## 2 Related Work and Challenges in Cooperative Manufacturing  
The challenges and the problems in the cooperative workcell are a broad subject of research, which can be seen from different angles. However, the most significant challenges are shown in Figure 2 and can be summarized as the following:  
* ABB-IRB-120. The system does not need to modify the standard robot hardware by adding any external sensors. Instead, it uses a virtual 3D representation of the cooperative workcell, to automatically control the robot speed based on the worker distance from it. The research in [9] is extensively discussing the different methodologies that can be used to grantee the physical and psychological safety in Human-Robot Interaction (HRI). The research in [10] introduces a neural network solution which dynamically compensates the robot path planning due to the changes in the work environment. * Worker Activity Recognition: worker activity recognition is a very essential requirement to achieve the cooperative manufacturing, this is because it is the very first step to physically communicate with the cobot. The recognition could be for acquiring the worker status (i.e., busy or free), or  
Figure 2: Challenges in Cooperative Manufacturing. for interacting with cobot during the manufacturing, for example to start or stop a cooperative task. The recognition of the worker activity can be also based on his natural movements during the manufacturing. For instance, picking and placing activity could be an indication that the worker is busy during that time, while holding a specific tool could express which operation is performed by the worker at the moment. The research in [11] proposes the use of the worker hand gestures to communicate with the cobot to add more flexibility to the manufacturing operations. Two modes of gesture recognition were defined due to this work. The first mode is an explicit gesture recognition, where a specific gesture which is not in the context of the manufacturing is detected. In other words, this mode of cooperation occurs when the worker is commanding the cobot directly via a set of a predefined gestures. The second mode is an implicit gesture recognition, where a specific gesture which is in the context of the manufacturing is detected, thus the cobot will perform a subsequent action. The research in [12] offers in detail a wide range of hand gestures which can be used by the worker during the cooperation with the cobot. * Cooperative Workcell Integration in the Industrial Enterprise: The workcell is the smallest building block of the industrial enterprise, therefore it is important to define a method which can connect all the cooperative workcells horizontally over the shop floor, and vertically over the different layers of the enterprise. The research in [13] proposes the idea of linking the concept of holonic manufacturing architecture to the international society of automation (ISA)-95 standard to obtain this integration. On the horizontal level, the research proposes a supervisor holon which is providing coordination services when it is needed to cooperate outside the boundaries of the cooperative workcell. On the vertical level, the research defines the responsibilities of various types of holons and distributes them over the different layers of the industrial enterprise. * Cooperative Workcell Self-organization: a form of self-organization must exist in the cooperative workcell to fulfill the purpose of the cooperative manufacturing. Self-organization is an adaptation mechanism which is used by the cooperative workcell to overcome the variations in the manufacturing environment. The articles in [13; 14; 15] mention four important sources of variations that can be found in the cooperative workcell. The first source comes from the variation in the production requirements such as the production customization and volume. The second source of variation is due to the change in the time which is taken from the worker to accomplish a specific task.