Challenge 4 of the CoLLaborate project involves the assembly of aeronautical structures by riveting, performed by Romaero S.A. The selection of the use case was performed taking into consideration the following aspects:
- The use case relies on the strong human – machine collaboration following the implementation of the robot in the production/assembly sequence
- The industrial activity consists of the manufacturing/assembly of a product with an aeronautical application
- The limitations of the existing collaborative robots is taken into account, in terms of dimensions and maximum lifting capability or exerted force.
The activity for this industrial use case is the assembly of the float of the CL-415 aircraft by riveting. The riveting process involves a truly collaborative sequence between two human operators. Replacing one of the operators with an industrial robot is challenging, but extremely useful for the project’s needs. The riveting process consists of placing a large number of permanent rivets for connecting all the components.
This operation can be further divided into a series of smaller steps:
- Step 2.1: a series of holes are performed on the to-be-assembled structure. After executing a given number of holes, extra temporary fasteners are added to the structure to increase its stability, to maintain the right position and to ensure that no relative movement occurs between the materials, as this would lead to misalignment of the holes.
- Step 2.2: Cleaning the holes – removing the metallic debris resulting in the drilling process
- Step 2.3: Placing the sealant in each hole, before inserting the rivet
- Step 2.4: Performing the permanent riveting
- Step 2.5: Removing the temporary fasteners and replacing them with permanent rivets In this process the position of the float is changed multiple times so that the workers have access to different regions of the float; the float is rotated around its axis so that it facilitates access.
The process to be addressed in this project is the actual riveting – Step 2.4 mentioned above.
Riveting requires a compressive force to be exerted on the rivet, from two sides simultaneously: an active force on the rivet head and a passive, counteracting force, on the rivet pin. The regions where this counteractive force needs to be applied are difficult to reach. As a consequence, the workers that perform this job need to have strong, long arms and be able to work for long periods of time in uncomfortable positions. In addition, they are subjected to vibrations induced by the pneumatic hammer which can be damaging, especially if the exposure times are long.
Following the addition of the sealant in the holes to be riveted, the actual riveting process begins. This process contains a sequence of operations that facilitate the collaborative work of the two technicians (although there is no visual contact between the two) for the matching of the positions of the riveting hammer and of the handheld bucking bar. This is a type of sensorial communication between the two workers which, most likely, will need to be reproduced by the robot. As the robot will replace only one of the technicians, the behavior of the machine will need to closely follow the human behavior so that the remaining human technician can perform his activity in a condition as similar as possible to the current condition, when no robot is in the loop.