# A State Validation System for Augmented Reality Based Maintenance Procedures
## 1 Introduction  
The term augmented reality (AR) comprises all the software and hardware technologies that confer to the ability to experience both the real world and computer's generated contents at the same time. Since artificial and physical objects are mixed together, the user can move in a hybrid space without constraints. AR is part of the mixed reality category, as depicted in the reality-virtuality continuum defined by [PERSON] and [PERSON] [1]. An indisputable advantage that can be attributed to AR compared to virtual reality (VR) is that the user can keep contact with the real world. This advantage provides two kinds of benefits: Firstly, since part of the environment that the user sees is real, it is not necessary to compute a virtual model of it; secondly, since the user's physical point of view is preserved, the physical and mental annoyances that usually occur in a detached full-immersive virtual world are avoided. Overall, with its capability of bridging the gap between real and virtual worlds, AR can be considered the best solution every time it is necessary to represent real and computer-generated elements within the same space. The concept of maintenance relates to the adequate care and actions required to ensure the correct operation of a given equipment. As a consequence of the technological improvement of such equipment, technicians involved in complex maintenance and repair tasks often need to refer to instruction manuals to correctly complete assigned procedures. It is widely recognized that the actionof continuously switching attention between two sources, such as the device under maintenance and the manual, might cause a high cognitive load [2]: This can lead to a higher occurrence of mistakes, thus repair times (and costs) can increase. AR can efficiently tackle this issue as digital contents are overlaid and correctly aligned with respect to the device to be maintained and can be conveyed to technicians while they are performing the procedure. Benefits of AR support in maintenance, repair and assembly tasks have been deeply analyzed in [3] and other researches such as [4], which proved the cost reduction and performance increase that can be achieved through the adoption of AR. Maintenance has been one of the most important domains for AR applications since its dawn [5]: AR applications enable industry technicians to receive visual and audio computer-generated aids while performing different kinds of activities, such as assembling, repairing, or maintenance procedures (Figure 1). Early examples of supporting technicians with AR-based systems, which can be dated back to the 1990's, were all based on special purpose hardware (e.g., Head Mounted Displays) [6; 7]. A set of possible high-impact applications for industrial AR were identified by [PERSON] [8] in 2004. Moreover, the benefits that AR could provide to maintenance, repair and assembly tasks have been thoroughly analyzed by [PERSON] and [PERSON] in [3]. Currently, maintenance, repair and assembly are identified as strategic application fields, since the reduction of associated costs represents a key goal in many domains, thus, any technological advancement is carefully considered in order to take the opportunity to reduce these costs. Moreover, augmented reality is considered one of the nine pillars of technological advancement comprised by the Industry 4.0 paradigm: to provide an exemplification of the impact of such technologies on Industry, it has been evaluated that Industry 4.0 in the next 5-10 years will boost productivity among all German manufacturing sectors by 90 billion euros to 150 billion euros, possibly driving an additional revenue growth of about 30 billion euros a year [9]. A typical maintenance task can benefit from AR since it is possible to provide assistance to the user through digital assets: 3D models, animations, audio tracks and video clips, which are available to the user to better understand how to perform the given task.