# Sensor-Guided Assembly of Segmented Structures with Industrial Robots
## 6 Experimental Results
### Panel Placement
#### 6.3.2 Placement with PBVS and Compliance Control  
After the panel is transported to the assembly nest, the panel will first be moved to about 5 cm above the nest based on the pose from the reference marker fixed on the assembly nest or the previously placed panel. Then, PBVS is implemented to match the \(3\times 8\) panel marker pose with respect to the reference marker pose, which is calibrated by placing the panel in the desired location manually.  
We used a motion capture system from PhaseSpace for accuracy verification. In the motion capture system, eight cameras are deployed around the testbed, which are able  
Figure 8: (**a**) Panel placement without compliance control risks high contact force (robot moves in the z direction for placing the panel in the assembly nest). (**b**) With compliance control, the placement force is regulated to the specified 200 N.  
Figure 7: Transport path generated by using the Resolved Motion method in Section 3.1 with user selected waypoints. (**a**) The user specifies a series of waypoints; from (**b**–**f**), the robot follows through the path generated by the input waypoints.  
to detect the 3D positions of the LEDs mounted on each panel. The resolution from this motion capture system is approximately 0.1 mm at a rate of 220 Hz [41].  
PBVS and compliance force control are used together for placement control as in Figure 3. In order to achieve sub-millimeter accuracy, the error gradually converges until the detected error is smaller than 1 mm. The PBVS gain is tuned for fast and smooth convergence. The motion and force convergence during placement is shown in Figure 9. The contact force in the \(z\)-direction converges to about \(-200\) N, and both the position and orientation errors converge to an acceptable level. In Figure 10, the second panel is assembled with respect to the first panel that was previously placed. From the view of the gripper camera, one set of \(4\times 4\) markers is firmly fixed to the placed panel (top), and another set of \(3\times 8\) markers is attached to the panel that is moved simultaneously with the gripper and therefore remains static in the image (bottom). The desired marker pose is obtained in advance. During the PBVS placement, the robot is guided to match the current reference marker estimated from the panel marker to the desired relative marker pose.  
We executed the placement process seven times for the two panels and report the planar error results in Figure 11 from the motion capture system. Our main focus is on the position error in the \((x,y)\) axes. The \(z\) direction is controlled by the contact force against the assembly nest. The results of the first panel are all within 1 mm with the mean error \((-0.24,0.48)\) mm and standard deviation \((0.5,0.45)\) mm. The placement of the second panel shows one run violating the 1 mm accuracy requirement in the \(x\)-axis. This is likely due to camera calibration error. The mean error is \((0.78,0.52)\) mm with standard deviation \((0.31,0.18)\) mm.  
Figure 9: Under force and vision feedback, the placement force converges to the specified 200 N to ensure secure seating in the nest, and the position and orientation alignment is achieved tow within 1 mm and 0.1 degree, respectively.