Understanding Assembly Process Quality through Product Complexity and Cognitive Demand: Preliminary Experimental Evidence

This paper investigates how psychophysiological and attentional demands vary across the different phases of manual assembly processes with different levels of product complexity. Within the Industry 5.0 perspective, understanding when operators experience higher mental demand is essential for improving assembly process quality in terms of design, instructions, training, and human- centered support systems to avoid errors and defects. An experimental study was conducted on two assembly processes: a mechanical equipment and a tile cutter, representing lower and higher complexity products, respectively. Participants performed the assemblies while multimodal data were collected through eye-tracking, electrodermal activity (EDA) biosensor, and functional near-infrared spectroscopy (fNIRS). Each assembly process was decomposed into macro-phases, and psychophysiological metrics were analyzed separately for each one. Results showed that the simpler product exhibited limited differences among macro-phases. In contrast, the more complex product revealed clearer heterogeneity, with some phases emerging as attentional and cognitive bottlenecks, particularly through eye-tracking measures. These findings confirmed that assembly complexity is not uniformly distributed across process steps and demonstrate the value of a multimodal, phase-based analysis for identifying critical moments where targeted support interventions, such as improved instructions, cognitive assistance, or collaborative technologies, may be most beneficial for process quality improvement.