# A Practical and Effective Layout for a Safe Human-Robot Collaborative Assembly Task
## 1 Introduction  
In the last years, collaborative robotics is becoming a more and more relevant topic. Historically, to ensure safety, the workers have been kept physically separated from the robot systems [1]. Nowadays, the basic idea of collaborative robotics is to ensure safe robotic systems to allow the removal of the fences that divide the working area of the robots from that of human operators, so that robots and humans can work together to accomplish a task. The introduction of a new class of robot designed to work side-by-side with humans, named \"cobots\", increases the chances for a successful collaboration [2, 3]. The existing cobots are mostly anthropomorphic manipulators, which are equipped with passive and active safety features. The passive features are design choices that intrinsically reduce the risk of injury in the case of undesired contact, such as lightweight materials and rounded edges. The most advanced cobots mount also a set of sensors and dedicated software packages to actively handle unexpected contacts by stopping the robot if the joint torques or the contact forces exceed the defined thresholds. However, these features, that come with the cobot out of the box, may not guarantee safe collaborative robotics. In fact, the workspace sharing can introduce new hazards and risks for the workers that must be taken into account when designing the application [4, 5]. The design of the collaborative robotics systems is principally related to the task. In fact, according to the International Federation of Robotics (IFR), one may require a different level of interaction depending on the human and robot activities [6]. The most common examples of collaborative robotics provide a shared workspace where the robot and employee work sequentially. This type of application is also identified as sequential collaboration. On the other hand, the most technically challenging application requires aresponsive collaboration, which means that the human and robot work at the same time in the shared workspace and they have to coordinate to accomplish the task safely [7]. Regardless of the level of collaboration, if the human and robot workspaces intersect, the industrial application must be designed according to the related standards [8]. In particular, two different approaches can be implemented to guarantee safety: The power and force limiting (PFL) and the speed and separation monitoring (SSM). The PFL combines an accurate risk assessment with risk reduction measures so that the contact between the operator and the robot does not result in harm to the operator. Therefore, in addition to the passive safety design methods, one can actively control the energy of the moving parts of the robot by limiting forces, torques, velocities or momentum within a given range. In contrast, the SSM does not specify limitation to the robotic system as long as it maintains a protective separation distance from the operator. In practice, if the operator is within the robot workspace, the SSM translates into online robot controlling to generate alternative collision-free paths. In this case, the use of human tracking systems is needed for the minimum distance monitoring and collision avoidance algorithms shall be implemented to update in real-time the robot trajectory. Concerning the tracking device, wearable systems and 3D vision systems are the best alternatives [9]. The first one would limit the human operator's movements with suits and sensors, while the use of cameras does not require wearing dedicated components. On the other hand, the data obtained from vision systems are sensitive to problems such as occlusions of the sensor and variable light conditions [10]. The signals coming from sensors are used to apply control strategies on the robot. Two basic approaches are possible: \"Global\" (or \"long-range\") and \"local\" (or \"short-range\") methods [11; 12]. In global methods, a complete knowledge of the workspace is required.