Visual servoing control for contour following

To support the scanning procedure a visual servoing control able to move the robot along the deburring path, keeping the camera (and the laser) at a fixed distance from the wheel and centered on the edge, is required (Fig. 1).

At each camera frame, ten points are measured on the deburring edge by way of a set of edge detectors, and the least square fitting line is computed. To ensure robustness the 3-parameters line equation and a method to automatically discard outliers are used.

The orientation of this line and the distance from the camera optical center are fed into a visual servoing loop that controls the motion of the robot. In particular, this control acts on the absolute velocity of the camera frame v_cx, v_cy, v_cz and ω_cz to keep the optical axis on the edge and one of the axis of the image frame parallel to the contour. Further, the velocity along the optical axis is used to keep the desired distance between the image plane and the wheel surface. To accomplish such a decomposition of the robot motion in the xy-plane and along the optical axis, the partitioned visual servoing control methodology has been selected.

Preliminary analysis of the deburring profile have been performed, exploiting the control system just described to drive the robot along the path (Fig. 2).

Figure 2 - Six frames extracted from an edge analysis

The results of one of these experiments are reported in Figs. 3-5. In particular, Figs. 3 show the x, y-errors on the image plane (the components of the vector e) and the orientation error. Note that these errors are very small (apart from the initial transient that represents the approach to the contour).

On the other hand, Figs. 4 show the control variables.

Finally, Fig. 5 shows the reconstructed deburring profile.

Figure 3 - The visual servoing errors in the image plane

Figure 4 - The control actions generated by the visual servoing laws

Figure 5 - The deburring proflle

Walk-through programming in contact

An hybrid admittance-force control scheme, that allows the walk-through programming in free motion and in contact with and hard surface, has been realised and tested in simulation.

The behavior of the control system is as follows: during the walk-through programming in free motion the robot in controlled by way of the admittance filters, when a contact is detected the surface normal is immediately estimated and the control along the normal direction is switched to force control (in order to ensure the contact at a desired force value).