Buffer

The buffer is used to temporarily store poles that cannot yet form part of an assembly until those poles can be used at a later stage. Poles being stored in the buffe r are i n an unriveted state.

The following points have been considered for the design of the buffer:

 Since a SCARA robot would typically be used to interact with the buffer, only four degrees of freedom should be required to retrieve poles or place poles into the buffer.

 It should be possible to pick up any pole at any time in any order, i.e. pole s should not block access to other poles, e.g. should not be stacked on top of one another, unless they are identical and can thus be used interchangeably.

 Since poles have common trip connectors protruding upward (as was shown in Figure 10), not even poles of the same kind can be stacked on top of one another.

 Poles should be placed repeatable and should not move in case the buffer gets bumped into.

 The buffer itself should ideally not consist of any actuators or controllers to make retrieval of poles possible.

 People should be able to easily insert or remove poles by hand in case of manual override mode or when the system has to be cleared after a restart.

It would have been possible to utilize the existing hardware present in the lab to form a buffer: Unused poles could have been stored on empty pallets which would then be sent

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to the pallet magazine until poles on those pallets would be required at a later stage . However, the logistical, practical and economic aspects of this approach we re far from desirable and other buffer concepts were therefore considered.

The concept that was chosen is shown in Figure 18. It was designed such that all poles lie on the same plane, to simplify calibration, which in turn simplifies reconfigurations. When all fixture positions are lying on the same plane, it further simplifies the task of choosing a reference point and expressing the coordinates of the individual fixtures relative to that reference point.

Figure 18 Exploded view of buffer

As shown in the figure above, the buffer provides space to store up to 18 si ngl e -pole s. For stacking, the robot needs to grip the poles from above which is why the buffe r has been designed such that poles can be removed and inserted from above. The top -most layer contains several holes which have the shape of the outline of the circuit bre ake rs, so that breakers are restricted from moving horizontally. These holes have chamfered edges to aid the robot placing the poles. The top-most plate is product-specific as the holes are custom-made to match the outline of the poles to be stored. The rest of the buffer is product-independent. Therefore, when the shape of a circuit breaker mode l i s modified, then only the topmost plate of the buffer needs to change to accommodate poles with a different shape. The rest of the buffer hardware does not need to be changed. When loaded into the buffer, poles will be resting on the middl e pl ate which has holes big enough for the gripper fingers to fit through in a fully opened and fully closed state. The entire buffer rests on raisers so that the tips of the gripper fingers would not collide with the table while picking up a pole.

The advantages of the chosen concept are:

 Poles can be placed in repeatable positions.  Poles of various thicknesses can be stored.

 Poles will not move in case that the buffer is pushed.

Product-specific constraining plate

Chamfered perimeter to aid insertion of poles

Plate on which poles rest Re-usable for various products Raisers to prevent collisions

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 Storing all poles horizontally, which is the same orientation that they are transported on the pallets, allows them to be handled by 4-DOF robots.

 The buffer was inexpensive and easy to manufacture.

 No complications occurred while installing the buffer and after removal, the buffer could be repositioned at its previous location without requiring re - calibration.

 Choosing a reference point on the buffer for calibration purposes, and calibrating the buffer's workspace coordinates was done without problems. The disadvantages of the chosen concept are:

 The contours of the top-most layer are dependent on the outer geometry of the poles that have to be stored. This means that only one type of circuit breaker can be stored in the buffer.

 A relatively large footprint was claimed by the buffer when installed horizontally. A vertical design would occupy a fraction of the floor space.

3.5 Conclusion

A background on the case study was given in this chapter, along with the design specifications for the stacking and buffering station. The mechanical design aspects of the transportation system, fixtures, grippers and buffer was then described. Chapte r 4 provides the details of how control software for the station controller was chosen.

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