## Materials and Methods  
### Assembly Task and Measurement System  
The human-robot collaborative assembly task used in this study was the desktop version of the common building blocks task. The robot used was a 6-degrees-of-freedom collaborative robot UR3 produced by Universal Robots (Odense, Denmark). The UR3 can work as the second member of a two-person team, which conforms with the situations to be explored in this study. In addition, the robot is equipped with 15 adjustable safety tools including power sensing, which enables the UR3 to maintain a high degree of safety during operation or when working in close proximity with the operator. The workbench layout is as displayed in Figure 1. The operator sat in front of the workbench. The point 12 cm from the leading edge of the workbench was the assembly point. Moreover, a parts bin was constructed 38 cm to the right using scattered orange blocks. The appearance of the blocks and their manner of assembly were similar to those of Lego blocks; thus, the assembly task involved only alignment and downward force once the blocks had been removed from the parts bin. Another parts bin containing assembled blocks was placed 38 cm to the left of the operator, and the point 34 cm in front of the assembly point was the handover point of the UR3 robot.  
The experimental operation is illustrated in Figure 2. Before the experiment was started, the operator sat in front of the workbench and placed both hands at the assembly point of the workbench. When the operation was started, the operator used the right hand to grab an orange block from the parts bin and passed the block to the left hand, which fixed the block on the assembly point. Subsequently,  
Figure 1: Workbench layout (**left**) and assembled blocks (**right**).  
the operator's right hand reached to the parts bin again to pick up a second orange block and return to the assembly point. The second block was assembled onto the first block. A similar procedure was applied for the third block. After assembling three orange blocks, the operator's right hand extended from the assembly point to the handover point, grabbed a green block being handed over by the robotic gripper, and returned to the assembly point, where the green block was assembled onto the semi-finished product of orange blocks. Upon completion of the assembly task, the finished product was placed in the parts bin on the left side. A completely assembled product is displayed on the right side in Figure 1.  
This study used PhaseSpace motion capture to capture motion data from the experiment. To measure the task operation of the operator, one light-emitting diode (LED) spotlight of the PhaseSpace was fixed to the joint of the operator's middle finger on the right hand, and another spotlight was fixed to the tool center point of the robot's outer gripper; the sampling frequency was set to 240 Hz. During the experiment, the spatial position and time information of the two LEDs were collected using Vizard Virtual Reality Software's built-in real-time Python application programming interface for capturing PhaseSpace data, and the handover time point was calculated according to the handover prediction model being tested in that experiment. Subsequently, the gripper of the UR3 robot was moved to the handover point according to the calculation result transmitted by the network; this completed the handover action. The robot returned to its initial state after completing the handover task and was manually loaded with a new assembly block by an assistant for the next assembly cycle. Vizard also recorded the position and speed of the LEDs on the UR3 gripper and the back of the operator's right hand. The data were stored in an Excel file at a recording frequency of 60 Hz.  
Figure 2: Experimental workflow diagram with an inset figure showing a close-up actual human–robot physical interaction.