# Dynamic Mixed Reality Assembly Guidance Using Optical Recognition Methods
## 1 Introduction  
Assembly is considered a key step of the entire fabrication process, on the one hand because it is the costliest phase, accounting for 30-40% of total manufacturing costs on average [1, 2, 3]; on the other, because the efficiency and accuracy in the execution of assembly tasks may affect the final quality of the product, in addition to total production time and cost. Unlike other industrial processes such as cutting, milling and forming, which have been highly automated in the years, assembly is in large part still conducted manually today [4]. Assembly sequences are frequently lengthy and complex, involving many components to be assembled in a particular order and often dealing with fine adjustment operations to ensure an acceptable result that meets project requirements. These factors, together with others related to the assembly system (e.g., workplaces with high noise and poor ergonomics), the product (e.g., many or similar components) and the worker (e.g., skills, training level, and experience) can affect the process and lead to human errors [5]. Assembly errors must be avoided, as they could result in production wastes and defects, with increased time and additional costs to be faced. To this purpose, different methods are used in industry as support and guidance to the human operator. Among others, are training sessions, which can be time-consuming and expensive, especially if realized through the consultation of paper manuals and reference tables that could lead to time inefficiencies and distractions. Moreover, paper-based assembly instructions have become more difficultto provide and locate inside organizations, due to increasing product variants derived from growing demand for manufacturing flexibility [6]. Further, using written instructions, the worker needs to switch the attention between the task being performed and the instructions given, having to remember the information read, with increased cognitive load [7]. Augmented Reality (AR) technology has made great strides over the last decades and the development of AR solutions to support manual assembly has experienced a steady increase [8]. Interactive and real-time assembly instructions can be visualized by the operator for guidance and training, also giving the opportunity to receive feedback on the process, thanks to the capability of AR to overlay virtual information on the real environment [9]. According to recent studies, the use of AR in assembly tasks can reduce execution times and errors, enhancing the correct interpretation of instructions [10], and solving problems related to paper-based information, such as lack of flexibility [7]. The operator does not continuously switch the attention between the information and the assembly, thus enhancing ergonomics [11]. Furthermore, new workers can be trained by AR systems, without sacrificing the time of other employees to train them. Although AR is widely used for this kind of applications, other tools can be implemented among the Extended Reality (XR) ones, which refers to technology-mediated experiences enabled via a wide spectrum of hardware and software, including sensory interfaces, applications, and infrastructures. These immersive technologies are ever more used to assist workers and provide support in enhancing industrial processes such as assembly [12]. In particular, Mixed Reality (MR) is showing similar trends to AR. MR is a combination of a virtual world and the real world, which are merged to construct an enriched environment that the user is allowed to interact with [13]. In industry, the use of MR, mostly implemented by means of eye-wear devices [13], has proven that new workers can be trained more effectively, efficiently, and safely, if compared to traditional methods. Instructions given to workers by animations, text, and audio [14] enhance their skills and knowledge, improving information processing during maintenance operations and increasing recall in short and long-term memory [15].