## 2 Materials and Methods
### RTMN 2.0--Extension of RTMN
#### 2.2.4 The HRC Model  
The authors have developed an HRC model that covers the following aspects of HRC: HRC task types, HRC modes, combining the HRC task types and HRC modes, workspace, and decision making. The authors proposed hardware and software solutions for each HRC task type. The authors address every aspect one by one in the following sections. Combining Collaboration Task Types and HRC Modes  
Based on the literature ([10; 11; 12; 13; 14; 15; 16; 25; 26; 27; 28; 29; 30]), the authors consolidated the different classifications of the HRC task types and HRC modes. In the literature, five HRC modes (fence, SMS, HG, SSM, and PFL) and five HRC task types (cell, coexistence, synchronized, cooperation, and collaboration) are commonly defined. The HRC modes can ensure the safety of people even when unexpected human intrusions occur. However, the HRC task types do not by definition ensure the safety of people. This classification focuses on how closely humans and robots work together, and only the people planned for the task are considered. In reality, no matter how the authors define the task type, humans can act differently than expected, and human intrusions can occur. Therefore, to ensure safety, each task type was required to have a minimum safety mode enabled. For this reason, the authors present a  
Figure 2: RTMN modeling elements—Part B: HRC tasks. new classification that integrates the HRC task types, HRC modes, and safety levels. Safety level 0 indicates no risk to the human from the robot. It has the lowest safety requirements, as the robot is within a physical fence. The safety levels go up to level 4, which requires the highest safety requirements not only from hardware but also from software. The authors also distinguish the collaboration levels (0-5): the higher, the more collaboration between human and robot. For each task, the authors have defined general safety requirements. A solution design for the hardware and software level is proposed for each HRC task type. This makes it easier for process engineers to ensure safety when implementing HRC processes. The overall approach is presented in Figure 5.  
Figure 4: RTMN 2.0 sequence flow connection rules. Figure 3: RTMN modeling elements—Part C: other notations. In our new classification, we have five categories: coexistence fence (CF), sequential cooperation SMS (SS), teaching HG (TH), parallel cooperation SSM (PS), and collaboration PFL (CP). The \"cell\" of the human robot task type refers to the robot working autonomously, surrounded by physical fences without human involvement, so we only needed to model the robot tasks and no human tasks need to be modeled. In this case, we do not need to create a separate HR task type for it, as it has only robot tasks. We added a new task type, \"Teaching\", which is when a human takes control of the robot and demonstrates what the robot should do, then the robot learns and repeats it. The reason for adding it is to cover this missing collaboration task type and then to be able to map it to the collaboration mode HG. Coexistence Fence (CF)  
This type (Figure 6) combines the task type \"coexistence\" and the collaboration mode \"fence\". This task has the lowest HRC level 0 and should activate at least safety level 0 (fence) in the background. We recommend using an industrial robot, since humans and robots do not work together in the same workplace. The fence for the robot can be in the form of a physical barrier or certified hardware such as a light curtain/2D scanner. As long as fence mode is insured, this task is safe. No additional software functionalities are needed. Sequential Cooperation SMS (SS)  
This type (Figure 7) combines the task type \"synchronized\" and collaboration mode \"safety-rated monitored stop\". \"safety-rated monitored stop\" means \"condition where the robot is stopped with drive power active, while a monitoring system with a specified sufficient safety performance ensures that the robot does not move\", as described in [54].