## 4 Use Case
### Design  
The aim of this process is to classify the different manufactured parts (Figure 3), to assemble the parts and the covers, and to put them on a tray for inspection and delivery. For this purpose, the parts and the covers are spread in different containers. The operator will prepare the batches according to the manufacturing orders, extracting the parts from the containers, placing them on the trays, and assembling the covers. Productivity and safety were limited by manual processes in the traditional industry. The introduction of automation and intelligent collaborative robots in the industrial manufacturing processes is resulting in a rapid increase in productivity, major material and energy savings, and safer working conditions [42]. Thus, in this use case robots will assist the operator during the tasks, and inspection systems will verify whether parts, covers, and batches are correct. The overall assembly process is composed of the following steps (Figure 4 shows the flowchart of the process):  
1. Batch preparation and individual inspection: 1. Once the operator and the robot are on the assembly table, the system indicates to the operator the batch and the first part to take. 2. Following the instructions, the operator puts the part in the buffer. 3. The robot verifies whether the part is correct with an on-board camera [43]. If the part is correct (type, dimensions, and color), it is picked and placed in the tray. If not, the robot puts away the wrong part. 4. The process is repeated for all the parts of the batch. Related to the wrong part, the program has a list of pending parts; thus as long as a part is still on this list, it is requested again. It will only be deleted if it is seen correctly in the buffer. 2. Covers assembly and batch inspection: 1. When all the positions of the template of the tray are completed, the robot verifies again that all the parts are the required ones and that they are in the right position. 2. The operator puts the covers inside the holes. 3. The robot verifies whether all the covers have been placed. If not, it notifies to the operator that the covers are not ok.
3. Batch distribution: 1. The robot takes the tray and puts it on the conveyor belt for delivery. 2. On the other side of the conveyor, another robot receives the tray with assembled parts. Figure 3: Design of the manufactured parts and the covers for the use case. To carry out this process, two different collaborative robots (cobots) and a conveyor belt have been selected instead of an autonomous mobile platform due to the spatial limitations and in order to save costs. Thus, the following components are necessary:  
* A table for the containers of parts and trays. * The assembly robot. * An assembly table with the containers of covers and discarded parts as well as a buffer where the operator will put the parts which the robot will move to the tray. * A vision system for part location and inspection. * A conveyor belt where the robot will place the tray once completed. * The delivery robot which is waiting the tray at the other side of the conveyor. Figure 5 shows the particularized architecture for the use case process:  
* Control system. The unit controller is a _Raspberry Pi 3 B+_. * Robots and other cell components. Two light weigh collaborative robots with grippers have been selected, the _Omron TM5-900_ and the _Universal Robots UIR5e_. The first one is used in the assembly operation as it has an on-board vision system which is used for the inspection. The tool of this robot is the _Robotiq 2F-140_ gripper. The second robot is used for the delivery of the trays with the _Robotiq Hand-e_ gripper as tool. The conveyor belt is own-manufacture and it is controlled by the _Siemens S7-1200_ PLC. Moreover, it contains presence sensors in order to control the position of the trays. * Safety system. The movements of the operators inside the working area of the robots are monitored by the _Sick microScan 3 Core_ scanner.