## 4 Features and Issues of Existing Applications
### Data Collection  
The physical assets from which existing applications collect digital data are summarized in panel (a) of Figure 9, where a single application can collect data from two or more kinds of assets. First, human workers are important physical assets in all of the 25 existing applications, which is quite natural since the scope of our survey is limited to DT applications that utilize digital data collected from human workers. Figure 7: Tasks for human workers of existing applications. In addition, nine applications collected digital data from industrial robots [4; 8; 28; 29; 30; 35; 37; 38; 39]. To be precise, these applications targeted HRC environments where human workers and robots share workspaces. Human workers have the dexterity required to perform delicate tasks, while robots are powerful, fast, and accurate, so they help achieve higher productivity for simple and repetitive tasks. HRC combines these advantages of human workers and robots, an essential part of a smart factory [35; 54; 53]. Nevertheless, the coexistence of human workers and robots in a single workspace can cause safety issues, such as collisions between them. Therefore, HRC can be a promising application area for human-focused DT. Some researchers considered other types of physical assets for their applications. For instance, [PERSON] et al. [49] and [PERSON] et al. [36] thought of facilities as physical assets for their applications. Materials and tools can also be used as physical assets for human-focused DT applications for OSH [14; 27; 35]. Panel (b) of Figure 9 shows the devices used to collect digital data in existing applications, where wearables and inertial measurement unit (IMU)-based motion capture (MoCap) systems are the most frequently used devices. Wearables are small electronic devices that can be attached to the human body or clothing and typically enable the detection of signals from the human body and provide connectivity to other ICT devices [59]. IMU is a specific sort of wearable device that can measure the kinematic parameters of a human body, such as the external load and joint angles. IMU sensors, as well as wearables, can hinder human workers from performing their job since these sensors must be attached to the human body or clothing. Nevertheless, IMUs have been widely adopted to measure factors related to the posture and motion of humans, due to their accuracy and reliability [60; 61]. Depth cameras and Microsoft Kinect are also popular data collection devices for human-focused DT for OSH [4; 14; 29; 30; 32; 34; 35; 37; 38]. Depth images obtained using depth cameras contain 3D information about how far an object is from the camera. Kinect is a well-known commercial device that provides a depth camera and related software libraries. Figure 8: Experimental environments of existing applications. Figure 9: Data collection procedure of existing applications. A 2D depth image can be converted into a 3D point cloud, a set of points scattered in 3D space, or a 3D skeleton that consists of significant joints and connection lines between them, by applying appropriate algorithms. Consequently, a depth camera and Kinect enable the existence, location, and posture of scene objects, including human workers and robots, to be detected. In recent years, depth cameras and Kinect have emerged as valuable data collection tools for ergonomic assessment and healthcare [62; 63]. Two research papers utilized optical MoCap systems [15; 39]. Typically, optical MoCap systems are used to collect data about the posture of humans using several cameras, which can detect the markers attached to the human body or clothing. A tiny piece of retroreflective material or light emitting diode can be used as a marker, and a marker for the optical MoCap system represents the position of a specific body segment. Therefore, an optical MoCap system can be used to collect data required to create a 3D skeleton and to measure joint angles. However, the markers or marker-attached body suits can be cumbersome to human workers.