# A Robust \(H_{\infty}\) Application for Motor-Link Control Systems of Industrial Manipulators
## 6 Experimental Results  
The validity of the proposed control solution has been finally tested on the real robot.  
The test has been performed in the Grugliasco COMAU plant. The robot has been positioned in such a way to guarantee, for all the experiment duration, an inertia condition equals to 1547 Kgm\({}^{2}\), with its wrist axes in \([0\ 0-90^{\circ}]\) orientation.  
The considered axis of the robot, corresponding to model (12), has been moved in different positions by reproducing a spot-welding trajectory, which is among the most difficult tasks, since it is composed by isolated and short movements. For this reason, robots intended to perform them should be structurally very rigid. They have to stop with small settling time values and without triggering structure vibrations. From a mechanical point of view, the first axis of the kinematic chain should be characterized by a ratio between stiffness and inertial load as high as possible, which would result, from a control point of view, in models having resonant zeros at higher frequency. Figure 18 shows the new N220 Comau robot involved in spot operations.  
Figure 18: Comau robot N220.  
Figure 17: Simulated Real Position Profile (red line); Reference Position Profile (blue line).  
The results reported in Figure 19 confirm the simulation ones: the axis is able to track the entire reference path, without any significant vibration, as it can be evaluated also by the analysis of the motor current profile, reported in Figure 20.  
Relevant for our analysis is the tracking error, computed as the difference between the reference position and the measured one. Figure 21 compares the time-history of the tracking error with the velocity one for the entire robot motion, while Figure 22 provides a zoom in of the results in a shorter time interval. From such figures, it is possible to appreciate how the tracking error behavior reproduces the velocity one very well, thus indicating that the controller is behaving as expected.  
The experimental results also show that the implemented controller is able to counteract possible disturbances deriving from the coupling with the other axes of the mechanical structure.  
Figure 19: Experimental Position Profile (red line); Reference Position Profile (blue line).  
Figure 20: Real Current Profile (blue line) and Reference Current Profile (red line).  
As previously discussed, the robot is not equipped with sensors on the link side. In order to evaluate the link response, it is possible to adopt some software solution able to estimate the link behavior. Figures 23 and 24 show the estimate of the link behavior, obtained by a virtual gyroscope developed by COMAU as a strictly confidential instrument, used to evaluate in an approximate but reliable way the behavior of a link, avoiding the addition of sensors that are not part of the standard equipment.  
The results shown in Figures 23 and 24 confirm that the link is moving in a satisfying way. In particular, Figure 24 shows the absence of any relevant vibration of the link. The actual difference between the velocity profile values and the measured/estimated ones is mainly due to the characteristics of the applied point-to-point motion, which is a very closely spaced. In addition, it is worth remembering that the main objective of the applied nested loops control scheme is the position: a slightly imprecise velocity tracking is considered as absolutely acceptable in practice in point-to-point motions, when corresponding to a satisfying position tracking.  
Figure 21: (**a**) Tracking Error, (**b**) Velocity: reference profile (blue line), measured velocity (red line).  
Figure 22: (**a**) Tracking Error, (**b**) Velocity: reference profile (blue line), measured velocity (red line) in a shorter time interval.