# A Practical and Effective Layout for a Safe Human-Robot Collaborative Assembly Task
## 2 Design of Collaborative Layout and Application  
The case study is an assembly cell which receives three parts to be assembled as input and produces an assembly made of the same parts. The parts consist of cubes of different sizes, while the assembly is obtained by sticking the cubes together, as shown in Figure 1. The entire task can be divided in two main operations: Collecting the components to be assembled and assembling them. Concerning the first operation, the components are supposed to be stored so that a robot can easily pick them up. The second operation is supposed to be too complex or too variable for a full automation solution, so that the human presence is required. For this reason, the operator should be able to take the parts to be assembled from a certain area which must be a portion of the robot workspace, since the robot has the role to provide the parts to the operator. With this aim, the workspace of the robot is divided in two zones, as shown in Figure 2. The parts are stored in the \"pick zone\", where no human presence is expected. In the \"collaborative zone\", where the human and robot can potentially come to contact as their workspaces intersect, the operator takes the components to be assembled.  
The human worker and robot perform seven operations in parallel to carry out the assembly task, as reported in Figure 3. The task, which is also identified as a cycle, starts when the operator takes the cube positioned in station A. As the human hand enters in the collaborative zone, the robot task begins by replacing the cube in A with a new one from station a. In parallel, the operator puts the glue on the first cube, takes the second cube by station B, and prepares both cubes to be glued. The next step for the robot is taking a cube from station B and placing it in station B, to replace the one taken by the operator. Meanwhile, the human picks up the last cube in station C and assembles the tower of cubes. Finally, the robot replaces the cube in station C and returns in the home position, while the worker checks the edges of the assembly to remove the exceeding glue before delivering. Concerning the human task, the phases in which he takes the components happen in the collaborative zone, while the other phases are carried out on the assembly table, outside of the robot workspace. On the other hand, the \"pick\" stages of the robot task identify the robot motion in the pick zone, i.e., outside the collaborative zone, while the \"place\" stages identify the robot motion within the collaborative zone. The execution of these operations involves a responsive collaboration. In fact, as the human and robot carry out the subtasks in parallel, the operator can access the collaborative zone at any time, even if the robot is placing a component.  
Figure 1: Assembly process.  
Figure 2: Collaborative assembly cell.  
The laboratory setup of the collaborative assembly cell is visible in Figure 4. Three PCs, two Microsoft Kinect v2, a router, and a collaborative robot UR3 from Universal Robots equipped with a Robotiq gripper 2F-85 are used. The three PCs are connected via Ethernet network and communicate using the TCP/IP protocol. The implemented algorithms are written in MATLAB environment. The two Kinect sensors are displaced at 1.5 m from the human, so that their field of view covers the entire workbench from different angles. Each Kinect can feedback depth data up to 30 Hz [30]. Two PCs are connected to the Microsoft Kinect v2 cameras, to obtain the data acquired by each sensor. The third PC is connected to the controller of the UR3 robot. Further information related to the control strategy will be given in the next section.