# Trends of Human-Robot Collaboration in Industry Contexts: Handover, Learning, and Metrics
## 2 Human-Robot Collaboration Concepts
### Safety in HRC  
One major concern regarding the application of human robot collaboration in industrial contexts is the fundamental problem of safety for both human operators and robots. Safety plays an essential role that should never be forgotten when studying human-robot co-working: every coexistence between a human and a machine entails a prior human safety. In fact, some standard ISO's have been established (ISO 10218-1 and 10218-2) to ensure the operator safety when working with collaborative robots ([3]). These authors detail several safety measures when presenting the design of some cobots (collaborative robots), inferring that a robot must satisfy at least one of the four criteria presented and deeply described in [5], as follows: (i) _safety-rated monitored stop_--the cobot completely stops if an operator enters the workspace, (ii) _hand guiding_--the cobot is compliant and moved manually by the human, (iii) _speed and separation monitoring_--the closer a human operator becomes the slower the cobot moves, and/or (iv) _power and force limiting_--the cobot is programmed to operate only within tolerable levels of force and torque. These four levels of safety strategies, derived from ISO 10218-1 and 10218-2 standards are also explored in [4].  
The norms ISO 10218-1 and 10218-2 define several levels of safety strategies. The first two levels, with simpler definitions, level 1 and 2, as illustrated in Figure 4 are named \"Safety-rated monitored stop\" and \"Hand guiding\". The other two levels, which encompass more complex requirements on the robot side, are level 3 and 4, named \"Speed and separation monitoring\" and \"Power and force limiting\" respectively, are illustrated in Figure 5.  
Safety emerges as a still open research field, despite the numerous studies that already had tackled this particular issue. We refer the reader to the following reviews on the requirements and challenges of safety in HRC [38; 39; 40; 41; 42; 43; 5]. Some reported studies approach the safety problem by assuming the human operators and the robots share the same workspace but perform different tasks. In these cases, the robot should be able to track the motion of the human operator to estimate the distance between them and, thereafter, avoid any collision. For example, in order to avoid possible collisions, before any movement, a robot should plan its own trajectory and verify if it is all clear. Keeping a predefined distance between the robot and the human is a safety measurement that will, in all motions, interfere with the robot's path planning. Different types of sensors and several strategies have been adopted to avoid potential collisions by jointly considering aspects of human monitoring and motion planning [44; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54; 55; 56; 57; 58]. When it comes to physical human-robot interaction (pHRI), ref. [12] presents an approach to safe oriented robot control. The proposed application is based on an elaboration of physical collision detection and robot reaction by using a collision avoidance methodology. In certain safe coexistence conditions, the human has the possibility to activate or deactivate, using gestures, this safe physical collaboration approach.