# Vision-Based Path Guidance to Achieve Dies-Free Roller Hemming Process
## 2 Experimental Apparatus and Methodology
### Hand-Eye Sensor Module  
It is well known that if the rollers can correctly follow the curve of the work piece and keep good and stable contact with the sheet metal surface during the hemming process, most of the defects caused by improper force can be effectively eliminated or at least minimized. For this reason, it is very important to eliminate position deviation that is introduced and caused by manufacturing tolerances, the sheet metal deflection deformation, and the assembling tolerances of the working platform, especially when there is no special die to offer data for aligning the parts in the system. In order to overcome the above-mentioned problems, the compensation of the hemming path must depend on reconstruction and analysis of the three-dimensional (3D) data after the work piece is positioned. Obtaining reconstruction data and coordinate accuracy will affect the final product quality, mainly in the rolled-over edges, i.e., the roll-in, roll-out, warp, and recoil by the rollers. Therefore, we proposed and designed a hand-eye sensor module for roller hemming to achieve the purpose of reconstructing the 3D shape data of the incoming materials.  
In 2013, [PERSON] [12] designed a new portable vision-based measurement system for the hemming process. It was demonstrated that quick and accurate analysis of the geometry and deformation of hemmed samples can be achieved. Following [PERSON]'s original design, an enhanced vision measurement module, namely the quick-change sensor module as shown in Figure 3, was proposed and constructed. This quick-change sensor module can be easily installed onto and released from the robotic arm through the quick-change interface. Once it is installed on to the robotic arm, this sensor module can be used to scan over time to determine the position and posture of the sheet metal work piece. The sensor module consists of a laser emitter and a CMOS image receiver. The laser emitter projects a line pattern onto object surface, from which the reflecting light of the projected pattern on the contour of work piece can then be captured by the CMOS image receiver to determine and estimate coordinates of the projected line. A series of coordinates are acquired by simultaneously scanning over the hemming path, and thus forming the 3D model of the metal sheet around the hemming path.  
The quick-change sensor module has a built-in battery to avoid the process of module restart and reinitializing due to lack of power each time it is disconnected and reconnected to the robot arm. The quick-change interface as illustrated in Figure 4 not only serves as the interface for the sensor module but also for the hemming tool. It is divided into two parts, namely the main plate and the tool plate. The main plate is installed on the flange  
Figure 2: The hemming tool: (**a**) the traditional single roller; (**b**) an active double-roller with different shapes of upper rollers.  
surface of the robot arm, and the tool plate is installed on the top of the sensor module and the hemming tool. With this quick-change interface, the robot arm can quickly replace the hemming tool or sensing module within 5 s through the combination and separation function of the main board and the tool board.