## 3 Results
### Industry 4.0 & Key Performance Indicators  
#### 3.2.1.1 Q2: What new Industry 4.0 digital technologies are relevant to assembly operations? How to measure the improvement? How to make the most out of them? [PERSON] et al. investigated in [53] the impact of the 4 th industrial revolution on assembly systems design. The dimensions to consider are six: balancing, sequencing, material feeding, ergonomic risk, equipment selection and learning effect. The evolution of the industrial environment in European countries leads to an aging workforce, re-shoring of production facilities and more efficient and distributed communication networks. In this environment, nine are the enabling technologies of Industry 4.0 that have the most potential to affect assembly systems: big data, IoT, real-time optimization, cloud computing, Cyber Physical Systems, machine learning, Augmented Reality, cobots and additive manufacturing. The integration of these technologies in the design and management of assembly processes leads to what [PERSON] et al. define as \"_AS40_\": Assembly Systems 4.0. The main  
Figure 6: Key aspects of Industry 4.0 technologies for assembly operations and Key Performance Indicators (KPIs), and main conclusions of the analysis. characteristics of _AS40_ are assembly control systems, aided assembly, intelligent storage management, late customization, product and process traceability and self-configured workstation layout [53]. [PERSON] et al. looked into how assembly system configuration would be affected by Industry 4.0 principles, understood as four incremental stages or steps to achieve the 4 th revolution: connectivity, information, knowledge and _smart_, which involves \"predictive and automated decision making processes, with possible self-adjustments and reconfiguration of the production system\". The new paradigm would reduce the costs of assembly automation; reduce setup costs and learning curves; enable the assembly of small quantities of large products in flow lines; enable the assembly of very different products in the same system; better traceability of failures and defects; and smarter material handling. In the last stage of Industry 4.0 (_smart_), assembly systems would be Self-Adapting Smart Systems (SASS), and together with continuous support to operators (OSS), flexibility, agility and productivity would be greatly increased [54]. According to [PERSON] et al. in [7], the main goal of flexible assembly systems in the Industry 4.0 era is to address the mass customization demand paradigm. At this moment, operational, tactical and strategical issues remain unsolved for implementing \"Assembly 4.0\". A key aspect is the social effect of Assembly 4.0: the assembly workforce is expected to shrink--at least, in Western countries-, but additional technological job positions will appear, partially offsetting the operator reduction. The workforce would experiment a net decrease, thus increasing the productivity per employee. Therefore, the role of people in A4.0 will be increasingly important, which calls for future research that considers human operators back at the center of the production systems of the future [7]. When looking ahead in the evolution of assembly systems into the 4 th Industrial revolution, [PERSON] et al. identify challenges when integrating new and existing technologies: uncertainty on the synergies of the I4.0 Key Enabling Technologies; the human-automation collaboration; incorporating Artificial Intelligence into assembly systems; and finding, developing and keeping the Assembly 4.0 human specialists. On top of the technical knowledge, Industry 4.0 operators will need a new set of non-technical skills, so education centers and companies will need to work together to meet this demand [55]. Developing an Assembly 4.0 system in a controlled environment, such as a Learning Factory, allows to better understand the complexity of such a system. The drone factory developed by [PERSON] et al.