile, even more dangerous at a robot payload up to 1,100 pounds High-grade workpieces require maxi-mum protection against loss or damage, too

Risk of workpiece loss or damage is evaluated with the help of the Failure Mode Effect Analysis (FMEA) which has become part and parcel of a methodical handling task approach Risk evaluation is a future-oriented method for analyzing potential hazards and the prob-ability of such hazards Beyond mere damage repair this method is

a significant step towards far-sighted and safe gripper design and construction

Grafik 3.25 Evaluating risks and eliminating defects

152

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&IGURE RELEASES



Losing our grip on a coffee-cup or accidentally letting a jar of mixed pickles slip from our hands in the supermarket does not great harm Losing the grip on workpieces during handling, ... within the gripper

of a robot rotating with an action radius of three feet at full speed turning into a kind of projectile, even more dangerous at a robot payload up to 1,100 pounds High-grade... addition to placing them in stacks the workpieces can be placed in rows if the gripper is first turned into a horizontal position by a

rotary unit or by the kinematics Although