# Comparing Augmented Reality-Assisted Assembly Functions--A Case Study on Dougong Structure
## 2 Related Work  
[PERSON] and [PERSON] [3] proposed one of the earliest AR-assisted systems for aircraft mainframe assembly where users accessed the CAD data on the shop floor using a head-mounted device (HMD). In this study, it was reported that the effectiveness of the assistance received through AR was reasonably limited. Not only was the user's field of vision restricted, but also the latency of showing images in the HMD was severe. Recently [PERSON] et al. [4] studied AR-assisted assembly of Lego bricks using an electronic projection device containing a camera to display instructional information. Compared to a traditional assembly process using paper-based instructions, the assembly speed increased, and the users expressed a positive response to the system. Similarly, [PERSON] et al. [5] concluded that when an AR-assisted system was used to train novice workers in a product assembly, it took less time and had a lower mental workload, based on workers' responses. In contrast, [PERSON] et al. [6] reported that AR-assisted assembly of 3D puzzles did not show advantages over the use of paper-based guidelines. Their AR system highlighted the required assembly parts in colors for each assembly step. After a part to be assembled is selected, the system displays the 3D model of that part and a set of instructions to assemble it on the screen. The experimental results showed that a reduced assembly time was not statistically significant. A possible reason was that the users were unfamiliar with the system's operation. A prolonged time spent on the identification of parts might have also induced the user's negative response to the system. In summary, previous studies have shown mixed results on AR-assisted functions for manual assembly. [PERSON] et al. [7] evaluated how various forms of instructional information in an AR-assisted system affect the total assembly time and total number of errors made. The errors of the manual assembly were classified into the part orientation and position errors. The results of user tests showed that the visual features used to explain a particular assembly operation must correspond to its relative difficulty level. Two additional error measures will be used to analyze the assembly process in this study. [PERSON] et al. [8] studied the difference between the efficiency of Lego assembly using paper-based descriptions and an AR-assisted system equipped with an HMD and a projector. Figure 1: Dougong architecture. The authors divided the assembly process into four motion-based steps: reach the part, grasp the part, move the part, and position and assemble the part. The experiments proposed by this study adopts the similar idea of decomposing a manual process into operation steps. The experimental results showed that locating assembly positions is slower using HMD compared to the tablet and paper-based instructions. The in situ projection instructions led to lower cognitive load based on the NASA-TLX questionnaire compared to HMD [9]. [PERSON] and [PERSON] [10] applied AR technology to assist the manual assembly of furniture. In their experiment, a computer screen displayed 3D animation of the parts to be assembled in each step. The experimental results showed that the assembly time was reduced. A possible reason was that showing the parts enhanced the user's spatial reasoning compared to the paper-based assembly instructions. [PERSON] et al. [11] compared video and AR assistance in the manual assembly of LEGOTM models. The experimental results showed fewer errors in the AR-assisted assembly; however, the time taken by both methods was of no significant difference. Subjective surveys also indicated that AR assistance has a higher perceived ease of use. [PERSON] and [PERSON] [12] evaluated a prototype AR user interface designed to assist users in the psychomotor phase of procedural tasks. A series of within-subject experiments was conducted to compare the AR prototype with 3D graphics-based instructions.