# Recognition of Manual Welding Positions from Depth Hole Image Remotely Sensed by RGB-D Camera
## 2 Previous Research  
### Shipbuilding Man-Hours and Welding Positions  
The shipbuilding industry's production management plan is based on construction man-hours, as such, estimating the required man-hours can help set the exact construction schedule and reduce costs. Since the beginning of time, there have been ongoing studies to establish an efficient production plan in the shipbuilding industry. Scheduling systems that consider the load of various assembly processes [5] or those that use hierarchical architecture and spatial scheduling [6] have been developed. Furthermore, a system for managing and predicting a shipyard's production plan through simulation has always piqued the interest of researchers [7].  
Shipyards have relied on the observation method to directly measure the working time required for each process or task. Recently, studies have been underway to establish a set of formulas for the whole manufacturing process [8] or to predict the man-hour for each process [9]. Previous studies have emphasized the importance of welding by carefully considering the effects of welding operations when planning production and calculating man-hours. Since the shipbuilding process requires welding in most assembly shops, the man-hour required for the welding operation occupies a large portion. For this reason, related studies have been conducted to obtain the man-hour of welding operation in the assembly process [1,10]. These studies attempted to categorize postures by identifying a difference in man-hours based on working postures. Manual welding, in particular, which plays an important role in the assembly process, demonstrates that the working time varies depending on the welding position.  
Many welding operations in the shipbuilding industry are currently being automated. However, as the assembly process progresses from sub-assembly to grand-assembly, the proportion of manual welding increases because automatic welding becomes difficult to apply due to limited space or complicated structural layouts.  
Manual welding requires various working postures. As a representative example, the American Welding Society (AWS) stipulated four general welding positions as shown in Figure 2: flat, horizontal, vertical, and overhead. Although the above four positions are applied to welding in the shipbuilding assembly process, many different positions are observed depending on the work characteristics.  
In the sub-assembly stage, the flat position ratio is high, while the vertical, horizontal, and overhead positions increase as the process progresses to the grand-assembly stage. In the grand-assembly stage, the proportion of welding posture that puts strain on the welder's muscles increases as the working space narrows and the surrounding structure becomes more complicated. As a result, these welding positions are difficult to maintain for an extended period of time, resulting in decreased work quality or increased man-hours. Furthermore, when working for a long time, a welding position can threaten the safety and health of the worker.  
Since the welding man-hour depends on the welding position, the first step is to recognize which welding position an arbitrary working posture belongs. Then, the man-hour required for block assembly can be accurately calculated. Previous studies have attempted methods such as the observation method [11], rapid upper limb assessment (RULA) [12], and electromyography [13]. However, most of them were limited to the analysis of worker's disease or safety. The purpose of this study is to suggest an automation method that remotely recognizes the working posture of a welder in the actual field in real-time, analyzes the welding position, and links it with man-hours.